Role of LDB1 in the transition from chromatin looping to transcription activation

GATA2 and PU.1 Collaborate To Activate the Expression of the Mouse Ms4a2 Gene, Encoding FcεRIβ, through Distinct Mechanisms

GATA factors GATA1 and GATA2 and ETS factor PU.1 are known to function antagonistically during hematopoietic development. In mouse mast cells, however, these factors are coexpressed and activate the expression of the Ms4a2 gene encoding the β chain of the high-affinity IgE receptor (FcεRI). The present study showed that these factors cooperatively regulate Ms4a2 gene expression through distinct mechanisms. Although GATA2 and PU.1 contributed almost equally to Ms4a2 gene expression, gene ablation experiments revealed that simultaneous knockdown of both factors showed neither a synergistic nor an additive effect. A chromatin immunoprecipitation analysis showed that they shared DNA binding to the +10.4-kbp region downstream of the Ms4a2 gene with chromatin looping factor LDB1, whereas the proximal -60-bp region was exclusively bound by GATA2 in a mast cell-specific manner. Ablation of PU.1 significantly reduced the level of GATA2 binding to both the +10.4-kbp and -60-bp regions. Surprisingly, the deletion of the +10.4-kbp region by genome editing completely abolished the Ms4a2 gene expression as well as the cell surface expression of FcεRI. These results suggest that PU.1 and LDB1 play central roles in the formation of active chromatin structure whereas GATA2 directly activates the Ms4a2 promoter.

Enhancer Dysfunction in 3D Genome and Disease

Spatiotemporal patterns of gene expression depend on enhancer elements and other factors during individual development and disease progression. The rapid progress of high-throughput techniques has led to well-defined enhancer chromatin properties. Various genome-wide methods have revealed a large number of enhancers and the discovery of three-dimensional (3D) genome architecture showing the distant interacting mechanisms of enhancers that loop to target gene promoters. Whole genome sequencing projects directed at cancer have led to the discovery of substantial enhancer dysfunction in misregulating gene expression and in tumor initiation and progression. Results from genome-wide association studies (GWAS) combined with functional genomics analyses have elucidated the functional impacts of many cancer risk-associated variants that are enriched within the enhancer regions of chromatin. Risk variants dysregulate the expression of enhancer variant-associated genes via 3D genomic interactions. Moreover, these enhancer variants often alter the chromatin binding affinity for cancer-relevant transcription factors, which in turn leads to aberrant expression of the genes associated with cancer susceptibility. In this review, we investigate the extent to which these genetic regulatory circuits affect cancer predisposition and how the recent development of genome-editing methods have enabled the determination of the impacts of genomic variation and alteration on cancer phenotype, which will eventually lead to better management plans and treatment responses to human cancer in the clinic.

Rotational symmetry of the structured Chip/LDB-SSDP core module of the Wnt enhanceosome

The Chip/LIM-domain binding protein (LDB)-single-stranded DNA-binding protein (SSDP) (ChiLS) complex controls numerous cell-fate decisions in animal cells, by mediating transcription of developmental control genes via remote enhancers. ChiLS is recruited to these enhancers by lineage-specific LIM-domain proteins that bind to its Chip/LDB subunit. ChiLS recently emerged as the core module of the Wnt enhanceosome, a multiprotein complex that primes developmental control genes for timely Wnt responses. ChiLS binds to NPFxD motifs within Pygopus (Pygo) and the Osa/ARID1A subunit of the BAF chromatin remodeling complex, which could synergize with LIM proteins in tethering ChiLS to enhancers. Chip/LDB and SSDP both contain N-terminal dimerization domains that constitute the bulk of their structured cores. Here, we report the crystal structures of these dimerization domains, in part aided by DARPin chaperones. We conducted systematic surface scanning by structure-designed mutations, followed by in vitro and in vivo binding assays, to determine conserved surface residues required for binding between Chip/LDB, SSDP, and Pygo-NPFxD. Based on this, and on the 4:2 (SSDP-Chip/LDB) stoichiometry of ChiLS, we derive a highly constrained structural model for this complex, which adopts a rotationally symmetrical SSDP2-LDB2-SSDP2 architecture. Integrity of ChiLS is essential for Pygo binding, and our mutational analysis places the NPFxD pockets on either side of the Chip/LDB dimer, each flanked by an SSDP dimer. The symmetry and multivalency of ChiLS underpin its function as an enhancer module integrating Wnt signals with lineage-specific factors to operate context-dependent transcriptional switches that are pivotal for normal development and cancer.

The enhancer RNA ARIEL activates the oncogenic transcriptional program in T-cell acute lymphoblastic leukemia

The oncogenic transcription factor TAL1 regulates the transcriptional program in T-ALL. ARID5B is one of the critical downstream targets of TAL1, which further activates the oncogenic regulatory circuit in T-ALL cells. Here, we elucidated the molecular functions of the noncoding RNA, ARID5B-inducing enhancer associated long noncoding RNA (ARIEL), in T-ALL pathogenesis. We demonstrated that ARIEL is specifically activated in TAL1 ⁺ T-ALL cases, and its expression is associated with ARID5B enhancer activity. ARIEL recruits mediator proteins to the ARID5B enhancer, promotes enhancer-promoter interactions, and activates the expression of ARID5B, thereby positively regulating the TAL1-induced transcriptional program and the MYC oncogene. The TAL1 complex coordinately regulates the expression of ARIEL Knockdown of ARIEL inhibits cell growth and survival of T-ALL cells in culture and blocks disease progression in a murine xenograft model. Our results indicate that ARIEL plays an oncogenic role as an enhancer RNA in T-ALL.

