COEXISTENCE OF SOMATIC HYPERMUTATION AND GENE CONVERSION IN HYPERVARIABLE REGIONS OF SINGLE IG-KAPPA CLONES

In the rabbit, recent investigations have provided evidence that gene conversion leads to the generation of diversity of heavy chain rearran ged V-H-D-H-J(H) genes. No data have been published on a similar mecha nism for rabbit light chains. In our laboratory, we initially infected rabbits with Trypanosoma brucei, which stimulates B-cell hyperplasia and hypergammaglobulinaemia. The heterozygous rabbits exhibited the C kappa 1 b4 and b9 kappa light chain allotypes. After reverse transcrip tion of mRNA, and cloning and sequencing of cDNA, the V kappa-J kappa- C kappa genes provided evidence for both somatic hypermutation and gen e conversion. We saw that in each of the b4 and b9 kappa light chain c DNA, CDR1 and CDR3 carried both point mutation and provisional gene co nversion traits. In the CDR2 region, point mutation and gene conversio n inserts were observed in the b4 genes, with only gene conversion in two b9 genes. In the CDR regions, although some genes exhibited only s omatic hypermutation or gene conversion, others showed linkage of both somatic hypermutation and gene conversion in the same sequence. This also marks the first time that somatic hypermutation and gene conversi on in the same cloned CDR region has been observed in V kappa 1 genes; however, it has been seen earlier in rabbit heavy chain VH sequences. Furthermore, the addition of several codons to the CDR3 segment by ge ne conversion may have provided a mechanism for length variation. In a ddition, we demonstrated that J kappa and framework region segments co ntained examples of somatic hypermutation. Confirmation of gene conver sion necessitates that donor sequences be identified as providing the templated inserts. Thus after cloning two pseudogenes we found putativ e CDR3 donor segments for two CDR3 rearranged genes. The results offer additional mechanisms for the generation of diversity among rearrange d rabbit kappa light chain genes. Whether there is a relationship or i nfluence of gene conversion upon somatic hypermutation or vice versa i s not discernable at present.