Aberrant DNA methylation reprogramming in bovine SCNT preimplantation embryos

Disruption of NNAT, NAP1L5 and MKRN3 DNA methylation and transcription in rabbit parthenogenetic fetuses

Parthenogenetically activated oocytes cannot develop to term in mammals due to lack of paternal gene expression. Disruption of imprinted gene expression and DNA methylation status in parthenogenetic fetuses has been reported in mice and pigs, but not in rabbits. In this study, the genomic imprinting status of the paternally expressed genes Neuronatin (NNAT), Nucleosome assembly protein 1-like 5 (NAP1L5), and Makorin ring finger protein 3 (MKRN3) was compared between rabbit parthenogenetic (PA) and normally fertilized fetuses (Con) using quantitative real-time PCR (qRT-PCR) and bisulfite sequencing PCR (BSP). The results revealed a significantly reduced expression of NNAT, NAP1L5, and MKRN3 in rabbit PA fetuses compared with Con fetuses (p<0.05). In addition, the BSP results demonstrated hypermethylation in the differentially methylated regions (DMRs) of NNAT, NAP1L5, and MKRN3 in rabbit PA fetuses. Taken together, these results suggest that hypermethylation of DMRs is associated with decreased NNAT, NAP1L5, and MKRN3 expression, which may be responsible for developmental failure of rabbit PA fetuses.

Proper reprogramming of imprinted and non-imprinted genes in cloned cattle gametogenesis

Epigenetic abnormalities in cloned animals are caused by incomplete reprogramming of the donor nucleus during the nuclear transfer step (first reprogramming). However, during the second reprogramming step that occurs only in the germline cells, epigenetic errors not corrected during the first step are repaired. Consequently, epigenetic abnormalities in the somatic cells of cloned animals should be erased in their spermatozoa or oocytes. This is supported by the fact that offspring from cloned animals do not exhibit defects at birth or during postnatal development. To test this hypothesis in cloned cattle, we compared the DNA methylation level of two imprinted genes (H19 and PEG3) and three non-imprinted genes (XIST, OCT4 and NANOG) and two repetitive elements (Satellite I and Satellite II) in blood and sperm DNAs from cloned and non-cloned bulls. We found no differences between cloned and non-cloned bulls. We also analyzed the DNA methylation levels of four repetitive elements (Satellite I, Satellite II, Alpha-satellite and Art2) in oocytes recovered from cloned and non-cloned cows. Again, no significant differences were observed between clones and non-clones. These results suggested that imprinted and non-imprinted genes and repetitive elements were properly reprogramed during gametogenesis in cloned cattle; therefore, they contributed to the soundness of cloned cattle offspring.

Reprogramming towards totipotency is greatly facilitated by synergistic effects of small molecules

Animal cloning has been achieved in many species by transplanting differentiated cell nuclei to unfertilized oocytes. However, the low efficiencies of cloning have remained an unresolved issue. Here we find that the combination of two small molecules, trichostatin A (TSA) and vitamin C (VC), under culture condition with bovine serum albumin deionized by ion-exchange resins, dramatically improves the cloning efficiency in mice and 15% of cloned embryos develop to term by means of somatic cell nuclear transfer (SCNT). The improvement was not observed by adding the non-treated, rather than deionized, bovine serum. RNA-seq analyses of SCNT embryos at the two-cell stage revealed that the treatment with TSA and VC resulted in the upregulated expression of previously identified reprogramming-resistant genes. Moreover, the expression of early-embryo-specific retroelements was upregulated by the TSA and VC treatment. The enhanced gene expression was relevant to the VC-mediated reduction of histone H3 lysine 9 methylation in SCNT embryos. Our study thus shows a simply applicable method to greatly improve mouse cloning efficiency, and furthers our understanding of how somatic nuclei acquire totipotency.

Summary: The optimized culture condition with small molecules is sufficient to allow highly efficient mouse cloning by removing epigenetic barriers to reprogramming.

Epigenetic Status of H19-Igf2 Imprinted Genes and Loss of 5-Hydroxymethylcytosine in the Brain of Cloned Goats

In mammals, the imprinted genes play vital roles in development and are generally controlled by DNA methylation at imprinting control regions (ICRs). Recently, it was discovered that 5-hydroxymethylcytosine (5-hmC) is a stable epigenetic modification; however, its functions in cloned animal genomes have not yet been fully elucidated. In this study, we interrogated and quantified the 5-hmC levels in the brain of cloned goats and discovered upregulation of Uhrf1 (p < 0.001), Dnmt1 (p < 0.05), Dnmt3a (p < 0.05), Igf2 (p < 0.01), and H19 (p < 0.05) and downregulation of Dnmt3b (p < 0.001), Tet1 (p < 0.001), Tet2 (p < 0.05), Tet3 (p < 0.001), Mecp2 (p < 0.05), and Igf2r (p < 0.05) in deceased cloned goat tissues compared with the normal controls. We demonstrated that DNA methylation was increased at H19 ICR (51.33% ± 2.03% vs. 93.07% ± 3.06%; p < 0.01) and that DNA was hypomethylated at Igf2 ICR (4.57% ± 1.48% vs. 7.63% ± 1.83%; p > 0.05) in the brain of deceased cloned goats. Finally, we showed that within the cloned goat brain genome, the amount of genome-wide 5-hmC was significantly decreased (0.083% ± 0.026% vs. 0.024% ± 0.007%; p < 0.05), whereas the 5-hmC levels within H19 and Igf2 CCGG sites were not significantly altered (0.17% ± 0.09% vs. 0.03% ± 0.01%; p > 0.05) in the brain of deceased cloned goats. Our data bring further experimental evidence regarding the abnormalities in 5-hmC and advance our current understanding of the role of 5-hmC in cloned animals.

Altered DNA methylation associated with an abnormal liver phenotype in a cattle model with a high incidence of perinatal pathologies

Cloning enables the generation of both clinically normal and pathological individuals from the same donor cells, and may therefore be a DNA sequence-independent driver of phenotypic variability. We took advantage of cattle clones with identical genotypes but different developmental abilities to investigate the role of epigenetic factors in perinatal mortality, a complex trait with increasing prevalence in dairy cattle. We studied livers from pathological clones dying during the perinatal period, clinically normal adult clones with the same genotypes as perinatal clones and conventional age-matched controls. The livers from deceased perinatal clones displayed histological lesions, modifications to quantitative histomorphometric and metabolic parameters such as glycogen storage and fatty acid composition, and an absence of birth-induced maturation. In a genome-wide epigenetic analysis, we identified DNA methylation patterns underlying these phenotypic alterations and targeting genes relevant to liver metabolism, including the type 2 diabetes gene TCF7L2. The adult clones were devoid of major phenotypic and epigenetic abnormalities in the liver, ruling out the effects of genotype on the phenotype observed. These results thus provide the first demonstration of a genome-wide association between DNA methylation and perinatal mortality in cattle, and highlight epigenetics as a driving force for phenotypic variability in farmed animals.