Point mutations in the N-ras oncogene in malignant melanoma and congenital naevi

DNA from formalin-fixed and paraffin-processed samples from 100 melanocytic lesions (39 malignant melanomas, 18 cases of dysplastic naevi, and 43 congenital naevi) was extracted, and the sequences around codons 12/13 and 61 of the N-ras oncogene were amplified using the polymerase chain reaction. The amplified product was then analysed both by dot-blotting and by direct sequencing for point mutations. By the dot-blotting technique, mutations were seen in 18 of 100 lesions. These were in one of five distant metastases (20%), in one of three nodal metastases (33%), in four of 31 (13%) primary melanomas, in none of 18 dysplastic naevi, and in 12 of 43 (28%) congenital naevi, all at codon 61. On direct sequencing, nine of 18 mutations were confirmed, in two of 31 (6%) primary tumours, one distant metastasis, and six of 43 (14%) congenital naevi. Of the 23 superficial spreading melanomas examined, eight were on sun-exposed skin. A superficial spreading melanoma, in which the N-ras mutation at codon 61 was confirmed, was on non-exposed skin, and an unconfirmed mutation was from an exposed site. One of three nodular melanomas with a confirmed mutation was on a light-exposed site, and the other two nodular melanomas were from non-exposed areas. All four lentigo maligna melanomas were from exposed sites, and one of these had an unconfirmed mutation. The only acral lentiginous melanoma, which had no mutation, was from a sun-exposed area. Ten of the 43 congenital or early onset naevi were on sun-exposed sites; six unconfirmed and five confirmed mutations were from lesions on non-sun-exposed sites (5/33; 15%), and one confirmed mutation was from a lesion on a sun-exposed area (1/10; 10%). Our results do not, therefore, support the hypothesis that N-ras mutations are specifically associated with primary melanomas arising on continually sun-exposed skin. The overall pattern suggests that N-ras mutations in isolation in melanocytic lesions are not frequent. This is the first report of N-ras mutations in congenital/early onset melanocytic naevi, and shows a relatively high incidence in comparison with that observed in melanomas in this study.