Disruption of parental-specific expression of imprinted genes in uniparental fetuses

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.

Haploid androgenetic development of bovine embryos reveals imbalanced WNT signaling and impaired cell fate differentiation†

Haploid embryos have contributed significantly to our understanding of the role of parental genomes in development and can be applied to important biotechnology for human and animal species. However, development to the blastocyst stage is severely hindered in bovine haploid androgenetic embryos (hAE). To further our understanding of such developmental arrest, we performed a comprehensive comparison of the transcriptomic profile of morula-stage embryos, which were validated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) of transcripts associated with differentiation in haploid and biparental embryos. Among numerous disturbances, results showed that pluripotency pathways, especially the wingless-related integration site (WNT) signaling, were particularly unbalanced in hAE. Moreover, transcript levels of KLF4, NANOG, POU5F1, SOX2, CDX2, CTNNBL1, AXIN2, and GSK3B were noticeably altered in hAE, suggesting disturbance of pluripotency and canonical WNT pathways. To evaluate the role of WNT on hAE competence, we exposed early Day-5 morula stage embryos to the GSK3B inhibitor CHIR99021. Although no alterations were observed in pluripotency and WNT-related transcripts, exposure to CHIR99021 improved their ability to reach the blastocysts stage, confirming the importance of the WNT pathway in the developmental outcome of bovine hAE.

Dysregulated Gene Expression of Imprinted and X-Linked Genes: A Link to Poor Development of Bovine Haploid Androgenetic Embryos

Mammalian uniparental embryos are efficient models for genome imprinting research and allow studies on the contribution of the paternal and maternal genomes to early embryonic development. In this study, we analyzed different methods for production of bovine haploid androgenetic embryos (hAE) to elucidate the causes behind their poor developmental potential. Results indicate that hAE can be efficiently generated by using intracytoplasmic sperm injection and oocyte enucleation at telophase II. Although androgenetic haploidy does not disturb early development up to around the 8-cell stage, androgenetic development is disturbed after the time of zygote genome activation and hAE that reach the morula stage are less capable to reach the blastocyst stage of development. Karyotypic comparisons to parthenogenetic- and ICSI-derived embryos excluded chromosomal segregation errors as causes of the developmental constraints of hAE. However, analysis of gene expression indicated abnormal levels of transcripts for key long non-coding RNAs involved in X chromosome inactivation and genomic imprinting of the KCNQ1 locus, suggesting an association with X chromosome and some imprinted loci. Moreover, transcript levels of methyltransferase 3B were significantly downregulated, suggesting potential anomalies in hAE establishing de novo methylation. Finally, the methylation status of imprinted control regions for XIST and KCNQ1OT1 genes remained hypomethylated in hAE at the morula and blastocyst stages, confirming their origin from spermatozoa. Thus, our results exclude micromanipulation and chromosomal abnormalities as major factors disturbing the normal development of bovine haploid androgenotes. In addition, although the cause of the arrest remains unclear, we have shown that the inefficient development of haploid androgenetic bovine embryos to develop to the blastocyst stage is associated with abnormal expression of key factors involved in X chromosome activity and genomic imprinting.

Developmental peculiarities in placentae of ovine uniparental conceptuses

Genomic imprinting is an epigenetic phenomenon regulating mono-allelic expression of genes depending on their parental origin. Defective genomic imprinting is involved in several placental disorders, such as intrauterine growth restriction and pre-eclampsia. Uniparental embryos, having maternal-only or paternal-only genomes (parthenogenotes [PAR] and androgenotes [AND], respectively), are useful models to study placentation. The aim of this work was to reveal the effect of parental genome (maternal and paternal) on placentation. To do this, uniparental (AND and PAR) and biparental (CTR) in vitro produced sheep embryos transferred to recipient females were collected at day 20 of pregnancy and their placentae were analyzed. qPCR analysis showed that imprinted genes (H19, IGF2R and DLK1) were expressed accordingly to their parental origin while the expression f DNA methyltransferases () was disregulated, especially in PAR (P < 0.05). AND placentae were significantly hypomethylated compared to both PAR and CTR (P = 0.023). Chorion-allantoid of AND showed impaired development of vessels and reduced mRNA expression of vasculogenetic factors (ANG2 P = 0.05; VEGFR2 P< 0.001; TIE2 P < 0.001). Morphologically, PAR placentae were characterized by abnormal structure of the trophoectodermal epithelium and reduced total number (P<0.03) of Trophoblastic Binucleate Cells. A reduced implantation rate of both classes of uniparental embryos (P<0.03) was also noted. Our results provide new insights into the characterization of uniparental embryos and demonstrate the complementary role of parental genomes for the correct establishment of pregnancy. Thus, our findings may suggest new targets to improve our understanding of the origin of imprinting-related placental dysfunction.

