Mutation of breast cancer 2, early onset (BRCA2) has been identified as a vital risk factor for esophageal cancer (EC). To date, several proteins have been reported as BRCA2-interacting proteins and are associated with multiple biological processes. This study's aim was to identify a novel interactive protein of BRCA2 and to explore its functional roles in EC. A yeast two-hybrid screening was performed to identify a novel BRCA2-interacting protein. Glutathione-S-transferase (GST) pull-down analysis was performed to find out how the binding domain of BRCA2 interacts with LIM domains containing 1 (LIMD1). The interaction between LIMD1 and BRCA2 at the endogenous level was confirmed by using coimmunoprecipitation and immunobloting. Furthermore, two different sequences of short hairpin RNAs (shRNAs) against LIMD1 were transfected into the human EC cell line ECA109. Afterward, the effects of LIMD1 suppression on the centrosome localization of BRCA2 and cell division were analyzed using an immunofluorescence microscope. Results showed that LIMD1 was a novel BRCA2-interacting protein, and LIMD1 interacted with the conserved region of BRCA2 (amino acids 2,750-3,094) in vitro. Importantly, after interfering with the protein expression of LIMD1 in ECA109 cells, the centrosome localization of BRCA2 was significantly abolished and abnormal cell division was significantly increased. These results suggested that LIMD1 is a novel BRCA2-interacting protein and is involved in the centrosome localization of BRCA2 and suppression of LIMD1, causing abnormal cell division in EC cells.

The DNA damage response (DDR) is a designation for a number of pathways that protects our DNA from various damaging agents. In normal cells, the DDR is extremely important for maintaining genome integrity, but in cancer cells these mechanisms counteract therapy-induced DNA damage. Inhibition of the DDR could therefore be used to increase the efficacy of anti-cancer treatments. Hyperthermia is an example of such a treatment-it inhibits a sub-pathway of the DDR, called homologous recombination (HR). It does so by inducing proteasomal degradation of BRCA2 -one of the key HR factors. Understanding the precise mechanism that mediates this degradation is important for our understanding of how hyperthermia affects therapy and how homologous recombination and BRCA2 itself function. In addition, mechanistic insight into the process of hyperthermia-induced BRCA2 degradation can yield new therapeutic strategies to enhance the effects of local hyperthermia or to inhibit HR. Here, we investigate the mechanisms driving hyperthermia-induced BRCA2 degradation. We find that BRCA2 degradation is evolutionarily conserved, that BRCA2 stability is dependent on HSP90, that ubiquitin might not be involved in directly targeting BRCA2 for protein degradation via the proteasome, and that BRCA2 degradation might be modulated by oxidative stress and radical scavengers.

BRCA2 encodes a protein with a fundamental role in homologous recombination that is essential for normal development. Carrier status of mutations in BRCA2 is associated with familial breast and ovarian cancer, while bi-allelic BRCA2 mutations can cause Fanconi anemia (FA), a cancer predisposition syndrome with cellular cross-linker hypersensitivity. Cancers associated with BRCA2 mutations can acquire chemo-resistance on relapse. We modeled acquired cross-linker resistance with an FA-derived BRCA2-mutated acute myeloid leukemia (AML) platform. Associated with acquired cross-linker resistance was the expression of a functional BRCA2 protein variant lacking exon 5 and exon 7 (BRCA2ΔE5+7), implying a role for BRCA2 splicing for acquired chemo-resistance. Integrated network analysis of transcriptomic and proteomic differences for phenotyping of BRCA2 disruption infers impact on transcription and chromatin remodeling in addition to the DNA damage response. The striking overlap with transcriptional profiles of FA patient hematopoiesis and BRCA mutation associated ovarian cancer helps define and explicate the 'BRCAness' profile.

