Molecular pathology:Molecular pathology:Physiopathology effect of Physiopathology effect of

MutationsMutations

Dr Pupak Derakhshandeh, PhD

Ass Prof of Medical Science of Tehran University

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MutationsMutations

• changes to the either changes to the either DNADNA or or RNARNA

• caused by copying errors in the genetic caused by copying errors in the genetic material:material:

– Cell divisionCell division

– UltravioletUltraviolet

– IonizingIonizing radiationradiation

– chemical chemical mutagens mutagens

– VirusesViruses

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By aspect of phenotype affectedBy aspect of phenotype affectedMorphological mutationsMorphological mutations

• usually affect the outward appearance of an usually affect the outward appearance of an individualindividual

• Mutations can change the height of a plant Mutations can change the height of a plant or change it from smooth to rough seeds. or change it from smooth to rough seeds.

• Biochemical mutations result in lesions Biochemical mutations result in lesions stopping the enzymatic pathwaystopping the enzymatic pathway

• Often, morphological mutants are the direct Often, morphological mutants are the direct result of a mutation due to the enzymatic result of a mutation due to the enzymatic pathwaypathway

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Special classesSpecial classesConditional mutationConditional mutation

• wild-type or less severe phenotype under certain wild-type or less severe phenotype under certain "permissive" environmental conditions "permissive" environmental conditions

• a mutant phenotype under certain "restrictive" a mutant phenotype under certain "restrictive" conditionsconditions

• For example: a temperature-sensitive mutation For example: a temperature-sensitive mutation can cause cell death at high temperature can cause cell death at high temperature (restrictive condition), but might have no (restrictive condition), but might have no deletirious consequences at a lower temperature deletirious consequences at a lower temperature (permissive condition). (permissive condition).

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NomenclatureNomenclature• Nomenclature of mutations specify the type of Nomenclature of mutations specify the type of

mutationmutation• and base or amino acid changesand base or amino acid changes

• Amino acid substitution: (e.g. D111E) Amino acid substitution: (e.g. D111E) – The The first letterfirst letter is the one letter code of the is the one letter code of the

wildtype amino acidwildtype amino acid– the numberthe number is the position of the amino acid is the position of the amino acid

from the N terminus from the N terminus – the second letterthe second letter is the one letter code of the is the one letter code of the

amino acid present in the mutationamino acid present in the mutation– If If the second letter is 'X',the second letter is 'X', any amino acid may any amino acid may

replace the wild typereplace the wild type

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NomenclatureNomenclature

• Amino acid deletion: (e.g. ΔF508)Amino acid deletion: (e.g. ΔF508)– The Greek symbol Δ or 'delta'The Greek symbol Δ or 'delta'

indicates a deletionindicates a deletion– The letter refersThe letter refers to the amino acid to the amino acid

present in the wildtype present in the wildtype – the numberthe number is the position from the is the position from the

N terminus of the amino acid were it N terminus of the amino acid were it to be present as in the wildtypeto be present as in the wildtype

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Harmful mutationsHarmful mutations• Changes in DNA caused by mutation can cause errors in Changes in DNA caused by mutation can cause errors in

protein protein sequencesequence– creating partially or completely non-functional proteinscreating partially or completely non-functional proteins

• To function correctly, each cell depends on thousands of To function correctly, each cell depends on thousands of proteins to function in the right places at the right timesproteins to function in the right places at the right times

• a mutation alters a protein that plays a critical role in the a mutation alters a protein that plays a critical role in the bodybody

• A condition caused by mutations in one or more genes is A condition caused by mutations in one or more genes is called a called a genetic disordergenetic disorder

• only a small percentage of mutations cause genetic only a small percentage of mutations cause genetic disordersdisorders

• most have no impact on healthmost have no impact on health– For example, some mutations alter a gene's DNA base For example, some mutations alter a gene's DNA base

sequence but don’t change the function of the protein sequence but don’t change the function of the protein made by the genemade by the gene

