Biochemical characterization of a Naegleria TET-like oxygenase and its application in single molecule sequencing of 5-methylcytosine

A highly sensitive signal-on biosensor based on restriction enzyme-mediated molecular switch for detection of TET1

Ten-eleven translocation 1 (TET1) is a member of the TET enzyme family of dioxygenases, which plays an important role in active DNA demethylation. Therefore, the sensitive TET1 detection could help us better understand DNA methylation-demethylation in epigenetics. Here we report a detection method that consists of electrode fabrication, TET1 modification, DNA digestion, signal-on oxidoreduction, and current peak monitoring. An exquisitely designed 5'end-G-rich oligodeoxynucleotide was synthesized bearing a methylated cytosine (5-mC), which formed into hairpin dsDNA with the MspI recognition sequence (C^mCGG/GGCC). Then hairpin dsDNA was fabricated onto gold nanoparticles modified glassy carbon electrode (DNA/AuNPs/GCE) via Au-S bond. The combination uses of restriction enzyme MspI and hemin converted fabricated-dsDNA into peroxidase-mimicking DNAzyme, thereby promoting the reduction of H₂O₂ with a current peak. However, the current peak was extremely decreased once TET1 and T4 β-GT were used in advance. We confirmed a delicately linear relationship matching between the current difference and TET1 activity from 0.7 to 10.5 ng μL^-1 with a detection limit of 0.027 ng μL^-1, which outcompeted the former methods at least one order of magnitudes. The TET1 activity evaluation in the existence of Bobcat339 was also tested as the proof of concept of inhibitors screening. Our strategy provides a novel, label-free, and sensitive electrochemical approach that enables us to complete both TET1 activity evaluation and potential TET1 inhibitors screening.

Dynamic changes in genomic 5-hydroxymethyluracil and N6-methyladenine levels in the Drosophila melanogaster life cycle and in response to different temperature conditions

In this study, the level of DNA modifications was investigated in three developmental stages of Drosophila melanogaster (larvae, pupae, imago) and in an in vitro model (Schneider 2 cells). Analysis was carried out using two-dimensional ultra-performance liquid chromatography with tandem mass spectrometry. Our method made it possible, for the first time, to analyze a broad spectrum of DNA modifications in the three stages of Drosophila. Each stage was characterized by a specific modification pattern, and the levels of these compounds fluctuated throughout the D. melanogaster life cycle. The level of DNA modification was also compared between insects bred at 25 °C (optimal temperature) and at 18 °C, and the groups differed significantly. The profound changes in N6-methyladenine and 5-hydroxymethyluracil levels during the Drosophila life cycle and as a result of breeding temperature changes indicate that these DNA modifications can play important regulatory roles in response to environmental changes and/or biological conditions. Moreover, the supplementation of Schneider 2 cells with 1 mM L-ascorbic acid caused a time-dependent increase in the level of 5-(hydroxymethyl)-2'-deoxyuridine. These data suggest that a certain pool of this compound may arise from the enzymatic activity of the dTET protein.

Chemical and enzymatic modifications of 5-methylcytosine at the intersection of DNA damage, repair, and epigenetic reprogramming

The DNA of all living organisms is persistently damaged by endogenous reactions including deamination and oxidation. Such damage, if not repaired correctly, can result in mutations that drive tumor development. In addition to chemical damage, recent studies have established that DNA bases can be enzymatically modified, generating many of the same modified bases. Irrespective of the mechanism of formation, modified bases can alter DNA-protein interactions and therefore modulate epigenetic control of gene transcription. The simultaneous presence of both chemically and enzymatically modified bases in DNA suggests a potential intersection, or collision, between DNA repair and epigenetic reprogramming. In this paper, we have prepared defined sequence oligonucleotides containing the complete set of oxidized and deaminated bases that could arise from 5-methylcytosine. We have probed these substrates with human glycosylases implicated in DNA repair and epigenetic reprogramming. New observations reported here include: SMUG1 excises 5-carboxyuracil (5caU) when paired with A or G. Both TDG and MBD4 cleave 5-formyluracil and 5caU when mispaired with G. Further, TDG not only removes 5-formylcytosine and 5-carboxycytosine when paired with G, but also when mispaired with A. Surprisingly, 5caU is one of the best substrates for human TDG, SMUG1 and MBD4, and a much better substrate than T. The data presented here introduces some unexpected findings that pose new questions on the interactions between endogenous DNA damage, repair, and epigenetic reprogramming pathways.

