New Xlinked recessive mutation

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New X-linked recessive mutation
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Duchenne muscular dystrophy
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Gower maneuver
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Duchenne and Becker muscular dystrophy

• Most common form of muscular dystrophy
• About 1 per 4000 male births
• Duchenne muscular dystrophy
• Onset before 6 yr of progressive proximal muscle
  weakness, cardiomyopathy, skeletal deformities
  (secondary to weakness), may have mental
  retardation, usually die by 20 yr of cardiac or
  respiratory complications
• Elevated serum creatine kinase myopathic
  electromyogram, abnormal EKG
• Becker muscular dystrophy
• Onset by 20 or 30 of progressive proximal
  weakness, survival normal to near normal
• X-linked recessive, but up to 2.5 of
  heterozygotes are symptomatic

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Duchenne and Becker dystrophy

• Caused by mutations of the dystrophin gene
• Located at Xp21
• Largest known gene (over 2,300,000 bases)
• About 70 of mutations are deletions
• About 20 of mutations are small or point
  mutations
• About 5 of mutations are duplications
• About 15 - 25 are new mutations Genetic
  disease with no prior family history
• Precise nature of mutation, small mutations
  determines disease severity

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Becker muscular dystrophy allelic heterogeneity
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Dystrophin and associated proteins
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Dystrophin immunocytochemistry
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Multiplex Polymerase chain reaction
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The deletion endpoints affect the reading frame
for protein translation
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Multiplex PCR gel
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Female heterozygotes can develop symptoms in
X-linked recessive disorders X inactivation
or Lyonization
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All Calico cats are females
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X-inactivation in anhidrotic ectodermal dysplasia
Affected males Lack sweat glands Missing or
pointed teeth Fine hair/alopecia Mental
retardation Mutation in ED1 gene Heterozygous
females Patchy loss of sweat glands revealed by
sweat test
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X inactivation

• A process of dosage compensation
• With supernumerary X chromosomes, all but one X
  chromosome is inactivated
• When a female carries an X linked recessive
  mutation, some cells will be functionally
  hemizygous for that allele and may be affected by
  the trait

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X inactivation (Lyonization)

• In normal females, during early development one X
  chromosome is randomly inactivated
• Both X chromosomes are used in trophoblast and
  other tissues early in development - probably why
  Turners fetuses usually die in utero
• The same X chromosome is stably inactivated in
  all descendants of the cell in which the
  particular X was selected
• Inactivation depends in part upon the protein
  product of the Xist (X inactive specific
  transcript) gene at Xq13
• Virtually all X chromosome genes except for those
  involved in X inactivation are transcriptionally
  repressed
• Females are mosaic with respect to whether
  maternal or paternal X chromosome is active in a
  given cell
• Skewed X inactivation can lead to females being
  symptomatic for X-linked recessive disorders or
  traits

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X inactivation
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Barr body
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A dramatic example of Allelic heterogeneity

• Different clinical syndromes from mutations in
  the same gene

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Androgen insensitivity syndrome
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Androgen insensitivity syndrome (Testicular
feminization)

• 46,XY phenotypic females except for
• Blind vaginal pouch, no internal female genital
  structures
• Inguinal testes (higher abdominal temperature
  predisposes to seminoma development)
• Breast development though often ample is
  immature, e.g. areolae pale
• Pubic and axillary hair do not develop at puberty
• May present because of failure to menstruate
• Caused by mutation of androgen receptor Xq11-12
  that results in premature termination of protein

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Androgen receptor
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The androgen receptor is a ligand-activated
transcription factor
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Androgen insensitivity syndrome

• Management
• Raise as normal females but need to advise that
  will be unable to conceive
• Removal of undescended testicular tissue to
  prevent seminoma

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X-linked Spinal and Bulbar Muscular Atrophy
(Kennedy disease)

• Clinical features Progressive, late onset
  weakness and atrophy of extremities and bulbar
  muscles, gonadal atrophy, gynecomastia
• Inheritance X-linked recessive
• Chromosome Xq12
• Affected gene Androgen receptor gene
• Mutation Expansion from 20 CAG repeats to 40 or
  more repeats, resulting in expansion in receptor
  protein of polyglutamine stretch

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Androgen receptor gene trinucleotide expansion
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X-linked spinal and bulbar muscular atrophy
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X Linked dominant inheritance

• Affected males with normal mates have no affected
  sons all daughters are affected.
• Both sons and daughters of an affected mother
  have a 50 risk of inheriting the trait (same as
  autosomal dominant).
• Males tend to be more severely affected, or
  under-represented due to early lethality
• Example X-linked hypophosphatemic rickets

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X linked dominant, affected father
2 affected daughters 2 normal sons
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X linked dominant, normal father, affected mother
1 affected daughter 1 normal daughter 1 affected
son 1 normal son (pseudo autosomal dominant)
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Rett syndrome (RTT)

• Only seen in females
• Normal development until 6 to 18 months
• Then lose speech and purposeful hand movements
• Microcephaly, seizures, autism, ataxia,
  intermittent hyperventilation, and stereotypic
  hand-wringing movements
• Later, the condition stabilizes and patients
  usually survive into adulthood

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Rett syndrome, contd

• Affected gene identified by positional cloning
• Xq28
• Mutations found in the MeCP2 gene
• Protein is one of several proteins that
  recognizes and binds to methylated CpG bases in
  DNA
• Methylation of these residues prevents gene
  transcription
• The pattern of methylation is restricted to
  specific regions of chromosomes

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Rett Syndrome contd

• The methylation pattern is stably transmitted
  even after cell division
• This is one mechanism of imprinting, a form of
  epigenetic inheritance
• The effect of the mutation in the MeCP2 gene
  likely causes RTT syndrome because of the
  derepression of as yet unidentified genes that
  are normally subject to methylation
• Mutations of MeCP2 are lethal to male fetuses,
  explaining the restriction of RTT to females.

