Changes in DNA methylation and imprinting disorders in E9.5 mouse fetuses and placentas derived from vitrified eight-cell embryos

Cryopreservation, cryoprotectants, and potential risk of epigenetic alteration

The cryopreservation of gametes and embryos has increased notably over the past 20 years and is now an essential part of assisted reproductive technologies (ARTs). However, because the cryopreservation process is un-physiological for human cells, gametes, and embryos, cryobiologists have suggested diverse methods to successfully cryopreserve human gametes and embryos in order to maintain their viability and assure successful pregnancy. During the first period of early development, major waves of epigenetic reprogramming-crucial for the fate of the embryo-occur. Recently, concerns relating to the increased incidence of epigenetic anomalies and genomic-imprinting disorders have been reported after ARTs and cryopreservation. Epigenetic reprogramming is particularly susceptible to environmental and un-physiological conditions such as ovarian stimulation, embryo culture, and cryopreservation that might collectively affect epigenetics dysregulation. Additionally, recent literature suggests that epigenetic and transcriptomic profiles are sensitive to the stress induced by vitrification, osmotic shock, oxidative stress, rapid temperature and pH changes, and cryoprotectants; it is therefore critical to have a more comprehensive understanding of the potential induced perturbations of epigenetic modifications that may be associated with vitrification. The aim of this paper is to present a critical evaluation of the association of gamete and embryo cryopreservation, use of cryoprotectants, and epigenetic dysregulations with potential long-term consequences for offspring health.

Long-term storage does not affect the DNA methylation profiles of vitrified-warmed human embryos

With the widespread application of embryo cryopreservation in assisted reproductive techniques, it is necessary to assess the safety of long-term cryopreservation of human embryos and it is unclear whether storage time has an impact on the DNA methylation profiles of human embryos. Nine women who received IVF treatment were recruited for this study. The retrieved eight-cell human embryos were classified into three groups including fresh embryos, cryopreserved embryos stored for 3 years, and cryopreserved embryos stored for 8 years. Single-cell whole-genome bisulfite sequencing (scWGBS) was conducted. The genome-wide methylation pattern of the fresh and two cryopreserved groups were similar. In addition, the methylation level in different genomic regions showed comparable patterns and no significant differences were observed in the methylation level of imprinted genes among the three groups. A total of 587 differentially methylated regions (DMRs) in the 3-year group and 540 DMRs in the 8-year group were identified comparing to fresh group. However, they were not enriched in promoters and had a similar genome-wide distributions, suggesting that these DMRs may not contribute to the changes in corresponding gene expressions. Our study illustrated that long-term cryopreservation will not affect the DNA methylation profiles of human eight-cell embryos at single-cell level.

Effect of paternal aging and vitrification on mitochondrial DNA copy number and telomere length of mouse blastocysts

In this study, we examined the effects of paternal aging on the mitochondrial DNA copy number (mt-cn), telomere length (TL), and gene expression in mouse embryos. The effects of vitrification on the mt-cn and TL of the embryos derived from young and aged male parents (YF and AF, respectively) were examined. C57BL/6N male mice were used for embryo production at 13-23 and 50-55 weeks of age. Two-cell stage embryos were collected from the oviducts of superovulated female mice (8-15 weeks old) and cultured for 24 h until the 8-cell stage, followed by embryo vitrification. Fresh and vitrified-warmed embryos were incubated for 2 days until the blastocyst stage, and mt-cn and TL were investigated. The cell-free mitochondrial DNA copy number (cf-mt-cn) in the spent culture medium (SCM) of the embryos was then investigated. RNA sequencing of blastocysts revealed that metabolic pathways, including oxidative phosphorylation and mTOR pathways, were enriched in differentially expressed genes. The mt-cn and TL of AF-derived blastocysts were lower and shorter, respectively, than those of YF-derived blastocysts. Paternal aging did not affect the blastocyst rate after vitrification. Vitrification of the 8-cell stage embryos did not affect the mt-cn of the blastocysts. However, it increased the cf-mt-cn (cell-free mt-cn) in the SCM of both YF- and AF-derived embryos. Vitrification did not affect the TL of either YF- or AF-derived embryos. Thus, paternal aging affected the mt-cn and TL of the embryos, but vitrification did not affect these parameters in either age groups.