Assay for TET1 activity and its inhibitors screening with signal amplification by both nanoparticles and Ru(III) redox recycling

Ten-eleven translocation protein 1 (TET1) is one member of TET proteins family which plays a key role in dynamic DNA methylation-demethylation process. Herein, a novel biosensor was constructed for TET1 detection and its inhibitors screening utilizing restriction digestion of endonuclease enzyme MspI. Half-methylated oligonucleotide (5mC DNA) was used as target and Ru(NH₃)₆³⁺ as electrochemical signal probe. After the treatment by TET1 and T4 β-glucosyltransferase (T4 β-GT), target oligonucleotide would not be recognized and digested. If there was no TET1, the target would be digested and the response of biosensor decreased greatly. The current difference of biosensor with and without the incubation with TET1 was therefore dependent on the concentration of TET1. To increase sensitivity of the biosensor, nanostructured film at electrode surface and nanoparticles modified oligonucleotides were employed as signal amplification elements for Ru(NH₃)₆³⁺ recycling. Finally, this biosensor showed high performance with a wide linear range of TET1 concentration from 3.5-21 ng/μL and a low detection limit of 0.33 ng/μL, which is superior to other existing methods. The inhibition effects of Bobcat339 on TET1 was successfully proved by our biosensor with an IC₅₀ of 38 μM. Not only that, but the feasibility of the biosensor for inhibitors screening was evaluated and further confirmed by other compounds including two anticancer drugs and three active ingredients of traditional Chinese medicine.

Phage-encoded ten-eleven translocation dioxygenase (TET) is active in C5-cytosine hypermodification in DNA

Chemical tailoring of canonical bases expands the functionality of DNA in the same manner that posttranscriptional and -translational modifications enhance functional diversity in RNA and proteins. We describe the activities of ten-eleven translocation dioxygenase (TET)-like iron(II)- and 2-oxo-glutarate–dependent 5mC dioxygenases that are encoded by several bacteriophages to enable hypermodification of C5-methyl cytosine bases in their DNA. Phage TETs act on methylation marks deposited within GpC sequences by functionally-associated cytosine 5-methyltransferases. The hydroxymethyl groups installed are further elaborated by tailoring enzymes, thereby decorating the phage DNA with diverse, complex modifications. These modifications are predicted to have protective roles against host defenses during viral infection.

TET/JBP (ten-eleven translocation/base J binding protein) enzymes are iron(II)- and 2-oxo-glutarate–dependent dioxygenases that are found in all kingdoms of life and oxidize 5-methylpyrimidines on the polynucleotide level. Despite their prevalence, few examples have been biochemically characterized. Among those studied are the metazoan TET enzymes that oxidize 5-methylcytosine in DNA to hydroxy, formyl, and carboxy forms and the euglenozoa JBP dioxygenases that oxidize thymine in the first step of base J biosynthesis. Both enzymes have roles in epigenetic regulation. It has been hypothesized that all TET/JBPs have their ancestral origins in bacteriophages, but only eukaryotic orthologs have been described. Here we demonstrate the 5mC-dioxygenase activity of several phage TETs encoded within viral metagenomes. The clustering of these TETs in a phylogenetic tree correlates with the sequence specificity of their genomically cooccurring cytosine C5-methyltransferases, which install the methyl groups upon which TETs operate. The phage TETs favor Gp5mC dinucleotides over the 5mCpG sites targeted by the eukaryotic TETs and are found within gene clusters specifying complex cytosine modifications that may be important for DNA packaging and evasion of host restriction.

