Determining the Compaction State of Genes Using DNA FISH

DNA fluorescence in situ hybridization (FISH) enables the visualization of chromatin architecture and the interactions between genomic loci at a single-cell level, complementary to genome-wide methods such as Hi-C. DNA FISH uses fluorescent-labeled DNA probes targeted to the loci of interest, allowing for the analysis of their spatial positioning and proximity with microscopy. Here, we describe an optimized experimental procedure for DNA FISH, from probe design and sample preparation through imaging and image quantification. This protocol can be readily applied to querying the spatial positioning of genomic loci of interest.

Pattern recognition in the landscape of seemingly random chimeric transcripts

The molecular and functional diversity generated by chimeric transcripts (CTs) that are derived from two genes is indicated to contribute to tumor cell survival. Several gaps yet exist. The present research is a systematic study of the spectrum of CTs identified in RNA sequencing datasets of 160 ovarian cancer samples in the The Cancer Genome Atlas (TCGA) (https://portal.gdc.cancer.gov). Structural annotation revealed complexities emerging from chromosomal localization of partner genes, differential splicing and inclusion of regulatory, untranslated regions. Identification of phenotype-specific associations further resolved a dynamically modulated mesenchymal signature during transformation. On an evolutionary background, protein-coding CTs were indicated to be highly conserved, while non-coding CTs may have evolved more recently. We also realized that the current premise postulating structural alterations or neighbouring gene readthrough generating CTs is not valid in instances wherein the parental genes are genomically distanced. In addressing this lacuna, we identified the essentiality of specific spatiotemporal arrangements mediated gene proximities in 3D space for the generation of CTs. All these features together suggest non-random mechanisms towards increasing the molecular diversity in a cell through chimera formation either in parallel or with cross-talks with the indigenous regulatory network.

3D organization of regulatory elements for transcriptional regulation in Arabidopsis

BACKGROUND

Although spatial organization of compartments and topologically associating domains at large scale is relatively well studied, the spatial organization of regulatory elements at fine scale is poorly understood in plants.

RESULTS

Here we perform high-resolution chromatin interaction analysis using paired-end tag sequencing approach. We map chromatin interactions tethered with RNA polymerase II and associated with heterochromatic, transcriptionally active, and Polycomb-repressive histone modifications in Arabidopsis. Analysis of the regulatory repertoire shows that distal active cis-regulatory elements are linked to their target genes through long-range chromatin interactions with increased expression of the target genes, while poised cis-regulatory elements are linked to their target genes through long-range chromatin interactions with depressed expression of the target genes. Furthermore, we demonstrate that transcription factor MYC2 is critical for chromatin spatial organization, and propose that MYC2 occupancy and MYC2-mediated chromatin interactions coordinately facilitate transcription within the framework of 3D chromatin architecture. Analysis of functionally related gene-defined chromatin connectivity networks reveals that genes implicated in flowering-time control are functionally compartmentalized into separate subdomains via their spatial activity in the leaf or shoot apical meristem, linking active mark- or Polycomb-repressive mark-associated chromatin conformation to coordinated gene expression.

CONCLUSION

The results reveal that the regulation of gene transcription in Arabidopsis is not only by linear juxtaposition, but also by long-range chromatin interactions. Our study uncovers the fine scale genome organization of Arabidopsis and the potential roles of such organization in orchestrating transcription and development.

NSD2 E1099K drives relapse in pediatric acute lymphoblastic leukemia by disrupting 3D chromatin organization

BACKGROUND

The NSD2 p.E1099K (EK) mutation is shown to be enriched in patients with relapsed acute lymphoblastic leukemia (ALL), indicating a role in clonal evolution and drug resistance.

RESULTS

To uncover 3D chromatin architecture-related mechanisms underlying drug resistance, we perform Hi-C on three B-ALL cell lines heterozygous for NSD2 EK. The NSD2 mutation leads to widespread remodeling of the 3D genome, most dramatically in terms of compartment changes with a strong bias towards A compartment shifts. Systematic integration of the Hi-C data with previously published ATAC-seq, RNA-seq, and ChIP-seq data show an expansion in H3K36me2 and a shrinkage in H3K27me3 within A compartments as well as increased gene expression and chromatin accessibility. These results suggest that NSD2 EK plays a prominent role in chromatin decompaction through enrichment of H3K36me2. In contrast, we identify few changes in intra-topologically associating domain activity. While compartment changes vary across cell lines, a common core of decompacting loci are shared, driving the expression of genes/pathways previously implicated in drug resistance. We further perform RNA sequencing on a cohort of matched diagnosis/relapse ALL patients harboring the relapse-specific NSD2 EK mutation. Changes in patient gene expression upon relapse significantly correlate with core compartment changes, further implicating the role of NSD2 EK in genome decompaction.

