The functional consequences of somatic non-coding mutations in ovarian cancer (OC) are unknown. To identify regulatory elements (RE) and genes perturbed by acquired non-coding variants, here we establish epigenomic and transcriptomic landscapes of primary OCs using H3K27ac ChIP-seq and RNA-seq, and then integrate these with whole genome sequencing data from 232 OCs. We identify 25 frequently mutated regulatory elements, including an enhancer at 6p22.1 which associates with differential expression of ZSCAN16 (P = 6.6 × 10-4) and ZSCAN12 (P = 0.02). CRISPR/Cas9 knockout of this enhancer induces downregulation of both genes. Globally, there is an enrichment of single nucleotide variants in active binding sites for TEAD4 (P = 6 × 10-11) and its binding partner PAX8 (P = 2×10-10), a known lineagespecific transcription factor in OC. In addition, the collection of cis REs associated with PAX8 comprise the most frequently mutated set of enhancers in OC (P = 0.003). These data indicate that non-coding somatic mutations disrupt the PAX8 transcriptional network during OC development.

Transcriptional regulation is highly disease and cell-type specific. We performed H3K27ac chromatin immunoprecipitation and transcriptomic sequencing in primary tumors for the four different subtypes of invasive epithelial ovarian cancer (OC). Histotype-specific regulatory elements (REs) were enriched in enhancers (P<0.001). In silico prediction of putative target genes for histotype-specific REs identified genes ( WFDC2 , P=5.5x10 -5 ) and pathways (PI3K-Akt signaling, P<0.002) known to be involved in OC development. Some genes (e.g. PAX8 and CA125 ) are associated with super-enhancers (SEs) in all OCs, while others are histotype-specific, including PPP1R3B which is associated with SEs specific to clear cell OC. Integrated analysis of active chromatin landscapes with somatic single nucleotide variants (SNVs) from whole genome sequencing (WGS) of 232 primary OCs identified frequently mutated REs, including the KLF6 promoter (P=8.2x10 -8 ) and a putative enhancer at chromosome 6p22.1 (P<0.05). In high-grade serous OCs, somatic SNVs clustered in binding sites for the PAX8 binding partner TEAD4 (P=6x10 -11 ), while the collection of cis regulatory elements associated with PAX8 was the most frequently mutated set of enhancers in OC (P=0.003).Functional analyses supported our findings: Knockdown of PPP1R3B in clear cell OC cells significantly reduced intracellular glycogen content, a signature feature of this histotype; and stable knockout of a 635 bp region in the 6p22.1 enhancer induced downregulation of two predicted target genes, ZSCAN16 and ZSCAN12 (P=6.6 x 10 -4 and P=0.02). In summary, we have characterized histotype-specific epigenomic and transcriptomic landscapes in OC and defined likely functional REs based on somatic mutation analysis of ovarian tumors.

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 (log2 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.

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