High-resolution nucleosome mapping of targeted regions using BAC-based enrichment

SATB1-mediated chromatin landscape in T cells

The regulatory circuits that define developmental decisions of thymocytes are still incompletely resolved. SATB1 protein is predominantly expressed at the CD4⁺CD8⁺cell stage exerting its broad transcription regulation potential with both activatory and repressive roles. A series of post-translational modifications and the presence of potential SATB1 protein isoforms indicate the complexity of its regulatory potential. The most apparent mechanism of its involvement in gene expression regulation is via the orchestration of long-range chromatin loops between genes and their regulatory elements. Multiple SATB1 perturbations in mice uncovered a link to autoimmune diseases while clinical investigations on cancer research uncovered that SATB1 has a promoting role in several types of cancer and can be used as a prognostic biomarker. SATB1 is a multivalent tissue-specific factor with a broad and yet undetermined regulatory potential. Future investigations on this protein could further uncover T cell-specific regulatory pathways and link them to (patho)physiology.

A versatile platform for locus-scale genome rewriting and verification

Routine rewriting of loci associated with human traits and diseases would facilitate their functional analysis. However, existing DNA integration approaches are limited in terms of scalability and portability across genomic loci and cellular contexts. We describe Big-IN, a versatile platform for targeted integration of large DNAs into mammalian cells. CRISPR/Cas9-mediated targeting of a landing pad enables subsequent recombinase-mediated delivery of variant payloads and efficient positive/negative selection for correct clones in mammalian stem cells. We demonstrate integration of constructs up to 143 kb, and an approach for one-step scarless delivery. We developed a staged pipeline combining PCR genotyping and targeted capture sequencing for economical and comprehensive verification of engineered stem cells. Our approach should enable combinatorial interrogation of genomic functional elements and systematic locus-scale analysis of genome function.

Runx proteins and transcriptional mechanisms that govern memory CD8 T cell development

Adaptive immunity to intracellular pathogens and tumors is mediated by antigen-experienced CD8 T cells. Individual naive CD8 T cells have the potential to differentiate into a diverse array of antigen-experienced subsets that exhibit distinct effector functions, life spans, anatomic positioning, and potential for regenerating an entirely new immune response during iterative pathogenic exposures. The developmental process by which activated naive cells undergo diversification involves regulation of chromatin structure and transcription but is not entirely understood. This review examines how alterations in chromatin structure, transcription factor binding, extracellular signals, and single-cell gene expression explain the differential development of distinct effector (T_EFF ) and memory (T_MEM ) CD8 T cell subsets. Special emphasis is placed on how Runx proteins function with additional transcription factors to pioneer changes in chromatin accessibility and drive transcriptional programs that establish the core attributes of cytotoxic T lymphocytes, subdivide circulating and non-circulating T_MEM cell subsets, and govern terminal differentiation. The discussion integrates the roles of specific cytokine signals, transcriptional circuits and how regulation of individual nucleosomes and RNA polymerase II activity can contribute to the process of differentiation. A model that integrates many of these features is discussed to conceptualize how activated CD8 T cells arrive at their fates.

Nucleosome Clutches are Regulated by Chromatin Internal Parameters

Nucleosomes cluster together when chromatin folds in the cell to form heterogeneous groups termed "clutches". These structural units add another level of chromatin regulation, for example during cell differentiation. Yet, the mechanisms that regulate their size and compaction remain obscure. Here, using our chromatin mesoscale model, we dissect clutch patterns in fibers with different combinations of nucleosome positions, linker histone density, and acetylation levels to investigate their role in clutch regulation. First, we isolate the effect of each chromatin parameter by studying systems with regular nucleosome spacing; second, we design systems with naturally-occurring linker lengths that fold onto specific clutch patterns; third, we model gene-encoding fibers to understand how these combined factors contribute to gene structure. Our results show how these chromatin parameters act together to produce different-sized nucleosome clutches. The length of nucleosome free regions (NFRs) profoundly affects clutch size, while the length of linker DNA has a moderate effect. In general, higher linker histone densities produce larger clutches by a chromatin compaction mechanism, while higher acetylation levels produce smaller clutches by a chromatin unfolding mechanism. We also show that it is possible to design fibers with naturally-occurring DNA linkers and NFRs that fold onto specific clutch patterns. Finally, in gene-encoding systems, a complex combination of variables dictates a gene-specific clutch pattern. Together, these results shed light into the mechanisms that regulate nucleosome clutches and suggest a new epigenetic mechanism by which chromatin parameters regulate transcriptional activity via the three-dimensional folded state of the genome at a nucleosome level.

