N6-methyladenine is incorporated into mammalian genome by DNA polymerase

Recent advances in high-performance liquid chromatography tandem mass spectrometry techniques for analysis of DNA damage and epigenetic modifications

Environmental exposure would cause DNA damage and epigenetic modification changes, potentially resulting in physiological dysfunction, thereby triggering diseases and even cancer. DNA damage and epigenetic modifications are thus promising biomarkers for environmental exposures and disease states. Benefiting from its high sensitivity and accuracy, high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) is considered the "gold standard technique" for investigating epigenetic DNA modifications. This review summarizes the recent advancements of UHPLC-MS/MS-based technologies for DNA damage and epigenetic modifications analysis, mainly focusing on the innovative methods developed for UHPLC-MS/MS-related pretreatment technologies containing efficient genomic DNA digestion and effective removal of the inorganic salt matrix, and the new strategies for improving detection sensitivity of liquid chromatography-mass spectrometry. Moreover, we also summarized the novel hyphenated techniques of the advanced UHPLC-MS/MS coupled with other separation and analysis methods for the measurement of DNA damage and epigenetic modification changes in special regions and fragments of chromosomes.

Adenine methylation is very scarce in the Drosophila genome and not erased by the ten-eleven translocation dioxygenase

N6-methyladenine (6mA) DNA modification has recently been described in metazoans, including in Drosophila, for which the erasure of this epigenetic mark has been ascribed to the ten-eleven translocation (TET) enzyme. Here, we re-evaluated 6mA presence and TET impact on the Drosophila genome. Using axenic or conventional breeding conditions, we found traces of 6mA by LC-MS/MS and no significant increase in 6mA levels in the absence of TET, suggesting that this modification is present at very low levels in the Drosophila genome but not regulated by TET. Consistent with this latter hypothesis, further molecular and genetic analyses showed that TET does not demethylate 6mA but acts essentially in an enzymatic-independent manner. Our results call for further caution concerning the role and regulation of 6mA DNA modification in metazoans and underline the importance of TET non-enzymatic activity for fly development.

6mA-Sniper: Quantifying 6mA sites in eukaryotes at single-nucleotide resolution

While N6-methyldeoxyadenine (6mA) modification is a fundamental regulation in prokaryotes, its prevalence and functions in eukaryotes are controversial. Here, we report 6mA-Sniper to quantify 6mA sites in eukaryotes at single-nucleotide resolution, and delineate a 6mA profile in Caenorhabditis elegans with 2034 sites. Twenty-six of 39 events with Mnl I restriction endonuclease sites were verified, demonstrating the feasibility of this method. The levels of 6mA sites pinpointed by 6mA-Sniper are generally increased after Pseudomonas aeruginosa infection, but decreased in strains with the removal of METL-9, the dominant 6mA methyltransferase. The enrichment of these sites on specific motif of [GC]GAG, the selective constrains on them, and their coordinated changes with METL-9 levels thus support an active shaping of the 6mA profile by methyltransferase. Moreover, for regions marked by 6mA sites that emerged after infection, an enrichment of up-regulated genes was detected, possibly mediated through a mutual exclusive cross-talk between 6mA and H3K27me3 modification. We thus highlight 6mA regulation as a previously neglected regulator in eukaryotes.

Single-Nucleotide Resolution Mapping of N6-Methyladenine in Genomic DNA

N6-Methyladenine (6mA) is a naturally occurring DNA modification in both prokaryotes and eukaryotes. Herein, we developed a deaminase-mediated sequencing (DM-seq) method for genome-wide mapping of 6mA at single-nucleotide resolution. The method capitalizes on the selective deamination of adenine, but not 6mA, in DNA mediated by an evolved adenine deaminase, ABE8e. By employing this method, we achieved genome-wide mapping of 6mA in Escherichia coli and in mammalian mitochondrial DNA (mtDNA) at single-nucleotide resolution. We found that the 6mA sites are mainly located in the GATC motif in the E. coli genome. We also identified 17 6mA sites in mtDNA of HepG2 cells, where all of the 6mA sites are distributed in the heavy strand of mtDNA. We envision that DM-seq will be a valuable tool for uncovering new functions of 6mA in DNA and for exploring its potential roles in mitochondria-related human diseases.

Simultaneous RNA and DNA Adductomics Using Single Data-Independent Acquisition Mass Spectrometry Analysis

Adductomics studies are used for the detection and characterization of various chemical modifications (adducts) of nucleic acids and proteins. The advancements in liquid chromatography coupled with high-resolution tandem mass spectrometry (HRMS/MS) have resulted in efficient methods for qualitative and quantitative adductomics. We developed an HRMS-based method for the simultaneous analysis of RNA and DNA adducts in a single run and demonstrated its application using Baltic amphipods, useful sentinels of environmental disturbances, as test organisms. The novelty of this method is screening for RNA and DNA adducts by a single injection on an Orbitrap HRMS instrument using full scan and data-independent acquisition. The MS raw files were processed with an open-source program, nLossFinder, to identify and distinguish RNA and DNA adducts based on the characteristic neutral loss of ribonucleosides and 2'-deoxyribonucleosides, respectively. In the amphipods, in addition to the nearly 150 putative DNA adducts characterized earlier, we detected 60 putative RNA adducts. For the structural identification of the detected RNA adducts, the MODOMICS database was used. The identified RNA adducts included simple mono- and dimethylation and other larger functional groups on different ribonucleosides and deaminated product inosine. However, 54 of these RNA adducts are not yet structurally identified, and further work on their characterization may uncover new layers of information related to the transcriptome and help understand their biological significance. Considering the susceptibility of nucleic acids to environmental factors, including pollutants, the developed multi-adductomics methodology with further advancement has the potential to provide biomarkers for diagnostics of pollution effects in biota.