DNA damage, malfunctions in DNA repair, and genomic instability are processes that intersect at the crossroads of carcinogenesis. Underscoring the importance of DNA repair in breast and ovarian tumorigenesis is the familial inherited cancer predisposition gene BRCA2. The role of BRCA2 in DNA double-strand break repair was first revealed based on its interaction with RAD51, a central player in homologous recombination. The RAD51 protein forms a nucleoprotein filament on single-stranded DNA, invades a DNA duplex, and initiates a search for homology. Once a homologous DNA sequence is found, the DNA is used as a template for the high-fidelity repair of the DNA break. Many of the biochemical features that allow BRCA2 to choreograph the activities of RAD51 have been elucidated and include: targeting RAD51 to single-stranded DNA while inhibiting binding to dsDNA, reducing the ATPase activity of RAD51, and facilitating the displacement of the single-strand DNA binding protein, Replication Protein A. These reinforcing activities of BRCA2 culminate in the correct positioning of RAD51 onto a processed DNA double-strand break and initiate its faithful repair by homologous recombination. In this review, I will address current biochemical data concerning the BRCA2 protein and highlight unanswered questions regarding BRCA2 function in homologous recombination and cancer.

The meiosis-specific synaptonemal complex protein SYCP3 has been reported to be aberrantly expressed in tumours. However, in contrast to its well-defined function in meiosis, its possible role in mitotic cells is entirely unknown. Here, we show that SYCP3 is expressed in a range of primary tumours and that it impairs chromosomal integrity in mitotic cells. Expression of SYCP3 inhibits the homologous recombination (HR) pathway mediated by RAD51, inducing hypersensitivity to DNA-damaging agents such as a poly(ADP-ribose) polymerase (PARP) inhibitor and chromosomal instability. SYCP3 forms a complex with BRCA2 and inhibits its role in HR. These findings highlight a new mechanism for chromosomal instability in cancer and extend the range of PARP-inhibitor sensitive tumours to those expressing SYCP3.

Missense (amino-acid changing) variants found in cancer predisposition genes often create difficulties when clinically interpreting genetic testing results. Although bioinformatics has developed approaches to predicting the impact of these variants, many of these approaches have not been readily applicable in the clinical setting. Bioinformatics approaches for predicting the impact of these variants have not yet found their footing in clinical practice because 1) interpreting the medical relevance of predictive scores is difficult; 2) the relationship between bioinformatics "predictors" (sequence conservation, protein structure) and cancer susceptibility is not understood.

RAD51 is a recombinase involved in the homologous recombination of double-strand breaks in DNA. RAD51 forms oligomers by binding to another molecule of RAD51 via an 'FxxA' motif, and the same recognition sequence is similarly utilised to bind BRCA2. We have tabulated the effects of mutation of this sequence, across a variety of experimental methods and from relevant mutations observed in the clinic. We use mutants of a tetrapeptide sequence to probe the binding interaction, using both isothermal titration calorimetry and X-ray crystallography. Where possible, comparison between our tetrapeptide mutational study and the previously reported mutations is made, discrepancies are discussed and the importance of secondary structure in interpreting alanine scanning and mutational data of this nature is considered.

PALB2 (partner and localizer of BRCA2) was initially identified as a binding partner of BRCA2. It interacts also with BRCA1 forming a complex promoting DNA repair by homologous recombination. Germline pathogenic variants in BRCA1, BRCA2 and PALB2 DNA repair genes are associated with high risk of developing breast cancer. Mutation screening in these breast cancer predisposition genes is routinely performed and allows the identification of individuals who carry pathogenic variants and are at risk of developing the disease. However, variants of uncertain significance (VUSs) are often detected and establishing their pathogenicity and clinical relevance remains a central challenge for the risk assessment of the carriers and the clinical decision-making process. Many of these VUSs are missense variants leading to single amino acid substitutions, whose impact on protein function is uncertain. Typically, VUSs are rare and due to the limited genetic, clinical, and pathological data the multifactorial approaches used for classification cannot be applied. Thus, these variants can only be characterized through functional analyses comparing their effect with that of normal and mutant gene products used as positive and negative controls. The two missense variants BRCA2:c.91T >G (p.Trp31Gly) and PALB2:c.3262C >T (p.Pro1088Ser) were detected in two breast cancer probands originally ascertained at Breast Cancer Units of Institutes located in Milan and Bergamo (Northern Italy), respectively. These variants were located in the BRCA2-PALB2 interacting domains, were predicted to be deleterious by in silico analyses, and were very rare and clinically not classified. Therefore, we initiate to study their functional effect by exploiting a green fluorescent protein (GFP)-reassembly in vitro assay specifically designed to test the BRCA2-PALB2 interaction. This functional assay proved to be easy to develop, robust and reliable. It also allows testing variants located in different genes. Results from these functional analyses showed that the BRCA2:p.Trp31Gly and the PALB2:p.Pro1088Ser prevented the BRCA2-PALB2 binding. While caution is warranted when the interpretation of the clinical significance of rare VUSs is based on functional studies only, our data provide initial evidences in favor of the possibility that these variants are pathogenic.

