S100A6 protein expression is different in Spitz nevi and melanomas.

The Spitz nevus is a benign melanocytic lesion that can be identified reliably in many cases by conventional histopathological criteria. However, there are subsets of Spitz nevi and of malignant melanoma that closely resemble each other and represent diagnostic challenges. S100 proteins are of interest because of their involvement in neoplastic processes and their genes are clustered in chromosome 1q21. Chromosome 1 contains mutations in several types of tumors, including melanomas. The expression of different S100 proteins (A2, A6 and A8/A9 or A12) was examined in 42 Spitz nevi, 105 melanomas, and 73 melanocytic nevi to test the hypothesis that their expression differs among these entities and may contribute to the distinction between these entities. The results showed an up-regulation of S100A6 protein in Spitz nevi, melanomas, and melanocytic nevi but with a different percentage of positivity and pattern of immunoreactivity. The differences between these three entities were statistically significant (P <.001). All 42 Spitz nevi (100%) showed strong and diffuse S100A6 protein expression, both in junctional and in dermal components of the nevi. Thirty-three percent of melanomas expressed S100A6 (35/105). The expression was mainly weak (30/35) and patchy in the dermal component and was negative or minimal in the junctional component. Fifty-six percent of different subtypes of melanocytic nevi (41/73) expressed S100A6, almost all of them weakly (40/41) and in the dermal component. Normal intraepidermal melanocytes were negative. The melanocytic cells in these three entities did not express S100A2, S100A8/A9 or A12. However, an up-regulation of S100A2 and S100A8/A9 or A12 proteins was observed in normal keratinocytes in the epidermis overlying Spitz nevi and melanomas, without differences. In summary, a simple immunohistochemical test for S100A6 protein differentiated between Spitz nevi, melanomas, and melanocytic nevi. This marker could be used when the distinction is very difficult or controversial in routine studies, especially when there is a junctional component. Further molecular analyses of the S100A6 protein and gene should be performed to study the underlying genetic bases for such differences.