Stabilization of RAD51 nucleoprotein filaments by the C-terminal region of BRCA2

Structure-based approaches in synthetic lethality strategies

Evolution has fostered robust DNA damage response (DDR) mechanisms to combat DNA lesions. However, disruptions in this intricate machinery can render cells overly reliant on the remaining functional but often less accurate DNA repair pathways. This increased dependence on error-prone pathways may result in improper repair and the accumulation of mutations, fostering genomic instability and facilitating the uncontrolled cell proliferation characteristic of cancer initiation and progression. Strategies based on the concept of synthetic lethality (SL) leverage the inherent genomic instability of cancer cells by targeting alternative pathways, thereby inducing selective death of cancer cells. This review emphasizes recent advancements in structural investigations of pivotal SL targets. The significant contribution of structure-based methodologies to SL research underscores their potential impact in characterizing the growing number of SL targets, largely due to advances in next-generation sequencing. Harnessing these approaches is essential for advancing the development of precise and personalized SL therapeutic strategies.

BRCA2 stabilises RAD51 and DMC1 nucleoprotein filaments through a conserved interaction mode

BRCA2 is essential for DNA repair by homologous recombination in mitosis and meiosis. It interacts with recombinases RAD51 and DMC1 to facilitate the formation of nucleoprotein filaments on resected DNA ends that catalyse recombination-mediated repair. BRCA2's BRC repeats bind and disrupt RAD51 and DMC1 filaments, whereas its PhePP motifs bind recombinases and stabilise their nucleoprotein filaments. However, the mechanism of filament stabilisation has hitherto remained unknown. Here, we report the crystal structure of a BRCA2-DMC1 complex, revealing how core interaction sites of PhePP motifs bind to recombinases. The interaction mode is conserved for RAD51 and DMC1, which selectively bind to BRCA2's two distinct PhePP motifs via subtly divergent binding pockets. PhePP motif sequences surrounding their core interaction sites protect nucleoprotein filaments from BRC-mediated disruption. Hence, we report the structural basis of how BRCA2's PhePP motifs stabilise RAD51 and DMC1 nucleoprotein filaments for their essential roles in mitotic and meiotic recombination.

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

Homologous recombination (HR) is a highly conserved tool for the removal of DNA double-strand breaks (DSBs) and the preservation of stalled and damaged DNA replication forks. Successful completion of HR requires the tumor suppressor BRCA2. Germline mutations in BRCA2 lead to familial breast, ovarian, and other cancers, underscoring the importance of this protein for maintaining genome stability. BRCA2 harbors two distinct DNA binding domains, one that possesses three oligonucleotide/oligosaccharide binding (OB) folds (known as the OB-DBD), and with the other residing in the C-terminal recombinase binding domain (termed the CTRB-DBD) encoded by the last gene exon. Here, we employ a combination of genetic, biochemical, and cellular approaches to delineate contributions of these two DNA binding domains toward HR and the maintenance of stressed DNA replication forks. We show that OB-DBD and CTRB-DBD confer ssDNA and dsDNA binding capabilities to BRCA2, respectively, and that BRCA2 variants mutated in either DNA binding domain are impaired in the ability to load the recombinase RAD51 onto ssDNA pre-occupied by RPA. While the CTRB-DBD mutant is modestly affected for HR, 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 DNA binding domains in genome maintenance.

BRCA2 C-terminal clamp restructures RAD51 dimers to bind B-DNA for replication fork stability

Tumor suppressor protein BRCA2 acts with RAD51 in replication-fork protection (FP) and homology-directed DNA break repair (HDR). Critical for cancer etiology and therapy resistance, BRCA2 C-terminus was thought to stabilize RAD51-filaments after they assemble on single-stranded (ss)DNA. Here we determined the detailed crystal structure for BRCA2 C-terminal interaction-domain (TR2i) with ATP-bound RAD51 prior to DNA binding. In contrast to recombinogenic RAD51-filaments comprising extended ATP-bound RAD51 dimers, TR2i unexpectedly reshapes ATP-RAD51 into a unique dimer conformation accommodating double-stranded B-DNA binding unsuited for HDR initiation. Structural, biochemical, and molecular results with interface-guided mutations uncover TR2i's FP mechanism. Proline-driven secondary-structure stabilizes residue triads and spans the RAD51 dimer engaging pivotal interactions of RAD51 M210 and BRCA2 S3291/P3292, the cyclin-dependent kinase (CDK) phosphorylation site that toggles between FP during S-phase and HDR in G2. TR2i evidently acts as an allosteric clamp switching RAD51 from ssDNA to double-stranded and B-DNA binding enforcing FP over HDR.

Mechanism of BCDX2-mediated RAD51 nucleation on short ssDNA stretches and fork DNA

Homologous recombination (HR) factors are crucial for DSB repair and processing stalled replication forks. RAD51 paralogs, including RAD51B, RAD51C, RAD51D, XRCC2 and XRCC3, have emerged as essential tumour suppressors, forming two subcomplexes, BCDX2 and CX3. Mutations in these genes are associated with cancer susceptibility and Fanconi anaemia, yet their biochemical activities remain unclear. This study reveals a linear arrangement of BCDX2 subunits compared to the RAD51 ring. BCDX2 shows a strong affinity towards single-stranded DNA (ssDNA) via unique binding mechanism compared to RAD51, and a contribution of DX2 subunits in binding branched DNA substrates. We demonstrate that BCDX2 facilitates RAD51 loading on ssDNA by suppressing the cooperative requirement of RAD51 binding to DNA and stabilizing the filament. Notably, BCDX2 also promotes RAD51 loading on short ssDNA and reversed replication fork substrates. Moreover, while mutants defective in ssDNA binding retain the ability to bind branched DNA substrates, they still facilitate RAD51 loading onto reversed replication forks. Our study provides mechanistic insights into how the BCDX2 complex stimulates the formation of BRCA2-independent RAD51 filaments on short stretches of ssDNA present at ssDNA gaps or stalled replication forks, highlighting its role in genome maintenance and DNA repair.

Integrated pan-cancer analysis and experimental verification of the roles of meiotic nuclear divisions 1 in breast cancer

INTRODUCTION

The aberrant up-regulation of meiotic nuclear division 1 (MND1) in somatic cells is considered as one of the driving factors of oncogenesis, whereas its expression and role in breast invasive cancer (BRCA) remain unclear. Hence, this study embarked on a comprehensive evaluation of MND1 across various cancers and identified its roles in BRCA.

METHODS

Based on publicly available databases, including but not limited to UCSC Xena, TCGA, GTEx, GEO, STRING, GeneMANIA, and CancerSEA, we evaluated the expression patterns, genomic features, and biological functions of MND1 from a pan-cancer viewpoint and delved into the implications of MND1 in the prognosis and treatment of BRCA. Further molecular biology experiments were undertaken to identify the role of MND1 in proliferation, migration, and apoptosis in BRCA cells.

RESULTS

Elevated levels of MND1 were notably observed in a wide array of tumor types, especially in BRCA, COAD, HNSC, LIHC, LUAD, LUSC, STAD, and UCEC. Elevated MND1 expression was markedly associated with shortened OS in several tumors, including BRCA (HR = 1.52 [95%CI, 1.10-2.09], P = 0.011). The up-regulation of MND1 in BRCA was validated in external cohorts and clinical samples. Survival analyses demonstrated that elevated MND1 expression was associated with decreased survival for patients with BRCA. Co-expressed genes of MND1 were identified, and subsequent pathway analyses based on significantly associated genes indicated that MND1 plays key roles in DNA replication, cell cycle regulation, and DNA damage repair. The observed abnormal elevation and activation of MND1 led to increased proliferation and migration, along with decreased apoptosis in BRCA cells.