TET-TDG Active DNA Demethylation at CpG and Non-CpG Sites

In mammalian genomes, cytosine methylation occurs predominantly at CG (or CpG) dinucleotide contexts. As part of dynamic epigenetic regulation, 5-methylcytosine (mC) can be erased by active DNA demethylation, whereby ten-eleven translocation (TET) enzymes catalyze the stepwise oxidation of mC to 5-hydroxymethylcytosine (hmC), 5-formylcytosine (fC), and 5-carboxycytosine (caC), thymine DNA glycosylase (TDG) excises fC or caC, and base excision repair yields unmodified cytosine. In certain cell types, mC is also enriched at some non-CG (or CH) dinucleotides, however hmC is not. To provide biochemical context for the distribution of modified cytosines observed in biological systems, we systematically analyzed the activity of human TET2 and TDG for substrates in CG and CH contexts. We find that while TET2 oxidizes mC more efficiently in CG versus CH sites, this context preference can be diminished for hmC oxidation. Remarkably, TDG excision of fC and caC is only modestly dependent on CG context, contrasting its strong context dependence for thymine excision. We show that collaborative TET-TDG oxidation-excision activity is only marginally reduced for CA versus CG contexts. Our findings demonstrate that the TET-TDG-mediated demethylation pathway is not limited to CG sites and suggest a rationale for the depletion of hmCH in genomes rich in mCH.

Liquid Chromatography-Mass Spectrometry Analysis of Cytosine Modifications

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a widely used technique in the global analysis of epigenetic DNA modifications. The high-resolution chromatographic separation along with sensitive MS detection permits the identification and quantification of deoxyribonucleosides with precision and reliability. Although there have been tremendous advances in LC and MS instrumentation in recent years, sample preparation has not experienced a similar rate of development and is often a bottleneck to chemical analysis. Here we present a protocol for identification and quantification of cytosine modifications that combines a robust and efficient method to generate single nucleosides from genomic DNA samples followed by direct LC-MS/MS analysis.

Fluorescence assay based on the thioflavin T-induced conformation switch of G-quadruplexes for TET1 detection

A simple and label-free fluorescence method is developed for the highly sensitive detection of TET1 based on ThT/G-quadruplexes in combination with the specific design of oligonucleotides.

Enzymatic Hydroxylation and Excision of Extended 5-Methylcytosine Analogues

Methylation of cytosine to 5-methylcytosine (mC) is a prevalent reversible epigenetic mark in vertebrates established by DNA methyltransferases (MTases); the methylation mark can be actively erased via a multi-step demethylation mechanism involving oxidation by Ten-eleven translocation (TET) enzyme family dioxygenases, excision of the latter oxidation products by thymine DNA (TDG) or Nei-like 1 (NEIL1) glycosylases followed by base excision repair to restore the unmodified state. Here we probed the activity of the mouse TET1 (mTET1) and Naegleria gruberi TET (nTET) oxygenases with DNA substrates containing extended derivatives of the 5-methylcytosine carrying linear carbon chains and adjacent unsaturated CC bonds. We found that the nTET and mTET1 enzymes were active on modified mC residues in single-stranded and double-stranded DNA in vitro, while the extent of the reactions diminished with the size of the extended group. Iterative rounds of nTET hydroxylations of ssDNA proceeded with high stereo specificity and included not only the natural alpha position but also the adjoining carbon atom in the extended side chain. The regioselectivity of hydroxylation was broken when the reactive carbon was adjoined with an sp¹ or sp² system. We also found that NEIL1 but not TDG was active with bulky TET-oxidation products. These findings provide important insights into the mechanism of these biologically important enzymatic reactions.

Differentiated Visualization of Single-Cell 5-Hydroxymethylpyrimidines with Microfluidic Hydrogel Encoding

