Persistent double strand break accumulation does not precede cell death in an Olaparib-sensitive BRCA-deficient colorectal cancer cell model

Systemic DNA Damage and Repair Activity Vary by Race in Breast Cancer Survivors

Non-Hispanic Black breast cancer survivors have poorer outcomes and higher mortality rates than White survivors, but systemic biological mechanisms underlying these disparities are unclear. We used circulating leukocytes as a surrogate for measuring systemic mechanisms, which might be different from processes in the target tissue (e.g., breast). We investigated race-based differences in DNA damage and repair, using a novel CometChip assay, in circulating leukocytes from breast cancer survivors who had completed primary cancer therapy and were cancer free. We observed novel race-based differences in systemic DNA damage and repair activity in cancer survivors, but not in cells from healthy volunteers. Basal DNA damage in leukocytes was higher in White survivors, but Black survivors showed a much higher induction after bleomycin treatment. Double-strand break repair activity was also significantly different between the races, with cells from White survivors showing more sustained repair activity compared to Black leukocytes. These results suggest that cancer and cancer therapy might have long-lasting effects on systemic DNA damage and repair mechanisms that differ in White survivors and Black survivors. Findings from our preliminary study in non-cancer cells (circulating leukocytes) suggest systemic effects beyond the target site, with implications for accelerated aging-related cancer survivorship disparities.

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.

The Influence of PARP, ATR, CHK1 Inhibitors on Premature Mitotic Entry and Genomic Instability in High-Grade Serous BRCAMUT and BRCAWT Ovarian Cancer Cells

Olaparib is a poly (ADP-ribose) polymerase inhibitor (PARPi) that inhibits PARP1/2, leading to replication-induced DNA damage that requires homologous recombination repair. Olaparib is often insufficient to treat BRCA-mutated (BRCA^MUT) and BRCA wild-type (BRCA^WT) high-grade serous ovarian carcinomas (HGSOCs). We examined the short-term (up to 48 h) efficacy of PARPi treatment on a DNA damage response pathway mediated by ATR and CHK1 kinases in BRCA^MUT (PEO-1) and BRCA^WT (SKOV-3 and OV-90) cells. The combination of ATRi/CHK1i with PARPi was not more cytotoxic than ATR and CHK1 monotherapy. The combination of olaparib with inhibitors of the ATR/CHK1 pathway generated chromosomal abnormalities, independent on BRCA^MUT status of cells and formed of micronuclei (MN). However, the beneficial effect of the PARPi:ATRi combination on MN was seen only in the PEO1 BRCA^MUT line. Monotherapy with ATR/CHK1 inhibitors reduced BrdU incorporation due to a slower rate of DNA synthesis, which resulted from elevated levels of replication stress, while simultaneous blockade of PARP and ATR caused beneficial effects only in OV-90 cells. Inhibition of ATR/CHK1 increased the formation of double-strand breaks as measured by increased γH2AX expression at collapsed replication forks, resulting in increased levels of apoptosis. Our findings indicate that ATR and CHK1 inhibitors provoke premature mitotic entry, leading to genomic instability and ultimately cell death.

Fanconi Anemia Pathway in Colorectal Cancer: A Novel Opportunity for Diagnosis, Prognosis and Therapy

Colorectal cancer (CRC) is the third most commonly diagnosed malignancy and has the second highest mortality rate globally. Thanks to the advent of next-generation sequencing technologies, several novel candidate genes have been proposed for CRC susceptibility. Germline biallelic mutations in one or more of the 22 currently recognized Fanconi anemia (FA) genes have been associated with Fanconi anemia disease, while germline monoallelic mutations, somatic mutations, or the promoter hypermethylation of some FANC genes increases the risk of cancer development, including CRC. The FA pathway is a substantial part of the DNA damage response system that participates in the repair of DNA inter-strand crosslinks through homologous recombination (HR) and protects genome stability via replication fork stabilization, respectively. Recent studies revealed associations between FA gene/protein tumor expression levels (i.e., FANC genes) and CRC progression and drug resistance. Moreover, the FA pathway represents a potential target in the CRC treatment. In fact, FANC gene characteristics may contribute to chemosensitize tumor cells to DNA crosslinking agents such as oxaliplatin and cisplatin besides exploiting the synthetic lethal approach for selective targeting of tumor cells. Hence, this review summarizes the current knowledge on the function of the FA pathway in DNA repair and genomic integrity with a focus on the FANC genes as potential predisposition factors to CRC. We then introduce recent literature that highlights the importance of FANC genes in CRC as promising prognostic and predictive biomarkers for disease management and treatment. Finally, we represent a brief overview of the current knowledge around the FANC genes as synthetic lethal therapeutic targets for precision cancer medicine.

