NEWS AND V IEWS

more likely to inhibit the oncogenic effects of the mutant. Thus, when meta-analysis is per-formed, a significant association might not be detected owing to the opposing effects in these different populations, in spite of real biological impact. This situation may also apply not only to oncogenic mutational events like KRAS2, but also to haploinsufficiency or inherited suscep-tibility loci. For example, in a BRCA1-positive population, individuals with a more actively transcribed BRCA1 on the wild-type chromo-some may have a lower risk of developing breast cancer than individuals with a less actively tran-scribed wild-type allele, potentially explaining some of the variability observed in these high-risk families. Stratification of affected individu-als by relative expression level of alleles might be necessary to increase the power of these stud-ies to identify real associations in the human population.

The major question now is how prevalent context-dependent susceptibility will be in the human genome. Other context-dependent situations do exist in the literature. For exam-ple, investigations of genetic susceptibility of malignant peripheral nerve sheath tumors in a compound knockout mouse model revealed linkage on two chromosomes, but only when stratified by the parental origin of the muta-tion (that is, whether the compound knockout mutations were passed down from the mother or the father11). Combining the two classes of mutant mice in the genetic analysis did not reveal any genetic association whatsoever. These data, combined with the lung suscep-tibility inversion results of To et al., suggest that it may sometimes be necessary to incor-porate more biological information into asso-ciation studies to gain clear insights into the underlying human biology. Where available,

mouse models and experimental strategies like those described by To et al. will continue to be extremely useful for investigations into the underlying mechanisms of cancer susceptibi-lity. In the meantime, these results provide a possible explanation for some of the inherent biological variability that confounds associa-tion studies despite the heroic efforts of the epidemiological community.

1. Pisani, P., Parkin, D.M. & Ferlay, J. Int. J. Cancer 55, 891–903 (1993).

2. Gariboldi, M. et al. Nat. Genet. 3, 132–136 (1993).3. Manenti, G. et al. Oncogene 23, 4495–4504 (2004).4. To, M. et al. Nat. Genet. 38, 926–930 (2006).5. Nagase, H. et al. Nat. Genet. 10, 424–429 (1995).6. Johnson, L. et al. Nature 410, 1111–1116 (2001).7. Manenti, G. et al. Cancer Res. 57, 4164–4166

(1997).8. Zhang, Z. et al. Nat. Genet. 29, 25–33 (2001).9. Salanti, G., Sanderson, S. & Higgins, J.P. Genet. Med. 7,

13–20 (2005).10. Lo, H.S. et al. Genome Res. 13, 1855–1862 (2003).11. Reilly, K.M. et al. Cancer Res. 66, 62–68 (2006).

The difference between simple and complex leavesElizabeth A Kellogg

The shape of leaves is highly variable. A new study shows that changes in plant homeobox genes seem to underlie some of the variation, although a major regulator of those genes remains unchanged.

Elizabeth A. Kellogg is in the Department of Biology, University of Missouri-St. Louis, One University Boulevard, St. Louis, Missouri 63121, USA. e-mail: tkellogg@umsl.edu

Even a non-botanist can distinguish spin-ach from parsley. The spinach leaf is shaped something like a spoon, with smooth edges and an undivided leaf blade, whereas the parsley leaf blade is dissected into many tiny segments, or leaflets. The last several years have brought an increasing understanding of the genetic basis of the very obvious dif-ferences between a simple leaf (like spinach) and a complex one (like parsley). On page 942 of this issue, Hay and Tsiantis1 describe results of experiments comparing leaf devel-opment in Arabidopsis thaliana (simple leaves, spinach-like) and its not-too-distant relative, Cardamine hirsuta (complex leaves, parsley-like; Fig. 1). The authors not only advance our understanding of how dispa-rate leaf forms are controlled, but they do so using a new and powerful experimental approach that may be called a ‘reciprocal genetic transplant experiment’, by analogy to similar experiments in plant ecology.

