Specific bisulfite modification of CTG triplet repeats of the androgen receptor gene, a gene associated with the triplet repeat disease X-linked spinal and bulbar muscular atrophy (Kennedy disease)

Expansion of triplet (CAG)n repeats has been associated with triplet-repeat diseases such as X-linked spinal and bulbar muscular atrophy (SBMA). To elucidate the molecular mechanism of the down-regulated expression of the human androgen receptor gene (hAR) in SBMA patients, we speculated that a certain percentage of the CAG triplets are methylated. We employed the bisulfite method to determine methylation of CAG triplets. Although the bisulfite reaction modified the cytosines in the plus strand CAG repeats, a certain percentage of the minus strand CTG repeats were not modified both in genomic DNA from patients and in bacterial plasmids. This unexpected modification was not seen in unexpanded triplets. Thus, we conclude that a peculiar secondary structure around the expanded CAG or CTG triplets protects itself against bisulfite chemical modification. This specific DNA structural property may account for the pathogenesis of CAG triplet repeat diseases.