Abstract 829: 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 contains small subsets of cells that display tumor-propagating stem-like phenotypes (i.e. glioma stem cells or GSCs) that act as critical determinants of 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 its 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 dynamically regulated and reversible by the ten-eleven translocation (TET) family of deoxygenases that catalyze the conversion of 5mC to 5-hydroxymethylcytosine (5hmC). Levels of 5hmC closely align with the differentiation state of cells; terminally differentiated cells contain high levels of 5hmC and less differentiated cells have 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. This strongly suggests that tumor suppressing epigenetic mechanisms are driven by TET-dependent 5hmC generation. We now show that SOX2, a reprogramming transcription factor that induces GSCs, represses the TET2 and decreases 5hmC in multiple GSC isolates. TET2 repression using two independent shRNA hairpins efficiently decreased 5hmc levels and significantly enhanced self-renewal capacity and tumor growth capacity of low-passage GSCs. We also show that low concentrations of vitamin C, a TET enzyme cofactor, significantly increases 5hmC in low-passage GSCs. This epigenetic change induced by vitamin C associated with increased GSC death induced by temozolomide (TMZ), the main chemotherapeutic used to treat GBM. These findings support a mechanism by which SOX2 induces loss of 5hmC in GSCs by repressing TET2 and show that vitamin C can reverse this epigenetic tumor promoting pathway and potentially enhance GBM chemotherapeutics. Citation Format: Sweta Sudhir, Hernando Lopez-Bertoni, Maria Lugo-Fagundo, Bachchu Lal, John Laterra. SOX2-mediated 5hmC dysregulation in GBM stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 829.