Cloning, characterization, and expression in Escherichia coli of the gene coding for the CpG DNA methylase from Spiroplasma sp. strain MQ1(M.SssI)

Plasma DNMT1 Activity for Assessing Tumor Burden and Predicting Neoadjuvant Therapy Response in Breast Cancer

DNA methylation is mediated by DNA methyltransferases (DNMTs), and the stability of their activity is essential for cellular fate. DNMT1 is considered one of the most promising targets for research. However, current detection techniques are limited in accurately quantifying its activity in peripheral blood. Here, a reaction system is developed known as DNMT1 Identification by Variable Activity (DIVA) for the highly sensitive detection of DNMT1 activity in the peripheral blood of breast cancer patients. DIVA can detect DNMT1 at levels as low as 10-7 U mL-1, with minimal time and cost. This method is applied to analyze 271 clinical samples, successfully evaluating tumor burden in patients staged I-IV. Finally, this method is utilized to assess the prognosis of 22 patients undergoing neoadjuvant therapy, demonstrating good consistency with ultrasound imaging results. It is believed that DIVA could serve as an effective auxiliary technique for both the early detection of breast cancer and evaluation of neoadjuvant therapy.

Systematic analysis of specificities and flanking sequence preferences of bacterial DNA-(cytosine C5)-methyltransferases reveals mechanisms of enzyme- and sequence-specific DNA readout

DNA-(cytosine C5)-methyltransferases (MTases) represent a large group of evolutionary related enzymes with specific DNA interaction. We systematically investigated the specificity and flanking sequence preferences of six bacterial enzymes of this class and many MTase mutants. We observed high (>1000-fold) target sequence specificity reflecting strong evolutionary pressure against unspecific DNA methylation. Strong flanking sequence preferences (∼100-fold) were observed which changed for methylation of near-cognate sites suggesting that the DNA structures in the transition states of the methylation of these sites differ. Mutation of amino acids involved in DNA contacts led to local changes of specificity and flanking sequence preferences, but also global effects indicating that larger conformational changes occur upon transition state formation. Based on these findings, we conclude that the transition state of the DNA methylation reaction precedes the covalent enzyme-DNA complex conformations with flipped target base that are resolved in structural studies. Moreover, our data suggest that alternative catalytically active conformations exist whose occupancy is modulated by enzyme-DNA contacts. Sequence dependent DNA shape analyses suggest that MTase flanking sequence preferences are caused by flanking sequence dependent modulation of the DNA conformation. Likely, many of these findings are transferable to other DNA MTases and DNA interacting proteins.

Targeted transcriptional downregulation of MYC using epigenomic controllers demonstrates antitumor activity in hepatocellular carcinoma models

Dysregulation of master regulator c-MYC (MYC) plays a central role in hepatocellular carcinoma (HCC) and other cancers but remains an elusive target for therapeutic intervention. MYC expression is epigenetically modulated within naturally occurring DNA loop structures, Insulated Genomic Domains (IGDs). We present a therapeutic approach using an epigenomic controller (EC), a programmable epigenomic mRNA medicine, to precisely modify MYC IGD sub-elements, leading to methylation of MYC regulatory elements and durable downregulation of MYC mRNA transcription. Significant antitumor activity is observed in preclinical models of HCC treated with the MYC-targeted EC, as monotherapy or in combination with tyrosine kinase or immune checkpoint inhibitors. These findings pave the way for clinical development of MYC-targeting epigenomic controllers in HCC patients and provide a framework for programmable epigenomic mRNA therapeutics for cancer and other diseases.

Epigenetic control of adaptive or homeostatic splicing during interval-training activities

Interval-training activities induce adaptive cellular changes without altering their fundamental identity, but the precise underlying molecular mechanisms are not fully understood. In this study, we demonstrate that interval-training depolarization (ITD) of pituitary cells triggers distinct adaptive or homeostatic splicing responses of alternative exons. This occurs while preserving the steady-state expression of the Prolactin and other hormone genes. The nature of these splicing responses depends on the exon's DNA methylation status, the methyl-C-binding protein MeCP2 and its associated CA-rich motif-binding hnRNP L. Interestingly, the steady expression of the Prolactin gene is also reliant on MeCP2, whose disruption leads to exacerbated multi-exon aberrant splicing and overexpression of the hormone gene transcripts upon ITD, similar to the observed hyperprolactinemia or activity-dependent aberrant splicing in Rett Syndrome. Therefore, epigenetic control is crucial for both adaptive and homeostatic splicing and particularly the steady expression of the Prolactin hormone gene during ITD. Disruption in this regulation may have significant implications for the development of progressive diseases.

Genome wide inherited modifications of the tomato epigenome by trans-activated bacterial CG methyltransferase

BACKGROUND

Epigenetic variation is mediated by epigenetic marks such as DNA methylation occurring in all cytosine contexts in plants. CG methylation plays a critical role in silencing transposable elements and regulating gene expression. The establishment of CG methylation occurs via the RNA-directed DNA methylation pathway and CG methylation maintenance relies on METHYLTRANSFERASE1, the homologue of the mammalian DNMT1.

PURPOSE

Here, we examined the capacity to stably alter the tomato genome methylome by a bacterial CG-specific M.SssI methyltransferase expressed through the LhG4/pOP transactivation system.

RESULTS

Methylome analysis of M.SssI expressing plants revealed that their euchromatic genome regions are specifically hypermethylated in the CG context, and so are most of their genes. However, changes in gene expression were observed only with a set of genes exhibiting a greater susceptibility to CG hypermethylation near their transcription start site. Unlike gene rich genomic regions, our analysis revealed that heterochromatic regions are slightly hypomethylated at CGs only. Notably, some M.SssI-induced hypermethylation persisted even without the methylase or transgenes, indicating inheritable epigenetic modification.

CONCLUSION

Collectively our findings suggest that heterologous expression of M.SssI can create new inherited epigenetic variations and changes in the methylation profiles on a genome wide scale. This open avenues for the conception of epigenetic recombinant inbred line populations with the potential to unveil agriculturally valuable tomato epialleles.

Conversion of the CG specific M.MpeI DNA methyltransferase into an enzyme predominantly methylating CCA and CCC sites

We used structure guided mutagenesis and directed enzyme evolution to alter the specificity of the CG specific bacterial DNA (cytosine-5) methyltransferase M.MpeI. Methylation specificity of the M.MpeI variants was characterized by digestions with methylation sensitive restriction enzymes and by measuring incorporation of tritiated methyl groups into double-stranded oligonucleotides containing single CC, CG, CA or CT sites. Site specific mutagenesis steps designed to disrupt the specific contacts between the enzyme and the non-substrate base pair of the target sequence (5'-CG/5'-CG) yielded M.MpeI variants with varying levels of CG specific and increasing levels of CA and CC specific MTase activity. Subsequent random mutagenesis of the target recognizing domain coupled with selection for non-CG specific methylation yielded a variant, which predominantly methylates CC dinucleotides, has very low activity on CG and CA sites, and no activity on CT sites. This M.MpeI variant contains a one amino acid deletion (ΔA323) and three substitutions (N324G, R326G and E305N) in the target recognition domain. The mutant enzyme has very strong preference for A and C in the 3' flanking position making it a CCA and CCC specific DNA methyltransferase.