Shaping leaves with KNOXShoot apical meristems from which leaf growth is initiated are maintained in part by the activity of Knotted-like homeobox (KNOX) genes, which seem to prevent differentiation of cells. An early indicator of leaf formation is loss of expression of KNOX genes in a small set of cells on the flanks of the meristem2. In a plant with simple leaves, such as A. thaliana, the KNOX genes then remain off throughout leaf development. In plants that form complex leaves, however, KNOX genes are reactivated in the leaf blade, delaying differentia-tion and promoting formation of leaflets3.

Hay and Tsiantis reasoned that the mor-phological differences between A. thaliana and C. hirsuta could be caused by differential regula-tion of KNOX genes. First, they reduced KNOX gene expression in C. hirsuta by RNA interference and verified that fewer leaflets were formed on the leaves. In addition, reduced KNOX expres-sion correlated with larger cells and reduced cell division, as measured by activity of HISTONE4. In another experiment, overexpression of KNOX genes created extra leaflets (a leaf even more like parsley) in C. hirsuta and made A. thaliana leaves more complex. Thus, KNOX proteins are both necessary and sufficient for creating com-plex leaves in C. hirsuta. They also showed that, as in A. thaliana, the protein ASYMMETRIC

LEAVES1 (AS1), a myb-domain transcription factor in the ARP class, is responsible for regu-lating the expression of KNOX genes and thereby affecting leaf form.

Reciprocal transplantation experimentSo what might be different about C. hirsuta KNOX genes? It could be the gene itself, or its regulation. The ideal experiment is to place the C. hirsuta gene and promoter individually in A. thaliana, and then do the reciprocal experi-ment: place the A. thaliana gene and promoter in C. hirsuta. The first experiment, transforming A. thaliana with heterologous genes or promoters, is a staple of comparative developmental genetics and is a common way to infer gene function4,5. This is effectively a ‘common garden’ experiment, similar to the experiments devised by Clausen, Keck and Hiesey6 to dissect ecotypic differentia-tion. Place several disparate plants (or genes) in the same (cellular) environment. If they look or function the same way in the new environment, the differences are due to the environment and not the plant (or gene) itself. Hay and Tsiantis used this approach to verify that the biochemical function of AS1 is apparently conserved between A. thaliana and C. hirsuta. When AS1 from C. hirsuta is moved into an A. thaliana as1 mutant plant, the A. thaliana plant looks normal.

NATURE GENETICS | VOLUME 38 | NUMBER 8 | AUGUST 2006 865

©20

06 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

ureg

enet

ics

NEWS AND V IEWS

The reciprocal transplant experiment, placing A. thaliana genes and promoters into another species, is rarely done because transforming non-model species is generally hard or impos-sible – indeed, much more difficult than the clas-sic reciprocal transplants done in an ecological setting6. But Hay and Tsiantis demonstrate the power of pursuing this approach. The authors attached the promoters of each of the KNOX genes of A. thaliana (SHOOTMERISTEMLESS and BREVIPEDICELLUS) to a reporter gene and introduced the constructs into both A. thaliana and C. hirsuta. They then performed the same experiment with the promoters of the corre-sponding C. hirsuta genes. Thus, they were able to compare the expression patterns of the C. hirsuta promoters with those of the A. thaliana promot-ers for each gene in each cellular environment. For both genes transformed into both species, the A. thaliana promoters drove gene expression only in the meristem, whereas the C. hirsuta pro-moters drove expression in the meristem and in the leaves. Thus, some aspect of the promoter itself—not its cellular environment—carries the information necessary to determine its expres-sion in leaves and thus differential leaf shape.

Common mechanism?What is different, then, between the two species? Downregulation of KNOX genes correlates with the site of leaf initiation in both species, and regulation of KNOX genes by ARP genes is the same. However, subtle changes in the KNOX

loci themselves change the strength and/or tim-ing of that interaction, such that KNOX genes are reactivated in the leaf blade, leading to the production of leaflets.

