The Role of SOX9 in Medulloblastoma

Abstract: Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Overall survival is about 70% and in cases where current treatment fails, the disease recurs and most often is fatal. At the molecular level, MB can be divided into four defined subgroups: WNT, SHH, Group 3 and Group 4. Amplification of MYC family genes is common in MB and correlates with poor prognosis and tumor relapse.In this thesis we showed how MYCN initiates brain tumors when transduced in neural stem cells (NSCs). Prior to transduction, NSCs were isolated from different brain regions and at various time points. While overexpression of wild-type MYCN did not generate any tumors, orthotopic transplantation of MYCNT58A-expressing forebrain, brain stem and cerebellar NSCs induced diffuse malignant glioma, PNET-like tumors and MB, respectively. Interestingly, MYCNT58A-expressing cerebellar NSCs induced SHH-dependent MB from embryonic cells but SHH-independent MB from postnatal cells. We further showed that cerebellar NSCs transduced with both MYCNT58A and transcription factor SOX9 developed tumors faster and promoted distant migration into the forebrain.The function and regulation of SOX9 in MB cells is poorly understood. We identified SOX9 protein as target of FBW7 ubiquitin ligase and demonstrated the effects of SOX9 on MB cells migration, metastasis and drug resistance. We further blocked PI3K pathway to destabilize SOX9 which sensitized cells to cytostatic treatment.We used a (TetOFF) transgenic mouse model of MYCN-induced MB (GTML) and crossed it with a (TetON) transgene which allowed us to specifically target rare SOX9-positive cells in the tumor. In this system, MB develops spontaneously and SOX9-negative tumor cells can be killed off by doxycycline. The few remaining SOX9-positive cancer cells were able to promote distant MB recurrences. Such a pattern of relapse was recently shown for Group 3 and 4 human MB where about 90% of the recurrences were distant.In summary, this thesis demonstrates that MYCN can generate various types of brain tumors depending on the timing and location of its expression. It further defines the existence of a rare population of SOX9-expressing MB cells that are involved in causing distant MB recurrences. Finally, it describes how SOX9 is stabilized in MB cells and increases MB migration and therapy resistance.