Pyrimidine catabolism is required to prevent the accumulation of 5-methyluridine in RNA

5-Methylated cytosine is a frequent modification in eukaryotic RNA and DNA influencing mRNA stability and gene expression. Here we show that free 5-methylcytidine (5mC) and 5-methyl-2'-deoxycytidine are generated from nucleic acid turnover in Arabidopsis thaliana, and elucidate how these cytidines are degraded, which is unclear in eukaryotes. First CYTIDINE DEAMINASE produces 5-methyluridine (5mU) and thymidine which are subsequently hydrolyzed by NUCLEOSIDE HYDROLASE 1 (NSH1) to thymine and ribose or deoxyribose. Interestingly, far more thymine is generated from RNA than from DNA turnover, and most 5mU is directly released from RNA without a 5mC intermediate, since 5-methylated uridine (m5U) is an abundant RNA modification (m5U/U ∼1%) in Arabidopsis. We show that m5U is introduced mainly by tRNA-SPECIFIC METHYLTRANSFERASE 2A and 2B. Genetic disruption of 5mU degradation in the NSH1 mutant causes m5U to occur in mRNA and results in reduced seedling growth, which is aggravated by external 5mU supplementation, also leading to more m5U in all RNA species. Given the similarities between pyrimidine catabolism in plants, mammals and other eukaryotes, we hypothesize that the removal of 5mU is an important function of pyrimidine degradation in many organisms, which in plants serves to protect RNA from stochastic m5U modification.

Methods and applications for single-cell and spatial multi-omics

The joint analysis of the genome, epigenome, transcriptome, proteome and/or metabolome from single cells is transforming our understanding of cell biology in health and disease. In less than a decade, the field has seen tremendous technological revolutions that enable crucial new insights into the interplay between intracellular and intercellular molecular mechanisms that govern development, physiology and pathogenesis. In this Review, we highlight advances in the fast-developing field of single-cell and spatial multi-omics technologies (also known as multimodal omics approaches), and the computational strategies needed to integrate information across these molecular layers. We demonstrate their impact on fundamental cell biology and translational research, discuss current challenges and provide an outlook to the future.

Randomly incorporated genomic N6-methyldeoxyadenosine delays zygotic transcription initiation in a cnidarian

N6-methyldeoxyadenosine (6mA) is a chemical alteration of DNA, observed across all realms of life. Although the functions of 6mA are well understood in bacteria and protists, its roles in animal genomes have been controversial. We show that 6mA randomly accumulates in early embryos of the cnidarian Hydractinia symbiolongicarpus, with a peak at the 16-cell stage followed by clearance to background levels two cell cycles later, at the 64-cell stage-the embryonic stage at which zygotic genome activation occurs in this animal. Knocking down Alkbh1, a putative initiator of animal 6mA clearance, resulted in higher levels of 6mA at the 64-cell stage and a delay in the initiation of zygotic transcription. Our data are consistent with 6mA originating from recycled nucleotides of degraded m6A-marked maternal RNA postfertilization. Therefore, while 6mA does not function as an epigenetic mark in Hydractinia, its random incorporation into the early embryonic genome inhibits transcription. In turn, Alkbh1 functions as a genomic 6mA "cleaner," facilitating timely zygotic genome activation. Given the random nature of genomic 6mA accumulation and its ability to interfere with gene expression, defects in 6mA clearance may represent a hitherto unknown cause of various pathologies.

A novel N6-Deoxyadenine methyltransferase METL-9 modulates C. elegans immunity via dichotomous mechanisms

N6-Methyldeoxyadenine (6mA) has been rediscovered as a DNA modification with potential biological function in metazoans. However, the physiological function and regulatory mechanisms regarding the establishment, maintenance and removal of 6mA in eukaryotes are still poorly understood. Here we show that genomic 6mA levels change in response to pathogenic infection in Caenorhabditis elegans (C. elegans). We further identify METL-9 as the methyltransferase that catalyzes DNA 6mA modifications upon pathogen infection. Deficiency of METL-9 impairs the induction of innate immune response genes and renders the animals more susceptible to pathogen infection. Interestingly, METL-9 functions through both 6mA-dependent and -independent mechanisms to transcriptionally regulate innate immunity. Our findings reveal that 6mA is a functional DNA modification in immunomodulation in C. elegans.

