Open Chromatin Domains Research Articles

Global Mapping of Open Chromatin Regulatory Elements by Formaldehyde-Assisted Isolation of Regulatory Elements Followed by Sequencing (FAIRE-seq).

Genetic information is organized in a complex structure composed of DNA and proteins together designated chromatin. Chromatin plays a dynamic role in transcriptional processes in that alteration of the interaction between its components results in the deregulation of cellular transcriptional program. Modification of epigenetic marks, variation in the precise positioning of nucleosomes, and consequent mobilization of nucleosomes regulate the access of various transcriptional factors to its underlying DNA template. Nucleosome-depleted regions, also designated open chromatin domains, are associated with active DNA regulatory elements, including promoters, enhancers, silencers, and insulators. Here, we describe the protocol of a rapid and simple technique entitled FAIRE (formaldehyde-assisted isolation of regulatory elements). Combined with high-throughput sequencing (FAIRE-seq), this procedure allows isolation of nucleosome-free regions and their mapping along the genome, thereby providing a global view of cell-specific regulatory elements.

Dissection of open chromatin domain formation by site-specific recombination in Drosophila

Drosophila polytene interphase chromosomes provide an ideal test system to study the rules that define the structure of chromatin domains. We established a transgenic condensed chromatin domain cassette for the insertion of large pieces of DNA by site specific recombination. Insertion of this cassette into open chromatin generated a condensed domain, visible as an extra band on polytene chromosomes. Site specific recombination of DNA sequence variants into this ectopic band allowed us to compare their capacity for open chromatin formation by cytogenetic methods. We demonstrate that the 61C7-8 interband DNA maintains its open chromatin conformation and epigenetic state at an ectopic position. By deletion analysis we mapped the sequences essential for open chromatin formation to a 490 bp fragment in the proximal part of the 17 kb interband sequence. This fragment overlaps binding sites of the chromatin protein Chriz, the histone kinase Jil-1 and the boundary element protein CP190. It also overlaps a promoter region that locates in between the Rev1 and Med30 transcription units.

DNA double-strand breaks promote methylation of histone H3 on lysine 9 and transient formation of repressive chromatin.

Dynamic changes in histone modification are critical for regulating DNA double-strand break (DSB) repair. Activation of the Tip60 acetyltransferase by DSBs requires interaction of Tip60 with histone H3 methylated on lysine 9 (H3K9me3). However, how H3K9 methylation is regulated during DSB repair is not known. Here, we demonstrate that a complex containing kap-1, HP1, and the H3K9 methyltransferase suv39h1 is rapidly loaded onto the chromatin at DSBs. Suv39h1 methylates H3K9, facilitating loading of additional kap-1/HP1/suv39h1 through binding of HP1's chromodomain to the nascent H3K9me3. This process initiates cycles of kap-1/HP1/suv39h1 loading and H3K9 methylation that facilitate spreading of H3K9me3 and kap-1/HP1/suv39h1 complexes for tens of kilobases away from the DSB. These domains of H3K9me3 function to activate the Tip60 acetyltransferase, allowing Tip60 to acetylate both ataxia telangiectasia-mutated (ATM) kinase and histone H4. Consequently, cells lacking suv39h1 display defective activation of Tip60 and ATM, decreased DSB repair, and increased radiosensitivity. Importantly, activated ATM rapidly phosphorylates kap-1, leading to release of the repressive kap-1/HP1/suv39h1 complex from the chromatin. ATM activation therefore functions as a negative feedback loop to remove repressive suv39h1 complexes at DSBs, which may limit DSB repair. Recruitment of kap-1/HP1/suv39h1 to DSBs therefore provides a mechanism for transiently increasing the levels of H3K9me3 in open chromatin domains that lack H3K9me3 and thereby promoting efficient activation of Tip60 and ATM in these regions. Further, transient formation of repressive chromatin may be critical for stabilizing the damaged chromatin and for remodeling the chromatin to create an efficient template for the DNA repair machinery.

