X-Linked InheritanceX-Linked Inheritance

X-linked recessive disordersX-linked recessive disorders

• Responsible gene on X chromosomeResponsible gene on X chromosome • For females, both copies of the X For females, both copies of the X

chromosome must be affectedchromosome must be affected

• Males, Males, hemizygoushemizygous for the X chromosome, for the X chromosome, much more likely to be affectedmuch more likely to be affected

Some CommonSex-Linked Recessive Disorders

• Duchenne and Becker Muscular Dystrophy

• Hemophilia A

• Glucose-6-phosphate dehydrogenase deficiency

• Color blindness

Hemophilia AHemophilia AX x

Maternal

X

YPa

tern

al

unaffected non-carrier

XY

Xx

XY

XX

unaffected carrier

affected

1/4 1/4

1/4 1/4

1/2

1/2

F

1/2

1/2

M

Predict possible fetal outcomes

Haemophilia AHaemophilia AAn X-linked recessive diseaseAn X-linked recessive disease

Caused by mutation in the clotting factor VIII Caused by mutation in the clotting factor VIII gene (F8) on chromosome Xq28gene (F8) on chromosome Xq28

Incidence: 1/5,000 males birthsIncidence: 1/5,000 males births

Clinical symptomsClinical symptoms

Haemorrhage into joints and muscles, Haemorrhage into joints and muscles,

easy bruising, and prolonged bleeding easy bruising, and prolonged bleeding from wounds.from wounds.

X-inactivation, Dosage compensation, and the X-inactivation, Dosage compensation, and the expression of X-linked genesexpression of X-linked genes

Same amount of X-linked gene products Same amount of X-linked gene products between males and femalesbetween males and females

MalesMales One X chromosomeOne X chromosome

FemalesFemales Two X chromosomesTwo X chromosomes

And yet, the mean amounts of gene products of X-linked And yet, the mean amounts of gene products of X-linked genes are the same in males as in femalesgenes are the same in males as in females

HOW?HOW? Through X chromosome inactivationThrough X chromosome inactivation

The molecular mechanism behind X-The molecular mechanism behind X-inactivationinactivation

The key player is the X-linked gene XISTThe key player is the X-linked gene XIST X(inactive)-specific transcriptX(inactive)-specific transcript Chromosome Xq13.2Chromosome Xq13.2

XIST is transcribed to produce a XIST is transcribed to produce a non-coding RNAnon-coding RNA that “coats” the X-chromosome and inactivates it that “coats” the X-chromosome and inactivates it

XIST is uniquely expressed from the inactive XXIST is uniquely expressed from the inactive X XIST XIST RNA does not travel over to any other X RNA does not travel over to any other X

chromosome in the nucleus (i.e., chromosome in the nucleus (i.e., ciscis action). action). Barr bodiesBarr bodies are inactive X chromosomes are inactive X chromosomes

"painted" with "painted" with XIST XIST RNA. RNA.

Transcription of Transcription of XISTXIST ceasesceases on the other X on the other X chromosome allowing all of its hundreds of chromosome allowing all of its hundreds of other genes to be expressed. The shut-down of other genes to be expressed. The shut-down of the the XISTXIST locus on the active X chromosome is locus on the active X chromosome is done by methylating done by methylating XISTXIST regulatory regulatory sequences.sequences.

So methylation permanently blocks So methylation permanently blocks XISTXIST expression and permits the continued expression and permits the continued expression of all the other X-linked genes.expression of all the other X-linked genes.

XIST

The XIST gene on one of the two X-chromosomes is randomly inactivated by DNA methylation

The active XIST is transcribed and its RNA product coats the X-chromosome

X with inactive XIST

X with active XIST

The histones on the coated X undergo methylation which causes the chromosome to condense, forming a Barr body, and renders it inactive

The uncoated X is left transcriptionally

active

Barr body

X inactivation by X inactivation-specific transcript (Xist)

Barr bodies

Expression of X-linked Genes in Expression of X-linked Genes in

HeterozyotesHeterozyotes Inactivation is random, established when embryo Inactivation is random, established when embryo

< 100 cells < 100 cells fraction of cells in carrier female fraction of cells in carrier female with normal or mutant allele tend to be variablewith normal or mutant allele tend to be variable

Thus, clinical variation in expression of X-linked Thus, clinical variation in expression of X-linked disorders is common in heterozygotes ranging disorders is common in heterozygotes ranging from normal to affectedfrom normal to affected

A A manifesting heterozygotemanifesting heterozygote is a female in whom is a female in whom the deleterious allele is on the active X in most the deleterious allele is on the active X in most or all of cells (an extreme e.g., of unbalanced or or all of cells (an extreme e.g., of unbalanced or skewed X-inactivation) skewed X-inactivation)

X chromosome InactivationX chromosome Inactivation

Inactivation is not always randomInactivation is not always random A structurally abnormal X is preferentially inactivated, e.g., A structurally abnormal X is preferentially inactivated, e.g.,

isochromosome Xisochromosome X E.g., extraembryonic membranes (that go on to form the amnion, E.g., extraembryonic membranes (that go on to form the amnion,

placenta, and umbilical cord). In all the cells of the placenta, and umbilical cord). In all the cells of the extraembryonic membranes, it is father's X chromosome that is extraembryonic membranes, it is father's X chromosome that is inactivated.inactivated.

