Abstract A35: Precision targeting of Homologous Recombination-deficient cancers using microRNA synthetic lethality
Authors: Srinivasan Gayathri , Williamson Elizabeth , Hromas Robert 
Published: December 2017

Abstract
Introduction: Because cancer often occurs from defects in DNA repair, cancer cells become addicted to the remaining repair pathways for survival and proliferation. This addiction of cancer cells to residual DNA repair pathways can be exploited therapeutically by inhibiting the residual repair pathway, which is termed synthetic lethality. An example for this is using PARP1 inhibitors as drugs that target cancer cells with an inherited deficiency in BRCA1 or 2. Indeed, PARP1 inhibitors are now being tested as a new treatment option for many other malignancies. In addition to BRCA1/2-deficient malignancies, we here show another type of malignancy, BAP1 mutant mesothelioma, which can be targeted by synthetic lethality. Similar to BRCA1/2, BAP1 is an HR component and these cells rely on other DNA repair pathways for survival. We report here a type of PARP1 regulation, with an endogenous microRNA, that may be subverted for therapy of HR-deficient malignancies. MicroRNAs (miRNA) are small (~22 nucleotides) non-coding RNAs that regulate gene expression by binding to the 3'UTR of mRNA with complete/partial complementarity. MiR223-3p targets the 3'UTR of PARP1 mRNA, and thereby abrogates PARP1 protein levels.
Hypothesis: We hypothesize that miR223-3p can be used to induce synthetic lethality in BRCA1/2 and BAP1 deficient malignancies by targeting the destruction of PARP1. By exploiting the addiction of HR-deficient malignancies to other DNA repair pathways, miR223-3p can be used as a therapeutic for such cancers.
Methods: By using colony formation assay, we found that the BRCA1/2 and BAP1 mutant cells can be killed by miR223-3p. We will assess the efficiency of HR and other DNA repair pathways in these cancer cells to study the molecular mechanisms of toxicity in these cells induced by miR223-3p. We will also modify the nucleotide sequence of miR223-3p to enhance PARP1 inhibition. Finally, we will package miR223-3p in either liposomes or exosomes to deliver to BRCA1/2 and BAP1 deficient malignancies in vivo.
Results: Using the 3 UTR of PARP1 driving a luciferase promoter, we demonstrated that miR223-3p over-expression could reduce luciferase expression from this reporter construct indicating that PARP1 mRNA is a direct target of miR223-3p. BAP1 deficient MPMs and BRCA1/2 mutant malignancies are addicted to other DNA repair pathways for survival and these cancers are effectively targeted by miR223-3p mediated destruction of PARP1. We found that the BAP1 and BRCA1/2 deficient cancer cells have repressed levels of miR223-3p since they cannot tolerate repressed PARP1 levels in cells. When miR223-3p is restored in these cells, they showed decreased active replication forks and deficient fork restart due to destruction of PARP1. We also found that miR223-3p is 120 fold more effective in killing HR deficient malignancies than olaparib, an FDA approved PARP1 inhibitor for BRCA1/2 deficient ovarian cancer.
Conclusions: Packaging miR223-3p in a nanoparticle can be a potential therapeutic for HR-deficient malignancies and it can confer less risk of toxicity to normal cells since this microRNA is naturally present in mammalian cells.
Citation Format: Gayathri Srinivasan, Elizabeth Williamson, Robert Hromas. Precision targeting of Homologous Recombination-deficient cancers using microRNA synthetic lethality [abstract]. In: Proceedings of the AACR Precision Medicine Series: Opportunities and Challenges of Exploiting Synthetic Lethality in Cancer; Jan 4-7, 2017; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2017;16(10 Suppl):Abstract nr A35.