A simple algorithm for quantifying DNA methylation levels on multiple independent CpG sites in bisulfite genomic sequencing electropherograms

DNA methylation at cytosines is a widely studied epigenetic modification. Methylation is commonly detected using bisulfite modification of DNA followed by PCR and additional techniques such as restriction digestion or sequencing. These additional techniques are either laborious, require specialized equipment, or are not quantitative. Here we describe a simple algorithm that yields quantitative results from analysis of conventional four-dye-trace sequencing. We call this method Mquant and we compare it with the established laboratory method of combined bisulfite restriction assay (COBRA). This analysis of sequencing electropherograms provides a simple, easily applied method to quantify DNA methylation at specific CpG sites.     

BSMAP: whole genome bisulfite sequence MAPping program

Background  

Bisulfite sequencing is a powerful technique to study DNA cytosine methylation. Bisulfite treatment followed by PCR amplification specifically converts unmethylated cytosines to thymine. Coupled with next generation sequencing technology, it is able to detect the methylation status of every cytosine in the genome. However, mapping high-throughput bisulfite reads to the reference genome remains a great challenge due to the increased searching space, reduced complexity of bisulfite sequence, asymmetric cytosine to thymine alignments, and multiple CpG heterogeneous methylation.     

GPU-BSM: A GPU-Based Tool to Map Bisulfite-Treated Reads

Cytosine DNA methylation is an epigenetic mark implicated in several biological processes. Bisulfite treatment of DNA is acknowledged as the gold standard technique to study methylation. This technique introduces changes in the genomic DNA by converting cytosines to uracils while 5-methylcytosines remain nonreactive. During PCR amplification 5-methylcytosines are amplified as cytosine, whereas uracils and thymines as thymine. To detect the methylation levels, reads treated with the bisulfite must be aligned against a reference genome. Mapping these reads to a reference genome represents a significant computational challenge mainly due to the increased search space and the loss of information introduced by the treatment. To deal with this computational challenge we devised GPU-BSM, a tool based on modern Graphics Processing Units. Graphics Processing Units are hardware accelerators that are increasingly being used successfully to accelerate general-purpose scientific applications. GPU-BSM is a tool able to map bisulfite-treated reads from whole genome bisulfite sequencing and reduced representation bisulfite sequencing, and to estimate methylation levels, with the goal of detecting methylation. Due to the massive parallelization obtained by exploiting graphics cards, GPU-BSM aligns bisulfite-treated reads faster than other cutting-edge solutions, while outperforming most of them in terms of unique mapped reads.     

Identification and resolution of artifacts in bisulfite sequencing

Bisulfite sequencing has become the most widely used application to detect 5-methylcytosine (5-MeC) in DNA, and provides a reliable way of detecting any methylated cytosine at single-molecule resolution in any sequence context. The process of bisulfite treatment exploits the different sensitivity of cytosine and 5-MeC to deamination by bisulfite under acidic conditions, in which cytosine undergoes conversion to uracil while 5-MeC remains unreactive. In this article, we address the more commonly encountered experimental artifacts associated with bisulfite sequencing, and provide methods for the detection and elimination of these artifacts. In particular, we focus on conditions that inhibit complete bisulfite-mediated conversion of cytosines in a target sequence, and demonstrate the necessity of complete protein removal from DNA samples prior to bisulfite treatment. We also include a brief summary of the experimental protocol for bisulfite treatment and tips for designing polymerase chain reaction (PCR) primers to amplify from bisulfite-treated DNA.     

The bisulfite genomic sequencing used in the analysis of epigenetic states, a technique in the emerging environmental genotoxicology research

Methylation at position 5 of cytosine in DNA is an important event in epigenetic changes of cells, the methylation being linked to the control of gene functions. The DNA methylation can be analyzed by bisulfite genomic sequencing, and a large body of data have now been accumulated, based on which causation of diseases, for example cancer, and many other manifestations of cellular activities have been discussed intensively. This article gives an extensive account of the chemical aspects of bisulfite modification of cytosine and 5-methylcytosine in DNA. Various factors that affect the action of bisulfite are discussed, and a recent progress from our laboratory is explained. Conventional procedures for the bisulfite treatment consist incubation of single-stranded DNA with sodium bisulfite under acidic conditions. This treatment converts cytosine into uracil, but 5-methylcytosine remains unchanged. Amplification by polymerase chain reaction (PCR) of the bisulfite-treated DNA followed by sequencing can result in revealing the positions of 5-methylcytosine in the gene. We have discovered that the whole procedure can be significantly speeded up by the use of a highly concentrated bisulfite solution, 10 M ammonium bisulfite. Another recent finding is that urea, which has been often added to the reaction mixture with the purpose of facilitating the bisulfite-mediated deamination of cytosine in DNA, may not work as anticipated: we have observed that urea does not show such promoting actions in our treatments of DNA. A laboratory protocol for quantifying bisulfite, suitably simple for routine practice to ensure valid experiments, is described.     