The adverse effects of vitrification on mouse embryo development and metabolic phenotype in offspring

Embryo vitrification is a standard procedure in assisted reproductive technology. Previous studies have shown that frozen embryo transfer is associated with an elevated risk of adverse maternal and neonatal outcomes. This study aimed to explore the effects of mouse blastocyst vitrification on the phenotype of vitrified-warmed blastocysts, their intrauterine and postnatal development, and the long-term metabolic health of the derived offspring. The vitrified-warmed blastocysts (IVF + VT group) exhibited reduced mitochondrial activity, increased apoptotic levels, and decreased cell numbers when compared to the fresh blastocysts (IVF group). Implantation rates, live pup rates, and crown-rump length at E18.5 were not different between the two groups. However, there was a significant decrease in fetal weight and fetal/placental weight ratio in the IVF + VT group. Furthermore, the offspring of the IVF + VT group at an age of 36 weeks had reduced whole energy consumption, impaired glucose and lipid metabolism when compared with the IVF group. Notably, RNA-seq results unveiled disturbed hepatic gene expression in the offspring from vitrified-warmed blastocysts. This study revealed the short-term negative impacts of vitrification on embryo and fetal development and the long-term influence on glucose and lipid metabolism that persist from the prenatal stage into adulthood in mice.

Exploring the effect of cryopreservation in assisted reproductive technology and potential epigenetic risk

Since the birth of the first baby by in vitro fertilization in 1978, more than 9 million children have been born worldwide using medically assisted reproductive treatments. Fertilization naturally takes place in the maternal oviduct where unique physiological conditions enable the early healthy development of the embryo. During this dynamic period of early development major waves of epigenetic reprogramming, crucial for the normal fate of the embryo, take place. Increasingly, over the past 20 years concerns relating to the increased incidence of epigenetic anomalies in general, and genomic-imprinting disorders in particular, have been raised following assisted reproduction technology (ART) treatments. Epigenetic reprogramming is particularly susceptible to environmental conditions during the periconceptional period and non-physiological conditions such as ovarian stimulation, in vitro fertilization and embryo culture, as well as cryopreservation procedure, might have the potential to independently or collectively contribute to epigenetic dysregulation. Therefore, this narrative review offers a critical reappraisal of the evidence relating to the association between embryo cryopreservation and potential epigenetic regulation and the consequences on gene expression together with long-term consequences for offspring health and wellbeing. Current literature suggests that epigenetic and transcriptomic profiles are sensitive to the stress induced by vitrification, in terms of osmotic shock, temperature and pH changes, and toxicity of cryoprotectants, it is therefore, critical to have a more comprehensive understanding and recognition of potential unanticipated iatrogenic-induced perturbations of epigenetic modifications that may or may not be a consequence of vitrification.

Assisted reproductive technology and imprinting errors: analyzing underlying mechanisms from epigenetic regulation

With the increasing maturity and widespread application of assisted reproductive technology (ART), more attention has been paid to the health outcomes of offspring following ART. It is well established that children born from ART treatment are at an increased risk of imprinting errors and imprinting disorders. The disturbances of genetic imprinting are attributed to the overlap of ART procedures and important epigenetic reprogramming events during the development of gametes and early embryos, but the detailed mechanisms are hitherto obscure. In this review, we summarized the DNA methylation-dependent and independent mechanisms that control the dynamic epigenetic regulation of imprinted genes throughout the life cycle of a mammal, including erasure, establishment, and maintenance. In addition, we systematically described the dysregulation of imprinted genes in embryos conceived through ART and discussed the corresponding underlying mechanisms according to findings in animal models. This work is conducive to evaluating and improving the safety of ART.

