A human globin enhancer causes both discrete and widespread alterations in chromatin structure

The human β-globin enhancer LCR HS2 plays a role in forming a TAD by activating chromatin structure at neighboring CTCF sites

The human β-globin locus control region (LCR) hypersensitive site 2 (HS2) is one of enhancers for transcription of the β-like globin genes in erythroid cells. Our previous study showed that the LCR HS2 has active chromatin structure before transcriptional induction of the β-globin gene, while another enhancer LCR HS3 is activated by the induction. To compare functional difference between them, we deleted each HS (ΔHS2 and ΔHS3) from the human β-globin locus in hybrid MEL/ch11 cells. Deletion of either HS2 or HS3 dramatically diminished the β-globin transcription and disrupted locus-wide histone H3K27ac and chromatin interaction between LCR HSs and gene. Surprisingly, ΔHS2 weakened interactions between CTCF sites forming the β-globin topologically associating domain (TAD), while ΔHS3 did not. CTCF occupancy and chromatin accessibility were reduced at the CTCF sites in the ΔHS2 locus. To further characterize the HS2, we deleted the maf-recognition elements for erythroid activator NF-E2 at HS2. This deletion decreased the β-globin transcription and enhancer-promoter interaction, but did not affect interactions between CTCF sites for the TAD. In light of these results, we propose that the HS2 has a role in forming a β-globin TAD by activating neighboring CTCF sites and this role is beyond typical enhancer activity.

An H4K16 histone acetyltransferase mediates decondensation of the X chromosome in C. elegans males

Background

In C. elegans, in order to equalize gene expression between the sexes and balance X and autosomal expression, two steps are believed to be required. First, an unknown mechanism is hypothesized to upregulate the X chromosome in both sexes. This mechanism balances the X to autosomal expression in males, but creates X overexpression in hermaphrodites. Therefore, to restore the balance, hermaphrodites downregulate gene expression twofold on both X chromosomes. While many studies have focused on X chromosome downregulation, the mechanism of X upregulation is not known.

Results

To gain more insight into X upregulation, we studied the effects of chromatin condensation and histone acetylation on gene expression levels in male C. elegans. We have found that the H4K16 histone acetyltransferase MYS-1/Tip60 mediates dramatic decondensation of the male X chromosome as measured by FISH. However, RNA-seq analysis revealed that MYS-1 contributes only slightly to upregulation of gene expression on the X chromosome. These results suggest that the level of chromosome decondensation does not necessarily correlate with the degree of gene expression change in vivo. Furthermore, the X chromosome is more sensitive to MYS-1-mediated decondensation than the autosomes, despite similar levels of H4K16ac on all chromosomes, as measured by ChIP-seq. H4K16ac levels weakly correlate with gene expression levels on both the X and the autosomes, but highly expressed genes on the X chromosome do not contain exceptionally high levels of H4K16ac.

Conclusion

These results indicate that H4K16ac and chromosome decondensation influence regulation of the male X chromosome; however, they do not fully account for the high levels of gene expression observed on the X chromosomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-016-0097-x) contains supplementary material, which is available to authorized users.

Differential contribution of cis-regulatory elements to higher order chromatin structure and expression of the CFTR locus

Higher order chromatin structure establishes domains that organize the genome and coordinate gene expression. However, the molecular mechanisms controlling transcription of individual loci within a topological domain (TAD) are not fully understood. The cystic fibrosis transmembrane conductance regulator (CFTR) gene provides a paradigm for investigating these mechanisms.CFTR occupies a TAD bordered by CTCF/cohesin binding sites within which are cell-type-selective cis-regulatory elements for the locus. We showed previously that intronic and extragenic enhancers, when occupied by specific transcription factors, are recruited to the CFTR promoter by a looping mechanism to drive gene expression. Here we use a combination of CRISPR/Cas9 editing of cis-regulatory elements and siRNA-mediated depletion of architectural proteins to determine the relative contribution of structural elements and enhancers to the higher order structure and expression of the CFTR locus. We found the boundaries of the CFTRTAD are conserved among diverse cell types and are dependent on CTCF and cohesin complex. Removal of an upstream CTCF-binding insulator alters the interaction profile, but has little effect on CFTR expression. Within the TAD, intronic enhancers recruit cell-type selective transcription factors and deletion of a pivotal enhancer element dramatically decreases CFTR expression, but has minor effect on its 3D structure.

Erythroid activator NF-E2, TAL1 and KLF1 play roles in forming the LCR HSs in the human adult β-globin locus

The β-like globin genes are developmental stage specifically transcribed in erythroid cells. The transcription of the β-like globin genes requires erythroid specific activators such as GATA-1, NF-E2, TAL1 and KLF1. However, the roles of these activators have not fully elucidated in transcription of the human adult β-globin gene. Here we employed hybrid MEL cells (MEL/ch11) where a human chromosome containing the β-globin locus is present and the adult β-globin gene is highly transcribed by induction. The roles of erythroid specific activators were analyzed by inhibiting the expression of NF-E2, TAL1 or KLF1 in MEL/ch11 cells. The loss of each activator decreased the transcription of human β-globin gene, locus wide histone hyperacetylation and the binding of other erythroid specific activators including GATA-1, even though not affecting the expression of other activators. Notably, sensitivity to DNase I was reduced in the locus control region (LCR) hypersensitive sites (HSs) with the depletion of activators. These results indicate that NF-E2, TAL1 and KLF1, all activators play a primary role in HSs formation in the LCR. It might contribute to the transcription of human adult β-globin gene by allowing the access of activators and cofactors. The roles of activators in the adult β-globin locus appear to be different from the roles in the early fetal locus.