CONCLUSIONS

In spite of cell-context-dependent changes mediated by EK, there appears to be a shared transcriptional program dependent on compartment shifts which could explain phenotypic differences across EK cell lines. This core program is an attractive target for therapeutic intervention.

Genomic rearrangements and evolutionary changes in 3D chromatin topologies in the cotton tribe (Gossypieae)

BACKGROUND

Analysis of the relationship between chromosomal structural variation (synteny breaks) and 3D-chromatin architectural changes among closely related species has the potential to reveal causes and correlates between chromosomal change and chromatin remodeling. Of note, contrary to extensive studies in animal species, the pace and pattern of chromatin architectural changes following the speciation of plants remain unexplored; moreover, there is little exploration of the occurrence of synteny breaks in the context of multiple genome topological hierarchies within the same model species.

RESULTS

Here we used Hi-C and epigenomic analyses to characterize and compare the profiles of hierarchical chromatin architectural features in representative species of the cotton tribe (Gossypieae), including Gossypium arboreum, Gossypium raimondii, and Gossypioides kirkii, which differ with respect to chromosome rearrangements. We found that (i) overall chromatin architectural territories were preserved in Gossypioides and Gossypium, which was reflected in their similar intra-chromosomal contact patterns and spatial chromosomal distributions; (ii) the non-random preferential occurrence of synteny breaks in A compartment significantly associate with the B-to-A compartment switch in syntenic blocks flanking synteny breaks; (iii) synteny changes co-localize with open-chromatin boundaries of topologically associating domains, while TAD stabilization has a greater influence on regulating orthologous expression divergence than do rearrangements; and (iv) rearranged chromosome segments largely maintain ancestral in-cis interactions.

CONCLUSIONS

Our findings provide insights into the non-random occurrence of epigenomic remodeling relative to the genomic landscape and its evolutionary and functional connections to alterations of hierarchical chromatin architecture, on a known evolutionary timescale.

Consistencies and contradictions in different polymer models of chromatin architecture

Genetic information is stored in very long DNA molecules, which are folded to form chromatin, a similarly long polymer fibre that is ultimately organised into chromosomes. The organisation of chromatin is fundamental to many cellular functions, from the expression of the genetic information to cell division. As a long polymer, chromatin is very flexible and may adopt a myriad of shapes. Globally, the polymer physics governing chromatin dynamics is very well understood. But chromatin is not uniform and regions of it, with chemical modifications and bound effectors, form domains and compartments through mechanisms not yet clear. Polymer models have been successfully used to investigate these mechanisms to explain cytological observations and build hypothesis for experimental validation. Many different approaches to conceptualise chromatin in polymer models can be envisioned and each reflects different aspects. Here, we compare recent approaches that aim at reproducing prominent features of interphase chromatin organisation: the compartmentalisation into eu- and heterochromatin compartments, the formation of a nucleolus, chromatin loops and the rosette and Rabl conformations of interphase chromosomes. We highlight commonalities and contradictions that point to a modulation of the mechanisms involved to fine degree. Consolidating models will require the inclusion of yet hidden or neglected parameters.

Easy Hi-C: A Low-Input Method for Capturing Genome Organization

Chromosome conformation capture technology and its derivatives have been widely used to study genome organization. Among them, Hi-C (chromosome conformation capture coupling with high-throughput sequencing) is popular in dissecting chromatin architecture on the genome-wide level. However, the intrinsic limitations prevent its application when it comes to rare samples. Here, we present easy Hi-C, a biotin-free technology that dramatically reduces DNA loss and is suitable for low-input samples.

Nickel-induced alterations to chromatin structure and function

Nickel (Ni), a heavy metal is prevalent in the atmosphere due to both natural and anthropogenic activities. Ni is a carcinogen implicated in the development of lung and nasal cancers in humans. Furthermore, Ni exposure is associated with a number of chronic lung diseases in humans including asthma, chronic bronchitis, emphysema, pulmonary fibrosis, pulmonary edema and chronic obstructive pulmonary disease (COPD). While Ni compounds are weak mutagens, a number of studies have demonstrated the potential of Ni to alter the epigenome, suggesting epigenomic dysregulation as an important underlying cause for its pathogenicity. In the eukaryotic nucleus, the DNA is organized in a three-dimensional (3D) space through assembly of higher order chromatin structures. Such an organization is critically important for transcription and other biological activities. Accumulating evidence suggests that by negatively affecting various cellular regulatory processes, Ni could potentially affect chromatin organization. In this review, we discuss the role of Ni in altering the chromatin architecture, which potentially plays a major role in Ni pathogenicity.

