Heterozygous KMT2D Loss and Medulloblastoma

项目总结 髓母细胞瘤(MB)是儿童最常见的原发恶性脑肿瘤。经常诱发甲胎蛋白 通过细胞信号通路的改变，如声波刺猬和无翼通路，它们具有 已经被广泛地定性了。然而，目前对甲基溴的治疗会导致严重的终身副作用和 没能治愈很多病人。因此，对新的机械性理解的需求尚未得到满足，这将是 有助于设计一种基于机制的甲基溴治疗方法。表观遗传异常，这是可遗传的 基因表达或细胞表型的异常而不伴随DNA序列的改变，是一种 肿瘤发生的主要因素。表观遗传修饰物通常含有DNA改变，如突变和 缺失，在人类MB中。然而，表观遗传修饰物在甲基溴发育中的作用在很大程度上仍不清楚。 组蛋白赖氨酸甲基化是组蛋白翻译后修饰的一种，是表观遗传学和 基因表达的转录调节，并被组蛋白甲基转移酶和组蛋白甲基转移酶可逆地修饰 去甲基酶。在组蛋白赖氨酸甲基化中，组蛋白H3赖氨酸4(H3K4)的甲基化是关键基因激活 表观基因组标记。例如，单甲基H3K4是增强剂的标志，增强剂通过相互作用激活基因 用基因启动子。此外，三甲基H3K4占据了人类所有基因调节区的75%， 而广谱的三甲基H3K4是一个基因激活信号，表示肿瘤抑制基因和细胞识别基因。 我们之前已经报道过H3K4甲基转移酶KMT2D(也称为MLL4、ALR和MLL2；a 转录辅活化子)是维甲酸诱导人神经细胞分化所必需的 NT2/D1干细胞。值得注意的是，我们的另一项研究表明，小鼠大脑中Kmt2d的纯合缺失 在控制运动协调和平衡的大脑区域小脑中发展了自发的MB。 值得注意的是，我们的额外结果显示，Kmt2d杂合性缺失(单等位基因)高度促进了MB。 基于这些令人信服的发现，我们的长期目标是确定骨质疏松症杂合性缺失的致癌作用 Kmt2d在MB发病机制中的作用我们的中心假设是Kmt2d杂合性缺失导致表观基因组 下调肿瘤抑制基因的改变，从而促进MB。在这里，我们建议学习到1) 利用基因工程小鼠模型研究杂合子Kmt2d缺失对MB的促进作用； 2)确定杂合性Kmt2d缺失促进MB的分子机制；以及3)确定 Kmt2d杂合性缺失导致表观基因组改变。因为KMT2D是最常见的突变之一 MB中的基因和MB中的大多数KMT2D突变是杂合和截断的，我们建议的研究 具有重要意义和临床意义。此外，我们使用基因工程小鼠模型的研究将 确定体内Kmt2d杂合性丢失的MB促进作用。此外，我们的结果将揭示 以前未被认识的MB发病的表观遗传学机制，并提供了有价值的信息 为合理设计一种治疗MB的方法。

PROJECT SUMMARY Medulloblastoma (MB) is the most common malignant primary brain tumor in children. MB is frequently induced by the alterations of cellular signaling pathways, such as sonic hedgehog and wingless pathways, which have been extensively characterized. Nevertheless, current treatment of MB causes severe life-long side effects and fails to cure many patients. Thus, there is an unmet need for a new mechanistic understanding that would be helpful for designing a mechanism-based approach for MB treatment. Epigenetic aberrations, which are heritable aberrations in gene expression or cellular phenotypes without accompanying changes in DNA sequences, are a major factor for tumorigenesis. Epigenetic modifiers often harbor DNA alterations, such as mutations and deletions, in human MB. However, the roles of epigenetic modifiers in MB development remain largely unknown. Histone lysine methylation, a type of histone posttranslational modification, is a hallmark of epigenetic and transcriptional regulation of gene expression and is reversibly modified by histone methyltransferases and demethylases. Of histone lysine methylation, methylations at histone H3 lysine 4 (H3K4) are key gene-activating epigenomic marks. For example, monomethyl H3K4 is a mark for enhancers, which activate genes by interacting with gene promoters. In addition, trimethyl H3K4 occupies as much as 75 % of all human gene-regulatory regions, and broad trimethyl H3K4 is a gene-activating signature that denotes tumor suppressor and cell identity genes. We have previously reported that the H3K4 methyltransferase KMT2D (also called MLL4, ALR, and MLL2; a transcriptional coactivator) is required for retinoic acid-induced neuronal differentiation of human neuron-lineage NT2/D1 stem cells. Notably, our other study showed that homozygous loss of Kmt2d in the mouse brain developed spontaneous MB in the cerebellum, a brain region that controls motor coordination and balance. Strikingly, our additional results showed that heterozygous loss (single-allelic) of Kmt2d highly promoted MB. Based on these compelling findings, our long-term goal is to define the oncogenic role of heterozygous loss of Kmt2d in MB pathogenesis. Our central hypothesis is that heterozygous loss of Kmt2d causes epigenomic alterations to downregulate tumor suppressor genes and thereby promotes MB. Here, we propose to study to 1) characterize the MB-promoting effect of heterozygous Kmt2d loss using genetically engineered mouse models; 2) define the molecular mechanism by which heterozygous Kmt2d loss promotes MB; and 3) determine how heterozygous Kmt2d loss causes epigenomic alterations. Because KMT2D is one of the most frequently mutated genes in MB and a majority of KMT2D mutations in MB are heterozygous and truncations, our proposed studies are significant and clinically relevant. In addition, our studies using genetically engineered mouse models will define an in vivo MB-promoting role for heterozygous Kmt2d loss. Furthermore, our results will uncover the previously unappreciated epigenetic mechanism underlying MB pathogenesis and provide valuable information for the rational design of a therapeutic approach for MB treatment.