Endometrial cancer is the most common gynecological cancer in the United States. Research efforts in this disease have focused on identifying key genetic aberrations such as somatic mutations, but no effective targeted therapies have been established. By contrast, "mutations" at the RNA level caused by RNA editing, an important epigenetic control, have received little attention in endometrial cancer and indeed, cancer in general. To date, the importance of individual RNA editing events has been documented in several cancer types; however a comprehensive investigation has not been attempted in any cancer type. Strikingly, the preliminary analyses by this research team reveal that the overexpression of major RNA editing enzymes significantly correlates with poor patient survival in endometrial cancer. The long-term goal is to elucidate the role of RNA editing in the pathophysiology of endometrial cancer and develop novel biomarkers or treatment strategies. The current objective is to systematically identify "driver" RNA editing events and elucidate theirassociated mechanisms in this disease. The central hypothesis is that some RNA editing events play a crucial role in the initiation and progression of endometrial tumors. The rationale is that the key RNA-editing-induced nucleotide changes identified will represent promising biomarkers or therapeutic targets for endometrial cancer and such a study will have major impacts across the breadth of oncology. Guided by strong preliminary data, this hypothesis will be tested by pursuing two specific aims: Aim 1. Detect RNA editing sites associated with tumor over-editing activity or clinical variables in endometrial cancer. The Cancer Genome Atlas (TCGA) sequencing data will be analyzed to identify RNA editing sites that have significant over-editing activity in endometrial tumors or correlate with important clinical variables. The observed patterns of RNA editing will then be validated using orthogonal approaches in both TCGA samples and a large, independent, in-house cohort of endometrial tumor samples. The preliminary results have demonstrated a substantial number of such RNA editing sites. Aim 2. Identify driver RNA editing events in the pathogenesis of endometrial tumors. The functional impact of RNA editing events identified in Aim 1 will be examined through a novel two-step functional assessment strategy using highly sensitive cell viability assays. The genomic aberrations associated with these editing events will be determined through disequilibrium analyses on TCGA/in-house genomics data. For the RNA editing driver events, functional proteomics in selected endometrial tumor cell lines will be used to identify their affected signaling pathways. This project is innovative because it represents a systematic approach to identifying key epigenetic changes as the molecular basis of endometrial cancer. It is significant because the knowledge gained will vertically advance the understanding of endometrial tumorigenesis and facilitate the development and implementation of a novel class of biomarkers or therapeutic targets. In addition, the computational and experimental methods established will be readily applicable to other cancer types.