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"