The molecular genetic background leading to the formation of the human erythroid-specific Xga/CD99 blood groups

The Xg^a and CD99 antigens of the human Xg blood group system show a unique and sex-specific phenotypic relationship. The phenotypic relationship is believed to result from transcriptional coregulation of the XG and CD99 genes, which span the pseudoautosomal boundary of the X and Y chromosomes. However, the molecular genetic background responsible for these blood groups has remained undetermined. During the present investigation, we initially conducted a pilot study aimed at individuals with different Xg^a/CD99 phenotypes; this used targeted next-generation sequencing of the genomic areas relevant to XG and CD99 This was followed by a large-scale association study that demonstrated a definite association between a single nucleotide polymorphism (SNP) rs311103 and the Xg^a/CD99 blood groups. The G and C genotypes of SNP rs311103 were associated with the Xg(a+)/CD99H and Xg(a-)/CD99L phenotypes, respectively. The rs311103 genomic region with the G genotype was found to have stronger transcription-enhancing activity by reporter assay, and this occurred specifically with erythroid-lineage cells. Such activity was absent when the same region with the C genotype was investigated. In silico analysis of the polymorphic rs311103 genomic regions revealed that a binding motif for members of the GATA transcription factor family was present in the rs311103[G] region. Follow-up investigations showed that the erythroid GATA1 factor is able to bind specifically to the rs311103[G] region and markedly stimulates the transcriptional activity of the rs311103[G] segment. The present findings identify the genetic basis of the erythroid-specific Xg^a/CD99 blood group phenotypes and reveal the molecular background of their formation.

DNA·RNA triple helix formation can function as a cis-acting regulatory mechanism at the human β-globin locus

We have identified regulatory mechanisms in which an RNA transcript forms a DNA duplex·RNA triple helix with a gene or one of its regulatory elements, suggesting potential auto-regulatory mechanisms in vivo. We describe an interaction at the human β-globin locus, in which an RNA segment embedded in the second intron of the β-globin gene forms a DNA·RNA triplex with the HS2 sequence within the β-globin locus control region, a major regulator of globin expression. We show in human K562 cells that the triplex is stable in vivo. Its formation causes displacement from HS2 of major transcription factors and RNA Polymerase II, and consequently in loss of factors and polymerase that bind to the human ε- and γ-globin promoters, which are activated by HS2 in K562 cells. This results in reduced expression of these genes. These effects are observed when a small length of triplex-forming RNA is introduced into cells, or when a full-length intron-containing human β-globin transcript is expressed. Related results are obtained in human umbilical cord blood-derived erythroid progenitor-2 cells, in which β-globin expression is similarly affected by triplex formation. These results suggest a model in which RNAs conforming to the strict sequence rules for DNA·RNA triplex formation may participate in feedback regulation of genes in cis.

SRC-3 Coactivator Governs Dynamic Estrogen-Induced Chromatin Looping Interactions during Transcription

Enhancers are thought to activate transcription by physically contacting promoters via looping. However, direct assays demonstrating these contacts are required to mechanistically verify such cellular determinants of enhancer function. Here, we present versatile cell-free assays to further determine the role of enhancer-promoter contacts (EPCs). We demonstrate that EPC is linked to mutually stimulatory transcription at the enhancer and promoter in vitro. SRC-3 was identified as a critical looping determinant for the estradiol-(E2)-regulated GREB1 locus. Surprisingly, the GREB1 enhancer and promoter contact two internal gene body SRC-3 binding sites, GBS1 and GBS2, which stimulate their transcription. Utilizing time-course 3C assays, we uncovered SRC-3-dependent dynamic chromatin interactions involving the enhancer, promoter, GBS1, and GBS2. Collectively, these data suggest that the enhancer and promoter remain "poised" for transcription via their contacts with GBS1 and GBS2. Upon E2 induction, GBS1 and GBS2 disengage from the enhancer, allowing direct EPC for active transcription.

GATA transcription factors in development and disease

The GATA family of transcription factors is of crucial importance during embryonic development, playing complex and widespread roles in cell fate decisions and tissue morphogenesis. GATA proteins are essential for the development of tissues derived from all three germ layers, including the skin, brain, gonads, liver, hematopoietic, cardiovascular and urogenital systems. The crucial activity of GATA factors is underscored by the fact that inactivating mutations in most GATA members lead to embryonic lethality in mouse models and are often associated with developmental diseases in humans. In this Primer, we discuss the unique and redundant functions of GATA proteins in tissue morphogenesis, with an emphasis on their regulation of lineage specification and early organogenesis.

Fetal γ-globin genes are regulated by the BGLT3 long noncoding RNA locus

Long noncoding RNAs (lncRNAs) are increasingly being appreciated as participants in regulation of important cellular processes, including transcription. Because lncRNAs are highly cell type specific, they have the potential to contribute to the unique transcriptional repertoire of diverse cells, but underlying mechanisms are unclear. We studied BGLT3, an erythroid lncRNA encoded downstream of ^Aγ-globin (HBG1). BGLT3 and γ-globin genes are dynamically cotranscribed in erythroid cells in vivo. Deletion of BGLT3 using CRISPR/Cas9 editing shows that it specifically contributes to regulation of γ-globin genes. We used reduction or overexpression of the RNA and inhibition of transcription through the locus by CRISPRi to distinguish functions of the transcript vs the underlying sequence. Transcription of the BGLT3 locus is critical for looping between the γ-globin genes and BGLT3 sequences. In contrast, the BGLT3 transcript is dispensable for γ-globin/BGLT3 looping but interacts with the mediator complex on chromatin. Manipulation of the BGLT3 locus does not compromise γ-globin gene long-range looping interactions with the β-globin locus control region (LCR). These data reveal that BGLT3 regulates γ-globin transcription in a developmental stage-specific fashion together with the LCR by serving as a separate means to increase RNA Pol II density at the γ-globin promoters.