Whole genome amplification of sodium bisulfite-treated DNA allows the accurate estimate of methylated cytosine density in limited DNA resources

Sodium bisulfite modification-based fine mapping of methylated cytosines represents the gold standard technique for DNA methylation studies. A major problem with this approach, however is that it results in considerable DNA degradation, and large quantities of genomic DNA material are needed if numerous genomic regions are to be profiled. In this study, we examined whether whole genome amplification (WGA) techniques can be applied to sodium bisulfite-treated DNA and whether WGA would bias DNA methylation results. Sodium bisulfite-treated DNA was amplified using a standard WGA method: optimized primer-extension preamplification (PEP) with degenerate primers. Following the PCR of bisulfite-treated DNA, the DNA methylation profiles of specific DNA fragments were assessed using three approaches: (i) direct sequencing of the overall product; (ii) the sequencing of cloned PCR products; and (iii) methylation-sensitive single nucleotide primer extension (MS-SNuPE)--and compared with those obtained from bisulfite-treated DNA not subjected to WGA. Our data indicates that the DNA methylation profiles obtained from WGA of sodium bisulfite-treated DNA are consistent with those obtained from non-WGA DNA. The average difference in methylation percentage calculated from the two sets of template using MS-SNuPE was 4%. If our results are replicated on other genomic loci, WGA may become a useful technique in DNA methylation studies.     

Detection of Cytosine Methylation in Ancient DNA from Five Native American Populations Using Bisulfite Sequencing

While cytosine methylation has been widely studied in extant populations, relatively few studies have analyzed methylation in ancient DNA. Most existing studies of epigenetic marks in ancient DNA have inferred patterns of methylation in highly degraded samples using post-mortem damage to cytosines as a proxy for cytosine methylation levels. However, this approach limits the inference of methylation compared with direct bisulfite sequencing, the current gold standard for analyzing cytosine methylation at single nucleotide resolution. In this study, we used direct bisulfite sequencing to assess cytosine methylation in ancient DNA from the skeletal remains of 30 Native Americans ranging in age from approximately 230 to 4500 years before present. Unmethylated cytosines were converted to uracils by treatment with sodium bisulfite, bisulfite products of a CpG-rich retrotransposon were pyrosequenced, and C-to-T ratios were quantified for a single CpG position. We found that cytosine methylation is readily recoverable from most samples, given adequate preservation of endogenous nuclear DNA. In addition, our results indicate that the precision of cytosine methylation estimates is inversely correlated with aDNA preservation, such that samples of low DNA concentration show higher variability in measures of percent methylation than samples of high DNA concentration. In particular, samples in this study with a DNA concentration above 0.015 ng/μL generated the most consistent measures of cytosine methylation. This study presents evidence of cytosine methylation in a large collection of ancient human remains, and indicates that it is possible to analyze epigenetic patterns in ancient populations using direct bisulfite sequencing approaches.     

Whole Methylome Analysis by Ultra-Deep Sequencing Using Two-Base Encoding

Methylation, the addition of methyl groups to cytosine (C), plays an important role in the regulation of gene expression in both normal and dysfunctional cells. During bisulfite conversion and subsequent PCR amplification, unmethylated Cs are converted into thymine (T), while methylated Cs will not be converted. Sequencing of this bisulfite-treated DNA permits the detection of methylation at specific sites. Through the introduction of next-generation sequencing technologies (NGS) simultaneous analysis of methylation motifs in multiple regions provides the opportunity for hypothesis-free study of the entire methylome. Here we present a whole methylome sequencing study that compares two different bisulfite conversion methods (in solution versus in gel), utilizing the high throughput of the SOLiD™ System. Advantages and disadvantages of the two different bisulfite conversion methods for constructing sequencing libraries are discussed. Furthermore, the application of the SOLiD™ bisulfite sequencing to larger and more complex genomes is shown with preliminary in silico created bisulfite converted reads.     

