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1.  High-Density Nucleosome Occupancy Map of Human Chromosome 9p21–22 Reveals Chromatin Organization of the Type I Interferon Gene Cluster 
Genome-wide investigations have dramatically increased our understanding of nucleosome positioning and the role of chromatin in gene regulation, yet some genomic regions have been poorly represented in human nucleosome maps. One such region is represented by human chromosome 9p21–22, which contains the type I interferon gene cluster that includes 16 interferon alpha genes and the single interferon beta, interferon epsilon, and interferon omega genes. A high-density nucleosome mapping strategy was used to generate locus-wide maps of the nucleosome organization of this biomedically important locus at a steady state and during a time course of infection with Sendai virus, an inducer of interferon gene expression. Detailed statistical and computational analysis illustrates that nucleosomes in this locus exhibit preferences for particular dinucleotide and oligomer DNA sequence motifs in vivo, which are similar to those reported for lower eukaryotic nucleosome–DNA interactions. These data were used to visualize the region's chromatin architecture and reveal features that are common to the organization of all the type I interferon genes, indicating a common nucleosome-mediated gene regulatory paradigm. Additionally, this study clarifies aspects of the dynamic changes that occur with the nucleosome occupying the transcriptional start site of the interferon beta gene after virus infection.
PMCID: PMC4150395  PMID: 24673249
2.  Genome and metagenome sequencing: Using the human methyl-binding domain to partition genomic DNA derived from plant tissues1 
Applications in Plant Sciences  2014;2(11):apps.1400064.
• Premise of the study: Variation in the distribution of methylated CpG (methyl-CpG) in genomic DNA (gDNA) across the tree of life is biologically interesting and useful in genomic studies. We illustrate the use of human methyl-CpG-binding domain (MBD2) to fractionate angiosperm DNA into eukaryotic nuclear (methyl-CpG-rich) vs. organellar and prokaryotic (methyl-CpG-poor) elements for genomic and metagenomic sequencing projects.
• Methods: MBD2 has been used to enrich prokaryotic DNA in animal systems. Using gDNA from five model angiosperm species, we apply a similar approach to identify whether MBD2 can fractionate plant gDNA into methyl-CpG-depleted vs. enriched methyl-CpG elements. For each sample, three gDNA libraries were sequenced: (1) untreated gDNA, (2) a methyl-CpG-depleted fraction, and (3) a methyl-CpG-enriched fraction.
• Results: Relative to untreated gDNA, the methyl-depleted libraries showed a 3.2–11.2-fold and 3.4–11.3-fold increase in chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA), respectively. Methyl-enriched fractions showed a 1.8–31.3-fold and 1.3–29.0-fold decrease in cpDNA and mtDNA, respectively.
• Discussion: The application of MBD2 enabled fractionation of plant gDNA. The effectiveness was particularly striking for monocot gDNA (Poaceae). When sufficiently effective on a sample, this approach can increase the cost efficiency of sequencing plant genomes as well as prokaryotes living in or on plant tissues.
PMCID: PMC4222543  PMID: 25383266
endophyte; enrichment; genome sequencing; metagenome; microbiome; methyl-binding domain
3.  BA Fast Highly Multiplexed Solution to NGS Library Prep with Low Nanogram DNA Input 
As the quantity of data generated per next generation sequencing (NGS) run increases and the time required per run decreases, the ability to quickly produce and track large numbers of libraries is becoming increasingly important. In addition, the ability to produce high quality libraries from limited starting material and multiple sample types, including FFPE is essential. To overcome these challenges and to minimize the bottleneck of NGS library prep, we have developed a fast, streamlined DNA library preparation method using novel reagents and adaptors. This method accommodates a wide range of sample input quantities and types including genomic, ChIP and fragmented DNA (e.g. FFPE). Data analysis of libraries constructed from as little as 250 pg of ChIP DNA show high complexity and significant overlap of target peaks with libraries made from10 ng of DNA. We have extended the utility of this library prep method by developing additional adaptor and primer reagents. These include a dual barcoding approach that is compatible with Illumina library prep and our novel NEBNext adaptor. This approach enables multiplexing of up to 96 different samples, which can be used to increase the number of samples per flow cell, and/or to identify specific samples/libraries in a lab. Together, the simple, streamlined workflow and dual barcode approach, significantly reduces the turn-around time, enabling high throughput processing of samples for clinical analysis and large scale genomics studies.
PMCID: PMC4162240
4.  A Method for Selectively Enriching Microbial DNA from Contaminating Vertebrate Host DNA 
PLoS ONE  2013;8(10):e76096.
