huntington’s disease: from pathology and genetics to ... · 192 s. imarisio and others initiator...

Biochem. J. (2008) 412, 191–209 (Printed in Great Britain) doi:10.1042/BJ20071619 191

REVIEW ARTICLEHuntington’s disease: from pathology and genetics to potential therapiesSara IMARISIO*†, Jenny CARMICHAEL*†, Viktor KOROLCHUK*, Chien-Wen CHEN*†, Shinji SAIKI*, Claudia ROSE*,Gauri KRISHNA*†, Janet E. DAVIES*, Evangelia TTOFI*, Benjamin R. UNDERWOOD* and David C. RUBINSZTEIN*1

*Department of Medical Genetics, University of Cambridge, Cambridge Institute for Medical Research, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 0XY, U.K., and †Departmentof Genetics, University of Cambridge, Cambridge CB2 3EH, U.K.

Huntington’s disease (HD) is a devastating autosomal dominantneurodegenerative disease caused by a CAG trinucleotide repeatexpansion encoding an abnormally long polyglutamine tract inthe huntingtin protein. Much has been learnt since the mutationwas identified in 1993. We review the functions of wild-typehuntingtin. Mutant huntingtin may cause toxicity via a range ofdifferent mechanisms. The primary consequence of the mutationis to confer a toxic gain of function on the mutant proteinand this may be modified by certain normal activities that areimpaired by the mutation. It is likely that the toxicity of mutanthuntingtin is revealed after a series of cleavage events leadingto the production of N-terminal huntingtin fragment(s) containingthe expanded polyglutamine tract. Although aggregation of themutant protein is a hallmark of the disease, the role of aggregationis complex and the arguments for protective roles of inclusions

are discussed. Mutant huntingtin may mediate some of its toxicityin the nucleus by perturbing specific transcriptional pathways.HD may also inhibit mitochondrial function and proteasomeactivity. Importantly, not all of the effects of mutant huntingtinmay be cell-autonomous, and it is possible that abnormalitiesin neighbouring neurons and glia may also have an impact onconnected cells. It is likely that there is still much to learn aboutmutant huntingtin toxicity, and important insights have alreadycome and may still come from chemical and genetic screens.Importantly, basic biological studies in HD have led to numerouspotential therapeutic strategies.

Key words: huntingtin, Huntington’s disease, neurodegenerativedisease, polyglutamine tract.

INTRODUCTION

Huntington’s disease (HD) is a devastating autosomal dominantneurodegenerative disorder named after George Huntington, whoprovided a classic account of the condition in 1872 in The Medicaland Surgical Reporter [1]. However, the first definite descriptionof HD by Charles Oscar Waters in 1841 provides a lucid picture ofone of its main clinical features, chorea, and its hereditary nature[2]: “It consists essentially in a spasmodic action of all thevoluntary muscles of the system, of involuntary and more or lessirregular motions of the extremities, face and trunk . . . The diseaseis markedly hereditary . . . The first indications of its appearanceare spasmodic twitching of the extremities, generally of the fingerswhich gradually extend and involve all the involuntary muscles.This derangement of muscular action is by no means uniform;in some cases it exists to a greater, in others to a lesser, extent,but in all cases gradually induces a state of more or less perfectdementia. When speaking of the manifestly hereditary nature ofthe disease, I should perhaps have remarked that I have neverknown a case of it to occur in a patient, one or both of whoseancestors were not, within the third generation at farthest, thesubject of this distressing malady . . . ”

Although Waters stated that “the singular disease rarely, veryrarely indeed, makes its appearance before adult life, and attacks

after the age of 45 are also very rare”, this reflects the peak of theincidence distribution, since HD can present at any age.

The pathology of HD reveals striking neurodegeneration inthe corpus striatum and shrinkage of the brain. These featureswere initially described by Meynert (1877) [3] and Jelgersma(1907) [4]. The obvious loss of the caudate and putamen (corpusstriatum) has led to the widely held belief that these neurons aremost vulnerable to the mutation and also that loss of these specificneuronal populations can account for the motor, psychiatric andcognitive features of disease. More recent studies suggest thatthere is also widespread cortical loss/dysfunction in early HD [5].This raises the possibility that some of the features of HD may bedriven by cortical dysfunction and the speculation that some ofthe striatal loss may be a secondary consequence of perturbationsto cortico–striatal pathways.

GENETICS OF HD

The gene responsible for HD (HTT) was discovered in 1993and encodes a 350 kDa ubiquitously expressed protein calledhuntingtin [6]. The causative mutation is an abnormal expansionof a tract of uninterrupted CAG trinucleotide repeats withinthe coding sequence of the gene, 17 codons downstream of the

Abbreviations used: BDNF, brain-derived neurotrophic factor; CREB, cAMP-response-element-binding protein; CBP, CREB-binding protein; EGFP,enhanced green fluorescent protein; ER, endoplasmic reticulum; GIT1, GPCR (G-protein-coupled receptor) kinase-interacting protein 1; GLT-1, glutamatetransporter-1; Hap, huntingtin-associated protein; HD, Huntington’s disease; HDAC, histone deacetylase; HEAT, huntingtin, elongation factor 3, thePR65/A subunit of protein phosphatase 2A and the lipid kinase Tor; Hip, huntingtin-interacting protein; hsp, heat-shock protein; LMP, low-molecular-masspolypeptide; MnSOD, manganese superoxide dismutase; MSN, medium spiny neuron; NES, nuclear export signal; NLS, nuclear localization signal;NMDA, N-methyl-D-aspartate; NRSE, neuron-restrictive silencer element; NRSF, neuron-restrictive silencer factor; OGG1, 7,8-dihydro-8-oxoguanine-DNAglycosylase; 8-OHdG, 8-hydroxy-2′-deoxyguanosine; PGC-1α, peroxisome-proliferator-activated receptor γ co-activator-1α; PSD, postsynaptic density;REST, repressor element-1 silencing transcription factor; ROS, reactive oxygen species; SCA, spinocerebellar ataxia; SH3, Src homology 3; siRNA, shortinterfering RNA; Sp1, specificity protein 1; TAFII, TBP-associated factor; TBP, TATA-box-binding protein; TF, transcription factor; UPS, ubiquitin–proteasomesystem.

1 To whom correspondence should be addressed (email [email protected]).

c© The Authors Journal compilation c© 2008 Biochemical Society

192 S. Imarisio and others

initiator ATG codon in exon 1. CAG is a codon for glutamine,and the mutation leads to an abnormally expanded polyglutaminetract in huntingtin [6]. There are now nine diseases that are knownto be caused by expanded CAG-encoded polyglutamine tracts,including many of the dominant SCAs (spinocerebellar ataxias):SCA1, 2, 3, 6, 7 and 17.

In normal individuals, the number of CAG repeats is 35 orfewer, with 17–20 repeats found most commonly [7]. Repeatsbetween 27 and 35 are rare and are not associated with disease,but are meiotically unstable and can expand into the disease rangeof 36 and above, when transmitted through the paternal line. Mostadult-onset cases have 40–50 CAGs, whereas expansions of 50and more repeats generally cause the juvenile form of the disease.Incomplete penetrance has been observed in individuals with 36–41 repeats, but the estimates of penetrance for this group areimprecise [8,9].

There is a strong inverse relationship between the age of onset ofHD and the number of CAG repeats. Longer repeats are correlatedwith an earlier age of onset [10]. However, there is a wide variationin the age of onset with a given CAG repeat number, and the CAGrepeat number itself has poor predictive power on the age of onsetfor any given individual. Only approx. 70% of the variance in theage of onset of HD can be accounted for by the number of CAGrepeats. The residual variance is represented by other modifyinggenes and environmental factors [11–18].

Many trinucleotide-repeat disorders, including HD, are charac-terized by the phenomenon of anticipation, where the age ofonset decreases and the disease severity increases in successivegenerations. This phenomenon can be explained by meioticinstability (which increases the number of CAG repeats) thatappears to be greater in spermatogenesis than oogenesis; anticip-ation is mainly observed when the mutation is inherited throughthe paternal line [19–21].

In contrast with some of the inherited dominant ataxias wherethe clinical course is more severe in homozygotes [22], HD waspreviously believed to be one of the rare genetic diseases whichdemonstrated ‘complete dominance’, i.e. heterozygotes were asbadly affected as homozygotes. However, more recent clinical andmolecular studies have suggested that, although homozygosity forthe HD mutation does not influence the age of onset of symptoms,homozygosity is associated with a more aggressive disease course[23,24].

GENETIC MECHANISM OF ACTION OF THE CAG MUTATION

Genetic data in humans and transgenic animal models suggest thatpolyglutamine mutations confer a deleterious gain-of-functionon the target proteins [10,25,26]. HD is an autosomal dominantcondition: one mutated gene is sufficient to cause the disease, inspite of the presence of a normal gene inherited from the otherparent. In humans, loss of one of the two HTT genes occurs inWolf–Hirschorn syndrome as a result of a terminal deletion ofone chromosome 4, involving the loss of one HTT gene [27], andhas also occurred with a balanced translocation with a breakpointbetween exons 40 and 41 which physically disrupts the HTT gene[28]. Hemizygous inactivation of huntingtin does not cause anabnormal HD-like phenotype. In addition, mice with only onefunctioning Htt gene do not show features of the disease [29–31].

Gain-of-function is also suggested by studies showing that theexpanded CAG repeat is toxic itself. Expression of expandedpolyglutamine peptides alone in Drosophila models has beenshown to cause neurodegeneration [32]. Ordway et al. [33]created a mouse model where a 146 CAG repeat sequence wasinserted into the hypoxanthine phosphoribosyltransferase (Hprt)gene, which is not involved in any CAG-repeat disorders, and

inactivation of the Hprt gene alone does not have any deleteriouseffects in mice. These mutant mice produced a polyglutamine-expanded form of the hypoxanthine phosphoribosyltransferaseprotein and developed a late-onset neurological phenotype thatprogressed to premature death [33]. Transgenic overexpressionof polyglutamine expansions, either in the context of the full-length huntingtin protein or only exon 1 of Htt, also produceneurodegeneration in mice and Drosophila [34]. Although theprimary consequence of the HD mutation is to confer gain-of-function, this does not preclude the possibility that disease severitymay be modified by certain loss-of-function effects [35].

HUNTINGTIN

Wild-type huntingtin is found mostly in the cytoplasm, althougha small proportion of the protein is intranuclear [37]. The proteinis known to be associated with the plasma membrane, endocytic(both clathrin-coated and non-coated) and autophagic vesicles,endosomal compartments, the ER (endoplasmic reticulum), theGolgi apparatus, mitochondria and microtubules [37–42].

Although the polyglutamine repeat in huntingtin has receivedattention for its pathogenic properties when expanded, it ispossibly not essential for normal function [43–45]. Anotherfeature of huntingtin protein structure is the presence of multipleHEAT (huntingtin, elongation factor 3, the PR65/A subunitof protein phosphatase 2A and the lipid kinase Tor) repeatsequences; 28–36 of these motifs are predicted to be distributedalong the entire length of the huntingtin protein [46,47] (Figure 1).A HEAT repeat is a degenerate ∼50-amino-acid sequencecomprising two anti-parallel α-helices forming a hairpin. HEATmotifs are usually involved in protein–protein interactions andare found in proteins that often play roles in intracellulartransport (including nucleocytoplasmic shuttling), microtubuledynamics and chromosome segregation. These proteins are alsocharacterized by high helical content (>50%) and frequentlyform superhelical structures with continuous hydrophobic cores[48–50]. Indeed, characterization of full-length huntingtin by bio-physical methods suggests that the protein is an elongated super-helical solenoid with a diameter of ∼200 Å (1 Å = 0.1 nm) [46].

It is unknown whether huntingtin contains NLSs (nuclear loc-alization signals). However, a conserved NES (nuclear export sig-nal) is found near its C-terminus [51]. In addition, the N-terminal17-amino-acid sequence of huntingtin has been suggested toact as a NES owing to its binding to the nuclear exporter Tpr(translocated promoter region). Expansion of the polyglutaminerepeat interferes with this interaction causing accumulation ofmutant huntingtin in the nucleus [51,52]. The 17-amino-acid N-terminal stretch of huntingtin has been recognized as playingan important role, together with a cluster of the first threeHEAT repeats flanked by positively charged regions (amino acidsresidues 172–372), in targeting huntingtin to various intracellularmembrane-bound organelles, including the plasma membrane,mitochondria, endosomal/autophagic vesicles, the Golgiapparatus and the ER [38,41,42]. Finally, several lysine residueswithin the same 17-amino-acid sequence immediately beforethe polyglutamine repeat, appear to compete for SUMOylationand ubiquitination, post-translational modifications that couldregulate the half-life, localization and nuclear export of wild-type huntingtin, as well as modifying the toxicity of the mutantprotein [53–55].

Cys214 of huntingtin is palmitoylated by Hip (huntingtin-interacting protein) 14, a palmitoyltransferase that regulatestrafficking and function of huntingtin as well as several otherneuronal proteins [56,57]. Palmitoylation could potentially playa role in the development of pathology as huntingtin with an


Huntington’s disease 193

Figure 1 Huntingtin and its normal cellular roles

(A) Linear structure of the huntingtin molecule. The locations of the main huntingtin polypeptide sequence features are shown, including the polyglutamine (polyQ) and polyproline (polyP) sequences,NES and clusters of HEAT motifs (blue bars). Sites of post-translational modifications such as ubiquitination, SUMOylation, palmitoylation, phosphorylation and cleavage by proteases are alsoshown. (B) Probable three-dimensional structure of huntingtin as an elongated superhelical solenoid containing multiple HEAT repeats. The structure has been modelled on another HEAT repeatprotein that has a molecular mass similar to that of huntingtin [47]. (C) Proposed cellular functions of wild-type huntingtin. Relevant interacting partners of huntingtin are shown in yellow. See thetext for details.

expanded polyglutamine repeat is a much poorer Hip14 substratecompared with wild-type protein [57].

Both wild-type and mutant huntingtin are cleaved by variousintracellular proteases, including caspases 1, 3, 6, 7 and 8, calpainand an unidentified aspartic protease [58–62]. Although theimportance of huntingtin proteolysis for its physiological functionremains to be elucidated, the role of mutant huntingtin cleavagein the progress of the disease is well established, as the full-lengthmutant protein is less toxic than its N-terminal fragments [63,64](see below).

HUNTINGTIN, A PLAYER OF MANY GAMES

Despite substantial efforts directed towards understandinghuntingtin function during last 14 years, its normal cellular roles

remain poorly defined. This is predominantly due to the large sizeof the protein that makes isolation and analysis difficult, the lack ofobvious homology with other proteins, ubiquitous localizationwithin the cell and promiscuous interactions with more than 200partners identified to date [43,44,65,66].

We will briefly describe several of the most well-studiedcellular roles attributed to huntingtin (also see Figure 1). Althoughthese diverse functions are currently considered to be relativelyindependent of each other, this view may change with an advance-ment of our knowledge about this interesting protein. Already,huntingtin is beginning to emerge as a scaffold protein orchestrat-ing converging intracellular trafficking and signalling pathways.

Huntingtin is essential for normal embryonic development,as the loss of protein causes increased apoptosis and disruptedtransport of maternal nutrients into the fetus, leading to lethality of



mouse embryos around day 8.5 [29–31]. Knockdown of hunting-tin expression in zebrafish embryos also produces a variety ofdevelopmental defects, including disruption of iron homoeostasis[67]. In addition, the protein is required in adult neurons and testisfor cellular viability [68]. The high levels of cell death in HTT-knockout animals suggests that the protein may have an anti-apoptotic role [31]. This idea is supported by the observationsthat overexpression of wild-type huntingtin protects againstvarious apoptotic insults including those caused by starvation,mitochondrial toxins and overexpression of mutant huntingtinwith an expanded polyglutamine repeat [69–71]. One possiblemolecular explanation for the anti-apoptotic capability of wild-type huntingtin is that it binds and sequesters the pro-apoptoticprotein Hip1 that together with HIPPI (Hip1 protein interactor)can activate pro-caspase 8 [72]. Also, huntingtin may also inhibitcaspase 3 directly [73].

Huntingtin is involved in transcription regulation by interactingwith an array of transcriptional factors and other proteins involvedin the regulation of mRNA production [43,44,65,66]. Huntingtinhas also been shown to interact with tryptophan (WW) domain-containing proteins implicated in non-receptor signalling andpre-mRNA splicing [74]. On the basis of this, and by analogywith other HEAT domain-containing proteins (e.g. importins)that interact with transcriptional regulatory proteins and facilitatetheir transport between cytoplasm and nucleus, huntingtin has aproposed role in nucleocytoplasmic shuttling of transcriptionalregulators and mRNA [47,51,75]. However, this function of wild-type huntingtin remains largely speculative, as most research hasfocused so far on the perturbations of transcriptional activityby mutant huntingtin. The most well-established example ofhuntingtin functioning as a transcriptional regulator is its rolein the production of BDNF (brain-derived neurotrophic factor),which does not require the nuclear translocation of huntingtin andis performed in the cytosol. In this case, huntingtin binds and se-questers REST (repressor element-1 silencing transcriptionfactor)/NRSF (neuron-restrictive silencer factor), a transcrip-tion factor that binds to NRSE (neuron-restrictive silencerelement), an upstream DNA element found in ∼2000 genesincluding BDNF. Thus huntingtin acts as a positive transcriptionalregulator of NRSE-regulated genes such as BDNF [76].

The role of huntingtin in vesicle trafficking was originallyproposed on the basis of its localization to endocytic/endosomalvesicles in axons and synaptic terminals and from its interactionwith a number of endocytic/trafficking proteins, including α-adaptin, Hip1, Hip14, Hap (huntingtin-associated protein) 1,Hap40, PACSIN1 (protein kinase C and casein kinase substrate inneurons-1) and SH3GL3 [SH3 (Src homology 3)-domain Grb2-like 3] (endophilin 3) (reviewed in [43,44,66,77]). Recently, thislist has been extended to bona fide endocytic proteins, such asclathrin and dynamin [65]. However, the role of huntingtin inendocytosis, despite being proposed by many, remains elusive, asthe effect of huntingtin knockdown on the endocytosis of plasmamembrane receptors has yet to be shown.

In contrast, the function of huntingtin as a facilitator of long- andshort-range transport along microtubules is documented in mam-malian cells, Drosophila and mouse models [39,78–81]. Hunting-tin interacts directly, as well as via its binding partner Hap1,with the dynein/dynactin microtubule-based motor complexresponsible for retrograde cellular trafficking. In addition, Hap1binds another molecular motor, kinesin, and thus could play arole (independently or as part of a complex with huntingtin) inanterograde axonal transport. Huntingtin in complex with anotherpartner Hap40 was shown to be important for movement of Rab5-positive endosomes along microtubules. As a result, knockdownof huntingtin inhibits movement of vesicles and mitochondria

along neuronal projections [39,78–82]. Importantly, in additionto its role in the transcriptional regulation of BDNF, huntingtin isessential for efficient axonal transport of vesicles containing thispro-survival factor and thus controls neurotrophic support andendurance of neuronal cells [78].

As discussed above, huntingtin is highly expressed presynap-tically, where it interacts with many proteins involved in synapticvesicle exocytosis and recycling [43,44,66,77]. However, with theexception of the above examples, the role of huntingtin itself inthese processes remains to be established. Huntingtin also seemsto be important for normal synaptic transmission as part of the pro-tein machinery localized to PSD (postsynaptic density), an elec-tron-dense dendritic part of the synapse. Here, huntingtin interactsdirectly with the SH3 domain of a key regulator of postsynapticactivity, PSD-95, which in turn forms complexes with NMDA (N-methyl-D-aspartate) and kainate receptors belonging to the familyof ionotropic glutamate receptors [83]. Huntingtin was shown tonegatively regulate the activity of glutamate receptors [84]. AsPSD-95 is also involved in relaying the signal from glutamatereceptors to proteins such as SynGAP (synaptic GTPase-activat-ing protein), huntingtin could potentially also modulate this signaltransduction thus regulating synaptic plasticity [84,85]. Finally,huntingtin has been implicated in mGluR1 (metabotropic gluta-mate receptor 1) signalling via its interaction with optineurin [86].

