Biochemical studies on human Rad51-mediated homologous recombination

Distinct roles of the two BRCA2 DNA-binding domains in DNA damage repair and replication fork preservation

Homologous recombination (HR) removes DNA double-strand breaks (DSBs) and preserves stressed DNA replication forks. Successful HR execution requires the tumor suppressor BRCA2, which harbors distinct DNA-binding domains (DBDs): one that possesses three oligonucleotide/oligosaccharide-binding (OB) folds (OB-DBD) and another residing in the C-terminal recombinase binding domain (CTRB-DBD). Here, we employ multi-faceted approaches to delineate the contributions of these domains toward HR and replication fork maintenance. We show that OB-DBD and CTRB-DBD confer single-strand DNA (ssDNA)- and dsDNA-binding capabilities, respectively, and that BRCA2 variants mutated in either domain are impaired in their ability to load the recombinase RAD51 onto ssDNA pre-occupied by RPA. While the CTRB-DBD mutant is modestly affected by DNA break repair, it exhibits a strong defect in the protection of stressed replication forks. In contrast, the OB-DBD is indispensable for both BRCA2 functions. Our study thus defines the unique contributions of the two BRCA2 DBDs in genome maintenance.

Reconstitution of the antagonistic effect between C1orf112/FIRRM-FIGNL1 and BRCA2 on RAD51 filament stabilization

C1orf112/FIRRM is a recently identified DNA damage repair factor that regulates RAD51 in homologous recombination through interacting with the anti-recombinase FIGNL1. Here, we describe steps for purifying C1orf112/FIRRM, FIGNL1, miBRCA2, and RAD51 proteins from Escherichia coli or Saccharomyces cerevisiae cells. We then detail procedures for reconstituting the disassembly of RAD51 filament by C1orf112/FIRRM-FIGNL1 in vitro and the antagonistic effect between C1orf112/FIRRM-FIGNL1 and miBRCA2 on RAD51 filament stabilization. For complete details on the use and execution of this protocol, please refer to Zhou et al. (2023).1.

DNA binding and RAD51 engagement by the BRCA2 C-terminus orchestrate DNA repair and replication fork preservation

The tumor suppressor BRCA2 participates in DNA double-strand break repair by RAD51-dependent homologous recombination and protects stressed DNA replication forks from nucleolytic attack. We demonstrate that the C-terminal Recombinase Binding (CTRB) region of BRCA2, encoded by gene exon 27, harbors a DNA binding activity. CTRB alone stimulates the DNA strand exchange activity of RAD51 and permits the utilization of RPA-coated ssDNA by RAD51 for strand exchange. Moreover, CTRB functionally synergizes with the Oligonucleotide Binding fold containing DNA binding domain and BRC4 repeat of BRCA2 in RPA-RAD51 exchange on ssDNA. Importantly, we show that the DNA binding and RAD51 interaction attributes of the CTRB are crucial for homologous recombination and protection of replication forks against MRE11-mediated attrition. Our findings shed light on the role of the CTRB region in genome repair, reveal remarkable functional plasticity of BRCA2, and help explain why deletion of Brca2 exon 27 impacts upon embryonic lethality.

Design, synthesis and properties of peptide inhibitors based on BRCA1856-871

Overexpression of RAD51 protein was found to increase drug resistance in breast cancer cells. Breast cancer susceptibility gene 1 (BRCA1) protein can specifically bind to RAD51 protein and regulate the expression level of RAD51 protein. Based on previous studies, eight modified peptides were obtained by modifying the N-terminus of the key peptide segment 856-871 of BRCA1 with nicotinic acid (NA) and its derivatives. The interaction of BRCA1^856-871 and modified peptides with the RAD51^158-180 target peptide was investigated by fluorescence and circular dichroism spectroscopies. The results showed that the binding ability of 2-TFM-NA-PP to RAD51^158-180 was significantly enhanced. BRCA1^856-871 and modified peptides were studied by in vitro cell experiments. The results showed that the antitumor activity of 5-TFM-NA-PP was significantly enhanced compared with BRCA1^856-871.

The F-Box Domain-Dependent Activity of EMI1 Regulates PARPi Sensitivity in Triple-Negative Breast Cancers

The BRCA1-BRCA2-RAD51 axis is essential for homologous recombination repair (HRR) and is frequently disrupted in breast cancers. PARP inhibitors (PARPis) are used clinically to treat BRCA-mutated breast tumors. Using a genetic screen, we identified EMI1 as a modulator of PARPi sensitivity in triple-negative breast cancer (TNBC) cells. This function requires the F-box domain of EMI1, through which EMI1 assembles a canonical SCF ubiquitin ligase complex that constitutively targets RAD51 for degradation. In response to genotoxic stress, CHK1-mediated phosphorylation of RAD51 counteracts EMI1-dependent degradation by enhancing RAD51's affinity for BRCA2, leading to RAD51 accumulation. Inhibition of RAD51 degradation restores HRR in BRCA1-depleted cells. Human breast cancer samples display an inverse correlation between EMI1 and RAD51 protein levels. A subset of BRCA1-deficient TNBC cells develop resistance to PARPi by downregulating EMI1 and restoring RAD51-dependent HRR. Notably, reconstitution of EMI1 expression reestablishes PARPi sensitivity both in cellular systems and in an orthotopic mouse model.

Protein dynamics during presynaptic-complex assembly on individual single-stranded DNA molecules

Homologous recombination is a conserved pathway for repairing double-stranded breaks, which are processed to yield single-stranded DNA overhangs that serve as platforms for presynaptic-complex assembly. Here we use single-molecule imaging to reveal the interplay between Saccharomyces cerevisiae RPA, Rad52 and Rad51 during presynaptic-complex assembly. We show that Rad52 binds RPA-ssDNA and suppresses RPA turnover, highlighting an unanticipated regulatory influence on protein dynamics. Rad51 binding extends the ssDNA, and Rad52-RPA clusters remain interspersed along the presynaptic complex. These clusters promote additional binding of RPA and Rad52. Our work illustrates the spatial and temporal progression of the association of RPA and Rad52 with the presynaptic complex and reveals a new RPA-Rad52-Rad51-ssDNA intermediate, with implications for how the activities of Rad52 and RPA are coordinated with Rad51 during the later stages of recombination.