Reduced Representation Bisulfite Sequencing Libraries Research Articles

Blood and Bone-Derived DNA Methylation Ages Predict Mortality After Geriatric Hip Fracture: A Pilot Study.

The purpose of this study was to (1) perform the first analysis of bone-derived DNA methylation, (2) compare DNA methylation clocks derived from bone with those derived from whole blood, and (3) establish a relationship between DNA methylation age and 1-year mortality within the geriatric hip fracture population. Patients ≥65 years old who presented to a Level-I trauma center with a hip fracture were prospectively enrolled from 2020 to 2021. Preoperative whole blood and intraoperative bone samples were collected. Following DNA extraction, RRBS (reduced representation bisulfite sequencing) libraries for methylation clock analysis were prepared. Sequencing data were analyzed using computational algorithms previously described by Horvath et al. to build a regression model of methylation (biological) age for each tissue type. Student t tests were used to analyze differences (Δ) in methylation age versus chronological age. Correlation between blood and bone methylation ages was expressed using the Pearson R coefficient. Blood and bone samples were collected from 47 patients. DNA extraction, sequencing, and methylation analysis were performed on 24 specimens from 12 subjects. Mean age at presentation was 85.4 ± 8.65 years. There was no difference in DNA extraction yield between the blood and bone samples (p = 0.935). The mean follow-up duration was 12.4 ± 4.3 months. The mortality cohort (4 patients, 33%) showed a mean ΔAgeBone of 18.33 ± 6.47 years and mean ΔAgeBlood of 16.93 ± 4.02 years. In comparison, the survival cohort showed a significantly lower mean ΔAgeBone and ΔAgeBlood (7.86 ± 6.7 and 7.31 ± 7.71 years; p = 0.026 and 0.039, respectively). Bone-derived methylation age was strongly correlated with blood-derived methylation age (R = 0.81; p = 0.0016). Bone-derived DNA methylation clocks were found to be both feasible and strongly correlated with those derived from whole blood within a geriatric hip fracture population. Mortality was independently associated with the DNA methylation age, and that age was approximately 17 years greater than chronological age in the mortality cohort. The results of the present study suggest that prevention of advanced DNA methylation may play a key role in decreasing mortality following hip fracture. Prognostic Level I. See Instructions for Authors for a complete description of levels of evidence.

Beyond the base pairs: comparative genome-wide DNA methylation profiling across sequencing technologies.

Deoxyribonucleic acid (DNA) methylation plays a key role in gene regulation and is critical for development and human disease. Techniques such as whole-genome bisulfite sequencing (WGBS) and reduced representation bisulfite sequencing (RRBS) allow DNA methylation analysis at the genome scale, with Illumina NovaSeq 6000 and MGI Tech DNBSEQ-T7 being popular due to their efficiency and affordability. However, detailed comparative studies of their performance are not available. In this study, we constructed 60 WGBS and RRBS libraries for two platforms using different types of clinical samples and generated approximately 2.8 terabases of sequencing data. We systematically compared quality control metrics, genomic coverage, CpG methylation levels, intra- and interplatform correlations, and performance in detecting differentially methylated positions. Our results revealed that the DNBSEQ platform exhibited better raw read quality, although base quality recalibration indicated potential overestimation of base quality. The DNBSEQ platform also showed lower sequencing depth and less coverage uniformity in GC-rich regions than did the NovaSeq platform and tended to enrich methylated regions. Overall, both platforms demonstrated robust intra- and interplatform reproducibility for RRBS and WGBS, with NovaSeq performing better for WGBS, highlighting the importance of considering these factors when selecting a platform for bisulfite sequencing.

Improve-RRBS: a novel tool to correct the 3′ trimming of reduced representation sequencing reads

Abstract Motivation Reduced Representation Bisulfite Sequencing (RRBS) is a popular approach to determine DNA methylation of the CpG-rich regions of the genome. However, we observed that false positive differentially methylated sites (DMS) are also identified using the standard computational analysis. Results During RRBS library preparation the MspI digested DNA undergo end-repair by a cytosine at the 3′ end of the fragments. After sequencing, Trim Galore cuts these end-repaired nucleotides. However, Trim Galore fails to detect end-repair when it overlaps with the 3′ end of the sequencing reads. We found that these non-trimmed cytosines bias methylation calling, thus, can identify DMS erroneously. To circumvent this problem, we developed improve-RRBS, which efficiently identifies and hides these cytosines from methylation calling with a false positive rate of maximum 0.5%. To test improve-RRBS, we investigated four datasets from four laboratories and two different species. We found non-trimmed 3′ cytosines in all datasets analyzed and as much as >50% of false positive DMS under certain conditions. By applying improve-RRBS, these DMS completely disappeared from all comparisons. Availability and implementation Improve-RRBS is a freely available python package https://pypi.org/project/iRRBS/ or https://github.com/fothia/improve-RRBS to be implemented in RRBS pipelines.

