Experimental animal models for human melanoma development have been difficult to establish because of the unique architectural and functional characteristics of human skin hampering investigations on etiology, diagnosis, and prevention of the disease. Epidemiological observations suggest a role for ultraviolet (UV) light irradiation in the etiology of human melanoma but direct evidence is still missing. The histological diagnosis of precursor dysplastic nevi and biologically early primary melanomas continuous to be difficult and controversial, even among experts, and there is currently little understanding of gene aberrations and their importance for melanoma development. To better understand development of human melanoma and to detect melanocytic lesions at the earliest stages with new molecular markers, we have developed a human skin graft model in immunodeficient RAG-1 mice in which human skill from newborns is exposed to the chemical carcinogen 7,12-dimethyl(a)benzanthracene (DMBA) followed by UV irradiation for up to 10 months. Preliminary studies with this model have revealed melanocytic hyperplasia, lentigo, dysplasia, and melanoma. Using a refined grafting technique we are now able to graft skin from adults and will select donors who have a high susceptibility for developing dysplastic nevi and/or melanomas to determine the optimal UV wavelength for induction of melanocytic lesions and to develop new criteria for melanocyte transformation in vivo, including proliferation, escape of melanocytes from control by keratinocytes, loss of melanocyte contact from basement membrane and morphological changes associated with melanocytic dysplasia. We will then define new molecular markers for transformation using microarrays for analyses of mRNA expression in cells from microdissected lesions or short-term cultures. These initial screening approaches are followed by immunohistochemistry and in situ hybridization analyses to provided information on gene expression in single cells at the RNA and protein levels, respectively. To investigate the role of major check point genes, we will transduce melanocytes in vitro with either the E6 gene or human papilloma virus for p53 sequestration or with H-ra for ras activation. Transduced melanocytes will then be included into skin reconstructs consisting of dermis and epidermis prior to grafting in vivo and long-term UV irradiation. Our unique in vitro/in vivo models are suitable to investigate etiology, early detection, and prevention of human melanoma.
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