BRCA1 functions independently of homologous recombination in DNA interstrand crosslink repair

Brca1 is required for DNA repair by homologous recombination (HR) and normal embryonic development. Here we report that deletion of the DNA damage responsefactor 53BP1 overcomes embryonic lethality in Brca1-nullizygous mice, and rescues HR deficiency, as measured by hypersensitivity to PARP (polyADPribose polymerase) inhibition. However, Brca1,53BP1 double-deficient cells are hypersensitive to DNA interstrand crosslinks (ICLs), indicating that BRCA1 has an additional role in DNA cross-link repair that is distinct from HR. Disruption of the non-homologous end-joining (NHEJ) factor, Ku, promotes DNA repair in Brca1-deficient cells; however deletion of either Ku or 53BP1 exacerbates genomic instability in cells lacking FANCD2, a mediator of the Fanconi Anemia pathway for ICL repair. Brca1 therefore has two separate roles in ICL repair, whereas FANCD2 provides a key activity that can not be bypassed by ablation of 53BP1 or Ku. B cells were stimulated to undergo class switch recombination in vitro. Chromatin from B cells was harvested 72 hours post-stimulation and used for RPA ChIP to study the extent of resection of DNA DSBs.