Mitotic inheritance of DNA methylation: more than just copy and paste

Monitoring and quantifying replication fork dynamics with high-throughput methods

Before each cell division, eukaryotic cells must replicate their chromosomes to ensure the accurate transmission of genetic information. Chromosome replication involves more than just DNA duplication; it also includes chromatin assembly, inheritance of epigenetic marks, and faithful resumption of all genomic functions after replication. Recent progress in quantitative technologies has revolutionized our understanding of the complexity and dynamics of DNA replication forks at both molecular and genomic scales. Here, we highlight the pivotal role of these novel methods in uncovering the principles and mechanisms of chromosome replication. These technologies have illuminated the regulation of genome replication programs, quantified the impact of DNA replication on genomic mutations and evolutionary processes, and elucidated the mechanisms of replication-coupled chromatin assembly and epigenome maintenance.

Epigenetic analyses in forensic medicine: future and challenges

The possibility of using epigenetics in forensic investigation has gradually risen over the last few years. Epigenetic changes with their dynamic nature can either be inherited or accumulated throughout a lifetime and be reversible, prompting investigation of their use across various fields. In forensic sciences, multiple applications have been proposed, such as the discrimination of monozygotic twins, identifying the source of a biological trace left at a crime scene, age prediction, determination of body fluids and tissues, human behavior association, wound healing progression, and determination of the post-mortem interval (PMI). Despite all these applications, not all the studies considered the impact of PMI and post-sampling effects on the epigenetic modifications and the tissue-specificity of the epigenetic marks.This review aims to highlight the substantial forensic significance that epigenetics could support in various forensic investigations. First, basic concepts in epigenetics, describing the main epigenetic modifications and their functions, in particular, DNA methylation, histone modifications, and non-coding RNA, with a particular focus on forensic applications, were covered. For each epigenetic marker, post-mortem stability and tissue-specificity, factors that should be carefully considered in the study of epigenetic biomarkers in the forensic context, have been discussed. The advantages and limitations of using post-mortem tissues have been also addressed, proposing directions for these innovative strategies to analyze forensic specimens.

Iterative oxidation by TET1 is required for reprogramming of imprinting control regions and patterning of mouse sperm hypomethylated regions

Ten-eleven translocation (TET) enzymes iteratively oxidize 5-methylcytosine (5mC) to generate 5-hydroxymethylcytosine (5hmC), 5-formylcytosine, and 5-carboxylcytosine to facilitate active genome demethylation. Whether these bases are required to promote replication-coupled dilution or activate base excision repair during mammalian germline reprogramming remains unresolved due to the inability to decouple TET activities. Here, we generated two mouse lines expressing catalytically inactive TET1 (Tet1-HxD) and TET1 that stalls oxidation at 5hmC (Tet1-V). Tet1 knockout and catalytic mutant primordial germ cells (PGCs) fail to erase methylation at select imprinting control regions and promoters of meiosis-associated genes, validating the requirement for the iterative oxidation of 5mC for complete germline reprogramming. TET1V and TET1HxD rescue most hypermethylation of Tet1-/- sperm, suggesting the role of TET1 beyond its oxidative capability. We additionally identify a broader class of hypermethylated regions in Tet1 mutant mouse sperm that depend on TET oxidation for reprogramming. Our study demonstrates the link between TET1-mediated germline reprogramming and sperm methylome patterning.

Methylation entropy landscape of Chinese long-lived individuals reveals lower epigenetic noise related to human healthy aging

The transition from ordered to noisy is a significant epigenetic signature of aging and age-related disease. As a paradigm of healthy human aging and longevity, long-lived individuals (LLI, >90 years old) may possess characteristic strategies in coping with the disordered epigenetic regulation. In this study, we constructed high-resolution blood epigenetic noise landscapes for this cohort by a methylation entropy (ME) method using whole genome bisulfite sequencing (WGBS). Although a universal increase in global ME occurred with chronological age in general control samples, this trend was suppressed in LLIs. Importantly, we identified 38,923 genomic regions with LLI-specific lower ME (LLI-specific lower entropy regions, for short, LLI-specific LERs). These regions were overrepresented in promoters, which likely function in transcriptional noise suppression. Genes associated with LLI-specific LERs have a considerable impact on SNP-based heritability of some aging-related disorders (e.g., asthma and stroke). Furthermore, neutrophil was identified as the primary cell type sustaining LLI-specific LERs. Our results highlight the stability of epigenetic order in promoters of genes involved with aging and age-related disorders within LLI epigenomes. This unique epigenetic feature reveals a previously unknown role of epigenetic order maintenance in specific genomic regions of LLIs, which helps open a new avenue on the epigenetic regulation mechanism in human healthy aging and longevity.

