Hypervariable minisatellites in mouse DNA
Hypervariable minisatellite loci provide highly informative genetic markers in mammalian genomes. Hybridisation probes based on a G-rich core sequence simultaneously detect many minisatellite loci in human DNA, generating individual specific and highly variable DNA fingerprints with a wide range of applications. Human minisatellite probes cross-hybridise to mouse DNA, generating DNA fingerprints as complex and variable as those of man. Inbred strains of mice have strain-specific DNA fingerprints which can be analysed using recombinant inbred strains. Analysis of the BXD recombinant inbred series using human minisatellite probes 33.6 and 33.15 revealed that the cross- hybridising loci show variation in germline stability; one locus in particular, Ms6-hm, detected by probe 33.6, exhibited multiallelism across the BXD strains, and heterozygosity among contemporary C57BL/6J inbred mice. Ms6-hm alleles were cloned from C57BL/6J DNA by cross-hybridisation to probe 33.6; on propagation in E.coli the majority of minisatellite repeat units were lost. DNA sequence analysis revealed that Ms6-hm consists of a homogeneous array of the repeat unit GGGCA which has evolved by amplification from within a member of the MT (mouse transcript) family of interspersed repetitive elements. Ms6-hm is flanked by two additional, diverged, MT elements, and there is further evidence that MT elements may be associated with other unstable regions of the mouse genome. Multiallelism and heterozygosity at Ms6-hm (which maps near brown on chromosome 4) result from a high germline mutation rate to new length alleles (2.5% per gamete). Mice mosaic for cells carrying common non-parental Ms6-hm alleles in somatic tissue, and in some cases also in the germline, provide evidence for additional, early developmental, mutation events at this locus. Such somatic mutant Ms6-hm alleles provide innocuous and informative markers with which to analyse the lineage relationships of cells in early mouse development. In four mosaic mice the fraction of cells containing the non-parental allele has been shown to be indistinguishable in different adult tissues, suggesting that the mutation events preceded the allocation of the somatic lineages, and that the same pool of primitive ectoderm cells contributes equally to all somatic tissues. Under low-stringency hybridisation conditions the minisatellite repeat array of Ms6-hm cross-hybridises to other unstable minisatellite loci in the mouse genome to generate a novel and highly individual specific DNA fingerprint; at least one cross-hybridising locus is also somatically unstable during early mouse development.