Melanocytes are considered cells-of-origin of melanomas. Because endogenous heterogeneity of melanocytes in the developmental lineages (e.g. progenitors or progenies) or in the anatomical areas (e.g. hairy vs glabrous skin) would be the base of inter-tumor heterogeneity in melanoma subtypes which should be originated from distinct melanocyte subpopulations, refining characterization of them should facilitate understanding development of melanomas.
Here we genetically, phenotypically and functionally studied heterogeneity in normal epidermal melanocytes in human hairy (breast) skin utilizing single cell RNA-seq. Four epidermal melanocyte subpopulations were newly determined by single cell molecular profiling, including ones with progenitor/precursor-like characteristics as well as ones with progeny-like characteristics.
One subpopulation with the exclusive expression of Neurotrophic tyrosine kinase receptor type 2 (NTRK2) showed differential gene expression profile which was inferred to be corresponding to activated progenitor/precursor melanocytes, eg, activated cell cycle and enhanced ribosome biogenesis with relatively undifferentiated cell status.
Analysis of NTRK2+ melanocytes in the human hairy skin by immunohistochemistry or flow cytometry showed that they were located both in the inter-follicular epidermis and in the hair follicle at various proportions depending to anatomical areas of the hairy skin and were augmented in the epidermis of tanned area than the less tanned area of the fresh human skin.
We utilized NTRK2 as a cell surface marker to sort viable NTRK2+ or - melanocytes from fresh human skin and evaluated phenotypes and functions of them in vitro. Interestingly NTRK2+ melanocytes showed distinct colony-formation and enhanced radio-resistance against ultraviolet B (UVB) compared to NTRK2- ones in vitro, indicating possible association with melanomagenesis in the context of sun-exposure.