The roles of DNA methylation on pH dependent i-motif (iM) formation in rice

I-motifs (iMs) are four-stranded non-B DNA structures containing C-rich DNA sequences. The formation of iMs is sensitive to pH conditions and DNA methylation, although the extent of which is still unknown in both humans and plants. To investigate this, we here conducted iMab antibody-based immunoprecipitation and sequencing (iM-IP-seq) along with bisulfite sequencing using CK (original genomic DNA without methylation-related treatments) and hypermethylated or demethylated DNA at both pH 5.5 and 7.0 in rice, establishing a link between pH, DNA methylation and iM formation on a genome-wide scale. We found that iMs folded at pH 7.0 displayed higher methylation levels than those formed at pH 5.5. DNA demethylation and hypermethylation differently influenced iM formation at pH 7.0 and 5.5. Importantly, CG hypo-DMRs (differentially methylated regions) and CHH (H = A, C and T) hyper-DMRs alone or coordinated with CG/CHG hyper-DMRs may play determinant roles in the regulation of pH dependent iM formation. Thus, our study shows that the nature of DNA sequences alone or combined with their methylation status plays critical roles in determining pH-dependent formation of iMs. It therefore deepens the understanding of the pH and methylation dependent modulation of iM formation, which has important biological implications and practical applications.

ATM and ATR, two central players of the DNA damage response, are involved in the induction of systemic acquired resistance by extracellular DNA, but not the plant wound response

Background

The plant immune response to DNA is highly self/nonself-specific. Self-DNA triggered stronger responses by early immune signals such as H2O2 formation than nonself-DNA from closely related plant species. Plants lack known DNA receptors. Therefore, we aimed to investigate whether a differential sensing of self-versus nonself DNA fragments as damage- versus pathogen-associated molecular patterns (DAMPs/PAMPs) or an activation of the DNA-damage response (DDR) represents the more promising framework to understand this phenomenon.

Results

We treated Arabidopsis thaliana Col-0 plants with sonicated self-DNA from other individuals of the same ecotype, nonself-DNA from another A. thaliana ecotype, or nonself-DNA from broccoli. We observed a highly self/nonself-DNA-specific induction of H2O2 formation and of jasmonic acid (JA, the hormone controlling the wound response to chewing herbivores) and salicylic acid (SA, the hormone controlling systemic acquired resistance, SAR, to biotrophic pathogens). Mutant lines lacking Ataxia Telangiectasia Mutated (ATM) or ATM AND RAD3-RELATED (ATR) - the two DDR master kinases - retained the differential induction of JA in response to DNA treatments but completely failed to induce H2O2 or SA. Moreover, we observed H2O2 formation in response to in situ-damaged self-DNA from plants that had been treated with bleomycin or SA or infected with virulent bacteria Pseudomonas syringae pv. tomato DC3000 or pv. glycinea carrying effector avrRpt2, but not to DNA from H2O2-treated plants or challenged with non-virulent P. syringae pv. glycinea lacking avrRpt2.

Conclusion

We conclude that both ATM and ATR are required for the complete activation of the plant immune response to extracellular DNA whereas an as-yet unknown mechanism allows for the self/nonself-differential activation of the JA-dependent wound response.

Controllable assembly of dendritic DNA nanostructures for ultrasensitive detection of METTL3-METTL14 m6A methyltransferase activity in cancer cells and human breast tissues

N⁶-Methyladenosine (m⁶A) is a reversible chemical modification in eukaryotic messenger RNAs and long noncoding RNAs. The aberrant expression of RNA methyltransferase METTL3-METTL14 complex may change the m⁶A methylation level and cause multiple diseases including cancers. The conventional METTL3-METTL14 assays commonly suffer from time-consuming procedures and poor sensitivity. Herein, we develop a controllable amplification machinery based on MazF-activated terminal deoxynucleotidyl transferase (TdT)-assisted dendritic DNA structure assembly for ultrasensitive detection of METTL3-METTL14 complex activity in cancer cells and breast tissues. The presence of METTL3-METTL14 complex catalyzes the formation of m⁶A in detection probe, effectively preventing the cleavage of methylated detection probes by MazF. The methylated detection probes with 3'-OH termini can function as the primers for template-free polymerization catalyzed by TdT on magnetic beads (MBs), producing long chains of poly-thymidine (poly-T) sequences. Then poly-T sequences hybridize with signal probes that contain poly-adenine (poly-A) sequence, inducing TdT-mediated polymerization and the subsequent hybridization with more poly-A signal probes for generating dendritic DNA nanostructures assembled on MBs. After magnetic separation and elevated temperature treatment, the signal probes are disassembled from MBs to generate a high fluorescence signal. This method possesses excellent specificity and high sensitivity with a limit of detection (LOD) of 2.61 × 10^-15 M, and it can accurately quantify cellular METTL3-METTL14 complex at single-cell level. Furthermore, it can screen inhibitors, evaluate kinetic parameters, and discriminate breast cancer tissues from normal tissues.

Chemoenzymatic labeling of DNA methylation patterns for single-molecule epigenetic mapping

DNA methylation, specifically, methylation of cytosine (C) nucleotides at the 5-carbon position (5-mC), is the most studied and significant epigenetic modification. Here we developed a chemoenzymatic procedure to fluorescently label non-methylated cytosines in CpG context, allowing epigenetic profiling of single DNA molecules spanning hundreds of thousands of base pairs. We used a CpG methyltransferase with a synthetic S-adenosyl-l-methionine cofactor analog to transfer an azide to cytosines instead of the natural methyl group. A fluorophore was then clicked onto the DNA, reporting on the amount and position of non-methylated CpGs. We found that labeling efficiency was increased up to 2-fold by the addition of a nucleosidase, presumably by degrading the inactive by-product of the cofactor after labeling, preventing its inhibitory effect. We used the method to determine the decline in global DNA methylation in a chronic lymphocytic leukemia patient and then performed whole-genome methylation mapping of the model plant Arabidopsis thaliana. Our genome maps show high concordance with published bisulfite sequencing methylation maps. Although mapping resolution is limited by optical detection to 500–1000 bp, the labeled DNA molecules produced by this approach are hundreds of thousands of base pairs long, allowing access to long repetitive and structurally variable genomic regions.

Impacts of Mycoplasma agalactiae restriction-modification systems on pan-epigenome dynamics and genome plasticity

DNA methylations play an important role in the biology of bacteria. Often associated with restriction modification (RM) systems, they are important drivers of bacterial evolution interfering in horizontal gene transfer events by providing a defence against foreign DNA invasion or by favouring genetic transfer through production of recombinogenic DNA ends. Little is known regarding the methylome of the Mycoplasma genus, which encompasses several pathogenic species with small genomes. Here, genome-wide detection of DNA methylations was conducted using single molecule real-time (SMRT) and bisulphite sequencing in several strains of Mycoplasma agalactiae, an important ruminant pathogen and a model organism. Combined with whole-genome analysis, this allowed the identification of 19 methylated motifs associated with three orphan methyltransferases (MTases) and eight RM systems. All systems had a homolog in at least one phylogenetically distinct Mycoplasma spp. Our study also revealed that several superimposed genetic events may participate in the M. agalactiae dynamic epigenomic landscape. These included (i) DNA shuffling and frameshift mutations that affect the MTase and restriction endonuclease content of a clonal population and (ii) gene duplication, erosion, and horizontal transfer that modulate MTase and RM repertoires of the species. Some of these systems were experimentally shown to play a major role in mycoplasma conjugative, horizontal DNA transfer. While the versatility of DNA methylation may contribute to regulating essential biological functions at cell and population levels, RM systems may be key in mycoplasma genome evolution and adaptation by controlling horizontal gene transfers.

