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Abstract

Malignant melanoma remains the most deadly form of skin cancer due to its quick metastatic spread and the development of resistance to available treatment. The cause of melanoma is still under investigation but environmental factors, such as ultraviolet radiation, have been associated with the initiation of melanoma. Moreover, studies have revealed that the melanocytic lineage is predisposed to malignant transformation due to its developmental program. Melanocytes are derived from the embryonic neural crest, which utilizes processes such as the epithelial-to-mesenchymal transition (EMT) during their normal development to spatially migrate and complete terminal differentiation. Therefore, recent work in the field of melanoma has focused on investigating embryonic and neural crest-related genes since they may be reactivated during melanomagenesis and metastatic spread. To date, limited studies have suggested an important role of the embryonic stem cell marker, sex determining region Y-box 2 (SOX2), in melanoma; however a lack of detailed analyses and understanding of its function remains. In this study, SOX2 was found to be highly expressed in primary melanomas compared to melanocytic nevi. Additionally, using fluorescence in situ hybridization analysis, genomic SOX2 was found to be significantly amplified in both primary melanomas and metastatic melanomas compared to melanocytic nevi. Gain-of-function studies revealed that melanoma cells lost differentiation marker expression upon SOX2 overexpression in vitro. The dedifferentiated phenotype displayed can be in part explained by SOX2 binding to the promoter region of the microphthalmia-associated transcription factor-M (MITF-M), thereby repressing its transcription. Additionally, SOX2 was shown to be required for melanoma cell migration and invasion in vitro. Invasion-related EMT markers were upmodulated upon SOX2 overexpression and immunohistochemical analysis revealed high SOX2 expression in deep regions of primary melanomas and in stroma-infiltrating melanoma cells. In vitro enhanced SOX2 expression could be induced by TGF-β, indicating TGF-β signaling as an upstream regulator of SOX2 expression in melanoma. This study proposes that TGF-β1 induces SOX2 expression, which may lead to melanoma progression by: i) SOX2 binding and repressing the MITF-M promoter, which may influence the dedifferentiation of human melanoma cells, and ii) SOX2 inducing high expression of ZEB1 and TWIST1, which promotes a mesenchymal phenotype. In line with the described phenotypic alterations, this work revealed that SOX2 enhances melanoma cell migration and invasion and depletion of this transcription factor results in loss of cellular motility. Therefore, I identified SOX2 as a key player in the complex molecular network that governs invasion-related processes and I revealed a role for SOX2 as an invasion-related marker with potential clinical application.