Glyphosate disturbs various epigenetic processes in vitro and in vivo - A mini review

Glyphosate in the concentrations corresponding to environmental or occupational exposure has been shown to induce epigenetic changes potentially involved in carcinogenesis. This substance (1) changes the global methylation in various cell types and organisms and is responsible for the methylation of different promoters of individual genes, such as TP53 and P21 in human PBMCs, (2) decreases H3K27me3 methylation and H3 acetylation and increases H3K9 methylation and H4 acetylation in rats, (3) increases the expression of P16, P21, CCND1 in human PBMCs, and the expression of EGR1, JUN, FOS, and MYC in HEK293 cells, but decreases TP53 expression in human PBMCs, (4) changes the expression of genes DNMT1, HDAC3, TET1, TET2, TET3 involved in chromatin architecture, e.g. in fish Japanese medaka, (5) alters the expression of various small, single-stranded, non-coding RNA molecules engaged in post-transcriptional regulation of gene expression, such as miRNA 182-5p in MCF10A cells, miR-30 and miR-10 in mammalian stem cells, as well as several dozen of murine miRNAs. Epigenetic changes caused by glyphosate can persist over time and can be passed on to the offsprings in the next generation; in the third generation they can result in some disorders development, such as prostate disease or obesity. Some epigenetic mechanisms have indicated a potential risk of breast cancer development in human as a result of the exposure to glyphosate. It should be emphasized that the majority of reported epigenetic changes have not yet been associated with the final metabolic effects, which may depend on many other factors.

Comparative 3D genome architecture in vertebrates

Background

The three-dimensional (3D) architecture of the genome has a highly ordered and hierarchical nature, which influences the regulation of essential nuclear processes at the basis of gene expression, such as gene transcription. While the hierarchical organization of heterochromatin and euchromatin can underlie differences in gene expression that determine evolutionary differences among species, the way 3D genome architecture is affected by evolutionary forces within major lineages remains unclear. Here, we report a comprehensive comparison of 3D genomes, using high resolution Hi-C data in fibroblast cells of fish, chickens, and 10 mammalian species.

Results

This analysis shows a correlation between genome size and chromosome length that affects chromosome territory (CT) organization in the upper hierarchy of genome architecture, whereas lower hierarchical features, including local transcriptional availability of DNA, are selected through the evolution of vertebrates. Furthermore, conservation of topologically associating domains (TADs) appears strongly associated with the modularity of expression profiles across species. Additionally, LINE and SINE transposable elements likely contribute to heterochromatin and euchromatin organization, respectively, during the evolution of genome architecture.

Conclusions

Our analysis uncovers organizational features that appear to determine the conservation and transcriptional regulation of functional genes across species. These findings can guide ongoing investigations of genome evolution by extending our understanding of the mechanisms shaping genome architecture.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01301-7.

3D chromatin architecture and transcription regulation in cancer

Chromatin has distinct three-dimensional (3D) architectures important in key biological processes, such as cell cycle, replication, differentiation, and transcription regulation. In turn, aberrant 3D structures play a vital role in developing abnormalities and diseases such as cancer. This review discusses key 3D chromatin structures (topologically associating domain, lamina-associated domain, and enhancer-promoter interactions) and corresponding structural protein elements mediating 3D chromatin interactions [CCCTC-binding factor, polycomb group protein, cohesin, and Brother of the Regulator of Imprinted Sites (BORIS) protein] with a highlight of their associations with cancer. We also summarise the recent development of technologies and bioinformatics approaches to study the 3D chromatin interactions in gene expression regulation, including crosslinking and proximity ligation methods in the bulk cell population (ChIA-PET and HiChIP) or single-molecule resolution (ChIA-drop), and methods other than proximity ligation, such as GAM, SPRITE, and super-resolution microscopy techniques.

Epigenetics of alcohol-related liver diseases

Alcohol-related liver disease (ARLD) is a primary cause of chronic liver disease in the United States. Despite advances in the diagnosis and management of ARLD, it remains a major public health problem associated with significant morbidity and mortality, emphasising the need to adopt novel approaches to the study of ARLD and its complications. Epigenetic changes are increasingly being recognised as contributing to the pathogenesis of multiple disease states. Harnessing the power of innovative technologies for the study of epigenetics (e.g., next-generation sequencing, DNA methylation assays, histone modification profiling and computational techniques like machine learning) has resulted in a seismic shift in our understanding of the pathophysiology of ARLD. Knowledge of these techniques and advances is of paramount importance for the practicing hepatologist and researchers alike. Accordingly, in this review article we will summarise the current knowledge about alcohol-induced epigenetic alterations in the context of ARLD, including but not limited to, DNA hyper/hypo methylation, histone modifications, changes in non-coding RNA, 3D chromatin architecture and enhancer-promoter interactions. Additionally, we will discuss the state-of-the-art techniques used in the study of ARLD (e.g. single-cell sequencing). We will also highlight the epigenetic regulation of chemokines and their proinflammatory role in the context of ARLD. Lastly, we will examine the clinical applications of epigenetics in the diagnosis and management of ARLD.

