Quantification of modified nucleotides and nucleosides by isotope dilution mass spectrometry

Epigenetic modifications are closely related to certain disorders of the organism, including the development of tumors. One of the main epigenetic modifications is the methylation of DNA cytosines, 5-methyl-2'-deoxycycytidine. Furthermore, 5-mdC can be oxidized to form three new modifications, 5-(hydroxymethyl)-2'-deoxycytidine, 5-formyl-2'-deoxycytidine, and 5-carboxy-2'-deoxycytidine. The coupling of liquid chromatography with tandem mass spectrometry has been widely used for the total determination of methylated DNA cytosines in samples of biological and clinical interest. These methods are based on the measurement of the free compounds (e.g., urine) or after complete hydrolysis of the DNA (e.g., tissues) followed by a preconcentration, derivatization, and/or clean-up step. This review highlights the main advances in the quantification of modified nucleotides and nucleosides by isotope dilution using isotopically labeled analogs combined with liquid or gas chromatography coupled to mass spectrometry reported in the last 20 years. The different possible sources of labeled compounds are indicated. Special emphasis has been placed on the different types of chromatography commonly used (reverse phase and hydrophilic interaction liquid chromatography) and the derivatization methods developed to enhance chromatographic resolution and ionization efficiency. We have also revised the application of bidimensional chromatography and indicated significant biological and clinical applications of these determinations.

Exploring the ageing methylome in the model insect, Nasonia vitripennis

BACKGROUND

The ageing process is a multifaceted phenomenon marked by the gradual deterioration of cellular and organismal functions, accompanied by an elevated susceptibility to diseases. The intricate interplay between genetic and environmental factors complicates research, particularly in complex mammalian models. In this context, simple invertebrate organisms have been pivotal, but the current models lack detectable DNA methylation limiting the exploration of this critical epigenetic ageing mechanism. This study introduces Nasonia vitripennis, the jewel wasp, as an innovative invertebrate model for investigating the epigenetics of ageing. Leveraging its advantages as a model organism and possessing a functional DNA methylation system, Nasonia emerges as a valuable addition to ageing research.

RESULTS

Whole-genome bisulfite sequencing unveiled dynamic alterations in DNA methylation, with differentially methylated CpGs between distinct time points in both male and female wasps. These changes were associated with numerous genes, enriching for functions related to telomere maintenance, histone methylation, and mRNA catabolic processes. Additionally, other CpGs were found to be variably methylated at each timepoint. Sex-specific effects on epigenetic entropy were observed, indicating differential patterns in the loss of epigenetic stability over time. Constructing an epigenetic clock containing 19 CpGs revealed a robust correlation between epigenetic age and chronological age.

CONCLUSIONS

Nasonia vitripennis emerges as a promising model for investigating the epigenetics of ageing, shedding light on the intricate dynamics of DNA methylation and their implications for age-related processes. This research not only expands the repertoire of ageing models but also opens avenues for deeper exploration of epigenetic mechanisms in the context of ageing.

DNA methylation 6 mA and histone methylation involved in multi-/trans-generational reproductive effects in Caenorhabditis elegans induced by Atrazine

