Forced expression of reprogramming factors can convert somatic cells into induced pluripotent stem cells (iPSCs). Here we studied genome topology dynamics during reprogramming of different somatic ...cell types with highly distinct genome conformations. We find large-scale topologically associated domain (TAD) repositioning and alterations of tissue-restricted genomic neighborhoods and chromatin loops, effectively erasing the somatic-cell-specific genome structures while establishing an embryonic stem-cell-like 3D genome. Yet, early passage iPSCs carry topological hallmarks that enable recognition of their cell of origin. These hallmarks are not remnants of somatic chromosome topologies. Instead, the distinguishing topological features are acquired during reprogramming, as we also find for cell-of-origin-dependent gene expression patterns.
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•The 3D genome topology of four somatic cell types varies greatly and differs from ESCs•The 3D genomes of iPSCs from different founders and of ESCs are overall highly similar•Early-passage iPSCs show subtle but reproducible founder-dependent 3D differences•The distinctive topology features of iPSCs are acquired during reprogramming
Krijger et al. report that the reprogramming of four somatic cell types with highly distinct 3D genomes results in pluripotent cells with largely identical, ESC-like, genome conformations carrying founder-dependent topological hallmarks. The latter are not remnants of somatic chromosome topologies but are acquired during reprogramming in a cell-of-origin-dependent manner.
The relationship between 3D organization of the genome and gene-regulatory networks is poorly understood. Here, we examined long-range chromatin interactions genome-wide in mouse embryonic stem cells ...(ESCs), iPSCs, and fibroblasts and uncovered a pluripotency-specific genome organization that is gradually reestablished during reprogramming. Our data confirm that long-range chromatin interactions are primarily associated with the spatial segregation of open and closed chromatin, defining overall chromosome conformation. Additionally, we identified two further levels of genome organization in ESCs characterized by colocalization of regions with high pluripotency factor occupancy and strong enrichment for Polycomb proteins/H3K27me3, respectively. Underlining the independence of these networks and their functional relevance for genome organization, loss of the Polycomb protein Eed diminishes interactions between Polycomb-regulated regions without altering overarching chromosome conformation. Together, our data highlight a pluripotency-specific genome organization in which pluripotency factors such as Nanog and H3K27me3 occupy distinct nuclear spaces and reveal a role for cell-type-specific gene-regulatory networks in genome organization.
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•Long-range chromatin contacts in pluripotent cells differ from those in somatic cells•Distal genomic regions with extensive Oct4/Sox2/Nanog binding colocalize in ESCs•Distal Polycomb protein/H3K27me3-enriched genomic regions frequently interact in ESCs•Spatial clustering of Polycomb/H3K27me3-enriched genomic regions requires Eed
Independent interaction networks involving chromatin structure, pluripotency factors, and Polycomb proteins mediate genome organization in pluripotent cells.
Abstract
It is becoming increasingly clear that chromosome organization plays an important role in gene regulation. High-resolution methods such as 4C, Capture-C and promoter capture Hi-C (PCHiC) ...enable the study of chromatin loops such as those formed between promoters and enhancers or CTCF/cohesin binding sites. An important aspect of 4C/Capture-C/PCHiC analyses is the reliable identification of chromatin loops, preferably not based on visual inspection of a DNA contact profile, but on reproducible statistical analysis that robustly scores interaction peaks in the non-uniform contact background. Here, we present peakC, an R package for the analysis of 4C/Capture-C/PCHiC data. We generated 4C data for 13 viewpoints in two tissues in at least triplicate to test our methods. We developed a non-parametric peak caller based on rank-products. Sampling analysis shows that not read depth but template quality is the most important determinant of success in 4C experiments. By performing peak calling on single experiments we show that the peak calling results are similar to the replicate experiments, but that false positive rates are significantly reduced by performing replicates. Our software is user-friendly and enables robust peak calling for one-vs-all chromosome capture experiments. peakC is available at: https://github.com/deWitLab/peakC.