CLOuD9: CRISPR-Cas9-Mediated Technique for Reversible Manipulation of Chromatin Architecture

The spatial organization of the genome plays a critical role in cell-specific biological functions such as gene expression. Existing genome-wide technologies reveal a dynamic interplay between chromatin looping and gene regulation, but the mechanisms by which regulatory interactions between genetic elements are established or maintained remain unclear. Here, we present CLOuD9, a CRISPR-based technology that can create de novo, pairwise chromatin interactions in cells. This technique for chromatin loop reorganization employs dCas9-targeting and ABI1-PYL heterodimerization. It is reversible, but can also establish epigenetic memory under certain conditions, which provides a way to dissect gene regulation mechanisms.

Circular Chromosome Conformation Capture Sequencing (4C-Seq ) in Primary Adherent Cells

The three-dimensional structure of the genome is highly organized and is an important aspect of gene regulation. Chromatin interactions can be identified using chromosome conformation capture-based techniques, which rely on proximity ligation. Of these techniques, circular chromosome conformation capture sequencing (4C-seq) is used to identify all chromatin interactions occurring with a single chromosomal location (one versus all). Here we describe a 4C-seq protocol that has been optimized for primary adherent cells, for which the first digestion step is inefficient using standard 4C-seq protocols. It can, however, also be applied to other cell or tissue types. This protocol utilizes a standard DNA library preparation method using a commercial kit, and includes a description of the data processing steps.

Advances in profiling chromatin architecture shed light on the regulatory dynamics underlying brain disorders

Understanding the exquisitely complex nature of the three-dimensional organization of the genome and how it affects gene regulation remains a central question in biology. Recent advances in sequencing- and imaging-based approaches in decoding the three-dimensional chromatin landscape have enabled a systematic characterization of gene regulatory architecture. In this review, we outline how chromatin architecture provides a reference atlas to predict the functional consequences of non-coding variants associated with human traits and disease. High-throughput perturbation assays such as massively parallel reporter assays (MPRA) and CRISPR-based genome engineering in combination with a reference atlas opened an avenue for going beyond observational studies to experimentally validating the regulatory principles of the genome. We conclude by providing a suggested path forward by calling attention to barriers that can be addressed for a more complete understanding of the regulatory landscape of the human brain.

In Situ HiC

In situ HiC uses the relative frequency of DNA-DNA ligation events to reconstruct the three-dimensional architecture of a genome. As such, restriction enzyme digested ends of genomic DNA within fixed nuclei are tagged with biotinylated dNTPs. DNA-DNA ligation events generated via proximity ligation are then captured, amplified and next generation sequenced to determine their linear genomic position, but also their three-dimensional relationship. Here, we describe these steps in detail.

Hexavalent chromium disrupts chromatin architecture

Accessibility of DNA elements and the orchestration of spatiotemporal chromatin-chromatin interactions are critical mechanisms in the regulation of gene transcription. Thus, in an ever-changing milieu, cells mount an adaptive response to environmental stimuli by modulating gene expression that is orchestrated by coordinated changes in chromatin architecture. Correspondingly, agents that alter chromatin structure directly impact transcriptional programs in cells. Heavy metals, including hexavalent chromium (Cr(VI)), are of special interest because of their ability to interact directly with cellular protein, DNA and other macromolecules, resulting in general damage or altered function. In this review we highlight the chromium-mediated mechanisms that promote disruption of chromatin architecture and how these processes are integral to its carcinogenic properties. Emerging evidence shows that Cr(VI) targets nucleosomal architecture in euchromatin, particularly in genomic locations flanking binding sites of the essential transcription factors CTCF and AP1. Ultimately, these changes contribute to an altered chromatin state in critical gene regulatory regions, which disrupts gene transcription in functionally relevant biological processes.

3D promoter architecture re-organization during iPSC-derived neuronal cell differentiation implicates target genes for neurodevelopmental disorders

Neurodevelopmental disorders are thought to arise from interrupted development of the brain at an early age. Genome-wide association studies (GWAS) have identified hundreds of loci associated with susceptibility to neurodevelopmental disorders; however, which noncoding variants regulate which genes at these loci is often unclear. To implicate neuronal GWAS effector genes, we performed an integrated analysis of transcriptomics, epigenomics and chromatin conformation changes during the development from Induced pluripotent stem cell-derived neuronal progenitor cells (NPCs) into neurons using a combination of high-resolution promoter-focused Capture-C, ATAC-seq and RNA-seq. We observed that gene expression changes during the NPC-to-neuron transition were highly dependent on both promoter accessibility changes and long-range interactions which connect distal cis-regulatory elements (enhancer or silencers) to developmental-stage-specific genes. These genome-scale promoter-cis-regulatory-element atlases implicated 454 neurodevelopmental disorder-associated, putative causal variants mapping to 600 distal targets. These putative effector genes were significantly enriched for pathways involved in the regulation of neuronal development and chromatin organization, with 27 % expressed in a stage-specific manner. The intersection of open chromatin and chromatin conformation revealed development-stage-specific gene regulatory architectures during neuronal differentiation, providing a rich resource to aid characterization of the genetic and developmental basis of neurodevelopmental disorders.

