Sharif Jafar, Muto Masahiro, Takebayashi Shin-ichiro, Suetake Isao, Iwamatsu Akihiro, Endo Takaho A, Shinga Jun, Mizutani-Koseki Yoko, Toyoda Tetsuro, Okamura Kunihiro, Tajima Shoji, Mitsuya Kohzoh, Okano Masaki, Koseki Haruhiko
Tohoku University Biomedical Engineering Research Organization (TUBERO), 2-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan.
Nature. 2007 Dec 6;450(7171):908-12. doi: 10.1038/nature06397. Epub 2007 Nov 11.
DNA methyltransferase (cytosine-5) 1 (Dnmt1) is the principal enzyme responsible for maintenance of CpG methylation and is essential for the regulation of gene expression, silencing of parasitic DNA elements, genomic imprinting and embryogenesis. Dnmt1 is needed in S phase to methylate newly replicated CpGs occurring opposite methylated ones on the mother strand of the DNA, which is essential for the epigenetic inheritance of methylation patterns in the genome. Despite an intrinsic affinity of Dnmt1 for such hemi-methylated DNA, the molecular mechanisms that ensure the correct loading of Dnmt1 onto newly replicated DNA in vivo are not understood. The Np95 (also known as Uhrf1 and ICBP90) protein binds methylated CpG through its SET and RING finger-associated (SRA) domain. Here we show that localization of mouse Np95 to replicating heterochromatin is dependent on the presence of hemi-methylated DNA. Np95 forms complexes with Dnmt1 and mediates the loading of Dnmt1 to replicating heterochromatic regions. By using Np95-deficient embryonic stem cells and embryos, we show that Np95 is essential in vivo to maintain global and local DNA methylation and to repress transcription of retrotransposons and imprinted genes. The link between hemi-methylated DNA, Np95 and Dnmt1 thus establishes key steps of the mechanism for epigenetic inheritance of DNA methylation.
DNA甲基转移酶（胞嘧啶-5）1（Dnmt1）是负责维持CpG甲基化的主要酶，对基因表达调控、寄生DNA元件沉默、基因组印记和胚胎发育至关重要。Dnmt1在S期需要将新复制的与DNA母链上甲基化的CpG相对的CpG甲基化，这对基因组中甲基化模式的表观遗传继承至关重要。尽管Dnmt1对这种半甲基化DNA具有内在亲和力，但确保Dnmt1在体内正确加载到新复制DNA上的分子机制尚不清楚。Np95（也称为Uhrf1和ICBP90）蛋白通过其SET和RING指相关（SRA）结构域结合甲基化的CpG。在这里，我们表明小鼠Np95定位于复制性异染色质取决于半甲基化DNA的存在。Np95与Dnmt1形成复合物，并介导Dnmt1加载到复制性异染色质区域。通过使用Np95缺陷的胚胎干细胞和胚胎，我们表明Np95在体内对于维持全局和局部DNA甲基化以及抑制逆转座子和印记基因的转录至关重要。因此，半甲基化DNA、Np95和Dnmt1之间的联系确立了DNA甲基化表观遗传继承机制的关键步骤。
