DNA demethylation is necessary for the epigenetic reprogramming of somatic cell nuclei

Nuclear transplantation experiments in amphibia and mammals have shown that oocyte and egg cytoplasm can extensively reprogram somatic cell nuclei with new patterns of gene expression and new pathways of cell differentiation(1-3); however, very little is known about the molecular mechanism of nuclear reprogramming. Here we have used nuclear and DNA transfer from mammalian somatic cells to analyse the mechanism of activation of the stem cell marker gene oct4 by Xenopus oocytes. We find that the removal of nuclear protein accelerates the rate of reprogramming, but even more important is the demethylation of somatic cell DNA. DNA demethylation seems to precede gene reprogramming, and is absolutely necessary for oct4 transcription. Reprogramming by oocytes occurs in the absence of DNA replication and RNA/protein synthesis. It is also selective, operating only on the promoter, but not enhancers, of oct4; both a putative Sp1/Sp3 and a GGGAGGG binding site are required for demethylation and transcription. We conclude that the demethylation of promoter DNA may be a necessary step in the epigenetic reprogramming of somatic cell nuclei.