Clinical and public health research using methylated DNA immunoprecipitation (MeDIP): a comparison of commercially available kits to examine differential DNA methylation across the genome

The methylated DNA immunoprecipitation method (MeDIP) is a genome-wide, high-resolution approach that detects DNA methylation with oligonucleotide tiling arrays or high throughput sequencing platforms. A simplified high-throughput MeDIP assay will enable translational research studies in clinics and populations, which will greatly enhance our understanding of the human methylome. We compared three commercial kits, MagMeDIP Kit TM (Diagenode), Methylated-DNA IP Kit (Zymo Research) and Methylamp™ Methylated DNA Capture Kit (Epigentek), in order to identify which one has better reliability and sensitivity for genomic DNA enrichment. Each kit was used to enrich two samples, one from fresh tissue and one from a cell line, with two different DNA amounts. The enrichment efficiency of each kit was evaluated by agarose gel band intensity after Nco I digestion and by reaction yield of methylated DNA. A successful enrichment is expected to have a 1:4 to 10:1 conversion ratio and a yield of 80% or higher. We also evaluated the hybridization efficiency to genome-wide methylation arrays in a separate cohort of tissue samples. We observed that the MagMeDIP kit had the highest yield for the two DNA amounts and for both the tissue and cell line samples, as well as for the positive control. In addition, the DNA was successfully enriched from a 1:4 to 10:1 ratio. Therefore, the MagMeDIP kit is a useful research tool that will enable clinical and public health genome-wide DNA methylation studies.     

Clinical and public health research using methylated DNA Immunoprecipitation (MeDIP): A comparison of commercially available kits to examine differential DNA methylation across the genome

The methylated DNA immunoprecipitation method (MeDIP) is a genome-wide, high-resolution approach that detects DNA methylation with oligonucleotide tiling arrays or high throughput sequencing platforms. A simplified high-throughput MeDIP assay will enable translational research studies in clinics and populations, which will greatly enhance our understanding of the human methylome. We compared three commercial kits, MagMeDIP Kit TM (Diagenode), Methylated-DNA IP Kit (Zymo Research) and Methylamp? Methylated DNA Capture Kit (Epigentek), in order to identify which one has better reliability and sensitivity for genomic DNA enrichment. Each kit was used to enrich two samples, one from fresh tissue and one from a cell line, with two different DNA amounts. The enrichment efficiency of each kit was evaluated by agarose gel band intensity after Nco I digestion and by reaction yield of methylated DNA. A successful enrichment is expected to have a 1:4 to 10:1 conversion ratio and a yield of 80% or higher. We also evaluated the hybridization efficiency to genome-wide methylation arrays in a separate cohort of tissue samples. We observed that the MagMeDIP kit had the highest yield for the two DNA amounts and for both the tissue and cell line samples, as well as for the positive control. In addition, the DNA was successfully enriched from a 1:4 to 10:1 ratio. Therefore, the MagMeDIP kit is a useful research tool that will enable clinical and public health genome-wide DNA methylation studies.     

Exome localization of complex disease association signals

Background

Recent progress in high-throughput technologies has greatly contributed to the development of DNA methylation profiling. Although there are several reports that describe methylome detection of whole genome bisulfite sequencing, the high cost and heavy demand on bioinformatics analysis prevents its extensive application. Thus, current strategies for the study of mammalian DNA methylomes is still based primarily on genome-wide methylated DNA enrichment combined with DNA microarray detection or sequencing. Methylated DNA enrichment is a key step in a microarray based genome-wide methylation profiling study, and even for future high-throughput sequencing based methylome analysis.

Results

In order to evaluate the sensitivity and accuracy of methylated DNA enrichment, we investigated and optimized a number of important parameters to improve the performance of several enrichment assays, including differential methylation hybridization (DMH), microarray-based methylation assessment of single samples (MMASS), and methylated DNA immunoprecipitation (MeDIP). With advantages and disadvantages unique to each approach, we found that assays based on methylation-sensitive enzyme digestion and those based on immunoprecipitation detected different methylated DNA fragments, indicating that they are complementary in their relative ability to detect methylation differences.

Conclusions

Our study provides the first comprehensive evaluation for widely used methodologies for methylated DNA enrichment, and could be helpful for developing a cost effective approach for DNA methylation profiling.     

Standardization and quality controls for the methylated DNA immunoprecipitation technique

MeDIP (Methylated DNA Immunoprecipitation) is a relatively recent technique aimed to enrich the methylated fraction of DNA with an antibody directed against 5-methyl-cytosine. MeDIP processed samples are suitable for investigation of the methylation status of specific genomic loci and for performing genome-wide screening when hybridized to DNA methylation microarrays or analyzed by deep sequencing. Here, we describe a standardization protocol and quality controls to assess the specificity, reproducibility and efficiency of the MeDIP procedure. These may have utility when comparing results between samples and experiments within laboratories and between laboratories.     