Atrazine (ATR), a widely used triazine herbicide, is an environmental endocrine disruptor that can cause health problems. However, whether there are multi/trans-generational reproductive impacts of ATR have not been studied. Therefore, in this study, Caenorhabditis elegans was used as a preferable model organism to identify the multi/trans-generational reproductive toxicity of ATR. Only parental C.elegans (P0) were exposed to different concentrations (0.0004-40 mg/L) for 48 h and the subsequent offspring (F1-F5) were grown under ATR-free conditions and ATR conditions.The results showed that ATR exposure during P0 decreased fecundity, including a reduction in fertilized eggs, oocytes, and ovulation rate, delayed gonadal development, and decreased the relative area of gonad arm and germ cell number. Furthermore, continuous ATR exposure (P0-F5) causes a significant increase in reproductive toxicity in subsequent generations, although no significant toxicity occurred in the P0 generation after exposure to environmental-related concentrations, suggesting that ATR exposure might have cumulative effects. Likewise, parental exposure to ATR caused transgenerational toxicity impairments. Interestingly, only reproductive toxicity, not development toxicity, was transmitted to several generations (F1-F4), and the F2 generation showed the most notable changes. QRT-PCR results showed that genes expression related to DNA methylation 6 mA (damt-1, nmad-1) and histone H3 methylation (mes-4, met-2, set-25, set-2, and utx-1) can also be passed on to offspring. The function of H3K4 and H3K9 methylation were explored by using loss-of-function mutants for set-2, set-25, and met-2. Transmissible reproductive toxicity was absent in met-2(n4256), set-2(ok952), and set-25(n5021) mutants, which suggests that the histone methyltransferases H3K4 and H3K9 activity are indispensable for the transgenerational effect of ATR. Finally, the downstream genes of DNA methylation and histone H3 methylation were determined. ATR upregulated the expression of ZC317.7, hsp-6, and hsp-60. Mitochondrial stress in parental generation dependent transcription 6 mA modifiers may establish these epigenetic marks in progeny.

Mechanisms of DNA methylation and histone modifications

The field of genetics has expanded a lot in the past few decades due to the accessibility of human genome sequences, but still, the regulation of transcription cannot be explicated exclusively by the sequence of DNA of an individual. The coordination and crosstalk between chromatin factors which are conserved is indispensable for all living creatures. The regulation of gene expression has been dependent on the methylation of DNA, post-translational modifications of histones, effector proteins, chromatin remodeler enzymes that affect the chromatin structure and function, and other cellular activities such as DNA replication, DNA repair, proliferation and growth. The mutation and deletion of these factors can lead to human diseases. Various studies are being performed to identify and understand the gene regulatory mechanisms in the diseased state. The information from these high throughput screening studies is able to aid the treatment developments based on the epigenetics regulatory mechanisms. This book chapter will discourse on various modifications and their mechanisms that take place on histones and DNA that regulate the transcription of genes.

Global analysis of cytosine and adenine DNA modifications across the tree of life

Interpreting the function and metabolism of enzymatic DNA modifications requires both position-specific and global quantities. Sequencing-based techniques that deliver the former have become broadly accessible, but analytical methods for the global quantification of DNA modifications have thus far been applied mostly to individual problems. We established a mass spectrometric method for the sensitive and accurate quantification of multiple enzymatic DNA modifications. Then, we isolated DNA from 124 archean, bacterial, fungal, plant, and mammalian species, and several tissues and created a resource of global DNA modification quantities. Our dataset provides insights into the general nature of enzymatic DNA modifications, reveals unique biological cases, and provides complementary quantitative information to normalize and assess the accuracy of sequencing-based detection of DNA modifications. We report that only three of the studied DNA modifications, methylcytosine (5mdC), methyladenine (N6mdA) and hydroxymethylcytosine (5hmdC), were detected above a picomolar detection limit across species, and dominated in higher eukaryotes (5mdC), in bacteria (N6mdA), or the vertebrate central nervous systems (5hmdC). All three modifications were detected simultaneously in only one of the tested species, Raphanus sativus. In contrast, these modifications were either absent or detected only at trace quantities, across all yeasts and insect genomes studied. Further, we reveal interesting biological cases. For instance, in Allium cepa, Helianthus annuus, or Andropogon gerardi, more than 35% of cytosines were methylated. Additionally, next to the mammlian CNS, 5hmdC was also detected in plants like Lepidium sativum and was found on 8% of cytosines in the Garra barreimiae brain samples. Thus, identifying unexpected levels of DNA modifications in several wild species, our resource underscores the need to address biological diversity for studying DNA modifications.