The influence of early environment and micronutrient availability on developmental epigenetic programming: lessons from the placenta

DNA methylation is the most commonly studied epigenetic mark in humans, as it is well recognised as a stable, heritable mark that can affect genome function and influence gene expression. Somatic DNA methylation patterns that can persist throughout life are established shortly after fertilisation when the majority of epigenetic marks, including DNA methylation, are erased from the pre-implantation embryo. Therefore, the period around conception is potentially critical for influencing DNA methylation, including methylation at imprinted alleles and metastable epialleles (MEs), loci where methylation varies between individuals but is correlated across tissues. Exposures before and during conception can affect pregnancy outcomes and health throughout life. Retrospective studies of the survivors of famines, such as those exposed to the Dutch Hunger Winter of 1944-45, have linked exposures around conception to later disease outcomes, some of which correlate with DNA methylation changes at certain genes. Animal models have shown more directly that DNA methylation can be affected by dietary supplements that act as cofactors in one-carbon metabolism, and in humans, methylation at birth has been associated with peri-conceptional micronutrient supplementation. However, directly showing a role of micronutrients in shaping the epigenome has proven difficult. Recently, the placenta, a tissue with a unique hypomethylated methylome, has been shown to possess great inter-individual variability, which we highlight as a promising target tissue for studying MEs and mixed environmental exposures. The placenta has a critical role shaping the health of the fetus. Placenta-associated pregnancy complications, such as preeclampsia and intrauterine growth restriction, are all associated with aberrant patterns of DNA methylation and expression which are only now being linked to disease risk later in life.

Aberrant DNA N6 -methyladenine incorporation via adenylate kinase 1 is suppressed by ADAL deaminase-dependent 2'-deoxynucleotide pool sanitation

Intracellular decay of N⁶ -methyladenine (m6A)-containing RNA potentially induces aberrant N⁶ -methyl-2'-adenine (6mdA) misincorporation into DNA. Biophysically, misincorporated 6mdA may destabilize the DNA duplex in a manner similar to bona fide methylated 6mdA DNA, thereby affecting DNA replication and transcription. Utilizing heavy stable isotope labeling and ultrasensitive UHPLC-MS/MS assay, we demonstrate that intracellular m6A-RNA decay does not generate free 6mdA species, nor lead to any misincorporated DNA 6mdA in most mammalian cell lines tested, unveiling the existence of a sanitation mechanism that prevents 6mdA misincorporation. Depletion of deaminase ADAL increases the levels of free 6mdA species, concomitant with the presence of DNA-misincorporated 6mdA resulting from intracellular RNA m6A decay, suggesting that ADAL catabolizes 6mdAMP in vivo. Furthermore, we show that the overexpression of adenylate kinase 1 (AK1) promotes 6mdA misincorporation, while AK1 knockdown diminishes 6mdA incorporation, in ADAL-deficient cells. We conclude that ADAL together with other factors (such as MTH1) contributes to 2'-deoxynucleotide pool sanitation in most cells but compromised sanitation (e.g., in NIH3T3 cells) and increased AK1 expression may facilitate aberrant 6mdA incorporation. This sanitation mechanism may provide a framework for the maintenance of the epigenetic 6mdA landscape.

Navigating the pitfalls of mapping DNA and RNA modifications

Chemical modifications to nucleic acids occur across the kingdoms of life and carry important regulatory information. Reliable high-resolution mapping of these modifications is the foundation of functional and mechanistic studies, and recent methodological advances based on next-generation sequencing and long-read sequencing platforms are critical to achieving this aim. However, mapping technologies may have limitations that sometimes lead to inconsistent results. Some of these limitations are technical in nature and specific to certain types of technology. Here, however, we focus on common (yet not always widely recognized) pitfalls that are shared among frequently used mapping technologies and discuss strategies to help technology developers and users mitigate their effects. Although the emphasis is primarily on DNA modifications, RNA modifications are also discussed.

Potential role of N6-adenine DNA methylation in alternative splicing and endosymbiosis in Paramecium bursaria

N⁶-adenine DNA methylation (6mA), a rediscovered epigenetic mark in eukaryotic organisms, diversifies in abundance, distribution, and function across species, necessitating its study in more taxa. Paramecium bursaria is a typical model organism with endosymbiotic algae of the species Chlorella variabilis. This consortium therefore serves as a valuable system to investigate the functional role of 6mA in endosymbiosis, as well as the evolutionary importance of 6mA among eukaryotes. In this study, we report the first genome-wide, base pair-resolution map of 6mA in P. bursaria and identify its methyltransferase PbAMT1. Functionally, 6mA exhibits a bimodal distribution at the 5' end of RNA polymerase II-transcribed genes and possibly participates in transcription by facilitating alternative splicing. Evolutionarily, 6mA co-evolves with gene age and likely serves as a reverse mark of endosymbiosis-related genes. Our results offer new insights for the functional diversification of 6mA in eukaryotes as an important epigenetic mark.

Identification and quantification of DNA N6-methyladenine modification in mammals: A challenge to modern analytical technologies

DNA N⁶-methyladenine modification (6mA) is a predominant epigenetic mark in prokaryotes but rarely present in multicellular metazoa. The analytical technologies have been developed for sensitive detection of 6mA, including ultra-high performance liquid chromatography coupled with mass spectrometry (UHPLC-MS/MS) and single molecule real-time sequencing (SMRTseq). However, it remains challenging to detect 6mA at global level and/or in the context of sequence in multicellular metazoa (including mammals). This mini-review brings insights into current dilemma and potential solutions for the identification and quantifications of 6mA in mammals.

