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The majority of simple nucleotide repeats that occur frequently throughout the genome are not associated with disease. However, some are of great importance.

In 1991, an expansion in a trinucleotide (CAG) repeat in the androgen receptor gene was identified in X‐linked spinal and bulbar muscular atrophy a.k.a. Kennedy’s disease. The repeat normally 13–30 CAGs, lengthens to 40 or more CAGs in this condition.

Huntington’s disease (see Chapter 7 for detail) and spinocerebellar ataxias – SCA1‐3, Friedreich’s ataxia are others. An expanded hexanucleotide repeat occurs in familial amyotrophic lateral sclerosis with fronto‐temporal dementia.

Disorders can exhibit anticipation – severity worsens in successive generations. This correlates with the increased number of repeats.