Hypersensitive quantification of global 5-hydroxymethylcytosine by chemoenzymatic tagging

Construction of a glycosylation-mediated fluorescent biosensor for label-free measurement of site-specific 5-hydroxymethylcytosine in cancer cells with zero background signal

BACKGROUND

5-hydroxymethylcytosine (5hmC) as an epigenetic modification can regulate gene expression, and its abnormal level is related with various tumor invasiveness and poor prognosis. Nevertheless, the current methods for 5hmC assay usually involve expensive instruments/antibodies, radioactive risk, high background, laborious bisulfite treatment procedures, and non-specific/long amplification time.

RESULTS

We develop a glycosylation-mediated fluorescent biosensor based on helicase-dependent amplification (HDA) for label-free detection of site-specific 5hmC in cancer cells with zero background signal. The glycosylated 5hmC-DNA (5ghmC) catalyzed by β-glucosyltransferase (β-GT) can be cleaved by AbaSI restriction endonuclease to generate two dsDNA fragments with sticky ends. The resultant dsDNA fragments are complementary to the biotinylated probes and ligated by DNA ligases, followed by being captured by magnetic beads. After magnetic separation, the eluted ligation products act as the templates to initiate HDA reaction, generating abundant double-stranded DNA (dsDNA) products within 20 min. The dsDNA products are measured in a label-free manner with SYBR Green I as an indicator. This biosensor can measure 5hmC with a detection limit of 2.75 fM and a wide linear range from 1 × 10-14 to 1 × 10-8 M, and it can discriminate as low as 0.001% 5hmC level in complex mixture. Moreover, this biosensor can measure site-specific 5hmC in cancer cells, and distinguish tumor cells from normal cells.

SIGNIFICANCE

This biosensor can achieve a zero-background signal without the need of either 5hmC specific antibody or bisulfite treatment, and it holds potential applications in biological research and disease diagnosis.

Identification and quantification of N6-methyladenosine by chemical derivatization coupled with 19F NMR spectroscopy

N 6-Methyladenosine (6mA) is a well-known prokaryotic DNA modification that has been shown to play epigenetic roles in eukaryotic DNA. Accurate detection and quantification of 6mA are prerequisites for molecular understanding of the impact of 6mA modification on DNA. However, the existing methods have several problems, such as high false-positive rate, time-consuming and complex operating procedures. Chemical sensors for the selective detection of 6mA modification are rarely reported in the literature. Fluorinated phenylboronic acid combined with 19F NMR analysis is an effective method for determining DNA or RNA modification. In this study, we presented a simple and fast chemical method for labelling the 6th imino group of 6mA using a boric-acid-derived probe. Besides, the trifluoromethyl group of trifluoromethyl phenylboronic acid (2a) could detect 6mA modification through 19F NMR. Combined with this sensor system, 6mA modification could be detected well and quickly in 6 types of deoxynucleoside mixtures and DNA samples. Taken together, the method developed in the current study has potential for specific detection of 6mA in biological samples.

Hydroxymethylation-Specific Ligation-Mediated Single Quantum Dot-Based Nanosensors for Sensitive Detection of 5-Hydroxymethylcytosine in Cancer Cells

5-Hydroxymethylcytosine (5hmC) modification is a key epigenetic regulator of cellular processes in mammalian cells, and its misregulation may lead to various diseases. Herein, we develop a hydroxymethylation-specific ligation-mediated single quantum dot (QD)-based fluorescence resonance energy transfer (FRET) nanosensor for sensitive quantification of 5hmC modification in cancer cells. We design a Cy5-modified signal probe and a biotinylated capture probe for the recognition of specific 5hmC-containing genes. 5hmC in target DNA can be selectively converted by T4 β-glucosyltransferase to produce a glycosyl-modified 5hmC, which cannot be cleaved by methylation-insensitive restriction enzyme MspI. The glycosylated 5hmC DNA may act as a template to ligate a signal probe and a capture probe, initiating hydroxymethylation-specific ligation to generate large amounts of biotin-/Cy5-modified single-stranded DNAs (ssDNAs). The assembly of biotin-/Cy5-modified ssDNAs onto a single QD through streptavidin-biotin interaction results in FRET and consequently the generation of a Cy5 signal. The nanosensor is very simple without the need for bisulfite treatment, radioactive reagents, and 5hmC-specific antibodies. Owing to excellent specificity and high amplification efficiency of hydroxymethylation-specific ligation and near-zero background of a single QD-based FRET, this nanosensor can quantify 5hmC DNA with a limit of detection of 33.61 aM and a wider linear range of 7 orders of magnitude, and it may discriminate the single-nucleotide difference among 5hmC, 5-methylcytosine, and unmodified cytosine. Moreover, this nanosensor can distinguish as low as a 0.001% 5hmC DNA in complex mixtures, and it can monitor the cellular 5hmC level and discriminate cancer cells from normal cells, holding great potential in biomedical research and clinical diagnostics.

