A potent, broadly neutralizing human monoclonal antibody that efficiently protects hACE2-transgenic mice from infection with the Wuhan, BA.5, and XBB.1.5 SARS-CoV-2 variants.

The COVID-19 pandemic has uncovered the high genetic variability of the SARS-CoV-2 virus and its ability to evade the immune responses that were induced by earlier viral variants. Only a few monoclonal antibodies that have been reported to date are capable of neutralizing a broad spectrum of SARS-CoV-2 variants. Here, we report the isolation of a new broadly neutralizing human monoclonal antibody, iC1. The antibody was identified through sorting the SARS-CoV-1 RBD-stained individual B cells that were isolated from the blood of a vaccinated donor following a breakthrough infection. In vitro, iC1 potently neutralizes pseudoviruses expressing a wide range of SARS-CoV-2 Spike variants, including those of the XBB sublineage. In an hACE2-transgenic mouse model, iC1 provided effective protection against the Wuhan strain of the virus as well as the BA.5 and XBB.1.5 variants. Therefore, iC1 can be considered as a potential component of the broadly neutralizing antibody cocktails resisting the SARS-CoV-2 mutation escape.

Time origin and structural analysis of the induced CRISPR/cas9 megabase-sized deletions and duplications involving the Cntn6 gene in mice.

In a previous study using one-step CRISPR/Cas9 genome editing in mouse zygotes, we created five founders carrying a 1,137 kb deletion and two founders carrying the same deletion, plus a 2,274 kb duplication involving the Cntn6 gene (encoding contactin-6). Using these mice, the present study had the following aims: (i) to establish stage of origin of these rearrangements; (ii) to determine the fate of the deleted DNA fragments; and (iii) to estimate the scale of unpredicted DNA changes accompanying the rearrangements. The present study demonstrated that all targeted deletions and duplications occurred at the one-cell stage and more often in one pronucleus only. FISH analysis revealed that there were no traces of the deleted DNA fragments either within chromosome 6 or on other chromosomes. These data were consistent with the Southern blot analysis showing that chromosomes with deletion often had close to expected sizes of removed DNA fragments. High-throughput DNA sequencing of two homozygotes for duplication demonstrated that there were no unexpected significant or scale DNA changes either at the gRNA and joint sites or other genome sites. Thus, our data suggested that CRISPR/Cas9 technology could generate megabase-sized deletions and duplications in mouse gametes at a reasonably specific level.

Targeted genomic integration of EGFP under tubulin beta 3 class III promoter and mEos2 under tryptophan hydroxylase 2 promoter does not produce sufficient levels of reporter gene expression.

Neuronal tracing is a modern technology that is based on the expression of fluorescent proteins under the control of cell type-specific promoters. However, random genomic integration of the reporter construct often leads to incorrect spatial and temporal expression of the marker protein. Targeted integration (or knock-in) of the reporter coding sequence is supposed to provide better expression control by exploiting endogenous regulatory elements. Here we describe the generation of two fluorescent reporter systems: enhanced green fluorescent protein (EGFP) under pan-neural marker class III ß-tubulin (Tubb3) promoter and mEos2 under serotonergic neuron-specific tryptophan hydroxylase 2 (Tph2) promoter. Differentiation of Tubb3-EGFP embryonic stem (ES) cells into neurons revealed that though Tubb3-positive cells express EGFP, its expression level is not sufficient for the neuronal tracing by routine fluorescent microscopy. Similarly, the expression levels of mEos2-TPH2 in differentiated ES cells was very low and could be detected only on messenger RNA level using polymerase chain reaction-based methods. Our data shows that the use of endogenous regulatory elements to control transgene expression is not always beneficial compared with the random genomic integration.

Allele-Specific Biased Expression of the CNTN6 Gene in iPS Cell-Derived Neurons from a Patient with Intellectual Disability and 3p26.3 Microduplication Involving the CNTN6 Gene.

