Epigenetic modifications during oocyte growth correlates with extended parthenogenetic development in the mouse

Roles of endogenous retroviral elements in the establishment and maintenance of imprinted gene expression

DNA methylation (DNAme) has long been recognized as a host defense mechanism, both in the restriction modification systems of prokaryotes as well as in the transcriptional silencing of repetitive elements in mammals. When DNAme was shown to be implicated as a key epigenetic mechanism in the regulation of imprinted genes in mammals, a parallel with host defense mechanisms was drawn, suggesting perhaps a common evolutionary origin. Here we review recent work related to this hypothesis on two different aspects of the developmental imprinting cycle in mammals that has revealed unexpected roles for long terminal repeat (LTR) retroelements in imprinting, both canonical and noncanonical. These two different forms of genomic imprinting depend on different epigenetic marks inherited from the mature gametes, DNAme and histone H3 lysine 27 trimethylation (H3K27me3), respectively. DNAme establishment in the maternal germline is guided by transcription during oocyte growth. Specific families of LTRs, evading silencing mechanisms, have been implicated in this process for specific imprinted genes. In noncanonical imprinting, maternally inherited histone marks play transient roles in transcriptional silencing during preimplantation development. These marks are ultimately translated into DNAme, notably over LTR elements, for the maintenance of silencing of the maternal alleles in the extraembryonic trophoblast lineage. Therefore, LTR retroelements play important roles in both establishment and maintenance of different epigenetic pathways leading to imprinted expression during development. Because such elements are mobile and highly polymorphic among different species, they can be coopted for the evolution of new species-specific imprinted genes.

High estrogen during ovarian stimulation induced loss of maternal imprinted methylation that is essential for placental development via overexpression of TET2 in mouse oocytes

BACKGROUND

Ovarian stimulation (OS) during assisted reproductive technology (ART) appears to be an independent factor influencing the risk of low birth weight (LBW). Previous studies identified the association between LBW and placenta deterioration, potentially resulting from disturbed genomic DNA methylation in oocytes caused by OS. However, the mechanisms by which OS leads to aberrant DNA methylation patterns in oocytes remains unclear.

METHODS

Mouse oocytes and mouse parthenogenetic embryonic stem cells (pESCs) were used to investigate the roles of OS in oocyte DNA methylation. Global 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) levels were evaluated using immunofluorescence or colorimetry. Genome-wide DNA methylation was quantified using an Agilent SureSelectXT mouse Methyl-Seq. The DNA methylation status of mesoderm-specific transcript homologue (Mest) promoter region was analyzed using bisulfite sequencing polymerase chain reaction (BSP). The regulatory network between estrogen receptor alpha (ERα, ESR1) and DNA methylation status of Mest promoter region was further detected following the knockdown of ERα or ten-eleven translocation 2 (Tet2).

RESULTS

OS resulted in a significant decrease in global 5mC levels and an increase in global 5hmC levels in oocytes. Further investigation revealed that supraphysiological β-estradiol (E2) during OS induced a notable decrease in DNA 5mC and an increase in 5hmC in both oocytes and pESCs of mice, whereas inhibition of estrogen signaling abolished such induction. Moreover, Tet2 may be a direct transcriptional target gene of ERα, and through the ERα-TET2 axis, supraphysiological E2 resulted in the reduced global levels of DNA 5mC. Furthermore, we identified that MEST, a maternal imprinted gene essential for placental development, lost its imprinted methylation in parthenogenetic placentas originating from OS, and ERα and TET2 combined together to form a protein complex that may promote Mest demethylation.

CONCLUSIONS

In this study, a possible mechanism of loss of DNA methylation in oocyte caused by OS was revealed, which may help increase safety and reduce epigenetic abnormalities in ART procedures.

Single-cell multi-omics profiling reveals key regulatory mechanisms that poise germinal vesicle oocytes for maturation in pigs

The molecular mechanisms controlling the transition from meiotic arrest to meiotic resumption in mammalian oocytes have not been fully elucidated. Single-cell omics technology provides a new opportunity to decipher the early molecular events of oocyte growth in mammals. Here we focused on analyzing oocytes that were collected from antral follicles in different diameters of porcine pubertal ovaries, and used single-cell M&T-seq technology to analyze the nuclear DNA methylome and cytoplasmic transcriptome in parallel for 62 oocytes. 10× Genomics single-cell transcriptomic analyses were also performed to explore the bi-directional cell-cell communications within antral follicles. A new pipeline, methyConcerto, was developed to specifically and comprehensively characterize the methylation profile and allele-specific methylation events for a single-cell methylome. We characterized the gene expressions and DNA methylations of individual oocyte in porcine antral follicle, and both active and inactive gene's bodies displayed high methylation levels, thereby enabled defining two distinct types of oocytes. Although the methylation levels of Type II were higher than that of Type I, Type II contained nearly two times more of cytoplasmic transcripts than Type I. Moreover, the imprinting methylation patterns of Type II were more dramatically divergent than Type I, and the gene expressions and DNA methylations of Type II were more similar with that of MII oocytes. The crosstalk between granulosa cells and Type II oocytes was active, and these observations revealed that Type II was more poised for maturation. We further confirmed Insulin Receptor Substrate-1 in insulin signaling pathway is a key regulator on maturation by in vitro maturation experiments. Our study provides new insights into the regulatory mechanisms between meiotic arrest and meiotic resumption in mammalian oocytes. We also provide a new analytical package for future single-cell methylomics study.

Why Is It So Difficult To Have Competent Oocytes from In vitro Cultured Preantral Follicles?

The developmental competence of oocytes is acquired gradually during follicular development, mainly through oocyte accumulation of RNA molecules and proteins that will be used during fertilization and early embryonic development. Several attempts to develop in vitro culture systems to support preantral follicle development up to maturation are reported in the literature, but oocyte competence has not yet been achieved in human and domestic animals. The difficulties to have fertilizable oocytes are related to thousands of mRNAs and proteins that need to be synthesized, long-term duration of follicular development, size of preovulatory follicles, composition of in vitro culture medium, and the need of multi-step culture systems. The development of a culture system that maintains bidirectional communication between the oocyte and granulosa cells and that meets the metabolic demands of each stage of follicle growth is the key to sustain an extended culture period. This review discusses the physiological and molecular mechanisms that determine acquisition of oocyte competence in vitro, like oocyte transcriptional activity, follicle and oocyte sizes, and length and regulation of follicular development in murine, human, and domestic animal species. The state of art of in vitro follicular development and the challenges to have complete follicular development in vitro are also highlighted.

