ATM and MSH2 control blunt DNA end joining in immunoglobulin class switch recombination

Class switch recombination (CSR) produces secondary immunoglobulin isotypes and requires AID-dependent DNA deamination of intronic switch (S) regions within the immunoglobulin heavy chain ( Igh ) gene locus. Non-canonical repair of deaminated DNA by mismatch repair (MMR) or base excision repair (BER) creates DNA breaks that permit recombination between distal S regions. ATM-dependent phosphorylation of AID at serine-38 (pS38-AID) promotes its interaction with APE1, a BER protein, suggesting that ATM regulates CSR through BER. However, pS38-AID may also function in MMR during CSR, although the mechanism remains unknown. To examine whether ATM modulates BER- and/or MMR-dependent CSR, Atm -/- mice were bred to mice deficient for the MMR gene Msh2 . Surprisingly, the predicted Mendelian frequencies of Atm -/- Msh2 -/- adult mice were not obtained. To generate ATM and MSH2-deficient B cells, Atm was conditionally deleted on an Msh2 -/- background using a floxed ATM allele [ Atm f ] and B cell-specific Cre recombinase expression ( CD23-cre ) to produce a deleted ATM allele ( Atm D ). As compared to Atm D/D and Msh2 -/- mice and B cells, Atm D/D Msh2 -/- mice and B cells display a reduced CSR phenotype. Interestingly, Sμ-Sγ1 junctions from Atm D/D Msh2 -/- B cells that were induced to switch to IgG1 in vitro showed a significant loss of blunt end joins and an increase in insertions as compared to wildtype, Atm D/D , or Msh2 -/- B cells. This data indicates that the absence of both ATM and MSH2 blocks non-homologous end joining (NHEJ), leading to inefficient CSR. We propose a model whereby ATM and MSH2 function cooperatively to regulate end-joining during CSR through pS38-AID. Summary Loss of the DNA repair genes Atm and Msh2 produces a novel synthetic lethality in mice. B cell specific deletion of Atm on an Msh2 -/- background reduces Ig CSR and inhibits NHEJ.