title: Unfaithful maintenance of methylation imprints due to loss of maternal nuclear Dnmt1 during somatic cell nuclear transfer.
abstract: The low success rate of somatic cell nuclear transfer (SCNT) in mammalian cloning is largely due to imprinting problems. However, little is known about the mechanisms of reprogramming imprinted genes during SCNT. Parental origin-specific DNA methylation regulates the monoallelic expression of imprinted genes. In natural fertilization, methylation imprints are established in the parental germline and maintained throughout embryonic development. However, it is unclear whether methylation imprints are protected from global changes of DNA methylation in cloned preimplantation embryos. Here, we demonstrate that cloned porcine preimplantation embryos exhibit demethylation at differentially methylated regions (DMRs) of imprinted genes; in particular, demethylation occurs during the first two cell cycles. By RNAi-mediated knockdown, we found that Dnmt1 is required for the maintenance of methylation imprints in porcine preimplantation embryos. However, no clear signals were detected in the nuclei of oocytes and preimplantation embryos by immunofluorescence. Thus, Dnmt1 is present at very low levels in the nuclei of porcine oocytes and preimplantation embryos and maintains methylation imprints. We further showed that methylation imprints were rescued in nonenucleated metaphase II (MII) oocytes. Our results indicate that loss of Dnmt1 in the maternal nucleus during SCNT significantly contributes to the unfaithful maintenance of methylation imprints in cloned embryos.
authors.affiliation: College of Life Science, Northeast Agricultural University of China, Harbin, China.
authors.forename: Yanchang
authors.initials: Y
authors.lastname: Wei
authors.forename: Yanjun
authors.initials: Y
authors.lastname: Huan
authors.forename: Yongqian
authors.initials: Y
authors.lastname: Shi
authors.forename: Zhongfeng
authors.initials: Z
authors.lastname: Liu
authors.forename: Gerelchimeg
authors.initials: G
authors.lastname: Bou
authors.forename: Yibo
authors.initials: Y
authors.lastname: Luo
authors.forename: Li
authors.initials: L
authors.lastname: Zhang
authors.forename: Cairong
authors.initials: C
authors.lastname: Yang
authors.forename: Qingran
authors.initials: Q
authors.lastname: Kong
authors.forename: Jiangtian
authors.initials: J
authors.lastname: Tian
authors.forename: Ping
authors.initials: P
authors.lastname: Xia
authors.forename: Qing-Yuan
authors.initials: QY
authors.lastname: Sun
authors.forename: ZhongHua
authors.initials: Z
authors.lastname: Liu
mesh_terms: D000818:Animals; D001755:Blastocyst; D000076123:DNA (Cytosine-5-)-Methyltransferase 1; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000235:genetics / Q000378:metabolism*; D019175:DNA Methylation; D005260:Female; D018392:Genomic Imprinting; D008297:Male; D051381:Rats
chemical_list: D000076123:DNA (Cytosine-5-)-Methyltransferase 1; D004248:DNA (Cytosine-5-)-Methyltransferases; C000619382:Dnmt1 protein, rat