Inactivation is not completeInactivation is not complete Some genes are known to escape inactivation (i.e. those with a Some genes are known to escape inactivation (i.e. those with a

functional homolog on the Y, e.g., genes located in the functional homolog on the Y, e.g., genes located in the pseudoautosomal region, pseudoautosomal region, still others are specific to X chrstill others are specific to X chr.) .)

Inactivation is not permanentInactivation is not permanent reversed in development of germ cells (not passed on to reversed in development of germ cells (not passed on to

gametes)gametes)

If an X bears a piece of autosome(translocation) then the untranslocated X is always inactivated since the cell needs both copies of the autosomal genes to be active

If the translocated X has a mutant allele, all the woman’s cells will be mutant

X-autosome translocationX-autosome translocation

There is normally a 50% chance thata particular X will be inactivated in a cell

from a female

Functional Mosaicism Resulting from X-Functional Mosaicism Resulting from X-inactivationinactivation

Females are mosaics wrt their X-linked Females are mosaics wrt their X-linked genesgenes

Mosaicism is readily detected for some Mosaicism is readily detected for some disorders e.g., DMDdisorders e.g., DMD

Immunostaining for dystrophin in muscle specimens. A, A normal female (magnification ×480).

B, A male with Duchenne muscular dystrophy (×480).

C, A carrier female (×240).

Staining creates the bright lines seen here encircling individual muscle fibers. Muscle from DMD patients lacks dystrophin staining. Muscle from DMD carriers exhibits both positive and negative patches of dystrophin immunostaining, reflecting X inactivation

Example: hemophilia A

P Predict possible fetal outcomes

X X

X Y

X

Y

X x

X x

x Y

m a t e r n a l

pate

rnal

1 2 m ale

1 2 female

1 4 fem ale carriers1 4

fem ale non-carriers

1 4 m ale affected1 4

m ale unaffected

m aternal

paternal

xX fem ale carrier1 4

x1 2

X1 2

Y1 2

X1 2

xY m ale affected1 4

XX fem ale non-carrier1 4X1

2

Y1 2

XY m ale unaffected1 4

Homozygous Affected FemalesHomozygous Affected Females

Consanguinity in an X-linked recessive pedigree for red-green color blindness, resulting in a homozygous affected female

New Mutation in X-linked DisordersNew Mutation in X-linked Disorders

• For a sex-linked recessive disorder with zero fitness, such as Duchenne muscular dystrophy, 1/3 of disease alleles are in males and are lost with each generation. Thus, 1/3 of disease alleles must be replaced with a new mutation in each generation

• DMD is said to be genetic lethal because affected males usually fail to reproduce

• For hemophilia, in which reproduction is reduced but not eliminated, a proportionately smaller fraction of cases will be due to new mutation

Males are more commonly affected than females.

The gene responsible is transmitted from an affected man through his daughters, who are seldom affected. Each daughter is an obligatory heterozygous carrier. Each of the carrier daughter's sons has a 50% chance of inheriting it.

No male to male transmission occurs.

The affected males in a pedigree are usually related through females.

Heterozygous female carriers are usually unaffected, but some may express the condition with variable severity (“Lyonization”).

Characteristics of Sex-LinkedRecessive Inheritance

X-Linked Dominant InheritanceX-Linked Dominant Inheritance

• Responsible gene on X chromosome Responsible gene on X chromosome

• The phenotype is regularly expressed in The phenotype is regularly expressed in heterozygotesheterozygotes

• Affected fathersAffected fathers transmit the disorder to transmit the disorder to ALLALL of their of their daughters daughters nonenone of their sons of their sons

• The pattern of inheritance through females is no The pattern of inheritance through females is no different from AD patterndifferent from AD pattern

• Each child of an affected female has a 50% chance Each child of an affected female has a 50% chance of inheriting the trait, regardless of sexof inheriting the trait, regardless of sex

• Rare X-linked dominant phenotypes are about twice Rare X-linked dominant phenotypes are about twice as common in females, though the expression is as common in females, though the expression is much milder in females who are almost always much milder in females who are almost always heterozygousheterozygous

X-Linked Dominant InheritanceX-Linked Dominant Inheritance

• X-linked hypophosphatemic rickets, also called vitamin D-resistant rickets, in which ability of kidney tubules to reabsorb filtered phosphate is impaired• Serum phosphate level is less depressed and rickets less severe in heterozygous females as compared to affected males• The defective gene product appears to be a member of a family of endopeptidases, but the pathogenic mechanism is not known