Highly Sensitive Fluorescence Detection of Global 5-Hydroxymethylcytosine from Nanogram Input with Strongly Emitting Copper Nanotags

Quantitative analysis of 5-hydroxymethylcytosine (5hmC) has remarkable clinical significance to early cancer diagnosis; however, it is limited by the requirement in current assays for large amounts of starting material and expensive instruments requring expertise. Herein, we present a highly sensitive fluorescence method, termed hmC-TACN, for global 5hmC quantification from several nanogram inputs based on terminal deoxynucleotide transferase (TdT)-assisted formation of fluorescent copper (Cu) nanotags. In this method, 5hmC is labeled with click tags by T4 phage β-glucosyltransferase (β-GT) and cross-linked with a random DNA primer via click chemistry. TdT initiates the template-free extension along the primer at the modified 5hmC site and then generates a long polythymine (T) tail, which can template the production of strongly emitting Cu nanoparticles (CuNPs). Consequently, an intensely fluorescent tag containing numerous CuNPs can be labeled onto the 5hmC site, providing the sensitive quantification of 5hmC with a limit of detection (LOD) as low as 0.021% of total nucleotides (S/N = 3). With only a 5 ng input (∼1000 cells) of genomic DNA, global 5hmC levels were accurately determined in mouse tissues, human cell lines (including normal and cancer cells of breast, lung, and liver), and urines of a bladder cancer patient and healthy control. Moreover, as few as 100 cells can also be distinguished between normal and cancer cells. The hmC-TACN method has great promise of being cost effective and easily mastered, with low-input clinical utility, and even for the microzone analysis of tumor models.

Integration of Epigenetic Mechanisms into Non-Genotoxic Carcinogenicity Hazard Assessment: Focus on DNA Methylation and Histone Modifications

Epigenetics involves a series of mechanisms that entail histone and DNA covalent modifications and non-coding RNAs, and that collectively contribute to programing cell functions and differentiation. Epigenetic anomalies and DNA mutations are co-drivers of cellular dysfunctions, including carcinogenesis. Alterations of the epigenetic system occur in cancers whether the initial carcinogenic events are from genotoxic (GTxC) or non-genotoxic (NGTxC) carcinogens. NGTxC are not inherently DNA reactive, they do not have a unifying mode of action and as yet there are no regulatory test guidelines addressing mechanisms of NGTxC. To fil this gap, the Test Guideline Programme of the Organisation for Economic Cooperation and Development is developing a framework for an integrated approach for the testing and assessment (IATA) of NGTxC and is considering assays that address key events of cancer hallmarks. Here, with the intent of better understanding the applicability of epigenetic assays in chemical carcinogenicity assessment, we focus on DNA methylation and histone modifications and review: (1) epigenetic mechanisms contributing to carcinogenesis, (2) epigenetic mechanisms altered following exposure to arsenic, nickel, or phenobarbital in order to identify common carcinogen-specific mechanisms, (3) characteristics of a series of epigenetic assay types, and (4) epigenetic assay validation needs in the context of chemical hazard assessment. As a key component of numerous NGTxC mechanisms of action, epigenetic assays included in IATA assay combinations can contribute to improved chemical carcinogen identification for the better protection of public health.

