BRCA1 and 2 proteins play integral functions in DNA homologous recombination repair (HRR). In normal cells, the HRR pathway is activated in response to DNA double-stranded breaks (1). In BRCA 1/2-deficient cells, HRR is faulty secondary to loss of BRCA function, and therefore, other more error-prone DNA repair pathways are activated. These less perfect mechanisms are felt to be accountable, in part, for carcinogenesis. Similarly, tumors with defective HRR mechanisms are more susceptible to the direct DNA damaging effects of chemotherapy.

Homologous recombination repair dysfunction can be exploited as a therapeutic strategy by the use of poly (ADP-ribose) polymerase (PARP) inhibitors, which inhibit PARP proteins, most commonly PARP1 and 2. As part of the base excision repair (BER) pathway, PARP1 attaches long polymers of ADP-ribose on itself, so that, XRCC1 and other repair proteins have the ability to rapidly locate single-stranded DNA breaks (2–4). Newer evidence reveals that the exact role of PARP1 in the BER pathway is perhaps more indirect and not yet clearly defined (5). Recent studies have also shown that PARP1 is more versatile, and has been implicated in other DNA repair pathways, such as the non-homologous end-joining (NHEJ) repair pathway (6, 7).

Several mechanisms by which PARP inhibition in HRR-deficient cells lead to cell death have been investigated. Most notably, the concept of synthetic lethality explains combinatory lethal effects of BER and HR repair dysfunction, whereas alone, HR or BER pathway disruptions are not lethal to the tumor cell (8). Additionally, other potential mechanisms have been explored including trapping of inhibited PARP1 at sites of DNA damage preventing other repair proteins access, failure to initiate HRR by PARP-dependent BRCA1 recruitment, and activation of the error-prone NHEJ repair pathway leading to genomic instability and subsequent cell death (9). Knowledge of PARP activity has led to effective treatment strategies for BRCA 1/2-germline mutation related tumors.

BRCA 1/2-mutation related ovarian and breast cancers account for 5–10% of all female ovarian and breast cancers (10, 11). Ovarian cancers in the setting of BRCA 1/2-germline mutations can present with more aggressive, high-grade histologies, but are frequently responsive to chemotherapy, particularly platinum-based regimens, leading to an improved 5 years survival (12). The chemotherapy-sensitive mechanism is felt to be related to the intimate relationship between BRCA 1/2 proteins and defective HRR, as discussed above. Recent studies have demonstrated that women with BRCA-related ovarian cancers fare much better than sporadic ovarian cancers (13–16). A study, published by the National Israeli Study of Ovarian Cancer, showed women with BRCA mutations had a median survival of 55.7 months compared to 37.9 months in sporadic ovarian cancers (p = 0.002) (15). This may be in part explained by the standard use of carboplatin-based therapies for ovarian malignancies as the DNA damage induced by the platinum should be more efficacious in the DNA repair-deficient BRCA-related tumors.

Contrary to the more convincing outcomes in BRCA 1/2-related ovarian cancers, the outcomes of BRCA mutation-related breast cancers are less clear. Women with BRCA1 mutations typically develop breast cancer at an earlier age than BRCA2-related and sporadic breast cancers. BRCA1-related breast cancers tend to also be higher grade, hormone receptor-negative, and HER-2-negative, or “triple negative” (17), and also frequently express a basal phenotype (18–26). Patients with BRCA-mutated breast cancers generally respond to therapy as well as sporadic cancers; however, the risk of second ipsilateral or contralateral primaries may be as high as 3–5% per year, compared to 0.5–1% per year risk, seen in sporadic breast cancers (17). In contrast to ovarian cancer, platinum chemotherapy is not standardly administered to patients with breast cancer. The use of platinum agents has been evaluated in a small series which have demonstrated high efficacy in breast cancer in particular in the setting of a BRCA mutation. Silver et al. evaluated the use of neoadjuvant platinum-containing chemotherapy in patients with triple negative breast cancer (TNBC) (N = 28), and found those more likely to be platinum-sensitive were those with low BRCA1 gene expression (27). Likewise, in BRCA-mutated breast cancer patients who received cisplatin in the neoadjuvant setting showed a high rate of pathologic complete response (pCR) in a small series. Ten of 12 patients achieved pCR (83%). When non-platinum-containing regimens were used, the pCR rate was 14% (28). These studies highlight the rationale to further explore the use of platinum-containing regimens, specifically for patients with TNBC and BRCA mutations.

