Genome-wide silencing screen in mesothelioma cells reveals that loss of function of BAP1 induces chemoresistance to ribonucleotide reductase inhibition: implication for therapy

Introduction Loss of function of BRCA1 associated protein 1 (BAP1) is observed in about 50% of malignant pleural mesothelioma (MPM) cases. The aim of this study was to investigate whether this aspect could be exploited for targeted therapy. Methods A genetically engineered model was established expressing either functional or nonfunctional BAP1 and whole-genome siRNA screens were performed assessing impaired survival between the two cell lines. Cytotoxity induced by gemcitabine and hydroxyurea were assessed in a panel of BAP1-WT and BAP1-mut/del cell lines. Functional studies were carried out in BAP1 mut/del cell line reconstituted with BAP1 WT or BAP1 C91A (catalytically dead mutant) and in BAP1 WT cell line upon siRNA-mediated knock-down of BAP1. Results The whole-genome siRNA screen unexpectedly revealed 11 hits (FDR<0.05) more cytotoxic for BAP1-proficient cells. Two actionable targets, RRM1 and RRM2, were validated and their inhibition mediated by gemcitabine or hydroxyurea respectively, was more cytotoxic in BAP1-proficient cell lines. Upregulation of RRM2 upon gemcitabine and hydroxyurea was more profound in BAP1 mut/del cell lines. Increased lethality mediated by gemcitabine and hydroxyurea was observed in NCI-H2452 cells reconstituted with BAP1 WT but not with C91A mutant and upregulation of RRM2 in NCI-H2452-BAP1 WT spheroids was modest compared to control or C91A mutant. Finally, the opposite was observed after BAP1 knockdown in BAP1-proficient SPC111 cell line. Conclusion We found that BAP1 is involved in the regulation of RRM2 levels during replication stress. These observations reveal a potential therapeutic approach where MPM patients to be stratified depending on BAP status for gemcitabine treatment.