The N6-methyladenine DNA demethylase ALKBH1 promotes gastric carcinogenesis by disrupting NRF1 binding capacity

DNA N6-methyladenine (6mA) is an epigenetic modification that regulates various biological processes. Here, we show that gastric cancer (GC) cells and tumors display a marked reduction in 6mA levels compared with normal gastric tissues and cells. 6mA is abundant in the surrounding transcription start sites and occurs at consensus motifs. Among the 6mA regulators, ALKBH1, a demethylase, is significantly overexpressed in GC tissues compared with adjacent normal tissues. Moreover, high ALKBH1 expression is associated with poor survival of patients with GC. ALKBH1 knockout in mice impairs chemically induced gastric carcinogenesis. Mechanistically, ALKBH1 mediates DNA 6mA demethylation to repress gene expression. In particular, the 6mA sites are enriched in NRF1 binding sequences and targeted for demethylation by ALKBH1. ALKBH1-induced 6mA demethylation inhibits NRF1-driven transcription of downstream targets, including multiple genes involved in the AMP-activated protein kinase (AMPK) signaling pathway. Accordingly, ALKBH1 suppresses AMPK signaling, causing a metabolic shift toward the Warburg effect, which facilitates tumorigenesis.

Genome-wide deposition of 6-methyladenine in human DNA reduces the viability of HEK293 cells and directly influences gene expression

While cytosine-C5 methylation of DNA is an essential regulatory system in higher eukaryotes, the presence and relevance of 6-methyladenine (m6dA) in human cells is controversial. To study the role of m6dA in human DNA, we introduced it in human cells at a genome-wide scale at GANTC and GATC sites by expression of bacterial DNA methyltransferases and observed concomitant reductions in cell viability, in particular after global GANTC methylation. We identified several genes that are directly regulated by m6dA in a GANTC context. Upregulated genes showed m6dA-dependent reduction of H3K27me3 suggesting that the PRC2 complex is inhibited by m6dA. Genes downregulated by m6dA showed enrichment of JUN family transcription factor binding sites. JUN binds m6dA containing DNA with reduced affinity suggesting that m6dA can reduce the recruitment of JUN transcription factors to target genes. Our study documents that global introduction of m6dA in human DNA has physiological effects. Furthermore, we identified a set of target genes which are directly regulated by m6dA in human cells, and we defined two molecular pathways with opposing effects by which artificially introduced m6dA in GANTC motifs can directly control gene expression and phenotypes of human cells.

Structural insights into DNA N6-adenine methylation by the MTA1 complex

N⁶-methyldeoxyadenine (6mA) has recently been reported as a prevalent DNA modification in eukaryotes. The Tetrahymena thermophila MTA1 complex consisting of four subunits, namely MTA1, MTA9, p1, and p2, is the first identified eukaryotic 6mA methyltransferase (MTase) complex. Unlike the prokaryotic 6mA MTases which have been biochemically and structurally characterized, the operation mode of the MTA1 complex remains largely elusive. Here, we report the cryogenic electron microscopy structures of the quaternary MTA1 complex in S-adenosyl methionine (SAM)-bound (2.6 Å) and S-adenosyl homocysteine (SAH)-bound (2.8 Å) states. Using an AI-empowered integrative approach based on AlphaFold prediction and chemical cross-linking mass spectrometry, we further modeled a near-complete structure of the quaternary complex. Coupled with biochemical characterization, we revealed that MTA1 serves as the catalytic core, MTA1, MTA9, and p1 likely accommodate the substrate DNA, and p2 may facilitate the stabilization of MTA1. These results together offer insights into the molecular mechanism underpinning methylation by the MTA1 complex and the potential diversification of MTases for N⁶-adenine methylation.

High-precision mapping reveals rare N6-deoxyadenosine methylation in the mammalian genome

N⁶-deoxyadenosine methylation (6mA) is the most widespread type of DNA modification in prokaryotes and is also abundantly distributed in some unicellular eukaryotes. However, 6mA levels are remarkably low in mammals. The lack of a precise and comprehensive mapping method has hindered more advanced investigations of 6mA. Here, we report a new method MM-seq (modification-induced mismatch sequencing) for genome-wide 6mA mapping based on a novel detection principle. We found that modified DNA bases are prone to form a local open region that allows capture by antibody, for example, via a DNA breathing or base-flipping mechanism. Specified endonuclease or exonuclease can recognize the antibody-stabilized mismatch-like structure and mark the exact modified sites for sequencing readout. Using this method, we examined the genomic positions of 6mA in bacteria (E. coli), green algae (C. reinhardtii), and mammalian cells (HEK239T, Huh7, and HeLa cells). In contrast to bacteria and green algae, human cells possess a very limited number of 6mA sites which are sporadically distributed across the genome of different cell types. After knocking out the RNA m⁶A methyltransferase METTL3 in mouse ES cells, 6mA becomes mostly diminished. Our results imply that rare 6mA in the mammalian genome is introduced by RNA m⁶A machinery via a non-targeted mechanism.