DNA methylation profiling to determine the primary sites of metastatic cancers using formalin-fixed paraffin-embedded tissues

Identifying the primary site of metastatic cancer is critical to guiding the subsequent treatment. Approximately 3–9% of metastatic patients are diagnosed with cancer of unknown primary sites (CUP) even after a comprehensive diagnostic workup. However, a widely accepted molecular test is still not available. Here, we report a method that applies formalin-fixed, paraffin-embedded tissues to construct reduced representation bisulfite sequencing libraries (FFPE-RRBS). We then generate and systematically evaluate 28 molecular classifiers, built on four DNA methylation scoring methods and seven machine learning approaches, using the RRBS library dataset of 498 fresh-frozen tumor tissues from primary cancer patients. Among these classifiers, the beta value-based linear support vector (BELIVE) performs the best, achieving overall accuracies of 81-93% for identifying the primary sites in 215 metastatic patients using top-k predictions (k = 1, 2, 3). Coincidentally, BELIVE also successfully predicts the tissue of origin in 81-93% of CUP patients (n = 68).

Reduced Representation Bisulfite Sequencing (RRBS).

Reduced representation bisulfite sequencing (RRBS) enriches CpG-rich genomic regions using the MspI restriction enzyme-which cuts DNA at all CCGG sites, regardless of their DNA methylation status at the CG site-and enables the measurement of DNA methylation levels at 5%~10% of all CpG sites in the mammalian genome. RRBS has been utilized in a large number of studies as a cost-effective method to investigate DNA methylation patterns, mainly at gene promoters and CpG islands. Here, we describe protocols for gel-free preparation of RRBS libraries, quality control, sequencing, and data analysis. Our protocols typically require nine cycles of polymerase chain reaction (PCR) amplification to obtain a sufficient amount of library for sequencing when 100ng of genomic DNA is used as a starting material; moreover, it takes 3days to complete library preparation and quality control procedures for up to eight samples.

A murine MODEL of e-cigarettes exposure to evaluate the carcinogenic effects of vaping on the bladder urothelium.

566 Background: Data evaluating the carcinogenic potential of e-cigarette (ECS) by-products on the bladder urothelium is scarce, and to date, no group has studied the direct impact of ECS on the urothelium at the genomic and epigenetic level. While it has been published that ECS exposure causes bladder hyperplasia in mice, no bladder tumors formed and the field is therefore lacking appropriate preclinical models to be able to study ECS related bladder cancer. Importantly, whole organ mapping studies of tumor bearing human bladders have identified DNA methylation field changes in “adjacent normal” urothelium as early events in bladder carcinogenesis. Whether ECS exposure results in early field alterations of DNA methylation in the urothelium is unknown. Methods: Mice (4 females, 4 males; 35 weeks old) were exposed to 12% nicotine (120mg/ml) in 1:1 polyethylene glycol: vegetable glycerol (PG/VG) or vehicle (PG/VG) daily: 3 hr sessions, 3 sec vape duration, 10 min interval between vapes. Nicotine and cotinine serum concentrations were measured at 3 hrs, and then at 5 days using LC-MS. Bladders were harvested after 4 weeks. DNA was extracted from microdissected urothelial layer. Reduced representation bisulfite sequencing (RRBS) was performed (Zymo Research) using 100ng of DNA input and paired-end chemistry. Alignment was performed using Bismark. Number of loci covered by each sample was determined and data was filtered to only include loci with > 10 reads. Differentially methylated loci were determined. Adjusted p-value < 0.01 was used as threshold to generate heatmaps. Gene Set Enrichment Analysis was performed to contextualize the DNA methylation changes. Results: Acute and repeated exposure resulted in significantly higher levels of nicotine and cotinine serum concentration in the nicotine group vs. the vehicle group without significant adverse events. No tumors were observed on gross visualization at the time of urothelium harvest. The number of loci in the RRBS libraries with > 10 reads ranged from 130,000 to 250,000. 53 loci were differentially methylated between nicotine and vehicle. These loci did not correspond to the top hyper/hypo methylated genes in premalignant bladder tissue from recently reported whole-cystectomy mapping. Conclusions: We established an effective and safe murine model of ECS exposure closely resembling the human experience of vaping. Significant changes of DNA methylation were observed in mice exposed to ECS vapor vs. a vehicle control after four weeks of daily exposure. Ongoing experiments include longer vaping exposures as well as comparison of RRBS to a recently developed commercial mouse methylarray. Understanding epigenetic and genomic changes as a result of ECS exposure in preclinical models can serve as a foundation for studies developing assays to identify ECS users whose bladders contain early field defects that may place them at higher risk for bladder cancer.

