DNA methylation analysis and editing in single mammalian oocytes

Transgenerational Epigenetic DNA Methylation Editing and Human Disease

During gestation, maternal (F0), embryonic (F1), and migrating primordial germ cell (F2) genomes can be simultaneously exposed to environmental influences. Accumulating evidence suggests that operating epi- or above the genetic DNA sequence, covalent DNA methylation (DNAme) can be recorded onto DNA in response to environmental insults, some sites which escape normal germline erasure. These appear to intrinsically regulate future disease propensity, even transgenerationally. Thus, an organism's genome can undergo epigenetic adjustment based on environmental influences experienced by prior generations. During the earliest stages of mammalian development, the three-dimensional presentation of the genome is dramatically changed, and DNAme is removed genome wide. Why, then, do some pathological DNAme patterns appear to be heritable? Are these correctable? In the following sections, I review concepts of transgenerational epigenetics and recent work towards programming transgenerational DNAme. A framework for editing heritable DNAme and challenges are discussed, and ethics in human research is introduced.

CRISPR/Cas9 technology: applications in oocytes and early embryos

CRISPR/Cas9, a highly versatile genome-editing tool, has garnered significant attention in recent years. Despite the unique characteristics of oocytes and early embryos compared to other cell types, this technology has been increasing used in mammalian reproduction. In this comprehensive review, we elucidate the fundamental principles of CRISPR/Cas9-related methodologies and explore their wide-ranging applications in deciphering molecular intricacies during oocyte and early embryo development as well as in addressing associated diseases. However, it is imperative to acknowledge the limitations inherent to these technologies, including the potential for off-target effects, as well as the ethical concerns surrounding the manipulation of human embryos. Thus, a judicious and thoughtful approach is warranted. Regardless of these challenges, CRISPR/Cas9 technology undeniably represents a formidable tool for genome and epigenome manipulation within oocytes and early embryos. Continuous refinements in this field are poised to fortify its future prospects and applications.

Toward the Development of Epigenome Editing-Based Therapeutics: Potentials and Challenges

The advancement in epigenetics research over the past several decades has led to the potential application of epigenome-editing technologies for the treatment of various diseases. In particular, epigenome editing is potentially useful in the treatment of genetic and other related diseases, including rare imprinted diseases, as it can regulate the expression of the epigenome of the target region, and thereby the causative gene, with minimal or no modification of the genomic DNA. Various efforts are underway to successfully apply epigenome editing in vivo, such as improving target specificity, enzymatic activity, and drug delivery for the development of reliable therapeutics. In this review, we introduce the latest findings, summarize the current limitations and future challenges in the practical application of epigenome editing for disease therapy, and introduce important factors to consider, such as chromatin plasticity, for a more effective epigenome editing-based therapy.

Targeted DNA Methylation in Mouse Early Embryos

In germ cell lines, including early preimplantation embryos, centromeres and pericentromeres are known to show a marked hypomethylation pattern compared to somatic cells. Elucidation of the biological function of this region-specific DNA hypomethylation state, region-specific epigenomic manipulation is essential as an analytical method. We have applied genome editing to show that region-specific DNA methylation can be effectively introduced by a fusion protein, TALE, which recognizes pericentromeres, and SssI, a bacterial CpG methyltransferase. This makes it possible to increase the DNA methylation state of the pericentromeres, which is normally about 20%, to about 60-75%, enabling comparative analysis of the developmental processes of normal embryos with hypomethylated pericentromeres and embryos that have been epigenetically edited to be hypermethylated. In this chapter, we describe a method for introducing DNA methylation into pericentromeres of early mouse embryos by expressing TALE-SssI fusion protein and a method for detecting DNA methylation.

Integration of parallel metabolomics and transcriptomics reveals metabolic patterns in porcine oocytes during maturation

Well-controlled metabolism is the prerequisite for optimal oocyte development. To date, numerous studies have focused mainly on the utilization of exogenous substrates by oocytes, whereas the underlying mechanism of intrinsic regulation during meiotic maturation is less characterized. Herein, we performed an integrated analysis of parallel metabolomics and transcriptomics by isolating porcine oocytes at three time points, cooperatively depicting the global picture of the metabolic patterns during maturation. In particular, we identified the novel metabolic features during porcine oocyte meiosis, such as the fall in bile acids, the active one-carbon metabolism and a progressive decline in nucleotide metabolism. Collectively, the current study not only provides a comprehensive multiple omics data resource, but also may facilitate the discovery of molecular biomarkers that could be used to predict and improve oocyte quality.

