Mutation at a hypervariable mouse minisatellite.

'Minisatellites' are a class of tandemly repeated sequences ubiquitous to eukaryotic genomes. A subset of minisatellites have been found to be highly variable in tandem repeat copy number. This variability makes such loci highly informative markers for genetic mapping, establishing family relationships in humans and other animals, and individual identification from forensic samples. One highly unstable mouse minisatellite locus, Ms6- hm, has been previously identified in mouse DNA fingerprints by crosshybridization with the human minisatellite probe 33.6. Ms6-hm showed a high germline mutation rate to new length alleles (2.5% per gamete) causing multi-allelism and heterozygosity even within inbred strains. Mice mosaic for cells carrying a non-parental allele in somatic tissue are also seen (2.8% of mice). At reduced stringency Ms6-hm detects other loci, one of which, Hm-2, also appeared to be highly unstable This work describes the characterization of this locus. Hm-2 has been cloned, localized to chromosome 9 using recombinant inbred strains, and sequenced. The repeat sequence, (GGCA)n, is short and the largest Hm-2 alleles can have over 5000 tandem repeats. Like Ms6-hm, Hm-2 shows a high germline mutation rate (3.6% per gamete). The incidence of mosaicism at Hm-2 (20.4% of adult mice) is much greater than at Ms6-hm, however, making Hm-2 an ideal locus for further study of somatic mutation events. Analysis of Hm-2 in embryonic and extra-embryonic tissues has shown that many mutation events occur early in development to produce in some cases divergence in Hm-2 genotype between the embryo and trophoblast. In others, mosaicism was shared between the embryo and trophoblast suggesting that these mutations arose very early in development before the fifth cell division following fertilization. The degree of mosaicism in 60 mosaics has been calculated and comparison of this data with computer simulations suggests that the somatic mutation rate is not constant throughout development but rather that mutations are biased towards the first two cell divisions after fertilization.