Mutations occurring at the human minisatellite MS1 integrated in haploid yeast are similar to MS1 mutations in humans

Mechanisms of human minisatellite mutation in yeast

Minisatellites are tandem repeat loci, with repeat units ranging in size from 5 bp to 100 bp. The total lengths of repeat arrays vary from about 0.5 kb to 30 kb, and excessive variability in allele length at human minisatellite loci is the result of germline-specific complex recombination events generating new length alleles. Minisatellite alleles also mutate to new lengths in somatic cells, but this occurs at a much lower rate than in the germline. Since recombination is involved in minisatellite mutation, the yeast Saccharomyces cerevisiae is a suitable model organism that has been employed to further dissect the molecular basis of mutation events at human minisatellites. These studies have shown that the mutational behaviour of a minisatellite in meiosis is not determined by the intrinsic properties of the repeat array, but are highly dependent on the position of the minisatellite in the genome. The processes for minisatellite mutation in yeast and humans are identical in the sense that mutation is indeed driven by meiotic recombination, but differ with regard to the types of structural changes that are generated by the recombination events. Tetrad analyses showed that inter-allelic transfers of repeats occur by conversion and not crossing over, and that several chromatids can be involved in successive recombination events in one meiosis, resulting in mutant alleles in several spores. It has been demonstrated that the genes SPO11 and RAD50, involved in the initiation of recombination events, are required for human minisatellite mutation in yeast meiosis. Intrinsic properties of the repeat array appear to determine the stability of human minisatellites in yeast mitosis, since mitotic mutation rates in yeast are highly variable between minisatellites. The repair genes RAD27 and DNA2 stabilise human minisatellites in yeast mitosis, while RAD5 has no effect on mitotic stability. MSH2 depresses human minisatellite frequency in meiotic cells of yeast.

Two modes of germline instability at human minisatellite MS1 (locus D1S7): complex rearrangements and paradoxical hyperdeletion

Minisatellite MS1 (locus D1S7) is one of the most unstable minisatellites identified in humans. It is unusual in having a short repeat unit of 9 bp and in showing somatic instability in colorectal carcinomas, suggesting that mitotic replication or repair errors may contribute to repeat-DNA mutation. We have therefore used single-molecule polymerase chain reaction to characterize mutation events in sperm and somatic DNA. As with other minisatellites, high levels of instability are seen only in the germline and generate two distinct classes of structural change. The first involves large and frequently complex rearrangements that most likely arise by recombinational processes, as is seen at other minisatellites. The second pathway generates primarily, if not exclusively, single-repeat changes restricted to sequence-homogeneous regions of alleles. Their frequency is dependent on the length of uninterrupted repeats, with evidence of a hyperinstability threshold similar in length to that observed at triplet-repeat loci showing expansions driven by dynamic mutation. In contrast to triplet loci, however, the single-repeat changes at MS1 exclusively involve repeat deletion, and can be so frequent--as many as 0.7-1.3 mutation events per sperm cell for the longest homogeneous arrays--that alleles harboring these long arrays must be extremely ephemeral in human populations. The apparently impossible existence of alleles with deletion-prone uninterrupted repeats therefore presents a paradox with no obvious explanation.

Cis-regulation of inter-allelic exchanges in mutation at human minisatellite MS205 in yeast

Tandemly repeated DNA is a major component of the human genome, and includes loci contributing to human disease. Minisatellites include the most variable human loci described to date, and the mechanisms by which this variation is generated in humans have been studied in detail. Integration of human minisatellites into yeast not only provides a model for further dissecting the molecular basis of length change mutation at these loci, but also more generally allows the study of complex recombinational events in yeast. We have used human minisatellite MS205 integrated into yeast to study the structural details of length change mutations. Apart from showing that mutation at this locus in yeast has features similar to those observed at some minisatellites in humans, including meiosis-specificity, and polarity, in which exchange events are localised to one extremity of the array, we here, for the first time, directly demonstrate that a flanking element in yeast regulates the mutation process. The results therefore support the hypothesis that flanking initiators are involved in minisatellite mutation in humans. Furthermore, mutant alleles showed more complex rearrangements in one orientation than the other. The data also suggest that the mutational pathway for deletions might be different from the pathway generating inter-allelic exchanges and duplications.