Several inherited neurodegenerative diseases are referred to as "triplet repeat disorders" because they are caused by the aberrant expansion of three consecutive nucleotides near or within specific disease genes. CAG and CTG repeats, for example, are found in certain forms of myotonic dystrophy, spinocerebellar ataxia, and Hunting-ton's disease. Numerous hypotheses have been proposed to explain how these DNA repeats contribute to disease pathogenesis, including toxic or inhibitory interactions of the expanded encoded RNA or protein with the normal protein.
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