Navigating the pitfalls of mapping DNA and RNA modifications

Bisulfite-Free and Quantitative Detection of DNA Methylation at Single-Base Resolution by eROS1-seq

5-Methylcytosine (5mC) is the most significant DNA modification present in mammalian genomes. Understanding the roles of 5mC in diverse biological processes requires quantitative detection at single-base resolution. In this study, we engineered the repressor of the silencing 1 (ROS1) protein derived from Arabidopsis thaliana to enhance its 5mC glycosylase/lyase activity, resulting in the creation of the engineered ROS1 (eROS1) protein. Leveraging the unique properties of eROS1, we introduced a method termed engineered ROS1 sequencing (eROS1-seq) for bisulfite-free and quantitative detection of 5mC in DNA at single-base resolution. In eROS1-seq, the eROS1 protein selectively cleaves 5mC while leaving unmodified cytosine (C) intact, followed by the incorporation of dTTP, which subsequently results in sequencing as thymine (T). This method effectively differentiates between C and 5mC. Unlike conventional bisulfite sequencing (BS-seq), which predominantly converts cytosines, eROS1-seq specifically transforms 5mC into T, thereby avoiding potential imbalances in the nucleobase composition of the sequencing library. Using eROS1-seq, we successfully achieved quantitative and site-specific detection of 5mC in the genomic DNA of lung cancer tissue. Overall, the eROS1-seq approach is bisulfite-free and straightforward, making it a valuable tool for the quantitative detection of 5mC at single-base resolution.

Past, Present, and Future of RNA Modifications in Infectious Disease Research

In early 2024, the National Academies of Sciences, Engineering, and Medicine (NASEM) released a roadmap for the future of research into mapping ribonucleic acid (RNA) modifications, which underscored the importance of better defining these diverse chemical changes to the RNA macromolecule. As nearly all mature RNA molecules harbor some form of modification, we must understand RNA modifications to fully appreciate the functionality of RNA. The NASEM report calls for massive mobilization of resources and investment akin to the transformative Human Genome Project of the early 1990s. Like the Human Genome Project, a concerted effort in improving our ability to assess every single modification on every single RNA molecule in an organism will change the way we approach biological questions, accelerate technological advance, and improve our understanding of the molecular world. Consequently, we are also at the start of a revolution in defining the impact of RNA modifications in the context of host-microbe and even microbe-microbe interactions. In this perspective, we briefly introduce RNA modifications to the infection biologist, highlight key aspects of the NASEM report and exciting examples of RNA modifications contributing to host and pathogen biology, and finally postulate where infectious disease research may benefit from this exciting new endeavor in globally mapping RNA modifications.

A DNA Polymerase Variant Senses the Epigenetic Marker 5-Methylcytosine by Increased Misincorporation

Dysregulation of DNA methylation is associated with human disease, particularly cancer, and the assessment of aberrant methylation patterns holds great promise for clinical diagnostics. However, DNA polymerases do not effectively discriminate between processing 5-methylcytosine (5 mC) and unmethylated cytosine, resulting in the silencing of methylation information during amplification or sequencing. As a result, current detection methods require multi-step DNA conversion treatments or careful analysis of sequencing data to decipher individual 5 mC bases. To overcome these challenges, we propose a novel DNA polymerase-mediated 5 mC detection approach. Here, we describe the engineering of a thermostable DNA polymerase variant derived from Thermus aquaticus with altered fidelity towards 5 mC. Using a screening-based evolutionary approach, we have identified a DNA polymerase that exhibits increased misincorporation towards 5 mC during DNA synthesis. This DNA polymerase generates mutation signatures at methylated CpG sites, allowing direct detection of 5 mC by reading an increased error rate after sequencing without prior treatment of the sample DNA.

Challenges in identifying mRNA transcript starts and ends from long-read sequencing data

Long-read sequencing (LRS) technologies have the potential to revolutionize scientific discoveries in RNA biology through the comprehensive identification and quantification of full-length mRNA isoforms. Despite great promise, challenges remain in the widespread implementation of LRS technologies for RNA-based applications, including concerns about low coverage, high sequencing error, and robust computational pipelines. Although much focus has been placed on defining mRNA exon composition and structure with LRS data, less careful characterization has been done of the ability to assess the terminal ends of isoforms, specifically, transcription start and end sites. Such characterization is crucial for completely delineating full mRNA molecules and regulatory consequences. However, there are substantial inconsistencies in both start and end coordinates of LRS reads spanning a gene, such that LRS reads often fail to accurately recapitulate annotated or empirically derived terminal ends of mRNA molecules. Here, we describe the specific challenges of identifying and quantifying mRNA terminal ends with LRS technologies and how these issues influence biological interpretations of LRS data. We then review recent experimental and computational advances designed to alleviate these problems, with ideal use cases for each approach. Finally, we outline anticipated developments and necessary improvements for the characterization of terminal ends from LRS data.