Hyperactive DNA Cutting for Unbiased UHPLC-MS/MS Quantification of Epigenetic DNA Marks by Engineering DNase I Mutants

Epigenetic DNA modifications, such as 5-methylcytosine, 5-hydroxymethylcytosine, and 5-formylcytosine, are associated with a variety of diseases and potential biomarkers for cancer diagnosis and therapy. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays are considered to be the gold standard for qualitative and quantitative detection of DNA modifications. DNA digestion for converting long DNA polymer into 2'-deoxynucleosides is an important preprocessing step to achieve sensitive and accurate LC-MS/MS quantification. Here, we showed that, as stimulated by divalent metal ions, Mg²⁺ and Mn²⁺, the engineered human DNase I Q9R:E13R:N74K mutant can efficiently digest DNA in the presence of monovalent metal ions at a high concentration (e.g., 1 M NaCl), showing hyperactivity on DNA cutting. We also found that the engineered DNase I mutants display exceptional DNA-cutting activity over a wider pH range (5.5-9.5). Due to their hyperactivity and high salt tolerance, the engineered DNase I mutants cut DNA 5mC and dC efficiently. Benefitting from this DNA-cutting hyperactivity, we demonstrated an LC-MS/MS assay for unbiased and accurate quantification of DNA 5mC.

DNA and histone modifications as potent diagnostic and therapeutic targets to advance non-small cell lung cancer management from the perspective of 3P medicine

Lung cancer has a very high mortality in females and males. Most (~ 85%) of lung cancers are non-small cell lung cancers (NSCLC). When lung cancer is diagnosed, most of them have either local or distant metastasis, with a poor prognosis. In order to achieve better outcomes, it is imperative to identify the molecular signature based on genetic and epigenetic variations for different NSCLC subgroups. We hypothesize that DNA and histone modifications play significant roles in the framework of predictive, preventive, and personalized medicine (PPPM; 3P medicine). Epigenetics has a significant impact on tumorigenicity, tumor heterogeneity, and tumor resistance to chemotherapy, targeted therapy, and immunotherapy. An increasing interest is that epigenomic regulation is recognized as a potential treatment option for NSCLC. Most attention has been paid to the epigenetic alteration patterns of DNA and histones. This article aims to review the roles DNA and histone modifications play in tumorigenesis, early detection and diagnosis, and advancements and therapies of NSCLC, and also explore the connection between DNA and histone modifications and PPPM, which may provide an important contribution to improve the prognosis of NSCLC. We found that the success of targeting DNA and histone modifications is limited in the clinic, and how to combine the therapies to improve patient outcomes is necessary in further studies, especially for predictive diagnostics, targeted prevention, and personalization of medical services in the 3P medicine approach. It is concluded that DNA and histone modifications are potent diagnostic and therapeutic targets to advance non-small cell lung cancer management from the perspective of 3P medicine.

LC-MS/MS for Assessing the Incorporation and Repair of N2-Alkyl-2'-deoxyguanosine in Genomic DNA

Understanding the occurrence, repair, and biological consequences of DNA damage is important in environmental toxicology and risk assessment. The most common way to assess DNA damage elicited by exogenous sources in a laboratory setting is to expose cells or experimental animals with chemicals that modify DNA. Owing to the lack of reaction specificities of DNA damaging agents, the approach frequently does not allow for induction of a specific DNA lesion. Herein, we employed metabolic labeling to selectively incorporate N2-methyl-dG (N2-MedG) and N2-n-butyl-dG (N2-nBudG) into genomic DNA of cultured mammalian cells, and investigated how the levels of the two lesions in cellular DNA are modulated by different DNA repair factors. Our results revealed that nucleotide excision repair (NER) exert moderate effects on the removal of N2-MedG and N2-nBudG from genomic DNA. We also observed that DNA polymerases κ and η contribute to the incorporation of N2-MedG into genomic DNA and modulate its repair in human cells. In addition, loss of ALKBH3 resulted in higher frequencies of N2-MedG and N2-nBuG incorporation into genomic DNA, suggesting a role of oxidative dealkylation in the reversal of these lesions. Together, our study provided new insights into the repair of minor-groove N2-alkyl-dG lesions in mammalian cells.

DNA N6-Methyladenine Modification in Eukaryotic Genome

DNA methylation is treated as an important epigenetic mark in various biological activities. In the past, a large number of articles focused on 5 mC while lacking attention to N6-methyladenine (6 mA). The presence of 6 mA modification was previously discovered only in prokaryotes. Recently, with the development of detection technologies, 6 mA has been found in several eukaryotes, including protozoans, metazoans, plants, and fungi. The importance of 6 mA in prokaryotes and single-celled eukaryotes has been widely accepted. However, due to the incredibly low density of 6 mA and restrictions on detection technologies, the prevalence of 6 mA and its role in biological processes in eukaryotic organisms are highly debated. In this review, we first summarize the advantages and disadvantages of 6 mA detection methods. Then, we conclude existing reports on the prevalence of 6 mA in eukaryotic organisms. Next, we highlight possible methyltransferases, demethylases, and the recognition proteins of 6 mA. In addition, we summarize the functions of 6 mA in eukaryotes. Last but not least, we summarize our point of view and put forward the problems that need further research.