Evidence for non-CpG methylation in mammals

In mammals, the existence of cytosine methylation on non-CpG sequences is controversial. Here, we adapted a LuminoMetric-based Assay (LUMA) to determine global non-CpG methylation levels in rodent and human tissues. We observed that < 1% cytosines in non-CpG motifs were methylated in 3T3-L1 fibroblasts, whereas 7–13% cytosines in non-CpG motifs were methylated in mouse tissues or embryonic fibroblasts. Analysis of cytosine methylation in human, rat, and mouse tissues by bisulfite sequencing revealed non-CpG methylation levels up to 7.5% of all non-CpG cytosines. These levels dropped to 1.5% when a second round of PCR was performed prior to bisulfite sequencing, providing an explanation for the common underestimation of non-CpG methylation levels. Collectively, our results provide evidence that non-CpG methylation exists at substantial levels in mammals.     

Kismeth: Analyzer of plant methylation states through bisulfite sequencing

Background  

There is great interest in probing the temporal and spatial patterns of cytosine methylation states in genomes of a variety of organisms. It is hoped that this will shed light on the biological roles of DNA methylation in the epigenetic control of gene expression. Bisulfite sequencing refers to the treatment of isolated DNA with sodium bisulfite to convert unmethylated cytosine to uracil, with PCR converting the uracil to thymidine followed by sequencing of the resultant DNA to detect DNA methylation. For the study of DNA methylation, plants provide an excellent model system, since they can tolerate major changes in their DNA methylation patterns and have long been studied for the effects of DNA methylation on transposons and epimutations. However, in contrast to the situation in animals, there aren't many tools that analyze bisulfite data in plants, which can exhibit methylation of cytosines in a variety of sequence contexts (CG, CHG, and CHH).     

An optimized rapid bisulfite conversion method with high recovery of cell-free DNA

Background

Methylation analysis of cell-free DNA is a encouraging tool for tumor diagnosis, monitoring and prognosis. Sensitivity of methylation analysis is a very important matter due to the tiny amounts of cell-free DNA available in plasma. Most current methods of DNA methylation analysis are based on the difference of bisulfite-mediated deamination of cytosine between cytosine and 5-methylcytosine. However, the recovery of bisulfite-converted DNA based on current methods is very poor for the methylation analysis of cell-free DNA.

Results

We optimized a rapid method for the crucial steps of bisulfite conversion with high recovery of cell-free DNA. A rapid deamination step and alkaline desulfonation was combined with the purification of DNA on a silica column. The conversion efficiency and recovery of bisulfite-treated DNA was investigated by the droplet digital PCR. The optimization of the reaction results in complete cytosine conversion in 30 min at 70 °C and about 65% of recovery of bisulfite-treated cell-free DNA, which is higher than current methods.

Conclusions

The method allows high recovery from low levels of bisulfite-treated cell-free DNA, enhancing the analysis sensitivity of methylation detection from cell-free DNA.

     

Enhanced Reduced Representation Bisulfite Sequencing for Assessment of DNA Methylation at Base Pair Resolution

DNA methylation pattern mapping is heavily studied in normal and diseased tissues. A variety of methods have been established to interrogate the cytosine methylation patterns in cells. Reduced representation of whole genome bisulfite sequencing was developed to detect quantitative base pair resolution cytosine methylation patterns at GC-rich genomic loci. This is accomplished by combining the use of a restriction enzyme followed by bisulfite conversion. Enhanced Reduced Representation Bisulfite Sequencing (ERRBS) increases the biologically relevant genomic loci covered and has been used to profile cytosine methylation in DNA from human, mouse and other organisms. ERRBS initiates with restriction enzyme digestion of DNA to generate low molecular weight fragments for use in library preparation. These fragments are subjected to standard library construction for next generation sequencing. Bisulfite conversion of unmethylated cytosines prior to the final amplification step allows for quantitative base resolution of cytosine methylation levels in covered genomic loci. The protocol can be completed within four days. Despite low complexity in the first three bases sequenced, ERRBS libraries yield high quality data when using a designated sequencing control lane. Mapping and bioinformatics analysis is then performed and yields data that can be easily integrated with a variety of genome-wide platforms. ERRBS can utilize small input material quantities making it feasible to process human clinical samples and applicable in a range of research applications. The video produced demonstrates critical steps of the ERRBS protocol.     