The Chromosome Conformation Capture (3C) in Drosophila melanogaster

The discovery of the DNA double helix by Watson and Crick in 1953 was the first report showing that the genomic information is not contained in a stretched linear molecule. After that, a huge advance in the knowledge of the structure of the eukaryotic genome in the nuclear space has been made over the last decades, bringing us to the widely accepted concept that the genome is packaged into hierarchical levels of higher-order three-dimensional structures. The spatial organization of the eukaryotic genome has direct influence on fundamental nuclear processes that include transcription, replication, and DNA repair. The idea that structural alterations of chromosomes may cause disease goes back to the early nineteenth century. Big effort has been devoted to the study of the three-dimensional architecture of the genome and its functional implications. In this chapter, I will describe the chromosome conformation capture (3C), one of the first techniques used to detect and measure the frequency of interactions between genomic sequences that are kept in spatial proximity in the nucleus.

H4K20me3 methyltransferase SUV420H2 shapes the chromatin landscape of pluripotent embryonic stem cells

Heterochromatin, a densely packed chromatin state that is transcriptionally silent, is a critical regulator of gene expression. However, it is unclear how the repressive histone modification H4K20me3 or the histone methyltransferase SUV420H2 regulates embryonic stem (ES) cell fate by patterning the epigenetic landscape. Here, we report that depletion of SUV420H2 leads to a near-complete loss of H4K20me3 genome wide, dysregulated gene expression and delayed ES cell differentiation. SUV420H2-bound regions are enriched with repetitive DNA elements, which are de-repressed in SUV420H2 knockout ES cells. Moreover, SUV420H2 regulation of H4K20me3-marked heterochromatin controls chromatin architecture, including fine-scale chromatin interactions in pluripotent ES cells. Our results indicate that SUV420H2 plays a crucial role in stabilizing the three-dimensional chromatin landscape of ES cells, as loss of SUV420H2 resulted in A/B compartment switching, perturbed chromatin insulation, and altered chromatin interactions of pericentric heterochromatin and surrounding regions, indicative of localized decondensation. In addition, depletion of SUV420H2 resulted in compromised interactions between H4K20me3 and gene-regulatory regions. Together, these findings describe a new role for SUV420H2 in regulating the chromatin landscape of ES cells.

Spatial patterns of CTCF sites define the anatomy of TADs and their boundaries

BACKGROUND

Topologically associating domains (TADs) are genomic regions of self-interaction. Additionally, it is known that TAD boundaries are enriched in CTCF binding sites. In turn, CTCF sites are known to be asymmetric, whereby the convergent configuration of a pair of CTCF sites leads to the formation of a chromatin loop in vivo. However, to date, it has been unclear how to reconcile TAD structure with CTCF-based chromatin loops.

RESULTS

We approach this problem by analysing CTCF binding site strengths and classifying clusters of CTCF sites along the genome on the basis of their relative orientation. Analysis of CTCF site orientation classes as a function of their spatial distribution along the human genome reveals that convergent CTCF site clusters are depleted while divergent CTCF clusters are enriched in the 5- to 100-kb range. We then analyse the distribution of CTCF binding sites as a function of TAD boundary conservation across seven primary human blood cell types. This reveals divergent CTCF site enrichment at TAD boundaries. Furthermore, convergent arrays of CTCF sites separate the left and right sections of TADs that harbour internal CTCF sites, resulting in unequal TAD 'halves'.

CONCLUSIONS

The orientation-based CTCF binding site cluster classification that we present reconciles TAD boundaries and CTCF site clusters in a mechanistically elegant fashion. This model suggests that the emergent structure of nuclear chromatin in the form of TADs relies on the obligate alternation of divergent and convergent CTCF site clusters that occur at different length scales along the genome.

