Somatic Nuclear Transfer Research Articles

Quiescence Loosens Epigenetic Constraints in Bovine Somatic Cells and Improves Their Reprogramming into Totipotency.

Reprogramming by nuclear transfer (NT) cloning forces cells to lose their lineage-specific epigenetic marks and reacquire totipotency. This process often produces molecular anomalies that compromise clone development. We hypothesized that quiescence alters the epigenetic status of somatic NT donor cells and elevates their reprogrammability. To test this idea, we compared chromatin composition and cloning efficiency of serum-starved quiescent (G0) fibroblasts versus nonstarved mitotically selected (G1) controls. We show that G0 chromatin contains reduced levels of Polycomb group proteins EED, SUZ12, PHC1, and RING2, as well as histone variant H2A.Z. Using quantitative confocal immunofluorescence microscopy and fluorometric enzyme-linked immunosorbent assay, we further show that G0 induced DNA and histone hypomethylation, specifically at H3K4me3, H3K9me2/3 and H3K27me3, but not H3K9me1. Collectively, these changes resulted in a more relaxed G0 chromatin state. Following NT, G0 donors developed into blastocysts that retained H3K9me3 hypomethylation, both in the inner cell mass and trophectoderm. G0 blastocysts from different cell types and cell lines developed significantly better into adult offspring. In conclusion, serum starvation induced epigenetic changes, specifically hypotrimethylation, that provide a mechanistic correlate for increased somatic cell reprogrammability.

Porcine Pluripotent Stem Cells Derived from IVF Embryos Contribute to Chimeric Development In Vivo.

Although the pig is considered an important model of human disease and an ideal animal for the preclinical testing of cell transplantation, the utility of this model has been hampered by a lack of genuine porcine embryonic stem cells. Here, we derived a porcine pluripotent stem cell (pPSC) line from day 5.5 blastocysts in a newly developed culture system based on MXV medium and a 5% oxygen atmosphere. The pPSCs had been passaged more than 75 times over two years, and the morphology of the colony was similar to that of human embryonic stem cells. Characterization and assessment showed that the pPSCs were alkaline phosphatase (AKP) positive, possessed normal karyotypes and expressed classic pluripotent markers, including OCT4, SOX2 and NANOG. In vitro differentiation through embryonic body formation and in vivo differentiation via teratoma formation in nude mice demonstrated that the pPSCs could differentiate into cells of the three germ layers. The pPSCs transfected with fuw-DsRed (pPSC-FDs) could be passaged with a stable expression of both DsRed and pluripotent markers. Notably, when pPSC-FDs were used as donor cells for somatic nuclear transfer, 11.52% of the reconstructed embryos developed into blastocysts, which was not significantly different from that of the reconstructed embryos derived from porcine embryonic fibroblasts. When pPSC-FDs were injected into day 4.5 blastocysts, they became involved in the in vitro embryonic development and contributed to the viscera of foetuses at day 50 of pregnancy as well as the developed placenta after the chimeric blastocysts were transferred into recipients. These findings indicated that the pPSCs were porcine pluripotent cells; that this would be a useful cell line for porcine genetic engineering and a valuable cell line for clarifying the molecular mechanism of pluripotency regulation in pigs.

Pluripotent stem cells and their use in hearing loss

Throughout its half a century of development, stem cell research has included two main fields: embryonic stem (ES) cell research and the reprogramming of body somatic cells. In the present review we focused on stem cell reprogramming and its relation with otolaryngology. The human body somatic cells are transformed into pluripotent cells by three basic methods: the somatic nuclear transfer method, the somatic cell fusion method (getting cellular pluripotent capacity in cellular reprogramming), and by transcription factors influencing the body somatic cells to generate reprogrammed induced pluripotent stem (iPS) cells. ES cells and iPS cells have pluripotency and differentiate into cells originating from the three germ layers; they are preferred for cellular treatment, drug development, and disease modeling research. Because of ethical restrictions in obtaining ES cells, iPS cells are an alternative pluripotent cell source and patient-specific autologous pluripotent cells are obtained by this method. Cellular treatment and regenerative medicine with pluripotent cells are currently developing and we aimed to raise awareness about this topic in our paper on using iPS cell technology in the biological treatment of hearing loss, which is an important area of research in otolaryngology.

Epigenetic Aberrations Are Not Specific to Transcription Factor-Mediated Reprogramming.