Copy number variations (CNVs) of the human CNTN6 gene caused by megabase-scale microdeletions or microduplications in the 3p26.3 region are often the cause of neurodevelopmental disorders, including intellectual disability and developmental delay. Surprisingly, patients with different copy numbers of this gene display notable overlapping of neuropsychiatric symptoms. The complexity of the study of human neuropathologies is associated with the inaccessibility of brain material. This problem can be overcome through the use of reprogramming technologies that permit the generation of induced pluripotent stem (iPS) cells from fibroblasts and their subsequent in vitro differentiation into neurons. We obtained a set of iPS cell lines derived from a patient carrier of the CNTN6 gene duplication and from two healthy donors. All iPS cell lines displayed the characteristics of pluripotent cells. Some iPS cell lines derived from the patient and from healthy donors were differentiated in vitro by exogenous expression of the Ngn2 transcription factor or by spontaneous neural differentiation of iPS cells through the neural rosette stage. The obtained neurons showed the characteristics of mature neurons as judged by the presence of neuronal markers and by their electrophysiological characteristics. Analysis of allele-specific expression of the CNTN6 gene in these neuronal cells by droplet digital PCR demonstrated that the level of expression of the duplicated allele was significantly reduced compared to that of the wild-type allele. Importantly, according to the sequencing data, both copies of the CNTN6 gene, which were approximately 1 Mb in size, showed no any additional structural rearrangements.

Unexpected phenotypic effects of a transgene integration causing a knockout of the endogenous Contactin-5 gene in mice.

Contactins (Cntn1-6) are a family of neuronal membrane proteins expressed in the brain. They are required for establishing cell-to-cell contacts between neurons and for the growth and maturation of the axons. In humans, structural genomic variations in the Contactin genes are implicated in neurodevelopmental disorders. In addition, population genetic studies associate Contactins loci with obesity and hypertension. Cntn5 knockout mice were first described in 2003, but showed no gross physiological or behavioral abnormalities (just minor auditory defects). We report a novel Cntn5 knockout mouse line generated by a random transgene integration as an outcome of pronuclear microinjection. Investigation of the transgene integration site revealed that the 6Kbp transgene construct coding for the human granulocyte-macrophage colony-stimulating factor (hGMCSF) replaced 170 Kbp of the Cntn5 gene, including four exons. Reverse transcription PCR analysis of the Cntn5 transcripts in the wild-type and transgenic mouse lines showed that splicing of the transgene leads to a set of chimeric hGMCSF-Cntn5 transcript variants, none of which encode functional Cntn5 protein due to introduction of stop codons. Although Cntn5 knockout animals displayed no abnormalities in behavior, we noted that they were leaner, with less body mass and fat percentage than wild-type animals. Their cardiovascular parameters (heart rate, blood pressure and blood flow speed) were elevated compared to controls. These findings link Cntn5 deficiency to obesity and hypertension.

Alternative dominance of the parental genomes in hybrid cells generated through the fusion of mouse embryonic stem cells with fibroblasts.

For the first time, two types of hybrid cells with embryonic stem (ES) cell-like and fibroblast-like phenotypes were produced through the fusion of mouse ES cells with fibroblasts. Transcriptome analysis of 2,848 genes differentially expressed in the parental cells demonstrated that 34-43% of these genes are expressed in hybrid cells, consistent with their phenotypes; 25-29% of these genes display intermediate levels of expression, and 12-16% of these genes maintained expression at the parental cell level, inconsistent with the phenotype of the hybrid cell. Approximately 20% of the analyzed genes displayed unexpected expression patterns that differ from both parents. An unusual phenomenon was observed, namely, the illegitimate activation of Xist expression and the inactivation of one of two X-chromosomes in the near-tetraploid fibroblast-like hybrid cells, whereas both Xs were active before and after in vitro differentiation of the ES cell-like hybrid cells. These results and previous data obtained on heterokaryons suggest that the appearance of hybrid cells with a fibroblast-like phenotype reflects the reprogramming, rather than the induced differentiation, of the ES cell genome under the influence of a somatic partner.

Generation of megabase-scale deletions, inversions and duplications involving the Contactin-6 gene in mice by CRISPR/Cas9 technology.