Circulating tumor DNA methylation as markers for early detection of pancreatic ductal adenocarcinoma (PDAC).

395 Background: PDAC is a cancer of high mortality and low survival. Its early detection is critical due to symptoms often occur only at advanced stages. However there is no reliable screening tool to identify high-risk patients. ctDNA methylation has recently emerged as a promising new target to differentiate PDAC plasma from normal plasma for its early detection. Methods: Reduced representation bisulfite sequencing libraries were made in 46 PDAC tissues, 30 para-PDAC tissues and 20 PDAC plasmas to screen PDAC-specific markers, which was done by quantifying and comparing methylation levels of genomic regions and individual CpG sites between those groups. Markers were validated in plasma samples from 84 PDAC patients and 64 normal controls to propose a blood classifier. The best-performing markers were developed into a targeted sequencing panel, which was tested on a larger collection of plasma samples from patients of a variety of pancreatic diseases to build and validate a PDAC-predicting model. Results: We profiled genome-wide methylation patterns of tissues samples to identify 171 PDAC-specific markers. We reiterated training and cross-validating PDAC classification models using SVM method, and achieved an average sensitivity of 86% and specificity of 88%. To prove the feasibility of a non-invasive detection in plasma, a targeted methylation assay using those markers was tested on PDAC and normal plasmas, and yielded an average sensitivity of 68.4% and a specificity of 85.8%. We refined the panel by selecting the most discriminatory markers and built the version II of the panel, which was named PANcreatic Cancer Detection Assay, or PANDA, for a more efficient target capture, which was validated in an independent cohort of plasma samples that included 94 PDAC cases, 25 chronic pancreatitis (CP) cases and 80 normal samples from multiple centers. The PANDA achieved an AUC of 0.906 when classifying PDAC from normal, and an AUC of 0.882 when separating PDAC from CPs, both of which are more accurate than CA19-9, the conventional blood marker for PDAC. We further integrated test subjects’ age and their CA19-9 level as features into the PANDA model, which further elevated their AUC to 0.882 and 0.933 when classifying PDAC plasma from either CP plasma or normal plasma, respectively. Conclusions: We have developed PANDA, an NGS based target assay covering PDAC-specific DNA methylation targets by screening and validation on PDAC tissues and plasmas. Combined with age and CA19-9 blood level, PANDA has shown encouraging results to classify PDAC plasma from non-malignant diseases, demonstrating its potential to be optimized into non-invasive diagnostics for blood-based early PDAC screening.

DNA methylome profiling identifies novel methylated genes in epithelial ovarian cancer patients with platinum resistance.

Platinum-based chemotherapy is widely used for epithelial ovarian cancer (EOC). As high as 20-25% of EOC patients will not respond to the initial chemotherapy. Accumulated evidences have implied that DNA methylation may serve as a potential bio-marker for chemotherapy-resistant phenotypic screening; however, the pattern underlying primary platinum resistance remains unclear. Reduced representation bisulfite sequencing (RRBS) analysis was performed to identify differences in methylation status between primary platinum-resistant patients Progression free survival (PFS) (PFS < 6 months, n = 8) and extreme sensitive patients (PFS ≥ 24 months, n = 8). The Qubit 3.0 Fluorometer was used for the quantification of RRBS library. The RRBS library was sequenced on Illumina HiSeq2500 sequencer as 50 bp paired-end reads. After screening, 94 valid hyper-/hypo-methylated regions were identified to be located within 94 gene promoter and exon regions (adjusted q ≤ 0.5), which were primarily associated with cell-cell adhesion, B cell activation and lymphocyte activation according to GO analysis. The 19 differentially methylated regions (DMR) located in the promoter region including TRC-GCA11-1, LOC105370912, ANO7P1, DHX4,MSH2, CDCP2, CCNL1, ARHGAP42P2, PRDM13, LOC101928344, USP29, ZIC5,IL1RAPL1, EVX2, ABR, MGRN1, UBALD1, LINC00261, and ISL2 were identified according to the order of P-values from low to high, of which MSH2, LINC00261, MGRN1, ZIC5, EVX2, CCNL1, and DHX40 were presented to play a variety of roles in cancers process based on the previous studies. DNA methylome profiling based on RRBS assay is an effective method for screening aberrantly methylated genes in primary platinum-resistant patients, which may serve as a potential epigenetic bio-marker for the prediction of primary platinum resistance.

