Dynamics of Dnmt1 Methyltransferase Expression and Intracellular Localization during Oogenesis and Preimplantation Development

Abstract

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.

Gamete imprinting: setting epigenetic patterns for the next generation

“…It was shown that DNMT1o was the only form of DNMT1 expressed in oocytes and preimplantation mouse embryos (Ratnam et al 2002) and it was initially postulated to play an important role in the acquisition of methylation patterns in oocytes. However, when Dnmt1o was subsequently knocked out in a mouse model, examination of gene-specific DNA methylation, as well as nuclear transplantation experiments, definitively showed that DNMT1o is not required for the establishment of methylation patterns in oocytes (Howell et al 2001).…”

Alternative splicing and expression analysis of bovine DNA methyltransferase 1

“…Dnmt1o is produced by the usage of an oocyte-specific promoter and first exon (Mertineit et al, 1998), which is not present in Dnmt1s and limits the expression of Dnmt1o to oocytes and early embryos. Furthermore, the activity of Dnmt1o is regulated by a nuclear sequestration mechanism that controls the retention of Dnmt1o in the cytoplasm of embryonic cells at all stages of early development with the exception of the 8-cell stage when it translocates into the nucleus maintaining DNA methylation patterns of imprinted genes (Ratnam et al, 2002). Dnmt1s (190,000 Da) expression initiates at the peri-implantation stage and is ubiquitous to all somatic cells (Mertineit et al, 1998;Trasler et al, 1996).…”

Erase–Maintain–Establish: Natural Reprogramming of the Mammalian Epigenome

“…To allow for efficient methylation removal, the maintenance DNA methyltransferase-1 (DNMT1) is largely excluded from the nuclei of the cleavage-stage embryos (Howell et al 2001;Ratnam et al 2002;Branco et al 2008;Hirasawa et al 2008). However, this compartmentalization and nuclear exclusion cannot be complete, as genetic deletion of both maternal and zygotic Dnmt1 results in imprinting defects, the result of complete ICR demethylation (Hirasawa et al 2008).…”