Influence of oocyte nuclei on demethylation of donor genome in cloned bovine embryos

Early and delayed aspects of nuclear reprogramming during cloning

The successful production of viable progeny following adult somatic cell nuclear transfer (cloning) provides exciting new opportunities for basic research for investigating early embryogenesis, for the propagation of valuable or endangered animals, for the production of genetically engineered animals, and possibly for developing therapeutically valuable stem cells. Successful cloning requires efficient reprogramming of gene expression to silence donor cell gene expression and activate an embryonic pattern of gene expression. Recent observations indicate that reprogramming may be initiated by early events that occur soon after nuclear transfer, but then continues as development progresses through cleavage and probably to gastrulation. Because reprogramming is slow and progressive, cloned embryos have dramatically altered characteristics in comparison with fertilized embryos. Events that occur early following nuclear transfer may be essential prerequisites for the later events. Additionally, the later reprogramming events may be inhibited by sub-optimum culture environments that exist because of the altered characteristics of cloned embryos. By addressing the unique requirements of cloned embryos, the entire process of reprogramming may be accelerated, thus increasing cloning efficiency.

Death losses due to stillbirth, neonatal death and diseases in cloned cattle derived from somatic cell nuclear transfer and their progeny: a result of nationwide survey in Japan

To obtain the data concerning death losses due to stillbirth, neonatal death and diseases in cloned cattle derived from somatic cell nuclear transfer (SCNT) and their progeny produced by Japanese institutions, a nationwide survey was carried out in July-August, 2006. As a result, lifetime data concerning 482 SCNT cattle (97.5% of cattle produced in the country at that time) and 202 progeny of SCNT cattle were accumulated and the death loss of these cattle was analyzed. Although 1/3 of delivered SCNT calves died during the perinatal period due to stillbirth and neonatal death, incidence of death loss due to diseases in SCNT cattle surviving more than 200 days after birth seems to be the same as these in conventionally bred cattle. In contrast, progeny of SCNT cattle showed the same level in death loss as observed in conventionally bred cattle throughout their lifetime. These results suggest that robust health would be expected in SCNT cattle surviving to adulthood and their progeny.

Modifications of chemically induced-enucleated nuclear transfer technique by reverse-order nuclear transfer in mouse

To improve the developmental potential of somatic cell cloned embryos derived from demecolcine (DC) induced-enucleated nuclear transfer (INT), we modified the INT procedures by transferring donor nuclei into recipient cytoplasts prior to the induced enucleation of the recipient cytoplasts, and we called this modified INT technique as reverse-order and induced-enucleated nuclear transfer (RINT). Standard nuclear transfer (SNT) and INT were performed as controls. The dynamic changes of maternal and transferred donor nuclei in the RINT oocytes were monitored to evaluate the feasibility of this new nuclear transfer (NT) technique by timed immunofluorescence. Timed immunofluorescence showed that RINT is feasible because none of the transferred donor nuclei were expelled with the second polar body (Pb) in the RINT oocytes, while 42.2% of the oocytes showed extrusion of all maternal chromosome and spindles with the second Pb at 60 min after activation and DC treatment. Although there was no difference in cleavage rate (86.6% vs. 82.1%), the rates of successful enucleation and blastocyst formation were significantly increased in RINT compared with INT (44.1% vs. 27.5% and 43.3% vs. 12.8%, respectively; p < 0.01). Compared with SNT, there was no difference in cleavage rate (86.6% vs. 78.4%), but the blastocyst developmental rate was significantly increased in the RINT group (43.3% vs. 25.3%; p < 0.01). Blastocysts derived from RINT had a higher total cell number than those from SNT (45.1 +/- 3 vs. 37.6 +/- 4; p < 0.05). Our results provide evidence that RINT is feasible and may provide a more efficient and simple method for NT than INT.

