Abstract B010: 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). Levels of 5hmC closely track with the differentiation state of cells with terminally differentiated cells containing high levels of 5hmC and less differentiated cells having very low levels. Multiple studies found negative correlations between 5hmC levels and glioma grade and loss of 5hmC correlates with poor prognosis of GBM patients, strongly suggesting that these enzymes activate tumor suppressing mechanisms. Our preliminary data shows that exogenous 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 enhanced 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 partially rescues the reduction in 5hmC expression observed in GSCs expressing transgenic SOX2, thus implicating miR-10b-5p as a critical mediator of SOX2-induced onco-methylation, GSC induction and glioma malignancy. Our preliminary results support a mechanism in which SOX2 represses TET2 leading to 5hmC loss by directly activating miR-10b-5p transcription and predicts that targeting this novel mechanism of epigenetic dysregulation by inhibiting miR-10b-5p in vivo will can lead to pre-clinical GBM therapeutics. Citation Format: Hernando Lopez-Bertoni. SOX2-mediated 5hmC dysregulation in GBM Stem Cells [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B010. doi:10.1158/1535-7163.TARG-19-B010