SummarySomatic cells can be reprogrammed to pluripotency using different methods. In comparison with pluripotent cells obtained through somatic nuclear transfer, induced pluripotent stem cells (iPSCs) exhibit a higher number of epigenetic errors. Furthermore, most of these abnormalities have been described to be intrinsic to the iPSC technology. Here, we investigate whether the aberrant epigenetic patterns detected in iPSCs are specific to transcription factor-mediated reprogramming. We used germline stem cells (GSCs), which are the only adult cell type that can be converted into pluripotent cells (gPSCs) under defined culture conditions, and compared GSC-derived iPSCs and gPSCs at the transcriptional and epigenetic level. Our results show that both reprogramming methods generate indistinguishable states of pluripotency. GSC-derived iPSCs and gPSCs retained similar levels of donor cell-type memory and exhibited comparable numbers of reprogramming errors. Therefore, our study demonstrates that the epigenetic abnormalities detected in iPSCs are not specific to transcription factor-mediated reprogramming.

RNA-Seq transcriptome analysis of porcine cloned and in vitro fertilized blastocysts

Somatic nuclear transfer technology has become increasingly promising in biomedicine and agriculture. Whereas the approach remains inefficient and underlying mechanisms remain ambiguous. Although cloned embryos have similar in vitro developmental capacity as in vitro fertilized (IVF) embryos before implantation, they appeared to have much lower full-term developmental efficiency in pig and cattle, and thus it would be reasonable to postulate that profound distinction at the molecular level should exist between them. Herein, RNA sequencing technique was used to screen differentially expressed genes in cloned and IVF blastocysts, and in total 628 differentially expressed transcripts were obtained, among which, 280 transcripts are up-regulated and 348 transcripts are down-regulated in cloned blastocysts. Moreover, one statistically significant pathway associated with endoplasmic reticulum (ER) protein processing was enriched, and some ER-stress markers such as ATF4, ATF6, PDIA3, HSPA1B, HSP40 and HSP90 between cloned and IVF blastocysts were suggested. Additionally, some developmentally important genes such as lipid metabolism related genes (MGLL, DDHD2 and FADS2) and epigenetic modification genes (DNMT1, KDM5C and MBD3L5) were found differentially expressed between cloned and IVF embryos.

Tissue engineering and regenerative medicine in basic research: a year in review of 2014.

Tissue engineering and regenerative medicine (TERM) remains to be one of the fastest growing fields, which covers a wide scope of topics of both basic and applied biological researches. This overview article summarized the advancements in basic researches of TERM area, including stem cell biology, cell engineering, somatic nuclear transfer, genomic editing, discovery of new tissue progenitor/stem cells, and immunomodulation of stem cells and tissue regeneration. It reflects the cutting-edge achievements in basic researches, which will lay solid scientific foundation for future TERM translational researches.

Can human embryos resulting from Somatic Cell Nuclear Transfers (SCNT) be used to treat human infertility? future implications in Assisted Reproductive Technologies (ART)

Can human embryos resulting from Somatic Cell Nuclear Transfers (SCNT) be used to treat human infertility? future implications in Assisted Reproductive Technologies (ART)

Epigenetic/Genetic Mismatch: Using Transdifferentiation as a Potential Cancer Therapy to Exploit the Cell Type Specificity of Cancer

Every cell type capable of proliferation can be malignantly transformed. However, there appears to be no naturally occurring universal set of genetic mutations capable of converting every cell type to a malignant state. Any specific cell type is generally resistant to transformation by the cancer mutations accumulated by cells of different lineages, presumably due to epigenetic differences. Evidence for this idea derives from experiments in which the developmental fates of cancer cells are altered to reduce malignancy. Reprogramming cancer cells to more primitive developmental states using pluripotency factors (IPS) or somatic nuclear transfer suppresses the malignant phenotype, as does subsequent directed differentiation to mature cells of lineages distinct from the originating cell. Direct transdifferentiation to an alternative cell fate also reduces tumorigenicity. In contrast, after reprogramming, cells induced to redifferentiate toward the original tumor cell type are tumorigenic. In these types of experiments an epigenetic/genetic mismatch often results in suppression of malignancy or cell death. Elucidating the specific transcription and cell signaling network incompatibilities will identify new targets for cancer therapy. Moreover, novel strategies to induce an incompatible transdifferentiated state, in which expression of thousands of genes are altered, will prove useful in controlling malignancies that otherwise easily evolve resistance to single target-based therapeutics. Engineering small molecules, genetic vectors, cytokines, growth factors, targeted extracellular vesicles, and cell fusion will help realize transdifferentiation-based therapeutics for cancer.