Methylation status of putative differentially methylated regions of porcine IGF2 and H19

This study was designed to identify the putative differentially methylated regions (DMRs) of the porcine imprinted genes insulin-like growth factor 2 and H19 (IGF2-H19), and to assess the genomic imprinting status of IGF2-H19 by identifying the methylation patterns of these regions in germ cells, and in tissues from porcine fetuses, an adult pig, as well as cloned offspring produced by somatic cell nuclear transfer (SCNT). Porcine IGF2-H19 DMRs exhibit a normal monoallelic methylation pattern (i.e., either the paternally- or the maternally derived allele is methylated) similar to the pattern observed for the same genes in the human and mice genomes. Examination of the methylation patterns of the IGF2-H19 DMRs revealed that the zinc finger protein binding sites CTCF1 and 2 did not exhibit differential methylation in both control and cloned offspring. In contrast, the CTCF3 and DMR2 loci of the IGF2 gene showed abnormal methylation in cloned offspring, but a normal differential or moderate methylation pattern in tissues from control offspring and an adult pig. Our data thus suggest that regulation of genomic imprinting at the porcine IGF2-H19 loci is conserved among species, and that the abnormal methylation pattern in the regulatory elements of imprinted genes may lead to an alteration in the coordinated expression of genes required for successful reprogramming, which, in consequence, may contribute to the low efficiency of porcine genome reprogramming induced by nuclear transfer.

Nuclear remodeling and nuclear reprogramming for making transgenic pigs by nuclear transfer

A better understanding of the cellular and molecular events that occur when a nucleus is transferred to the cytoplasm of an oocyte will permit the development of improved procedures for performing nuclear transfer and cloning. In some cases it appears that the gene(s) are reprogrammed, while in other cases there appears to be little effect on gene expression. Not only does the pattern of gene expression need to be reprogrammed, but other structures within the nucleus also need to be remodeled. While nuclear transfer works and transgenic and knockout animals can be created, it still is an inefficient process. However, even with the current low efficiencies this technique has proved very valuable for the production of animals that might be useful for tissue or organ transplantation to humans.

Diploidized eggs reprogram adult somatic cell nuclei to pluripotency in nuclear transfer in medaka fish (Oryzias latipes)

Reprogramming of adult somatic cell nuclei to pluripotency has been unsuccessful in non-mammalian animals, primarily because of chromosomal aberrations in nuclear transplants, which are considered to be caused by asynchrony between the cell cycles of the recipient egg and donor nucleus. In order to normalize the chromosomal status, we used diploidized eggs by retention of second polar body release, instead of enucleated eggs, as recipients in nuclear transfer of primary culture cells from the caudal fin of adult green fluorescent protein gene (GFP) transgenic medaka fish (Oryzias latipes). We found that 2.7% of the reconstructed embryos grew into adults that expressed GFP in various tissues in the same pattern as in the donor fish. Moreover, these fish were diploid, fertile and capable of passing the marker gene to the next generation in Mendelian fashion. We hesitate to call these fish 'clones' because we used non-enucleated eggs as recipients; in effect, they may be chimeras consisting of cells derived from diploid recipient nuclei and donor nuclei. In either case, fish adult somatic cell nuclei were reprogrammed to pluripotency and differentiated into a variety of cell types including germ cells via the use of diploidized recipient eggs.

Epigenetic dedifferentiation of somatic cells into pluripotency: cellular alchemy in the age of regenerative medicine?

Ever since the derivation of the first human embryonic stem cell line, hopes have persisted for the treatment of a wide range of cellular degenerative diseases. However, significant immuno-incompatibility between donor cells and recipient patients remains an unsolved challenge. Currently, three main strategies are investigated in humans to create autologous pluripotent stem cells: somatic cell nuclear transfer, cell fusion and cell extract incubation. All methods exploit the fact that a somatic genome is amenable to epigenetic dedifferentiation into a more plastic state, presumably through direct exposure to and manipulation by heterologous transcriptional factors. Epigenetic reprogramming includes profound modifications of chromatin structure, but the responsible mechanisms that work in toti- and pluripotent cells remain largely unknown. This review presents a brief introduction to stem cell terminology and epigenetics, followed by a critical examination of the predominant methodologies involved. Finally, the search for specific reprogramming factors is discussed, and obstacles for the clinical implementation of reprogrammed cells are addressed.

Somatic cell nuclei in cloning: strangers traveling in a foreign land

The recent successes in producing cloned offspring by somatic cell nuclear transfer are nothing short of remarkable. This process requires the somatic cell chromatin to substitute functionally for both the egg and the sperm genomes, and indeed the processing of the transferred nuclei shares aspects in common with processing of both parental genomes in normal fertilized embryos. Recent studies have yielded new information about the degree to which this substitution is accomplished. Overall, it has become evident that multiple aspects of genome processing and function are aberrant, indicating that the somatic cell chromatin only infrequently manages the successful transition to a competent surrogate for gamete genomes. This review focuses on recent results revealing these limitations and how they might be overcome.

