Methylation patterns in 5′ terminal regions of pluripotency-related genes in bovine in vitro fertilized and cloned embryos

Astaxanthin enhances the development of bovine cloned embryos by inhibiting apoptosis and improving DNA methylation reprogramming of pluripotency genes

Low cloning efficiency limits the wide application of somatic cell nuclear transfer technology. Apoptosis and incomplete DNA methylation reprogramming of pluripotency genes are considered as the main causes for low cloning efficiency. Astaxanthin (AST), a powerfully antioxidative and antiapoptotic carotenoid, is recently shown to improve the development of early embryos, however, the potential role of AST during the development of cloned embryos remains unclear. This study displayed that treating cloned embryos with AST significantly increased the blastocyst rate and total blastocyst cell number in a concentration dependent manner, and also alleviated the damage of H2O2 to the development of cloned embryos. In addition, compared with the control group, AST significantly reduced the apoptotic cell number and rate in cloned blastocysts, and the significantly upregulated expression of anti-apoptotic gene Bcl2l1 and antioxidative genes (Sod1 and Gpx4) and downregulated transcription of pro-apoptotic genes (Bax, P53 and Caspase3) were observed in the AST group. Moreover, AST treatment facilitated DNA demethylation of pluripotency genes (Pou5f1, Nanog and Sox2), in accompany with the improved transcription levels of DNA methylation reprogramming genes (Tet1, Tet3, Dnmt1, Dnmt3a and Dnmt3b) in cloned embryos, and then, the significantly upregulated expression levels of embryo development related genes including Pou5f1, Nanog, Sox2 and Cdx2 were observed in comparison with the control group. In conclusion, these results revealed that astaxanthin enhanced the developmental potential of bovine cloned embryos by inhibiting apoptosis and improving DNA methylation reprogramming of pluripotency genes, and provided a promising approach to improve cloning efficiency.

DNA methylation and miRNA-1296 act in concert to mediate spatiotemporal expression of KPNA7 during bovine oocyte and early embryonic development

BACKGROUND

Epigenetic regulation of oocyte-specific maternal factors is essential for oocyte and early embryonic development. KPNA7 is an oocyte-specific maternal factor, which controls transportation of nuclear proteins important for early embryonic development. To elucidate the epigenetic mechanisms involved in the controlled expression of KPNA7, both DNA methylation associated transcriptional silencing and microRNA (miRNA)-mediated mRNA degradation of KPNA7 were examined.

RESULTS

Comparison of DNA methylation profiles at the proximal promoter of KPNA7 gene between oocyte and 6 different somatic tissues identified 3 oocyte-specific differentially methylated CpG sites. Expression of KPNA7 mRNA was reintroduced in bovine kidney-derived CCL2 cells after treatment with the methylation inhibitor, 5-aza-2'-deoxycytidine (5-Aza-CdR). Analysis of the promoter region of KPNA7 gene in CCL2 cells treated with 5-Aza-CdR showed a lighter methylation rate in all the CpG sites. Bioinformatic analysis predicted 4 miRNA-1296 binding sites in the coding region of KPNA7 mRNA. Ectopic co-expression of miRNA-1296 and KPNA7 in HEK293 cells led to reduced expression of KPNA7 protein. Quantitative real time PCR (RT-qPCR) analysis revealed that miRNA-1296 is expressed in oocytes and early stage embryos, and the expression reaches a peak level in 8-cell stage embryos, coincident with the time of embryonic genome activation and the start of declining of KPNA7 expression.

CONCLUSIONS

These results suggest that DNA methylation may account for oocyte-specific expression of KPNA7, and miRNA-1296 targeting the coding region of KPNA7 is a potential mechanism for KPNA7 transcript degradation during the maternal-to-zygotic transition.

