Evolution and population genetics of the H-ras minisatellite and cancer predisposition

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Abstract
Case–control studies have implicated rare length H-ras minisatellite alleles in cancer risk. In Europeans, this locus has four common alleles, and a larger number of rare alleles; possession of a rare allele has been identified as a risk factor responsible for 5–10% of some cancers. This unusual model of predisposition has been controversial in case–control studies, but also makes characteristic predictions about the population genetics of the locus, which we examine in this study. Using minisatellite variant repeat (‘MVR’) mapping, and compound haplotypes composed of the minisatellite and surrounding substitutional polymorphisms, we have reconstructed the main steps in the evolution of this locus in human populations. MVR-calibrated measurements of allele length yield rare allele frequencies significantly higher than most previous studies, and show that most other analyses have not distinguished two common alleles of 84 and 85 repeat units. Alleles classified as ‘rare’ in European populations predominate (70%) in the African sample studied. Small-pool PCR (SP–PCR) analysis on common alleles in sperm DNA gives an estimate for the germline minisatellite mutation rate of about 0.05% (95% confidence upper limit 0.15%). Overall, our results do not reflect a locus subject to frequent mutation and strong selection, and are difficult to reconcile with the proposed cancer predisposition. Restricted variation at this locus is most simply explained by low mutation rate, and although definitive case–control studies can only be performed using methods capable of reproducibly distinguishing rare and common alleles, our work suggests that most studies to date have not resolved alleles adequately.

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