Complex leaves have originated multiple times in evolution, and in all cases KNOX genes are turned off in leaf primordia and reactivated in leaflets3. The pattern of complex leaf develop-ment is also affected by ARP proteins7. The regu-lation of KNOX genes by ARP proteins may thus be quite general among flowering plants. The great variation in leaf shape seen in the grocery store or in the field may then be controlled by when and where in leaf development ARP pro-teins affect the expression of KNOX genes. ARP proteins have recently been shown to interact with a histone chaperone to prevent KNOX gene activation8, suggesting an epigenetic component to the regulatory mechanism. We can speculate that complex leaves might evolve rather easily from simple ones via modest changes in KNOX loci that affect the strength or timing of interac-tions with conserved regulatory proteins. A small step in a KNOX locus may be a giant leap for leaf morphology.

1. Hay, A., and Tsiantis, M. Nat. Genet. 38, 942–947 (2006).

2. Smith, L.G., Greene, B., Veit, B. & Hake, S. Development 116, 21–30 (1992).

3. Bharathan, G. et al. Science 296, 1858–1860 (2002).

4. Whipple, C.J. et al. Development 131, 6083–6091 (2004).

5. Yoon, H-S. & Baum, D.A. Proc. Natl. Acad. Sci. USA 101,

6524–6529 (2004).6. Clausen, J., Keck, D.D. & Hiesey, W.M. Carnegie Inst.

Washington Publ. 520, 15–18 (1940).7. Kim, M., McCormick, S., Timmermans, M. & Sinha, N.

Nature 424, 438–443 (2003).8. Phelps-Durr, T.L., Thomas, J., Vahab, P. & Timmermans,

M.C.P. Plant Cell 17, 2886–2898 (2005).

Nucleic acid by-products and chronic inflammation Marta E Alarcón-Riquelme

Aicardi-Goutières syndrome (AGS) is an autosomal recessive condition of progressive encephalopathy of early childhood, very similar to congenital viral infections. Two new papers identify four genes mutated in AGS and implicate the processing of DNA and RNA by-products in its pathogenesis.

Marta E. Alarcón-Riquelme is in the Department of Genetics and Pathology, Rudbeck Laboratory, Uppsala University, Dag Hammarsjölds väg 20, 751 85, Uppsala, Sweden. e-mail: marta.alarcon@genpat.uu.se

AGS was described in 1984 by Jean Aicardi and Françoise Goutières as an autosomal recessive condition that presented clini-cally as a progressive encephalopathy of early childhood. Affected individuals had calcification of the basal ganglia and a dis-tinct type of leukodystrophy1. Later studies showed that individuals with the disease had

systemically elevated levels of interferon alpha and lymphocytosis in the cerebrospinal fluid, accompanied by imbalances in cyto-kine production. The disease is very similar to congenital viral infections, but its reces-sive inheritance pattern and the presence of strong consanguinity indicated a genetic etiology, a hypothesis that was strengthened by the absence of detected virus in affected individuals2,3. On pages 910 and 917 of this issue, Yanick Crow, Andrew Jackson and colleagues show that mutations in TREX1 (ref. 4) or in genes encoding subunits of RNaseH2 (ref. 5) cause AGS, providing key insights into the nature of the disease.

Connections to lupusThere is interesting clinical overlap between AGS and the autoimmune disease systemic lupus erythematosus (SLE), a disease also characterized by increased production of interferon alpha. Some individuals with AGS eventually develop a disease very similar to SLE at an early age. In addition to encephalitis, these individuals have circulating antinuclear antibodies and other autoantibody specifici-ties common in SLE, such as antiphospho-lipid antibodies, as well as thrombocytopenia, hemolytic anemia and skin disease reminis-cent of SLE6,7. Treatments typically used for lupus, such as prednisone and azathioprine,

866 VOLUME 38 | NUMBER 8 | AUGUST 2006 | NATURE GENETICS

Figure 1 Artistic representation of Cardamine hirsuta. Illustration provided by Rosemary Wise.

©20

06 N

atur

e P

ublis

hing

Gro

up

http

://w

ww

.nat

ure.

com

/nat

ureg

enet

ics