Conservation of methylation reprogramming in mammalian development: Aberrant reprogramming in cloned embryos

TLDR

Mouse embryos undergo genome‑wide methylation reprogramming, with demethylation early in preimplantation followed by remethylation. The study investigates whether genome‑wide methylation reprogramming is conserved in mammals and occurs in embryos cloned from highly methylated somatic donor nuclei. Normal bovine, rat, and pig zygotes exhibit active demethylation of the paternal genome, with bovine embryos undergoing further demethylation until the eight‑cell stage before de novo methylation at the 16‑cell stage, whereas cloned embryos show only an initial demethylation followed by precocious de novo methylation and nuclear reorganization resembling differentiated cells, leading to highly methylated nuclei in morulae and suggesting incomplete reprogramming underlies low cloning efficiency.

Abstract

Mouse embryos undergo genome-wide methylation reprogramming by demethylation in early preimplantation development, followed by remethylation thereafter. Here we show that genome-wide reprogramming is conserved in several mammalian species and ask whether it also occurs in embryos cloned with the use of highly methylated somatic donor nuclei. Normal bovine, rat, and pig zygotes showed a demethylated paternal genome, suggesting active demethylation. In bovine embryos methylation was further reduced during cleavage up to the eight-cell stage, and this reduction in methylation was followed by de novo methylation by the 16-cell stage. In cloned one-cell embryos there was a reduction in methylation consistent with active demethylation, but no further demethylation occurred subsequently. Instead, de novo methylation and nuclear reorganization of methylation patterns resembling those of differentiated cells occurred precociously in many cloned embryos. Cloned, but not normal, morulae had highly methylated nuclei in all blastomeres that resembled those of the fibroblast donor cells. Our study shows that epigenetic reprogramming occurs aberrantly in most cloned embryos; incomplete reprogramming may contribute to the low efficiency of cloning.