Erg Lysine Methylation Promotes Prostate Cancer Progression In Erg Transgenic Mice

The TMPRSS2-ERG gene fusion occurs frequently in prostate cancers and leads to over-expression of the ETS transcription factor ERG. We have recently described a novel mechanism cooperating with ERG fusion and enhancing ERG oncogenic activity. We found that the protein methyltransferase Enhancer of zest homolog 2 (EZH2) interacts with ERG and catalyzes methylation of a specific lysine residue in the ERG DNA binding domain. Lysine methylation of ERG alters intra-domain dynamics leading to increased chromatin binding and transcriptional activity. These events result in the formation of ERG/EZH2 co-activator complexes on selected gene promoters and enhancers and in broad transcriptional reprogramming in prostate epithelial cells. In this study we examined whether ERG methylation and ERG/EZH2 crosstalk were associated with ERG-driven tumor progression in genetically engineered mouse models represented by mice with prostate-specific expression of ERG (Pb-Cre4; Rosa26ERG/ERG) and mice with combined prostate-specific expression of ERG and deletion of PTEN (Pb-Cre4; Ptenflox/flox; Rosa26ERG/ERG). Only the combined ERG/PTEN mice exhibit progressive disease and develop invasive adenocarcinomas, whereas ERG mice fail to do so. We detected ERG methylation exclusively in ERG/PTEN mice. Enhanced methylation was linked to increased expression and AKT-induced phosphorylation of EZH2 at Serine 21 (pS21). Consistently, we observed higher promoter occupancy by ERG/EZH2 complexes and increased expression of selected ERG/EZH2 co-regulated genes in ERG/PTEN mice. Thus, enhanced ERG methylation and EZH2 activation occur in mice with combined ERG gain and PTEN loss and are concomitant with the emergence of an invasive phenotype. Systemic treatment with pharmacological inhibitors of EZH2, such as GSK343 blocked ERG methylation and expression of ERG/EZH2 co-regulated genes in ERG/PTEN mice. Moreover, GSK343 significantly reduced prostate volume, Ki67 immuno-staining and areas of invasive adenocarcinomas compared to control mice. Relevantly, we found preferential upregulation of ERG/EZH2 co-regulated genes in human prostate cancers exhibiting combined ERG over-expression and PTEN loss. These data establish the association of ERG methylation with enhanced ERG oncogenic activity and provide mechanistic insights into the synergy between ERG gain and PTEN loss in human tumors. Furthermore, these results establish the efficacy of EZH2 inhibitors in antagonizing ERG oncogenic activity in the ERG/PTEN model providing a strong rationale for developing new therapeutic strategies for the management of ERG fusion positive prostate cancers. Citation Format: Marita Zoma, Dheeraj Shinde, Domenico Albino, Simone Mosole, Jacopo Sgrignani, Andrea Cavalli, Carlo V. Catapano, Giuseppina M. Carbone. ERG lysine methylation promotes prostate cancer progression in ERG transgenic mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-201.