中外学者两篇Nature文章获干细胞研究新突破

　　

生物通报道：干细胞研究近年来已成为国内外生命科学研究领域的重要方向之一，近期Nature公布了两项这一领域的新成果，分别由中科院上海生命科学研究院，以及英国剑桥大学完成。

在第一篇文章中，研究人员揭示了Tet3 DNA双加氧酶在卵细胞重编程中的作用，获得了卵细胞重编程的机制研究的突破性进展。

受精是精子和卵细胞融合为一个合子（受精卵）的过程，是动物个体发育的起点。然而，受精并不是简单的精卵结合。为了形成一个具有发育全能性的早期胚胎，卵细胞需要对来源于精子的父本基因组进行一系列的重编程（reprogramming），其中最为重要的一项就是基因组DNA的去甲基化。这种在合子中，父本基因组上的特异去甲基化为何发生以及怎么发生，一直是表观遗传学领域重要的有待解释的问题之一。

在这篇文章中，研究人员揭示了Tet3 DNA双加氧酶在卵细胞重编程中的作用：卵细胞来源的母源因子Tet3加氧酶负责父本基因组DNA胞嘧啶甲基的氧化修饰，从而启动DNA的去甲基化，进一步激活Oct4和Nanog等全能性基因的表达。卵细胞内特异性敲除Tet3的母鼠生育力显著下降，其大部分胚胎在着床后发生退化，被母体吸收。此外，Tet3在动物克隆过程中对移入卵细胞的供体细胞DNA的重编程也发挥着重要的作用。

这一发现提示，动物克隆和自然受精过程很可能采用了同样的重编程机制。该研究成果使人们对早期胚胎发育中的重编程过程有了更清晰的认识，也为提高动物克隆效率带来了新的理论依据，有可能在分子机制上为不孕不育症提供新的诠释。

第二篇文章中，研究人员创造了只带有一套染色体的小鼠胚胎干细胞，这将有助于科学家们深入分析小鼠发育过程中的神经网路。

这种单倍体小鼠为哺乳动物系统遗传学研究开启了一扇新窗口，也提供了一种更方便快捷分析基因网路的新方法。

（生物通：万纹）

原文摘要：

The role of Tet3 DNA dioxygenase in epigenetic reprogramming by oocytes

Sperm and eggs carry distinctive epigenetic modifications that are adjusted by reprogramming after fertilization1. The paternal genome in a zygote undergoes active DNA demethylation before the first mitosis2, 3. The biological significance and mechanisms of this paternal epigenome remodelling have remained unclear4. Here we report that, within mouse zygotes, oxidation of 5-methylcytosine (5mC) occurs on the paternal genome, changing 5mC into 5-hydroxymethylcytosine (5hmC). Furthermore, we demonstrate that the dioxygenase Tet3 (ref. 5) is enriched specifically in the male pronucleus. In Tet3-deficient zygotes from conditional knockout mice, paternal-genome conversion of 5mC into 5hmC fails to occur and the level of 5mC remains constant. Deficiency of Tet3 also impedes the demethylation process of the paternal Oct4 and Nanog genes and delays the subsequent activation of a paternally derived Oct4 transgene in early embryos. Female mice depleted of Tet3 in the germ line show severely reduced fecundity and their heterozygous mutant offspring lacking maternal Tet3 suffer an increased incidence of developmental failure. Oocytes lacking Tet3 also seem to have a reduced ability to reprogram the injected nuclei from somatic cells. Therefore, Tet3-mediated DNA hydroxylation is involved in epigenetic reprogramming of the zygotic paternal DNA following natural fertilization and may also contribute to somatic cell nuclear reprogramming during animal cloning.

Derivation of haploid embryonic stem cells from mouse embryos

Most animals are diploid, but haploid-only and male-haploid (such as honeybee and ant) species have been described1. The diploid genomes of complex organisms limit genetic approaches in biomedical model species such as mice. To overcome this problem, experimental induction of haploidy has been used in fish2, 3. Haploid development in zebrafish has been applied for genetic screening2. Recently, haploid pluripotent cell lines from medaka fish (Oryzias latipes) have also been established3. In contrast, haploidy seems less compatible with development in mammals4, 5. Although haploid cells have been observed in egg cylinder stage parthenogenetic mouse embryos6, most cells in surviving embryos become diploid. Here we describe haploid mouse embryonic stem cells and show their application in forward genetic screening.