NHJ-1 regulates canonical non-homologous end joining in Caenorhabditis elegans

Canonical non-homologous end joining (cNHEJ) is a near-universally conserved pathway for the repair of DNA double-strand breaks (DSBs). While the cNHEJ pathway encompasses more than a dozen factors in vertebrates and is similarly complex in other eukaryotes, in the nematode C. elegans the entire known cNHEJ toolkit consists of two proteins that comprise the Ku ring complex, cku-70 and cku-80, and the terminal ligase lig-4. Here, we report the discovery of nhj-1 as the fourth cNHEJ factor in C. elegans. Observing a difference in the phenotypic response to ionizing radiation (IR) between two lines of the wild type N2 strain, we mapped the locus causative of IR-sensitivity to a candidate on chromosome V. Using CRISPR-Cas9 mutagenesis, we show that disrupting the nhj-1 sequence induces IR-sensitivity in an IR-resistant background. Double mutants of nhj-1 and the cNHEJ factors lig-4 or cku-80 do not exhibit additive IR-sensitivity, arguing that nhj-1 is a member of the cNHEJ pathway. Furthermore, like the loss of lig-4, the loss of nhj-1 in the com-1 genetic background, in which meiotic DSBs are repaired by cNHEJ instead of homologous recombination, increased the number of DAPI-staining bodies in diakinesis, consistent with increased chromosome fragmentation in the absence of cNHEJ repair. Finally, we show that NHJ-1 localizes to many somatic nuclei in the L1 larva, but not the primordial germline, which is in accord with a role in the predominantly somatically active cNHEJ. Although nhj-1 shares no sequence homology with other known eukaryotic cNHEJ factors and is taxonomically restricted to the Rhadbitid family, its discovery underscores the evolutionary plasticity of even highly conserved pathways, and may represent a springboard for further characterization of cNHEJ in C. elegans.