Role of EZH2 in Medulloblastoma Tumorigenesis

描述（申请人提供）：髓母细胞瘤是儿童最常见的恶性脑肿瘤。尽管采用手术、放疗和化疗治疗，但这些高毒性治疗的结果并不理想，长期发病率很高。髓母细胞瘤由4个不同的分子亚群组成（Wnt, Sonic Hedgehog， Group 3和Group 4）。其中，第4组成神经管细胞瘤是最常见的亚组，但其潜在的生物学特征是最少的。我们最近证实了Zeste同源物2的增强子（Enhancer of Zeste Homolog 2, EZH2）在第4组髓母细胞瘤中的关键作用。EZH2是PRC2染色质重塑复合体的催化核心蛋白，催化组蛋白3赖氨酸27 （H3K27me3）的三甲基化，介导参与细胞命运决定、分化和癌症的基因的表观遗传沉默。我们证明了EZH2在第4组髓母细胞瘤患者中过表达，调节髓母细胞瘤细胞的增殖和自我更新能力，并且H3K27me3标记在与不良预后相关的第4组髓母细胞瘤患者中富集。然而，EZH2介导的髓母细胞瘤发生控制的机制尚不清楚。我们的初步数据表明，EZH2抑制神经元分化关键调控因子的表达并促进神经干细胞的转化，这提出了一种有趣的可能性，即异常的EZH2表达加强了神经干细胞和肿瘤干细胞的分化阻断并维持了多能状态。髓母细胞瘤中EZH2调控的确切基因表达程序尚不清楚，小脑中EZH2异常表达的影响也不确定。我们假设EZH2通过抑制小脑干细胞的分化和维持多能状态来介导成神经管细胞瘤的发生。我们的目标是研究EZH2对成神经管细胞瘤发生的生物学影响，并测试潜在的针对EZH2的新型治疗分子。为了实现我们的假设，我们将首先研究EZH2通过改变H3K27me3组蛋白核心关键启动子的染色质占用来抑制髓母细胞瘤神经元分化程序的基因表达。接下来，我们将利用一种新的小鼠成神经管细胞瘤模型，探讨在第4组特异性小脑干细胞中异常增加EZH2表达将抑制小鼠小脑分化并诱导肿瘤形成的概念。最后，我们将使用患者衍生的髓母细胞瘤异种移植模型在体内测试三种临床相关的抑制剂。这项工作的成功完成将确定EZH2在成神经管细胞瘤中的作用，并确定在临床相关的体内靶向治疗这种酶的潜力

DESCRIPTION (provided by applicant): Medulloblastoma is the most common malignant brain tumor that afflicts children. Despite therapy with surgery, radiation and chemotherapy, outcomes of these highly toxic treatments are sub-optimal with significant long-term morbidity. Medulloblastoma consists of 4 distinct molecular subgroups (Wnt, Sonic Hedgehog, Group 3 and Group 4). Among these, Group 4 medulloblastoma is the most common subgroup but its underlying biology is the least characterized. We have recently demonstrated a critical role for Enhancer of Zeste Homolog 2 (EZH2) in Group 4 medulloblastoma. EZH2 is the catalytic core protein of the PRC2 chromatin-remodeling complex, which catalyzes the trimethylation of histone3 lysine27 (H3K27me3) and mediates epigenetic silencing of genes involved in cell fate decisions, differentiation and cancer. We demonstrated that EZH2 is overexpressed in Group 4 medulloblastoma patients, regulates the proliferation and self-renewal capacity of medulloblastoma cells and that the H3K27me3 mark is enriched in Group 4 medulloblastoma patients associated with adverse outcomes. However, the mechanisms underlying EZH2 mediated control of medulloblastoma tumorigenesis are poorly understood. Our preliminary data demonstrates that EZH2 suppresses expression of key regulators of neuronal differentiation and promotes transformation of neural stem cells, raising the intriguing possibility that aberrant EZH2 expression enforces a neuronal differentiation block and maintains pluripotent state in neural and tumor stem cells. The exact gene expression programs regulated by EZH2 in medulloblastoma are unknown and the impact of abnormal EZH2 expression in the cerebellum is undetermined. We hypothesize that EZH2 mediates medulloblastoma tumorigenesis by inhibiting differentiation of cerebellar stem cells and maintaining a pluripotent state. Our objective is to examine the biological impact of EZH2 on medulloblastoma tumorigenesis and test potential novel therapeutic molecules targeting EZH2. To pursue our hypothesis we will first investigate the proposition that EZH2 suppresses gene expression of neuronal differentiation programs in medulloblastoma by altering chromatin occupancy of the H3K27me3 histone core at key promoters. Next we will pursue concept that aberrantly increased EZH2 expression in Group 4 specific cerebellar stem cells will inhibit differentiation and induce tumor formation in the murine cerebellum using a novel mouse model of medulloblastoma. Finally we will test three clinically relevant inhibitors in vivo using patient derived xenograft models of medulloblastoma. Successful completion of the proposed work will determine the role of EZH2 in medulloblastoma and establish the potential of therapeutically targeting this enzyme in clinically relevant in vivo models.