Role of EZH2 in Medulloblastoma Tumorigenesis

摘要 髓母细胞瘤是困扰儿童的最常见的恶性脑瘤。尽管接受了治疗 手术、放疗和化疗，这些高毒性治疗的结果是次优的 严重的长期发病率。髓母细胞瘤由4个不同的分子亚群组成(Wnt，Sonic 刺猬、第3组和第4组)。其中，4组髓母细胞瘤最为常见 亚群，但其潜在的生物学特征是最不典型的。我们最近展示了一种关键的 Zust Homolog 2增强子(EZH2)在第4组髓母细胞瘤中的作用EZH2是催化核心 PRC2染色质重塑复合体的蛋白质，它催化组蛋白3的三甲基化 Lysine27(H3K27me3)并介导参与细胞命运决定的基因的表观遗传沉默， 分化和癌症。我们证明了EZH2在第4组髓母细胞瘤中过表达 调节髓母细胞瘤细胞的增殖和自我更新能力，以及 H3K27me3标记在第4组髓母细胞瘤患者中丰富，并伴有不良结局。 然而，EZH2介导的控制髓母细胞瘤发生的机制是 人们对此知之甚少。我们的初步数据表明，EZH2抑制了KEY的表达 调节神经元分化并促进神经干细胞的转化，提高 耐人寻味的可能性是EZH2的异常表达会导致神经元分化受阻 维持神经和肿瘤干细胞的多能状态。精确的基因表达程序 EZH2在髓母细胞瘤中的调控尚不清楚，EZH2异常表达对髓母细胞瘤的影响 小脑尚未确定。我们推测EZH2通过以下途径介导髓母细胞瘤的发生 抑制小脑干细胞分化，维持多能状态。我们的目标是 检测EZH2对髓母细胞瘤发生的生物学影响并测试潜在的新基因 靶向EZH2的治疗分子。为了继续我们的假设，我们将首先调查这个命题 EZH2通过抑制髓母细胞瘤神经分化程序基因表达 改变关键启动子上H3K27me3组蛋白核心的染色质占有率。接下来，我们将继续 在第4组特异性小脑干细胞中异常增加EZH2表达的概念将抑制 用一种新的小鼠模型诱导小鼠小脑分化和肿瘤形成 髓母细胞瘤。最后，我们将在体内测试三种临床相关的抑制剂，使用患者来源的 髓母细胞瘤异种移植模型的建立。 拟议工作的成功完成将确定EZH2在髓母细胞瘤和 在临床相关的活体模型中建立治疗靶向这种酶的可能性。

ABSTRACT 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.