Cancer cells exhibit HR defects, increased proliferation and checkpoint aberrations. Tumour suppressor proteins, BRCA2 and p53 counteract such aberrant proliferation by checkpoint regulation. Intriguingly, chemo-resistant cancer cells, exhibiting mutated BRCA2 and p53 protein survive even with increased DNA damage accumulation. Such cancer cells show upregulation of RAD52 tumour suppressor protein implying that RAD52 might be providing survival advantage to cancer cells. To understand this paradoxical condition of a tumour suppressor protein facilitating cancer cell survival, in the current study, we investigate the role of RAD52 overexpression in BRCA2 deficient cells. We provide evidence that RAD52 protein alleviates HR inhibition imposed by p53 in BRCA2 deficient cells. In addition, we study the role of RAD52 protein during short replication stress in BRCA2 deficient cells. BRCA2 deficient cells exhibit excessive origin firing and checkpoint evasion in the presence of prevailing DNA damage. Interestingly, overexpression of RAD52 rescues the excessive origin firing and checkpoint defects observed in BRCA2 deficient cells, indicating RAD52 protein compensates for the loss of BRCA2 function. We show that RAD52 protein, just as BRCA2, interacts with pCHK1 checkpoint protein and helps maintain the checkpoint control in BRCA2 deficient cells during DNA damage response.

BRCA2 localizes to centrosomes between G1 and prophase and is removed from the centrosomes during mitosis, but the underlying mechanism is not clear. Here we show that BRCA2 is cleaved into two fragments by membrane type-1 matrix metalloproteinase (MT1-MMP), and that knockdown of MT1-MMP prevents the removal of BRCA2 from centrosomes during metaphase. Mass spectrometry mapping revealed that the MT1-MMP cleavage site of human BRCA2 is between Asn-2135 and Leu-2136 ((2132)LSNN/LNVEGG(2141)), and the point mutation L2136D abrogated MT1-MMP cleavage. Our data demonstrate that MT1-MMP proteolysis of BRCA2 regulates the abundance of BRCA2 on centrosomes.

The BRCA2 gene encodes a large multidomain protein of 3418 residues. Studies to elucidate the mechanisms by which BRCA2 prevents tumour formation have been largely restricted by the size of the protein. Based on secondary structure predictions we have cloned regions across the BRCA2 gene and determined the solubility of the proteins they encode. The fragment consisting of amino acids 290-456 BRCA2 was found predominantly in the soluble portion of the cell lysate and was purified by ion exchange and nickel-NTA affinity chromatography. CD spectroscopy revealed secondary structure elements consistent with a folded peptide and limited proteolysis was used to identify a potential novel domain.

The breast cancer suppressor BRCA2 controls the recombinase RAD51 in the reactions that mediate homologous DNA recombination, an essential cellular process required for the error-free repair of DNA double-stranded breaks. The primary mode of interaction between BRCA2 and RAD51 is through the BRC repeats, which are ∼35 residue peptide motifs that interact directly with RAD51 in vitro. Human BRCA2, like its mammalian orthologues, contains 8 BRC repeats whose sequence and spacing are evolutionarily conserved. Despite their sequence conservation, there is evidence that the different human BRC repeats have distinct capacities to bind RAD51. A previously published crystal structure reports the structural basis of the interaction between human BRC4 and the catalytic core domain of RAD51. However, no structural information is available regarding the binding of the remaining seven BRC repeats to RAD51, nor is it known why the BRC repeats show marked variation in binding affinity to RAD51 despite only subtle sequence variation. To address these issues, we have performed fluorescence polarisation assays to indirectly measure relative binding affinity, and applied computational simulations to interrogate the behaviour of the eight human BRC-RAD51 complexes, as well as a suite of BRC cancer-associated mutations. Our computational approaches encompass a range of techniques designed to link sequence variation with binding free energy. They include MM-PBSA and thermodynamic integration, which are based on classical force fields, and a recently developed approach to computing binding free energies from large-scale quantum mechanical first principles calculations with the linear-scaling density functional code onetep. Our findings not only reveal how sequence variation in the BRC repeats directly affects affinity with RAD51 and provide significant new insights into the control of RAD51 by human BRCA2, but also exemplify a palette of computational and experimental tools for the analysis of protein-protein interactions for chemical biology and molecular therapeutics.

