Incomplete reactivation of<i>Oct4</i>-related genes in mouse embryos cloned from somatic nuclei

TLDR

Cloned embryos from somatic nuclei reach the blastocyst stage but die after implantation, a fate similar to Oct4‑knockout embryos that also lack pluripotent cells. The study tests whether somatic‑cell clones fail to generate pluripotent cells because key genes such as Oct4 are not fully reactivated. An in silico screen identified Oct4‑related genes with expression patterns matching Oct4 to assess their reactivation in cloned blastocysts. Only 62 % of cumulus‑cell–derived cloned blastocysts expressed all 11 Oct4‑related genes, whereas ES‑cell–derived clones and normal embryos did, and this incomplete reactivation correlated with lower developmental success to term, implicating gene reactivation failure in embryonic lethality.

Abstract

The majority of cloned animals derived by nuclear transfer from somatic cell nuclei develop to the blastocyst stage but die after implantation. Mouse embryos that lack an Oct4 gene, which plays an essential role in control of developmental pluripotency, develop to the blastocyst stage and also die after implantation, because they lack pluripotent embryonic cells. Based on this similarity, we posited that cloned embryos derived from differentiated cell nuclei fail to establish a population of truly pluripotent embryonic cells because of faulty reactivation of key embryonic genes such as Oct4. To explore this hypothesis, we used an in silico approach to identify a set of Oct4-related genes whose developmental expression pattern is similar to that of Oct4. When expression of Oct4 and 10 Oct4-related genes was analyzed in individual cumulus cell-derived cloned blastocysts, only 62% correctly expressed all tested genes. In contrast to this incomplete reactivation of Oct4-related genes in somatic clones, ES cell-derived cloned blastocysts and normal control embryos expressed these genes normally. Notably, the contrast between expression patterns of the Oct4-related genes correlated with efficiency of embryonic development of somatic and ES cell-derived cloned blastocysts to term. These observations suggest that failure to reactivate the full spectrum of these Oct4-related genes may contribute to embryonic lethality in somatic-cell clones.