RAS MUTATIONS IN HUMAN-MELANOMA - A MARKER OF MALIGNANT PROGRESSION

In this study we address whether there is an association between ras m utations and disease progression in malignant melanoma. DNA was extrac ted from 100 paraffin-embedded melanomas and sequences around the 12th , 13th and 61st codons of N-, H-, and K-ras were amplified using the p olymerase chain reaction and probed for single base pair mutations usi ng synthetic oligonucleotide probes. Thirty-six melanomas contained mu tations, which in 25 cases (69%) occurred at the 61st codon of N-ras. The results from dot blot hybridizations were confirmed by subcloning and sequencing the polymerase chain reaction products from two tumors. No ras mutations were found in Clark's level I melanomas, whereas 19% of level II and 45% of the more advanced primary tumors contained ras mutations (Chi squared test: p < 0.05). The median Breslow thickness of primary melanomas with ras mutations was 0.72 mm, significantly thi cker than the 0.42 mm of melanomas without mutations (Mann-Whitney U t est, p = 0.042). Ras mutations were found more frequently in primary t umors from continuously exposed skin (56%) than tumors from intermitte ntly or non-sun exposed sites (21%). Fifty percent of locally recurren t and 47% of metastatic melanomas had ras mutations. We conclude that ras mutations occur in a subset of melanomas from sun-exposed skin as a feature of tumor progression.