Abstract 152: SOX2-mediated 5hmC dysregulation in GBM stem cells

Abstract

Abstract Primary brain tumors are among the most devastating forms of cancer and glioblastoma (GBM) represents the most aggressive and lethal form of the disease. We now know that GBM contain small subsets of cells that display tumor-propagating stem-like phenotypes (i.e. glioma stem cells or GSCs) that act as critical determinants of GBM resistance to current treatments and tumor recurrence for which there is no proven therapy. Altered patterns of DNA methylation are widely reported in human GBM. However, substantial knowledge gaps remain in our understanding of the molecular mechanisms responsible for this epigenetic dysregulation, its downstream consequences and role in the GBM tumor propagating phenotype. Understanding and ultimately targeting the epigenetic mechanisms that induce and maintain these tumor-propagating cell subsets is critical to improving GBM therapy and patient outcomes. DNA methylation generally occurs in cytosine-guanine (CpG) sequences and is established by DNMTs, which catalyze the conversion of cytosine to 5-methylcytosine (5mC). DNA methylation is a reversible process and is partially mediated by the ten-eleven translocation (TET) family of enzymes which function as deoxygenases to catalyze the conversion of 5mC to 5-hydroxymethylcytosine (5hmC). Multiple studies found negative correlations between 5hmC levels and glioma grade and loss of 5hmC correlates with poor prognosis of GBM patients. TET enzymes and regulation of 5mC and 5hmC have the capacity to function on a global scale to regulate tumor suppressing mechanisms. We have found that SOX2 represses the TET2 demethylase and decreases 5hmC, the enzymatic product catalyzed by TET proteins, in GSCs. TET2 repression using 2 independent shRNA hairpins efficiently decreases 5hmc levels and significantly enhances self-renewal capacity and tumor growth capacity of low-passage GSC isolates. We also show that SOX2 induces expression of miR-10b-5p, a known onco-miR predicted to target TET2, and inhibiting this miRNA rescued the reduction in 5hmC expression observed in GSCs, thus identifying miR-10b-5p as a critical mediator of SOX2-induced onco-methylation, GSC induction and glioma malignancy. Our results identify a novel targetable pathway by which SOX2 regulates the epigenetic landscape of GBM-propagating cells. This novel mechanism combined with our recent reports that SOX2 regulates DNMTs and the HMGA1 chromatin remodeling protein identified a multi-dimensional epigenetic mechanism by which oncogenic signals control GBM cell stemness and tumor propagation. Citation Format: Hernando M. Lopez-Bertoni. SOX2-mediated 5hmC dysregulation in GBM stem cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 152.