MCPyV infection is associated with clinical outcomes in patients with Merkel cell carcinoma. These findings lend support to the hypothesis that viral infection is frequently associated with the pathogenesis of Merkel cell carcinoma.

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Purpose: Merkel cell carcinoma (MCC) is rare skin cancer that is often associated with Merkel cell polyomavirus (MCPyV) infection. Polyomaviruses repress tumor suppressor proteins, thus influencing cell-cycle progression, but the effect of MCPyV on the key cell-cycle regulating proteins is poorly understood.Experimental Design: We evaluated expression of the MCPyV large T-antigen (LTA), Ki-67, and the key putative tumor suppressor proteins, the retinoblastoma protein (RB and phospho-RB) and p53, and their regulatory proteins (cyclin D1, cyclin E, p16, p21, p27, and MDM2) by using immunohistochemistry from tumors of 91 MCC patients identified from a population-based nationwide cohort. Tumor MCPyV DNA was measured by using quantitative PCR, and TP53 mutations were identified with sequencing.Results: MCPyV LTA expression was strongly associated with presence of MCPyV DNA in tumor, and it was almost invariably associated with tumor RB expression (P < 0.0001 for both comparisons). Both MCC LTA and RB expression were strongly associated with favorable MCC-specific and overall survival in univariable analyses (P 0.01 for all four analyses). Presence of MCPyV LTA was also associated with the female gender, the intermediate type of tumor histology, location of the tumor in a limb, cell proliferation rate, and absence of p53 expression. TP53 mutations were detected only in MCPyV DNA-negative tumors.Conclusions: MCPyV DNA-positive MCC has several clinical and molecular features that differ from MCPyV DNA-negative cancers. MCPyV-associated MCCs express RB, but may not harbor TP53 mutations. These findings provide further support that MCPyV causes the majority of MCCs.