Construction of an APE1-Mediated Cascade Signal Amplification Platform for Homogeneously Sensitive and Rapid Measurement of DNA Methyltransferase in Escherichia coli Cells

DNA methylation is an essential genomic epigenetic behavior in both eukaryotes and prokaryotes. Deregulation of DNA methyltransferase (Dam MTase) can change the DNA methylation level and cause various diseases. Herein, we develop an apurinic/apyrimidinic endonuclease 1 (APE1)-mediated cascade signal amplification platform for homogeneously sensitive and rapid measurement of Dam MTase in Escherichia coli cells. This assay involves a partial double-stranded DNA (dsDNA) substrate and two hairpin signal probes (HP1 and HP2) that are modified with Cy5 and BHQ2 at two ends, respectively. When Dam MTase is present, it methylates the dsDNA substrate, and subsequently, endonuclease DpnI cleaves the methylated substrate, yielding trigger probe 1. Hybridization of trigger probe 1 with HP1 forms a partial dsDNA containing an apurinic/apyrimidinic (AP) site, which is cleaved by APE1 to induce the cyclic cleavage of HP1 and the production of abundant trigger probe 2. Subsequent hybridization of trigger probe 2 with HP2 forms a partial dsDNA with an AP site, inducing the cyclic cleavage of HP2 by APE1. Consequently, cyclic cleavage of HP1 and HP2 induces the generation of abundant Cy5 molecules, which are easily measured by single-molecule imaging. This assay can be performed homogeneously and rapidly within 2 h, which is the shortest among the reported amplification-based assays. Moreover, it exhibits good selectivity and high sensitivity, and it can discriminate Dam MTase from other enzymes and screen inhibitors. Importantly, it can accurately measure the Dam MTase activity in serum and E. coli cells, with promising applications in clinical diagnosis and drug discovery.

Z-DNA as a Tool for Nuclease-Free DNA Methyltransferase Assay

Methylcytosines in mammalian genomes are the main epigenetic molecular codes that switch off the repertoire of genes in cell-type and cell-stage dependent manners. DNA methyltransferases (DMT) are dedicated to managing the status of cytosine methylation. DNA methylation is not only critical in normal development, but it is also implicated in cancers, degeneration, and senescence. Thus, the chemicals to control DMT have been suggested as anticancer drugs by reprogramming the gene expression profile in malignant cells. Here, we report a new optical technique to characterize the activity of DMT and the effect of inhibitors, utilizing the methylation-sensitive B-Z transition of DNA without bisulfite conversion, methylation-sensing proteins, and polymerase chain reaction amplification. With the high sensitivity of single-molecule FRET, this method detects the event of DNA methylation in a single DNA molecule and circumvents the need for amplification steps, permitting direct interpretation. This method also responds to hemi-methylated DNA. Dispensing with methylation-sensitive nucleases, this method preserves the molecular integrity and methylation state of target molecules. Sparing methylation-sensing nucleases and antibodies helps to avoid errors introduced by the antibody’s incomplete specificity or variable activity of nucleases. With this new method, we demonstrated the inhibitory effect of several natural bio-active compounds on DMT. All taken together, our method offers quantitative assays for DMT and DMT-related anticancer drugs.

Evaluation of the Properties of the DNA Methyltransferase from Aeropyrum pernix K1

Little is known regarding the DNA methyltransferases (MTases) in hyperthermophilic archaea. In this study, we focus on an MTase from Aeropyrum pernix K1, a hyperthermophilic archaeon that is found in hydrothermal vents and whose optimum growth temperature is 90°C to 95°C. From genomic sequence analysis, A. pernix K1 has been predicted to have a restriction-modification system (R-M system). The restriction endonuclease from A. pernix K1 (known as ApeKI from New England BioLabs Inc. [catalog code R06435]) has been described previously, but the properties of the MTase from A. pernix K1 (M.ApeKI) have not yet been clarified. Thus, we demonstrated the properties of M.ApeKI. In this study, M.ApeKI was expressed in Escherichia coli strain JM109 and affinity purified using its His tag. The recognition sequence of M.ApeKI was determined by methylation activity and bisulfite sequencing (BS-seq). High-performance liquid chromatography (HPLC) was used to detect the position of the methyl group in methylated cytosine. As a result, it was clarified that M.ApeKI adds the methyl group at the C-5 position of the second cytosine in 5'-GCWGC-3'. Moreover, we also determined that the MTase optimum temperature was over 70°C and that it is strongly tolerant to high temperatures. M.ApeKI is the first highly thermostable DNA (cytosine-5)-methyltransferase to be evaluated by experimental evidence. IMPORTANCE In general, thermophilic bacteria with optimum growth temperatures over or equal to 60°C have been predicted to include only N⁴-methylcytosine or N⁶-methyladenine as methylated bases in their DNA, because 5-methylcytosine is susceptible to deamination by heat. However, from this study, A. pernix K1, with an optimum growth temperature at 95°C, was demonstrated to produce a DNA (cytosine-5)-methyltransferase. Thus, A. pernix K1 presumably has 5-methylcytosine in its DNA and may produce an original repair system for the expected C-to-T mutations. M.ApeKI was demonstrated to be tolerant to high temperatures; thus, we expect that M.ApeKI may be valuable for the development of a novel analysis system or epigenetic editing tool.

Enzymatic approaches for profiling cytosine methylation and hydroxymethylation

Background

In mammals, modifications to cytosine bases, particularly in cytosine-guanine (CpG) dinucleotide contexts, play a major role in shaping the epigenome. The canonical epigenetic mark is 5-methylcytosine (5mC), but oxidized versions of 5mC, including 5-hydroxymethylcytosine (5hmC), are now known to be important players in epigenomic dynamics. Understanding the functional role of these modifications in gene regulation, normal development, and pathological conditions requires the ability to localize these modifications in genomic DNA. The classical approach for sequencing cytosine modifications has involved differential deamination via the chemical sodium bisulfite; however, bisulfite is destructive, limiting its utility in important biological or clinical settings where detection of low frequency populations is critical. Additionally, bisulfite fails to resolve 5mC from 5hmC.

Scope of review

To summarize how enzymatic rather than chemical approaches can be leveraged to localize and resolve different cytosine modifications in a non-destructive manner.

Major conclusions

Nature offers a suite of enzymes with biological roles in cytosine modification in organisms spanning from bacteriophages to mammals. These enzymatic activities include methylation by DNA methyltransferases, oxidation of 5mC by TET family enzymes, hypermodification of 5hmC by glucosyltransferases, and the generation of transition mutations from cytosine to uracil by DNA deaminases. Here, we describe how insights into the natural reactivities of these DNA-modifying enzymes can be leveraged to convert them into powerful biotechnological tools. Application of these enzymes in sequencing can be accomplished by relying on their natural activity, exploiting their ability to discriminate between cytosine modification states, reacting them with functionalized substrate analogs to introduce chemical handles, or engineering the DNA-modifying enzymes to take on new reactivities. We describe how these enzymatic reactions have been combined and permuted to localize DNA modifications with high specificity and without the destructive limitations posed by chemical methods for epigenetic sequencing.

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Chemical sequencing methods damage DNA and can confound cytosine modifications.

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DNA modifying enzymes offer non-destructive and selective biotechnological tools.

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DNA deaminases, methyltransferases, oxygenases and glucosyltransferases can be used.