Chromosome Conformation Capture (3C and Higher) with Erythroid Samples

Chromosome conformation capture (3C) allows for the determination of the proximity in nuclei of DNA sequences that are linearly distant from one another in the genome. Proximity that is above that expected from random interaction provides evidence for potential long-range functional interactions such as between enhancers and their target genes. Many controls are required to convincingly demonstrate increased frequency of interaction between sequences and stringent functional tests must also be applied. Here, we present methodology suitable for 3C experiments that can also be applied as the basis for related 4C, 5C, and Hi-C approaches. These procedures are widely applicable to erythroid cell lines, progenitor cells, and tissues.

The LDB1 Complex Co-opts CTCF for Erythroid Lineage-Specific Long-Range Enhancer Interactions

Lineage-specific transcription factors are critical for long-range enhancer interactions, but direct or indirect contributions of architectural proteins such as CCCTC-binding factor (CTCF) to enhancer function remain less clear. The LDB1 complex mediates enhancer-gene interactions at the β-globin locus through LDB1 self-interaction. We find that an LDB1-bound enhancer upstream of carbonic anhydrase 2 (Car2) activates its expression by interacting directly with CTCF at the gene promoter. Both LDB1 and CTCF are required for enhancer-Car2 looping, and the domain of LDB1 contacted by CTCF is necessary to rescue Car2 transcription in LDB1-deficient cells. Genome-wide studies and CRISPR/Cas9 genome editing indicate that LDB1-CTCF enhancer looping underlies activation of a substantial fraction of erythroid genes. Our results provide a mechanism by which long-range interactions of architectural protein CTCF can be tailored to achieve a tissue-restricted pattern of chromatin loops and gene expression.

Transcription factor Hoxb5 reprograms B cells into functional T lymphocytes

Deletion of master regulators of the B cell lineage reprograms B cells into T cells. Here we found that the transcription factor Hoxb5, which is expressed in uncommitted hematopoietic progenitor cells but is not present in cells committed to the B cell or T cell lineage, was able to reprogram pro-pre-B cells into functional early T cell lineage progenitors. This reprogramming started in the bone marrow and was completed in the thymus and gave rise to T lymphocytes with transcriptomes, hierarchical differentiation, tissue distribution and immunological functions that closely resembled those of their natural counterparts. Hoxb5 repressed B cell 'master genes', activated regulators of T cells and regulated crucial chromatin modifiers in pro-pre-B cells and ultimately drove the B cell fate-to-T cell fate conversion. Our results provide a de novo paradigm for the generation of functional T cells through reprogramming in vivo.

Chromatin Immunoprecipitation (ChIP) with Erythroid Samples

Chromatin immunoprecipitation (ChIP) allows determination of the locations to which a select protein is bound in chromatin. Chemical crosslinking of DNA and protein with bi-functional reagents such as formaldehyde and precipitation of the protein with a specific antibody permit PCR amplification (ChIP) or sequencing (ChIP-seq) to identify the bound sites. Here, we present methodology for these approaches that are widely applicable to erythroid cell lines, progenitor cells, and tissues.

Static and Dynamic DNA Loops form AP-1-Bound Activation Hubs during Macrophage Development

The three-dimensional arrangement of the human genome comprises a complex network of structural and regulatory chromatin loops important for coordinating changes in transcription during human development. To better understand the mechanisms underlying context-specific 3D chromatin structure and transcription during cellular differentiation, we generated comprehensive in situ Hi-C maps of DNA loops in human monocytes and differentiated macrophages. We demonstrate that dynamic looping events are regulatory rather than structural in nature and uncover widespread coordination of dynamic enhancer activity at preformed and acquired DNA loops. Enhancer-bound loop formation and enhancer activation of preformed loops together form multi-loop activation hubs at key macrophage genes. Activation hubs connect 3.4 enhancers per promoter and exhibit a strong enrichment for activator protein 1 (AP-1)-binding events, suggesting that multi-loop activation hubs involving cell-type-specific transcription factors represent an important class of regulatory chromatin structures for the spatiotemporal control of transcription.

LDB1-mediated enhancer looping can be established independent of mediator and cohesin

Mechanistic studies in erythroid cells indicate that LDB1, as part of a GATA1/TAL1/LMO2 complex, brings erythroid-expressed genes into proximity with enhancers for transcription activation. The role of co-activators in establishing this long-range interaction is poorly understood. Here we tested the contributions of the RNA Pol II pre-initiation complex (PIC), mediator and cohesin to establishment of locus control region (LCR)/β-globin proximity. CRISPR/Cas9 editing of the β-globin promoter to eliminate the RNA Pol II PIC by deleting the TATA-box resulted in loss of transcription, but enhancer-promoter interaction was unaffected. Additional deletion of the promoter GATA1 site eliminated LDB1 complex and mediator occupancy and resulted in loss of LCR/β-globin proximity. To separate the roles of LDB1 and mediator in LCR looping, we expressed a looping-competent but transcription-activation deficient form of LDB1 in LDB1 knock down cells: LCR/β-globin proximity was restored without mediator core occupancy. Further, Cas9-directed tethering of mutant LDB1 to the β-globin promoter forced LCR loop formation in the absence of mediator or cohesin occupancy. Moreover, ENCODE data and our chromatin immunoprecipitation results indicate that cohesin is almost completely absent from validated and predicted LDB1-regulated erythroid enhancer-gene pairs. Thus, lineage specific factors largely mediate enhancer-promoter looping in erythroid cells independent of mediator and cohesin.