Depletion of oocyte dynamin-related protein 1 shows maternal-effect abnormalities in embryonic development

Eggs contain about 200,000 mitochondria that generate adenosine triphosphate and metabolites essential for oocyte development. Mitochondria also integrate metabolism and transcription via metabolites that regulate epigenetic modifiers, but there is no direct evidence linking oocyte mitochondrial function to the maternal epigenome and subsequent embryo development. Here, we have disrupted oocyte mitochondrial function via deletion of the mitochondrial fission factor Drp1. Fission-deficient oocytes exhibit a high frequency of failure in peri- and postimplantation development. This is associated with altered mitochondrial function, changes in the oocyte transcriptome and proteome, altered subcortical maternal complex, and a decrease in oocyte DNA methylation and H3K27me3. Transplanting pronuclei of fertilized Drp1 knockout oocytes to normal ooplasm fails to rescue embryonic lethality. We conclude that mitochondrial function plays a role in establishing the maternal epigenome, with serious consequences for embryo development.

Oocyte mitochondria – Key regulators of oocyte function and potential therapeutic targets for improving fertility

The development of oocytes and early embryos is dependent on mitochondrial ATP production. This reliance on mitochondrial activity, together with the exclusively maternal inheritance of mitochondria in development, places mitochondria as central regulators of both fertility and transgenerational inheritance mechanisms. Mitochondrial mass and mtDNA content massively increase during oocyte growth. They are highly dynamic organelles and oocyte maturation is accompanied by mitochondrial trafficking around subcellular compartments. Due to their key roles in generation of ATP and reactive oxygen species, oocyte mitochondrial defects have largely been linked with energy deficiency and oxidative stress. Pharmacological treatments and mitochondrial supplementation have been proposed to improve oocyte quality and fertility by enhancing ATP generation and reducing reactive oxygen species levels. More recently, the role of mitochondria-derived metabolites in controlling epigenetic modifiers has provided a mechanistic basis for mitochondria-nuclear crosstalk, allowing adaptation of gene expression to specific metabolic states. Here, we discuss the multi-faceted mechanisms by which mitochondrial function influence oocyte quality, as well as longer-term developmental events within and across generations.

The Epigenetics of Gametes and Early Embryos and Potential Long-Range Consequences in Livestock Species-Filling in the Picture With Epigenomic Analyses

The epigenome is dynamic and forged by epigenetic mechanisms, such as DNA methylation, histone modifications, chromatin remodeling, and non-coding RNA species. Increasing lines of evidence support the concept that certain acquired traits are derived from environmental exposure during early embryonic and fetal development, i.e., fetal programming, and can even be "memorized" in the germline as epigenetic information and transmitted to future generations. Advances in technology are now driving the global profiling and precise editing of germline and embryonic epigenomes, thereby improving our understanding of epigenetic regulation and inheritance. These achievements open new avenues for the development of technologies or potential management interventions to counteract adverse conditions or improve performance in livestock species. In this article, we review the epigenetic analyses (DNA methylation, histone modification, chromatin remodeling, and non-coding RNAs) of germ cells and embryos in mammalian livestock species (cattle, sheep, goats, and pigs) and the epigenetic determinants of gamete and embryo viability. We also discuss the effects of parental environmental exposures on the epigenetics of gametes and the early embryo, and evidence for transgenerational inheritance in livestock.

Maternal DNMT3A-dependent de novo methylation of the paternal genome inhibits gene expression in the early embryo

De novo DNA methylation (DNAme) during mammalian spermatogenesis yields a densely methylated genome, with the exception of CpG islands (CGIs), which are hypomethylated in sperm. While the paternal genome undergoes widespread DNAme loss before the first S-phase following fertilization, recent mass spectrometry analysis revealed that the zygotic paternal genome is paradoxically also subject to a low level of de novo DNAme. However, the loci involved, and impact on transcription were not addressed. Here, we employ allele-specific analysis of whole-genome bisulphite sequencing data and show that a number of genomic regions, including several dozen CGI promoters, are de novo methylated on the paternal genome by the 2-cell stage. A subset of these promoters maintains DNAme through development to the blastocyst stage. Consistent with paternal DNAme acquisition, many of these loci are hypermethylated in androgenetic blastocysts but hypomethylated in parthenogenetic blastocysts. Paternal DNAme acquisition is lost following maternal deletion of Dnmt3a, with a subset of promoters, which are normally transcribed from the paternal allele in blastocysts, being prematurely transcribed at the 4-cell stage in maternal Dnmt3a knockout embryos. These observations uncover a role for maternal DNMT3A activity in post-fertilization epigenetic reprogramming and transcriptional silencing of the paternal genome.

The paternal genome in mice undergoes widespread DNA methylation loss post-fertilization. Here, the authors apply allele-specific analysis of WGBS data to show that a number of genomic regions are simultaneously de novo methylated on the paternal genome dependent on maternal DNMT3A activity, which induces transcriptional silencing of this allele in the early embryo.

Polyploidy of semi-cloned embryos generated from parthenogenetic haploid embryonic stem cells

In mammals, the fusion of two gametes, an oocyte and a spermatozoon, during fertilization forms a totipotent zygote. There has been no reported case of adult mammal development by natural parthenogenesis, in which embryos develop from unfertilized oocytes. The genome and epigenetic information of haploid gametes are crucial for mammalian development. Haploid embryonic stem cells (haESCs) can be established from uniparental blastocysts and possess only one set of chromosomes. Previous studies have shown that sperm or oocyte genome can be replaced by haESCs with or without manipulation of genomic imprinting for generation of mice. Recently, these remarkable semi-cloning methods have been applied for screening of key factors of mouse embryonic development. While haESCs have been applied as substitutes of gametic genomes, the fundamental mechanism how haESCs contribute to the genome of totipotent embryos is unclear. Here, we show the generation of fertile semi-cloned mice by injection of parthenogenetic haESCs (phaESCs) into oocytes after deletion of two differentially methylated regions (DMRs), the IG-DMR and H19-DMR. For characterizing the genome of semi-cloned embryos further, we establish ESC lines from semi-cloned blastocysts. We report that polyploid karyotypes are observed in semi-cloned ESCs (scESCs). Our results confirm that mitotically arrested phaESCs yield semi-cloned embryos and mice when the IG-DMR and H19-DMR are deleted. In addition, we highlight the occurrence of polyploidy that needs to be considered for further improving the development of semi-cloned embryos derived by haESC injection.