The emerging roles of eRNAs in transcriptional regulatory networks

Following reports by ENCyclopedia Of DNA Elements (ENCODE; GENCODE) Consortium and others, it is now fairly evident that the majority (70-80%) of the mammalian genome has the potential to be transcribed into non-protein-coding RNAs (ncRNAs). Critical to our understanding of genetic processes is the mechanism by which ncRNAs exert their roles. Accordingly, ncRNAs are shown to regulate the expression of protein-coding loci (i.e., genes) at the transcriptional as well as post-transcriptional stages. We recently reported on a widespread transcription at the DNA enhancer elements in myogenic cells. In our study, we found certain enhancer RNAs (eRNAs) regulate chromatin accessibility of the transcriptional machinery at loci encoding master regulators of myogenesis (i.e., MyoD/MyoG), thus suggesting their significance and site-specific impact in cellular programming. Here, we examine recent discoveries pertinent to the proposed role(s) of eRNAs in regulating gene expression. We will highlight consistencies, discuss confounding observations, and consider a lack of critical information in a way to prioritize future objectives.

CTCF demarcates chicken embryonic α-globin gene autonomous silencing and contributes to adult stage-specific gene expression

Genomic loci composed of more than one gene are frequently subjected to differential gene expression, with the chicken α-globin domain being a clear example. In the present study we aim to understand the globin switching mechanisms responsible for the epigenetic silencing of the embryonic π gene and the transcriptional activation of the adult α(D) and α(A) genes at the genomic domain level. In early stages, we describe a physical contact between the embryonic π gene and the distal 3' enhancer that is lost later during development. We show that such a level of regulation is achieved through the establishment of a DNA hypermethylation sub-domain that includes the embryonic gene and the adjacent genomic sequences. The multifunctional CCCTCC-binding factor (CTCF), which is located upstream of the α(D) gene promoter, delimits this sub-domain and creates a transition between the inactive sub-domain and the active sub-domain, which includes the adult α(D) gene. In avian-transformed erythroblast HD3 cells that are induced to differentiate, we found active DNA demethylation of the adult α(D) promoter, coincident with the incorporation of 5-hydroxymethylcytosine (5hmC) and concomitant with adult gene transcriptional activation. These results suggest that autonomous silencing of the embryonic π gene is needed to facilitate an optimal topological conformation of the domain. This model proposes that CTCF is contributing to a specific chromatin configuration that is necessary for differential α-globin gene expression during development.

Altered expression of nuclear and cytoplasmic histone H1 in pulmonary artery and pulmonary artery smooth muscle cells in patients with IPAH

The pathogenesis of idiopathic pulmonary hypertension is poorly understood. This paper utilized histology-based Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI MS) to identify as-yet unknown proteins that may be associated with the structural changes in the pulmonary arterial walls of patients with IPAH. The technology identified significant increases in two fragments of histone H1 in the IPAH cases compared to controls. This finding was further examined using immunofluorescence techniques applied to sections from IPAH and control pulmonary arteries. In addition, cultured pulmonary artery smooth muscle cells (PASMCs) were utilized for Western analysis of histone H1 and importin β and importin 7, immunoprecipitation and assessment of nucleosomal repeat length (NRL). Immunofluorescence techniques revealed that nuclear expression of histone H1 was decreased and the chromatin was less compact in the IPAH cases than in the controls; furthermore, some cases showed a marked increase in cytoplasmic histone H1 expression. Using nuclear and cytoplasmic fractions of cultured PASMCs, we confirmed the reduction in histone H1 in the nucleus and an increase in the cytoplasm in IPAH cells compared to controls. Immunoprecipitation demonstrated a decreased association of histone H1 with importin β while importin 7 was unchanged in the IPAH cells compared to controls. The assessment of NRL revealed that the distance between nucleosomes was increased by ~20 bp in IPAH compared to controls. We conclude that at least two factors contribute to the reduction in nuclear histone H1-fragmentation of the protein and decreased import of histone H1 into the nucleus by importins. We further suggest that the decreased nuclear H1 contributes the less compact nucleosomal pattern in IPAH and this, in turn, contributes to the increase in NRL.

POU1F1-mediated activation of hGH-N by deoxyribonuclease I hypersensitive site II of the human growth hormone locus control region

The human growth hormone gene (hGH-N) is regulated by a distal locus control region (LCR) composed of five deoxyribonuclease I hypersensitive sites (HSs). The region encompassing HSI and HSII contains the predominant pituitary somatotrope-specific hGH-N activation function of the LCR. This activity was attributed primarily to POU1F1 (Pit-1) elements at HSI, as linkage to HSI was sufficient for properly regulated hGH-N expression in transgenic mice, while HSII alone had no activity. However, the presence of HSII in conjunction with HSI further enhanced hGH-N transgene expression, indicating additional determinants of pituitary hGH-N activation in the HSII region, but limitations of transgenic models and previous ex vivo systems have prevented the characterization of HSII. In the present study, we employ a novel minichromosome model of the hGH-N regulatory domain and show that HSII confers robust POU1F1-dependent activation of hGH-N in this system. This effect was accompanied by POU1F1-dependent histone acetylation and methylation throughout the minichromosome LCR/hGH-N domain. A series of in vitro DNA binding experiments revealed that POU1F1 binds to multiple sites at HSII, consistent with a direct role in HSII function. Remarkably, POU1F1 binding was localized in part to the 3' untranslated region of a primate-specific LINE-1 (long interspersed nuclear element 1) retrotransposon, suggesting that its insertion during primate evolution may have conferred function to the HSII region in the context of pituitary GH gene regulation. These observations clarify the function of HSII, expanding the role of POU1F1 in hGH LCR activity, and provide insight on the molecular evolution of the LCR.