BiSearch: primer-design and search tool for PCR on bisulfite-treated genomes

Bisulfite genomic sequencing is the most widely used technique to analyze the 5-methylation of cytosines, the prevalent covalent DNA modification in mammals. The process is based on the selective transformation of unmethylated cytosines to uridines. Then, the investigated genomic regions are PCR amplified, subcloned and sequenced. During sequencing, the initially unmethylated cytosines are detected as thymines. The efficacy of bisulfite PCR is generally low; mispriming and non-specific amplification often occurs due to the T richness of the target sequences. In order to ameliorate the efficiency of PCR, we developed a new primer-design software called BiSearch, available on the World Wide Web. It has the unique property of analyzing the primer pairs for mispriming sites on the bisulfite-treated genome and determines potential non-specific amplification products with a new search algorithm. The options of primer-design and analysis for mispriming sites can be used sequentially or separately, both on bisulfite-treated and untreated sequences. In silico and in vitro tests of the software suggest that new PCR strategies may increase the efficiency of the amplification.     

Study of Tissue-Specific CpG Methylation of DNA in Extended Genomic Loci

Modern approaches for studies on genome functioning include investigation of its epigenetic regulation. Methylation of cytosines in CpG dinucleotides is an inherited epigenetic modification that is responsible for both functional activity of certain genomic loci and total chromosomal stability. This review describes the main approaches for studies on DNA methylation. Under consideration are site-specific approaches based on bisulfite sequencing and methyl-sensitive PCR, whole-genome approaches aimed at searching for new methylation hot spots, and also mapping of unmethylated CpG sites in extended genomic loci.     

Bismark: a flexible aligner and methylation caller for Bisulfite-Seq applications

SUMMARY: A combination of bisulfite treatment of DNA and high-throughput sequencing (BS-Seq) can capture a snapshot of a cell's epigenomic state by revealing its genome-wide cytosine methylation at single base resolution. Bismark is a flexible tool for the time-efficient analysis of BS-Seq data which performs both read mapping and methylation calling in a single convenient step. Its output discriminates between cytosines in CpG, CHG and CHH context and enables bench scientists to visualize and interpret their methylation data soon after the sequencing run is completed. Availability and implementation: Bismark is released under the GNU GPLv3+ licence. The source code is freely available from www.bioinformatics.bbsrc.ac.uk/projects/bismark/.     

The Behaviour of 5-Hydroxymethylcytosine in Bisulfite Sequencing

Background

We recently showed that enzymes of the TET family convert 5-mC to 5-hydroxymethylcytosine (5-hmC) in DNA. 5-hmC is present at high levels in embryonic stem cells and Purkinje neurons. The methylation status of cytosines is typically assessed by reaction with sodium bisulfite followed by PCR amplification. Reaction with sodium bisulfite promotes cytosine deamination, whereas 5-methylcytosine (5-mC) reacts poorly with bisulfite and is resistant to deamination. Since 5-hmC reacts with bisulfite to yield cytosine 5-methylenesulfonate (CMS), we asked how DNA containing 5-hmC behaves in bisulfite sequencing.

Methodology/Principal Findings

We used synthetic oligonucleotides with different distributions of cytosine as templates for generation of DNAs containing C, 5-mC and 5-hmC. The resulting DNAs were subjected in parallel to bisulfite treatment, followed by exposure to conditions promoting cytosine deamination. The extent of conversion of 5-hmC to CMS was estimated to be 99.7%. Sequencing of PCR products showed that neither 5-mC nor 5-hmC undergo C-to-T transitions after bisulfite treatment, confirming that these two modified cytosine species are indistinguishable by the bisulfite technique. DNA in which CMS constituted a large fraction of all bases (28/201) was much less efficiently amplified than DNA in which those bases were 5-mC or uracil (the latter produced by cytosine deamination). Using a series of primer extension experiments, we traced the inefficient amplification of CMS-containing DNA to stalling of Taq polymerase at sites of CMS modification, especially when two CMS bases were either adjacent to one another or separated by 1–2 nucleotides.