Developmental and Degenerative Characterization of Porcine Parthenogenetic Fetuses during Early Pregnancy

Simple Summary

To increase the early implantation rate, oocytes and zygotes have been subjected to various artificial stimulations before and/or after in vitro fertilization, nuclear transfer, or sperm (spermatid) injection, etc. However, the stimulation process may induce parthenogenetic development. It is difficult to identify whether the embryo or fetus is normally fertilized or parthenogenetically activated in early pregnancy. In the present study, the porcine parthenotes originated from electric stimulation implanted and developed normally during the first month, in a manner similar to artificially inseminated embryos and fetuses. There were no statistical differences in the formation of the major organs such as the brain, liver, kidney, or heart in both groups. However, the implanted parthenotes radically ceased their development and degenerated after one month. It can be postulated that the parthenotes are one of the reasons for the gap between early pregnancy and delivery rate in assisted reproduction techniques.

Abstract

The difference between early pregnancy and delivery rate is quite large in assisted reproduction techniques (ARTs), including animal cloning. However, it is not clear why the implanted fetuses aborted after the early pregnancy stage. In the present study, we tried to evaluate the developmental and morphological characteristics of porcine parthenogenetically activated (PA) embryos or fetuses by electric stimulation during the early pregnancy period. The implanted PA and artificially inseminated (AI) embryos and fetuses were collected at day 26 and 35 after embryo transfer, respectively. The developmental and morphological parameters in the PA embryos at day 26 were similar to the AI embryos. The size, weight, formation of major organs, and apoptotic cells were not statistically different in both embryos at day 26. However, the PA fetuses at day 35 showed ceased fetal development and degenerated with abnormal morphologies in their organs. The day 35 PA fetuses showed significantly higher apoptotic cells and lower methylation status in three differentially methylated regions of the H19 gene compared to their comparators. Therefore, the normal development of PA embryos and fetuses during early gestation could lead to these pregnancies being misinterpreted as normal and become one of the main reasons for the gap between early pregnancy and delivery rate.

Temporal regulation of prenatal embryonic development by paternal imprinted loci

Paternal imprinted genes (H19 and Gtl2) are pivotal for prenatal embryonic development in mice. Nongrowing oocytes and sperm- or oocyte-originated haploid embryonic stem cells (haESCs) carrying both H19-DMR (differentially DNA-methylated region) and IG (intergenic)-DMR deletions that partially mimic paternal imprinting of H19-Igf2 and Dlk1-Dio3 can be employed as sperm replacement to efficiently support full-term embryonic development. However, how H19-DMR and IG-DMR act together to regulate embryonic development is still largely unknown. Here, using androgenetic haESC (AG-haESC)-mediated semi-cloned (SC) technology, we showed that paternal H19-DMR and IG-DMR are not essential for pre-implantation development of SC embryos generated through injection of AG-haESCs into oocytes. H19-DMR plays critical roles before 12.5 days of gestation while IG-DMR is essential for late-gestation of SC embryos. Interestingly, we found that combined deletions of H19 and H19-DMR can further improve the efficiency of normal development of SC embryos at mid-gestation compared to DKO SC embryos. Transcriptome and histology analyses revealed that H19 and H19-DMR combined deletions rescue the placental defects. Furthermore, we showed that H19, H19-DMR and IG-DMR deletions (TKO) give rise to better prenatal and postnatal embryonic development of SC embryos compared to DKO. Together, our results indicate the temporal regulation of paternal imprinted loci during embryonic development.

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.

Technical advances contribute to the study of genomic imprinting

Genomic imprinting in mammals was discovered over 30 years ago through elegant embryological and genetic experiments in mice. Imprinted genes show a monoallelic and parent of origin-specific expression pattern; the development of techniques that can distinguish between expression from maternal and paternal chromosomes in mice, combined with high-throughput strategies, has allowed for identification of many more imprinted genes, most of which are conserved in humans. Undoubtedly, technical progress has greatly promoted progress in the field of genomic imprinting. Here, we summarize the techniques used to discover imprinted genes, identify new imprinted genes, define imprinting regulation mechanisms, and study imprinting functions.

Methylome Dynamics of Bovine Gametes and in vivo Early Embryos

DNA methylation undergoes drastic fluctuation during early mammalian embryogenesis. The dynamics of global DNA methylation in bovine embryos, however, have mostly been studied by immunostaining. We adopted the whole genome bisulfite sequencing (WGBS) method to characterize stage-specific genome-wide DNA methylation in bovine sperm, immature oocytes, oocytes matured in vivo and in vitro, as well as in vivo developed single embryos at the 2-, 4-, 8-, and 16-cell stages. We found that the major wave of genome-wide DNA demethylation was complete by the 8-cell stage when de novo methylation became prominent. Sperm and oocytes were differentially methylated in numerous regions (DMRs), which were primarily intergenic, suggesting that these non-coding regions may play important roles in gamete specification. DMRs were also identified between in vivo and in vitro matured oocytes, suggesting environmental effects on epigenetic modifications. In addition, virtually no (less than 1.5%) DNA methylation was found in mitochondrial DNA. Finally, by using RNA-seq data generated from embryos at the same developmental stages, we revealed a weak inverse correlation between gene expression and promoter methylation. This comprehensive analysis provides insight into the critical features of the bovine embryo methylome, and serves as an important reference for embryos produced in vitro, such as by in vitro fertilization and cloning. Lastly, these data can also provide a model for the epigenetic dynamics in human early embryos.

