RAG1/2 destabilizes the lymphocyte genome by mobilizing DNA

The RAG1/2 recombinase plays an essential role in adaptive immunity by inducing physiologic DNA double-strand breaks (DSBs) at immunoglobulin (Ig) and T cell receptor genes during V(D)J recombination in developing lymphocytes. In contrast, aberrant RAG1/2 activity promotes lymphocyte malignancies by causing translocations and DNA deletions in cancer genes, and the same enzyme has also been postulated to mediate DNA insertions by transposition. However, whether RAG1/2 alters the B cell genome by inducing insertions during V(D)J recombination has not been determined. To explore this possibility, we examined RAG1/2-mediated chromosomal rearrangements in primary murine B cells using next generation sequencing methods. In addition to RAG1/2-dependent DNA damage at non-Ig loci, we find that RAG1/2 induces aberrant DNA insertions by releasing cleaved Ig fragments that are then re-integrated into independently generated DSBs on heterologous chromosomes. Similar insertions were also found in human cancers indicating that this process can destabilize the genome.