Triple negative breast cancers account for ~20% of all breast cancers and are associated with an aggressive clinical picture (20, 25, 29). Due to lack of hormone receptor or HER-2 expression, and no other known target for tailored therapy, the only current treatment option is chemotherapy. Over 80% of hereditary BRCA1-mutated cancers are TNBCs. Several studies have investigated a potential role for BRCA1 inactivation in sporadic TNBC given the similar clinical outcomes and histological characteristics among these cancers and hereditary BRCA1-mutated breast cancers. Breast cancers developing in patients with BRCA1 mutations, in addition to frequently being triple negative, also often express basal markers (18–22, 25, 26). Gene microarray expression profiling has shown considerable similarities between BRCA1-mutated tumors and basal tumors (25). This shared phenotype has been termed “BRCAness” (26). What is unknown is whether the basal phenotype is a result of the BRCA loss or if the BRCA loss results in the basal phenotype (6).

Recently, Lehmann and colleagues delved further into the characterization of TNBC. They performed an analysis of gene expression profiles of 587 TNBC cases and identified six separate subtypes of TNBC. These six subtypes were: basal-like 1 (BL1), basal-like 2 (BL2), immunomodulatory (IM), mesenchymal (M), mesenchymal stem-like (MSL), and luminal androgen receptor (LAR) subtype. Additional analysis of TNBC cell lines, representative of each of these identified subsets, revealed differential responses to various therapeutic agents. Both the BL1 and BL2 groups showed increased gene expression involved in DNA damage response, and showed higher response to cisplatin (30). In a follow-up study, Masuda et al. presented neoadjuvant chemotherapy response data in each of the aforementioned TNBC subtypes (31). In 130 TNBC patients, who received standard anthracycline- and taxane-based chemotherapy, the BL1 subtype achieved a pCR most frequently (52%). In contrast, the pCR in the BL2 subtype was 0%. The molecular differences in BL1 and BL2 may explain these differential responses. Specifically, the BL1 subtype involves the cell cycle, DNA replication reactome, and the BRCA pathway, among others, whereas the BL2 subtype involves growth factor, glycolysis, and gluconeogenesis pathways. This work demonstrates that even within “basal-like breast cancer (BLBC),” there may be a great deal of heterogeneity.

Telli and colleagues recently presented a study evaluating gemcitabine, carboplatin, and iniparib, a compound initially believed to have PARP inhibitory effects, in the neoadjuvant treatment of triple negative and BRCA-mutated breast cancer (32). This study demonstrated a pCR of 36% overall, with a pCR in BRCA 1/2-mutation carriers of 47%. Furthermore, patients who were both triple negative and had a BRCA 1/2-mutation, had a pCR of 56%. Although only 10 patients were classified as BL1 or BL2, there were an equal number of responders and non-responders to the neoadjuvant platinum regimen. It is also notable that only one patient classified as basal-like had a known BRCA mutation, whereas, there were BRCA-mutated tumors that were classified as IM, M, MSL, and unspecified (32). Although basal-like TNBC has become nearly synonymous with BRCAness, this study found that the basal-like subtype of TNBC was neither particularly responsive to the treatment combination, nor had a higher number of BRCA-germline mutations. In this study, the homologous recombination deficiency (HRD) score appeared to be more predictive of platinum response, as compared to TNBC intrinsic subtyping (30). The HRD assay has been developed to evaluate for loss of heterozygosity (LOH), which has been shown to be predictive of response to platinum in BRCA-related and sporadic cancers (33). While, this data is hypothesis-generating and thought-provoking, larger, prospective studies will be needed before any formal conclusions can be drawn.

In sporadic basal tumors, there are data that show reduced BRCA1 mRNA expression. It is felt that epigenetic modification of the BRCA gene, such as promoter hypermethylation, is responsible for this (34–36). Interestingly, no tumors showed both BRCA1 mutation and BRCA1 promoter methylation suggesting that these events are mutually exclusive in The Cancer Genome Atlas (TCGA) research network data (37). The association between BRCA1-mutated and BLBCs provides an important rationale to include this frequently encountered patient population in studies geared toward manipulation of the characteristic faulty DNA repair mechanisms in BRCA1-mutated tumors. As we move into an era where genomic analyses of tumors is becoming the norm, it will be important to link the genome, methylome, and proteome to clinical characteristics and outcomes.

Similarly, there are many commonalities among BRCA 1/2-mutated cancers and sporadic epithelial ovarian cancers (EOCs). Although only 5–10% of ovarian cancers are directly attributable to a germline mutation in BRCA1 or 2, there is a growing body of evidence to suggest that additional mechanisms of BRCA dysfunction are involved in the pathogenesis of ovarian cancer (26, 38, 39). One study demonstrated alterations of BRCA1 and/or 2 in up to 82% of examined ovarian cancers (n = 92) (40). Methylation of the BRCA1 promoter has been demonstrated in up to 14% of sporadic breast and up to 30% of sporadic ovarian cancers (26, 35, 41–46). LOH has been described in ovarian tumors and may have multiple possible mechanisms leading to malignancy including co-existing LOH of BRCA1 and p53, and hypermethylation acting in a synergistic fashion (33, 47–51). In contrast, BRCA2 methylation has not been found to be a significant contributor (39, 52). Identifying and manipulating these BRCA-like deficiencies in DNA repair in sporadic ovarian cancers is of great importance and provides rationale for including these patients in clinical trials designed for BRCA-related malignancies.

