In aggressive variants of non-Hodgkin lymphomas, Ezh2 is strongly expressed and polycomb repressive complex PRC1.4 dominates over PRC1.2

Abstract

Polycomb group (PcG) proteins are important for the regulation of hematopoiesis by regulating chromatin compaction and silencing genes related to differentiation and cell cycle. Overexpression of enhancer of zeste homologue 2 (Ezh2) and Bmi-1/PCGF4 has been implicated in solid organ cancers, while Mel-18/PCGF2 has been reported as a tumor suppressor. Detailed expression profiles of PcG proteins and their diagnostic significance in malignant lymphomas are still unknown. In this study, we analyzed the expression levels of Ezh2, Bmi-1, Mel-18, and Ki67 in 197 Hodgkin's and non-Hodgkin's lymphoma patient samples and in lymphoma cell lines using immunohistochemistry, fluorescent immunocytochemistry, and Western blotting. Immunohistochemical staining showed that Ezh2 expression was significantly increased in aggressive compared to indolent subtypes of B cell neoplasms (P = 0.000-0.030), while no significant differences in Bmi-1 expression were found between these subtypes. Compared to the normal counterpart, T cell lymphomas showed significant overexpression of Bmi-1 (P = 0.011) and Ezh2 (P = 0.000). The Ki67 labeling index showed a positive correlation with Ezh2 expression in B cell lymphomas (correlation coefficient (Co) = 0.983, P = 0.000) and T/NK cell lymphomas (Co = 0.629, P = 0.000). Fluorescent immunohistochemical staining showed coexpression of Ezh2 and Ki67 in the same tumor cells, indicating that Ezh2 expression correlates with cell proliferation. Both B and T/NK cell neoplasms showed low expression of Mel-18 and high expression of both Bmi-1 and Ezh2. In conclusion, in aggressive lymphoma variants, Ezh2 is strongly expressed and polycomb repressive complex PRC1.4 dominates over PRC1.2. Coexpression of Bmi-1 and Ezh2 is a characteristic of aggressive lymphomas. Ezh2 correlates with the proliferation and aggressive nature of non-Hodgkin's lymphomas.