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DNA repair systemDNA repair system

• Often, gene mutations that could cause a genetic Often, gene mutations that could cause a genetic disorder disorder

• repaired by the repaired by the DNA repairDNA repair system of the cell system of the cell• Each cell has a number of pathways through Each cell has a number of pathways through

which enzymes recognize and repair mistakes in which enzymes recognize and repair mistakes in DNADNA

• Because DNA can be damaged or mutated in Because DNA can be damaged or mutated in many ways:many ways:– the process of DNA repair is an important way the process of DNA repair is an important way

in which the body protects itself from diseasein which the body protects itself from disease

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Beneficial mutationsBeneficial mutations• A very small percentage of all mutations :A very small percentage of all mutations :

– have a positive effecthave a positive effect• lead to new versions of proteins that help an organism and lead to new versions of proteins that help an organism and

its future generations better adapt to changes in their its future generations better adapt to changes in their environment:environment:– For example, a specfic 32 base pair deletion in human For example, a specfic 32 base pair deletion in human

CChemokinehemokine R Receptoreceptor CCR5 (CCR5-32) confers CCR5 (CCR5-32) confers HIVHIV resistance to resistance to homozygoteshomozygotes

– delays delays AIDSAIDS onset in onset in heterozygotesheterozygotes– The CCR5 mutation is more common in those of The CCR5 mutation is more common in those of

European descentEuropean descent– One theory for the One theory for the etiology etiology of the relatively high of the relatively high

frequency of CCR5-32 in the european population is frequency of CCR5-32 in the european population is that it conferred resistance to the that it conferred resistance to the bubonic plaquebubonic plaque in in mid-14th century Europemid-14th century Europe

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Selection at the Selection at the CCR5CCR5 locus locus• CCR5CCR532/CCR532/CCR53232 homozygotes are resistant homozygotes are resistant

to HIV and AIDSto HIV and AIDS

• The high frequency and wide distribution of The high frequency and wide distribution of the the 3232 allele suggest past selection by an allele suggest past selection by an unknown agentunknown agent

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The Role of the Chemokine Receptor The Role of the Chemokine Receptor Gene CCR5 and Its Allele Gene CCR5 and Its Allele

(del32 CCR5)(del32 CCR5) • Since the late 1970sSince the late 1970s

• 8.4 million people worldwide8.4 million people worldwide

• including 1.7 million children, have died including 1.7 million children, have died of AIDSof AIDS

• an estimated 22 million people are an estimated 22 million people are infected with human immunodeficiency infected with human immunodeficiency virus (HIVvirus (HIV)

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CCR5 and Its Allele ( del32 CCR5)CCR5 and Its Allele ( del32 CCR5)

T-cell line (Tl)

monocyte/macrophage (M),

a circulating T-cell (T)

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Mutations In multicellular Mutations In multicellular organismsorganisms

• can be subdivided into:can be subdivided into:

– Germline mutationsGermline mutations

• can be passed on to descendantscan be passed on to descendants

– Somatic mutationsSomatic mutations

• cannot be transmitted to descendants cannot be transmitted to descendants in animalsin animals

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Germ & Somatic cellGerm & Somatic cell

• a mutation is present in a a mutation is present in a germ cellgerm cell– can give rise to offspring that carries the can give rise to offspring that carries the

mutation in all of its cellsmutation in all of its cells– Such mutations will be present in all descendants of this Such mutations will be present in all descendants of this

cellcell– This is the case in This is the case in hereditary diseasehereditary disease

• a mutation can occur in a a mutation can occur in a somatic cellsomatic cell of an of an organismorganism

• certain mutations can cause the cell to become certain mutations can cause the cell to become malignantmalignant– cause cause cancercancer

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ClassificationClassificationBy effect on structureBy effect on structure

• Gene mutations have varying effects on Gene mutations have varying effects on health:health:

– where they occur where they occur

– whether they alter the function of whether they alter the function of essential proteinsessential proteins

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Structurally, mutations can be Structurally, mutations can be classified as:classified as:

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Point mutationsPoint mutations

• caused by chemicals/malfunction of DNA caused by chemicals/malfunction of DNA replicationreplication

• exchange a single exchange a single nucleotide nucleotide for anotherfor another

• Most common is the Most common is the transitiontransition that that exchanges a purine for a purine (A exchanges a purine for a purine (A ↔↔ G) G)

• or a pyrimidine for a pyrimidine, (C or a pyrimidine for a pyrimidine, (C ↔↔ T) T)

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TransitionTransition

• caused by:caused by:

– Nitrous acidNitrous acid

• base mispairingbase mispairing– 5-bromo-2-deoxyuridine (BrdU):5-bromo-2-deoxyuridine (BrdU):

• mutagenic base analogs mutagenic base analogs

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base analogbase analog (C (C ↔↔ T) T)

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Bromodeoxyuridine & Thymine

CH3

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TransversionTransversion

• Less common Less common

• exchanges a purine for a pyrimidineexchanges a purine for a pyrimidine

• or a pyrimidine for a purine (C/T or a pyrimidine for a purine (C/T ↔↔ A/G) A/G)

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Point mutations that occur within Point mutations that occur within the the proteinprotein coding region of a gene coding region of a gene– depending upon what the erroneous depending upon what the erroneous codoncodon

codes for: codes for: • Silent mutationsSilent mutations::

– which code for the same amino acid which code for the same amino acid • Missense mutationsMissense mutations : :

– which code for a different amino acidwhich code for a different amino acid• Nonsense mutationsNonsense mutations : :

– which code for a stop and can truncate the which code for a stop and can truncate the proteinprotein

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InsertionsInsertions• add one or more extra nucleotides into the DNAadd one or more extra nucleotides into the DNA

– usually caused by usually caused by transposable elementstransposable elements– or errors during replication of repeating elements or errors during replication of repeating elements

(e.g. AT repeats)(e.g. AT repeats)• in the non/coding region of a gene may alter:in the non/coding region of a gene may alter:

– splicing splicing of the of the mRNA (splice site mutation)mRNA (splice site mutation)– or cause a shift in the or cause a shift in the reading frame (frame shift) reading frame (frame shift)

• significantly alter the gene productsignificantly alter the gene product• Insertions can be reverted by excision of the Insertions can be reverted by excision of the

Transposable element Transposable element

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DeletionDeletion

• remove one or more nucleotides from remove one or more nucleotides from the DNAthe DNA

• Like insertions, these mutations can Like insertions, these mutations can alter the alter the reading frame reading frame of the geneof the gene

• DeletionsDeletions of large chromosomal regions, of large chromosomal regions, leading to loss of the genes within those leading to loss of the genes within those regionsregions

• They are irreversibleThey are irreversible

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Deletions/insertions/duplicationsDeletions/insertions/duplications

• Out of frameOut of frame

• In frameIn frame

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Deletions/insertions/duplicationsDeletions/insertions/duplicationsOut of frame: Out of frame:

result in frameshifts giving rise to stop result in frameshifts giving rise to stop codons.codons.

no protein product or truncated protein no protein product or truncated protein product product

deletions/insertions in DMD patients : deletions/insertions in DMD patients : truncated dystrophins of decreased stabilitytruncated dystrophins of decreased stability

RB1 gene - usually no protein product in RB1 gene - usually no protein product in retinoblastomaretinoblastoma

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Deletions/insertions/duplicationsDeletions/insertions/duplicationsIn frame:In frame:

loss or gain of amino acid(s) loss or gain of amino acid(s) depending on the size and may give rise depending on the size and may give rise

to altered protein product with changed to altered protein product with changed propertiesproperties

eg CF Delta F508 loss of single amino eg CF Delta F508 loss of single amino acidacid

