Exploiting Acquired Resistance Mechanisms to MEK Inhibition in RAS-Activated Neuroblastoma

High-risk neuroblastoma is characterized by an aggressively metastatic phenotype and five-year survival rates of approximately 40%. Half of all high-risk patients experience disease relapse which remains incurable. Recent studies have identified an enrichment of mutations in the RAS-MAPK pathway upon relapse that are potentially sensitive to MEK1/2 inhibition with drugs such as trametinib. Although trametinib is a potent MEK1/2 inhibitor, single-agent therapy invariably encounters de novo or acquired bypass mechanisms that allow for disease progression. The central goal of this dissertation was to contribute to the understanding of compensatory signaling mechanisms adopted by RAS-MAPK aberrant neuroblastomas in response to MEK1/2 inhibition. Here, we interrogated the role of the Hippo pathway protein YAP in intrinsic trametinib resistance and discovered that YAP1 gene knockout sensitized neuroblastoma cells to trametinib. Further exploration into this mechanism showed that significantly reduced expression of E2F and MYCN gene signatures promoted G1 phase cell cycle arrest. This study also investigated novel TEAD palmitoylation inhibitors as inhibitors of YAP activity in combination with trametinib. Of the three compounds tested, one showed synergy with trametinib across three RAS-hyperactivated neuroblastoma cell lines. To evaluate other signaling adaptations driving trametinib resistance, we performed a high-throughput screen to identify synergistic trametinib drug combinations in six RAS-hyperactivated neuroblastoma cell lines. The top drug targets with broad efficacy in at least three cell lines were HMG-CoA reductase inhibitors, or statins, and three epigenetic inhibitors. The principal finding of these studies was the observation of synergy between trametinib and two statins in two neuroblastoma cell lines. However, no synergy was detected between trametinib and epigenetic inhibitors, suggesting a false positive result. Altogether, this dissertation provides evidence of two synergistic trametinib drug combinations in RAS-hyperactivated neuroblastoma that can be explored in the clinic. These findings underscore the importance of YAP activity in response to trametinib in RAS-driven neuroblastomas, as well as the potential for harnessing the pleiotropic effects of TEAD palmytoilation inhibition and/or statins in a trametinib combination. Continued functional characterization of potential targets of resistance will build upon these efforts to improve clinical responses of relapsed neuroblastoma to trametinib combinatorial therapies and contribute to the larger field of MEK inhibitor bypass mechanisms in cancer