P50-associated COX-2 extragenic RNA (PACER) activates COX-2 gene expression by occluding repressive NF-κB complexes.

Deregulated expression of COX-2 has been causally linked to development, progression, and outcome of several types of human cancer. We describe a novel fundamental level of transcriptional control of COX-2 expression. Using primary human mammary epithelial cells and monocyte/macrophage cell lines, we show that the chromatin boundary/insulator factor CTCF establishes an open chromatin domain and induces expression of a long non-coding RNA within the upstream promoter region of COX-2. Upon induction of COX-2 expression, the lncRNA associates with p50, a repressive subunit of NF-κB, and occludes it from the COX-2 promoter, potentially facilitating interaction with activation-competent NF-κB p65/p50 dimers. This enables recruitment of the p300 histone acetyltransferase, a domain-wide increase in histone acetylation and assembly of RNA Polymerase II initiation complexes. Our findings reveal an unexpected mechanism of gene control by lncRNA-mediated repressor occlusion and identify the COX-2-lncRNA, PACER, as a new potential target for COX-2-modulation in inflammation and cancer.DOI: http://dx.doi.org/10.7554/eLife.01776.001.

Dissection of open chromatin domain formation by site specific recombination inDrosophila

Drosophila polytene interphase chromosomes provide an ideal test system to study the rules that define the structure of chromatin domains. We established a transgenic condensed chromatin domain cassette for the insertion of large pieces of DNA by site-specific recombination. Insertion of this cassette into open chromatin generated a condensed domain, visible as an extra band on polytene chromosomes. Site-specific recombination of DNA sequence variants into this ectopic band allowed us to compare their capacity for open chromatin formation by cytogenetic methods. We demonstrate that the 61C7-8 interband DNA maintains its open chromatin conformation and epigenetic state at an ectopic position. By deletion analysis, we mapped the sequences essential for open chromatin formation to a 490-bp fragment in the proximal part of the 17-kb interband sequence. This fragment overlaps binding sites for the chromatin protein Chriz (also known as Chro), the histone kinase Jil-1 and the boundary element protein CP190. It also overlaps a promoter region that locates between the Rev1 and Med30 transcription units.

Distinct Distribution of Ectopically Expressed Histone Variants H2A.Bbd and MacroH2A in Open and Closed Chromatin Domains

BackgroundIt becomes increasingly evident that nuclesomes are far from being identical to each other. This nucleosome diversity is due partially to the existence of histone variants encoded by separate genes. Among the known histone variants the less characterized are H2A.Bbd and different forms of macroH2A. This is especially true in the case of H2A.Bbd as there are still no commercially available antibodies specific to H2A.Bbd that can be used for chromatin immunoprecipitation (ChIP).MethodsWe have generated HeLa S3 cell lines stably expressing epitope-tagged versions of macroH2A1.1, H2A.Bbd or canonical H2A and analyzed genomic distribution of the tagged histones using ChIP-on-chip technique.ResultsThe presence of histone H2A variants macroH2A1.1 and H2A.Bbd has been analyzed in the chromatin of several segments of human chromosomes 11, 16 and X that have been chosen for their different gene densities and chromatin status. Chromatin immunoprecipitation (ChIP) followed by hybridization with custom NimbleGene genomic microarrays demonstrated that in open chromatin domains containing tissue-specific along with housekeeping genes, the H2A.Bbd variant was preferentially associated with the body of a subset of transcribed genes. The macroH2A1.1 variant was virtually absent from some genes and underrepresented in others. In contrast, in closed chromatin domains which contain only tissue-specific genes inactive in HeLa S3 cells, both macroH2A1.1 and H2A.Bbd histone variants were present and often colocalized.ConclusionsGenomic distribution of macro H2A and H2A.Bbd does not follow any simple rule and is drastically different in open and closed genomic domains.