The effects of melatonin on bovine uniparental embryos development in vitro and the hormone secretion of COCs

Melatonin is a unique multifunctional molecule that mediates reproductive functions in animals. In this study, we investigated the effects of melatonin on bovine parthenogenetic and androgenetic embryonic development, oocyte maturation, the reactive oxygen species (ROS) levels in parthenogenetic and androgenetic embryos and cumulus—oocyte complexes (COCs) hormone secretion with melatonin supplementation at four concentrations (0, 10, 20, and 30 pmol/mL), respectively. The results showed that melatonin significantly promoted the rates of bovine parthenogenetic and androgenetic embryonic cleavage and morula and blastocysts development (P < 0.05). The rate of cleavage was higher in the androgenetic embryo than that in the parthenogenetic embryo. Compared with the parthenogenetic embryos, the androgenetic embryos had a poor developmental competence from morula to blastocyst stage. Moreover, the levels of ROS were significantly lower in the parthenogenetic and androgenetic embryoes with melatonin-treated group than that of the control group (P < 0.05). Melatonin supplemented significantly increased the maturation rate of oocyte in vitro (P < 0.05). More importantly, melatonin significantly promoted the secretion of progesterone and estradiol by COCs (P < 0.05). To reveal the regulatory mechanism of melatonin on steroids synthesis, we found that steroidogenic genes (CYP11A1, CYP19A1 and StAR) were upregulated, suggesting that melatonin regulated estradiol and progesterone secretion through mediating the expression of steroidogenic genes (CYP11A1, CYP19A1 and StAR). In addition, MT1 and MT2 were identified in bovine early parthenogenetic and androgenetic embryos using western blot. It could be concluded that melatonin had beneficial effects on bovine oocyte in vitro maturation, COC hormone secretion, early development of subsequent parthenogenetic and androgenetic embryos. It is inferred that melatonin could be used to enhance the efficiency of in vitro developed embryos.

The Influence of Polyploidy and Genome Composition on Genomic Imprinting in Mice

Genomic imprinting is an epigenetic mechanism that switches the expression of imprinted genes involved in normal embryonic growth and development in a parent-of-origin-specific manner. Changes in DNA methylation statuses from polyploidization are a well characterized epigenetic modification in plants. However, how changes in ploidy affect both imprinted gene expression and methylation status in mammals remains unclear. To address this, we used quantitative real time PCR to analyze expression levels of imprinted genes in mouse tetraploid fetuses. We used bisulfite sequencing to assess the methylation statuses of differentially methylated regions (DMRs) that regulate imprinted gene expression in triploid and tetraploid fetuses. The nine imprinted genes H19, Gtl2, Dlk1, Igf2r, Grb10, Zim1, Peg3, Ndn, and Ipw were all unregulated; in particular, the expression of Zim1 was more than 10-fold higher, and the expression of Ipw was repressed in tetraploid fetuses. The methylation statuses of four DMRs H19, intergenic (IG), Igf2r, and Snrpn in tetraploid and triploid fetuses were similar to those in diploid fetuses. We also performed allele-specific RT-PCR sequencing to determine the alleles expressing the three imprinted genes Igf2, Gtl2, and Dlk1 in tetraploid fetuses. These three imprinted genes showed monoallelic expression in a parent-of-origin-specific manner. Expression of non-imprinted genes regulating neural cell development significantly decreased in tetraploid fetuses, which might have been associated with unregulated imprinted gene expression. This study provides the first detailed analysis of genomic imprinting in tetraploid fetuses, suggesting that imprinted gene expression is disrupted, but DNA methylation statuses of DMRs are stable following changes in ploidy in mammals.

Cell proliferation potency is independent of FGF4 signaling in trophoblast stem cells derived from androgenetic embryos

We previously established trophoblast stem cells from mouse androgenetic embryos (AGTS cells). In this study, to further characterize AGTS cells, we compared cell proliferation activity between trophoblast stem (TS) cells and AGTS cells under fibroblast growth factor 4 (FGF4) signaling. TS cells continued to proliferate and maintained mitotic cell division in the presence of FGF4. After FGF4 deprivation, the cell proliferation stopped, the rate of M-phase cells decreased, and trophoblast giant cells formed. In contrast, some of AGTS cells continued to proliferate, and the rate of M-phase cells did not decrease after FGF4 deprivation, although the other cells differentiated into giant cells. RO3306, an ATP competitor that selectively inhibits CDK1, inhibited the cell proliferation of both TS and AGTS cells. Under RO3306 treatment, cell death was induced in AGTS cells but not in TS cells. These results indicate that RO3306 caused TS cells to shift mitotic cell division to endoreduplication but that some of AGTS cells did not shift to endoreduplication and induced cell death. In conclusion, the paternal genome facilitated the proliferation of trophoblast cells without FGF4 signaling.

