Analysis of germline mutation spectra at the Huntington's disease locus supports a mitotic mutation mechanism

Trinucleotide repeat disease alleles can undergo 'dynamic' mutations in whi ch repeat number may change when a gene is transmitted from parent to offsp ring. By typing >3500 sperm, we determined the size distribution of Hunting ton's disease (HD) germline mutations produced by 26 individuals from the V enezuelan cohort with CAG/CTG repeat numbers ranging from 37 to 62, Both th e mutation frequency and mean change in allele size increased with increasi ng somatic repeat number. The mutation frequencies averaged 82% and, for in dividuals with at least 50 repeats, 98%, The extraordinarily high mutation frequency levels are most consistent with a mutation process that occurs th roughout germline mitotic divisions, rather than resulting from a single me iotic event. In several cases, the mean change in repeat number differed si gnificantly among individuals with similar somatic allele sizes. This indiv idual variation could not be attributed to age in a simple way or to 'cis' sequences, suggesting the influence of genetic background or other factors. A familial effect is suggested in one family where both the father and son gave highly unusual spectra compared with other individuals matched for ag e and repeat number. A statistical model based on incomplete processing of Okazaki fragments during DNA replication was found to provide an excellent fit to the data but variation in parameter values among individuals suggest s that the molecular mechanism might be more complex.