Brain tumors represent a leading cause of cancer mortality, of which medulloblastoma (MB) and glioblastoma (GBM) represent the most frequent malignant pediatric and adult brain tumors, respectively. The identification of a rare clonal population of cells, termed cancer stem cells (CSCs) or brain tumor-initiating cells (BTICs), as having the ability to initiate, proliferate, and maintain tumor growth has offered a developmental framework for studying MB and GBM. Evidence in support of cell signaling programs carried forward from brain development into oncogenesis have provided opportunities for BTIC-directed therapies targeting the key BTIC property of self-renewal. Given that neural stem cells (NSCs) must maintain a relative balance between self-renewal and differentiation, brain tumorigenesis may be conceptualized as a disease of unregulated BTIC self-renewal. In this work, I aim to demonstrate the re-emergence of self-renewal genes that regulate NSCs in BTICs, use the Wnt pathway as a model by which these genes may be regulated in a context-specific manner, and identify clinically tractable therapies directed at the overall BTIC self-renewal signaling machinery. Specifically, in Chapter 2, I describe the presence of a shared signaling program between NSCs and MB BTICs consisting of Bmi1 and FoxG1. In Chapter 3, I provide evidence in support of a context-specific tumor suppressive function for activated Wnt/β-catenin signaling in MB. Lastly, in Chapter 4, I demonstrate a CD133-AKT-Wnt signaling axis in which CD133 functions as a putative cell surface receptor for AKT-dependent Wnt activation in GBM. Overall, the body of this thesis offers a mechanistic model by which BTICs may be regulated and targeted to impair tumor growth and improve overall survivorship in childhood MB and adult GBM. / Thesis / Doctor of Philosophy (PhD)

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