Absence of ras gene mutations in UV-induced malignant melanomas correlates with a dermal origin of melanocytes in Monodelphis domestica

The South American opossum, Monodelphis domestica, has been used as a model system to study ultraviolet (UV)-induced genetic alterations that lead to the development of melanoma. Suckling young of Monodelphis exposed to multiple doses of UVB radiation can develop benign or malignant melanomas later as adults. Point mutations predominantly at codon 61 of the N-ras gene have been found in melanomas from sun-exposed body sites in humans. To determine if similar mutations are associated with UV-induced melanoma in Monodelphis, the nucleotide sequence of a Monodelphis N-ras cDNA was determined, and the occurrence of ras mutations in melanomas from UV-irradiated opossums was investigated. Single-strand conformation polymorphism analysis revealed no mutations in either the Monodelphis N-ras or H-ras genes in any of 24 primary malignant melanoma samples analyzed in this study. The disparate association of ras mutations with melanoma in humans and Monodelphis may be explained by differences in nucleotide sequence at codon 61 of the N-ras gene as well as differences in skin architecture between the two species. These results support the contention that a mutationally activated N-ras gene contributes to the vertical growth phase, which is specific to the progression of malignant melanoma in humans.