5-Hydroxymethyluracil ( 5hmU ) is found in the genomes of a diverse range of organisms as another kind of 5-hydroxymethylpyrimidine, with the exception of 5-hydroxymethylcytosine ( 5hmC ). The biological function of 5hmU has not been well explored due to lacking both specific 5hmU recognition and single-cell analysis methods. Here we report differentiated visualization of single-cell 5hmU and 5hmC with microfluidic hydrogel encoding (sc 5hmU / 5hmC -microgel). Single cells and their genomic DNA after cell lysis can be encapsulated in individual agarose microgels. The 5hmU sites are then specifically labeled with thiophosphate for the first time, followed by labeling 5hmC with azide glucose. These labeled bases are each encoded into respective DNA barcode primers by chemical cross-linking. In situ amplification is triggered for single-molecule fluorescence visualization of single-cell 5hmU and 5hmC . On the basis of the sc 5hmU / 5hmC -microgel, we reveal cell type-specific molecular signatures of these two bases with remarkable single-cell heterogeneity. Utilizing machine learning algorithms to decode four-dimensional signatures of 5hmU / 5hmC , we visualize the discrimination of nontumorigenic, carcinoma and highly invasive breast cell lines. This strategy provides a new route to analyze and decode single-cell DNA epigenetic modifications.

Structural basis for effects of CpA modifications on C/EBPβ binding of DNA

CCAAT/enhancer binding proteins (C/EBPs) regulate gene expression in a variety of cells/tissues/organs, during a range of developmental stages, under both physiological and pathological conditions. C/EBP-related transcription factors have a consensus binding specificity of 5'-TTG-CG-CAA-3', with a central CpG/CpG and two outer CpA/TpG dinucleotides. Methylation of the CpG and CpA sites generates a DNA element with every pyrimidine having a methyl group in the 5-carbon position (thymine or 5-methylcytosine (5mC)). To understand the effects of both CpG and CpA modification on a centrally-important transcription factor, we show that C/EBPβ binds the methylated 8-bp element with modestly-increased (2.4-fold) binding affinity relative to the unmodified cognate sequence, while cytosine hydroxymethylation (particularly at the CpA sites) substantially decreased binding affinity (36-fold). The structure of C/EBPβ DNA binding domain in complex with methylated DNA revealed that the methyl groups of the 5mCpA/TpG make van der Waals contacts with Val285 in C/EBPβ. Arg289 recognizes the central 5mCpG by forming a methyl-Arg-G triad, and its conformation is constrained by Val285 and the 5mCpG methyl group. We substituted Val285 with Ala (V285A) in an Ala-Val dipeptide, to mimic the conserved Ala-Ala in many members of the basic leucine-zipper family of transcription factors, important in gene regulation, cell proliferation and oncogenesis. The V285A variant demonstrated a 90-fold binding preference for methylated DNA (particularly 5mCpA methylation) over the unmodified sequence. The smaller side chain of Ala285 permits Arg289 to adopt two alternative conformations, to interact in a similar fashion with either the central 5mCpG or the TpG of the opposite strand. Significantly, the best-studied cis-regulatory elements in RNA polymerase II promoters and enhancers have variable sequences corresponding to the central CpG or reduced to a single G:C base pair, but retain a conserved outer CpA sequence. Our analyses suggest an important modification-dependent CpA recognition by basic leucine-zipper transcription factors.

Bisulfite-free direct detection of 5-methylcytosine and 5-hydroxymethylcytosine at base resolution

Bisulfite sequencing has been the gold standard for mapping DNA modifications including 5-methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC) for decades1-4. However, this harsh chemical treatment degrades the majority of the DNA and generates sequencing libraries with low complexity2,5,6. Here, we present a bisulfite-free and base-level-resolution sequencing method, TET-assisted pyridine borane sequencing (TAPS), for detection of 5mC and 5hmC. TAPS combines ten-eleven translocation (TET) oxidation of 5mC and 5hmC to 5-carboxylcytosine (5caC) with pyridine borane reduction of 5caC to dihydrouracil (DHU). Subsequent PCR converts DHU to thymine, enabling a C-to-T transition of 5mC and 5hmC. TAPS detects modifications directly with high sensitivity and specificity, without affecting unmodified cytosines. This method is nondestructive, preserving DNA fragments over 10 kilobases long. We applied TAPS to the whole-genome mapping of 5mC and 5hmC in mouse embryonic stem cells and show that, compared with bisulfite sequencing, TAPS results in higher mapping rates, more even coverage and lower sequencing costs, thus enabling higher quality, more comprehensive and cheaper methylome analyses.

