Updated: 12/25/2018

Trinucleotide Repeat Expansion Diseases

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Topic
Overview
 
Disease
Nucleotide Repeat Gene affected Chromosome
Fragile X syndrome
  • CGG 
  • FMR1
  • Sex X chromosome
Friedrich ataxia
  • GAA 
  • FXN
  • 9
Huntington's disease
  • CAG
  • HTT
  • 4
Myotonic dystrophy - type 1
  • CTG 
  • DMPK
  • 19
 
Fragile X Syndrome
  • Inheritance pattern
    • X-linked dominant 
  • Repeat expansion
    • CGG repeat on FMR1 gene
  • Presentation
    • mental retardation
    • autism
    • long face large jaw
    • large ears
    • large testicles (macroorchidism)
Friedrich's Ataxia
  • Inheritance pattern
    • autosomal recessive
  • Repeat expansion
    • GAA repeat on chromosome 9
      • results in
        • defect in frataxin (an iron binding protein) that leads to impaired mitochondrial function
        • degeneration of various spinal cord tracts
  • Presentation
    • neurological findings
      • muscle weakness
      • loss of deep tendon reflexes
      • loss of vibratory sensation and proprioception
      • clumsy gait with falls, nystagmus
    • other findings
      • pes cavus
      • diabetes mellitus
      • hypertrophic cardiomyopathy
Huntington's Disease
  • Inheritance pattern
    •  autosomal dominant
  • Repeat expansion
    • CAG repeat on chromosome 4
  • Presentation
    • neurologic findings
      • caudate atrophy with enlarged ventricles on head CT
      • decreased GABA and Ach
      • increased dopamine
      • NMDA mediated excitotoxicity
      • movement disorder
      • aggression/pyschosis
      • depression
      • dementia
      • occurs typically a young patient (age 20 - 50)
  • mnemonic
    • HUNT 4 an animal, put it in a CAGe
Myotonic Dystrophy - Type 1
  • Inheritance pattern
    • autosomal dominant
  • Repeat expansion
    • CTG repeat on DMPK gene
      • results in
        • abnormal expression of myotonin protein kinase
  • Presentation
    • myotonia
    • muscle wasting
    • cataracts
    • testicular atrophy
    • arrhythmia
    • frontal balding
    • classically "can't release a doorknob/handshake"

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EXPERT COMMENTS (24)
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