Another important mechanism of BRCAness in ovarian cancers is the presence of somatic mutations in BRCA1 and 2 (53). Hennessy and colleagues performed BRCA1/2 sequencing on 235 unselected ovarian cancers and found that 19% of the sample had detectable mutations in BRCA1 (N = 31) or BRCA2 (N = 13). In the 28 samples, where germline DNA was also available, 42.9% of the BRCA1 mutations and 28.6% of the BRCA2 mutations were somatic. Of interest, somatic BRCA 1/2-mutations in breast cancer appear to be less frequent. In the TCGA BLBC cohort, about 20% had either germline (N = 12) or somatic (N = 8) BRCA 1/2-mutations. Another study evaluated 77 TNBC samples and only one harbored a somatic BRCA mutation (54). This potentially explains the seemingly higher activity of single agent PARP inhibitors, discussed later, in sporadic ovarian cancer as compared to sporadic TNBC.

The Cancer Genome Atlas network recently published findings again demonstrating the four distinct molecular signatures in breast cancer from diverse genetic and epigenetic alterations: luminal A, luminal B, basal-like, and HER-2 enriched subtypes (55). Strikingly, BLBCs were notably different than the other three subtypes based on comprehensive analyses using multiple platforms. As expected, these cancers also often (80%) lacked expression of ER, PR, and HER-2 identifying as TNBCs. Specifically, most BLBCs showed a high frequency of TP53 deleterious mutations (80%), as well as, loss of RB1 and BRCA1. PIK3CA mutations (~9%) were also a common feature of BLBC. Analyses also highlighted increased MYC activation as a BLBC characteristic.

The BLBC mutation spectrum reported in the TCGA was similar to that identified in previously described serous ovarian cancers (56) and BLBC were more similar to serous ovarian carcinomas than to other subtypes of breast cancer. One gene, in particular, TP53, had a >10% mutation frequency in both basal-like breast and serous ovarian cancers. As well, both tumors when compared to luminal showed increased BRCA1 inactivation, RB1 loss, cyclin E1 amplification, high expression of AKT3, and MYC amplification. These molecular commonalities strongly suggest shared driving events in tumorigenesis, and similarly, show support for shared treatment strategies for TNBCs and high-grade serous ovarian cancers. Of note, p53 mutations have been described to have high frequency in BRCA mutation-related cancers as well (57, 58).

Bryant et al. and Farmer et al. demonstrated synthetic lethality in BRCA2-deficient cells with the use of two different PARP inhibitors (59, 60). PARP inhibitors have also shown efficacy preclinically in cells lacking other HRR proteins, such as RAD51, ATR, ATM, CHK1, and FANCA or FANCC (61). These studies have given basis for clinical trials in both BRCA-deficient cancer populations, as well as, those with malignancies sharing qualities of BRCAness or HRR-deficiency, such as basal-like or TNBC and serous ovarian cancer.

BRCA 1/2-deficient cancers are known to be hypersensitive to platinum agents which are thought account for, in part, better overall prognosis for those patients with BRCA 1/2-germline mutation-related breast and ovarian cancer. Not all patients respond to platinum, however, and indeed, it is likely that the majority of tumors will eventually become platinum-resistant. Additionally, not all patients with BRCA 1/2-germline mutations or those with an expected BRCAness phenotype respond to PARP inhibition. Several mechanisms of resistance to both agents have been hypothesized and are likely to be multifactorial in etiology. Current evidence suggests that secondary mutations occur in the BRCA1 or BRCA2 gene restoring the wild type BRCA 1/2 open reading frame which may provide return of DNA repair through a functional HR pathway. These reversion mutations are thought to lead to platinum resistance, as well as PARP inhibitor resistance (83–87). It is imperative that these secondary mutations are identified to help modulate therapeutic management of these populations. Of interest, PARP inhibitor resistance may, in fact, not affect subsequent therapy response, including subsequent platinum regimens (88).

Poly (ADP-ribose) polymerase inhibitors have shown promising activity as both monotherapy and in combination with cytotoxic chemotherapy in BRCA 1/2-mutated cancers. More recently, this concept has been implicated in sporadic high-grade serous ovarian cancers and TNBCs. Like platinum agents, PARP inhibitors have been efficacious in this population. Published data from the TCGA network further support this therapeutic strategy by showcasing the genomic similarities between high-grade serous ovarian cancers and TNBCs. It may be worthwhile in the future to study new drug therapies in tandem in these two populations. New strategies are needed to combat tumor resistance mechanisms, such as secondary mutations that revert BRCA genes to wild type, to both platinum agents and PARP inhibitors. Future directions for PARP inhibition include when best to use these agents, in what combinations, and precisely, how to define the optimal populations that will get the most benefit.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.