Dramatic replacement of histone variants during genome remodeling in nuclear-transferred embryos.

The genome of differentiated somatic nuclei is remodeled to a totipotent state when they are transplanted into enucleated oocytes. To clarify the mechanism of this genome remodeling, we analyzed changes in the composition of core histone variants in nuclear-transferred embryos, since recent evidence has revealed that chromatin structure can be remodeled as a result of variant histone replacement. We found that the donor cell-derived histone H3 variants H3.1, H3.2, and H3.3, as well as H2A and H2A.Z, were rapidly eliminated from the chromatin of nuclei transplanted into enucleated oocytes. Accompanying this removal, oocyte-stored histone H3 variants and H2A.X were incorporated into the transplanted nuclei, while the incorporation of H2A and H2A.Z was minimal or not detected. The incorporation of these variant histones was DNA replication-independent. These results suggest that most core histone H2A and H3 components are dynamically exchanged between donor nuclei and recipient cytoplasm, which further suggests that replacement of donor cell histones with oocyte-stored histones may play a key role in genome remodeling in nuclear-transferred embryos. In addition, the incorporation patterns of all of the histone variants in the nuclear-transferred embryos were virtually the same as in the fertilized embryos. Only the incorporation pattern of H3.1 differed; it was incorporated into the transplanted donor nuclei, but not in the pronuclei of fertilized embryos. This result suggests that the incorporation of H3.1 has a detrimental effect on the process of genome remodeling and contributes to the low success rate of somatic nuclear cloning.