Abstract A31: Targeting APC loss using synthetic lethality in Colorectal Cancer
Authors: Shailes Hannah , Bridge Gemma , Foxler Daniel , Sharp Tyson V. , Silver Andrew , Martin Sarah A. 
Published: December 2017

Abstract
Background: Mutations in the tumor suppressor gene Adenomatous Polyposis Coli (APC) are found in 80% of sporadic colorectal cancer (CRC) tumors and are also responsible for the inherited form of CRC, Familial Adenomatous Polyposis (FAP). APC mutations typically occur early in the development of CRC, followed by mutations in KRAS, SMAD2/3/4 and TP53. The majority of mutations in APC occur in a region known as the mutation cluster region (MCR). This region is responsible for negatively regulating levels of B-catenin and therefore the level of Wnt signaling activation. In APC mutated tumors, B-catenin is not tightly regulated resulting in hyperactivation of the Wnt signaling pathway leading to uncontrolled cellular proliferation, differentiation, migration and apoptosis.
Aim: We aim to use the concept of synthetic lethality to identify novel therapeutic targets for the treatment of APC mutated CRC.
Results: In order to identify novel therapeutic targets for the treatment of APC mutated CRC, we have generated an in vitro model of APC mutant CRC using a lentiviral CRISPR-cas9 system. Using the APC wildtype colorectal carcinoma cell line RKO, we targeted the cells with a guide RNA targeting the start of the final exon of APC, which covers 80% of the coding region. These cell lines allow us to identify targets, which are synthetically lethal with the APC mutation. To identify novel potential drug targets, we have used two parallel screening approaches. Firstly, we have screened our cell lines with a library of siRNA silencing over 700 kinases. Typically kinases are easy to target pharmaceutically, with many kinase inhibitors already clinically available. Upon analysis, we have identified seven genes which show synthetic lethality with the APC mutation, whilst not harming the control wildtype APC cells. In parallel, we have screened over 1000 FDA-approved compounds to identify drugs which cause increased selective lethality in APC mutant cells. From screening our FDA-approved compounds, we have identified a number of compounds which display synthetic lethality with the APC mutation.
Conclusions: We have identified seven genes as potential therapeutic targets and a number of FDA-approved compounds, which could potentially be new selective therapies for 80% of CRC patients. Currently we are validating these findings and investigating the mechanism of synthetic lethality with APC mutation. To further validate our findings we are also exploring whether these results extend to other CRC cell lines with different mutational backgrounds, this will help us access how many patients may benefit from our novel therapeutic targets.
Acknowledgments: We would like to thank Bowel and Cancer Research and The Rosetree Trust for co-funding this project.
Citation Format: Hannah Shailes, Gemma Bridge, Daniel Foxler, Tyson V. Sharp, Andrew Silver, Sarah A. Martin. Targeting APC loss using synthetic lethality in Colorectal Cancer [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 A31.