DNA methylation is one of the epigenetic mechanisms and plays important roles during oogenesis and early embryo development in mammals. DNA methylation is basically known as adding a methyl group to the fifth carbon atom of cytosine residues within cytosine-phosphate-guanine (CpG) and non-CpG dinucleotide sites. This mechanism is composed of two main processes: de novo methylation and maintenance methylation, both of which are catalyzed by specific DNA methyltransferase (DNMT) enzymes. To date, six different DNMTs have been characterized in mammals defined as DNMT1, DNMT2, DNMT3A, DNMT3B, DNMT3C, and DNMT3L. While DNMT1 primarily functions in maintenance methylation, both DNMT3A and DNMT3B are essentially responsible for de novo methylation. As is known, either maintenance or de novo methylation processes appears during oocyte and early embryo development terms. The aim of the present study is to investigate spatial and temporal expression levels and subcellular localizations of the DNMT1, DNMT3A, and DNMT3B proteins in the mouse germinal vesicle (GV) and metaphase II (MII) oocytes, and early embryos from 1-cell to blastocyst stages. We found that there are remarkable differences in the expressional levels and subcellular localizations of the DNMT1, DNMT3A and DNMT3B proteins in the GV and MII oocytes, and 1-cell, 2-cell, 4-cell, 8-cell, morula, and blastocyst stage embryos. The fluctuations in the expression of DNMT proteins in the analyzed oocytes and early embryos are largely compatible with DNA methylation changes and genomic imprintestablishment appearing during oogenesis and early embryo development. To understand precisemolecular biological meaning of differently expressing DNMTs in the early developmental periods, further studies are required.
Keywords: DNA methylation; DNA methyltransferase; Early embryo; Oocyte.