Dynamics of TET family expression in porcine preimplantation embryos is related to zygotic genome activation and required for the maintenance of NANOG

Dynamic changes in DNA methylation are observed during embryo development. Recent studies show that the TET family is involved in these changes by converting 5-methylcytosine (5mec) to 5-hydroxymethylcytosine (5hmec). Specifically, TET3 is responsible for the conversion in the early stages, and then TET1 is a key regulator at later stages of embryo development. From previous mouse reports and our preliminary data in porcine embryos, we hypothesized that TET1 becomes the main regulator at the time of the maternal to zygotic transition (MZT). Transcript abundance of TET3 was high only at the zygote and 2-cell stage. The abundance of TET1 mRNA was high in the blastocysts and TET1 protein was present at the 4-cell stage and the blastocysts. The dynamic was similar in porcine somatic cell nuclear transfer (SCNT) embryos however; abnormally upregulated TET3 was detected at the 4-cell stage. When transcription or translation was blocked at the 2-cell stage, TET3 mRNA remained high at the 4-cell stage suggesting that degradation of TET3 is related to the MZT. Downregulation of TET3 before fertilization resulted in the reduction of 5hmec in zygotes indicating that TET3 is a key molecule for 5hmec synthesis. This misregulation of 5hmec in zygotes also affected the level of NANOG expression in the blastocysts. We show here that the porcine TET family shows dynamic expression patterns during embryogenesis, and is responsible for the appearance of 5hmec in the zygotes by TET3. This appearance of 5hmec in zygote is important for the expression of NANOG in the blastocysts.