Project Abstract Merkel cell carcinoma (MCC) is one of the most aggressive skin cancers. Clonal integration of Merkel cell polyomavirus (MCPyV) genome into the host DNA has been observed in ~80% of MCCs, and represents a key causal factor for MCC development. LT and sT encoded by MCPyV genome have been shown to support not only viral replication but also MCPyV-induced tumorigenesis. Immune suppression is another important risk factor for the development of MCPyV-associated MCC. MCC has a nearly 50% mortality rate. The incidence of MCC has increased by >95% in the US since 2000. MCC is highly prone to metastasis. The metastatic cancers are more difficult to treat and can often be fatal. Thus, there is a need to better understand the oncogenic mechanisms of MCPyV and MCC in order to develop new strategies to prevent and treat this highly lethal skin cancer. MCC tumors are usually detected in the human dermis, which maintain a hypoxic microenvironment. Hypoxia supports tumor progression partly by driving metabolic adaptation, angiogenesis and metastasis, through upregulation of hypoxia-regulated genes. Importantly, we found that a large number of hypoxia genes are highly induced in MCC, suggesting that the hypoxic skin microenvironment represents an important starting point for MCC progression and metastatic spread. Some of these genes such as carbonic anhydrase 9 (CA9) and vascular endothelial growth factor A (VEGF-A) are critical for promoting tumor growth and metastasis. However, how MCPyV infected cells and MCC tumor cells respond to hypoxia and the impact of hypoxia-regulated gene expression on MCPyV infection and MCC tumorigenesis remain largely unknown. We showed that MCPyV oncogene LT is associated with epigenetic reader BRD4, which functionally interacts with HIF-1α, a key regulator of hypoxic responses, to control the transcription of hypoxic genes such as CA9 and VEGF-A. We also found that MCPyV sT can induce hypoxic gene expression. Building on these findings, we hypothesize that collaborative interactions between MCPyV LT and sT with their host partners, such as BRD4 and HIF-1α, collectively regulate hypoxia gene expression in MCPyV-infected and MCC origin cells to promote viral infection and MCC tumorigenesis. In this project, we propose to determine how MCPyV interacts with the host cells in the hypoxic skin environment (Aim 1), characterize the impact of LT-BRD4-HIF-1α interaction on hypoxic gene expression in skin cells (Aim 2), and investigate the function of MCPyV oncogenes in controlling MCC hypoxic reprograming and the impact on MCC tumorigenesis (Aim 3). These results will fill the gap in our understanding of hypoxic response mechanism in MCPyV-infected cells and associated MCC. Our study may provide new insights into viral and cellular factors that support hypoxia-mediated metabolic reprogramming during MCPyV infection and MCC oncogenic development, revealing new strategies to improve therapeutic intervention of MCPyV-induced ...