In some genes loss or gain of a single In some genes loss or gain of a single amino acid: mildamino acid: mild

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In frame:In frame:In some regions of RB1 a single amino acid In some regions of RB1 a single amino acid

loss:loss:rise to mild retinoblastoma or incomplete rise to mild retinoblastoma or incomplete

penetrancepenetrance BMD patients: BMD patients:

Some times in-frame deletions/duplications Some times in-frame deletions/duplications DMD deletions:DMD deletions:

mostly disrupt the reading framemostly disrupt the reading frame

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Deletions/insertions/duplicationsDeletions/insertions/duplications

In untranslated regions:In untranslated regions:

these might affect these might affect transcription/expression and/or stability transcription/expression and/or stability of the message:of the message:Fragile XFragile XMD expansionsMD expansions

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Large-scale mutations in Large-scale mutations in chromosomal structurechromosomal structure

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AmplificationsAmplifications (gene duplications)(gene duplications)

• leading to multiple copies of all leading to multiple copies of all chromosomal regionschromosomal regions

• double-minute chromosomes:double-minute chromosomes:– Sometimes, so many copies of the amplified Sometimes, so many copies of the amplified

region are producedregion are produced– they can actually form their own small pseudo-they can actually form their own small pseudo-

chromosomes chromosomes

• increasing the dosage of the genesincreasing the dosage of the genes

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AmplificationsAmplifications

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Chromosomal translocations:Chromosomal translocations:

• Fusion genes:Fusion genes:– Mutations: to juxtapose previously Mutations: to juxtapose previously

separate pieces of DNAseparate pieces of DNA– potentially bringing together separate potentially bringing together separate

genes to form functionally distinct (e.g. genes to form functionally distinct (e.g. bcr-abl)bcr-abl)

• Chromosomal translocation:Chromosomal translocation:– interchange of genetic parts from interchange of genetic parts from

nonhomologous chromosomesnonhomologous chromosomes

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Lethal mutationsLethal mutations

• lead to a phenotype:

– incapable of effective reproduction

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Interstitial deletionsInterstitial deletions::• an intra-chromosomal deletion:an intra-chromosomal deletion:

– removes a segment of DNA from a single chromosomeremoves a segment of DNA from a single chromosome– For example, cells isolated from a human For example, cells isolated from a human

astrocytomaastrocytoma, a type of brain tumor, a type of brain tumor– have a chromosomal deletion removing sequences have a chromosomal deletion removing sequences

between the "between the "ffused used iin n gglioblastoma" (lioblastoma" (figfig) gene and ) gene and the the rreceptor tyreceptor tyrososine kinase "ine kinase "rosros", producing a fusion ", producing a fusion protein (protein (FIG-ROSFIG-ROS))

– The abnormal FIG-ROS fusion protein has The abnormal FIG-ROS fusion protein has constitutively active kinase activity constitutively active kinase activity

– causes oncogenic transformation (a transformation causes oncogenic transformation (a transformation from normal cells to cancer cells) from normal cells to cancer cells)

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AstrocytomaAstrocytoma• a primary tumor of the central nervous system a primary tumor of the central nervous system • develops from the large, star-shaped glial cells known as develops from the large, star-shaped glial cells known as

astrocytesastrocytes• Most frequently astrocytomas occur in the brainMost frequently astrocytomas occur in the brain• but occasionally they appear along the spinal cordbut occasionally they appear along the spinal cord• occur most often in middle-aged menoccur most often in middle-aged men• Symptoms of an astrocytoma, similar to other brain Symptoms of an astrocytoma, similar to other brain

tumors:tumors:– depend on the precise location of the growthdepend on the precise location of the growth– For instance, if the frontal lobe is affectedFor instance, if the frontal lobe is affected

• mood swings and changes in personality may occurmood swings and changes in personality may occur• a temporal lobe tumor is more typically associated a temporal lobe tumor is more typically associated

with speech and coordination difficultieswith speech and coordination difficulties