BACKGROUND: Copy Number Variation (CNV) of the human CNTN6 gene (encoding the contactin-6 protein), caused by deletions or duplications, is responsible for severe neurodevelopmental impairments, often in combination with facial dysmorphias. Conversely, deleterious point mutations of this gene do not show any clinical phenotypes. The aim of this study is to generate mice carrying large deletions, duplications and inversions involving the Cntn6 gene as a new experimental model to study CNV of the human CNTN6 locus. RESULTS: To generate large chromosomal rearrangements on mouse chromosome 6, we applied CRISPR/Cas9 technology in zygotes. Two guide RNAs (gRNAs) (flanking a DNA fragment of 1137 Mb) together with Cas9 mRNA and single-stranded DNA oligonucleotides (ssODN) were microinjected into the cytoplasm of 599 zygotes of F1 (C57BL x CBA) mice, and 256 of them were transplanted into oviducts of CD-1 females. As a result, we observed the birth of 41 viable F0 offspring. Genotyping of these mice was performed by PCR analysis and sequencing of PCR products. Among the 41 F0 offspring, we identified seven mice with deletions, two animals carrying duplications of the gene and four carrying inversions. Interestingly, two F0 offspring had both deletions and duplications. It is important to note that while three of seven deletion carriers showed expected sequences at the new joint sites, in another three, we identified an absence of 1-10 nucleotides at the CRISPR/Cas9 cut sites, and in one animal, 103 bp were missing, presumably due to error-prone non-homologous end joining. In addition, we detected the absence of 5 and 13 nucleotides at these sites in two F0 duplication carriers. Similar sequence changes at CRISPR/Cas9 cut sites were observed at the right and left boundaries of inversions. Thus, megabase-scale deletions, duplications and inversions were identified in 11 F0 offspring among 41 analyzed, i.e., approximately 25% efficiency. All genetically modified F0 offspring were viable and able to transmit these large chromosomal rearrangements to the next generation. CONCLUSIONS: Using CRISPR/Cas9 technology, we created mice carrying megabase-scale deletions, duplications, and inversions involving the full-sized Cntn6 gene. These mice became founders of new mouse lines, which may be more appropriate experimental models of CNV in the human 3p26.3 region than Сntn6 knockout mice.

Comparison of American mink embryonic stem and induced pluripotent stem cell transcriptomes.

BACKGROUND: Recently fibroblasts of many mammalian species have been reprogrammed to pluripotent state using overexpression of several transcription factors. This technology allows production of induced pluripotent stem (iPS) cells with properties similar to embryonic stem (ES) cells. The completeness of reprogramming process is well studied in such species as mouse and human but there is not enough data on other species. We produced American mink (Neovison vison) ES and iPS cells and compared these cells using transcriptome analysis. RESULTS: We report the generation of 10 mink ES and 22 iPS cell lines. The majority of the analyzed cell lines had normal diploid chromosome number. The only ES cell line with XX chromosome set had both X-chromosomes in active state that is characteristic of pluripotent cells. The pluripotency of ES and iPS cell lines was confirmed by formation of teratomas with cell types representing all three germ layers. Transcriptome analysis of mink embryonic fibroblasts (EF), two ES and two iPS cell lines allowed us to identify 11831 assembled contigs which were annotated. These led to a number of 6891 unique genes. Of these 3201 were differentially expressed between mink EF and ES cells. We analyzed expression levels of these genes in iPS cell lines. This allowed us to show that 80% of genes were correctly reprogrammed in iPS cells, whereas approximately 6% had an intermediate expression pattern, about 7% were not reprogrammed and about 5% had a "novel" expression pattern. We observed expression of pluripotency marker genes such as Oct4, Sox2 and Rex1 in ES and iPS cell lines with notable exception of Nanog. CONCLUSIONS: We had produced and characterized American mink ES and iPS cells. These cells were pluripotent by a number of criteria and iPS cells exhibited effective reprogramming. Interestingly, we had showed lack of Nanog expression and consider it as a species-specific feature.

Generation of mouse chimeras with high contribution of tetraploid embryonic stem cells and embryonic stem cell-fibroblast hybrid cells.

The in vitro long-term cultivation of embryonic stem (ES) cells derived from pre-implantation embryos offers the unique possibility of combining ES cells with pre-implantation embryos to generate chimeras, thus facilitating the creation of a bridge between in vitro and in vivo investigations. Genomic manipulation using ES cells and homologous recombination is one of the most outstanding scientific achievements, resulting in the generation of animals with desirable genome modifications. As such, the generation of ES cells with different ploidy via cell fusion also deserves much attention because this approach allows for the production of chimeras that contain somatic cells with various ploidy. Therefore, this is a powerful tool that can be used to study the role of polyploidy in the normal development of mammals.

Comparison of the three-dimensional organization of sperm and fibroblast genomes using the Hi-C approach.

