Identification and molecular characterisation of drug-resistance melanoma stem cells

Abstract

Cutaneous melanoma is a malignancy of melanocytes in the skin. Although cutaneous melanoma represents a minority of skin cancer diagnoses, it is over-represented in skin cancer mortalities as it becomes difficult to treat after metastasis. The introduction of small molecule BRAF and MEK kinase inhibitors has significantly improved patient outcomes, but a large population of metastatic melanoma patients experience aggressive tumour recurrence. Such recurrence is thought to be driven by melanoma stem cells, which are slow cycling, dedifferentiated and express a range of drug resistance markers. To further understand the molecular and functional properties of melanoma stem cells, we have generated a A375 melanoma reporter line stably expressing eGFP upon activation of the human OCT4 promoter for marking melanoma stem cell population. Our flow cytometry analysis identified a rare population of BRAF/MEK-inhibitor treated OCT4-eGFP+ melanoma cells which express high levels of embryonic and melanoma stem cell markers not present in untreated populations. This rare population of eGFP+ cells are also enriched with PD-L1 which has been shown to bestow melanoma stem cells with immunosuppressive effect to enhance their self-renewal capacity and tumorigenicity as well as acquired resistance to PD-1 monoclonal antibody treatment. Interestingly, these eGFP+ cells upregulate expression of FSTL1, a secreted glycoprotein which has modest role in melanoma tumorgenicity. Administration of FSTL1 neutralising antibodies has been reported to reduce tumorigenicity in many cancer types. Based on these findings, we hypothesise that this rare population of drug resistance melanoma cells could fulfil the functions of melanoma stem cells and express a druggable secretory protein FSTL1. If so, this small population of cells could be characterised to identify new druggable targets to specifically target melanoma stem cell populations resistant to traditional BRAF and MEK inhibitors and PD-1 monoclonal antibodies.