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Astrocytoma & AstrocyteAstrocytoma & Astrocyte

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AA: anaplastic astrocytomas(60.6%)

GBM: glioblastoma multiforme (65%)

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• Chromosomal inversions:Chromosomal inversions:

• Reversing the orientation of a Reversing the orientation of a chromosomal segmentchromosomal segment

• Loss of heterozygosity:Loss of heterozygosity:

– loss of one allele:loss of one allele:

• either by a deletioneither by a deletion

• recombination recombination eventevent

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By effect on functionBy effect on function

• Loss-of-function mutationsLoss-of-function mutations

• Gain-of-function mutationsGain-of-function mutations

• Dominant negative mutationsDominant negative mutations

• Lethal mutationsLethal mutations

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Loss-of-function mutationsLoss-of-function mutations

• Wild type alleles typically encode a product Wild type alleles typically encode a product necessary for a specific biological functionnecessary for a specific biological function

• If a mutation occurs in that allele, the If a mutation occurs in that allele, the function for which it encodes is also lostfunction for which it encodes is also lost

• The degree to which the function is lost can The degree to which the function is lost can varyvary

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Loss-of-function mutationsLoss-of-function mutations

• gene product having less or no function:gene product having less or no function:– Phenotypes associated with such mutations are Phenotypes associated with such mutations are

most oftenmost often recessive recessive::– to produce the to produce the wild typewild type phenotype! phenotype!

• Exceptions are when the organism is Exceptions are when the organism is haploidhaploid• or when the reduced dosage of a normal gene or when the reduced dosage of a normal gene

product is not enough for a normal product is not enough for a normal phenotype phenotype (haploinsufficiency)(haploinsufficiency)

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Loss-of-function mutationsLoss-of-function mutations

• mutant allele will act as a mutant allele will act as a dominantdominant::• the wild type allele may not the wild type allele may not

compensate for the loss-of-function compensate for the loss-of-function alleleallele

• the phenotype of the heterozygote will the phenotype of the heterozygote will be equal to that of the loss-of-function be equal to that of the loss-of-function mutant (as mutant (as homozygotehomozygote))– to produce the to produce the mutantmutant phenotype ! phenotype !

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Loss-of-function mutationsLoss-of-function mutations

• Null allele:Null allele:– When the allele has a complete loss of function When the allele has a complete loss of function

• it is often called an it is often called an amorphicamorphic mutation mutation • Leaky mutationsLeaky mutations::

– If some function may remain, but not at the If some function may remain, but not at the level of the wild type allelelevel of the wild type allele

• The degree to which the function is lost can varyThe degree to which the function is lost can vary

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Gain-of-function mutationsGain-of-function mutations• change the gene product such that it gains a change the gene product such that it gains a

new and abnormal functionnew and abnormal function

• These mutations usually have These mutations usually have dominant dominant phenotypesphenotypes

• Often called a Often called a neomorphic neomorphic mutation mutation (A* B)(A* B)

*Normal allele*Normal allele

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Gain-of-function mutationsGain-of-function mutations• Although it would be expected that most Although it would be expected that most

mutations would lead to a mutations would lead to a loss of functionloss of function

• A mutation in which dominance is caused A mutation in which dominance is caused by by changing the specificity or expression changing the specificity or expression pattern of a gene or gene product (pattern of a gene or gene product (Gain-Gain-of-functionof-function!),!), rather than simply by rather than simply by reducing or eliminating the normal activity reducing or eliminating the normal activity of that gene or gene productof that gene or gene product

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Dominant negative mutationsDominant negative mutations

• Dominant negative mutations:Dominant negative mutations:– antimorphicantimorphic mutations mutations (B A)(B A)– an altered gene product that acts an altered gene product that acts

antagonistically to the wild-type alleleantagonistically to the wild-type allele– These mutations usually result in an These mutations usually result in an

altered molecular function (often altered molecular function (often inactive):inactive):• DominantDominant• or or semi-dominantsemi-dominant phenotype phenotype

