Genome-wide analysis reveals TET- and TDG-dependent 5-methylcytosine oxidation dynamics.

TET dioxygenases successively oxidize 5-methylcytosine (5mC) in mammalian genomes to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC). 5fC/5caC can be excised and repaired to regenerate unmodified cytosines by thymine-DNA glycosylase (TDG) and base excision repair (BER) pathway, but it is unclear to what extent and at which part of the genome this active demethylation process takes place. Here, we have generated genome-wide distribution maps of 5hmC/5fC/5caC using modification-specific antibodies in wild-type and Tdg-deficient mouse embryonic stem cells (ESCs). In wild-type mouse ESCs, 5fC/5caC accumulates to detectable levels at major satellite repeats but not at nonrepetitive loci. In contrast, Tdg depletion in mouse ESCs causes marked accumulation of 5fC and 5caC at a large number of proximal and distal gene regulatory elements. Thus, these results reveal the genome-wide view of iterative 5mC oxidation dynamics and indicate that TET/TDG-dependent active DNA demethylation process occurs extensively in the mammalian genome.

DNMT1 can induce primary germ layer differentiation through de novo DNA methylation.

DNA methyltransferases and Ten-Eleven Translocation (TET) proteins regulate the DNA methylation and demethylation cycles during mouse embryonic development. Although DNMT1 mainly plays a role in the maintenance of DNA methylation after DNA replication, it is also reported to possess de novo methyltransferase capacity. However, its physiological significance remains unclear. Here, we demonstrate that full-length DNMT1 (FL) and a mutant lacking the N-terminus necessary for its maintenance activity (602) confer the differentiation potential of mouse Dnmt1, Dnmt3a, and Dnmt3b (Dnmts-TKO) embryonic stem cells (ESCs). Both FL and 602 inhibit the spontaneous differentiation of Dnmts-TKO ESCs in the undifferentiated state. Dnmts-TKO ESCs showed loss of DNA methylation and de-repression of primitive endoderm-related genes, but these defects were partially restored in Dnmts-TKO + FL and Dnmts-TKO + 602 ESCs. Upon differentiation, Dnmts-TKO + FL ESCs show increased 5mC and 5hmC levels across chromosomes, including pericentromeric regions. In contrast, Dnmts-TKO + 602 ESCs didn't accumulate 5mC, and sister chromatids showed 5hmC asynchronously. Furthermore, in comparison with DNMT1_602, DNMT1_FL effectively promoted commitment to the epiblast-like cells and beyond, driving cell-autonomous mesendodermal and germline differentiation through embryoid body-based methods. With precise target selectivity achieved by its N-terminal region, DNMT1 may play a role in gene regulation leading to germline development.

Myocardial extracellular volume quantification in cardiac amyloidosis: a comparative study between cardiac computed tomography and magnetic resonance imaging.

OBJECTIVES: Myocardial extracellular volume (ECV) on computed tomography (CT), an alternative to cardiac magnetic resonance (CMR), has significant practical clinical advantages. However, the consistency between ECVs quantified via CT and CMR in cardiac amyloidosis (CA) has not been investigated sufficiently. Therefore, the current study investigated the application of CT-ECV in CA with CMR-ECV as the reference standard. METHODS: We retrospectively evaluated 31 patients with CA who underwent cardiac CT and CMR. Pearson correlation analysis was performed to investigate correlations between CT-ECV and CMR-ECV at each segment. Further, correlations between ECV and clinical parameters were assessed. RESULTS: There were no significant differences in the mean global ECVs between CT scan and CMR (51.3% ± 10.2% vs 50.0% ± 10.5%). CT-ECV was correlated with CMR-ECV at the septal (r = 0.88), lateral (r = 0.80), inferior (r = 0.79), anterior (r = 0.77) segments, and global (r = 0.87). In both CT and CMR, the ECV had a weak to strong correlation with high-sensitivity cardiac troponin T level, a moderate correlation with global longitudinal strain, and an inverse correlation with left ventricular ejection fraction. Further, the septal ECV and global ECV had a slightly higher correlation with the clinical parameters. CONCLUSIONS: Cardiac CT can quantify myocardial ECV and yield results comparable to CMR in patients with CA. Moreover, a significant correlation between CT-ECV and clinical parameters was observed. Thus, CT-ECV can be an imaging biomarker and alternative to CMR-ECV. CLINICAL RELEVANCE STATEMENT: Cardiac CT can quantify myocardial ECV and yield results comparable to CMR in patients with CA, and CT-ECV can be used clinically as an imaging biomarker and alternative to CMR-ECV. KEY POINTS: • A significant correlation was found between CT myocardial extracellular volume and cardiac MR myocardial extracellular volume in patients with cardiac amyloidosis. • In CT and cardiac MR, the myocardial extracellular volume correlated well with high-sensitivity cardiac troponin T level, global longitudinal strain, and left ventricular ejection fraction. • CT myocardial extracellular volume can be an imaging biomarker and alternative to cardiac MR myocardial extracellular volume.

