Epigenetic mechanisms regulating the Igf2/H19 and Kcnq1 locus

印记是对正常孟德尔遗传学的一种奇特的挑战。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从母亲和父亲的等位基因中平等地表达。然而，印迹基因仅在一条染色体上以依赖于亲本的方式表达。由于沉默启动子和活跃启动子存在于单个细胞核中，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表观遗传机制限制基因表达的明确情况。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，由于亲本的限制性表达，由印迹基因决定的表型不仅容易受到基因本身突变的影响，而且还容易受到控制调控的表观遗传程序的破坏。因此，印迹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。本部门正在研究小鼠7号染色体远端的一组基因。人类11p15.5染色体上的共链区在基因组组织和单等位基因表达模式中是保守的。具体来说，我们正在剖析H19基因的母体特异性表达和Igf2基因的父亲特异性表达的分子基础。这两个基因的印迹突变缺失与Beckwith Wiedemann综合征（BWS）和Wilms肿瘤有关。H19和Igf2的表达都依赖于两个基因下游的一组共享的增强子元件。我们在H19启动子上游发现了一个2.4 kb的ICR（印迹控制区）。使用条件删除和插入突变，我们已经确定了与该元件相关的三个功能。首先，这个元素的作用是区分它所插入的任何染色体的亲本起源。具体来说，该区域内的CpGs在父系遗传后变得高度甲基化。其次，该元件作为ctcf依赖的甲基化敏感转录绝缘子发挥作用。通过重组附近启动子和增强子元件的远程相互作用，这种绝缘子能够指导附近基因的亲本特异性激活。最后，当父系遗传时，该ICR也作为发育调节的沉默元件。具体来说，甲基化的ICR诱导邻近序列染色质结构的变化，从而影响基因表达。我们目前的目标是鉴定和表征与ICR相互作用的蛋白质因子和非编码rna，并建立与母系和父系染色体相关的染色质结构。我们正在解决这些问题，在生殖细胞中，印迹已经建立，在体细胞组织中，Igf2和H19的表达对正常、健康的细胞功能至关重要。

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. Expression of both H19 and Igf2 is dependent upon a shared set of enhancer elements downstream of both genes. We have identified a 2.4 kb ICR (for Imprinting Control Region) upstream of the H19 promoter. Using conditional deletion and insertional mutagenesis we have identified three functions associated with this element. First, this element acts to distinguish the parental origin of any chromosome into which it is inserted. Specifically, the CpGs within this region become hypermethylated upon paternal inheritance. Second, this element functions as a CTCF-dependent, methylation-sensitive transcriptional insulator. By reorganizing the long-range interactions of nearby promoter and enhancer elements, this insulator is able to direct parental-specific activation of nearby genes. Finally, this ICR also acts as a developmentally regulated silencer element when paternally inherited. Specifically, the methylated ICR induces changes in chromatin structure of neighboring sequences that impacts gene expression. Our current goals are to identify and characterize the protein factors and non-coding RNAs that interact with the ICR and establish the chromatin structures associated with the maternal and paternal chromosomes. We are addressing these issues both in germ cells, where the imprints are established, and in somatic tissues where expression of Igf2 and H19 are most critical for normal, healthy cell function. A second focus of our research is to generate mouse models for cardiac arrhythmias. We first focused on uncovering the biological function of the imprinted Kcnq1 gene, located just upstream of Igf2. More recently, we have generated a mouse model for Calsequestrin2 deficiency. We demonstrate that calsequestrin2 is not essential for cardiac calcium ion storage, which can be maintained by an expansion of the sarcoplasmic reticulum (SR) volume and surface area. Rather, the primary function of calsequestrin appears to be the regulation of the SR calcium ion release channel during conditions of beta-adrenergic stimulation. The loss of calsequestrin2 thus results in premature calcium ion release from the SR, leading to voltage changes that result in premature contraction of cardiomyocytes and thus arrhythmia. The validity of this mouse model has been recently confirmed by demonstration that drugs that we used to successfully ameliorate the mouse arrhythmias were highly effective in pilot studies on human patients. In the past year, we have demonstrated that the arrhythmias associated with calsequestrin2-deficiency worsen signficantly with age. We have recently generated and are now analyzing conditional alleles of calsequestrin 2. Using these models we have analyzed the effect of late-onset loss of calsequestrin 2 gene function, thus modeling a common human condition. Our results indicate that the phenotypes associated with loss of gene function late in development are much more severe. Thus we we believe that the the developing heart has mechanisms for coping aberrant regulation of Ca++ metabolism that can permanently protect the heart. We are initiating genomic approaches that will identify these mechanism and then evaluate whether these mechanisms represent therapeutic targets. We are also now determining the effect of restoration of calsequestrin 2 gene function to animals that have developed in the absence of any active calsequestrin 2 gene. Together these experiments will help us understand how calsequestin 2 gene activity regulates sarcoplasmic reticulum structure and also help us develop novel therapies for human patients with both congenital and acquired deficiencies in Ca++ excitation-contraction coupling.