Evaluation of candidate biomarkers to predict cancer cell sensitivity or resistance to PARP-1 inhibitor treatment.

Impaired DNA damage response pathways may create vulnerabilities of cancer cells that can be exploited therapeutically. One such selective vulnerability is the sensitivity of BRCA1- or BRCA2-defective tumors (hence defective in DNA repair by homologous recombination, HR) to inhibitors of the poly(ADP-ribose) polymerase-1 (PARP-1), an enzyme critical for repair pathways alternative to HR. While promising, treatment with PARP-1 inhibitors (PARP-1i) faces some hurdles, including (1) acquired resistance, (2) search for other sensitizing, non-BRCA1/2 cancer defects and (3) lack of biomarkers to predict response to PARP-1i. Here we addressed these issues using PARP-1i on 20 human cell lines from carcinomas of the breast, prostate, colon, pancreas and ovary. Aberrations of the Mre11-Rad50-Nbs1 (MRN) complex sensitized cancer cells to PARP-1i, while p53 status was less predictive, even in response to PARP-1i combinations with camptothecin or ionizing radiation. Furthermore, monitoring PARsylation and Rad51 foci formation as surrogate markers for PARP activity and HR, respectively, supported their candidacy for biomarkers of PARP-1i responses. As to resistance mechanisms, we confirmed the role of the multidrug resistance efflux transporters and its reversibility. More importantly, we demonstrated that shRNA lentivirus-mediated depletion of 53BP1 in human BRCA1-mutant breast cancer cells increased their resistance to PARP-1i. Given the preferential loss of 53BP1 in BRCA-defective and triple-negative breast carcinomas, our findings warrant assessment of 53BP1 among candidate predictive biomarkers of response to PARPi. Overall, this study helps characterize genetic and functional determinants of cellular responses to PARP-1i and contributes to the search for biomarkers to exploit PARP inhibitors in cancer therapy.

Low-dose DNA damage and replication stress responses quantified by optimized automated single-cell image analysis.

Maintenance of genome integrity is essential for homeostasis and survival as impaired DNA damage response (DDR) may predispose to grave pathologies such as neurodegenerative and immunodeficiency syndromes, cancer and premature aging. Therefore, accurate assessment of DNA damage caused by environmental or metabolic genotoxic insults is critical for contemporary biomedicine. The available physical, flow cytometry and sophisticated scanning approaches to DNA damage estimation each have some drawbacks such as insufficient sensitivity, limitation to analysis of cells in suspension, or high costs and demand for trained personnel. Here we present an option how to transform a regular fluorescence microscope and personal computer with common software into a functional alternative to high-throughput screening devices. In two detailed protocols we introduce a new semi-automatic procedure allowing for very sensitive, quantitative, rapid and simple fluorescence image analysis in thousands of adherent cells per day. Sensitive DNA breakage estimation through analysis of phosphorylated histone H2AX (gamma-H2AX), and homologous recombination (HR) assessed by a new RPA/Rad51 dual-marker approach illustrate the advantages and applicability of this technique. Our present data on assessment of low radiation doses, repair kinetics, spontaneous DNA damage in cancer cells, as well as constitutive and replication stress-induced HR events and their dependence on upstream factors within the DDR machinery document the versatility of the method. We believe this affordable approach may facilitate mechanistic insights into the role of low-dose DNA damage in human diseases, and generally promote both basic and translational research in many areas of biomedicine where suitable fluorescence markers are available.

Aberrations of the MRE11-RAD50-NBS1 DNA damage sensor complex in human breast cancer: MRE11 as a candidate familial cancer-predisposing gene.

The MRE11, RAD50, and NBS1 genes encode proteins of the MRE11-RAD50-NBS1 (MRN) complex critical for proper maintenance of genomic integrity and tumour suppression; however, the extent and impact of their cancer-predisposing defects, and potential clinical value remain to be determined. Here, we report that among a large series of approximately 1000 breast carcinomas, around 3%, 7% and 10% tumours showed aberrantly reduced protein expression for RAD50, MRE11 and NBS1, respectively. Such defects were more frequent among the ER/PR/ERBB2 triple-negative and higher-grade tumours, among familial (especially BRCA1/BRCA2-associated) rather than sporadic cases, and the NBS1 defects correlated with shorter patients' survival. The BRCA1-associated and ER/PR/ERBB2 triple-negative tumours also showed high incidence of constitutively active DNA damage signalling (gammaH2AX) and p53 aberrations. Sequencing the RAD50, MRE11 and NBS1 genes of 8 patients from non-BRCA1/2 breast cancer families whose tumours showed concomitant reduction/loss of all three MRN-complex proteins revealed two germline mutations in MRE11: a missense mutation R202G and a truncating mutation R633STOP (R633X). Gene transfer and protein analysis of cell culture models with mutant MRE11 implicated various destabilization patterns among the MRN complex proteins including NBS1, the abundance of which was restored by re-expression of wild-type MRE11. We propose that germline mutations qualify MRE11 as a novel candidate breast cancer susceptibility gene in a subset of non-BRCA1/2 families. Our data have implications for the concept of the DNA damage response as an intrinsic anti-cancer barrier, various components of which become inactivated during cancer progression and also represent the bulk of breast cancer susceptibility genes discovered to date.