BRCA1 and BRCA2 are prominently associated with inherited breast and ovarian cancer. The encoded proteins function in DNA damage responses, but no functional link between BRCA1 and BRCA2 has been established. We show here that PALB2 physically and functionally connects BRCA1 and BRCA2 into a DNA damage response network that also includes the RAD51 recombinase. PALB2 directly binds BRCA1, as determined with bacterially expressed fragments of each protein. Furthermore, PALB2 independently interacts with BRCA1 and BRCA2 through its NH₂ and COOH termini, respectively. Critically, two point mutants (L21P and L24P) of the PALB2 coiled-coil domain or an NH₂-terminal deletion (Δ1-70) disrupt its interaction with BRCA1. We have reconstituted PALB2-deficient cells with PALB2Δ1-70, PALB2-L21P, or PALB2-L24P, or with COOH-terminally truncated PALB2 that is deficient for interaction with BRCA2. Using extracts from these cells, we find that PALB2 mediates the physical interaction of BRCA2 with a COOH-terminal fragment of BRCA1. Analysis of the assembly of foci in these cells by BRCA1, PALB2, BRCA2, and RAD51 suggests that BRCA1 recruits PALB2, which in turn organizes BRCA2 and RAD51. Resistance to mitomycin C and the repair of DNA double-strand breaks by homologous recombination require the interaction of PALB2 with both BRCA1 and BRCA2. These results suggest that BRCA1 and BRCA2 cooperate in DNA damage responses in a PALB2-dependent manner, and have important implications for the genesis of breast/ovarian cancer and for chemotherapy with DNA interstrand cross-linking agents. (Mol Cancer Res 2009;7(7):1110–8)

Precise genome editing/correction of DNA double-strand breaks (DSBs) induced by CRISPR-Cas9 by homology-dependent repair (HDR) is limited by the competing error-prone non-homologous end-joining (NHEJ) DNA repair pathway. Here, we define a safer and efficient system that promotes HDR-based precise genome editing, while reducing NHEJ locally, only at CRISPR-Cas9-induced DSBs. We fused a dominant-negative mutant of 53BP1, DN1S, to Cas9 nucleases, and the resulting Cas9-DN1S fusion proteins significantly block NHEJ events specifically at Cas9 cut sites and improve HDR frequency; HDR frequency reached 86% in K562 cells. Cas9-DN1S protein maintains this effect in different human cell types, including leukocyte adhesion deficiency (LAD) patient-derived immortalized B lymphocytes, where nearly 70% of alleles were repaired by HDR and 7% by NHEJ. Our CRISPR-Cas9-DN1S system is clinically relevant to improve the efficiencies of precise gene correction/insertion, significantly reducing error-prone NHEJ events at the nuclease cleavage site, while avoiding the unwanted effects of global NHEJ inhibition.

Heterozygous carriers of germ-line mutations in the BRCA2/FANCD1, PALB2/FANCN and RAD51C/FANCO DNA repair genes have an increased lifetime risk of developing breast, ovarian and other cancers; bi-allelic mutations in these genes clinically manifest as Fanconi anemia (FA). Here, we demonstrate that RAD51C is part of a novel protein complex that contains PALB2 and BRCA2. Further, the PALB2 WD40 domain can directly and independently bind RAD51C and BRCA2. To understand the role of these homologous recombination (HR) proteins in DNA repair, we functionally characterize effects of missense mutants of the PALB2 WD40 domain that have been reported in breast cancer patients. In contrast to large truncations of PALB2, which display a complete loss of interaction, the L939W, T1030I and L1143P missense mutants/variants of the PALB2 WD40 domain are associated with altered patterns of direct binding to the RAD51C, RAD51 and BRCA2 HR proteins in biochemical assays. Further, the T1030I missense mutant is unstable, whereas the L939W and L1143P proteins are stable but partially disrupt the PALB2–RAD51C–BRCA2 complex in cells. Functionally, the L939W and L1143P mutants display a decreased capacity for DNA double-strand break-induced HR and an increased cellular sensitivity to ionizing radiation. As further evidence for the functional importance of the HR complex, RAD51C mutants that are associated with cancer susceptibility and FA also display decreased complex formation with PALB2. Together, our results suggest that three different cancer susceptibility and FA proteins function in a DNA repair pathway based upon the PALB2 WD40 domain binding to RAD51C and BRCA2.

The cellular response to DNA damage is composed of cell cycle checkpoint and DNA repair mechanisms that serve to ensure proper replication of the genome prior to cell division. The function of the DNA damage response during DNA replication in S-phase is critical to this process. Recent evidence has suggested a number of interrelationships of DNA replication and cellular DNA damage responses. These include S-phase checkpoints which suppress replication initiation or elongation in response to DNA damage. Also, many components of the DNA damage response are required either for the stabilization of, or for restarting, stalled replication forks. Further, translesion synthesis permits DNA replication to proceed in the presence of DNA damage and can be coordinated with subsequent repair by homologous recombination (HR). Finally, cohesion of sister chromatids is established coincident with DNA replication and is required for subsequent DNA repair by homologous recombination. Here we review these processes, all of which occur at, or are related to, the advancing replication fork. We speculate that these multiple interdependencies of DNA replication and DNA damage responses integrate the many steps necessary to ensure accurate duplication of the genome.