Solid-state nanopore analysis of human genomic DNA shows unaltered global 5-hydroxymethylcytosine content associated with early-stage breast cancer

5-Hydroxymethylcytosine (5hmC), the first oxidized form of the well-known epigenetic modification 5-methylcytosine, is an independent regulator of gene expression and therefore a potential marker for disease. Here, we report on methods developed for a selective solid-state nanopore assay that enable direct analysis of global 5hmC content in human tissue. We first describe protocols for preparing genomic DNA derived from both healthy breast tissue and stage 1 breast tumor tissue and then use our approach to probe the net abundance of the modified base in each cohort. Then, we employ empirical data to adjust for the impact of nanopore diameter on the quantification. Correcting for variations in nanopore diameter among the devices used for analysis reveals no detectable difference in global 5hmC content between healthy and tumor tissue. These results suggest that 5hmC changes may not be associated with early-stage breast cancer and instead are a downstream consequence of the disease.

Detection of N6-Methyladenine in Eukaryotes

DNA N⁶-methyladenine (6mA) is a chemical modification at the N⁶-positon of adenine. In the last decades, 6mA had been found in genome from numerous prokaryotic species, but only existed in a few lower eukaryotes. In prokaryotes, 6mA plays an important role in restriction-modification, DNA replication, and DNA mismatch repair. Because of the too low abundance of 6mA, it was long-stalled whether 6mA existed in multicellular eukaryotes and playing any functions, particularly in mammals. In recent years, partially benefitting from the advances in analytical methods, 6mA was found in the genomes from Drosophila melanogaster, Chlamydomonas algae, Caenorhabditis elegans, zebrafish, Xenopus laevis and mouse embryonic stem cells and even in the human genome. The 6mA was dynamic changed in early embryonic development of fly and zebrafish and much more enriched in gene body of transposons in fly, repetitive regions in zebrafish, around the transcription start sites in Chlamydomonas, and widespread distribution in C. elegans, indicating 6mA probably playing different functions in different species. Meanwhile, 6mA methylases and demethylases were found in fly, worm, and Chlamydomonas. In this chapter, we will briefly review the distribution, regulation, and function of 6mA in eukaryotes and focus on the advances of 6mA analysis methods, especially LC-MS/MS, immunoprecipitation, next-generation sequencing, and single-molecule real-time sequencing technology.

Quantitative analysis of global 5-methyl- and 5-hydroxymethylcytosine in TET1 expressed HEK293T cells

DNA methylation at the 5-position of cytosine bases (5-methylcytosine, 5mC) in genomic DNA is representative epigenetic modification and is involved in many cellular processes, including gene expression and embryonic development. The hydroxylation of 5mC provide 5-hydroxymethylcytosine (5hmC), the so-called sixth base rediscovered recently in mammalian cells, is also considered to act as an epigenetic regulator. We report herein the immunochemical assessment of 5hmC achieved by an enzyme-linked immunosorbent assay (ELISA) using our linker technology. The keys to this assay are 1) the immobilization of genomic DNA with the bifunctional linker molecule, and 2) quantitative analysis by using guaranteed standard samples containing defined amounts of 5hmC. We succeeded in the sensitive and quantitative detection of 5hmC as well as 5mC in HEK293T cells transfected with TET1, and also monitored the effect of ascorbate on the TET1 catalyzed conversion of 5mC to 5hmC. Our linker technology enables the rapid and stable immobilization of genomic samples and thus contributes to the realization of a reproducible 5hmC evaluation method.

Hydroxymethylation and tumors: can 5-hydroxymethylation be used as a marker for tumor diagnosis and treatment?

5-Methylcytosine (5mC) is considered as a common epigenetic modification that plays an important role in the regulation of gene expression. At the same time, 5-hydroxymethylcytosine (5hmC) has been found as an emerging modification of cytosine bases of recent years. Unlike 5mC, global 5hmC levels vary from tissues that have differential distribution both in mammalian tissues and in the genome. DNA hydroxymethylation is the process that 5mC oxidates into 5hmC with the catalysis of TET (ten-eleven translocation) enzymes. It is an essential option of DNA demethylation, which modulates gene expression by adjusting the DNA methylation level. Various factors can regulate the demethylation of DNA, such as environmental toxins and mental stress. In this review, we summarize the progress in the formation of 5hmC, and obtaining 5hmC in a cell-free DNA sample presents multiple advantages and challenges for the subject. Furthermore, the clinical potential for 5hmC modification in dealing with cancer early diagnosis, prognostic evaluation, and prediction of therapeutic effect is also mentioned.