Maternal histone variants and their chaperones promote paternal genome activation and boost somatic cell reprogramming.

The mammalian egg employs a wide spectrum of epigenome modification machinery to reprogram the sperm nucleus shortly after fertilization. This event is required for transcriptional activation of the paternal/zygotic genome and progression through cleavage divisions. Reprogramming of paternal nuclei requires replacement of sperm protamines with canonical and non-canonical histones, covalent modification of histone tails, and chemical modification of DNA (notably oxidative demethylation of methylated cytosines). In this essay we highlight the role maternal histone variants play during developmental reprogramming following fertilization. We discuss how reduced maternal histone variant incorporation in somatic nuclear transfer experiments may explain the reduced viability of resulting embryos and how knowledge of repressive and activating maternal factors may be used to improve somatic cell reprogramming.

Transgenic expression of human cytoxic T-lymphocyte associated antigen4-immunoglobulin (hCTLA4Ig) by porcine skin for xenogeneic skin grafting.

Porcine skin is frequently used as a substitute of human skin to cover large wounds in clinic practice of wound care. In our previous work, we found that transgenic expression of human cytoxicT-lymphocyte associated antigen4-immunoglobulin (hCTLA4Ig) in murine skin graft remarkably prolonged its survival in xenogeneic wounds without extensive immunosuppression in recipients, suggesting that transgenic hCTLA4Ig expression in skin graft may be an effective and safe method to prolong xenogeneic skin graft survival. In this work, using a transgene construct containing hCTLA4Ig coding sequence under the drive of human Keratine 14 (k14) promoter, hCTLA4Ig transgenic pigs were generated by somatic nuclear transfer. The derived transgenic pigs were healthy and exhibited no signs of susceptibility to infection. The hCTLA4Ig transgene was stably transmitted through germline over generations, and thereby a transgenic pig colony was established. In the derived transgenic pigs, hCTLA4Ig expression in skin was shown to be genetically stable over generations, and detected in heart, kidney and corneal as well as in skin. Transgenic hCTLA4Ig protein in pigs exhibited expected biological activity as it suppressed human lymphocyte proliferation in human mixed lymphocyte culture to extents comparable to those of commercially purchased purified hCTLA4Ig protein. In skin grafting from pigs to rats, transgenic porcine skin grafts exhibited remarkably prolonged survival compared to the wild-type skin grafts derived from the same pig strain (13.33 ± 3.64 vs. 6.25 ± 2.49 days, P < 0.01), further indicating that the transgenic hCTLA4Ig protein was biologically active and capable of extending porcine skin graft survival in xenogeneic wounds. The transgenic pigs generated in this work can be used as a reproducible resource to provide porcine skin grafts with extended survival for wound coverage, and also as donors to investigate the impacts of hCTLA4Ig on xenotransplantation of other organs (heart, kidney and corneal) due to the ectopic transgenic hCTLA4Ig expression.

Porcine Circovirus Type 2 Infection in a Piglet Born from a Surrogate Mother

A 4-week-old male piglet being maintained in a research facility was found dead without any previous clinical signs. The piglet had been born from a surrogate mother after somatic nuclear transfer as part of a xenotransplantation study. Ovaries for nuclear transfer were obtained from a private farm outside the research facility. Histopathologically, multifocal to coalescing granulomatous myocarditis was observed in the heart, characterized by infiltration of lymphocytes, macrophages and multinucleated giant cells, and by myocardial necrosis and fibrosis. Lymphoid tissues showed marked lymphoid depletion with infiltration by histiocytes or giant cells. Immunohistochemistry showed PCV-2 antigens in necrotic myocytes, macrophages and multinucleated giant cells in the heart, as well as in macrophages and giant cells in lymphoid depleted areas of lymphoid tissues. Reproductive failure associated with PCV-2 in aborted or stillborn piglets is frequently characterized by myocarditis, and similar lesions were observed in this 4-week-old piglet with PCV-2 infection. The PCV-2 infection in this piglet may have been due to contamination or infection of an ovary from the pig farm.