Rapid Multiplexed Reduced Representation Bisulfite Sequencing Library Prep (rRRBS).

DNA methylation is a common mechanism of epigenetic regulation involved in transcriptional modulation and genome stability. With the evolution of next-generation sequencing technologies, establishing quantitative genome-wide DNA methylation profiles is becoming routine in many laboratories. However, many of these approaches take several days to accomplish and use subjective PCR methods to amplify sequencing libraries, which can induce amplification bias. Here we propose a rapid Reduced Representation Bisulfite Sequencing (rRRBS) protocol to minimize PCR amplification bias and reduce total time of multiplexed library construction. In this modified approach, the precise quantification of the final library amplification step is accomplished and monitored by qPCR, instead of using standard PCR and gel electrophoresis, to determine the appropriate number of cycles to perform. The main advantages of this rRRBS method are: i) Reduced amount of amplification enzyme used for library prep, ii) Reduced number of PCR cycles resulting in less PCR amplification bias, and iii) Preparation of quality multiplexed rRRBS libraries in only ~2 days.

Library Preparation for Multiplexed Reduced Representation Bisulfite Sequencing with a Universal Adapter.

Reduced representation bisulfite sequencing (RRBS) is an efficient approach for estimating cytosine methylation and hydroxymethylation at single nucleotide resolution with a focus on CpG sites located in CpG islands. Commonly used methods for multiplexing RRBS libraries involve a different indexed adapter, which is expensive to generate, for each library. Here, we describe a library preparation method that utilizes a universal adapter and labels samples with unique indexed PCR primers, significantly reducing the cost of multiplexed RRBS.

5 Gene expression analysis and DNA methylation patterns of porcine somatic cell nuclear transfer blastocysts with high and low incidence of apoptosis

Somatic cell NT (SCNT) efficiency remains poor, preventing the technology from being regularly used in the agricultural industry. It is believed that faulty epigenetic reprogramming of SCNT embryos leads to the low overall success. A clear apoptotic signature is associated with inappropriate gene expression and epigenomic aberrancies in many experimental cell culture systems, and we hypothesised that an apoptosis biomarker could be used to effectively separate properly reprogrammed porcine SCNT embryos from those that are destined to fail due to incomplete reprogramming. Therefore, our objective was to evaluate global gene expression and DNA methylation patterns in high- and low-apoptosis individual embryos in an effort to characterise the extent of genomic reprogramming that had taken place. Porcine SCNT blastocysts on Day 6 of development were stained with a nontoxic, noninvasive caspase activity reporter, and the top and bottom 20% of detected caspase activity were classified as high and low apoptosis, respectively (3 replicate cloning sessions; n=13 embryos per group). Genomic DNA and total RNA were isolated from each individual blastocyst. The RNA sequencing libraries were prepared using the Ovation SoLo RNA-Seq system (NuGen, San Carlos, CA, USA). Reduced representation bisulfite sequencing libraries were prepared for DNA methylation analysis using a modification of the single-cell reduced representation bisulfite sequencing global DNA methylation analysis approach detailed by Guo et al. (2015 Nat. Protoc. 10, 645-59). The RNA sequencing analysis using EdgeR (https://bioconductor.org/packages/release/bioc/html/edgeR.html) revealed 175 total differentially expressed genes (fold change ≥1.5; false discovery rate ≤0.05) between the high- and low-apoptosis SCNT embryos. This list of differentially expressed genes was used to perform enrichment analysis to identify overrepresented Gene Ontology (GO) terms or Kyoto Encyclopedia of Genes and Genomes pathways (DAVID Ease version 6.8 (https://david.ncifcrf.gov/) against the Sus scrofa background genome). However, no significantly enriched GO terms or pathways were identified (false discovery rate P&amp;gt;0.05). Analysis of global DNA methylation patterns between high- and low-apoptosis SCNT embryos using MethylKit (Akalin et al. 2012Genome Biol. 13, R87) revealed 335 differentially methylated 100-bp regions with at least 25% difference in methylation (adjusted P ≤ 0.01). Gene transcription start sites associated with these regions were used for enrichment analysis; again, no significant enrichment of GO terms or Kyoto Encyclopedia of Genes and Genomes pathways was identified. Principal component analysis of CpG methylation showed the low-apoptosis embryos clustering more tightly than the high-apoptosis embryos, which were highly scattered. Ongoing comparisons of high- and low-apoptosis cloned embryos with naturally fertilized embryos produced invivo may provide more information about which embryos were properly reprogrammed. Although we are still pursuing a link between reprogramming and gene expression in high- and low-apoptosis embryos, we conclude that these data support a model of stochastic epigenetic reprogramming following SCNT and reinforce the necessity of identifying embryos most likely to be successful due to proper epigenetic reprogramming in order to increase SCNT efficiency.