Age-related advanced glycation end-product accumulation impairs mitochondrial regulation after vitrification†

Vitrification is an important assisted reproductive technology, although it induces mitochondrial dysfunction in embryos. Herein, we aimed to investigate whether age-associated accumulation of advanced glycation end-products (AGEs) in oocytes impairs the recovery of embryos from cryopreservation-induced mitochondrial dysfunction/damage. Mouse eight-cell stage embryos developed in vitro were vitrified and warmed and incubated up to the blastocyst stage. AGE levels in oocytes were higher in both aged mice and AGE accumulation mouse models (MGO-mice) than those in young and control mice. In addition, the level of SIRT1 upregulation was lower for embryos of aged and MGO-mice than that for embryos of young and control mice. The highest mitochondrial DNA (mtDNA) content was detected in blastocysts derived from vitrified embryos of aged and MGO-mice. The spent culture medium of blastocysts derived from both aged and MGO-mice contained higher mtDNA content than that of the blastocysts derived from young and control mice. EX527 increased mtDNA content in the spent culture medium of vitrified embryos derived from young mice. In addition, p62 aggregate levels were higher in vitrified embryos of control mice than those in vitrified embryos of MGO-mice. The SIRT1 activator, resveratrol, increased p62 aggregation levels in vitrified embryos derived from young and aged mice, whereas vitrification did not affect p62 aggregation levels in embryos from aged mice. Therefore, age-associated AGE accumulation induces decreased responsive SIRT1 upregulation following vitrified-warmed treatment and impairs mitochondrial quality control activity in vitrified embryos.

Long-Term Effects of ART on the Health of the Offspring

Assisted reproductive technologies (ART) significantly increase the chance of successful pregnancy and live birth in infertile couples. The different procedures for ART, including in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), intrauterine insemination (IUI), and gamete intrafallopian tube transfer (GIFT), are widely used to overcome infertility-related problems. In spite of its inarguable usefulness, concerns about the health consequences of ART-conceived babies have been raised. There are reports about the association of ART with birth defects and health complications, e.g., malignancies, high blood pressure, generalized vascular functional disorders, asthma and metabolic disorders in later life. It has been suggested that hormonal treatment of the mother, and the artificial environment during the manipulation of gametes and embryos may cause genomic and epigenetic alterations and subsequent complications in the health status of ART-conceived babies. In the current study, we aimed to review the possible long-term consequences of different ART procedures on the subsequent health status of ART-conceived offspring, considering the confounding factors that might account for/contribute to the long-term consequences.

Cryopreservation of Ovarian and Testicular Tissue and the Influence on Epigenetic Pattern

Ovarian tissue cryopreservation (OTC) or testicular tissue cryopreservation (TTC) are effective and often the only options for fertility preservation in female or male patients due to oncological, medical, or social aspects. While TTC and resumption of spermatogenesis, either in vivo or in vitro, has still be considered an experimental approach in humans, OTC and autotransplantation has been applied increasingly to preserve fertility, with more than 200 live births worldwide. However, the cryopreservation of reproductive cells followed by the resumption of gametogenesis, either in vivo or in vitro, may interfere with sensitive and highly regulated cellular processes. In particular, the epigenetic profile, which includes not just reversible modifications of the DNA itself but also post-translational histone modifications, small non-coding RNAs, gene expression and availability, and storage of related proteins or transcripts, have to be considered in this context. Due to complex reprogramming and maintenance mechanisms of the epigenome in germ cells, growing embryos, and offspring, OTC and TTC are carried out at very critical moments early in the life cycle. Given this background, the safety of OTC and TTC, taking into account the epigenetic profile, has to be clarified. Cryopreservation of mature germ cells (including metaphase II oocytes and mature spermatozoa collected via ejaculation or more invasively after testicular biopsy) or embryos has been used successfully for many years in medically assisted reproduction (MAR). However, tissue freezing followed by in vitro or in vivo gametogenesis has become more attractive in the past, while few human studies have analysed the epigenetic effects, with most data deriving from animal studies. In this review, we highlight the potential influence of the cryopreservation of immature germ cells and subsequent in vivo or in vitro growth and differentiation on the epigenetic profile (including DNA methylation, post-translational histone modifications, and the abundance and availability of relevant transcripts and proteins) in humans and animals.