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.

Cancer-causing missense mutations in the 3418 amino acid BRCA2 breast and ovarian cancer suppressor protein frequently affect a short (∼340 residue) segment in its carboxyl-terminal domain (DBD). Here, we identify a shared molecular mechanism underlying their pathogenicity. Pathogenic BRCA2 missense mutations cluster in the DBD's helical domain (HD) and OB1-fold motifs, which engage the partner protein DSS1. Pathogenic - but not benign - DBD mutations weaken or abolish DSS1-BRCA2 assembly, provoking mutant BRCA2 oligomers that are excluded from the cell nucleus, and disable DNA repair by homologous DNA recombination (HDR). DSS1 inhibits the intracellular oligomerization of wildtype, but not mutant, forms of BRCA2. Remarkably, DSS1 expression corrects defective HDR in cells bearing pathogenic BRCA2 missense mutants with weakened, but not absent, DSS1 binding. Our findings identify a DSS1-mediated intracellular protein assembly mechanism that is disrupted by cancer-causing BRCA2 missense mutations, and suggest an approach for its therapeutic correction.

The purpose of the present study was to investigate the clinical significance of BRCA1/BRCA2 DNA repair associated (BRCA1/BRCA2) gene expression in patients with sporadic gastric cancer (GC) who had received postoperative adjuvant chemotherapy. Breast cancer type 1 and 2 susceptibility protein (BRCA1 and BRCA2) expression and BRCA1/BRCA2 mRNA expression were evaluated using immunohistochemistry (IHC) and in-situ hybridization (ISH) on tissue GC microarray tissues, in addition to reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The results were analyzed for clinicopathological associations. A total of 367 cases of sporadic GC (stages II and III) were subjected to BRCA1 and BRCA2 expression analysis, and for BRCA1 and BRCA2 IHC, 360 cases were informative. A total of 61 cases (16.9%) displayed a loss of BRCA1 and 63 (17.5%) displayed a loss of BRCA2. BRCA1 and BRCA2 ISH results were obtained in 364 cases, of which 98 (26.9%) presented with low expression of BRCA1 mRNA and 148 (40.7%) with low expression of BRCA2 mRNA. In 72 of the 367 cases, BRCA1 and BRCA2 mRNA expression levels were assessed using RT-qPCR, of which 50 (69.4%) and 56 (77.8%) displayed low expression of BRCA1 and BRCA2, respectively. Positive IHC expression of BRCA2 was associated with advanced tumor stage; however, BRCA1 expression was not associated with any clinicopathological parameters. Associations between the RT-qPCR and ISH methods were not significant for either BRCA1 or BRCA2. The results of Kaplan-Meier survival analysis with stage subgrouping revealed no significant differences with regard to survival rate. Of the multivariate analyses, neither BRCA1 nor BRCA2 IHC results were independent prognostic factors. In summary, the present study indicated that BRCA1 and BRCA2, as assessed by IHC, may be used as clinicopathological biomarkers to evaluate the prognosis of sporadic GC.

Cis-acting regulatory single nucleotide polymorphisms (SNPs) at specific loci may modulate penetrance of germline mutations at the same loci by introducing different levels of expression of the wild-type allele. We have previously reported that BRCA2 shows differential allelic expression and we hypothesize that the known variable penetrance of BRCA2 mutations might be associated with this mechanism.

