ATR kinase activity promotes antibody class switch recombination in B cells through cell cycle regulation without suppressing DSB resection and microhomology usage.

Class switch recombination (CSR) changes the effector functions of antibodies and is carried out by classical and alternative nonhomologous end joining (c-NHEJ and A-EJ) of repetitive switch (S) region double-strand breaks (DSBs). The master DNA damage response (DDR) kinase ataxia-telangiectasia mutated (ATM) is critical for CSR in part by suppressing S region DSB resection. However, whether another related DDR kinase ATM- and Rad3-related (ATR) plays similar role in CSR remains elusive. In this study, we investigated the requirement for ATR kinase activity on CSR in both c-NHEJ competent and deficient B cell lines with high-throughput sequencing of S-S junctions. We found that ATR kinase inhibition efficiently blocked both c-NHEJ- and A-EJ-mediated CSR without affecting germline transcription and activation-induced cytosine deaminase expression. In contrast to ATM, ATR does not suppress S region DSB resection and microhomology usage. In addition, ATR kinase inhibition did not affect Cas9-generated DSB end joining by either c-NHEJ and A-EJ. ATR kinase-inhibited stimulated B cells proliferate much slower than controls and exhibited altered cell cycle profile with increased G1 and G2/M phase cells. In summary, our data revealed a role for ATR in promoting both c-NHEJ- and A-EJ-mediated CSR through regulating cell proliferation upon damage without negatively influencing DSB end-joining features.