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Gene-distal enhancers are critical for tissue-specific gene expression, but their genomic determinants within a specific lineage at different stages of development are unknown. Here we profile chromatin state maps, transcription factor occupancy, and gene expression profiles during human erythroid development at fetal and adult stages. Comparative analyses of human erythropoiesis identify developmental stage-specific enhancers as primary determinants of stage-specific gene expression programs. We find that erythroid master regulators GATA1 and TAL1 act cooperatively within active enhancers but confer little predictive value for stage specificity. Instead, a set of stage-specific co-regulators collaborates with master regulators and contributes to differential gene expression. We further identify and validate IRF2, IRF6, and MYB as effectors of an adult-stage expression program. Thus, the combinatorial assembly of lineage-specific master regulators and transcriptional co-regulators within developmental stage-specific enhancers determines gene expression programs and temporal regulation of transcriptional networks in a mammalian genome.
Erythropoiesis in mammals occurs in three waves consisting of primitive progenitors in the yolk sac, definitive precursors in the fetal liver and later in the postnatal bone marrow (McGrath and Palis, 2008; Orkin and Zon, 2008). Several transcription factors, such as GATA1 and TAL1 (or SCL), are essential for erythroid development and are recognized as the erythroid ‘master’ regulators (Cantor and Orkin, 2002). These lineage-specifying master regulators, together with other transcription factors and cofactors, act within complexes on chromatin, establish transcriptional networks, and orchestrate differentiation (Kim and Bresnick, 2007). Master regulators of different lineages often cross-antagonize each other’s activity during lineage specification (Graf and Enver, 2009). However, it is less clear how master regulators control programs at different stages of development within the same cell lineage.
A gene regulatory network consists of trans-acting regulators and cis-acting elements within core promoters and gene-distal enhancers whose interaction with each other control tissue- and developmental stage-specific programs (Bulger and Groudine, 2011). Genome-wide studies suggest that enhancers and promoters exhibit distinct chromatin ‘signatures’. The characteristic signature for enhancers consists of monomethylation of histone H3 lysine 4 (H3K4me1), acetylation of histones (H3K9ac and H3K27ac), and binding of the acetyltransferase p300 (Heintzman et al., 2007; Koch et al., 2007; Visel et al., 2009). Studies comparing various lineages indicate that enhancers are associated with highly cell-type-specific histone modifications and strongly correlate to global cell-type-specific programs (Blow et al., 2010; Creyghton et al., 2010; Ernst et al., 2011; Ghisletti et al., 2010; Heintzman et al., 2009; Mikkelsen et al., 2010; Rada-Iglesias et al., 2011). Hence, it has been suggested that enhancers are the primary determinants of cell type specificity in gene expression. However, the majority of studies have employed cells from very different lineages and/or immortalized cell lines. It remains unknown the degree of overlap between cells within a specific lineage at different stages of differentiation or similar cells at different stages of development.
We reasoned that comparative profiling of closely related cell types corresponding to distinct developmental stages should delineate regulatory networks directly related to the associated gene expression programs. Classification of the trans- and cis-regulatory elements that are either shared or stage-specific should clarify their relative importance and prioritize functional candidates. We have focused on an ex vivo maturation system for human fetal and adult erythropoiesis. Primary hematopoietic stem/progenitor cells (HSPCs) can be propagated and induced for erythroid differentiation with a set of defined cytokines ex vivo. This system has been widely used in molecular analysis of erythropoiesis (Migliaccio et al., 2009).
Here, we report the comparative investigation of genome-wide chromatin state maps, TF occupancy, and gene expression profiles from developing red cell precursors at two developmental stages. Distal enhancers, not promoters, are marked with highly stage-specific histone modifications and DNase I hypersensitivity, strongly correlate to stage-specific gene expression changes, and are functionally active in a stage-specific manner. Master regulators GATA1 and TAL1 act cooperatively within active enhancers but have little predictive value for stage-specific enhancer activity. In contrast, a set of stage-specific cofactors and signaling pathways collaborate with these regulators and account for the stage specificity. Two such cofactors, the interferon regulatory factors IRF2 and IRF6, are essential for activation of adult erythroid programs through cooperation with master regulators and cohesin-mediator complexes at distal enhancers.