Restricted development of mouse triploid fetuses with disorganized expression of imprinted genes

Eukaryotic species commonly contain a diploid complement of chromosomes. The diploid state appears to be advantageous for mammals because it enables sexual reproduction and facilitates genetic recombination. Nonetheless, the effects of DNA ploidy on mammalian ontogeny have yet to be understood. The present study shows phenotypic features and expression patterns of imprinted genes in tripronucleate diandric and digynic triploid (DAT and DGT) mouse fetuses on embryonic day 10.5 (E10.5). Measurement of crown–rump length revealed that the length of DGT fetuses (1.87 ± 0.13 mm; mean ± standard error of the mean) was much smaller than that of diploid fetuses (4.81 ± 0.05 mm). However, no significant difference was observed in the crown–rump length between diploid and DAT fetuses (3.86 ± 0.43 mm). In DGT fetuses, the expression level of paternally expressed genes, Igf2, Dlk1, Ndn, and Peg3, remained significantly reduced and that of maternally expressed genes, Igf2r and Grb10, increased. Additionally, in DAT fetuses, the Igf2 mRNA expression level was approximately twice that in diploid fetuses, as expected. These results provide the first demonstration that imprinted genes in mouse triploid fetuses show distinctive expression patterns independent of the number of parental-origin haploid sets. These data suggest that both DNA ploidy and asymmetrical functions of parental genomes separately influence mammalian ontogeny.

Development and imprinted gene expression in uniparental preimplantation mouse embryos in vitro

Increasing numbers of reports show that imprinted genes play a crucial role in fetal development, and uniparental embryos, which possess two paternally or two maternally derived pronuclei, are excellent tools for investigating the biological significance of imprinted genes. In the present study, to examine the in vitro developmental ability and expression pattern of eight imprinted genes in uniparental embryos, we produced androgenones, gynogenones, and parthenogenones using enucleation. Our data confirmed the previously observed restriction in haploid androgenetic development potential and first indicated that diploid androgenetic embryos were arrested in the 3/4-cell stage. Some imprinted genes were expressed in androgenetic, gynogenetic, and parthenogenetic blastocysts, suggesting that they were unable to maintain their imprinted expression status in uniparental embryos and that both paternal and maternal alleles are required for the specific expression of some imprinted genes.

Disruption of imprinted gene expression and DNA methylation status in porcine parthenogenetic fetuses and placentas

Parthenogenetically activated oocytes cannot develop to term in mammals due to the lack of paternal gene expression and failed X chromosome inactivation (XCI). To further characterize porcine parthenogenesis, the expression of 18 imprinted genes was compared between parthenogenetic (PA) and normally fertilized embryos (Con) using quantitative real-time PCR (qRT-PCR). The results revealed that maternally expressed genes were over-expressed, whereas paternally expressed genes were significantly reduced in PA fetuses and placentas. The results of bisulfite sequencing PCR (BSP) demonstrated that PRE-1 and Satellite were hypermethylated in both Con and PA fetuses and placentas, while XIST DMRs were hypomethylated only in PA samples. Taken together, these results suggest that the aberrant methylation profile of XIST DMRs and abnormal imprinted gene expression may be responsible for developmental failure and impaired growth in porcine parthenogenesis.

Genomic imprinting analysis of Igf2/H19 in porcine cloned fetuses using parthenogenetic somatic cells as nuclear donors

To gain insight into parthenogenesis in pigs, we report for the first time that using parthenogenetic somatic cells as nuclear donors (PSCNT), the porcine parthenogenetic fetus can develop to gestational day 39. Weight and morphological analysis revealed that PSCNT fetuses were smaller and developmentally retarded when compared to normally fertilized controls. Quantitative gene expression analysis indicated that in PSCNT fetuses, H19 was over-expressed, whereas Igf2 was significantly reduced (p < 0.05) compared with their controls. In addition, bisulfite-sequencing PCR results demonstrated that H19 differentially DNA methylated regions (DMRs) were hypomethylated in PSCNT fetuses, while Igf2 DMRs were hypermethylated in both PSCNT and control fetuses. Our results suggest that extended development of the porcine parthenogenetic fetus can be accomplished using PSCNT and that abnormal DNA methylation of H19 DMRs might contribute to the critical barrier of parthenogenesis in pigs.

Autophagy and apoptosis: parent-of-origin genome-dependent mechanisms of cellular self-destruction

Functional genomic imprinting is necessary for the transfer of maternal resources to mammalian embryos. Imprint-free embryos are unable to establish a viable placental vascular network necessary for the transfer of resources such as nutrients and oxygen. How the parental origin of inherited genes influences cellular response to resource limitation is currently not well understood. Because such limitations are initially realized by the placenta, we studied how maternal and paternal genomes influence the cellular self-destruction responses of this organ specifically. Here, we show that cellular autophagy is prevalent in androgenetic (i.e. having only a paternal genome) placentae, while apoptosis is prevalent in parthenogenetic (i.e. having only a maternal genome) placentae. Our findings indicate that the parental origin of inherited genes determines the placenta's cellular death pathway: autophagy for androgenotes and apoptosis for parthenogenotes. The difference in time of arrest between androgenotes and parthenogenotes can be attributed, at least in part, to their placentae's selective use of these two cell death pathways. We anticipate our findings to be a starting point for general studies on the parent-of-origin regulation of autophagy. Furthermore, our work opens the door to new studies on the involvement of autophagy in pathologies of pregnancy in which the restricted transfer of maternal resources is diagnosed.

