Analysis Of Imprinting On Mouse Distal Chromosome 7

小鼠远端染色体 7 上的印记分析

• 批准号: 6813784
• 负责人: Karl Eric Pfeifer
• 金额: --
• 依托单位: EUNICE KENNEDY SHRIVER NATIONAL INSTITUTE OF CHILD HEALTH & HUMAN DEVELOPMENT
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/6813784
• 关键词: DNA methylation; animal genetic material tag; animal population genetics; catecholamines; chromatin; cytogenetics; epinephrine; functional /structural genomics; gene expression; genetic regulation; genetically modified animals; genomic imprinting; laboratory mouse; long QT syndrome; norepinephrine; nucleic acid sequence; nucleic acid structure

Imprinting represents a curious defiance of normal Mendelian genetics. Mammals inherit two complete sets of chromosomes, one from the mother and one from the father, and most autosomal genes will be expressed equally from maternal and paternal alleles. Imprinted genes, however, are expressed from only one chromosome in a parent-of-origin dependent manner. Because silent and active promoters are present in a single nucleus, the differences in activity cannot be explained by transcription factor abundance. Thus the transcriptional of imprinted genes represents a clear situation in which epigenetic mechanisms restrict gene expression. Therefore imprinted genes are good models for understanding the role of DNA modifications and chromatin structure in maintaining appropriate patterns of gene expression. Further, because of parent-of-origin restricted expression, phenotypes determined by imprinted genes are not only susceptible to mutations of the genes themselves but also to disruptions in the epigenetic programs controlling regulation. Thus imprinted genes are frequently associated with human diseases, including disorders affecting cell growth, development, and behavior. Our Section is investigating a cluster of genes on the distal end of mouse chromosome 7. The syntenic region in humans on chromosome 11p15.5 is conserved in genomic organization and in monoallelic expression patterns. Specifically we are dissecting the molecular basis for the maternal specific expression of the H19 gene and the paternal specific expression of the Igf2 gene. Loss of imprinting mutations in these two genes is associated with Beckwith Wiedemann Syndrome (BWS) and with Wilms' tumor. We have demonstrated that sequences upstream of the H19 promoter are required for imprinted expression of H19 transgenes. These sequences are called the H19DMR (for differentially methylated region) because they are specifically hypermethylated only on the paternal chromosome. We have deleted this region from the endogenous locus and shown that mice inheriting this mutation paternally show biallelic expression of H19 while mice inheriting the mutation through the maternal germline show loss of repression of the normally silent Igf2 allele. Thus the H19DMR is a parent-of-origin specific silencer. By constructing alleles in which we could delete this element in specific cells and at specific developmental time points we were able to demonstrate that the DMR silences H19 and Igf2 by distinct mechanisms. Specifically, we demonstrate that the DMR contains a methylation-sensitive transcriptional insulator. Upon paternal inheritance, the DMR is methylated and the insulator is thereby inactivated, thus permitting expression of the Igf2 gene. Upon maternal inheritance, the unmethylated insulator is active and Igf2 transciption is blocked. In contrast, the methylated paternal H19DMR silences the H19 gene by directing epigenetic modifications of the H19 promoter that directly interfere with transcriptional activation. Based on these genetic studies, we have devised model systems where we imprint normally non-imprinted loci (e.g. Afp) in order to more precisely define the molecular basis for imprinting and monoallelic expression. These experiments have led to the surprising discovery that DNA methylation, although crucial for correct transcriptonal regulation, is not the primary gametic imprint. A second focus of our research is to uncover the biological function of the Kcnq1 gene, also in this locus. This gene has been identified independently by groups looking for genes important in the etiology of BWS, a disease with parent-of-origin inheritance patterns, and for genes important in Long QT syndromes (LQTS) mapping to 11p15.5, a disease with no parent-of-origin effects. We have elucidated the complex developmental regulation of imprinting of this gene so to resolve this apparent paradox. Recently, we have developed a model for inherited LQTS by generating mice deficient in Kcnq1. In vivo ECGs from these mice show abnormal T-wave and P-wave morphologies and prolongation of the QT and JT intervals. However, ECGs of isolated hearts are normal. These changes are indicative of cardiac repolarization defects that are dependent upon some extracardiac signal. Further studies demonstrate that beta-adrenergic stimulation is the primary extracardiac signal and the molecular basis for this effect is being dissected. To address the role of beta-adrenergic stimulation in LQTS and in cardiac development and function more generally, we have developed a mouse model in which the cre recombinase enzyme is expressed in place of the Pnmt gene. Pnmt encodes the enzyme converting norepinephrine to epinephrine. Thus mice homozygous for this allele cannot make any epinephrine and thus offer a good genetic system for identifying the specific role of this hormone. Moreover, the cre recombinase expressed under control of the Pnmt promoter will, in the appropriate genetic background, mark B-adrenergic synthesizing cells and all their descendants so that the fate of these cells can be assayed. These experiments demonstrate the major source of epinephrine (and norepinephrine) in the developing embryo is actually the heart. Thus the heart supplies the catecholamines to the midgestation embryo, the only developmental timepoint when these hormones are absolutely essential for life. We have generated transgenic mice where the catecholamine synthesizing cardiac cells are marked for easy purification.

