DNA methylation is a major epigenetic modification of vertebrate genomes that is mostly associated with transcriptional repression. During embryogenesis, DNA methylation together with other ...epigenetic factors plays an essential role in selecting and maintaining cell identity. Recent technological advances are now allowing for the exploration of this mark at unprecedented resolution. This has resulted in a wealth of studies describing the developmental roles of DNA methylation in various vertebrate model systems. It is now evident that in certain contexts DNA methylation can act as a key regulator of cell identity establishment, whereas in many other cases the quantity of DNA methylation will merely reflect other upstream regulatory changes. For example, a number of studies have indicated that DNA methylation might be dispensable for pluripotency stages of embryonic development. Nevertheless, targeted deposition and removal of DNA methylation by DNMTs and TET proteins, respectively, appears to be required for vertebrate gastrulation. Here we review the roles of DNA methylation in the establishment and maintenance of cell identity during development, with a special emphasis on insights obtained from in vivo studies.
Complete sequences of myriad eukaryotic genomes, including several human genomes, are now available, and recent dramatic developments in DNA sequencing technology are opening the floodgates to vast ...volumes of sequence data. Yet, despite knowing for several decades that a significant proportion of cytosines in the genomes of plants and animals are present in the form of methylcytosine, until very recently the precise locations of these modified bases have never been accurately mapped throughout a eukaryotic genome. Advanced "next-generation" DNA sequencing technologies are now enabling the global mapping of this epigenetic modification at single-base resolution, providing new insights into the regulation and dynamics of DNA methylation in genomes.
Single-cell RNA sequencing has been widely adopted to estimate the cellular composition of heterogeneous tissues and obtain transcriptional profiles of individual cells. Multiple approaches for ...optimal sample dissociation and storage of single cells have been proposed as have single-nuclei profiling methods. What has been lacking is a systematic comparison of their relative biases and benefits.
Here, we compare gene expression and cellular composition of single-cell suspensions prepared from adult mouse kidney using two tissue dissociation protocols. For each sample, we also compare fresh cells to cryopreserved and methanol-fixed cells. Lastly, we compare this single-cell data to that generated using three single-nucleus RNA sequencing workflows. Our data confirms prior reports that digestion on ice avoids the stress response observed with 37 °C dissociation. It also reveals cell types more abundant either in the cold or warm dissociations that may represent populations that require gentler or harsher conditions to be released intact. For cell storage, cryopreservation of dissociated cells results in a major loss of epithelial cell types; in contrast, methanol fixation maintains the cellular composition but suffers from ambient RNA leakage. Finally, cell type composition differences are observed between single-cell and single-nucleus RNA sequencing libraries. In particular, we note an underrepresentation of T, B, and NK lymphocytes in the single-nucleus libraries.
Systematic comparison of recovered cell types and their transcriptional profiles across the workflows has highlighted protocol-specific biases and thus enables researchers starting single-cell experiments to make an informed choice.
Current high-throughput DNA sequencing technologies enable acquisition of billions of data points through which myriad biological processes can be interrogated, including genetic variation, chromatin ...structure, gene expression patterns, small RNAs and protein-DNA interactions. Here we describe the MethylC-sequencing (MethylC-seq) library preparation method, a 2-d protocol that enables the genome-wide identification of cytosine DNA methylation states at single-base resolution. The technique involves fragmentation of genomic DNA followed by adapter ligation, bisulfite conversion and limited amplification using adapter-specific PCR primers in preparation for sequencing. To date, this protocol has been successfully applied to genomic DNA isolated from primary cell culture, sorted cells and fresh tissue from over a thousand plant and animal samples.
Regulation of gene expression by DNA methylation is crucial for defining cellular identities and coordinating organism-wide developmental programs in many organisms. In plants, modulation of DNA ...methylation in response to environmental conditions represents a potentially robust mechanism to regulate gene expression networks; however, examples of dynamic DNA methylation are largely limited to gene imprinting. Here we report an unexpected role for DNA methylation in regulation of the Arabidopsis thaliana immune system. Profiling the DNA methylomes of plants exposed to bacterial pathogen, avirulent bacteria, or salicylic acid (SA) hormone revealed numerous stress-induced differentially methylated regions, many of which were intimately associated with differentially expressed genes. In response to SA, transposon-associated differentially methylated regions, which were accompanied by up-regulation of 21-nt siRNAs, were often coupled to transcriptional changes of the transposon and/or the proximal gene. Thus, dynamic DNA methylation changes within repetitive sequences or transposons can regulate neighboring genes in response to SA stress.
Two waves of DNA methylation reprogramming occur during mammalian embryogenesis; during preimplantation development and during primordial germ cell (PGC) formation. However, it is currently unclear ...how evolutionarily conserved these processes are. Here we characterise the DNA methylomes of zebrafish PGCs at four developmental stages and identify retention of paternal epigenetic memory, in stark contrast to the findings in mammals. Gene expression profiling of zebrafish PGCs at the same developmental stages revealed that the embryonic germline is defined by a small number of markers that display strong developmental stage-specificity and that are independent of DNA methylation-mediated regulation. We identified promoters that are specifically targeted by DNA methylation in somatic and germline tissues during vertebrate embryogenesis and that are frequently misregulated in human cancers. Together, these detailed methylome and transcriptome maps of the zebrafish germline provide insight into vertebrate DNA methylation reprogramming and enhance our understanding of the relationships between germline fate acquisition and oncogenesis.