The US FDA and animal cloning: Risk and regulatory approach

The Food and Drug Administration's (FDA's) Center for Veterinary Medicine issued a voluntary request to producers of livestock clones not to introduce food from clones or their progeny into commerce until the agency had assessed whether production of cattle, swine, sheep, or goats by somatic cell nuclear transfer (SCNT) posed any unique risks to the animal(s) involved in the process, humans, or other animals by consuming food from those animals, compared with any other assisted reproductive technology (ART) currently in use. Following a comprehensive review, no anomalies were observed in animals produced by cloning that have not also been observed in animals produced by other ARTs and natural mating. Further systematic review on the health of, and composition of meat and milk from, cattle, swine, and goat clones and the progeny of cattle and sheep did not result in the identification of any food-consumption hazards. The agency therefore concluded that food from cattle, swine, and goat clones was as safe to eat as food from animals of those species derived by conventional means. The agency also concluded that food from the progeny of the clone of any species normally consumed for food is as safe to eat as those animals. The article also describes the methodology used by the agency to analyze data and draw these conclusions, the plans the agency has proposed to manage any identified risks, and the risk communication approaches the agency has used.

Comparison of the Growth Performances of Offspring Produced by a Pair of Cloned Cattle and Their Nuclear Donor Animals

In the present study, the growth performance of a calf produced by mating a somatic cell cloned dam and sire was compared with that of its full siblings produced by mating the cattle used as nuclear donors for the cloned animals. The somatic cell cloned dam and sire were derived from cultured cumulus cells and ear cells, respectively. The cloned dam was artificially inseminated with semen from the cloned sire. A female calf was produced that was reared under general group feeding conditions. The calf was subjected to a clinical examination and to hematology, serum biochemistry, and telomere length analyses; all of these tests indicated that the calf was normal. The growth characteristics (body weight and shoulder height) of the calf fell within the range of the full siblings of the same sex produced by mating the animals used as the nuclear donors of clones. These findings suggest that the same breeding performance is expected from mating a cloned dam and sire as from mating the animals used as nuclear donors for the clones.

Animal cloning and the FDA—the risk assessment paradigm under public scrutiny

The evidence gathered thus far--ultimately to be published in the Draft Risk Assessment on Animal Cloning--indicates that there are no unique risks associated with animal cloning.

Interspecies somatic cell nuclear transfer and preliminary data for horse-cow/mouse iSCNT

Nuclear transfer (NT) experiments in mammals have demonstrated that adult cells are genetically equivalent to early embryonic cells and the reversal of the differentiated state of a cell to another that has characteristics of the undifferentiated embryonic state can be defined as nuclear reprogramming. The feasibility of interspecies somatic cell NT (iSCNT) has been demonstrated by blastocyst formation and the production of offspring in a number of studies. Embryo and oocyte availability is a major limiting factor in conducting NT to obtain, blastocysts for both reproductive NT studies in genetically endangered animals and in embryonic stem cell derivation for species such as the horse and human. One approach to generate new embryonic stem cells in human as disease models, or in species where embryos and oocytes are not widely available, is to use oocytes from another species. Utilization of oocytes for recipient cytoplasts from other species that are accessible and abundant, such as the cow and rabbit, would greatly benefit ongoing research on reprogramming and stem cell sciences. The use of iSCNT is an exciting possibility for species with limited availability of oocytes as well as for endangered or exotic species where assisted reproduction is needed. However, the mechanisms involved in nuclear reprogramming by the oocyte are still unknown and the extent of the "universality" of ooplasmic reprogramming of development remains under investigation.

Active demethylation of individual genes in intracytoplasmic sperm injection rabbit embryos

Intracytoplasmic sperm injection (ICSI), as an assisted reproduction technique, has been widely used in animal and human. However, its possible effect on epigenetic changes has not been well studied. To investigate whether ICSI can induce aberrant DNA methylation changes in rabbit preimplantation embryos, we examined the methylation status of the SP-A promoter region and the satellite sequence Rsat IIE by bisulfite-sequencing technology. The SP-A promoter region was extensively demethylated before the first round of DNA replication commences, and the unmethylated status was maintained until morula when dynamic remethylation occurred. A similar but more moderate demethylation process was observed in satellite sequence Rsat IIE. These results are in contrast with the previous reports of no active demethylation in normal rabbit embryos, suggesting that the active demethylation we observed may be induced by ICSI.