Tet1 regulates epigenetic remodeling of the pericentromeric heterochromatin and chromocenter organization in DNA hypomethylated cells.

Pericentromeric heterochromatin (PCH), the constitutive heterochromatin of pericentromeric regions, plays crucial roles in various cellular events, such as cell division and DNA replication. PCH forms chromocenters in the interphase nucleus, and chromocenters cluster at the prophase of meiosis. Chromocenter clustering has been reported to be critical for the appropriate progression of meiosis. However, the molecular mechanisms underlying chromocenter clustering remain elusive. In this study, we found that global DNA hypomethylation, 5hmC enrichment in PCH, and chromocenter clustering of Dnmt1-KO ESCs were similar to those of the female meiotic germ cells. Tet1 is essential for the deposition of 5hmC and facultative histone marks of H3K27me3 and H2AK119ub at PCH, as well as chromocenter clustering. RING1B, one of the core components of PRC1, is recruited to PCH by TET1, and PRC1 plays a critical role in chromocenter clustering. In addition, the rearrangement of the chromocenter under DNA hypomethylated condition was mediated by liquid-liquid phase separation. Thus, we demonstrated a novel role of Tet1 in chromocenter rearrangement in DNA hypomethylated cells.

Role of Tet proteins in enhancer activity and telomere elongation.

DNA methylation at the C-5 position of cytosine (5mC) is one of the best-studied epigenetic modifications and plays important roles in diverse biological processes. Iterative oxidation of 5mC by the ten-eleven translocation (Tet) family of proteins generates 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC). 5fC and 5caC are selectively recognized and excised by thymine DNA glycosylase (TDG), leading to DNA demethylation. Functional characterization of Tet proteins has been complicated by the redundancy between the three family members. Using CRISPR/Cas9 technology, we generated mouse embryonic stem cells (ESCs) deficient for all three Tet proteins (Tet triple knockout [TKO]). Whole-genome bisulfite sequencing (WGBS) analysis revealed that Tet-mediated DNA demethylation mainly occurs at distally located enhancers and fine-tunes the transcription of genes associated with these regions. Functional characterization of Tet TKO ESCs revealed a role for Tet proteins in regulating the two-cell embryo (2C)-like state under ESC culture conditions. In addition, Tet TKO ESCs exhibited increased telomere-sister chromatid exchange and elongated telomeres. Collectively, our study reveals a role for Tet proteins in not only DNA demethylation at enhancers but also regulating the 2C-like state and telomere homeostasis.

Ribosomal stress induces 2-cell embryo-like state transition of the mouse ESCs through p53 activation.

Embryonic stem cells (ESCs) maintain a pluripotent state and genome integrity in long-term culture. A rare population of ESCs showing 2-cell embryo-specific gene expression is believed to play critical roles in sustainable pluripotency and genome stability. However, the molecular mechanism controlling this transition to a 2-cell embryo-like (2CL) state remains unclear. We carried out screening to search for the factors involved in 2CL state induction and found a ribosomal RNA processing factor, Pum3 to be a candidate. Increased 2CL state population accompanied with an accumulation of pre-ribosomal RNA and activated p53 in the Pum3-KO ESC. Furthermore, the increase of 2CL state cells in the Pum3-KO ESCs was completely abrogated by the deletion of p53. The DNA damage induced by the Ultraviolet light (UV) irradiation and Zeocin promoted the transition to a 2CL state in a p53-dependent manner. Thus, our study provides new insights into a 2CL state transition mechanism through stress-dependent p53 activation of ESCs.

Nucleus-localized adiponectin is survival gatekeeper through miR-214-mediated AIFM2 regulation.

