REARRANGEMENT OF UPSTREAM D-H AND V-H GENES TO A REARRANGED IMMUNOGLOBULIN VARIABLE REGION GENE INSERTED INTO THE DQ52-J(H) REGION OF THE IMMUNOGLOBULIN HEAVY-CHAIN LOCUS

We investigated gene rearrangements in the mutant IgH locus of a mouse strain generated by insertion of a rearranged heavy chain variable region gene (V-T15) into the DQ52-J(H) region through gene targeting. In more than half of the B cells of heterozygous mutant mice, the mutant IgH locus was silenced by the rearrangement of an endogenous D-H or D-H and V-H gene to the inserted V-T15 gene. In these cases, a functional V(H)D(H)J(H) gene was present on the wild-type allele. The silencing rearrangement appeared to be mediated by recombination signal sequence (RSS)-like elements present in the ''recipient'' V-T15 gene. Among the many such elements on the inserted V-T15 gene, which apparently met the requirement for an RSS with respect to nucleotide sequence, only two were observed in the actual rearrangements. This indicates that targeting of the recombination machinery involves sequences in addition to the RSS motifs as they have been characterized so far. In homozygous mutant mice, most B cells appeared to carry the intact V-T15 gene on both mutant IgH alleles, although single-cell polymerase chain reaction revealed that silencing rearrangements occured frequently in B cell progenitors in the bone marrow. This observation indicates that once silencing rearrangements are initiated in a cell, they involve both V-T15 genes in most cases, reminiscent of normal D-H-J(H), rearrangement. B cells which did not initiate such rearrangements develop to populate the peripheral B cell compartment.