Ratnam Sarayu, Mertineit Carmen, Ding Feng, Howell Carina Y, Clarke Hugh J, Bestor Timothy H, Chaillet J Richard, Trasler Jacquetta M
Department of Human Genetics, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
Dev Biol. 2002 May 15;245(2):304-14. doi: 10.1006/dbio.2002.0628.
The imprinting of mammalian genes depends on the maintenance of DNA methylation patterns during pre- and postimplantation development. Dnmt1o is a variant form of the somatically expressed Dnmt1 cytosine methyltransferase that is synthesized and stored in the oocyte cytoplasm and trafficks to the eight-cell nucleus during preimplantation development, where it maintains DNA methylation patterns on alleles of imprinted genes. Transcripts encoding Dnmt1 are present in preimplantation embryos, suggesting that Dnmt1 protein is also expressed in the preimplantation embryo, and may account for maintenance methylation at preimplantation stages other than the eight-cell embryo. However, using an antibody that detects Dnmt1, but not Dnmt1o, no Dnmt1 protein was detected on immunoblots or by immunocytochemical staining in wildtype preimplantation embryos. Moreover, Dnmt1 protein produced in the oocyte from a modified Dnmt1 allele, Dnmt1(1s/1o), trafficked to nuclei of eight-cell embryos, but not to nuclei of other stages. The highly restricted nuclear localization patterns of oocyte-derived Dnmt1o and Dnmt1 during preimplantation development add further support to the notion that DNA methyltransferases other than Dnmt1 are required for maintaining imprints during preimplantation development.
哺乳动物基因的印记取决于着床前和着床后发育过程中DNA甲基化模式的维持。Dnmt1o是体细胞表达的Dnmt1胞嘧啶甲基转移酶的一种变体形式，它在卵母细胞细胞质中合成并储存，在着床前发育过程中运输到八细胞期胚胎的细胞核，在那里维持印记基因等位基因上的DNA甲基化模式。编码Dnmt1的转录本存在于着床前胚胎中，这表明Dnmt1蛋白也在着床前胚胎中表达，并且可能在八细胞胚胎以外的着床前阶段负责维持甲基化。然而，使用一种能检测Dnmt1而不是Dnmt1o的抗体，在野生型着床前胚胎的免疫印迹或免疫细胞化学染色中未检测到Dnmt1蛋白。此外，由修饰的Dnmt1等位基因Dnmt1(1s/1o)在卵母细胞中产生的Dnmt1蛋白运输到八细胞胚胎的细胞核，但不运输到其他阶段的细胞核。着床前发育过程中卵母细胞来源的Dnmt1o和Dnmt1高度受限的核定位模式进一步支持了这样一种观点，即着床前发育过程中维持印记需要Dnmt1以外的DNA甲基转移酶。
