Abstract
Glioblastoma (GBM) is among the most aggressive and frequent forms of adult brain cancer with a very poor survival rate despite decades of research and a myriad of clinical trials. Similar to other tumors, such as bladder and lung cancer, GBM is reported to have a higher incidence in males than females. This consistent sex difference in incidence demands understanding at a molecular level. RNA sequencing of a murine model of male and female mesenchymal glioblastoma revealed significant sex differences in gene expression. We hypothesized that sex-dependent differences in gene expression and tumorigenic phenotype are achieved through differential stretch/super enhancer (SE) usage. Genome-wide mapping of SEs with ChIP-seq for H3K27Ac and transposon “Calling Cards” for Brd4 revealed robust sex differences in SE usage that correlated with differential gene expression. The functional significance of differential SE usage was demonstrated by pharmacological (JQ1) and genetic inhibition of Brd4, which reproducibly abrogated the sex differences in SE usage, gene expression, and critical elements of the cancer phenotype, including stem-like cell frequency and in vivo tumorigenesis. These data establish for the first time that cellular sex-identity is driven by differential SE usage and accounts for critical sex differences in GBM phenotype. Increasing our knowledge of these sex-specific genetic and epigenetic mechanisms will lead to a greater understanding of cancer biology and its relationship to normal development, as well as identify novel therapeutic targets to improve outcome for all patients with GBM and potentially other cancers that exhibit substantial sex differences in incidence or outcome.