Background: Transformed melanocytes in atypical nevi, which are thought to be precursors of melanoma, are frequently deleted on chromosomes 1p, 9q, and 9p21 (p16 locus). Single UV irradiation induces histological changes that are similar to those of atypical nevi and, in part, of melanoma in situ. Objective: To determine the effects of UV irradiation on benign melanocytic nevi in vivo. Design: We investigated one half of a symmetric nevus 1 week after a single UV exposure with 4 times the patient's minimal erythema dose and compared it with the nonirradiated, shielded half of the same nevus. Two to 3 areas containing 5 to 30 melanocytes in 7 nevi were microdissected (a total of 18 areas in each nonirradiated and irradiated part), followed by a single-step DNA extraction. Extracted genomic DNA was amplified using a polymerase chain reaction with polymorphic markers DIS450 (1p), D9S12 (9q), IFNA, and D9S171 (9p21) and subjected to autoradiography. Observations: Two, 3, 2, and 2 of 18 areas were homozygous for D1S450, D9S12, IFNA, and D9S171, respectively. No allelic loss could be demonstrated in either nonirradiated or irradiated nevi. Conclusions: Acute histological changes demonstrated in melanocytic nevi after UV irradiation are not followed by allelic loss on identical chromosomal areas found in dysplastic melanocytes of atypical nevi. This finding supports the hypothesis that initial nonspecific genetic events may occur after UV irradiation, followed by an increase in various repair mechanisms potentially leading to specific genetic damage and loss of heterozygosity; however, loss of heterozygosity is not detectable at an early stage.
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