Medulloblastoma (MB) is among the most malignant of pediatric brain tumors, having an average 5-year survival rate of only 50%. MB is believed to arise mostly from the undifferentiated neural stem/progenitor cells (CPCs) of the external granule layer cells of the cerebellum. Although developmentally important signaling pathways such as Wnt and Hedgehog are known to be activated in some MBs, the mechanism of tumorigenesis remains unknown for the majority of human MBs. Thus, one challenge for studies of MB is to understand the mechanisms that regulate most MBs. Our previous studies with human medulloblastoma tumor samples, a tissue culture system of CPCs, and an orthotopic intracranial mouse model system suggested that the transcription factor REST is a critical player in a major percentage of human MBs. On the basis of our current findings, we propose to test the hypothesis that the mechanism of REST-mediated MB tumorigenesis involves blocking the neuronal differentiation of CPCs by arresting them in a self-renewal mode (maintenance of stemness) and also by repressing the transcription of other target genes relevant in MB tumorigenesis. We further propose that the abnormal overexpression of REST seen in many MBs is due in part to mutations in the REST gene promoter. Thus, the proposed studies will yield critical information relevant to our long- term goals of studying the genesis of REST-mediated MB tumorigenesis and producing mechanism-based animal models that can be used both to identify new, physiologically relevant targets for therapy and to test existing and new drugs for MB. The experiments proposed here are in accordance with the recommendations of the NCI Brain Tumor Progress Review Group. Furthermore, because our proposed project involves neural stem/progenitor cells and medulloblastoma, it may be suitable for Program Announcement PA- 05-086 (Stem Cells and Cancer).