Mitochondrial DNA Mutation in Microglia Can Be Treated by SCNT Cloning and Not by Reprogramming of Olfactory Ensheathing Cells in the Multiple Sclerosis Treatment

Background: Multiple sclerosis (MS) is a demyelinating chronic inflammatory neurological disease of the central nervous system (CNS), characterized by activation of microglia. Mitochondrial mutations (mtDNA.mutat) and dysfunctions in microglial cells are thought to centrally contribute to the damaging effects of neuroinflammation seen in MS. Objective: The somatic nuclear transfer (SCNT) technology offers a more practical mode of therapy in MS, This method would attempt to dilute and/or progressively replace the (mtDNA.mutat) and activated microglia with cloned olfactory ensheathing cells (OEC) with remyelinating and scavenging properties which would attempt to limit the progression of MS. Proposed Methods: Applying SCNT-derived embryonic stem (ES) cells based therapy by cloning olfactory ensheathing cells (OEC), engineered with an autologous nuclear component of the recipient OEC with a healthy donor oocyte. The inner cell mass of the subsequently developed blastocyst would be the source to generate the microglia. The novel proposed transcribrial route device offers a painless mode of cell transplantation to the brain. Expected Results: This mode of generating SCNT cloned glia and its transplantation to the brain is expected to replace the (mtDNA.mutat) and activated microglia of the patients with MS and use the regenerative and remyelinating and properties of the OEC’s, as has been seen in recent clinical trials in paraplegic patients with spinal cord injuries. Conclusion: The use of SCNT in contrast to the reprogramming adult somatic cell is expected to develop isogenic ES cell-based therapies for the prevention and treatment of MS associated with mtDNA mutations that may open a new avenue of designer’s targeted cell therapy unique for the patients with MS. The proposed “transcribrial device” to access the brain can be an advantageous route of delivery of cloned cells to the brain.

Cloned Microglias with Novel Delivery System in Multiple Sclerosis

Multiple Sclerosis (MS) is a chronic inflammatory neurological disease of the Central Nervous System (CNS), characterized by demyelination and activation of microglia. Mitochondrial mutations and dysfunctions in microglial cells are thought to contribute to the detrimental effects of neuroinflammation seen in MS. The Somatic Nuclear Transfer (SCNT) technology offers a more practical mode of therapy in MS, this method would attempt to dilute and/or progressively replace the mutated and activated microglia with cloned Olfactory Ensheathing Cells (OEC) with remyelinating and scavenging properties which would attempt to limit the progression of MS. Applying SCNTderived Embryonic Stem (ES) cells based therapy by cloning Olfactory Ensheathing Cells (OEC), engineered with an autologous nuclear component of the recipient OEC with a healthy donor oocyte. The inner cell mass of the subsequently developed blastocyst would be the source to generate the radial microglia to be used for cell therapy in MS. The novel proposed transcribrial route device offers a painless mode of cell transplantation to the brain. This mode of generating cloned glia and its transplantation to the brain is expected to replace the mutated and activated microglia of the patients with MS and use the regenerative and remyelinating and scavenging properties of the OEC’s, as has been seen in clinical trials in patients with spinal cord injuries. The use of SCNT to develop isogenic ES cellbased therapies for the prevention and treatment of MS associated with mtDNA mutations may open a new avenue of designer’s targeted cell therapy unique for the patients with MS. The proposed “transcribrial device” to access the brain can be an advantageous route of delivery of cloned cells to the brain.

Abnormal dynamic changes in β-tubulin in somatic nuclear transfer cloned mouse embryos

The efficiency of somatic cell nuclear transfer (SCNT) cloning remains low, thus limiting the applications of this technique. In this study, we used immunochemistry and confocal microscopy to detect the microtubule component, β-tubulin, in SCNT, parthenogenetic (PA), and intracytoplasmic sperm injection (ICSI) embryos before the first mitotic division. β-Tubulin is the component subunit of microtubule, which plays critical roles in regulating localization of cellular organelles, and the growth, maturation and fertilization of oocytes. Our results demonstrated similar changes of spindle patterns in PA and ICSI embryos. The second meiotic division resumed 1 h post-treatment, and the cytoplasmic asters (CAs) disappeared. After about 4-6 h of treatment, pronuclei formed with the midbodies connecting each other. Meanwhile, the CAs reappeared and a microtubule network developed in the cytoplasm. However, SCNT embryos showed abnormal multipolar spindles, and the pseudopronuclei that contained many nucleoli existed after 6 h of SrCl2 activation. Enucleated oocytes alone did not form spindle-like structures when they were artificially activated for 6 h, indicating that somatic cell chromosomes might be necessary for spindle formation in SCNT embryos. These results demonstrated abnormal changes of β-tubulin in mouse SCNT embryos, compared with PA and ICSI embryos.

