Somatic mutation processes at a human minisatellite

Germline instability at human minisatellites frequently involves complexinter-allelic transfers of repeat units usually restricted to one end ofthe repeat array and apparently regulated by flanking DNA. In contrast,nothing is known about the structural basis of somatic instability atminisatellites. An electrophoretic size-enrichment strategy was thereforedeveloped at minisatellite MS32 (D1S8) to enable rare abnormal-lengthmutants to be detected, validated and quantitated in blood DNA by singlemolecule PCR. Structural analysis of rare mutant alleles in blood revealedsimple deletions/duplications of repeat unit blocks located at random alongthe tandem repeat array, a mode of mutation completely different from thatseen in sperm. Furthermore, allele-specific suppression of sperminstability at MS32 did not affect somatic instability. These data suggestthat conversion-based minisatellite mutation in sperm is completelygermline-specific and most likely meiotic in origin. Somatic instabilityappears to occur by a separate pathway involving replication slippage or,more likely, intra-allelic unequal crossing over.