Chemical probes targeting epigenetic proteins: Applications beyond oncology

Epigenetic chemical probes are potent, cell-active, small molecule inhibitors or antagonists of specific domains in a protein; they have been indispensable for studying bromodomains and protein methyltransferases. The Structural Genomics Consortium (SGC), comprising scientists from academic and pharmaceutical laboratories, has generated most of the current epigenetic chemical probes. Moreover, the SGC has shared about 4 thousand aliquots of these probes, which have been used primarily for phenotypic profiling or to validate targets in cell lines or primary patient samples cultured in vitro. Epigenetic chemical probes have been critical tools in oncology research and have uncovered mechanistic insights into well-established targets, as well as identify new therapeutic starting points. Indeed, the literature primarily links epigenetic proteins to oncology, but applications in inflammation, viral, metabolic and neurodegenerative diseases are now being reported. We summarize the literature of these emerging applications and provide examples where existing probes might be used.

SCL/TAL1: a multifaceted regulator from blood development to disease

SCL/TAL1 (stem cell leukemia/T-cell acute lymphoblastic leukemia [T-ALL] 1) is an essential transcription factor in normal and malignant hematopoiesis. It is required for specification of the blood program during development, adult hematopoietic stem cell survival and quiescence, and terminal maturation of select blood lineages. Following ectopic expression, SCL contributes to oncogenesis in T-ALL. Remarkably, SCL’s activities are all mediated through nucleation of a core quaternary protein complex (SCL:E-protein:LMO1/2 [LIM domain only 1 or 2]:LDB1 [LIM domain-binding protein 1]) and dynamic recruitment of conserved combinatorial associations of additional regulators in a lineage- and stage-specific context. The finely tuned control of SCL’s regulatory functions (lineage priming, activation, and repression of gene expression programs) provides insight into fundamental developmental and transcriptional mechanisms, and highlights mechanistic parallels between normal and oncogenic processes. Importantly, recent discoveries are paving the way to the development of innovative therapeutic opportunities in SCL+ T-ALL.

Reciprocal regulation of γ-globin expression by exo-miRNAs: Relevance to γ-globin silencing in β-thalassemia major

Induction of fetal hemoglobin (HbF) is a promising strategy in the treatment of β-thalassemia major (β-TM). The present study shows that plasma exosomal miRNAs (exo-miRs) are involved in γ-globin regulation. Exosomes shuttle miRNAs and mediate cell-cell communication. MiRNAs are regulators of biological processes through post-transcriptional targeting. Compared to HD (Healthy Donor), β-TM patients showed increased levels of plasma exosomes and the majority of exosomes had cellular origin from CD34+ cells. Further, HD and β-TM exosomes showed differential miRNA expressions. Among them, deregulated miR-223-3p and miR-138-5p in β-TM exosomes and HD had specific targets for γ-globin regulator and repressor respectively. Functional studies in K562 cells showed that HD exosomes and miR-138-5p regulated γ-globin expression by targeting BCL11A. β-TM exosomes and miR-223-3p down regulated γ-globin expression through LMO2 targeting. Importantly, miR-223-3p targeting through sponge repression resulted in γ-globin activation. Further, hnRNPA1 bound to stem-loop structure of pre-miR-223 and we found that hnRNPA1 knockdown or mutagenesis at miR-223-3p stem-loop sequence resulted in less mature exo-miR-223-3p levels. Altogether, the study shows for the first time on the important clinical evidence that differentially expressed exo-miRNAs reciprocally control γ-globin expressions. Further, the hnRNPA1-exo-miR-223-LMO2 axis may be critical to γ-globin silencing in β-TM.

LIM domain-binding 1 maintains the terminally differentiated state of pancreatic β cells

The recognition of β cell dedifferentiation in type 2 diabetes raises the translational relevance of mechanisms that direct and maintain β cell identity. LIM domain-binding protein 1 (LDB1) nucleates multimeric transcriptional complexes and establishes promoter-enhancer looping, thereby directing fate assignment and maturation of progenitor populations. Many terminally differentiated endocrine cell types, however, remain enriched for LDB1, but its role is unknown. Here, we have demonstrated a requirement for LDB1 in maintaining the terminally differentiated status of pancreatic β cells. Inducible ablation of LDB1 in mature β cells impaired insulin secretion and glucose homeostasis. Transcriptomic analysis of LDB1-depleted β cells revealed the collapse of the terminally differentiated gene program, indicated by a loss of β cell identity genes and induction of the endocrine progenitor factor neurogenin 3 (NEUROG3). Lineage tracing confirmed that LDB1-depleted, insulin-negative β cells express NEUROG3 but do not adopt alternate endocrine cell fates. In primary mouse islets, LDB1 and its LIM homeodomain-binding partner islet 1 (ISL1) were coenriched at chromatin sites occupied by pancreatic and duodenal homeobox 1 (PDX1), NK6 homeobox 1 (NKX6.1), forkhead box A2 (FOXA2), and NK2 homeobox 2 (NKX2.2) - factors that co-occupy active enhancers in 3D chromatin domains in human islets. Indeed, LDB1 was enriched at active enhancers in human islets. Thus, LDB1 maintains the terminally differentiated state of β cells and is a component of active enhancers in both murine and human islets.