The distinctive roles of erythroid specific activator GATA-1 and NF-E2 in transcription of the human fetal γ-globin genes

GATA-1 and NF-E2 are erythroid specific activators that bind to the β-globin locus. To explore the roles of these activators in transcription of the human fetal stage specific γ-globin genes, we reduced GATA-1 and p45/NF-E2 using shRNA in erythroid K562 cells. GATA-1 or p45/NF-E2 knockdown inhibited the transcription of the γ-globin genes, hypersensitive site (HS) formation in the LCR and chromatin loop formation of the β-globin locus, but histone acetylation across the locus was decreased only in the case of GATA-1 knockdown. In p45/NF-E2 knockdown cells, GATA-1 binding was maintained at the LCR HSs and γ-globin promoter, but NF-E2 binding at the LCR HSs was reduced by GATA-1 knockdown regardless of the amount of p45/NF-E2 in K562 cells. These results indicate that histone acetylation is dependent on GATA-1 binding, but the binding of GATA-1 is not sufficient for the γ-globin transcription, HS formation and chromatin loop formation and NF-E2 is required. This idea is supported by the distinctive binding pattern of CBP and Brg1 in the β-globin locus. Furthermore GATA-1-dependent loop formation between HS5 and 3′HS1 suggests correlation between histone modifications and chromatin looping.

An evolutionarily conserved TNF-alpha-responsive enhancer in the far upstream region of human CCL2 locus influences its gene expression

Comparative cross-species genomic analysis has served as a powerful tool to discover novel noncoding regulatory regions that influence gene expression in several cytokine loci. In this study, we have identified several evolutionarily conserved regions (ECRs) that are shared between human, rhesus monkey, dog, and horse and that are upstream of the promoter regions that have been previously shown to play a role in regulating CCL2 gene expression. Of these, an ECR that was ~16.5 kb (-16.5 ECR) upstream of its coding sequence contained a highly conserved NF-κB site. The region encompassing the -16.5 ECR conferred TNF-α responsiveness to homologous and heterologous promoters. In vivo footprinting demonstrated that specific nucleotide residues in the -16.5 ECR were protected or became hypersensitive after TNF-α treatment. The footprinted regions were found to bind NF-κB subunits in vitro and in vivo. Mutation/deletion of the conserved NF-κB binding site in the -16.5 ECR led to loss of TNF-α responsiveness. After TNF-α stimulation, the -16.5 ECR showed increased sensitivity to nuclease digestion and loss of histone signatures that are characteristic of a repressive chromatin. Chromosome conformation capture assays indicated that -16.5 ECR physically interacts with the CCL2 proximal promoter after TNF-α stimulation. Taken together, these results suggest that the -16.5 ECR may play a critical role in the regulation of CCL2.

Characterization of histone H3K27 modifications in the β-globin locus

Histone H3K27 is acetylated or methylated in the environment of nuclear chromatin. Here, to characterize the modification pattern of H3K27 in locus control region (LCR) and to understand the correlation of various H3K27 modifications with transcriptional activity of genes, we analyzed the human β-globin locus using the ChIP assay. The LCR of the human β-globin locus was enriched by H3K27ac and H3K27me1 in erythroid K562 cells. The highly transcribed globin genes were hyperacetylated at H3K27, but the repressed globin genes were highly dimethylated at this lysine in these cells. However, in non-erythroid 293FT cells, the β-globin locus was marked by a high level of H3K27me3. EZH2 and SUZ12, subunits of polycomb repressive complex 2, were comparably detected with the H3K27me3 pattern in K562 and 293FT cells. In addition, H3K27ac, H3K27me1 and H3K27me3 were established in an enhancer-dependent manner in a model minichromosomal locus containing an enhancer and its target gene. Taken together, these results show that H3K27 modifications have distinctive correlations with the chromatin state or transcription level of genes and are influenced by an enhancer.

High nucleosome occupancy is encoded at human regulatory sequences

Active eukaryotic regulatory sites are characterized by open chromatin, and yeast promoters and transcription factor binding sites (TFBSs) typically have low intrinsic nucleosome occupancy. Here, we show that in contrast to yeast, DNA at human promoters, enhancers, and TFBSs generally encodes high intrinsic nucleosome occupancy. In most cases we examined, these elements also have high experimentally measured nucleosome occupancy in vivo. These regions typically have high G+C content, which correlates positively with intrinsic nucleosome occupancy, and are depleted for nucleosome-excluding poly-A sequences. We propose that high nucleosome preference is directly encoded at regulatory sequences in the human genome to restrict access to regulatory information that will ultimately be utilized in only a subset of differentiated cells.

