Immunoglobulin class switch recombination is impaired in Atm-deficient mice

Immunoglobulin class switch recombination (Ig CSR) involves DNA double strand breaks (DSBs) at recombining switch regions and repair of these breaks by nonhomologous end-joining. Because the protein kinase ataxia telengiectasia (AT) mutated (ATM) plays a critical role in DSB repair and AT patients show abnormalities of Ig isotype expression, we assessed the role of ATM in CSR by examining ATM-deficient mice. In response to T cell-dependent antigen (Ag), Atm^-/- mice secreted substantially less Ag-specific IgA, IgG1, IgG2b, and IgG3, and less total IgE than Atm^+/+ controls. To determine whether Atm^-/- B cells have an intrinsic defect in their ability to undergo CSR, we analyzed in vitro responses of purified B cells. Atm ^-/- cells secreted substantially less IgA, IgG1, IgG2a, IgG3, and IgE than wild-type (WT) controls in response to stimulation with lipopolysaccharide, CD40 ligand, or anti-IgD plus appropriate cytokines. Molecular analysis of in vitro responses indicated that WT and Atm^-/- B cells produced equivalent amounts of germline IgG1 and IgE transcripts, whereas Atm^-/- B cells produced markedly reduced productive IgGl and IgE transcripts. The reduction in isotype switching by Atm^-/- B cells occurs at the level of genomic DNA recombination as measured by digestion-circularization PCR. Analysis of sequences at CSR sites indicated that there is greater microhomology at the μ-γ1 switch junctions in ATM B cells than in wild-type B cells, suggesting that ATM function affects the need or preference for sequence homology in the CSR process. These findings suggest a role of ATM in DNA DSB recognition and/or repair during CSR.