Loss of Membranous Expression of β-Catenin Is Associated with Tumor Progression in Cutaneous Melanoma and Rarely Caused by Exon 3 Mutations

Abstract

β-Catenin plays a fundamental role in the regulation of the E-cadherin–catenin cell adhesion complex. It also plays a role in the Wnt signaling pathway by activating T-cell factor– and lymphoid enhancer factor–regulated gene transcription. The level of β-catenin in cells is tightly controlled in a multiprotein complex, and mutations in the glycogen synthase kinase 3β (GSK-3β) phosphorylation sites of the β-catenin gene (CTNNB1) result in nuclear and/or cytoplasmic accumulation of β-catenin and constitutive transactivation of T-cell factor and lymphoid enhancer factor target genes, a mechanism occurring in many cancers. Melanoma cell lines may harbor β-catenin mutations; in vivo, however, cellular accumulation of β-catenin is rarely caused by CTNNB1 mutations. In our study, 43 primary cutaneous melanoma and 30 metastases were screened for CTNNB1 exon 3 mutations by using a denaturing gradient gel electrophoresis technique and sequencing. β-Catenin mutations were found in 2 primary melanomas and 1 metastatic melanoma and were not correlated with nuclear accumulation of β-catenin in these cases. Cellular expression of β-catenin was evaluated by immun-ohistochemistry and by reverse polymerase chain reaction (RT-PCR) in 80 and 70 cases, respectively. Immunohistochemistry revealed a significant loss of membranous β-catenin staining between the primary and metastatic melanomas as well as between radial and vertical growth phase. RT-PCR showed a significant inverse correlation between the amount of RNA and the proportion of cells with membranous expression of β-catenin (P = .0015); no correlation existed between the amount of RNA and the number of cells with nuclear or cytoplasmic expression of β-catenin. In conclusion, nuclear expression of β-catenin is seen in cutaneous melanoma but, in contrast to the case of many other cancers, does not correlate with tumor stage or mutation status. A combination of immunohistochemistry and RT-PCR showed that down-regulation of membranous β-catenin was associated with an increased amount of β-catenin RNA in primary or metastatic melanoma. Our results suggest that posttranslational events, rather than CTNNB1 mutations, are responsible for the altered distribution of β-catenin in cutaneous melanoma.