Critical assessment of nanopore sequencing for the detection of multiple forms of DNA modifications

While nanopore sequencing is increasingly used for mapping DNA modifications, it is important to recognize false positive calls as they can mislead biological interpretations. To assist biologists and methods developers, we describe a framework for rigorous evaluation that highlights the use of false discovery rate with rationally designed negative controls capturing both general background and confounding modifications. Our critical assessment across multiple forms of DNA modifications highlights that while nanopore sequencing performs reliably for high-abundance modifications, including 5-methylcytosine (5mC) at CpG sites in mammalian cells and 5-hydroxymethylcytosine (5hmC) in mammalian brain cells, it makes a significant proportion of false positive detections for low-abundance modifications, such as 5mC at CpH sites, 5hmC and N6-methyldeoxyadenine (6mA) in most mammal cell types. This study highlights the urgent need to incorporate this framework in future methods development and biological studies, and advocates prioritizing nanopore sequencing for mapping abundant over rare modifications in biomedical applications.

The interplay between epitranscriptomic RNA modifications and neurodegenerative disorders: Mechanistic insights and potential therapeutic strategies

Neurodegenerative disorders encompass a group of age-related conditions characterized by the gradual decline in both the structure and functionality of the central nervous system (CNS). RNA modifications, arising from the epitranscriptome or RNA-modifying protein mutations, have recently been observed to contribute significantly to neurodegenerative disorders. Specific modifications like N6-methyladenine (m6A), N1-methyladenine (m1A), 5-methylcytosine (m5C), pseudouridine and adenosine-to-inosine (A-to-I) play key roles, with their regulators serving as crucial therapeutic targets. These epitranscriptomic changes intricately control gene expression, influencing cellular functions and contributing to disease pathology. Dysregulation of RNA metabolism, affecting mRNA processing and noncoding RNA biogenesis, is a central factor in these diseases. This review underscores the complex relationship between RNA modifications and neurodegenerative disorders, emphasizing the influence of RNA modification and the epitranscriptome, exploring the function of RNA modification enzymes in neurodegenerative processes, investigating the functional consequences of RNA modifications within neurodegenerative pathways, and evaluating the potential therapeutic advancements derived from assessing the epitranscriptome.

DNA Adductomics: A Narrative Review of Its Development, Applications, and Future

DNA adductomics is the global study of all DNA adducts and was first proposed in 2006 by the Matsuda group. Its development has been greatly credited to the advances in mass spectrometric techniques, particularly tandem and multiple-stage mass spectrometry. In fact, liquid chromatography-mass spectrometry (LC-MS)-based methods are virtually the sole technique with practicality for DNA adductomic studies to date. At present, DNA adductomics is primarily used as a tool to search for DNA adducts, known and unknown, providing evidence for exposure to exogenous genotoxins and/or for the molecular mechanisms of their genotoxicity. Some DNA adducts discovered in this way have the potential to predict cancer risks and/or to be associated with adverse health outcomes. DNA adductomics has been successfully used to identify and determine exogenous carcinogens that may contribute to the etiology of certain cancers, including bacterial genotoxins and an N-nitrosamine. Also using the DNA adductomic approach, multiple DNA adducts have been observed to show age dependence and may serve as aging biomarkers. These achievements highlight the capability and power of DNA adductomics in the studies of medicine, biological science, and environmental science. Nonetheless, DNA adductomics is still in its infancy, and great advances are expected in the future.