MUTANT HUNTINGTIN AND HD PATHOGENESIS

Huntingtin cleavage: a probable rate-limiting step

There is now strong support for the idea that mutant huntingtincleavage resulting in an N-terminal fragment containing thepolyglutamine expansion is a key step in pathogenesis (Figure 2).N-terminal mutant huntingtin fragments are sufficient to produceHD-like abnormal clinical syndromes in model animals [87–89]and intranuclear inclusions [88,89]. Mutant huntingtin may becleaved into a repertoire of different fragments by different pro-teases, including caspases, calpains and an as yet uncharacterizedaspartic endopeptidase. Two cleavage sites at residues 513 and552 are susceptible to caspase 3, producing N-terminal fragmentsof polyglutamine huntingtin of approx. 70 and 75 kDa respect-ively [90]. The cleavage site at residue 552 is also susceptibleto caspase 2 [62]. A slightly larger peptide fragment, 80 kDa insize, is produced by caspase 6 cleavage at residue 586. The pro-teolysis and subsequent toxicity of the mutant protein can bemodified (usually suppressed) as a result of phosphorylationof huntingtin by protein kinases, including Akt, Cdk5 (cyclin-dependent kinase 5) and ERK1 (extracellular-signal-regulatedkinase 1) [91–93]. Importantly, recent data strongly suggest thatinhibition of caspase 6 cleavage of mutant huntingtin rescuesboth the behavioural and neuropathological HD phenotype inmice expressing full-length mutant Htt transgenes [64]. Thusthe cleavage events may be rate-limiting steps in pathogenesisallowing conversion of comparatively non-toxic or benign full-length mutant huntingtin into toxic fragments.

In addition to caspases, calpains can also cleave huntingtin. Themost N-terminal calpain cleavage site is at residue 536, whichwould lead to the formation of a 72 kDa N-terminal fragmentof huntingtin as an intermediate product. This fragment maybe cleaved further to generate a 47 kDa product, which is smallenough in size to shuttle in and out of the nucleus [59]. Huntingtinhas also been reported to be a substrate for unidentified asparticendopeptidases [61]. This protease generates smaller N-terminalfragments of huntingtin compared with caspases and calpains andmay be a crucial factor for the formation of intranuclear inclusions[61,94].



Figure 2 Mutant huntingtin induces many different toxic pathways, someof which may interlink

For example, mutant huntingtin can be cleaved by calpains, which may result in toxic fragmentproduction. These toxic fragments may induce excitotoxicity, which will increase intracytosoliccalcium levels, which will increase calpain activity and result in further toxic fragment production.Cdk5, cyclin-dependent kinase 5; htt, huntingtin; Qn, polyglutamine.

Huntingtin aggregation and its relationship with toxicity

The formation of neuronal intranuclear and intracytoplasmicinclusions of mutant huntingtin are pathological hallmarks of HD[95], and aggregates are a feature of all the known polyglutaminediseases. There has been considerable debate whether these repre-sent toxic or protective species, or epiphenomena. In humanbrains, the density of inclusions in the cortex correlates with repeatlength [95,96], consistent with in vitro data. However, there is littlecorrelation between inclusion burden and the areas of the brainmost affected in HD [97,98].

In mammalian cell culture systems, there is a strong correlationbetween aggregate formation and cellular toxicity, and celldeath follows the formation of aggregates in many cases [99–101]. In HD mice expressing mutant Htt exon 1, intranuclearneuronal inclusions were detected before or near the onsetof behavioural changes [87,102]. Overexpression of molecularchaperone(s) including hsps (heat-shock proteins) 70, 40, 104and the chaperonin TRiC (tail-less complex polypeptide 1 ringcomplex) reduced both aggregation and cell death in HD cellularand/or mouse models [100,103–113]. However, these chaperonesmay be reducing the number of large inclusions by preventingoligomer formation, and it may be the oligomeric precursors thatare the most toxic species.

Certain studies have reported a dissociation between aggregateformation and toxicity. When R6/1 HD exon 1 mice were crossedwith tissue transglutaminase-knockout mice, this resulted inpartial rescue of the brain and body weight loss and early mortalityof the phenotype, but an increase in intranuclear inclusions [114].

Overexpression of CA150, a transcription factor, rescued neuronaltoxicity, while it increased neuritic aggregation without reducingnuclear inclusions [115]. Furthermore, promotion of inclusionformation with a small molecule in a cell culture model ofHD rescued huntingtin-mediated proteasome dysfunction [116].The most striking dissociation between aggregate formation andtoxicity was the demonstration that cells that formed huntingtininclusions had an improved survival compared with those thatdid not form inclusions [117]. This suggests that cells with largeinclusions are less compromised than cells with diffuse mutanthuntingtin. However, the study did not compare the toxicityin cells with aggregates with wild-type cells. Also, cells withdiffuse huntingtin are likely to contain oligomeric structures. Sucholigomeric forms may be highly reactive because of their largersurface area to volume ratios, compared with large inclusions, andthis may correlate with toxicity. However, in vivo, certain largeinclusions may exert toxic effects if they block neuronal processes,as this may impair anterograde and retrograde transport.

PERTURBATION OF TRANSCRIPTION AS A POSSIBLE DISEASEMECHANISM

The idea that the nucleus may be an important site for huntingtintoxicity was suggested by studies proposing that the full-length wild-type protein was mainly localized to the cytosol,whereas the cleaved mutated molecule redistributed to the nuclearcompartment [118]. Furthermore, nuclear localization of mutanthuntingtin appeared to enhance its toxicity both in cell cultureand in mice [119–121]. A number of transcriptional regulatorscontain glutamine-rich activating domains, important for the inter-action between transcription factors and transcription regulators.This led to the possibility that proteins carrying polyglutaminestretches could associate with transcription factors, leading totranscriptional alterations [122,123].

CBP [CREB (cAMP-response-element-binding protein)-bind-ing protein] and Sp1 (specificity protein 1) have been identifiedas two major transcriptional regulators affected by polyglutamineproteins. CBP is an important transcription co-activator and isa major mediator of survival signals in neurons. It has histoneacetyltransferase activity, which is important for allowing tran-scription factors access to DNA. The C-terminal glutamine-rich domain of CBP can mediate its interaction with mutatedhuntingtin. The interaction causes cellular toxicity and CBPrelocalization from the nucleus into huntingtin aggregates [124–126]. Interestingly, further studies showed that CBP–huntingtinbinding is primarily mediated by CBP’s acetyltransferase domainand that the interaction depends on huntingtin’s polyglutaminetract and proline-rich region. Moreover, huntingtin can bind toother proteins with acetyltransferase domains, modulating theiractivity [127].

The connection between histone acetylation and neurodegen-eration led to further investigations testing the potential of HDAC(histone deacetylase) inhibitors for therapy. In a Drosophilapolyglutamine model, treatment with butyrate or SAHA(suberoylanilide hydroxamic acid) rescued neurodegeneration[127]. Similar results obtained in yeast and in cell culture providedevidence for the role of histone acetylation in neurodegeneration[128,129]. Further studies have reported beneficial effects ofHDAC inhibitors in HD mouse models [130,131].

Sp1 is a sequence-specific transcription activator which binds toCG-rich regions of DNA. It contains a glutamine-rich activationdomain, through which it binds to and regulates molecules ofthe transcriptional machinery, such as TF (transcription factor)IID, a multiprotein complex composed of TBP (TATA-box-binding protein) and multiple TAFIIs (TBP-associated factors).



Moreover, Sp1 interacts with different molecules of the TFIIDcomplex, particularly binding TAFII130 through the glutamine-rich domain, supporting the idea that the glutamine interface playsa fundamental role in recruiting components of the transcrip-tional machinery and subsequently RNA polymerase II [132].A specific interaction occurs between the N-terminus of mutanthuntingtin and Sp1 [133], which interferes with Sp1-driven generegulation. Indeed, mutant huntingtin interacts with Sp1, disrupt-ing the specific Sp1–TAFII130 complex and altering the express-ion of certain Sp1 neuronal target genes, including the dopamineD2 receptor [134]. Sp1–TAFII130 overexpression is able tocounterbalance mutant huntingtin toxicity and suppression ofdopamine D2 receptor regulation in cell culture. However, thissituation may be more complex, as recent studies using cellularand transgenic HD models demonstrated that reduction of Sp1could be neuroprotective [135]. In particular, Qui et al. [135]showed an increase in Sp1 expression levels in different experi-mental models of HD, suggesting that suppression of Sp1 couldbe beneficial for HD pathology, while an increase in Sp1 levelsmay enhance mutant huntingtin toxicity.

Recent studies have shown that huntingtin also interacts withmembers of the core transcriptional machinery other than TFIIDand TFIIF, affecting gene transcription in a polyglutamine-dependent manner [136].

Another pathway by which huntingtin affects transcriptionregulation involves the transcriptional regulation of BDNF, whichis important for the survival of striatal neurons and for the activityof cortico–striatal synapses. Studies using in vitro and in vivomodels showed that wild-type huntingtin, but not the mutatedform, modulates BDNF expression in the cortex by regulating itstranscription. [35,137]. The expression of BDNF is regulated byREST/NRSF, which recognizes and binds to the NRSE withinthe BDNF promoter [138–140]. Wild-type huntingtin is ableto bind and sequester the cytosolic REST/NRSF, limiting itstranslocation to the nucleus and allowing BDNF transcription.Mutated huntingtin does not bind REST/NRSF effectively,leading to its accumulation in the nucleus. This leads to transcrip-tional repression of NRSE-sensitive genes, such as BDNF [76].

A new insight is emerging connecting impaired energymetabolism and transcription as contributors to HD pathologyvia PGC-1α (peroxisome-proliferator-activated receptor γ co-activator-1α). PGC-1α [141] is a transcriptional co-activatorinvolved in different metabolic programmes that mainly acts as afundamental regulator of mitochondrial biogenesis and respiration[142]. Mice lacking PGC-1α show defects in brown adipose tissueas well as a pattern of neurodegeneration not unlike that seen inHD [143,144]. The possibility that PGC-1α could have a rolein HD was suggested by observations of reduced levels of PGC-1α mRNA expression in human and mouse HD brains, andexperiments showing that overexpression of PGC-1α reversed theeffects of mutant huntingtin in cell models and in HD mice. PGC-1α expression is regulated directly by the CREB–TAF4 complex,which is impaired by mutant huntingtin, abolishing its ability tobind to the PGC-1α promoter [145]. An alternative possibilityis that mutated huntingtin binds directly to PGC-1α, affectingits ability to up-regulate expression of its downstream targets[145,146].

ELEVATED ROS (REACTIVE OXYGEN SPECIES) ANDMITOCHONDRIAL DYSFUNCTION IN HD

ROS and metabolic mitochondrial dysfunction have been implic-ated in many neurodegenerative diseases [147–149]. Mitochon-dria are the major source of ROS production [150] and, at thesame time, are also a key target for ROS damage. The respiratory

chain (especially complex I and the Q cycle operating in complexIII) generates superoxide, which is converted into hydrogen per-oxide by MnSOD (manganese superoxide dismutase) [151,152].Hydrogen peroxide can react with the available iron to producethe extremely reactive hydroxyl radical [153]. Superoxide alsoreacts with nitric oxide to produce the dangerous peroxynitrite[154], which inhibits the respiratory chain [155] and inactivatesaconitase and MnSOD [156,157]. Superoxide can also directlyinactivate certain Fe–S proteins such as aconitase [158,159].To protect the cell against ROS damage, mitochondria containa variety of antioxidant systems. These include non-enzymaticcomponents, such as α-tocopherol, coenzyme Q10 or glutathione,as well as enzymatic components such as MnSOD, catalase andglutathione peroxidase [160,161]. However, excessive productionof ROS or a disruption of the antioxidant mechanisms can leadto oxidative damage to mitochondrial protein, lipid and DNA[162].

Evidence from post-mortem brains of HD patients andtransgenic mouse models suggests that mitochondrial metabolicdysfunction could play a role in HD pathogenesis [163–165].Mitochondrial impairment and oxidative stress have even beendetected in asymptomatic HD carriers [166], indicating that thismay be an early step in disease development. It is, however, notclear whether metabolic mitochondrial dysfunction is a primarycause in HD or a secondary consequence underlying neuronalloss [167]. One possible mechanism of how mutant huntingtincould lead to mitochondrial impairment is through direct asso-ciation with the outer mitochondrial membrane, which wasshown in brain mitochondria from transgenic mice expressinga pathological CAG-repeat and isolated mitochondria fromlymphoblasts of HD patients [168–171].

In addition to energy production and metabolism, mitochondriaalso play an important role in cellular calcium homoeostasisand apoptosis [172,173], and isolated mitochondria from HDmice also showed decreased membrane potential, depolarized atlower calcium loads compared with controls [170] and were moresensitive to calcium-induced cytochrome c release [168]. Theseeffects could be reproduced by incubating normal mitochondriawith mutant huntingtin in vitro.

A relationship between the number of CAG repeats and mito-chondrial ATP production has been reported [174]. In huntingtinstriatal cells, the ATP/ADP production decreases as repeatnumbers increase, whether in the normal or the disease-causingrange. The decreased ATP/ADP ratio was linked to enhancedcalcium influx through NMDA receptors. Impaired energy meta-bolism probably leads to reduced ATP production, with a con-comitant reduced mitochondrial membrane potential and a highervulnerability to NMDA-mediated calcium influx and excito-toxicity [175,176]. Calcium influx could trigger further freeradical production, exacerbating cell damage. There is also apotentiating effect of mutant huntingtin on NMDA receptoractivity as NMDA-evoked currents and NMDA-mediated calciumtransients were significantly increased in striatal neurons fromYAC72 transgenic mice compared with wild-type controls,which could lead to an increased vulnerability to excitotoxicity[177,178]. Also, calcium influx through the NMDA receptorresults in impaired mitochondrial function and increased oxidativestress [179,180].

Brain mitochondria have a higher concentration of lipids withpolyunsaturated acyls, which are more sensitive to oxidative dam-age than other lipids [181]. An increase in striatal lipid peroxid-ation was observed in HD transgenic mice which paralleledthe worsening of the neurological phenotype [182]. The overalleffects of lipid peroxidation probably decrease membrane fluidity,making it easier for phospholipids to exchange between the two



halves of the bilayer. This would increase the leakiness of themembrane to substances that do not normally cross it other thanthrough specific channels, and also cause damage and inactivationof membrane proteins, receptors, enzymes and ion channels [183].Products of the lipid peroxidation process, such as 4-hydroxy-hexenal and 4-hydroxynonenal, have also been shown to facilitatethe induction of mitochondrial permeability transition [184],which could lead to cell death by release of apoptogenic factors.

Mutant huntingtin has been shown to directly impair the motil-ity of mitochondria, with aggregates probably acting as ‘physi-cal roadblocks’ for mitochondrial transport [81,185]. Aggregatesmay impair the passage of mitochondria along neuronalprocesses, causing them to accumulate adjacent to aggregates andbecome immobilized [185]. This may heighten glutamate excito-toxicity and alter calcium handling owing to the inability totransverse the neurite.

ROS can cause direct damage to DNA, and an enhanced ROSproduction may lead to accumulation of somatic mutations [186].8-OHdG (8-hydroxy-2′-deoxyguanosine) is a biomarker foroxidative DNA damage and increased levels of this ROS-damagedguanine nucleotide were found in mtDNA (mitochondrial DNA)from HD post-mortem parietal cortex [187] as well as in R6/2HD transgenic mice [188]. Also, increased oxidative damageto total DNA was found in caudate and frontal cortex of HDpost-mortem brain [189]. 8-OHdG can cause nucleotide basemispairing, resulting in DNA point mutations, probably leadingto respiratory dysfunction, higher rates of ROS production andhigher susceptibility to apoptotic stimuli [190–192].

In addition to showing meiotic instability, the HD mutationalso shows somatic instability. Different CAG repeat lengths areseen in different neurons. Whether or not this has an impact ondisease severity is not certain, but this phenomenon is certainly anattractive contributor to pathology. A recent study showed that theage-dependent CAG somatic mutation events associated with HDoccur in the process of removing oxidized base lesions, and arelargely mediated by the single base excision repair enzyme, OGG1(7,8-dihydro-8-oxoguanine-DNA glycosylase) [193]. OGG1 isactivated in response to oxidative DNA lesions and results insomatic instability. This initiates a potential positive-feedbackloop, since longer CAG stretches will lead to even more oxidativedamage and hence more OGG1 activity [193].

ROS may also result in the formation of protein carbonyls;oxidatively modified proteins and enhanced protein carbonyllevels have been found in the striatum of R6/2 mice [194–196].These modified proteins are generally dysfunctional owing to lossof catalytic or structural integrity, which may lead to decreasedactivities of key metabolic enzymes and disturbed cellularsignalling systems [195,197,198].

It is interesting to consider that many of the pathogenic processproposed in HD pathogenesis may interact, and this potentialcross-talk may lead to various types of positive-feedback loops(see Figure 2 for examples).

EVIDENCE FOR AND AGAINST THE IMPAIRMENT OF THE UPS(UBIQUITIN–PROTEASOME SYSTEM) IN HD

It has been proposed that the UPS is impaired in HD and that thiscontributes to the disease mechanism. However, this is contro-versial, and conflicting results have been obtained from differentassays performed in a variety of different HD model systems.Some groups have demonstrated decreased proteasome activity[199,200], some have shown no change in activity [201,202] andothers have even demonstrated an increase in proteasome activity[203,204] in response to mutant huntingtin expression.

The UPS consists of multiple components and is not onlyimportant for protein turnover, but also essential for normalcellular and physiological function [205,206]. At the centre ofthe UPS is the 20S catalytic core of the proteasome. This is abarrel-shaped multisubunit complex that has three main proteo-lytic activities: chymotrypsin, trypsin and peptidyl-glutamyl thatcleave after hydrophobic, basic and acidic residues respectively[207]. 19S regulatory particles (also termed PA700) bind eitherside of the 20S core proteasome to form the 26S proteasome[208]. A cascade of enzymes act to covalently attach multipleubiquitin molecules to target proteins, which mark them fordegradation [209]. Polyubiquitin chains are recognized by the19S regulatory particle, which facilitates protein degradationby ATP-dependent de-ubiquitination and unfolding of the targetprotein, and opening the outer rings of the 20S core proteasome.The activity of the proteasome can be altered by its associationwith a number of regulatory molecules and complexes such asthe PA28 family of proteasome activators, which enhance thedegradation of short peptides [210] and Rad23, which is thought toshuttle ubiquitinated proteins to the proteasome [211]. In addition,the catalytic activity of the proteasome can also be modulatedby alterations in subunit composition in response to cellularstimuli [e.g. IFNγ (interferon γ ) induction of immunoproteasomesubunits LMP (low-molecular-mass polypeptide) 2 and LMP7 tobias proteolysis in favour of producing short peptides suitable forMHC-1 presentation at the cell surface] [212].

Although responsible for the degradation of short-lived anddamaged proteins, the UPS indirectly regulates other cellularactivities. The UPS also has a role in cell signalling throughthe degradation of many key regulatory proteins, protein subunitsand transcription factors such as p53 and IκB (inhibitor of nuclearfactor κB). Recently, it has been proposed that the proteasome hasa role in normal synaptic function and plasticity, and is involvedin the NMDA-dependent remodelling of the protein compositionof synapses [213]. Thus impairment of the UPS is likely to have adetrimental effect on the function of the cell and indeed the wholeorganism.

