Super-enhancers demarcate regions of master regulators involved in cellular differentiation and tumorigenesis. While many trait-associated variants discovered through genome wide association studies are enriched in regulatory elements, the role of super-enhancers in disease susceptibility is less well understood. We evaluated the role of super-enhancers, defined by H3K27ac chromatin immunoprecipitation sequencing (ChIP-seq) analysis of epithelial ovarian cancers (EOC) and precursor cells, in mediating the genetic risk of EOC. We characterized super-enhancer-risk SNP intersections to identify a series of risk-associated loci where multiple candidate causal alleles overlap with ovarian super-enhancers (approximately 33% of candidate causal loci). In particular, a risk locus at 3q25 marked by SNP rs7651446, (odds ratio = 1.59, p = 1.5x10-34), is tagged by 79 candidate causal risk SNPs, more than half of which coincide with super-enhancers detected in EOC precursor cells (ovarian and fallopian tube epithelia) but not in ovarian cancer cell lines or primary tumors suggesting that loss of activity of this super-enhancer is associated with EOC development.
We hypothesized that SNPs at the 3q25 locus modulate activity of super-enhancers that regulate in cis the expression of genes involved in the identity of normal tissue precursors, and that these genes are downregulated during tumorigenesis. We tested the effect of decreasing the binding affinity of BRD4, a protein that binds super-enhancer regions, using a small molecule inhibitor (JQ1). We measured the expression of genes at this locus following JQ1 treatment in EOC precursor cells, identifying three candidate genes, LEKR1, SSR3, and TiPARP, that show decreased expression due to BRD4 binding inhibition. For SSR3 and TIPARP, these expression level changes mirrored the differential expression observed between normal precursor cells and primary high grade serous ovarian cancer. We then established stable shRNA-mediated knockdown models for SSR3 and TIPARP in normal EOC precursor cell lines to establish a role for these genes in ovarian cancer development. In summary, we have identified a potential role for super-enhancers at a subset of EOC risk loci, and present a functional pipeline for identification of the target gene/genes and evaluation of their role in neoplastic transformation.
Citation Format: Kevin C. Vavra, Rosario I. Corona, Ji-Heui Seo, Simon G. Coetzee, Janet M. Lee, Matthew L. Freedman, Paul D. Pharoah, Dennis J. Hazelett, Simon A. Gayther, Kate Lawrenson. The role of tissues specific super-enhancers in mediating the genetic risk of ovarian cancer [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 1498.