Efficient generation of epigenetic disease model mice by epigenome editing using the piggyBac transposon system

BACKGROUND

Epigenome-edited animal models enable direct demonstration of disease causing epigenetic mutations. Transgenic (TG) mice stably expressing epigenome-editing factors exhibit dramatic and stable changes in target epigenome modifications. Successful germline transmission of a transgene from founder mice to offspring will yield a sufficient number of epigenome-edited mice for phenotypic analysis; however, if the epigenetic mutation has a detrimental phenotypic effect, it can become difficult to obtain the next generation of animals. In this case, the phenotype of founder mice must be analyzed directly. Unfortunately, current TG mouse production efficiency (TG founders per pups born) is relatively low, and improvements would increase the versatility of this technology.

RESULTS

In the current study, we describe an approach to generate epigenome-edited TG mice using a combination of both the dCas9-SunTag and piggyBac (PB) transposon systems. Using this system, we successfully generated mice with demethylation of the differential methylated region of the H19 gene (H19-DMR), as a model for Silver-Russell syndrome (SRS). SRS is a disorder leading to growth retardation, resulting from low insulin-like growth factor 2 (IGF2) gene expression, often caused by epimutations at the H19-IGF2 locus. Under optimized conditions, the efficiency of TG mice production using the PB system was approximately threefold higher than that using the conventional method. TG mice generated by this system showed demethylation of the targeted DNA region and associated changes in gene expression. In addition, these mice exhibited some features of SRS, including intrauterine and postnatal growth retardation, due to demethylation of H19-DMR.

CONCLUSIONS

The dCas9-SunTag and PB systems serve as a simple and reliable platform for conducting direct experiments using epigenome-edited founder mice.

Viable offspring derived from single unfertilized mammalian oocytes

In mammals, parthenogenesis is limited because of problems arising from genomic imprinting. Here, we report live mammalian offspring derived from single unfertilized eggs. This was achieved by the targeted DNA methylation rewriting of seven imprinting control regions. By designing guide RNAs with protospacer adjacent motif (PAM) sequences matching one allele but not the other, dCas9-Dnmt3a or dCpf1-Tet1 enables targeted DNA methylation editing in an allele-specific manner. The success of parthenogenesis in mammals opens many opportunities in agriculture, research, and medicine.

In mammals, a new life starts with the fusion of an oocyte and a sperm cell. Parthenogenesis, a way of generating offspring solely from female gametes, is limited because of problems arising from genomic imprinting. Here, we report live mammalian offspring derived from single unfertilized oocytes, which was achieved by targeted DNA methylation rewriting of seven imprinting control regions. Oocyte coinjection of catalytically inactive Cas9 (dCas9)-Dnmt3a or dCpf1-Tet1 messenger RNA (mRNA) with single-guide RNAs (sgRNAs) targeting specific regions induced de novo methylation or demethylation, respectively, of the targeted region. Following parthenogenetic activation, these edited regions showed maintenance of methylation as naturally established regions during early preimplantation development. The transfer of modified parthenogenetic embryos into foster mothers resulted in significantly extended development and finally in the generation of viable full-term offspring. These data demonstrate that parthenogenesis can be achieved by targeted epigenetic rewriting of multiple critical imprinting control regions.

Integrative Genome-Wide DNA Methylome and Transcriptome Analysis of Ovaries from Hu Sheep with High and Low Prolific

DNA methylation plays an important role in biological processes by affecting gene expression. However, how DNA methylation regulates phenotypic variation in Hu sheep remains unclear. Therefore, we generated genome-wide DNA methylation and transcriptomic profiles in the ovaries of Hu sheep with different prolificacies and genotypes (FecBB and FecB+). Results showed that ovary DNA methylome and transcriptome were significantly different between high prolificacy and low prolificacy Hu sheep. Comparative methylome analyses identified 10,644, 9,594, and 12,214 differentially methylated regions and 87, 1,121, and 2,375 genes, respectively, showing differential expression levels in three different comparison groups. Female reproduction-associated differentially methylated regions-related genes and differentially expressed genes were enriched, thereby the respective interaction networks were constructed. Furthermore, systematical integrative analyses revealed a negative correlation between DNA methylation around the transcriptional start site and gene expression levels, which was confirmed by testing the expression of integrin β2 subunit (ITGB2) and lysosome-associated protein transmembrane-4 beta (LAPTM4B) in vivo and in vitro. These findings demonstrated that DNA methylation influences the propensity for prolificacy by affecting gene expression in the ovaries, which may contribute to a greater understanding of the epigenome and transcriptome that will be useful for animal breeding.

