De novo mutations and allelic diversity at minisatellite locus D7S22 investigated by allele-specific four-state MVR-PCR analysis

We have studied the allelic diversity and de novo mutations at the hypervar iable minisatellite locus D7S22, A four-state minisatellite variant repeat unit mapping by PCR (MVR-PCR) method was developed for this purpose, and a substitution polymorphism close to the repeat array was used to design alle le-specific flanking primers to study individual haplotypes in genomic DNA, A total of 150 alleles from different allele size groups and flanking hapl otypes were mapped. On average, MVR-codes extending 65 repeats (2.4 kb) int o the repeat array were obtained. The interspersion patterns of variant rep eats were highly polymorphic. However, subgroups of alleles close in size a nd with identical flanking haplotype revealed common MVR-code characteristi cs indicating a close evolutionary relationship. Unlike the situation in ma ny other hypervariable minisatellites, no polarized variability was reveale d at this minisatellite locus. Fifty four small families with D7S22 de novo mutations were analysed by MVR-PCR, The sites where the length change occu rred were revealed in 22 cases, while in 32 cases the mutation obviously oc curred further into the repeat array. In agreement with a non-polar distrib ution of the allelic variation, there was no evidence for a hypermutable ho t spot for mutation within the repeat array. Comparison of MVR-codes in the mutant and progenitor in gain mutations indicated that at least one, possi bly four cases, reflected inter-allelic events. Together with evidence from DNA sequencing of alleles of <2 kb, this indicates that as many as half of the gain mutations might be inter-allelic events in D7S22, Based on these results, different factors which might affect the mutation rate are discuss ed.