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Methyl cytosine
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Fragile X syndrome
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A Fragile X chromosome
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Fragile X syndrome (Martin-Bell)

• Most common inherited cause of mental retardation
• Clinical syndrome
• Large head, long facies, large ears
• Macroorchidism
• Moderate to severe mental retardation
• One third of females with the mutation have
  mental retardation
• Cytology
• Experimentally induced fragility of tip of long
  arm of X (q28)

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Fragile X (FMR1)

• Prevalence
• 1/1200 males, 1/2500 females
• Incompletely penetrant
• All daughters of transmitting male (normal
  intelligence) have normal intelligence
• Anticipation (more severe disease in subsequent
  generations)
• Sons born to such daughters have a 40 of FMR1
  and daughters of these women have a 16 chance
  of being retarded
• 50 of sons born to a retarded mother will have
  FMRI

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Fragile X syndrome Sherman paradox
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Fragile X

• Mutation is an expansion of a CGG trinucleotide
  repeat of the FRAXA gene at Xq28, readily
  detected by molecular testing
• Normal repeat length is 6 - 54 repeats
• Normal transmitting males have 60 - 200 repeats
• Affecteds have 200 - thousands of repeats
• Extreme expansions occur when mother is
  transmitting
• Repeat occurs in 5 untranslated region of gene
• Pathologic expansions usually associated with
  methylation that inhibits its expression
• FMRI protein is an RNA binding protein

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FMR gene and trinucleotide expansion
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Pelizaeus-Merzbacher disease

• An X-linked disorder of central nervous system
  (CNS) myelin
• Recessive in most families
• Caused by mutations of the proteolipid protein
  (PLP) gene

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Myelin ultrastructure
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PMD The clinical spectrum

• Very severe (connatal)
• Nystagmus (oscillatory eye movements) at birth
• Stridor
• Seizures
• Severe hypotonia and quadriparesis
• Severe cognitive impairment Usually never verbal
• Visual impairment
• Normal peripheral nerves
• Death in first or second decade
• Mutations cause PLP to misfold in the endoplasmic
  reticulum, which activates biochemical pathways
  that kill oligodendrocytes (apoptosis)

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PMD The clinical spectrum

• Classical
• Nystagmus (oscillatory eye movements) at birth
• Moderate hypotonia and quadriparesis
• Mild to moderate cognitive impairment Usually
  understand well but have more trouble with speech
• Variable visual impairment
• Normal peripheral nerves
• Survival into sixth or seventh decade is possible
• Most often caused by duplication of PLP gene

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Pelizaeus-Merzbacher disease
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Pelizaeus-Merzbacher disease
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Oligodendrocytes are membrane factories

• During the peak of myelination, each
  oligodendrocyte makes about 175 X 10-9 mg protein
  per day 3 times the weight of the perikaryon
• Each oligodendrocyte makes an area of myelin
  membrane about 1000 times the surface area of its
  perikaryon

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Proteolipid protein structure
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Pelizaeus-Merzbacher diseaseFISH
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No PLP is not as bad as too much PLP

• Null syndrome
• Nystagmus
• Spastic quadriparesis, usually worse in legs
• Ambulation often possible
• Ataxia
• Athetosis
• Peripheral neuropathy
• Mild to moderate cognitive impairment usually
  learn to speak
• Lack of PLP is not deleterious to oligodendrocytes

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The Big PictureIt helps to understand the
biology and biochemistry as well as genetics of
the disease
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Cell biology influences whether mutations are
recessive or dominant

• Females heterozygous for a severe mutation
  usually do not develop neurologic symptoms and
  signs
• Apoptosis of oligodendrocytes that have
  inactivated healthy X chromosome occurs as in
  affected males
• Dying cells are replaced by oligodendrocytes that
  have inactivated the abnormal X chromosome

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Hairy ears Not Y linked
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Y inheritance

• The handful of characterized genes on the Y
  chromosome either participate in determining male
  phenotype or testicular function
• Unequal exchange of a critical region of the Y
  chromosome result in karyotypically normal XX
  males or XY females (sex reversal)
• A gene, TDF (testis determining factor, aka SRY
  for sex determining region of Yp11.3) that
  encodes a probable transcription control factor
  lies in this region
• A cascade of genes, including TDF, participate in
  determining testicular development. Some of these
  other genes have homology to SRY and lie on
  autosomes.
• In principal a Y linked mutation would show ONLY
  male to male transmission, however mutations of
  known Y genes cause sex reversal or male
  sterility, therefore the only cases of
  male-to-male transmission occur with autosomal
  dominant genes