Histone H2A.Z Suppression of Interferon-Stimulated Transcription and Antiviral Immunity Is Modulated by GCN5 and BRD2

Type I interferon (IFN)-stimulated gene (ISG) expression requires interaction between a transcription factor complex, ISGF3, and target gene promoters to initiate transcription and protection against infection. To uncover chromatin regulatory features of this antiviral immune response, IFN-induced nucleosome and histone dynamics of human ISG loci were examined. ISGF3 recruitment after IFN stimulation was accompanied by nucleosome reorganization at promoters and gene bodies. IFN stimulation induced loss of core histones H2B, H3, and H4, as well as H2A.Z at ISG promoters. A strong correlation was found between H2A.Z occupancy and ISGF3 target sites, and IFN-stimulated H2A.Z removal requires STAT1, STAT2, and IRF9. Neither INO80 nor SWI/SNF participate in IFN-driven H2A.Z eviction, but GCN5 and BRD2 are required. Interference with H2A.Z expression enhanced ISGF3 recruitment to ISG promoters, ISG mRNA expression, and IFN-stimulated antiviral immunity. This indicates that H2A.Z nucleosomes at ISG promoters restrict optimal ISGF3 engagement and modulate the biological response to IFN.

Comprehensive nucleosome mapping of the human genome in cancer progression

Altered chromatin structure is a hallmark of cancer, and inappropriate regulation of chromatin structure may represent the origin of transformation. Important studies have mapped human nucleosome distributions genome wide, but the role of chromatin structure in cancer progression has not been addressed. We developed a MNase-Transcription Start Site Sequence Capture method (mTSS-seq) to map the nucleosome distribution at human transcription start sites genome-wide in primary human lung and colon adenocarcinoma tissue. Here, we confirm that nucleosome redistribution is an early, widespread event in lung (LAC) and colon (CRC) adenocarcinoma. These altered nucleosome architectures are consistent between LAC and CRC patient samples indicating that they may serve as important early adenocarcinoma markers. We demonstrate that the nucleosome alterations are driven by the underlying DNA sequence and potentiate transcription factor binding. We conclude that DNA-directed nucleosome redistributions are widespread early in cancer progression. We have proposed an entirely new hierarchical model for chromatin-mediated genome regulation.

dCATCH-Seq: improved sequencing of large continuous genomic targets with double-hybridization

BACKGROUND

Targeted sequencing is a powerful tool with broad application in both basic and translational sciences. Relatively low on-target rates for most current targeted sequencing studies influence the required coverage and data quality for subsequent applications.

RESULTS

We present an improved targeted sequencing method that uses two rounds of in solution hybridization with probes synthesized from genomic clone templates, termed dCATCH-Seq. Independent captures of two large continuous genomic regions across three cell types within the human major histocompatibility complex (MHC) that spans ~3.5 Mb and a ~250 kb region on chromosome 11 demonstrated that dCATCH-Seq was highly reproducible with ~95% capture specificity. Comprehensive analyses of sequencing data generated using the dCATCH-Seq approach also showed high accuracy in the detection of genetic variants and HLA typing. The double hybridization capture approach can also be coupled with bisulfite sequencing for DNA methylation profiling of both CpG and non-CpG sites.

CONCLUSIONS

Altogether, dCATCH-Seq is a powerful and scalable targeted sequencing approach to investigate both genetic and epigenetic features.

