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 hypervariable 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 allele-specific flanking primers to study individual haplotypes in genomic DNA. A total of 150 alleles from different allele size groups and flanking haplotypes were mapped. On average, MVR-codes extending 65 repeats (2.4 kb) into the repeat array were obtained. The interspersion patterns of variant repeats were highly polymorphic. However, subgroups of alleles close in size and with identical flanking haplotype revealed common MVR-code characteristics indicating a close evolutionary relationship. Unlike the situation in many other hypervariable minisatellites, no polarized variability was revealed at this minisatellite locus. Fifty four small families with D7S22 de novo mutations were analysed by MVR-PCR. The sites where the length change occurred were revealed in 22 cases, while in 32 cases the mutation obviously occurred further into the repeat array. In agreement with a non-polar distribution of the allelic variation, there was no evidence for a hypermutable hot spot for mutation within the repeat array. Comparison of MVR-codes in the mutant and progenitor in gain mutations indicated that at least one, possibly 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 discussed.