β Cells led astray by transcription factors and the company they keep

Pancreatic β cells have one of the highest protein secretion burdens in the body, as these cells must synthesize and secrete insulin in proportion to postprandial rises in blood glucose. Remarkably, it is now becoming clear that adult β cells retain plasticity and can dedifferentiate into embryonic fates or adopt alternate islet endocrine cell identities. This property is especially important, because changes in cell fate alter β cell function and could form the basis for defects in insulin secretion that occur early in the pathogenesis of the most prevalent form of β cell dysfunction, type 2 diabetes. In this issue, three different studies provide complementary perspectives on how the transcription factors NK2 homeobox 2 (NKX2.2), paired box 6 (PAX6), and LIM domain-binding protein 1 (LDB1) serve to maintain mature adult β cell identity, revealing clues as to how adult β cells can partially dedifferentiate or become reprogrammed into other islet endocrine cells.

Insight into GATA1 transcriptional activity through interrogation of cis elements disrupted in human erythroid disorders

Whole-exome sequencing has been incredibly successful in identifying causal genetic variants and has revealed a number of novel genes associated with blood and other diseases. One limitation of this approach is that it overlooks mutations in noncoding regulatory elements. Furthermore, the mechanisms by which mutations in transcriptionalcis-regulatory elements result in disease remain poorly understood. Here we used CRISPR/Cas9 genome editing to interrogate three such elements harboring mutations in human erythroid disorders, which in all cases are predicted to disrupt a canonical binding motif for the hematopoietic transcription factor GATA1. Deletions of as few as two to four nucleotides resulted in a substantial decrease (>80%) in target gene expression. Isolated deletions of the canonical GATA1 binding motif completely abrogated binding of the cofactor TAL1, which binds to a separate motif. Having verified the functionality of these three GATA1 motifs, we demonstrate strong evolutionary conservation of GATA1 motifs in regulatory elements proximal to other genes implicated in erythroid disorders, and show that targeted disruption of such elements results in altered gene expression. By modeling transcription factor binding patterns, we show that multiple transcription factors are associated with erythroid gene expression, and have created predictive maps modeling putative disruptions of their binding sites at key regulatory elements. Our study provides insight into GATA1 transcriptional activity and may prove a useful resource for investigating the pathogenicity of noncoding variants in human erythroid disorders.

Integrated genome-scale analysis of the transcriptional regulatory landscape in a blood stem/progenitor cell model

New genome-wide maps for 17 TFs, 3 histone modifications, DNase I sites, Hi-C, and Promoter Capture Hi-C in a stem/progenitor model. Integrated analysis shows that chromatin loops in a stem/progenitor model are characterized by specific TF occupancy patterns.

Chromatin looping as a target for altering erythroid gene expression

The β-hemoglobinopathies are the most common monogenic disorders in humans, with symptoms arising after birth when the fetal γ-globin genes are silenced and the adult β-globin gene is activated. There is a growing appreciation that genome organization and the folding of chromosomes are key determinants of gene transcription. Underlying this function is the activity of transcriptional enhancers that increase the transcription of target genes over long linear distances. To accomplish this, enhancers engage in close physical contact with target promoters through chromosome folding or looping that is orchestrated by protein complexes that bind to both sites and stabilize their interaction. We find that enhancer activity can be redirected with concomitant changes in gene transcription. Both targeting the β-globin locus control region (LCR) to the γ-globin gene in adult erythroid cells by tethering and epigenetic unmasking of a silenced γ-globin gene lead to increased frequency of LCR/γ-globin contacts and reduced LCR/β-globin contacts. The outcome of these manipulations is robust, pancellular γ-globin transcription activation with a concomitant reduction in β-globin transcription. These examples show that chromosome looping may be considered a therapeutic target for gene activation in β-thalassemia and sickle cell disease.

Lhx1 functions together with Otx2, Foxa2, and Ldb1 to govern anterior mesendoderm, node, and midline development

Costello et al. demonstrate that Smad4/Eomes-dependent Lhx1 expression in the epiblast marks the entire definitive endoderm lineage, the anterior mesendoderm, and midline progenitors. In proteomic experiments, they characterize a complex comprised of Lhx1, Otx2, and Foxa2 as well as the chromatin-looping protein Ldb1.

Gene regulatory networks controlling functional activities of spatially and temporally distinct endodermal cell populations in the early mouse embryo remain ill defined. The T-box transcription factor Eomes, acting downstream from Nodal/Smad signals, directly activates the LIM domain homeobox transcription factor Lhx1 in the visceral endoderm. Here we demonstrate Smad4/Eomes-dependent Lhx1 expression in the epiblast marks the entire definitive endoderm lineage, the anterior mesendoderm, and midline progenitors. Conditional inactivation of Lhx1 disrupts anterior definitive endoderm development and impedes node and midline morphogenesis in part due to severe disturbances in visceral endoderm displacement. Transcriptional profiling and ChIP-seq (chromatin immunoprecipitation [ChIP] followed by high-throughput sequencing) experiments identified Lhx1 target genes, including numerous anterior definitive endoderm markers and components of the Wnt signaling pathway. Interestingly, Lhx1-binding sites were enriched at enhancers, including the Nodal-proximal epiblast enhancer element and enhancer regions controlling Otx2 and Foxa2 expression. Moreover, in proteomic experiments, we characterized a complex comprised of Lhx1, Otx2, and Foxa2 as well as the chromatin-looping protein Ldb1. These partnerships cooperatively regulate development of the anterior mesendoderm, node, and midline cell populations responsible for establishment of the left–right body axis and head formation.

3CPET: finding co-factor complexes from ChIA-PET data using a hierarchical Dirichlet process

Various efforts have been made to elucidate the cooperating proteins involved in maintaining chromatin interactions; however, many are still unknown. Here, we present 3CPET, a tool based on a non-parametric Bayesian approach, to infer the set of the most probable protein complexes involved in maintaining chromatin interactions and the regions that they may control, making it a valuable downstream analysis tool in chromatin conformation studies. 3CPET does so by combining data from ChIA-PET, transcription factor binding sites, and protein interactions. 3CPET results show biologically significant and accurate predictions when validated against experimental and simulation data.