A comparison of nucleosome organization in Drosophila cell lines

Changes in the distribution of nucleosomes along the genome influence chromatin structure and impact gene expression by modulating the accessibility of DNA to transcriptional machinery. However, the role of genome-wide nucleosome positioning in gene expression and in maintaining differentiated cell states remains poorly understood. Drosophila melanogaster cell lines represent distinct tissue types and exhibit cell-type specific gene expression profiles. They thus could provide a useful tool for investigating cell-type specific nucleosome organization of an organism's genome. To evaluate this possibility, we compared genome-wide nucleosome positioning and occupancy in five different Drosophila tissue-specific cell lines, and in reconstituted chromatin, and then tested for correlations between nucleosome positioning, transcription factor binding motifs, and gene expression. Nucleosomes in all cell lines were positioned in accordance with previously known DNA-nucleosome interactions, with helically repeating A/T di-nucleotide pairs arranged within nucleosomal DNAs and AT-rich pentamers generally excluded from nucleosomal DNA. Nucleosome organization in all cell lines differed markedly from in vitro reconstituted chromatin, with highly expressed genes showing strong nucleosome organization around transcriptional start sites. Importantly, comparative analysis identified genomic regions that exhibited cell line-specific nucleosome enrichment or depletion. Further analysis of these regions identified 91 out of 16,384 possible heptamer sequences that showed differential nucleosomal occupation between cell lines, and 49 of the heptamers matched one or more known transcription factor binding sites. These results demonstrate that there is differential nucleosome positioning between these Drosophila cell lines and therefore identify a system that could be used to investigate the functional significance of differential nucleosomal positioning in cell type specification.

Differential Nucleosome Occupancies across Oct4-Sox2 Binding Sites in Murine Embryonic Stem Cells

The binding sequence for any transcription factor can be found millions of times within a genome, yet only a small fraction of these sequences encode functional transcription factor binding sites. One of the reasons for this dichotomy is that many other factors, such as nucleosomes, compete for binding. To study how the competition between nucleosomes and transcription factors helps determine a functional transcription factor site from a predicted transcription factor site, we compared experimentally-generated in vitro nucleosome occupancy with in vivo nucleosome occupancy and transcription factor binding in murine embryonic stem cells. Using a solution hybridization enrichment technique, we generated a high-resolution nucleosome map from targeted regions of the genome containing predicted sites and functional sites of Oct4/Sox2 regulation. We found that at Pax6 and Nes, which are bivalently poised in stem cells, functional Oct4 and Sox2 sites show high amounts of in vivo nucleosome displacement compared to in vitro. Oct4 and Sox2, which are active, show no significant displacement of in vivo nucleosomes at functional sites, similar to nonfunctional Oct4/Sox2 binding. This study highlights a complex interplay between Oct4 and Sox2 transcription factors and nucleosomes among different target genes, which may result in distinct patterns of stem cell gene regulation.

Targeted sequencing of large genomic regions with CATCH-Seq

Current target enrichment systems for large-scale next-generation sequencing typically require synthetic oligonucleotides used as capture reagents to isolate sequences of interest. The majority of target enrichment reagents are focused on gene coding regions or promoters en masse. Here we introduce development of a customizable targeted capture system using biotinylated RNA probe baits transcribed from sheared bacterial artificial chromosome clone templates that enables capture of large, contiguous blocks of the genome for sequencing applications. This clone adapted template capture hybridization sequencing (CATCH-Seq) procedure can be used to capture both coding and non-coding regions of a gene, and resolve the boundaries of copy number variations within a genomic target site. Furthermore, libraries constructed with methylated adapters prior to solution hybridization also enable targeted bisulfite sequencing. We applied CATCH-Seq to diverse targets ranging in size from 125 kb to 3.5 Mb. Our approach provides a simple and cost effective alternative to other capture platforms because of template-based, enzymatic probe synthesis and the lack of oligonucleotide design costs. Given its similarity in procedure, CATCH-Seq can also be performed in parallel with commercial systems.

Poly-dA:dT tracts form an in vivo nucleosomal turnstile

Nucleosomes regulate many DNA-dependent processes by controlling the accessibility of DNA, and DNA sequences such as the poly-dA:dT element are known to affect nucleosome binding. We demonstrate that poly-dA:dT tracts form an asymmetric barrier to nucleosome movement in vivo, mediated by ATP-dependent chromatin remodelers. We theorize that nucleosome transit over poly-A elements is more energetically favourable in one direction, leading to an asymmetric arrangement of nucleosomes around these sequences. We demonstrate that different arrangements of poly-A and poly-T tracts result in very different outcomes for nucleosome occupancy in yeast, mouse, and human, and show that yeast takes advantage of this phenomenon in its promoter architecture.