DNA methylation analysis and editing in single mammalian oocytes

Mammalian oocytes carry specific nongenetic information, including DNA methylation to the next generation, which is important for development and disease. However, evaluation and manipulation of specific methylation for both functional analysis and therapeutic purposes remains challenging. Here, we demonstrate evaluation of specific methylation in single oocytes from its sibling first polar body (PB1) and manipulation of specific methylation in single oocytes by microinjection-mediated dCas9-based targeted methylation editing. We optimized a single-cell bisulfite sequencing approach with high efficiency and demonstrate that the PB1 carries similar methylation profiles at specific regions to its sibling oocyte. By bisulfite sequencing of a single PB1, the methylation information regarding agouti viable yellow (A ^vy )-related coat color, as well as imprinting linked parthenogenetic development competency, in a single oocyte can be efficiently evaluated. Microinjection-based dCas9-Tet/Dnmt-mediated methylation editing allows targeted manipulation of specific methylation in single oocytes. By targeted methylation editing, we were able to reverse A ^vy -related coat color, generate full-term development of bimaternal mice, and correct familial Angelman syndrome in a mouse model. Our work will facilitate the investigation of specific methylation events in oocytes and provides a strategy for prevention and correction of maternally transmitted nongenetic disease or disorders.

Oocytes from stem cells

Folliculogenesis describes the process of activating an oocyte-containing primordial follicle from the ovarian reserve and its development to the mature ovulatory stage. This process is highly complex and is controlled by extra- and intra-ovarian signaling events. Oocyte competence and capacity for fertilization to support a viable pregnancy are acquired during folliculogenesis. Cancer and cancer-based therapies can negatively affect this process, compromising fertility. Currently, preservation of fertility in these patients remains limited to surrogacy, oocyte freezing, oocyte donation, or in vitro maturation (IVM). Recent reports of stem cells being used to produce fully competent oocytes and subsequently healthy offspring in mice have opened up a novel avenue for fertility preservation. However, translating these findings into human health first relies on enhancing our understanding of follicle growth and mimicking its intricacies in vitro. Indeed, the future of oocytes from stem cells in humans comes with many possibilities but currently faces several technical and ethical obstacles.

Chromatin condensation of Xist genomic loci during oogenesis in mice

Repression of maternal Xist (Xm-Xist) during preimplantation in mouse embryos is essential for establishing imprinted X chromosome inactivation. Nuclear transplantation (NT) studies using nuclei derived from non-growing (ng) and full-grown (fg) oocytes have indicated that maternal-specific repressive modifications are imposed on Xm-Xist during oogenesis, as well as on autosomal imprinted genes. Recent studies have revealed that histone H3 lysine 9 trimethylation (H3K9me3) enrichments on Xm-Xist promoter regions are involved in silencing at the preimplantation stages. However, whether H3K9me3 is imposed on Xm-Xist during oogenesis is not known. Here, we dissected the chromatin states in ng and fg oocytes and early preimplantation stage embryos. Chromatin immunoprecipitation experiments against H3K9me3 revealed that there was no significant enrichment within the Xm-Xist region during oogenesis. However, NT embryos with ng nuclei (ngNT) showed extensive Xm-Xist derepression and H3K9me3 hypomethylation of the promoter region at the 4-cell stage, which corresponds to the onset of paternal Xist expression. We also found that the chromatin state at the Xist genomic locus became markedly condensed as oocyte growth proceeded. Although the condensed Xm-Xist genomic locus relaxed during early preimplantation phases, the extent of the relaxation across Xm-Xist loci derived from normally developed oocytes was significantly smaller than those of paternal-Xist and ngNT-Xist genomic loci. Furthermore, Xm-Xist from 2-cell metaphase nuclei became derepressed following NT. We propose that chromatin condensation is associated with imprinted Xist repression and that skipping of the condensation step by NT leads to Xist activation during the early preimplantation phase.

Summary: The analysis of chromatin state and H3K9me3 levels in mouse oocytes and early embryos provides insights into the dynamics of Xist repression and activation during early development and reprogramming.

Development of parthenote following in vivo transfer of embryos in Capra hircus

The aim of this study is to generate parthenogenetic embryos from chemically activated in vitro matured caprine oocytes and to study the in vivo developmental potency of such embryos. The parthenogenetic embryos (2-8 and 16 cells to morula stage) were surgically transferred in 26 recipients. Pregnancy in recipients following embryo transfer was monitored by ultrasonography. The recipient aborted a foetus on day 34 post transfer. Sexing of parthenogenetic foetus showed a single band of amelogenin gene indicating female cell DNA. Microsatellite analysis revealed that the recipient has not contributed genetically to the parthenogenetic foetus confirming the identity of aborted foetus of parthenogenetic origin. The authors believe that this is the first authentic report on in vivo development of parthenogenetic foetus in Capra hircus.

Molecules and mechanisms controlling the active DNA demethylation of the mammalian zygotic genome

The active DNA demethylation in early embryos is essential for subsequent development. Although the zygotic genome is globally demethylated, the DNA methylation of imprinted regions, part of repeat sequences and some gamete-specific regions are maintained. Recent evidence has shown that multiple proteins and biological pathways participate in the regulation of active DNA demethylation, such as TET proteins, DNA repair pathways and DNA methyltransferases. Here we review the recent understanding regarding proteins associated with active DNA demethylation and the regulatory networks controlling the active DNA demethylation in early embryos.

Maternal effect genes: Findings and effects on mouse embryo development

Stored maternal factors in oocytes regulate oocyte differentiation into embryos during early embryonic development. Before zygotic gene activation (ZGA), these early embryos are mainly dependent on maternal factors for survival, such as macromolecules and subcellular organelles in oocytes. The genes encoding these essential maternal products are referred to as maternal effect genes (MEGs). MEGs accumulate maternal factors during oogenesis and enable ZGA, progression of early embryo development, and the initial establishment of embryonic cell lineages. Disruption of MEGs results in defective embryogenesis. Despite their important functions, only a few mammalian MEGs have been identified. In this review we summarize the roles of known MEGs in mouse fertility, with a particular emphasis on oocytes and early embryonic development. An increased knowledge of the working mechanism of MEGs could ultimately provide a means to regulate oocyte maturation and subsequent early embryonic development.