Chromatin Architectures Are Associated with Response to Dark Treatment in the Oil Crop Sesamum indicum, Based on a High-Quality Genome Assembly

Eukaryotic chromatin is tightly packed into hierarchical structures, allowing appropriate gene transcription in response to environmental and developmental cues. Here, we provide a chromosome-scale de novo genome assembly of sesame with a total length of 292.3 Mb and a scaffold N50 of 20.5 Mb, containing estimated 28,406 coding genes using Pacific Biosciences long reads combined with a genome-wide chromosome conformation capture (Hi-C) approach. Based on this high-quality reference genome, we detected changes in chromatin architectures between normal growth and dark-treated sesame seedlings. Gene expression level was significantly higher in 'A' compartment and topologically associated domain (TAD) boundary regions than in 'B' compartment and TAD interior regions, which is coincident with the enrichment of H4K3me3 modification in these regions. Moreover, differentially expressed genes (DEGs) induced by dark treated were enriched in the changed TAD-related regions and genomic differential contact regions. Gene Ontology (GO) enrichment analysis of DEGs showed that genes related to 'response to stress' and 'photosynthesis' functional categories were enriched, which corresponds to dark treatment. These results suggested that chromatin organization is associated with gene transcription in response to dark treatment in sesame. Our results will facilitate the understanding of regulatory mechanisms in response to environmental cues in plants.

Polycomb Group Proteins Regulate Chromatin Architecture in Mouse Oocytes and Early Embryos

In mammals, chromatin organization undergoes drastic reorganization during oocyte development. However, the dynamics of three-dimensional chromatin structure in this process is poorly characterized. Using low-input Hi-C (genome-wide chromatin conformation capture), we found that a unique chromatin organization gradually appears during mouse oocyte growth. Oocytes at late stages show self-interacting, cohesin-independent compartmental domains marked by H3K27me3, therefore termed Polycomb-associating domains (PADs). PADs and inter-PAD (iPAD) regions form compartment-like structures with strong inter-domain interactions among nearby PADs. PADs disassemble upon meiotic resumption from diplotene arrest but briefly reappear on the maternal genome after fertilization. Upon maternal depletion of Eed, PADs are largely intact in oocytes, but their reestablishment after fertilization is compromised. By contrast, depletion of Polycomb repressive complex 1 (PRC1) proteins attenuates PADs in oocytes, which is associated with substantial gene de-repression in PADs. These data reveal a critical role of Polycomb in regulating chromatin architecture during mammalian oocyte growth and early development.

Clustered CTCF binding is an evolutionary mechanism to maintain topologically associating domains

BACKGROUND

CTCF binding contributes to the establishment of a higher-order genome structure by demarcating the boundaries of large-scale topologically associating domains (TADs). However, despite the importance and conservation of TADs, the role of CTCF binding in their evolution and stability remains elusive.

RESULTS

We carry out an experimental and computational study that exploits the natural genetic variation across five closely related species to assess how CTCF binding patterns stably fixed by evolution in each species contribute to the establishment and evolutionary dynamics of TAD boundaries. We perform CTCF ChIP-seq in multiple mouse species to create genome-wide binding profiles and associate them with TAD boundaries. Our analyses reveal that CTCF binding is maintained at TAD boundaries by a balance of selective constraints and dynamic evolutionary processes. Regardless of their conservation across species, CTCF binding sites at TAD boundaries are subject to stronger sequence and functional constraints compared to other CTCF sites. TAD boundaries frequently harbor dynamically evolving clusters containing both evolutionarily old and young CTCF sites as a result of the repeated acquisition of new species-specific sites close to conserved ones. The overwhelming majority of clustered CTCF sites colocalize with cohesin and are significantly closer to gene transcription start sites than nonclustered CTCF sites, suggesting that CTCF clusters particularly contribute to cohesin stabilization and transcriptional regulation.

CONCLUSIONS

Dynamic conservation of CTCF site clusters is an apparently important feature of CTCF binding evolution that is critical to the functional stability of a higher-order chromatin structure.

EVR: reconstruction of bacterial chromosome 3D structure models using error-vector resultant algorithm

BACKGROUND

More and more 3C/Hi-C experiments on prokaryotes have been published. However, most of the published modeling tools for chromosome 3D structures are targeting at eukaryotes. How to transform prokaryotic experimental chromosome interaction data into spatial structure models is an important task and in great need.

RESULTS

We have developed a new reconstruction program for bacterial chromosome 3D structure models called EVR that exploits a simple Error-Vector Resultant (EVR) algorithm. This software tool is particularly optimized for the closed-loop structural features of prokaryotic chromosomes. The parallel implementation of the program can utilize the computing power of both multi-core CPUs and GPUs.

CONCLUSIONS

EVR can be used to reconstruct the bacterial 3D chromosome structure based on the contact frequency matrix derived from 3C/Hi-C experimental data quickly and precisely.