Enhanced DNA repair of bleomycin-induced 3'-phosphoglycolate termini at the transcription start sites of actively transcribed genes in human cells

The anti-tumour agent, bleomycin, cleaves DNA to give 3'-phosphoglycolate and 5'-phosphate termini. The removal of 3'-phosphoglycolate to give 3'-OH ends is a very important step in the DNA repair of these lesions. In this study, next-generation DNA sequencing was utilised to investigate the repair of these 3'-phosphoglycolate termini at the transcription start sites (TSSs) of genes in HeLa cells. The 143,600 identified human TSSs in HeLa cells comprised 82,596 non-transcribed genes and 61,004 transcribed genes; and the transcribed genes were divided into quintiles of 12,201 genes comprising the top 20%, 20-40%, 40-60%, 60-80%, 80-100% of expressed genes. Repair of bleomycin-induced 3'-phosphoglycolate termini was enhanced at actively transcribed genes. The top 20% and 20-40% quintiles had a very similar level of enhanced repair, the 40-60% quintile was intermediate, while the 60-80% and 80-100% quintiles were close to the low level of enhancement found in non-transcribed genes. There were also interesting differences regarding bleomycin repair on the sense and antisense strands of DNA at TSSs. The sense strand had highly enhanced repair between 0 and 250bp relative to the TSS, while for the antisense strand highly enhanced repair was between 150 and 450bp. Repair of DNA damage is a major mechanism of resistance to anti-tumour drugs and this study provides an insight into this process in human tumour cells.

The anti-tumor drug bleomycin preferentially cleaves at the transcription start sites of actively transcribed genes in human cells

The genome-wide pattern of DNA cleavage at transcription start sites (TSSs) for the anti-tumor drug bleomycin was examined in human HeLa cells using next-generation DNA sequencing. It was found that actively transcribed genes were preferentially cleaved compared with non-transcribed genes. The 143,600 identified human TSSs were split into non-transcribed genes (82,596) and transcribed genes (61,004) for HeLa cells. These transcribed genes were further split into quintiles of 12,201 genes comprising the top 20, 20-40, 40-60, 60-80, and 80-100 % of expressed genes. The bleomycin cleavage pattern at highly transcribed gene TSSs was greatly enhanced compared with purified DNA and non-transcribed gene TSSs. The top 20 and 20-40 % quintiles had a very similar enhanced cleavage pattern, the 40-60 % quintile was intermediate, while the 60-80 and 80-100 % quintiles were close to the non-transcribed and purified DNA profiles. The pattern of bleomycin enhanced cleavage had peaks that were approximately 200 bp apart, and this indicated that bleomycin was identifying the presence of phased nucleosomes at TSSs. Hence bleomycin can be utilized to detect chromatin structures that are present at actively transcribed genes. In this study, for the first time, the pattern of DNA damage by a clinically utilized cancer chemotherapeutic agent was performed on a human genome-wide scale at the nucleotide level.

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.

Multiplexed direct genomic selection (MDiGS): a pooled BAC capture approach for highly accurate CNV and SNP/INDEL detection

Despite declining sequencing costs, few methods are available for cost-effective single-nucleotide polymorphism (SNP), insertion/deletion (INDEL) and copy number variation (CNV) discovery in a single assay. Commercially available methods require a high investment to a specific region and are only cost-effective for large samples. Here, we introduce a novel, flexible approach for multiplexed targeted sequencing and CNV analysis of large genomic regions called multiplexed direct genomic selection (MDiGS). MDiGS combines biotinylated bacterial artificial chromosome (BAC) capture and multiplexed pooled capture for SNP/INDEL and CNV detection of 96 multiplexed samples on a single MiSeq run. MDiGS is advantageous over other methods for CNV detection because pooled sample capture and hybridization to large contiguous BAC baits reduces sample and probe hybridization variability inherent in other methods. We performed MDiGS capture for three chromosomal regions consisting of ∼550 kb of coding and non-coding sequence with DNA from 253 patients with congenital lower limb disorders. PITX1 nonsense and HOXC11 S191F missense mutations were identified that segregate in clubfoot families. Using a novel pooled-capture reference strategy, we identified recurrent chromosome chr17q23.1q23.2 duplications and small HOXC 5′ cluster deletions (51 kb and 12 kb). Given the current interest in coding and non-coding variants in human disease, MDiGS fulfills a niche for comprehensive and low-cost evaluation of CNVs, coding, and non-coding variants across candidate regions of interest.