(Project Leader: Andrei Goga) Hepatocellular carcinoma (HCC) and hepatoblastoma (HB) are the major forms of primary human liver cancer. Together they represent amongst the top four leading causes of cancer-related death worldwide, nearly 700,000 deaths per year, and HCC has been increasing in recent years in developed nations. Unfortunately most existing therapeutics have only limited activity against HCC. Despite the general lack of activity for most conventional therapeutics, hepatocytes can readily take-up nucleic acids, raising the possibility that circulating extracellular RNAs (exRNAs) play a role in liver tumor formation and regression, and may serve as novel biomarkers. We seek to define the variety of exRNAs found in primary liver tumors as they form and regress, and determine whether they play a role in regulating tumor formation at distant sites of metastasis. To do this, we have developed several new conditional transgenic models of liver cancer that accurately model both liver cancer histology, gene expression and miRNA expression found in human cancers. Furthermore, we have identified a miRNA 'megacluster' comprised of 29 miRNAs that is located at an imprinted locus on human 14q32 (mouse distal chromosome 12) and is over-expressed in our liver cancer models. Over-expression ofthis miRNA cluster in human liver cancer is associated with poorly differentiated histology and shorter patient survival. In our preliminary studies we found that oncogenic miRNAs can contribute to liver tumor formation, and their inhibition attenuates tumor formation. We now seek to: 1) Characterize small exRNAs using conditional transgenic models of MYC and/or RAS driven liver cancer and compare these to exRNAs found in the plasma of patients with HCC, 2) Determine if small exRNAs can regulated gene expression at distant sites, and 3) Determine if small exRNAs can act at distant sites to increase tumor metastasis. The overarching goal of this project is the identification and functional characterization of small exRNAs regulated by oncogene signaling in primary liver cancer. Results from these studies are expected to result and rapid translation offindings to new patient diagnostics and therapeutics.