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.

Analysis of imprinted gene expression and implantation in haploid androgenetic mouse embryos

The successful development of mammalian embryos requires both parental genomes. Nuclear transfer techniques have been adapted to generate uniparental embryos, which possess two sets of paternal or maternal genomes. These embryos fail to develop to term because of abnormal imprinted gene expression, which is not regulated by Mendelian inheritance. Uniparental embryos provide us with an important model to investigate imprinted gene function and ontogenesis. To evaluate the pre- and post-developmental ability of haploid androgenetic mouse embryos, and to analyse the expression of imprinted genes Igf2r, Asb4 and Mest in haploid androgenetic/gynogenetic blastocysts, we produced the haploid mouse embryos using the enucleation technique, examined their development at 6.5 dpc and quantified gene expression by quantitative real-time PCR. The results demonstrated that the developmental potential of haploid embryos was severely impaired and revealed that the haploid androgenones could induce the deciduas reaction, but failed to retain a live foetus at 6.5 dpc. Expression of imprinted genes Igf2r and Asb4 was unregulated in haploid androgenetic/gynogenetic blastocysts.

Comparative expression analysis of embryonic development-related genes at different stages of parthenogenetic and in vitro fertilized embryos in caprine

Aberrant gene expression occurs in parthenogenetic embryos due to abnormal epigenetic modifications in the genome that probably diminish viability and enhance developmental abnormalities in these embryos. In the present study, five developmentally important genes (HPRT1, Cx43, Sox2, Mest and IGF2R) were analysed at different stages in parthenotes (haploid and diploid) and compared with similar stages in in vitro fertilized (IVF) embryos. The results indicated that in haploid parthenotes expression of HPRT1 was upregulated (P < 0.05) only at the 2-4-cell stage whereas Cx43 expression was significantly (P < 0.05) downregulated in all stages as compared with the control. However, expression of this gene was upregulated (P < 0.05) in 2-4-cell and morula stages of diploid parthenotes. Expression of Sox2 was significantly (P < 0.05) downregulated in morula stage haploid parthenotes, whereas it was upregulated (P < 0.05) in 8-16-cell stage diploid embryos. The expression of Mest was upregulated (P < 0.05) at the 2-4-cell stage of both haploid and diploid parthenotes, whereas it was downregulated in 8-16-cell stage diploid embryos as compared with control. IGF2R expression was upregulated (P < 0.05) only in morula stage haploid and diploid parthenote as compared with control. These results indicate that parthenogenetic embryos showed aberrant gene expression of developmentally important genes such as HPRT1, Cx43, Sox2, Mest and IGF2R in comparison with IVF embryos, this finding may be one of the major reasons for the poor developmental competence of parthenogenetic embryos.

Genomic imprinting in farm animals

The mouse is the first species in which genomic imprinting was studied. Imprinting research in farm species has lagged behind owing to a lack of sequencing and genetic background information, as well as long generation intervals and high costs in tissue collection. Since the creation of Dolly, the first cloned mammal from an adult sheep, studies on genomic imprinting in domestic species have accelerated because animals from cloning and other assisted reproductive technologies exhibit phenotypes of imprinting disruptions. Although this review focuses on new developments in farm animals, most of the imprinting mechanism information was derived from the mouse.

In vivo and in vitro differentiation of uniparental embryonic stem cells into hematopoietic and neural cell types

The biological role of genomic imprinting in adult tissue is central to the consideration of transplanting uniparental embryonic stem (ES) cell-derived tissues. We have recently shown that both maternal (parthenogenetic/gynogenetic) and paternal (androgenetic) uniparental ES cells can differentiate, both in vivo in chimeras and in vitro, into adult-repopulating hematopoietic stem and progenitor cells. This suggests that, at least in some tissues, the presence of two maternal or two paternal genomes does not interfere with stem cell function and tissue homeostasis in the adult. Here, we consider implications of the contribution of uniparental cells to hematopoiesis and to development of other organ systems, notably neural tissue for which consequences of genomic imprinting are associated with a known bias in development and behavioral disorders. Our findings so far indicate that there is little or no limit to the differentiation potential of uniparental ES cells outside the normal developmental paradigm. As a potentially donor MHC-matching source of tissue, uniparental transplants may provide not only a clinical resource but also a unique tool to investigate aspects of genomic imprinting in adults.