We employed a serum-free two-phase liquid culture system to expand and differentiate primary fetal or adult stage human CD34+ hematopoietic stem/progenitor cells (HSPCs) ex vivo (Sankaran et al., 2008). In this experimental context, highly enriched populations of stage-matched, differentiating, primary proerythroblasts (ProEs) were generated (Figure 1A). We selected four time points (day 0, CD34+ HSPCs; day 3, 5, and 7, differentiating ProEs) that represented similar stages of differentiation (Figure 1A; Figure S1A,B). Adult CD34+ HSPC-derived ProEs expressed predominantly adult hemoglobin (β-globin). Conversely, fetal ProEs expressed largely fetal hemoglobin (γ-globin) (Figure 1B), indicating that the ex vivo system faithfully recapitulates stage-specificity.
We determined the mRNA expression profiles in fetal and adult HSPCs and differentiating ProEs by Affymetrix microarray (Figure 1A, Table S1). Upon differentiation of CD34+ HSPCs to day 5 ProEs, 858 and 1137 genes were significantly upregulated (fold change > 1.5, FDR < 0.05) at fetal and adult stages, respectively (Figure S1C). 333 genes were upregulated at both stages (Figure S1D). These genes are involved in heme biosynthesis, iron homeostasis, and erythrocyte differentiation (Figure S1E), suggesting that a common set of gene signatures required for erythroid functions is activated at both stages. More importantly, comparative transcriptome profiling revealed distinct gene expression programs at different stages of erythroid maturation. 1039 and 1291 genes linked to distinct functional annotations were differentially expressed in fetal and adult day 5 ProEs, respectively (Figure 1C; Figure S1F,G). 93.7% of the differentially expressed genes had expression changes less than 4-fold, suggesting that the majority of these genes differed to a modest extent between fetal and adult stages (Figure 1C,D).
To investigate the basis of the distinct programs, we generated genome-wide maps for chromatin state and transcription factor (TF) occupancy by the ChIP-seq method. We profiled 9 histone modifications (H3K4me1/me2/me3, H3K9me3, H3K37me3, H3K36me2/me3, H3K9ac, and H3K27ac) and 6 TFs (GATA1, TAL1, NFE2, CTCF, RAD21, and RNA polymerase II) in fetal and adult day 5 ProEs. GATA1, TAL1, and NFE2 are critical hematopoietic regulators (Orkin and Zon, 2008). CTCF is required for transcriptional insulation (Phillips and Corces, 2009). RAD21 is a component of the cohesin complex and regulates chromatin architecture between enhancers and promoters (Kagey et al., 2010). Most TFs were expressed comparably between fetal and adult ProEs, except that TAL1 and NFE2 were slightly upregulated in adult ProEs (Figure S2A). We identified between 17106 and 71785 binding peaks for histone marks, and between 5664 and 22835 peaks for TFs (Table S2; Experimental Procedures). 95% (55 out of 58) and 97% (56 out of 58) of ChIP-seq peaks were validated by ChIP-qPCR in fetal and adult ProEs, respectively (Figure S2B–H).
The genomic distributions of the profiled histone marks were consistent with their known functions. H3K4me3, which is associated with transcriptional initiation, was distributed primarily near proximal promoters (Figure 1E). H3K4me1/me2, H3K9ac and H3K27ac, which are associated with open chromatin and distal elements, showed diverse distributions across promoters, intragenic, and intergenic regions. H3K36me3, which is associated with transcriptional elongation, was found primarily across gene bodies. Of note, the global distributions of histone marks were highly similar at fetal and adult stages. The distributions of profiled TFs were also comparable (Figure 1E; Table S2).
To explore the relationships between cis-regulatory elements, chromatin signatures, and TF occupancy, we focused on the β-globin gene cluster (Figure 2). The β-globin locus contains the embryonic (ε), fetal (Gγ, Aγ) and adult (δ, β) globin genes that are expressed sequentially during development. The switch from fetal (γ) to adult (β) expression provides a paradigm for tissue- and developmental stage-specific transcription (Sankaran et al., 2010). A locus control region (LCR) containing erythroid-specific cis-regulatory elements is located upstream and controls expression of the linked globin genes (Li et al., 2002a).
