Bate-Eya Laurel T, Gierman Hinco J, Ebus Marli E, Koster Jan, Caron Huib N, Versteeg Rogier, Dolman M Emmy M, Molenaar Jan J
Department of Translational Medicine, Prinses Máxima Center for Pediatric Oncology, Utrecht, The Netherlands.
Department of Pediatric Oncology, Emma Kinderziekenhuis, Amsterdam, The Netherlands.
Eur J Cancer. 2017 Apr;75:63-72. doi: 10.1016/j.ejca.2016.12.019. Epub 2017 Feb 17.
Neuroblastoma is predominantly characterised by chromosomal rearrangements. Next to V-Myc Avian Myelocytomatosis Viral Oncogene Neuroblastoma Derived Homolog (MYCN) amplification, chromosome 7 and 17q gains are frequently observed. We identified a neuroblastoma patient with a regional 7q36 gain, encompassing the enhancer of zeste homologue 2 (EZH2) gene. EZH2 is the histone methyltransferase of lysine 27 of histone H3 (H3K27me3) that forms the catalytic subunit of the polycomb repressive complex 2. H3K27me3 is commonly associated with the silencing of genes involved in cellular processes such as cell cycle regulation, cellular differentiation and cancer. High EZH2 expression correlated with poor prognosis and overall survival independent of MYCN amplification status. Unexpectedly, treatment of 3 EZH2-high expressing neuroblastoma cell lines (IMR32, CHP134 and NMB), with EZH2-specific inhibitors (GSK126 and EPZ6438) resulted in only a slight G1 arrest, despite maximum histone methyltransferase activity inhibition. Furthermore, colony formation in cell lines treated with the inhibitors was reduced only at concentrations much higher than necessary for complete inhibition of EZH2 histone methyltransferase activity. Knockdown of the complete protein with three independent shRNAs resulted in a strong apoptotic response and decreased cyclin D1 levels. This apoptotic response could be rescued by overexpressing EZH2ΔSET, a truncated form of wild-type EZH2 lacking the SET transactivation domain necessary for histone methyltransferase activity. Our findings suggest that high EZH2 expression, at least in neuroblastoma, has a survival function independent of its methyltransferase activity. This important finding highlights the need for studies on EZH2 beyond its methyltransferase function and the requirement for compounds that will target EZH2 as a complete protein.
神经母细胞瘤主要特征为染色体重排。除了V-Myc禽成髓细胞瘤病毒癌基因神经母细胞瘤衍生同源物（MYCN）扩增外，还经常观察到7号染色体和17号染色体长臂（17q）增加。我们鉴定出一名神经母细胞瘤患者，其7号染色体长臂36区（7q36）存在局部增加，包含zeste同源物2（EZH2）基因的增强子。EZH2是组蛋白H3赖氨酸27（H3K27me3）的组蛋白甲基转移酶，它构成了多梳抑制复合物2的催化亚基。H3K27me3通常与参与细胞周期调控、细胞分化和癌症等细胞过程的基因沉默相关。EZH2高表达与不良预后和总生存期相关，且与MYCN扩增状态无关。出乎意料的是，用EZH2特异性抑制剂（GSK126和EPZ6438）处理3种EZH2高表达的神经母细胞瘤细胞系（IMR32、CHP134和NMB），尽管最大程度抑制了组蛋白甲基转移酶活性，但仅导致轻微的G1期阻滞。此外，用抑制剂处理的细胞系中的集落形成仅在远高于完全抑制EZH2组蛋白甲基转移酶活性所需浓度时才减少。用三种独立的短发夹RNA（shRNA）敲低完整蛋白导致强烈凋亡反应并降低细胞周期蛋白D1水平。这种凋亡反应可通过过表达EZH2ΔSET来挽救，EZH2ΔSET是野生型EZH2的截短形式，缺乏组蛋白甲基转移酶活性所需的SET反式激活结构域。我们的研究结果表明，至少在神经母细胞瘤中，EZH2高表达具有独立于其甲基转移酶活性的生存功能。这一重要发现凸显了对EZH2甲基转移酶功能之外进行研究的必要性，以及对能够靶向完整蛋白形式的EZH2的化合物的需求。