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Permuting enzymes with various activities can reveal distinct cytosine states.

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Engineered enzymes utilizing unnatural co-substrates expand sequencing scope.

Lowering DNA binding affinity of SssI DNA methyltransferase does not enhance the specificity of targeted DNA methylation in E. coli

Targeted DNA methylation is a technique that aims to methylate cytosines in selected genomic loci. In the most widely used approach a CG-specific DNA methyltransferase (MTase) is fused to a sequence specific DNA binding protein, which binds in the vicinity of the targeted CG site(s). Although the technique has high potential for studying the role of DNA methylation in higher eukaryotes, its usefulness is hampered by insufficient methylation specificity. One of the approaches proposed to suppress methylation at unwanted sites is to use MTase variants with reduced DNA binding affinity. In this work we investigated how methylation specificity of chimeric MTases containing variants of the CG-specific prokaryotic MTase M.SssI fused to zinc finger or dCas9 targeting domains is influenced by mutations affecting catalytic activity and/or DNA binding affinity of the MTase domain. Specificity of targeted DNA methylation was assayed in E. coli harboring a plasmid with the target site. Digestions of the isolated plasmids with methylation sensitive restriction enzymes revealed that specificity of targeted DNA methylation was dependent on the activity but not on the DNA binding affinity of the MTase. These results have implications for the design of strategies of targeted DNA methylation.

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.

Editing DNA Methylation in Mammalian Embryos

DNA methylation in mammals is essential for numerous biological functions, such as ensuring chromosomal stability, genomic imprinting, and X-chromosome inactivation through transcriptional regulation. Gene knockout of DNA methyltransferases and demethylation enzymes has made significant contributions to analyzing the functions of DNA methylation in development. By applying epigenome editing, it is now possible to manipulate DNA methylation in specific genomic regions and to understand the functions of these modifications. In this review, we first describe recent DNA methylation editing technology. We then focused on changes in DNA methylation status during mammalian gametogenesis and preimplantation development, and have discussed the implications of applying this technology to early embryos.

Rapid and Definitive Analysis of In Vitro DNA Methylation by Nano-electrospray Ionization Mass Spectrometry

CpG methylation of DNA is an epigenetic marker that is highly related to the regulation of transcription initiation. For analysis of CpG methylation in genomic DNA sequences, bisulfite-induced modification in combination with polymerase chain reaction (PCR) is usually utilized, but it cannot be straightforwardly applied to methylated short- and middle-sized DNAs, such as < 500 base pairs (bp), which are often utilized in structural biology studies. In the present study, we applied nano-electrospray ionization mass spectrometry (nano-ESI-MS) for the characterization of methylated DNA with < 400 bp prepared in vitro. First, double-stranded DNA oligomers were methylated with recombinant M.SssI DNA methylase, which has been reported to modify completely and exclusively CpG sites in the sequence. The fragments generated by the digestion with methylation-insensitive restriction nuclease were then analyzed to identify the methylation levels by nano-ESI-MS, without liquid chromatography (LC) separation. By methylation-insensitive nuclease digestion, we divided the DNA strands into several fragments, and nano-ESI-MS enabled the accurate analysis of methylation levels in the DNA fragments with a relatively small amount of DNA sample prepared under optimized conditions. Furthermore, it was revealed that M.SssI methylase hardly modifies the CpG sites closely positioned at the ends of linear DNA. The present method is similar to the strategy for post-translational modification analysis of proteins and is promising for the rapid and definitive characterization of methylated DNA that may be used in structural biology studies.

A model of pulldown alignments from SssI-treated DNA improves DNA methylation prediction

Background

Protein pulldown using Methyl-CpG binding domain (MBD) proteins followed by high-throughput sequencing is a common method to determine DNA methylation. Algorithms have been developed to estimate absolute methylation level from read coverage generated by affinity enrichment-based techniques, but the most accurate one for MBD-seq data requires additional data from an SssI-treated Control experiment.

Results

Using our previous characterizations of Methyl-CpG/MBD2 binding in the context of an MBD pulldown experiment, we build a model of expected MBD pulldown reads as drawn from SssI-treated DNA. We use the program BayMeth to evaluate the effectiveness of this model by substituting calculated SssI Control data for the observed SssI Control data. By comparing methylation predictions against those from an RRBS data set, we find that BayMeth run with our modeled SssI Control data performs better than BayMeth run with observed SssI Control data, on both 100 bp and 10 bp windows. Adapting the model to an external data set solely by changing the average fragment length, our calculated data still informs the BayMeth program to a similar level as observed data in predicting methylation state on a pulldown data set with matching WGBS estimates.

Conclusion

In both internal and external MBD pulldown data sets tested in this study, BayMeth used with our modeled pulldown coverage performs better than BayMeth run without the inclusion of any estimate of SssI Control pulldown, and is comparable to – and in some cases better than – using observed SssI Control data with the BayMeth program. Thus, our MBD pulldown alignment model can improve methylation predictions without the need to perform additional control experiments.

Electronic supplementary material

The online version of this article (10.1186/s12859-019-3011-2) contains supplementary material, which is available to authorized users.

Sources of artifact in measurements of 6mA and 4mC abundance in eukaryotic genomic DNA

BACKGROUND

Directed DNA methylation on N6-adenine (6mA), N4-cytosine (4mC), and C5-cytosine (5mC) can potentially increase DNA coding capacity and regulate a variety of biological functions. These modifications are relatively abundant in bacteria, occurring in about a percent of all bases of most bacteria. Until recently, 5mC and its oxidized derivatives were thought to be the only directed DNA methylation events in metazoa. New and more sensitive detection techniques (ultra-high performance liquid chromatography coupled with mass spectrometry (UHPLC-ms/ms) and single molecule real-time sequencing (SMRTseq)) have suggested that 6mA and 4mC modifications could be present in a variety of metazoa.

RESULTS

Here, we find that both of these techniques are prone to inaccuracies, which overestimate DNA methylation concentrations in metazoan genomic DNA. Artifacts can arise from methylated bacterial DNA contamination of enzyme preparations used to digest DNA and contaminating bacterial DNA in eukaryotic DNA preparations. Moreover, DNA sonication introduces a novel modified base from 5mC that has a retention time near 4mC that can be confused with 4mC. Our analyses also suggest that SMRTseq systematically overestimates 4mC in prokaryotic and eukaryotic DNA and 6mA in DNA samples in which it is rare. Using UHPLC-ms/ms designed to minimize and subtract artifacts, we find low to undetectable levels of 4mC and 6mA in genomes of representative worms, insects, amphibians, birds, rodents and primates under normal growth conditions. We also find that mammalian cells incorporate exogenous methylated nucleosides into their genome, suggesting that a portion of 6mA modifications could derive from incorporation of nucleosides from bacteria in food or microbiota. However, gDNA samples from gnotobiotic mouse tissues found rare (0.9-3.7 ppm) 6mA modifications above background.

CONCLUSIONS

Altogether these data demonstrate that 6mA and 4mC are rarer in metazoa than previously reported, and highlight the importance of careful sample preparation and measurement, and need for more accurate sequencing techniques.