Dissecting the regulatory switches of development: lessons from enhancer evolution in Drosophila

Cis-regulatory modules are non-protein-coding regions of DNA essential for the control of gene expression. One class of regulatory modules is embryonic enhancers, which drive gene expression during development as a result of transcription factor protein binding at the enhancer sequences. Recent comparative studies have begun to investigate the evolution of the sequence architecture within enhancers. These analyses are illuminating the way that developmental biologists think about enhancers by revealing their molecular mechanism of function.

Differential requirement of a distal regulatory region for pre-initiation complex formation at globin gene promoters

Although distal regulatory regions are frequent throughout the genome, the molecular mechanisms by which they act in a promoter-specific manner remain to be elucidated. The human β-globin locus constitutes an extremely well-established multigenic model to investigate this issue. In erythroid cells, the β-globin locus control region (LCR) exerts distal regulatory function by influencing local chromatin organization and inducing high-level expression of individual β-like globin genes. Moreover, in transgenic mice expressing the entire human β-globin locus, deletion of LCR-hypersensitive site 2 (HS2) can alter β-like globin gene expression. Here, we show that abnormal expression of human β-like globin genes in the absence of HS2 is associated with decreased efficacy of pre-initiation complex formation at the human ɛ- and γ-promoters, but not at the β-promoter. This promoter-specific phenomenon is associated with reduced long-range interactions between the HS2-deleted LCR and human γ-promoters. We also find that HS2 is dispensable for high-level human β-gene transcription, whereas deletion of this hypersensitive site can alter locus chromatin organization; therefore the functions exerted by HS2 in transcriptional enhancement and locus chromatin organization are distinct. Overall, our data delineate one mechanism whereby a distal regulatory region provides promoter-specific transcriptional enhancement.

Enhancer requirement for histone methylation linked with gene activation

Enhancers cause a high level of transcription and activation of chromatin structure at target genes. Hyperacetylation of histones H3 and H4, a mark of active chromatin, is established broadly across target loci by enhancers that function over long distances. In the present study, we studied the role of an enhancer in methylation of various lysine residues on H3 by comparing a model gene locus having an active enhancer with one in which the enhancer has been inactivated within the context of minichromosomes. The intact enhancer affected histone methylation at K4, K9 and K36 in distinct ways depending on the methylation level and the location in the locus. All three lysine residues were highly tri-methylated in the coding region of the gene linked to the active enhancer but not the inactive enhancer. However di-methylation of K9 and K36 was not affected by the enhancer. The enhancer region itself was marked by mono-methylation at K4 and K9, distinguishing it from the methyl marks in the gene coding region. These results indicate that an enhancer has roles in establishing active histone methylation patterns linked with gene transcription rather than removing methylation linked with gene inactivation.

The apolipoprotein CIII enhancer regulates both extensive histone modification and intergenic transcription of human apolipoprotein AI/CIII/AIV genes but not apolipoprotein AV

The apolipoprotein (apo) AI/CIII/AIV/AV cluster genes are expressed at different levels in the liver and intestine. The apoCIII enhancer, a common regulatory element, regulates the tissue-specific expression of apoAI, apoCIII, and apoAIV but not apoAV. To study this regulation at the chromatin level, the histone modifications and intergenic transcription in the human apoAI/CIII/AIV/AV cluster were investigated in HepG2 and Caco-2 cells and in the livers of transgenic mice carrying the human gene cluster constructs with or without the apoCIII enhancer. We found that both the promoters and the intergenic regions of the apoAI/CIII/AIV genes were hyperacetylated and formed an open subdomain that did not include the apoAV gene. Hepatic and intestinal intergenic transcripts were identified to transcribe bidirectionally with strand preferences along the cluster. The deletion of the apoCIII enhancer influenced both histone modification and intergenic transcription in the apoAI/CIII/AIV gene region. These results demonstrate that the apoCIII enhancer contributes to the maintenance of an active chromatin subdomain of the apoAI/CIII/AIV genes, but not apoAV.

The role of transcriptional activator GATA-1 at human beta-globin HS2

GATA-1 is an erythroid activator that binds β-globin gene promoters and DNase I hypersensitive sites (HSs) of the β-globin locus control region (LCR). We investigated the direct role of GATA-1 interaction at the LCR HS2 enhancer by mutating its binding sites within minichromosomes in erythroid cells. Loss of GATA-1 in HS2 did not compromise interaction of NF-E2, a second activator that binds to HS2, nor was DNase I hypersensitivity at HS2 or the promoter of a linked ε-globin gene altered. Reduction of NF-E2 using RNAi confirmed the overall importance of this activator in establishing LCR HSs. However, recruitment of the histone acetyltransferase CBP and RNA pol II to HS2 was diminished by GATA-1 loss. Transcription of ε-globin was severely compromised with loss of RNA pol II from the transcription start site and reduction of H3 acetylation and H3K4 di- and tri-methylation in coding sequences. In contrast, widespread detection of H3K4 mono-methylation was unaffected by loss of GATA-1 in HS2. These results support the idea that GATA-1 interaction in HS2 has a prominent and direct role in co-activator and pol II recruitment conferring active histone tail modifications and transcription activation to a target gene but that it does not, by itself, play a major role in establishing DNase I hypersensitivity.