LMO2 Oncoprotein Stability in T-Cell Leukemia Requires Direct LDB1 Binding

LMO2 is a component of multisubunit DNA-binding transcription factor complexes that regulate gene expression in hematopoietic stem and progenitor cell development. Enforced expression of LMO2 causes leukemia by inducing hematopoietic stem cell-like features in T-cell progenitor cells, but the biochemical mechanisms of LMO2 function have not been fully elucidated. In this study, we systematically dissected the LMO2/LDB1-binding interface to investigate the role of this interaction in T-cell leukemia. Alanine scanning mutagenesis of the LIM interaction domain of LDB1 revealed a discrete motif, R(320)LITR, required for LMO2 binding. Most strikingly, coexpression of full-length, wild-type LDB1 increased LMO2 steady-state abundance, whereas coexpression of mutant proteins deficient in LMO2 binding compromised LMO2 stability. These mutant LDB1 proteins also exerted dominant negative effects on growth and transcription in diverse leukemic cell lines. Mass spectrometric analysis of LDB1 binding partners in leukemic lines supports the notion that LMO2/LDB1 function in leukemia occurs in the context of multisubunit complexes, which also protect the LMO2 oncoprotein from degradation. Collectively, these data suggest that the assembly of LMO2 into complexes, via direct LDB1 interaction, is a potential molecular target that could be exploited in LMO2-driven leukemias resistant to existing chemotherapy regimens.

Demystifying the secret mission of enhancers: linking distal regulatory elements to target genes

Enhancers are short regulatory sequences bound by sequence-specific transcription factors and play a major role in the spatiotemporal specificity of gene expression patterns in development and disease. While it is now possible to identify enhancer regions genomewide in both cultured cells and primary tissues using epigenomic approaches, it has been more challenging to develop methods to understand the function of individual enhancers because enhancers are located far from the gene(s) that they regulate. However, it is essential to identify target genes of enhancers not only so that we can understand the role of enhancers in disease but also because this information will assist in the development of future therapeutic options. After reviewing models of enhancer function, we discuss recent methods for identifying target genes of enhancers. First, we describe chromatin structure-based approaches for directly mapping interactions between enhancers and promoters. Second, we describe the use of correlation-based approaches to link enhancer state with the activity of nearby promoters and/or gene expression. Third, we describe how to test the function of specific enhancers experimentally by perturbing enhancer–target relationships using high-throughput reporter assays and genome editing. Finally, we conclude by discussing as yet unanswered questions concerning how enhancers function, how target genes can be identified, and how to distinguish direct from indirect changes in gene expression mediated by individual enhancers.

An ancient Pygo-dependent Wnt enhanceosome integrated by Chip/LDB-SSDP

TCF/LEF factors are ancient context-dependent enhancer-binding proteins that are activated by β-catenin following Wnt signaling. They control embryonic development and adult stem cell compartments, and their dysregulation often causes cancer. β-catenin-dependent transcription relies on the NPF motif of Pygo proteins. Here, we use a proteomics approach to discover the Chip/LDB-SSDP (ChiLS) complex as the ligand specifically binding to NPF. ChiLS also recognizes NPF motifs in other nuclear factors including Runt/RUNX2 and Drosophila ARID1, and binds to Groucho/TLE. Studies of Wnt-responsive dTCF enhancers in the Drosophila embryonic midgut indicate how these factors interact to form the Wnt enhanceosome, primed for Wnt responses by Pygo. Together with previous evidence, our study indicates that ChiLS confers context-dependence on TCF/LEF by integrating multiple inputs from lineage and signal-responsive factors, including enhanceosome switch-off by Notch. Its pivotal function in embryos and stem cells explain why its integrity is crucial in the avoidance of cancer.

The Isl1/Ldb1 Complex Orchestrates Genome-wide Chromatin Organization to Instruct Differentiation of Multipotent Cardiac Progenitors

Cardiac stem/progenitor cells hold great potential for regenerative therapies; however, the mechanisms regulating their expansion and differentiation remain insufficiently defined. Here we show that Ldb1 is a central regulator of genome organization in cardiac progenitor cells, which is crucial for cardiac lineage differentiation and heart development. We demonstrate that Ldb1 binds to the key regulator of cardiac progenitors, Isl1, and protects it from degradation. Furthermore, the Isl1/Ldb1 complex promotes long-range enhancer-promoter interactions at the loci of the core cardiac transcription factors Mef2c and Hand2. Chromosome conformation capture followed by sequencing identified specific Ldb1-mediated interactions of the Isl1/Ldb1 responsive Mef2c anterior heart field enhancer with genes that play key roles in cardiac progenitor cell function and cardiovascular development. Importantly, the expression of these genes was downregulated upon Ldb1 depletion and Isl1/Ldb1 haplodeficiency. In conclusion, the Isl1/Ldb1 complex orchestrates a network for heart-specific transcriptional regulation and coordination in three-dimensional space during cardiogenesis.

