HNRNPU facilitates antibody class-switch recombination through C-NHEJ promotion and R-loop suppression
Ahmed M. Refaat,
Mikiyo Nakata,
Afzal Husain,
Hidetaka Kosako,
Tasuku Honjo,
Nasim A. Begum
Affiliations
Ahmed M. Refaat
Department of Immunology and Genomic Medicine, Center for Cancer Immunotherapy and Immunobiology, Kyoto University Graduate School of Medicine, Kyoto 606-8501, Japan; Zoology Department, Faculty of Science, Minia University, El-Minia 61519, Egypt
Mikiyo Nakata
Department of Immunology and Genomic Medicine, Center for Cancer Immunotherapy and Immunobiology, Kyoto University Graduate School of Medicine, Kyoto 606-8501, Japan
Afzal Husain
Department of Biochemistry, Faculty of Life Sciences, Aligarh Muslim University, Aligarh, Uttar Pradesh 202002, India
Hidetaka Kosako
Division of Cell Signaling, Institute of Advanced Medical Sciences, University of Tokushima, Tokushima 770-8503, Japan
Tasuku Honjo
Department of Immunology and Genomic Medicine, Center for Cancer Immunotherapy and Immunobiology, Kyoto University Graduate School of Medicine, Kyoto 606-8501, Japan; Corresponding author
Nasim A. Begum
Department of Immunology and Genomic Medicine, Center for Cancer Immunotherapy and Immunobiology, Kyoto University Graduate School of Medicine, Kyoto 606-8501, Japan
Summary: B cells generate functionally different classes of antibodies through class-switch recombination (CSR), which requires classical non-homologous end joining (C-NHEJ) to join the DNA breaks at the donor and acceptor switch (S) regions. We show that the RNA-binding protein HNRNPU promotes C-NHEJ-mediated S-S joining through the 53BP1-shieldin DNA-repair complex. Notably, HNRNPU binds to the S region RNA/DNA G-quadruplexes, contributing to regulating R-loop and single-stranded DNA (ssDNA) accumulation. HNRNPU is an intrinsically disordered protein that interacts with both C-NHEJ and R-loop complexes in an RNA-dependent manner. Strikingly, recruitment of HNRNPU and the C-NHEJ factors is highly sensitive to liquid-liquid phase separation inhibitors, suggestive of DNA-repair condensate formation. We propose that HNRNPU facilitates CSR by forming and stabilizing the C-NHEJ ribonucleoprotein complex and preventing excessive R-loop accumulation, which otherwise would cause persistent DNA breaks and aberrant DNA repair, leading to genomic instability.