Transcriptomes of Dravet syndrome iPSC derived GABAergic cells reveal dysregulated pathways for chromatin remodeling and neurodevelopment

Dravet syndrome (DS) is an early onset refractory epilepsy typically caused by de novo heterozygous variants in SCN1A encoding the α-subunit of the neuronal sodium channel Na_v1.1. The syndrome is characterized by age-related progression of seizures, cognitive decline and movement disorders. We hypothesized that the distinct neurodevelopmental features in DS are caused by the disruption of molecular pathways in Na_v1.1 haploinsufficient cells resulting in perturbed neural differentiation and maturation. Here, we established DS-patient and control induced pluripotent stem cell derived neural progenitor cells (iPSC NPC) and GABAergic inter-neuronal (iPSC GABA) cells. The DS-patient iPSC GABA cells showed a shift in sodium current activation and a perturbed response to induced oxidative stress. Transcriptome analysis revealed specific dysregulations of genes for chromatin structure, mitotic progression, neural plasticity and excitability in DS-patient iPSC NPCs and DS-patient iPSC GABA cells versus controls. The transcription factors FOXM1 and E2F1, positive regulators of the disrupted pathways for histone modification and cell cycle regulation, were markedly up-regulated in DS-iPSC GABA lines. Our study highlights transcriptional changes and disrupted pathways of chromatin remodeling in Na_v1.1 haploinsufficient GABAergic cells, providing a molecular framework that overlaps with that of neurodevelopmental disorders and other epilepsies.

Spatial chromatin architecture alteration by structural variations in human genomes at the population scale

BACKGROUND

The number of reported examples of chromatin architecture alterations involved in the regulation of gene transcription and in disease is increasing. However, no genome-wide testing has been performed to assess the abundance of these events and their importance relative to other factors affecting genome regulation. This is particularly interesting given that a vast majority of genetic variations identified in association studies are located outside coding sequences. This study attempts to address this lack by analyzing the impact on chromatin spatial organization of genetic variants identified in individuals from 26 human populations and in genome-wide association studies.

RESULTS

We assess the tendency of structural variants to accumulate in spatially interacting genomic segments and design an algorithm to model chromatin conformational changes caused by structural variations. We show that differential gene transcription is closely linked to the variation in chromatin interaction networks mediated by RNA polymerase II. We also demonstrate that CTCF-mediated interactions are well conserved across populations, but enriched with disease-associated SNPs. Moreover, we find boundaries of topological domains as relatively frequent targets of duplications, which suggest that these duplications can be an important evolutionary mechanism of genome spatial organization.

CONCLUSIONS

This study assesses the critical impact of genetic variants on the higher-order organization of chromatin folding and provides insight into the mechanisms regulating gene transcription at the population scale, of which local arrangement of chromatin loops seems to be the most significant. It provides the first insight into the variability of the human 3D genome at the population scale.

LaminA/C regulates epigenetic and chromatin architecture changes upon aging of hematopoietic stem cells

BACKGROUND

The decline of hematopoietic stem cell (HSC) function upon aging contributes to aging-associated immune remodeling and leukemia pathogenesis. Aged HSCs show changes to their epigenome, such as alterations in DNA methylation and histone methylation and acetylation landscapes. We previously showed a correlation between high Cdc42 activity in aged HSCs and the loss of intranuclear epigenetic polarity, or epipolarity, as indicated by the specific distribution of H4K16ac.

RESULTS

Here, we show that not all histone modifications display a polar localization and that a reduction in H4K16ac amount and loss of epipolarity are specific to aged HSCs. Increasing the levels of H4K16ac is not sufficient to restore polarity in aged HSCs and the restoration of HSC function. The changes in H4K16ac upon aging and rejuvenation of HSCs are correlated with a change in chromosome 11 architecture and alterations in nuclear volume and shape. Surprisingly, by taking advantage of knockout mouse models, we demonstrate that increased Cdc42 activity levels correlate with the repression of the nuclear envelope protein LaminA/C, which controls chromosome 11 distribution, H4K16ac polarity, and nuclear volume and shape in aged HSCs.

CONCLUSIONS

Collectively, our data show that chromatin architecture changes in aged stem cells are reversible by decreasing the levels of Cdc42 activity, revealing an unanticipated way to pharmacologically target LaminA/C expression and revert alterations of the epigenetic architecture in aged HSCs.

CTCF maintains regulatory homeostasis of cancer pathways

BACKGROUND

CTCF binding to DNA helps partition the mammalian genome into discrete structural and regulatory domains. Complete removal of CTCF from mammalian cells causes catastrophic genome dysregulation, likely due to widespread collapse of 3D chromatin looping and alterations to inter- and intra-TAD interactions within the nucleus. In contrast, Ctcf hemizygous mice with lifelong reduction of CTCF expression are viable, albeit with increased cancer incidence. Here, we exploit chronic Ctcf hemizygosity to reveal its homeostatic roles in maintaining genome function and integrity.

RESULTS

We find that Ctcf hemizygous cells show modest but robust changes in almost a thousand sites of genomic CTCF occupancy; these are enriched for lower affinity binding events with weaker evolutionary conservation across the mouse lineage. Furthermore, we observe dysregulation of the expression of several hundred genes, which are concentrated in cancer-related pathways, and are caused by changes in transcriptional regulation. Chromatin structure is preserved but some loop interactions are destabilized; these are often found around differentially expressed genes and their enhancers. Importantly, the transcriptional alterations identified in vitro are recapitulated in mouse tumors and also in human cancers.