Forced expression of DNA methyltransferases during oocyte growth accelerates the establishment of methylation imprints but not functional genomic imprinting

In mammals, genomic imprinting governed by DNA methyltransferase DNMT3A and its cofactor DNMT3L is essential for functional gametes. Oocyte-specific methylation imprints are established during oocyte growth concomitant with DNMT3A/DNMT3L expression, although the mechanisms of oocyte-specific imprinting are not fully understood. To determine whether the presence of DNMT3A/DNMT3L in oocytes is sufficient for acquisition of methylation imprints, we produced transgenic mice to induce DNMT3A/DNMT3L expression prematurely in oogenesis and analyzed DNA methylation imprints. The results showed that 2- to 4-fold greater expression of DNMT3A/DNMT3L was achieved in non-growing (ng) oocytes versus fully grown oocytes derived from wild-type mice, but the analyzed imprint domains were not methylated. Thus, the presence of DNMT3A/DNMT3L in ng oocytes is insufficient for methylation imprints, and imprinted regions are resistant to DNMT3A/DNMT3L in ng oocytes. In contrast, excess DNMT3A/DNMT3L accelerated imprint acquisition at Igf2r, Lit1, Zac1 and Impact but not Snrpn and Mest in growing oocytes. Therefore, DNMT3A/DNMT3L quantity is an important factor for imprint acquisition. Transcription at imprinted domains is proposed to be involved in de novo methylation; however, transcription at Lit1, Snrpn and Impact was observed in ng oocytes. Thus, transcription cannot induce DNMT3A catalysis at imprinted regions even if DNMT3A/DNMT3L is present. However, the accelerated methylation imprints in oocytes, with the exception of Igf2r, were erased during embryogenesis. In conclusion, a sufficient amount of DNMT3A/DNMT3L and a shift from the resistant to permissive state are essential to establish oocyte-specific methylation imprints and that maintenance of the acquired DNA methylation imprints is essential for functional imprinting.

Concise review: parthenote stem cells for regenerative medicine: genetic, epigenetic, and developmental features

Embryonic stem cells (ESCs) have the potential to provide unlimited cells and tissues for regenerative medicine. ESCs derived from fertilized embryos, however, will most likely be rejected by a patient's immune system unless appropriately immunomatched. Pluripotent stem cells (PSCs) genetically identical to a patient can now be established by reprogramming of somatic cells. However, practical applications of PSCs for personalized therapies are projected to be unfeasible because of the enormous cost and time required to produce clinical-grade cells for each patient. ESCs derived from parthenogenetic embryos (pESCs) that are homozygous for human leukocyte antigens may serve as an attractive alternative for immunomatched therapies for a large population of patients. In this study, we describe the biology and genetic nature of mammalian parthenogenesis and review potential advantages and limitations of pESCs for cell-based therapies.

The epigenetics of early development: inferences from stem cells

Approximately 200 cell types and multiple tissues are established throughout the development of the zygote to an adult mammal. During this process, the cellular genome remains fixed, yet the transcriptome of each of the cell types become widely divergent. This review discusses the epigenetics of preimplantation embryos and the use of embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs) as cell-culture models for the inner cell mass (ICM) and epiblast, respectively. Differential patterns of transcription are set up during development by the action of key transcription factors and epigenetics, which are involved in the establishment and maintenance of stable transcriptional states during development. In early embryos, for example, changes in the epigenome consist of alterations to the methylation of CpG dinucleotides and post-translational modification of histones within chromatin. In addition, histone replacement occurs broadly in zygotes. The ICM of the blastocyst, on the other hand, has the amazing ability to contribute to every tissue and cell type present in the adult body. Therefore, ESCs are arguably the most important cell-culture model available to developmental biologists. The advantages and risks of using ESCs to model ICM pluripotency are therefore discussed.

Epigenetic regulation of genomic imprinting from germ line to preimplantation

Genomic imprinting is an epigenetic process that distinguishes parental alleles, resulting in parent-specific expression of a gene or cluster of genes. Imprints are acquired during gametogenesis when genome-wide epigenetic remodeling occurs. These imprints must then be maintained during preimplantation development, when another wave of genome-wide epigenetic remodeling takes place. Thus, for imprints to persist as parent-specific epigenetic marks, coordinated factors and processes must be involved to both recognize an imprint and protect it from genome-wide remodeling. Parent-specific DNA methylation has long been recognized as a primary epigenetic mark demarcating a genomic imprint. Recent work has advanced our understanding of how and when parent-specific DNA methylation is erased and acquired in the germ line as well as maintained during preimplantation development. Epigenetic factors have also been identified that are recruited to imprinted regions to protect them from genome-wide DNA demethylation during preimplantation development. Intriguingly, asynchrony in epigenetic reprogramming appears to be a recurrent theme with asynchronous acquisition between male and female germ lines, between different imprinted genes, and between the two parental alleles of a gene. Here, we review recent advancements and discuss how they impact our current understanding of the epigenetic regulation of genomic imprinting.

Effect of actin polymerization inhibitor during oocyte maturation on parthenogenetic embryo development and ploidy in Capra hircus

This study was designed to observe the effect of cytochalasin B (CCB) concentrations on ploidy and early development of parthenogenetic embryos in a caprine species. Caprine oocytes were matured in the presence of different concentrations of CCB (5, 10, 15, and 20 μg/ml) and activated by 7% ethanol followed by incubation with 2 mM DMAP. For embryos fertilized in vitro, oocytes were matured in maturation medium without CCB. The cleavage rate and further embryo development were significantly higher (P < 0.05) when oocytes were treated in this way. The percentage of embryos showed higher diploid values in 15 μg/ml CCB (83.66 ± 1.13), followed by 20 (72.22 ± 1.22), 10 (68.57 ± 1.17), and 5 μg/ml (62.00 ± 2.48). These results indicate that CCB with a concentration of 15 μg/ml in maturation medium can be used for the production of diploid parthenogenetic embryos in the caprine species.

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.

