Somatic hypermutation of VkappaJkappa rearrangements: targeting of RGYW motifs on both DNA strands and preferential selection of mutated codons within RGYW motifs.

Productive and nonproductive VkappaJkappa gene rearrangements from individual peripheral blood B cells were analyzed for the pattern and distribution of mutations. The eight RGYW motifs and their inverse repeats, WRCY, were present in germ-line Vkappa genes significantly more often than anticipated by random chance (1.6-fold and 1.4-fold, respectively) and were also mutated in nonproductive rearrangements significantly more often than expected, with a frequency 1.96 fold greater than that of non-RGYW / WRCY motifs. As a result, 50 % of all mutations in nonproductive VkappaJkappa rearrangements occurred in RGYW / WRCY motifs. Each RGYW tetramer and its corresponding WRCY contained mutations at comparable frequencies. Furthermore, mutations of G and C were significantly more frequent in RGYW / WRCY but not in other tetranucleotides. Finally, mutations in codons contained within RGYW / WRCY were significantly more frequent in complementarity-determining regions but not framework regions of productive compared to nonproductive rearrangements and were increased by a factor that was significantly greater than for mutations in other motifs. These results indicate that the mutational machinery targets the overrepresented RGYW motifs in Vkappa genes on both DNA strands and that the resulting replacement mutations are preferentially selected into the productive repertoire.