The idea that the UPS may be impaired in polyglutamineexpansion disorders initially came from studies showing the la-belling of polyglutamine aggregates with antibodies raised againstubiquitin and proteasome subunits in cell models [100,214],transgenic mice [87] and human post-mortem samples [95]. Fromthese observations, the sequestration hypothesis was proposed.It suggested that the sequestration of UPS components inaggregates and the altered subcellular localization of proteasomesmight affect UPS activity. However, in contrast with the seques-tration hypothesis, inhibition of the proteasome has been demon-strated in cells co-expressing a GFP (green fluorescent protein)–degron (GFP tagged to ubiquitin) construct and pathogenic Httexon 1 constructs in the absence of visible aggregates [215],and there is some evidence to suggest that some molecules arenot sequestered tightly into aggregates, but are only looselyassociated and can diffuse freely [216]. Another model toaccount for the impairment of the proteasome in HD camefrom both in vitro and cell model data suggesting that expandedpolyglutamine-containing proteins are not easily degraded by theeukaryotic proteasome, which can only accommodate unfoldedproteins [217,218]. As these studies show that the proteasomecannot cleave between successive glutamine residues in apolyglutamine tract, the choking hypothesis proposes that proteinscontaining expanded polyglutamine tracts may get ‘stuck’ inthe proteasome and block the entry of other substrates into thebarrel of the 20S catalytic core. Although it has been shown thatsynthetically generated polyglutamine aggregates do not inhibit26S proteasome function in vitro [215], it has recently been shown



that fibrillar species purified from HD transgenic mouse andhuman HD post-mortem brains do decrease proteasome activityin vitro [219].

The first study to measure proteasome activity in HD cell mod-els directly used a fluorigenic substrate specific for the chymotryp-sin activity of the proteasome [200]. A shift in chymotryspinactivity was demonstrated from cytosolic fractions to aggregate-containing precipitated fractions derived from lysates from botha stable HD cell model (expressing huntingtin exon 1 with a 150polyglutamine repeat) and brain lysates derived from R6/1 mice[200]. Chymotrypsin activity was reduced in the cytosolic fractionand increased in precipitated fractions derived from lysatesof polyglutamine-expressing cells compared with control cells[200]. This suggested the altered localization of proteasomes toaggregates. The authors also demonstrated reduced degradationof the endogenous proteasome substrate, p53 [200]. This studystrongly suggested the impairment of the UPS in HD. Subse-quently, these data were supported further by a study in cellsusing a reporter molecule comprising EGFP (enhanced GFP)fused to a short sequence that targets the protein for proteasomedegradation (termed degrons) [199]. When this EGFP–degronreporter was co-expressed with mutant huntingtin in cells, EGFPfluorescence was increased more than 2-fold compared with cellsexpressing wild-type huntingtin. This observation implicates amajor impairment of the proteasome function because >50% de-crease of chymotrypsin-like activity is required to obtain a 50 %increase of GFP fluorescence [199]. Similar results were foundwith the �F508 mutant cystic fibrosis membrane conductanceregulator, an unrelated protein sharing only the propensity toaggregate, suggesting that proteasome impairment is causedby aggregate formation [199]. Consistent with these data, areduction of chymotrypsin and peptidyl-glutamyl activities hasbeen demonstrated in lysates from human HD post-mortem brainsand HD patient skin fibroblasts [220].

Using the co-expression of NLS- or NES-tagged EGFP–degrons and NES or NLS mutant polyglutamine constructs, aglobal impairment of the UPS was demonstrated, regardless ofthe intracellular locations of the proteins containing the expandedpolyglutamine tracts or the degron reporters [215]. The authorsalso examined two hypotheses proposing mechanisms for theinhibition of proteasome activity. Contrary to the sequestrationhypothesis being the only mechanism, they demonstrated protea-some inhibition in the absence of visible aggregates. They alsoshowed that synthetic protein aggregates do not inhibit activityof the 26S proteasome function in vitro, suggesting that UPSimpairment is unlikely to be caused solely by blocking theproteasome. Indeed, the decreases in nuclear proteasome functionby extranuclear mutant polyglutamine and vice versa, argued thatthe observed effects were independent of interactions betweenmutant protein and the proteasome. Nevertheless, one cannotdiscount either of these models, as fibrillar forms of hunt-ingtin purified from transgenic mouse and human post-mortembrains do inhibit the 26S proteasome in vitro [219]. Furthermore,an accumulation of proteasome substrates may occur in thepresence of normal proteasome function, owing to abnormalitiesin ubiquitination, de-ubiquitination or compromise of the activ-ities of various shuttling proteins that may be required to trafficubiquitinated proteins to the proteasome.

Data contrary to the above, suggesting that the proteasome isnot impaired in polyglutamine expansion disorders, come from avariety of sources. SH-SY5Y cells stably expressing mutant hunt-ingtin did not show a difference in the degradation of fluorigenicpeptides, compared with cells expressing wild-type huntingtin[201]. Also, an increase (not decrease, as expected) in the chymo-trypsin and trypsin activities of the proteasome was observed

in lysates derived from the cortex and striatum of the HD94conditional mouse model of HD [203]. This was attributed toan increase in the levels of the proteasome subunits LMP2 andLMP7 and the induction of the immunoproteasome. Increasedproteasomal chymotrypsin-like activity has also been observedin brain lysates from the R6/2 model of HD compared with non-transgenic littermates [204]. However, this study found no changein overall 26S proteasome activity and showed that the nuclearproteasome activator PA28 (also termed REGγ ) is not involvedin polyglutamine pathology [204]. This is in contrast with datademonstrating the reversal of proteasome dysfunction in mutant-huntingtin-expressing striatal neurons and rescue of cell death byPA28 overexpression [221].

One of the caveats of studies in cell culture and manytransgenic models is that they express artificially high levels ofmutant proteins, which may induce proteasome dysfunction eitherdirectly or indirectly. The role of the proteasome in vivo hasrecently been tested using the knockin mouse model of SCA7 (apolyglutamine expansion disorder caused by mutations in ataxin7) crossed with a transgenic mouse expressing an EGFP–degronreporter [202]. The authors observed an increase in levels of thereporter in neurons at late stages of the disease. However, this wasnot due to inhibition of proteasome activity, but instead correlatedwith an increase in mRNA coding the EGFP–degron reporter[202].

Thus it still remains unclear whether the UPS is impaired in HD.Conflicting data are likely to occur as many of the experimental ap-proaches used to assess UPS function have caveats. For instance,studies of UPS function have been performed in many differentmodels of HD (stable, inducible and transient cell models,transgenic Drosophila, transgenic mice and human post-mortemsamples). These models may represent different stages of thehuman disease and express HTT transgenes of different sizes(e.g. full-length huntingtin or smaller exon 1 fragments) at dif-ferent levels under the control of different promoters. In addition,the various reporters used are likely to be assessing the activity ofdifferent components of the UPS. One problem with assays of iso-lated proteasome activity using small fluorigenic peptides isthat modest changes in proteasome number/activity may not berate-limiting for substrate clearance. It is likely that ubiquitinconjugation, and, in some situations, transport of ubiquitinatedproteins to the proteasome, may be more important physiologicalregulators, and these will not be measured using these substrates.This has been partially circumvented by using EGFP–degronreporters and measuring levels of endogenous UPS substratessuch as p53. However, levels of artificial EGFP–degron reportersmay be affected by changes in mRNA encoding the reporter [202].Likewise, protein levels of endogenous proteasome substratessuch as p53 are likely to be affected not only by UPS activity, butalso by changes in transcription elicited by mutant huntingtin[26]. In order to try to overcome these problems, UPS functionhas recently been assessed using polyubiquitin chains as anendogenous biomarker [222]. The amount of polyubiquitinchains within a cell was shown be a faithful indicator of UPSfunction and, using this approach, impairment of the UPS wasdemonstrated in brain lysates from R6/2 HD transgenic mice, theHdhQ150/Q150 knockin model of HD and human HD post-mortembrains [222]. One question raised by this study is whether theincreased numbers of ubiquitin chains are necessarily due to pro-teasome dysfunction, or an increase in the ubiquitination rate ofsubstrates. In other words, although the amounts of polyubiquitinchains will increase when proteasome function is disrupted, theycan also increase in the context of normal proteasome activityor under conditions where substrate degradation is enhanced(for instance, if induction of ubiquitination exceeds substrate



clearance). Thus, although this study is consistent with datasuggesting impaired proteasome function in HD, it is still notdefinitive, and the conflicting results of previous studies mustbe properly resolved before it is proposed that the UPS is trulyimpaired in HD.

THE UPS AND DEGRADATION OF MUTANT HUNTINGTIN

One strategy for the treatment of polyglutamine expansion dis-orders is to decrease levels of the toxic mutant protein. This couldbe achieved by increasing the clearance of the mutant protein.Indeed, induction of autophagy by treatment with the mTOR(mammalian target of rapamycin) inhibitor rapamycin has beendemonstrated to reduce aggregation and attenuate toxicity in HDcell and mouse models [223].

It is unclear whether proteins with an expanded polyglutaminetract are good proteasome substrates. Huntingtin interacts with thehuman ubiquitin-conjugating enzyme E2-25K, which requiresthe polyglutamine domain [55]. Parkin, an E3-ubiquitin ligase,also co-localizes with mutant huntingtin aggregates in HD mouseand human brains, and overexpression of parkin enhances theclearance of the mutant protein [224]. These data suggest thathuntingtin may be a proteasome substrate. Consistent with this,proteasome inhibitors such as lactacystin and epoxomycin preventmutant huntingtin clearance in a conditional HD mouse or cellmodels after its expression is stopped [225]. Proteasome inhibitionalso increases mutant huntingtin aggregation and toxicity in HDcell models [100,200,225–227]. As the proteasome is unable tocleave between glutamine residues within polyglutamine tracts[217,218], up-regulation of proteasome activity would possiblyreduce the levels of proteins with polyglutamine expansions andassociated flanking sequences, producing increased levels of longisolated polyglutamine tracts. Such products are predicted to bemore toxic than the inputs that have flanking sequences. However,such products have been shown to be degraded by puromycin-sensitive aminopeptidase, albeit very slowly and inefficiently[228]. It is unclear whether the substrate capacity of puromycin-sensitive aminopeptidase could be overwhelmed if proteasomeactivity were increased. In addition, modulation of the protea-some may not be a good therapeutic strategy. The proteasome hasa key regulatory role, and altering its rate of degradation may havemany side effects. One may be able to use chemical chaperonessuch as trehalose or Congo Red to increase the degradationof polyglutamine-containing proteins, as these agents shift theequilibrium towards increasing the levels of soluble mono-meric proteasome-accessible species and away from aggregates[229,230]. This may make the polyglutamine proteins moreaccessible to the proteasome.

NON-CELL-AUTONOMOUS PROCESSES IN HD

Much of the focus on pathogenic mechanisms in HD has focusedon cell-autonomous mechanisms. Although less attention hasbeen focused on the pathological role of huntingtin in cell–cell interactions, a number of studies suggest that non-cell-auto-nomous mechanisms may also contribute to disease pathogenesis.

MSNs (medium spiny neurons), which are particularlyvulnerable to the HD mutation, are innervated by glutamatergicaxons, and overstimulation of glutamate receptors can inducecell death via excitotoxicity [231]. HD transgenic mouse modelsshow increased NMDA receptor activity in neurons [178,232].The abundant glutamatergic afferents to MSNs and the NMDAreceptor subunit composition in MSNs [233,234] may contributeto their preferential vulnerability in HD, especially when the gluta-

matergic input is increased or the clearance of extracellularglutamate is decreased. Clearance of extracellular excitatoryneurotransmitters is largely performed by glutamate transporters[GLT-1 (glutamate transporter-1) and GLAST (glutamate aspart-ate transporter)] in astrocytes, the major subtype of glia [235].Huntingtin can reduce the expression level of GLT-1 in the brainsof HD transgenic mice and Drosophila [236,237]. A decreasedactivity of GLT-1-dependent glutamate uptake in astrocytes leadsto an increase of glutamate concentration extracellularly. This maythen lead to increased and deleterious calcium entry in striatalneurons. All of these events may contribute to the activationof ATP- and calcium-dependent deleterious enzymes such ascalpains, caspases and endonucleases [238].

Elegant glial–neuron co-culture experiments showed that N-terminal huntingtin in glia promoted the death of cultured neuronsthat did not express huntingtin [239]. Huntingtin may affectvarious functions of glial cells, including their production ofchemokines and neurotrophic factors. The neurodegenerationcaused by overexpressed N-terminal huntingtin in neurons in vitrois reduced in the presence of glial cells. The conclusion might bethat glial dysfunction contributes more to pathology than glialdegeneration itself.

Another type of non-cell autonomous mechanism in HDcomes from studies in mouse models expressing toxic mutanthuntingtin fragments either in all neurons in the brain or onlyin cortical pyramidal neurons, which are vulnerable in HD [240].Restriction of huntingtin expression to cortical pyramidal neuronswas sufficient to produce nuclear accumulation, but insufficient toproduce neuropathology or motor deficits. However, expression inall of the neurons showed both progressive motor deficits and HDcortical pathology via nuclear accumulation, aggregation, reactivegliosis, dysmorphic neurites and dark neuron degeneration.One attractive possibility is that mutant huntingtin in corticalinterneurons attenuates the ability of these neurons to mediateinhibition on their target pyramidal neurons and that this lossof inhibition contributes to pathology. However, these data donot preclude a combination of cell-autonomous and non-cell-autonomous mechanisms working in concert.

GENETIC SCREENS REVEAL MANY NOVEL PATHWAYS IN HDPATHOGENESIS

HD pathology may be a result of the cumulative effect of a varietyof pathway perturbations. Many candidate-based approaches forHD treatment have identified target pathways, but searches fornovel modifiers of HD pathology may allow us to identifyfurther targets for therapeutic intervention as well as gain a betterunderstanding of the pathology. One way of identifying suchtargets is through genetic or chemical screens.

Yeast two-hybrid screens have been used extensively for identi-fying huntingtin interactors [55,241–247]. Hap1, Hip2, Hip1 andCBS (cystathionine β-synthase), as well as many other proteins,have been identified as mutant huntingtin protein interactors. Astudy based on a vast network of 186 protein–protein interactions[248], identified GIT1 [GPCR (G-protein-coupled receptor)kinase-interacting protein 1], as a novel interactor. Interestingly,GIT1 associates with wild-type huntingtin in mammalian cells,but, in pathogenic circumstances, GIT1 localizes to mutant hunt-ingtin aggregates and is required for their formation. In HD brains,GIT1 is cleaved, resulting in altered function.

Recently, Kaltenbach et al. [249] performed a yeast two-hybrid screen, as well as affinity pull-downs with MS, to identifyhuntingtin interactors. Out of 234 novel protein targets identi-fied, an arbitrary set of 60 genes encoding interacting proteinswere tested for their ability to behave as genetic modifiers of



neurodegeneration in a Drosophila model of HD. This high-content validation assay showed that 27 of 60 orthologues testedwere high-confidence genetic modifiers involved in a varietyof pathways such as synaptic transmission, signal transduction,transcription and cytoskeletal organization. This study providespowerful evidence that huntingtin interactors are a particularlyenriched source of HD modifiers.

Many screens have attempted to identify genetic and pharm-acological modifiers of aggregate formation and clearance. A cell-free filter retardation assay identified benzothiazole derivatives asinhibitors of the formation of HD-insoluble aggregates [250],whereas a cell-based screen identified a lead compound that wasable to specifically clear mutant huntingtin protein, but not normalhuntingtin [251]. Yamamoto et al. [252] used a gene array in a cellline that established transcriptional changes induced by the mutanthuntingtin protein. Genes that were up-regulated were targetedusing siRNA (short interfering RNA) molecules to decreaseexpression. Of these up-regulated genes, 23 were required forclearance of the mutant huntingtin protein. Activation of IRS-2(insulin receptor substrate 2), which is involved in signalling frominsulin and IGF-1 (insulin-like growth factor 1), enhanced theclearance of aggregate-prone proteins.

A genome-wide RNAi (RNA interference) screen has been usedto identify loss-of-function enhancers of aggregation in a Caen-orhabditis elegans model of polyglutamine disease [253]. The186 genes that enhanced aggregation were involved in a varietyof pathways, such as RNA metabolism, protein synthesis, proteinfolding, protein degradation and protein trafficking, reflecting adiversity of potential cellular pathways that may have an impacton polyglutamine disease pathogenesis.

A number of cell-based assays have aimed to identify modul-ators of polyglutamine toxicity [254,255]. A screen of 4850haploid mutants in a yeast (Saccharomyces cerevisiae) modelidentified 52 enhancers and 28 suppressors of mutant Htt exon1-Q53-induced toxicity [256,257]. The enhancers are involvedin cellular processes such as protein folding, response to stressand the UPS [257]. Suppressors of toxicity are involved intranscription, protein aggregation, vesicle transport, vacuolardegradation and the kynurenine pathway, which is involved intryptophan degradation [256]. These studies suggest that yeastmodels may provide important insights into the biology of HD.

Rescue of cellular toxicity has also been studied in the nematodeworm. A C. elegans model expressing N-terminal huntingtincarrying a stretch of 150 glutamine residues in the glutamatergicASH sensory neurons leads to their degeneration by day 8 [258].In a genetic screen, for the enhancement of neurodegeneration,loss of pqe-1 (polyglutamine enhancer 1) gene functionenhances neurodegeneration and pqe-1 overexpression rescuescellular toxicity. This gene encodes a putative glutamine-richRNA exonuclease, which may rescue toxicity by sequestratingpolyglutamine-expanded proteins.

Drosophila models of neurodegenerative diseases have recentlybeen established and genetic screens in Drosophila have provideda number of modifiers. These include hsp40-like J domainsproteins (dHDJ1 and dTPR2) [259]. Molecular chaperoneshave been implicated in the rescue of other polyglutamine-induced pathogeneses in Drosophila, as well as in other models[260–262]. A novel modifier has been identified, dMLF1, theDrosophila homologue of human MLF1 (myeloid leukaemiafactor 1), which suppresses toxicity by potentially inhibitingmutant polyglutamine protein aggregation [263]. A SCA1Drosophila model expressing ataxin-1(82Q) under the gmr driver,leads to severe external eye abnormality and reduced retinalthickness [264]. Genetic screens with this model identifiedgenes involved in protein-folding/heat-shock response, cellular

detoxification, nuclear transport, RNA processing and tran-scriptional cofactors as modifiers of polyglutamine pathogene-sis. Recently, a screen of a Drosophila model of SCA3 identifiedthe miRNA bantam a suppressor of toxicity [265], revealing afurther possible modifier pathway.

THERAPY FOR HD

HD has a number of features which make it a comparativelytractable problem, compared with neurodegenerative diseaseswhich do not have Mendelian inheritance. Its autosomal dominantnature and single type of mutation allows most people at risk tobe potentially identified before symptoms develop, making pre-symptomatic treatment a feasible possibility. This is importantbecause a significant amount of neuronal loss has already occurredby the time most neurodegenerative diseases present clinically,lowering the rate of loss is potentially easier than repairingdamage after it has occurred. The development of therapeuticstrategies for HD may have wider relevance, most obviously forthe eight other polyglutamine diseases, but even possibly for otherneurodegenerative conditions characterized by abnormalities ofprotein conformation.

There are no disease-modifying treatments for HD in routineclinical use, and current treatment is therefore symptomatic.Although many trials have concentrated on mechanisms andoutcomes associated with movement disorder, patients report thattheir quality of life is more often decreased by psychiatric mani-festations of their condition, including depression, irritability andapathy [266]. Rates of depression may be as high as 40 %, andsuicide may occur in as many as 10% [267,268]. Obsessivecompulsive symptoms are also common [269]. Given their fre-quency and impact, it is surprising that the evidence base fortreatment of psychiatric disturbance is limited to case studies. SS-RIs [selective serotonin (5-hydroxytryptamine)-re-uptake inhib-itor] or mirtazapine may be preferred for depression as they have amore favourable anticholinergic profile compared with some otherantidepressants [270]. Improvement in depression and obsessionalthinking has also been reported with olanzapine and sertraline[271,272]. Risperidone and amisulpiride may have a role in thetreatment of psychosis in HD [273,274], while there are reportsof quetiapine helping with behavioural disturbance [275].

