Malignant pleural mesothelioma (MPM) remains a lethal cancer due to difficulties in early diagnosis, a scarcity of targets for innovative treatments, inherent resistance to chemotherapy, and an immune-suppressive tumor microenvironment. Previous research has demonstrated that asbestos fibers induce DNA damage in cells, thereby contributing to the molecular diversity seen in MPM. It has been reported that 10-15% of MPM samples contain alterations in genes associated with homologous recombination (HR) or other DNA repair mechanisms. We proposed that these genetic alterations might create a susceptibility in certain MPMs to treatments that target the DNA damage response (DDR) pathways.

We assembled a panel consisting of 22 MPM human cell lines and 3 murine cell lines sourced from commercial suppliers or developed from patients at Azienda Ospedaliera SS. Antonio e Biagio in Alessandria and San Luigi Hospital in Orbassano, Italy. Molecular characterization involved whole-genome sequencing and comprehensive methylome profiling of the cell lines. Long-term cell proliferation assays lasting 7 to 15 days were conducted using various concentrations of the following DDR inhibitors: berzosertib and ceralasertib (ATR inhibitors), AZD0156 (an ATM inhibitor), olaparib (a PARP inhibitor), adavosertib (a WEE1 inhibitor), and rabusertib (a CHEK1 inhibitor). We quantified RAD51 nuclear foci after treatment with DDR inhibitors or exposure to ionizing radiation to evaluate DNA damage and HR activation.

The paucity of preclinical MPM models has hindered advancements in identifying targets and effective therapies for this uncommon tumor type. By gathering a substantial collection of MPM models, we discovered that 10 out of 22 human and 2 out of 3 studied murine MPM lines exhibited remarkable sensitivity to all DDR inhibitors. We identified genomic alterations of unclear significance impacting 112 HR and 132 DDR genes; however, these did not correlate with sensitivity to the drugs. Additionally, the deficiency of BAP1 protein did not serve as a predictor for drug response. MPMs that failed to form RAD51 foci after exposure to ionizing radiation, which are known to induce double strand breaks, were among the most DDR-sensitive cell lines. Intriguingly, drug refractory cell lines showed either sarcomatoid or biphasic histology, whereas drug sensitive lines showed epithelioid features, suggesting a previously unrecognized role of tumor histology in modulating DDR in MPM.

These results represent a preclinical rationale for designing clinical trials with DDR for MPM patients.