Circularly permuted variants of two CG-specific prokaryotic DNA methyltransferases

The highly similar prokaryotic DNA (cytosine-5) methyltransferases (C5-MTases) M.MpeI and M.SssI share the specificity of eukaryotic C5-MTases (5'-CG), and can be useful research tools in the study of eukaryotic DNA methylation and epigenetic regulation. In an effort to improve the stability and solubility of complementing fragments of the two MTases, genes encoding circularly permuted (CP) variants of M.MpeI and M.SssI were created, and cloned in a plasmid vector downstream of an arabinose-inducible promoter. MTase activity of the CP variants was tested by digestion of the plasmids with methylation-sensitive restriction enzymes. Eleven of the fourteen M.MpeI permutants and six of the seven M.SssI permutants had detectable MTase activity as indicated by the full or partial protection of the plasmid carrying the cpMTase gene. Permutants cp62M.MpeI and cp58M.SssI, in which the new N-termini are located between conserved motifs II and III, had by far the highest activity. The activity of cp62M.MpeI was comparable to the activity of wild-type M.MpeI. Based on the location of the split sites, the permutants possessing MTase activity can be classified in ten types. Although most permutation sites were designed to fall outside of conserved motifs, and the MTase activity of the permutants measured in cell extracts was in most cases substantially lower than that of the wild-type enzyme, the high proportion of circular permutation topologies compatible with MTase activity is remarkable, and is a new evidence for the structural plasticity of C5-MTases. A computer search of the REBASE database identified putative C5-MTases with CP arrangement. Interestingly, all natural circularly permuted C5-MTases appear to represent only one of the ten types of permutation topology created in this work.

A novel HER2 gene body enhancer contributes to HER2 expression

The transcriptional regulation of the human epidermal growth factor receptor-2 (HER2) contributes to an enhanced HER2 expression in HER2-positive breast cancers with HER2 gene amplification and HER2-low or HER2-negative breast cancers following radiotherapy or endocrine therapy, and this drives tumorigenesis and the resistance to therapy. Epigenetic mechanisms are critical for transcription regulation, however, such mechanisms in the transcription regulation of HER2 are limited to the involvement of tri-methylated histone 3 lysine 4 (H3K4me3) and acetylated histone 3 lysine 9 (H3K9ac) at the HER2 promoter region. Here, we report the identification of a novel enhancer in the HER2 3’ gene body, which we have termed HER2 gene body enhancer (HGE). The HGE starts from the 3’ end of intron 19 and extends into intron 22, possesses enhancer histone modification marks in specific cells and enhances the transcriptional activity of the HER2 promoters. We also found that TFAP2C, a known regulator of HER2, binds to HGE and is required for its enhancer function and that DNA methylation in the HGE region inhibits the histone modifications characterizing enhancer and is inversely correlated with HER2 expression in breast cancer samples. The identification of this novel enhancer sheds a light on the roles of epigenetic mechanisms in HER2 transcription, in both HER2-positive breast cancer samples and individuals with HER2-low or HER2-negative breast cancers undergoing radiotherapy or endocrine therapy.

Inhibition studies of DNA methyltransferases by maleimide derivatives of RG108 as non-nucleoside inhibitors

AIM

DNA methyltransferases (DNMTs) are important drug targets for epigenetic therapy of cancer. Nowadays, non-nucleoside DNMT inhibitors are in development to address high toxicity of nucleoside analogs. However, these compounds still have low activity in cancer cells and mode of action of these compounds remains unclear.

MATERIALS & METHODS

In this work, we studied maleimide derivatives of RG108 by biochemical, structural and computational approaches to highlight their inhibition mechanism on DNMTs.

RESULTS

Findings demonstrated a correlation between cytotoxicity on mesothelioma cells of these compounds and their inhibitory potency against DNMTs. Noncovalent and covalent docking studies, supported by crystallographic (apo structure of M.HhaI) and differential scanning fluorimetry assays, provided detailed insights into their mode of action and revealed essential residues for the stabilization of such compounds inside DNMTs. [Formula: see text].

Targeted DNA methylation in vivo using an engineered dCas9-MQ1 fusion protein

Comprehensive studies have shown that DNA methylation plays vital roles in both loss of pluripotency and governance of the transcriptome during embryogenesis and subsequent developmental processes. Aberrant DNA methylation patterns have been widely observed in tumorigenesis, ageing and neurodegenerative diseases, highlighting the importance of a systematic understanding of DNA methylation and the dynamic changes of methylomes during disease onset and progression. Here we describe a facile and convenient approach for efficient targeted DNA methylation by fusing inactive Cas9 (dCas9) with an engineered prokaryotic DNA methyltransferase MQ1. Our study presents a rapid and efficient strategy to achieve locus-specific cytosine modifications in the genome without obvious impact on global methylation in 24 h. Finally, we demonstrate our tool can induce targeted CpG methylation in mice by zygote microinjection, thereby demonstrating its potential utility in early development.

Understanding how DNA methylation regulates gene expression requires the capacity to deploy it to regions of interest. The authors generate a highly rapid and locus-specific CpG methylation tool by fusing dCas9 to MQ1 DNA methyltransferase and show efficacy at multiple sites in vitro and in vivo.

Detection and discrimination of maintenance and de novo CpG methylation events using MethylBreak

Understanding the principles governing the establishment and maintenance activities of DNA methyltransferases (DNMTs) can help in the development of predictive biomarkers associated with genetic disorders and diseases. A detection system was developed that distinguishes and quantifies methylation events using methylation-sensitive endonucleases and molecular beacon technology. MethylBreak (MB) is a 22-mer oligonucleotide with one hemimethylated and two unmethylated CpG sites, which are also recognition sites for Sau96I and SacII, and is attached to a fluorophore and a quencher. Maintenance methylation was quantified by fluorescence emission due to the digestion of SacII when the hemimethylated CpG site is methylated, which inhibits Sau96I cleavage. The signal difference between SacII digestion of both MB substrate and maintenance methylated MB corresponds to de novo methylation event. Our technology successfully discriminated and measured both methylation activities at different concentrations of MB and achieved a high correlation coefficient of R²=0.997. Additionally, MB was effectively applied to normal and cancer cell lines and in the analysis of enzymatic kinetics and RNA inhibition of recombinant human DNMT1.

Towards improved butanol production through targeted genetic modification of Clostridium pasteurianum

Declining fossil fuel reserves, coupled with environmental concerns over their continued extraction and exploitation have led to strenuous efforts to identify renewable routes to energy and fuels. One attractive option is to convert glycerol, a by-product of the biodiesel industry, into n-butanol, an industrially important chemical and potential liquid transportation fuel, using Clostridium pasteurianum. Under certain growth conditions this Clostridium species has been shown to predominantly produce n-butanol, together with ethanol and 1,3-propanediol, when grown on glycerol. Further increases in the yields of n-butanol produced by C. pasteurianum could be accomplished through rational metabolic engineering of the strain. Accordingly, in the current report we have developed and exemplified a robust tool kit for the metabolic engineering of C. pasteurianum and used the system to make the first reported in-frame deletion mutants of pivotal genes involved in solvent production, namely hydA (hydrogenase), rex (Redox response regulator) and dhaBCE (glycerol dehydratase). We were, for the first time in C. pasteurianum, able to eliminate 1,3-propanediol synthesis and demonstrate its production was essential for growth on glycerol as a carbon source. Inactivation of both rex and hydA resulted in increased n-butanol titres, representing the first steps towards improving the utilisation of C. pasteurianum as a chassis for the industrial production of this important chemical.

