CtIP-mediated DNA resection is dispensable for IgH class switch recombination by alternative end-joining

Proc Natl Acad Sci U S A. 2020 Oct 13;117(41):25700-25711. doi: 10.1073/pnas.2010972117. Epub 2020 Sep 28.


To generate antibodies with different effector functions, B cells undergo Immunoglobulin Heavy Chain (IgH) class switch recombination (CSR). The ligation step of CSR is usually mediated by the classical nonhomologous end-joining (cNHEJ) pathway. In cNHEJ-deficient cells, a remarkable ∼25% of CSR can be achieved by the alternative end-joining (Alt-EJ) pathway that preferentially uses microhomology (MH) at the junctions. While A-EJ-mediated repair of endonuclease-generated breaks requires DNA end resection, we show that CtIP-mediated DNA end resection is dispensable for A-EJ-mediated CSR using cNHEJ-deficient B cells. High-throughput sequencing analyses revealed that loss of ATM/ATR phosphorylation of CtIP at T855 or ATM kinase inhibition suppresses resection without altering the MH pattern of the A-EJ-mediated switch junctions. Moreover, we found that ATM kinase promotes Alt-EJ-mediated CSR by suppressing interchromosomal translocations independent of end resection. Finally, temporal analyses reveal that MHs are enriched in early internal deletions even in cNHEJ-proficient B cells. Thus, we propose that repetitive IgH switch regions represent favored substrates for MH-mediated end-joining contributing to the robustness and resection independence of A-EJ-mediated CSR.

Keywords: CtIP; alternative end-joining; class switch recombination.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Motifs
  • Animals
  • Ataxia Telangiectasia Mutated Proteins / genetics
  • Ataxia Telangiectasia Mutated Proteins / metabolism
  • B-Lymphocytes / metabolism
  • Carrier Proteins / chemistry
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism*
  • Cell Cycle Proteins / chemistry
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism*
  • DNA End-Joining Repair*
  • Immunoglobulin Class Switching*
  • Immunoglobulin Heavy Chains / genetics*
  • Immunoglobulin Heavy Chains / metabolism
  • Mice
  • Phosphorylation
  • Recombination, Genetic


  • Carrier Proteins
  • Cell Cycle Proteins
  • CtIP protein, mouse
  • Immunoglobulin Heavy Chains
  • Ataxia Telangiectasia Mutated Proteins