Immunoglobulin variable region exons are assembled in developing B cells by V(D)J recombination. Once mature, these cells undergo class-switch recombination (CSR) when activated by antigen. CSR changes the heavy chain constant region exons (Ch) expressed with a given variable region exon from Cmu to a downstream Ch (for example, Cgamma, Cepsilon or Calpha), thereby switching expression from IgM to IgG, IgE or IgA. Both V(D)J recombination and CSR involve the introduction of DNA double-strand breaks and their repair by means of end joining. For CSR, double-strand breaks are introduced into switch regions that flank Cmu and a downstream Ch, followed by fusion of the broken switch regions. In mammalian cells, the 'classical' non-homologous end joining (C-NHEJ) pathway repairs both general DNA double-strand breaks and programmed double-strand breaks generated by V(D)J recombination. C-NHEJ, as observed during V(D)J recombination, joins ends that lack homology to form 'direct' joins, and also joins ends with several base-pair homologies to form microhomology joins. CSR joins also display direct and microhomology joins, and CSR has been suggested to use C-NHEJ. Xrcc4 and DNA ligase IV (Lig4), which cooperatively catalyse the ligation step of C-NHEJ, are the most specific C-NHEJ factors; they are absolutely required for V(D)J recombination and have no known functions other than C-NHEJ. Here we assess whether C-NHEJ is also critical for CSR by assaying CSR in Xrcc4- or Lig4-deficient mouse B cells. C-NHEJ indeed catalyses CSR joins, because C-NHEJ-deficient B cells had decreased CSR and substantial levels of IgH locus (immunoglobulin heavy chain, encoded by Igh) chromosomal breaks. However, an alternative end-joining pathway, which is markedly biased towards microhomology joins, supports CSR at unexpectedly robust levels in C-NHEJ-deficient B cells. In the absence of C-NHEJ, this alternative end-joining pathway also frequently joins Igh locus breaks to other chromosomes to generate translocations.
V(D)J recombination assembles immunoglobulin (Ig) heavy or light chain (IgH or IgL) variable region exons in developing bone marrow B cells, while class switch recombination (CSR) exchanges IgH constant region exons in peripheral B cells. Both processes employ 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/c-myc translocations of peripheral B cell lymphomas. Collaboration between these processes also has been proposed to initiate translocations. However, occurrence of V(D)J recombination in peripheral B cells is controversial. Here, we report that activated NHEJ-deficient splenic B cells accumulate V(D)J recombination-associated IgL chromosomal breaks, as well as CSR-associated IgH breaks, often in the same cell. Moreover, IgL breaks frequently are joined to IgH breaks to form translocations, a phenomenon associated with specific IgH/IgL co-localization. IgH and c-myc also co-localize in these cells; correspondingly, introduction of frequent c-myc DSBs robustly promotes IgH/c-myc translocations. Our studies reveal peripheral B cells that attempt secondary V(D)J recombination and elucidate a role for mechanistic factors in promoting recurrent translocations in tumors.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.