A major neurological symptom associated with HD is chorea.Tetrabenazine depletes dopamine from central neurons. The firstrelatively large randomized control trial of its use in HD [276]found that tetrabenazine did significantly improve the UHDRS(unified HD rating scale) and global improvement assessments,but was associated with an increased incidence of adverse effects,including one suicide. Atypical antipsychotics are often used inthe clinic, although the evidence from a larger trial of clozapineshowed disappointing efficacy with significant side effects [277].More encouraging results are reported for olanzapine, but fromsmall open-label studies [278]. Strikingly, there are limiteddata providing conclusive support for any cognitive-enhancingtherapies in HD. A study in a small number of HD patientstreated with rivastigmine suggested possible motor and cognitivebenefit [279]. These results were not supported by a further smalltrial of donepezil [280]. Given the limited proven efficacy ofsymptomatic treatments, an important part of treatment is co-ordinating appropriate social, paramedical and palliative care forHD patients.

Although further work towards developing and validatingsymptomatic treatments is clearly justified, the steadily increasingknowledge base around potential pathways leading toneurodegeneration in HD provides the possibility to develop



Figure 3 Potential disease-modifying strategies for HD

HD toxicity may be ameliorated by direct modification of the mutant gene or protein. Strategies which seek to achieve this include repression of mutant gene expression, inhibition of aggregation ormisfolding, inhibition of the cleavage of the protein to form toxic fragments and increased clearance of the mutant protein by up-regulating autophagy. Alternative strategies depend on mitigating thedeleterious effects of the mutant protein by stabilizing mitochondria or correcting transcriptional dysregulation. More general neuroprotective strategies which may be important include attempts todecrease excitotoxic cell death or enhance neurotrophin release. See the text for more details.

rational mechanism-based therapies that may slow the neurode-generation and neurological dysfunction at the core of this disease.We have selected a few possible examples of such therapeuticstrategies that have been initiated largely in cell and animalmodels, before conducting studies in humans (Figure 3).

Preventing mutant gene expression is an attractive strategy,as it aims to remove the primary culprit: the toxic mutant pro-tein. Human individuals with only one working copy of the HTTgene suffer no obvious adverse consequences. It has been possibleto use siRNA in mouse models of HD [281] and other poly-glutamine diseases [282] to decrease mutant protein expressionand aggregation and prolong survival. These trials used directintraventricular injection of the siRNA, a technique that may berelatively less acceptable to human sufferers. There are safetyissues that will need to be addressed for this therapeutic approach.First, the knockdown must be specific to the mutant form of theprotein and the wild-type should ideally be unaffected (as it mayhave anti-apoptotic functions). Although it may be possible totarget the mutant allele to some extent using single nucleotidepolymorphisms, these are likely to show interindividual variationand therefore require a library of siRNAs [283]. Other safetyconcerns centre on both off-target effects (decreasing expressionof genes other than the HTT gene) and inactivation of tumour-suppressor genes. These current technical difficulties and safetyconcerns mean that, although these techniques are potentiallyexciting, their use in clinical trials may be somewhat more distant.

If it is not possible to silence production of the mutant protein,then enhancing its clearance may be an alternative (or adjunct).Mutant huntingtin is cleared by autophagy, a process involving theformation of double-membrane structures called autophagosomesaround a portion of cytoplasm. These autophagosomes ultimatelyfuse with lysosomes, where their contents are degraded. Thestrategy of up-regulating autophagy to increase clearance ofmutant protein has shown promise in cell, Drosophila and mouse

models of HD where the increase in clearance shows a preferencefor the mutant form of the protein [223]. Drugs which can be usedto up-regulate autophagy (rapamycin, carbamazepine, sodiumvalproate) are already U.S. FDA (Food and Drug Administration)-approved and some have a long track record in treating humanCNS (central nervous system) disease. This approach is also onewhere the principle has been shown to work in animal and cellmodels expressing other aggregate-prone proteins that lead tohuman disease, including tau and mutant α-synuclein [284].

Ideally, we would like to develop drugs that target putativepathological mechanisms. One process that may be rate-limitingis huntingtin cleavage by caspases and other proteolytic enzymes.Indeed, the importance of cleavage of huntingtin and the roleof apoptosis in HD have been described above. Inhibitors of theenzymes which cleave mutant huntingtin to its toxic N-terminalfragment(s) (including caspases 2, 3 and 6 and calpain) mayprovide potential targets for therapeutic intervention. Given therole of caspases in induction of apoptosis their inhibition may bedoubly attractive. Broad-spectrum caspase inhibitors increasedsurvival in an exon 1 mouse model, but required intracerebraladministration [285]. However, there may be cancer risks withlong-term caspase inhibition. Minocycline may prevent apoptosisby inhibiting the mitochondrial permeability transition and is acaspase inhibitor (although it probably has a range of properties).Despite mixed results in mouse models [286,287], its historyof long-term use in humans as an antibiotic has encouragedhuman trials. A small trial in humans has suggested benefit [288]and good tolerability has been reported in larger trials, the finaloutcome of which is awaited [289].

The potential importance of mitochondrial dysfunction andits implication in cell death in HD has already been described.Compounds which enhance mitochondrial stability have been in-vestigated. Improvements in survival, neuropathology and motorperformance have been reported in mouse models treated with



creatine [290], and early results from its use in human suffererssuggest good tolerability [291].

Other potential treatments that have been investigated includetransglutaminase inhibitors. Transglutaminases belong to a familyof closely related proteins that catalyse the cross-linking of aglutamine residue of a protein/peptide substrate to a lysine residueof a protein/peptide co-substrate with the formation of a GGEL[Nε-(γ -L-glutamyl)-L-lysine] cross-link. These bonds may beimportant in the formation of aggregates and the toxicity of mutanthuntingtin. Cystamine is a transglutaminase inhibitor whichimproves survival, motor phenotype and neuropathology in mousemodels [292], and preliminary dose-finding and tolerability trialsin human sufferers have been completed [293].

Chaperones help proteins to adopt more stable conformationsand prevent aggregation, and their production is increased inheat-shock responses. Chaperones may be induced by chemicalinitiators of heat-shock responses such as geldanamycin which isprotective in cell models of HD [294]. Small-molecule ‘chemicalchaperones’ such as trehalose have a similar effect and have shownbeneficial effects in mouse models of HD [295].

Mutant huntingtin binds a number of transcription factors, ofwhich the best described is CBP [125]. CBP acetylates histonesand thereby exposes the DNA sequence to allow transcription.Given that the mutant huntingtin therefore results in decreasedhistone acetylation, HDAC inhibitors have been investigated as atherapeutic strategy. These drugs have shown efficacy in bothDrosophila [127] and mouse models [130] and one (phenyl-butyrate) has long-term safety data in humans in the treatmentof ornithine transcarbamylase deficiency [296]. However, currentHDAC inhibitors have major side effects.

Excitotoxic cell death in HD is implicated by the reproductionof HD pathology by NMDA agonists and the increased sensitiv-ity of NMDA receptors in the presence of mitochondrialdysfunction. As a result, there has been interest in NMDA receptorantagonists as a potential therapeutic strategy. Murine modelsshow increased survival with the NMDA receptor antagonistsriluzole [297] and remacemide (alone and an additive effectwith coenzyme Q10) [298]. Amantadine, riluzole, lamotragineand remacemide have all been studied in human randomizedcontrolled trials. The results for both amantadine and riluzole havebeen mixed, although, in both cases, the results from the largestand best-designed trials have been disappointing [299,300]. Arelatively large (n = 55) double-blind randomized control trialof lamotragine for 30 months found no significant difference inprimary or secondary response variables [301]. In one of thelargest trials in HD to date, 347 patients were randomized to eithercoenzyme Q10 or remacemide, both or neither for 30 months.Sadly, the encouraging results from the mouse study were notreplicated, with no treatment arm showing advantage over placebo[302]. Memantine (an NMDA antagonist licensed in the U.K.for Alzheimer’s disease) has shown some promise in an open-label trial, but this remains to be confirmed using more rigorousmethodology [222]. It should be noted that many of the com-pounds described act via more than one mechanism. For example,creatine is a transglutaminase inhibitor as well as having mito-chondrial effects, and coenzyme Q10 may have effects onmitochondrial stability and amelioration of excitotoxicity as wellas being an antioxidant.

One way to repair neuronal loss in HD may be with trans-plantation. Studies in HD mouse models transplanting either fetalstriatal grafts [303] or wild-type cortex [304] showed the potentialfor graft survival and some modest improvements in phenotype.Trials have been carried out in humans, mostly using tissue fromhuman fetuses [305–309], although one used porcine material[310]. Results from these trials have been encouraging in terms of

graft survival and largely also in terms of safety (although, in onestudy, of the seven patients transplanted, three developed subduralhaematomas [311]). Although graft survival and function has beenpromising using proxy measures of glucose uptake [312] and post-mortem examination [313], the symptomatic benefits have beenless clear-cut (although the trials have been largely associatedwith stability or slowed decline of motor symptoms). Althoughthis approach may have potential, there is still much to do in termsof determining the optimal source and storage method of grafttissue (including the possibility of stem cell sources), choice ofrecipient, location of graft, graft susceptibility to disease process,rating of outcome, use of immunosuppression and study design(including the possible use of sham surgery to provide control).The results of longer-term follow-up trials currently underwayare awaited. Also, grafting striatum may have potential value inalleviating motor symptoms, but is unlikely to be able to have asignificant impact on the cognitive features of this disease whichare likely to be due to the extensive cortical damage.

Neuronal loss in HD results in decreased availability of neuro-trophic factors to adjacent neurons and subsequently furtherneuronal loss. The relationship between growth factors and mutanthuntingtin appears to be antagonistic, with each being down-regulated by the presence of the other [35,314]. Difficultiescrossing the blood–brain barrier and side effects associated withparenteral administration have resulted in trials of centrallyadministered neurotrophic factors. Cells engineered to expressneurotrophins can be encased in capsules which protect them fromhost immune defences, but allow the release of neurotrophins.This approach has been used successfully in primate toxin models[315] (which may bear only distant resemblance to human HD)and Phase I trials have been conducted in humans [316]. Safetyresults from this trial were reassuring, and, although the clinicalbenefits were not significant, the results were encouraging andfurther data are awaited.

In the 14 years since the causative gene in HD was discovered,huge advances have been made in understanding the biologyof the disease and in designing rational therapeutic strategieswhich work well in animal models. Despite this, there remains nodisease-modifying treatment. The challenge for the next decadewill be translating laboratory data into clinical treatments, but,given the progress of the past, it is a challenge which can beviewed with optimism. Furthermore, it remains a challenge toconduct clinical trials in human HD, given its insidious onsetand slow multifaceted progression. A further challenge will beto learn how to conduct powerful and cost-efficient trials toexplore the growing repertoire of strategies emerging from morebasic research studies. These may be considerably simplified bythe identification of suitable biomarkers for disease progression[317].

We thank Michael Jardine and James Tweedley for help with illustrations. Work inD. C. R.’s laboratory on HD is funded by the MRC (Medial Research Council), theWellcome Trust (Senior Fellowship to D. C. R..), Action Medical Research (ResearchTraining Fellowship to B. R. U.), Sackler scholarship (B. R. U.), EU (European Union)[EUROSCA (European Integrated Project on Spinocerebellar Ataxias) and TAMAHUD(Identification of Early Disease Markers, Novel Pharmacologically Tractable Targets andSmall Molecule Phenotypic Modulators in Huntington’s Disease)] and NIHR (NationalInstitute of Health Research) Biomedical Research Centre (Addenbrooke’s Hospital).

REFERENCES

1 Huntington, G. (1872) On Chorea. In Medical and Surgical Reporter, vol. 26,pp. 320–321, Philadelphia

2 Water, C. O. (1842) In Practice of Medicine, vol. 2 (R. Dunglison, ed.), pp. 312, Lee andBlanchard, Philadelphia

3 Meynert, T. (1877) Discussion to Fritsch. Psychiatry 4, 47



4 Jelgersma, G. (1907) Die anatomischen Veranderungen bei. Paralysis agitans undchronischer Chorea. Verh. Ges. Dtsch. Naturforsch. Aerzte 2, 383–388

5 Rosas, H. D., Liu, A. K., Hersch, S., Glessner, M., Ferrante, R. J., Salat, D. H., van derKouwe, A., Jenkins, B. G., Dale, A. M. and Fischl, B. (2002) Regional and progressivethinning of the cortical ribbon in Huntington’s disease. Neurology 58, 695–701

6 The Huntington’s Disease Collaborative Research Group (1993) A novel gene containinga trinucleotide repeat that is expanded and unstable on Huntington’s diseasechromosomes. Cell 72, 971–983

7 Myers, R. H. (2004) Huntington’s disease genetics. NeuroRx 1, 255–2628 McNeil, S. M., Novelletto, A., Srinidhi, J., Barnes, G., Kornbluth, I., Altherr, M. R.,

Wasmuth, J. J., Gusella, J. F., MacDonald, M. E. and Myers, R. H. (1997) Reducedpenetrance of the Huntington’s disease mutation. Hum. Mol. Genet. 6, 775–779

9 Quarrell, O. W., Rigby, A. S., Barron, L., Crow, Y., Dalton, A., Dennis, N., Fryer, A. E.,Heydon, F., Kinning, E., Lashwood, A. et al. (2007) Reduced penetrance alleles forHuntington’s disease: a multi-centre direct observational study. J. Med. Genet. 44, e68

10 Ross, C. A. (1995) When more is less: pathogenesis of glutamine repeatneurodegenerative diseases. Neuron 15, 493–496

11 Rubinsztein, D. C., Leggo, J., Chiano, M., Dodge, A., Norbury, G., Rosser, E. andCraufurd, D. (1997) Genotypes at the GluR6 kainate receptor locus are associated withvariation in the age of onset of Huntington disease. Proc. Natl. Acad. Sci. U.S.A. 94,3872–3876

12 MacDonald, M. E., Vonsattel, J. P., Shrinidhi, J., Couropmitree, N. N., Cupples, L. A.,Bird, E. D., Gusella, J. F. and Myers, R. H. (1999) Evidence for the GluR6 geneassociated with younger onset age of Huntington’s disease. Neurology 53, 1330–1332

13 Kehoe, P., Krawczak, M., Harper, P. S., Owen, M. J. and Jones, A. L. (1999) Age of onsetin Huntington disease: sex specific influence of apolipoprotein E genotype and normalCAG repeat length. J. Med. Genet. 36, 108–111

14 Rosenblatt, A., Brinkman, R. R., Liang, K. Y., Almqvist, E. W., Margolis, R. L., Huang,C. Y., Sherr, M., Franz, M. L., Abbott, M. H., Hayden, M. R. and Ross, C. A. (2001)Familial influence on age of onset among siblings with Huntington disease. Am. J.Med. Genet. 105, 399–403

15 Chattopadhyay, B., Ghosh, S., Gangopadhyay, P. K., Das, S. K., Roy, T., Sinha, K. K., Jha,D. K., Mukherjee, S. C., Chakraborty, A., Singhal, B. S. et al. (2003) Modulation of age atonset in Huntington’s disease and spinocerebellar ataxia type 2 patients originated fromeastern India. Neurosci. Lett. 345, 93–96

16 Chattopadhyay, B., Baksi, K., Mukhopadhyay, S. and Bhattacharyya, N. P. (2005)Modulation of age at onset of Huntington disease patients by variations in TP53 andhuman caspase activated DNase (hCAD) genes. Neurosci. Lett. 374, 81–86

17 Djousse, L., Knowlton, B., Hayden, M. R., Almqvist, E. W., Brinkman, R. R., Ross, C. A.,Margolis, R. L., Rosenblatt, A., Durr, A., Dode, C. et al. (2004) Evidence for a modifier ofonset age in Huntington disease linked to the HD gene in 4p16. Neurogenetics 5,109–114

18 Wexler, N. S., Lorimer, J., Porter, J., Gomez, F., Moskowitz, C., Shackell, E., Marder, K.,Penchaszadeh, G., Roberts, S. A., Gayan, J. et al. (2004) Venezuelan kindreds reveal thatgenetic and environmental factors modulate Huntington’s disease age of onset.Proc. Natl. Acad. Sci. U.S.A. 101, 3498–3503

19 Ranen, N. G., Stine, O. C., Abbott, M. H., Sherr, M., Codori, A. M., Franz, M. L., Chao,N. I., Chung, A. S., Pleasant, N., Callahan, C. et al. (1995) Anticipation and instability ofIT-15 (CAG)n repeats in parent-offspring pairs with Huntington disease. Am. J.Hum. Genet. 57, 593–602

20 Kremer, B., Almqvist, E., Theilmann, J., Spence, N., Telenius, H., Goldberg, Y. P. andHayden, M. R. (1995) Sex-dependent mechanisms for expansions and contractions ofthe CAG repeat on affected Huntington disease chromosomes. Am. J. Hum. Genet. 57,343–350

21 Trottier, Y., Biancalana, V. and Mandel, J. L. (1994) Instability of CAG repeats inHuntington’s disease: relation to parental transmission and age of onset. J. Med. Genet.31, 377–382

22 Gusella, J. F. and MacDonald, M. E. (2000) Molecular genetics: unmaskingpolyglutamine triggers in neurodegenerative disease. Nat. Rev. Neurosci. 1, 109–115

23 Maglione, V., Cannella, M., Gradini, R., Cislaghi, G. and Squitieri, F. (2006) Huntingtinfragmentation and increased caspase 3, 8 and 9 activities in lymphoblasts withheterozygous and homozygous Huntington’s disease mutation. Mech. Ageing Dev. 127,213–216

24 Squitieri, F., Gellera, C., Cannella, M., Mariotti, C., Cislaghi, G., Rubinsztein, D. C.,Almqvist, E. W., Turner, D., Bachoud-Levi, A. C., Simpson, S. A. et al. (2003)Homozygosity for CAG mutation in Huntington disease is associated with a more severeclinical course. Brain 126, 946–955

25 Perutz, M. F. (1999) Glutamine repeats and neurodegenerative diseases: molecularaspects. Trends Biochem. Sci. 24, 58–63

26 Sugars, K. L. and Rubinsztein, D. C. (2003) Transcriptional abnormalities in Huntingtondisease. Trends Genet. 19, 233–238

27 Harper, P. S. (1996) New genes for old diseases: the molecular basis of myotonicdystrophy and Huntington’s disease. The Lumleian Lecture 1995. J. R. Coll.Phys. London 30, 221–231

28 Ambrose, C. M., Duyao, M. P., Barnes, G., Bates, G. P., Lin, C. S., Srinidhi, J.,Baxendale, S., Hummerich, H., Lehrach, H., Altherr, M. et al. (1994) Structure andexpression of the Huntington’s disease gene: evidence against simple inactivation due toan expanded CAG repeat. Somat. Cell Mol. Genet. 20, 27–38

29 Duyao, M. P., Auerbach, A. B., Ryan, A., Persichetti, F., Barnes, G. T., McNeil, S. M.,Ge, P., Vonsattel, J. P., Gusella, J. F., Joyner, A. L. et al. (1995) Inactivation of the mouseHuntington’s disease gene homolog Hdh. Science 269, 407–410

30 Nasir, J., Floresco, S. B., O’Kusky, J. R., Diewert, V. M., Richman, J. M., Zeisler, J.,Borowski, A., Marth, J. D., Phillips, A. G. and Hayden, M. R. (1995) Targeted disruptionof the Huntington’s disease gene results in embryonic lethality and behavioral andmorphological changes in heterozygotes. Cell 81, 811–823

