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16 Improvement of porcine cloned embryo developmental competence via KDM4A overexpression and H3K9me3 methyltransferase inhibitor treatment

Aberrant epigenetic reprogramming is a major reason for the developmental failure of somatic cell nuclear transfer (SCNT) embryos. Histone H3 lysine 9 trimethylation (H3K9me3), a histone marker for transcriptional repression, is considered a key barrier to the development of cloned embryos. In the present study, we found that H3K9me3 was much higher in SCNT embryos than in IVF embryos at the 4-cell and 2-cell stages; H3K9me3 demethylase KDM4A mRNA was injected into cloned embryos 5 h after activation. The intensity of H3K9me3 modification decreased obviously after microinjection of KDM4A mRNA, and the developmental efficiency of porcine cloned embryos was enhanced significantly compared with control (32.2 ± 4.2 vs. 21.0 ± 3.5; P < 0.05). Moreover, we evaluated the effect of chaetocin, an inhibitor of histone methyltransferase suv39h1/2, on SCNT embryo development. The results showed that 10 nM chaetocin not only suppressed the H3K9me3 modification in porcine embryonic fibroblast but also downregulated the expression of SUV39H1, SUV39H2, and KDM4D. However, treatment of cloned embryos with 10 nM chaetocin efficiently decreased the H3K9me3 level. Importantly, chaetocin treatment at the 4-cell stage for 6 h significantly increased the blastocyst rate (57.8 ± 4.3 vs. 43.5 ± 1.8; P < 0.05) compared with the control group. Furthermore, because a recent study showed that a high level of H3K9me3 appears in the reprogramming resistance region of nuclear transfer embryos during zygogene activation (ZGA), we examined the expression levels of ZGA-related genes. The qPCR results showed that the expression of ZGA-related genes increased significantly in SCNT embryos with chaetocin treatment compared with the control. These results suggested that chaetocin treatment can improve the efficiency of SCNT reprogramming during ZGA. In summary, our results suggested that H3K9me3 acts as an epigenetic barrier in porcine SCNT reprogramming and that a suv39h1/2 inhibitor can effectively reduce the H3K9me3 level in the early reprogramming phase and further improve the in vitro developmental competence. Due to the existence of a variety of abnormal epigenetic mechanisms during somatic cell reprogramming, the combined use of small-molecule inhibitors is required in future studies. Data analyses were performed using SPSS software. Significance was set at P < 0.05 unless otherwise specified. The results are expressed as the mean ± standard deviation. This work was supported by the National Natural Science Foundation of China (grant number 31601942); the Postdoctoral Science Foundation of Heilongjiang Province (grant number LBH-Z17010); and the Fund for the National Key Research and Development program of China-Stem Cell and Translational Research (2016YFA0100200).