Bisulfite-free and base-resolution analysis of 5-methylcytidine and 5-hydroxymethylcytidine in RNA with peroxotungstate

5-Methylcytosine (5mC) and 5-hydroxymethylcytosine (5hmC), two of the best-studied DNA modifications, play crucial roles in normal development and disease in mammals. Although 5-methylcytidine (m5C) and 5-hydroxymethylcytidine (hm5C) have also been identified in RNA, their distribution and biological function in RNA remain largely unexplored, due to the lack of suitable sequencing methods. Here, we report a base-resolution sequencing method for hm5C in RNA. We applied the selective oxidation of hm5C to trihydroxylated-thymine (thT) mediated by peroxotungstate. thT was subsequently converted to T during cDNA synthesis using a thermostable group II intron reverse transcriptase (TGIRT). Base-resolution analysis of the hm5C sites in RNA was performed using Sanger sequencing. Furthermore, in combination with the TET enzyme oxidation of m5C to hm5C in RNA, we expand the use of peroxotungstate oxidation to detect m5C in RNA at base-resolution. By using this method, we confirmed three known m5C sites in human tRNA, demonstrating the applicability of our method in analyzing real RNA samples.

Insights into the Biochemistry, Evolution, and Biotechnological Applications of the Ten-Eleven Translocation (TET) Enzymes

A tight link exists between patterns of DNA methylation at carbon 5 of cytosine and differential gene expression in mammalian tissues. Indeed, aberrant DNA methylation results in various human diseases, including neurologic and immune disorders, and contributes to the initiation and progression of various cancers. Proper DNA methylation depends on the fidelity and control of the underlying mechanisms that write, maintain, and erase these epigenetic marks. In this Perspective, we address one of the key players in active demethylation: the ten-eleven translocation enzymes or TETs. These enzymes belong to the Fe²⁺/α-ketoglutarate-dependent dioxygenase superfamily and iteratively oxidize 5-methylcytosine (5mC) in DNA to produce 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxycytosine. The latter three bases may convey additional layers of epigenetic information in addition to being intermediates in active demethylation. Despite the intense interest in understanding the physiological roles TETs play in active demethylation and cell regulation, less has been done, in comparison, to illuminate details of the chemistry and factors involved in regulating the three-step oxidation mechanism. Herein, we focus on what is known about the biochemical features of TETs and explore questions whose answers will lead to a more detailed understanding of the in vivo modus operandi of these enzymes. We also summarize the membership and evolutionary history of the TET/JBP family and highlight the prokaryotic homologues as a reservoir of potentially diverse functionalities awaiting discovery. Finally, we spotlight sequencing methods that utilize TETs for mapping 5mC and its oxidation products in genomic DNA and comment on possible improvements in these approaches.

Highly Selective 5-Formyluracil Labeling and Genome-wide Mapping Using (2-Benzimidazolyl)Acetonitrile Probe

Chemical modifications to nucleobases have a great influence on various cellular processes, by making gene regulation more complex, thus indicating their profound impact on aspects of heredity, growth, and disease. Here, we provide the first genome-wide map of 5-formyluracil (5fU) in living tissues and evaluate the potential roles for 5fU in genomics. We show that an azido derivative of (2-benzimidazolyl)acetonitrile has high selectivity for enriching 5fU-containing genomic DNA. The results have demonstrated the feasibility of using this method to determine the genome-wide distribution of 5fU. Intriguingly, most 5fU sites were found in intergenic regions and introns. Also, distribution of 5fU in human thyroid carcinoma tissues is positively correlated with binding sites of POLR2A protein, which indicates that 5fU may distributed around POLR2A-binding sites.