31 Zeitlin, S., Liu, J. P., Chapman, D. L., Papaioannou, V. E. and Efstratiadis, A. (1995)Increased apoptosis and early embryonic lethality in mice nullizygous for theHuntington’s disease gene homologue. Nat. Genet. 11, 155–163

32 Marsh, J. L., Walker, H., Theisen, H., Zhu, Y. Z., Fielder, T., Purcell, J. and Thompson,L. M. (2000) Expanded polyglutamine peptides alone are intrinsically cytotoxic andcause neurodegeneration in Drosophila. Hum. Mol. Genet. 9, 13–25

33 Ordway, J. M., Tallaksen-Greene, S., Gutekunst, C. A., Bernstein, E. M., Cearley, J. A.,Wiener, H. W., Dure, 4th, L. S., Lindsey, R., Hersch, S. M., Jope, R. S. et al. (1997)Ectopically expressed CAG repeats cause intranuclear inclusions and a progressive lateonset neurological phenotype in the mouse. Cell 91, 753–763

34 Rubinsztein, D. C. (2002) Lessons from animal models of Huntington’s disease.Trends Genet. 18, 202–209

35 Zuccato, C., Ciammola, A., Rigamonti, D., Leavitt, B. R., Goffredo, D., Conti, L.,MacDonald, M. E., Friedlander, R. M., Silani, V., Hayden, M. R. et al. (2001) Loss ofhuntingtin-mediated BDNF gene transcription in Huntington’s disease. Science 293,493–498

36 Reference deleted37 Kegel, K. B., Meloni, A. R., Yi, Y., Kim, Y. J., Doyle, E., Cuiffo, B. G., Sapp, E., Wang, Y.,

Qin, Z. H., Chen, J. D. et al. (2002) Huntingtin is present in the nucleus, interacts withthe transcriptional corepressor C-terminal binding protein, and represses transcription.J. Biol. Chem. 277, 7466–7476

38 Kegel, K. B., Sapp, E., Yoder, J., Cuiffo, B., Sobin, L., Kim, Y. J., Qin, Z.-H., Hayden,M. R., Aronin, N., Scott, D. L. et al. (2005) Huntingtin associates with acidicphospholipids at the plasma membrane. J. Biol. Chem. 280, 36464–36473

39 Caviston, J. P., Ross, J. L., Antony, S. M., Tokito, M. and Holzbaur, E. L. F. (2007)Huntingtin facilitates dynein/dynactin-mediated vesicle transport. Proc. Natl. Acad.Sci. U.S.A. 104, 10045–10050

40 Strehlow, A. N. T., Li, J. Z. and Myers, R. M. (2007) Wild-type huntingtin participates inprotein trafficking between the Golgi and the extracellular space. Hum. Mol. Genet. 16,391–409

41 Rockabrand, E., Slepko, N., Pantalone, A., Nukala, V. N., Kazantsev, A., Marsh, J. L.,Sullivan, P. G., Steffan, J. S., Sensi, S. L. and Thompson, L. M. (2007) The first 17amino acids of huntingtin modulate its sub-cellular localization, aggregation and effectson calcium homeostasis. Hum. Mol. Genet. 16, 61–77

42 Atwal, R. S., Xia, J., Pinchev, D., Taylor, J., Epand, R. M. and Truant, R. (2007)Huntingtin has a membrane association signal that can modulate huntingtinaggregation, nuclear entry and toxicity. Hum. Mol. Genet. 16, 2600–2615

43 Harjes, P. and Wanker, E. E. (2003) The hunt for huntingtin function: interaction partnerstell many different stories. Trends Biochem. Sci. 28, 425–433

44 Li, S.-H. and Li, X.-J. (2004) Huntingtin-protein interactions and the pathogenesis ofHuntington’s disease. Trends Genet. 20, 146–154

45 Clabough, E. B. D. and Zeitlin, S. O. (2006) Deletion of the triplet repeat encodingpolyglutamine within the mouse Huntington’s disease gene results in subtlebehavioral/motor phenotypes in vivo and elevated levels of ATP with cellular senescencein vitro. Hum. Mol. Genet. 15, 607–623

46 Li, W., Serpell, L. C., Carter, W. J., Rubinsztein, D. C. and Huntington, J. A. (2006)Expression and characterization of full-length human huntingtin, an elongated HEATrepeat protein. J. Biol. Chem. 281, 15916–15922

47 Takano, H. and Gusella, J. (2002) The predominantly HEAT-like motif structure ofhuntingtin and its association and coincident nuclear entry with dorsal, anNF-κB/Rel/dorsal family transcription factor. BMC Neurosci. 3, 15

48 Andrade, M. A., Petosa, C., O’Donoghue, S. I., Muller, C. W. and Bork, P. (2001)Comparison of ARM and HEAT protein repeats. J. Mol. Biol. 309, 1–18

49 Perry, J. and Kleckner, N. (2003) The ATRs, ATMs, and TORs are giant HEAT repeatproteins. Cell 112, 151–155

50 Neuwald, A. and Hirano, T. (2000) HEAT repeats associated with condensins, cohesins,and other complexes involved in chromosome-related functions. Genome Res. 10,1445–1452



51 Xia, J., Lee, D., Taylor, J., Vandelft, M. and Truant, R. (2003) Huntingtin contains a highlyconserved nuclear export signal. Hum. Mol. Genet. 12, 1393–1404

52 Cornett, J., Cao, F., Wang, C.-E., Ross, C. A., Bates, G. P., Li, S.-H. and Li, X.-J. (2005)Polyglutamine expansion of huntingtin impairs its nuclear export. 37, 198–204

53 Dohmen, R. J. (2004) SUMO protein modification. Biochim. Biophys. Acta 1695,113–131

54 Steffan, J. S., Agrawal, N., Pallos, J., Rockabrand, E., Trotman, L. C., Slepko, N., Illes, K.,Lukacsovich, T., Zhu, Y.-Z., Cattaneo, E. et al. (2004) SUMO modification of huntingtinand Huntington’s disease pathology. Science 304, 100–104

55 Kalchman, M. A., Graham, R. K., Xia, G., Koide, H. B., Hodgson, J. G., Graham, K. C.,Goldberg, Y. P., Gietz, R. D., Pickart, C. M. and Hayden, M. R. (1996) Huntingtin isubiquitinated and interacts with a specific ubiquitin-conjugating enzyme. J. Biol. Chem.271, 19385–19394

56 Huang, K., Yanai, A., Kang, R., Arstikaitis, P., Singaraja, R. R., Metzler, M., Mullard, A.,Haigh, B., Gauthier-Campbell, C., Gutekunst, C.-A. et al. (2004) Huntingtin-interactingprotein HIP14 is a palmitoyl transferase involved in palmitoylation and trafficking ofmultiple neuronal proteins. Neuron 44, 977–986

57 Yanai, A., Huang, K., Kang, R., Singaraja, R. R., Arstikaitis, P., Gan, L., Orban, P. C.,Mullard, A., Cowan, C. M., Raymond, L. A. et al. (2006) Palmitoylation of huntingtin byHIP14 is essential for its trafficking and function. 9, 824–831

58 Kim, Y. J., Yi, Y., Sapp, E., Wang, Y., Cuiffo, B., Kegel, K. B., Qin, Z. H., Aronin, N. andDiFiglia, M. (2001) Caspase 3-cleaved N-terminal fragments of wild-type and mutanthuntingtin are present in normal and Huntington’s disease brains, associate withmembranes, and undergo calpain-dependent proteolysis. Proc. Natl. Acad. Sci. U.S.A.98, 12784–12789

59 Gafni, J. and Ellerby, L. M. (2002) Calpain activation in Huntington’s disease.J. Neurosci. 22, 4842–4849

60 Wellington, C. L., Ellerby, L. M., Gutekunst, C. A., Rogers, D., Warby, S., Graham, R. K.,Loubser, O., van Raamsdonk, J., Singaraja, R., Yang, Y. Z. et al. (2002) Caspase cleavageof mutant huntingtin precedes neurodegeneration in Huntington’s disease. J. Neurosci.22, 7862–7872

61 Lunkes, A., Lindenberg, K. S., Ben-Haiem, L., Weber, C., Devys, D., Landwehrmeyer,G. B., Mandel, J. L. and Trottier, Y. (2002) Proteases acting on mutant huntingtingenerate cleaved products that differentially build up cytoplasmic and nuclearinclusions. Mol. Cell 10, 259–269

62 Hermel, E., Gafni, J., Propp, S. S., Leavitt, B. R., Wellington, C. L., Young, J. E., Hackam,A. S., Logvinova, A. V., Peel, A. L., Chen, S. F. et al. (2004) Specific caspase interactionsand amplification are involved in selective neuronal vulnerability in Huntington’sdisease. Cell Death Differ. 11, 424–438

63 Gafni, J., Hermel, E., Young, J. E., Wellington, C. L., Hayden, M. R. and Ellerby, L. M.(2004) Inhibition of calpain cleavage of huntingtin reduces toxicity: accumulation ofcalpain/caspase fragments in the nucleus. J. Biol. Chem. 279, 20211–20220

64 Graham, R. K., Deng, Y., Slow, E. J., Haigh, B., Bissada, N., Lu, G., Pearson, J.,Shehadeh, J., Bertram, L., Murphy, Z. et al. (2006) Cleavage at the caspase-6 site isrequired for neuronal dysfunction and degeneration due to mutant huntingtin. Cell 125,1179–1191

65 Kaltenbach, L. S., Romero, E., Becklin, R. R., Chettier, R., Bell, R., Phansalkar, A.,Strand, A., Torcassi, C., Savage, J., Hurlburt, A. et al. (2007) Huntingtin interactingproteins are genetic modifiers of neurodegeneration. PLoS Genet. 3, e82

66 Borrell-Pages, M., Zala, D., Humbert, S. and Saudou, F. (2006) Huntington’s disease:from huntingtin function and dysfunction to therapeutic strategies. Cell. Mol. Life Sci.63, 2642–2660

67 Lumsden, A. L., Henshall, T. L., Dayan, S., Lardelli, M. T. and Richards, R. I. (2007)Huntingtin-deficient zebrafish exhibit defects in iron utilization and development.Hum. Mol. Genet. 16, 1905–1920

68 Dragatsis, I., Levine, M. S. and Zeitlin, S. (2000) Inactivation of Hdh in the brain andtestis results in progressive neurodegeneration and sterility in mice. Nat. Genet. 26,300–306

69 Rigamonti, D., Sipione, S., Goffredo, D., Zuccato, C., Fossale, E. and Cattaneo, E. (2001)Huntingtin’s neuroprotective activity occurs via inhibition of procaspase-9 processing.J. Biol. Chem. 276, 14545–14548

70 Ho, L. W., Brown, R., Maxwell, M., Wyttenbach, A. and Rubinsztein, D. C. (2001) Wildtype huntingtin reduces the cellular toxicity of mutant huntingtin in mammalian cellmodels of Huntington’s disease. J. Med. Genet. 38, 450–452

71 Rigamonti, D., Bauer, J. H., De-Fraja, C., Conti, L., Sipione, S., Sciorati, C., Clementi, E.,Hackam, A., Hayden, M. R., Li, Y. et al. (2000) Wild-type huntingtin protects fromapoptosis upstream of caspase-3. J. Neurosci. 20, 3705–3713

72 Gervais, F. G., Singaraja, R., Xanthoudakis, S., Gutekunst, C. A., Leavitt, B. R.,Metzler, M., Hackam, A. S., Tam, J., Vaillancourt, J. P., Houtzager, V. et al. (2002)Recruitment and activation of caspase-8 by the huntingtin-interacting protein Hip-1 anda novel partner HIPPI. Nat. Cell Biol. 4, 95–105

73 Zhang, Y., Leavitt, B. R., van Raamsdonk, J. M., Dragatsis, I., Goldowitz, D., MacDonald,M. E., Hayden, M. R. and Friedlander, R. M. (2006) Huntingtin inhibits caspase-3activation. EMBO J. 25, 5896–5906

74 Faber, P., Barnes, G., Srinidhi, J., Chen, J., Gusella, J. and MacDonald, M. (1998)Huntingtin interacts with a family of WW domain proteins. Hum. Mol. Genet. 7,1463–1474

75 Truant, R., Atwal, R. S. and Burtnik, A. (2007) Nucleocytoplasmic trafficking andtranscription effects of huntingtin in Huntington’s disease. Prog. Neurobiol. 83,211–227

76 Zuccato, C., Tartari, M., Crotti, A., Goffredo, D., Valenza, M., Conti, L., Cataudella, T.,Leavitt, B. R., Hayden, M. R., Timmusk, T. et al. (2003) Huntingtin interacts withREST/NRSF to modulate the transcription of NRSE-controlled neuronal genes. 35,76–83

77 Smith, R., Brundin, P. and Li, J. Y. (2005) Synaptic dysfunction in Huntington’s disease:a new perspective. Cell. Mol. Life Sci. 62, 1901–1912

78 Gauthier, L. R., Charrin, B. C., Borrell-Pages, M., Dompierre, J. P., Rangone, H.,Cordelieres, F. P., De Mey, J., MacDonald, M. E., Lessmann, V. et al. (2004) Huntingtincontrols neurotrophic support and survival of neurons by enhancing BDNF vesiculartransport along microtubules. Cell 118, 127–138

79 Gunawardena, S., Her, L. S., Brusch, R. G., Laymon, R. A., Niesman, I. R.,Gordesky-Gold, B., Sintasath, L., Bonini, N. M. and Goldstein, L. S. (2003) Disruption ofaxonal transport by loss of huntingtin or expression of pathogenic polyQ proteins inDrosophila. Neuron 40, 25–40

80 Pal, A., Severin, F., Lommer, B., Shevchenko, A. and Zerial, M. (2006) Huntingtin–HAP40 complex is a novel Rab5 effector that regulates early endosome motility and isup-regulated in Huntington’s disease. J. Cell Biol. 172, 605–618

81 Trushina, E., Dyer, R. B., Badger, 2nd, J. D., Ure, D., Eide, L., Tran, D. D., Vrieze, B. T.,Legendre-Guillemin, V., McPherson, P. S., Mandavilli, B. S. et al. (2004) Mutanthuntingtin impairs axonal trafficking in mammalian neurons in vivo and in vitro.Mol. Cell. Biol. 24, 8195–8209

82 McGuire, J. R., Rong, J., Li, S.-H. and Li, X.-J. (2006) Interaction of huntingtin-associated protein-1 with kinesin light chain: implications in intracellular trafficking inneurons. J. Biol. Chem. 281, 3552–3559

83 Fan, M. M. and Raymond, L. A. (2007) N-methyl-D-aspartate (NMDA) receptor functionand excitotoxicity in Huntington’s disease. Prog. Neurobiol. 81, 272–293

84 Sun, Y., Savanenin, A., Reddy, P. H. and Liu, Y. F. (2001) Polyglutamine-expandedhuntingtin promotes sensitization of N-methyl-D-aspartate receptors via post-synapticdensity 95. J. Biol. Chem. 276, 24713–24718

85 Kim, J. H., Liao, D., Lau, L. F. and Huganir, R. L. (1998) SynGAP: a synaptic RasGAP thatassociates with the PSD-95/SAP90 protein family. Neuron 20, 683–691

86 Anborgh, P. H., Godin, C., Pampillo, M., Dhami, G. K., Dale, L. B., Cregan, S. P.,Truant, R. and Ferguson, S. S. (2005) Inhibition of metabotropic glutamate receptorsignaling by the huntingtin-binding protein optineurin. J. Biol. Chem. 280,34840–34848

87 Davies, S. W., Turmaine, M., Cozens, B. A., DiFiglia, M., Sharp, A. H., Ross, C. A.,Scherzinger, E., Wanker, E. E., Mangiarini, L. and Bates, G. P. (1997) Formation ofneuronal intranuclear inclusions underlies the neurological dysfunction in micetransgenic for the HD mutation. Cell 90, 537–548

88 Schilling, G., Becher, M. W., Sharp, A. H., Jinnah, H. A., Duan, K., Kotzuk, J. A., Slunt,H. H., Ratovitski, T., Cooper, J. K., Jenkins, N. A. et al. (1999) Intranuclear inclusionsand neuritic aggregates in transgenic mice expressing a mutant N-terminal fragment ofhuntingtin. Hum. Mol. Genet. 8, 397–407

89 Palfi, S., Brouillet, E., Jarraya, B., Bloch, J., Jan, C., Shin, M., Conde, F., Li, X. J.,Aebischer, P., Hantraye, P. and Deglon, N. (2007) Expression of mutated huntingtinfragment in the putamen is sufficient to produce abnormal movement in non-humanprimates. Mol. Ther. 15, 1444–1451

90 Wellington, C. L., Ellerby, L. M., Hackam, A. S., Margolis, R. L., Trifiro, M. A., Singaraja,R., McCutcheon, K., Salvesen, G. S., Propp, S. S., Bromm, M. et al. (1998) Caspasecleavage of gene products associated with triplet expansion disorders generatestruncated fragments containing the polyglutamine tract. J. Biol. Chem. 273, 9158–9167

91 Luo, S., Vacher, C., Davies, J. E. and Rubinsztein, D. C. (2005) Cdk5 phosphorylation ofhuntingtin reduces its cleavage by caspases: implications for mutant huntingtin toxicity.J. Cell Biol. 169, 647–656

92 Humbert, S., Bryson, E. A., Cordelieres, F. P., Connors, N. C., Datta, S. R., Finkbeiner, S.,Greenberg, M. E. and Saudou, F. (2002) The IGF-1/Akt pathway is neuroprotective inHuntington’s disease and involves huntingtin phosphorylation by Akt. Dev. Cell 2,831–837

93 Schilling, B., Gafni, J., Torcassi, C., Cong, X., Row, R. H., LaFevre-Bernt, M. A., Cusack,M. P., Ratovitski, T., Hirschhorn, R., Ross, C. A. et al. (2006) Huntingtin phosphorylationsites mapped by mass spectrometry: modulation of cleavage and toxicity. J. Biol. Chem.281, 23686–23697



94 Tanaka, Y., Igarashi, S., Nakamura, M., Gafni, J., Torcassi, C., Schilling, G., Crippen, D.,Wood, J. D., Sawa, A., Jenkins, N. A. et al. (2006) Progressive phenotype and nuclearaccumulation of an amino-terminal cleavage fragment in a transgenic mouse model withinducible expression of full-length mutant huntingtin. Neurobiol. Dis. 21,381–391

95 DiFiglia, M., Sapp, E., Chase, K. O., Davies, S. W., Bates, G. P., Vonsattel, J. P. andAronin, N. (1997) Aggregation of huntingtin in neuronal intranuclear inclusions anddystrophic neurites in brain. Science 277, 1990–1993

96 Becher, M. W., Kotzuk, J. A., Sharp, A. H., Davies, S. W., Bates, G. P., Price, D. L. andRoss, C. A. (1998) Intranuclear neuronal inclusions in Huntington’s disease anddentatorubral and pallidoluysian atrophy: correlation between the density of inclusionsand IT15 CAG triplet repeat length. Neurobiol. Dis. 4, 387–397

97 Gutekunst, C. A., Li, S. H., Yi, H., Mulroy, J. S., Kuemmerle, S., Jones, R., Rye, D.,Ferrante, R. J., Hersch, S. M. and Li, X. J. (1999) Nuclear and neuropil aggregates inHuntington’s disease: relationship to neuropathology. J. Neurosci. 19,2522–2534

98 Kuemmerle, S., Gutekunst, C. A., Klein, A. M., Li, X. J., Li, S. H., Beal, M. F., Hersch,S. M. and Ferrante, R. J. (1999) Huntington aggregates may not predict neuronal deathin Huntington’s disease. Ann. Neurol. 46, 842–849

99 Hackam, A. S., Singaraja, R., Wellington, C. L., Metzler, M., McCutcheon, K., Zhang, T.,Kalchman, M. and Hayden, M. R. (1998) The influence of huntingtin protein size onnuclear localization and cellular toxicity. J. Cell Biol. 141, 1097–1105

