Seven new MHC recombinant strains defining new H-2 haplotypes

A highly polymorphic microsatellite in the class II Eb gene allows tracing of major histocompatibility complex evolution in mouse

A hallmark of major histocompatibility complex (MHC) genes is their extraordinarily high level of polymorphism. Polymorphic residues on MHC molecules determine which peptide ligands they bind and present to effector T lymphocytes. Although the genetic mechanisms responsible for MHC polymorphism have been delineated, the timetable and the pathway of their diversification remain unclear. To trace MHC evolution, we have characterized a highly polymorphic microsatellite containing tandem repeats (TRs) of two tetranucleotide units, TGGA and GGCA, located at the 3' end of the second intron in the class II Eb gene of mouse. On the basis of length as well as sequence variations, 11 TR alleles were defined in 55 inbred mouse strains, which included MHC recombinant haplotypes and haplotypes derived from different subspecies of mouse. In this extensive sampling, a striking concordance was observed between the serologically identified class II proteins and the associated TR alleles. Examination of several strains carrying the same MHC haplotypes as well as strains carrying recombinant MHC haplotypes indicates that TR alleles are extremely stable. These observations suggest that TR polymorphism predates the separation of various subspecies of mouse. On the basis of sequence divergence, a genealogical tree has been constructed to depict evolution of the different TR alleles. Finally, evidence is presented that suggests this microsatellite polymorphism is generated by slipped-strand mispairing during DNA replication.

Multiple sites of crossing over within the Eb recombinational hotspot in the mouse

The Eb gene of the mouse major histocompatibility complex (MHC) contains a well-documented hotspot of recombination. Twelve cases of intra-Eb recombination derived from the b, d, k and s alleles of the Eb gene were sequenced to more precisely position the sites of meiotic recombination. This analysis was based on positioning recombination breakpoints between nucleotide polymorphisms found in the sequences of parental haplotypes. All twelve cases of recombination mapped within the second intron of the Eb gene. Six of these recombinants, involving the k and s haplotypes, mapped to two adjoining DNA segments of 394 and 955 base pairs (bp) in the 3' half of the intron. In an additional two cases derived by crossing over between the d and s alleles, breakpoints were positioned to adjoining segments of 28 and 433 bp, also in the 3' half of the intron. Finally, four b versus k recombinants were mapped to non-contiguous segments of DNA covering 2.9 kb and 1005 bp of the intron. An analysis of the map positions of crossover breakpoints defined in this study suggests that the second intron of the Eb gene contains a recombinational hotspot of approximately 800-1000 bp which contains at least two closely linked recombinationally active sites or segments. Further examination of the sequence data also suggests that the postulated location for the recombinational hotspot corresponds almost precisely to an 812 bp sequence that shows nucleotide sequence similarity to the MT family of middle repetitive DNA.