CONCLUSIONS

MND1 emerges as a promising biomarker for diagnostic and therapeutic targeting in various cancers, including BRCA. The abnormal up-regulation and activation of MND1 are linked to carcinogenesis and poor prognosis among BRCA patients, which may be attributed to its involvement in HR-dependent ALT, warranting further scrutiny.

MEILB2-BRME1 forms a V-shaped DNA clamp upon BRCA2-binding in meiotic recombination

DNA double-strand break repair by homologous recombination has a specialised role in meiosis by generating crossovers that enable the formation of haploid germ cells. This requires meiosis-specific MEILB2-BRME1, which interacts with BRCA2 to facilitate loading of recombinases onto resected DNA ends. Here, we report the crystal structure of the MEILB2-BRME1 2:2 core complex, revealing a parallel four-helical assembly that recruits BRME1 to meiotic double-strand breaks in vivo. It forms an N-terminal β-cap that binds to DNA, and a MEILB2 coiled-coil that bridges to C-terminal ARM domains. Upon BRCA2-binding, MEILB2-BRME1 2:2 complexes dimerize into a V-shaped 2:4:4 complex, with rod-like MEILB2-BRME1 components arranged at right-angles. The β-caps located at the tips of the MEILB2-BRME1 limbs are separated by 25 nm, allowing them to bridge between DNA molecules. Thus, we propose that BRCA2 induces MEILB2-BRME1 to function as a DNA clamp, connecting resected DNA ends or homologous chromosomes to facilitate meiotic recombination.

The RAD51 S181P mutation shortens lifespan of female mice

RAD51 is critical to the homologous recombination (HR) pathway that repairs DNA double strand breaks (DSBs) and protects replication forks (RFs). Previously, we showed that the S181P (SP) mutation in RAD51 causes defective RF maintenance but is proficient for DSB repair. Here we report that SP/SP female mice exhibit a shortened lifespan compared to +/+ females but not males. Histological analysis found that most mice in this study died from lymphoma, independent of genotype and sex. We propose that a potential cause for shortened lifespan in SP/SP females is due to the RF defect.

DMC1 and RAD51 bind FxxA and FxPP motifs of BRCA2 via two separate interfaces

In vertebrates, the BRCA2 protein is essential for meiotic and somatic homologous recombination due to its interaction with the RAD51 and DMC1 recombinases through FxxA and FxPP motifs (here named A- and P-motifs, respectively). The A-motifs present in the eight BRC repeats of BRCA2 compete with the A-motif of RAD51, which is responsible for its self-oligomerization. BRCs thus disrupt RAD51 nucleoprotein filaments in vitro. The role of the P-motifs is less studied. We recently found that deletion of Brca2 exons 12-14 encoding one of them (the prototypical 'PhePP' motif), disrupts DMC1 but not RAD51 function in mouse meiosis. Here we provide a mechanistic explanation for this phenotype by solving the crystal structure of the complex between a BRCA2 fragment containing the PhePP motif and DMC1. Our structure reveals that, despite sharing a conserved phenylalanine, the A- and P-motifs bind to distinct sites on the ATPase domain of the recombinases. The P-motif interacts with a site that is accessible in DMC1 octamers and nucleoprotein filaments. Moreover, we show that this interaction also involves the adjacent protomer and thus increases the stability of the DMC1 nucleoprotein filaments. We extend our analysis to other P-motifs from RAD51AP1 and FIGNL1.

RAD51 separation of function mutation disables replication fork maintenance but preserves DSB repair

Homologous recombination (HR) protects replication forks (RFs) and repairs DNA double-strand breaks (DSBs). Within HR, BRCA2 regulates RAD51 via two interaction regions: the BRC repeats to form filaments on single-stranded DNA and exon 27 (Ex27) to stabilize the filament. Here, we identified a RAD51 S181P mutant that selectively disrupted the RAD51-Ex27 association while maintaining interaction with BRC repeat and proficiently forming filaments capable of DNA binding and strand invasion. Interestingly, RAD51 S181P was defective for RF protection/restart but proficient for DSB repair. Our data suggest that Ex27-mediated stabilization of RAD51 filaments is required for the protection of RFs, while it seems dispensable for the repair of DSBs.

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.

The dePARylase NUDT16 promotes radiation resistance of cancer cells by blocking SETD3 for degradation via reversing its ADP-ribosylation

Poly(ADP-ribosyl)ation (PARylation) is a critical posttranslational modification that plays a vital role in maintaining genomic stability via a variety of molecular mechanisms, including activation of replication stress and the DNA damage response. The nudix hydrolase NUDT16 was recently identified as a phosphodiesterase that is responsible for removing ADP-ribose units and that plays an important role in DNA repair. However, the roles of NUDT16 in coordinating replication stress and cell cycle progression remain elusive. Here, we report that SETD3, which is a member of the SET-domain containing protein (SETD) family, is a novel substrate for NUDT16, that its protein levels fluctuate during cell cycle progression, and that its stability is strictly regulated by NUDT16-mediated dePARylation. Moreover, our data indicated that the E3 ligase CHFR is responsible for the recognition and degradation of endogenous SETD3 in a PARP1-mediated PARylation-dependent manner. Mechanistically, we revealed that SETD3 associates with BRCA2 and promotes its recruitment to stalled replication fork and DNA damage sites upon replication stress or DNA double-strand breaks, respectively. Importantly, depletion of SETD3 in NUDT16-deficient cells did not further exacerbate DNA breaks or enhance the sensitivity of cancer cells to IR exposure, suggesting that the NUDT16-SETD3 pathway may play critical roles in the induction of tolerance to radiotherapy. Collectively, these data showed that NUDT16 functions as a key upstream regulator of SETD3 protein stability by reversing the ADP-ribosylation of SETD3, and NUDT16 participates in the resolution of replication stress and facilitates HR repair.

BRCA2 promotes genomic integrity and therapy resistance primarily through its role in homology-directed repair

Tumor suppressor BRCA2 functions in homology-directed repair (HDR), the protection of stalled replication forks, and the suppression of replicative gaps, but their relative contributions to genome integrity and chemotherapy response are under scrutiny. Here, we report that mouse and human cells require a RAD51 filament stabilization motif in BRCA2 for fork protection and gap suppression but not HDR. In mice, the loss of fork protection/gap suppression does not compromise genome stability or shorten tumor latency. By contrast, HDR deficiency increases spontaneous and replication stress-induced chromosome aberrations and tumor predisposition. Unlike with HDR, fork protection/gap suppression defects are also observed in Brca2 heterozygous cells, likely due to reduced RAD51 stabilization at stalled forks/gaps. Gaps arise from PRIMPOL activity, which is associated with 5-hydroxymethyl-2'-deoxyuridine sensitivity due to the formation of SMUG1-generated abasic sites and is exacerbated by poly(ADP-ribose) polymerase (PARP) inhibition. However, HDR proficiency has the major role in mitigating sensitivity to chemotherapeutics, including PARP inhibitors.