Rare and misincorporated DNA N6-methyladenine is a hallmark of cytotoxic stresses for selectively stimulating the stemness and proliferation of glioblastoma cells

The entity of DNA N6-methyladenine (6mA) in mammals remains elusive and subsequently its roles in diseases are poorly understood. Here we exploited a bacterial DNA contamination-free and ultrasensitive UHPLC-MS/MS assay to reassess DNA 6mA in human glioblastomas and unveiled that DNA 6mA (~0.08 ppm) is extremely rare. By the use of two independent heavy stable isotope-labeling strategies, we further prove that the observed 6mA is solely generated by DNA polymerase-mediated misinocorporation. In vitro experiments point toward that the generation of misincorporated DNA 6mA is associated with the cellular stresses-caused release of RNA N6-methyladenine (m6A) nucleoside, which is profoundly inhibited by hypoxia milieu. Consistently, compared with normal brain tissues, DNA 6mA decreases in hypoxic human gliomas. Our data also strongly support that rare DNA 6mA rather than relatively abundant DNA 5-methylcytosine and 5-hydroxymethylcytosine is a hallmark of poor prognosis of IDH1/2 mutation-absent glioblastoma patients, reflecting the incidence of cytotoxic stresses and subsequent release of m6A nucleoside. The released m6A nucleoside may selectively preserve a subset of the glioblastoma cells and stimulate their stemness and proliferation. Noteworthily, demethylation-inhibiting IDH1 mutation increases the DNA 6mA content in human gliomas, but the depletion of the demethylase candidate ALKBH1 fails to do so, together suggesting the presence of other unknown 6mA demethylase for erasing misincorporated DNA 6mA. This is the first report on the identification of the misincorporated 6mA together with its origin and roles in diseases.

Means, mechanisms and consequences of adenine methylation in DNA

N⁶-methyl-2'-deoxyadenosine (6mA or m⁶dA) has been reported in the DNA of prokaryotes and eukaryotes ranging from unicellular protozoa and algae to multicellular plants and mammals. It has been proposed to modulate DNA structure and transcription, transmit information across generations and have a role in disease, among other functions. However, its existence in more recently evolved eukaryotes remains a topic of debate. Recent technological advancements have facilitated the identification and quantification of 6mA even when the modification is exceptionally rare, but each approach has limitations. Critical assessment of existing data, rigorous design of future studies and further development of methods will be required to confirm the presence and biological functions of 6mA in multicellular eukaryotes.

Critical assessment of DNA adenine methylation in eukaryotes using quantitative deconvolution

The discovery of N⁶-methyldeoxyadenine (6mA) across eukaryotes led to a search for additional epigenetic mechanisms. However, some studies have highlighted confounding factors that challenge the prevalence of 6mA in eukaryotes. We developed a metagenomic method to quantitatively deconvolve 6mA events from a genomic DNA sample into species of interest, genomic regions, and sources of contamination. Applying this method, we observed high-resolution 6mA deposition in two protozoa. We found that commensal or soil bacteria explained the vast majority of 6mA in insect and plant samples. We found no evidence of high abundance of 6mA in Drosophila, Arabidopsis, or humans. Plasmids used for genetic manipulation, even those from Dam methyltransferase mutant Escherichia coli, could carry abundant 6mA, confounding the evaluation of candidate 6mA methyltransferases and demethylases. On the basis of this work, we advocate for a reassessment of 6mA in eukaryotes.

In Vitro Evaluation of DNA Damage Effect Markers toward Five Nitrogen Mustards Based on Liquid Chromatography-Tandem Mass Spectrometry

Endogenous DNA lesions frequently occur due to internal effects such as oxidative stress, inflammation, endogenous alkylation, and epigenetic modifications. However, exposure to chemical toxicants from the environment, diet, or drugs can also induce significant endogenous DNA damage. The quantification of endogenous DNA damage effect markers might reflect the actual DNA damage level of chemical toxicants. Herein, we report a liquid chromatography-triple quadrupole tandem mass spectrometry (LC-QqQ MS/MS) method for simultaneous determination of eight representative endogenous DNA damage biomarkers, including five endogenous DNA damage effect markers (oxidative damage, 8-oxo-dG; lipid peroxidation, εdA and N²-Et-dG; inflammation, 5-Cl-dC; and endogenous alkylation, O⁶-Me-dG), and three epigenetic modifications (5-m-dC, 5-hm-dC, and N⁶-Me-dA). The method validation was performed, and the linear range was 0.05 pg to 2 ng (on-column), the limit of detection was 0.02 pg (on-column), and the precision, accuracy, matrix effect, and recovery were all between 85 and 115%. We then applied this method to evaluate endogenous DNA damage to human embryonic lung fibroblast cells exposed to five nitrogen mustards [NMs, i.e., HN1, HN2, HN3, chlorambucil (CB), and cyclophosphamide (CTX)], where curcumin exposure was used as a control due to its inability to induce the formation of endogenous DNA adducts. The amounts of eight DNA adducts in the low-, middle-, and high-concentration exposure groups of five NMs were almost all significantly different from those in the blank group (P < 0.05). We obtained a positive correlation between the contents of eight DNA damage biomarkers and the inhibition dose of five NMs, except for N²-Et-dG and 5-Cl-dC. Via further principal component analysis and partial least squares discriminant analysis, we clustered all NMs into three units with different cytotoxicity levels, that is, HN2 and HN1 (highly toxic), HN3 and CB (moderately toxic), and CTX (less toxic). Moreover, for the same concentration of HN1/2/3 exposure groups, as the cytotoxicity increased according to the order of HN3 < HN1 < HN2, the contents of 8-oxo-dG, 5-m-dC, 5-hm-dC, and N⁶-Me-dA increased, whereas the content of O⁶-Me-dG decreased. Therefore, the contents of these DNA damage effect markers were somewhat related to the cytotoxicity and concentration of NMs. We hope that this method will provide an alternative evaluation approach for the toxicological effects of NMs and the safety of the medication.