Df31 Protein and snoRNAs Maintain Accessible Higher-Order Structures of Chromatin

Packaging of DNA into nucleosomes and the formation of higher-order chromatin structures determine DNA accessibility and activity of genome domains. We identified an RNA-dependent mechanism maintaining the open chromatin structure within euchromatic regions in Drosophila cells. The mechanism of reversible chromatin opening, reconstituted in vitro, depends on the Drosophila decondensation factor 31 (Df31) that specifically binds to RNA and localizes to euchromatic regions. Df31 is capable to tether a heterogeneous pool of short, single-stranded RNAs to chromatin. This class of chromatin-associated RNA (caRNA) is stably linked to chromatin and is largely composed of snoRNAs, which are preferentially bound by Df31. We suggest that the Df31-mediated linkage of snoRNAs and chromatin, forms a RNA-chromatin network resulting in the establishment of open chromatin domains. Analysis of caRNAs in human cells also reveals a strong enrichment of snoRNAs, implying a conserved role for these molecules in higher-order structures of chromatin.

The GATA1-HS2 Enhancer Allows Persistent and Position-Independent Expression of a β-globin Transgene

Gene therapy of genetic diseases requires persistent and position-independent expression of a therapeutic transgene. Transcriptional enhancers binding chromatin-remodeling and modifying complexes may play a role in shielding transgenes from repressive chromatin effects. We tested the activity of the HS2 enhancer of the GATA1 gene in protecting the expression of a β-globin minigene delivered by a lentiviral vector in hematopoietic stem/progenitor cells. Gene expression from proviruses carrying GATA1-HS2 in both LTRs was persistent and resistant to silencing at most integration sites in the in vivo progeny of human hematopoietic progenitors and murine long-term repopulating stem cells. The GATA1-HS2-modified vector allowed correction of murine β-thalassemia at low copy number without inducing clonal selection of erythroblastic progenitors. Chromatin immunoprecipitation studies showed that GATA1 and the CBP acetyltransferase bind to GATA1-HS2, significantly increasing CBP-specific histone acetylations at the LTRs and β-globin promoter. Recruitment of CBP by the LTRs thus establishes an open chromatin domain encompassing the entire provirus, and increases the therapeutic efficacy of β-globin gene transfer by reducing expression variegation and epigenetic silencing.

The human insulin gene is part of a large open chromatin domain specific for human islets

Knowledge of how insulin (INS) gene expression is regulated will lead to better understanding of normal and abnormal pancreatic beta cell function. We have mapped histone modifications over the INS region, coupled with an expression profile, in freshly isolated islets from multiple human donors. Unlike many other human genes, in which active modifications tend to be concentrated within 1 kb around the transcription start site, these marks are distributed over the entire coding region of INS as well. Moreover, a region of approximately 80 kb around the INS gene, which contains the {tyrosine hydroxylase (TH)-(INS)-insulin-like growth factor 2 antisense (IGF2AS)-insulin-like growth factor 2 (IGF2)} gene cluster, unusually is marked by almost uniformly elevated levels of histone acetylation and H3K4 dimethylation, extending both downstream into IGF2 and upstream beyond the TH gene. This is accompanied by islet specific coordinate expression with INS of the neighboring TH and IGF2 genes. The presence of islet specific intergenic transcripts suggests their possible function in the maintenance of this unusual large open chromatin domain.

Topoisomerase IIα-dependent induction of a persistent DNA damage response in response to transient etoposide exposure

Cytotoxicity of the topoisomerase II (topoII) poison etoposide has been ascribed to the persistent covalent trapping of topoII in DNA cleavage complexes that become lethal as cells replicate their DNA. However, short term etoposide treatment also leads to subsequent cell death, suggesting that the lesions that lead to cytotoxicity arise rapidly and prior to the onset DNA replication. In the present study 1 h treatment with 25 μM etoposide was highly toxic and initiated a double-stranded DNA damage response as reflected by the recruitment of ATM, MDC1 and DNA-PKcs to γH2AX foci. While most DNA breaks were rapidly repaired upon withdrawal of the etoposide treatment, the repair machinery remained engaged in foci for at least 24 h following withdrawal. TopoII siRNA ablation showed the etoposide toxicity and γH2AX response to correlate with the inability of the cell to correct topoIIα-initiated DNA damage. γH2AX induction was resistant to the inhibition of DNA replication and transcription, but was increased by pre-treatment with the histone deacetylase inhibitor trichostatin A. These results link the lethality of etoposide to the generation of persistent topoIIα-dependent DNA defects within topologically open chromatin domains.