Adiponectin secreted from adipocytes into plasma has anti-aging, anti-obesity and anti-inflammation effects. Here, we detected intracellular adiponectin localized in the nuclei of human and mouse pluripotent stem cells, mouse germ cells and some somatic cells. Nucleus-localized (Nu) adiponectin protein is characterized by an N-terminal truncated monomer form in a native state, compared with intact multimer forms of cytoplasm-localized (Cy) adiponectin protein. Doxycycline-induced over-expression of ADIPONECTIN caused cell death in human and mouse Nu-type pluripotent stem cells. Genome-wide gene expression analysis indicated that apoptosis by ADIPONECTIN over-expression was induced in accompany with upregulation of AIFM2 and MEG3. Upregulation of AIFM2 and MEG3 and down-regulation of miR-214-3p verified by qPCR analyses after ADIPONECTIN over-expression indicated that the MEG3/miR-214/AIFM2 pathway played a role in the apoptotic cell death of pluripotent cells. Adiponectin-induced cell death was rescued by the treatment with miR-214-3p mimic. Global data analysis shows that Nu adiponectin has a role in microRNA-mediated post-transcription regulation, cell-cell interactions and chromatin remodeling as a survival gatekeeper.

Role of Tet1 in erasure of genomic imprinting.

Genomic imprinting is an allele-specific gene expression system that is important for mammalian development and function. The molecular basis of genomic imprinting is allele-specific DNA methylation. Although it is well known that the de novo DNA methyltransferases Dnmt3a and Dnmt3b are responsible for the establishment of genomic imprinting, how the methylation mark is erased during primordial germ cell (PGC) reprogramming remains unclear. Tet1 is one of the ten-eleven translocation family proteins, which have the capacity to oxidize 5-methylcytosine (5mC), specifically expressed in reprogramming PGCs. Here we report that Tet1 has a critical role in the erasure of genomic imprinting. We show that despite their identical genotype, progenies derived from mating between Tet1 knockout males and wild-Peg10 and Peg3, which exhibit aberrant hypermethylation in the paternal allele of differential methylated regions (DMRs). RNA-seq reveals extensive dysregulation of imprinted genes in the next generation due to paternal loss of Tet1 function. Genome-wide DNA methylation analysis of embryonic day 13.5 PGCs and sperm of Tet1 knockout mice revealed hypermethylation of DMRs of imprinted genes in sperm, which can be traced back to PGCs. Analysis of the DNA methylation dynamics in reprogramming PGCs indicates that Tet1 functions to wipe out remaining methylation, including imprinted genes, at the late reprogramming stage. Furthermore, we provide evidence supporting the role of Tet1 in the erasure of paternal imprints in the female germ line. Thus, our study establishes a critical function of Tet1 in the erasure of genomic imprinting.

Tet1 controls meiosis by regulating meiotic gene expression.

Meiosis is a germ-cell-specific cell division process through which haploid gametes are produced for sexual reproduction. Before the initiation of meiosis, mouse primordial germ cells undergo a series of epigenetic reprogramming steps, including the global erasure of DNA methylation at the 5-position of cytosine (5mC) in CpG-rich DNA. Although several epigenetic regulators, such as Dnmt3l and the histone methyltransferases G9a and Prdm9, have been reported to be crucial for meiosis, little is known about how the expression of meiotic genes is regulated and how their expression contributes to normal meiosis. Using a loss-of-function approach in mice, here we show that the 5mC-specific dioxygenase Tet1 has an important role in regulating meiosis in mouse oocytes. Tet1 deficiency significantly reduces female germ-cell numbers and fertility. Univalent chromosomes and unresolved DNA double-strand breaks are also observed in Tet1-deficient oocytes. Tet1 deficiency does not greatly affect the genome-wide demethylation that takes place in primordial germ cells, but leads to defective DNA demethylation and decreased expression of a subset of meiotic genes. Our study thus establishes a function for Tet1 in meiosis and meiotic gene activation in female germ cells.

Nanog co-regulated by Nodal/Smad2 and Oct4 is required for pluripotency in developing mouse epiblast.