Single-Cell Analysis Using Machine Learning Techniques and Its Application to Medical Research

In recent years, the diversity of cancer cells in tumor tissues as a result of intratumor heterogeneity has attracted attention. In particular, the development of single-cell analysis technology has made a significant contribution to the field; technologies that are centered on single-cell RNA sequencing (scRNA-seq) have been reported to analyze cancer constituent cells, identify cell groups responsible for therapeutic resistance, and analyze gene signatures of resistant cell groups. However, although single-cell analysis is a powerful tool, various issues have been reported, including batch effects and transcriptional noise due to gene expression variation and mRNA degradation. To overcome these issues, machine learning techniques are currently being introduced for single-cell analysis, and promising results are being reported. In addition, machine learning has also been used in various ways for single-cell analysis, such as single-cell assay of transposase accessible chromatin sequencing (ATAC-seq), chromatin immunoprecipitation sequencing (ChIP-seq) analysis, and multi-omics analysis; thus, it contributes to a deeper understanding of the characteristics of human diseases, especially cancer, and supports clinical applications. In this review, we present a comprehensive introduction to the implementation of machine learning techniques in medical research for single-cell analysis, and discuss their usefulness and future potential.

Aberrant DNA Methylation Mediates the Transgenerational Risk of Metabolic and Chronic Disease Due to Maternal Obesity and Overnutrition

Maternal obesity is a rapidly evolving universal epidemic leading to acute and long-term medical and obstetric health issues, including increased maternal risks of gestational diabetes, hypertension and pre-eclampsia, and the future risks for offspring's predisposition to metabolic diseases. Epigenetic modification, in particular DNA methylation, represents a mechanism whereby environmental effects impact on the phenotypic expression of human disease. Maternal obesity or overnutrition contributes to the alterations in DNA methylation during early life which, through fetal programming, can predispose the offspring to many metabolic and chronic diseases, such as non-alcoholic fatty liver disease, obesity, diabetes, and chronic kidney disease. This review aims to summarize findings from human and animal studies, which support the role of maternal obesity in fetal programing and the potential benefit of altering DNA methylation to limit maternal obesity related disease in the offspring.

Variably methylated retrotransposons are refractory to a range of environmental perturbations

The agouti viable yellow (Avy) allele is an insertional mutation in the mouse genome caused by a variably methylated intracisternal A particle (VM-IAP) retrotransposon. Avy expressivity is sensitive to a range of early-life chemical exposures and nutritional interventions, suggesting that environmental perturbations can have long-lasting effects on the methylome. However, the extent to which VM-IAP elements are environmentally labile with phenotypic implications is unknown. Using a recently identified repertoire of VM-IAPs, we assessed the epigenetic effects of different environmental contexts. A longitudinal aging analysis indicated that VM-IAPs are stable across the murine lifespan, with only small increases in DNA methylation detected for a subset of loci. No significant effects were observed after maternal exposure to the endocrine disruptor bisphenol A, an obesogenic diet or methyl donor supplementation. A genetic mouse model of abnormal folate metabolism exhibited shifted VM-IAP methylation levels and altered VM-IAP-associated gene expression, yet these effects are likely largely driven by differential targeting by polymorphic KRAB zinc finger proteins. We conclude that epigenetic variability at retrotransposons is not predictive of environmental susceptibility.

