VHDJH formation and DJH replacement during pre‐B differentiation: non‐random usage of gene segments.

The Abelson murine leukemia virus (A‐MuLV) transformed cell line 300‐19 was derived from the bone marrow of an adult NIH/Swiss outbred mouse. The original 300‐19 clonal isolate carried DHH rearrangements of both JH alleles, a molecular genotype characteristic of early pre‐B cells. During propagation in culture, the 300‐19 line frequently generates secondary rearrangements of its JH alleles including rearrangements which append VH segments to the pre‐existing DJH complexes to form complete VHDJH variable region genes and secondary D to JH rearrangements which replace the pre‐existing DJH rearrangement by joining an upstream D to a downstream JH. The two types of secondary rearrangement events occur at approximately equal frequency. Approximately 30% of the VH to DJH joins lead to the production of mu heavy chains providing support for a regulated model of allelic exclusion. Like pre‐B cell lines from other origins, the 300‐19 line preferentially utilized VH gene segments from the more JH‐proximal (3′) families to form VHDJH rearrangements. However, the VH segments preferentially employed by 300‐19 were from a different family than those previously demonstrated to be utilized by pre‐B lines of BALB/c origin; we relate these different utilization patterns to differences in the organization of the more 3′ VH families between the two strains. The initial DJH rearrangements of the 300‐19 line employed more 3′ (JH‐proximal) D segments; however, the DJH replacements preferentially employed the most 5′ D segment. We discuss this phenomenon in the context of a mechanism which may target recombinase to regions of the chromosome more 5′ to the D locus (VH‐containing regions) once an initial DJH complex is formed.