Steps Toward Safe Cell Therapy Using Induced Pluripotent Stem Cells

The enthusiasm for producing patient-specific human embryonic stem cells using somatic nuclear transfer has somewhat abated in recent years because of ethical, technical, and political concerns. However, the interest in generating induced pluripotent stem cells (iPSCs), in which pluripotency can be obtained by transcription factor transduction of various somatic cells, has rapidly increased. Human iPSCs are anticipated to open enormous opportunities in the biomedical sciences in terms of cell therapies for regenerative medicine and stem cell modeling of human disease. On the other hand, recent reports have emphasized the pitfalls of iPSC technology, including the potential for genetic and epigenetic abnormalities, tumorigenicity, and immunogenicity of transplanted cells. These constitute serious safety-related concerns for iPSC-based cell therapy. However, preclinical data supporting the safety and efficacy of iPSCs are also accumulating. In this Review, recent achievements and future tasks for safe iPSC-based cell therapy are summarized, using regenerative medicine for repair strategies in the damaged central nervous system (CNS) as a model. Insights on safety and preclinical use of iPSCs in cardiovascular repair model are also discussed.

Direct Reprogramming into Desired Cell Types by Defined Factors

In the field of developmental biology, the concept that cells, once terminally differentiated, are fixed in their cell fate was long believed to be true. However, Dr. Gurdon and colleagues challenged this fundamental doctrine and demonstrated that cellular reprogramming and cell fate conversion are possible by somatic nuclear transfer and cell fusion. The Weintraub laboratory discovered in the 1980s that a single transcription factor, MyoD, can convert fibroblasts into skeletal muscle cells, and subsequent studies also demonstrated that several transcription factors are lineage converting factors within the blood cell lineage. In 2006, Takahashi and Yamanaka discovered that transduction of the four stem cell-specific transcription factors Oct4, Sox2, Klf4, and c-Myc can reprogram mouse fibroblast cells into a pluripotent state. In 2007, they demonstrated that the same four factors similarly reprogram human somatic cells into pluripotent stem cells. These discoveries by Dr. Yamanaka and colleagues fundamentally changed research in the fields of disease modeling and regenerative medicine and also inspired the next stage of cellular reprogramming, i.e., the generation of desired cell types without reverting to stem cells by overexpression of lineage-specific transcription factors. Recent studies demonstrated that a diverse range of cell types, such as pancreatic β cells, neurons, neural progenitors, cardiomyocytes, and hepatocytes, can be directly induced from somatic cells by combinations of specific factors. In this article, I review the pioneering works of cellular reprogramming and discuss the recent progress and future perspectives of direct reprogramming technology.

Mitochondrial DNA copy number is regulated in a tissue specific manner by DNA methylation of the nuclear-encoded DNA polymerase gamma A

DNA methylation is an essential mechanism controlling gene expression during differentiation and development. We investigated the epigenetic regulation of the nuclear-encoded, mitochondrial DNA (mtDNA) polymerase γ catalytic subunit (PolgA) by examining the methylation status of a CpG island within exon 2 of PolgA. Bisulphite sequencing identified low methylation levels (<10%) within exon 2 of mouse oocytes, blastocysts and embryonic stem cells (ESCs), while somatic tissues contained significantly higher levels (>40%). In contrast, induced pluripotent stem (iPS) cells and somatic nuclear transfer ESCs were hypermethylated (>20%), indicating abnormal epigenetic reprogramming. Real time PCR analysis of 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) immunoprecipitated DNA suggests active DNA methylation and demethylation within exon 2 of PolgA. Moreover, neural differentiation of ESCs promoted de novo methylation and demethylation at the exon 2 locus. Regression analysis demonstrates that cell-specific PolgA expression levels were negatively correlated with DNA methylation within exon 2 and mtDNA copy number. Finally, using chromatin immunoprecipitation (ChIP) against RNA polymerase II (RNApII) phosphorylated on serine 2, we show increased DNA methylation levels are associated with reduced RNApII transcriptional elongation. This is the first study linking nuclear DNA epigenetic regulation with mtDNA regulation during differentiation and cell specialization.