Environment-driven reprogramming of gamete DNA methylation occurs during maturation and is transmitted intergenerationally in Atlantic Salmon

An epigenetic basis for transgenerational plasticity in animals is widely theorized, but convincing empirical support is limited by taxa-specific differences in the presence and role of epigenetic mechanisms. In teleost fishes, DNA methylation generally does not undergo extensive reprogramming and has been linked with environmentally induced intergenerational effects, but solely in the context of early life environmental differences. Using whole-genome bisulfite sequencing, we demonstrate that differential methylation of sperm occurs in response to captivity during the maturation of Atlantic Salmon (Salmo salar), a species of major economic and conservation significance. We show that adult captive exposure further induces differential methylation in an F1 generation that is associated with fitness-related phenotypic differences. Some genes targeted with differential methylation were consistent with genes differential methylated in other salmonid fishes experiencing early-life hatchery rearing, as well as genes under selection in domesticated species. Our results support a mechanism of transgenerational plasticity mediated by intergenerational inheritance of DNA methylation acquired late in life for salmon. To our knowledge, this is the first-time environmental variation experienced later in life has been directly demonstrated to influence gamete DNA methylation in fish.

Long-term nanoplastics exposure results in multi and trans-generational reproduction decline associated with germline toxicity and epigenetic regulation in Caenorhabditis elegans

The environmental risk from long-term plastic pollution is growing. We investigated the multi and trans-generational reproductive toxicity of nanoplastics (NPs) in Caenorhabditis elegans and the underlying mechanisms over five generations. Following a single maternal exposure (F0) to NPs (100 nm; 1, 10, 50, or 100 mg/L) for 72 h, the subsequent generations (F1-F4) were cultured under NPs-free conditions. We showed that the total brood size was significantly reduced across all offspring generations (F1-F4). NPs accumulated in the intestine of C. elegans in the F0 generation, but not in the germline system, and not observed in subsequent generations. Chromosomal aberrations in oocytes and germline cell apoptosis were significantly elevated in the NPs-exposed F0 generation and in subsequent unexposed generations. Likewise, the expression of ced-3 was increased across generations, regulated by hypomethylation in the promoter region of ced-3 after maternal NPs exposure. Finally, NPs exposure reduced the expression of epigenesis-related genes met-2, set-2, and spr-5 and the trans-generational effects of maternal NPs exposure were not observed in met-2, set-2, and spr-5 RNAi worms. We demonstrate that a single long-term maternal NPs exposure can cause multi and trans-generational reproduction decline in C. elegans, which may be associated with germline toxicity and epigenetic regulation.

Global DNA methylation levels in white blood cells of patients with chronic heroin use disorder. A prospective study

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Opioid abstinence for 21 days does not affect global DNA methylation levels in white blood cells.

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All participants in the study completed a 21-day “dry” detoxification program.

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Findings do not rule out the possibility of site-specific methylation changes.

Background

Increasing scientific evidence shows the significant role of epigenetic mechanisms in drug use disorder, abstinence and relapse. Studies on human subjects are limited compared to those on animals, for various reasons such as poly-substance abuse, high drop-out rate and technical difficulties.

Objectives

Our goal was to evaluate whether a monitored abstinence period of 21 days could induce changes in global DNA methylation in chronic heroin users.

Method

In the current study, we present data on global DNA methylation on a set of 18 male patients with chronic heroin use disorder, carefully selected based on inclusion and exclusion criteria, who were hospitalized and closely monitored during a 21-day detoxification program, one of the few where no opioid agonist is administered. The participants were sampled twice, once upon enrolment to the program and once upon completion.

Results

According to our results, no difference in global DNA methylation was detected between samples collected upon enrolment and samples collected upon completion of the program.

Conclusion

The findings of this study do not rule out the possibility that the 21-day abstinence period was not long enough to observe changes in global DNA methylation, or that abstinence induced site-specific methylation changes (but not global changes), that certainly merit further evaluation.