Positive and negative regulators of RAD51/DMC1 in homologous recombination and DNA replication

RAD51 recombinase plays a central role in homologous recombination (HR) by forming a nucleoprotein filament on single-stranded DNA (ssDNA) to catalyze homology search and strand exchange between the ssDNA and a homologous double-stranded DNA (dsDNA). The catalytic activity of RAD51 assembled on ssDNA is critical for the DNA-homology-mediated repair of DNA double-strand breaks in somatic and meiotic cells and restarting stalled replication forks during DNA replication. The RAD51-ssDNA complex also plays a structural role in protecting the regressed/reversed replication fork. Two types of regulators control RAD51 filament formation, stability, and dynamics, namely positive regulators, including mediators, and negative regulators, so-called remodelers. The appropriate balance of action by the two regulators assures genome stability. This review describes the roles of positive and negative RAD51 regulators in HR and DNA replication and its meiosis-specific homolog DMC1 in meiotic recombination. We also provide future study directions for a comprehensive understanding of RAD51/DMC1-mediated regulation in maintaining and inheriting genome integrity.

Mechanisms of synthetic lethality between BRCA1/2 and 53BP1 deficiencies and DNA polymerase theta targeting

A synthetic lethal relationship exists between disruption of polymerase theta (Polθ), and loss of either 53BP1 or homologous recombination (HR) proteins, including BRCA1; however, the mechanistic basis of these observations are unclear. Here we reveal two distinct mechanisms of Polθ synthetic lethality, identifying dual influences of 1) whether Polθ is lost or inhibited, and 2) the underlying susceptible genotype. Firstly, we find that the sensitivity of BRCA1/2- and 53BP1-deficient cells to Polθ loss, and 53BP1-deficient cells to Polθ inhibition (ART558) requires RAD52, and appropriate reduction of RAD52 can ameliorate these phenotypes. We show that in the absence of Polθ, RAD52 accumulations suppress ssDNA gap-filling in G2/M and encourage MRE11 nuclease accumulation. In contrast, the survival of BRCA1-deficient cells treated with Polθ inhibitor are not restored by RAD52 suppression, and ssDNA gap-filling is prevented by the chemically inhibited polymerase itself. These data define an additional role for Polθ, reveal the mechanism underlying synthetic lethality between 53BP1, BRCA1/2 and Polθ loss, and indicate genotype-dependent Polθ inhibitor mechanisms.

Structural basis for stabilisation of the RAD51 nucleoprotein filament by BRCA2

The BRCA2 tumour suppressor protein preserves genomic integrity via interactions with the DNA-strand exchange RAD51 protein in homology-directed repair. The RAD51-binding TR2 motif at the BRCA2 C-terminus is essential for protection and restart of stalled replication forks. Biochemical evidence shows that TR2 recognises filamentous RAD51, but existing models of TR2 binding to RAD51 lack a structural basis. Here we used cryo-electron microscopy and structure-guided mutagenesis to elucidate the mechanism of TR2 binding to nucleoprotein filaments of human RAD51. We find that TR2 binds across the protomer interface in the filament, acting as a brace for adjacent RAD51 molecules. TR2 targets an acidic-patch motif on human RAD51 that serves as a recruitment hub in fission yeast Rad51 for recombination mediators Rad52 and Rad55-Rad57. Our findings provide a structural rationale for RAD51 filament stabilisation by BRCA2 and reveal a common recruitment mechanism of recombination mediators to the RAD51 filament.

C1orf112 teams up with FIGNL1 to facilitate RAD51 filament disassembly and DNA interstrand cross-link repair

The recombinase RAD51 plays a core role in DNA repair by homologous recombination (HR). The assembly and disassembly of RAD51 filament need to be orderly regulated by mediators such as BRCA2 and anti-recombinases. To screen for potential regulators of RAD51, we perform RAD51 proximity proteomics and identify factor C1orf112. We further find that C1orf112 complexed with FIGNL1 facilitates RAD51 filament disassembly in the HR step of Fanconi anemia (FA) pathway. Specifically, C1orf112 physically interacts with FIGNL1 and enhances its protein stability. Meanwhile, the RAD51 filament disassembly activity of FIGNL1 is directly stimulated by C1orf112. BRCA2 directly interacts with C1orf112-FIGNL1 complex and functions upstream of this complex to protect RAD51 filament from premature disassembly. C1orf112- and FIGNL1-deficient cells are primarily sensitive to DNA interstrand cross-link (ICL) agents. Thus, these findings suggest an important function of C1orf112 in RAD51 regulation in the HR step of ICL repair by FA pathway.

Visualization of direct and diffusion-assisted RAD51 nucleation by full-length human BRCA2 protein

Homologous recombination (HR) is essential for error-free repair of DNA double-strand breaks, perturbed replication forks (RFs), and post-replicative single-stranded DNA (ssDNA) gaps. To initiate HR, the recombination mediator and tumor suppressor protein BRCA2 facilitates nucleation of RAD51 on ssDNA prior to stimulation of RAD51 filament growth by RAD51 paralogs. Although ssDNA binding by BRCA2 has been implicated in RAD51 nucleation, the function of double-stranded DNA (dsDNA) binding by BRCA2 remains unclear. Here, we exploit single-molecule (SM) imaging to visualize BRCA2-mediated RAD51 nucleation in real time using purified proteins. We report that BRCA2 nucleates and stabilizes RAD51 on ssDNA either directly or through an unappreciated diffusion-assisted delivery mechanism involving binding to and sliding along dsDNA, which requires the cooperative action of multiple dsDNA-binding modules in BRCA2. Collectively, our work reveals two distinct mechanisms of BRCA2-dependent RAD51 loading onto ssDNA, which we propose are critical for its diverse functions in maintaining genome stability and cancer suppression.

Structure and function of the RAD51B-RAD51C-RAD51D-XRCC2 tumour suppressor

Homologous recombination is a fundamental process of life. It is required for the protection and restart of broken replication forks, the repair of chromosome breaks and the exchange of genetic material during meiosis. Individuals with mutations in key recombination genes, such as BRCA2 (also known as FANCD1), or the RAD51 paralogues RAD51B, RAD51C (also known as FANCO), RAD51D, XRCC2 (also known as FANCU) and XRCC3, are predisposed to breast, ovarian and prostate cancers1-10 and the cancer-prone syndrome Fanconi anaemia11-13. The BRCA2 tumour suppressor protein-the product of BRCA2-is well characterized, but the cellular functions of the RAD51 paralogues remain unclear. Genetic knockouts display growth defects, reduced RAD51 focus formation, spontaneous chromosome abnormalities, sensitivity to PARP inhibitors and replication fork defects14,15, but the precise molecular roles of RAD51 paralogues in fork stability, DNA repair and cancer avoidance remain unknown. Here we used cryo-electron microscopy, AlphaFold2 modelling and structural proteomics to determine the structure of the RAD51B-RAD51C-RAD51D-XRCC2 complex (BCDX2), revealing that RAD51C-RAD51D-XRCC2 mimics three RAD51 protomers aligned within a nucleoprotein filament, whereas RAD51B is highly dynamic. Biochemical and single-molecule analyses showed that BCDX2 stimulates the nucleation and extension of RAD51 filaments-which are essential for recombinational DNA repair-in reactions that depend on the coupled ATPase activities of RAD51B and RAD51C. Our studies demonstrate that BCDX2 orchestrates RAD51 assembly on single stranded DNA for replication fork protection and double strand break repair, in reactions that are critical for tumour avoidance.