Epigenetics in the Uterine Environment: How Maternal Diet and ART May Influence the Epigenome in the Offspring with Long-Term Health Consequences

The societal burden of non-communicable disease is closely linked with environmental exposures and lifestyle behaviours, including the adherence to a poor maternal diet from the earliest preimplantation period of the life course onwards. Epigenetic variations caused by a compromised maternal nutritional status can affect embryonic development. This review summarises the main epigenetic modifications in mammals, especially DNA methylation, histone modifications, and ncRNA. These epigenetic changes can compromise the health of the offspring later in life. We discuss different types of nutritional stressors in human and animal models, such as maternal undernutrition, seasonal diets, low-protein diet, high-fat diet, and synthetic folic acid supplement use, and how these nutritional exposures epigenetically affect target genes and their outcomes. In addition, we review the concept of thrifty genes during the preimplantation period, and some examples that relate to epigenetic change and diet. Finally, we discuss different examples of maternal diets, their effect on outcomes, and their relationship with assisted reproductive technology (ART), including their implications on epigenetic modifications.

Epigenetic Mechanisms of ART-Related Imprinting Disorders: Lessons From iPSC and Mouse Models

The rising frequency of ART-conceived births is accompanied by the need for an improved understanding of the implications of ART on gametes and embryos. Increasing evidence from mouse models and human epidemiological data suggests that ART procedures may play a role in the pathophysiology of certain imprinting disorders (IDs), including Beckwith-Wiedemann syndrome, Silver-Russell syndrome, Prader-Willi syndrome, and Angelman syndrome. The underlying molecular basis of this association, however, requires further elucidation. In this review, we discuss the epigenetic and imprinting alterations of in vivo mouse models and human iPSC models of ART. Mouse models have demonstrated aberrant regulation of imprinted genes involved with ART-related IDs. In the past decade, iPSC technology has provided a platform for patient-specific cellular models of culture-associated perturbed imprinting. However, despite ongoing efforts, a deeper understanding of the susceptibility of iPSCs to epigenetic perturbation is required if they are to be reliably used for modelling ART-associated IDs. Comparing the patterns of susceptibility of imprinted genes in mouse models and IPSCs in culture improves the current understanding of the underlying mechanisms of ART-linked IDs with implications for our understanding of the influence of environmental factors such as culture and hormone treatments on epigenetically important regions of the genome such as imprints.

Successful vitrification of early-stage porcine cloned embryos

The objective of this study was to investigate the survival and development of porcine cloned embryos vitrified by Cryotop carrier at the zygote, 2- and 4-cell stages. The quality of resultant blastocysts was evaluated according to their total cell number, apoptotic cell rate, reactive oxygen species (ROS) production, glutathione (GSH) content and mRNA expression levels of genes related to embryonic development. The survival rates of zygotes, 2- and 4-cell embryos after vitrification did not differ from those of their fresh counterparts. Vitrification still resulted in significantly decreased blastocyst formation rates of these early-stage embryos. Moreover, the total cells, apoptotic rate, ROS and GSH levels in resultant blastocysts were unaffected by vitrification. The mRNA expression levels of PCNA, CPT1, POU5F1 and DNMT3B in the blastocysts derived from vitrified early-stage embryos were significantly higher than those in the fresh blastocysts, but there was no change in expression of CDX2 and DNMT3A genes. In conclusion, our data demonstrate that the early-stage porcine cloned embryos including zygotes, 2- and 4-cells can be successfully vitrified, with respectable blastocyst yield and quality.

Implications of Assisted Reproductive Technologies for Pregnancy Outcomes in Mammals

Development of assisted reproductive technologies has been driven by the goals of reducing the incidence of infertility, increasing the number of offspring from genetically elite animals, facilitating genetic manipulation, aiding preservation and long-distance movement of germplasm, and generating research material. Superovulation is associated with reduced fertilization rate and alterations in endometrial function. In vitro production of embryos can have a variety of consequences. Most embryos produced in vitro are capable of establishing pregnancy and developing into healthy neonatal animals. However, in vitro production is associated with reduced ability to develop to the blastocyst stage, increased incidence of failure to establish pregnancy, placental dysfunction, and altered fetal development. Changes in the developmental program mean that some consequences of being produced in vitro can extend into adult life. Reduced competence of the embryo produced in vitro to develop to the blastocyst stage is caused largely by disruption of events during oocyte maturation and fertilization. Conditions during embryo culture can affect embryo freezability and competence to establish pregnancy after transfer. Culture conditions, including actions of embryokines, can also affect the postnatal phenotype of the resultant progeny.