The molecular basis of cell memory in mammals: The epigenetic cycle

Cell memory refers to the capacity of cells to maintain their gene expression program once the initiating environmental signal has ceased. This exceptional feature is key during the formation of mammalian organisms, and it is believed to be in part mediated by epigenetic factors that can endorse cells with the landmarks required to maintain transcriptional programs upon cell duplication. Here, we review current literature analyzing the molecular basis of epigenetic memory in mammals, with a focus on the mechanisms by which transcriptionally repressive chromatin modifications such as methylation of DNA and histone H3 are propagated through mitotic cell divisions. The emerging picture suggests that cellular memory is supported by an epigenetic cycle in which reversible activities carried out by epigenetic regulators in coordination with cell cycle transition create a multiphasic system that can accommodate both maintenance of cell identity and cell differentiation in proliferating stem cell populations.

Effects of DNA Methylation on Gene Expression and Phenotypic Traits in Cattle: A Review

Gene expression in cells is determined by the epigenetic state of chromatin. Therefore, the study of epigenetic changes is very important to understand the regulatory mechanism of genes at the molecular, cellular, tissue and organ levels. DNA methylation is one of the most studied epigenetic modifications, which plays an important role in maintaining genome stability and ensuring normal growth and development. Studies have shown that methylation levels in bovine primordial germ cells, the rearrangement of methylation during embryonic development and abnormal methylation during placental development are all closely related to their reproductive processes. In addition, the application of bovine male sterility and assisted reproductive technology is also related to DNA methylation. This review introduces the principle, development of detection methods and application conditions of DNA methylation, with emphasis on the relationship between DNA methylation dynamics and bovine spermatogenesis, embryonic development, disease resistance and muscle and fat development, in order to provide theoretical basis for the application of DNA methylation in cattle breeding in the future.

The patterns and participants of parental histone recycling during DNA replication in Saccharomyces cerevisiae

Epigenetic information carried by histone modifications not only reflects the state of gene expression, but also participates in the maintenance of chromatin states and the regulation of gene expression. Recycling of parental histones to daughter chromatin after DNA replication is vital to mitotic inheritance of epigenetic information and the maintenance of cell identity, because the locus-specific modifications of the parental histones need to be maintained. To assess the precision of parental histone recycling, we developed a synthetic local label-chasing system in budding yeast Saccharomyces cerevisiae. Using this system, we observed that parental histone H3 can be recycled to their original position, thereby recovering their position information after DNA replication at all tested loci, including heterochromatin boundary, non-transcribed region, and actively transcribed regions. Moreover, the recycling rate appears to be affected by local chromatin environment. We surveyed a number of potential regulatory factors and observed that histone H3-H4 chaperon Asf1 contributed to parental histone recycling, while the eukaryotic replisome-associated components Mcm2 and Dpb3 displayed compounding effects in this process. In addition, the FACT complex also plays a role in the recycling of parental histones and helps to stabilize the nucleosomes.

Mechanisms of chromatin-based epigenetic inheritance

Multi-cellular organisms such as humans contain hundreds of cell types that share the same genetic information (DNA sequences), and yet have different cellular traits and functions. While how genetic information is passed through generations has been extensively characterized, it remains largely obscure how epigenetic information encoded by chromatin regulates the passage of certain traits, gene expression states and cell identity during mitotic cell divisions, and even through meiosis. In this review, we will summarize the recent advances on molecular mechanisms of epigenetic inheritance, discuss the potential impacts of epigenetic inheritance during normal development and in some disease conditions, and outline future research directions for this challenging, but exciting field.

H3K27me3 shapes DNA methylome by inhibiting UHRF1-mediated H3 ubiquitination

DNA methylation and histone lysine tri-methylation at H3K27 (H3K27me3) are two chromatin modifications for transcriptional gene silencing, which play important roles in diverse biological processes, including cell fate determination and cell lineage commitment. These two marks are largely mutually exclusive and target distinct sets of genes in the mammalian genome. However, how H3K27me3 shapes the DNA methylome remains elusive. Here, we report that the loss of H3K27me3 modification leads to increased DNA methylation at previously marked H3K27me3 sites, indicating that H3K27me3 negatively regulates DNA methylation. Genome-wide analysis of H3 ubiquitination, essential for recruitment and activation of DNA methyltransferase DNMT1, reveals the absence of H3 ubiquitination at H3K27me3 marked nucleosomes. Moreover, loss of H3K27me3 modification induces an increase in H3K18 ubiquitination at the corresponding hyper-methylated loci. Importantly, we show that H3K27me3 directly inhibits UHRF1-mediated H3 ubiquitination toward nucleosomes in a defined biochemical assay. Taken together, our findings reveal a general mechanism for H3K27me3-mediated shaping of the mammalian DNA methylome via modulation of H3 ubiquitination.