The significance of mitochondria for embryo development in cloned farm animals

The role of mitochondria in remodeling of the donor cell nucleus in cloned animals has gained increased attention, as mitochondria interact in direct or indirect ways with the donor cell nuclear DNA. Mitochondria comprise 1% of the genetic material that is contributed to the developing embryo by the recipient oocyte and provide the energy that is required for embryo development. In this review we compare mitochondria distribution in various species and the importance of mitochondria distribution for embryo development. We also compare the inheritance pattern of mitochondria in cloned embryos that remains unresolved, as the donor cell nucleus is typically transferred with surrounding cytoplasm including mitochondria which become destroyed in some but not all species. We review the role of mitochondria in cloned farm animals with emphasis on nucleo-cytoplasmic interactions and consequences for embryo development.

X chromosome reactivation and regulation in cloned embryos

Somatic cell nuclear transfer embryos exhibit extensive epigenetic abnormalities, including aberrant methylation and abnormal imprinted gene expression. In this study, a thorough analysis of X chromosome inactivation (XCI) was performed in both preimplantation and postimplantation nuclear transfer embryos. Cloned blastocysts reactivated the inactive somatic X chromosome, possibly in a gradient fashion. Analysis of XCI by Xist RNA and Eed protein localization revealed heterogeneity within cloned embryos, with some cells successfully inactivating an X chromosome and others failing to do so. Additionally, a significant proportion of cells contained more than two X chromosomes, which correlated with an increased incidence of tetraploidy. Imprinted XCI, normally found in preimplantation embryos and extraembryonic tissues, was not observed in blastocysts or placentae from later stage clones, although fetuses recapitulated the Xce effect. We conclude that, although SCNT embryos can reactivate, count, and inactivate X chromosomes, they are not able to regulate XCI consistently. These results illustrate the heterogeneity of epigenetic changes found in cloned embryos.

Maternal and environmental factors in early cloned embryo development

Cloning by somatic cell nuclear transfer (SCNT) in mammals has revealed the remarkable ability of an oocyte to reprogram somatic cell nuclei and induce them to recapitulate the developmental program. Despite the success, cloning remains very inefficient. This review summarizes recent observations from cloning in mice that reveal some of the likely causes for the present inefficiency. One cause appears to be the slow pace of reprogramming combined with the early onset of genome transcription, which together cause cloned embryos to elaborate many somatic cell characteristics even before the first cleavage division. The altered phenotypes of cloned embryos render standard embryo culture conditions grossly sub-optimum. Another cause appears to be a hitherto unappreciated contribution of spindle-associated factors to early embryo development. As current procedures remove the spindle and associated factors, cloned embryos lack these factors. These observations are providing new insight into basic mammalian embryology. They also reveal possible changes to protocols that could improve the overall success of cloning.

DNA methylation patterns in human tripronucleate zygotes

In mammals, the dynamic reprogramming of DNA methylation begins during gametogenesis and continues through embryogenesis. Recently, immunofluorescence staining with an antibody against 5-methylcytosine (anti-5-MeC) has revealed active demethylation of the male pronucleus in zygotes beginning at 4-6 h after fertilization. In this study, we characterized the DNA methylation patterns in mouse zygotes and in human tripronucleate (3 PN) zygotes discarded after conventional fertilization or following ICSI. Pronuclei were subjected to fluorescence in-situ hybridization to identify the X and/or Y chromosomes and then stained with anti-5-MeC. In diandric 3 PN zygotes from conventional IVF, we consistently observed one strongly and two weakly stained pronuclei. In contrast, the majority of 3 PN ICSI zygotes, mainly digynic zygotes, displayed two strongly and one weakly stained pronuclei. Two zygotes from ICSI failed to show any staining difference among the three pronuclei. Our results indicate that the active demethylation of male pronuclei occurs in both mouse and human zygotes. It is possible that the abnormal methylation patterns resulting from a dysfunctional cytoplasm may occur in a small number of oocytes and may affect embryonic viability.

