Inhibited, trapped or adducted: the optimal selective synthetic lethal mix for BRCAness

Structure-specific nucleases in genome dynamics and strategies for targeting cancers

Nucleases are a super family of enzymes that hydrolyze phosphodiester bonds present in genomes. They widely vary in substrates, causing differentiation in cleavage patterns and having a diversified role in maintaining genetic material. Through cellular evolution of prokaryotic to eukaryotic, nucleases become structure-specific in recognizing its own or foreign genomic DNA/RNA configurations as its substrates, including flaps, bubbles, and Holliday junctions. These special structural configurations are commonly found as intermediates in processes like DNA replication, repair, and recombination. The structure-specific nature and diversified functions make them essential to maintaining genome integrity and evolution in normal and cancer cells. In this article, we review their roles in various pathways, including Okazaki fragment maturation during DNA replication, end resection in homology-directed recombination repair of DNA double-strand breaks, DNA excision repair and apoptosis DNA fragmentation in response to exogenous DNA damage, and HIV life cycle. As the nucleases serve as key points for the DNA dynamics, cellular apoptosis, and cancer cell survival pathways, we discuss the efforts in the field in developing the therapeutic regimens, taking advantage of recently available knowledge of their diversified structures and functions.

Clinical BRCA1/2 Reversion Analysis Identifies Hotspot Mutations and Predicted Neoantigens Associated with Therapy Resistance

Reversion mutations in BRCA1 or BRCA2 are associated with resistance to PARP inhibitors and platinum. To better understand the nature of these mutations, we collated, codified, and analyzed more than 300 reversions. This identified reversion "hotspots" and "deserts" in regions encoding the N and C terminus, respectively, of BRCA2, suggesting that pathogenic mutations in these regions may be at higher or lower risk of reversion. Missense and splice-site pathogenic mutations in BRCA1/2 also appeared less likely to revert than truncating mutations. Most reversions were <100 bp deletions. Although many deletions exhibited microhomology, this was not universal, suggesting that multiple DNA-repair processes cause reversion. Finally, we found that many reversions were predicted to encode immunogenic neopeptides, suggesting a route to the treatment of reverted disease. As well as providing a freely available database for the collation of future reversion cases, these observations have implications for how drug resistance might be managed in BRCA-mutant cancers. SIGNIFICANCE: Reversion mutations in BRCA genes are a major cause of clinical platinum and PARP inhibitor resistance. This analysis of all reported clinical reversions suggests that the position of BRCA2 mutations affects the risk of reversion. Many reversions are also predicted to encode tumor neoantigens, providing a potential route to targeting resistance.This article is highlighted in the In This Issue feature, p. 1426.

Controversial issues in the neoadjuvant treatment of triple-negative breast cancer

Triple-negative breast cancer (TNBC), as a collective group of heterogenous tumours, displays the highest rate of distant recurrence and lowest survival from metastatic disease across breast cancer subtypes. However, a subset of TNBC display impressive primary tumour response to neoadjuvant chemotherapy, translating to reduction in future relapse and increased overall survival. Maximizing early treatment response is crucial to improving the outlook in this subtype. Numerous systemic therapy strategies are being assessed in the neoadjuvant setting and the current paradigm of generic chemotherapy components in regimens for high-risk breast cancers, regardless of biological subtype, is changing. Therapeutic approaches with evidence of benefit include platinum drugs, polyadenosine diphosphate ribose polymerase (PARP) inhibitors, immunotherapy and second adjuvant therapy for those not achieving pathological complete response. Importantly, molecular testing can identify subgroups within TNBC, such as deoxyribonucleic acid (DNA) homologous recombination repair deficiency, lymphocyte-predominant tumours, and TNBC type 4 molecular subtypes. Clinical trials that address the interaction between these biomarkers and treatment approaches are a priority, to identify subgroups benefiting from additional therapy.

Triple-negative breast cancer: recent treatment advances

Triple-negative breast cancer (TNBC) is a breast cancer subtype renowned for its capacity to affect younger women, metastasise early despite optimal adjuvant treatment and carry a poor prognosis. Neoadjuvant therapy has focused on combinations of systemic agents to optimise pathological complete response. Treatment algorithms now guide the management of patients with or without residual disease, but metastatic TNBC continues to harbour a poor prognosis. Innovative, multi-drug combination systemic therapies in the neoadjuvant and adjuvant settings have led to significant improvements in outcomes, particularly over the past decade. Recently published advances in the treatment of metastatic TNBC have shown impressive results with poly (ADP-ribose) polymerase (PARP) inhibitors and immunotherapy agents. Immunotherapy agents in combination with traditional systemic chemotherapy have been shown to alter the natural history of this devastating condition, particularly in patients whose tumours are positive for programmed cell death ligand 1 (PD-L1).

Exploiting DNA repair defects in breast cancer: from chemotherapy to immunotherapy

Introduction: Impaired DNA damage response (DDR) and subsequent genomic instability are associated with the carcinogenic process itself, but it also results in sensitivity of tumor cells to certain drugs and can be exploited to treat cancer by inducing deadly mutations or mitotic catastrophe. Exploiting DDR defects in breast cancer cells has been one of the main strategies in both conventional chemotherapy, targeted therapies, or immunotherapies. Areas covered: In this review, the authors first discuss DDR mechanisms in healthy cells and DDR defects in breast cancer, then focus on current therapies and developments in the treatment of DDR-deficient breast cancer. Expert opinion: Among conventional chemotherapeutics, platinum-based regimens, in particular, seem to be effective in DDR-deficient patients. PARP inhibitors represent one of the successful models of translational research in this area and clinical data showed high efficacy and reasonable toxicity with these agents in patients with breast cancer and BRCA mutation. Recent studies have underlined that some subtypes of breast cancer are highly immunogenic. Promising activity has been shown with immunotherapeutic agents, particularly in DDR-deficient breast cancers. Chemotherapeutics, DNA-repair pathway inhibitors, and immunotherapies might result in further improved outcomes in certain subsets of patients with breast cancer and DDR.