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Dominant negative mutationsDominant negative mutations

• In humans:In humans:– Marfan syndromeMarfan syndrome is an example of a is an example of a

dominant negative mutation dominant negative mutation – occurring in an occurring in an autosomal dominantautosomal dominant

diseasedisease– the defective glycoprotein product of the the defective glycoprotein product of the

fibrillin gene (FBN1):fibrillin gene (FBN1):» antagonizes the product of the normal antagonizes the product of the normal

allele allele

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Fibrillin geneFibrillin gene

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True dominant to wild type

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Types of dominant mutation (10%)Types of dominant mutation (10%)

• Muller (1932) quantitative changes to a pre-Muller (1932) quantitative changes to a pre-existing WT character:existing WT character:

• AmorphAmorph

• HypomorphHypomorph

• HypermorphHypermorph

• AntimorphAntimorph

• neomorphneomorph

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Null / leaky Allele

Marfan

Gain-of-function mutations

Dominant negative mutations

Collagen/ Fibrillin

(Genetic mechanisms)

RAS

PKU/

globin

LDL Receptor

PMP-22>Charcot-Marie-Tooth

P53

(Semi- and Dominant)

BCR-ABL

in cis (hem S+ b23Val>Ile)

globin

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Molecular and Genetic classification of Molecular and Genetic classification of dominant mutationdominant mutation

1.1. reduced gene dosage, expression, or protein reduced gene dosage, expression, or protein activity (haploinsufficiency) activity (haploinsufficiency)

2.2. increased gene dosageincreased gene dosage3.3. ectopic or temporally altered mRNA expressionectopic or temporally altered mRNA expression4.4. increased or constitutive protein activityincreased or constitutive protein activity5.5. dominant negative effectsdominant negative effects6.6. altered structural proteinsaltered structural proteins7.7. toxic protein alterationstoxic protein alterations8.8. new protein functionsnew protein functions

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Reduced gene dosage, expression, or Reduced gene dosage, expression, or protein activity (haploinsufficiency) protein activity (haploinsufficiency)

• Inactivation of one of a pair of allelesInactivation of one of a pair of alleles– Mutation > loss of function:Mutation > loss of function:

– Deletion, Ch Translocation, truncation,…Deletion, Ch Translocation, truncation,…

–Type I collagenType I collagen–globinsglobins–LDL-ReceptorLDL-Receptor

• Regulatory genes:Regulatory genes:–PAX3PAX3

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Waardenburg SyndromeWaardenburg Syndrome (PAX3)(PAX3)

• DeafnessDeafness• pigmentary anomaliespigmentary anomalies• white forelockwhite forelock• heterochromia iridisheterochromia iridis• partial albinismpartial albinism• Prominent broad nasal root Prominent broad nasal root • Hypertrichosis of the medial part of the Hypertrichosis of the medial part of the

eyebrows eyebrows

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heterochromia iridisheterochromia iridis

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Increased DosageIncreased Dosage

• Increase gene dosage to three copies affect Increase gene dosage to three copies affect phenotype others, than reduction to one phenotype others, than reduction to one copy (+21, +18, +13, XXY, than X0,…)copy (+21, +18, +13, XXY, than X0,…)

• Critical genes are importantCritical genes are important

• PMP-22: duplication >Charcot-Marie-PMP-22: duplication >Charcot-Marie-Tooth disease:Tooth disease:

– Haploinsufficient > different phenotype Haploinsufficient > different phenotype of Increased Dosage!of Increased Dosage!

