Melanoma is the deadliest type of skin cancer that arises from transformed melanocytes. Due to its aggressiveness, melanoma has a high propensity to metastasize and invade other essential organs to seed distant tumors. The main goal of this study was to define when metastatic melanoma cells leave the primary tumors and reach the circulating system to potentially initiate metastases. To achieve this, a spontaneous melanoma metastatic mouse model (Dct-Grm1/K5-Edn3) has been created where metastasis to the lungs is 80% penetrant. The Dct-Grm1/K5-Edn3mice were crossed to TYR-CreERT2/mT/mGmice to indelibly label tyrosinase-expressing cells within the primary tumors by topical application of 4-Hydroxytamoxifen (4HT), which induces the expression of green fluorescence protein (GFP). In vivolineage tracing of GFP-labeled cells showed that tyrosinase-expressing cells derived from the primary tumor, or their progeny, can seed successful metastases in the lung thereby demonstrating their tumor initiating capacity. To establish the timing of metastatic cell intravasation, tyrosinase-expressing cells were labeled in the mice at three different stages during tumor progression: pre-nevus stage, nevus stage and mature tumor stage. As expected, GFP-labeled cells were found entering blood vessels at the time tumors had formed. They were also observed entering blood vessels at the nevus stage. Interestingly, GFP-labeled cells were also found in close association with blood vessels prior to the appearance of detectable nevi at sites where tumors generally develop. These results indicate that tyrosinase-positive cells have the ability to disseminate very early and continue to do so during the process of melanomagenesis. Further characterization of the early aggressive cells in melanoma will allow for the development of new prognostic tests and novel therapeutic strategies to eliminate metastasis.
