Neuroendocrine tumors arise in multiple tissues, including the skin, lung, prostate, and pancreas, and share common neural and endocrine phenotypes. Discovering their mechanism of carcinogenesis is challenging due to our inability to directly observe tumor formation, as well as the mutational heterogeneity of established tumors. Unlike tumors that arise through random mutational processes, Merkel cell carcinoma (MCC) is a neuroendocrine skin cancer that can be caused by infection with Merkel cell polyomavirus (MCPyV) and whose mechanisms can be studied in vitro. MCPyV contains two oncogenes, the small T (ST) and large T (LT) antigens. The impact of ST on cell migration and LT on the cell cycle are known, but there is no genome-wide, quantitative understanding of how ST and LT reprogram normal cells into a neuroendocrine tumor prone to metastasis. Our preliminary RNA-sequencing data from normal cells expressing MCPyV oncogenes indicates that LT transcriptionally alters invasion and migration-related pathways such as WNT and TGF-beta signaling, in a manner that is similar to expression changes in human MCC tumors and metastases. Using this system, we can also measure gene expression dynamics and directly observe the temporal sequence of molecular events underlying carcinogenesis in a way that cannot be achieved through static tumor measurements alone. The viral etiology of MCC therefore provides us with a unique opportunity to systematically investigate and quantify the regulatory circuits driving neuroendocrine tumor formation and metastasis. In Aim 1, we will reconstruct the host transcriptional networks and signaling pathways utilized by MCPyV to promote cell motility and invasion, determine the roles of LT, WNT, and TGF-beta in promoting metastasis, and predict the optimal strategy for inhibiting these mechanisms. In collaboration with metastasis experts at the University of Arizona, we will test our predictions in a series of MCC model systems, including cell-based migration and invasion assays and mouse xenograft studies. In Aim 2, we will measure protein dynamics of neural regulators activated by ST and LT and integrate them with gene expression and ATAC-seq data to build a network model of the cell fate transition induced by MCPyV. Our work will provide a systems-level analysis of regulatory circuits underlying metastasis and cellular reprograming in neuroendocrine tumors, and will identify new therapies for Merkel cell carcinoma that may be applicable to other tumor types.