Inhibition of G9a methyltransferase stimulates fetal hemoglobin production by facilitating LCR/γ-globin looping

Induction of fetal hemoglobin (HbF) production in adult erythrocytes can reduce the severity of sickle cell disease and β-thalassemia. Transcription of β-globin genes is regulated by the distant locus control region (LCR), which is brought into direct gene contact by the LDB1/GATA-1/TAL1/LMO2-containing complex. Inhibition of G9a H3K9 methyltransferase by the chemical compound UNC0638 activates fetal and represses adult β-globin gene expression in adult human hematopoietic precursor cells, but the underlying mechanisms are unclear. Here we studied UNC0638 effects on β-globin gene expression using ex vivo differentiation of CD34(+) erythroid progenitor cells from peripheral blood of healthy adult donors. UNC0638 inhibition of G9a caused dosed accumulation of HbF up to 30% of total hemoglobin in differentiated cells. Elevation of HbF was associated with significant activation of fetal γ-globin and repression of adult β-globin transcription. Changes in gene expression were associated with widespread loss of H3K9me2 in the locus and gain of LDB1 complex occupancy at the γ-globin promoters as well as de novo formation of LCR/γ-globin contacts. Our findings demonstrate that G9a establishes epigenetic conditions preventing activation of γ-globin genes during differentiation of adult erythroid progenitor cells. In this view, manipulation of G9a represents a promising epigenetic approach for treatment of β-hemoglobinopathies.

Chromatin looping and eRNA transcription precede the transcriptional activation of gene in the β-globin locus

Enhancers are closely positioned with actively transcribed target genes by chromatin looping. Non-coding RNAs are often transcribed on active enhancers, referred to as eRNAs (enhancer RNAs). To explore the kinetics of enhancer–promoter looping and eRNA transcription during transcriptional activation, we induced the β-globin locus by chemical treatment and analysed cross-linking frequency between the β-globin gene and locus control region (LCR) and the amount of eRNAs transcribed on the LCR in a time course manner. The cross-linking frequency was increased after chemical induction but before the transcriptional activation of gene in the β-globin locus. Transcription of eRNAs was increased in concomitant with the increase in cross-linking frequency. These results show that chromatin looping and eRNA transcription precedes the transcriptional activation of gene. Concomitant occurrence of the two events suggests functional relationship between them.

Chromatin looping between enhancer and promoter was generated after chemical induction but before the transcriptional activation of gene in the β-globin locus. Transcription of enhancer RNAs was increased in concomitant with the increase of chromatin looping in this locus.

Enhancer-promoter interaction facilitated by transiently forming G-quadruplexes

G-quadruplexes are guanine-rich nucleic acid sequences capable of forming a four-stranded structure through Hoogsteen hydrogen bonding. G-quadruplexes are highly concentrated near promoters and transcription start sites suggesting a role in gene regulation. They are less often found on the template than non-template strand where they either inhibit or enhance transcription, respectively. However, their potential role in enhancers and other distal regulatory elements has not been assessed yet. Here we show that DNAse hypersensitive (DHS) cis-regulatory elements are also enriched in Gs and their G-content correlate with that of their respective promoters. Besides local G4s, the distal cis regions may form G-quadruplexes together with the promoters, each contributing half a G4. This model is supported more for the non-template strand and we hypothesised that the G4 forming capability of the promoter and the enhancer non-template strand could facilitate their binding together and making the DHS regions accessible for the transcription factory.

Chromatin signatures of cancer

Studies by cancer genome consortiums have identified frequent mutations in chromatin regulatory factors and histone proteins in human cancer, implicating them as major mediators in the pathogenesis of both hematological malignancies and solid tumors. Here, Morgan and Shilatifard review recent advances in our understanding of the role of chromatin in cancer, focusing on transcriptional regulatory complexes, enhancer-associated factors, histone point mutations, and alterations in heterochromatin-interacting factors.

Changes in the pattern of gene expression play an important role in allowing cancer cells to acquire their hallmark characteristics, while genomic instability enables cells to acquire genetic alterations that promote oncogenesis. Chromatin plays central roles in both transcriptional regulation and the maintenance of genomic stability. Studies by cancer genome consortiums have identified frequent mutations in genes encoding chromatin regulatory factors and histone proteins in human cancer, implicating them as major mediators in the pathogenesis of both hematological malignancies and solid tumors. Here, we review recent advances in our understanding of the role of chromatin in cancer, focusing on transcriptional regulatory complexes, enhancer-associated factors, histone point mutations, and alterations in heterochromatin-interacting factors.

Long-range gene regulation and novel therapeutic applications

An intimate relationship exists between nuclear architecture and gene activity. Unraveling the fine-scale three-dimensional structure of the genome and its impact on gene regulation is a major goal of current epigenetic research, one with direct implications for understanding the molecular mechanisms underlying human phenotypic variation and disease susceptibility. In this context, the novel revolutionary genome editing technologies and emerging new ways to manipulate genome folding offer new promises for the treatment of human disorders.

An autoregulatory pathway establishes the definitive chromatin conformation at the pit-1 locus

The transcription factor Pit-1 (POU1-F1) plays a dominant role in cell lineage expansion and differentiation in the anterior pituitary. Prior studies of the mouse Pit-1 (mPit-1) gene revealed that this master regulatory locus is activated at embryonic day 13.5 (E13.5) by an early enhancer (EE), whereas its subsequent expression throughout adult life is maintained by a more distal definitive enhancer (DE). Here, we demonstrate that the sequential actions of these two enhancers are linked to corresponding shifts in their proximities to the Pit-1 promoter. We further demonstrate that the looping of the definitive enhancer to the mPit-1 promoter is critically dependent on a self-sustaining autoregulatory mechanism mediated by the Pit-1 protein. These Pit-1-dependent actions are accompanied by localized recruitment of CBP and enrichment for H3K27 acetylation within the Pit-1 locus. These data support a model in which the sequential actions of two developmentally activated enhancers are linked to a corresponding shift in higher-order chromatin structures. This shift establishes an autoregulatory circuit that maintains durable expression of Pit-1 throughout adult life.