Altered cell cycle gene expression and apoptosis in post-implantation dog parthenotes

Mature oocytes can be parthenogenetically activated by a variety of methods and the resulting embryos are valuable for studies of the respective roles of paternal and maternal genomes in early mammalian development. In the present study, we report the first successful development of parthenogenetic canine embryos to the post-implantation stage. Nine out of ten embryo transfer recipients became pregnant and successful in utero development of canine parthenotes was confirmed. For further evaluation of these parthenotes, their fetal development was compared with artificially inseminated controls and differentially expressed genes (DEGs) were compared using ACP RT-PCR, histological analysis and immunohistochemistry. We found formation of the limb-bud and no obvious differences in histological appearance of the canine parthenote recovered before degeneration occurred; however canine parthenotes were developmentally delayed with different cell cycle regulating-, mitochondria-related and apoptosis-related gene expression patterns compared with controls. In conclusion, our protocols were suitable for activating canine oocytes artificially and supported early fetal development. We demonstrated that the developmental abnormalities in canine parthenotes may result from defective regulation of apoptosis and aberrant gene expression patterns, and provided evidence that canine parthenotes can be a useful tool for screening and for comparative studies of imprinted genes.

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.

Epigenetically immature oocytes lead to loss of imprinting during embryogenesis

Loss of imprinting (LOI) is occasionally observed in human imprinting disorders. However, the process behind the LOI is not fully understood. To gain a better understanding, we produced embryos and pups from mouse oocytes that lacked a complete methylation imprint using a method that involved transferring the nuclei of growing oocytes into the cytoplasm of enucleated fully grown oocytes following in vitro fertilization (IVF). We then analyzed the imprinting statuses. Our findings show that the incomplete methylation imprint derived from growing oocytes results in epigenetic mosaicism or a loss of methylation imprint (LOM) at maternal alleles in embryos. In some embryos, both hypo- and hypermethylated maternal Kcnq1ot1 alleles were detected, whereas either hypo- or hypermethylated maternal Kcnq1ot1 alleles were detected in others. Such tendencies were also observed at the Igf2r and Mest loci. Gene expression levels of imprinted genes were linked with their methylation statuses in some but not all embryos. Possible explanations of the inconsistency between the data from DNA methylation and gene expression include epigenetic mosaicism in embryos. Pups were successfully produced from growing oocytes at a quite low frequency. They exhibited an obese phenotype and LOI with respect to Igf2r, Snrpn and Mest. Our finding suggests the possibility that LOI/LOM at maternal alleles in human concepti could be derived from epigenetically immature/mutated oocytes.

Parthenogenetic chimaerism/mosaicism with a Silver-Russell syndrome-like phenotype

Introduction

We report a 34-year-old Japanese female with a Silver-Russell syndrome (SRS)-like phenotype and a mosaic Turner syndrome karyotype (45,X/46,XX).

Methods/Results

Molecular studies including methylation analysis of 17 differentially methylated regions (DMRs) on the autosomes and the XIST-DMR on the X chromosome and genome-wide microsatellite analysis for 96 autosomal loci and 30 X chromosomal loci revealed that the 46,XX cell lineage was accompanied by maternal uniparental isodisomy for all chromosomes (upid(AC)mat), whereas the 45,X cell lineage was associated with biparentally derived autosomes and a maternally derived X chromosome. The frequency of the 46,XX upid(AC)mat cells was calculated as 84% in leukocytes, 56% in salivary cells, and 18% in buccal epithelial cells.

Discussion

The results imply that a parthenogenetic activation took place around the time of fertilisation of a sperm missing a sex chromosome, resulting in the generation of the upid(AC)mat 46,XX cell lineage by endoreplication of one blastomere containing a female pronucleus and the 45,X cell lineage by union of male and female pronuclei. It is likely that the extent of overall (epi)genetic aberrations exceeded the threshold level for the development of SRS phenotype, but not for the occurrence of other imprinting disorders or recessive Mendelian disorders.

Generation of histocompatible androgenetic embryonic stem cells using spermatogenic cells

Androgenetic embryonic stem (aES) cells, produced by pronuclear transplantation, offer an important autologous pluripotent stem cell source. However, the isolation of aES cells, particularly individual-specific aES cells, with the use of fertilized embryos has limited the practical applications of this technology in humans. In this study, we applied a new approach, essentially described as somatic cell nuclear transfer, and generated three aES cell line types with the use of spermatogenic cells including primary spermatocytes, round spermatids, and mature spermatozoa as donor cells, omitting the need to use fertilized embryos. Although abnormality of chimeras and absent germline competency indicated that all three types of aES cells exhibited limited pluripotency, the epigenetic status of the aES cell lines tended to resemble normal ES cells during long-term culture, and some parental-specific imprinted genes were expressed at levels comparable to those of normal ES cells. Furthermore, the histocompatibility of the aES cells was investigated by transplanting the differentiation progenies of the aES cells into major histocompatibility (MHC)-matched and -mismatched recipient mice. The results indicated that these aES cells were histocompatible with MHC-matched mice after transplantation. Our study provides evidence that MHC-competent autologous aES cells could be generated from different spermatogenic cells using nuclear transfer into oocytes, a process that could avoid the use of fertilized embryos.