BACKGROUND: The three-dimensional organization of the genome is tightly connected to its biological function. The Hi-C approach was recently introduced as a method that can be used to identify higher-order chromatin interactions genome-wide. The aim of this study was to determine genome-wide chromatin interaction frequencies using the Hi-C approach in mouse sperm cells and embryonic fibroblasts. RESULTS: The obtained data demonstrate that the three-dimensional genome organizations of sperm and fibroblast cells show a high degree of similarity both with each other and with the previously described mouse embryonic stem cells. Both A- and B-compartments and topologically associated domains are present in spermatozoa and fibroblasts. Nevertheless, sperm cells and fibroblasts exhibit statistically significant differences between each other in the contact probabilities of defined loci. Tight packaging of the sperm genome results in an enrichment of long-range contacts compared with the fibroblasts. However, only 30% of the differences in the number of contacts are based on differences in the densities of their genome packages; the main source of the differences is the gain or loss of contacts that are specific for defined genome regions. We find that the dependence of the contact probability on genomic distance for sperm is close to the dependence predicted for the fractal globular folding of chromatin. CONCLUSIONS: Overall, we can conclude that the three-dimensional structure of the genome is passed through generations without being dramatically changed in sperm cells.

Methodological strategies for transgene copy number quantification in goats (Capra hircus) using real-time PCR.

Taking into account the importance of goats as transgenic models, as well as the rarity of copy number (CN) studies in farm animals, the present work aimed to evaluate methodological strategies for accurate and precise transgene CN quantification in goats using quantitative polymerase chain reaction (qPCR). Mouse and goat lines transgenic for human granulocyte-colony stimulating factor were used. After selecting the best genomic DNA extraction method to be applied in mouse and goat samples, intra-assay variations, accuracy and precision of CN quantifications were assessed. The optimized conditions were submitted to mathematical strategies and used to quantify CN in goat lines. The findings were as follows: validation of qPCR conditions is required, and amplification efficiency is the most important. Absolute and relative quantifications are able to produce similar results. For normalized absolute quantification, the same plasmid fragment used to generate goat lines must be mixed with wild-type goat genomic DNA, allowing the choice of an endogenous reference gene for data normalization. For relative quantifications, a resin-based genomic DNA extraction method is strongly recommended when using mouse tail tips as calibrators to avoid tissue-specific inhibitors. Efficient qPCR amplifications (≥95%) allow reliable CN measurements with SYBR technology. TaqMan must be used with caution in goats if the nucleotide sequence of the endogenous reference gene is not yet well understood. Adhering to these general guidelines can result in more exact CN determination in goats. Even when working under nonoptimal circumstances, if assays are performed that respect the minimum qPCR requirements, good estimations of transgene CN can be achieved.

Dynamics of recombinant hG-CSF in transgenic goat: preliminary study in the founder during hormonally induced lactation.

This study aimed to characterize the dynamic of human granulocyte colony-stimulating factor (hG-CSF) during artificial lactation in a transgenic founder goat and to assess its potential ectopic expression and health. The female secreted 93.9 to 1,474.6 µg hG-CSF per mL of milk. Two peaks of serum hG-CSF (3,470 and 7,390 pg/mL) were detected in the first half of the lactation. Outside of the lactation, hG-CSF was absent from serum, indicating no ectopic expression. During the treatment to induce lactation, transgenic female presented increased neutrophil and lymphocyte blood counts when compared to nontransgenic female. Despite transient neutrophilia, serum biochemistry profiles indicated normal liver and renal functions. Thus, transgenic goat expressed hG-CSF in quantities sufficient for a commercial bioreactor and remained clinically healthy.

Reprogramming somatic cells by fusion with embryonic stem cells does not cause silencing of the Dlk1-Dio3 region in mice.

AIM: To examine the imprinted Dlk1-Dio3 locus in pluripotent embryonic stem (ES) cell/fibroblast hybrid cells. METHODS: Gtl2, Rian, and Mirg mRNA expression in mouse pluripotent ES cell/fibroblast hybrid cells was examined by real-time reverse transcription-polymerase chain reaction. Pyrosequencing and bisulfate sequencing were used to determine the DNA methylation level of the Dlk1-Dio3 locus imprinting control region. RESULTS: The selected hybrid clones had a near-tetraploid karyotype and were highly pluripotent judging from their capacity to generate chimeric embryos and adult chimeras. Our data clearly demonstrate that Gtl2, Rian, and Mirg, which are imprinted genes within the Dlk1-Dio3 locus, are active in all examined ES cell/fibroblast hybrid clones. In spite of interclonal variability, the expression of the imprinted genes is comparable to that of ES cells and fibroblasts. Quantitative analysis of the DNA methylation status of the intergenic differentially methylated region (IG DMR) within the Dlk1-Dio3 locus by pyrosequencing and bisulfite sequencing clearly showed that the DNA methylation status of the imprinted region in the tested hybrid clones was comparable to that of both ES cells and fibroblasts. CONCLUSION: Reprogramming process in a hybrid cell system is achieved without marked alteration of the imprinted Dlk1-Dio3 locus.

