Trans-chromosomal recombination within the Ig heavy chain switch region in B lymphocytes

Somatic DNA rearrangements in B lymphocytes, including V(D)J gene rearrangements and isotype switching, generally occur in cis, i. e., intrachromosomally. We showed previously, however, that 3 to 7% of IgA heavy chains have the V(H) and Cα regions encoded in trans. To determine whether the trans-association of V(H) and Cα occurred by trans-chromosomal recombination, by trans-splicing, or by trans-chromosomal gene conversion, we generated and analyzed eight IgA-secreting rabbit hybridomas with trans-associated V(H) and Cα heavy chains. By ELISA and by nucleotide sequence analysis we found that the V(H) and Cα regions were encoded by genes that were in trans in the germline. We cloned the rearranged VDJ-Cα gene from a fosmid library of one hybridoma and found that the expressed V(H) and Cα genes were juxtaposed. Moreover, the juxtaposed V(H) and Cα genes originated from different IgH alleles. From the same hybridoma, we also identified a fosmid clone with the other expected product of a trans-chromosomal recombination. The recombination breakpoint occurred within the Sμ/Sα region, indicating that the trans-association of V(H) and Cα genes occurred by trans-chromosomal recombination during isotype switching. We conclude that trans-chromosomal recombination occurs at an unexpectedly high frequency (7%) within the IgH locus of B lymphocytes in normal animals, which may explain the high incidence of B-cell tumors that arise from oncogene translocation into the IgH locus.