The exploration of N6-deoxyadenosine methylation in mammalian genomes

N⁶-methyladenine (N⁶-mA, m⁶dA, or 6mA), a prevalent DNA modification in prokaryotes, has recently been identified in higher eukaryotes, including mammals. Although 6mA has been well-studied in prokaryotes, the function and regulatory mechanism of 6mA in eukaryotes are still poorly understood. Recent studies indicate that 6mA can serve as an epigenetic mark and play critical roles in various biological processes, from transposable-element suppression to environmental stress response. Here, we review the significant advances in methodology for 6mA detection and major progress in understanding the regulation and function of this non-canonical DNA methylation in eukaryotes, predominantly mammals.

Mitochondrial nucleoid in cardiac homeostasis: bidirectional signaling of mitochondria and nucleus in cardiac diseases

Metabolic function and energy production in eukaryotic cells are regulated by mitochondria, which have been recognized as the intracellular 'powerhouses' of eukaryotic cells for their regulation of cellular homeostasis. Mitochondrial function is important not only in normal developmental and physiological processes, but also in a variety of human pathologies, including cardiac diseases. An emerging topic in the field of cardiovascular medicine is the implication of mitochondrial nucleoid for metabolic reprogramming. This review describes the linear/3D architecture of the mitochondrial nucleoid (e.g., highly organized protein-DNA structure of nucleoid) and how it is regulated by a variety of factors, such as noncoding RNA and its associated R-loop, for metabolic reprogramming in cardiac diseases. In addition, we highlight many of the presently unsolved questions regarding cardiac metabolism in terms of bidirectional signaling of mitochondrial nucleoid and 3D chromatin structure in the nucleus. In particular, we explore novel techniques to dissect the 3D structure of mitochondrial nucleoid and propose new insights into the mitochondrial retrograde signaling, and how it regulates the nuclear (3D) chromatin structures in mitochondrial diseases.

Quantification and mapping of DNA modifications

Apart from the four canonical nucleobases, DNA molecules carry a number of natural modifications. Substantial evidence shows that DNA modifications can regulate diverse biological processes. Dynamic and reversible modifications of DNA are critical for cell differentiation and development. Dysregulation of DNA modifications is closely related to many human diseases. The research of DNA modifications is a rapidly expanding area and has been significantly stimulated by the innovations of analytical methods. With the recent advances in methods and techniques, a series of new DNA modifications have been discovered in the genomes of prokaryotes and eukaryotes. Deciphering the biological roles of DNA modifications depends on the sensitive detection, accurate quantification, and genome-wide mapping of modifications in genomic DNA. This review provides an overview of the recent advances in analytical methods and techniques for both the quantification and genome-wide mapping of natural DNA modifications. We discuss the principles, advantages, and limitations of these developed methods. It is anticipated that new methods and techniques will resolve the current challenges in this burgeoning research field and expedite the elucidation of the functions of DNA modifications.

Effects of Adenine Methylation on the Structure and Thermodynamic Stability of a DNA Minidumbbell

DNA methylation is a prevalent regulatory modification in prokaryotes and eukaryotes. N1-methyladenine (m1A) and N6-methyladenine (m6A) have been found to be capable of altering DNA structures via disturbing Watson-Crick base pairing. However, little has been known about their influences on non-B DNA structures, which are associated with genetic instabilities. In this work, we investigated the effects of m1A and m6A on both the structure and thermodynamic stability of a newly reported DNA minidumbbell formed by two TTTA tetranucleotide repeats. As revealed by the results of nuclear magnetic resonance spectroscopic studies, both m1A and m6A favored the formation of a T·m1A and T·m6A Hoogsteen base pair, respectively. More intriguingly, the m1A and m6A modifications brought about stabilization and destabilization effects on the DNA minidumbbell, respectively. This work provides new biophysical insights into the effects of adenine methylation on the structure and thermodynamic stability of DNA.

Conjoint expression and purification strategy for acquiring proteins with ultra-low DNA N6-methyladenine backgrounds in Escherichia coli

DNA N⁶-methyladenine (6mA), a kind of DNA epigenetic modification, is widespread in eukaryotes and prokaryotes. An enzyme activity study coupled with 6mA detection using ultra-high-performance liquid chromatography-quadruple mass spectrometry (UHPLC-MS/MS) is commonly applied to investigate 6mA potentially related enzymes in vitro. However, the protein expressed in a common Escherichia coli (E. coli) strain shows an extremely high 6mA background due to minute co-purified bacterial DNA, though it has been purified to remove DNA using multiple strategies. Furthermore, as occupied by DNA with abundant 6mA, the activity of 6mA-related proteins will be influenced seriously. Here, to address this issue, we for the first time construct a derivative of E. coli Rosetta (DE3) via the λRed knockout system specifically for the expression of 6mA-related enzymes. The gene dam encoding the 6mA methyltransferase (MTase) is knocked out in the newly constructed strain named LAMBS (low adenine methylation background strain). Contrasting with E. coli Rosetta (DE3), LAMBS shows an ultra-low 6mA background on the genomic DNA when analyzed by UHPLC-MS/MS. We also demonstrate an integral strategy of protein purification, coupled with the application of LAMBS. As a result, the purified protein expressed in LAMBS exhibits an ultra-low 6mA background comparing with the one expressed in E. coli Rosetta (DE3). Our integral strategy of protein expression and purification will benefit the in vitro investigation and application of 6mA-related proteins from eukaryotes, although these proteins are elusive until now.

