X-Chromosome Inactivation and X-Linked Diseases

X-Chromosome Inactivation

• In females, one X chromosome is randomly inactivated early in embryonic development.
• This inactivation is permanent and clonally propagated.
• The inactive X chromosome appears as a Barr body.
• Inactivation can lead to mosaicism, with different cell lineages expressing different X-linked alleles.

X-Linked Diseases

• Genes located on the X chromosome are called X-linked genes.
• X-linked diseases have a characteristic pattern of inheritance, with males being hemizygous for the X chromosome and females being heterozygous.
• X-linked diseases can be dominant or recessive, with different severity and expression patterns between males and females.
• X-linked recessive diseases are often observed in males due to hemizygosity, while females may have a milder phenotype due to random X inactivation.
• X-linked dominant diseases can affect both males and females, with males usually exhibiting more severe symptoms.

X-Linked Recessive (XLR) Diseases

• Males are more likely to be affected.
• Females are usually carriers but may exhibit symptoms due to skewed X inactivation.
• Examples include Hemophilia A and Duchenne Muscular Dystrophy.

X-Linked Dominant (XLD) Diseases

• Both males and females can be affected.
• Males usually have more severe symptoms.
• Fathers cannot transmit the disease to their sons.
• Example: Vitamin-D resistant rickets (hypophosphatemic rickets).

Extra-Nuclear Inheritance (Mitochondrial DNA Disorders)

• Mitochondria contain their own circular DNA (mtDNA), which is maternally inherited.
• Some diseases are caused by mutations in mtDNA, often affecting skeletal/cardiac muscle and neurological function.
• Examples: Leber's Hereditary Optic Atrophy.

Triplet Repeat Disorders

• Sequences of three base pair (bp) repeats are common in the genome.
• Triplet repeat disorders are caused by unstable expansion of these repeats.
• The number of repeats can influence disease severity and age of onset.
• There are over 30 known diseases caused by triplet repeat expansion, with varying inheritance patterns and repeat sequences.

Fragile X Syndrome

• This is the most common inherited cause of intellectual disability.
• Caused by expansion of a CGG repeat in the 5' UTR of the FMR1 gene.
• The gene encodes an RNA-binding protein involved in synapse formation.
• Premutations (50-100 repeats) are clinically normal but can expand to full mutations during female meiosis, leading to a maternal parent of origin effect.
• Full mutations (>200 repeats) lead to silencing of the FMR1 gene, resulting in the syndrome.

Huntington's Disease

• Caused by expansion of a CAG repeat in the coding region of the huntingtin gene.
• The repeat expansion leads to an altered protein that causes neuronal damage.
• Over 36-40 repeats are associated with disease.
• Expansion primarily occurs during male meiosis, indicating a paternal parent of origin effect.
• This is an autosomal dominant disorder.

Anticipation in Triplet Repeat Disorders

• Expansion of the repeat during transmission can lead to earlier onset and increased severity of the disease in subsequent generations.
• This phenomenon is known as anticipation.
• The Sherman paradox describes a specific case of anticipation where a father transmits a premutation to his daughter, who later expands the repeat and gives birth to a child with a full mutation.

Parent of Origin Effects

• The identity of the parent transmitting the mutated gene can influence the expansion of the repeat and the severity of the disease.
• In Fragile X, premutation expansion occurs during female meiosis, making it a maternally transmitted disorder.
• In Huntington's Disease, premutation expansion occurs during male meiosis, making it a paternally transmitted disorder.