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The derivative of (2-benzimidazolyl)acetonitrile (azi-BIAN) can selectivity label 5fU

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Azi-BIAN can selectively label and pull down 5fU in the genome for NGS

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The first genome-wide map of 5fU in mammalian genomic DNA

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5fU is highly enriched at intergenic regions and introns

Sequencing 5-Hydroxymethyluracil at Single-Base Resolution

5-hydroxymethyluracil (5hmU) is formed through oxidation of thymine both enzymatically and non-enzymatically in various biological systems. Although 5hmU has been reported to affect biological processes such as protein-DNA interactions, the consequences of 5hmU formation in genomes have not been yet fully explored. Herein, we report a method to sequence 5hmU at single-base resolution. We employ chemical oxidation to transform 5hmU to 5-formyluracil (5fU), followed by the polymerase extension to induce T-to-C base changes owing to the inherent ability of 5fU to form 5fU:G base pairing. In combination with the Illumina next generation sequencing technology, we developed polymerase chain reaction (PCR) conditions to amplify the T-to-C base changes and demonstrate the method in three different synthetic oligonucleotide models as well as part of the genome of a 5hmU-rich eukaryotic pathogen. Our method has the potential capability to map 5hmU in genomic DNA and thus will contribute to promote the understanding of this modified base.

Detecting and interpreting DNA methylation marks

The generation, alteration, recognition, and erasure of epigenetic modifications of DNA are fundamental to controlling gene expression in mammals. These covalent DNA modifications include cytosine methylation by AdoMet-dependent methyltransferases and 5-methylcytosine oxidation by Fe(II)-dependent and α-ketoglutarate-dependent dioxygenases. Sequence-specific transcription factors are responsible for interpreting the modification status of specific regions of chromatin. This review focuses on recent developments in characterizing the functional and structural links between the modification status of two DNA bases: 5-methylcytosine and 5-methyluracil (thymine).

Epigenetic modification of nucleic acids: from basic studies to medical applications

The epigenetic modification of nucleic acids represents one of the most significant areas of study in the field of nucleic acids because it makes gene regulation more complex and heredity more complicated, thus indicating its profound impact on aspects of heredity, growth, and diseases. The recent characterization of epigenetic modifications of DNA and RNA using chemical labelling strategies has promoted the discovery of these modifications, and the newly developed single-base or single-cell resolution mapping strategies have enabled large-scale epigenetic studies in eukaryotes. Due to these technological breakthroughs, several new epigenetic marks have been discovered that have greatly extended the scope and impact of epigenetic modifications in nucleic acids over the past few years. Because epigenetics is reversible and susceptible to environmental factors, it could potentially be a promising direction for clinical medicine research. In this review, we have comprehensively discussed how these epigenetic marks are involved in disease, including the pathogenesis, prevention, diagnosis and treatment of disease. These findings have revealed that the epigenetic modification of nucleic acids has considerable significance in various areas from methodology to clinical medicine and even in biomedical applications.

A method for the efficient and selective identification of 5-hydroxymethyluracil in genomic DNA

Recently, 5-hydroxymethyluracil (5hmU) was identified in mammalian genomic DNA as an oxidative product of thymine by the ten-eleven translocation (TET) proteins. While the biological role of this modification remains unclear, identifying its genomic location will assist in elucidating function. Here we present a rapid and robust method to selectively tag and enrich genomic regions containing 5hmU. This method involves the selective glucosylation of 5hmU residues by the base J glucosyltransferase from trypanosomes creating glucosylhydroxymethyluracil (base J). The base J can then be efficiently and selectively pulled down by antibodies against base J or by J-binding protein 1. DNA that is enriched is suitable for analysis by quantitative PCR or sequencing. We utilized this tagging reaction to provide proof of concept for the enrichment of 5hmU containing DNA from a pool that contains modified and unmodified DNA. Furthermore, we demonstrate that the base J pull-down assay identifies 5hmU at specific regions of the trypanosome genome involved in transcriptional repression. The method described here will allow for a greater understanding of the functional role and dynamics of 5hmU in biology.

Synthesis and biophysical analysis of modified thymine-containing DNA oligonucleotides

We report the synthesis of a 5-formyl-2'-deoxyuridine (5fU) phosphoramidite and the preparation of oligonucleotides comprising all known, naturally observed eukaryotic thymidine modifications. Biophysical characterization of the synthetic oligonucleotides indicates that 5fU, but not the other T-derivatives, can alter DNA structures.