The profiled histone marks and TFs showed spatial and temporal density distributions that are qualitatively consistent with their known functions within the β-globin gene cluster (Figure 2). In fetal ProEs, the active fetal globin genes (Gγ and Aγ) were associated strongly with activating histone marks (H3K4me2/me3, H3K9ac, and H3K27ac). Conversely, these marks were enriched only at the adult globin genes (δ and β) in adult ProEs, indicating that the fetal-to-adult globin switch is reflected in a switch of chromatin landscape within the cluster (Figure 2). The upstream LCR was enriched with active chromatin marks in both fetal and adult ProEs. The peaks overlapped with known DNase I hypersensitive sites (DHS) (Figure 2), consistent with function of the LCR at both stages. In contrast with a dynamic chromatin profile within the β-globin locus, the occupancy of several TFs was largely comparable. GATA1 and TAL1 predominantly occupied HS1-4 in both fetal and adult ProEs. The profiles of several TFs were consistent with their known functions within the locus. NFE2 predominantly occupied the HS2 within the LCR and was associated with transcriptional activation (Andrews et al., 1993). CTCF occupied the HS5 at both stages, consistent with its insulator function within the LCR (Bell et al., 2001; Li et al., 2002b).
To investigate the regulatory mechanisms for stage-specificity, we first characterized the chromatin features around RefSeq-annotated promoters. We used H3K4me3 and H3K27me3 to categorize promoters into four groups: active (marked by H3K4me3-only), bivalent (marked by both H3K4me3 and H3K27me3) (Bernstein et al., 2006; Boyer et al., 2006), repressed (H3K27me3-only), and null (no H3K4me3/H3K27me3) (Figure 3A, Table S3). The promoter activities highly correlated with mRNA levels and were similar between fetal and adult ProEs (Figure 3A,B). Comparative analyses of the individual promoter categories revealed significant overlaps between fetal and adult cells (Figure 3C), indicating that the promoter activities are largely invariant. Clustering analysis of all ChIP-seq densities at proximal promoters indicated that the activating histone marks (H3K4me1/me2/me3, H3K9ac, and H3K27ac) highly correlated with each other and negatively correlated with repressive marks (H3K9me3 and H3K27me3). Most profiled histone marks and TFs displayed strong colocalization between fetal and adult cells (Figure 3D), indicating that their binding to the proximal promoters is largely invariant. Heatmap analysis revealed that several profiled TFs, such as GATA1, NFE2, and CTCF, were associated predominantly with active promoters at both stages (Figure 3E).
To directly examine the correlation between chromatin signatures, TF occupancy and mRNA expression, we focused on genes differentially expressed between fetal and adult day 5 ProEs. Surprisingly, no obvious changes in chromatin signatures and TF occupancy were found within the promoters of differentially expressed genes (Figure S3A), suggesting that the modest expression changes (Figure 1C,D) are not accompanied by changes in promoter activities. It is important to note that the small subset of genes showing the strongest differences in expression appear to be more likely than the rest to show changes in H3K4me3 in their promoters (data not shown). Similarly, no significant changes in TF occupancy were found in promoters that remained unchanged (‘invariant’ promoters) or changed their activities (‘variant’ promoters) at fetal and adult stages (Figure S3B,C). Thus, chromatin state and TF occupancy at proximal promoters are largely invariant on a global scale during fetal and adult erythropoiesis.
Studies of cells representing different lineages have indicated that gene-distal enhancers are marked with highly cell-type-specific histone modifications and strongly correlate to cell-type-specific gene expression. We next investigated whether enhancers also contribute to stage-specific gene expression programs during fetal and adult erythropoiesis. To identify distal enhancers, we used histone marks H3K4me1, H3K9ac, H3K27ac, and H3K27me3 to distinguish distal elements from proximal promoters (Experimental Procedures). Genome-wide analysis identified 8947 and 11709 active enhancers (that is, marked by H3K4me1, H3K9ac or H3K27ac, DHS, and absence of H3K27me3) in fetal and adult ProEs, respectively. Enhancer regions that overlapped the same chromatin marks in both cells were identified as “common” enhancers (total 4360) and the remainder as “fetal-only” (total 2594) or “adult-only” (total 5730) enhancers (Figure 4A, Table S4). Representative common and developmental stage-specific enhancers are shown (Figure 4B; Figure S4D).