Lactogenic Hormonal Induction of Long Distance Interactions between β-Casein Gene Regulatory Elements

Lactogenic hormone regulation of beta-casein gene expression in mammary epithelial cells provides an excellent model in which to study the mechanisms by which steroid and peptide hormone signaling control gene expression. Prolactin- and glucocorticoid-mediated induction of beta-casein gene expression involves two principal regulatory regions, a proximal promoter and a distal enhancer located in the mouse approximately -6 kb upstream of the transcription start site. Using a chromosome conformation capture assay and quantitative real time PCR, we demonstrate that a chromatin loop is created in conjunction with the recruitment of specific transcription factors and p300 in HC11 mammary epithelial cells. Stimulation with both prolactin and hydrocortisone is required for the induction of these long range interactions between the promoter and enhancer, and no DNA looping was observed in nontreated cells or cells treated with each of the hormones separately. The lactogenic hormone-induced interaction between the proximal promoter and distal enhancer was confirmed in hormone-treated primary three-dimensional mammary acini cultures. In addition, the developmental regulation of DNA looping between the beta-casein regulatory regions was observed in lactating but not in virgin mouse mammary glands. Furthermore, beta-casein mRNA induction and long range interactions between these regulatory regions were inhibited in a progestin-dependent manner following stimulation with prolactin and hydrocortisone in HC11 cells expressing human PR-B. Collectively, these data suggest that the communication between these regulatory regions with intervening DNA looping is a crucial step required to both create and maintain active chromatin domains and regulate transcription.

Targeted Deletion of Multiple CTCF-Binding Elements in the Human C-MYC Gene Reveals a Requirement for CTCF in C-MYC Expression

BackgroundInsulators and domain boundaries both shield genes from adjacent enhancers and inhibit intrusion of heterochromatin into transgenes. Previous studies examined the functional mechanism of the MYC insulator element MINE and its CTCF binding sites in the context of transgenes that were randomly inserted into the genome by transfection. However, the contribution of CTCF binding sites to both gene regulation and maintenance of chromatin has not been tested at the endogenous MYC gene.Methodology/Principal FindingsTo determine the impact of CTCF binding on MYC expression, a series of mutant human chromosomal alleles was prepared in homologous recombination-efficient DT40 cells and individually transferred by microcell fusion into murine cells. Functional tests reported here reveal that deletion of CTCF binding elements within the MINE does not impact the capacity of this locus to correctly organize an ‘accessible’ open chromatin domain, suggesting that these sites are not essential for the formation of a competent, transcriptionally active locus. Moreover, deletion of the CTCF site at the MYC P2 promoter reduces transcription but does not affect promoter acetylation or serum-inducible transcription. Importantly, removal of either CTCF site leads to DNA methylation of flanking sequences, thereby contributing to progressive loss of transcriptional activity.ConclusionsThese findings collectively demonstrate that CTCF-binding at the human MYC locus does not repress transcriptional activity but is required for protection from DNA methylation.

The impact of nucleosome positioning on the organization of replication origins in eukaryotes

The distribution of DNA replication origins (ORIs) on eukaryotic chromosomes is nonrandom, but the reasons behind this are not well understood. Previous studies have suggested a prominent role of transcriptional activity in determining the ORI organization. Here, we identify nucleosome occupancy as a likely candidate to set up ORI distribution. Combining genome-wide data on nucleosome positioning and ORI organization in yeast and humans, we demonstrate that open chromatin domains, characterized by nucleosome depletion, are preferentially permissive for replication. However, contrary to priori claims, the impact of transcriptional activity is considerably weaker than previously proposed and could partly be explained by our nucleosome exclusion model. We propose that the ORI organization imposed by nucleosome positioning is phylogenetically widespread in eukaryotes.

