Cytoplasmic beta-catenin is lacking in a subset of melanoma-associated naevi, but is detectable in naevus-associated melanomas: potential implications for melanoma tumorigenesis?

BACKGROUND: An excess of intracellular beta-catenin protein is triggered by various genetic alterations in melanoma cell lines, and has been suggested to play a role in melanoma tumorigenesis.
OBJECTIVES: To investigate the role played in vivo by beta-catenin in melanoma tumorigenesis, we compared the cytoplasmic detection of beta-catenin in benign melanocytic cells vs. malignant melanoma cells presumably generated from these benign melanocytic cells. For this purpose, melanocytic naevi occurring in association with melanoma, which were suggested to be melanoma precursors, were compared with their associated melanoma for beta-catenin cytoplasmic immunoreactivity.
METHODS: Fifty-seven consecutive cases of primary cutaneous melanoma were considered, and 15 of them were found to be associated with a melanocytic naevus portion. The naevus portion showed features of acquired melanocytic naevus (total 12 cases: five dysplastic, seven intradermal) or congenital growth pattern naevus (total three cases: one superficial, two deep). All specimens were immunohistochemically investigated for beta-catenin.
RESULTS: Virtually all primary cutaneous melanomas, including those associated with a naevus portion, showed cytoplasmic beta-catenin positivity. However, the intradermal naevus portion was consistently cytoplasmic beta-catenin negative, while both the dysplastic and the congenital naevus portions were cytoplasmic beta-catenin positive.
CONCLUSIONS: Beta-catenin excess may play a role in melanoma tumorigenesis, because beta-catenin cytoplasmic reactivity was found in primary cutaneous melanoma but not in its associated intradermal naevus precursor. As, however, beta-catenin cytoplasmic reactivity was detected not only in primary cutaneous melanoma but also in its associated dysplastic/congenital naevus precursors, beta-catenin stabilization alone is not sufficient to play a decisive role for melanoma onset.