X-chromosome inactivation

• In female cells, one of the two X-chromosomes is inactivated, this can occur on the maternal or paternal chromosome
• X-chromosome inactivation occurs early in embryonic life, the inactivation is random and permanent and is clonally propagated
• The inactive X chromosome appears as a dark-staining mass known as a Barr body

X-Linked Diseases

• Genes carried on the X chromosome are “X-Linked”
• May be dominant or recessive
• Males are hemizygous for X
• Random X inactivation occurs in females – X-linked traits are variably expressed
  • Balanced X-inactivation
  • Skewed X-inactivation
    • ‘Manifesting heterozygote’ – the normal X chromosome is preferentially inactivated
    • Unbalanced inactivation -- the mutant X chromosome is preferentially inactivated: clinically unaffected

X-Linked Recessive

• XLR diseases are rarely expressed clinically in females
• Males are affected in half the offspring from a carrier female and all carrier females
• Females are carriers in half the offspring from a carrier female

X-Linked Recessive Diseases: Examples

• Haemophilia A:
  • Deficiency of factor VIII
  • Haemophilia A is characterised by soft-tissue bleeding
• Duchenne Muscular Dystrophy
  • A form of muscular dystrophy; onset at ~5 years, progressive muscle weakness, death in early 20s
  • death occurs due to respiratory or cardiac complications

X-Linked Dominant

• Example: vitamin-D resistant rickets
  • Low blood and high urinary phosphate
  • Short stature and bony deformities
  • Absence of male-to-male transmission
  • Daughters of affected males may be affected
• Expression of XLD symptoms are more severe and constant in males than in females

Extra-nuclear inheritance

• The mitochondrion contains a ~16kb circular DNA genome – mtDNA
• mtDNA is maternally inherited
• Some human diseases are characterised by abnormal mtDNA
• Mitochondrial diseases often involve skeletal/cardiac muscle impairment and neurological problems

Triplet Repeat Disorders

• Sequences of three bp repeats are found throughout the genome
• Repeat sequences may be polymorphic
• Triplet repeat disorders are due to unstable expansion in number of repeats

Triplet Repeat Expansion Diseases

• At least 30 diseases are known to result from triplet repeat expansion
• Characteristics/differences
  • Inheritance : AD, AR, X-linked
  • Base sequence of repeat
    • eg CGG in Fragile-X; CAG Huntingtons
  • Number of repeats, presymptomatic vs. affected

Fragile X Syndrome

• Most common form of inherited mental retardation
• ~ 1/2000 - 1/4000 individuals are affected
  • Females are also affected
• Fragile X is responsible for 4-8% of males with mental retardation
• Named for a characteristic break observed on the X chromosome (cytogenetics)
• Molecular defect in FMR1 (Fragile X Mental Retardation 1) gene
• FMR1 is an RNA binding protein involved in formation of synapses
• FMR1 has CGG ‘triplet repeat’ in the 5’ UTR
• During meiosis in the female, the repeat is unstable and can increase in length – expansion
• Expansion beyond 200 copies prevents translation of FMR1 protein

Fragile X Syndrome: Repeat Number Ranges

• 10 – 50 CGG repeats - normal
  • 50 – 100 repeats - premutation
  • 200 – 2000 repeats - full mutation

Fragile X: Inheritance

• Grandfather (I-1) has premutation -- is a ‘normal transmitting male’
• Expansion of repeats during meiosis in the Mother (II-2) results in a full mutant gene being transmitted to the son (III-1)
• ‘modified’ X-linked inheritance
• Premutation : upper limit of normal, but capable of expanding to disease

Anticipation, the Sherman paradox and parent of origin effects

• Anticipation & the Sherman paradox:

  • Expansion of mutation from one generation to the next
  • Sometimes seen clinically as: reduced age of onset and / or increased severity down the pedigree

  Parent of origin effect

  • Premutation – expansion occurs:
    • in fragile X, premutation only expands in female : risk
  • only if inherited from mother  sometimes described as being “maternally transmitted “

Huntington’s disease

• CAG triplet repeat in protein coding region  polyQ region in protein
• Over 28 repeats leads to unstable expansion
• Expansion beyond ~ 36 -40 repeats leads to expression of a protein that causes neuronal damage
• Paternal parent of origin effect (Expansion of repeat number occurs in males)
• Autosomal dominant