Reduced Representation Bisulfite Sequencing in Maize.

DNA methylation is an epigenetic modification that regulates plant development (Law and Jacobsen, 2010). Whole genome bisulfite sequencing (WGBS) is a state-of-the-art method for profiling genome-wide methylation patterns with single-base resolution ( Cokus et al., 2008 ). However, for an organism with a large genome, e.g., the 2.1 Gb genome of maize, WGBS may be very expensive. Reduced representation bisulfite sequencing (RRBS) has been developed in mammalian studies ( Smith et al., 2009 ). By digesting the genome with MspI with a size selection range of approximately 40-220 bp, CG-rich regions covering only ~1% of the human genome can be specifically sequenced. However, unlike mammalian genomes, plant genomes do not exhibit clear CpG islands. Therefore the original RRBS protocol is not suitable for plants. Accordingly, we developed an in silico pipeline to select specific enzymes to generate a region of interest (ROI)-enriched, e.g., promoter-enriched, reduced representation genome in plants ( Hsu et al., 2017 ). By digesting the maize genome with MseI and selecting 40-300 bp segments, we sequenced about one-fourth of the maize genome while preserving 84.3% of the promoter information. The protocol has been successfully established in maize and can be broadly used in any genome. Our in silico pipeline is combined with the RRBS library preparation protocol, allowing for the computational analysis and experimental validation.

A streamlined method for analysing genome-wide DNA methylation patterns from low amounts of FFPE DNA

BackgroundFormalin fixed paraffin embedded (FFPE) tumor samples are a major source of DNA from patients in cancer research. However, FFPE is a challenging material to work with due to macromolecular fragmentation and nucleic acid crosslinking. FFPE tissue particularly possesses challenges for methylation analysis and for preparing sequencing-based libraries relying on bisulfite conversion. Successful bisulfite conversion is a key requirement for sequencing-based methylation analysis.MethodsHere we describe a complete and streamlined workflow for preparing next generation sequencing libraries for methylation analysis from FFPE tissues. This includes, counting cells from FFPE blocks and extracting DNA from FFPE slides, testing bisulfite conversion efficiency with a polymerase chain reaction (PCR) based test, preparing reduced representation bisulfite sequencing libraries and massively parallel sequencing.ResultsThe main features and advantages of this protocol are:An optimized method for extracting good quality DNA from FFPE tissues.An efficient bisulfite conversion and next generation sequencing library preparation protocol that uses 50 ng DNA from FFPE tissue.Incorporation of a PCR-based test to assess bisulfite conversion efficiency prior to sequencing.ConclusionsWe provide a complete workflow and an integrated protocol for performing DNA methylation analysis at the genome-scale and we believe this will facilitate clinical epigenetic research that involves the use of FFPE tissue.

Rapid and affordable genome-wide bisulfite DNA sequencing by XmaI-reduced representation bisulfite sequencing.

