Embryonic development following somatic cell nuclear transfer impeded by persisting histone methylation.

Mammalianoocytescanreprogramsomaticcellsintoatotipotentstateenablinganimalcloningthroughso-matic cell nuclear transfer (SCNT). However, the ma-jority of SCNT embryos fail to develop to term due toundefined reprogramming defects. Here, we identifyhistoneH3lysine9trimethylation(H3K9me3)ofdonorcell genome as a major barrier for efficient repro-gramming by SCNT. Comparative transcriptomeanalysis identified reprogramming resistant regions(RRRs) that are expressed normally at 2-cell mouseembryos generated by in vitro fertilization (IVF) butnot SCNT. RRRs are enriched for H3K9me3 in donorsomatic cells and its removal by ectopically ex-pressed H3K9me3 demethylase Kdm4d not onlyreactivates the majority of RRRs, but also greatly im-proves SCNT efficiency. Furthermore, use of donorsomatic nuclei depleted of H3K9 methyltransferasesmarkedly improves SCNT efficiency. Our study thusidentifies H3K9me3 as a critical epigenetic barrierin SCNT-mediated reprogramming and provides apromising approach for improving mammalian clon-ing efficiency.