Identification of epigenetic barrier that prevents reprogramming in somatic cell nuclear transfer

Mammalian oocytes can reprogram somatic cells into totipotent state, which allows animal cloning through somatic cell nuclear transfer (SCNT). However, the great majority of SCNT embryos fail to develop to term due to poorly defined reprogramming defects. Here we demonstrate that histone H3 lysine 9 trimethylation (H3K9me3) in donor nuclei is a major epigenetic barrier that prevents efficient nuclear reprogramming in mouse oocytes. Comparative transcriptome analysis of early embryos revealed reprogramming resistant regions (RRRs) where transcriptional activation at 2-cell embryos is inhibited by SCNT compared to in vitro fertilization (IVF). RRRs significantly overlap with H3K9me3 enrichment in donor somatic cells. Importantly, removal of the H3K9me3 by ectopic expression of an H3K9me3 demethylase Kdm4d in recipient oocytes not only reactivates most RRRs, but also greatly improves development of SCNT embryos. Furthermore, the use of Suv39h1/2-depleted somatic nuclei as donors also greatly improves the development of SCNT embryos. Our study thus reveals H3K9me3 as an epigenetic barrier in SCNT-mediated reprogramming and provides a feasible method for improving mammalian cloning efficiency. Overall design: Here we perform RNA-seq based transcriptome profiling in Donor (cumulus cells), in vitro fertilized (IVF) embryos at 1- and 2-cell stages, somatic cell nuclear transfer (SCNT) embryos at 1- and 2-cell stages, Kdm4d over-expressed 2-cell SCNT embryos, and catalytic domain mutated Kdm4d over-expressed 2-cell SCNT embryos with duplicates. 

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Matoba S, Liu Y, Lu F, Iwabuchi KA, Shen L, Inoue A, Zhang Y