Current evidence for a role of epigenetic mechanisms in response to ionizing radiation in an ecotoxicological context

The issue of potential long-term or hereditary effects for both humans and wildlife exposed to low doses (or dose rates) of ionising radiation is a major concern. Chronic exposure to ionising radiation, defined as an exposure over a large fraction of the organism's lifespan or even over several generations, can possibly have consequences in the progeny. Recent work has begun to show that epigenetics plays an important role in adaptation of organisms challenged to environmental stimulae. Changes to so-called epigenetic marks such as histone modifications, DNA methylation and non-coding RNAs result in altered transcriptomes and proteomes, without directly changing the DNA sequence. Moreover, some of these environmentally-induced epigenetic changes tend to persist over generations, and thus, epigenetic modifications are regarded as the conduits for environmental influence on the genome. Here, we review the current knowledge of possible involvement of epigenetics in the cascade of responses resulting from environmental exposure to ionising radiation. In addition, from a comparison of lab and field obtained data, we investigate evidence on radiation-induced changes in the epigenome and in particular the total or locus specific levels of DNA methylation. The challenges for future research and possible use of changes as an early warning (biomarker) of radiosensitivity and individual exposure is discussed. Such a biomarker could be used to detect and better understand the mechanisms of toxic action and inter/intra-species susceptibility to radiation within an environmental risk assessment and management context.

Advances in detection and quantification of methylcytosine and its derivatives

Methylation of the fifth carbon atom in cytosine is an epigenetic modification of deoxyribonucleic acid that plays important roles in numerous cellular processes and disease pathogenesis. Three additional states of cytosine, that is, 5-hydroxymethylcytosine, 5-formylcytosine and 5-carboxylcytosine, have been identified and associated with the diagnosis and/or prognosis of diseases. However, accurate measurement of those intermediates is a challenge since their global levels are relatively low. A number of innovative methods have been developed to detect and quantify these compounds in biological samples, such as blood, tissue and urine, etc. This review focuses on recent advancement in detection and quantification of four cytosine modifications, based on which, the development, diagnosis, and prognosis of diseases could be monitored through non-invasive procedures.

Caenorhabditis elegans as an emerging model system in environmental epigenetics

The roundworm Caenorhabitis elegans has been an established model organism for the study of genetics and developmental biology, including studies of transcriptional regulation, since the 1970s. This model organism has continued to be used as a classical model system as the field of transcriptional regulation has expanded to include scientific advances in epigenetics and chromatin biology. In the last several decades, C. elegans has emerged as a powerful model for environmental toxicology, particularly for the study of chemical genotoxicity. Here, we outline the utility and applicability of C. elegans as a powerful model organism for mechanistic studies of environmental influences on the epigenome. Our goal in this article is to inform the field of environmental epigenetics of the strengths and limitations of the well-established C. elegans model organism as an emerging model for medium-throughput, in vivo exploration of the role of exogenous chemical stimuli in transcriptional regulation, developmental epigenetic reprogramming, and epigenetic memory and inheritance. As the field of environmental epigenetics matures, and research begins to map mechanisms underlying observed associations, new toolkits and model systems, particularly manipulable, scalable in vivo systems that accurately model human transcriptional regulatory circuits, will provide an essential experimental bridge between in vitro biochemical experiments and mammalian model systems. Environ. Mol. Mutagen. 59:560-575, 2018. © 2018 Wiley Periodicals, Inc.

