ROLE OF HOMOLOGY-DIRECTED RECOMBINATION - PREDOMINANTLY PRODUCTIVE REARRANGEMENTS OF V(H)81X IN NEWBORNS BUT NOT IN ADULTS

In the neonate, Ig V-D-J junctions often occur at regions of short sequence homology, resulting in one to two predominant junctional sequences for most-V-D and D-J recombinations. We have proposed that this mechanism of homology-directed recombination may play a role in the non-random usage of V(H) genes observed in fetal and neonatal life, since use of the short homologies at V-D junctions would preferentially make productive rearrangements for the overutilized 7183 and Q52 V(H) genes, and would make predominantly non-productive rearrangements for the underutilized V(H)J558 gene family. Here we test this hypothesis for the 81X gene from the V(H)7183 family. Since pre-B cells which have rearranged the 81X gene do not appear to undergo the normal clonal proliferation before light chain rearrangement, analysis of the percentage of productive versus non-productive rearrangements for this V(H) gene is not skewed by the expansion of pre-B cells with productively rearranged IgH alleles. If V-D-J rearrangements were random, one would predict that only one-third of the rearrangements would be in-frame. This is close to what we observed for the 81X gene in adult bone marrow. In contrast, we show that 62% of all 81X rearrangements in fetal/newborn pre-B cells were productive. Forty-one percent of all the neonatal pre-B sequences containing DFL16 or DSP2 used homology-directed recombination to create the predominantly observed V-D junctional sequences, and 93% of those sequences were productive. This is consistent with our hypothesis that the mechanism of homology-directed recombination would result in an increased proportion of productive 81X rearrangements in the newborn. Therefore, we suggest that in fetal and neonatal life, when N regions are lacking, V(H)7183 and V(H)Q52 genes are more likely to undergo productive rearrangements than other V(H) families and thus are much more likely to contribute to the early B cell repertoire.