Medulloblastoma is the most common malignant brain tumor in children, and the most aggressive molecular subgroups have poorly understood biology and no targeted therapies. These subgroups are characterized by increased levels of transcription factors MYCN or MYC, as well as independence from sonic hedgehog (SHH) signaling. We have recently developed a model of SHH-independent, murine NMYC-driven medulloblastoma in which high levels of NMYC protein drives oncogenesis. Previous data show that modulation of the translational apparatus downstream of the mammalian target of rapamycin (mTOR) is required for MYC-driven oncogenesis and that there is a synergistic relationship between MYC and the mTOR axis to promote tumor formation. mTOR signals through two primary outputs, rpS6 kinase (S6K) and the translation initiation factor eIF4E. A new class of clinical mTOR active site inhibitors disrupts signaling through both effectors, whereas the allosteric binder rapamycin disrupts only S6K. These mechanistically distinct activities have enormous therapeutic implications-in our medulloblastoma model, active site inhibitors of mTOR, but not rapamycin, demonstrate efficacy in vitro. Taken together, this supports the idea that signaling through S6K is dispensable, whereas eIF4E is required for MYC-driven oncogenesis. We hypothesize that MYCN cooperates with eIF4E downstream of mTOR signaling to induce proliferation of cerebellar neurospheres, and that mTOR kinase inhibition is a critical therapeutic strategy for SHH-independent medulloblastoma. We will investigate this hypothesis through the use of both pharmacological approaches and genetic approaches, the latter to eliminate the off-target effects of inhibitors, to validate the significace of mTOR active site inhibition as opposed to mTOR allosteric inhibition. First, we will determine how MYCN influences the translational apparatus (Aim 1). Using genetic approaches, we will separately evaluate the importance of S6K and eIF4E for MYCN-driven oncogenesis (Aim 2). Finally, we will use clinical inhibitors of mTOR to evaluate S6K (using allosteric inhibitors of mTOR) and eIF4E (using mTOR active site inhibitors) as potential therapeutic targets in both cell lines and our in vivo mouse models (Aim 3). Successful completion of these aims elucidates fundamental and targetable mechanisms underlying MYCN's role in the most aggressive subtypes of medulloblastoma.
Medulloblastoma is the deadliest brain tumor in children, and the most aggressive types of medulloblastoma have poor survival and no targeted therapies. An emerging class of drugs, known as mTOR inhibitors, may provide an effective means of treating this disease. However, there are two distinct types of mTOR inhibitors, and we have limited understanding as to which types will work best in which diseases. Our hope is to better understand the biology of the most aggressive forms of medulloblastoma, and in doing so, determine which type of mTOR inhibitor will be most suitable for the safe and efficacious treatment of children with aggressive medulloblastoma.
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