Targeted De-Methylation of the FOXP3-TSDR Is Sufficient to Induce Physiological FOXP3 Expression but Not a Functional Treg Phenotype

CD4+ regulatory T cells (Tregs) are key mediators of immunological tolerance and promising effector cells for immuno-suppressive adoptive cellular therapy to fight autoimmunity and chronic inflammation. Their functional stability is critical for their clinical utility and has been correlated to the demethylated state of the TSDR/CNS2 enhancer element in the Treg lineage transcription factor FOXP3. However, proof for a causal contribution of the TSDR de-methylation to FOXP3 stability and Treg induction is so far lacking. We here established a powerful transient-transfection CRISPR-Cas9-based epigenetic editing method for the selective de-methylation of the TSDR within the endogenous chromatin environment of a living cell. The induced de-methylated state was stable over weeks in clonal T cell proliferation cultures even after expression of the editing complex had ceased. Epigenetic editing of the TSDR resulted in FOXP3 expression, even in its physiological isoform distribution, proving a causal role for the de-methylated TSDR in FOXP3 regulation. However, successful FOXP3 induction was not associated with a switch towards a functional Treg phenotype, in contrast to what has been reported from FOXP3 overexpression approaches. Thus, TSDR de-methylation is required, but not sufficient for a stable Treg phenotype induction. Therefore, targeted demethylation of the TSDR may be a critical addition to published in vitro Treg induction protocols which so far lack FOXP3 stability.

Manipulating the Epigenome in Nuclear Transfer Cloning: Where, When and How

The nucleus of a differentiated cell can be reprogrammed to a totipotent state by exposure to the cytoplasm of an enucleated oocyte, and the reconstructed nuclear transfer embryo can give rise to an entire organism. Somatic cell nuclear transfer (SCNT) has important implications in animal biotechnology and provides a unique model for studying epigenetic barriers to successful nuclear reprogramming and for testing novel concepts to overcome them. While initial strategies aimed at modulating the global DNA methylation level and states of various histone protein modifications, recent studies use evidence-based approaches to influence specific epigenetic mechanisms in a targeted manner. In this review, we describe-based on the growing number of reports published during recent decades-in detail where, when, and how manipulations of the epigenome of donor cells and reconstructed SCNT embryos can be performed to optimize the process of molecular reprogramming and the outcome of nuclear transfer cloning.

DNA methylation as an epigenetic biomarker in imprinting disorders

Epigenetic modifications control gene expression and enable the same genotype to lead to various phenotypes, thus exhibiting extensive variability in human cells function. DNA methylation is one of the most often investigated epigenetic modifications, playing a key part in genomic imprinting. Genomic imprinting is an epigenetic process by which the male and the female germ cells confer specific marks (imprints). Maternal chromatin marks differ from paternal ones, leading to expression of specific genes from only one allele. Disturbance in imprinting process results in epimutations, which are epigenetic defects, including DNA methylation changes. These abnormalities are identified in a group of imprinting disorders, associated with abnormal growth, development, behaviour and metabolism. Epimutations can occur spontaneously without any accompanying variant in DNA genomic sequence (a primary epimutation), whose defect can be a result of environmental factors. They can also be caused by changes in DNA sequence of genes involved in imprinting process (a secondary epimutation). DNA methylation in imprinting control regions is a very useful epigenetic biomarker and its detection is applied in the diagnostics of imprinting disorders. At present, various techniques for DNA methylation analysis are employed, which allow for investigations of one to several imprinted loci or the whole genome. DNA methylation studies are important not only in medical molecular diagnostics but are crucial in the search for therapies that would restore normal epigenetic status in patients.

Contiguous and stochastic CHH methylation patterns of plant DRM2 and CMT2 revealed by single-read methylome analysis

Cytosine methylome data is commonly generated through next-generation sequencing, with analyses averaging methylation states of individual reads. We propose an alternative method of analysing single-read methylome data. Using this method, we identify patterns relating to the mechanism of two plant non-CG-methylating enzymes, CMT2 and DRM2. CMT2-methylated regions show higher stochasticity, while DRM2-methylated regions have higher variation among cells. Based on these patterns, we develop a classifier that predicts enzyme activity in different species and tissues. To facilitate further single-read analyses, we develop a genome browser, SRBrowse, optimised for visualising and analysing sequencing data at single-read resolution.

