Role of Tet proteins in 5mC to 5hmC conversion, ES-cell self-renewal and inner cell mass specification

The enzymes responsible for methylating DNA, a ubiquitous epigenetic modification, have been extensively studied. Less is known about demethylation. Here, Tet1, an enzyme that catalyses the conversion of 5-methylcytosine of DNA to 5-hydroxyl-methylcytosine, is shown to have a role in mouse embryonic stem-cell maintenance by maintaining the expression of Nanog and preserving the Nanog promoter in a hypomethylated state. This suggests that Tet1 has a role in regulating DNA methylation, as well as in embryonic stem-cell self-renewal and inner cell mass specification. TET1 is an enzyme that catalyses the conversion of 5-methylcytosine of DNA to 5-hydroxymethylcytosine, raising the possibility that it is involved in mediating DNA demethylation. These authors show that Tet1 is involved in mouse embryonic stem cell maintenance and specification of the inner cell mass. It is required to maintain both the expression of Nanog in embryonic stem cells and the Nanog promoter in a hypomethylated state, supporting a role for Tet1 in regulating DNA methylation. DNA methylation is one of the best-characterized epigenetic modifications1,2,3,4. Although the enzymes that catalyse DNA methylation have been characterized, enzymes responsible for demethylation have been elusive5. A recent study indicates that the human TET1 protein could catalyse the conversion of 5-methylcytosine (5mC) of DNA to 5-hydroxymethylcytosine (5hmC), raising the possibility that DNA demethylation may be a Tet1-mediated process6. Here we extend this study by demonstrating that all three mouse Tet proteins (Tet1, Tet2 and Tet3) can also catalyse a similar reaction. Tet1 has an important role in mouse embryonic stem (ES) cell maintenance through maintaining the expression of Nanog in ES cells. Downregulation of Nanog via Tet1 knockdown correlates with methylation of the Nanog promoter, supporting a role for Tet1 in regulating DNA methylation status. Furthermore, knockdown of Tet1 in pre-implantation embryos results in a bias towards trophectoderm differentiation. Thus, our studies not only uncover the enzymatic activity of the Tet proteins, but also demonstrate a role for Tet1 in ES cell maintenance and inner cell mass cell specification.