New strategy to address DNA-methyl transferase activity in ovarian cancer cell cultures by monitoring the formation of 5-methylcytosine using HPLC-UV

Methylation of mammalian genomic DNA is catalyzed by DNA methyltransferases (DNMTs). Aberrant expression and activity of these enzymes has been reported to play an important role in the initiation and progression of tumors and its response to chemotherapy. Therefore, there is a great interest in developing strategies to detect human DNMTs activity. We propose a simple, antibody-free, label-free and non-radioactive analytical strategy in which methyltransferase activity is measured trough the determination of the 5-methylcytosine (5mC) content in DNA by a chromatographic method (HPLC-UV) previously developed. For this aim, a correlation between the enzyme activity and the concentration of 5mC obtained by HPLC-UV is previously obtained under optimized conditions using both, un-methylated and hemi-methylated DNA substrates and the prokaryotic methyltransferase M.SssI as model enzyme. The evaluation of the methylation yield in un-methylated known sequences (a 623bp PCR-amplicon) turned to be quantitative (110%) in experiments conducted in-vitro. Methylation of hemi-methylated and low-methylated sequences could be also detected with the proposed approach. The application of the methodology to the determination of the DNMTs activity in nuclear extracts from human ovarian cancer cells has revealed the presence of matrix effects (also confirmed by standard additions) that hampered quantitative enzyme recovery. The obtained results showed the high importance of adequate sample clean-up steps.

Crystal structure of human nucleosome core particle containing enzymatically introduced CpG methylation

Cytosine methylation, predominantly of the CpG sequence in vertebrates, is one of the major epigenetic modifications crucially involved in the control of gene expression. Due to the difficulty of reconstituting site-specifically methylated nucleosomal DNA at crystallization quality, most structural analyses of CpG methylation have been performed using chemically synthesized oligonucleotides, There has been just one recent study of nucleosome core particles (NCPs) reconstituted with nonpalindromic human satellite 2-derived DNAs. Through the preparation of a 146-bp palindromic α-satellite-based nucleosomal DNA containing four CpG dinucleotide sequences and its enzymatic methylation and restriction, we reconstituted a 'symmetric' human CpG-methylated nucleosome core particle (NCP). We solved the crystal structures of the CpG-methylated and unmodified NCPs at 2.6 and 3.0 Å resolution, respectively. We observed the electron densities of two methyl groups, among the eight 5-methylcytosines introduced in the CpG-fully methylated NCP. There were no obvious structural differences between the CpG-methylated 'symmetric NCP' and the unmodified NCP. The preparation of a crystallization-grade CpG-methylated NCP provides a platform for the analysis of CpG-methyl reader and eraser proteins.

TET-catalyzed oxidation of intragenic 5-methylcytosine regulates CTCF-dependent alternative splicing

Intragenic 5-methylcytosine and CTCF mediate opposing effects on pre-mRNA splicing: CTCF promotes inclusion of weak upstream exons through RNA polymerase II pausing, whereas 5-methylcytosine evicts CTCF, leading to exon exclusion. However, the mechanisms governing dynamic DNA methylation at CTCF-binding sites were unclear. Here, we reveal the methylcytosine dioxygenases TET1 and TET2 as active regulators of CTCF-mediated alternative splicing through conversion of 5-methylcytosine to its oxidation derivatives. 5-hydroxymethylcytosine and 5-carboxylcytosine are enriched at an intragenic CTCF-binding sites in the CD45 model gene and are associated with alternative exon inclusion. Reduced TET levels culminate in increased 5-methylcytosine, resulting in CTCF eviction and exon exclusion. In vitro analyses establish the oxidation derivatives are not sufficient to stimulate splicing, but efficiently promote CTCF association. We further show genomewide that reciprocal exchange of 5-hydroxymethylcytosine and 5-methylcytosine at downstream CTCF-binding sites is a general feature of alternative splicing in naïve and activated CD4(+) T cells. These findings significantly expand our current concept of the pre-mRNA "splicing code" to include dynamic intragenic DNA methylation catalyzed by the TET proteins.

Single-base resolution analysis of active DNA demethylation using methylase-assisted bisulfite sequencing

Active DNA demethylation in mammals involves TET-mediated iterative oxidation of 5-methylcytosine (5mC)/5-hydroxymethylcytosine (5hmC) and subsequent excision repair of highly oxidized cytosine bases 5-formylcytosine (5fC)/5-carboxylcytosine (5caC) by thymine DNA glycosylase (TDG). However, quantitative and high-resolution analysis of active DNA demethylation activity remains challenging. Here, we describe M.SssI methylase-assisted bisulfite sequencing (MAB-seq), a method that directly maps 5fC/5caC at single-base resolution. Genome-wide MAB-seq allows systematic identification of 5fC/5caC in Tdg-depleted embryonic stem cells, thereby generating a base-resolution map of active DNA demethylome. A comparison of 5fC/5caC and 5hmC distribution maps indicates that catalytic processivity of TET enzymes correlates with local chromatin accessibility. MAB-seq also reveals strong strand asymmetry of active demethylation within palindromic CpGs. Integrating MAB-seq with other base-resolution mapping methods enables quantitative measurement of cytosine modification states at key transitioning steps of the active DNA demethylation cascade and reveals a regulatory role of 5fC/5caC excision repair in this step-wise process.

Directed evolution of improved zinc finger methyltransferases

The ability to target DNA methylation toward a single, user-designated CpG site in vivo may have wide applicability for basic biological and biomedical research. A tool for targeting methylation toward single sites could be used to study the effects of individual methylation events on transcription, protein recruitment to DNA, and the dynamics of such epigenetic alterations. Although various tools for directing methylation to promoters exist, none offers the ability to localize methylation solely to a single CpG site. In our ongoing research to create such a tool, we have pursued a strategy employing artificially bifurcated DNA methyltransferases; each methyltransferase fragment is fused to zinc finger proteins with affinity for sequences flanking a targeted CpG site for methylation. We sought to improve the targeting of these enzymes by reducing the methyltransferase activity at non-targeted sites while maintaining high levels of activity at a targeted site. Here we demonstrate an in vitro directed evolution selection strategy to improve methyltransferase specificity and use it to optimize an engineered zinc finger methyltransferase derived from M.SssI. The unusual restriction enzyme McrBC is a key component of this strategy and is used to select against methyltransferases that methylate multiple sites on a plasmid. This strategy allowed us to quickly identify mutants with high levels of methylation at the target site (up to ∼80%) and nearly unobservable levels of methylation at a off-target sites (<1%), as assessed in E. coli. We also demonstrate that replacing the zinc finger domains with new zinc fingers redirects the methylation to a new target CpG site flanked by the corresponding zinc finger binding sequences.

Rapid and reliable method for identification of associated endonuclease cleavage and recognition sites

One barrier to cross during genetic engineering is the restriction-modification system found in many bacteria. In this study, we developed a fast and reliable method for mapping the recognition and cleavage site of the restriction endonucleases. Clostridium pasteurianum, a model organism for the study of nitrogen fixation, has been found to harbour at least two restriction-modification systems including the restriction endonucleases CpaPI, which is an isoschizomer of MboI and CpaAI. Dam-methylated DNA was used to isolate the activity of CpaAI. Exposing freshly prepared cell lysate to known nucleotide fragments and directly sequencing the pool of digested nucleotide fragments enabled identification of the cleavage sites in the fragments. By aligning the sequences adjacent to the cleavage site, it was possible to identify the recognition sequence. Using this method, we successfully located all CpaAI recognition and cleavage sites within the template sequence. By modifying DNA with both Dam and CpG methylases (M.SssI) and thereby preventing digestion by CpaPI and CpaAI, no further endonuclease activity was detected.