A positive role for NLI/Ldb1 in long-range beta-globin locus control region function

Long-range interactions between distant regulatory elements, such as enhancers, and their target genes underlie the specificity of gene expression in many developmentally regulated gene families. NLI/Ldb1, a widely expressed nuclear factor, is a potential mediator of long-range interactions. Here, we show that NLI/Ldb1 and erythroid-binding partners GATA-1/SCL/LMO2 bind in vivo to the beta-globin locus control region (LCR). The C-terminal LIM interaction domain of NLI is required for formation of the complex on chromatin. Loss of the LIM domain converts NLI into a dominant-negative inhibitor of globin gene expression, and knockdown of NLI by using shRNA results in failure to activate beta-globin expression. Kinetic studies reveal that the NLI/GATA-1/SCL/LMO2 complex is detected at the beta-globin promoter coincident with RNA Pol II recruitment, beta-globin transcription, and chromatin loop formation during erythroid differentiation, providing evidence that NLI facilitates long-range gene activation.

Nucleosome and transcription activator antagonism at human beta-globin locus control region DNase I hypersensitive sites

Locus control regions are regulatory elements that activate distant genes and typically consist of several DNase I hypersensitive sites coincident with clusters of transcription activator binding sites. To what extent nucleosomes and activators occupy these sites together or exclusively has not been extensively studied in vivo. We analyzed the chromatin structure of human β-globin locus control region hypersensitive sites in erythroid cells expressing embryonic and fetal globin genes. Nucleosomes were variably depleted at hypersensitive sites HS1-HS4 and at HS5 which flanks the 5′ of the locus. In lieu of nucleosomes, activators were differentially associated with these sites. Erythroid–specific GATA-1 resided at HS1, HS2 and HS4 but the NF-E2 hetero-dimer was limited to HS2 where nucleosomes were most severely depleted. Histones H3 and H4 were hyperacetylated and H3 was di-methylated at K4 across the LCR, however, the H3 K4 MLL methyltransferase component Ash2L and histone acetyltransferases CBP and p300 occupied essentially only HS2 and the NF-E2 motif in HS2 was required for Ash2L recruitment. Our results indicate that each hypersensitive site in the human β-globin LCR has distinct structural features and suggest that HS2 plays a pivotal role in LCR organization at embryonic and fetal stages of globin gene expression.

Linear distance from the locus control region determines epsilon-globin transcriptional activity

Enhancer elements modulate promoter activity over vast chromosomal distances, and mechanisms that ensure restrictive interactions between promoters and enhancers are critical for proper control of gene expression. The human beta-globin locus control region (LCR) activates expression of five genes in erythroid cells, including the proximal embryonic epsilon- and the distal adult beta-globin genes. To test for possible distance sensitivity of the genes to the LCR, we extended the distance between the LCR and genes by 2.3 kbp within the context of a yeast artificial chromosome, followed by the generation of transgenic mice (TgM). In these TgM lines, epsilon-globin gene expression decreased by 90%, while the more distantly located gamma- or beta-globin genes were not affected. Remarkably, introduction of a consensus EKLF binding site into the epsilon-globin promoter rendered its expression distance insensitive; when tested in an EKLF-null genetic background, expression of the mutant epsilon-globin gene was severely compromised. Thus, the epsilon-globin gene differs in its distance sensitivity to the LCR from the other beta-like globin genes, which is, at least in part, determined by the transcription factor EKLF.

Chromatin domains and regulation of transcription

Compartmentalization and compaction of DNA in the nucleus is the characteristic feature of eukaryotic cells. A fully extended DNA molecule has to be compacted 100,000 times to fit within the nucleus. At the same time it is critical that various DNA regions remain accessible for interaction with regulatory factors and transcription/replication factories. This puzzle is solved at the level of DNA packaging in chromatin that occurs in several steps: rolling of DNA onto nucleosomes, compaction of nucleosome fiber with formation of the so-called 30 nm fiber, and folding of the latter into the giant (50-200 kbp) loops, fixed onto the protein skeleton, the nuclear matrix. The general assumption is that DNA folding in the cell nucleus cannot be uniform. It has been known for a long time that a transcriptionally active chromatin fraction is more sensitive to nucleases; this was interpreted as evidence for the less tight compaction of this fraction. In this review we summarize the latest results on structure of transcriptionally active chromatin and the mechanisms of transcriptional regulation in the context of chromatin dynamics. In particular the significance of histone modifications and the mechanisms controlling dynamics of chromatin domains are discussed as well as the significance of spatial organization of the genome for functioning of distant regulatory elements.