Developmental programming of CpG island methylation profiles in the human genome

CpG island-like sequences are commonly thought to provide the sole signals for designating constitutively unmethylated regions in the genome, thus generating open chromatin domains within a sea of global repression. Using a new database obtained from comprehensive microarray analysis, we show that unmethylated regions (UMRs) seem to be formed during early embryogenesis, not as a result of CpG-ness, but rather through the recognition of specific sequence motifs closely associated with transcription start sites. This same system probably brings about the resetting of pluripotency genes during somatic cell reprogramming. The data also reveal a new class of nonpromoter UMRs that become de novo methylated in a tissue-specific manner during development, and this process may be involved in gene regulation. In short, we show that UMRs are an important aspect of genome structure that have a dynamic role in development.

Early replication timing of the chicken α-globin gene domain correlates with its open chromatin state in cells of different lineages

The vertebrate α-globin gene domain is an open chromatin domain overlapping a neighboring house-keeping gene. The tissue-specific cluster of α-globin genes and the overlapping housekeeping gene share the same replication origin. We have studied the replication timing of chicken α-globin genes in cells of different lineages using the FISH-based approach and found that α-globin genes replicate early both in erythroid and in non-erythroid cells, i.e. regardless of their transcriptional activity. Early replication timing of chicken α-globin genes in cells of different lineages was in good correlation with the open chromatin configuration of the α-globin gene domain in both erythroid and non-erythroid cells. We propose that active transcription of the housekeeping gene overlapping the α-globin gene domain enables an access of Origin Recognition Complex (ORC) proteins to the replication origin resulting in early replication of α-globin genes even in non-erythroid cells.

A multifunctional reporter mouse line for Cre‐ and FLP‐dependent lineage analysis

The Cre/lox and FLP/FRT recombination systems have been used extensively for both conditional knockout and cell lineage analysis in mice. Here we report a new multifunctional Cre/FLP dual reporter allele (R26(NZG)) that exhibits strong and apparently ubiquitous marker expression in embryos and adults. The reporter construct, which is driven by the CAG promoter, was knocked into the ROSA26 locus providing an open chromatin domain for consistent expression and avoiding site-of-integration effects often observed with transgenic reporters. R26(NZG) directs Cre-dependent nuclear-localized beta-galactosidase (beta-gal) expression, and can be converted into a Cre-dependent EGFP reporter (R26(NG)) by germline excision of the FRT-flanked nlslacZ cassette. Alternatively, germline excision of the floxed PGKNEO cassette in R26(NZG) generates an FLP-dependent EGFP reporter (R26(ZG)) that expresses beta-gal in FLP-nonexpressing cells. Finally, by the simultaneous use of both Cre and FLP deleters, R26(NZG) allows lineage relationships to be interrogated with greater refinement than is possible with single recombinase reporter systems.

Transcription is required for establishment of germline methylation marks at imprinted genes

Genomic imprinting requires the differential marking by DNA methylation of genes in male and female gametes. In the female germline, acquisition of methylation imprint marks depends upon the de novo methyltransferase Dnmt3a and its cofactor Dnmt3L, but the reasons why specific sequences are targets for Dnmt3a and Dnmt3L are still poorly understood. Here, we investigate the role of transcription in establishing maternal germline methylation marks. We show that at the Gnas locus, truncating transcripts from the furthest upstream Nesp promoter disrupts oocyte-derived methylation of the differentially methylated regions (DMRs). Transcription through DMRs in oocytes is not restricted to this locus but occurs across the prospective DMRs at many other maternally marked imprinted domains, suggesting a common requirement for transcription events. The transcripts implicated here in gametic methylation are protein-coding, in contrast to the noncoding antisense transcripts involved in the monoallelic silencing of imprinted genes in somatic tissues, although they often initiate from alternative promoters in oocytes. We propose that transcription is a third essential component of the de novo methylation system, which includes optimal CpG spacing and histone modifications, and may be required to create or maintain open chromatin domains to allow the methylation complex access to its preferred targets.

