Purpose: Uveal melanoma (UM) cells in high-metastatic risk tumors have an undifferentiated molecular signature indicative of a more primitive cellular phenotype. Given mounting evidence for the existence of cancer stem cells (CSC), the authors investigated whether UM cell lines retain a population of self-renewing tumorigenic cells.
Methods: Single-cell cloning and spheroid culture studies were used to study the presence of a CSC-like population in two cell lines derived from the primary tumor (Mel270) and metastatic liver lesion (Omm2.5) of the same patient.
Results: Mel270 and Omm2.5 cells exhibited distinct clonal morphologies in adherent culture akin to holoclones, meroclones, and paraclones. Holoclones were large colonies of tightly packed small cells, which could be serially passaged (> 10 generations) to produce colonies of all three types; paraclones were small colonies of flattened cells that could be passaged for only one or two generations to produce further paraclones. Mel270 and Omm2.5 cells surviving cisplatin treatment produced significantly more holoclones than untreated cells (P < 0.05), suggesting enrichment for this CSC-like subpopulation. Mel270 and Omm2.5 cells also formed melanomaspheres (MS) when grown at clonal density in nonadherent culture. MS possessed self-renewal capacity to generate further MS and when replated to adherent culture yielded colonies of all three types. Mel270 and Omm2.5 holoclones and MS also demonstrated antigenic heterogeneity expressing markers associated with both a primitive migratory neural crest phenotype, and a more differentiated phenotype.
Conclusions: These data suggest the presence in UM cell lines, of a CSC-like subpopulation with enhanced self-renewal and proliferative capabilities that could more appropriately model therapeutic efficacy.