ZNF143 provides sequence specificity to secure chromatin interactions at gene promoters

Chromatin interactions connect distal regulatory elements to target gene promoters guiding stimulus- and lineage-specific transcription. Few factors securing chromatin interactions have so far been identified. Here, by integrating chromatin interaction maps with the large collection of transcription factor-binding profiles provided by the ENCODE project, we demonstrate that the zinc-finger protein ZNF143 preferentially occupies anchors of chromatin interactions connecting promoters with distal regulatory elements. It binds directly to promoters and associates with lineage-specific chromatin interactions and gene expression. Silencing ZNF143 or modulating its DNA-binding affinity using single-nucleotide polymorphisms (SNPs) as a surrogate of site-directed mutagenesis reveals the sequence dependency of chromatin interactions at gene promoters. We also find that chromatin interactions alone do not regulate gene expression. Together, our results identify ZNF143 as a novel chromatin-looping factor that contributes to the architectural foundation of the genome by providing sequence specificity at promoters connected with distal regulatory elements.

Chromatin interactions can connect distal regulatory elements to promoters via protein factors, but few such factors have been identified. Here, the authors show that zinc-finger protein ZNF143 is a sequence-specific chromatin-looping factor that connects promoters with distal regulatory elements.

Enhancer-bound LDB1 regulates a corticotrope promoter-pausing repression program

Substantial evidence supports the hypothesis that enhancers are critical regulators of cell-type determination, orchestrating both positive and negative transcriptional programs; however, the basic mechanisms by which enhancers orchestrate interactions with cognate promoters during activation and repression events remain incompletely understood. Here we report the required actions of LIM domain-binding protein 1 (LDB1)/cofactor of LIM homeodomain protein 2/nuclear LIM interactor, interacting with the enhancer-binding protein achaete-scute complex homolog 1, to mediate looping to target gene promoters and target gene regulation in corticotrope cells. LDB1-mediated enhancer:promoter looping appears to be required for both activation and repression of these target genes. Although LDB1-dependent activated genes are regulated at the level of transcriptional initiation, the LDB1-dependent repressed transcription units appear to be regulated primarily at the level of promoter pausing, with LDB1 regulating recruitment of metastasis-associated 1 family, member 2, a component of the nucleosome remodeling deacetylase complex, on these negative enhancers, required for the repressive enhancer function. These results indicate that LDB1-dependent looping events can deliver repressive cargo to cognate promoters to mediate promoter pausing events in a pituitary cell type.

Early stages of induction of anterior head ectodermal properties in Xenopus embryos are mediated by transcriptional cofactor ldb1

BACKGROUND

Specific molecules involved in early inductive signaling from anterior neural tissue to the placodal ectoderm to establish a lens-forming bias, as well as their regulatory factors, remain largely unknown. In this study, we sought to identify and characterize these molecules.

RESULTS

Using an expression cloning strategy to isolate genes with lens-inducing activity, we identified the transcriptional cofactor ldb1. This, together with evidence for its nuclear dependence, suggests its role as a regulatory factor, not a direct signaling molecule. We propose that ldb1 mediates induction of early lens genes in our functional assay by transcriptional activation of lens-inducing signals. Gain-of-function assays demonstrate that the inductive activity of the anterior neural plate on head ectodermal structures can be augmented by ldb1. Loss-of-function assays show that knockdown of ldb1 leads to decreased expression of early lens and retinal markers and subsequently to defects in eye development.

CONCLUSIONS

The functional cloning, expression pattern, overexpression, and knockdown data show that an ldb1-regulated mechanism acts as an early signal for Xenopus lens induction.

The structure of an LIM-only protein 4 (LMO4) and Deformed epidermal autoregulatory factor-1 (DEAF1) complex reveals a common mode of binding to LMO4

LIM-domain only protein 4 (LMO4) is a widely expressed protein with important roles in embryonic development and breast cancer. It has been reported to bind many partners, including the transcription factor Deformed epidermal autoregulatory factor-1 (DEAF1), with which LMO4 shares many biological parallels. We used yeast two-hybrid assays to show that DEAF1 binds both LIM domains of LMO4 and that DEAF1 binds the same face on LMO4 as two other LMO4-binding partners, namely LIM domain binding protein 1 (LDB1) and C-terminal binding protein interacting protein (CtIP/RBBP8). Mutagenic screening analysed by the same method, indicates that the key residues in the interaction lie in LMO4_LIM2 and the N-terminal half of the LMO4-binding domain in DEAF1. We generated a stable LMO4_LIM2-DEAF1 complex and determined the solution structure of that complex. Although the LMO4-binding domain from DEAF1 is intrinsically disordered, it becomes structured on binding. The structure confirms that LDB1, CtIP and DEAF1 all bind to the same face on LMO4. LMO4 appears to form a hub in protein-protein interaction networks, linking numerous pathways within cells. Competitive binding for LMO4 therefore most likely provides a level of regulation between those different pathways.

Enhancer function: mechanistic and genome-wide insights come together

Enhancers establish spatial or temporal patterns of gene expression that are critical for development, yet our understanding of how these DNA cis-regulatory elements function from a distance to increase transcription of their target genes and shape the cellular transcriptome has been gleaned primarily from studies of individual genes or gene families. High-throughput sequencing studies place enhancer-gene interactions within the 3D context of chromosome folding, inviting a new look at enhancer function and stimulating provocative new questions. Here, we integrate these whole-genome studies with recent mechanistic studies to illuminate how enhancers physically interact with target genes, how enhancer activity is regulated during development, and the role of noncoding RNAs transcribed from enhancers in their function.