Ectopic expression of Fgf3 leads to aberrant lineage segregation in the mouse parthenote preimplantation embryos

BACKGROUND

Parthenogenetic mammalian embryos were reported to die in utero no later than the 25-somite stage due to abnormal development of both embryonic and extraembryonic lineages. Interestingly, it has been shown that parthenogenetic ICM cells tend to differentiate more into primitive endoderm cells and less into epiblast and ES cells. Hence we are interested in studying the molecular mechanisms underlying lineage defects of parthenotes.

RESULTS

We found that parthenote inner cell masses (ICMs) contained decreased numbers of Sox2(+) /Nanog(+) epiblast cells but increased numbers of Gata4(+) primitive endoderm cells, indicating an unusual lineage segregation. We demonstrate for the first time that the increased Gata4 level in parthenotes may be explained by the strong up-regulation of Fgf3 and Fgfr2 phosphorylation. Inhibition of Fgfr2 activation by SU5402 in parthenotes restored normal Nanog and Gata4 levels without affecting Fgf3, indicating that Fgf3 is upstream of Fgfr2 activation. In parthenote trophectoderm, we detected normal Cdx2 but ectopic Gata4 expression and reduced Elf5 and Tbr2(Eomes) levels.

CONCLUSIONS

Taken together, our work provides for the first time the insight into the molecular mechanisms of the developmental defects of parthenogenetic embryos in both the trophectoderm and ICM.

Porcine nuclei in early growing stage do not possess meiotic competence in matured oocytes

To determine whether the nuclei of early growing stage porcine oocytes can mature to the MII stage, we examined meiotic competence of nuclei that had been fused with enucleated GV oocytes using the nuclear transfer method. In vitro matured oocytes were enucleated and then fused with early growing oocytes (30-40 μm in diameter) from 5 to 7-wk-old piglets using the hemagglutinating virus of Japan (HVJ). Reconstructed oocytes were cultured for 24 h to the MII stage. Although these oocytes extruded the first polar body, they did not contain normal haploid chromosomes, and the spindles were misaligned or absent at the metaphase II (MII) stage. Furthermore, maturation promoting factor (MPF) activity levels were low in oocytes reconstructed with early growing oocytes at metaphase I (MI) and MII. In contrast, mitogen-activated protein kinase (MAPK) activity was detected between the MI and MII stages, although at slightly lower levels. In conclusion, the nuclei of early growing oocytes did not accomplish normal meiotic division in matured oocytes due to misaligned or absent spindle formation.

An unbiased assessment of the role of imprinted genes in an intergenerational model of developmental programming

Environmental factors during early life are critical for the later metabolic health of the individual and of future progeny. In our obesogenic environment, it is of great socioeconomic importance to investigate the mechanisms that contribute to the risk of metabolic ill health. Imprinted genes, a class of functionally mono-allelic genes critical for early growth and metabolic axis development, have been proposed to be uniquely susceptible to environmental change. Furthermore, it has also been suggested that perturbation of the epigenetic reprogramming of imprinting control regions (ICRs) may play a role in phenotypic heritability following early life insults. Alternatively, the presence of multiple layers of epigenetic regulation may in fact protect imprinted genes from such perturbation. Unbiased investigation of these alternative hypotheses requires assessment of imprinted gene expression in the context of the response of the whole transcriptome to environmental assault. We therefore analyse the role of imprinted genes in multiple tissues in two affected generations of an established murine model of the developmental origins of health and disease using microarrays and quantitative RT–PCR. We demonstrate that, despite the functional mono-allelicism of imprinted genes and their unique mechanisms of epigenetic dosage control, imprinted genes as a class are neither more susceptible nor protected from expression perturbation induced by maternal undernutrition in either the F1 or the F2 generation compared to other genes. Nor do we find any evidence that the epigenetic reprogramming of ICRs in the germline is susceptible to nutritional restriction. However, we propose that those imprinted genes that are affected may play important roles in the foetal response to undernutrition and potentially its long-term sequelae. We suggest that recently described instances of dosage regulation by relaxation of imprinting are rare and likely to be highly regulated.

Environmental perturbations during early life are known to affect one's risk of metabolic disease many years later. Furthermore, that risk can be inherited by future generations, although the mechanisms responsible are poorly understood. Imprinted genes are unusual as only one of the two copies is expressed in a parent-of-origin–specific manner. As only one copy is active, imprinted gene dosage has been hypothesised to be uniquely vulnerable to environmental change. Therefore, it has been suggested that imprinted genes may play an important role in the developmental origins of health and disease. Alternatively, the opposite may be true—imprinted genes may be more tightly safeguarded from perturbation. To test these two hypotheses, we analysed the expression of imprinted genes in the context of all active genes in two affected generations of a mouse model of the developmental origins of health and disease. Our data show that imprinted genes as a class are neither more nor less susceptible to expression change, but a subset of imprinted genes may be involved in the adaptation of the conceptus. Furthermore, imprints in the developing germline are not affected and imprinted genes are largely stable in the second generation. This is important, as it is the first time that this hypothesis has been tested in an unbiased fashion.

Imprinting of Genes and the Barker Hypothesis

Several common adult diseases appear to be related to impaired fetal growth and this may be caused either by nutritional inadequacies at particular stages of pregnancy or by variation in alleles at specific growth loci. Little is known about the genes involved in the underlying mechanism. This review proposes that at least some of the effects have their origins at imprinted loci, genes that are unusual because they are expressed from only one parental allele. Many imprinted genes are crucial for fetal growth and determine birthweight. They can be disrupted in the early embryo by environmental influences and these disruptions can be inherited through many cell cycles into adult tissues. Their disruption can affect specific organs during fetal development and disruption could affect adult disease in a variety of direct and indirect means. Imprinted genes may be particularly vulnerable to disruption as they are functionally haploid and their expression is regulated by different means from the rest of the genome. Thus many imprinted genes provide plausible candidates for programming adult disease and warrant further study in this context.

Defects in imprinting and genome-wide DNA methylation are not common in the in vitro fertilization population

OBJECTIVE

To examine an IVF cohort for imprinted and genome-wide DNA methylation abnormalities.

DESIGN

Retrospective study.

SETTING

Research laboratory.

PATIENT(S)

DNA samples from a previously described IVF cohort that comprised 66 IVF-conceived prepubertal children (IVF, n = 34; intracytoplasmic sperm injection, n = 32) and 69 matched naturally conceived controls.

