BACKGROUND: Tumor recurrence is the main cause of death for children with medulloblastoma, the most common malignant childhood brain tumor. The MYCN oncogene is a poor prognosis marker and is amplified in the molecularly defined SHH and Group 4 subgroups but rarely in WNT and Group 3 subgroups of human medulloblastoma. Recent findings on childhood brain tumor relapse mechanisms suggest spatiotemporal differences within these four subgroups. SOX9 is a transcription factor that is important for glial fate determination in the brain but has also been found to promote tumor metastasization. We previously showed how expression of SOX9 correlates well with human SHH tumors but only few scattered SOX9-positive cells are found in SHH-independent Group 3 and Group 4 human medulloblastoma. METHODS: In order to study recurrence processes experimentally, we used a previously described transgenic Tet-OFF (Glt1-tTA) inducible model of MYCN-driven SHH-independent medulloblastoma (GTML mouse). To recreate metastatic recurrence we further used a Tet-ON (SOX9-rtTA) model that drives MYCN expression from the SOX9 promoter upon doxycycline treatment. RESULTS: By crossing the GTML Tet-OFF model with a Tet-ON transgene we managed to study rare SOX9-positive tumor cells after SOX9-negative tumor cells were first depleted using doxycycline. SOX9-positive GTML cells were tumorigenic and reinitiated distant recurrences over time. The SOX9-positive cells further showed an increased resistance to MYCN-targeted therapies. Relapses showed similar histopathology but presented generally higher levels SOX9 as compared to primary GTML tumors. A similar correlation was found in Group 3 and Group 4 medulloblastoma patients where isolated metastases had consistently higher SOX9 levels as compared to corresponding primary tumors. Finally, we overexpressed SOX9 in normal cerebellar stem cells transduced with mutationally stabilized MYCN-T58A and injected them back into the cerebellum of adult mice. Surprisingly, SOX9-positive MYCN-T58A brain tumors migrated and developed in the forebrain in contrast to the cerebellar stem cells transduced with MYCN-T58A only. CONCLUSIONS: Our findings suggest that increased levels of SOX9 drives migration in MYCN-driven medulloblastoma. Rare SOX9-positive tumor cells show an increased therapy resistance and are alone capable of reinitiating childhood brain tumors. Further characterization of SOX9-positive cells in Group 3 and Group 4 tumors could help us understand what drives metastatic medulloblastoma relapse and could lead to new therapies directed against these particularly serious cell types. SECONDARY CATEGORY: Tumor Biology.