Combinatorial epigenetics, “junk DNA”, and the evolution of complex organisms

At certain evolutionary junctures, two or more mutations participating in the build-up of a new complex function may be required to become available simultaneously in the same individuals. How could this happen in higher organisms whose populations are small compared to those of microbes, and in which chances of combined nearly simultaneous highly specific favorable mutations are correspondingly low? The question can in principle be answered for regulatory evolution, one of the basic processes of evolutionary change. A combined resetting of transcription rates in several genes could occur in the same individual. It is proposed that, in eukaryotes, changes in epigenetic trends and epigenetically transforming encounters between alternative chromatin structures could arise frequently enough so as to render probable particular conjunctions of changed transcription rates. Such conjunctions could involve mutational changes with low specificity requirements in gene-associated regions of non-protein-coding sequences. The effects of such mutations, notably when they determine the use of histone variants and covalent modifications of histones, can be among those that migrate along chromatin. Changes in chromatin structure are often cellularly inheritable over at least a limited number of generations of cells, and of individuals when the germ line is involved. SINEs and LINEs, which have been considered "junk DNA", are among the repeat sequences that would appear liable to have teleregulatory effects on the function of a nearby promoter, through changes in their numbers and distribution. There may also be present preexisting unstably inheritable epigenetic trends leading to cellular variegation, trends endemic in a cell population based on DNA sequences previously established in the neighborhood. Either way, epigenetically conditioned teleregulatory trends may display only limited penetrance. The imposition at a distance of new chromatin structures with regulatory impact can occur in cis as well as in trans, and is examined as intrachromosomally spreading teleregulation and interchromosomal "gene kissing". The chances for two or more particular epigenetically determined regulatory trends to occur together in a cell are increased thanks to the proposed low specificity requirements for most of the pertinent sequence changes in intergenic and intronic DNA or in the distribution of middle repetitive sequences that have teleregulatory impact. Inheritable epigenetic changes ("epimutations") with effects at a distance would then perdure over the number of generations required for "assimilation" of the several regulatory novelties through the occurrence and selection, gene by gene, of specific classical mutations. These mutations would have effects similar to the epigenetic effects, yet would provide stability and penetrance. The described epigenetic/genetic partnership may well at times have opened the way toward certain complex new functions. Thus, the presence of "junk DNA", through co-determining the (higher or lower) order and the variants of chromatin structure with regulatory effects at a distance, might make an important contribution to the evolution of complex organisms.

Beta-globin intergenic transcription and histone acetylation dependent on an enhancer

Histone acetyltransferases are associated with the elongating RNA polymerase II (Pol II) complex, supporting the idea that histone acetylation and transcription are intertwined mechanistically in gene coding sequences. Here, we studied the establishment and function of histone acetylation and transcription in noncoding sequences by using a model locus linking the beta-globin HS2 enhancer and the embryonic epsilon-globin gene in chromatin. An intact HS2 enhancer that recruits RNA Pol II is required for intergenic transcription and histone H3 acetylation and K4 methylation between the enhancer and target gene. RNA Pol II recruitment to the target gene TATA box is not required for the intergenic transcription or intergenic histone modifications, strongly implying that they are properties conferred by the enhancer. However, Pol II recruitment at HS2, intergenic transcription, and intergenic histone modification are not sufficient for transcription or modification of the target gene: these changes require initiation at the TATA box of the gene. The results suggest that intergenic and genic transcription complexes are independent and possibly differ from one another.

A modular enhancer is differentially regulated by GATA and NFAT elements that direct different tissue-specific patterns of nucleosome positioning and inducible chromatin remodeling

We investigated alternate mechanisms employed by enhancers to position and remodel nucleosomes and activate tissue-specific genes in divergent cell types. We demonstrated that the granulocyte-macrophage colony-stimulating factor (GM-CSF) gene enhancer is modular and recruits different sets of transcription factors in T cells and myeloid cells. The enhancer recruited distinct inducible tissue-specific enhanceosome-like complexes and directed nucleosomes to different positions in these cell types. In undifferentiated T cells, the enhancer was activated by inducible binding of two NFAT/AP-1 complexes which disrupted two specifically positioned nucleosomes (N1 and N2). In myeloid cells, the enhancer was remodeled by GATA factors which constitutively displaced an upstream nucleosome (N0) and cooperated with inducible AP-1 elements to activate transcription. In mast cells, which express both GATA-2 and NFAT, these two pathways combined to activate the enhancer and generate high-level gene expression. At least 5 kb of the GM-CSF locus was organized as an array of nucleosomes with fixed positions, but the enhancer adopted different nucleosome positions in T cells and mast cells. Furthermore, nucleosomes located between the enhancer and promoter were mobilized upon activation in an enhancer-dependent manner. These studies reveal that distinct tissue-specific mechanisms can be used either alternately or in combination to activate the same enhancer.

Enhancer blocking by chicken beta-globin 5'-HS4: role of enhancer strength and insulator nucleosome depletion

The 5'-HS4 chicken beta-globin insulator functions as a positional enhancer blocker on chromatinized episomes in human cells, blocking the HS2 enhancer of the human beta-globin locus control region from activating a downstream epsilon-globin gene. 5'-HS4 interrupted formation of a domain of histone H3 and H4 acetylation encompassing the 6-kb minilocus and inhibited transfer of RNA polymerase from the enhancer to the gene promoter. We found that the enhancer blocking phenotype was amplified when the insulated locus contained a weakened HS2 enhancer in which clustered point mutations eliminated interaction of the transcription factor GATA-1. The GATA-1 mutation compromised recruitment of histone acetyltransferases and RNA polymerase II to HS2. Enhancer blocking correlated with a significant depletion of nucleosomes in the core region of the insulator as revealed by micrococcal nuclease and DNase I digestion studies. Nucleosome depletion at 5'-HS4 was dependent on interaction of the insulator protein CCCTC-binding factor (CTCF) and was required for enhancer blocking. These findings provide evidence that a domain of active chromatin is formed by spreading from an enhancer to a target gene and can be blocked by a nucleosome-free gap in an insulator.