100 Wyttenbach, A., Carmichael, J., Swartz, J., Furlong, R. A., Narain, Y., Rankin, J. andRubinsztein, D. C. (2000) Effects of heat shock, heat shock protein 40 (HDJ-2), andproteasome inhibition on protein aggregation in cellular models of Huntington’s disease.Proc. Natl. Acad. Sci. U.S.A. 97, 2898–2903

101 Lunkes, A. and Mandel, J. L. (1998) A cellular model that recapitulates major pathogenicsteps of Huntington’s disease. Hum. Mol. Genet. 7, 1355–1361

102 Morton, A. J., Lagan, M. A., Skepper, J. N. and Dunnett, S. B. (2000) Progressiveformation of inclusions in the striatum and hippocampus of mice transgenic for thehuman Huntington’s disease mutation. J. Neurocytol. 29, 679–702

103 Tam, S., Geller, R., Spiess, C. and Frydman, J. (2006) The chaperonin TRiC controlspolyglutamine aggregation and toxicity through subunit-specific interactions.Nat. Cell Biol. 8, 1155–1162

104 Mitsui, K., Nakayama, H., Akagi, T., Nekooki, M., Ohtawa, K., Takio, K., Hashikawa, T.and Nukina, N. (2002) Purification of polyglutamine aggregates and identification ofelongation factor-1α and heat shock protein 84 as aggregate-interacting proteins.J. Neurosci. 22, 9267–9277

105 Kitamura, A., Kubota, H., Pack, C. G., Matsumoto, G., Hirayama, S., Takahashi, Y.,Kimura, H., Kinjo, M., Morimoto, R. I. and Nagata, K. (2006) Cytosolic chaperoninprevents polyglutamine toxicity with altering the aggregation state. Nat. Cell Biol. 8,1163–1170

106 Jana, N. R., Dikshit, P., Goswami, A., Kotliarova, S., Murata, S., Tanaka, K. andNukina, N. (2005) Co-chaperone CHIP associates with expanded polyglutamineprotein and promotes their degradation by proteasomes. J. Biol. Chem. 280,11635–11640

107 Chuang, J. Z., Zhou, H., Zhu, M., Li, S. H., Li, X. J. and Sung, C. H. (2002)Characterization of a brain-enriched chaperone, MRJ, that inhibits huntingtinaggregation and toxicity independently. J. Biol. Chem. 277, 19831–19838

108 Jana, N. R., Tanaka, M., Wang, G. and Nukina, N. (2000) Polyglutaminelength-dependent interaction of Hsp40 and Hsp70 family chaperones with truncatedN-terminal huntingtin: their role in suppression of aggregation and cellular toxicity.Hum. Mol. Genet. 9, 2009–2018

109 Vacher, C., Garcia-Oroz, L. and Rubinsztein, D. C. (2005) Overexpression of yeasthsp104 reduces polyglutamine aggregation and prolongs survival of a transgenic mousemodel of Huntington’s disease. Hum. Mol. Genet. 14, 3425–3433

110 Carmichael, J., Chatellier, J., Woolfson, A., Milstein, C., Fersht, A. R. and Rubinsztein,D. C. (2000) Bacterial and yeast chaperones reduce both aggregate formation and celldeath in mammalian cell models of Huntington’s disease. Proc. Natl. Acad. Sci. U.S.A.97, 9701–9705

111 Wyttenbach, A., Sauvageot, O., Carmichael, J., Diaz-Latoud, C., Arrigo, A. P. andRubinsztein, D. C. (2002) Heat shock protein 27 prevents cellular polyglutamine toxicityand suppresses the increase of reactive oxygen species caused by huntingtin.Hum. Mol. Genet. 11, 1137–1151

112 Goswami, A., Dikshit, P., Mishra, A., Mulherkar, S., Nukina, N. and Jana, N. R. (2006)Oxidative stress promotes mutant huntingtin aggregation and mutanthuntingtin-dependent cell death by mimicking proteasomal malfunction. Biochem.Biophys. Res. Commun. 342, 184–190

113 Schiffer, N. W., Broadley, S. A., Hirschberger, T., Tavan, P., Kretzschmar, H. A., Giese, A.,Haass, C., Hartl, F. U. and Schmid, B. (2007) Identification of anti-prion compounds asefficient inhibitors of polyglutamine protein aggregation in a zebrafish model.J. Biol. Chem. 282, 9195–9203

114 Mastroberardino, P. G., Iannicola, C., Nardacci, R., Bernassola, F., De Laurenzi, V.,Melino, G., Moreno, S., Pavone, F., Oliverio, S., Fesus, L. and Piacentini, M. (2002)‘Tissue’ transglutaminase ablation reduces neuronal death and prolongs survival in amouse model of Huntington’s disease. Cell Death Differ. 9, 873–880

115 Arango, M., Holbert, S., Zala, D., Brouillet, E., Pearson, J., Regulier, E., Thakur, A. K.,Aebischer, P., Wetzel, R., Deglon, N. and Neri, C. (2006) CA150 expression delaysstriatal cell death in overexpression and knock-in conditions for mutant huntingtinneurotoxicity. J. Neurosci. 26, 4649–4659

116 Bodner, R. A., Outeiro, T. F., Altmann, S., Maxwell, M. M., Cho, S. H., Hyman, B. T.,McLean, P. J., Young, A. B., Housman, D. E. and Kazantsev, A. G. (2006)Pharmacological promotion of inclusion formation: a therapeutic approach forHuntington’s and Parkinson’s diseases. Proc. Natl. Acad. Sci. U.S.A. 103,4246–4251

117 Arrasate, M., Mitra, S., Schweitzer, E. S., Segal, M. R. and Finkbeiner, S. (2004)Inclusion body formation reduces levels of mutant huntingtin and the risk of neuronaldeath. Nature 431, 805–810

118 Martindale, D., Hackam, A., Wieczorek, A., Ellerby, L., Wellington, C., McCutcheon, K.,Singaraja, R., Kazemi-Esfarjani, P., Devon, R., Kim, S. U. et al. (1998) Length ofhuntingtin and its polyglutamine tract influences localization and frequency ofintracellular aggregates. Nat. Genet. 18, 150–154

119 Saudou, F., Finkbeiner, S., Devys, D. and Greenberg, M. E. (1998) Huntingtin acts in thenucleus to induce apoptosis but death does not correlate with the formation ofintranuclear inclusions. Cell 95, 55–66

120 Ross, C. A. and Poirier, M. A. (2004) Protein aggregation and neurodegenerativedisease. Nat. Med. 10, (Suppl.), S10–S17

121 Schilling, G., Savonenko, A. V., Klevytska, A., Morton, J. L., Tucker, S. M., Poirier, M.,Gale, A., Chan, N., Gonzales, V. and Slunt, H. H. (2004) Nuclear-targeting of mutanthuntingtin fragments produces Huntington’s disease-like phenotypes in transgenic mice.Hum. Mol. Genet. 13, 1599–1610

122 Perutz, M. F., Johnson, T., Suzuki, M. and Finch, J. T. (1994) Glutamine repeats as polarzippers: their possible role in inherited neurodegenerative diseases. Proc. Natl. Acad.Sci. U.S.A. 91, 5355–5358

123 Gerber, H. P., Seipel, K., Georgiev, O., Hofferer, M., Hug, M., Rusconi, S. and Schaffner,W. (1994) Transcriptional activation modulated by homopolymeric glutamine andproline stretches. Science 263, 808–811

124 Kazantsev, A., Preisinger, E., Dranovsky, A., Goldgaber, D. and Housman, D. (1999)Insoluble detergent-resistant aggregates form between pathological and nonpathologicallengths of polyglutamine in mammalian cells. Proc. Natl. Acad. Sci. U.S.A. 96,11404–11409

125 Nucifora, Jr, F. C., Sasaki, M., Peters, M. F., Huang, H., Cooper, J. K., Yamada, M.,Takahashi, H., Tsuji, S., Troncoso, J., Dawson, V. L. et al. (2001) Interference byhuntingtin and atrophin-1 with CBP-mediated transcription leading to cellular toxicity.Science 291, 2423–2428

126 Steffan, J. S., Kazantsev, A., Spasic-Boskovic, O., Greenwald, M., Zhu, Y. Z., Gohler, H.,Wanker, E. E., Bates, G. P., Housman, D. E. and Thompson, L. M. (2000) TheHuntington’s disease protein interacts with p53 and CREB-binding protein and repressestranscription. Proc. Natl. Acad. Sci. U.S.A. 97, 6763–6768

127 Steffan, J. S., Bodai, L., Pallos, J., Poelman, M., McCampbell, A., Apostol, B. L.,Kazantsev, A., Schmidt, E., Zhu, Y. Z., Greenwald, M. et al. (2001) Histone deacetylaseinhibitors arrest polyglutamine-dependent neurodegeneration in Drosophila. Nature413, 739–743

128 McCampbell, A., Taye, A. A., Whitty, L., Penney, E., Steffan, J. S. and Fischbeck, K. H.(2001) Histone deacetylase inhibitors reduce polyglutamine toxicity. Proc. Natl. Acad.Sci. U.S.A. 98, 15179–15184

129 Hughes, R. E., Lo, R. S., Davis, C., Strand, A. D., Neal, C. L., Olson, J. M. and Fields, S.(2001) Altered transcription in yeast expressing expanded polyglutamine. Proc. Natl.Acad. Sci. U.S.A. 98, 13201–13206

130 Ferrante, R. J., Kubilus, J. K., Lee, J., Ryu, H., Beesen, A., Zucker, B., Smith, K., Kowall,N. W., Ratan, R. R., Luthi-Carter, R. and Hersch, S. M. (2003) Histone deacetylaseinhibition by sodium butyrate chemotherapy ameliorates the neurodegenerativephenotype in Huntington’s disease mice. J. Neurosci. 23, 9418–9427

131 Hockly, E., Richon, V. M., Woodman, B., Smith, D. L., Zhou, X., Rosa, E., Sathasivam, K.,Ghazi-Noori, S., Mahal, A., Lowden, P. A. et al. (2003) Suberoylanilide hydroxamic acid,a histone deacetylase inhibitor, ameliorates motor deficits in a mouse model ofHuntington’s disease. Proc. Natl. Acad. Sci. U.S.A. 100, 2041–2046

132 Tanese, N. and Tjian, R. (1993) Coactivators and TAFs: a new class of eukaryotictranscription factors that connect activators to the basal machinery. Cold Spring HarborSymp. Quant. Biol. 58, 179–185

133 Yu, Z. X., Li, S. H., Nguyen, H. P. and Li, X. J. (2002) Huntingtin inclusions do notdeplete polyglutamine-containing transcription factors in HD mice. Hum. Mol. Genet.11, 905–914



134 Dunah, A. W., Jeong, H., Griffin, A., Kim, Y. M., Standaert, D. G., Hersch, S. M.,Mouradian, M. M., Young, A. B., Tanese, N. and Krainc, D. (2002) Sp1 and TAFII130transcriptional activity disrupted in early Huntington’s disease. Science 296, 2238–2243

135 Qiu, Z., Norflus, F., Singh, B., Swindell, M. K., Buzescu, R., Bejarano, M., Chopra, R.,Zucker, B., Benn, C. L., DiRocco, D. P. et al. (2006) Sp1 is up-regulated in cellular andtransgenic models of Huntington disease, and its reduction is neuroprotective.J. Biol. Chem. 281, 16672–16680

136 Zhai, W., Jeong, H., Cui, L., Krainc, D. and Tjian, R. (2005) In vitro analysis ofhuntingtin-mediated transcriptional repression reveals multiple transcription factortargets. Cell 123, 1241–1253

137 Hodgson, J. G., Agopyan, N., Gutekunst, C. A., Leavitt, B. R., LePiane, F., Singaraja, R.,Smith, D. J., Bissada, N., McCutcheon, K., Nasir, J. et al. (1999) A YAC mouse model forHuntington’s disease with full-length mutant huntingtin, cytoplasmic toxicity, andselective striatal neurodegeneration. Neuron 23, 181–192

138 Chong, J. A., Tapia-Ramirez, J., Kim, S., Toledo-Aral, J. J., Zheng, Y., Boutros, M. C.,Altshuller, Y. M., Frohman, M. A., Kraner, S. D. and Mandel, G. (1995) REST: amammalian silencer protein that restricts sodium channel gene expression to neurons.Cell 80, 949–957

139 Reference deleted140 Schoenherr, C. J. and Anderson, D. J. (1995) The neuron-restrictive silencer factor

(NRSF): a coordinate repressor of multiple neuron-specific genes. Science 267,1360–1363

141 Puigserver, P., Wu, Z., Park, C. W., Graves, R., Wright, M. and Spiegelman, B. M. (1998)A cold-inducible coactivator of nuclear receptors linked to adaptive thermogenesis. Cell92, 829–839

142 Puigserver, P. and Spiegelman, B. M. (2003) Peroxisome proliferator activatedreceptor-γ coactivator 1α (PGC-1α): transcriptional coactivator and metabolicregulator. Endocr. Rev. 24, 78–90

143 Lin, J., Wu, H., Tarr, P. T., Zhang, C. Y., Wu, Z., Boss, O., Michael, L. F., Puigserver, P.,Isotani, E., Olson, E. N. et al. (2002) Transcriptional co-activator PGC-1α drives theformation of slow-twitch muscle fibres. Nature 418, 797–801

144 Leone, T. C., Lehman, J. J., Finck, B. N., Schaeffer, P. J., Wende, A. R., Boudina, S.,Courtois, M., Wozniak, D. F., Sambandam, N., Bernal-Mizrachi, C. et al. (2005) PGC-1αdeficiency causes multi-system energy metabolic derangements: muscle dysfunction,abnormal weight control and hepatic steatosis. PLoS Biol. 3, e101

145 Cui, L., Jeong, H., Borovecki, F., Parkhurst, C. N., Tanese, N. and Krainc, D. (2006)Transcriptional repression of PGC-1α by mutant huntingtin leads to mitochondrialdysfunction and neurodegeneration. Cell 127, 59–69

146 Weydt, P., Pineda, V. V., Torrence, A. E., Libby, R. T., Satterfield, T. F., Lazarowski, E. R.,Gilbert, M. L., Morton, G. J., Bammler, T. K., Strand, A. D. et al. (2006) Thermoregulatoryand metabolic defects in Huntington’s disease transgenic mice implicate PGC-1α inHuntington’s disease neurodegeneration. Cell Metab. 4, 349–362

147 Trushina, E. and McMurray, C. T. (2007) Oxidative stress and mitochondrial dysfunctionin neurodegenerative diseases. Neuroscience 145, 1233–1248

148 Lin, M. T. and Beal, M. F. (2006) Mitochondrial dysfunction and oxidative stress inneurodegenerative diseases. Nature 443, 787–795

149 Beal, M. F. (2005) Mitochondria take center stage in aging and neurodegeneration.Ann. Neurol. 58, 495–505

150 Raha, S. and Robinson, B. H. (2000) Mitochondria, oxygen free radicals, disease andageing. Trends Biochem. Sci. 25, 502–508

151 Chance, B., Sies, H. and Boveris, A. (1979) Hydroperoxide metabolism in mammalianorgans. Physiol. Rev. 59, 527–605

152 Guidot, D. M., McCord, J. M., Wright, R. M. and Repine, J. E. (1993) Absence of electrontransport (Rho 0 state) restores growth of a manganese superoxide dismutase-deficientSaccharomyces cerevisiae in hyperoxia: evidence for electron transport as a majorsource of superoxide generation in vivo. J. Biol. Chem. 268, 26699–26703

153 Floyd, R. A. and Carney, J. M. (1992) Free radical damage to protein and DNA:mechanisms involved and relevant observations on brain undergoing oxidative stress.Ann. Neurol. 32, (Suppl.), S22–S27

154 Huie, R. E. and Padmaja, S. (1993) The reaction of NO with superoxide. Free RadicalRes. Commun. 18, 195–199

155 Radi, R., Rodriguez, M., Castro, L. and Telleri, R. (1994) Inhibition of mitochondrialelectron transport by peroxynitrite. Arch. Biochem. Biophys. 308, 89–95

156 Castro, L., Rodriguez, M. and Radi, R. (1994) Aconitase is readily inactivated byperoxynitrite, but not by its precursor, nitric oxide. J. Biol. Chem. 269, 29409–29415

157 MacMillan-Crow, L. A., Crow, J. P. and Thompson, J. A. (1998) Peroxynitrite-mediatedinactivation of manganese superoxide dismutase involves nitration and oxidation ofcritical tyrosine residues. Biochemistry 37, 1613–1622

158 Imlay, J. A. (2003) Pathways of oxidative damage. Annu. Rev. Microbiol. 57, 395–418159 Gardner, P. R., Raineri, I., Epstein, L. B. and White, C. W. (1995) Superoxide radical and

iron modulate aconitase activity in mammalian cells. J. Biol. Chem. 270, 13399–13405

160 Balaban, R. S., Nemoto, S. and Finkel, T. (2005) Mitochondria, oxidants, and aging. Cell120, 483–495

161 Finkel, T. (2003) Oxidant signals and oxidative stress. Curr. Opin. Cell Biol. 15, 247–254162 Sohal, R. S. (2002) Role of oxidative stress and protein oxidation in the aging process.

Free Radical Biol. Med. 33, 37–44163 Browne, S. E. and Beal, M. F. (2004) The energetics of Huntington’s disease.

