X inactivation, DNA deletion, and microsatellite instability in common acquired melanocytic nevi

We have investigated several molecular characteristics of common acquired melanocytic nevi to clarify their relationship to malignant melanoma, which is characterized by clonality and the progressive accumulation of DNA deletions. Twenty-four common acquired nevi were subjected to analysis for loss of heterozygosity at four loci on chromosome 9p and six loci on 10q that are commonly deleted in melanoma, but no deletions were seen. X inactivation analysis was performed in lesions from females, using the methylation-sensitive restriction HpaII site in the CAG microsatellite repeat (HUMARA) in exon 1 of the androgen receptor (AR) gene. In 14 melanomas, 11 (92%) were confirmed to have skewed X inactivation, consistent with monoclonality, as were 16 (80%) of 20 benign nevi. One nevus (5%) and 4 (33%) of 12 melanomas also showed loss of heterozygosity at HUMARA. One nevus showed an additional allele, consistent with low level microsatellite instability, at one of the 11 loci that were examined. Common melanocytic nevi, therefore, arise by apparently clonal proliferation, but they do not share chromosomal deletions that are characteristic of melanoma. However, skewed X inactivation patterns were seen in some samples of adjacent microdissected normal epidermis.