Therapeutic disruption of RAD52-ssDNA complexation via novel drug-like inhibitors

RAD52 protein is a coveted target for anticancer drug discovery. Similar to poly-ADP-ribose polymerase (PARP) inhibitors, pharmacological inhibition of RAD52 is synthetically lethal with defects in genome caretakers BRCA1 and BRCA2 (∼25% of breast and ovarian cancers). Emerging structure activity relationships for RAD52 are complex, making it challenging to transform previously identified disruptors of the RAD52-ssDNA interaction into drug-like leads using traditional medicinal chemistry approaches. Using pharmacophoric informatics on the RAD52 complexation by epigallocatechin (EGC), and the Enamine in silico REAL database, we identified six distinct chemical scaffolds that occupy the same physical space on RAD52 as EGC. All six were RAD52 inhibitors (IC50 ∼23-1200 μM) with two of the compounds (Z56 and Z99) selectively killing BRCA-mutant cells and inhibiting cellular activities of RAD52 at micromolar inhibitor concentrations. While Z56 had no effect on the ssDNA-binding protein RPA and was toxic to BRCA-mutant cells only, Z99 inhibited both proteins and displayed toxicity towards BRCA-complemented cells. Optimization of the Z99 scaffold resulted in a set of more powerful and selective inhibitors (IC50 ∼1.3-8 μM), which were only toxic to BRCA-mutant cells. RAD52 complexation by Z56, Z99 and its more specific derivatives provide a roadmap for next generation of cancer therapeutics.

BRCA2 promotes genomic integrity and therapy resistance primarily through its role in homology-directed repair

Highlights

Gap suppression requires BRCA2 C-terminal RAD51 binding in mouse and human cells Brca2 heterozygosity in mice results in fork protection and gap suppression defects Gap suppression mitigates sensitivity to hmdU, but only when HDR is unperturbedHDR deficiency is the primary driver of chemotherapeutic sensitivity.

eTOC blurb

Lim et al . report that gap suppression as well as fork protection require BRCA2 stabilization of RAD51 filaments in human and mouse cells but have minimal impact on genome integrity, oncogenesis, and drug resistance. BRCA2 suppression of PRIMPOL-mediated replication gaps confers resistance to the nucleotide hmdU, incorporation of which leads to cytotoxic abasic sites.This effect is diminished when HDR is abrogated.

Summary

Tumor suppressor BRCA2 functions in homology-directed repair (HDR), protection of stalled replication forks, and suppression of replicative gaps. The relative contributions of these pathways to genome integrity and chemotherapy response are under scrutiny. Here, we report that mouse and human cells require a RAD51 filament stabilization motif in BRCA2 for both fork protection and gap suppression, but not HDR. Loss of fork protection and gap suppression do not compromise genome instability or shorten tumor latency in mice or cause replication stress in human mammary cells. By contrast, HDR deficiency increases spontaneous and replication stress-induced chromosome aberrations and tumor predisposition. Unlike with HDR, fork protection and gap suppression defects are also observed in Brca2 heterozygous mouse cells, likely due to reduced RAD51 stabilization at stalled forks and gaps. Gaps arise from PRIMPOL activity, which is associated with sensitivity to 5-hydroxymethyl-2’-deoxyuridine due to the formation of abasic sites by SMUG1 glycosylase and is exacerbated by poly(ADP-ribose) polymerase inhibition. However, HDR deficiency ultimately modulates sensitivity to chemotherapeutics, including PARP inhibitors.

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.

Dipeptidyl peptidase 9 triggers BRCA2 degradation and promotes DNA damage repair

N-terminal sequences are important sites for post-translational modifications that alter protein localization, activity, and stability. Dipeptidyl peptidase 9 (DPP9) is a serine aminopeptidase with the rare ability to cleave off N-terminal dipeptides with imino acid proline in the second position. Here, we identify the tumor-suppressor BRCA2 as a DPP9 substrate and show this interaction to be induced by DNA damage. We present crystallographic structures documenting intracrystalline enzymatic activity of DPP9, with the N-terminal Met1-Pro2 of a BRCA21-40 peptide captured in its active site. Intriguingly, DPP9-depleted cells are hypersensitive to genotoxic agents and are impaired in the repair of DNA double-strand breaks by homologous recombination. Mechanistically, DPP9 targets BRCA2 for degradation and promotes the formation of RAD51 foci, the downstream function of BRCA2. N-terminal truncation mutants of BRCA2 that mimic a DPP9 product phenocopy reduced BRCA2 stability and rescue RAD51 foci formation in DPP9-deficient cells. Taken together, we present DPP9 as a regulator of BRCA2 stability and propose that by fine-tuning the cellular concentrations of BRCA2, DPP9 alters the BRCA2 interactome, providing a possible explanation for DPP9's role in cancer.

BRCA2 BRC missense variants disrupt RAD51-dependent DNA repair

Pathogenic mutations in the BRCA2 tumor suppressor gene predispose to breast, ovarian, pancreatic, prostate, and other cancers. BRCA2 maintains genome stability through homology-directed repair (HDR) of DNA double-strand breaks (DSBs) and replication fork protection. Nonsense or frameshift mutations leading to truncation of the BRCA2 protein are typically considered pathogenic; however, missense mutations resulting in single amino acid substitutions can be challenging to functionally interpret. The majority of missense mutations in BRCA2 have been classified as Variants of Uncertain Significance (VUS) with unknown functional consequences. In this study, we identified three BRCA2 VUS located within the BRC repeat region to determine their impact on canonical HDR and fork protection functions. We provide evidence that S1221P and T1980I, which map to conserved residues in the BRC2 and BRC7 repeats, compromise the cellular response to chemotherapeutics and ionizing radiation, and display deficits in fork protection. We further demonstrate biochemically that S1221P and T1980I disrupt RAD51 binding and diminish the ability of BRCA2 to stabilize RAD51-ssDNA complexes. The third variant, T1346I, located within the spacer region between BRC2 and BRC3 repeats, is fully functional. We conclude that T1346I is a benign allele, whereas S1221P and T1980I are hypomorphic disrupting the ability of BRCA2 to fully engage and stabilize RAD51 nucleoprotein filaments. Our results underscore the importance of correctly classifying BRCA2 VUS as pathogenic variants can impact both future cancer risk and guide therapy selection during cancer treatment.

Double-stranded DNA binding function of RAD51 in DNA protection and its regulation by BRCA2

RAD51 and the breast cancer suppressor BRCA2 have critical functions in DNA double-strand (dsDNA) break repair by homologous recombination and the protection of newly replicated DNA from nucleolytic degradation. The recombination function of RAD51 requires its binding to single-stranded DNA (ssDNA), whereas binding to dsDNA is inhibitory. Using reconstituted MRE11-, EXO1-, and DNA2-dependent nuclease reactions, we show that the protective function of RAD51 unexpectedly depends on its binding to dsDNA. The BRC4 repeat of BRCA2 abrogates RAD51 binding to dsDNA and accordingly impairs the function of RAD51 in protection. The BRCA2 C-terminal RAD51-binding segment (TR2) acts in a dominant manner to overcome the effect of BRC4. Mechanistically, TR2 stabilizes RAD51 binding to dsDNA, even in the presence of BRC4, promoting DNA protection. Our data suggest that RAD51's dsDNA-binding capacity may have evolved together with its function in replication fork protection and provide a mechanistic basis for the DNA-protection function of BRCA2.