Heatmap analysis revealed that most of the profiled TFs associated strongly with active enhancers at both stages (Figure 4C). The binding intensities of the master regulators, such as GATA1 and TAL1, were higher at common enhancers as compared to fetal- or adult-only enhancers (Figure 4C). We also detected preferential association of activating histone marks (H3K4me1/2, H3K9ac, and H3K27ac) and RNA Pol II with common enhancers at both stages. To determine whether the chromatin signatures for distal enhancers correlate with transcriptional activity, we profiled enhancer peak densities around differentially expressed genes (Figure 4D) (Experimental Procedures). Remarkably, the peak densities of fetal-only enhancers were significantly higher at genes upregulated in fetal day 5 ProEs (F5-high genes) compared to adult day 5 (A5)-high genes, and vice versa. Common enhancers associated comparably with both F5- and A5-high genes (Figure 4D). Both common and stage-specific enhancers were associated positively with mRNA levels in F5 and A5 ProEs (Figure S4A), and were highly enriched among genes activated during erythropoiesis (Figure S4B). This analysis also indicated that enhancer peaks were highly enriched in both intragenic regions (TSS to TES) and gene-distal regions, indicating that a significant subset of enhancers is located within gene bodies.
To functionally test whether the distal open chromatin marks identified erythroid cell- and developmental stage-specific enhancers, we performed transient enhancer reporter assays in erythroleukemia K562 cells and stable reporter assays in primary fetal or adult erythroid progenitor cells. Genomic fragments containing putative enhancers markedly enhanced reporter expression in K562 cells (Figure S4C). More importantly, fetal- or adult-only enhancers displayed differential activities consistent with their stage-specificity (Figure 4E). Thus, distal regions that show changes in open chromatin marks during fetal and adult erythropoiesis are likely to be enriched for stage-specific enhancers.
Enhancers often function through direct enhancer-promoter contact by DNA loop formation. We next determined whether the identified enhancers interact with proximal promoters by chromosome conformation capture (Dekker et al., 2002). The frequency of interaction between the IRF2 promoter and a putative downstream enhancer was significantly higher than other tested regions (Figure 4F). Importantly, the interaction frequency increased 2-fold in adult relative to fetal ProEs, consistent with an adult stage-specificity. Conversely, a representative fetal-only enhancer within the COL4A5 gene displayed increased interaction frequency in fetal ProEs. In contrast, a representative common enhancer within the ZFPM1 gene had comparable interaction frequency in both cells (Figure 4F; Figure S4D).
To directly assess whether stage-specific enhancers contribute to differential gene expression patterns, we mapped enhancers to target genes. We reasoned that selecting enhancers with the most ‘biased’ (for stage-specific enhancers) or ‘unbiased’ (for common enhancers) activities would illustrate how distal enhancers and the associated trans-acting regulators contribute to the developmental stage-specific gene regulatory networks. To this end, we first normalized the ChIP-seq datasets by ‘MAnorm’ (Shao et al., 2012). In this approach, ChIP-seq datasets from different samples were quantitatively compared after rescaling of ChIP-seq signals by a set of common peaks (Experimental Procedures). We then selected biased stage-specific enhancers by a more stringent threshold requiring a fold change ≥ 2, which represents the ratio of enhancer binding intensities between fetal and adult samples or vice versa. These analyses identified of 967 and 2024 high-confidence (HC) fetal-only and adult-only enhancers, respectively (Figure S5A,B, Table S5). Similarly, we identified 2970 high-confidence common enhancers (fold change < 2). Compared to enhancers from previous overlap analysis (Figure 4A), the high-confidence stage-specific enhancers displayed more distinct stage-specific patterns for histone marks (Figure S5C,D), consistent with their stage specificity. Finally, we mapped enhancers to target genes by two approaches including ‘nearest neighbor gene’ and ‘proximal neighbor genes’ (Figure S6A,B). By comparing the gene expression patterns of enhancer targets, we observed no significant difference between two approaches and choose the ‘nearest neighbor gene’ approach for the remaining analyses (Figure S6A; Table S7).
We then assessed the relationship between enhancer binding and mRNA expression. Strikingly, genes targeted by HC-fetal-only enhancers were significantly enriched for F5-high genes and depleted for A5-high genes (P = 4.8 × 10−22). Genes targeted by HC-adult-only enhancers were significantly enriched for A5-high genes (P = 1.9 × 10−48). In contrast, genes targeted by HC-common enhancers were enriched for both F5- and A5-high genes and had little predictive value (Figure 5A). These analyses demonstrate that the stage-specific enhancers highly correlate with changes in expression of the linked genes. Notably, we found that the greater the expression level of a gene increased in fetal or adult ProEs, the more likely it was to be associated with fetal- or adult-stage-specific enhancers (Figure 5B).