To develop a reduced representation bisulfite sequencing (RRBS) approach for rapid and affordable genome-wide DNA methylation analysis. We have selected restriction endonuclease XmaI to produce RRBS library fragments. After digestion and partial fill-in DNA fragments were ligated to barcoded adapters, bisulfite converted, size-selected, and sequenced on the Ion Torrent Personal Genome Machine. XmaI-RRBS results were compared with the previously published RRBS data. We have developed an XmaI-RRBS method for rapid and affordable genome-wide DNA methylation analysis, with library preparation taking only 4 days and sequencing possible within 4 h. We have also addressed several challenges in order to further improve the RRBS technology. XmaI-RRBS may be performed on degraded DNA samples and is compatible with the bench-top next-generation sequencing machines.

Generating Multiple Base-Resolution DNA Methylomes Using Reduced Representation Bisulfite Sequencing.

Reduced representation bisulfite sequencing (RRBS) is an effective technique for profiling genome-wide DNA methylation patterns in eukaryotes. RRBS couples size selection, bisulfite conversion, and second-generation sequencing to enrich for CpG-dense regions of the genome. The progressive improvement of second-generation sequencing technologies and reduction in cost provided an opportunity to examine the DNA methylation patterns of multiple genomes. Here, we describe a protocol for sequencing multiple RRBS libraries in a single sequencing reaction to generate base-resolution methylomes. Furthermore, we provide a brief guideline for base-calling and data analysis of multiplexed RRBS libraries. These strategies will be useful to perform large-scale, genome-wide DNA methylation analysis.

Abstract P139: Stability of DNA Methylation Profiles in Human DNA Samples in Long Term Storage

Introductions: Historic DNA samples represent a potentially highly valuable resource for epigenetic analysis. Reduced representation bisulfite sequencing (RRBS) is a cost-effective, near genome-wide method for quantifying DNA methylation levels at single base resolution. It is unknown whether historic DNA samples stored for a long time (15-20 years) under various conditions could maintain stable methylation profiles as determined by RRBS. Methods: We used 5 groups of DNA samples (n=4 in each group) to compare and evaluate the stability of DNA methylation profiles under standard storage conditions of 4°C since 1996. Group 1 had been extracted from EDTA-anticoagulated blood and stored at 4°C since 1996; Group 2 had been diluted from group 1 to 10ng/μl and stored at -20°C since 2009; Group 3 were extracted from citrate-anticoagulated blood stored at -80°C since 1996; Group 4 were extracted from fresh EDTA-anticoagulated blood; Group 5 were extracted from fresh citrate-anticoagulated blood. The samples in groups 1, 2 and 3 were from the same subjects. All DNA samples were processed for RRBS libraries and then sequenced. Results: The global methylation level of group 1 was lower than group 2 (36.88±4.12% vs 43.66±1.29%, p=0.04), but was not statistically different from group 3 (44.31±1.46%, p=0.07). The methylation level of groups 4 and 5 (40.34±3.73% and 38.64±1.94%) was not significantly different. In addition, group 4 and 5 were not significantly different from group 1. The correlation of methylation levels for all CpG sites between individual samples in group 1 (0.987±0.001) was higher than that in group 2 and group 3 (0.984±0.002, p=0.01, and 0.969±0.015, p=0.01) and not significantly different from group 4 or 5. For CpG sites within CpG islands (CGI), the correlation between individual samples was not significantly different between any of the 5 groups. Moreover, the samples from the same subject show high correlation levels between groups 1, 2, and 3 for all CpG sites (0.987±0.006) and CpG sites within CGI (0.983±0.009). Conclusion: DNA samples stored at 4°C for prolonged periods of time are amenable to RRBS analysis. In addition, there were no significant differences in the DNA methylation profiles between citrate- and EDTA-anticoagulated blood.

Reduced representation bisulfite sequencing design for assessing the methylation of human CpG islands in large samples

The reduced representation bisulfite sequencing (RRBS) method has been developed for the high-throughput analysis of DNA methylation based on the sequencing of genomic libraries treated with sodium bisulfite by next-generation approaches. In contrast to whole-genome sequencing, the RRBS approach elaborates specific endonucleases to prepare libraries in order to produce pools of CpG-rich DNA fragments. The original RRBS technology based on the use of the MspI libraries allows one to increase the relative number of CpG islands in the pools of genomic fragments compared to whole-genome bisulfite sequencing. Nevertheless, this technology is rarely used due to the high cost compared with bisulfite methylation analysis with hybridization microarrays and significant residual amount of data represented by the sequences of genomic repeats that complicates the alignment and is not of particular interest for developing DNA methylation markers, which is often the main goal of biomedical research. We have developed an algorithm for estimating the likelihood that recognition sites of restriction endonucleases will be represented in CpG islands and present a method of reducing the effective size of the RRBS library without a significant loss of the CpG islands based on the use of the XmaI endonuclease for library preparation. In silico analysis demonstrates that the optimum range of the XmaI-RRBS fragment lengths is 110-200 base pairs. The sequencing of this library allows one to assess the methylation status of over 125000 CpG dinucleotides, of which over 90000 belong to CpG islands.