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Increased Dosage in Increased Dosage in Charcot-Marie-Tooth disease:Charcot-Marie-Tooth disease:

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Ectopic or Temporally altered Ectopic or Temporally altered mRNA ExpressionmRNA Expression

• Point mutation in Point mutation in • Alters binding of the transacting factorAlters binding of the transacting factor

– Abrogate the normal switch from Abrogate the normal switch from expression of :expression of : to to and and

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HPFH HPFH as a δβ-globin Diseaseas a δβ-globin Disease

• Large deletions at the β-globin locusLarge deletions at the β-globin locus

• from the region close to the human Aγ gene from the region close to the human Aγ gene to well downstream of the human β-globinto well downstream of the human β-globin

• gene and including deletion of the structural gene and including deletion of the structural δ- and β-globin genesδ- and β-globin genes

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HPFHHPFH

• Heterozygotes:Heterozygotes:

– a normal level of HbA2a normal level of HbA2

– even higher levels of HbF (15 to 30 %) even higher levels of HbF (15 to 30 %)

• Homozygotes: Homozygotes:

– clinically normalclinically normal

– albeit with reduced MCV and MCHalbeit with reduced MCV and MCH

• Compound heterozygotes with b thalassemia:Compound heterozygotes with b thalassemia:

– clinically very mildclinically very mild

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Why mutations of structural Why mutations of structural proteins are frequently dominant?proteins are frequently dominant?

• Admixture of normal and abnormal structure Admixture of normal and abnormal structure components will disrupt the overall structurecomponents will disrupt the overall structure

• Biochemical analysis: Biochemical analysis:

– Abnormal mRNAAbnormal mRNA

– Cellular processingCellular processing

– SecretionSecretion

– Without mature FibrillsWithout mature Fibrills

• Type I Collagen, Fibrillin in MarfanType I Collagen, Fibrillin in Marfan

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Toxic protein alterationsToxic protein alterations

• Usually missense mutations:Usually missense mutations:

– Disrupt normal function Disrupt normal function

– Lead to toxic products or precursorsLead to toxic products or precursors

• Sickle cell mutations (hem S, Sickle cell mutations (hem S, 6Glu>Val)*6Glu>Val)*

• * Although : recessive* Although : recessive

• Coinheritance in Coinheritance in ciscis (hem S+ (hem S+ 23Val>Ile)23Val>Ile)– Sickling to manifest in the heterozygote!Sickling to manifest in the heterozygote!

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Toxic protein alterationsToxic protein alterations

• Various point mutations in rhodopsinVarious point mutations in rhodopsin

– Slow degeneration of rod photoreceptor Slow degeneration of rod photoreceptor outer segmentouter segment

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New protein functionsNew protein functions

• Creation of new , adventageus protein functions by Creation of new , adventageus protein functions by mutation:mutation:

– The life blood the evolutionThe life blood the evolution

– Occurs over protracted time scaleOccurs over protracted time scale

– Protein with truly new function: rareProtein with truly new function: rare

– Usually pathologicalUsually pathological

– Juxtaposition of domains from different proteins.Juxtaposition of domains from different proteins.

• Generate new function: ABL-BCR (9;22) Generate new function: ABL-BCR (9;22) Philadelphia translocationPhiladelphia translocation

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A gene affecting brain sizeA gene affecting brain size

Microcephaly (MCPH)Microcephaly (MCPH)• Small (~430 cc v Small (~430 cc v

~1,400 cc) but ~1,400 cc) but otherwise ~normal otherwise ~normal brain, only mild brain, only mild mental retardationmental retardation

• MCPH5MCPH5 shows shows Mendelian autosomal Mendelian autosomal recessive inheritancerecessive inheritance ASPM-/ASPM- control

Bond Bond et al.et al. (2002) (2002) Nature GenetNature Genet. 32, 316-320. 32, 316-320

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Other mechanismOther mechanism

• Genomic imprinting:Genomic imprinting:• If a gene is transcribed only from the ch If a gene is transcribed only from the ch

originating from one of the two parentsoriginating from one of the two parents• The locus is hemizygousThe locus is hemizygous• Mutation of the allele on the active chromosome Mutation of the allele on the active chromosome

