Founder effect in spinal and bulbar muscular atrophy (SBMA)

We analyzed the polymorphic (CAG)(n) and (GGC)(n) repeats of the androgen receptor gene in 113 unrelated X-linked spinal and bulbar muscular atrophy (SBMA) X chromosomes and 173 control X chromosomes in Japanese males. The control chromosomes had an average CAG repeat number of 21 ± 3 with a range from 14-32 repeat units, and SBMA chromosomes had a range from 40-55 with a median of 47 ± 3 copies. The control chromosomes had seven different alleles of the (GGC)(n) repeat with the range of 11 to 17; the most frequent size of (GGC)(n) was 16 (79%), while (GGC)₁₇ was very rare (1%). However, in SBMA chromosomes only two alleles were seen; the most frequent size of (GGC)(n) was 16 (61%) followed by 17 (39%). (GGC)(n) size distribution was significantly different between SBMA and control chromosomes (P < 0.0001), indicating the presence of linkage disequilibrium. There was no allelic association between the (CAG)(n) and (GGC)(n) microsatellites among control subjects as well as SBMA patients, which suggests that a founder effect makes a more significant contribution to generation of Japanese SBMA chromosomes than new mutations.