Abstract 1422: Clinical association of miR-155-5p with breast cancer and its relevance for treatment with PARP inhibitors

Abstract INTRODUCTION. miR-155-5p is a well-known oncogenic microRNA frequently overexpressed in human cancers, including breast cancer (BC), but its putative role as biomarker remains controversial. Thus, we aimed at evaluating miR-155-5p levels in a large BC cohort, and its association with clinical parameters. Then, since miR-155-5p was found to be epigenetically controlled by BRCA1, and to increase the tumor mutation load, we analysed miR-155-5p correlation with defects in Homologous Recombination (HR) genes (i.e. HRD), and its capability to alter the response to the first-in-class PARP (Poly-[ADP-ribose]-polymerase-1 (PARP-1)) inhibitor Olaparib (AZD2281). METHODS. We measured miR-155-5p expression in a retrospective cohort of breast cancer patients (CSS cohort, n=283; median follow-up=81 months) by RT-qPCR. Then, we used the TCGA Breast Cancer (TCGA-BC) dataset (n=1095) to validate our results, and evaluate the association between miR-155-5p and mutations in HR genes. Last, we ectopically modulated miR-155-5p in three wtBRCA1/2 cell lines (i.e. MDA-MB-231, MDA-MB-468, MDA-MB-453) and one mutBRCA1 cell line (MDA-MB-436) by using mirVana miR-155-5p mimic or inhibitor, and we measured cell viability before and after treatment with Olaparib or vehicle by PrestoBlue viability reagent. RESULTS. High miR-155-5p levels were associated with unfavourable prognostic indicators including high tumor grade (overall P=0.0007; G2 vs G3 P=0.0053), reduced expression of ER (r= -0.243; P=0.0002) and PgR (r= -0.240; P<0.0001), and high Ki-67 expression (r=0.215; P=0.0005). Consistently, miR-155-5p was differentially expressed across the BC subgroups identified by the surrogate molecular classification in the CSS-cohort (P=0.0008) and by the PAM50 panel in the TCGA-BC dataset (P=0.0001), with the Triple Negative and HER2-amplified tumors showing the highest levels. The analysis of the TCGA BRCA dataset also found higher miR-155 levels in the patient subgroup carrying somatic mutations in HR genes compared to the subgroup without somatic mutations in HR genes (P<0.0001). Last, the in vitro drug testing confirmed that mutBRCA1 MDA-MB-436 cells, exhibiting miR-155-5p endogenous levels 1000-fold higher than the other TNBC cell lines, had a higher sensitivity to a short-term (72hrs) treatment with Olaparib alone (IC50 25.80, 95%CI 24.52-27.15) compared to the other cell lines. Importantly, ectopic overexpression of miR-155-5p in wtBRCA1/2 cell lines, followed by Olaparib exposure, resulted in a significant reduction in viability (p<0.001) compared to Olaparib alone in all cell lines. However, miR-155-5p inhibition in BRCA1mut MDA-MB-436 cell line did not affect the higher sensitivity to PARP inhibition, suggesting that sensitivity to PARPi is not limited to HRD. CONCLUSIONS. Overall, our data point to a role of miR-155-5p in Homologous Recombination Deficiency (HRD), and suggest a putative novel mechanism of synthetic lethality mediated by PARP inhibition and miR-155-5p to promote cancer cell death. Citation Format: Barbara Pasculli, Raffaela Barbano, Andrea Fontana, Tommaso Biagini, Maria Pia Di Viesti, Michelina Rendina, Vanna Maria Valori, Maria Morritti, Sara Bravaccini, Sara Ravaioli, Evaristo Maiello, Paolo Graziano, Roberto Murgo, Massimiliano Copetti, Tommaso Mazza, Vito Michele Fazio, Manel Esteller, Paola Parrella. Clinical association of miR-155-5p with breast cancer and its relevance for treatment with PARP inhibitors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1422.