Genome‐wide variation in DNA methylation is associated with stress resilience and plumage brightness in a wild bird

Individuals often differ in their ability to cope with challenging environmental and social conditions. Evidence from model systems suggests that patterns of DNA methylation are associated with variation in coping ability. These associations could arise directly if methylation plays a role in controlling the physiological response to stressors by, among other things, regulating the release of glucocorticoids in response to challenges. Alternatively, the association could arise indirectly if methylation and resilience have a common cause, such as early‐life conditions. In either case, methylation might act as a biomarker for coping ability. At present, however, relatively little is known about whether variation in methylation is associated with organismal performance and resilience under natural conditions. We studied genome‐wide patterns of DNA methylation in free‐living female tree swallows (Tachycineta bicolor) using methylated DNA immunoprecipitation (MeDIP) and a tree swallow genome that was assembled for this study. We identified areas of the genome that were differentially methylated with respect to social signal expression (breast brightness) and physiological traits (ability to terminate the glucocorticoid stress response through negative feedback). We also asked whether methylation predicted resilience to a subsequent experimentally imposed challenge. Individuals with brighter breast plumage and higher stress resilience had lower methylation at differentially methylated regions across the genome. Thus, widespread differences in methylation predicted both social signal expression and the response to future challenges under natural conditions. These results have implications for predicting individual differences in resilience, and for understanding the mechanistic basis of resilience and its environmental and social mediators.     

Genome-wide mapping of DNA methylation in Nile Tilapia

Cytosine DNA methylation is crucial for gene regulation and maintenance of genome stability. However, the detailed nile tilapia methylome remains uncharacterized. In this study, we present the first high-resolution methylome of tilapia gonad generated using methylated DNA immunoprecipitation (MeDIP) and high-throughput sequencing. In the ovary, 265 and 56 methylation peaks were identified in the genebody and promoter region of 145 genes, respectively. In the testis, 293 and 80 methylation peaks were identified in the genebody and promoter region of 144 genes. Furthermore, 8 and 49 genes showed differentially higher and lower promoter-region methylation rates, respectively, in the ovary relative to those of the testis. Quantitative PCR results revealed that the expression level of fibroblast growth factor 16 (fgf16), sialidase-3-like, fibroblast growth factor 20, aromatase (cyp19a), estrogen receptor, and gonadotropin receptor II precursor were negatively correlated to their methylation levels in the ovary and testis. The methylated levels of cyp19a and fgf16 were validated by bisulfite sequencing PCR technology, and the results were consistent with the MeDIP results. Thus, apart from generating the first methylation map, this study produced a candidate gene repository that provides additional options to explore the relationship between DNA methylation and sex differentiation or maintenance.     

Examination of the specificity of DNA methylation profiling techniques towards 5-methylcytosine and 5-hydroxymethylcytosine

DNA cytosine-5 methylation is a well-studied epigenetic pathway implicated in gene expression control and disease pathogenesis. Different technologies have been developed to examine the distribution of 5-methylcytosine (5mC) in specific sequences of the genome. Recently, substantial amounts of 5-hydroxymethylcytosine (5hmC), most likely derived from enzymatic oxidation of 5mC by TET1, have been detected in certain mammalian tissues. Here, we have examined the ability of several commonly used DNA methylation profiling methods to distinguish between 5mC and 5hmC. We show that techniques based on sodium bisulfite treatment of DNA are incapable of distinguishing between the two modified bases. In contrast, techniques based on immunoprecipitation with anti-5mC antibody (methylated DNA immunoprecipitation, MeDIP) or those based on proteins that bind to methylated CpG sequences (e.g. methylated-CpG island recovery assay, MIRA) do not detect 5hmC and are specific for 5mC unless both modified bases occur in the same DNA fragment. We also report that several methyl-CpG binding proteins including MBD1, MBD2 and MBD4 do not bind to sequences containing 5hmC. Selective mapping of 5hmC will require the development of unique tools for the detection of this modified base.     

Folic acid stimulation of neural stem cell proliferation is associated with altered methylation profile of PI3K/Akt/CREB

Proliferation of neural stem cells (NSCs) is required for development and repair in the nervous system. NSC amplification in vitro is a necessary step towards using NSC transplantation therapy to treat neurodegenerative diseases. Folic acid (FA) has been shown to act through DNA methyltransferase to stimulate NSC proliferation. To elucidate the underlying mechanism, the effect of FA on the methylation profiles in neonatal rat NSCs was assessed by methylated DNA immunoprecipitation (MeDIP) and methylated DNA immunoprecipitation-DNA microarray (MeDIP-Chip). Differentially methylated regions (DMRs) were determined by quantitative differentially methylated regions analysis, and genes carrying at least three DMRs were selected for pathway analysis. Gene network analysis revealed links with steroid biosynthesis, fatty acid elongation and the PI3K/Akt/CREB, neuroactive ligand–receptor interaction, Jak-STAT and MAPK signaling pathways. Moreover, Akt3 acted as a hub in the network, in which 14 differentially methylated genes converged to the PI3K/Akt/CREB signaling pathway. These findings indicate that FA stimulates NSC proliferation by modifying DNA methylation levels in the PI3K/Akt/CREB pathway.     

