Prospective Isolation According to Melanin Pigment Content of Melanoma Cells with Heterogeneous Potentials for Disease Propagation.

Intra-tumoral heterogeneity poses a major challenge to treating and managing cancer patients. A characteristic feature of melanoma is its composition of cancer cells with typically heterogeneous content of melanin pigment, the production of which is a hallmark of normal melanocytic differentiation but of poorly understood consequence in melanoma cells, as prospective assessment of pigment heterogeneity in melanoma cells has been experimentally challenging. Here, we describe a novel flow cytometric method for high purity separation of viable melanoma cells based on their melanin content, exploiting the light scattering properties of melanin. By fluorescence-activated cell sorting, we show that cells with low-pigment content (LPCs) in melanoma cell lines and patient tumors are usually far more abundant than high-pigment cells (HPCs) and have substantially increased potentials for colony formation in vitro and tumor formation in vivo. In RNAseq analysis, HPCs showed P53 activation and perturbed cell cycling, whereas LPCs displayed upregulation of MYC-associated transcription and activated ribosome biogenesis. In proof-of-concept studies, the latter was targeted by topoisomerase 2 beta targeting with CX-5461, which induced senescent HPC phenotypes and irreversible loss of clonogenic activity. These data indicate an 'inverted pyramid' hierarchical model of melanoma cell propagation wherein abundant LPCs frequently renew their own malignant potential to propagate disease but also infrequently generate HPCs that spontaneously lose this ability in a manner that might be exploited as an anti-melanoma strategy.