Activation-induced cytidine deaminase selectively catalyzed active DNA demethylation in pluripotency gene and improved cell reprogramming in bovine SCNT embryo

DNA methylation in mammals is an epigenetic marker and necessary for normal embryogenesis. The global genomic demethylation of 5-methylcytosine occurs during the first cell cycle following fertilization. Activation-induced cytidine deaminase (AID), which is well-known for the function in antibody diversification, has been implicated to play a role in active demethylation, but its role in cell reprogramming and its crosstalk with other DNA demethylation mechanism need to be clarified. In this study, the dynamic epigenetic regulation of cell pluripotency and embryo development by AID in bovine preimplantation embryos was investigated. The analysis of an AID overexpressing transgenic cell line showed that AID overexpression did not change the global genomic methylation but did change the methylation status of the promoters of the OCT4, NANOG and SOX2 genes, thereby causing changes in their expression. The siRNA-mediated AID knockdown in early embryonic development indicated that AID interference did not affect oocyte maturation or the following embryo development after in vitro fertilization but influenced the DNA methylation status of OCT4 and NANOG. To clarify the role of AID in preimplantation embryos, SCNT embryos were obtained using AID-overexpressing cells as nuclear donors. Compared to the control group, the cleavage and blastocyst rates were both significantly improved in the AID-overexpression group. The expression of OCT4 and NANOG was increased in the SCNT embryos, whereas the methylation levels of their promoters were reduced. In conclusion, this study demonstrated that AID selectively catalyzes DNA demethylation of pluripotency genes to play a role in regulation their expression, improves bovine SCNT embryo development by increased expression levels.

Epigenetics, embryo quality and developmental potential

It is very important for embryologists to understand how parental inherited genomes are reprogrammed after fertilisation in order to obtain good-quality embryos that will sustain further development. In mammals, it is now well established that important epigenetic modifications occur after fertilisation. Although gametes carry special epigenetic signatures, they should attain embryo-specific signatures, some of which are crucial for the production of healthy embryos. Indeed, it appears that proper establishment of different epigenetic modifications and subsequent scaffolding of the chromatin are crucial steps during the first cleavages. This ‘reprogramming’ is promoted by the intimate contact between the parental inherited genomes and the oocyte cytoplasm after fusion of the gametes. This review introduces two main epigenetic players, namely histone post-translational modifications and DNA methylation, and highlights their importance during early embryonic development.

Placental development during early pregnancy in sheep: effects of embryo origin on fetal and placental growth and global methylation

The origin of embryos including those created through assisted reproductive technologies might have profound effects on placental and fetal development, possibly leading to compromised pregnancies associated with poor placental development. To determine the effects of embryo origin on fetal size, and maternal and fetal placental cellular proliferation and global methylation, pregnancies were achieved through natural mating (NAT), or transfer of embryos generated through in vivo (NAT-ET), IVF, or in vitro activation (IVA). On Day 22 of pregnancy, fetuses were measured and placental tissues were collected to immunologically detect Ki67 (a marker of proliferating cells) and 5-methyl cytosine followed by image analysis, and determine mRNA expression for three DNA methyltransferases. Fetal length and labeling index (proportion of proliferating cells) in maternal caruncles (maternal placenta) and fetal membranes (fetal placenta) were less (P < 0.001) in NAT-ET, IVF, and IVA than in NAT. In fetal membranes, expression of 5-methyl cytosine was greater (P < 0.02) in IVF and IVA than in NAT. In maternal caruncles, mRNA expression for DNMT1 was greater (P < 0.01) in IVA compared with the other groups, but DNMT3A expression was less (P < 0.04) in NAT-ET and IVA than in NAT. In fetal membranes, expression of mRNA for DNMT3A was greater (P < 0.01) in IVA compared with the other groups, and was similar in NAT, NAT-ET, and IVF groups. Thus, embryo origin might have specific effects on growth and function of ovine uteroplacental and fetal tissues through regulation of tissue growth, DNA methylation, and likely other mechanisms. These data provide a foundation for determining expression of specific factors regulating placental and fetal tissue growth and function in normal and compromised pregnancies, including those achieved with assisted reproductive technologies.

Methylation profile of the promoters of Nanog and Oct4 in ICSI human embryos

STUDY QUESTION

What is the methylation status of the Nanog and Oct4 promoters in human gametes and ICSI embryos and is abnormal reprogramming of their methylation associated with developmental failure of ICSI embryos?

SUMMARY ANSWER

Developmental failure of human ICSI embryos is associated with high methylation of the Oct4 promoter.

WHAT IS KNOWN ALREADY

Nanog and Oct4 genes play critical roles in the establishment and maintenance of pluripotency during normal early embryonic development, and both are negatively regulated through the methylation of their promoters.

STUDY DESIGN, SIZE AND DURATION

We analysed the methylation profile of Nanog and Oct4 promoters in 5 control sperm from normally fertile men, 70 metaphase II oocytes, 21 4-cell control ICSI embryos, 7 control blastocysts and 45 ICSI embryos arrested at 2- to 8-cell stage following prolonged culture.