DNA Methylation: Bisulphite Modification and Analysis

Epigenetics describes the heritable changes in gene function that occur independently to the DNA sequence. The molecular basis of epigenetic gene regulation is complex, but essentially involves modifications to the DNA itself or the proteins with which DNA associates. The predominant epigenetic modification of DNA in mammalian genomes is methylation of cytosine nucleotides (5-MeC). DNA methylation provides instruction to gene expression machinery as to where and when the gene should be expressed. The primary target sequence for DNA methylation in mammals is 5''-CpG-3'' dinucleotides (Figure 1). CpG dinucleotides are not uniformly distributed throughout the genome, but are concentrated in regions of repetitive genomic sequences and CpG "islands" commonly associated with gene promoters (Figure 1). DNA methylation patterns are established early in development, modulated during tissue specific differentiation and disrupted in many disease states including cancer. To understand the biological role of DNA methylation and its role in human disease, precise, efficient and reproducible methods are required to detect and quantify individual 5-MeCs.This protocol for bisulphite conversion is the "gold standard" for DNA methylation analysis and facilitates identification and quantification of DNA methylation at single nucleotide resolution. The chemistry of cytosine deamination by sodium bisulphite involves three steps (Figure 2). (1) Sulphonation: The addition of bisulphite to the 5-6 double bond of cytosine (2) Hydrolic Deamination: hydrolytic deamination of the resulting cytosine-bisulphite derivative to give a uracil-bisulphite derivative (3) Alkali Desulphonation: Removal of the sulphonate group by an alkali treatment, to give uracil. Bisulphite preferentially deaminates cytosine to uracil in single stranded DNA, whereas 5-MeC, is refractory to bisulphite-mediated deamination. Upon PCR amplification, uracil is amplified as thymine while 5-MeC residues remain as cytosines, allowing methylated CpGs to be distinguished from unmethylated CpGs by presence of a cytosine "C" versus thymine "T" residue during sequencing.DNA modification by bisulphite conversion is a well-established protocol that can be exploited for many methods of DNA methylation analysis. Since the detection of 5-MeC by bisulphite conversion was first demonstrated by Frommer et al.¹ and Clark et al.², methods based around bisulphite conversion of genomic DNA account for the majority of new data on DNA methylation. Different methods of post PCR analysis may be utilized, depending on the degree of specificity and resolution of methylation required. Cloning and sequencing is still the most readily available method that can give single nucleotide resolution for methylation across the DNA molecule.     

Alcohol exposure alters DNA methylation profiles in mouse embryos at early neurulation

Alcohol exposure during development can cause variable neurofacial deficit and growth retardation known as fetal alcohol spectrum disorders (FASD). The mechanism underlying FASD is not fully understood. However, alcohol, which is known to affect methyl donor metabolism, may induce aberrant epigenetic changes contributing to FASD. Using a tightly controlled whole-embryo culture, we investigated the effect of alcohol exposure (88mM) at early embryonic neurulation on genome-wide DNA methylation and gene expression in the C57BL/6 mouse. The DNA methylation landscape around promoter CpG islands at early mouse development was analyzed using MeDIP (methylated DNA immunoprecipitation) coupled with microarray (MeDIP-chip). At early neurulation, genes associated with high CpG promoters (HCP) had a lower ratio of methylation but a greater ratio of expression. Alcohol-induced alterations in DNA methylation were observed, particularly in genes on chromosomes 7, 10, and X; remarkably, a >10 fold increase in the number of genes with increased methylation on chromosomes 10 and X was observed in alcohol-exposed embryos with a neural tube defect phenotype compared to embryos without a neural tube defect. Significant changes in methylation were seen in imprinted genes, genes known to play roles in cell cycle, growth, apoptosis, cancer, and in a large number of genes associated with olfaction. Altered methylation was associated with significant (p     

Genome-wide high throughput analysis of DNA methylation in eukaryotes

Cytosine methylation is the quintessential epigenetic mark. Two well-established methods, bisulfite sequencing and methyl-DNA immunoprecipitation (MeDIP) lend themselves to the genome-wide analysis of DNA methylation by high throughput sequencing. Here we provide an overview and brief review of these methods. We summarize our experience with MeDIP followed by high throughput Illumina/Solexa sequencing, exemplified by the analysis of the methylated fraction of the Neurospora crassa genome ("methylome"). We provide detailed methods for DNA isolation, processing and the generation of in vitro libraries for Illumina/Solexa sequencing. We discuss potential problems in the generation of sequencing libraries. Finally, we provide an overview of software that is appropriate for the analysis of high throughput sequencing data generated by Illumina/Solexa-type sequencing by synthesis, with a special emphasis on approaches and applications that can generate more accurate depictions of sequence reads that fall in repeated regions of a chosen reference genome.     

