To better characterize the biology of the multiple primary melanoma (MPM), we explored the global microRNA profile of 21 single and multiple primary melanomas, including multiple tumors from the same patient and 3 benign nevi.

Malignant cutaneous melanoma is a potentially lethal form of skin cancer whose worldwide incidence has been increasing constantly over the past decades.A fraction of all cutaneous melanoma patients (up to 8%) develop multiple melanomas during their lifetime, usually at a young age and within 3 years from the first tumor/diagnosis, a condition known as multiple primary melanoma (MPM). Patients affected by multiple primary melanoma could have a genetically determined susceptibility, though germline mutations in hereditary melanoma genes are rarely detected in these patients.To better characterize the biology of this subset of MPM, we explored the global microRNA profile of 21 single and multiple primary melanomas, including multiple tumors from the same patient and 3 benign nevi. We applied an unsupervised smallRNA sequencing approach to demonstrate that multiple primary melanomas have a distinctive miRNA expression. Indeed, 22 miRNAs were differentially expressed in MPM compared to single cutaneous melanoma, including key miRNAs involved in epithelial-mesenchymal transition (EMT). Moreover, the comparison between the first and the second tumor from the same patient showed that the miRNA profile changed in the second tumor (37 differentially expressed miRNAs). We validated the differential expression of miR-25-3p, miR-149-5p, miR-92b-3p, miR-211-5p, miR-125a-5p, miR-125b-5p, miR-205-5p, miR-200b-3p in a larger cohort of single and MPM patients (N=29). We identified the molecular pathways where they act the miRNAs that are dysregulated in MPM. Overall, we demonstrated a more differentiated and less metastasis-prone status of MPM compared to single primary tumors and provided insights into multiple melanoma molecular pathogenesis.