Analysis of ras oncogenes in malignant melanoma and precursor lesions: correlation of point mutations with differentiation phenotype.

This study examined noncultured and cultured melanomas and related precursor specimens for (i) mutated ras genes using polymerase chain reaction (PCR) methodology, (ii) correlation of mutated ras genes with differentiation related phenotypic characteristics, (iii) expression of ras-encoded p21 proteins in tissues by immunoperoxidase analysis, (iv) quantitative expression of mutated and wild-type ras encoded p21 proteins by flow cytometry, and (v) correlation between p21 expression, the occurrence of ras mutations, and cell cycle kinetics. The results of these studies are (1) 24% of cultured malignant melanomas have activated ras genes, with N-ras being activated ten times as frequently as Harvey (Ha)-ras. Each example of an activated ras gene showed a mutation at the 61st codon of the protein, with the exception of one melanoma which showed a mutation at codon 13 of the N-ras gene; (2) all the melanomas displaying an activated ras gene had a similar cell surface phenotype and appear to come from a similar phase of differentiation; (3) 5-6% of noncultured primary and metastatic melanomas have mutated ras genes; (4) no ras gene mutations were found in any precursor lesion, specifically normal nevi and dysplastic nevi; (5) immunoperoxidase analysis of paraffin-embedded specimens indicated no quantitative or qualitative alterations in p21 expression that correlate with tumor progression; (6) there were no observable differences in p21 expression between melanoma cells growing exponentially or in plateau phase, or between melanoma cells with or without ras mutations; nor were any cell kinetic differences found between cells with and without mutated ras genes. These studies suggest that the role of ras mutations may be limited to an indirect involvement in the transformation of a subset of melanomas.