DNA demethylation pathways: Additional players and regulators

DNA demethylation can occur passively by "dilution" of methylation marks by DNA replication, or actively and independently of DNA replication. Direct conversion of 5-methylcytosine (5mC) to cytosine (C), as originally proposed, does not occur. Instead, active DNA methylation involves oxidation of the methylated base by ten-eleven translocations (TETs), or deamination of the methylated or a nearby base by activation induced deaminase (AID). The modified nucleotide, possibly together with surrounding nucleotides, is then replaced by the BER pathway. Recent data clarify the roles and the regulation of well-known enzymes in this process. They identify base excision repair (BER) glycosylases that may cooperate with or replace thymine DNA glycosylase (TDG) in the base excision step, and suggest possible involvement of DNA damage repair pathways other than BER in active DNA demethylation. Here, we review these new developments.

Distinctive Klf4 mutants determine preference for DNA methylation status

Reprogramming of mammalian genome methylation is critically important but poorly understood. Klf4, a transcription factor directing reprogramming, contains a DNA binding domain with three consecutive C2H2 zinc fingers. Klf4 recognizes CpG or TpG within a specific sequence. Mouse Klf4 DNA binding domain has roughly equal affinity for methylated CpG or TpG, and slightly lower affinity for unmodified CpG. The structural basis for this key preference is unclear, though the side chain of Glu446 is known to contact the methyl group of 5-methylcytosine (5mC) or thymine (5-methyluracil). We examined the role of Glu446 by mutagenesis. Substituting Glu446 with aspartate (E446D) resulted in preference for unmodified cytosine, due to decreased affinity for 5mC. In contrast, substituting Glu446 with proline (E446P) increased affinity for 5mC by two orders of magnitude. Structural analysis revealed hydrophobic interaction between the proline's aliphatic cyclic structure and the 5-methyl group of the pyrimidine (5mC or T). As in wild-type Klf4 (E446), the proline at position 446 does not interact directly with either the 5mC N4 nitrogen or the thymine O4 oxygen. In contrast, the unmethylated cytosine's exocyclic N4 amino group (NH₂) and its ring carbon C5 atom hydrogen bond directly with the aspartate carboxylate of the E446D variant. Both of these interactions would provide a preference for cytosine over thymine, and the latter one could explain the E446D preference for unmethylated cytosine. Finally, we evaluated the ability of these Klf4 mutants to regulate transcription of methylated and unmethylated promoters in a luciferase reporter assay.

Quantification of Oxidized 5-Methylcytosine Bases and TET Enzyme Activity

In eukaryotic DNA, cytosine can be enzymatically modified to yield up to four epigenetic base variants. DNA methyltransferases convert cytosine to 5-methylcytosine (mC), which plays critical roles in gene regulation during development. Ten-eleven translocation (TET) enzymes can sequentially oxidize mC to three products: 5-hydroxymethylcytosine (hmC), 5-formylcytosine (fC), and 5-carboxylcytosine (caC). These oxidized bases have been found in numerous mammalian cell types, where they potentially carry out independent epigenetic functions and aid in DNA demethylation. To gain insight into the mechanisms and functions of TET family enzymes, rigorous approaches are needed to quantify genomic cytosine modifications in cells and track TET enzyme activity in vitro. Here, we present tools developed by our lab and others to report on each of the five forms of cytosine (unmodified, mC, hmC, fC, and caC) with high specificity and sensitivity. We provide detailed protocols for qualitative and quantitative analysis of cytosine modifications in genomic DNA by dot blotting and LC-MS/MS. We then describe methods for generating synthetic oligonucleotide substrates for biochemical studies, provide optimized reaction conditions, and introduce several chemoenzymatic assays, as well as HPLC, mass spectrometry, and scintillation counting methods to quantify cytosine modifications in vitro. These approaches enable mechanistic studies of TET activity, which are key to understanding the role of these enzymes in epigenetic regulation.

