Tucker K L, Beard C, Dausmann J, Jackson-Grusby L, Laird P W, Lei H, Li E, Jaenisch R
Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142, USA.
Genes Dev. 1996 Apr 15;10(8):1008-20. doi: 10.1101/gad.10.8.1008.
Embryonic stem (ES) cells homozygous for a disruption of the DNA (cytosine-5)-methyltransferase gene (Dnmt) proliferate normally with their DNA highly demethylated but die upon differentiation. Expression of the wild-type Dnmt cDNA in mutant male ES cells caused an increase in methylation of bulk DNA and of the Xist and Igf2 genes to normal levels, but did not restore the methylation of the imprinted genes H19 and Igf2r. These cells differentiated normally in vitro and contributed substantially to adult chimeras. While the Xist gene was not expressed in the remethylated male ES cells, no restoration of the normal expression profile was seen for H19, Igf2r, or Igf2. This indicates that ES cells can faithfully reestablish normal methylation and expression patterns of nonimprinted genes but lack the ability to restore those of imprinted genes. Full restoration of monoallelic methylation and expression was imposed on H19, Igf2, and Igf2r upon germ-line transmission. These results are consistent with the presence of distinct de novo DNA methyltransferase activities during oogenesis and spermatogenesis, which specifically recognize imprinted genes but are absent in the postimplantation embryo and in ES cells.
DNA（胞嘧啶-5）-甲基转移酶基因（Dnmt）缺失的纯合胚胎干细胞（ES细胞）在DNA高度去甲基化的情况下能正常增殖，但在分化时死亡。在突变雄性ES细胞中野生型Dnmt cDNA的表达导致总体DNA以及Xist和Igf2基因的甲基化增加至正常水平，但未恢复印记基因H19和Igf2r的甲基化。这些细胞在体外正常分化，并对成年嵌合体有显著贡献。虽然Xist基因在重新甲基化的雄性ES细胞中不表达，但H19、Igf2r或Igf2的正常表达谱未恢复。这表明ES细胞能够忠实地重新建立非印记基因的正常甲基化和表达模式，但缺乏恢复印记基因甲基化和表达模式的能力。在种系传递时，H19、Igf2和Igf2r的单等位基因甲基化和表达得以完全恢复。这些结果与在卵子发生和精子发生过程中存在独特的从头DNA甲基转移酶活性一致，这些活性特异性识别印记基因，但在植入后胚胎和ES细胞中不存在。