Upregulation of imprinted genes in mice: an insight into the intensity of gene expression and the evolution of genomic imprinting

Imprinted genes are expressed monoallelically because one of the two copies is silenced epigentically in a parent-of-origin pattern. This pattern of expression is controlled by differential marking of parental alleles by DNA methylation and chromatin modifications, including both suppressive and permissive histone acetylation and methylation. Suppressive histone modifications mark silenced alleles of imprinted genes, while permissive histone modifications mark the active alleles, suggesting the possibility that imprinted genes would show upregulation in gene expression. However, it is currently unknown whether imprinted genes show such upregulation. To address this question in mice, we estimated the intensity of expression of 59 genes relative to the rest of the genome by analyzing microarray data. Expression levels of 24 genes were validated using quantitative real-time PCR (qPCR). Expression of imprinted genes was found to be upreguled in various adult and embryonic mouse tissues. Consistent with their functions in growth and development, imprinted genes were found to be highly expressed in extraembryonic tissues and progressively upregulated during early embryonic development. In conclusion, upregulation of imprinted genes found in this study is similar to the dosage compensation (twofold upregulation) recently reported for X-linked genes. It has been proposed that the twofold upregulation of X-linked genes has been coupled with low transcriptional variation (noise) which could lead to deleterious effects on the organism. Results of this study suggest a general need for imprinted genes in the mouse to be upregulated to certain levels in order to avoid deleterious effects of variation in gene expression.

Involvement of insulin-like growth factor 2 in angiogenic factor transcription in Bi-maternal mouse conceptuses

Imprinted genes in which only one of the two parental chromosome copies is expressed have a substantial effect on mammalian ontogenesis. On mouse distal chromosome 7, the paternally expressed gene insulin-like growth factor 2 (Igf2) is separated by approximate 100 kb from the maternally expressed non-coding gene H19. However, there is limited knowledge of the manner in which Igf2 transcription affects the other genes involved in embryonic development. To clarify this, we performed quantitative gene expression analysis for representative angiogenic factors-Vegf, Flt1, Flt4, Flk1, Ang1, Ang2, Tie1, and Tie2-for 3 types of bi-maternal conceptuses containing genomes with non-growing (ng) and fully grown (fg) oocytes. The genetic backgrounds of the ng oocytes were 1) the wild type (ng(wt)), 2) mutant mice carrying a 3-kb deletion of the H19 transcription unit (ng(H19Delta3-KO)/fg) and 3) mutant mice carrying a 13-kb deletion in the H19 transcription unit, including the germline-derived differentially methylated region on chromosome 7 (ng(H19Delta13-KO)/fg). In the ng(wt)/fg and ng(H19Delta3-KO)/fg placentae, Vegf and Flt1 were upregulated compared with the mean value for the wt placenta, whereas in the ng(H19Delta13-KO)/fg placenta, these transcriptional levels were restored. In the fetus, however, only 2 genes among the 8 genes analyzed were significantly changed in the bi-maternal fetuses, indicating that the effects of the Igf2 mRNA level on angiogenic factor transcription in the fetus differed from those in the placenta. Our results indicated that the Igf2 mRNA level affects transcription of angiogenic factors in both bi-maternal placentae and fetuses.

Proteomic analysis of parthenogenetic and in vitro fertilized porcine embryos

Proteomic data from embryos are essential for the completion of whole proteome catalog due to embryo-specific expression of certain proteins. In this study, using reverse phase LC-MS/MS combined with 1-D SDS-PAGE, we identified 1625 mammalian and 735 Sus scrofa proteins from porcine zygotes that included both cytosolic and membranous proteins. We also found that the global protein profiles of parthenogenetically activated (PA) and in vitro fertilized (IVF) zygotes were similar but differences in expression of individual proteins were also evident. These differences were not due to culture conditions, polyspermy or non-activation of oocytes, as the same culture method was used in both groups, the frequency of polyspermy was 24.3+/-3.0% and the rates of oocyte activation did not differ (p>0.05) between PA and IVF embryos. Consistent with proteomic data, fluorescent Hoechst 33 342 staining and terminal deoxynucleotidyl transferase dUTP nick end labeling assay also revealed that PA embryos were of poor quality as they contained less cells per blastocyst and were more predisposed to apoptosis (p<0.05), although their in vitro development rates were similar. To our knowledge, this is the first report on global peptide sequencing and quantification of protein in PA and IVF embryos by LC-MS/MS that may be useful as a reference map for future studies.