DNA Base Flipping: A General Mechanism for Writing, Reading, and Erasing DNA Modifications

The modification of DNA bases is a classic hallmark of epigenetics. Four forms of modified cytosine-5-methylcytosine, 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine-have been discovered in eukaryotic DNA. In addition to cytosine carbon-5 modifications, cytosine and adenine methylated in the exocyclic amine-N4-methylcytosine and N6-methyladenine-are other modified DNA bases discovered even earlier. Each modified base can be considered a distinct epigenetic signal with broader biological implications beyond simple chemical changes. Since 1994, crystal structures of proteins and enzymes involved in writing, reading, and erasing modified bases have become available. Here, we present a structural synopsis of writers, readers, and erasers of the modified bases from prokaryotes and eukaryotes. Despite significant differences in structures and functions, they are remarkably similar regarding their engagement in flipping a target base/nucleotide within DNA for specific recognitions and/or reactions. We thus highlight base flipping as a common structural framework broadly applied by distinct classes of proteins and enzymes across phyla for epigenetic regulations of DNA.

Adenine methylation in eukaryotes: Apprehending the complex evolutionary history and functional potential of an epigenetic modification

While N⁶‐methyladenosine (m⁶A) is a well‐known epigenetic modification in bacterial DNA, it remained largely unstudied in eukaryotes. Recent studies have brought to fore its potential epigenetic role across diverse eukaryotes with biological consequences, which are distinct and possibly even opposite to the well‐studied 5‐methylcytosine mark. Adenine methyltransferases appear to have been independently acquired by eukaryotes on at least 13 occasions from prokaryotic restriction‐modification and counter‐restriction systems. On at least four to five instances, these methyltransferases were recruited as RNA methylases. Thus, m⁶A marks in eukaryotic DNA and RNA might be more widespread and diversified than previously believed. Several m⁶A‐binding protein domains from prokaryotes were also acquired by eukaryotes, facilitating prediction of potential readers for these marks. Further, multiple lineages of the AlkB family of dioxygenases have been recruited as m⁶A demethylases. Although members of the TET/JBP family of dioxygenases have also been suggested to be m⁶A demethylases, this proposal needs more careful evaluation.

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Structure of Naegleria Tet-like dioxygenase (NgTet1) in complexes with a reaction intermediate 5-hydroxymethylcytosine DNA

The family of ten-eleven translocation (Tet) dioxygenases is widely distributed across the eukaryotic tree of life, from mammals to the amoeboflagellate Naegleria gruberi. Like mammalian Tet proteins, the Naegleria Tet-like protein, NgTet1, acts on 5-methylcytosine (5mC) and generates 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC) in three consecutive, Fe(II)- and α-ketoglutarate-dependent oxidation reactions. The two intermediates, 5hmC and 5fC, could be considered either as the reaction product of the previous enzymatic cycle or the substrate for the next cycle. Here we present a new crystal structure of NgTet1 in complex with DNA containing a 5hmC. Along with the previously solved NgTet1–5mC structure, the two complexes offer a detailed picture of the active site at individual stages of the reaction cycle. In the crystal, the hydroxymethyl (OH-CH₂-) moiety of 5hmC points to the metal center, representing the reaction product of 5mC hydroxylation. The hydroxyl oxygen atom could be rotated away from the metal center, to a hydrophobic pocket formed by Ala212, Val293 and Phe295. Such rotation turns the hydroxyl oxygen atom away from the product conformation, and exposes the target CH₂ towards the metal-ligand water molecule, where a dioxygen O₂ molecule would occupy to initiate the next round of reaction by abstracting a hydrogen atom from the substrate. The Ala212-to-Val (A212V) mutant profoundly limits the product to 5hmC, probably because the reduced hydrophobic pocket size restricts the binding of 5hmC as a substrate.

The Mechanisms of Generation, Recognition, and Erasure of DNA 5-Methylcytosine and Thymine Oxidations

One of the most fundamental questions in the control of gene expression in mammals is how the patterns of epigenetic modifications of DNA are generated, recognized, and erased. This includes covalent cytosine methylation of DNA and its associated oxidation states. An array of AdoMet-dependent methyltransferases, Fe(II)- and α-ketoglutarate-dependent dioxygenases, base excision glycosylases, and sequence-specific transcription factors is responsible for changing, maintaining, and interpreting the modification status of specific regions of chromatin. This review focuses on recent developments in characterizing the functional and structural links between the modification status of two DNA bases 5-methylcytosine and thymine (5-methyluracil).