Differential Chemotherapeutic Sensitivity for Breast Tumors With “BRCAness”: A Review

BRCA1 or BRCA2 mutations predispose to cancer development, primarily through their loss of role in the repair of DNA double‐strand breaks. They play a key role in homologous recombination repair, which is a conservative, error‐free DNA repair mechanism. When mutated, other alternative, error‐prone mechanisms for DNA repair take over, leading to genomic instability. Somatic mutations are rare in sporadic breast tumors, but expression of BRCA1 and BRCA2 genes can be downregulated in other mechanistic ways. These tumors have similar features in terms of their phenotypic and genotypic profiles, which are normally regulated by these genes, and mutations lead to defective DNA repair capacity, called “BRCAness.” Attempts have been made to exploit this differentially expressed feature between tumors and normal tissues by treatment with DNA‐damaging chemotherapy agents. Cells with this functional BRCA deficiency should be selectively susceptible to DNA‐damaging drugs. Preclinical and early clinical (primarily retrospective) evidence supports this approach. In contrast, there is emerging evidence of relative resistance of tumors containing BRCA1 or BRCA2 mutations (or BRCAness) to taxanes. In this review, we summarize the data supporting differential chemotherapeutic sensitivity on the basis of defective DNA repair. If confirmed with available, clinically applicable techniques, this differential chemosensitivity could lead to treatment choices in breast cancer that have a more individualized biologic basis.     

Bi‐allelic alterations in DNA repair genes underpin homologous recombination DNA repair defects in breast cancer

Homologous recombination (HR) DNA repair‐deficient (HRD) breast cancers have been shown to be sensitive to DNA repair targeted therapies. Burgeoning evidence suggests that sporadic breast cancers, lacking germline BRCA1/BRCA2 mutations, may also be HRD. We developed a functional ex vivo RAD51‐based test to identify HRD primary breast cancers. An integrated approach examining methylation, gene expression, and whole‐exome sequencing was employed to ascertain the aetiology of HRD. Functional HRD breast cancers displayed genomic features of lack of competent HR, including large‐scale state transitions and specific mutational signatures. Somatic and/or germline genetic alterations resulting in bi‐allelic loss‐of‐function of HR genes underpinned functional HRD in 89% of cases, and were observed in only one of the 15 HR‐proficient samples tested. These findings indicate the importance of a comprehensive genetic assessment of bi‐allelic alterations in the HR pathway to deliver a precision medicine‐based approach to select patients for therapies targeting tumour‐specific DNA repair defects. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

BRCA1 and BRCA2 mutations sensitize to chemotherapy in patient-derived pancreatic cancer xenografts

Background:

Germline mutations of the BRCA tumour suppressors have been associated with increased risk of pancreatic cancer. Clinical evidence suggests that these patients may be more sensitive to treatment with cisplatin. As the frequency of germline BRCA mutations is low, definitive experimental data to support the clinical observations are still missing.

Methods:

We tested gemcitabine and cisplatin sensitivity of four BRCA1 and BRCA2 mutant and three BRCA1 and BRCA2 wild-type (WT) patient-derived pancreatic cancer xenografts.

Results:

We observed treatment sensitivity to gemcitabine and cisplatin in the BRCA WT and mutant models. The BRCA1 and BRCA2 mutant xenografts were significantly more sensitive to cisplatin although these models also showed sensitivity to gemcitabine. The BRCA1 and BRCA2 WT models showed sensitivity to gemcitabine but not cisplatin. Treatment sensitivity in the xenograft models closely resembled treatment response in the corresponding patients.

Discussion:

We have characterised a panel of xenografts derived from pancreatic cancer patients carrying germline BRCA mutations, and shown that their genetic features resemble the patient donor. Our results support further clinical testing of treatment regimens combining gemcitabine and platinum drugs in this patient population, as well as preclinical research aiming to identify mechanisms of cisplatin resistance in BRCA mutant pancreatic cancers.     

