Abstract AP24: CRISPR/CAS9–MEDIATED TRP53, BRCA1, BRCA2 AND PTEN KNOCKOUT TO GENERATE IMPROVED MURINE MODELS OF OVARIAN HIGH GRADE SEROUS CARCINOMA
Authors: Walton Josephine , Blagih Julianna , Farquharson Malcolm , Ennis Darren , Leung Elaine , Dowson Suzanne , Tookman Laura A. , Orange Clare , Athineos Dimitris , Mason Susan , Stevenson David , Blyth Karen , Strathdee Douglas , Balkwill Frances R. , Vousden Karen , Lockley Michelle , McNeish Iain A. 
Published: June 2017

Abstract
BACKGROUND: Ovarian high grade serous carcinoma (HGSC) is the commonest subtype of human ovarian cancer, and prognosis remains poor. Transplantable murine models of HGSC that recreate key mutations seen in the human disease are greatly needed. These models would assist investigation of the relationships between tumor genotype, chemotherapy response and immune microenvironment. ID8 is the most widely-used mouse model of ovarian cancer, but it has not been characterized in light of current understanding of HGSC biology.
METHODS: We performed whole exome sequencing of ID8, covering 194,000 exons at a mean depth of 400x with 90% exons sequenced &gt;50x. Using CRISPR/Cas9 gene editing, we have generated novel ID8 derivatives with single (Trp53-/-) and double (Trp53-/-;Brca2-/-, Trp53-/-;Brca1-/- and Trp53-/-;Pten-/-) deletions. We have characterized intra-peritoneal growth of these novel mutants, and investigated platinum and PARP inhibitor sensitivity and immune cell infiltration into the tumor microenvironment.
RESULTS: Whole exome sequencing showed no functional mutations in genes characteristic of HGSC (Trp53, Brca1, Brca2, Nf1, Rb1). In addition, mutations typical of clear cell (Arid1A, Pik3ca), low grade serous (Braf), endometrioid (Ctnnb1) and mucinous (Kras) carcinomas were notably absent. ID8 also demonstrated intact p53 function and homologous recombination in functional assays. We show that loss of p53 alone significantly increases tumor growth rate within the peritoneal cavity of C57Bl/6 mice, and has marked effects upon immune microenvironment. In particular, loss of p53 increases CCL2 expression and induces immunosuppressive myeloid cell infiltration into tumor and ascites. Trp53-/-;Brca2-/- and Trp53-/-;Brca1-/- cells show significantly increased sensitivity to the PARP inhibitor rucaparib in vitro compared to parental and Trp53-/- cells. In vivo, Trp53- /-;Brca2-/- tumors are more sensitive to platinum chemotherapy than Trp53-/-. They also exhibit slower intraperitoneal growth, with appearance of intra-tumoral CD3+ cell-rich tertiary lymphoid structures, a phenotype observed in human tumors with high mutational burden. Mice bearing Trp53-/-;Pten-/- tumors have significantly reduced survival compared to Trp53-/-. Full in vivo assessment of the Trp53-/-;Brca1-/- and new tripleTrp53-/-;Brca2-/- ;Pten-/- lines is on-going.
CONCLUSIONS: These models represent a new and simple tool to investigate the biology of HGSC. All cells will be made available to other researchers upon request.
Citation Format: Josephine Walton, Julianna Blagih, Malcolm Farquharson, Darren Ennis, Elaine Leung, Suzanne Dowson, Laura A. Tookman, Clare Orange, Dimitris Athineos, Susan Mason, David Stevenson, Karen Blyth, Douglas Strathdee, Frances R. Balkwill, Karen Vousden, Michelle Lockley, Iain A. McNeish. CRISPR/CAS9–MEDIATED TRP53, BRCA1, BRCA2 AND PTEN KNOCKOUT TO GENERATE IMPROVED MURINE MODELS OF OVARIAN HIGH GRADE SEROUS CARCINOMA [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr AP24.