cag repeat

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MNEMONIC: Diseases showing anticipation associated with Triplet Repeat Expansion

Anticipation: pattern of inheritance, symptoms of a genetic disorder become apparent at an earlier age as it is passed on to the next generation, an increase of severity of symptoms is also noted

Huntington Disease (HD)

  • Mnemonic: “HD es una CAGada” in Spanish, cagada means something bad, shitty, to be more precise. This is to remember the CAG trinucleotid repeat.
  • Autosomal Dominant, chromosome 4: “Huntin’ 4 food”
  • Chorea: purposeless movement of limbs, due to a loss of GABAergic neurons of neostriatum (caudate nucleus and putamen) of indirect pathway.
  • Personality changes, dementia, tendency for suicide

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Fragile X Syndrome

  • CGG trinucleotid repeat
  • X dominant, FMR 1
  • Mental retardation, large ears and jaw. post-pubertal macro-orchidism (males), attention deficit disorder (females)

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Friedreich Ataxia

  • GAA trinucleotid repeat
  • Autosomal recessive, Frataxin gene, chromosome 9
  • Neuronal degeneration: dorsal root ganglia, Clarke column (spinocerebellar tract), posterior column of spinal cord, dentate nucleus, Purkinje cells, Betz neurons, CN nuclei of VII, X, XII
  • Progressive gait & limb ataxia, arreflexia, hypertrophic cardiomyopathy, axonal sensory neuropathy, kyphoscoliosis, dysarthria, hand clumsiness, loss of sense of position, impaired vibratory sensation.

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MyoTonic DysTrophy

  • CTG trinucleotid repeat
  • Autosomal Dominant, MD1: chromosome 3; MD2: chromosome 19
  • Muscle loss, cardiac arrythmia, testicular atrophy, frontal baldness, cataracts.

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