INTERVENTION(S)

DNA methylation was examined at four imprinted gene loci (H19, SNRPN, KCNQ1OT1, and IGF2) and satellite 2 using methylation-sensitive quantitative polymerase chain reaction (MSQ-PCR) followed by bisulfite sequencing at H19, SNRPN, and KCNQ1OT1. Methylated DNA immunoprecipitation (MeDIP) microarray with validation using the Sequenom MassARRAY EpiTYPER(®) platform was also used.

MAIN OUTCOME MEASURE(S)

Percentage of DNA methylation by MSQ-PCR, differential methylation based on microarray signal intensity, and percentage DNA methylation as determined by Sequenom MassARRAY EpiTYPER were compared.

RESULT(S)

No differences in percentage of methylation between the IVF and control group were observed at H19, KCNQ1OT1, SNRPN, or IGF2. Absence of aberrant imprinting was confirmed using bisulfite sequencing. Methylation of satellite 2 repeats (a surrogate for global methylation) showed no difference between the IVF and control groups. MeDIP was used to screen for differences in promoter methylation. Subsequent quantification of methylation of eight candidate genes using the Sequenom MassARRAY EpiTYPER system did not reveal any differential methylation.

CONCLUSION(S)

Low-level imprinting errors are not common in the IVF population.

Downregulation of H19 improves the differentiation potential of mouse parthenogenetic embryonic stem cells

Parthenogenetic embryonic stem cells (P-ESCs) offer an alternative source of pluripotent cells, which hold great promise for autologous transplantation and regenerative medicine. P-ESCs have been successfully derived from blastocysts of several mammalian species. However, compared with biparental embryonic stem cells (B-ESCs), P-ESCs are limited in their ability to fully differentiate into all 3 germ layers. For example, it has been observed that there is a differentiation bias toward ectoderm derivatives at the expense of endoderm and mesoderm derivatives-muscle in particular-in chimeric embryos, teratomas, and embryoid bodies. In the present study we found that H19 expression was highly upregulated in P-ESCs with more than 6-fold overexpression compared with B-ESCs. Thus, we hypothesized that manipulation of the H19 gene in P-ESCs would alleviate their limitations and allow them to function like B-ESCs. To test this hypothesis we employed a small hairpin RNA approach to reduce the amount of H19 transcripts in mouse P-ESCs. We found that downregulation of H19 led to an increase of mesoderm-derived muscle and endoderm in P-ESCs teratomas similar to that observed in B-ESCs teratomas. This phenomenon coincided with upregulation of mesoderm-specific genes such as Myf5, Myf6, and MyoD. Moreover, H19 downregulated P-ESCs differentiated into a higher percentage of beating cardiomyocytes compared with control P-ESCs. Collectively, these results suggest that P-ESCs are amenable to molecular modifications that bring them functionally closer to true ESCs.

Sex-specific dynamics of global chromatin changes in fetal mouse germ cells

Mammalian germ cells undergo global reprogramming of DNA methylation during their development. Global DNA demethylation occurs around the time when the primordial germ cells colonize the embryonic gonads and this coincides with dynamic changes in chromatin composition. Global de novo DNA methylation takes place with remarkably different dynamics between the two sexes, prospermatogonia attaining methylation during fetal stages and oocytes attaining methylation postnatally. Our hypothesis was that dynamic changes in chromatin composition may precede or accompany the wave of global DNA de novo methylation as well. We used immunocytochemistry to measure global DNA methylation and chromatin components in male and female mouse fetal germ cells compared to control somatic cells of the gonad. We found that global DNA methylation levels sharply increased in male germ cells at 17.5 days post coitum, but remained low in female germ cells at all fetal stages. Global changes in chromatin composition: i, preceded global DNA methylation in fetal germ cells; ii, sex specifically occurred in male but not in female germ cells; iii, affected active and repressive histone marks and iv, included histone tail and histone globular domain modifications. Our data suggest that dynamic changes of chromatin composition may provide a framework for the pattern of male-specific de novo DNA methylation in prospermatogonia.

Ontological hypothesis of the cancer etiology: discord between cells' survival determinism and their disposition to biological altruism

During the last decades, scientific community has implicitly viewed cancer as a number of different diseases with the same underlying phenotype. Such a view was justified for the fact that some of the genetic and phenotypic similarities, observed in different types of tumors, were perpetuated via some distinct mechanisms. Nevertheless, this manuscript aims to interpret all of these differences in a context of the same underlying cause. To do so, the epigenetic and genetic alterations observed in cancers are initially interpreted in the context of their advantage for the evolution of the early eukaryotic organisms. Subsequently, the proposed premises are further discussed with respect to their propagation in the subsequent generations of the new eukaryotic species, as well as their role in the development of the higher organisms. In the subsequent section, the role of the proposed mechanism is discussed in the context of cancer, which is proposed to originate due to the analogous underlying mechanisms. Finally, the proposed mechanism is briefly discussed in parallel with some other contemporary theories of carcinogenesis, aiming to further support its validity. Thereby, the model presents an alternative interpretation of multiple cancer-related biomedical phenomena from the aspect of a proposed evolutionary mechanism.

Mammalian recombination hot spots: properties, control and evolution

Recombination, together with mutation, generates the raw material of evolution, is essential for reproduction and lies at the heart of all genetic analysis. Recent advances in our ability to construct genome-scale, high-resolution recombination maps and new molecular techniques for analysing recombination products have substantially furthered our understanding of this important biological phenomenon in humans and mice: from describing the properties of recombination hot spots in male and female meiosis to the recombination landscape along chromosomes. This progress has been accompanied by the identification of trans-acting systems that regulate the location and relative activity of individual hot spots.