Epigenetic reprogramming in early embryonic development: effects of in-vitro production and somatic nuclear transfer

A considerable proportion of offspring, in particular in ruminants and mice, born from nuclear transfer (NT)-derived and in-vitro-produced (IVP) embryos is affected by multiple abnormalities of which a high birthweight and an extended gestation length are the predominant features; a phenomenon that has been called 'large offspring syndrome' (LOS). The underlying mechanisms are largely unknown at present, but alterations of epigenetic modifications of embryonic and fetal gene expression patterns, primarily caused by alterations in DNA methylation are thought to be involved in this syndrome. In mammals, DNA methylation is essential for the regulation of transcription during development and differentiation. This review summarizes results from studies in which mRNA expression patterns from IVP and NT-derived embryos were compared with those of their in-vivo counterparts. Numerous aberrations have been found ranging from suppression of expression to de-novo overexpression or more frequently to a significant up- or down-regulation of a specific gene. These observations emphasize the need for further epigenetic studies during preimplantation embryo development to gain insight into the molecular regulation correlated with an undisturbed embryonic and fetal development. Understanding molecular mechanisms will aid improvements in biotechnologies applied to early embryos in all species, including humans.

The effect of interspecific oocytes on demethylation of sperm DNA

In contrast to mice, in sheep no genome-wide demethylation of the paternal genome occurs within the first postfertilization cell cycle. This difference could be due either to an absence of a sheep demethylase activity that is present in mouse ooplasm or to an increased protection of methylated cytosine residues in sheep sperm. Here, we use interspecies intracytoplasmic sperm injection to demonstrate that sheep sperm DNA can be demethylated in mouse oocytes. Surprisingly, mouse sperm can also be demethylated to a limited extent in sheep oocytes. Our results suggest that the murine demethylation process is facilitated either by a sperm-derived factor or by male pronuclear chromatin composition.

Epigenetic and genomic imprinting analysis in nuclear transfer derived Bos gaurus/Bos taurus hybrid fetuses

Somatic cell nuclear transfer (NT) in cattle is an inefficient process, whereby the production of calves is hindered by low pregnancy rates as well as fetal and placental abnormalities. Interspecies models have been previously used to facilitate the identification of single nucleotide polymorphisms (SNPs) within coding regions of genes to discriminate between parental alleles in the offspring. Here we report the use of a bovine interspecies model (Bos gaurus x Bos taurus) for the assessment and characterization of epigenetic modifications and genomic imprinting in Day 40-old female NT-derived fetuses and placenta. Analysis of NT and control pregnancies indicated disruption of genomic imprinting at the X inactivation-specific transcript (XIST) locus in the chorion, but not the fetus of clones, whereas proper allelic expression of the insulin-like growth factor II (IGF2) and gene trap locus 2 (GTL2) loci was maintained in both the fetus and placenta. Analysis of the XIST differentially methylated region (DMR) in clones indicated normal patterns of methylation; however, bisulfite sequencing of the satellite I repeat element and epidermal cytokeratin promoter indicated hypermethylation in the chorion of clones when compared with controls. No differences were detected in methylation levels in the fetus proper. These results indicate that the nuclear transfer process affects gene expression patterns in the trophectoderm- and inner cell mass-derived tissues to different extents.

Cloning: questions answered and unsolved

Cloning by the transfer of adult somatic cell nuclei to oocytes has produced viable offspring in a variety of mammalian species. The technology is still in its initial stages of development. Studies to date have answered several basic questions related to such issues as genome potency, life expectancy of clones, mitochondrial fates, and feasibility of inter-species nuclear transfer. They have also raised new questions related to the control of nuclear reprogramming and function. These questions are reviewed here.