The architecture of human chromosomes in interphase nuclei is still largely unknown. Microscopy studies have indicated that specific regions of chromosomes are located in close proximity to the ...nuclear lamina (NL). This has led to the idea that certain genomic elements may be attached to the NL, which may contribute to the spatial organization of chromosomes inside the nucleus. However, sequences in the human genome that interact with the NL in vivo have not been identified. Here we construct a high-resolution map of the interaction sites of the entire genome with NL components in human fibroblasts. This map shows that genome-lamina interactions occur through more than 1,300 sharply defined large domains 0.1-10 megabases in size. These lamina-associated domains (LADs) are typified by low gene-expression levels, indicating that LADs represent a repressive chromatin environment. The borders of LADs are demarcated by the insulator protein CTCF, by promoters that are oriented away from LADs, or by CpG islands, suggesting possible mechanisms of LAD confinement. Taken together, these results demonstrate that the human genome is divided into large, discrete domains that are units of chromosome organization within the nucleus.
The evolution of digits was an essential step in the success of tetrapods. Among the key players, Hoxd genes are coordinately regulated in developing digits, where they help organize growth and ...patterns. We identified the distal regulatory sites associated with these genes by probing the three-dimensional architecture of this regulatory unit in developing limbs. This approach, combined with in vivo deletions of distinct regulatory regions, revealed that the active part of the gene cluster contacts several enhancer-like sequences. These elements are dispersed throughout the nearby gene desert, and each contributes either quantitatively or qualitatively to Hox gene transcription in presumptive digits. We propose that this genetic system, which we call a “regulatory archipelago,” provides an inherent flexibility that may partly underlie the diversity in number and morphology of digits across tetrapods, as well as their resilience to drastic variations.
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► Hox genes active in digits integrate the input of multiple regulatory elements ► The global regulatory architecture of the HoxD locus involves flanking gene deserts ► Alterations in this regulatory structure may fine tune digital morphology in tetrapods
Multiple distal enhancer units are spatially coordinated to ensure Hox gene expression and proper development of digits. This regulatory arrangement suggests why digit patterning might be highly diverse between tetrapods yet robust within individuals.
Chromosomal rearrangements without gene fusions have been implicated in leukemogenesis by causing deregulation of proto-oncogenes via relocation of cryptic regulatory DNA elements. AML with ...inv(3)/t(3;3) is associated with aberrant expression of the stem-cell regulator EVI1. Applying functional genomics and genome-engineering, we demonstrate that both 3q rearrangements reposition a distal GATA2 enhancer to ectopically activate EVI1 and simultaneously confer GATA2 functional haploinsufficiency, previously identified as the cause of sporadic familial AML/MDS and MonoMac/Emberger syndromes. Genomic excision of the ectopic enhancer restored EVI1 silencing and led to growth inhibition and differentiation of AML cells, which could be replicated by pharmacologic BET inhibition. Our data show that structural rearrangements involving the chromosomal repositioning of a single enhancer can cause deregulation of two unrelated distal genes, with cancer as the outcome.
The 3D organization of our genome is an important determinant for the transcriptional output of a gene in (patho)physiological contexts. The spatial organization of linear chromosomes within nucleus ...is dominantly inferred using two distinct approaches, chromosome conformation capture (3C) and DNA fluorescent in situ hybridization (DNA–FISH). While 3C and its derivatives score genomic interaction frequencies based on proximity ligation events, DNA–FISH methods measure physical distances between genomic loci. Despite these approaches probe different characteristics of chromosomal topologies, they provide a coherent picture of how chromosomes are organized in higher‐order structures encompassing chromosome territories, compartments, and topologically associating domains. Yet, at the finer topological level of promoter–enhancer communication, the imaging‐centered and the 3C methods give more divergent and sometimes seemingly paradoxical results. Here, we compare and contrast observations made applying visual DNA–FISH and molecular 3C approaches. We emphasize that the 3C approach, due to its inherently competitive ligation step, measures only ‘relative’ proximities. A 3C interaction enriched between loci, therefore does not necessarily translates into a decrease in absolute spatial distance. Hence, we advocate caution when modeling chromosome conformations.
