STEM-11. A MAPK-DRIVEN Mir-124-sox9 AXIS IS CRITICAL FOR STEM CELL MAINTENANCE, PROGRESSION, AND THERAPY-RESISTANCE IN GLIOBLASTOMA

Glioblastoma (GBM) is the most common and aggressive malignant primary brain tumor. Genetic alterations in growth factor signaling pathways are found in 90% of GBMs. Advances in developmental and glioma biology suggest that common down-stream effector molecules in growth factor signaling pathways are critical for stem cell maintenance in the normal brain and GBM cells. It remains unclear whether differentiation therapies will be of therapeutic value for GBM patients. Here, we demonstrate that constitutive mitogen activated protein kinase (MAPK) activation in stem cells drives GBM formation and blocks neurogenesis in mice. Pharmacological inhibition of MAPK signaling restored neurogenesis in vivo and induced neuronal differentiation in GBM tumorspheres cultures established from murine GBMs and patient-derived tumors. Inhibition of MAPK signaling depleted SOX9 protein expression, and to a lesser extent SOX9 mRNA levels, in GBM cells. MicroRNA profiling experiments demonstrated that MAPK signaling regulates genome-wide expression of miRNAs, including the neuronal determinant miR-124. Pharmacological inhibition of MAPK signaling increased miR-124 levels in SOX9-expressing GBMs, but not SOX10-expressing proneural tumors. Using a doxycycline-inducible approach in vitro and in vivo, we demonstrated that miR-124 overexpression blocks SOX9 expression and induces neuronal differentiation in an EGFRvIII-driven GBM model and patient-derived xenografts. Neuronal differentiation resulted in apoptosis, reduced DNA repair capacity, and radiosensitized GBM cells. Doxycycline-mediated miR-124 overexpression resulted in complete regression for 1/3 of patient-derived xenografts. Mechanistic studies showed that SOX9 was a direct target of miR-124 and a major regulator of stem cell maintenance in GBM. Preliminary data showed that MAPK activation regulates transcriptional networks in SOX10-expressing proneural glioma, suggesting that distinct miRNAs regulate glioma aggressiveness in a subtype-specific manner. In conclusion, our results provide a mechanistic explanation for MAPK-dependent expansion of the stem cell pool during GBM initiation and demonstrate that enforcing neuronal differentiation represents a viable therapeutic strategy in glioma.