title: The ATPase activity of MLH1 is required to orchestrate DNA double-strand breaks and end processing during class switch recombination.
abstract: Antibody diversification through somatic hypermutation (SHM) and class switch recombination (CSR) are similarly initiated in B cells with the generation of U:G mismatches by activation-induced cytidine deaminase but differ in their subsequent mutagenic consequences. Although SHM relies on the generation of nondeleterious point mutations, CSR depends on the production of DNA double-strand breaks (DSBs) and their adequate recombination through nonhomologous end joining (NHEJ). MLH1, an ATPase member of the mismatch repair (MMR) machinery, is emerging as a likely regulator of whether a U:G mismatch progresses toward mutation or DSB formation. We conducted experiments on cancer modeled ATPase-deficient MLH1G67R knockin mice to determine the function that the ATPase domain of MLH1 mediates in SHM and CSR. Mlh1(GR/GR) mice displayed a significant decrease in CSR, mainly attributed to a reduction in the generation of DSBs and diminished accumulation of 53BP1 at the immunoglobulin switch regions. However, SHM was normal in these mice, which distinguishes MLH1 from upstream members of the MMR pathway and suggests a very specific role of its ATPase-dependent functions during CSR. In addition, we show that the residual switching events still taking place in Mlh1(GR/GR) mice display unique features, suggesting a role for the ATPase activity of MLH1 beyond the activation of the endonuclease functions of its MMR partner PMS2. A preference for switch junctions with longer microhomologies in Mlh1(GR/GR) mice suggests that through its ATPase activity, MLH1 also has an impact in DNA end processing, favoring canonical NHEJ downstream of the DSB. Collectively, our study shows that the ATPase domain of MLH1 is important to transmit the CSR signaling cascade both upstream and downstream of the generation of DSBs.
authors.affiliation: Department of Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
authors.forename: Richard
authors.initials: R
authors.lastname: Chahwan
authors.forename: Johanna M M
authors.initials: JM
authors.lastname: van Oers
authors.forename: Elena
authors.initials: E
authors.lastname: Avdievich
authors.forename: Chunfang
authors.initials: C
authors.lastname: Zhao
authors.forename: Winfried
authors.initials: W
authors.lastname: Edelmann
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Scharff
authors.forename: Sergio
authors.initials: S
authors.lastname: Roa
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000502:physiology*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology; D053903:DNA Breaks, Double-Stranded; D059766:DNA End-Joining Repair; D004260:DNA Repair; D017578:Immunoglobulin Class Switching / Q000235:genetics*; D051379:Mice; D000070957:MutL Protein Homolog 1; D009687:Nuclear Proteins / Q000502:physiology*; D011995:Recombination, Genetic; D027041:Somatic Hypermutation, Immunoglobulin
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C501485:Mlh1 protein, mouse; D009687:Nuclear Proteins; D000251:Adenosine Triphosphatases; D000070957:MutL Protein Homolog 1