Nanog, a core pluripotency factor, is required for stabilizing pluripotency of inner cell mass (ICM) and embryonic stem cells (ESCs), and survival of primordial germ cells in mice. Here, we have addressed function and regulation of Nanog in epiblasts of postimplantation mouse embryos by conditional knockdown (KD), chromatin immunoprecipitation (ChIP) using in vivo epiblasts, and protein interaction with the Nanog promoter in vitro. Differentiation of Nanog-KD epiblasts demonstrated requirement for Nanog in stabilization of pluripotency. Nanog expression in epiblast is directly regulated by Nodal/Smad2 pathway in a visceral endoderm-dependent manner. Notably, Nanog promoters switch from Oct4/Esrrb in ICM/ESCs to Oct4/Smad2 in epiblasts. Smad2 directly associates with Oct4 to form Nanog promoting protein complex. Collectively, these data demonstrate that Nanog plays a key role in stabilizing Epiblast pluripotency mediated by Nodal/Smad2 signaling, which is involved in Nanog promoter switching in early developing embryos.

Development of a chemically disclosed serum-free medium for mouse pluripotent stem cells.

Mouse embryonic stem cells (mESCs) have been widely used as a model system to study the basic biology of pluripotency and to develop cell-based therapies. Traditionally, mESCs have been cultured in a medium supplemented with fetal bovine serum (FBS). However, serum with its inconsistent chemical composition has been problematic for reproducibility and for studying the role of specific components. While some serum-free media have been reported, these media contain commercial additives whose detailed components have not been disclosed. Recently, we developed a serum-free medium, DA-X medium, which can maintain a wide variety of adherent cancer lines. In this study, we modified the DA-X medium and established a novel serum-free condition for both naïve mESCs in which all components are chemically defined and disclosed (DA-X-modified medium for robust growth of pluripotent stem cells: DARP medium). The DARP medium fully supports the normal transcriptome and differentiation potential in teratoma and the establishment of mESCs from blastocysts that retain the developmental potential in all three germ layers, including germ cells in chimeric embryos. Utility of chemically defined DA-X medium for primed mouse epiblast stem cells (mEpiSCs) revealed that an optimal amount of cholesterol is required for the robust growth of naïve-state mESCs, but is dispensable for the maintenance of primed-state mEpiSCs. Thus, this study provides reliable and reproducible culture methods to investigate the role of specific components regulating self-renewal and pluripotency in a wide range of pluripotent states.

Cardiac MRI T1 and T2 Mapping as a Quantitative Imaging Biomarker in Transthyretin Amyloid Cardiomyopathy.

RATIONALE AND OBJECTIVES: This study aimed to assess the utility of cardiac magnetic resonance imaging (MRI) T1 and T2 mapping as quantitative imaging biomarkers in transthyretin amyloid cardiomyopathy (ATTR-CM). MATERIALS AND METHODS: This study retrospectively evaluated 74 patients with confirmed wild-type ATTR-CM who underwent cardiac MRI, 99mTc-labeled pyrophosphate (99mTc-PYP) scintigraphy, and echocardiography. We assessed the quantitative disease parameters, for example, left ventricular ejection fraction (LVEF), and global longitudinal strain (GLS) by echocardiography, native T1, extracellular volume fraction (ECV), and native T2 value by cardiac MRI, heart-to-contralateral ratio (H/CL) by 99mTc-PYP, and high-sensitive cardiac troponin T. Myocardial native T2 of ≥50 ms was defined as myocardial edema. Correlations between the disease's quantitative parameters were evaluated, and the ECV was compared to other parameters in ATTR-CM with/without myocardial edema. RESULTS: ECV in all patients with ATTR-CM revealed a strong correlation with native T1 (r = 0.62), a moderate correlation with hs-TnT (r = 0.59), LVEF (r = -0.48), GLS (r = 0.58), and H/CL (r = 0.48). Correlations between ECV and other quantitative parameters decreased in ATTR-CM with myocardial edema except for H/CL. Meanwhile, the correlations increased in ATTR-CM without myocardial edema. CONCLUSION: The presence of myocardial edema affected the interpretation of ECV assessment, although ECV can be a comprehensive imaging biomarker for ATTR-CM. ECV showed a significant correlation with various quantitative disease parameters and can be a reliable disease monitoring marker in patients with ATTR-CM when myocardial edema was excluded.

REST and its downstream molecule Mek5 regulate survival of primordial germ cells.

