Epigenetic Regulation of Imprinting in Mouse Embryo

在小鼠和人类中，印迹的H19基因与相反的印迹的Igf2基因紧密相连。这些基因的印迹表达是由位于H19基因5'处的2kb差异甲基化区（DMR）介导的。在体外，当DMR位于增强子和启动子之间时，它作为阻止增强子与启动子接合的边界起作用。边界功能可能是由甲基化敏感的转录调节蛋白CTCF介导的。根据边界模型，CTCF与母体等位基因上的DMR结合，从而使母体H19基因独占获得增强子。在父系染色体上，高甲基化的DMR不能结合CTCF。这种超甲基化既抑制了父亲的H19启动子活性，又允许增强子只参与父亲的Igf2启动子。因此，该模型解释了观察到的母系H19表达和父系Igf2的相互表达。Specific Aim 1将在小鼠中使用RNA干扰（RNAi）来验证以下假设：(1)母体等位基因上的CTCF结合负责母体H19的独家表达；(2)父亲等位基因上的DNA超甲基化和相互作用的DNA甲基化结合蛋白介导H19的抑制，从而允许父亲Igf2的独家表达。此外，本目的目的是验证CTCF与母细胞DMR结合的假设，即阻止DNA甲基化，从而确保母细胞H19在胚胎中的表达。了解印迹基因的表达对辅助生殖技术（ART）的实践具有重要意义。在过去二十年中，抗逆转录病毒治疗的做法急剧增加，导致成千上万的儿童出生。植入前小鼠胚胎培养基的细微变化可导致正常印迹和母体表达的H19基因的印迹丢失。由此产生的双等位基因表达与父本等位基因上DMR甲基化的缺失相关。虽然尚不清楚这种变化是否发生在人类植入前胚胎的培养过程中（以及是否有长期后果），但ART是在几乎没有使用动物模型进行任何基础研究的情况下发生的。Specific Aim 2将使用小鼠作为模型系统来测试以下假设：(1)胚胎培养导致印迹基因的差异缺失，(2)这种缺失与DMR中特定胞嘧啶的DNA甲基化缺失有关，(3)植入后胚胎恢复正确DNA甲基化模式和印迹基因表达的能力与成功发育到足月相结合。

The imprinted H19 gene is closely linked to the oppositely imprinted Igf2 gene in both mouse and humans. The imprinted expression of these genes is mediated by a 2 kb differentially methylated region (DMR) located 5' to the H19 gene. In vitro when the DMR is situated between an enhancer and promoter, it functions as a boundary that blocks the enhancer from engaging the promoter. The boundary function is likely mediated by the methylation-sensitive transcriptional regulatory protein CTCF. According to the boundary model, CTCF binds to the DMR on the maternal allele, thereby allowing the maternal H19 gene exclusive access to the enhancers. On the paternal chromosome, the hypermethylated DMR cannot bind CTCF. This hypermethylation results in both suppressing paternal H19 promoter activity and permitting the enhancers to engage exclusively the paternal Igf2 promoter. The model thus accounts for the observed maternal H19 expression and the reciprocal paternal Igf2 expression. Specific Aim 1 will use RNA interference (RNAi) in the mouse to test the hypotheses that (1) CTCF binding on the maternal allele is responsible for the exclusive maternal H19 expression, and (2) DNA hypermethylation and the interacting DNA methyl-binding proteins on the paternal allele mediate the repression of H19 and hence permits the exclusive paternal Igf2 expression. In addition, this aim will test the hypothesis that CTCF binding to the maternal DMR in the oocyte prevents DNA methylation and thereby ensures maternal H19 expression in the embryo. Understanding the expression of imprinted genes has implications for the practice of Assisted Reproductive Technology (ART). The practice of ART has increased dramatically during the past two decades and has resulted in the birth of tens of thousands of children. Subtle changes in the culture media for preimplantation mouse embryos can result in loss-of-imprinting of the normally imprinted and maternally expressed H19 gene. The resulting biallelic expression correlates with the loss of methylation in the DMR on the paternal allele. Although it is not known if such changes occur during the course of culture of human preimplantation embryos (and if there are long-term consequences), ART is occurring in the virtual absence of any basic research using an animal model. Specific Aim 2 will use the mouse as a model system to test the hypotheses that (1) embryo culture results in the differential loss-of-imprinting of imprinted genes, (2) this loss-of- imprinting is linked with the loss of DNA methylation of specific cytosines in the DMR, and (3) following implantation the ability of the embryo to restore the correct DNA methylation pattern and imprinted gene expression is coupled with successful development to term.