X-linked Dominant Disorders with Male LethalityX-linked Dominant Disorders with Male Lethality

Some rare genetic defects expressed exclusively or Some rare genetic defects expressed exclusively or almost exclusively in females appear to be XD almost exclusively in females appear to be XD lethal in males before birth or early infancylethal in males before birth or early infancy

Typical pedigrees: transmission by affected female Typical pedigrees: transmission by affected female affected daughters, normal daughters, normal affected daughters, normal daughters, normal sons in equal proportions (1:1:1)sons in equal proportions (1:1:1)

Rett syndrome meets criteria for an XD that is Rett syndrome meets criteria for an XD that is usually lethal in hemizygous males. The syndrome usually lethal in hemizygous males. The syndrome is characterized by normal prenatal and neonatal is characterized by normal prenatal and neonatal growth and development, followed by rapid onset of growth and development, followed by rapid onset of neurological symptoms and loss of milestones neurological symptoms and loss of milestones between 6 and 18 months of age.between 6 and 18 months of age.

Rett syndrome cont.Rett syndrome cont.

Children become spastic and ataxic, develop autistic Children become spastic and ataxic, develop autistic features and irritable behavior with outbursts of features and irritable behavior with outbursts of crying, and demonstrate characteristic purposeless crying, and demonstrate characteristic purposeless wringing or flapping movements of hands and arms.wringing or flapping movements of hands and arms.

Head growth slows and microcephaly develops. Head growth slows and microcephaly develops. Seizures are common (~50%)Seizures are common (~50%)

Mental deterioration stops after a few years and the Mental deterioration stops after a few years and the patients can then survive for many decades with a patients can then survive for many decades with a stable but severe neurological disability.stable but severe neurological disability.

Most cases caused by spontaneous mutations in an Most cases caused by spontaneous mutations in an X-linked MECP2 gene encoding methyl CpG binding X-linked MECP2 gene encoding methyl CpG binding protein 2. ? Thought to reflect abnormalities in protein 2. ? Thought to reflect abnormalities in regulation of genes in developing brain.regulation of genes in developing brain.

Typical appearance and hand posture of girls with Rett syndrome

Rett syndrome cont.Rett syndrome cont.

Males who survive with the syndrome usually have Males who survive with the syndrome usually have two X chromosomes (as in 47,XXY or in a two X chromosomes (as in 47,XXY or in a 46,X,der(X) male with the male determining SRY 46,X,der(X) male with the male determining SRY gene translocated to an X) or are mosaic for a gene translocated to an X) or are mosaic for a mutation that is absent in most of their cellsmutation that is absent in most of their cells

There are a few apparently unaffected women There are a few apparently unaffected women who have given birth to more than one child with who have given birth to more than one child with Rett syndrome. ? X-inactivation pattern in a Rett syndrome. ? X-inactivation pattern in a heterozygous female. ? Germline mosaic heterozygous female. ? Germline mosaic

Pedigree pattern demonstrating an X-linked dominant disorder, lethal in males during the prenatal period.

Characteristics of X-Linked Dominant Characteristics of X-Linked Dominant InheritanceInheritance

• Affected fathersAffected fathers with normal mates have with normal mates have no no affected sonsaffected sons and and no normal daughtersno normal daughters

• For rare pehnotypes, affected females are about For rare pehnotypes, affected females are about twice as common as affected malestwice as common as affected males (unless (unless disease is lethal in males), but affected females disease is lethal in males), but affected females typically have milder (though variable) expressiontypically have milder (though variable) expression

• Both male and female offspring of a Both male and female offspring of a heterozygous female have a 50% risk of heterozygous female have a 50% risk of inheriting the phenotype. The pedigree pattern is inheriting the phenotype. The pedigree pattern is similar to AD inheritancesimilar to AD inheritance

Patterns of Pseudoautosomal InheritancePatterns of Pseudoautosomal Inheritance

Genes on pseudoautosomal region can regularly Genes on pseudoautosomal region can regularly exchange b/w the two sex chr’sexchange b/w the two sex chr’s

E.g., Dyschondrosteosis, a dominantly inherited E.g., Dyschondrosteosis, a dominantly inherited skeletal dysplasia with disproportionate short skeletal dysplasia with disproportionate short stature and deformity of the forearmstature and deformity of the forearm The responsible gene is pseudoautosomal that escapes The responsible gene is pseudoautosomal that escapes

X-inactivation, encodes a transcription factor likely X-inactivation, encodes a transcription factor likely involved in statureinvolved in stature

Either deletion/mutations Either deletion/mutations dyschondrosteosis in both dyschondrosteosis in both heterozygous males and females heterozygous males and females

Inheritance pattern of dyschondrosteosis. Arrow shows a male who inherited the trait on his Y chr. from his father. His father, however, had inherited the trait on his X chr. from his mother