Boboila, CristianYan, CatherineWesemann, Duane R.Jankovic, MilaWang, Jing H.Manis, JohnNussenzweig, AndreNussenzweig, MichelAlt, Frederick W.2019-10-052010Boboila, Cristian, Catherine Yan, Duane R. Wesemann, Mila Jankovic, Jing H. Wang, John Manis, Andre Nussenzweig, Michel Nussenzweig, and Frederick W. Alt. 2010. “Alternative End-Joining Catalyzes Class Switch Recombination in the Absence of Both Ku70 and DNA Ligase 4.” The Journal of Experimental Medicine 207 (2): 417–27. https://doi.org/10.1084/jem.20092449.0022-10071540-9538http://nrs.harvard.edu/urn-3:HUL.InstRepos:41483414The classical nonhomologous end-joining (C-NHEJ) DNA double-strand break (DSB) repair pathway employs the Ku70/80 complex (Ku) for DSB recognition and the XRCC4/DNA ligase 4 (Lig4) complex for ligation. During IgH class switch recombination (CSR) in B lymphocytes, switch (S) region DSBs are joined by C-NHEJ to form junctions either with short microhomologies (MHs; "MH-mediated" joins) or no homologies ("direct" joins). In the absence of XRCC4 or Lig4, substantial CSR occurs via "alternative" end-joining (A-EJ) that generates largely MH-mediated joins. Because upstream C-NHEJ components remain in XRCC4- or Lig4-deficient B cells, residual CSR might be catalyzed by C-NHEJ using a different ligase. To address this, we have assayed for CSR in B cells deficient for Ku70, Ku80, or both Ku70 and Lig4. Ku70- or Ku80-deficient B cells have reduced, but still substantial, CSR. Strikingly, B cells deficient for both Ku plus Lig4 undergo CSR similarly to Ku-deficient B cells, firmly demonstrating that an A-EJ pathway distinct from C-NHEJ can catalyze CSR end-joining. Ku-deficient or Ku-plus Lig4-deficient B cells are also biased toward MH-mediated CSR joins; but, in contrast to XRCC4- or Lig4-deficient B cells, generate substantial numbers of direct CSR joins. Our findings suggest that more than one form of A-EJ can function in CSR.en-USJournal Article2019-10-0510.1084/jem.20092449