Mechanisms promoting translocations in editing and switching peripheral B cells

Variable, diversity and joining gene segment (V(D) J) recombination assembles immunoglobulin heavy or light chain (IgH or IgL) variable region exons in developing bone marrow B cells, whereas class switch recombination (CSR) exchanges IgH constant region exons in peripheral B cells. Both processes use directed DNA double-strand breaks (DSBs) repaired by non-homologous end-joining (NHEJ). Errors in either V(D) J recombination or CSR can initiate chromosomal translocations, including oncogenic IgH locus (Igh) to c-myc (also known as Myc) translocations of peripheral B cell lymphomas. Collaboration between these processes has also been proposed to initiate translocations. However, the occurrence of V(D) J recombination in peripheral B cells is controversial. Here we show that activated NHEJ-deficient splenic B cells accumulate V(D) J-recombination-associated breaks at the lambda IgL locus (Igl), as well as CSR-associated Igh breaks, often in the same cell. Moreover, Igl and Igh breaks are frequently joined to form translocations, a phenomenon associated with specific Igh-Igl co-localization. Igh and c-myc also co-localize in these cells; correspondingly, the introduction of frequent c-myc DSBs robustly promotes Igh-c-myc translocations. Our studies show peripheral B cells that attempt secondary V(D) J recombination, and determine a role for mechanistic factors in promoting recurrent translocations in tumours.