Widespread 5-methylcytosine in the genomes of avian Coccidia and other apicomplexan parasites detected by an ELISA-based method

To date, little is known about cytosine methylation in the genomic DNA of apicomplexan parasites, although it has been confirmed that this important epigenetic modification exists in many lower eukaryotes, plants, and animals. In the present study, ELISA-based detection demonstrated that low levels of 5-methylcytosine (5-mC) are present in Eimeria spp., Toxoplasma gondii, Cryptosporidium spp., and Neospora caninum. The proportions of 5-mC in genomic DNA were 0.18 ± 0.02% in E tenella sporulated oocysts, 0.19 ± 0.01% in E. tenella second-generation merozoites, 0.22 ± 0.04% in T. gondii tachyzoites, 0.28 ± 0.03% in N. caninum tachyzoites, and 0.06 ± 0.01, 0.11 ± 0.01, and 0.09 ± 0.01% in C. andersoni, C. baileyi, and C. parvum sporulated oocysts, respectively. In addition, we found that the percentages of 5-mC in E. tenella varied considerably at different life stages, with sporozoites having the highest percentage of 5-mC (0.78 ± 0.10%). Similar stage differences in 5-mC were also found in E. maxima, E. necatrix, and E. acervulina, the levels of 5-mC in their sporozoites being 4.3-, 1.8-, 2.5-, and 2.0-fold higher than that of sporulated oocysts, respectively (p < 0.01). Furthermore, a total DNA methyltransferase-like activity was detected in whole cell extracts prepared from E. tenella sporozoites. In conclusion, genomic DNA methylation is present in these apicomplexan parasites and may play a role in the stage conversion of Eimeria.

Lamarck rises from his grave: parental environment-induced epigenetic inheritance in model organisms and humans

Organisms can change their physiological/behavioural traits to adapt and survive in changed environments. However, whether these acquired traits can be inherited across generations through non-genetic alterations has been a topic of debate for over a century. Emerging evidence indicates that both ancestral and parental experiences, including nutrition, environmental toxins, nurturing behaviour, and social stress, can have powerful effects on the physiological, metabolic and cellular functions in an organism. In certain circumstances, these effects can be transmitted across several generations through epigenetic (i.e. non-DNA sequence-based rather than mutational) modifications. In this review, we summarize recent evidence on epigenetic inheritance from parental environment-induced developmental and physiological alterations in nematodes, fruit flies, zebrafish, rodents, and humans. The epigenetic modifications demonstrated to be both susceptible to modulation by environmental cues and heritable, including DNA methylation, histone modification, and small non-coding RNAs, are also summarized. We particularly focus on evidence that parental environment-induced epigenetic alterations are transmitted through both the maternal and paternal germlines and exert sex-specific effects. The thought-provoking data presented here raise fundamental questions about the mechanisms responsible for these phenomena. In particular, the means that define the specificity of the response to parental experience in the gamete epigenome and that direct the establishment of the specific epigenetic change in the developing embryos, as well as in specific tissues in the descendants, remain obscure and require elucidation. More precise epigenetic assessment at both the genome-wide level and single-cell resolution as well as strategies for breeding at relatively sensitive periods of development and manipulation aimed at specific epigenetic modification are imperative for identifying parental environment-induced epigenetic marks across generations. Considering their diverse epigenetic architectures, the conservation and prevalence of the mechanisms underlying epigenetic inheritance in non-mammals require further investigation in mammals. Interpretation of the consequences arising from epigenetic inheritance on organisms and a better understanding of the underlying mechanisms will provide insight into how gene-environment interactions shape developmental processes and physiological functions, which in turn may have wide-ranging implications for human health, and understanding biological adaptation and evolution.

Human blood cell levels of 5-hydroxymethylcytosine (5hmC) decline with age, partly related to acquired mutations in TET2

