Targeting the translational apparatus in MYCN-driven pediatric solid tumors

描述（申请人提供）：髓母细胞瘤是儿童中最常见的恶性脑肿瘤，最具侵袭性的分子亚群对生物学知之甚少，没有靶向治疗。这些亚群的特点是转录因子MYCN或MYC水平升高，并且不依赖于sonic hedgehog （SHH）信号。我们最近开发了一种不依赖sh的、NMYC驱动的小鼠髓母细胞瘤模型，其中高水平的NMYC蛋白驱动肿瘤发生。先前的数据表明，MYC驱动的肿瘤发生需要调节哺乳动物雷帕霉素靶点（mTOR）下游的翻译装置，MYC和mTOR轴之间存在协同关系，以促进肿瘤的形成。mTOR信号通过两个主要输出，rpS6激酶（S6K）和翻译起始因子eIF4E。一类新的临床mTOR活性位点抑制剂通过这两种效应器破坏信号，而变构结合剂雷帕霉素仅破坏S6K。这些机制上不同的活性具有巨大的治疗意义——在我们的髓母细胞瘤模型中，mTOR的活性位点抑制剂，而不是雷帕霉素，在体外显示出疗效。综上所述，这支持了通过S6K信号传导是可缺性的观点，而通过eIF4E信号传导是myc驱动的肿瘤发生所必需的。我们假设MYCN与mTOR信号下游的eIF4E合作诱导小脑神经球的增殖，并且mTOR激酶抑制是shh非依赖性髓母细胞瘤的关键治疗策略。我们将通过使用药理学方法和遗传学方法来研究这一假设，后者消除抑制剂的脱靶效应，以验证mTOR活性位点抑制的重要性，而不是mTOR变构抑制。首先，我们将确定MYCN如何影响翻译装置（目的1）。使用遗传学方法，我们将分别评估S6K和eIF4E在mycn驱动的肿瘤发生中的重要性（目的2）。最后，我们将使用mTOR的临床抑制剂来评估S6K（使用mTOR的变构抑制剂）和eIF4E（使用mTOR活性位点抑制剂）作为细胞系和体内小鼠模型的潜在治疗靶点（目的3）。这些目标的成功完成阐明了MYCN在大多数侵袭性成神经管细胞瘤亚型中作用的基本和可靶向机制。

DESCRIPTION (provided by applicant): 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.