Improved in vitro development rates of sheep somatic nuclear transfer embryos by using a reverse-order zona-free cloning method

This paper describes a modified zona-free cloning protocol for examining the effects of short-term exposure of donor nuclei to maternal chromosomal components and associated factors. In vitro matured zona-free sheep oocytes were enucleated by micromanipulator-assisted aspiration either before or after fusion with adult fibroblast or granulosa donor cells. Subsequent kinetics of donor nuclei and maternal chromatin as well as in vitro embryo development rates were recorded. The effect of an additional activation stimulus in connection with the reverse-order cloning (fusion before enucleation) and the feasibility of manual enucleation by metal blade were also studied. As a result of the simultaneous fusion and activation, most donor nuclei remained in interphase but swelled in size. Maternal chromosomes reached anaphase II-telophase II stages within 1-2 h of activation, effectively facilitating telophase enucleation with both the micromanipulator-assisted aspiration and manual bisection. A significantly higher development rate to the blastocyst stage was achieved with the reverse-order protocol, suggesting further investigation into the possible role of oocyte nucleus-associated factors in reprogramming is warranted. Overall, the reverse-order zona-free cloning method was efficient in the production of transferable cloned sheep blastocysts and may offer yet another choice of methodology in the practical application of nuclear transfer technology.

Precise recapitulation of methylation change in early cloned embryos

Change of DNA methylation during preimplantation development is very dynamic, which brings this term to the most attractive experimental target for measuring the capability of cloned embryo to reprogram its somatic genome. However, one weak point is that the preimplantation stage carries little information on genomic sequences showing a site-specific re-methylation after global demethylation; these sequences, if any, may serve as an advanced subject to test how exactly the reprogramming/programming process is recapitulated in early cloned embryos. Here, we report a unique DNA methylation change occurring at bovine neuropeptide galanin gene sequence. The galanin gene sequence in early bovine embryos derived by in vitro fertilization (IVF) maintained a undermethylated status till the morula stage. By the blastocyst, certain CpG sites became methylated specifically, which may be an epigenetic sign for the galanin gene to start a differentiation programme. The same sequence was moderately methylated in somatic donor cell and, after transplanted into an enucleated oocyte by nuclear transfer (NT), came rapidly demethylated to a completion, and then, at the blastocyst stage, re-methylated at exactly the same CpG sites, as observed so in normal blastocysts. The precise recapitulation of normal methylation reprogramming and programming at the galanin gene sequence in bovine cloned embryos gives a cue for the potential of cloned embryo to superintend the epigenetic states of foreign genome, even after global demethylation.

Disruption of imprinted gene methylation and expression in cloned preimplantation stage mouse embryos

Cloning by somatic cell nuclear transfer requires that epigenetic information possessed by the donor nucleus be reprogrammed to an embryonic state. Little is known, however, about this remodeling process, including when it occurs, its efficiency, and how well epigenetic markings characteristic of normal development are maintained. Examining the fate of epigenetic information associated with imprinted genes during clonal development offers one means of addressing these questions. We examined transcript abundance, allele specificity of imprinted gene expression, and parental allele-specific DNA methylation in cloned mouse blastocysts. Striking disruptions were seen in total transcript abundance and allele specificity of expression for five imprinted genes. Only 4% of clones recapitulated a blastocyst mode of expression for all five genes. Cloned embryos also exhibited extensive loss of allele-specific DNA methylation at the imprinting control regions of the H19 and Snprn genes. Thus, epigenetic errors arise very early in clonal development in the majority of embryos, indicating that reprogramming is inefficient and that some epigenetic information may be lost.

Somatic cell-like features of cloned mouse embryos prepared with cultured myoblast nuclei

Cloning by somatic cell nuclear transfer requires silencing of the donor cell gene expression program and the initiation of the embryonic gene expression program (nuclear reprogramming). Failure to silence the donor cell program could lead to altered embryonic phenotypes. Cloned mouse embryos produced using myoblast nuclei fail to thrive in standard embryo culture media but flourish in somatic cell culture media favored by the donor myoblasts themselves, forming blastocysts at a significant rate, with robust morphologies, high total cell number, and a normal allocation of cells to the inner cell mass in most embryos. Myoblast cloned embryos continue expressing the GLUT4 glucose transporter, which is typically expressed in muscle but not in preimplantation stage embryos. Myoblast clones also exhibit precocious enrichment of GLUT1 at the cell surface. Both myoblast and cumulus cell cloned embryos exhibit enhanced rates of glucose uptake. These observations indicate that silencing of the donor cell genome during cloning either is incomplete or occurs progressively over the course of preimplantation development. As a result, cloned embryos initially exhibit many somatic cell-like characteristics. Tetraploid constructs, which possess a transplanted somatic cell genome plus the oocyte-derived chromosomes, exhibit a more embryonic-like pattern of gene expression and culture preference. We conclude that preimplantation stage cloned embryos have profoundly altered characteristics that are donor cell type specific and that exposure of cloned embryos to standard embryo culture conditions may lead to disruptions in basic homeostasis and inhibition of a range of essential processes including further nuclear reprogramming, contributing to cloned embryo demise.

