Merkel cell polyomavirus infection and host defence in patients with Merkel cell carcinoma

Background and purpose: Merkel cell carcinoma (MCC) is a rare and often aggressive skin cancer that usually arises in elderly individuals. MCC is frequently associated with ultraviolet radiation exposure and immunosuppressive conditions. The majority of MCCs harbor Merkel cell polyomavirus (MCPyV), a newly discovered human cancer virus. Several studies have indicated its importance in MCC tumorigenesis. The aim of the thesis was to evaluate the frequency of MCPyV infection in MCC and to investigate its associations with patient and tumor characteristics, and with survival. In addition, we aimed to investigate the associations between MCPyV infection with cell cycle regulatory protein expression, platelet-derived growth factor receptor (PDGFR) family protein expression, TP53, KIT and PDGFRA mutations, and their associations with clinicopathological factors. We also determined the frequency and type of leukocytes that infiltrate MCC, and their associations with the presence of MCPyV DNA in MCC and clinicopathological factors including disease outcome. Experimental design: The study was based on a population-wide MCC patient series from Finland, with the MCCs diagnosed in 1979 to 2004, and the corresponding archival formalinfixed paraffin-embedded tumor tissue samples. The patients were identified from the files of the Finnish Cancer Registry. MCPyV DNA was detected using polymerase chain reaction (PCR) and quantitative PCR, tumor infiltrating immune cells were identified and the expression of MCPyV large T (LT) antigen and other proteins was assessed by immunohistochemistry. Gene mutations were investigated using PCR and DNA sequencing. Protein expression was usually assessed from tissue microarray (TMA) sections, while the numbers of tumor infiltrating leukocytes were counted from full tumor tissue sections. The associations between the molecular and host response factors studied and the clinicopathological factors including survival were investigated using conventional statistical tests, such as Kaplan-Meier survival analyses and Cox’s proportional hazards models.