The precise expression of genes in time and space during embryogenesis is largely influenced by communication between enhancers and promoters, which is propagated and governed by the physical ...proximity of these elements in the nucleus. Here, we review how chromatin domains organize the genome by guiding enhancers to their target genes thereby preventing non-specific interactions with other neighboring regions. We also discuss the dynamics of chromatin interactions between enhancers and promoters, as well as the consequent changes in gene expression, that occur in pluripotent cells and during development. Finally, we evaluate how genomic changes such as deletions, inversions and duplications affect 3D chromatin configuration overall and lead to ectopic enhancer-promoter contacts, and thus gene misexpression, which can contribute to abnormal development and disease.
The causal relationship between 3D chromatin domains and gene regulation has been of considerable debate in recent years. Initial Hi-C studies profiling the 3D chromatin structure of the genome ...described evolutionarily conserved Topologically Associating Domains (TADs) that correlated with gene expression. Subsequent evidence from mouse models and human disease directly linked TADs to gene regulation. However, a number of focused genetic and genome-wide studies questioned the relevance of 3D chromatin domains for orchestrating gene expression, ultimately yielding a more multi-layered view of 3D chromatin structure and gene regulation. We review the evidence for and against the importance of 3D chromatin structure for gene regulation and argue for a more comprehensive classification of regulatory chromatin domains that integrates 3D chromatin structure with genomic, functional, and evolutionary conservation.
Mammalian organogenesis is a remarkable process. Within a short timeframe, the cells of the three germ layers transform into an embryo that includes most of the major internal and external organs. ...Here we investigate the transcriptional dynamics of mouse organogenesis at single-cell resolution. Using single-cell combinatorial indexing, we profiled the transcriptomes of around 2 million cells derived from 61 embryos staged between 9.5 and 13.5 days of gestation, in a single experiment. The resulting 'mouse organogenesis cell atlas' (MOCA) provides a global view of developmental processes during this critical window. We use Monocle 3 to identify hundreds of cell types and 56 trajectories, many of which are detected only because of the depth of cellular coverage, and collectively define thousands of corresponding marker genes. We explore the dynamics of gene expression within cell types and trajectories over time, including focused analyses of the apical ectodermal ridge, limb mesenchyme and skeletal muscle.
Spatial organization is an inherent property of the vertebrate genome to accommodate the roughly 2 m of DNA in the nucleus of a cell. In this nonrandom organization, topologically associating domains ...(TADs) emerge as a fundamental structural unit that is thought to guide regulatory elements to their cognate promoters. In this review we summarize the most recent findings about TADs and the boundary regions separating them. We discuss how the disruption of these structures by genomic rearrangements can result in gene misexpression and disease.
Recent advances in understanding spatial genome organization inside the nucleus have shown that chromatin is compartmentalized into megabase-scale units known as topologically associating domains ...(TADs). In further studies, TADs were linked to differing transcriptional activity, suggesting that they might provide a scaffold for gene regulation by promoting enhancer–promoter interaction and by insulating regulatory activities. One strong argument for this hypothesis was provided by the effects of disease-causing structural variations in congenital disease and cancer. By rearranging TADs, these mutations result in a rewiring of enhancer–promoter contacts, consecutive gene misexpression, and ultimately disease. However, not all rearrangements are equally effective in creating these effects. Here, we review several recent studies aiming to understand the mechanisms by which disease-causing mutations achieve gene misregulation. We will discuss which regulatory effects are to be expected by different disease mutations and how this new knowledge can be used for diagnostics in the clinic.
During embryogenesis, precise gene transcription in space and time requires that distal enhancers and promoters communicate by physical proximity within gene regulatory landscapes. To achieve this, ...regulatory landscapes fold in nuclear space, creating complex 3D structures that influence enhancer-promoter communication and gene expression and that, when disrupted, can cause disease. Here, we provide an overview of how enhancers and promoters construct regulatory landscapes and how multiple scales of 3D chromatin structure sculpt their communication. We focus on emerging views of what enhancer-promoter contacts and chromatin domains physically represent and how two antagonistic fundamental forces—loop extrusion and homotypic attraction—likely form them. We also examine how these same forces spatially separate regulatory landscapes by functional state, thereby creating higher-order compartments that reconfigure during development to enable proper enhancer-promoter communication.