Conclusions

We have confirmed that the widely used bisulfite sequencing technique does not distinguish between 5-mC and 5-hmC. Moreover, we show that CMS, the product of bisulfite conversion of 5-hmC, tends to stall DNA polymerases during PCR, suggesting that densely hydroxymethylated regions of DNA may be underrepresented in quantitative methylation analyses.     

MethylQuant: a sensitive method for quantifying methylation of specific cytosines within the genome

Here we present MethylQuant, a novel method that allows accurate quantification of the methylation level of a specific cytosine within a complex genome. This method relies on the well-established treatment of genomic DNA with sodium bisulfite, which converts cytosine into uracil without modifying 5-methyl cytosine. The region of interest is then PCR-amplified and quantification of the methylation status of a specific cytosine is performed by methylation-specific real-time PCR with SYBR Green I using one of the primers whose 3′ end discriminates between the methylation states of this cytosine. The presence of a locked nucleic acid at the 3′ end of the discriminative primer provides the specificity necessary for accurate and sensitive quantification, even when one of the methylation states is present at a level as low as 1% of the overall population. We demonstrate that accurate quantification of the methylation status of specific cytosines can be achieved in biological samples. The method is high-throughput, cost-effective, relatively simple and does not require any specific equipment other than a real-time PCR instrument.     

Profiling DNA methylation by melting analysis

The idea of modifying DNA with bisulfite has paved the way for a variety of polymerase chain reaction (PCR) methods for accurately mapping 5-methylcytosine at specific genes. Bisulfite selectively deaminates cytosine to uracil under conditions where 5-methylcytosine remains unreacted. Following conventional PCR amplification of bisulfite-treated DNA, original cytosines appear as thymine while 5-methylcytosines appear as cytosine. Because the relative thermostability of a DNA duplex increases with increasing content of G:C base pairs, PCR products originating from DNA templates with different contents of 5-methylcytosine differ in melting temperature, i.e., the temperature required to convert the double helix into random coils. We describe two methods that resolve differentially methylated DNA sequences on the basis of differences in melting temperature. The first method integrates PCR amplification of bisulfite-treated DNA and subsequent melting analysis by using a thermal cycler coupled with a fluorometer. By including in the reaction a PCR-compatible, fluorescent dye that specifically binds to double-stranded DNA, the melting properties of the PCR product can be examined directly in the PCR tube by continuous fluorescence monitoring during a temperature transition. The second method relies on resolution of alleles with different 5-methylcytosine contents by analysis of PCR products in a polyacrylamide gel containing a gradient of chemical denaturants. Optimal resolution of differences in melting temperature is achieved by a special design of PCR primers. Both methods allow resolution of "heterogeneous" methylation, i.e., the situation where the content and distribution of 5-methylcytosine in a target gene differ between different molecules in the same sample.     

The CHH motif in sugar beet satellite DNA: a modulator for cytosine methylation

Methylation of DNA is important for the epigenetic silencing of repetitive DNA in plant genomes. Knowledge about the cytosine methylation status of satellite DNAs, a major class of repetitive DNA, is scarce. One reason for this is that arrays of tandemly arranged sequences are usually collapsed in next‐generation sequencing assemblies. We applied strategies to overcome this limitation and quantified the level of cytosine methylation and its pattern in three satellite families of sugar beet (Beta vulgaris) which differ in their abundance, chromosomal localization and monomer size. We visualized methylation levels along pachytene chromosomes with respect to small satellite loci at maximum resolution using chromosome‐wide fluorescent in situ hybridization complemented with immunostaining and super‐resolution microscopy. Only reduced methylation of many satellite arrays was obtained. To investigate methylation at the nucleotide level we performed bisulfite sequencing of 1569 satellite sequences. We found that the level of methylation of cytosine strongly depends on the sequence context: cytosines in the CHH motif show lower methylation (44–52%), while CG and CHG motifs are more strongly methylated. This affects the overall methylation of satellite sequences because CHH occurs frequently while CG and CHG are rare or even absent in the satellite arrays investigated. Evidently, CHH is the major target for modulation of the cytosine methylation level of adjacent monomers within individual arrays and contributes to their epigenetic function. This strongly indicates that asymmetric cytosine methylation plays a role in the epigenetic modification of satellite repeats in plant genomes.