Neurochem. Res. 29, 531–546164 Tabrizi, S. J., Cleeter, M. W., Xuereb, J., Taanman, J. W., Cooper, J. M. and Schapira,

A. H. (1999) Biochemical abnormalities and excitotoxicity in Huntington’s disease brain.Ann. Neurol. 45, 25–32

165 Mangiarini, L., Sathasivam, K., Seller, M., Cozens, B., Harper, A., Hetherington, C.,Lawton, M., Trottier, Y., Lehrach, H., Davies, S. W. and Bates, G. P. (1996) Exon 1 of theHD gene with an expanded CAG repeat is sufficient to cause a progressive neurologicalphenotype in transgenic mice. Cell 87, 493–506

166 Saft, C., Zange, J., Andrich, J., Muller, K., Lindenberg, K., Landwehrmeyer, B., Vorgerd,M., Kraus, P. H., Przuntek, H. and Schols, L. (2005) Mitochondrial impairment in patientsand asymptomatic mutation carriers of Huntington’s disease. Mov. Disord. 20, 674–679

167 Orth, M. and Schapira, A. H. (2001) Mitochondria and degenerative disorders. Am. J.Med. Genet. 106, 27–36

168 Choo, Y. S., Johnson, G. V., MacDonald, M., Detloff, P. J. and Lesort, M. (2004) Mutanthuntingtin directly increases susceptibility of mitochondria to the calcium-inducedpermeability transition and cytochrome c release. Hum. Mol. Genet. 13, 1407–1420

169 Panov, A. V., Burke, J. R., Strittmatter, W. J. and Greenamyre, J. T. (2003) In vitro effectsof polyglutamine tracts on Ca2+-dependent depolarization of rat and humanmitochondria: relevance to Huntington’s disease. Arch. Biochem. Biophys. 410, 1–6

170 Panov, A. V., Gutekunst, C. A., Leavitt, B. R., Hayden, M. R., Burke, J. R., Strittmatter,W. J. and Greenamyre, J. T. (2002) Early mitochondrial calcium defects in Huntington’sdisease are a direct effect of polyglutamines. Nat. Neurosci. 5, 731–736

171 Sawa, A., Wiegand, G. W., Cooper, J., Margolis, R. L., Sharp, A. H., Lawler, Jr, J. F.,Greenamyre, J. T., Snyder, S. H. and Ross, C. A. (1999) Increased apoptosis ofHuntington disease lymphoblasts associated with repeat length dependentmitochondrial depolarization. Nat. Med. 5, 1194–1198

172 Carafoli, E. (2003) Historical review: mitochondria and calcium: ups and downs of anunusual relationship. Trends Biochem. Sci. 28, 175–181

173 Orrenius, S., Zhivotovsky, B. and Nicotera, P. (2003) Regulation of cell death: thecalcium–apoptosis link. Nat. Rev. Mol. Cell Biol. 4, 552–565

174 Seong, I. S., Ivanova, E., Lee, J. M., Choo, Y. S., Fossale, E., Anderson, M., Gusella,J. F., Laramie, J. M., Myers, R. H., Lesort, M. and MacDonald, M. E. (2005) HD CAGrepeat implicates a dominant property of huntingtin in mitochondrial energy metabolism.Hum. Mol. Genet. 14, 2871–2880

175 Novelli, A., Reilly, J. A., Lysko, P. G. and Henneberry, R. C. (1988) Glutamate becomesneurotoxic via the N-methyl-D-aspartate receptor when intracellular energy levels arereduced. Brain Res. 451, 205–212

176 Fagni, L., Lafon-Cazal, M., Rondouin, G., Manzoni, O., Lerner-Natoli, M. and Bockaert,J. (1994) The role of free radicals in NMDA-dependent neurotoxicity. Prog. Brain Res.103, 381–390

177 Zeron, M. M., Fernandes, H. B., Krebs, C., Shehadeh, J., Wellington, C. L., Leavitt, B. R.,Baimbridge, K. G., Hayden, M. R. and Raymond, L. A. (2004) Potentiation of NMDAreceptor-mediated excitotoxicity linked with intrinsic apoptotic pathway in YACtransgenic mouse model of Huntington’s disease. Mol. Cell. Neurosci. 25, 469–479

178 Zeron, M. M., Hansson, O., Chen, N., Wellington, C. L., Leavitt, B. R., Brundin, P.,Hayden, M. R. and Raymond, L. A. (2002) Increased sensitivity to N-methyl-D-aspartatereceptor-mediated excitotoxicity in a mouse model of Huntington’s disease. Neuron 33,849–860

179 Lafon-Cazal, M., Pietri, S., Culcasi, M. and Bockaert, J. (1993) NMDA-dependentsuperoxide production and neurotoxicity. Nature 364, 535–537

180 Reynolds, I. J. and Hastings, T. G. (1995) Glutamate induces the production of reactiveoxygen species in cultured forebrain neurons following NMDA receptor activation.J. Neurosci. 15, 3318–3327

181 Bolanos, J. P., Almeida, A., Fernandez, E., Medina, J. M., Land, J. M., Clark, J. B. andHeales, S. J. (1997) Potential mechanisms for nitric oxide-mediated impairment of brainmitochondrial energy metabolism. Biochem. Soc. Trans. 25, 944–949

182 Perez-Severiano, F., Rios, C. and Segovia, J. (2000) Striatal oxidative damage parallelsthe expression of a neurological phenotype in mice transgenic for the mutation ofHuntington’s disease. Brain Res. 862, 234–237

183 Halliwell, B. (2006) Oxidative stress and neurodegeneration: where are we now?J. Neurochem. 97, 1634–1658

184 Stavrovskaya, I. G. and Kristal, B. S. (2005) The powerhouse takes control of the cell: isthe mitochondrial permeability transition a viable therapeutic target against neuronaldysfunction and death? Free Radical Biol. Med. 38, 687–697



185 Chang, D. T., Rintoul, G. L., Pandipati, S. and Reynolds, I. J. (2006) Mutant huntingtinaggregates impair mitochondrial movement and trafficking in cortical neurons.Neurobiol. Dis. 22, 388–400

186 Smeitink, J., van den Heuvel, L. and DiMauro, S. (2001) The genetics and pathology ofoxidative phosphorylation. Nat. Rev. Genet. 2, 342–352

187 Polidori, M. C., Mecocci, P., Browne, S. E., Senin, U. and Beal, M. F. (1999) Oxidativedamage to mitochondrial DNA in Huntington’s disease parietal cortex. Neurosci. Lett.272, 53–56

188 Bogdanov, M. B., Andreassen, O. A., Dedeoglu, A., Ferrante, R. J. and Beal, M. F. (2001)Increased oxidative damage to DNA in a transgenic mouse model of Huntington’sdisease. J. Neurochem. 79, 1246–1249

189 Browne, S. E., Bowling, A. C., MacGarvey, U., Baik, M. J., Berger, S. C., Muqit, M. M.,Bird, E. D. and Beal, M. F. (1997) Oxidative damage and metabolic dysfunction inHuntington’s disease: selective vulnerability of the basal ganglia. Ann. Neurol. 41,646–653

190 Taylor, R. W. and Turnbull, D. M. (2005) Mitochondrial DNA mutations in humandisease. Nat. Rev. Genet. 6, 389–402

191 Liu, C. Y., Lee, C. F., Hong, C. H. and Wei, Y. H. (2004) Mitochondrial DNA mutation anddepletion increase the susceptibility of human cells to apoptosis. Ann. N.Y. Acad. Sci.1011, 133–145

192 Fraga, C. G., Shigenaga, M. K., Park, J. W., Degan, P. and Ames, B. N. (1990) Oxidativedamage to DNA during aging: 8-hydroxy-2′-deoxyguanosine in rat organ DNA andurine. Proc. Natl. Acad. Sci. U.S.A. 87, 4533–4537

193 Kovtun, I. V., Liu, Y., Bjoras, M., Klungland, A., Wilson, S. H. and McMurray, C. T.(2007) OGG1 initiates age-dependent CAG trinucleotide expansion in somatic cells.Nature 447, 447–452

194 Zourlidou, A., Gidalevitz, T., Kristiansen, M., Landles, C., Woodman, B., Wells, D. J.,Latchman, D. S., de Belleroche, J., Tabrizi, S. J., Morimoto, R. I. and Bates, G. P. (2007)Hsp27 overexpression in the R6/2 mouse model of Huntington’s disease: chronicneurodegeneration does not induce Hsp27 activation. Hum. Mol. Genet. 16,1078–1090

195 Perluigi, M., Poon, H. F., Maragos, W., Pierce, W. M., Klein, J. B., Calabrese, V., Cini, C.,De Marco, C. and Butterfield, D. A. (2005) Proteomic analysis of protein expression andoxidative modification in r6/2 transgenic mice: a model of Huntington disease.Mol. Cell. Proteomics 4, 1849–1861

196 Griffiths, H. R., Moller, L., Bartosz, G., Bast, A., Bertoni-Freddari, C., Collins, A., Cooke,M., Coolen, S., Haenen, G., Hoberg, A. M. et al. (2002) Biomarkers. Mol. Aspects Med.23, 101–208

197 Poon, H. F., Frasier, M., Shreve, N., Calabrese, V., Wolozin, B. and Butterfield, D. A.(2005) Mitochondrial associated metabolic proteins are selectively oxidized in A30Pα-synuclein transgenic mice: a model of familial Parkinson’s disease. Neurobiol. Dis.18, 492–498

198 Malorni, W., Rainaldi, G., Rivabene, R., Santini, M. T., Peterson, S. W., Testa, U. andDonelli, G. (1994) Cytoskeletal oxidative changes lead to alterations of specific cellsurface receptors. Eur. J. Histochem. 38, (Suppl. 1), 91–100

199 Bence, N. F., Sampat, R. M. and Kopito, R. R. (2001) Impairment of the ubiquitin–proteasome system by protein aggregation. Science 292, 1552–1555

200 Jana, N. R., Zemskov, E. A., Wang, G. and Nukina, N. (2001) Altered proteasomalfunction due to the expression of polyglutamine-expanded truncated N-terminalhuntingtin induces apoptosis by caspase activation through mitochondrial cytochrome crelease. Hum. Mol. Genet. 10, 1049–1059

201 Ding, Q., Lewis, J. J., Strum, K. M., Dimayuga, E., Bruce-Keller, A. J., Dunn, J. C. andKeller, J. N. (2002) Polyglutamine expansion, protein aggregation, proteasome activity,and neural survival. J. Biol. Chem. 277, 13935–13942

202 Bowman, A. B., Yoo, S. Y., Dantuma, N. P. and Zoghbi, H. Y. (2005) Neuronaldysfunction in a polyglutamine disease model occurs in the absence ofubiquitin–proteasome system impairment and inversely correlates with the degree ofnuclear inclusion formation. Hum. Mol. Genet. 14, 679–691

203 Diaz-Hernandez, M., Hernandez, F., Martin-Aparicio, E., Gomez-Ramos, P., Moran,M. A., Castano, J. G., Ferrer, I., Avila, J. and Lucas, J. J. (2003) Neuronal induction ofthe immunoproteasome in Huntington’s disease. J. Neurosci. 23, 11653–11661

204 Bett, J. S., Goellner, G. M., Woodman, B., Pratt, G., Rechsteiner, M. and Bates, G. P.(2006) Proteasome impairment does not contribute to pathogenesis in R6/2Huntington’s disease mice: exclusion of proteasome activator REGγ as a therapeutictarget. Hum. Mol. Genet. 15, 33–44

205 Goldberg, A. L. (2003) Protein degradation and protection against misfolded ordamaged proteins. Nature 426, 895–899

206 Ciechanover, A. (2006) The ubiquitin proteolytic system: from a vague idea, throughbasic mechanisms, and onto human diseases and drug targeting. Neurology 66,S7–S19

207 DeMartino, G. N. and Slaughter, C. A. (1999) The proteasome, a novel proteaseregulated by multiple mechanisms. J. Biol. Chem. 274, 22123–22126

208 DeMartino, G. N., Moomaw, C. R., Zagnitko, O. P., Proske, R. J., Chu-Ping, M., Afendis,S. J., Swaffield, J. C. and Slaughter, C. A. (1994) PA700, an ATP-dependent activator ofthe 20 S proteasome, is an ATPase containing multiple members of a nucleotide-bindingprotein family. J. Biol. Chem. 269, 20878–20884

209 Hershko, A. and Ciechanover, A. (1998) The ubiquitin system. Annu. Rev. Biochem. 67,425–479

210 Goldberg, A. L., Cascio, P., Saric, T. and Rock, K. L. (2002) The importance of theproteasome and subsequent proteolytic steps in the generation of antigenic peptides.Mol. Immunol. 39, 147–164

211 Madura, K. (2004) Rad23 and Rpn10: perennial wallflowers join the melee. TrendsBiochem. Sci. 29, 637–640

212 Fruh, K., Gossen, M., Wang, K., Bujard, H., Peterson, P. A. and Yang, Y. (1994)Displacement of housekeeping proteasome subunits by MHC-encoded LMPs: a newlydiscovered mechanism for modulating the multicatalytic proteinase complex. EMBO J.13, 3236–3244

213 Bingol, B. and Schuman, E. M. (2006) Activity-dependent dynamics and sequestrationof proteasomes in dendritic spines. Nature 441, 1144–1148

214 Cummings, C. J., Mancini, M. A., Antalffy, B., DeFranco, D. B., Orr, H. T. and Zoghbi,H. Y. (1998) Chaperone suppression of aggregation and altered subcellular proteasomelocalization imply protein misfolding in SCA1. Nat. Genet. 19, 148–154

215 Bennett, E. J., Bence, N. F., Jayakumar, R. and Kopito, R. R. (2005) Global impairment ofthe ubiquitin–proteasome system by nuclear or cytoplasmic protein aggregatesprecedes inclusion body formation. Mol. Cell 17, 351–365

216 Kim, S., Nollen, E. A., Kitagawa, K., Bindokas, V. P. and Morimoto, R. I. (2002)Polyglutamine protein aggregates are dynamic. Nat. Cell Biol. 4, 826–831

217 Holmberg, C. I., Staniszewski, K. E., Mensah, K. N., Matouschek, A. and Morimoto, R. I.(2004) Inefficient degradation of truncated polyglutamine proteins by the proteasome.EMBO J. 23, 4307–4318

218 Venkatraman, P., Wetzel, R., Tanaka, M., Nukina, N. and Goldberg, A. L. (2004)Eukaryotic proteasomes cannot digest polyglutamine sequences and release themduring degradation of polyglutamine-containing proteins. Mol. Cell 14, 95–104

219 Diaz-Hernandez, M., Valera, A. G., Moran, M. A., Gomez-Ramos, P., Alvarez-Castelao,B., Castano, J. G., Hernandez, F. and Lucas, J. J. (2006) Inhibition of 26S proteasomeactivity by huntingtin filaments but not inclusion bodies isolated from mouse and humanbrain. J. Neurochem. 98, 1585–1596

220 Seo, H., Sonntag, K. C. and Isacson, O. (2004) Generalized brain and skin proteasomeinhibition in Huntington’s disease. Ann. Neurol. 56, 319–328

221 Seo, H., Sonntag, K. C., Kim, W., Cattaneo, E. and Isacson, O. (2007) Proteasomeactivator enhances survival of huntington’s disease neuronal model cells. PLoS ONE 2,e238

222 Beister, A., Kraus, P., Kuhn, W., Dose, M., Weindl, A. and Gerlach, M. (2004) TheN-methyl-D-aspartate antagonist memantine retards progression of Huntington’sdisease. J. Neural Transm. Suppl. 68, 117–122

223 Ravikumar, B., Vacher, C., Berger, Z., Davies, J. E., Luo, S., Oroz, L. G., Scaravilli, F.,Easton, D. F., Duden, R., O’Kane, C. J. and Rubinsztein, D. C. (2004) Inhibition of mTORinduces autophagy and reduces toxicity of polyglutamine expansions in fly and mousemodels of Huntington disease. Nat. Genet. 36, 585–595

224 Tsai, Y. C., Fishman, P. S., Thakor, N. V. and Oyler, G. A. (2003) Parkin facilitates theelimination of expanded polyglutamine proteins and leads to preservation of proteasomefunction. J. Biol. Chem. 278, 22044–22055

225 Martin-Aparicio, E., Yamamoto, A., Hernandez, F., Hen, R., Avila, J. and Lucas, J. J.(2001) Proteasomal-dependent aggregate reversal and absence of cell death in aconditional mouse model of Huntington’s disease. J. Neurosci. 21, 8772–8781

226 Waelter, S., Boeddrich, A., Lurz, R., Scherzinger, E., Lueder, G., Lehrach, H. and Wanker,E. E. (2001) Accumulation of mutant huntingtin fragments in aggresome-like inclusionbodies as a result of insufficient protein degradation. Mol. Biol. Cell 12, 1393–1407

227 Ravikumar, B., Duden, R. and Rubinsztein, D. C. (2002) Aggregate-prone proteins withpolyglutamine and polyalanine expansions are degraded by autophagy.Hum. Mol. Genet. 11, 1107–1117

228 Bhutani, N., Venkatraman, P. and Goldberg, A. L. (2007) Puromycin-sensitiveaminopeptidase is the major peptidase responsible for digesting polyglutaminesequences released by proteasomes during protein degradation. EMBO J. 26,1385–1396

229 Sanchez, I., Mahlke, C. and Yuan, J. (2003) Pivotal role of oligomerization in expandedpolyglutamine neurodegenerative disorders. Nature 421, 373–379

230 Davies, J. E., Sarkar, S. and Rubinsztein, D. C. (2006) Trehalose reduces aggregateformation and delays pathology in a transgenic mouse model of oculopharyngealmuscular dystrophy. Hum. Mol. Genet. 15, 23–31

231 Li, W., Serpell, L. C., Carter, W. J., Rubinsztein, D. C. and Huntington, J. A. (2006)Expression and characterization of full-length human huntingtin, an elongated HEATrepeat protein. J. Biol. Chem. 281, 15916–15922



232 Cepeda, C., Hurst, R. S., Calvert, C. R., Hernandez-Echeagaray, E., Nguyen, O. K., Jocoy,E., Christian, L. J., Ariano, M. A. and Levine, M. S. (2003) Transient and progressiveelectrophysiological alterations in the corticostriatal pathway in a mouse model ofHuntington’s disease. J. Neurosci. 23, 961–969

233 Calabresi, P., Centonze, D., Pisani, A., Sancesario, G., Gubellini, P., Marfia, G. A. andBernardi, G. (1998) Striatal spiny neurons and cholinergic interneurons expressdifferential ionotropic glutamatergic responses and vulnerability: implications forischemia and Huntington’s disease. Ann. Neurol. 43, 586–597

234 Kuppenbender, K. D., Standaert, D. G., Feuerstein, T. J., Penney, Jr, J. B., Young, A. B.and Landwehrmeyer, G. B. (2000) Expression of NMDA receptor subunit mRNAs inneurochemically identified projection and interneurons in the human striatum.J. Comp. Neurol. 419, 407–421

235 Maragakis, N. J. and Rothstein, J. D. (2001) Glutamate transporters in neurologicdisease. Arch. Neurol. 58, 365–370

236 Lievens, J. C., Woodman, B., Mahal, A., Spasic-Boscovic, O., Samuel, D., Kerkerian-LeGoff, L. and Bates, G. P. (2001) Impaired glutamate uptake in the R6 Huntington’sdisease transgenic mice. Neurobiol. Dis. 8, 807–821

237 Behrens, P. F., Franz, P., Woodman, B., Lindenberg, K. S. and Landwehrmeyer, G. B.(2002) Impaired glutamate transport and glutamate–glutamine cycling: downstreameffects of the Huntington mutation. Brain 125, 1908–1922

238 Popoli, P., Blum, D., Martire, A., Ledent, C., Ceruti, S. and Abbracchio, M. P. (2007)Functions, dysfunctions and possible therapeutic relevance of adenosine A2A receptorsin Huntington’s disease. Prog. Neurobiol. 81, 331–348

239 Shin, J. Y., Fang, Z. H., Yu, Z. X., Wang, C. E., Li, S. H. and Li, X. J. (2005) Expression ofmutant huntingtin in glial cells contributes to neuronal excitotoxicity. J. Cell Biol. 171,1001–1012

240 Gu, X., Li, C., Wei, W., Lo, V., Gong, S., Li, S. H., Iwasato, T., Itohara, S., Li, X. J., Mody,I. et al. (2005) Pathological cell–cell interactions elicited by a neuropathogenic form ofmutant huntingtin contribute to cortical pathogenesis in HD mice. Neuron 46, 433–444

241 Li, X. J., Li, S. H., Sharp, A. H., Nucifora, Jr, F. C., Schilling, G., Lanahan, A., Worley, P.,Snyder, S. H. and Ross, C. A. (1995) A huntingtin-associated protein enriched in brainwith implications for pathology. Nature 378, 398–402

242 Kalchman, M. A., Koide, H. B., McCutcheon, K., Graham, R. K., Nichol, K., Nishiyama,K., Kazemi-Esfarjani, P., Lynn, F. C., Wellington, C., Metzler, M. et al. (1997) HIP1, ahuman homologue of S. cerevisiae Sla2p, interacts with membrane associatedhuntingtin in the brain. Nat. Genet. 16, 44–53

243 Wanker, E. E., Rovira, C., Scherzinger, E., Hasenbank, R., Walter, S., Tait, D., Colicelli, J.and Lehrach, H. (1997) HIP-I: a huntingtin interacting protein isolated by the yeasttwo-hybrid system. Hum. Mol. Genet. 6, 487–495

244 Boutell, J. M., Wood, J. D., Harper, P. S. and Jones, A. L. (1998) Huntingtin interactswith cystathionine β-synthase. Hum. Mol. Genet. 7, 371–378

245 Faber, P. W., Barnes, G. T., Srinidhi, J., Chen, J., Gusella, J. F. and MacDonald, M. E.(1998) Huntingtin interacts with a family of WW domain proteins. Hum. Mol. Genet. 7,1463–1474

246 Holbert, S., Dedeoglu, A., Humbert, S., Saudou, F., Ferrante, R. J. and Neri, C. (2003)Cdc42-interacting protein 4 binds to huntingtin: neuropathologic and biologicalevidence for a role in Huntington’s disease. Proc. Natl. Acad. Sci. U.S.A. 100,2712–2717

