Polymerase theta repairs persistent G1-induced DNA breaks in S-phase during class switch recombination

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Non-homologous end joining (NHEJ) is the main repair pathway for DNA double-strand breaks (DSBs) in G1 phase during immunoglobulin heavy chain (Igh) class switch recombination (CSR) in B lymphocytes.
• Alternative end-joining pathways are utilized when NHEJ or DSB end protection (SHLD1) is compromised.
• Polymerase theta (Pol θ) is implicated in repairing replication-associated DSBs during mitosis.

Purpose of the Study:

• To investigate the role of Pol θ in alternative DNA repair pathways during CSR in B cells lacking NHEJ components.
• To characterize the mechanisms and outcomes of Pol θ-dependent and Pol θ-independent repair pathways in the absence of NHEJ.

Main Methods:

• Primary B cells deficient in XRCC4 (NHEJ), SHLD1, and/or Pol θ were analyzed for CSR.
• Assessment of DNA repair pathway usage, end resection, inversion, and microhomology during CSR.
• Investigation of Pol θ-mediated repair timing and dependence on RHINO and PLK1.

Main Results:

• Two distinct repair pathways were identified: a Pol θ-independent productive pathway and a Pol θ-dependent unproductive pathway.
• The unproductive pathway is characterized by end resection, inversion, and microhomology.
• Pol θ-mediated repair in NHEJ-deficient cells occurs during the G1-to-S phase transition, independent of RHINO and PLK1.

Conclusions:

• In the absence of NHEJ, Pol θ repairs persistent G1-phase DSBs during S-phase, not mitosis.
• This highlights a distinct role for Pol θ in managing DNA breaks during the cell cycle in B lymphocytes.