IgH class switching and translocations use a robust non-classical end-joining pathway

Area of Science:

• Immunology
• Molecular Biology
• DNA Repair Mechanisms

Background:

• B cells mature through V(D)J recombination, assembling immunoglobulin variable regions.
• Upon antigen activation, mature B cells undergo class-switch recombination (CSR) to change antibody isotype (e.g., IgM to IgG).
• Both V(D)J recombination and CSR involve DNA double-strand breaks repaired by end-joining pathways.

Purpose of the Study:

• To investigate the role of classical non-homologous end joining (C-NHEJ) in class-switch recombination (CSR).
• To determine if C-NHEJ factors Xrcc4 and DNA ligase IV (Lig4) are critical for CSR.
• To identify alternative end-joining pathways involved in CSR.

Main Methods:

• Assayed CSR in mouse B cells deficient in Xrcc4 or Lig4.
• Analyzed DNA double-strand break repair and joining events during CSR.
• Investigated immunoglobulin heavy chain (Igh) locus integrity and chromosomal translocations.

Main Results:

• C-NHEJ is confirmed to catalyze CSR joins, as C-NHEJ-deficient B cells showed reduced CSR and increased Igh locus breaks.
• An alternative end-joining pathway, biased towards microhomology joins, supports CSR at significant levels in C-NHEJ-deficient cells.
• In the absence of C-NHEJ, this alternative pathway frequently causes translocations by joining Igh breaks to other chromosomes.

Conclusions:

• Classical non-homologous end joining (C-NHEJ) plays a critical role in class-switch recombination (CSR) by repairing DNA breaks.
• An alternative microhomology-biased end-joining pathway compensates for C-NHEJ deficiency, maintaining CSR but increasing translocation risk.
• Understanding these DNA repair pathways is crucial for B cell development and preventing genomic instability.