The Enhancer of zeste homologue 2 (EZH2) gene, a catalytic subunit of the Polycomb Repressive Complex 2 (PRC2) is a histone methyltransferase that targets lys-27 of histone H3 (H3K27me3). Previous studies have shown that EZH2 can via its histone methyltranseferase activity induce the silencing of known tumour suppressor genes. A functional role of EZH2 independent of its histone methyltransferase activity has been reported as well. In this study, we explored the functional role of EZH2 and therapeutic potential of inhibiting the histone methyltransferase activity of EZH2 in neuroblastoma with two known inhibitors GSK126 and EPZ6438 as well as the targeted downregulation of the EZH2 protein.
We identified in one neuroblastoma tumour a regional gain in the 7q36 region that encompasses the PRC2 group protein EZH2 and its concomitant overexpression. High EZH2 expression correlated with poor prognosis and overall survival independent of MYCN amplification patient status. Inhibition of the histone methyltransferase activity of EZH2 by small molecule inhibitors GSK126 and EPZ6438 in EZH2 high-expressing cell lines IMR32, CHP134 and NMB led to a slight G1 arrest of the cell cycle despite a strong decrease H3K27me3. Additionally, we showed a reduction of colony forming capacity of all three cell lines but only at high nanomolar concentrations of both compounds even though strong target-specific inhibition of EZH2 histone methyltransferase activity was observed already at low nanomolar concentrations of both compounds. Knockdown of EZH2 by three shRNA resulted in a strong apoptotic response and a decrease in the cycle D1 gene while simultaneously overexpressing EZH2ΔSET, a truncated form of the EZH2 wild-type gene lacking the SET transactivation domain and consequently histone methyltransferase activity rescued the apoptotic phenotypes.
Taken together, our findings suggest that EZH2 might play a pivotal role in the survival of neuroblastoma cell lines independent of its histone methyltransferase activity, paving the way for the development of therapeutics which specifically targets the EZH2 protein as a whole.