EZH2 Is Aberrantly Expressed and Plays a Pro-Proliferative Role Independent of Its Methyltransferase Activity in Natural Killer/T-Cell Lymphoma

Abstract 3498

Nasal-type Natural Killer/T-cell lymphoma (NKTL) is an aggressive lymphoid malignancy associated with very poor survival. A better understanding of the molecular abnormalities underlying this disease will lead to a better therapy. We recently performed whole genome gene expression studies and identify a number genes that are differentially expressed in NKTL as well as pathways which are activated in NKTL. EZH2, one of the genes identified in our study to be aberrantly over-expressed in NKTL, is a H3K27-specific histone methyltransferase and a component of the polycomb repressive complex 2 (PRC2), which plays a key role in the epigenetic maintenance of repressive chromatin mark. To the best of our knowledge, the mechanism of EZH2 overexpression in NKTL has not yet been described. In this study, we showed that EZH2 overexpression in NKTL can be attributed to a deregulated MYC-miRNA-EZH2 regulatory axis where MYC activation represses miRNAs that normally downregulate EZH2. He functionally demonstrated this relationship using NKTL cell lines while also demonstrating the correlation between MYC activation and EZH2 expression in clinical samples through the analysis of gene expression data as well as histological detection of nuclear MYC and EZH2 protein using a tissue microarray containing 35 NKTL clinical samples using immunohistochemistry.

We then investigated the functional role of EZH2 in NKTL. We showed that ectopic overexpression of EZH2 in both primary NK cells and NK cell lines led to a significant growth advantage. Conversely, knock-down of EZH2 in NK cell lines resulted in growth inhibition of tumor cells. Intriguingly, ectopic EZH2 mutant deficient for histone methyltransferase activity was also able to confer growth advantage and rescue the growth inhibition upon endogenous EZH2 depletion in NKTL cells, indicating an oncogenic role of EZH2 independent of its gene silencing activity. Indeed, EZH2 expression in clinical NKTL samples is associated with higher Ki67 staining implying a role in driving NKTL proliferation. We further demonstrated that EZH2 directly binds to the gene promoter of Cyclin D1 and EZH2 promotes the transcription of Cyclin D1 independent of its enzymatic activity. Consistent with its potential oncogenic role, depletion of EZH2 using an inhibitor called DZNep also induced significant growth inhibition in NKTL cells. Taken together, our study demonstrates an unconventional role of EZH2 in promoting oncogenic growth in NKTL and provides novel insights into the oncogenic function of EZH2 in human cancers. The pro-proliferative properties of EZH2 in NKTL support the rationale for using of EZH2 inhibitors in the treatment of NKTL. However, it is important to note that in some tumor, EZH2 may be mediating its oncogenic functions through non-enzymatic mechanism. This has critical implications on the choice of specific inhibitors of its enzymatic function or compounds that can deplete EZH2 as the most appropriate therapeutic approach. Since targeting of EZH2 is an active area of drug development at present, there is great potential for the development of better treatment modalities and this is especially important for aggressive cancers, such as NKTL, for which no effective curative treatment is currently available.