Role of epigenetic in leukemia: From mechanism to therapy

Epigenetic variations can play remarkable roles in different normal and abnormal situations. Such variations have been shown to have a direct role in the pathogenesis of various diseases either through inhibition of tumor suppressor genes or increasing the expression of oncogenes. Enzymes involving in epigenetic machinery are the main actors in tuning the epigenetic-based controls on gene expressions. Aberrant expression of these enzymes can trigger big chaos in the cellular gene expression networks and finally lead to cancer progression. This situation has been shown in different types of leukemia, where high or low levels of an epigenetic enzyme are partly or highly responsible for the involvement or progression of a disease. DNA hypermethylation, different histone modifications, and aberrant miRNA expressions are three main epigenetic variations, which have been shown to play a role in leukemia progression. Epigenetic based treatments now are considered as novel and effective therapies in order to decrease the abnormal epigenetic modifications in patient cells. Different epigenetic-based approaches have been developed and tested to inhibit or reverse the unusual expression of epigenetic agents in leukemia. Acute myeloid leukemia (AML), the most prevalent acute leukemia in adults, is anaggressive hematological malignancy arising in hematopoietic stem and progenitor cells. With the exception of a few specific AML subtypes, the mainstays of treatment have not significantly changed over the last 20 years, and are still based on standard cytotoxic chemotherapy. In this review, we will discuss the recent development of therapeutics specifically targeting these key epigenetic programs in AML, describe their mechanism of action and present their current clinical development. Finally, we will discuss the opportunities presented by epigenetically targeted therapy in AML and will highlight future challenges ahead for the AML community, to ensure that this novel therapeutics are optimally translated into clinical practice and result in clinical improvement for AML patients.

Evidence for novel epigenetic marks within plants

Variation in patterns of gene expression can result from modifications in the genome that occur without a change in the sequence of the DNA; such modifications include methylation of cytosine to generate 5-methylcytosine (5mC) resulting in the generation of heritable epimutation and novel epialleles. This type of non-sequence variation is called epigenetics. The enzymes responsible for generation of such DNA modifications in mammals are named DNA methyltransferases (DNMT) including DNMT1, DNMT2 and DNMT3. The later stages of oxidations to these modifications are catalyzed by Ten Eleven Translocation (TET) proteins, which contain catalytic domains belonging to the 2-oxoglutarate dependent dioxygenase family. In various mammalian cells/tissues including embryonic stem cells, cancer cells and brain tissues, it has been confirmed that these proteins are able to induce the stepwise oxidization of 5-methyl cytosine to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and finally 5-carboxylcytosine (5caC). Each stage from initial methylation until the end of the DNA demethylation process is considered as a specific epigenetic mark that may regulate gene expression. This review discusses controversial evidence for the presence of such oxidative products, particularly 5hmC, in various plant species. Whereas some reports suggest no evidence for enzymatic DNA demethylation, other reports suggest that the presence of oxidative products is followed by the active demethylation and indicate the contribution of possible TET-like proteins in the regulation of gene expression in plants. The review also summarizes the results obtained by expressing the human TET conserved catalytic domain in transgenic plants.

Global modulation in DNA epigenetics during pro-inflammatory macrophage activation

DNA methylation patterns create distinct gene-expression profiles. These patterns are maintained after cell division, thus enabling the differentiation and maintenance of multiple cell types from the same genome sequence. The advantage of this mechanism for transcriptional control is that chemical-encoding allows to rapidly establish new epigenetic patterns 'on-demand' through enzymatic methylation and demethylation of DNA. Here we show that this feature is associated with the fast response of macrophages during their pro-inflammatory activation. By using a combination of mass spectroscopy and single-molecule imaging to quantify global epigenetic changes in the genomes of primary macrophages, we followed three distinct DNA marks (methylated, hydroxymethylated and unmethylated), involved in establishing new DNA methylation patterns during pro-inflammatory activation. The observed epigenetic modulation together with gene-expression data generated for the involved enzymatic machinery may suggest that de-methylation upon LPS-activation starts with oxidation of methylated CpGs, followed by excision-repair of these oxidized bases and their replacement with unmodified cytosine.