SIGNIFICANCE AND IMPACT OF THE STUDY

Restriction-modification systems are important barriers to successful genetic modification in many bacterial species. In this study, we demonstrate an efficient and general applicable method for identifying endonuclease recognition and cleavage sites. For the study and the trails, the model organism for nitrogen fixation Clostridium pasteurianum was used. The method was proven to be reliable, and by modifying DNA at the identified sites, it is possible to prevent digestion.

DNA unmethylome profiling by covalent capture of CpG sites

Dynamic patterns of cytosine-5 methylation and successive hydroxylation are part of epigenetic regulation in eukaryotes, including humans, which contributes to normal phenotypic variation and disease risk. Here we present an approach for the mapping of unmodified regions of the genome, which we call the unmethylome. Our technique is based on DNA methyltransferase-directed transfer of activated groups and covalent biotin tagging of unmodified CpG sites followed by affinity enrichment and interrogation on tiling microarrays or next generation sequencing. Control experiments and pilot studies of human genomic DNA from cultured cells and tissues demonstrate that, along with providing a unique cross-section through the chemical landscape of the epigenome, the methyltransferase-directed transfer of activated groups-based approach offers high precision and robustness as compared with existing affinity-based techniques.

Association of self-DNA mediated TLR9-related gene, DNA methyltransferase, and cytokeratin protein expression alterations in HT29-cells to DNA fragment length and methylation status

To understand the biologic role of self-DNA bound to Toll-like Receptor 9 (TLR9), we assayed its effect on gene and methyltransferase expressions and cell differentiation in HT29 cells. HT29 cells were incubated separately with type-1 (normally methylated/nonfragmented), type-2 (normally methylated/fragmented), type-3 (hypermethylated/nonfragmented), or type-4 (hypermethylated/fragmented) self-DNAs. Expression levels of TLR9-signaling and proinflammatory cytokine-related genes were assayed by qRT-PCR. Methyltransferase activity and cell differentiation were examined by using DNA methyltransferase (DNMT1, -3A, -3B) and cytokeratin (CK) antibodies. Treatment with type-1 DNA resulted in significant increase in TLR9 expression. Type-2 treatment resulted in the overexpression of TLR9-related signaling molecules (MYD88A, TRAF6) and the IL8 gene. In the case of type-3 treatment, significant overexpression of NFkB, IRAK2, and IL8 as well as downregulation of TRAF6 was detected. Using type-4 DNA, TRAF6 and MYD88A gene expression was upregulated, while MYD88B, IRAK2, IL8, and TNFSF10 were all underexpressed. CK expression was significantly higher only after type-1 DNA treatment. DNMT3A expression could also be induced by type-1 DNA treatment. DNA structure may play a significant role in activation of the TLR9-dependent and even independent proinflammatory pathways. There may be a molecular link between TLR9 signaling and DNMT3A. The mode of self-DNA treatment may influence HT29 cell differentiation.

Electrochemical assay for the signal-on detection of human DNA methyltransferase activity

Strategies to detect human DNA methyltransferases are needed, given that aberrant methylation by these enzymes is associated with cancer initiation and progression. Here we describe a nonradioactive, antibody-free, electrochemical assay in which methyltransferase activity on DNA-modified electrodes confers protection from restriction for signal-on detection. We implement this assay with a multiplexed chip platform and show robust detection of both bacterial (SssI) and human (Dnmt1) methyltransferase activity. Essential to work with human methyltransferases, our unique assay design allows activity measurements on both unmethylated and hemimethylated DNA substrates. We validate this assay by comparison with a conventional radioactive method. The advantages of electrochemistry over radioactivity and fluorescence make this assay an accessible and promising new approach for the sensitive, label-free detection of human methyltransferase activity.

Cytosine-to-uracil deamination by SssI DNA methyltransferase

The prokaryotic DNA(cytosine-5)methyltransferase M.SssI shares the specificity of eukaryotic DNA methyltransferases (CG) and is an important model and experimental tool in the study of eukaryotic DNA methylation. Previously, M.SssI was shown to be able to catalyze deamination of the target cytosine to uracil if the methyl donor S-adenosyl-methionine (SAM) was missing from the reaction. To test whether this side-activity of the enzyme can be used to distinguish between unmethylated and C5-methylated cytosines in CG dinucleotides, we re-investigated, using a sensitive genetic reversion assay, the cytosine deaminase activity of M.SssI. Confirming previous results we showed that M.SssI can deaminate cytosine to uracil in a slow reaction in the absence of SAM and that the rate of this reaction can be increased by the SAM analogue 5'-amino-5'-deoxyadenosine. We could not detect M.SssI-catalyzed deamination of C5-methylcytosine ((m5)C). We found conditions where the rate of M.SssI mediated C-to-U deamination was at least 100-fold higher than the rate of (m5)C-to-T conversion. Although this difference in reactivities suggests that the enzyme could be used to identify C5-methylated cytosines in the epigenetically important CG dinucleotides, the rate of M.SssI mediated cytosine deamination is too low to become an enzymatic alternative to the bisulfite reaction. Amino acid replacements in the presumed SAM binding pocket of M.SssI (F17S and G19D) resulted in greatly reduced methyltransferase activity. The G19D variant showed cytosine deaminase activity in E. coli, at physiological SAM concentrations. Interestingly, the C-to-U deaminase activity was also detectable in an E. coli ung (+) host proficient in uracil excision repair.

CpG underrepresentation and the bacterial CpG-specific DNA methyltransferase M.MpeI

Cytosine methylation promotes deamination. In eukaryotes, CpG methylation is thought to account for CpG underrepresentation. Whether scarcity of CpGs in prokaryotic genomes is diagnostic for methylation is not clear. Here, we report that Mycoplasms tend to be CpG depleted and to harbor a family of constitutively expressed or phase variable CpG-specific DNA methyltransferases. The very CpG poor Mycoplasma penetrans and its constitutively active CpG-specific methyltransferase M.MpeI were chosen for further characterization. Genome-wide sequencing of bisulfite-converted DNA indicated that M.MpeI methylated CpG target sites both in vivo and in vitro in a locus-nonselective manner. A crystal structure of M.MpeI with DNA at 2.15-Å resolution showed that the substrate base was flipped and that its place in the DNA stack was taken by a glutamine residue. A phenylalanine residue was intercalated into the "weak" CpG step of the nonsubstrate strand, indicating mechanistic similarities in the recognition of the short CpG target sequence by prokaryotic and eukaryotic DNA methyltransferases.

Successes and failures in modular genetic engineering

Synthetic biology relies on engineering concepts such as abstraction, standardization, and decoupling to develop systems that address environmental, clinical, and industrial needs. Recent advances in applying modular design to system development have enabled creation of increasingly complex systems. However, several challenges to module and system development remain, including syntactic errors, semantic errors, parameter mismatches, contextual sensitivity, noise and evolution, and load and stress. To combat these challenges, researchers should develop a framework for describing and reasoning about biological information, design systems with modularity in mind, and investigate how to predictively describe the diverse sources and consequences of metabolic load and stress.