– Inactive the locusInactive the locus• Mutation of the other chromosome Mutation of the other chromosome

– No phenotypic effectNo phenotypic effect• Beckwith-wiedermann syndromeBeckwith-wiedermann syndrome

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Beckwith-wiedermann Beckwith-wiedermann syndrome (syndrome (BWS)BWS)

• The incidence of BWS :The incidence of BWS :

– 1:13700 live births 1:13700 live births

• The increased risk of tumor formation in The increased risk of tumor formation in BWS patients:BWS patients:

– 7.5% 7.5%

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• Studies of mutagenesis in many organisms Studies of mutagenesis in many organisms indicate that the majority (over 90%) of indicate that the majority (over 90%) of mutations are recessive to wild type mutations are recessive to wild type

• If If recessivenessrecessiveness represents the 'default' represents the 'default' state, what are the distinguishing features state, what are the distinguishing features that make a minority of mutations give that make a minority of mutations give rise to rise to dominantdominant or or semidominantsemidominant characters? characters?

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The concepts of dominance & The concepts of dominance & recessiverecessive

• Formulated by Mendel (1965)Formulated by Mendel (1965)• Why are some disease dominant and other Why are some disease dominant and other

recessive?recessive?• Dominance is not an intrinsic property of a Dominance is not an intrinsic property of a

genegene or or mutant allelemutant allele• Relationship between the phenotypes of 3 Relationship between the phenotypes of 3

genotypes (AA, AB, BB):genotypes (AA, AB, BB):– Dominant & Semi dominant (10%)Dominant & Semi dominant (10%)– Recessive (90%)Recessive (90%)

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Semi dominantSemi dominant

• Example of homozygous mutants:Example of homozygous mutants:

– Thalassemia, Familial Thalassemia, Familial hypercholesterolemiahypercholesterolemia

– Phenotype of the homozygotePhenotype of the homozygote

• More severity than heterozygoteMore severity than heterozygote

• Huntington:Huntington:

– True dominant to wild typeTrue dominant to wild type

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Dominant mutations are much rarer Dominant mutations are much rarer than recessive onesthan recessive ones

• Insertional inactivation by retroviral DNA Insertional inactivation by retroviral DNA in mouse genom:in mouse genom:

– 10-20:1 (Rec:Dom)10-20:1 (Rec:Dom)

• Wright et al.:Wright et al.:

– Physiology of the gene actionPhysiology of the gene action

• Fisher et al.:Fisher et al.:

– Accumulation of modifier alleles at other Accumulation of modifier alleles at other lociloci

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Alga Alga ChlamydomonasChlamydomonas

• Usually haploidUsually haploid

• In a diploid backgroundIn a diploid background

– Nevertheless : recessive behaviorNevertheless : recessive behavior

– Supporting: Wright ‘s theorySupporting: Wright ‘s theory

• Indeed, diploidy:Indeed, diploidy:

– Protects against recessive mutationsProtects against recessive mutations!!

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Why most inborn errors of Why most inborn errors of metabolism are recessive?metabolism are recessive?

• Metabolic pathway:Metabolic pathway:

– Not Not critical rate limiting stepscritical rate limiting steps

– Not Not qualitatively alteredqualitatively altered function function

– Perhaps: dominat mutations:Perhaps: dominat mutations:

• Developmental malformationsDevelopmental malformations

8989

Recessive to Dominant mutationsRecessive to Dominant mutations

• Caenorhabditis elegansCaenorhabditis elegans (C elegans): (C elegans):

• Recessive mutations at a series of loci Recessive mutations at a series of loci termed smg:termed smg:

– May alter the behavior of mutations May alter the behavior of mutations from from recessive to dominantrecessive to dominant

• It seems: Wt smg: encode proteins :It seems: Wt smg: encode proteins :

– Recognize and degrade mutant mRNA Recognize and degrade mutant mRNA species (species (surveillancesurveillance))