Intratumoral heterogeneity (ITH), defined by the presence of phenotypically-, genetically- and/or epigenetically-distinct sub-populations within a single tumour, is a feature of most human cancers, including melanoma. ITH has significant clinical implications if distinct sub-populations differ in their functional capacity to propagate malignant disease and/or respond to different treatments.
Here, we find that melanomas are highly heterogeneous for production of the pigment, melanin. Histologically, 84% of patient melanomas contain melanin and, of these, 91% are comprised of mixtures of cells with low and high pigment content (LPC and HPC, respectively). We observe an identical pattern of cellular heterogeneity in laboratory models of pigmented melanoma, including mouse and human cell lines and patient-derived xenografts (PDX). We developed a novel method to prospectively separate LPC from HPC melanoma cells using fluorescence-activated cell sorting (FACS). Using this approach we find that LPC cells are significantly enhanced in clonogenic potential in vitro and tumorigenic potential in vivo, compared to HPC cells, but that tumorigenic LPC cells are not rare. Moreover, LPC- but not HPC-derived cell populations can recapitulate the pigment heterogeneity of the parental culture. These findings are inconsistent with both classical CSC and plasticity models of melanoma propagation, and suggest that in most patients with pigmented melanomas effective therapies must eradicate the abundant tumorigenic LPC cell population. Future studies defining the mechanisms driving the hierarchy between LPC and HPC cells may identify novel opportunities for targeting tumorigenic LPC cells in melanotic melanomas.