MicroRNA Biogenesis and the Cancer Proteome

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Recent studies in human cancers have revealed defects in microRNA biogenesis that promote tumor aggressiveness and decrease survival via mechanisms that are poorly understood. Much of the control of protein expression by microRNAs occurs without alterations in mRNA expression, and single microRNAs may target dozens of mRNAs in a tissue specific manner. Thus, this broad-based decrease in microRNA synthesis presents a challenge to those trying to understand its downstream molecular effects on carcinogenesis. Here we propose a new strategy that will comprehensively identify the mechanisms by which defects in microRNA biogenesis decrease cancer survivorship. This approach involves the integration of genome-scale high throughput quantitative mass spectrometry analysis of the cancer proteome with genome-wide microRNA, mRNA and DNA analyses of human brain tumors (glioblastomas) that have intact or defective microRNA biogenesis. Novel algorithms that incorporate clinical variables will be used to generate an integrated genome- scale view of identified differences, and clinically-related targets will be investigated further using in vivo and in vitro models. Preliminary studies in our laboratory using aCGH and mRNA microarrays revealed deletion at the DICER1 locus and decreased DICER1 mRNA expression in a subset of glioblastomas, and this correlated with decreased microRNA expression and survival. Genome- wide analysis identified numerous microRNAs that were differentially expressed between glioblastomas with low versus high DICER1 expression. Cdk6, which promotes cell cycle progression, was a predicted target of several of these microRNAs. DICER1 knockdown in glioblastoma cells increased cell growth and upregulated Cdk6 protein expression. Importantly, Cdk6 protein was overexpressed in glioblastomas with low DICER1 expression, suggesting that Cdk6 upregulation may be one mechanism by which defective microRNA biogenesis contributes to increased tumor aggressiveness. This innovative and ambitious project will integrate mass spectrometry proteomics, genomics and clinical variables to comprehensively identify the mechanisms underlying the decreased cancer survivorship associated with dysregulated microRNA biogenesis.