BRCA2 and CDKN1A(p21,CIP1)-interacting protein (BCCIP) is an evolutionary conserved protein implicated in maintenance of genome stability and cell cycle progression. Two isoforms of BCCIP with distinct C-terminal domains exist in humans. We show that mammalian BCCIPβ, but not BCCIPα, forms a ternary complex with the ribosomal protein RPL23/uL14 and the pre-60S trans-acting factor eIF6. Complex formation is dependent on an intact C-terminal domain of BCCIPβ. Depletion of BCCIPβ reduces the pool of free RPL23, and decreases eIF6 levels in nucleoli. Overexpression of BCCIPβ leads to nucleoplasmic accumulation of extra-ribosomal RPL23 and stabilizes overexpressed RPL23, suggesting that BCCIPβ functions as nuclear chaperone for RPL23.

The BRCA2 tumor suppressor functions in repair of DNA by homologous recombination through regulating the action of Rad51. In turn, BRCA2 appears to be regulated by other interacting proteins. Dss1, a small interacting protein that binds to the C-terminal domain, has a profound effect on activity as deduced from studies on the BRCA2-related protein Brh2 in Ustilago maydis. Evidence accumulating in mammalian systems suggests that BCCIP, another small interacting protein that binds to the C-terminal domain of BRCA2, also serves to regulate homologous recombination activity. Here we were interested in testing the role of the putative U. maydis BCCIP ortholog Bcp1 in DNA repair and recombination. In keeping with the mammalian paradigm, Bcp1 bound to the C-terminal region of Brh2. Mutants deleted of the gene were extremely slow growing, showed a delay passing through S phase and exhibited sensitivity to hydroxyurea, but were otherwise normal in DNA repair and homologous recombination. In the absence of Bcp1 cells were unable to maintain the wild type morphology when challenged by a DNA replication stress. These results suggest that Bcp1 could be involved in coordinating morphogenetic events with DNA processing during replication.

The BCCIP (BRCA2- and CDKN1A-interacting protein) is an important cofactor for BRCA2 in tumor suppression. Although the low expression of BCCIP is observed in multiple clinically diagnosed primary tumor tissues such as ovarian cancer, renal cell carcinoma and colorectal carcinoma, the mechanism of how BCCIP is regulated in cells is still unclear. The human INO80/YY1 chromatin remodeling complex composed of 15 subunits catalyzes ATP-dependent sliding of nucleosomes along DNA. Here, we first report that BCCIP is a novel target gene of the INO80/YY1 complex by presenting a series of experimental evidence. Gene expression studies combined with siRNA knockdown data locked candidate genes including BCCIP of the INO80/YY1 complex. Silencing or over-expressing the subunits of the INO80/YY1 complex regulates the expression level of BCCIP both in mRNA and proteins in cells. Also, the functions of INO80/YY1 complex in regulating the transactivation of BCCIP were confirmed by luciferase reporter assays. Chromatin immunoprecipitation (ChIP) experiments clarify the enrichment of INO80 and YY1 at +0.17 kb downstream of the BCCIP transcriptional start site. However, this enrichment is significantly inhibited by either knocking down INO80 or YY1, suggesting the existence of both INO80 and YY1 is required for recruiting the INO80/YY1 complex to BCCIP promoter region. Our findings strongly indicate that BCCIP is a potential target gene of the INO80/YY1 complex.

The tumor suppressor FANCD1/BRCA2 is crucial for DNA homologous recombination repair (HRR). BRCA2 biallelic pathogenic variants result in a severe form of Fanconi anemia (FA) syndrome, whereas monoallelic pathogenic variants cause mainly hereditary breast and ovarian cancer predisposition. For decades, the co-occurrence in trans with a clearly pathogenic variant led to assume that the other allele was benign. However, here we show a patient with biallelic BRCA2 (c.1813dup and c.7796 A > G) diagnosed at age 33 with FA after a hypertoxic reaction to chemotherapy during breast cancer treatment. After DNA damage, patient cells displayed intermediate chromosome fragility, reduced survival, cell cycle defects, and significantly decreased RAD51 foci formation. With a newly developed cell-based flow cytometric assay, we measured single BRCA2 allele contributions to HRR, and found that expression of the missense allele in a BRCA2 KO cellular background partially recovered HRR activity. Our data suggest that a hypomorphic BRCA2 allele retaining 37-54% of normal HRR function can prevent FA clinical phenotype, but not the early onset of breast cancer and severe hypersensitivity to chemotherapy.