BRCA mutations and gastrointestinal cancers: When to expect the unexpected?

BRCA1/2 pathogenic variants are widely known as major risk factors mainly for breast and ovarian cancer, while their role in gastrointestinal (GI) malignancies such as colorectal cancer (CRC), gastric cancer and oesophageal cancer (OeC) is still not well established. The main objective of this review is to summarise the available evidence on this matter. The studies included in the review were selected from PubMed/GoogleScholar/ScienceDirect databases to identify published articles where BRCA1/2 pathogenic variants were assessed either as a risk factor or a prognostic/predictive factor in these malignancies. Our review suggests that BRCA1/2 might have a role as a risk factor for colorectal, gastric and OeC, albeit with differences among these diseases: In particular BRCA1 seems to be much more frequently mutated in CRC whereas BRCA2 appears to be much more closely associated with gastric and OeC. Early-onset cancer seems to be also associated with BRCA1/2 mutations and a few studies suggest a positive prognostic role of these mutations. The assessment of a potentially predictive role of these mutations is hampered by the fact that most patients with these diseases have been treated with platinum compounds, where it is expected that a higher probability of response should be seen. A few clinical trials focused on poly (ADP-ribose) polymerase inhibitors use in GI cancers are currently ongoing.

Unscheduled MRE11 activity triggers cell death but not chromosome instability in polymerase eta-depleted cells subjected to UV irradiation

The elimination of DNA polymerase eta (pol η) causes discontinuous DNA elongation and fork stalling in UV-irradiated cells. Such alterations in DNA replication are followed by S-phase arrest, DNA double-strand break (DSB) accumulation, and cell death. However, their molecular triggers and the relative timing of these events have not been fully elucidated. Here, we report that DSBs accumulate relatively early after UV irradiation in pol η-depleted cells. Despite the availability of repair pathways, DSBs persist and chromosome instability (CIN) is not detectable. Later on cells with pan-nuclear γH2AX and massive exposure of template single-stranded DNA (ssDNA), which indicate severe replication stress, accumulate and such events are followed by cell death. Reinforcing the causal link between the accumulation of pan-nuclear ssDNA/γH2AX signals and cell death, downregulation of RPA increased both replication stress and the cell death of pol η-deficient cells. Remarkably, DSBs, pan-nuclear ssDNA/γH2AX, S-phase arrest, and cell death are all attenuated by MRE11 nuclease knockdown. Such results suggest that unscheduled MRE11-dependent activities at replicating DNA selectively trigger cell death, but not CIN. Together these results show that pol η-depletion promotes a type of cell death that may be attractive as a therapeutic tool because of the lack of CIN.

Therapeutic opportunities for PLK1 inhibitors: Spotlight on BRCA1-deficiency and triple negative breast cancers

Polo-Like Kinases (PLKs) are central players of mitotic progression in Eukaryotes. Given the intimate relationship between cell cycle progression and cancer development, PLKs in general and PLK1 in particular have been thoroughly studied as biomarkers and potential therapeutic targets in oncology. The oncogenic properties of PLK1 overexpression across different types of human cancers are attributed to its roles in promoting mitotic entry, centrosome maturation, spindle assembly and cytokinesis. While several academic labs and pharmaceutical companies were able to develop potent and selective inhibitors of PLK1 (PLK1i) for preclinical research, such compounds have reached only limited success in clinical trials despite their great pharmacokinetics. Even though this could be attributed to multiple causes, the housekeeping roles of PLK1 in both normal and cancer cells are most likely the main reason for clinical trials failure and withdraw due to toxicities issues. Therefore, great efforts are being invested to position PLK1i in the treatment of specific types of cancers with revised dosages schemes. In this mini review we focus on two potential niches for PLK1i that are supported by recent evidence: triple negative breast cancers (TNBCs) and BRCA1-deficient cancers. On the one hand, we recollect several lines of strong evidence indicating that TNBCs are among the cancers with highest PLK1 expression and sensitivity to PLK1i. These findings are encouraging because of the limited therapeutics options available for TNBC patients, which rely mainly on classic chemotherapy. On the other hand, we discuss recent evidence that unveils synthetic lethality induction by PLK1 inhibition in BRCA1-deficient cancers cells. This previously unforeseen therapeutic link between PLK1 and BRCA1 is promising because it defines novel therapeutic opportunities for PLK1i not only for breast cancer (i.e. TNBCs with BRCA1 deficiencies), but also for other types of cancers with BRCA1-deficiencies, such as pancreatic and prostate cancers.