In mouse embryos, some primordial germ cells (PGCs) are eliminated by apoptosis, but the molecular pathways that lead to PGC survival versus apoptosis have not been fully characterized. Here, we found that REST (repressor element 1-silencing transcription factor), a transcription factor that binds a conserved regulatory element, NRSE/RE1, played a role in PGC survival. REST expression was higher in PGCs than in surrounding somatic cells. Moreover, in mouse embryos with a PGC-specific conditional REST mutation, the PGC population experienced more apoptosis and was significantly smaller than that in control embryos; these findings indicated that REST functioned in a cell-autonomous fashion that was critical for PGC survival. Several anti-apoptotic genes were among the previously identified REST-target gene candidates; moreover, some of these genes were downregulated in the REST-deficient PGCs. Mek5, which encodes a component in the a MAP kinase cascade, was one of these downregulated REST-target gene candidates, and a Mek5 mutation, like the REST mutation, caused an increase in PGC apoptosis; these finding suggested that REST promoted PGC survival via regulation of the Mek5 expression. Importantly, there were a normal number of PGCs in the REST mutants at birth, and both the male and female REST-mutant adults were fertile; these final observations revealed that the PGC population was very robust and could recover from a genetically induced reduction in cell number.

Conditional knockdown of Nanog induces apoptotic cell death in mouse migrating primordial germ cells.

The pluripotency factor Nanog is expressed in peri-implantation embryos and primordial germ cells (PGCs). Nanog-deficient mouse embryos die soon after implantation. To explore the function of Nanog in germ cells, Nanog RNA was conditionally knocked down in vivo by shRNA. Nanog shRNA transgenic (NRi-Tg) mice were generated through the formation of germline chimeras with NRi-Tg embryonic stem cells. In E12.5 Cre-induced ER-Cre/NRi-Tg and TNAP-Cre/NRi-Tg double-transgenic embryos, the number of alkaline phosphatase-positive and SSEA1-positive PGCs decreased significantly. In the E9.5 and E10.5 migrating Nanog-knockdown PGCs, TUNEL-positive apoptotic cell death became prominent in vivo and in vitro, despite Oct4 expression. Single-cell microarray analysis of E10.5 Nanog-knockdown PGCs revealed significant up- and downregulation of a substantial number of genes, including Tial1, Id1 and Suz12. These data suggest that Nanog plays a key role in the proliferation and survival of migrating PGCs as a safeguard of the PGC-specific molecular network.

A region-dependent allele-biased expression of Dopa decarboxylase in mouse brain.

Genomic imprinting is an epigenetic event in which genes are expressed only from either the paternal or maternal allele. Dopa decarboxylase (Ddc), is an imprinted gene that encodes an enzyme which catalyzes the conversion of L-dopa to dopamine. Although Ddc has been reported to be paternally expressed in embryonic and neonatal hearts, its expression pattern in the brain has been controversial. To visualize Ddc-expressing neurons, we established a knock-in mouse carrying a humanized Kusabira orange 1 (hKO1) reporter cassette at the Ddc locus (Ddc-hKO1). The expression of Ddc-hKO1 was detected in all known Ddc-positive cells in the brains of embryonic, neonatal, adult, and aged mice. We further developed an efficient purification method for Ddc-hKO1-positive neurons using a cell sorter. RNA sequencing analysis confirmed the enrichment of dopaminergic, serotonergic and cholinergic neurons in Ddc-hKO1-positive cell population recovered using this method. A detailed analysis of Ddc-hKO1 paternally and maternally derived heterozygous mice combined with immunostaining revealed that Ddc was preferentially expressed from the maternal allele in ventral tegmented area (VTA), substantia nigra pars compacta (SNc), and retrorubral field (RRF); while it was expressed from both alleles in dorsal raphe nucleus (DR). These results indicate that Ddc exhibit an allele-specific expression pattern in different brain regions, presumably reflecting the diverse regulatory mechanisms of imprinting.

Efficient reprogramming of human and mouse primary extra-embryonic cells to pluripotent stem cells.

