Exosomal melanotransferrin as a potential biomarker for metastatic melanoma

Cutaneous melanoma is the most lethal type of skin cancer. Early detection is crucial to improve the outcome of melanoma patients. The identification of noninvasive prognostic biomarkers for the follow-up of melanoma patients is still in demand for clinical use. We show here that exosomal melanotransferrin (MTf) fulfils the biomarker characteristics required to meet this demand. MTf is typically overexpressed in melanoma cells compared to other cell types - including cancer cells - and is efficiently sorted and secreted with nanovesicles, or so-called exosomes, due to its membrane-anchoring by a glycosylphosphatidylinositol. MTf is exposed on the surface of exosomes and is accessible for antibody recognition. An ELISA was set-up to quantify MTf after immobilization of nanovesicles through the exosomal constituent tetraspanins CD63. MTf was detected using low number of exosomes purified from melanoma cell line cultures, and MTf detection was abolished by phosphatidylinositol-specific phospholipase C (PI-PLC) treatment. This exosomal MTf ELISA was able to discriminate an equal number of assayed exosomes purified from two different melanoma cell lines (A-375 vs SK-MEL-28). Moreover, plasma samples from patients with melanoma and noncancer disease were assayed using this ELISA and exosomal MTf was validated as a potential melanoma biomarker.