PS1 - 169 Discovering the Treatment Refractory BTIC Population in Group 3 Medulloblastoma through Therapy Adapted Patient-Derived Human Mouse Xenograft (PDX) Model

Medulloblastoma (MB), the most common malignant pediatric brain tumor, is categorized into four molecular subgroups. Given the high rate of metastatic dissemination at diagnosis and recurrence in Group 3 MBs, these patients have the worst clinical outcome with a 5-year survivorship of approximately 50%. By adapting the existing COG (Children’s Oncology Group) Protocol for children with newly diagnosed high-risk MB, for treatment of immuno-deficient mice intracranially engrafted with human MB brain tumour initiating cells we aim to identify and characterize the treatment-refractory cell population in Group 3 MBs. Mice were sacrificed at multiple time points during the course of tumor development and therapy: (i) at engraftment; (ii) post-radiation; (iii) post-radiation and chemotherapy; and (iv) at MB recurrence. MB cell populations recovered separately from brains and spines were comprehensively profiled for gene expression analysis, stem cell and molecular features to generate a global, comparative profile of MB cells through therapy. We report a higher expression of CD133, Sox2 and Bmi1 in addition to increased self-renewal capacity following chemoradiotherapy treatment. The enrichment map constructed from global gene expression analysis showed an increase in pathways regulating self-renewal, DNA repair and chemoresistance post-therapy despite the apparent decrease in tumour size and vascularity. Additionally, from gene expression at MB recurrence, we identified a list of genes that negatively correlate with survival in patients diagnosed with Group 3 MB. A differential genomic profile of the “treatment-responsive” tumors against those that fail therapy may contribute to discovery of novel therapeutic approaches for the most aggressive subgroup of MB.