Phenotypic Correlates of the Class 2 Gene Expression Signature in Uveal Melanoma

Abstract

Purpose:: Using gene expression profiling, we previously showed that primary uveal melanomas cluster into two distinct molecular groups that predict metastatic risk. Class 1 tumors are at low risk and class 2 tumors are at high risk of metastasis. This molecular classification outperforms clinical, pathologic and chromosomal prognostic factors. Other groups have suggested that metastasizing uveal melanomas exhibit a vasculogenic phenotype. However, our previous work suggested that the class 2 tumors exhibit an epithelial-like phenotype. In this study, we compare markers of epithelial differentiation versus vasculogenesis/angiogenesis for association with class 2 gene expression signature.

Methods:: The study was performed on formalin-fixed, paraffin-embedded sections from 70 eyes enucleated for primary uveal melanoma, including 30 class 1 tumors and 29 class 2 tumors. The following clinicopathologic variables were analyzed: patient age, gender, tumor location, basal diameter, thickness, cell type, scleral invasion, and looping extracellular matrix patterns. Immunohistochemical staining was examined for E-cadherin, ß-catenin, cytokeratin-18, VE-cadherin, and the hypoxia marker HIF1α. Significance of association was evaluated using Fisher's exact test.

Results:: Significant associations were observed between class 2 signature and patient age (P=0.004), tumor pigmentation (P=0.01), epithelioid cell type (P<0.00001), looping extravascular matrix patterns (P=0.0004), membranous immunostaining for E-cadherin (P=0.04), and membranous immunostaining for ß-catenin (P=0.001). A trend toward significant association was observed between immunostaining for cytokeratin-18 and membranous immunostaining for ß-catenin (P=0.06).

Conclusions:: The class 2 molecular signature, which is the most accurate known predictor of uveal melanoma metastasis, is closely associated with markers of epithelial differentiation but not markers of vasculogenesis and angiogenesis. These findings support the idea that epithelioid morphology and looping extracellular matrix patterns are manifestations of epithelial differentiation, and they may have important implications for understanding the pathobiology of uveal melanoma.

Keywords: melanoma • immunohistochemistry • tumors