USF1 Recruits hSET1 Complex and Is Important for Maintaining Active Chromatin Domains in the β-Globin Locus.

The chicken β-globin insulator element acts as a barrier to the encroachment of chromosomal silencing. This barrier insulator prevents the spread of a 16 Kb domain of condensed chromatin which lies immediately upstream of the β-globin genes in the chicken genome. We have previously shown that the transcription factors USF1/2 mediate the barrier activity by binding to the 5′HS4 insulator element of the chicken β-globin locus and maintaining a local active chromatin structure. In the mouse β-globin locus, USFs also play a critical role in regulating the developmental stage-specific transcription of the β-major globin gene. To further understand the diverse roles of USF proteins, we undertook the biochemical isolation of USF1-containing multiprotein complexes. We found that USF1 associates with the hSET1 complex, which is an H3K4-specific methyltransferase. This complex exhibits histone H3 methyltransferase enzymatic activity. Consistent with the role of USF1 in recruiting histone modifying enzyme hSET1 to the 5′HS4 insulator, knockdown of USF1 expression in erythroid leukemia cell line 6C2 leads to the loss of histone H3K4 dimethylation at 5′HS4 of the chicken β-globin locus. In addition, USF1, hSET1, as well as H3K4 methylation are colocalized on HS2 of the locus control region (LCR) and the promoter of the β-major globin gene. Furthermore, the double ChIP analysis demonstrates that the histone methyltransferase hSET1 is recruited by transcription factor USF1 onto the mouse β-globin locus. The results suggest that hSET1 may play an important role in establishing H3K4 methylations on the active β-globin locus and reveal the mechanisms of USF and its associated proteins in maintaining the open chromatin domain and in regulation of globin gene transcription.

Noncoding transcription controls downstream promoters to regulate T-cell receptor α recombination

The T early alpha (TEA) promoter in the murine Tcra locus generates noncoding transcripts that extend across the 65 kb Jalpha array. Here, we have analyzed the significance of TEA transcription for Tcra locus regulation through the targeted introduction of a transcription terminator downstream of the TEA promoter. We demonstrate that noncoding transcription driven by this single promoter can instruct both positively and negatively the activity of downstream Jalpha promoters, and can similarly instruct alterations in Jalpha chromatin structure and Jalpha recombination. TEA transcription activates promoters associated with relatively proximal Jalpha segments and stimulates histone acetylation, histone methylation and chromatin accessibility in this region. In contrast, at more distal locations, TEA transcription inhibits promoter activity through transcriptional interference and suppresses chromatin accessibility. In combination, these effects target initial Valpha-to-Jalpha recombination to TEA-proximal Jalpha segments and promote the ordered usage of the Jalpha array. The ability of TEA transcription to coordinate the activity of multiple downstream promoters maximizes the biological potential of the Jalpha array and diversifies the Tcra repertoire.

Evolution of chromosome organization driven by selection for reduced gene expression noise

The distribution of genes on eukaryotic chromosomes is nonrandom, but the reasons behind this are not well understood. The commonly observed clustering of essential genes is a case in point. Here we model and test a new hypothesis. Essential proteins are unusual in that random fluctuations in abundance (noise) can be highly deleterious. We hypothesize that persistently open chromatin domains are sinks for essential genes, as they enable reduced noise by avoidance of transcriptional bursting associated with chromatin remodeling. Simulation of the model captures clustering and correctly predicts that (i) essential gene clusters are associated with low nucleosome occupancy (ii) noise-sensitive nonessential genes cluster with essential genes (iii) nonessential genes of similar knockout fitness are physically linked (iv) genes in domains rich in essential genes have low noise (v) essential genes are rare subtelomerically and (vi) essential gene clusters are preferentially conserved. We conclude that different noise characteristics of different genomic domains favors nonrandom gene positioning. This has implications for gene therapy and understanding transgenic phenotypes.