Epigenetic alteration may play a role in age-associated dysfunction of stem cells and predispose to the development of hematological cancers. We analyzed global levels of hematopoietic 5-hydroxymethylcytosine (5hmC) and 5-methylcytosine (5mC) in a cross-sectional study comprising 198 unrelated individuals from four age categories (neonates, 25-30, 70-75, and >90 years old) by liquid chromatography-electrospray ionization-tandem mass spectrometry with multiple reaction monitoring. X-chromosome inactivation (XCI) ratios and telomere length (TL) were measured in all individuals by polymerase chain reaction. Sequencing of epigenetic regulator genes (including TET2, DNMT3A, ASXL1, IDH1, IDH2, and WT1) was performed in the two older subcohorts. We found that global 5hmC levels declined with age in human blood cells (27.5% reduction from birth to old age, p < 0.0005). The levels of 5mC underwent a more modest reduction (2.4% drop) between newborns and the elderly (p < 0.0005). Low 5hmC was associated with increased skewing of XCI (age-adjusted p = 0.0304) and reduced TL (age-adjusted p = 0.0354), both surrogate markers of clonal dominance. Of the 100 individuals over the age of 70, 16 had somatic mutations in TET2, 14 in DNMT3A, and none in IDH1, IDH2, or WT1. Individuals with TET2 mutations had significantly lower 5hmC (relative to unmutated individuals), whereas DNMT3A-mutated subjects did not. However, mutations in TET2 cannot account solely for the decline in 5hmC levels observed with aging because unmutated older individuals also had lower 5hmC levels compared with younger individuals. This suggests that the age-associated decline in 5hmC is multifactorial. Larger prospective studies are needed to determine whether 5hmC reduction is a biomarker of hematological cancer development.

Insights into zinc and cadmium biology in the nematode Caenorhabditis elegans

Zinc is an essential metal that is involved in a wide range of biological processes, and aberrant zinc homeostasis is implicated in multiple human diseases. Cadmium is chemically similar to zinc, but it is a nonessential environmental pollutant. Because zinc deficiency and excess are deleterious, animals require homeostatic mechanisms to maintain zinc levels in response to dietary fluctuations. The nematode Caenorhabditis elegans is emerging as a powerful model system to investigate zinc trafficking and homeostasis as well as cadmium toxicity. Here we review genetic and molecular studies that have combined to generate a picture of zinc homeostasis based on the transcriptional control of zinc transporters in intestinal cells. Furthermore, we summarize studies of cadmium toxicity that reveal intriguing parallels with zinc biology.

A Transparent Window into Biology: A Primer on Caenorhabditis elegans

A little over 50 years ago, Sydney Brenner had the foresight to develop the nematode (round worm) Caenorhabditis elegans as a genetic model for understanding questions of developmental biology and neurobiology. Over time, research on C. elegans has expanded to explore a wealth of diverse areas in modern biology including studies of the basic functions and interactions of eukaryotic cells, host-parasite interactions, and evolution. C. elegans has also become an important organism in which to study processes that go awry in human diseases. This primer introduces the organism and the many features that make it an outstanding experimental system, including its small size, rapid life cycle, transparency, and well-annotated genome. We survey the basic anatomical features, common technical approaches, and important discoveries in C. elegans research. Key to studying C. elegans has been the ability to address biological problems genetically, using both forward and reverse genetics, both at the level of the entire organism and at the level of the single, identified cell. These possibilities make C. elegans useful not only in research laboratories, but also in the classroom where it can be used to excite students who actually can see what is happening inside live cells and tissues.

Epigenetically modified nucleotides in chronic heroin and cocaine treated mice

Epigenetic changes include the addition of a methyl group to the 5' carbon of the cytosine ring, known as DNA methylation, which results in the generation of the fifth DNA base, namely 5-methylcytosine. During active or passive demethylation, an intermediate modified base is formed, 5-hydroxymethylcytosine. We have currently quantified 5-methylcytosine and 5-hydroxymethylcytosine in the liver and brain of mice treated with cocaine or heroin, using liquid chromatography/tandem mass spectrometry (LC-MS/MS). Our results show that global 5-methylcytosine levels are not affected by heroin or cocaine administration, neither in the liver nor in the brain. However, 5-hydroxymethylcytosine levels are reduced in the liver following cocaine administration, while they are not affected by cocaine in the brain or by heroin administration in the liver and the brain. Elucidation of the epigenetic phenomena that takes place with respect to drug abuse and addiction, via quantitative analysis of different modified bases, may enable a better understanding of the underlying mechanisms and may lead to more personalized and effective treatment options.