SEQUENCE HOMOLOGIES, N-SEQUENCE INSERTION AND JH GENE UTILIZATION IN VHDJH JOINING - IMPLICATIONS FOR THE JOINING MECHANISM AND THE ONTOGENIC TIMING OF LY1-B CELL AND B-CLL PROGENITOR GENERATION

Sequence analysis of rearranged V(H)DJ(H) genes of B lineage cells from various stages of ontogeny indicates that short sequence homologies at the breakpoints of recombination contribute to V region gene assembly. Such homologies are regularly seen at DJ(H) junctions of neonatal pre-B cells, most of which do not contain N sequences. In the same cells, but not at later developmental stages, preferential usage of the J(H)1 element is observed. After birth, N sequence insertion increases with time and is always more prominent at the V(H)D border than the DJ(H) border. In pre-B cells from adult animals and in mature B cells, in cases where N sequences were not detectable, sequence homologies at the DJ(H) border were found in only half of the instances. This lower incidence could be due to N sequence addition to one of the recombining DNA ends and/or cellular selection. Inspection of V(H)DJ(H) junctions for N sequence insertion, sequence homologies at the DJ(H) border and J(H)1 usage allows the estimation of the timepoint in ontogeny at which particular B cell subsets are seeded into the immune system. Specifically, the present data show that the cells of the Ly1 B cell subset are generated not only neonatally but also beyond the first weeks of life. However, the DJ(H) junctions of the progenitors of chronic B cell leukemias which originate from the same B cell subset resemble those of neonatal pre-B cells, suggesting that these cells have already undergone a transforming event at this early developmental stage.