Conceptus elongation in cattle: Genes, models and questions

In ruminants, more than 30% of the embryonic loss observed after artificial insemination has an early origin that is coincident with the marked elongation of the conceptus that occurs before implantation. During this developmental phase, physiological interactions are established between the conceptus and the uterus which are essential for the establishment of pregnancy and the elongation process. Our molecular knowledge of elongating conceptuses in cattle has long been focused on its analysis in view of its interactions with the uterus with the elongating stages being defined, like the uterus stages, by days post insemination or conception. The gene clusters reported so far indicate important pathways, some being shared by the non-elongating conceptuses of other mammals. However, to identify the key components of the elongation process – that could be specific to ungulates – new models are needed. Somatic nuclear transfer could be one of them as it provides complementary insights on differentiation beyond the blastocyst stage. Nonetheless, other models are necessary to convert gene lists or networks in elongating phenotypes. This review partly summarizes information on these topics, but data on the impact of the uterus on the elongation process or on the differentiation of the embryonic tissues are reviewed elsewhere.

Avian Karyoplasts in Inactivated Chicken Germ Stem Cells (GSCs): Avian Somatic Nuclear Transfer Facilitated by a Novel Antibody Alignment Method.

The production of host chickens (Gallus gallus) with chimeric gonads producing gametes derived from transferred exotic donor-bird germ stem cells (GSCs), could potentially have a great impact on the management of threatened species. In this study a novel method of somatic nuclear transfer (snt) was developed using an antibody-biotin-avidin-antibody complex for alignment of somatic cells and GSCs for electrofusion. Quail GSC's were stained with the red fluorescent stains PKH-26 and ethidium bromide following irradiation with UV-light (250nm) for 60s creating an inactivated nucleus. Karyoplasts were produced by centrifugation of chicken fibroblasts at 10,000g on a 10% ficoll gradient and stained with the blue fluorescent stain Hoechst-33342. Inactivated GSCs and karyoplasts were stained with primary antibodies SSEA-1 (avian germ stem cell marker) and C1 (avian fibroblast specific marker), respectively. GSCs and karyoplasts were bound with secondary antibodies conjugated to biotin and avidin, respectively. To align the GSCs to karyoplasts, they were incubated at 37oc for 48 hours. The aligned cells were electrofused (two 2.5kV pulses at 10uF) and immediately resuspended in culture medium before fluorescent microscopy evaluation. Doublet cells were observed after incubation for up to 48 hours and following the first 2.5kV pulse. After the second 2.5kV fusion pulse, cells were observed to contain one each of a red and a blue nucleus, suggesting successful electrofusion of a single GSC and karyoplast. This method may be more efficient than random pearl chain formation for alignment of single GSCs and karyoplasts prior to electrofusion, providing a practical means of making the large numbers of snt-GSCs necessary for the production of chimeric gonads.

DNA methylation in stem cell renewal and multipotency

Owing to their potential for differentiation into multiple cell types, multipotent stem cells extracted from many adult tissues are an attractive stem cell resource for the replacement of damaged tissues in regenerative medicine. The requirements for cellular differentiation of an adult stem cell are a loss of proliferation potential and a gain of cell-type identity. These processes could be restricted by epigenetic modifications that prevent the risks of lineage-unrelated gene expression or the undifferentiated features of stem cells in adult somatic cells. In this review, we focus on the role of DNA methylation in controlling the transcriptional activity of genes important for self-renewal, the dynamism of CpG methylation of tissue-specific genes during several differentiation programs, and whether the multilineage potential of adult stem cells could be imposed early in the original precursor stem cells through CpG methylation. Additionally, we draw attention to the role of DNA methylation in adult stem cell differentiation by reviewing the reports on spontaneous differentiation after treatment with demethylating agents and by considering the evidence provided by reprogramming of somatic cells into undifferentiated cells (that is, somatic nuclear transfer or generation of induced pluripotent cells). It is clear from the evidence that DNA methylation is necessary for controlling stem cell proliferation and differentiation, but their exact contribution in each lineage program is still unclear. As a consequence, in a clinical setting, caution should be exerted before employing adult stem cells or their derivatives in regenerative medicine and appropriate tests should be applied to ensure the integrity of the genome and epigenome.