Active regulatory elements often contain low nucleosome density and DNase I hypersensitive sites (DHS). We examined whether stage-specific enhancers correlate with DHS at global scale using DNase-seq (Hesselberth et al., 2009). Strikingly, the enhancer activities highly correlate to DNase-seq intensities in a stage-dependent manner (Figure 5C). DNase-seq intensities were markedly increased in the HC-fetal-only enhancers as compared to the HC-adult-only enhancers in fetal erythroid cells, and vice versa. HC-common enhancers displayed comparable levels, suggesting that they are largely invariant (Figure 5C).
We next explored whether enhancer maps might reveal novel trans-regulatory factors involved in the stage-specific gene expression. We first observed that the most enriched TF motifs are recognized by erythroid master regulators (TAL::GATA1 and GATA1) in all three enhancer classes (Figure S6C), suggesting that binding of these regulators does not distinguish and/or predict stage selectivity. To identify TF motifs differentially enriched between the stage-specific enhancers, we enumerated instances of all known TF motifs and ranked them according to their relative enrichment in fetal- or adult-only enhancers. Among the motifs most enriched within the HC-fetal-only enhancers are recognition sites for known regulators of hematopoiesis, including EVI1, GATA2, and GATA1 (Figure 5D). The list of motifs contained other known regulators of cell cycle regulation, proliferation and apoptosis, such as the SRF, MAF, and AP1 families. The most enriched motifs within the HC-adult-only enhancers are recognized by the interferon regulatory factors (IRF2, IRF1, and IRF8), CBF, EGR, and MYB proteins (Figure 5D).
The motif enrichment analyses of developmental stage-specific enhancers identified candidate regulators whose roles in human erythropoiesis remain unexplored. We examined whether these candidate regulators contribute to erythroid enhancer functions. Expression of two of the nine IRF family genes, IRF2 and IRF6, was specifically and progressively activated during adult erythropoiesis (Figure S7A). We confirmed mRNA and protein expression in CD34+ HSPCs and differentiating ProEs (Figure 5E; Figure S7B). Interestingly, studies in knockout mice have implicated IRF2 in aspects of hematopoiesis (Matsuyama et al., 1993). Besides defective lymphoid development, adult Irf2-deficient mice are anemic due to ineffective erythropoiesis (Mizutani et al., 2008). IRF2 can activate or repress gene transcription depending on the cellular context (Harada et al., 1990; Lohoff et al., 2000). IRF6 has been linked to Van der Woude and popliteral pterygium syndromes but its role in hematopoiesis has not been explored (Kondo et al., 2002). The involvement of the proto-oncogene MYB in regulation of erythropoiesis has been established in mice containing null or hypomorphic Myb alleles (Emambokus et al., 2003; Mucenski et al., 1991). Although the mechanism remains elusive, MYB controls erythropoiesis by coordinating cell cycle regulation, differentiation, and globin gene expression (Emambokus et al., 2003; Sankaran et al., 2011). MYB expression was significantly higher in adult relative to fetal ProEs (Figure 5E; Figure S7A,B).
To establish whether these factors also regulate human erythropoiesis, we used loss-of-function assays in primary erythroid progenitors. Upon shRNA-mediated depletion of IRF2 or IRF6 expression, differentiating adult ProEs failed to activate a large portion of A5-high signature genes, whereas F5-high signature genes were slightly derepressed (Figure 5H; Figure S7C,D). Similarly, depletion of MYB expression downregulated A5-high genes. Moreover, loss of IRF2, IRF6 or MYB expression led to downregulation of the signature genes induced during adult erythropoiesis (A0-to-A5-high genes; Figure S7E), whereas the F0-to-F5-high signature genes remained largely unaffected (Figure S7F). Gene set enrichment analysis (GSEA) was used with A5-high signature genes. A marked decrease in expression of A5-high signature genes was notable in the shRNA-treated cells compared with controls, suggesting that these candidate regulators are indispensable for activation of adult-high signature genes (Figure 5I).
To directly assess the function of IRF2 in adult cells, we performed ChIP-seq analysis of IRF2 in adult ProEs. IRF2 is preferentially associated with adult-only enhancers, consistent with its role in adult enhancer functions (Figure 5J,K). IRF2 occupied 11.4% and 13.8% of HC-Adult-Only and HC-Common enhancers, respectively, whereas only 2.8% of HC-Fetal-Only enhancers were occupied (Figure 5L,M). Taken together, these data demonstrate crucial roles of several candidate regulators for the adult stage-specific program, and indicate that trans-regulatory factors can be identified by an integrative approach combining genomic profiling and motif enrichment analysis.
