Epigenetic Reprogramming During Somatic Cell Nuclear Transfer: Recent Progress and Future Directions

Abstract

Somatic cell nuclear transfer (SCNT) has broad applications but is limited by low cloning efficiency. In this review, we mainly focus on SCNT-mediated epigenetic reprogramming in livestock and also describe mice data for reference. This review presents the factors contributing to low cloning efficiency, demonstrates that incomplete epigenetic reprogramming leads to the low developmental potential of cloned embryos, and further describes the regulation of epigenetic reprogramming by long non-coding RNAs, which is a new research perspective in the field of SCNT-mediated epigenetic reprogramming. In conclusion, this review provides new insights into the epigenetic regulatory mechanism during SCNT-mediated nuclear reprogramming, which could have great implications for improving cloning efficiency.

Keywords: cloning efficiency; epigenetic modification; long non-coding RNA; nuclear reprogramming; somatic cell nuclear transfer.

FIGURE 1

Schematic illustration of the SCNT process. Somatic cells from the desired donor mammal are cultured for SCNT. Oocytes are recovered from the ovaries obtained from the slaughterhouse and allowed to mature into metaphase (M)II oocytes. MII oocytes are enucleated, and donor somatic cells are transferred into the perivitelline space of oocytes. After the fusion and activation of cell–cytoplast complexes, the reconstructed cloned embryos begin to develop, undergoing nuclear condensation and nuclear swelling, followed by pseudo-pronucleus, 2-cell, 4-cell, etc. stages and form blastocysts in vitro. Cloned embryos are transferred into surrogates and develop into cloned mammals.

FIGURE 2

Diagram of epigenetic modification changes during SCNT-mediated nuclear reprogramming. The data of pig embryos are adopted to describe epigenetic modification reprogramming. For DNA methylation, cloned embryos demonstrate delayed DNA demethylation and incomplete DNA remethylation of genome (the data shown here represent DNA methylation status at centromeric repeats, which partly reflects genome DNA status), high DNA methylation status of pluripotency-related gene Oct4, and low DNA methylation levels of tissue-specific gene Thy1, respectively. For histone modifications, low levels of histone acetylation (H3K9ac at the ZGA stage and H3K14ac at the blastocyst stage) and H3K4me3, and high levels of histone methylation (H3K9me3 after ZGA and H3K27me3 at the 2-cell stage) are observed in cloned embryos. For genomic imprinting, DNA methylation of H19/Igf2 is not maintained during SCNT. For XCI, DNA methylation of Xist is not fully established in female cloned embryos.

FIGURE 3

Strategy for improving epigenetic reprogramming during SCNT. The application of epigenetic modification drugs and the regulation of enzymes involved in epigenetic modification as well as associated genes can induce changes in DNA methylation, chromosome structure, histone modification, genomic imprinting, and XCI in cloned embryos close to those in normal fertilized embryos. Targeting epigenetic reprogramming to activate or silence genes can yield cloned embryos with high developmental competence.

FIGURE 4

Role of lncRNAs in the regulation of SCNT-mediated epigenetic reprogramming. LncRNAs interact with epigenetic modification enzymes to modulate DNA methylation, chromosome structure, histone modification, genomic imprinting, and XCI, thereby controlling gene expression and promoting the development of cloned embryos.