The Hippo/YAP signaling pathway is a crucial regulator of tissue growth, stem cell activity, and tumorigenesis. However, the mechanism by which YAP controls transcription remains to be fully ...elucidated. Here, we utilize global chromatin occupancy analyses to demonstrate that robust YAP binding is restricted to a relatively small number of distal regulatory elements in the genome. YAP occupancy defines a subset of enhancers and superenhancers with the highest transcriptional outputs. YAP modulates transcription from these elements predominantly by regulating promoter-proximal polymerase II (Pol II) pause release. Mechanistically, YAP interacts and recruits the Mediator complex to enhancers, allowing the recruitment of the CDK9 elongating kinase. Genetic and chemical perturbation experiments demonstrate the requirement for Mediator and CDK9 in YAP-driven phenotypes of overgrowth and tumorigenesis. Our results here uncover the molecular mechanisms employed by YAP to exert its growth and oncogenic functions, and suggest strategies for intervention.
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•YAP/TAZ binding is restricted to a subset of distal regulatory regions in the genome•YAP/TAZ occupancy confers potent transcriptional activity to superenhancer regions•YAP/TAZ regulate transcriptional elongation•YAP recruits the Mediator complex and CDK9-elongating kinase
The transcriptional coactivators YAP and TAZ are critical regulators of stem cell activity and tumorigenesis. Galli et al. show that YAP/TAZ binding is restricted to a relatively small number of the most potent enhancers in the genome. They show that YAP/TAZ regulate transcriptional elongation from these elements by recruiting the Mediator complex.
CCCTC-binding factor (CTCF) is an architectural protein involved in the three-dimensional (3D) organization of chromatin. In this study, we assayed the 3D genomic contact profiles of a large number ...of CTCF binding sites with high-resolution 4C-seq. As recently reported, our data also suggest that chromatin loops preferentially form between CTCF binding sites oriented in a convergent manner. To directly test this, we used CRISPR/Cas9 genome editing to delete core CTCF binding sites in three loci, including the CTCF site in the Sox2 super-enhancer. In all instances, CTCF and cohesin recruitment were lost, and chromatin loops with distal, convergent CTCF sites were disrupted or destabilized. Re-insertion of oppositely oriented CTCF recognition sequences restored CTCF and cohesin recruitment, but did not re-establish chromatin loops. We conclude that CTCF binding polarity plays a functional role in the formation of higher-order chromatin structure.
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•CTCF binding polarity determines chromatin looping•Inverted or disengaged CTCF sites do not necessarily form new chromatin loops•Cohesin recruitment to CTCF sites is independent of loop formation•Phenocopied linear but altered 3D chromatin landscape can affect gene expression
CCCTC-binding factor (CTCF) shapes the three-dimensional genome. Here, de Wit et al. provide direct evidence for CTCF binding polarity playing an underlying role in chromatin looping. Cohesin association persists, but inverted CTCF sites fail to loop, which can sometimes lead to long-range changes in gene expression.
Genetic variants identified by genome-wide association studies explain only a modest proportion of heritability, suggesting that meaningful associations lie 'hidden' below current thresholds. Here, ...we integrate information from association studies with epigenomic maps to demonstrate that enhancers significantly overlap known loci associated with the cardiac QT interval and QRS duration. We apply functional criteria to identify loci associated with QT interval that do not meet genome-wide significance and are missed by existing studies. We demonstrate that these 'sub-threshold' signals represent novel loci, and that epigenomic maps are effective at discriminating true biological signals from noise. We experimentally validate the molecular, gene-regulatory, cellular and organismal phenotypes of these sub-threshold loci, demonstrating that most sub-threshold loci have regulatory consequences and that genetic perturbation of nearby genes causes cardiac phenotypes in mouse. Our work provides a general approach for improving the detection of novel loci associated with complex human traits.
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