Challenges and strategies for generating therapeutic patient-specific hemangioblasts and hematopoietic stem cells from human pluripotent stem cells

Recent characterization of hemangioblasts differentiated from human embryonic stem cells (hESC) has further confirmed evidence from murine, zebrafish and avian experimental systems that hematopoietic and endothelial lineages arise from a common progenitor. Such progenitors may provide a valuable resource for delineating the initial developmental steps of human hemato-endotheliogenesis, which is a process normally difficult to study due to the very limited accessibility of early human embryonic/fetal tissues. Moreover, efficient hemangioblast and hematopoietic stem cell (HSC) generation from patient-specific pluripotent stem cells has enormous potential for regenerative medicine, since it could lead to strategies for treating a multitude of hematologic and vascular disorders. However, significant scientific challenges remain in achieving these goals, and the generation of transplantable hemangioblasts and HSC derived from hESC currently remains elusive. Our previous work has suggested that the failure to derive engraftable HSC from hESC is due to the fact that current methodologies for differentiating hESC produce hematopoietic progenitors developmentally similar to those found in the human yolk sac, and are therefore too immature to provide adult-type hematopoietic reconstitution. Herein, we outline the nature of this challenge and propose targeted strategies for generating engraftable human pluripotent stem cell-derived HSC from primitive hemangioblasts using a developmental approach. We also focus on methods by which reprogrammed somatic cells could be used to derive autologous pluripotent stem cells, which in turn could provide unlimited sources of patient-specific hemangioblasts and HSC.

Genomic imprinting in mammals: its life cycle, molecular mechanisms and reprogramming

Genomic imprinting, an epigenetic gene-marking phenomenon that occurs in the germline, leads to parental-origin-specific expression of a small subset of genes in mammals. Imprinting has a great impact on normal mammalian development, fetal growth, metabolism and adult behavior. The epigenetic imprints regarding the parental origin are established during male and female gametogenesis, passed to the zygote through fertilization, maintained throughout development and adult life, and erased in primordial germ cells before the new imprints are set. In this review, we focus on the recent discoveries on the mechanisms involved in the reprogramming and maintenance of the imprints. We also discuss the epigenetic changes that occur at imprinted loci in induced pluripotent stem cells.

The parental non-equivalence of imprinting control regions during mammalian development and evolution

In mammals, imprinted gene expression results from the sex-specific methylation of imprinted control regions (ICRs) in the parental germlines. Imprinting is linked to therian reproduction, that is, the placenta and imprinting emerged at roughly the same time and potentially co-evolved. We assessed the transcriptome-wide and ontology effect of maternally versus paternally methylated ICRs at the developmental stage of setting of the chorioallantoic placenta in the mouse (8.5dpc), using two models of imprinting deficiency including completely imprint-free embryos. Paternal and maternal imprints have a similar quantitative impact on the embryonic transcriptome. However, transcriptional effects of maternal ICRs are qualitatively focused on the fetal-maternal interface, while paternal ICRs weakly affect non-convergent biological processes, with little consequence for viability at 8.5dpc. Moreover, genes regulated by maternal ICRs indirectly influence genes regulated by paternal ICRs, while the reverse is not observed. The functional dominance of maternal imprints over early embryonic development is potentially linked to selection pressures favoring methylation-dependent control of maternal over paternal ICRs. We previously hypothesized that the different methylation histories of ICRs in the maternal versus the paternal germlines may have put paternal ICRs under higher mutational pressure to lose CpGs by deamination. Using comparative genomics of 17 extant mammalian species, we show here that, while ICRs in general have been constrained to maintain more CpGs than non-imprinted sequences, the rate of CpG loss at paternal ICRs has indeed been higher than at maternal ICRs during evolution. In fact, maternal ICRs, which have the characteristics of CpG-rich promoters, have gained CpGs compared to non-imprinted CpG-rich promoters. Thus, the numerical and, during early embryonic development, functional dominance of maternal ICRs can be explained as the consequence of two orthogonal evolutionary forces: pressure to tightly regulate genes affecting the fetal-maternal interface and pressure to avoid the mutagenic environment of the paternal germline.

The H19 locus: role of an imprinted non-coding RNA in growth and development

The H19 gene produces a non-coding RNA, which is abundantly expressed during embryonic development and down-regulated after birth. Although this gene was discovered over 20 years ago, its function has remained unclear. Only recently a role was identified for the non-coding RNA and/or its microRNA partner, first as a tumour suppressor gene in mice, then as a trans-regulator of a group of co-expressed genes belonging to the imprinted gene network that is likely to control foetal and early postnatal growth in mice. The mechanisms underlying this transcriptional or post-transcriptional regulation remain to be discovered, perhaps by identifying the protein partners of the full-length H19 RNA or the targets of the microRNA. This first in vivo evidence of a functional role for the H19 locus provides new insights into how genomic imprinting helps to control embryonic growth.

[Review on the genomic imprinting at the mammalian DLK1-DIO3 cluster.]

The mammalian imprinting domain DLK1-DIO3 is located on distal human chromosome 14, mouse chromosome 12 and sheep chromosome 18. This cluster contains three imprinted protein-coding genes (Dlk1, Rtl1, and Dio3), which were expressed from the paternally inherited chromosome and several imprinted noncoding RNA genes expressed from the maternally inherited allele, such as miRNAs, snoRNAs, and large noncoding RNA Gtl2. The altered gene dosage of DLK1-DIO3 cluster resulted in several severe abnormal phenotypes in human and mouse, even death, suggesting the importance of these genes for normal development. This review focuses on the function of imprinted genes on this domain and the mechanism of their imprinting regulation.

Autonomous silencing of the imprinted Cdkn1c gene in stem cells

Parent-of-origin specific expression of imprinted genes relies on the differential DNA methylation of specific genomic regions. Differentially methylated regions (DMRs) acquire DNA methylation either during gametogenesis (primary DMR) or after fertilisation when allele-specific expression is established (secondary DMR). Little is known about the function of these secondary DMRs. We investigated the DMR spanning Cdkn1c in mouse embryonic stem cells, androgenetic stem cells and embryonic germ stem cells. In all cases, expression of Cdkn1c was appropriately repressed in in vitro differentiated cells. However, stem cells failed to de novo methylate the silenced gene even after sustained differentiation. In the absence of maintained DNA methylation (Dnmt1(-/-)), Cdkn1c escapes silencing demonstrating the requirement for DNA methylation in long term silencing in vivo. We propose that postfertilisation differential methylation reflects the importance of retaining single gene dosage of a subset of imprinted loci in the adult.