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 [ Atmf ] and B cell-specific Cre recombinase expression ( CD23-cre ) to produce a deleted ATM allele ( AtmD ). As compared to AtmD/D and Msh2-/- mice and B cells, AtmD/DMsh2-/- mice and B cells display a reduced CSR phenotype. Interestingly, Sμ-Sγ1 junctions from AtmD/DMsh2-/- 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, AtmD/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. ### Competing Interest Statement The authors have declared no competing interest. * AID : activation induced cytidine deaminase A-EJ : alternative end joining APE1 : apurinic/apyrimadinic endonuclease 1 ATM : ataxia telangiectasia mutated BER : base excision repair pathway CSR : Class switch recombination DNA-PKcs : DNA-dependent protein kinase catalytic subunit DSB : double stranded DNA break E13.5 : mouse embryonic day 13.5 MMR : mismatch repair pathway pS38-AID : phosphorylation of serine 38 on AID MSH2 : mut S homolog 2 MLH1 : mutL homology 1 NHEJ : Nonhomologous end joining P0 : newborn mouse pup P28 : Post natal day 28 PMS2 : PMS1 homolog 2 SSA : single strand annealing SSB : single stranded DNA break UNG : uracil DNA glycosylase 1 XRCC4 : X-ray repair cross complementing