Merkel cell carcinoma (MCC) is an aggressive and lethal type of neuroendocrine skin cancer. Mutated Merkel cell polyomavirus (MCV) is commonly found in MCC, and leads to upregulation of the survivin oncogene. However, ∼20% of MCC tumors do not have detectable MCV, suggesting alternative etiologies for this tumor type. In this study, our aim was to evaluate microRNA (miRNA) expression profiles and their associations with MCV status and clinical outcomes in MCC. We showed that miRNA expression profiles were distinct between MCV-positive (MCV+) and MCV-negative (MCV−) MCCs and further validated that miR-203, miR-30a-3p, miR-769-5p, miR-34a, miR-30a-5p, and miR-375 were significantly different. We also identified a subset of miRNAs associated with tumor metastasis and MCC-specific survival. Functionally, overexpression of miR-203 was found to inhibit cell growth, induce cell cycle arrest, and regulate survivin expression in MCV− MCC cells, but not in MCV+ MCC cells. Our findings reveal a mechanism of survivin expression regulation in MCC cells, and provide insights into the role of miRNAs in MCC tumorigenesis.

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Telomerase activation through the induction of its catalytic component TERT is essential in carcinogenesis. The regulatory mechanism and clinical significance underlying cancer-specific TERT expression have been extensively investigated in various human malignancies, but little is known about these in Merkel cell carcinoma (MCC), an aggressive neuroendocrine skin tumor. Here we addressed these issues by determining TERT promoter mutations, gene amplification, mRNA expression and association with clinical variables in MCC. TERT mRNA was expressed in 6/6 MCC cell lines and 41 of 43 tumors derived from 35 MCC patients. Telomerase activity was detectable in all 6 cell lines and 11 tumors analyzed. TERT promoter mutations were identified in 1/6 cell lines and 4/35 (11.4%) MCC cases. The mutation exhibited UV signature and occurred in sun-exposed areas. Increased TERT gene copy numbers were observed in 1/6 cell lines and 11/14 (79%) tumors, and highly correlated with its mRNA expression (r = 0.7419, P = 0.0024). Shorter overall survival was significantly associated with higher TERT mRNA levels in MCC patients (P = 0.032). Collectively, TERT expression and telomerase activity is widespread in MCC, and may be attributable to TERT promoter mutations and gene amplification. Higher TERT expression predicts poor patient outcomes.

MicroRNA-375 (miR-375) is deregulated in multiple tumor types and regulates important targets involved in tumorigenesis and metastasis. This miRNA is highly expressed in Merkel cell carcinoma (MCC) compared to normal skin and other non-MCC skin cancers, and its expression is high in Merkel cell polyomavirus (MCPyV)-positive (MCPyV+) and low in MCPyV-negative (MCPyV−) MCC tumors. In this study, we characterized the function and target of miR-375 in MCPyV+ and MCPyV− MCC cell lines. Ectopic expression of miR-375 in MCPyV− MCC cells resulted in decreased cell proliferation and migration, as well as increased cell apoptosis and cell cycle arrest. However, in MCPyV+ MCC cells, inhibition of miR-375 expression reduced cell growth and induced apoptosis. Additionally, the expression of lactate dehydrogenase B (LDHB), a known target of miR-375, was inversely correlated with miR-375. Silencing of LDHB reduced cell growth in MCPyV− cell lines, while its silencing in MCPyV+ cell lines rescued the cell growth effect mediated by miR-375 inhibition. Together, our results suggest dual roles of miR-375 and LDHB in MCPyV and non-MCPyV-associated MCCs. We propose that LDHB could be a therapeutic target in MCC and different strategies should be applied in virus- and non-virus-associated MCCs.

Viruses can inhibit host autophagy through multiple mechanisms, and evasion of autophagy plays an important role in immune suppression and viral oncogenesis. Merkel cell polyomavirus (MCPyV) T‐antigens are expressed and involved in the pathogenesis of a large proportion of Merkel cell carcinoma (MCC). Yet, how MCPyV induces tumorigenesis is not fully understood. Herein, we show that MCPyV T‐antigens induce miR‐375, miR‐30a‐3p and miR‐30a‐5p expressions, which target multiple key genes involved in autophagy, including ATG7, SQSTM1 (p62) and BECN1. In MCC tumors, low expression of ATG7 and p62 are associated with MCPyV‐positive tumors. Ectopic expression of MCPyV small T‐antigen and truncated large T‐antigen (LT), but not the wild‐type LT, resulted in autophagy suppression, suggesting the importance of autophagy evasion in MCPyV‐mediated tumorigenesis. Torin‐1 treatment induced cell death, which was attenuated by autophagy inhibitor, but not pan‐caspase inhibitor, suggesting a potential role of autophagy in promoting cell death in MCC. Conceptually, our study shows that MCPyV oncoproteins suppress autophagy to protect cancer cells from cell death, which contribute to a better understanding of MCPyV‐mediated tumorigenesis and potential MCC treatment.