247 Holbert, S., Denghien, I., Kiechle, T., Rosenblatt, A., Wellington, C., Hayden, M. R.,Margolis, R. L., Ross, C. A., Dausset, J., Ferrante, R. J. and Neri, C. (2001) The Gln-Alarepeat transcriptional activator CA150 interacts with huntingtin: neuropathologic andgenetic evidence for a role in Huntington’s disease pathogenesis. Proc. Natl. Acad.Sci. U.S.A. 98, 1811–1816

248 Goehler, H., Lalowski, M., Stelzl, U., Waelter, S., Stroedicke, M., Worm, U., Droege, A.,Lindenberg, K. S., Knoblich, M., Haenig, C. et al. (2004) A protein interaction networklinks GIT1, an enhancer of huntingtin aggregation, to Huntington’s disease. Mol. Cell15, 853–865

249 Kaltenbach, L. S., Romero, E., Becklin, R. R., Chettier, R., Bell, R., Phansalkar, A., Strand,A., Torcassi, C., Savage, J., Hurlburt, A. et al. (2007) Huntingtin interacting proteins aregenetic modifiers of neurodegeneration. PLoS Genet. 3, e82

250 Heiser, V., Engemann, S., Brocker, W., Dunkel, I., Boeddrich, A., Waelter, S., Nordhoff, E.,Lurz, R., Schugardt, N., Rautenberg, S. et al. (2002) Identification of benzothiazoles aspotential polyglutamine aggregation inhibitors of Huntington’s disease by using anautomated filter retardation assay. Proc. Natl. Acad. Sci. U.S.A. 99, (Suppl. 4),16400–16406

251 Coufal, M., Maxwell, M. M., Russel, D. E., Amore, A. M., Altmann, S. M., Hollingsworth,Z. R., Young, A. B., Housman, D. E. and Kazantsev, A. G. (2007) Discovery of a novelsmall-molecule targeting selective clearance of mutant huntingtin fragments.J. Biomol. Screen. 12, 351–360

252 Yamamoto, A., Cremona, M. L. and Rothman, J. E. (2006) Autophagymediated clearanceof huntingtin aggregates triggered by the insulin-signaling pathway. J. Cell Biol. 172,719–731

253 Nollen, E. A., Garcia, S. M., van Haaften, G., Kim, S., Chavez, A., Morimoto, R. I. andPlasterk, R. H. (2004) Genome-wide RNA interference screen identifies previouslyundescribed regulators of polyglutamine aggregation. Proc. Natl. Acad. Sci. U.S.A. 101,6403–6408

254 Aiken, C. T., Tobin, A. J. and Schweitzer, E. S. (2004) A cell-based screen for drugs totreat Huntington’s disease. Neurobiol. Dis. 16, 546–555

255 Piccioni, F., Roman, B. R., Fischbeck, K. H. and Taylor, J. P. (2004) A screen for drugsthat protect against the cytotoxicity of polyglutamine-expanded androgen receptor.Hum. Mol. Genet. 13, 437–446

256 Giorgini, F., Guidetti, P., Nguyen, Q., Bennett, S. C. and Muchowski, P. J. (2005) Agenomic screen in yeast implicates kynurenine 3-monooxygenase as a therapeutic targetfor Huntington disease. Nat. Genet. 37, 526–531

257 Willingham, S., Outeiro, T. F., DeVit, M. J., Lindquist, S. L. and Muchowski, P. J. (2003)Yeast genes that enhance the toxicity of a mutant huntingtin fragment or α-synuclein.Science 302, 1769–1772

258 Faber, P. W., Voisine, C., King, D. C., Bates, E. A. and Hart, A. C. (2002)Glutamine/proline-rich PQE-1 proteins protect Caenorhabditis elegans neurons fromhuntingtin polyglutamine neurotoxicity. Proc. Natl. Acad. Sci. U.S.A. 99, 17131–17136

259 Kazemi-Esfarjani, P. and Benzer, S. (2000) Genetic suppression of polyglutamine toxicityin Drosophila. Science 287, 1837–1840

260 Jackson, G. R., Salecker, I., Dong, X., Yao, X., Arnheim, N., Faber, P. W., MacDonald,M. E. and Zipursky, S. L. (1998) Polyglutamine-expanded human huntingtin transgenesinduce degeneration of Drosophila photoreceptor neurons. Neuron 21, 633–642

261 Warrick, J. M., Chan, H. Y., Gray-Board, G. L., Chai, Y., Paulson, H. L. and Bonini, N. M.(1999) Suppression of polyglutamine-mediated neurodegeneration in Drosophila by themolecular chaperone HSP70. Nat. Genet. 23, 425–428

262 Warrick, J. M., Paulson, H. L., Gray-Board, G. L., Bui, Q. T., Fischbeck, K. H., Pittman,R. N. and Bonini, N. M. (1998) Expanded polyglutamine protein forms nuclearinclusions and causes neural degeneration in Drosophila. Cell 93, 939–949

263 Kazemi-Esfarjani, P. and Benzer, S. (2002) Suppression of polyglutamine toxicity by aDrosophila homolog of myeloid leukemia factor 1. Hum. Mol. Genet. 11, 2657–2672

264 Fernandez-Funez, P., Nino-Rosales, M. L., de Gouyon, B., She, W. C., Luchak, J. M.,Martinez, P., Turiegano, E., Benito, J., Capovilla, M., Skinner, P. J. et al. (2000)Identification of genes that modify ataxin-1-induced neurodegeneration. Nature 408,101–106

265 Bilen, J., Liu, N., Burnett, B. G., Pittman, R. N. and Bonini, N. M. (2006) MicroRNApathways modulate polyglutamine-induced neurodegeneration. Mol. Cell 24, 157–163

266 Thompson, J. C., Snowden, J. S., Craufurd, D. and Neary, D. (2002) Behavior inHuntington’s disease: dissociating cognition-based and mood-based changes.J. Neuropsychiatry Clin. Neurosci. 14, 37–43

267 Folstein, S. E., Chase, G. A., Wahl, W. E., McDonnell, A. M. and Folstein, M. F. (1987)Huntington disease in Maryland: clinical aspects of racial variation. Am. J. Hum. Genet.41, 168–179

268 Baliko, L., Csala, B. and Czopf, J. (2004) Suicide in Hungarian Huntington’s diseasepatients. Neuroepidemiology 23, 258–260

269 Anderson, K. E., Louis, E. D., Stern, Y. and Marder, K. S. (2001) Cognitive correlates ofobsessive and compulsive symptoms in Huntington’s disease. Am. J. Psychiatry 158,799–801

270 Bonelli, R. M. (2003) Mirtazapine in suicidal Huntington’s disease. Ann. Pharmacother.37, 452

271 Patzold, T. and Brune, M. (2002) Obsessive compulsive disorder in Huntington disease:a case of isolated obsessions successfully treated with sertraline. NeuropsychiatryNeuropsychol. Behav. Neurol. 15, 216–219

272 Paleacu, D., Anca, M. and Giladi, N. (2002) Olanzapine in Huntington’s disease.Acta Neurol. Scand. 105, 441–444

273 Madhusoodanan, S. and Brenner, R. (1998) Use of risperidone in psychosis associatedwith Huntington’s disease. Am. J. Geriatr. Psychiatry 6, 347–349

274 Saft, C., Andrich, J., Kraus, P. H. and Przuntek, H. (2005) Amisulpride in Huntington’sdisease. Psychiatr. Prax. 32, 363–366

275 Alpay, M. and Koroshetz, W. J. (2006) Quetiapine in the treatment of behavioraldisturbances in patients with Huntington’s disease. Psychosomatics 47, 70–72

276 Huntington Study Group (2006) Tetrabenazine as antichorea therapy in Huntingtondisease: a randomized controlled trial. Neurology 66, 366–372

277 van Vugt, J. P., Siesling, S., Vergeer, M., van der Velde, E. A. and Roos, R. A. (1997)Clozapine versus placebo in Huntington’s disease: a double blind randomisedcomparative study. J. Neurol. Neurosurg. Psychiatry 63, 35–39

278 Bonelli, R. M., Mahnert, F. A. and Niederwieser, G. (2002) Olanzapine for Huntington’sdisease: an open label study. Clin. Neuropharmacol. 25, 263–265

279 de Tommaso, M., Difruscolo, O., Sciruicchio, V., Specchio, N. and Livrea, P. (2007) Twoyears’ follow-up of rivastigmine treatment in Huntington disease. Clin. Neuropharmacol.30, 43–46



280 Cubo, E., Shannon, K. M., Tracy, D., Jaglin, J. A., Bernard, B. A., Wuu, J. and Leurgans,S. E. (2006) Effect of donepezil on motor and cognitive function in Huntington disease.Neurology 67, 1268–1271

281 Wang, Y. L., Liu, W., Wada, E., Murata, M., Wada, K. and Kanazawa, I. (2005)Clinico-pathological rescue of a model mouse of Huntington’s disease by siRNA.Neurosci. Res. 53, 241–249

282 Xia, H., Mao, Q., Eliason, S. L., Harper, S. Q., Martins, I. H., Orr, H. T., Paulson, H. L.,Yang, L., Kotin, R. M. and Davidson, B. L. (2004) RNAi suppresses polyglutamine-induced neurodegeneration in a model of spinocerebellar ataxia. Nat. Med. 10,816–820

283 Miller, V. M., Xia, H., Marrs, G. L., Gouvion, C. M., Lee, G., Davidson, B. L. and Paulson,H. L. (2003) Allele-specific silencing of dominant disease genes. Proc. Natl. Acad.Sci. U.S.A. 100, 7195–7200

284 Berger, Z., Ravikumar, B., Menzies, F. M., Oroz, L. G., Underwood, B. R., Pangalos,M. N., Schmitt, I., Wullner, U., Evert, B. O., O’Kane, C. J. and Rubinsztein, D. C. (2006)Rapamycin alleviates toxicity of different aggregate-prone proteins. Hum. Mol. Genet.15, 433–442

285 Sanchez, I., Xu, C. J., Juo, P., Kakizaka, A., Blenis, J. and Yuan, J. (1999) Caspase-8 isrequired for cell death induced by expanded polyglutamine repeats. Neuron 22,623–633

286 Wang, X., Zhu, S., Drozda, M., Zhang, W., Stavrovskaya, I. G., Cattaneo, E., Ferrante,R. J., Kristal, B. S. and Friedlander, R. M. (2003) Minocycline inhibitscaspase-independent and -dependent mitochondrial cell death pathways in models ofHuntington’s disease. Proc. Natl. Acad. Sci. U.S.A. 100, 10483–10487

287 Smith, D. L., Woodman, B., Mahal, A., Sathasivam, K., Ghazi-Noori, S., Lowden, P. A.,Bates, G. P. and Hockly, E. (2003) Minocycline and doxycycline are not beneficial in amodel of Huntington’s disease. Ann. Neurol. 54, 186–196

288 Bonelli, R. M., Hodl, A. K., Hofmann, P. and Kapfhammer, H. P. (2004) Neuroprotectionin Huntington’s disease: a 2-year study on minocycline. Int. Clin. Psychopharmacol. 19,337–342

289 Thomas, M., Ashizawa, T. and Jankovic, J. (2004) Minocycline in Huntington’s disease:a pilot study. Mov. Disord. 19, 692–695

290 Varma, H., Cheng, R., Voisine, C., Hart, A. C. and Stockwell, B. R. (2007) Inhibitors ofmetabolism rescue cell death in Huntington’s disease models. Proc. Natl. Acad.Sci. U.S.A. 104, 14525–14530

291 Ferrante, R. J., Andreassen, O. A., Jenkins, B. G., Dedeoglu, A., Kuemmerle, S., Kubilus,J. K., Kaddurah-Daouk, R., Hersch, S. M. and Beal, M. F. (2000) Neuroprotective effectsof creatine in a transgenic mouse model of Huntington’s disease. J. Neurosci. 20,4389–4397

292 Dedeoglu, A., Kubilus, J. K., Jeitner, T. M., Matson, S. A., Bogdanov, M., Kowall,N. W., Matson, W. R., Cooper, A. J., Ratan, R. R., Beal, M. F. et al. (2002) Therapeuticeffects of cystamine in a murine model of Huntington’s disease. J. Neurosci. 22,8942–8950

293 Dubinsky, R. and Gray, C. (2006) CYTE-I-HD: phase I dose finding and tolerability studyof cysteamine (Cystagon) in Huntington’s disease. Mov. Disord. 21, 530–533

294 Sittler, A., Lurz, R., Lueder, G., Priller, J., Lehrach, H., Hayer-Hartl, M. K., Hartl, F. U. andWanker, E. E. (2001) Geldanamycin activates a heat shock response and inhibitshuntingtin aggregation in a cell culture model of Huntington’s disease. Hum. Mol. Genet.10, 1307–1315

295 Tanaka, M., Machida, Y., Niu, S., Ikeda, T., Jana, N. R., Doi, H., Kurosawa, M., Nekooki,M. and Nukina, N. (2004) Trehalose alleviates polyglutamine-mediated pathology in amouse model of Huntington disease. Nat. Med. 10, 148–154

296 Burlina, A. B., Ogier, H., Korall, H. and Trefz, F. K. (2001) Long-term treatment withsodium phenylbutyrate in ornithine transcarbamylase-deficient patients.Mol. Genet. Metab. 72, 351–355

297 Schiefer, J., Landwehrmeyer, G. B., Luesse, H. G., Sprunken, A., Puls, C., Milkereit, A.,Milkereit, E. and Kosinski, C. M. (2002) Riluzole prolongs survival time and altersnuclear inclusion formation in a transgenic mouse model of Huntington’s disease.Mov. Disord. 17, 748–757

298 Ferrante, R. J., Andreassen, O. A., Dedeoglu, A., Ferrante, K. L., Jenkins, B. G., Hersch,S. M. and Beal, M. F. (2002) Therapeutic effects of coenzyme Q10 and remacemide intransgenic mouse models of Huntington’s disease. J. Neurosci. 22, 1592–1599

299 O’Suilleabhain, P. and Dewey, Jr, R. B. (2003) A randomized trial of amantadine inHuntington disease. Arch. Neurol. 60, 996–998

300 Huntington Study Group (2003) Dosage effects of riluzole in Huntington’s disease: amulticenter placebo-controlled study. Neurology 61, 1551–1556

301 Kremer, B., Clark, C. M., Almqvist, E. W., Raymond, L. A., Graf, P., Jacova, C., Mezei, M.,Hardy, M. A., Snow, B., Martin, W. and Hayden, M. R. (1999) Influence of lamotrigine onprogression of early Huntington disease: a randomized clinical trial. Neurology 53,1000–1011

302 Huntington Study Group (2001) A randomized, placebo-controlled trial of coenzyme Q10

and remacemide in Huntington’s disease. Neurology 57, 397–404303 Dunnett, S. B., Carter, R. J., Watts, C., Torres, E. M., Mahal, A., Mangiarini, L., Bates, G.

and Morton, A. J. (1998) Striatal transplantation in a transgenic mouse model ofHuntington’s disease. Exp. Neurol. 154, 31–40

304 van Dellen, A., Deacon, R., York, D., Blakemore, C. and Hannan, A. J. (2001) Anteriorcingulate cortical transplantation in transgenic Huntington’s disease mice.Brain Res. Bull. 56, 313–318

305 Sramka, M., Rattaj, M., Molina, H., Vojtassak, J., Belan, V. and Ruzicky, E. (1992)Stereotactic technique and pathophysiological mechanisms of neurotransplantation inHuntington’s chorea. Stereotact. Funct. Neurosurg. 58, 79–83

306 Madrazo, I., Franco-Bourland, R. E., Castrejon, H., Cuevas, C. and Ostrosky-Solis, F.(1995) Fetal striatal homotransplantation for Huntington’s disease: first two case reports.Neurol. Res. 17, 312–315

307 Kopyov, O. V., Jacques, S., Lieberman, A., Duma, C. M. and Eagle, K. S. (1998) Safety ofintrastriatal neurotransplantation for Huntington’s disease patients. Exp. Neurol. 149,97–108

308 Bachoud-Levi, A., Bourdet, C., Brugieres, P., Nguyen, J. P., Grandmougin, T., Haddad,B., Jeny, R., Bartolomeo, P., Boisse, M. F., Barba, G. D. et al. (2000) Safety andtolerability assessment of intrastriatal neural allografts in five patients with Huntington’sdisease. Exp. Neurol. 161, 194–202

309 Rosser, A. E., Barker, R. A., Harrower, T., Watts, C., Farrington, M., Ho, A. K., Burnstein,R. M., Menon, D. K., Gillard, J. H., Pickard, J. and Dunnett, S. B. (2002) Unilateraltransplantation of human primary fetal tissue in four patients with Huntington’s disease:NEST-UK safety report ISRCTN no. 36485475. J. Neurol. Neurosurg. Psychiatry 73,678–685

310 Schumacher, J. M., Ellias, S. A., Palmer, E. P., Kott, H. S., Dinsmore, J., Dempsey, P. K.,Fischman, A. J., Thomas, C., Feldman, R. G., Kassissieh, S. et al. (2000) Transplantationof embryonic porcine mesencephalic tissue in patients with PD. Neurology 54,1042–1050

311 Hauser, R. A., Furtado, S., Cimino, C. R., Delgado, H., Eichler, S., Schwartz, S., Scott, D.,Nauert, G. M., Soety, E., Sossi, V. et al. (2002) Bilateral human fetal striataltransplantation in Huntington’s disease. Neurology 58, 687–695

312 Gaura, V., Bachoud-Levi, A. C., Ribeiro, M. J., Nguyen, J. P., Frouin, V., Baudic, S.,Brugieres, P., Mangin, J. F., Boisse, M. F., Palfi, S. et al. (2004) Striatal neural graftingimproves cortical metabolism in Huntington’s disease patients. Brain 127, 65–72

313 Freeman, T. B., Cicchetti, F., Hauser, R. A., Deacon, T. W., Li, X. J., Hersch, S. M., Nauert,G. M., Sanberg, P. R., Kordower, J. H., Saporta, S. and Isacson, O. (2000) Transplantedfetal striatum in Huntington’s disease: phenotypic development and lack of pathology.Proc. Natl. Acad. Sci. U.S.A. 97, 13877–13882

314 Haque, N. S. and Isacson, O. (2000) Neurotrophic factors NGF and FGF-2 alter levels ofhuntingtin (IT15) in striatal neuronal cell cultures. Cell Transplant. 9, 623–627

315 Mittoux, V., Joseph, J. M., Conde, F., Palfi, S., Dautry, C., Poyot, T., Bloch, J., Deglon,N., Ouary, S., Nimchinsky, E. A. et al. (2000) Restoration of cognitive and motorfunctions by ciliary neurotrophic factor in a primate model of Huntington’s disease.Hum. Gene Ther. 11, 1177–1187

316 Bloch, J., Bachoud-Levi, A. C., Deglon, N., Lefaucheur, J. P., Winkel, L., Palfi, S.,Nguyen, J. P., Bourdet, C., Gaura, V., Remy, P. et al. (2004) Neuroprotective gene therapyfor Huntington’s disease, using polymer-encapsulated cells engineered to secrete humanciliary neurotrophic factor: results of a phase I study. Hum. Gene Ther. 15, 968–975

317 Underwood, B. R., Broadhurst, D., Dunn, W. B., Ellis, D. I., Michell, A. W., Vacher, C.,Mosedale, D. E., Kell, D. B., Barker, R. A., Grainger, D. J. and Rubinsztein, D. C. (2006)Huntington disease patients and transgenic mice have similar pro-catabolic serummetabolite profiles. Brain 129, 877–886

Received 29 November 2007/19 March 2008; accepted 19 March 2008Published on the Internet 14 May 2008, doi:10.1042/BJ20071619


huntington’s disease: from pathology and genetics to ... · 192 s. imarisio and others initiator...

Documents