PARTICIPANTS, MATERIALS, SETTING AND METHODS

Embryos and gametes were donated for research by patients from the Department of Reproductive Medicine at the Hôpital Femme Mère Enfant (Bron, France) and the Clinique du Tonkin (Villeurbanne, France) after giving their informed consent.

MAIN RESULTS

For both promoters, high methylation was observed in sperm cells. Although, in general, the promoters were unmethylated in oocytes, the methylation of some alleles was observed, particularly in oocytes from women with known infertility. Both gene promoters were hypomethylated in control blastocyst ICM (inner cell mass) and in control 2-8-cells embryos obtained from 6 out of 8 couples. However, they appeared highly methylated in embryos obtained from the other two couples. In most arrested ICSI embryos, the Nanog promoter was unmethylated while the Oct4 promoter was highly methylated. High methylation of the Oct4 promoter was significantly more pronounced in embryos from couples where a male factor was the only known cause of infertility. When the embryos were heterozygous for a G/A single nucleotide polymorphism, both alleles could be methylated, each likely representing a paternally inherited or a maternally inherited copy.

LIMITATIONS AND REASONS FOR CAUTION

The study was done on a limited number of oocytes and embryos and the gametes of the couples were not available.

WIDER IMPLICATIONS OF THE FINDINGS

These results provide new insight regarding the roles of epigenetic abnormalities in early developmental failure in humans.

STUDY FUNDING/COMPETING INTEREST(S)

No external funding was obtained for this study. There was no competing interest.

Somatic cell-induced hyperacetylation, but not hypomethylation, positively and reversibly affects the efficiency of in vitro cloned blastocyst production in cattle

5-Aza-2'-deoxycytidine (AzC), trichostatin A (TSA), and its natural mimetic, sodium butyrate (NaB), are antineoplastic drugs that can modify the epigenetic status of donor cells prior to somatic cell nuclear transfer (SCNT). In this study, we used fibroblast cells treated with these drugs to investigate the direct and indirect effects of induced changes in DNA methylation and acetylation of the lysine 9 residue of histone H3 (H3K9). Additionally, we assayed cellular characteristics (cell growth, cell proliferation, cell cycle progression, and apoptosis) and SCNT efficiency in response to these drugs as well as monitoring these effects 24 h after removing the drugs. We observed the following: (1) AzC, TSA, and NaB all showed dose-dependent effects on different cellular characteristics; (2) TSA and NaB induced H3K9 hyperacetylation accompanied by DNA hypermethylation, whereas AzC induced DNA hypomethylation with no effect on H3K9 hyperacetylation; (3) TSA and NaB improved cloning efficiency, whereas AzC reduced it; and (4) unlike AzC, the effects of TSA and NaB on cellular characteristics and SCNT efficiency were reversed following drug removal. Our results indicate that somatic cells treated with TSA and NaB show better survival and recovery rates following the removal of these drugs. Moreover, H3K9 hyperacetylation (induced with TSA and NaB), but not DNA hypomethylation (induced with AzC), favors cloning efficiency.

[The methylation status of PEG10 in placentas of cloned transgenic calves]

The low efficiency of somatic cell nuclear transfer (SCNT) is a significant barrier to the production of highly valuable transgenic livestock. It is generally believed that the principal cause of the low SCNT efficiency is the aberrant nuclear epigenetic reprogramming of donor somatic cell. DNA methylation is a major epigenetic modification of the genome and plays a crucial role in nuclear reprogramming during SCNT. In order to assess whether the abnormal epigenetic modifications of the imprinted gene in placenta are correlated with the development abnormality and death of the cloned transgenic calves, the DNA methylation patterns of PEG10 were compared in the placentas from different kinds of cattle. This comparison included transgenic cloned calves died during perinatal stage and showed developmental defects (Death group), transgenic cloned calves survived and lived on healthily (Live group) and the normal reproduced calves (N group) used as the control group analyzed by Bisulfite Sequencing PCR (BSP) method and Combined Bisulfite Restriction Analy-sis (COBRA). Comparing to the control group, PEG10 gene in the Death group showed abnormal hypermethylation, but was not significant different in methylation level from the Live group. It can be postulated from the results that the incom-plete or abnormal DNA methylation epigenetic reprogramming of imprinting gene in placenta may be one of the main causes of the abnormal development and death of the transgenic cloned cattle.