Enzymatic characterization of three human RNA adenosine methyltransferases reveals diverse substrate affinities and reaction optima

RNA methylations of varied RNA species (mRNA, tRNA, rRNA, non-coding RNA) generate a range of modified nucleotides, including N6-methyladenosine. Here we study the enzymology of three human RNA methyltransferases that methylate the adenosine amino group in diverse contexts, when it is: the first transcribed nucleotide after the mRNA cap (PCIF1), at position 1832 of 18S rRNA (MettL5-Trm112 complex), and within a hairpin in the 3′ UTR of the S-adenosyl-l-methionine synthetase (MettL16). Among these three enzymes, the catalytic efficiency ranges from PCIF1, with the fastest turnover rate of >230 h⁻¹ μM⁻¹ on mRNA cap analog, down to MettL16, which has the lowest rate of ∼3 h⁻¹ μM⁻¹ acting on an RNA hairpin. Both PCIF1 and MettL5 have a binding affinity (K_m) of ∼1 μM or less for both substrates of SAM and RNA, whereas MettL16 has significantly lower binding affinities for both (K_m >0.4 mM for SAM and ∼10 μM for RNA). The three enzymes are active over a wide pH range (∼5.4–9.4) and have different preferences for ionic strength. Sodium chloride at 200 mM markedly diminished methylation activity of MettL5-Trm112 complex, whereas MettL16 had higher activity in the range of 200 to 500 mM NaCl. Zinc ion inhibited activities of all three enzymes. Together, these results illustrate the diversity of RNA adenosine methyltransferases in their enzymatic mechanisms and substrate specificities and underline the need for assay optimization in their study.

Beta class amino methyltransferases from bacteria to humans: evolution and structural consequences

S-adenosyl-l-methionine dependent methyltransferases catalyze methyl transfers onto a wide variety of target molecules, including DNA and RNA. We discuss a family of methyltransferases, those that act on the amino groups of adenine or cytosine in DNA, have conserved motifs in a particular order in their amino acid sequence, and are referred to as class beta MTases. Members of this class include M.EcoGII and M.EcoP15I from Escherichia coli, Caulobacter crescentus cell cycle-regulated DNA methyltransferase (CcrM), the MTA1-MTA9 complex from the ciliate Oxytricha, and the mammalian MettL3-MettL14 complex. These methyltransferases all generate N6-methyladenine in DNA, with some members having activity on single-stranded DNA as well as RNA. The beta class of methyltransferases has a unique multimeric feature, forming either homo- or hetero-dimers, allowing the enzyme to use division of labor between two subunits in terms of substrate recognition and methylation. We suggest that M.EcoGII may represent an ancestral form of these enzymes, as its activity is independent of the nucleic acid type (RNA or DNA), its strandedness (single or double), and its sequence (aside from the target adenine).

A Role for N6-Methyladenine in DNA Damage Repair

The leading cause of mutation due to oxidative damage is 8-oxo-2'-deoxyguanosine (8-oxoG) mispairing with adenine (Ade), which can occur in two ways. First, guanine of a G:C DNA base pair can be oxidized. If not repaired in time, DNA polymerases can mispair Ade with 8-oxoG in the template. This 8-oxoG:A can be repaired by enzymes that remove Ade opposite to template 8-oxoG, or 8-oxoG opposite to Cyt. Second, free 8-oxo-dGTP can be misincorporated by DNA polymerases into DNA opposite template Ade. However, there is no known repair activity that removes 8-oxoG opposite to template Ade. We suggest that a major role of N6-methyladenine in mammalian DNA is minimizing incorporation of 8-oxoG opposite to Ade by DNA polymerases following adduct formation.

Biochemical and structural basis for YTH domain of human YTHDC1 binding to methylated adenine in DNA

The recently characterized mammalian writer (methyltransferase) and eraser (demethylase) of the DNA N6-methyladenine (N6mA) methyl mark act on single-stranded (ss) and transiently-unpaired DNA. As YTH domain-containing proteins bind N6mA-containing RNA in mammalian cells, we investigated whether mammalian YTH domains are also methyl mark readers of N6mA DNA. Here, we show that the YTH domain of YTHDC1 (known to localize in the nucleus) binds ssDNA containing N6mA, with a 10 nM dissociation constant. This binding is stronger by a factor of 5 than in an RNA context, tested under the same conditions. However, the YTH domains of YTHDF2 and YTHDF1 (predominantly cytoplasmic) exhibited the opposite effect with ∼1.5-2× stronger binding to ssRNA containing N6mA than to the corresponding DNA. We determined two structures of the YTH domain of YTHDC1 in complex with N6mA-containing ssDNA, which illustrated that YTHDC1 binds the methylated adenine in a single-stranded region flanked by duplexed DNA. We discuss the hypothesis that the writer-reader-eraser of N6mA-containining ssDNA is associated with maintaining genome stability. Structural comparison of YTH and SRA domains (the latter a DNA 5-methylcytosine reader) revealed them to be diverse members of a larger family of DNA/RNA modification readers, apparently having originated from bacterial modification-dependent restriction enzymes.