Abstract 1494: SOX18: A novel master regulator of high-grade serous ovarian tumorigenesis

High-grade serous ovarian cancer (HGSOC) is the most common and lethal subtype of ovarian cancer. Recent data indicate that fallopian tube secretory epithelial cells (FTSECs) represent the cell of origin for HGSOC. While many studies have characterized molecular features associated with HGSOC biology, the master transcription factors (TFs) that drive disease development are not known. To identify master TFs for HGSOC, we first performed an integrative analysis of gene expression profiles from 73 FTSEC samples and 394 HGSOCs from The Cancer Genome Atlas (TCGA) project. We then integrated these data with chromatin immunoprecipitation sequencing (ChIP-seq) analyses performed on primary HGSOC samples to select active chromatin regions marked by H3k27ac and to identify super-enhancer (SE) regions. These analyses identified overexpressed TFs that coincide with tumor-specific SEs, a hallmark of master TFs. We identified multiple putative master TFs including SOX18, which was highly overexpressed in HGSOCs relative to normal FTSECs (log 2 fold change in expression = 3.5, p = 2.5x10 -19 ). SOX18 is a TF that regulates development of blood and lymphatic vessels, but its association with HGSOC development has not been reported before. We examined the expression of SOX18 in ~8,000 tumors representing 17 tumor types from TCGA. SOX18 expression was significantly elevated in HGSOC relative to any other tumor types, suggesting it is highly specific to this cancer. We found that SOX18 is highly expressed in the HGSOC cell lines UWB1.289, Kuramochi, and EFO21, and we then performed lentiviral-mediated shRNA knockdown of SOX18 in these cell lines to establish the effects of SOX18 depletion on neoplastic phenotypes. In ongoing experiments, we are performing gene expression profiling and H3k27ac ChIP-seq analysis after SOX18 knockdown to identify SOX18 target genes and characterize the landscape of SOX18 binding sites in HGSOC. These studies will likely identify novel molecular biomarkers that may represent much-needed therapeutic targets for HGSOC. Citation Format: Annie Y. Liu, Kevin C. Vavra, Rosario I. Corona, Forough Abassi, Marcos Fonseca, Felipe Segato, Matthew L. Freedman, Simon A. Gayther, Houtan Noushmehr, Kate Lawrenson. SOX18: A novel master regulator of high-grade serous ovarian tumorigenesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1494.