In silico analysis identifies novel restriction enzyme combinations that expand reduced representation bisulfite sequencing CpG coverage.

BackgroundEpigenetics is the study of gene expression changes that are not caused by changes in the deoxyribonucleic acid (DNA) sequence. DNA methylation is an epigenetic mark occurring in C–phosphate–G sites (CpGs) that leads to local or regional gene expression changes. Reduced-representation bisulfite sequencing (RRBS) is a technique that is used to ascertain the DNA methylation of millions of CpGs at single-nucleotide resolution. The genomic coverage of RRBS is given by the restriction enzyme combination used during the library preparation and the throughput capacity of the next-generation sequencer, which is used to read the generated libraries. The four-nucleotide cutters, MspI and TaqαI, are restriction enzymes commonly used in RRBS that, when combined, achieve ~12% genomic coverage. The increase in throughput of next-generation sequencers allows for novel combinations of restriction enzymes that provide higher CpG coverage.ResultsWe performed a near-neighbor analysis of the four nucleotide sequences most frequently found within 50 nt of all genomic CpGs. This resulted in the identification of seven methylation-insensitive restriction enzymes (AluI, BfaI, HaeIII, HpyCH4V, MluCI, MseI, and MspI) that shared similar restriction conditions suitable for RRBS library preparation. We report that the use of two or three enzyme combinations increases the theoretical epigenome coverage to almost half of the human genome.ConclusionsWe provide the enzyme combinations that are more likely to increase the CpG coverage in human, rat, and mouse genomes.Electronic supplementary materialThe online version of this article (doi:10.1186/1756-0500-7-534) contains supplementary material, which is available to authorized users.

Mapping the zebrafish brain methylome using reduced representation bisulfite sequencing

Reduced representation bisulfite sequencing (RRBS) has been used to profile DNA methylation patterns in mammalian genomes such as human, mouse and rat. The methylome of the zebrafish, an important animal model, has not yet been characterized at base-pair resolution using RRBS. Therefore, we evaluated the technique of RRBS in this model organism by generating four single-nucleotide resolution DNA methylomes of adult zebrafish brain. We performed several simulations to show the distribution of fragments and enrichment of CpGs in different in silico reduced representation genomes of zebrafish. Four RRBS brain libraries generated 98 million sequenced reads and had higher frequencies of multiple mapping than equivalent human RRBS libraries. The zebrafish methylome indicates there is higher global DNA methylation in the zebrafish genome compared with its equivalent human methylome. This observation was confirmed by RRBS of zebrafish liver. High coverage CpG dinucleotides are enriched in CpG island shores more than in the CpG island core. We found that 45% of the mapped CpGs reside in gene bodies, and 7% in gene promoters. This analysis provides a roadmap for generating reproducible base-pair level methylomes for zebrafish using RRBS and our results provide the first evidence that RRBS is a suitable technique for global methylation analysis in zebrafish.

Proliferation-Dependent Alterations of the DNA Methylation Landscape Underlie Hematopoietic Stem Cell Aging

The functional potential of hematopoietic stem cells (HSCs) declines during aging, and in doing so, significantly contributes to hematopoietic pathophysiology in the elderly. To explore the relationship between age-associated HSC decline and the epigenome, we examined global DNA methylation of HSCs during ontogeny in combination with functional analysis. Although the DNA methylome is generally stable during aging, site-specific alterations of DNA methylation occur at genomic regions associated with hematopoietic lineage potential and selectively target genes expressed in downstream progenitor and effector cells. We found that age-associated HSC decline, replicative limits, and DNA methylation are largely dependent on the proliferative history of HSCs, yet appear to be telomere-length independent. Physiological aging and experimentally enforced proliferation of HSCs both led to DNA hypermethylation of genes regulated by Polycomb Repressive Complex 2. Our results provide evidence that epigenomic alterations of the DNA methylation landscape contribute to the functional decline of HSCs during aging.