The Mechanistic Understanding of RAD51 Defibrillation: A Critical Step in BRCA2-Mediated DNA Repair by Homologous Recombination

The cytotoxic action of anticancer drugs can be potentiated by inhibiting DNA repair mechanisms. RAD51 is a crucial protein for genomic stability due to its critical role in the homologous recombination (HR) pathway. BRCA2 assists RAD51 fibrillation and defibrillation in the cytoplasm and nucleus and assists its nuclear transport. BRC4 is a peptide derived from the fourth BRC repeat of BRCA2, and it lacks the nuclear localization sequence. Here, we used BRC4 to (i) reverse RAD51 fibrillation; (ii) avoid the nuclear transport of RAD51; and (iii) inhibit HR and enhance the efficacy of chemotherapeutic treatments. Specifically, using static and dynamic light scattering, transmission electron microscopy, and microscale thermophoresis, we show that BRC4 eroded RAD51 fibrils from their termini through a “domino” mechanism and yielded monomeric RAD51 with a cumulative nanomolar affinity. Using cellular assays (BxPC-3, pancreatic cancer), we show that a myristoylated BRC4 (designed for a more efficient cell entry) abolished the formation of nuclear RAD51 foci. The present study provides a molecular description of RAD51 defibrillation, an essential step in BRCA2-mediated homologous recombination and DNA repair.

Divergent binding mode for a protozoan BRC repeat to RAD51

Interaction of BRCA2 through ca. 30 amino acid residue motifs, BRC repeats, with RAD51 is a conserved feature of the double-strand DNA break repair by homologous recombination in eukaryotes. In humans the binding of the eight BRC repeats is defined by two sequence motifs, FxxA and LFDE, interacting with distinct sites on RAD51. Little is known of the interaction of BRC repeats in other species, especially in protozoans, where variable number of BRC repeats are found in BRCA2 proteins. Here, we have studied in detail the interactions of the two BRC repeats in Leishmania infantum BRCA2 with RAD51. We show LiBRC1 is a high-affinity repeat and determine the crystal structure of its complex with LiRAD51. Using truncation mutagenesis of the LiBRC1 repeat, we demonstrate that high affinity binding is maintained in the absence of an LFDE-like motif and suggest compensatory structural features. These observations point towards a divergent evolution of BRC repeats, where a common FxxA-binding ancestor evolved additional contacts for affinity maturation and fine-tuning.

Identification of the most common BRCA alterations through analysis of germline mutation databases: Is droplet digital PCR an additional strategy for the assessment of such alterations in breast and ovarian cancer families?

Breast and ovarian cancer represent two of the most common tumor types in females worldwide. Over the years, several non‑modifiable and modifiable risk factors have been associated with the onset and progression of these tumors, including age, reproductive factors, ethnicity, socioeconomic status and lifestyle factors, as well as family history and genetic factors. Of note, BRCA1 and BRCA2 are two tumor suppressor genes with a key role in DNA repair processes, whose mutations may induce genomic instability and increase the risk of cancer development. Specifically, females with a family history of breast or ovarian cancer harboring BRCA1/2 germline mutations have a 60‑70% increased risk of developing breast cancer and a 15‑40% increased risk for ovarian cancer. Different databases have collected the most frequent germline mutations affecting BRCA1/2. Through the analysis of such databases, it is possible to identify frequent hotspot mutations that may be analyzed with next‑generation sequencing (NGS) and novel innovative strategies. In this context, NGS remains the gold standard method for the assessment of BRCA1/2 mutations, while novel techniques, including droplet digital PCR (ddPCR), may improve the sensitivity to identify such mutations in the hereditary forms of breast and ovarian cancer. On these bases, the present study aimed to provide an update of the current knowledge on the frequency of BRCA1/2 mutations and cancer susceptibility, focusing on the diagnostic potential of the most recent methods, such as ddPCR.

BRCA2 C-Terminal RAD51-Binding Domain Confers Resistance to DNA-Damaging Agents

Breast cancer type 2 susceptibility (BRCA2) protein is crucial for initiating DNA damage repair after chemotherapy with DNA interstrand crosslinking agents or X-ray irradiation, which induces DNA double-strand breaks. BRCA2 contains a C-terminal RAD51-binding domain (CTRBD) that interacts with RAD51 oligomer-containing nucleofilaments. In this study, we investigated CTRBD expression in cells exposed to X-ray irradiation and mitomycin C treatment. Surprisingly, BRCA2 CTRBD expression in HeLa cells increased their resistance to X-ray irradiation and mitomycin C. Under endogenous BRCA2 depletion using shRNA, the sensitivities of the BRCA2-depleted cells with and without the CTRBD did not significantly differ. Thus, the resistance to X-ray irradiation conferred by an exogenous CTRBD required endogenous BRCA2 expression. BRCA2 CTRBD-expressing cells demonstrated effective RAD51 foci formation and increased homologous recombination efficiency, but not nonhomologous end-joining efficiency. To the best of our knowledge, our study is the first to report the ability of the BRCA2 functional domain to confer resistance to X-ray irradiation and mitomycin C treatment by increased homologous recombination efficiency. Thus, this peptide may be useful for protecting cells against X-ray irradiation or chemotherapeutic agents.

C. elegans as a model organism to study female reproductive health

Female reproductive health has been historically understudied and underfunded. Here, we present the advantages of using a free-living nematode, Caenorhabditis elegans, as an animal system to study fundamental aspects of female reproductive health. C. elegans is a powerful high-throughput model organism that shares key genetic and physiological similarities with humans. In this review, we highlight areas of pressing medical and biological importance in the 21st century within the context of female reproductive health. These include the decline in female reproductive capacity with increasing chronological age, reproductive dysfunction arising from toxic environmental insults, and cancers of the reproductive system. C. elegans has been instrumental in uncovering mechanistic insights underlying these processes, and has been valuable for developing and testing therapeutics to combat them. Adopting a convenient model organism such as C. elegans for studying reproductive health will encourage further research into this field, and broaden opportunities for making advancements into evolutionarily conserved mechanisms that control reproductive function.

Mechanism of mitotic recombination: insights from C. elegans

Homologous recombination (HR) plays a critical role in largely error-free repair of mitotic and meiotic DNA double-strand breaks (DSBs). DSBs are one of the most deleterious DNA lesions, which are repaired by non-homologous end joining (NHEJ), homologous recombination (HR) or, if compromised, micro-homology mediated end joining (MMEJ). If left unrepaired, DSBs can lead to cell death or if repaired incorrectly can result in chromosome rearrangements that drive cancer development. Here, we describe recent advances in the field of mitotic HR made using Caenorhabditis elegans roundworm, as a model system.

RAD52: Paradigm of Synthetic Lethality and New Developments

DNA double-strand breaks and inter-strand cross-links are the most harmful types of DNA damage that cause genomic instability that lead to cancer development. The highest fidelity pathway for repairing damaged double-stranded DNA is termed Homologous recombination (HR). Rad52 is one of the key HR proteins in eukaryotes. Although it is critical for most DNA repair and recombination events in yeast, knockouts of mammalian RAD52 lack any discernable phenotypes. As a consequence, mammalian RAD52 has been long overlooked. That is changing now, as recent work has shown RAD52 to be critical for backup DNA repair pathways in HR-deficient cancer cells. Novel findings have shed light on RAD52's biochemical activities. RAD52 promotes DNA pairing (D-loop formation), single-strand DNA and DNA:RNA annealing, and inverse strand exchange. These activities contribute to its multiple roles in DNA damage repair including HR, single-strand annealing, break-induced replication, and RNA-mediated repair of DNA. The contributions of RAD52 that are essential to the viability of HR-deficient cancer cells are currently under investigation. These new findings make RAD52 an attractive target for the development of anti-cancer therapies against BRCA-deficient cancers.

hMOB2 deficiency impairs homologous recombination-mediated DNA repair and sensitises cancer cells to PARP inhibitors

Monopolar spindle-one binder (MOBs) proteins are evolutionarily conserved and contribute to various cellular signalling pathways. Recently, we reported that hMOB2 functions in preventing the accumulation of endogenous DNA damage and a subsequent p53/p21-dependent G1/S cell cycle arrest in untransformed cells. However, the question of how hMOB2 protects cells from endogenous DNA damage accumulation remained enigmatic. Here, we uncover hMOB2 as a regulator of double-strand break (DSB) repair by homologous recombination (HR). hMOB2 supports the phosphorylation and accumulation of the RAD51 recombinase on resected single-strand DNA (ssDNA) overhangs. Physiologically, hMOB2 expression supports cancer cell survival in response to DSB-inducing anti-cancer compounds. Specifically, loss of hMOB2 renders ovarian and other cancer cells more vulnerable to FDA-approved PARP inhibitors. Reduced MOB2 expression correlates with increased overall survival in patients suffering from ovarian carcinoma. Taken together, our findings suggest that hMOB2 expression may serve as a candidate stratification biomarker of patients for HR-deficiency targeted cancer therapies, such as PARP inhibitor treatments.