Abnormal regulation of DNA methyltransferase expression in cloned mouse embryos

Cloning by somatic cell nuclear transfer is inefficient. This is evident in the significant attrition in the number of surviving cloned offspring at virtually all stages of embryonic and fetal development. We find that cloned preimplantation mouse embryos aberrantly express the somatic form of the Dnmt1 DNA (cytosine-5) methyltransferase, the expression of which is normally prevented by a posttranscriptional mechanism. Additionally, the maternal oocyte-derived Dnmt1o isoform undergoes little or none of its expected translocation to embryonic nuclei at the eight-cell stage. Such defects in the regulation of Dnmt1s and Dnmt1o expression and cytoplasmic-nuclear trafficking may prevent clones from completing essential early developmental events. Furthermore, aberrant Dnmt1 localization and expression may contribute to the defects in DNA methylation and the developmental abnormalities seen in cloned mammals.

Nuclear reprogramming of cloned embryos produced in vitro

Despite the fact that cloned animals derived from somatic cells have been successfully generated in a variety of mammalian species, there are still many unsolved problems with current cloning technology. Somatic cell nuclear transfer has shown several developmental aberrancies, including a high rate of abortion during early gestation and increased perinatal death. One cause of these developmental failures of cloned embryos may reside in the epigenetic reprogramming of somatic donor genome. In mammals, DNA methylation is an essential process in the regulation of transcription during embryonic development and is generally associated with gene silencing. A genome-wide demethylation may be a prerequisite for the formation of pluripotent stem cells that are important for later development. We analyzed methylation patterns in cloned bovine embryos to monitor the epigenetic reprogramming process of donor genomic DNA. Aberrant methylation profiles of cloned bovine embryos were observed in various genomic regions, except in single-copy gene sequences. The overall genomic methylation status of cloned embryos was quite different from that of normal embryos produced in vitro or in vivo. These results suggest that the developmental failures of cloned embryos may be due to incomplete epigenetic reprogramming of donor genomic DNA. We expect that advances in understanding the molecular events for reprogramming of donor genome will contribute to clarify the developmental defects of cloned embryos.

The recent history of somatic cloning in mammals

The history of somatic cell nuclear transfer (NT) in mammals is full of exciting experiments and findings regarding the technique and outcome of NT, despite only covering a period of 6 years. The production of Dolly, for the first time demonstrating cloning from an adult somatic cell, had a great impact on subsequent studies. However, the more progress we make, the more obvious it becomes how little we know about the processes during NT, specifically how reprogramming events occur. Therefore, it is certainly challenging to continue investigating every step of somatic cell NT more intensively, starting from the donor cell, (type, cell cycle, synchronization, population doublings) and continuing until the cloned offspring are born and even further, to see how and if NT has an influence on health, viability, quantitative traits, and reproduction of cloned individuals.

Limited demethylation leaves mosaic-type methylation states in cloned bovine pre-implantation embryos

Cloning by nuclear transfer (NT) has been riddled with difficulties: most clones die before birth and survivors frequently display growth abnormalities. The cross-species similarity in abnormalities observed in cloned fetuses/animals leads us to suspect the fidelity of epigenetic reprogramming of the donor genome. Here, we found that single-copy sequences, unlike satellite sequences, are demethylated in pre-implantation NT embryos. The differential demethylation pattern between genomic sequences was confirmed by analyzing single blastocysts. It suggests selective demethylation of other developmentally important genes in NT embryos. We also observed a reverse relationship between methylation levels and inner cell mass versus trophectoderm (ICM/TE) ratios, which was found to be a result of another type of differential demethylation occurring in NT blastocysts where unequal methylation was maintained between ICM and TE regions. TE-localized methylation aberrancy suggests a widespread gene dysregulation in an extra-embryonic region, thereby resulting in placental dysfunction familiar to cloned fetuses/animals. These differential demethylations among genomic sequences and between differently allocated cells produce varied overall, but specified, methylation patterns, demonstrating that epigenetic reprogramming occurs in a limited fashion in NT embryos.