STEM-21. CONTEXT-SPECIFIC TUMOR SUPPRESSIVE FUNCTION OF THE CANONICAL Wnt PATHWAY IN PEDIATRIC MEDULLOBLASTOMA HIGHLIGHTS A THERAPEUTIC STRATEGY FOR TREATMENT-REFRACTORY SUBGROUPS

Current molecular subgroups of childhood medulloblastoma (MB) recognize distinct disease entities of which activated Wnt signaling is associated with a distinct subgroup and the best overall outcome. In contrast, non-Wnt MBs are characterized by metastatic disease, increased rate of recurrence, and poor overall survivorship. Given the excellent clinical outcome in Wnt-driven MB, we aimed to convert treatment-resistant MB subgroups 3 and 4 into an ostensibly benign tumor. Activated Wnt signaling by way of Wnt agonists decreased in vitro self-renewal of primary MB cells. Comparative RNA-sequencing of control and transgenic lines containing a stabilized beta-catenin mutant demonstrated a reduction in self-renewal genes following beta-catenin overexpression, including Sox2 and Bmi1. In order to validate the therapy-sensitive nature of Wnt-activated cells, we developed stable human Group 3 and 4 patient-derived lines containing a 7XTOPFlash reporter to determine the presence of endogenous Wnt signaling. Rare subclonal Wnt-active cells demonstrated a reduced self-renewal and tumor-initiating capacity through in vivo limiting dilution assays when compared to bulk Wnt-inactive cells from Group 3 and 4 MBs. The therapeutic relevance of these findings were demonstrated with an in vivo survival advantage in mice with orthotopic injections of cells containing stabilized beta-catenin overexpression or endogenous Wnt-active cells. Resulting xenograft tumors were smaller in size, maintained a lower rate of proliferation, and reduction in MB self-renewal genes. To develop a rationale clinical therapeutic, we used a novel substrate-competitive peptide inhibitor for GSK. Treatment with our peptide inhibitor showed a significant reduction in tumor burden and metastatic disease with a corresponding increase in survival of patient-derived Group 3 and 4 tumors that were otherwise treatment-resistant. Our work establishes activated Wnt signaling as a novel treatment paradigm in childhood MB, identifies a rationale therapeutic approach for recurrent MB, and provides evidence for the context-specific tumor suppressive function of the canonical Wnt pathway.