印记代表了对正常孟德尔遗传学的一种奇怪的蔑视。哺乳动物遗传了两套完整的染色体，一套来自母亲，一套来自父亲，大多数常染色体基因将在母亲和父亲的等位基因中平等表达。然而，印记基因仅从一条染色体以亲本来源依赖的方式表达。因为沉默和活性启动子存在于单个核中，所以活性的差异不能用转录因子丰度来解释。因此，印迹基因的转录代表了一个明确的情况下，表观遗传机制限制基因表达。因此，印迹基因是理解DNA修饰和染色质结构在维持适当的基因表达模式中的作用的良好模型。此外，由于亲本来源的限制性表达，由印记基因决定的表型不仅容易受到基因本身突变的影响，而且容易受到控制调控的表观遗传程序的破坏。因此，印记基因经常与人类疾病有关，包括影响细胞生长、发育和行为的疾病。我们的部门正在研究小鼠7号染色体远端的一组基因。人类染色体11p15.5上的同线区域在基因组组织和单等位基因表达模式中是保守的。具体而言，我们正在解剖H19基因的母体特异性表达和Igf 2基因的父体特异性表达的分子基础。这两个基因中印记突变的缺失与Beckwith Wiedemann综合征（BWS）和Wilms肿瘤相关。我们已经证明，序列上游的H19启动子所需的印迹表达的H19转基因。这些序列被称为H19 DMR（差异甲基化区域），因为它们仅在父本染色体上特异性高甲基化。我们已经从内源性基因座中删除了该区域，并表明父系遗传该突变的小鼠显示H19的双等位基因表达，而通过母系种系遗传该突变的小鼠显示正常沉默的Igf 2等位基因的抑制丧失。因此，H19 DMR是亲本来源特异性沉默子。通过构建等位基因，我们可以在特定的细胞和特定的发育时间点删除这个元件，我们能够证明DMR通过不同的机制沉默H19和Igf 2。具体来说，我们证明了DMR包含一个甲基化敏感的转录绝缘子。在父系遗传时，DMR被甲基化，绝缘子因此失活，从而允许Igf 2基因的表达。在母系遗传时，未甲基化的绝缘子是活跃的，Igf 2的转录被阻断。相反，甲基化的父本H19 DMR通过指导H19启动子的表观遗传修饰而使H19基因沉默，所述表观遗传修饰直接干扰转录激活。基于这些遗传学研究，我们设计了模型系统，其中我们通常非印记位点（如AFP）的印记，以更精确地定义印记和单等位基因表达的分子基础。这些实验令人惊讶地发现，DNA甲基化虽然对正确的转录调控至关重要，但并不是主要的配子印记。 我们研究的第二个重点是揭示Kcnq 1基因的生物学功能，也在这个位点。该基因已被寻找BWS（一种具有起源父母遗传模式的疾病）病因学中重要基因的小组独立鉴定，并被寻找长QT综合征（LQTS）（一种无起源父母影响的疾病）中重要基因的小组独立鉴定。我们已经阐明了这个基因的印迹复杂的发育调控，以解决这个明显的矛盾。最近，我们通过产生Kcnq 1缺陷的小鼠开发了遗传性LQTS模型。这些小鼠的体内ECG显示异常T波和P波形态以及QT和JT间期延长。然而，离体心脏的ECG正常。这些变化表明心脏复极缺陷取决于一些心外信号。进一步的研究表明，β-肾上腺素能刺激是主要的心外信号，这种作用的分子基础正在解剖。为了解决β-肾上腺素能刺激在LQTS和心脏发育和功能中的作用，我们开发了一种小鼠模型，其中表达cre重组酶代替Pnmt基因。Pnmt编码将去甲肾上腺素转化为肾上腺素的酶。因此，这种等位基因纯合子的小鼠不能产生任何肾上腺素，从而为鉴定这种激素的特定作用提供了良好的遗传系统。此外，在Pnmt启动子控制下表达的cre重组酶将在适当的遗传背景下标记B-肾上腺素能合成细胞及其所有后代，从而可以测定这些细胞的命运。这些实验表明，发育中的胚胎中肾上腺素（和去甲肾上腺素）的主要来源实际上是心脏。因此，心脏向妊娠中期胚胎提供儿茶酚胺，这是这些激素对生命绝对必要的唯一发育时间点。我们已经产生了转基因小鼠，其中合成儿茶酚胺的心脏细胞被标记为易于纯化。