Our comparative investigation of epigenetic state maps and TF occupancy provided a unique system to explore the combinatorial regulation of erythroid enhancers. We performed k-means clustering to classify each enhancer on the basis of TF occupancy (Experimental Procedures). We observed four distinct clusters (AE1 to AE4) within adult enhancers characterized by distinct patterns of TF occupancy (Figure 6A, Table S6). Notably, AE1 was the largest cluster and occupied by the master regulators GATA1 and TAL1. AE2 was co-occupied by GATA1 and TAL1, together with the cohesin subunit RAD21. AE3 and AE4 were predominantly associated with CTCF and NFE2, respectively. To assess the temporal regulation of adult enhancers, we next compared TF occupancy between fetal and adult ProEs. Profiles of TF occupancy within the AE1 and AE3 clusters were largely invariant at both stages. In contrast, RAD21 and NFE2 profiles within the AE2 and AE4 clusters were variable, increasing from little or no binding in fetal ProEs to strong binding in adult ProEs (Figure 6A; Figure S8A). These analyses indicate that the variant enhancer clusters are associated with temporal changes in combinatorial regulatory patterns and may contribute to stage-specific transcription.
To test this hypothesis, we mapped each enhancer to its target gene and assessed the relationship between enhancer clustering and mRNA expression. Genes targeted by AE2 (GATA1 + TAL1 + cohesin) and AE4 (NFE2) were significantly enriched for A5-high genes relative to F5-high genes (P = 3.6 × 10−6 and 1.3 × 10−7, respectively) (Figure 6B, Table S7), suggesting that these variant enhancer clusters are strongly associated with adult-specific transcription. In contrast, the invariant clusters (AE1 and AE3) showed less enrichment and had little predictive value for stage selectivity (Figure 6B). To explore other candidate regulators that may contribute to the temporal changes in enhancer activity, we examined the enriched TF motifs within the enhancer clusters. Consistent with the TF occupancy patterns, motif analyses identified strong enrichment of motifs for the corresponding TFs in each enhancer cluster (Figure 6C). Notably, the IRF2 motif was more enriched in variant clusters (AE2 and AE4) than invariant clusters (Figure 6C), consistent with increased IRF2 occupancy within the variant clusters (Figure 6D,E), suggesting that it may contribute to the temporal changes in enhancer activity by differential enhancer association.
To gain further insight into the mechanistic role of IRF2 and IRF6 in regulating adult enhancers, we identified their interacting protein complexes by a proteomic screen in human erythroid cells (Figure 7A). In addition to known interacting proteins such as IRF2BP1, IRF2BP2, RELA, STAT1, and STAT2 (Childs and Goodbourn, 2003; Rouyez et al., 2005), we identified subunits of the mediator and cohesin complexes, implicating the association between IRF2/IRF6 and the mediator and cohesin complexes for function. Physical association between IRF2/IRF6 and the mediator subunits (MED1 and MED12) or the cohesin subunits (RAD21 and SMC1A) was confirmed by independent co-IP experiments in K562 cells and primary erythroid cells (Figure 7B,C). Of note, interaction between IRF2/IRF6 and the master regulators (GATA1 and TAL1), but not NFE2 or CTCF, was also observed, indicating that they may function in the same complexes. Importantly, IRF2 co-occupied a portion of GATA1 and TAL1 peaks at a global scale (Figure 7D). The overlap was significantly higher within adult enhancers (Figure 7E). In addition, both IRF2 and IRF6 genes were associated with adult-only enhancers occupied by GATA1 and TAL1 in a stage-dependent manner (Figure 7F). It remains unclear how IRF proteins contribute to adult-stage selectivity of CTCF- or NFE2-enriched enhancer cluster (AE3 and AE4; Figure 6A). We did not observe interaction between IRF proteins, CTCF, and NFE2 (Figure 7A,B). However, IRF proteins may act indirectly on NFE2 activity. For example, enforced expression of IRF2 in myeloid progenitor cells induced megakaryocytic maturation and enhanced NFE2 binding activity to its consensus DNA sequences (Stellacci et al., 2004).
