PO-494 Using functional genetic screens to understand resistance to PARPi in BRCA-deficient tumours

Abstract

Introduction Error-free repair of double-strand DNA breaks is achieved by homologous recombination (HR), and both BRCA1 and BRCA2 are crucial for this process. Inactivating germline mutations of BRCA1 and BRCA2 genes predispose to breast and ovarian cancers and result in HR deficiency. This defect can be specifically targeted by inhibition of Poly-(ADP-ribose) polymerase (PARP) 1, which leads to the selective killing of HR-deficient tumour cells. These observations have recently resulted in the approval of the first PARP inhibitors (PARPi) for the treatment of patients with germline BRCA-mutated tumours. Although this approach has shown promise, the efficacy of PARPi is limited due to drug resistance, with only a fraction of the BRCA1/2 mutation carriers responding to this therapy. Those who do respond eventually develop resistance and relapse. Although some drug resistance mechanisms have been characterised, many other mechanisms remain to be elucidated. Further investigation is needed to achieve a strategy to overcome drug resistance in order to improve this promising targeted therapy. Material and methods Our aim is to uncover novel mechanisms of resistance and to find promising therapeutic targets able to revert the resistant phenotype. To this end, we will conduct functional genetic screens in PARPi-sensitive and PARPi-resistant 2D and 3D models using shRNA and CRISPR libraries. Furthermore, we will combine multi-omics analysis of BRCA1/2-deficient mammary tumours that acquired PARPi resistance in vivo. Taken together, both approaches will yield a list of candidate genes that we further intend to validate and characterise, both in vitro and in vivo. Results and discussions Preliminary results yielded a list of candidate genes responsible for resistance or able to re-sensitise to PARPi. Hits are currently being validated in vitro . Furthermore, we intend to characterise the mechanism of action of the validated genes and to further understand its role in DNA repair. Additionally, we intend to validate our findings in vivo and to analyse the relevance of our results in the clinic. Conclusion With this project we intend to characterise new biological mechanisms that lead to resistance to PARPi and to find new tumour vulnerabilities that could be exploited in order to revert innate and acquired resistance to this therapy. Moreover, it will also allow us to get insights into the biological functions and molecular pathways in which BRCA1 and BRCA2 proteins are involved.