A complex of BRCA2 and PP2A-B56 is required for DNA repair by homologous recombination

Mutations in the tumour suppressor gene BRCA2 are associated with predisposition to breast and ovarian cancers. BRCA2 has a central role in maintaining genome integrity by facilitating the repair of toxic DNA double-strand breaks (DSBs) by homologous recombination (HR). BRCA2 acts by controlling RAD51 nucleoprotein filament formation on resected single-stranded DNA, but how BRCA2 activity is regulated during HR is not fully understood. Here, we delineate a pathway where ATM and ATR kinases phosphorylate a highly conserved region in BRCA2 in response to DSBs. These phosphorylations stimulate the binding of the protein phosphatase PP2A-B56 to BRCA2 through a conserved binding motif. We show that the phosphorylation-dependent formation of the BRCA2-PP2A-B56 complex is required for efficient RAD51 filament formation at sites of DNA damage and HR-mediated DNA repair. Moreover, we find that several cancer-associated mutations in BRCA2 deregulate the BRCA2-PP2A-B56 interaction and sensitize cells to PARP inhibition. Collectively, our work uncovers PP2A-B56 as a positive regulator of BRCA2 function in HR with clinical implications for BRCA2 and PP2A-B56 mutated cancers.

Non-Recombinogenic Functions of Rad51, BRCA2, and Rad52 in DNA Damage Tolerance

The DNA damage tolerance (DDT) response is aimed to timely and safely complete DNA replication by facilitating the advance of replication forks through blocking lesions. This process is associated with an accumulation of single-strand DNA (ssDNA), both at the fork and behind the fork. Lesion bypass and ssDNA filling can be performed by translation synthesis (TLS) and template switching mechanisms. TLS uses low-fidelity polymerases to incorporate a dNTP opposite the blocking lesion, whereas template switching uses a Rad51/ssDNA nucleofilament and the sister chromatid to bypass the lesion. Rad51 is loaded at this nucleofilament by two mediator proteins, BRCA2 and Rad52, and these three factors are critical for homologous recombination (HR). Here, we review recent advances showing that Rad51, BRCA2, and Rad52 perform some of these functions through mechanisms that do not require the strand exchange activity of Rad51: the formation and protection of reversed fork structures aimed to bypass blocking lesions, and the promotion of TLS. These findings point to the central HR proteins as potential molecular switches in the choice of the mechanism of DDT.

The RAD51 recombinase protects mitotic chromatin in human cells

The RAD51 recombinase plays critical roles in safeguarding genome integrity, which is fundamentally important for all living cells. While interphase functions of RAD51 in maintaining genome stability are well-characterised, its role in mitosis remains contentious. In this study, we show that RAD51 protects under-replicated DNA in mitotic human cells and, in this way, promotes mitotic DNA synthesis (MiDAS) and successful chromosome segregation. In cells experiencing mild replication stress, MiDAS was detected irrespective of mitotically generated DNA damage. MiDAS broadly required de novo RAD51 recruitment to single-stranded DNA, which was supported by the phosphorylation of RAD51 by the key mitotic regulator Polo-like kinase 1. Importantly, acute inhibition of MiDAS delayed anaphase onset and induced centromere fragility, suggesting a mechanism that prevents the satisfaction of the spindle assembly checkpoint while chromosomal replication remains incomplete. This study hence identifies an unexpected function of RAD51 in promoting genomic stability in mitosis.

Transcription-coupled DNA double-strand break repair

The genomic DNA is constantly under attack by cellular and/or environmental factors. Fortunately, the cell is armed to safeguard its genome by various mechanisms such as nucleotide excision, base excision, mismatch and DNA double-strand break repairs. While these processes maintain the integrity of the genome throughout, DNA repair occurs preferentially faster at the transcriptionally active genes. Such transcription-coupled repair phenomenon plays important roles to maintain active genome integrity, failure of which would interfere with transcription, leading to an altered gene expression (and hence cellular pathologies/diseases). Among the various DNA damages, DNA double-strand breaks are quite toxic to the cells. If DNA double-strand break occurs at the active gene, it would interfere with transcription/gene expression, thus threatening cellular viability. Such DNA double-strand breaks are found to be repaired faster at the active gene in comparison to its inactive state or the inactive gene, thus supporting the existence of a new phenomenon of transcription-coupled DNA double-strand break repair. Here, we describe the advances of this repair process.

Guardians of the Genome: BRCA2 and Its Partners

The tumor suppressor BRCA2 functions as a central caretaker of genome stability, and individuals who carry BRCA2 mutations are predisposed to breast, ovarian, and other cancers. Recent research advanced our mechanistic understanding of BRCA2 and its various interaction partners in DNA repair, DNA replication support, and DNA double-strand break repair pathway choice. In this review, we discuss the biochemical and structural properties of BRCA2 and examine how these fundamental properties contribute to DNA repair and replication fork stabilization in living cells. We highlight selected BRCA2 binding partners and discuss their role in BRCA2-mediated homologous recombination and fork protection. Improved mechanistic understanding of how BRCA2 functions in genome stability maintenance can enable experimental evidence-based evaluation of pathogenic BRCA2 mutations and BRCA2 pseudo-revertants to support targeted therapy.

Intrinsic Disorder and Phosphorylation in BRCA2 Facilitate Tight Regulation of Multiple Conserved Binding Events

The maintenance of genome integrity in the cell is an essential process for the accurate transmission of the genetic material. BRCA2 participates in this process at several levels, including DNA repair by homologous recombination, protection of stalled replication forks, and cell division. These activities are regulated and coordinated via cell-cycle dependent modifications. Pathogenic variants in BRCA2 cause genome instability and are associated with breast and/or ovarian cancers. BRCA2 is a very large protein of 3418 amino acids. Most well-characterized variants causing a strong predisposition to cancer are mutated in the C-terminal 700 residues DNA binding domain of BRCA2. The rest of the BRCA2 protein is predicted to be disordered. Interactions involving intrinsically disordered regions (IDRs) remain difficult to identify both using bioinformatics tools and performing experimental assays. However, the lack of well-structured binding sites provides unique functional opportunities for BRCA2 to bind to a large set of partners in a tightly regulated manner. We here summarize the predictive and experimental arguments that support the presence of disorder in BRCA2. We describe how BRCA2 IDRs mediate self-assembly and binding to partners during DNA double-strand break repair, mitosis, and meiosis. We highlight how phosphorylation by DNA repair and cell-cycle kinases regulate these interactions. We finally discuss the impact of cancer-associated variants on the function of BRCA2 IDRs and more generally on genome stability and cancer risk.

