Purpose
Merkel cell carcinoma (MCC) is an aggressive skin cancer with neuroendocrine differentiation. There is an unmet need for MCC-specific blood-based surrogate biomarkers of tumor burden; circulating cell-free micro(mi)RNA may serve this purpose. Experimental Design: Expression of miR-375 was quantified in 24 MCC and 23 non-MCC cell lines, 67 MCC and 58 non-MCC tumor tissues, sera of two preclinical MCC models, sera of 109 MCC patients and 30 healthy controls by nCounter®human-v2-miRNA expression or miR-375 specific real-time PCR assays. The patients’ sera consisted of two retrospective (discovery and training) and two prospective (validation) cohorts. Results: miR-375 expression was high in MCC cell lines and tissues compared to non-MCCs. It was readily detected in MCC conditioned medium and sera of preclinical models bearing MCC xenografts. miR-375 levels were higher in sera from tumor-bearing MCC patients than in tumor-free patients or healthy controls (p<0.0005). Moreover, miR-375 serum levels correlated with tumor stage in tumor-bearing (p=0.037) but not in tumor-free (p=0.372) MCC patients. miR-375 serum level showed high diagnostic accuracy to discriminate tumor-bearing and tumor-free MCC patients as demonstrated by receiver operating characteristic curve analysis in the retrospective cohorts (AUC=0.954 and 0.800) as well as in the prospective cohorts (AUC=0.929 and 0.959). miR-375 serum level reflected dynamic changes in tumor burden of MCC patients during therapeutic interventions. Conclusions: Circulating cell-free miR-375 proved as a surrogate marker for tumor burden in MCC without restriction to polyomavirus positivity, it thus appears to be useful for therapy monitoring and the follow-up of MCC patients.