BRCA1/2‐negative hereditary triple‐negative breast cancers exhibit BRCAness

BRCA1/2‐associated breast cancers are sensitive to poly(ADPribose) polymerase (PARP) inhibitors and platinum compounds mainly due to their deficiency in DNA repair via homologous recombination (HR). However, approximately only 15% of triple‐negative breast cancers (TNBCs) are BRCA1/2‐associated. TNBCs that exhibit BRCAness (a phenotype reflecting impaired HR in BRCA1/2‐negative tumors) are also regarded sensitive to PARP inhibitors and platinum compounds. Thus, we hypothesized that hereditary BRCA1/2‐negative TNBCs may exhibit BRCAness. To find a subset of hereditary BRCA1/2‐negative TNBCs among 360 TNBCs, we first identified a group of 41 hereditary TNBCs by analyzing the family histories of the patients. Next, we tested this group for the presence of germline BRCA1/2 mutations, and finally, we compared the expression levels of 120 genes involved in HR and five other major mechanisms of DNA damage repair between BRCA1/2‐associated and BRCA1/2‐negative subgroups of hereditary TNBCs using real‐time PCR arrays. Approximately 73% of the hereditary TNBCs were BRCA1/2‐associated and 27% were BRCA1/2‐negative. The expression levels of the analyzed genes showed no significant differences between these two subgroups indicating the BRCAness of the BRCA1/2‐negative hereditary TNBCs and thereby distinguishing a novel subset of TNBCs as a potential target for PARP inhibitors or platinum‐based therapy. The results show the significance of family history in selecting patients with TNBC for therapies directed at incompetent DNA repair (e.g., PARP inhibitors and/or platinum‐based therapies) and indicate that a relatively simple strategy for broadening the target group for these modes of treatment is to identify patients with hereditary TNBCs.     

Triple-negative breast cancer: BRCAness and concordance of clinical features with BRCA1-mutation carriers

Background:

BRCAness is defined as shared tumour characteristics between sporadic and BRCA-mutated cancers. However, how to exactly measure BRCAness and its frequency in breast cancer is not known. Assays to establish BRCAness would be extremely valuable for the clinical management of these tumours. We assessed BRCAness characteristics frequencies in a large cohort of triple-negative breast cancers (TNBCs).

Methods:

As a measure of BRCAness, we determined a specific BRCA1-like pattern by array Comparative Genomic Hybridisation (aCGH), and BRCA1 promoter methylation in 377 TNBCs, obtained from 3 different patient cohorts. Clinicopathological data were available for all tumours, BRCA1-germline mutation status and chemotherapy response data were available for a subset.

Results:

Of the tumours, 66–69% had a BRCA1-like aCGH profile and 27–37% showed BRCA1 promoter methylation. BRCA1-germline mutations and BRCA1 promoter methylation were mutually exclusive events (P=1 × 10⁻⁵). BRCAness was associated with younger age and grade 3 tumours. Chemotherapy response was significantly higher in BRCA1-mutated tumours, but not in tumours with BRCAness (63% (12 out of 19) vs 35% (18 out of 52) pathological complete remission rate, respectively).

Conclusion:

The majority of the TNBCs show BRCAness, and those tumours share clinicopathological characteristics with BRCA1-mutated tumours. A better characterisation of TNBC and the presence of BRCAness could have consequences for both hereditary breast cancer screening and the treatment of these tumours.     

Histone deacetylase inhibitor treatment induces ‘BRCAness’ and synergistic lethality with PARP inhibitor and cisplatin against human triple negative breast cancer cells

There is an unmet need to develop new, more effective and safe therapies for the aggressive forms of triple negative breast cancers (TNBCs). While up to 20% of women under 50 years of age with TNBC harbor germline mutations in BRCA1, and these tumors are sensitive to treatment with poly(ADP) ribose polymerase inhibitors, a majority of TNBCs lack BRCA1 mutations or loss of expression. Findings presented here demonstrate that by attenuating the levels of DNA damage response and homologous recombination proteins, pan-histone deacetylase inhibitor (HDI) treatment induces ‘BRCAness’ and sensitizes TNBC cells lacking BRCA1 to lethal effects of PARP inhibitor or cisplatin. Treatment with HDI also induced hyperacetylation of nuclear hsp90. Similar effects were observed following shRNA-mediated depletion of HDAC3, confirming its role as the deacetylase for nuclear HSP90. Furthermore, cotreatment with HDI and ABT-888 induced significantly more DNA strand breaks than either agent alone, and synergistically induced apoptosis of TNBC cells. Notably, co-treatment with HDI and ABT-888 significantly reduced in vivo tumor growth and markedly improved the survival of mice bearing TNBC cell xenografts. These findings support the rationale to interrogate the clinical activity of this novel combination against human TNBC, irrespective of its expression of mutant BRCA1.     

BET proteins regulate homologous recombination-mediated DNA repair: BRCAness and implications for cancer therapy