DNA samples derived from vertebrate skin, bodily cavities and body fluids contain both host and microbial DNA; the latter often present as a minor component. Consequently, DNA sequencing of a microbiome sample frequently yields reads originating from the microbe(s) of interest, but with a vast excess of host genome-derived reads. In this study, we used a methyl-CpG binding domain (MBD) to separate methylated host DNA from microbial DNA based on differences in CpG methylation density. MBD fused to the Fc region of a human antibody (MBD-Fc) binds strongly to protein A paramagnetic beads, forming an effective one-step enrichment complex that was used to remove human or fish host DNA from bacterial and protistan DNA for subsequent sequencing and analysis. We report enrichment of DNA samples from human saliva, human blood, a mock malaria-infected blood sample and a black molly fish. When reads were mapped to reference genomes, sequence reads aligning to host genomes decreased 50-fold, while bacterial and Plasmodium DNA sequences reads increased 8–11.5-fold. The Shannon-Wiener diversity index was calculated for 149 bacterial species in saliva before and after enrichment. Unenriched saliva had an index of 4.72, while the enriched sample had an index of 4.80. The similarity of these indices demonstrates that bacterial species diversity and relative phylotype abundance remain conserved in enriched samples. Enrichment using the MBD-Fc method holds promise for targeted microbiome sequence analysis across a broad range of sample types.
PMCID: PMC3810253  PMID: 24204593
5.  Discovering the RNA Transcription Landscape using Directional Approaches 
High-throughput complementary DNA sequencing (RNA-Seq) is a powerful technique that allows for sensitive digital quantification of transcript levels. Moreover, RNA-Seq enables the detection of non-canonical transcription start sites and termination sites, alternative splice isoforms and transcript mutation and edition. Standard “next-generation” RNA-sequencing approaches generally require double-stranded cDNA synthesis, which erases RNA strand information. In this approach, the synthesis of randomly primed double-stranded cDNA followed by addition of adaptors for sequencing leads to the loss of information about which strand was present in the original mRNA template. The polarity of the transcript is important for correct annotation of novel genes, identification of antisense transcripts with potential regulatory roles, and for correct determination of gene expression levels in the presence of antisense transcripts. Our objective was to address this need by developing a novel streamlined, low input method for Directional RNA-Sequencing that highly retains strand orientation information while maintaining even coverage of transcript expression. This method is based on second strand labeling and excision after adaptor ligation; allowing differential tagging of the first strand cDNA ends. As a result, we have enabled strand specific mRNA sequencing, as well as whole transcriptome sequencing (Total RNA-Seq) from ribosomal-depleted samples. Total RNA-Seq provides a much broader picture of expression dynamics including discovery of antisense transcripts. This work presents a streamlined, fast solution for complete RNA sequencing, with high quality data that illustrates the complexity and diversity of the RNA transcription landscape.
PMCID: PMC3635307
6.  A Fast Solution to NGS Library Prep with Low Nanogram DNA Input 
Next Generation Sequencing (NGS) has significantly impacted human genetics, enabling a comprehensive characterization of the human genome as well as a better understanding of many genomic abnormalities. By delivering massive DNA sequences at unprecedented speed and cost, NGS promises to make personalized medicine a reality in the foreseeable future. To date, library construction with clinical samples has been a challenge, primarily due to the limited quantities of sample DNA available. Our objective here was to overcome this challenge by developing NEBNext® Ultra DNA Library Prep Kit, a fast library preparation method. Specifically, we streamlined the workflow utilizing novel NEBNext reagents and adaptors, including a new DNA polymerase that has been optimized to minimize GC bias. As a result of this work, we have developed a simple method for library construction from an amount of DNA as low as 5 ng, which can be used for both intact and fragmented DNA. Moreover, the workflow is compatible with multiple NGS platforms.
PMCID: PMC3635320
7.  High-resolution nucleosome mapping of targeted regions using BAC-based enrichment 
Nucleic Acids Research  2013;41(7):e87.
We report a target enrichment method to map nucleosomes of large genomes at unprecedented coverage and resolution by deeply sequencing locus-specific mononucleosomal DNA enriched via hybridization with bacterial artificial chromosomes. We achieved ∼10 000-fold enrichment of specific loci, which enabled sequencing nucleosomes at up to ∼500-fold higher coverage than has been reported in a mammalian genome. We demonstrate the advantages of generating high-sequencing coverage for mapping the center of discrete nucleosomes, and we show the use of the method by mapping nucleosomes during T cell differentiation using nuclei from effector T-cells differentiated from clonal, isogenic, naïve, primary murine CD4 and CD8 T lymphocytes. The analysis reveals that discrete nucleosomes exhibit cell type-specific occupancy and positioning depending on differentiation status and transcription. This method is widely applicable to mapping many features of chromatin and discerning its landscape in large genomes at unprecedented resolution.