Developmental ability of trophoblast stem cells in uniparental mouse embryos

Neither parthenogenetic (PG) nor androgenetic (AG) mouse embryos survive after day 9.5 of pregnancy, owing to the inadequate growth of extraembryonic tissues, including the placenta. At day 9.5 of pregnancy, the placental structures are poorly developed in PG embryos, while trophoblast giant cells are abundant at the implantation site in AG embryos. These findings suggest that both parental genomes are required for placental development. To gain further insight into the trophoblast lineage in PG and AG embryos, we attempted to derive trophoblast stem (TS)-like cell lines from uniparental embryos. Furthermore, we sought to assess their ability to differentiate into cells of the trophoblast lineage by using gene expression analysis. Three cell lines that expressed marker genes for undifferentiated TS cells (Cdx2 and Errbeta) were derived from AG embryos. Under differentiation conditions, these cells expressed the trophoblast giant cell-specific genes, but did not express the spongiotrophoblast-specific genes. In contrast, none of the four cell lines from PG embryos expressed marker genes for undifferentiated TS cells, but they expressed Oct3/4, a marker gene for embryonic stem cells. Immunohistochemical analysis indicated that PG blastocysts expressed Oct3/4 and Cdx2 specifically in inner cell mass and the trophectoderm respectively. These results suggest that PG embryos do not possess TS cells, because of the lack of the developmental ability of trophoblast cells.

Correlation of expression and methylation of imprinted genes with pluripotency of parthenogenetic embryonic stem cells

Mammalian parthenogenetic embryos (pE) are not viable due to placental deficiency, presumably resulting from lack of paternally expressed imprinted genes. Pluripotent parthenogenetic embryonic stem (pES) cells derived from pE could advance regenerative medicine by avoiding immuno-rejection and ethical roadblocks. We attempted to explore the epigenetic status of imprinted genes in the generation of pES cells from parthenogenetic blastocysts, and its relationship to pluripotency of pES cells. Pluripotency was evaluated for developmental and differentiation potential in vivo, based on contributions of pES cells to chimeras and development to day 9.5 of pES fetuses complemented by tetraploid embryos (TEC). Consistently, pE and fetuses failed to express paternally expressed imprinted genes, but pES cells expressed those genes in a pattern resembling that of fertilized embryos (fE) and fertilized embryonic stem (fES) cells derived from fE. Like fE and fES cells, but unlike pE or fetuses, pES cells and pES cell-fetuses complemented by TEC exhibited balanced methylation of Snrpn, Peg1 and U2af1-rs1. Coincidently, global methylation increased in pE but decreased in pES cells, further suggesting dramatic epigenetic reprogramming occurred during isolation and culture of pES cells. Moreover, we identified decreased methylation of Igf2r, Snrpn, and especially U2af1-rs1, in association with increased contributions of pES cells to chimeras. Our data show that in vitro culture changes epigenetic status of imprinted genes during isolation of pES cells from their progenitor embryos and that increased expression of U2af1-rs1 and Snrpn and decreased expression of Igf2r correlate with pluripotency of pES cells.

Analysis of the expression of putatively imprinted genes in bovine peri-implantation embryos

The application of assisted reproductive technologies (ART) has been shown to induce changes in the methylation of the embryonic genome, leading to aberrant gene expression, including that of imprinted genes. Aberrant methylation and gene expression has been linked to the large offspring syndrome (LOS) in bovine embryos resulting in increased embryonic morbidity and mortality. In the bovine, limited numbers of imprinted genes have been studied and studies have primarily been restricted to pre-implantation stages. This study reports original data on the expression pattern of 8 putatively imprinted genes (Ata3, Dlk1, Gnas, Grb10, Magel2, Mest-1, Ndn and Sgce) in bovine peri-implantation embryos. Two embryonic developmental stages were examined, Day 14 and Day 21. The gene expression pattern of single embryos was recorded for in vivo, in vitro produced (IVP) and parthenogenetic embryos. The IVP embryos allow us to estimate the effect of in vitro procedures and the analysis of parthenogenetic embryos provides provisional information on maternal genomic imprinting. Among the 8 genes investigated, only Mest-1 showed differential expression in Day 21 parthenogenetic embryos compared to in vivo and IVP counterparts, indicating maternal imprinting of this gene. In addition, our expression analysis of single embryos revealed a more heterogeneous gene expression in IVP than in in vivo developed embryos, adding further to the hypothesis of transcriptional dysregulation induced by in vitro procedures, either by in vitro maturation, fertilization or culture. In conclusion, effects of genomic imprinting and of in vitro procedures for embryo production may influence the success of bovine embryo implantation.