On a chromosome far, far away: LCRs and gene expression

Transcription activation of a gene involves the ordered recruitment of components of the basal transcription machinery in concert with alterations in chromatin structure, including nucleosome remodeling and post-translational modification of histones. Enhancers and locus control regions (LCRs) that are remote from the genes they activate, recruit the complexes that carry out these alterations and, sometimes, recruit RNA polymerase II. The question of how these distant activators interact with their target genes has been of long-standing interest. Recent data indicate that LCRs mediate contact with their coordinate genes through the formation of domains of histone modification and of intra- and inter-chromosomal loops and that they might localize genes within nuclear regions that favor transcription.

Organizing the genome: enhancers and insulators

Enhancers can activate their target genes over large linear distances. Insulators can delimit the influence of an enhancer to an appropriate target. There are a number of intertwined mechanisms by which the regulatory functions of enhancers and insulators might be carried out at the level of the chromatin fiber. Recent evidence suggests that both enhancers and insulators participate in higher-order organization of chromatin in the nucleus and in localization of their regulated sequences to both subnuclear structures and compartments. Novel experimental approaches are helping to reveal the mechanisms underlying nuclear organization of developmentally regulated genes.

Heterogeneous nuclear ribonucleoprotein C1/C2, MeCP1, and SWI/SNF form a chromatin remodeling complex at the beta-globin locus control region

Locus control regions (LCRs) are regulatory DNA sequences that are situated many kilobases away from their cognate promoters. LCRs protect transgenes from position effect variegation and heterochromatinization and also promote copy-number dependence of the levels of transgene expression. In this work, we describe the biochemical purification of a previously undescribed LCR-associated remodeling complex (LARC) that consists of heterogeneous nuclear ribonucleoprotein C1/C2, nucleosome remodeling SWI/SNF, and nucleosome remodeling and deacetylating (NuRD)/MeCP1 as a single homogeneous complex. LARC binds to the hypersensitive 2 (HS2)-Maf recognition element (MARE) DNA in a sequence-specific manner and remodels nucleosomes. Heterogeneous nuclear ribonucleoprotein C1/C2, previously known as a general RNA binding protein, provides a sequence-specific DNA recognition element for LARC, and the LARC DNA-recognition sequence is essential for the enhancement of transcription by HS2. Independently of the initiation of transcription, LARC becomes associated with beta-like globin promoters.

The dynamics of chromatin remodeling at promoters

The nucleosome, the structural unit of chromatin, is known to play a central role in regulating gene transcription from promoters. The last seven years have spawned a vast amount of data on the enzymes that remodel and modify nucleosomes and the rules governing how transcription factors interact with the epigenetic code on histones. Yet despite this effort, there has yet to emerge a unifying mechanism by which nucleosomes are remodeled during gene regulation. Recent advances have allowed nucleosome dynamics on promoters to be studied in real time, dramatically changing how we think about gene regulation on chromatin templates.

Regulation of human fetal hemoglobin: new players, new complexities

The human globin genes are among the most extensively characterized in the human genome, yet the details of the molecular events regulating normal human hemoglobin switching and the potential reactivation of fetal hemoglobin in adult hematopoietic cells remain elusive. Recent discoveries demonstrate physical interactions between the beta locus control region and the downstream structural gamma- and beta-globin genes, and with transcription factors and chromatin remodeling complexes. These interactions all play roles in globin gene expression and globin switching at the human beta-globin locus. If the molecular events in hemoglobin switching were better understood and fetal hemoglobin could be more fully reactivated in adult cells, the insights obtained might lead to new approaches to the therapy of sickle cell disease and beta thalassemia by identifying specific new targets for molecular therapies.

Astrocyte- and hepatocyte-specific expression of genes from the distal serpin subcluster at 14q32.1 associates with tissue-specific chromatin structures

The distal serpin subcluster contains genes encoding alpha1-antichymotrypsin (ACT), protein C inhibitor (PCI), kallistatin (KAL) and the KAL-like protein, which are expressed in hepatocytes, but only the act gene is expressed in astrocytes. We show here that the tissue-specific expression of these genes associates with astrocyte- and hepatocyte-specific chromatin structures. In hepatocytes, we identified 12 Dnase I-hypersensitive sites (DHSs) that were distributed throughout the entire subcluster, with the promoters of expressed genes accessible to restriction enzyme digestion. In astrocytes, only six DHSs were located exclusively in the 5' flanking region of the act gene, with its promoter also accessible to restriction enzyme digestion. The acetylation of histone H3 and H4 was found throughout the subcluster in both cell types but this acetylation did not correlate with the expression pattern of these serpin genes. Analysis of histone modifications at the promoters of the act and pci genes revealed that methylation of histone H3 on lysine 4 correlated with their expression pattern in both cell types. In addition, inhibition of methyltransferase activity resulted in suppression of ACT and PCI mRNA expression. We propose that lysine 4 methylation of histone H3 correlates with the tissue-specific expression pattern of these serpin genes.