Bis-SNP: combined DNA methylation and SNP calling for Bisulfite-seq data

Bisulfite treatment of DNA followed by high-throughput sequencing (Bisulfite-seq) is an important method for studying DNA methylation and epigenetic gene regulation, yet current software tools do not adequately address single nucleotide polymorphisms (SNPs). Identifying SNPs is important for accurate quantification of methylation levels and for identification of allele-specific epigenetic events such as imprinting. We have developed a model-based bisulfite SNP caller, Bis-SNP, that results in substantially better SNP calls than existing methods, thereby improving methylation estimates. At an average 30× genomic coverage, Bis-SNP correctly identified 96% of SNPs using the default high-stringency settings. The open-source package is available at http://epigenome.usc.edu/publicationdata/bissnp2011.

Complementation between inactive fragments of SssI DNA methyltransferase

Background

Silencing mammalian genes by targeted DNA (cytosine-5) methylation of selected CG sites in the genome would be a powerful technique to analyze epigenomic information and to study the roles of DNA methylation in physiological and pathological states. A promising approach of targeted DNA methylation is based on the ability of split fragments of a monomeric DNA methyltransferase (C5-MTase) to associate and form active enzyme. A few C5-MTases of different specificities have been shown to possess the ability of fragment complementation, but a demonstration of this phenomenon for a C5-MTase, which has CG specificity and thus can be targeted to methylate any CG site, has been lacking. The purpose of this study was to test whether the CG-specific prokaryotic C5-MTase M.SssI shows the phenomenon of fragment complementation.

Results

We show that truncated inactive N-terminal fragments of M.SssI can assemble with truncated inactive C-terminal fragments to form active enzyme in vivo when produced in the same E. coli cell. Overlapping and non-overlapping fragments as well as fragments containing short appended foreign sequences had complementation capacity. In optimal combinations C-terminal fragments started between conserved motif VIII and the predicted target recognizing domain of M.SssI. DNA methyltransferase activity in crude extracts of cells with the best complementing fragment pairs was ~ 4 per cent of the activity of cells producing the full length enzyme. Fusions of two N-terminal and two C-terminal fragments to 21.6 kDa zinc finger domains only slightly reduced complementation ability of the fragments.

Conclusions

The CG-specific DNA methyltransferase M.SssI shows the phenomenon of fragment complementation in vivo in E. coli. Fusion of the split fragments to six unit zinc finger domains does not substantially interfere with the formation of active enzyme. These observations and the large number of complementing fragment combinations representing a wide range of MTase activity offer the possibility to develop M.SssI into a programmable DNA methyltransferase of high specificity.

Simultaneous single-molecule detection of endogenous C-5 DNA methylation and chromatin accessibility using MAPit

Bisulfite genomic sequencing provides a single-molecule view of cytosine methylation states. After deamination, each cloned molecule contains a record of methylation within its sequence. The full power of this technique is harnessed by treating nuclei with an exogenous DNMT prior to DNA extraction. This exogenous methylation marks regions of accessibility and footprints nucleosomes, as well as other DNA-binding proteins. Thus, each cloned molecule records not only the endogenous methylation present (at CG sites, in mammals), but also the exogenous (GC, when using the Chlorella virus protein M.CviPI). We term this technique MAPit, methylation accessibility protocol for individual templates.

Sulforaphane induction of p21(Cip1) cyclin-dependent kinase inhibitor expression requires p53 and Sp1 transcription factors and is p53-dependent

Sulforaphane (SFN) is an important cancer preventive agent derived from cruciferous vegetables. We show that SFN treatment suppresses normal human keratinocyte proliferation via a mechanism that involves increased expression of p21(Cip1). SFN treatment produces a concentration-dependent increase in p21(Cip1) promoter activity via a mechanism that involves stabilization of the p53 protein leading to increased p53 binding to the p21(Cip1) promoter p53 response elements. The proximal p21(Cip1) promoter GC-rich Sp1 factor binding elements are also required, as the SFN-dependent increase is lost when these sites are mutated. SFN treatment increases Sp1 binding to these elements, and the response is enhanced in the presence of exogenous Sp1 and reduced in the presence of ΔN-Sp3. CpG island methylation alters p21(Cip1) promoter activity some systems; however, expression in SFN-treated keratinocytes does not involve changes in proximal promoter methylation. The promoter is minimally methylated, and the methylation level is not altered by SFN treatment. This study indicates that SFN increases p21(Cip1) promoter transcription via a mechanism that involves SFN-dependent stabilization of p53 and increased p53 and Sp1 binding to their respective response elements in the p21(Cip1) promoter. These results are in marked contrast to the mechanisms observed in skin cancer cell lines and suggest that SFN may protect normal keratinocytes from damage while causing cancer cells to undergo apoptosis.

Transducing methyltransferase activity into electrical signals in a carbon nanotube-DNA device()

This study creates a device where the DNA is electronically integrated to serve as both the biological target and electrical transducer in a CNT-DNA-CNT device. We detect DNA binding and methylation by the methyltransferase M.SssI at the single molecule level. We demonstrate sequence-specific, reversible binding of M.SssI and protein-catalyzed methylation that alters the protein-binding affinity of the device. This device, which relies on the exquisite electrical sensitivity of DNA, represents a unique route for the specific, single molecule detection of enzymatic activity.

Simultaneous single-molecule mapping of protein-DNA interactions and DNA methylation by MAPit

Sites of protein binding to DNA are inferred from footprints or spans of protection against a probing reagent. In most protocols, sites of accessibility to a probe are detected by mapping breaks in DNA strands. As discussed in this unit, such methods obscure molecular heterogeneity by averaging cuts at a given site over all DNA strands in a sample population. The DNA methyltransferase accessibility protocol for individual templates (MAPit), an alternative method described in this unit, localizes protein-DNA interactions by probing with cytosine-modifying DNA methyltransferases followed by bisulfite sequencing. Sequencing individual DNA products after amplification of bisulfite-converted sequences permits assignment of the methylation status of every enzyme target site along a single DNA strand. Use of the GC-methylating enzyme M.CviPI allows simultaneous mapping of chromatin accessibility and endogenous CpG methylation. MAPit is therefore the only footprinting method that can detect subpopulations of molecules with distinct patterns of protein binding or chromatin architecture and correlate them directly with the occurrence of endogenous methylation. Additional advantages of MAPit methylation footprinting as well as considerations for experimental design and potential sources of error are discussed.

Interruption of intrachromosomal looping by CCCTC binding factor decoy proteins abrogates genomic imprinting of human insulin-like growth factor II

CCCTC binding factor (CTCF) mutants that cannot bind components of the polycomb repressive complex-2 (PRC2) do not form the chromatin loops that regulate monoallelic gene expression.

Monoallelic expression of IGF2 is regulated by CCCTC binding factor (CTCF) binding to the imprinting control region (ICR) on the maternal allele, with subsequent formation of an intrachromosomal loop to the promoter region. The N-terminal domain of CTCF interacts with SUZ12, part of the polycomb repressive complex-2 (PRC2), to silence the maternal allele. We synthesized decoy CTCF proteins, fusing the CTCF deoxyribonucleic acid–binding zinc finger domain to CpG methyltransferase Sss1 or to enhanced green fluorescent protein. In normal human fibroblasts and breast cancer MCF7 cell lines, the CTCF decoy proteins bound to the unmethylated ICR and to the IGF2 promoter region but did not interact with SUZ12. EZH2, another part of PRC2, was unable to methylate histone H3-K27 in the IGF2 promoter region, resulting in reactivation of the imprinted allele. The intrachromosomal loop between the maternal ICR and the IGF2 promoters was not observed when IGF2 imprinting was lost. CTCF epigenetically governs allelic gene expression of IGF2 by orchestrating chromatin loop structures involving PRC2.