title: IgH class switching and translocations use a robust non-classical end-joining pathway.
abstract: 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.
authors: Howard Hughes Medical Institute, of Genetics, Boston, Massachusetts 02115, USA.
authors: Catherine T
authors: CT
authors: Yan
authors: Cristian
authors: C
authors: Boboila
authors: Ellen Kris
authors: EK
authors: Souza
authors: Sonia
authors: S
authors: Franco
authors: Thomas R
authors: TR
authors: Hickernell
authors: Michael
authors: M
authors: Murphy
authors: Sunil
authors: S
authors: Gumaste
authors: Mark
authors: M
authors: Geyer
authors: Ali A
authors: AA
authors: Zarrin
authors: John P
authors: JP
authors: Manis
authors: Klaus
authors: K
authors: Rajewsky
authors: Frederick W
authors: FW
authors: Alt
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000166:cytology / Q000378:metabolism / Q000528:radiation effects; D001483:Base Sequence; D049109:Cell Proliferation; D002478:Cells, Cultured; D019457:Chromosome Breakage; D004268:DNA-Binding Proteins / Q000172:deficiency / Q000235:genetics; D050438:Genes, Immunoglobulin Heavy Chain / Q000235:genetics*; D017578:Immunoglobulin Class Switching / Q000235:genetics*; D007074:Immunoglobulin G / Q000235:genetics / Q000378:metabolism; D017404:In Situ Hybridization, Fluorescence; D051379:Mice; D011839:Radiation, Ionizing; D011995:Recombination, Genetic / Q000235:genetics*; D016615:Telomere / Q000235:genetics; D014178:Translocation, Genetic / Q000235:genetics*
chemical_list: D004268:DNA-Binding Proteins; D007074:Immunoglobulin G; C511788:XRCC4 protein, mouse