To further explore how IRF2 cooperates with the erythroid regulators within adult enhancers, we examined the TF occupancy and chromatin state at a set of representative adult enhancers (GATA1/IRF2 co-bound or GATA1 only bound enhancers; Figure 7G) in adult ProEs in the presence or absence of IRF2. Upon depletion of IRF2 expression, GATA1 occupancy was diminished at 7 out of 8 GATA1/IRF2 co-bound enhancers whereas it remained associated with GATA1 only bound enhancers (Figure 7G). Binding of cohesin subunit (RAD21) and mediator subunit (SMC1A) was abolished at GATA1/IRF2 co-bound enhancers, suggesting that IRF2 is indispensable for maintaining the configuration of at least a subset of adult stage-specific enhancers and contributes directly or indirectly to the recruitment of master regulators. Taken together, these data support a model in which the erythroid regulators GATA1 and TAL1 trans-activate IRF2 and IRF6 by occupying their distal enhancers during adult erythropoiesis. IRF2 and IRF6 then cooperate with these regulators, as well as the mediator and cohesin complexes, on a set of adult stage-specific enhancers and contribute to the differential activation of an adult program (Figure 7H).
To illustrate the temporal and stage-specific regulation of distal enhancers, gene expression patterns, and TF occupancy during fetal and adult erythropoiesis, we assembled the gene regulatory networks by connecting enhancer activity and gene expression changes (Enhancer-to-Gene) or TF occupancy and enhancer activity (TF-to-Enhancer). In the Enhancer-to-Gene network, we connected an edge from each enhancer to its corresponding target gene and focused on those enhancers associated with genes differentially expressed between fetal and adult ProEs (Figure 8A, Table S8). The Enhancer-to-Gene connections were largely exclusive and reflected stage-specificity. Adult stage-specific enhancers were predominantly associated with genes upregulated in adult erythropoiesis. Conversely, fetal stage-specific enhancers were highly associated with genes upregulated in fetal erythropoiesis. Common enhancers were associated with both gene groups and had little predictive value for gene expression changes.
To further delineate the relationship between TF occupancy and enhancer activity, we generated the TF-to-Enhancer network in which each edge was defined when at least one enrichment peak for the profiled TFs was present within the enhancer region (Figure 8B). Subsets of the common and stage-specific enhancers were associated with TFs in a highly stage-specific manner, suggesting that the temporal changes in TF occupancy control differential enhancer activity. Moreover, the high degree of co-occupancy by multiple TFs within each enhancer cluster suggests that they are coregulated by several TFs. Of note, because a large portion of enhancers were associated with yet to be determined TFs, the density and complexity of the combinatorial regulation is likely even greater than observed. Thus, our data suggest that gene-distal enhancers are major determinants of developmental stage specific expression. The combinatorial assembly of master regulators and transcriptional co-regulators at developmental stage-specific enhancers controls gene expression programs and temporal regulation of transcriptional networks during human erythropoiesis.
Our comparative analyses of erythropoiesis at two developmental stages has identified a set of developmental stage-specific enhancers that are marked with highly stage-specific histone marks and functionally active in a stage-specific manner. Compared to common enhancers, the stage-specific enhancers are more weakly associated with master regulators (GATA1 and TAL1) as well as active histone marks (Figure 4C). These data suggest that stage-specific enhancers may function to modulate expression of target genes (“fine-tuning”) rather than to control genes “on” or “off”. Consistent with this hypothesis, the vast majority of the differentially expressed genes display only modest changes in expression between fetal and adult stages (Figure 1C,D). Alternatively, the stage-specific enhancers may be bound/activated in only a proportion of the cells, while the common enhancers are bound/activated in all cells. Single cell analysis is needed to determine the frequency of expression of stage-specific genes and enhancer activities. Nevertheless, our findings suggest that distal enhancers are the major determinants of developmental stage-specific gene expression programs. Further characterization of these stage-specific enhancers provides a path to discovery of novel candidate regulators that contribute to the differential enhancer activity required for erythroid development.
Lineage-specifying master regulators are required for programming of global gene expression towards specific lineages and often exhibit dual functions by promoting their own lineage decisions while antagonizing factors favoring other choices. Genome-wide studies in multiple cell types have revealed pivotal roles of master regulators in establishing gene expression programs by diverse mechanisms including cooperation with other cofactors and chromatin modifying complexes (Cheng et al., 2009; Fujiwara et al., 2009; Kassouf et al., 2010; Pilon et al., 2011; Tijssen et al., 2011; Yu et al., 2009). Moreover, studies utilizing embryonic stem (ES) cells and multiple hematopoietic cell types have shown that master regulators of various cell lineages direct signaling transcription factors to cell-type-specific gene programs during differentiation and regeneration (Chen et al., 2008; Mullen et al., 2011; Trompouki et al., 2011). Here we demonstrate that master regulators act cooperatively within active enhancers but have little predictive value for stage-specific transcription. Instead, a set of stage-specific cofactors and signaling pathways, such as the interferon regulatory factors IRF2 and IRF6, collaborate with master regulators and confer stage specificity. Thus, we provide a model in which master regulators are actively involved in cell-type and developmental stage-specific transcriptional programs, but their specific roles are instructed and refined by developmental stage-specific cofactors to establish stage-specificity.