A 3,387 bp 5'-flanking sequence of the goat alpha-S1-casein gene provides correct tissue-specific expression of human granulocyte colony-stimulating factor (hG-CSF) in the mammary gland of transgenic mice.

A new expression vector containing the 1,944 bp 5'-flanking regulatory region together with exon 1 and intron 1 of the goat alpha-S1-casein gene (CSN1S1), the full-sized human granulocyte colony-stimulating factor gene (hGCSF) and the 3'-flanking sequence of the bovine CSN1S1, was created. The vector DNA was used for generation of four mouse transgenic lines. The transgene was integrated into chromosomes 8 and 12 of two founders as 2 and 5 copies, respectively. Tissue-specific secretion of hG-CSF into the milk of transgenic mice was in the range of 19-40 µg/ml. RT-PCR analysis of various tissues of the transgenic mice demonstrated that expression of hGCSF was detected in only the mammary gland in the progeny of all founders. Moreover, cells were shown to be positive for hG-CSF by immunofluorescent analysis in the mammary glands but not in any other tissues. There were no signs of mosaic expression in the mammary gland. Trace amounts of hG-CSF were detected in the serum of females of two transgenic lines during lactation only. However, no transgenic mice showed any changes in hematopoiesis based on the number of granulocytes in blood. Immunoblotting of hG-CSF in the milk of transgenic mice revealed two forms, presumably the glycosylated and non-glycosylated forms. The hematopoietic activity of hG-CSF in the milk of transgenic females is comparable to that of recombinant G-CSF. In general, the data obtained in this study show that the new expression vector is able to provide correct tissue-specific expression of hG-CSF with high biological activity in transgenic mice.

Reprogramming mediated by cell fusion technology.

This review is focused on recent advances in fusion-based reprogramming of cells of different pluripotent statuses or lineage origins. Recent findings are discussed from standpoints of both the developmental potency of hybrid cells generated by fusion of pluripotent embryonic stem (ES) cells, embryonal carcinoma (EC) cells, and somatic cells and epigenetic mechanisms and other aspects involved in the reprogramming process. Complete reprogramming occurs at least 5-7 days after fusion and includes at least two steps. (i) initiation at the heterokaryon stage and choice of the direction of reprogramming using an "all-or-none principle" to establish the dominance of one parental genome and (ii) "fixation" of the newly acquired expression profile by epigenetic mechanisms. The first step is realized without cell division, whereas the second requires cell proliferation. Reprogramming in hybrid cells is rapid and complete. Thus, cell fusion is a powerful tool for reprogramming.

Bidirectional reprogramming of mouse embryonic stem cell/fibroblast hybrid cells is initiated at the heterokaryon stage.

Immunofluorescent analysis of markers specific for parental genomes was used to study heterokaryons and hybrid cells soon after the fusion of diploid embryonic stem (ES) cells marked with green fluorescent protein and diploid fibroblasts labeled by blue fluorescent beads. Heterokaryons were identified by an analysis of parental mitochondrial DNAs. Within 20 h after fusion, most heterokaryons (up to 80%) had a fibroblast-like phenotype, being positive for typical fibroblast markers (collagen type I, fibronectin, lamin A/C) and for the modification me3H3K27 chromatin marking the inactive X chromosome but being negative for Oct4 and Nanog. Approximately 20% of heterokaryons had an alternative ES-like phenotype being positive for Oct4 and Nanog, with signs of reactivation of the previously inactive X-chromosome but negative for fibroblast markers. Hybrid cells having alternative phenotypes were easily identified from 24-48 h. The level of DNA methylation at the promoter of the fibroblast Oct4 allele in the ES-like hybrid cells at day 4 was similar to that of ES cells but at the same time, both parental Oct4 alleles were heavily methylated in fibroblast-like hybrid cells. Thus, bidirectional reprogramming initiated at the heterokaryon stage seems to lead to the formation of two types of hybrid cells with alternative dominance of the parental genomes. However, the further fates of two types of hybrid cells are different: ES-like hybrid cells form colonies at 4-6 days but no colonies are derived from the fibroblast-like hybrid cells. The latter grow as disconnected single cells and are unable to form colonies, like mouse embryonic fibroblasts.

Isolation of oogonia from ovaries of the sea urchin Strongylocentrotus nudus.