PMCID: PMC3627574  PMID: 23413004
8.  Nucleosome mapping across the CFTR locus identifies novel regulatory factors 
Nucleic Acids Research  2013;41(5):2857-2868.
Nucleosome positioning on the chromatin strand plays a critical role in regulating accessibility of DNA to transcription factors and chromatin modifying enzymes. Hence, detailed information on nucleosome depletion or movement at cis-acting regulatory elements has the potential to identify predicted binding sites for trans-acting factors. Using a novel method based on enrichment of mononucleosomal DNA by bacterial artificial chromosome hybridization, we mapped nucleosome positions by deep sequencing across 250 kb, encompassing the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR shows tight tissue-specific regulation of expression, which is largely determined by cis-regulatory elements that lie outside the gene promoter. Although multiple elements are known, the repertoire of transcription factors that interact with these sites to activate or repress CFTR expression remains incomplete. Here, we show that specific nucleosome depletion corresponds to well-characterized binding sites for known trans-acting factors, including hepatocyte nuclear factor 1, Forkhead box A1 and CCCTC-binding factor. Moreover, the cell-type selective nucleosome positioning is effective in predicting binding sites for novel interacting factors, such as BAF155. Finally, we identify transcription factor binding sites that are overrepresented in regions where nucleosomes are depleted in a cell-specific manner. This approach recognizes the glucocorticoid receptor as a novel trans-acting factor that regulates CFTR expression in vivo.
PMCID: PMC3597660  PMID: 23325854
11.  An RIG-I-Like RNA Helicase Mediates Antiviral RNAi Downstream of Viral siRNA Biogenesis in Caenorhabditis elegans 
PLoS Pathogens  2009;5(2):e1000286.
Dicer ribonucleases of plants and invertebrate animals including Caenorhabditis elegans recognize and process a viral RNA trigger into virus-derived small interfering RNAs (siRNAs) to guide specific viral immunity by Argonaute-dependent RNA interference (RNAi). C. elegans also encodes three Dicer-related helicase (drh) genes closely related to the RIG-I-like RNA helicase receptors which initiate broad-spectrum innate immunity against RNA viruses in mammals. Here we developed a transgenic C. elegans strain that expressed intense green fluorescence from a chromosomally integrated flock house virus replicon only after knockdown or knockout of a gene required for antiviral RNAi. Use of the reporter nematode strain in a feeding RNAi screen identified drh-1 as an essential component of the antiviral RNAi pathway. However, RNAi induced by either exogenous dsRNA or the viral replicon was enhanced in drh-2 mutant nematodes, whereas exogenous RNAi was essentially unaltered in drh-1 mutant nematodes, indicating that exogenous and antiviral RNAi pathways are genetically distinct. Genetic epistatic analysis shows that drh-1 acts downstream of virus sensing and viral siRNA biogenesis to mediate specific antiviral RNAi. Notably, we found that two members of the substantially expanded subfamily of Argonautes specific to C. elegans control parallel antiviral RNAi pathways. These findings demonstrate both conserved and unique strategies of C. elegans in antiviral defense.
Author Summary
The genome of Caenorhabditis elegans encodes three Dicer-related helicases (DRHs) highly homologous to the DExD/H box helicase domain found in two distinct families of virus sensors, Dicer ribonucleases and RIG-I-like helicases (RLRs). Dicer initiates the specific, RNAi-mediated viral immunity in plants, fungi and invertebrates by producing virus-derived small interfering RNAs (siRNAs). By contrast, mammalian RLRs trigger interferon production and broad-spectrum viral immunity, although one of the three RLRs may act as both a negative and positive regulator of viral immunity. In this study we developed a transgenic C. elegans strain for high-throughput genetic screens and identified 35 genes including drh-1 that are required for RNAi-mediated viral immunity. Genetic epistatic analyses demonstrate that drh-1 mediates RNAi immunity downstream of the production of viral siRNAs. Notably, we found that drh-2 functions as a negative regulator of the viral immunity. Thus, both nematode DRHs and mammalian RLRs participate in antiviral immune responses. Unlike mammalian RLRs, however, nematode DRH-1 employs an RNAi effector mechanism and is unlikely to be involved in direct virus sensing.
PMCID: PMC2629121  PMID: 19197349

Results 1-11 (11)