Metaboloepigenetics in cancer, immunity and cardiovascular disease

The influence of cellular metabolism on epigenetic pathways are well documented but misunderstood. Scientists have long known of the metabolic impact on epigenetic determinants. More often than not, that title role for DNA methylation was portrayed by the metabolite SAM or S-adenosylmethionine. Technically speaking there are many other metabolites that drive epigenetic processes that instruct seemingly distant - yet highly connect pathways - and none more so than our understanding of the cancer epigenome. Recent studies have shown that available energy link the extracellular environment to influence cellular responses. This focused review examines the recent interest in epigenomics and casts cancer, metabolism and immunity in unfamiliar roles - cooperating. There are not only language lessons from cancer research, we have come round to appreciate that reaching into areas previously thought of as too distinct are also object lessons in understanding health and disease. The Warburg effect is one such signature of how glycolysis influences metabolic shift during oncogenesis. That shift in metabolism - now recognised as central to proliferation in cancer biology - influence core enzymes that not only control gene expression but are also central to replication, condensation and the repair of nucleic acid. These nuclear processes rely on metabolism and with glucose at center stage the role of respiration and oxidative metabolism are now synonymous with the mitochondria as the powerhouses of metaboloepigenetics. The emerging evidence for metaboloepigenetics in trained innate immunity has revealed recognisable signalling pathways with antecedent extracellular stimulation. With due consideration to immunometabolism we discuss the striking signalling similarities influencing these core pathways. The immunometabolic-epigenetic axis in cardiovascular disease has deeply etched connections with inflammation and we examine the chromatin template as a carrier of epigenetic indices that determine the expression of genes influencing atherosclerosis and vascular complications of diabetes.

Natural Flavonoids Derived From Fruits Are Potential Agents Against Atherosclerosis

Atherosclerosis, as a chronic inflammatory response, is one of the main causes of cardiovascular diseases. Atherosclerosis is induced by endothelial cell dysfunction, migration and proliferation of smooth muscle cells, accumulation of foam cells and inflammatory response, resulting in plaque accumulation, narrowing and hardening of the artery wall, and ultimately leading to myocardial infarction or sudden death and other serious consequences. Flavonoid is a kind of natural polyphenol compound widely existing in fruits with various structures, mainly including flavonols, flavones, flavanones, flavanols, anthocyanins, isoflavones, and chalcone, etc. Because of its potential health benefits, it is now used in supplements, cosmetics and medicines, and researchers are increasingly paying attention to its role in atherosclerosis. In this paper, we will focus on several important nodes in the development of atherosclerotic disease, including endothelial cell dysfunction, smooth muscle cell migration and proliferation, foam cell accumulation and inflammatory response. At the same time, through the classification of flavonoids from fruits, the role and potential mechanism of flavonoids in atherosclerosis were reviewed, providing a certain direction for the development of fruit flavonoids in the treatment of atherosclerosis drugs.

An in silico pipeline for the discovery of multitarget ligands: A case study for epi-polypharmacology based on DNMT1/HDAC2 inhibition

The search for novel therapeutic compounds remains an overwhelming task owing to the time-consuming and expensive nature of the drug development process and low success rates. Traditional methodologies that rely on the one drug-one target paradigm have proven insufficient for the treatment of multifactorial diseases, leading to a shift to multitarget approaches. In this emerging paradigm, molecules with off-target and promiscuous interactions may result in preferred therapies. In this study, we developed a general pipeline combining machine learning algorithms and a deep generator network to train a dual inhibitor classifier capable of identifying putative pharmacophoric traits. As a case study, we focused on dual inhibitors targeting DNA methyltransferase 1 (DNMT) and histone deacetylase 2 (HDAC2), two enzymes that play a central role in epigenetic regulation. We used this approach to identify dual inhibitors from a novel large natural product database in the public domain. We used docking and atomistic simulations as complementary approaches to establish the ligand-interaction profiles between the best hits and DNMT1/HDAC2. By using the combined ligand- and structure-based approaches, we discovered two promising novel scaffolds that can be used to simultaneously target both DNMT1 and HDAC2. We conclude that the flexibility and adaptability of the proposed pipeline has predictive capabilities of similar or derivative methods and is readily applicable to the discovery of small molecules targeting many other therapeutically relevant proteins.

Epigenetic Regulation of Vascular Smooth Muscle Cell Phenotype Switching in Atherosclerotic Artery Remodeling: A Mini-Review

Atherosclerosis is a chronic inflammatory disease characterized by extensive remodeling of medium and large-sized arteries. Inward remodeling (=lumen shrinkage) of the vascular walls is the underlying cause for ischemia in target organs. Therefore, inward remodeling can be considered the predominant feature of atherosclerotic pathology. Outward remodeling (=lumen enlargement) is a physiological response compensating for lumen shrinkage caused by neointimal hyperplasia, but as a pathological response to changes in blood flow, outward remodeling leads to substantial arterial wall thinning. Thinned vascular walls are prone to rupture, and subsequent thrombus formation accounts for the majority of acute cardiovascular events. Pathological remodeling is driven by inflammatory cells which induce vascular smooth muscle cells to switch from quiescent to a proliferative and migratory phenotype. After decades of intensive research, the molecular mechanisms of arterial remodeling are starting to unfold. In this mini-review, we summarize the current knowledge of the epigenetic and transcriptional regulation of vascular smooth muscle cell phenotype switching from the contractile to the synthetic phenotype involved in arterial remodeling and discuss potential therapeutic options.