Detection of DNA methylation in the genome has been possible for decades; however, the ability to deliberately and specifically manipulate local DNA methylation states in the genome has been ...extremely limited. Consequently, this has impeded our understanding of the direct effect of DNA methylation on transcriptional regulation and transcription factor binding in the native chromatin context. Thus, highly specific targeted epigenome editing tools are needed to address this. Recent adaptations of genome editing technologies, including fusion of the DNMT3A DNA methyltransferase catalytic domain to catalytically inactive Cas9 (dC9-D3A), have aimed to alter DNA methylation at desired loci. Here, we show that these tools exhibit consistent off-target DNA methylation deposition in the genome, limiting their capabilities to unambiguously assess the functional consequences of DNA methylation. To address this, we developed a modular dCas9-SunTag (dC9Sun-D3A) system that can recruit multiple DNMT3A catalytic domains to a target site for editing DNA methylation. dC9Sun-D3A is tunable, specific, and exhibits much higher induction of DNA methylation at target sites than the dC9-D3A direct fusion protein. Importantly, genome-wide characterization of dC9Sun-D3A binding sites and DNA methylation revealed minimal off-target protein binding and induction of DNA methylation with dC9Sun-D3A, compared to pervasive off-target methylation by dC9-D3A. Furthermore, we used dC9Sun-D3A to demonstrate the binding sensitivity to DNA methylation for CTCF and NRF1 in situ. Overall, this modular dC9Sun-D3A system enables precise DNA methylation deposition with the lowest off-target DNA methylation levels reported to date, allowing accurate functional determination of the role of DNA methylation at single loci.
Variation in the presence or absence of transposable elements (TEs) is a major source of genetic variation between individuals. Here, we identified 23,095 TE presence/absence variants between 216 ...Arabidopsis accessions. Most TE variants were rare, and we find these rare variants associated with local extremes of gene expression and DNA methylation levels within the population. Of the common alleles identified, two thirds were not in linkage disequilibrium with nearby SNPs, implicating these variants as a source of novel genetic diversity. Many common TE variants were associated with significantly altered expression of nearby genes, and a major fraction of inter-accession DNA methylation differences were associated with nearby TE insertions. Overall, this demonstrates that TE variants are a rich source of genetic diversity that likely plays an important role in facilitating epigenomic and transcriptional differences between individuals, and indicates a strong genetic basis for epigenetic variation.
Seed germination involves progression from complete metabolic dormancy to a highly active, growing seedling. Many factors regulate germination and these interact extensively, forming a complex ...network of inputs that control the seed-to-seedling transition. Our understanding of the direct regulation of gene expression and the dynamic changes in the epigenome and small RNAs during germination is limited. The interactions between genome, transcriptome and epigenome must be revealed in order to identify the regulatory mechanisms that control seed germination.
We present an integrated analysis of high-resolution RNA sequencing, small RNA sequencing and MethylC sequencing over ten developmental time points in Arabidopsis thaliana seeds, finding extensive transcriptomic and epigenomic transformations associated with seed germination. We identify previously unannotated loci from which messenger RNAs are expressed transiently during germination and find widespread alternative splicing and divergent isoform abundance of genes involved in RNA processing and splicing. We generate the first dynamic transcription factor network model of germination, identifying known and novel regulatory factors. Expression of both microRNA and short interfering RNA loci changes significantly during germination, particularly between the seed and the post-germinative seedling. These are associated with changes in gene expression and large-scale demethylation observed towards the end of germination, as the epigenome transitions from an embryo-like to a vegetative seedling state.
This study reveals the complex dynamics and interactions of the transcriptome and epigenome during seed germination, including the extensive remodelling of the seed DNA methylome from an embryo-like to vegetative-like state during the seed-to-seedling transition. Data are available for exploration in a user-friendly browser at https://jbrowse.latrobe.edu.au/germination_epigenome .
DNA methylation is implicated in mammalian brain development and plasticity underlying learning and memory. We report the genome-wide composition, patterning, cell specificity, and dynamics of DNA ...methylation at single-base resolution in human and mouse frontal cortex throughout their lifespan. Widespread methylome reconfiguration occurs during fetal to young adult development, coincident with synaptogenesis. During this period, highly conserved non-CG methylation (mCH) accumulates in neurons, but not glia, to become the dominant form of methylation in the human neuronal genome. Moreover, we found an mCH signature that identifies genes escaping X-chromosome inactivation. Last, whole-genome single-base resolution 5-hydroxymethylcytosine (hmC) maps revealed that hmC marks fetal brain cell genomes at putative regulatory regions that are CG-demethylated and activated in the adult brain and that CG demethylation at these hmC-poised loci depends on Tet2 activity.
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