To Join or Not to Join: Decision Points Along the Pathway to Double-Strand Break Repair vs. Chromosome End Protection

The regulation of DNA double-strand breaks (DSBs) and telomeres are diametrically opposed in the cell. DSBs are considered one of the most deleterious forms of DNA damage and must be quickly recognized and repaired. Telomeres, on the other hand, are specialized, stable DNA ends that must be protected from recognition as DSBs to inhibit unwanted chromosome fusions. Decisions to join DNA ends, or not, are therefore critical to genome stability. Yet, the processing of telomeres and DSBs share many commonalities. Accordingly, key decision points are used to shift DNA ends toward DSB repair vs. end protection. Additionally, DSBs can be repaired by two major pathways, namely homologous recombination (HR) and non-homologous end joining (NHEJ). The choice of which repair pathway is employed is also dictated by a series of decision points that shift the break toward HR or NHEJ. In this review, we will focus on these decision points and the mechanisms that dictate end protection vs. DSB repair and DSB repair choice.

Understanding BRCA2 Function as a Tumor Suppressor Based on Domain-Specific Activities in DNA Damage Responses

BRCA2 is an essential genome stability gene that has various functions in cells, including roles in homologous recombination, G2 checkpoint control, protection of stalled replication forks, and promotion of cellular resistance to numerous types of DNA damage. Heterozygous mutation of BRCA2 is associated with an increased risk of developing cancers of the breast, ovaries, pancreas, and other sites, thus BRCA2 acts as a classic tumor suppressor gene. However, understanding BRCA2 function as a tumor suppressor is severely limited by the fact that ~70% of the encoded protein has not been tested or assigned a function in the cellular DNA damage response. Remarkably, even the specific role(s) of many known domains in BRCA2 are not well characterized, predominantly because stable expression of the very large BRCA2 protein in cells, for experimental purposes, is challenging. Here, we review what is known about these domains and the assay systems that are available to study the cellular roles of BRCA2 domains in DNA damage responses. We also list criteria for better testing systems because, ultimately, functional assays for assessing the impact of germline and acquired mutations identified in genetic screens are important for guiding cancer prevention measures and for tailored cancer treatments.

A fork in the road: Where homologous recombination and stalled replication fork protection part ways

In response to replication hindrances, DNA replication forks frequently stall and are remodelled into a four-way junction. In such a structure the annealed nascent strand is thought to resemble a DNA double-strand break and remodelled forks are vulnerable to nuclease attack by MRE11 and DNA2. Proteins that promote the recruitment, loading and stabilisation of RAD51 onto single-stranded DNA for homology search and strand exchange in homologous recombination (HR) repair and inter-strand cross-link repair also act to set up RAD51-mediated protection of nascent DNA at stalled replication forks. However, despite the similarities of these pathways, several lines of evidence indicate that fork protection is not simply analogous to the RAD51 loading step of HR. Protection of stalled forks not only requires separate functions of a number of recombination proteins, but also utilises nucleases important for the resection steps of HR in alternative ways. Here we discuss how fork protection arises and how its differences with HR give insights into the differing contexts of these two pathways.

BRCA1 and BRCA2 Tumor Suppressor Function in Meiosis

Meiosis is a specialized cell cycle that results in the production of haploid gametes for sexual reproduction. During meiosis, homologous chromosomes are connected by chiasmata, the physical manifestation of crossovers. Crossovers are formed by the repair of intentionally induced double strand breaks by homologous recombination and facilitate chromosome alignment on the meiotic spindle and proper chromosome segregation. While it is well established that the tumor suppressors BRCA1 and BRCA2 function in DNA repair and homologous recombination in somatic cells, the functions of BRCA1 and BRCA2 in meiosis have received less attention. Recent studies in both mice and the nematode Caenorhabditis elegans have provided insight into the roles of these tumor suppressors in a number of meiotic processes, revealing both conserved and organism-specific functions. BRCA1 forms an E3 ubiquitin ligase as a heterodimer with BARD1 and appears to have regulatory roles in a number of key meiotic processes. BRCA2 is a very large protein that plays an intimate role in homologous recombination. As women with no indication of cancer but carrying BRCA mutations show decreased ovarian reserve and accumulated oocyte DNA damage, studies in these systems may provide insight into why BRCA mutations impact reproductive success in addition to their established roles in cancer.

The phospho-dependent role of BRCA2 on the maintenance of chromosome integrity

Chromosomal instability is a hallmark of cancer. The tumor suppressor protein BRCA2 performs an important role in the maintenance of genome integrity particularly in interphase; as a mediator of homologous recombination DNA repair pathway, it participates in the repair of DNA double-strand breaks, inter-strand crosslinks and replicative DNA lesions. BRCA2 also protects stalled replication forks from aberrant degradation. Defects in these functions lead to structural chromosomal aberrations. BRCA2 is a large protein containing highly disordered regions that are heavily phosphorylated particularly in mitosis. The functions of these modifications are getting elucidated and reveal emerging activities in chromosome alignment, chromosome segregation and abscission during cell division. Defects in these activities result in numerical chromosomal aberrations. In addition to BRCA2, other factors of the DNA damage response (DDR) participate in mitosis in close association with cell cycle kinases and phosphatases suggesting that the maintenance of genome integrity functions of these factors extends beyond DNA repair. Here we will discuss the regulation of BRCA2 functions through phosphorylation by cell cycle kinases particularly in mitosis, and illustrate with some examples how BRCA2 and other DDR proteins partially rewire their interactions, essentially via phosphorylation, to fulfill mitotic specific functions that ensure chromosome stability.

Identification of the core motif of the BRCA2 C-terminal RAD51-binding domain by comparing canine and human BRCA2

Mammary tumors are the most common tumors in women and non-spayed female dogs. One of the reasons for mammary tumors is mutations of the tumor suppressor gene, BRCA2. BRCA2 participates in homologous recombination repair by interacting with the RAD51 recombinase. BRCA2 has two RAD51-binding domains, consisting of BRC repeats and the C-terminal RAD51-binding domain, respectively. Although several studies have addressed the function of the C-terminal RAD51-binding domain of human BRCA2, the amino acid sequences required for the RAD51-interaction activity remain unclear. In this study, the C-terminal RAD51-binding domains of canine and human BRCA2 were compared; the canine domain displayed a weaker interaction with RAD51. This difference was attributed to the C-terminal portion of the domain via a comparison between canine and human domains. Furthermore, peptides shorter than those previously reported displayed RAD51-interacting activity, and a core motif of this domain consisting of 25 amino acids was identified. Since a mutation (S3323N) was reported in the core motif of this domain, the effect of this mutation was evaluated. The mutant exhibited similar RAD51-binding activity as that of the wild-type protein, suggesting that the mutation was functionally neutral. These data suggested that the C-terminal portion of the BRCA2 C-terminal RAD51-binding domain influenced its RAD51-interaction activity, and a minimum core motif of 25 amino acids was identified in this domain. These data may help clarify BRCA2 function, as well as the tumorigenic effects of BRCA2 mutation.

A small-molecule inhibitor of the BRCA2-RAD51 interaction modulates RAD51 assembly and potentiates DNA damage-induced cell death

BRCA2 controls RAD51 recombinase during homologous DNA recombination (HDR) through eight evolutionarily conserved BRC repeats, which individually engage RAD51 via the motif Phe-x-x-Ala. Using structure-guided molecular design, templated on a monomeric thermostable chimera between human RAD51 and archaeal RadA, we identify CAM833, a 529 Da orthosteric inhibitor of RAD51:BRC with a K_d of 366 nM. The quinoline of CAM833 occupies a hotspot, the Phe-binding pocket on RAD51 and the methyl of the substituted α-methylbenzyl group occupies the Ala-binding pocket. In cells, CAM833 diminishes formation of damage-induced RAD51 nuclear foci; inhibits RAD51 molecular clustering, suppressing extended RAD51 filament assembly; potentiates cytotoxicity by ionizing radiation, augmenting 4N cell-cycle arrest and apoptotic cell death and works with poly-ADP ribose polymerase (PARP)1 inhibitors to suppress growth in BRCA2-wildtype cells. Thus, chemical inhibition of the protein-protein interaction between BRCA2 and RAD51 disrupts HDR and potentiates DNA damage-induced cell death, with implications for cancer therapy.