Bromodomain and Extra-Terminal (BET) proteins are historically involved in regulating gene expression and BRD4 was recently found to be involved in DNA damage regulation. Aims of our study were to assess BRD4 regulation in homologous recombination-mediated DNA repair and to explore novel clinical strategies through the combinations of the pharmacological induction of epigenetic BRCAness in BRCA1 wild-type triple negative breast cancer (TNBC) cells by means of BET inhibitors and compounds already available in clinic. Performing a dual approach (chromatin immunoprecipitation and RNA interference), the direct relationship between BRD4 and BRCA1/RAD51 expression was confirmed in TNBC cells. Moreover, BRD4 pharmacological inhibition using two BET inhibitors (JQ1 and GSK525762A) induced a dose-dependent reduction in BRCA1 and RAD51 levels and is able to hinder homologous recombination-mediated DNA damage repair, generating a BRCAness phenotype in TNBC cells. Furthermore, BET inhibition impaired the ability of TNBC cells to overcome the increase in DNA damage after platinum salts (i.e., CDDP) exposure, leading to massive cell death, and triggered synthetic lethality when combined with PARP inhibitors (i.e., AZD2281). Altogether, the present study confirms that BET proteins directly regulate the homologous recombination pathway and their inhibition induced a BRCAness phenotype in BRCA1 wild-type TNBC cells. Noteworthy, being this strategy based on drugs already available for human use, it is rapidly transferable and could potentially enable clinicians to exploit platinum salts and PARP inhibitors-based treatments in a wider population of TNBC patients and not just in a specific subgroup, after validating clinical trials.     

Inhibition of poly (ADP-Ribose) polymerase: A promising strategy targeting pancreatic cancer with BRCAness phenotype

The use of chemotherapeutic regimens for the treatment of pancreatic cancer is still limited because pancreatic cancer is usually diagnosed at an advanced stage as a refractory disease in which symptoms are difficult to recognize in the early stages. Furthermore, at advanced stages, there are important challenges to achieve clinical benefit and symptom resolution, even with the use of an expanded spectrum of anticancer drugs. Recently, a point of reduced susceptibility to conventional chemotherapies by breast cancer susceptibility gene (BRCA) mutations led to a new perspective for overcoming the resistance of pancreatic cancer within the framework of increased genome instability. Poly (ADP-Ribose) polymerase (PARP) -1 is an enzyme that can regulate intrinsic functions, such as response to DNA damage. Therefore, in an environment where germline mutations in BRCAs (BRCAness) inhibit homologous recombination in DNA damage, resulting in a lack of DNA damage response, a key role of PARP-1 for the adaptation of the genome instability could be further emphasized. Here, we summarized the key functional role of PARP-1 in genomic instability of pancreatic cancer with the BRCAness phenotype and listed clinical applications and outcomes of PARP-1 inhibitors to highlight the importance of targeting PARP-1 activity.     

Targeting deficient DNA damage repair in gastric cancer

Introduction: Over recent years our understanding of DNA damage repair has evolved leading to an expansion of therapies attempting to exploit DNA damage repair deficiencies across multiple solid tumours. Gastric cancer has been identified as a tumour where a subgroup of patients demonstrates deficiencies in the homologous recombination pathway providing a potential novel treatment approach for this poor prognosis disease.

Area covered: This review provides an overview of DNA damage repair and how this has been targeted to date in other tumour types exploiting the concept of synthetic lethality. This is followed by a discussion of how deficiencies in homologous recombination may be identified across tumour types and on recent progress in targeting DNA repair deficiencies in gastric cancer.

Expert opinion: Gastric cancer remains a difficult malignancy to treat and the possibility of targeting deficient DNA repair in a subgroup of patients is an exciting prospect. Future combinations with immunotherapy and radiotherapy are appealing and appear to have a sound biological rationale. However, much work remains to be done to understand the significance of the genetic and epigenetic alterations involved, to elucidate the optimum predictive signatures or biomarkers and to consider means of overcoming treatment resistance.     

BRCAness: finding the Achilles heel in ovarian cancer

Ovarian cancer is the leading cause of death among gynecological cancers. It exhibits great heterogeneity in tumor biology and treatment response. Germline mutations of DNA repair genes BRCA1/2 are the fundamental defects in hereditary ovarian cancer that expresses a distinct phenotype of high response rates to platinum agents, improved disease-free intervals and survival rates, and high-grade serous histology. The term "BRCAness" describes the phenotypic traits that some sporadic ovarian tumors share with tumors in BRCA1/2 germline mutation carriers and reflects similar causative molecular abnormalities. BRCA pathway studies and molecular profiling reveal BRCA-related defects in almost half of the cases of ovarian cancer. BRCA-like tumors are particularly sensitive to DNA-damaging agents (e.g., platinum agents) because of inadequate BRCA-mediated DNA repair mechanisms, such as nucleotide-excision repair and homologous recombination (HR). Additional inhibition of other DNA repair pathways leads to synthetic lethality in HR-deficient cells; this has been employed in the treatment of BRCA-like ovarian tumors with poly(ADP-ribose) polymerase inhibitors with promising results. This article presents a comprehensive review of the relevant literature on the role of BRCAness in ovarian cancer with respect to BRCA function, methods of BRCA epigenetic defect detection and molecular profiling, and the implications of BRCA dysfunction in the treatment of ovarian cancer.