The inactive X chromosome in the human female is enriched in 5-methylcytosine to an unusual degree and appears to contain more of this modified nucleotide than the remainder of the genome

By employing a procedure that combines ELISA and photoacoustic spectroscopy, we have examined the content of 5-methylcytosine (m⁵C) in DNA of individuals who differed from one another in the number of X chromosomes in their genomes. The results show that the human inactive X chromosome (Xi) contains very high amounts of this modified nucleotide. We estimate that in the 46, XX female there is more m⁵C in Xi (∼ 3.6 x 10⁷) than in all the remaining chromosomes put together (∼2.1 x 10⁷). Our results also suggest that nearly one-fifth of all cytosines in Xi are methylated and that, in addition to CpG methylation, there is extensive non-CpG methylation as well.     

A Bayesian deconvolution strategy for immunoprecipitation-based DNA methylome analysis

DNA methylation is an indispensible epigenetic modification required for regulating the expression of mammalian genomes. Immunoprecipitation-based methods for DNA methylome analysis are rapidly shifting the bottleneck in this field from data generation to data analysis, necessitating the development of better analytical tools. In particular, an inability to estimate absolute methylation levels remains a major analytical difficulty associated with immunoprecipitation-based DNA methylation profiling. To address this issue, we developed a cross-platform algorithm-Bayesian tool for methylation analysis (Batman)-for analyzing methylated DNA immunoprecipitation (MeDIP) profiles generated using oligonucleotide arrays (MeDIP-chip) or next-generation sequencing (MeDIP-seq). We developed the latter approach to provide a high-resolution whole-genome DNA methylation profile (DNA methylome) of a mammalian genome. Strong correlation of our data, obtained using mature human spermatozoa, with those obtained using bisulfite sequencing suggest that combining MeDIP-seq or MeDIP-chip with Batman provides a robust, quantitative and cost-effective functional genomic strategy for elucidating the function of DNA methylation.     

Different methylation pattern of melon satellite DNA sequences in hypocotyl and callus tissues

Satellite DNA sequences from Cucumis melo have been examined with respect to modification at CCGG sequences in hypocotyls and in callus tissues. For this purpose, restriction fragments given by HpaII and MspI were compared (both enzymes recognize CCGG sequences but have different sensitivity to methylation at this site). Whereas the methylation level of satellite DNA sequences is on average higher in hypocotyls than in callus tissues, the comparison of partially methylated repeat units of satellite DNA reveals that in callus tissues, all methylated restriction sites are doubly methylated.     

Purification and various properties of uracil-DNA-glycosylase from human placenta

Uracil-DNA-glycosylase was isolated from human placenta and purified 2100-fold. The apparent Km value for non-methylated DNA substrate of the enzyme is 3.10(-7) M. However, Km for uracil-DNA-glycosylase was 3 times as low when methylated DNA was used as a substrate. It was shown that the initial rate of uracil excision was greater for the non-methylated than for the hypermethylated DNA. The experimental results indicate that the postreplicative methylation of DNA can interfere with uracil excision.     

On the role of steric clashes in methylation control of restriction endonuclease activity

Restriction-modification systems digest non-methylated invading DNA, while protecting host DNA against the endonuclease activity by methylation. It is widely believed that the methylated DNA would not ‘fit’ into the binding site of the endonuclease in the productive orientation, and thus steric clashes should account for most of the protection. We test this concept statistically by grafting methyl groups in silico onto non-methylated DNA in co-crystal structures with restriction endonucleases. Clash scores are significantly higher for protective than non-protective methylation (P < 0.05% according to the Wilcoxon rank sum test). Structural data alone are sufficient to distinguish between protective and non-protective DNA methylation with 90% confidence and decision thresholds of 1.1 Å and 48 ų for the most severe distance-based and cumulative volume-based clash with the protein, respectively (0.1 Å was deducted from each interatomic distance to allow for coordinate errors). The most severe clashes are more pronounced for protective methyl groups attached to the nitrogen atoms (N6-methyladenines and N4-methylcytosines) than for C5-methyl groups on cytosines. Cumulative clashes are comparable for all three types of protective methylation.