Application of Multilayer Evidence for Annotation of C-Terminal BRCA2 Variants

Simple Summary

The potential pathogenic role of germline BRCA2 c.9976A>T and c.10095delinsGAATTATATCT was evaluated in hereditary breast and ovarian cancer (HBOC) patients by investigating 2491 probands and verified in an independent cohort of 122,209 patients. Although the c.10095delinsGAATTATATCT variant was more prevalent among patients compared to control populations, no increased risk for cancer was found. No association between c.9976A>T and clinicopathological parameters or elevated risk for HBOC cases was detected. However, lung cancer was more prevalent in families carrying c.9976A>T compared to pathogenic BRCA1/BRCA2 carrier families. An increased frequency of pancreatic cancer was found in families where c.9976A>T occurred together with other pathogenic BRCA1 variants. The C-terminal stop codon variants showed no association with other pathogenic BRCA2 variants. No loss of heterozygosity (LOH) in tumor tissue and no allelic imbalance in RNA level were confirmed. The c.9976A>T variant may be considered as a potential risk for lung cancer, and a potential modifying factor in pancreatic cancer when it occurs along with the pathogenic BRCA1 variant, although this observation should be validated in a larger sample cohort.

Abstract

The clinical relevance of the BRCA2 C-terminal stop codon variants is controversial. The pathogenic role of the germline BRCA2 c.9976A>T and c.10095delinsGAATTATATCT variants in hereditary breast and ovarian cancer (HBOC) patients was evaluated. An association with clinicopathological parameters was performed in 2491 independent probands diagnosed with HBOC and in 122,209 cancer patients reported earlier. Loss-of-heterozygosity (LOH) in tumor samples and allelic imbalance in RNA extracted from peripheral blood cells were investigated. Neither c.10095delinsGAATTATATCT or c.9976A>T variants showed significant association with clinicopathological parameters or elevated risk for HBOC-associated tumors. Lung cancer was more prevalent in families carrying the c.9976A>T variant compared to pathogenic BRCA1 or BRCA2 carrier families. An increased prevalence of pancreatic cancer was found in families where c.9976A>T occurred together with other pathogenic BRCA1 variants. An increased risk for familial pancreatic, lung and upper aero-digestive tract cancers was confirmed in the validation set. Regarding BRCA2 C-terminal variants, no linkage with other pathogenic BRCA2 variants, no LOH in tumor tissue and no allelic imbalance in RNA level were confirmed. The c.9976A>T variant may be considered as a potential risk for lung cancer, and a potential modifying factor in pancreatic cancer when it occurs along with the pathogenic BRCA1 variant, although this observation should be validated in a larger sample cohort.

Advances in the Treatment of Ovarian Cancer Using PARP Inhibitors and the Underlying Mechanism of Resistance

The standard treatment for advanced ovarian cancer is cytoreductive surgery followed by cytotoxic chemotherapy. However, it has high risk of recurrence and poor prognosis. Poly(ADPribose) polymerase (PARP) inhibitors selectively target DNA double-strand breaks (DSBs) in tumor cells that cannot be repaired and induce the synthetic lethality of BRCA1/2 mutation cancers. PARP inhibitors are clinically used to treat recurrent ovarian cancer and show significant efficacy in ovarian cancer patients with homologous recombination repair (HRR) pathway defects. PARP inhibitors also have significant clinical benefits in patients without HR defects. With the increasingly extensive clinical application of PARP inhibitors, the possibility of acquiring drug resistance is high. Therefore, clinical strategies should be adopted to manage drug resistance of PARP inhibitors. This study aims to summarize the indications and toxicity of PARP inhibitors, the mechanism of action, targeted treatment of drug resistance, and potential methods to manage drug-resistant diseases. We used the term "ovarian cancer" and the names of each PARP inhibitor as keywords to search articles published in the Medical Subject Headings (MeSH) on Pubmed, along with the keywords "clinicaltrials.gov" and "google.com/patents" as well as "uspto.gov." The FDA has approved olaparib, niraparib, and rucaparib for the treatment of recurrent epithelial ovarian cancer (EOC). Talazoparib and veliparib are currently in early trials and show promising clinical results. The mechanism underlying resistance to PARP inhibitors and the clinical strategies to overcome them remain unclear. Understanding the mechanism of resistance to PARP inhibitors and their relationship with platinum resistance may help with the development of antiresistance therapies and optimization of the sequence of drug application in the future clinical treatment of ovarian cancer.

Rice OsBRCA2 Is Required for DNA Double-Strand Break Repair in Meiotic Cells

The mammalian BREAST CANCER 2 (BRCA2) gene is a tumor suppressor that plays a crucial role in DNA repair and homologous recombination (HR). Here, we report the identification and characterization of OsBRCA2, the rice orthologue of human BRCA2. Osbrca2 mutant plants exhibit normal vegetative growth but experience complete male and female sterility as a consequence of severe meiotic defects. Pairing, synapsis and recombination are impaired in osbrca2 male meiocytes, leading to chromosome entanglements and fragmentation. In the absence of OsBRCA2, localization to the meiotic chromosome axes of the strand-invasion proteins OsRAD51 and OsDMC1 is severely reduced and in vitro OsBRCA2 directly interacts with OsRAD51 and OsDMC1. These results indicate that OsBRCA2 is essential for facilitating the loading of OsRAD51 and OsDMC1 onto resected ends of programmed double-strand breaks (DSB) during meiosis to promote single-end invasions of homologous chromosomes and accurate recombination. In addition, treatment of osbrca2-1 seedlings with mitomycin C (MMC) led to hypersensitivity. As MMC is a genotoxic agent that creates DNA lesions in the somatic cells that can only be repaired by HR, these results suggest that OsBRCA2 has a conserved role in DSB repair and HR in rice.

Making Choices: DNA Replication Fork Recovery Mechanisms

DNA replication is laden with obstacles that slow, stall, collapse, and break DNA replication forks. At each obstacle, there is a decision to be made whether to bypass the lesion, repair or restart the damaged fork, or to protect stalled forks from further demise. Each "decision" draws upon multitude of proteins participating in various mechanisms that allow repair and restart of replication forks. Specific functions for many of these proteins have been described and an understanding of how they come together in supporting replication forks is starting to emerge. Many questions, however, remain regarding selection of the mechanisms that enable faithful genome duplication and how "normal" intermediates in these mechanisms are sometimes funneled into "rogue" processes that destabilize the genome and lead to cancer, cell death, and emergence of chemotherapeutic resistance. In this review we will discuss molecular mechanisms of DNA damage bypass and replication fork protection and repair. We will specifically focus on the key players that define which mechanism is employed including: PCNA and its control by posttranslational modifications, translesion synthesis DNA polymerases, molecular motors that catalyze reversal of stalled replication forks, proteins that antagonize fork reversal and protect reversed forks from nucleolytic degradation, and the machinery of homologous recombination that helps to reestablish broken forks. We will also discuss risks to genome integrity inherent in each of these mechanisms.