Simultaneous detection of 5-methylcytosine and 5-hydroxymethylcytosine at specific genomic loci by engineered deaminase-assisted sequencing

Cytosine modifications, particularly 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC), play crucial roles in numerous biological processes. Current analytical methods are often constrained to the separate detection of either 5mC or 5hmC, or the combination of both modifications. The ability to simultaneously detect C, 5mC, and 5hmC at the same genomic locations with precise stoichiometry is highly desirable. Herein, we introduce a method termed engineered deaminase-assisted sequencing (EDA-seq) for the simultaneous quantification of C, 5mC, and 5hmC at the same genomic sites. EDA-seq utilizes a specially engineered protein, derived from human APOBEC3A (A3A), known as eA3A-M5. eA3A-M5 exhibits distinct deamination capabilities for C, 5mC, and 5hmC. In EDA-seq, C undergoes complete deamination and is sequenced as T. 5mC is partially deaminated resulting in a mixed readout of T and C, and 5hmC remains undeaminated and is read as C. Consequently, the proportion of T readouts (P T) reflects the collective occurrences of C and 5mC, regulated by the deamination rate of 5mC (R 5mC). By determining R 5mC and P T values, we can deduce the precise levels of C, 5mC, and 5hmC at particular genomic locations. We successfully used EDA-seq to simultaneously measure C, 5mC, and 5hmC at specific loci within human lung cancer tissue and their normal counterpart. The results from EDA-seq demonstrated a strong concordance with those obtained from the combined application of BS-seq and ACE-seq methods. EDA-seq eliminates the need for bisulfite treatment, DNA oxidation or glycosylation and uniquely enables simultaneous quantification of C, 5mC and 5hmC at the same genomic locations.

Rockfish: A transformer-based model for accurate 5-methylcytosine prediction from nanopore sequencing

DNA methylation plays an important role in various biological processes, including cell differentiation, ageing, and cancer development. The most important methylation in mammals is 5-methylcytosine mostly occurring in the context of CpG dinucleotides. Sequencing methods such as whole-genome bisulfite sequencing successfully detect 5-methylcytosine DNA modifications. However, they suffer from the serious drawbacks of short read lengths and might introduce an amplification bias. Here we present Rockfish, a deep learning algorithm that significantly improves read-level 5-methylcytosine detection by using Nanopore sequencing. Rockfish is compared with other methods based on Nanopore sequencing on R9.4.1 and R10.4.1 datasets. There is an increase in the single-base accuracy and the F1 measure of up to 5 percentage points on R.9.4.1 datasets, and up to 0.82 percentage points on R10.4.1 datasets. Moreover, Rockfish shows a high correlation with whole-genome bisulfite sequencing, requires lower read depth, and achieves higher confidence in biologically important regions such as CpG-rich promoters while being computationally efficient. Its superior performance in human and mouse samples highlights its versatility for studying 5-methylcytosine methylation across varied organisms and diseases. Finally, its adaptable architecture ensures compatibility with new versions of pores and chemistry as well as modification types.

[Advances in mapping analysis of ribonucleic acid modifications through sequencing]

Over 170 chemical modifications have been discovered in various types of ribonucleic acids (RNAs), including messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA). These RNA modifications play crucial roles in a wide range of biological processes such as gene expression regulation, RNA stability maintenance, and protein translation. RNA modifications represent a new dimension of gene expression regulation known as the "epitranscriptome". The discovery of RNA modifications and the relevant writers, erasers, and readers provides an important basis for studies on the dynamic regulation and physiological functions of RNA modifications. Owing to the development of detection technologies for RNA modifications, studies on RNA epitranscriptomes have progressed to the single-base resolution, multilayer, and full-coverage stage. Transcriptome-wide methods help discover new RNA modification sites and are of great importance for elucidating the molecular regulatory mechanisms of epitranscriptomics, exploring the disease associations of RNA modifications, and understanding their clinical applications. The existing RNA modification sequencing technologies can be categorized according to the pretreatment approach and sequencing principle as direct high-throughput sequencing, antibody-enrichment sequencing, enzyme-assisted sequencing, chemical labeling-assisted sequencing, metabolic labeling sequencing, and nanopore sequencing technologies. These methods, as well as studies on the functions of RNA modifications, have greatly expanded our understanding of epitranscriptomics. In this review, we summarize the recent progress in RNA modification detection technologies, focusing on the basic principles, advantages, and limitations of different methods. Direct high-throughput sequencing methods do not require complex RNA pretreatment and allow for the mapping of RNA modifications using conventional RNA sequencing methods. However, only a few RNA modifications can be analyzed by high-throughput sequencing. Antibody enrichment followed by high-throughput sequencing has emerged as a crucial approach for mapping RNA modifications, significantly advancing the understanding of RNA modifications and their regulatory functions in different species. However, the resolution of antibody-enrichment sequencing is limited to approximately 100-200 bp. Although chemical crosslinking techniques can achieve single-base resolution, these methods are often complex, and the specificity of the antibodies used in these methods has raised concerns. In particular, the issue of off-target binding by the antibodies requires urgent attention. Enzyme-assisted sequencing has improved the accuracy of the localization analysis of RNA modifications and enables stoichiometric detection with single-base resolution. However, the enzymes used in this technique show poor reactivity, specificity, and sequence preference. Chemical labeling sequencing has become a widely used approach for profiling RNA modifications, particularly by altering reverse transcription (RT) signatures such as RT stops, misincorporations, and deletions. Chemical-assisted sequencing provides a sequence-independent RNA modification detection strategy that enables the localization of multiple RNA modifications. Additionally, when combined with the biotin-streptavidin affinity method, low-abundance RNA modifications can be enriched and detected. Nevertheless, the specificity of many chemical reactions remains problematic, and the development of specific reaction probes for particular modifications should continue in the future to achieve the precise localization of RNA modifications. As an indirect localization method, metabolic labeling sequencing specifically localizes the sites at which modifying enzymes act, which is of great significance in the study of RNA modification functions. However, this method is limited by the intracellular labeling of RNA and cannot be applied to biological samples such as clinical tissues and blood samples. Nanopore sequencing is a direct RNA-sequencing method that does not require RT or the polymerase chain reaction (PCR). However, challenges in analyzing the data obtained from nanopore sequencing, such as the high rate of false positives, must be resolved. Discussing sequencing analysis methods for various types of RNA modifications is instructive for the future development of novel RNA modification mapping technologies, and will aid studies on the functions of RNA modifications across the entire transcriptome.

