Characterizing c-Myc Dependent Lung Cancers



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MYC is one of the most commonly deregulated oncogenes in human cancer, including breast, colorectal and lung. While mutations in myc are rare, MYC is overexpressed and in some cases amplified in these (and other) cancers. Recent reports demonstrate the utility of various drugs in selectively targeting MYC-driven cancers. However, given the lack of consistency across tissue types, particularly lung cancer, a multimodal approach to delineate MYC-dependent lung cancers is required. My goal is to characterize MYC deregulation in lung cancer, investigate the degree of differential dependence on MYC in lung cancer, and to elucidate the mechanism for resistance to MYC inhibition. A large panel of clinically and molecularly annotated NSCLC lines was investigated for MYC mRNA, protein expression, and DNA copy number. In addition, publically available databases were interrogated to characterize the degree of MYC deregulation in lung cancer. Functional tests were performed on a large panel of NSCLC cell lines (n = 83) using four drugs that were recently shown to selectively target MYC-driven cancers. Further, we utilized the dominant negative mini-protein OMOMYC for functional classification. In all cases, drug effects were monitored by colony forming efficiency (CFE) assays. OMOMYC results were confirmed via xenograft experiments. Each of the four MYC inhibitors tested elicited a variable response in a subset of the 83 NSCLC cell lines, though the sensitive subset was not similar between any two drugs (highest correlation coefficient of 0.24). In order to determine which, if any, of the drugs targeted MYC-driven lung cancers, we stably expressed OMOMYC in a subset of the NSCLC cell line panel and performed functional assays. Most of the cell lines were sensitive to OMOMYC (with up to 100 fold reduction in CFE), compared to 3/8 that were totally resistant. The variability in the presence of OMOMYC showed a significant correlation with one of the four MYC inhibitors tested. These results support the idea that this sensitive subset represents a truly MYC-dependent class of lung cancers. Surprisingly, there was no correlation between MYC dependence and either MYC mRNA, protein expression or DNA copy number. OMOMYC levels were normalized in all cell lines tested and quantified using qRT-PCR. Additionally, in all cases, exogenous OMOMYC expression led to down regulation of MYC target genes as measured by both qRT-PCR and microarray. These data could be interpreted to suggest that the observed phenotype was the result of decreased MYC activity. Last, the NSCLC probed with OMOMYC showed a variable response in the Wnt pathway, with some cell lines showing a dramatic activation of the Wnt pathway upon OMOMYC induction. This activation proved to be functionally important, as dual inhibition of β-catenin and MYC proved more effective than either approach alone. To investigate the clinical significance of this approach, a subset of the original panel of NSCLC (n = 15) was screened with the MYC inhibitor 10058-F4, Wnt inhibitor Wnt-C59, or a combination of both drugs. Here, 8/15 cell lines displayed a statistically significant increase in sensitivity to MYC inhibition when Wnt pathway inhibition was added. We conclude: there is a subset of NSCLCs that demonstrates dramatic growth inhibition by a single MYC-inhibitor, and these data are phenocopied by the more specific MYC-dominant negative protein, OMOMYC. We further conclude that activation of the Wnt pathway serves as a compensatory response in some cell lines that confers resistance to MYC inhibition. In conclusion, simultaneously targeting MYC and the Wnt pathway elicits superior sensitivity in a subset of NSCLCs, and thus provides rationale for a combinatorial approach in a subset of lung cancer patients.

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