Loss of alpha-dystroglycan expression in cutaneous melanocytic lesions

The dystroglycan complex (DG), originally characterized in muscle and in genetic muscle diseases, links the epithelial cell cytoskeleton to the basement membrane.[1] DG has been shown to be involved in skin morphogenesis and in epithelial carcinogenesis, modulating cell differentiation and adhesion, assembly of the epithelial basement membrane and interactions with the extracellular matrix (ECM).[2] DG comprises an alpha (α-DG) and a beta (β-DG)-subunits, with the α-DG subunit being the extracellular, functional part of the complex and binding several ECM components. Loss of α-DG function has been reported in several epithelial- and neural-derived tumours and correlated with tumour grading, progression and clinical outcome.[3] In the skin, DG is localized at the dermo-epidermal junction and is produced by epidermal keratinocytes and dermal fibroblasts.[4] Tissue expression of the α-DG subunit has not been previously characterized in cutaneous melanocytic nevi and in malignant melanoma. In this study, expression of the DG complex was analysed by Western-blot in cell extracts from human normal melanocytes (hMel) and three melanoma cell lines (BLM, M14, IF6). Furthermore, tissue expression of α-DG was assessed by immunohistochemistry in a panel of archival melanocytic lesions, including melanocytic nevi (n = 20), cutaneous melanoma (n = 51) and melanoma metastases (n = 53). A polyclonal antibody to alpha-DG (clone VIA4-1) from Upstate Biotechnology and a monoclonal antibody to β-DG (clone 43DAG/8D5) from Novocastra were used for the analyses. β-DG molecule was detectable in both melanocytes and melanoma cells displaying a higher expression in IF6 melanoma cells compared to normal melanocytes. On the other hand, a decreased expression of α-DG was observed in all three melanoma cell lines compared to normal melanocytes (Fig. 1). At the tissue level, in benign melanocytic nevi α-DG was expressed by basal melanocytes at the dermo-epidermal junction and by epithelioid melanocytes, organized in nests, in the superficial dermis (Fig. 2a,b). In primary cutaneous melanoma, α-DG expression appeared to be lost in almost all cases. Loss of α-DG expression in melanoma was observed at all Clark invasion levels, both in the epidermis and dermo-epidermal junction as well as in the superficial to deep dermis (Fig. 2c–e). In melanoma metastases, expression of α-DG was evident in 20.8% of cases, being mainly focal and limited to spindle-cell like melanoma cells (Fig. 2f). Previous studies have only characterized the in-vitro expression of DG subunits by melanocytes and melanoma cells, as well as tissue expression of β-DG in melanocytic nevi.[5, 6] We report a differential pattern of α-DG expression between benign and malignant melanocytic tumours, suggesting a peculiar role of the DG complex in melanocyte biology and melanomagenesis. In benign melanocytic nevi, α-DG displayed a consistent expression pattern at the dermo-epidermal junction, where interactions with basal keratinocytes and ECM components are prominent in the epidermal melanin unit. In primary cutaneous melanoma, a loss of α-DG tissue expression was observed across all stages of progression. This finding is compatible with the loss of anchorage to the basement membrane in the early intraepidermal proliferation of transformed melanocytes due to an altered expression of ECM binding proteins.[7] Loss of the α-DG subunit in melanoma is likely to be caused by several post-translational mechanisms, such as an altered glycosylation pattern and/or proteolytic degradation of the membrane complex.[8] Loss of α-DG function results in a disruption of cell-to-ECM interactions and might promote tumour invasion and metastasis. However, the observed expression of α-DG on cells with spindle-cell like morphology in melanoma metastasis suggests that its restoration could favour the implant of cells in metastatic sites, as previously reported in other cancers.[9] Further in vitro and in vivo studies are warranted to assess the role of the DG complex in melanocyte-keratinocyte interactions and in human melanomagenesis.