Illegitimate V(D)J recombination involving nonantigen receptor loci in lymphoid malignancy

V(D)J recombination of antigen receptor loci (IGH, IGK, IGL, TCRA, TCRB, TCRG, and TCRD) is an essential mechanism that confers enormous diversity to the mammalian immune system. However, there are now at least six examples of intrachromosomal interstitial deletions caused by aberrant V(D)J recombination between nonantigen receptor loci; five of out these six are associated with lymphoid malignancy. The SIL-SCL fusion and deletions of CDKN2A, IKZF1, Notch1, and Bcl11b are all associated with lymphoid malignancy. These interstitial deletions seem to be species specific, as the deletions seen in mice are not seen in humans; the converse is true as well. Nucleotide sequence analysis of these rearrangements reveals the hallmarks of V(D)J recombination, including site specificity near cryptic heptamer signal sequences, exonucleolytic "nibbling" at the junction site, and nontemplated "N"-region nucleotide insertion at the junction site. Two of these interstitial deletions (murine Notch1 and Bcl11b deletions) have been detected, at low frequency, in tissues from healthy mice with no evidence of malignancy, similar to the finding of chromosomal translocations in the peripheral blood or tonsils of healthy individuals. The contention that these are mediated via V(D)J recombination is strengthened by in vivo assays using extrachromosomal substrates, and chromatin immunoprecipitation-sequence analysis which shows Rag2 binding at the sites of rearrangement. Although the efficiency of these "illegitimate" recombination events is several orders of magnitude less than that at bona fide antigen receptor loci, the consequence of such deletions, namely activation of proto-oncogenes or deletion of tumor suppressor genes, is devastating, and a major cause for lymphoid malignancy.