Practical clinical applications for current induced pluripotent stem cell (iPSC) technologies are hindered by very low generation efficiencies. Here, we demonstrate that newborn human (h) and mouse (m) extra-embryonic amnion (AM) and yolk-sac (YS) cells, in which endogenous KLF4/Klf4, c-MYC/c-Myc and RONIN/Ronin are expressed, can be reprogrammed to hiPSCs and miPSCs with efficiencies for AM cells of 0.02% and 0.1%, respectively. Both hiPSC and miPSCs are indistinguishable from embryonic stem cells in colony morphology, expression of pluripotency markers, global gene expression profile, DNA methylation status of OCT4 and NANOG, teratoma formation and, in the case of miPSCs, generation of germline transmissible chimeric mice. As copious amounts of human AM cells can be collected without invasion, and stored long term by conventional means without requirement for in vitro culture, they represent an ideal source for cell banking and subsequent 'on demand' generation of hiPSCs for personal regenerative and pharmaceutical applications.

Distinct requirements for the maintenance and establishment of mouse embryonic stem cells.

Mouse embryonic stem cells (ESCs) that maintain a sustainable pluripotent state are derived from the inner cell mass (ICM) of blastocysts, in which pluripotency is lost during differentiation in vivo. It is unclear when and how the ability to maintain pluripotency is acquired during the derivation of ESCs. We analyzed the required culture condition for the maintenance and establishment of ESCs in detail. Even at low concentration of the GSK3ß inhibitor and LIF (LowGiL), the expression levels of pluripotency markers and the chimera-producing ability of the cells were comparable with those of ESCs cultured in the presence of both inhibitors and LIF (2iL). However, blastocysts underwent spontaneous differentiation, and ESCs were not established under LowGiL condition. Time-course analysis showed that 2iL condition for three days from the initiation of culture was sufficient for the acquisition of permanent pluripotency. Although X chromosome-linked pluripotent genes were significantly up-regulated during the culture of both male and female blastocysts in 2iL condition, no such up-regulation was observed in LowGiL condition. In conclusion, 2iL-dependent activation of these X-linked genes at the earliest phase of ESC derivation is one of the molecular bases for the acquisition of permanent pluripotency.

Novel miRNA biomarkers for genotoxicity screening in mouse.

The genotoxic potential of drugs is a serious problem, and its evaluation is one of the most critical processes of drug development. Although the comet assay of compound-exposed tissue is a frequently used genotoxicity test, its high false-positive rate is a major complication, and we consistently obtained false-positive results using the comet assay of mouse liver for nine hepatotoxic non-genotoxins (NGTXs). To identify novel genotoxin (GTX)-specific biomarkers, we screened the expression of 750 microRNAs (miRNAs) in the livers of mice treated with GTXs or NGTXs. Three miRNAs, miR-22-3p, miR-409-3p, and miR-543-3p, were significantly down-regulated in GTX-treated mouse liver. In contrast, these three miRNAs were significantly up-regulated in plasma. A discrimination model based on the expression levels of these biomarkers successfully identified GTXs and NGTXs. This novel biomarker expression-based discrimination model analysis using both liver and plasma is effective for detecting genotoxicity with high sensitivity and reliability to support drug development.

Pluripotential competence of cells associated with Nanog activity.

Nanog is a novel pluripotential cell-specific gene that plays a crucial role in maintaining the undifferentiated state of early postimplantation embryos and embryonic stem (ES) cells. We have explored the expression pattern and function of Nanog and a Nanog-homologue, Nanog-ps1.Nanog-ps1 was mapped on Chromosome 7 and shown to be a pseudogene. Immunocytochemical analysis in vivo showed that the NANOG protein was absent in unfertilized oocytes, and was detected in cells of morula-stage embryos, the inner cell mass of blastocysts and the epiblast of E6.5 and E7.5 embryos, but not in primordial germ cells of early postimplantation embryos. In monkey and human ES cells, NANOG expression was restricted to undifferentiated cells. Furthermore, reactivation of the somatic cell-derived Nanog was tightly linked with nuclear reprogramming induced by cell hybridization with ES cells and by nuclear transplantation into enucleated oocytes. Notably, mouse Nanog (+/-) ES cells, which produced approximately half the amount of NANOG produced by wild-type ES cells, readily differentiated to multi-lineage cells in culture medium including LIF. The labile undifferentiated state was fully rescued by constitutive expression of exogenous Nanog. Thus, the activity of Nanog is tightly correlated with an undifferentiated state of cells even in nuclear reprogrammed somatic cells. Nanog may function as a key regulator for sustaining pluripotency in a dose-dependent manner.