中文摘要：

DNA甲基化是一种相对稳定且可遗传的表观遗传标记，在植物和动物细胞中均发现有DNA主动去甲基化现象，其机制在植物中已基本得到阐释，但在哺乳动物中尚未鉴定出一种有效的DNA去甲基化酶，并且DNA主动去甲基化途径也存在争议。文章综合分析了近期的文献资料，阐述了哺乳动物中发生DNA主动去甲基化的时空特异性，并从细胞和组织特异性角度介绍DNA主动去甲基化的可能通路和机制，即5－甲基胞嘧啶的氧化作用、5－甲基胞嘧啶脱氨基以及DNA修复等，旨在为破译表观遗传重编程过程提供理论依据。

英文摘要：

DNA methylation is a stable and heritable epigenetic mark, and it is one of the best characterized epigenetic modifications. Active DNA demethylation has been reported both in plant and animal cells, and the mechanism behind it is becoming clear in plant. Whereas a bonafide enzyme, which is responsible for active DNA demethylation, have not been identified in mammals, and active demethylation pathway is controversial. In the present review, we described that active DNA demethylation take place in a spatial- and temporal-specific way on the basis of recent literatures. Moreover, several candidate pathways such as oxygenation and deamination of 5-methyl cytosine and DNA repair pathways, which may be responsible for the active process were introduced on a cell- and tissue-specific view. The aim of this paper is to help researchers reveal the mechanism underlying this important event during epigenetic reprogramming in mammals.