Other resources

• Reading
  • Thompson & Thompson ch 5,12
  • ABC of clinical genetics Ch 6

Learning Objectives

• Explain X-chromosome inactivation in females
• Describe extra-nuclear inheritance (mitochondrial DNA disorders)
• Discuss triplet repeat disorders, parent of origin effects and anticipation (with examples: Huntington disease and Fragile X Syndrome)

Sex Determining Chromosomes

• Sex determined at fertilization
• Male: XY
• Female: XX
• Males are "hemizygous" for the X chromosome
• Equal amounts of proteins encoded by genes on the X chromosome are produced in males and females, despite females having twice the number of chromosomes

X-Chromosome Inactivation

• One X-chromosome is inactivated in female cells
• Occurs early in embryonic development
• Inactivation is random (either maternal or paternal X can be inactivated)
• Complete inactivation is permanent and clonally propagated
• Inactive X chromosome appears as a dark-staining mass (Barr body)

X-Linked Diseases

• X-linked genes are carried on the X chromosome
• Lack of male to male transmission
• X-linked disorders can be dominant or recessive
• Males are hemizygous for X, leading to full expression of mutant traits
• Random X inactivation occurs in females, causing variable expression of X-linked traits
  • Balanced: Equal inactivation of X chromosomes
  • Skewed: Preferential inactivation of either normal or mutant X chromosome
  • Manifesting heterozygote: The normal X chromosome is preferentially inactivated, leading to clinical expression of the trait
  • Unbalanced inactivation: The mutant X chromosome is preferentially inactivated, leading to clinically unaffected individuals

X-Linked Recessive Inheritance

• X-linked diseases are rarely expressed in heterozygous females and are therefore labelled as X-linked recessive (XLR).

X-Linked Dominant Inheritance

• X-linked dominant (XLD) disorders are expressed clinically in many females.
• XLD disorders are characterized relative to XLR disorders by the proportion of males and females who are affected.

Mitochondrial Inheritance

• Mitochondria have a circular DNA genome (~16kb) - mtDNA
• Mitochondrial inheritance is maternal
• Some diseases involve abnormal mtDNA, often affecting skeletal/cardiac muscles and neurological function
  • Leber’s Hereditary Optic Atrophy

Triplet Repeat Disorders

• Sequences of three base pair (bp) repeats are found throughout the genome
• These repeats can be polymorphic (variable number of repeats between individuals)
• Triplet repeat disorders are caused by unstable expansion in the number of repeats
• At least 30 diseases are known to arise from triplet repeat expansion
• Characteristics of Triplet Repeat Expansion:
  • Different inheritance patterns: Autosomal dominant (AD), Autosomal recessive (AR), X-linked
  • Variable base sequence of repeat
  • Variable number of repeats (presymptomatic vs. affected)

Fragile X Syndrome

• Most common form of inherited mental retardation
• Affects ~1/2000 - 1/4000 individuals
• Accounts for 4-8% of males with mental retardation
• Named for a characteristic break observed on the X chromosome (cytogenetics)
• Molecular defect in the FMR1 (Fragile X Mental Retardation 1) gene
  • FMR1 encodes an RNA binding protein involved in synapse formation
• FMR1 has a CGG triplet repeat in the 5’ UTR
• Expansion of the repeat during meiosis in females leads to a maternal parent of origin effect
• Expansion beyond 200 copies prevents translation of the FMR1 protein
• Repeats:
  • 10 - 50: Normal
  • 50 - 100: Premutation (can expand to disease)
  • 200 - 2000: Full mutation

Huntington's Disease

• CAG triplet repeat in the protein coding region
• PolyQ region in protein
• Over 28 repeats leads to unstable expansion
• Expansion beyond 36-40 repeats causes neuronal damage
• Paternal parent of origin effect (expansion occurs in males)
• Autosomal dominant inheritance pattern

Anticipation

• Expansion of the mutation from one generation to the next
• Can result in:
  • Reduced age of onset
  • Increased severity
• Parent of origin effect:
  • Fragile X premutation only expands in females
  • Huntington's expansion occurs in males