MethPhaser: methylation-based long-read haplotype phasing of human genomes

The assignment of variants across haplotypes, phasing, is crucial for predicting the consequences, interaction, and inheritance of mutations and is a key step in improving our understanding of phenotype and disease. However, phasing is limited by read length and stretches of homozygosity along the genome. To overcome this limitation, we designed MethPhaser, a method that utilizes methylation signals from Oxford Nanopore Technologies to extend Single Nucleotide Variation (SNV)-based phasing. We demonstrate that haplotype-specific methylations extensively exist in Human genomes and the advent of long-read technologies enabled direct report of methylation signals. For ONT R9 and R10 cell line data, we increase the phase length N50 by 78%-151% at a phasing accuracy of 83.4-98.7% To assess the impact of tissue purity and random methylation signals due to inactivation, we also applied MethPhaser on blood samples from 4 patients, still showing improvements over SNV-only phasing. MethPhaser further improves phasing across HLA and multiple other medically relevant genes, improving our understanding of how mutations interact across multiple phenotypes. The concept of MethPhaser can also be extended to non-human diploid genomes. MethPhaser is available at https://github.com/treangenlab/methphaser .

Enhanced ac4C detection in RNA via chemical reduction and cDNA synthesis with modified dNTPs

The functional analysis of epitranscriptomic modifications in RNA is constrained by a lack of methods that accurately capture their locations and levels. We previously demonstrated that the RNA modification N4-acetylcytidine (ac4C) can be mapped at base resolution through sodium borohydride reduction to tetrahydroacetylcytidine (tetrahydro-ac4C), followed by cDNA synthesis to misincorporate adenosine opposite reduced ac4C sites, culminating in C:T mismatches at acetylated cytidines (RedaC:T). However, this process is relatively inefficient, resulting in <20% C:T mismatches at a fully modified ac4C site in 18S rRNA. Considering that ac4C locations in other substrates including mRNA are unlikely to reach full penetrance, this method is not ideal for comprehensive mapping. Here, we introduce "RetraC:T" (reduction to tetrahydro-ac4C and reverse transcription with amino-dATP to induce C:T mismatches) as a method with enhanced ability to detect ac4C in cellular RNA. In brief, RNA is reduced through NaBH4 or the closely related reagent sodium cyanoborohydride (NaCNBH3) followed by cDNA synthesis in the presence of a modified DNA nucleotide, 2-amino-dATP, that preferentially binds to tetrahydro-ac4C. Incorporation of the modified dNTP substantially improved C:T mismatch rates, reaching stoichiometric detection of ac4C in 18S rRNA. Importantly, 2-amino-dATP did not result in truncated cDNA products nor increase mismatches at other locations. Thus, modified dNTPs are introduced as a new addition to the toolbox for detecting ac4C at base resolution.