Precise gene expression is orchestrated by enhancers and promoters communicating within intricate regulatory landscapes structured in 3D space. Robson et al. explore how 3D genome organization can act at different scales to critically control this communication, what these structures physically represent, and which mechanisms drive them.
The genome is organized in three-dimensional units called topologically associating domains (TADs), through a process dependent on the cooperative action of cohesin and the DNA-binding factor CTCF. ...Genomic rearrangements of TADs have been shown to cause gene misexpression and disease, but genome-wide depletion of CTCF has no drastic effects on transcription. Here, we investigate TAD function in vivo in mouse limb buds at the Sox9-Kcnj2 locus. We show that the removal of all major CTCF sites at the boundary and within the TAD resulted in a fusion of neighboring TADs, without major effects on gene expression. Gene misexpression and disease phenotypes, however, were achieved by redirecting regulatory activity through inversions and/or the repositioning of boundaries. Thus, TAD structures provide robustness and precision but are not essential for developmental gene regulation. Aberrant disease-related gene activation is not induced by a mere loss of insulation but requires CTCF-dependent redirection of enhancer-promoter contacts.
The application of massively parallel sequencing technology to the field of skeletal disorders has boosted the discovery of the underlying genetic defect for many of these diseases. It has also ...resulted in the delineation of new clinical entities and the identification of genes and pathways that had not previously been associated with skeletal disorders. These rapid advances have prompted the Nosology Committee of the International Skeletal Dysplasia Society to revise and update the last (2015) version of the Nosology and Classification of Genetic Skeletal Disorders. This newest and tenth version of the Nosology comprises 461 different diseases that are classified into 42 groups based on their clinical, radiographic, and/or molecular phenotypes. Remarkably, pathogenic variants affecting 437 different genes have been found in 425/461 (92%) of these disorders. By providing a reference list of recognized entities and their causal genes, the Nosology should help clinicians achieve accurate diagnoses for their patients and help scientists advance research in skeletal biology.
Mammalian genomes are organized into megabase-scale topologically associated domains (TADs). We demonstrate that disruption of TADs can rewire long-range regulatory architecture and result in ...pathogenic phenotypes. We show that distinct human limb malformations are caused by deletions, inversions, or duplications altering the structure of the TAD-spanning WNT6/IHH/EPHA4/PAX3 locus. Using CRISPR/Cas genome editing, we generated mice with corresponding rearrangements. Both in mouse limb tissue and patient-derived fibroblasts, disease-relevant structural changes cause ectopic interactions between promoters and non-coding DNA, and a cluster of limb enhancers normally associated with Epha4 is misplaced relative to TAD boundaries and drives ectopic limb expression of another gene in the locus. This rewiring occurred only if the variant disrupted a CTCF-associated boundary domain. Our results demonstrate the functional importance of TADs for orchestrating gene expression via genome architecture and indicate criteria for predicting the pathogenicity of human structural variants, particularly in non-coding regions of the human genome.
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•Disruptions of TADs lead to de novo enhancer-promoter interactions and misexpression•Misexpression occurs when CTCF-associated TAD boundary elements are disrupted•Structural variations disrupting TAD structures can cause malformation syndromes•Different phenotypes can result from one enhancer acting on different target genes
Disease-associated structural variants, when affecting CTCF-associated boundary elements, cause pathogenicity by disrupting the structure of topologically associated chromatin domains leading to ectopic promoter interactions and altered gene expression.
Structural variants (SVs) can result in changes in gene expression due to abnormal chromatin folding and cause disease. However, the prediction of such effects remains a challenge. Here we present a ...polymer-physics-based approach (PRISMR) to model 3D chromatin folding and to predict enhancer-promoter contacts. PRISMR predicts higher-order chromatin structure from genome-wide chromosome conformation capture (Hi-C) data. Using the EPHA4 locus as a model, the effects of pathogenic SVs are predicted in silico and compared to Hi-C data generated from mouse limb buds and patient-derived fibroblasts. PRISMR deconvolves the folding complexity of the EPHA4 locus and identifies SV-induced ectopic contacts and alterations of 3D genome organization in homozygous or heterozygous states. We show that SVs can reconfigure topologically associating domains, thereby producing extensive rewiring of regulatory interactions and causing disease by gene misexpression. PRISMR can be used to predict interactions in silico, thereby providing a tool for analyzing the disease-causing potential of SVs.
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