Cell-type-specific epigenetic marking of the IL2 gene at a distal cis-regulatory region in competent, nontranscribing T-cells

T-cells retain cell-type-specific programming for IL-2 inducibility through many rounds of division without being stimulated to transcribe the locus. To understand the layering of controls needed to poise this gene heritably for activation, we have used chromatin immunoprecipitation to map histone modifications across the murine IL2 locus, from −10.2 through +0.25 kb, in induction-competent and incompetent cells. In highly inducible EL4 T-lineage cells, stimulation with PMA/A23187 induced strong acetylation of histone H3 and H4, in parallel with transcriptional activation, from −4.6 through +0.25 kb. However, dimethylation of histone H3/K4 was already fully elevated across the same restricted domain before stimulation, with little change after stimulation. RNA polymerase II binding, in contrast, was only found at the known promoter region after stimulation. Similar patterns of histone modifications were seen also in normal IL-2-inducible T-lineage cells. However, neither acetylated histone H3, H4 nor dimethylated histone H3/K4 marking was detected, with or without stimulation, in expression-incompetent cells (NIH/3T3 or Scid.adh). These results identify a discrete new domain of IL2 regulatory sequence marked by dimethylated histone H3/K4 in expression-permissive T-cells even when they are not transcribing IL2, setting boundaries for histone H3 and H4 acetylation when the IL2 gene is transcriptionally activated.

Granulocyte-macrophage colony-stimulating factor enhancer activation requires cooperation between NFAT and AP-1 elements and is associated with extensive nucleosome reorganization

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) gene is activated by an NFAT-dependent enhancer forming an inducible DNase I hypersensitive (DH) site. The enhancer core comprising the DH site contains the GM330 and GM420 elements that bind NFAT and AP-1 cooperatively. Here we demonstrate that both elements are essential for enhancer activity and that Sp1 and AML1 sites in the enhancer become occupied in vivo only after activation. Chromatin structure analysis revealed that the GM-CSF enhancer core elements are divided between two adjacent nucleosomes that become destabilized and highly accessible after activation. Inducible chromatin reorganization was not restricted to the enhancer core but extended across a 3-kb domain of mobilized nucleosomes, within which the nucleosome repeat length was compressed from approximately 185 to 150 bp. The GM420 element is a high-affinity site that binds NFAT independently of AP-1 but depends on the linked AP-1 site for enhancer function. Nevertheless, just the NFAT motif from the GM420 element was sufficient to form a DH site within chromatin even in the absence of the AP-1 site. Hence, NFAT has the potential to cooperate with other transcription factors by promoting chromatin remodelling and increasing accessibility at inducible regulatory elements.

An insulator blocks spreading of histone acetylation and interferes with RNA polymerase II transfer between an enhancer and gene

We studied the mechanism by which an insulator interrupts enhancer signaling to a gene using stably replicated chromatin templates containing the human beta-globin locus control region HS2 enhancer and a target globin gene. The chicken beta-globin 5' HS4 (cHS4) insulator acted as a positional enhancer blocker, inhibiting promoter remodeling and transcription activation only when placed between the enhancer and gene. Enhancer blocking by cHS4 reduced histone hyperacetylation across a zone extending from the enhancer to the gene and inhibited recruitment of CBP and p300 to HS2. Enhancer blocking also led to accumulation of RNA polymerase II at HS2 and within cHS4, accompanied by its diminution at the gene promoter. The enhancer blocking effects were completely attributable to the CTCF binding site in cHS4. These findings provide experimental evidence for the involvement of spreading in establishment of a broad zone of histone modification by an enhancer, as well as for blocking by an insulator of the transfer of RNA polymerase II from an enhancer to a promoter.

Chromatin decondensation and nuclear reorganization of the HoxB locus upon induction of transcription

The colinearity of genes in Hox clusters suggests a role for chromosome structure in gene regulation. We reveal programmed changes in chromatin structure and nuclear organization upon induction of Hoxb expression by retinoic acid. There is an early increase in the histone modifications that are marks of active chromatin at both the early expressed gene Hoxb1, and also at Hoxb9 that is not expressed until much later. There is also a visible decondensation of the chromatin between Hoxb1 and Hoxb9 at this early stage. However, a further change in higher-order chromatin structure, looping out of genes from the chromosome territory, occurs in synchrony with the execution of the gene expression program. We suggest that higher-order chromatin structure regulates the expression of the HoxB cluster at several levels. Locus-wide changes in chromatin structure (histone modification and chromatin decondensation) may establish a transcriptionally poised state but are not sufficient for the temporal program of gene expression. The choreographed looping out of decondensed chromatin from chromosome territories may then allow for activation of high levels of transcription from the sequence of genes along the cluster.

Developmental stage differences in chromatin subdomains of the beta-globin locus

The mammalian beta-globin loci each contain a family of developmentally expressed genes, and a far upstream regulatory element, the locus control region (LCR). In adult murine erythroid cells, the LCR and the transcribed beta-globin genes exist within domains of histone acetylation and RNA polymerase II (pol II) is associated with them. In contrast, the silent embryonic genes lie between these domains within hypoacetylated chromatin, and pol II is not found there. We used chromatin immunoprecipitation and real-time PCR to analyze histone modification and pol II recruitment to the globin locus in human erythroid K562 cells that express the embryonic epsilon-globin gene but not the adult beta-globin gene. H3 and H4 acetylation and H3 K4 methylation were continuous over a 17-kb region including the LCR and the active epsilon-globin gene. The level of modification varied directly with the transcription of the epsilon-globin gene. In contrast, this region in nonerythroid HeLa cells lacked these modifications and displayed instead widespread H3 K9 methylation. pol II was also detected continuously from the LCR to the epsilon-globin gene. These studies reveal several aspects of chromatin structure and pol II distribution that distinguish the globin locus at embryonic and adult stages and suggest that both enhancer looping and tracking mechanisms may contribute to LCR-promoter communication at different developmental stages.