DNA methylation status of imprinted H19 and KvDMR1 genes in human placentas after conception using assisted reproductive technology

BACKGROUND

Assisted reproductive technologies (ARTs), such as in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), are thought to destabilize genomic imprints. Previous studies examining the association between ART and aberrant DNA methylation have been inconclusive.

METHOD

The DNA methylation status of H19 and KvDMR1was compared between newborns conceived through ART and those conceived naturally to evaluate the safety of ART. Placental tissues from 6 full-term, naturally conceived pregnancies (no gestational comorbidities) and six full-term ART pregnancies (no gestational complication) were collected. Genomic DNA (gDNA) and RNA were extracted from both groups. Real-time PCR was used to analyze the mRNA expression levels of H19 and KvDMR1 in the placenta for both groups. A whole-genome DNA methylation microarray was used to examine three placentas from full-term, naturally conceived pregnancies and three placentas from full-term IVF pregnancies.

RESULT

The expression level of H19 in the IVF group was significantly higher than that in the natural pregnancy group, whereas the expression level of KvDMR1 was significantly lower in the ART group than in the natural pregnancy group. Also, human ART manipulation resulted in placental gDNA methylation modifications. Conclusion: Abnormal methylation patterns were detected in phenotypically normal phenotype conceived by ART, which may occur due to imprinting errors in sperm/oocyte cells or side effects of in vitro embryo culture procedures. Further investigation is necessary to determine whether imprinted gene expression and DNA methylation can be regulated through other mechanisms.

KEYWORDS

Assisted reproductive technology (ART); placenta; methylation; H19; KvDMR1.

Conditions of embryo culture from days 5 to 7 of development alter the DNA methylome of the bovine fetus at day 86 of gestation

PURPOSE

We tested whether in vitro production (IVP) causes changes in DNA methylation in fetal liver and skeletal muscle and if exposure of cultured embryos to colony-stimulating factor 2 (CSF2) alters DNA methylation.

METHODS

Female fetuses were produced by artificial insemination or transfer of an IVP embryo. Embryos were treated from days 5 to 7 after fertilization with CSF2 or vehicle. DNA methylation in fetal liver and skeletal muscle was determined by post-bisulfite adaptor tagging-based sequencing. The degree of DNA methylation for CpG sites in 50-bp windows of the promoter region 500 bp upstream of the transcriptional start site was compared between treatments.

RESULTS

For liver, there were 12 genes (6% of those analyzed) in which DNA methylation was affected by treatment, with one 50-bp window per gene affected by treatment. For muscle, the degree of DNA methylation was affected by treatment for 32 windows (19% of the total windows analyzed) representing 28 distinct genes (23% of analyzed genes). For 19 of the 28 genes in muscle, the greatest deviation in DNA methylation was for the CSF2 group.

CONCLUSION

Results are consistent with alterations in the methylome being one of the mechanisms by which IVP can result in altered fetal development and postnatal function in the resultant offspring. In addition, results indicate that maternally derived cell-signaling molecules can regulate the pattern of DNA methylation.

DNA methylation, environmental exposures and early embryo development

The first crucial step in the developmental program occurs during pre-implantation, the time after the oocyte has been fertilized and before the embryo implants in the uterus. This period represents a vulnerable window as the epigenome undergoes dynamic changes in DNA methylation profiles. Alterations in the early embryonic reprogramming wave can impair DNA methylation patterns and induce permanent changes to the developmental program, leading to the onset of adverse health outcomes in offspring. Although there is an increasing body of evidence indicating that harmful exposures during pre-implantation embryo development can trigger lasting epigenetic alterations in offspring, the mechanisms are still not fully understood. Since physiological or pathological changes in DNA methylation can occur as a response to environmental cues, proper environmental milieu plays a critical role in the success of embryonic development. In this review, we depict the mechanisms behind the embryonic epigenetic reprogramming of DNA methylation and highlight how maternal environmental stressors (e.g., alcohol, heat stress, nutrient availability) during pre-implantation and assisted reproductive technology procedures affect development and DNA methylation marks.