Diploid parthenogenetic embryos adopt a maternal-type methylation pattern on both sets of maternal chromosomes

Epigenetic modifications are closely associated with embryo developmental potential. One of the epigenetic modifications thought to be involved in genomic imprinting is DNA methylation. Here we show that the maternally imprinted genes Snrpn and Peg1/Mest were nearly unmethylated or heavily methylated, respectively, in their differentially methylated regions (DMRs) at the two-cell stage in parthenogenetic embryos. However, both genes were gradually de novo methylated, with almost complete methylation of all CpG sites by the morula stage in parthenogenetic embryos. Unexpectedly, another maternally imprinted gene, Peg3, showed distinct dynamics of methylation during preimplantation development of diploid parthenogenetic embryos. Peg3 showed seemingly normal methylation patterns at the two-cell and morula stages, but was also strongly de novo methylated in parthenogenetic blastocysts. In contrast, the paternally imprinted genes H19 and Rasgrf1 showed complete unmethylation of their DMRs at the morula stage in parthenogenetic embryos. These results indicate that diploid parthenogenetic embryos adopt a maternal-type methylation pattern on both sets of maternal chromosomes and that the aberrantly homogeneous status of methylation imprints may partially account for developmental failure.

Appropriate expression of imprinted genes on mouse chromosome 12 extends development of bi-maternal embryos to term

Recently, we reported that the restored regulation of imprinted gene expression from two regions -H19 differentially methylated region (H19-DMR) and intergenic germline-derived DMR (IG-DMR) - is sufficient for accomplishing full-term development in mice. In the present study, we determined the developmental ability of the bi-maternal embryos (BMEs) containing the non-growing oocyte genome with the IG-DMR deletion (ng(Deltach12)) and fully-grown (fg) oocyte genome. Foetuses derived from ng(Deltach12)/fg BMEs were alive at E19.5 but could not survive further. Comparison with BMEs derived from Igf2+/- ng/fg genomes suggests that bi-allelic H19 expression might be involved in foetal development.

Coordinated diurnal regulation of genes from the Dlk1–Dio3 imprinted domain: implications for regulation of clusters of non-paralogous genes

The functioning of the genome is tightly related to its architecture. Therefore, understanding the relationship between different regulatory mechanisms and the organization of chromosomal domains is essential for understanding genome regulation. The majority of imprinted genes are assembled into clusters, share common regulatory elements, and, hence, represent an attractive model for studies of regulation of clusters of non-paralogous genes. Here, we investigated the relationship between genomic imprinting and diurnal regulation of genes from the imprinted domain of mouse chromosome 12. We compared gene expression patterns in C57BL/6 mice and congenic mice that carry the imprinted region from a Mus musculus molossinus strain MOLF/Ei. In the C57BL/6 mice, a putative enhancer/oscillator regulated the expression of only Mico1/Mico1os, whereas in the congenic mice its influence was spread onto Rtl1as, Dio3 and Dio3os, i.e. the distal part of the imprinted domain, resulting in coordinated diurnal variation in expression of five genes. Using additional congenic strains we determined that in C57BL/6 the effect of the putative enhancer/oscillator was attenuated by a linked dominant trans-acting factor located in the distal portion of chromosome 12. Our data demonstrate that (i) in adult organs, mRNA levels of several imprinted genes vary during the day, (ii) genetic variation may remove constraints on the influence of an enhancer and lead to spreading of its effect onto neighboring genes, thereby generating genotype-dependent expression patterns and (iii) different regulatory mechanisms within the same domain act independently and do not seem to interfere with each other.

Comparative genomic imprinting and expression analysis of six cattle genes

Imprinted genes are monoallelically expressed in a parent-of-origin-specific manner under epigenetic regulation. Although it is generally believed that genomic imprinting is conserved among mammalian species, there is accumulating evidence that suggests such an assumption is false. Identification of species-specific imprinted genes is necessary to understand the evolution of genomic imprinting and to elucidate mechanisms leading to allele-specific expression. In this study, we analyzed the imprinting status of the CD81, target of antiproliferation antibody 1 (TSSC4), and oxysterol-binding protein homologue 1 (OBPH1) genes clustered on bovine chromosome 29; the paternally expressed gene 10 and ankyrin repeat and suppressor of cytokine signaling box-containing protein 4 genes clustered on bovine chromosome 4; and the 5-hydroxytryptamine (serotonin) 2A receptor microdomain gene on bovine chromosome 12 using a sequencing-based approach. It was found that CD81 and OBPH1 showed biallelic expression in all cattle tissues examined, whereas TSSC4 showed monoallelic expression in placental tissues, like its mouse ortholog. Comparative expression analysis showed that the imprinting pattern of the CD81, TSSC4, and OBPH1 cluster was not conserved among mouse, human, and cattle. None of these genes were imprinted in all 3 species. The the paternally expressed gene 10 gene was imprinted in all 3 species, whereas ankyrin repeat and suppressor of cytokine signaling box-containing protein 4 gene, reported to be imprinted in mouse, was not imprinted in cattle. The the 5-hydroxytryptamine (serotonin) 2A receptor gene was not imprinted in cattle, and human imprinting data has shown conflicting results. It is more likely that imprinting in the genes examined in this study is species-specific. In addition, we studied the expression and tissue distribution of transcripts of these genes in 174 fetal and adult cattle tissues.