Genetic and epigenetic editing in nervous system

Numerous neuronal functions depend on the precise spatiotemporal regulation of gene expression, and the cellular machinery that contributes to this regulation is frequently disrupted in neurodevelopmental, neuropsychiatric, and neurological disease states. Recent advances in gene editing technology have enabled increasingly rapid understanding of gene sequence variation and gene regulatory function in the central nervous system. Moreover, these tools have provided new insights into the locus-specific functions of epigenetic modifications and enabled epigenetic editing at specific gene loci in disease contexts. Continued development of clustered regularly interspaced short palindromic repeats (CRISPR)-based tools has provided not only cell-specific modulation, but also rapid induction profiles that permit sophisticated interrogation of the temporal dynamics that contribute to brain health and disease. This review summarizes recent advances in genetic editing, transcriptional modulation, and epigenetic reorganization, with a focus on applications to neuronal systems and potential uses in brain disorders characterized by genetic sequence variation or transcriptional dysregulation.
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CRISPR Tools for Physiology and Cell State Changes: Potential of Transcriptional Engineering and Epigenome Editing

Given the large amount of genome-wide data that have been collected during the last decades, a good understanding of how and why cells change during development, homeostasis, and disease might be expected. Unfortunately, the opposite is true; triggers that cause cellular state changes remain elusive, and the underlying molecular mechanisms are poorly understood. Although genes with the potential to influence cell states are known, the historic dependency on methods that manipulate gene expression outside the endogenous chromatin context has prevented us from understanding how cells organize, interpret, and protect cellular programs. Fortunately, recent methodological innovations are now providing options to answer these outstanding questions, by allowing to target and manipulate individual genomic and epigenomic loci. In particular, three experimental approaches are now feasible due to DNA targeting tools, namely, activation and/or repression of master transcription factors in their endogenous chromatin context; targeting transcription factors to endogenous, alternative, or inaccessible sites; and finally, functional manipulation of the chromatin context. In this article, we discuss the molecular basis of DNA targeting tools and review the potential of these new technologies before we summarize how these have already been used for the manipulation of cellular states and hypothesize about future applications.

Maternal H3K27me3-dependent autosomal and X chromosome imprinting

Genomic imprinting and X-chromosome inactivation (XCI) are classic epigenetic phenomena that involve transcriptional silencing of one parental allele. Germline-derived differential DNA methylation is the best-studied epigenetic mark that initiates imprinting, but evidence indicates that other mechanisms exist. Recent studies have revealed that maternal trimethylation of H3 on lysine 27 (H3K27me3) mediates autosomal maternal allele-specific gene silencing and has an important role in imprinted XCI through repression of maternal Xist. Furthermore, loss of H3K27me3-mediated imprinting contributes to the developmental defects observed in cloned embryos. This novel maternal H3K27me3-mediated non-canonical imprinting mechanism further emphasizes the important role of parental chromatin in development and could provide the basis for improving the efficiency of embryo cloning.

Successful generation of epigenetic disease model mice by targeted demethylation of the epigenome

BACKGROUND

Epigenetic modifications, including DNA methylation, play an important role in gene silencing and genome stability. Consequently, epigenetic dysregulation can cause several diseases, such as cancer, obesity, diabetes, autism, and imprinting disorders.

RESULTS

We validate three methods for the generation of epigenome-edited mice using the dCas9-SunTag and single-chain variable fragment-TET1 catalytic domain. We generate model mice for Silver-Russell syndrome (SRS), an imprinting disorder, by target-specific DNA demethylation in the H19 differentially methylated region. Like SRS patients, these mice show H19 upregulation and Igf2 downregulation, leading to severe intrauterine and postnatal growth retardation.

CONCLUSION

This is the first report of an imprinting disease model animal generated by targeted demethylation of specific loci of the epigenome in fertilized eggs. Epigenome-edited animals are also useful for exploring the causative epimutations in epigenetic diseases.

Editing DNA Methylation in Mammalian Embryos

DNA methylation in mammals is essential for numerous biological functions, such as ensuring chromosomal stability, genomic imprinting, and X-chromosome inactivation through transcriptional regulation. Gene knockout of DNA methyltransferases and demethylation enzymes has made significant contributions to analyzing the functions of DNA methylation in development. By applying epigenome editing, it is now possible to manipulate DNA methylation in specific genomic regions and to understand the functions of these modifications. In this review, we first describe recent DNA methylation editing technology. We then focused on changes in DNA methylation status during mammalian gametogenesis and preimplantation development, and have discussed the implications of applying this technology to early embryos.