Unveiling microbial diversity: harnessing long-read sequencing technology

Long-read sequencing has recently transformed metagenomics, enhancing strain-level pathogen characterization, enabling accurate and complete metagenome-assembled genomes, and improving microbiome taxonomic classification and profiling. These advancements are not only due to improvements in sequencing accuracy, but also happening across rapidly changing analysis methods. In this Review, we explore long-read sequencing's profound impact on metagenomics, focusing on computational pipelines for genome assembly, taxonomic characterization and variant detection, to summarize recent advancements in the field and provide an overview of available analytical methods to fully leverage long reads. We provide insights into the advantages and disadvantages of long reads over short reads and their evolution from the early days of long-read sequencing to their recent impact on metagenomics and clinical diagnostics. We further point out remaining challenges for the field such as the integration of methylation signals in sub-strain analysis and the lack of benchmarks.

Comparative restriction enzyme analysis of methylation (CREAM) reveals methylome variability within a clonal in vitro cannabis population

The primary focus of medicinal cannabis research is to ensure the stability of cannabis lines for consistent administration of chemically uniform products to patients. In recent years, tissue culture has emerged as a valuable technique for genetic preservation and rapid multiplication of cannabis clones. However, there is concern that the physical and chemical conditions of the growing media can induce somaclonal variation, potentially impacting the viability and uniformity of clones. To address this concern, we developed Comparative Restriction Enzyme Analysis of Methylation (CREAM), a novel method to assess DNA methylation patterns and used it to study a population of 78 cannabis clones maintained in tissue culture. Through bioinformatics analysis of the methylome, we successfully detected 2,272 polymorphic methylated regions among the clones. Remarkably, our results demonstrated that DNA methylation patterns were preserved across subcultures within the clonal population, allowing us to distinguish between two subsets of clonal lines used in this study. These findings significantly contribute to our understanding of the epigenetic variability within clonal lines in medicinal cannabis produced through tissue culture techniques. This knowledge is crucial for understanding the effects of tissue culture on DNA methylation and ensuring the consistency and reliability of medicinal cannabis products with therapeutic properties. Additionally, the CREAM method is a fast and affordable technology to get a first glimpse at methylation in a biological system. It offers a valuable tool for studying epigenetic variation in other plant species, thereby facilitating broader applications in plant biotechnology and crop improvement.

Epigenetic marks or not? The discovery of novel DNA modifications in eukaryotes

DNA modifications add another layer of complexity to the eukaryotic genome to regulate gene expression, playing critical roles as epigenetic marks. In eukaryotes, the study of DNA epigenetic modifications has been confined to 5mC and its derivatives for decades. However, rapid developing approaches have witnessed the expansion of DNA modification reservoirs during the past several years, including the identification of 6mA, 5gmC, 4mC, and 4acC in diverse organisms. However, whether these DNA modifications function as epigenetic marks requires careful consideration. In this review, we try to present a panorama of all the DNA epigenetic modifications in eukaryotes, emphasizing recent breakthroughs in the identification of novel DNA modifications. The characterization of their roles in transcriptional regulation as potential epigenetic marks is summarized. More importantly, the pathways for generating or eliminating these DNA modifications, as well as the proteins involved are comprehensively dissected. Furthermore, we briefly discuss the potential challenges and perspectives, which should be taken into account while investigating novel DNA modifications.

Whole-Genome Mapping of Epigenetic Modification of 5-Formylcytosine at Single-Base Resolution by Chemical Labeling Enrichment and Deamination Sequencing

DNA cytosine methylation (5-methylcytosine, 5mC) is a predominant epigenetic modification that plays a critical role in a variety of biological and pathological processes in mammals. In active DNA demethylation, the 10-11 translocation (TET) dioxygenases can sequentially oxidize 5mC to generate three modified forms of cytosine, 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC). Beyond being a demethylation intermediate, recent studies have shown that 5fC has regulatory functions in gene expression and chromatin organization. While some methods have been developed to detect 5fC, genome-wide mapping of 5fC at base resolution is still highly desirable. Herein, we propose a chemical labeling enrichment and deamination sequencing (CLED-seq) method for detecting 5fC in genomic DNA at single-base resolution. The CLED-seq method utilizes selective labeling and enrichment of 5fC-containing DNA fragments, followed by deamination mediated by apolipoprotein B mRNA-editing catalytic polypeptide-like 3A (APOBEC3A or A3A) and sequencing. In the CLED-seq process, while all C, 5mC, and 5hmC are interpreted as T during sequencing, 5fC is still read as C, enabling the precise detection of 5fC in DNA. Using the proposed CLED-seq method, we accomplished genome-wide mapping of 5fC in mouse embryonic stem cells. The mapping study revealed that promoter regions enriched with 5fC overlapped with H3K4me1, H3K4me3, and H3K27ac marks. These findings suggest a correlation between 5fC marks and active gene expression in mESCs. In conclusion, CLED-seq is a straightforward, bisulfite-free method that offers a valuable tool for detecting 5fC in genomes at a single-base resolution.

