Interdependent and separable functions of C. elegans MRN-C complex members couple formation and repair of meiotic DSBs

Abstract

Faithful inheritance of genetic information through sexual reproduction relies on the formation of crossovers between homologous chromosomes during meiosis, which in turn relies on the formation and repair of numerous double-strand DNA breaks (DSBs). As DSBs pose a potential threat to the genome, mechanisms that ensure timely and error-free DSB repair are crucial for successful meiosis. Here we identify NBS-1, the Caenorhabditis elegans ortholog of the NBS1 subunit of the conserved MRE11-RAD50-NBS1/Xrs2 (MRN) complex, as a key mediator of DSB repair via homologous recombination (HR) during meiosis. Loss of nbs-1 leads to: severely reduced loading of recombinase RAD-51, ssDNA binding protein RPA and pro-crossover factor COSA-1 during meiotic prophase progression; aggregated and fragmented chromosomes at the end of meiotic prophase; and 100% progeny lethality. These phenotypes reflect a role for NBS-1 in processing of meiotic DSBs for HR that is shared with its interacting partners MRE-11-RAD-50 and COM-1 (ortholog of Com1/Sae2/CtIP). Unexpectedly, in contrast to MRE-11 and RAD-50, NBS-1 is not required for meiotic DSB formation. Meiotic defects of the nbs-1 mutant are partially suppressed by abrogation of the non-homologous end-joining (NHEJ) pathway, indicating a role for NBS-1 in antagonizing NHEJ during meiosis. Our data further reveal that NBS-1 and COM-1 play distinct roles in promoting HR and antagonizing NHEJ. We propose a model in which different components of the MRN-C complex work together to couple meiotic DSB formation with efficient and timely engagement of HR, thereby ensuring crossover formation and restoration of genome integrity prior to the meiotic divisions.\n\nSignificance StatementDouble-strand breaks (DSBs) are deleterious DNA lesions, and impairment of the DSB repair machinery can lead to devastating diseases such as the Nijmegen Breakage Syndrome (NBS). During meiosis, DSBs represent a \"necessary evil\": they are required to promote formation of crossovers between homologous chromosomes. Crossovers in turn ensure correct chromosome inheritance during gamete formation, which is essential for viability and normal development of embryos. During meiosis, numerous DSBs are actively created, so meiotic cells must ensure that all breaks are properly repaired to ensure crossover formation and restore genomic integrity. Here we identify C. elegans NBS-1 as essential to properly process meiotic DSBs, both to promote crossover formation and antagonize an error-prone DSB repair pathway, thereby ensuring faithful chromosome inheritance.