The presence of oogonia in the ovaries of adult females is typical in species with a broadcast spawning reproductive strategy, including invertebrates and lower vertebrates. In sea urchins, difficulties in the study of oogonia arise from the small number of these cells and the lack of specific markers for their identification. Therefore, more reliable methods are needed for identifying and manipulating oogonial cells in quantities sufficient for experimentation. Homologs of the DEAD-box RNA helicase vasa expressed in germline cells have been proposed for use as markers to detect germline cells in diverse species. We have developed a method for the isolation of sea urchin oogonia by using immunocytochemistry with vasa antibodies, together with reverse transcription and the polymerase chain reaction to detect the expression of Sp-vasa and Sp-nanos2 homologs and a morphological approach to identify germline cells in sea urchin ovaries and cell fractions isolated from the ovarian germinal epithelium. This method has allowed us to obtain 15%-18% of small oogonia with 70%-75% purity from the total amount of isolated germ cells. Our findings represent the first methodological basis for obtaining cell populations containing sea urchin oogonia; this method might be useful as a tool for further investigations of the early stages of sea urchin oogenesis.

Dominance of parental genomes in embryonic stem cell/fibroblast hybrid cells depends on the ploidy of the somatic partner.

Two dozen hybrid clones were produced by fusion of diploid embryonic stem (ES) cells positive for green fluorescent protein (GFP) with tetraploid fibroblasts derived from DD/c and C57BL-I(I)1RK mice. Cytogenetic analysis demonstrated that most cells from these hybrid clones contained near-hexaploid chromosome sets. Additionally, the presence of chromosomes derived from both parental cells was confirmed by polymerase chain reaction (PCR) analysis of polymorphic microsatellites. All hybrid cells were positive for GFP and demonstrated growth characteristics and fibroblast-like morphology. In addition, most hybrid cells were positive for collagen type I, fibronectin, and lamin A/C but were negative for Oct4 and Nanog proteins. Methylation status of the Oct4 and Nanog gene promoters was evaluated by bisulfite genomic sequencing analysis. The methylation sites (CpG-sites) of the Oct4 and Nanog gene promoters were highly methylated in hybrid cells, whereas the CpG-sites were unmethylated in the parental ES cells. Thus, the fibroblast genome dominated the ES genome in the diploid ES cell/tetraploid fibroblast hybrid cells. Immunofluorescent analysis of the pluripotent and fibroblast markers demonstrated that establishment of the fibroblast phenotype occurred shortly after fusion and that the fibroblast phenotype was further maintained in the hybrid cells. Fusion of karyoplasts and cytoplast derived from tetraploid fibroblasts with whole ES cells demonstrated that karyoplasts were able to establish the fibroblast phenotype of the reconstructed cells but not fibroblast cytoplasts. Thus, these data suggest that the dominance of parental genomes in hybrid cells of ES cell/somatic cell type depends on the ploidy of the somatic partner.

Allelic expression and DNA methylation profiles of promoters at the parental Oct4 and Nanog genes in Mus musculus ES cell/Mus caroli splenocyte hybrid cells.

Expression of the parental Oct4 and Nanog alleles and DNA methylation of their promoters were studied in a set of Mus musculus embryonic stem (ES) cell/M. caroli splenocyte hybrid cells containing a variable ratio of parental chromosomes 6 and 17. The transcripts of the reactivated splenocyte Oct4 and Nanog genes were revealed in all hybrid cell clones positive for M. caroli chromosomes 6 and 17. We found that 11 CpG sites in the Oct4 promoter were heavily methylated in M. caroli splenocytes (>80%), whereas M. musculus ES cells were essentially unmethylated (<1%). Analysis of the methylation status of the Oct4 promoter in seven hybrid cell clones showed that the splenocyte-derived promoter sequence lost DNA methylation so that its methylation level was comparable with that of the ES cells. Additionally, no preferential de novo methylation was seen in the Oct4 promoters of M. musculus and M. caroli in teratomas developed from two independent hybrid clones. The upstream region of Nanog was heavily methylated in mouse embryonic fibroblasts (66%) and less methylated in M. caroli splenocytes (24%). The Nanog promoter region was completely unmethylated in M. musculus ES cells. We found that both parental alleles of the Nanog gene promoter were essentially unmethylated in five examined hybrid clones. Thus, we have demonstrated that (1) the Oct4 and Nanog genes of splenocytes are activated, and their promoters undergo demethylation in ES cell hybrids; (2) these events are independent of the number and ratio of parental chromosomes carrying these genes.