NAT10 promotes renal ischemia-reperfusion injury via activating NCOA4-mediated ferroptosis

Ischemia-reperfusion injury (IRI) is a significant contributor to acute kidney injury (AKI) and is associated with substantial morbidity and mortality rates. In this study, we aimed to investigate the role of NAT10 and its ac4C RNA modification in IRI-induced renal injury. Our findings revealed that both the expression level of NAT10 and the RNA ac4C level in the kidneys were elevated in the IRI group compared to the sham group. Functionally, we observed that inhibition of NAT10 activity with Remodelin or the specific knockout of NAT10 in the kidney led to a significant attenuation of IRI-induced renal injury. Furthermore, in vitro experiments demonstrated that NAT10 inhibition and specific knockout of NAT10 in the kidney markedly suppressed global ac4C RNA modification, providing protection against hypoxia/reoxygenation-induced tubular epithelial cell injury and ferroptosis. Mechanistically, our study uncovered that NAT10 promoted ac4C RNA modification of NCOA4 mRNA, thereby enhancing its stability and contributing to IRI-induced ferroptosis in tubular epithelial cells (TECs). These findings underscore the potential of NAT10 and ac4C RNA modification as promising therapeutic targets for the treatment of AKI. Overall, our study sheds light on the critical involvement of NAT10 and ac4C RNA modification in the pathogenesis of IRI-induced renal injury, offering valuable insights for the development of novel AKI treatment strategies.

The rise of epitranscriptomics: recent developments and future directions

The epitranscriptomics field has undergone tremendous growth since the discovery that the RNA N6-methyladenosine (m6A) modification is reversible and is distributed throughout the transcriptome. Efforts to map RNA modifications transcriptome-wide and reshape the epitranscriptome in disease settings have facilitated mechanistic understanding and drug discovery in the field. In this review we discuss recent advancements in RNA modification detection methods and consider how these developments can be applied to gain novel insights into the epitranscriptome. We also highlight drug discovery efforts aimed at developing epitranscriptomic therapeutics for cancer and other diseases. Finally, we consider engineering of the epitranscriptome as an emerging direction to investigate RNA modifications and their causal effects on RNA processing at high specificity.

An Overview of Global, Local, and Base-Resolution Methods for the Detection of 5-Hydroxymethylcytosine in Genomic DNA

The discovery of 5-hydroxymethylcytosine (5hmC) as a common DNA modification in mammalian genomes has ushered in new areas of inquiry regarding the dynamic epigenome. The balance between 5hmC and its precursor, 5-methylcytosine (5mC), has emerged as a determinant of key processes including cell fate specification, and alterations involving these bases have been implicated in the pathogenesis of various diseases. The identification of 5hmC separately from 5mC initially posed a challenge given that legacy epigenetic sequencing technologies could not discriminate between these two most abundant modifications, a significant blind spot considering their potentially functionally opposing roles. The growing interest in 5hmC, as well as in the Ten-Eleven Translocation (TET) family enzymes that catalyze its generation and further oxidation to 5-formylcytosine (5fC) and 5-carboxycytosine (5caC), has spurred the development of versatile methods for 5hmC detection. These methods enable the quantification and localization of 5hmC in diverse biological samples and, in some cases, at the resolution of individual nucleotides. However, navigating this growing toolbox of methods for 5hmC detection can be challenging. Here, we detail existing and emerging methods for the detection, quantification, and localization of 5hmC at global, locus-specific, and base resolution levels. These methods are discussed in the context of their advantages and limitations, with the goal of providing a framework to help guide researchers in choosing the level of resolution and the associated method that could be most suitable for specific needs.

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.