Mutation or altered regulation of master TFs has been linked to hematologic malignancies. Our findings suggest that transcriptional co-regulators or signaling pathways may be essential for the proper functionality of master regulators, and thus may be potential targets for therapeutic interventions. Particularly, cell-type and stage-specific activity may offer a therapeutic option that is more selective and effective. The stage-specific activity of co-regulators may provide an opportunity to ‘reprogram’ hematopoietic differentiation by switching of gene expression programs. For example, LIN28B gene is specifically expressed in fetal cells and harbors a fetal stage-specific enhancer (Table S1; Figures 4E and S4C). Ectopic expression of LIN28B reprograms adult hematopoietic progenitors to fetal-like lymphocytes in a cell autonomous manner (Yuan et al., 2012), suggesting that LIN28B plays a critical role in regulating fetal identity. Our studies indicate that the developmental stage-specific enhancers may contribute to the differential expression of LIN28B gene, and interference with enhancer functionality may lead to altered gene expression. Manipulation of stage-selective cis- or trans-regulatory elements can highlight therapeutic interventions in hematologic disorders, such as sickle cell anemia and β-thalassemias, in which the fetal-to-adult hemoglobin switch is important for pathogenesis and its reversal can ameliorate disease severity (Xu et al., 2011). Thus, comparative analyses of enhancer signatures provide critical insights into global networks regulating transcription during the ontogeny and pathogenesis of human hematopoiesis.
Primary human fetal CD34+ HSPCs were isolated from second trimester fetal livers as described (Van Handel et al., 2010). Primary human adult CD34+ HSPCs were obtained from Yale Center of Excellence in Molecular Hematology (YCEMH). Primary fetal or adult erythroblasts were generated ex vivo as described (Sankaran et al., 2008; Xu et al., 2010). The K562-BirA, K562-FLAG-Bio-IRF2 (FB-IRF2), and K562-FLAG-Bio-IRF6 (FB-IRF6) stable cell lines were generated as described (Kim et al., 2009).
For ChIP-seq analysis using the HeliScope™ Single Molecule Sequencer, ChIP DNA was processed for 3′ polyA tailing, followed by 3′ ddATP-blocking as described (Hart et al., 2010). Processing samples by ligation, amplification and size selection are not required by Helicos sequencing. Peaks were called using Model-based Analysis for ChIP-Seq (MACS) (Zhang et al., 2008). See Supplemental Experimental Procedures.
mRNA profiling of primary fetal and adult HSPCs (CD34+; F0 and A0) and maturing erythroid cells (ProE; F3, F5, F7, A3, A5, and A7) were performed using Affymetrix human genome U133 Plus 2.0 arrays. See Supplemental Experimental Procedures.
Lentiviral RNAi was performed as described (Xu et al., 2010). For enhancer reporter assay in primary human erythroid progenitors, genomic fragments were cloned into a lentiviral vector in front of a TK minimal promoter driving GFP expression. See Supplemental Experimental Procedures.
We thank V.G. Sankaran, D. Higgs, D.A. Williams, L.I. Zon, E.H. Bresnick, and members of the Orkin laboratory for discussions. We thank Z. Herbert for assistance with Helicos sequencing, F. Abderazzaq and R. Rubio at CCCB sequencing facility at DFCI for assistance with Illumina HiSeq2000, E. Fox at Microarray Core at DFCI for microarray analyses, and R. Tomaino and S. Gygi at the Taplin Mass Spectrometry Facility for assistance with protein identification. This work was supported by funding from the National Institutes of Health (NIH) (S.H.O., G.C.Y., J.A.S., and H.K.A.M.), and NIH ARRA grant RCHL101553 (S.H.O.). S.H.O. is an Investigator of the Howard Hughes Medical Institute (HHMI). J.X. is an HHMI-Helen Hay Whitney Foundation fellow and supported by a NIDDK Career Development Award K01DK093543.
The cDNA microarray and ChIP-seq data were deposited in the Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo) under accession number GSE36994.
The authors declare no conflict of interest.
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