IgH class switching and translocations use a robust non-classical end-joining pathway.
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.
Yan
Catherine T
CT
Howard Hughes Medical Institute, of Genetics, Boston, Massachusetts 02115, USA.
Boboila
Cristian
C
Souza
Ellen Kris
EK
Franco
Sonia
S
Hickernell
Thomas R
TR
Murphy
Michael
M
Gumaste
Sunil
S
Geyer
Mark
M
Zarrin
Ali A
AA
Manis
John P
JP
Rajewsky
Klaus
K
Alt
Frederick W
FW
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*
D004268:DNA-Binding Proteins; D007074:Immunoglobulin G; C511788:XRCC4 protein, mouse