Trinucleotide repeats associated with human disease

Triplet repeat expansion diseases (TREDs) are characterized by the coincidence of disease manifestation with amplification of d(CAG. CTG), d(CGG.CCG) or d(GAA.TTC) repeats contained within specific genes. Amplification of triplet repeats continues in offspring of affected individuals, which generally results in progressive severity of the disease and/or an earlier age of onset, phenomena clinically referred to as 'anticipation'. Recent biophysical and biochemical studies reveal that five of the six d(CGG)n, d(CCG)n, (CAG)n, d(CTG)n and d(GAA)n complementary sequences that are associated with human disease form stable hairpin structures. Although the triplet repeat sequences d(GAC)n and d(GTC)n also form hairpins, repeats of the double-stranded forms of these sequences are conspicuously absent from DNA sequence databases and are not anticipated to be associated with human disease. With the exception of d(GAG)n and d(GTG)n, the remaining triplet repeat sequences are unlikely to form hairpin structures at physiological salt and temperature. The details of hairpin structures containing trinucleotide repeats are summarized and discussed with respect to potential mechanisms of triplet repeat expansion and d(CGG.CCG) n methylation/demethylation.