A phenotype-based cell culture model of melanoma cells that persist as residual disease after therapies leading to recurrence.

A crucial adaptability trait of stem-like melanoma cells that persist under all selection pressures, including therapies, is their ability to survive in deep quiescence. This ability coupled with their intrinsic ability to proliferate leads to recurrence. Here we describe an approach to modeling this trait in cell culture. A lack of glutamine proved to be a selection pressure for the highly metastatic human melanoma cell line A375SM, killing more than 99% of cells and selecting rare cells based on their ability to survive in deep quiescence. After 4 weeks, cells gradually exited quiescence and proliferated indefinitely. Interestingly, by not providing fresh glutamine-free medium at this stage, we could select rare cells that persist in deep quiescence as single cells. Alternatively, we could model deeper quiescence lasting longer than 4 weeks by increasing the severity of the selection pressure using dialyzed serum in medium. The cells selected in this manner were much more resistant to paclitaxel than was their parental cell line. We obtained similar results with the highly metastatic mouse melanoma cell line B16-BL6. Thus, our phenotype-based approach is suitable for modeling abnormal deep quiescence in melanoma that is responsible for therapy resistance, disease progression, and recurrence/metastasis.