Phenotypic Heterogeneity among Tumorigenic Melanoma Cells from Patients that Is Reversible and Not Hierarchically Organized

Abstract

Summary We investigated whether melanoma is hierarchically organized into phenotypically distinct subpopulations of tumorigenic and non-tumorigenic cells, or whether most melanoma cells retain tumorigenic capacity, irrespective of their phenotype. We found 28% of single melanoma cells obtained directly from patients formed tumors in NOD/SCID IL2Rγnull mice. All stage II, III, and IV melanomas obtained directly from patients had common tumorigenic cells. All tumorigenic cells appeared to have unlimited tumorigenic capacity upon serial transplantation. We were unable to find any large subpopulation of melanoma cells that lacked tumorigenic potential. None of 22 heterogeneously-expressed markers, including CD271 and ABCB5, enriched tumorigenic cells. Some melanomas metastasized in mice, irrespective of whether they arose from CD271- or CD271+ cells. Many markers appeared to be reversibly expressed by tumorigenic melanoma cells. Significance In cancers that follow a stem cell model, phenotypically distinct tumorigenic cells form abundant and phenotypically diverse non-tumorigenic progeny in a hierarchical manner that resembles normal stem cell differentiation. In contrast to this model, our results indicate that primary cutaneous or metastatic melanomas from patients have common and phenotypically diverse tumorigenic cells that undergo reversible phenotypic changes in vivo. Most of the phenotypic heterogeneity in melanoma is therefore not associated with a loss of tumorigenic potential or organized in stable hierarchies. These data suggest a phenotypic plasticity model in which phenotypic heterogeneity is driven largely by reversible changes within lineages of tumorigenic cells rather than by irreversible epigenetic or genetic changes.