Allele diversity and germline mutation at the insulin minisatellite

Previous analysis of germline mutation at highly unstable GC-rich minisatellites with continuous allele size distributions revealed similar meiotic recombinational mechanisms operating at all loci investigated. The insulin minisatellite has been studied intensively due to its associations with diabetes, polycystic ovary syndrome, obesity and birth size. Its bimodal allele size distribution in Caucasians suggests a much lower mutation rate and possible differences in the mutation process compared with highly unstable minisatellites. Mutation at the insulin minisatellite therefore was studied both indirectly from allele diversity surveys and directly by recovering de novo mutants from sperm DNA. Structural analysis of variant repeat distributions in 876 alleles identified 189 different alleles, almost all of which could be assigned to one of three very distinct lineages. Variation within a lineage was minor and due mainly to the gain or loss of one or a few repeat units. These events most probably arise by mitotic replication slippage at a frequency of perhaps 10(-3)per gamete. Sperm DNA analysis revealed a second class of mutation occurring at a frequency of approximately 2 x 10(-5)that involved highly complex intra- and inter-allelic rearrangements very similar to those seen at unstable minisatellites. These complex rearrangements were not seen in somatic DNA and are probably meiotic in origin. Minisatellite homozygosity did not reduce the frequency of these mutants in sperm. The insulin minisatellite therefore appears to evolve by two distinct processes: one involving slippage-like events and the second resulting in complex recombinational turnover of allele structure.