CONCLUSIONS

This multi-dimensional genomic and epigenomic profiling of a Ctcf hemizygous mouse model system shows that chronic depletion of CTCF dysregulates steady-state gene expression by subtly altering transcriptional regulation, changes which can also be observed in primary tumors.

Genomic positional conservation identifies topological anchor point RNAs linked to developmental loci

BACKGROUND

The mammalian genome is transcribed into large numbers of long noncoding RNAs (lncRNAs), but the definition of functional lncRNA groups has proven difficult, partly due to their low sequence conservation and lack of identified shared properties. Here we consider promoter conservation and positional conservation as indicators of functional commonality.

RESULTS

We identify 665 conserved lncRNA promoters in mouse and human that are preserved in genomic position relative to orthologous coding genes. These positionally conserved lncRNA genes are primarily associated with developmental transcription factor loci with which they are coexpressed in a tissue-specific manner. Over half of positionally conserved RNAs in this set are linked to chromatin organization structures, overlapping binding sites for the CTCF chromatin organiser and located at chromatin loop anchor points and borders of topologically associating domains (TADs). We define these RNAs as topological anchor point RNAs (tapRNAs). Characterization of these noncoding RNAs and their associated coding genes shows that they are functionally connected: they regulate each other's expression and influence the metastatic phenotype of cancer cells in vitro in a similar fashion. Furthermore, we find that tapRNAs contain conserved sequence domains that are enriched in motifs for zinc finger domain-containing RNA-binding proteins and transcription factors, whose binding sites are found mutated in cancers.

CONCLUSIONS

This work leverages positional conservation to identify lncRNAs with potential importance in genome organization, development and disease. The evidence that many developmental transcription factors are physically and functionally connected to lncRNAs represents an exciting stepping-stone to further our understanding of genome regulation.

Analysis of Chromatin Interactions Mediated by Specific Architectural Proteins in Drosophila Cells

Chromosome conformation capture assays have been established, modified, and enhanced for over a decade with the purpose of studying nuclear organization. A recently published method uses in situ Hi-C followed by chromatin immunoprecipitation (HiChIP) to enrich the overall yield of significant genome-wide interactions mediated by a specific protein. Here we applied a modified version of the HiChIP protocol to retrieve the significant contacts mediated by architectural protein CP190 in D. melanogaster cells.

Low-input Capture-C: A Chromosome Conformation Capture Assay to Analyze Chromatin Architecture in Small Numbers of Cells

Chromosome conformation capture (3C) techniques are crucial to understanding tissue-specific regulation of gene expression, but current methods generally require large numbers of cells. This protocol describes two new low-input Capture-C approaches that can generate high-quality 3C interaction profiles from 10,000-20,000 cells, depending on the resolution used for analysis.

Chromatin architecture: A new dimension in the dynamic control of gene expression

As the most recent evidence of eukaryotic cell complexity, genome architecture has astounded the scientific community and prompted a variety of technical and cognitive challenges. Several technologies have emerged and evidenced the integration of chromatin packaging and topology, epigenetic processes, and transcription for the pertinent regulation of gene expression. In the present addendum we present and discuss some of our recent research, directed toward the holistic comprehension of the processes by which plants respond to environmental and developmental stimuli. We propose that the study of genome topology and genomic interactions is essential for the understanding of the molecular mechanisms behind a phenotype. Even though our knowledge and understanding of genome architecture and hierarchy has improved substantially in the last few years -in Arabidopsis and other eukaryotes -, there is still a long way ahead in this relatively new field of study. For this, it is necessary to take advantage of the high resolution of the emerging available techniques, and perform integrative approaches with which it will be possible to depict the role of chromatin architecture in the regulation of transcription and ultimately, physiological processes.

The Chromatin Signature of Pluripotency: Establishment and Maintenance

The revolutionary discovery that somatic cells can be reprogrammed by a defined set transcription factors to induced pluripotent stem cells (iPSCs) changed dramatically the way we perceive cell fate determination. Importantly, iPSCs, similar to embryo-derived stem cells (ESCs), are characterized by a remarkable developmental plasticity and the capacity to self-renew "indefinitely" under appropriate culture conditions, opening new avenues for personalized therapy and disease modeling. Elucidating the molecular mechanisms that maintain, induce, or alter stem cell identity is crucial for a deeper understanding of cell fate determination and potential translational applications. Intense research over the last 10 years exploiting technological advances in epigenomics and genome editing has unraveled many of the mysteries of pluripotent identity enabling novel and efficient ways to manipulate it for biomedical purposes. In this review, we focus on the chromatin and epigenetic characteristics that distinguish stem cells from somatic cells and their dynamic changes during differentiation and reprogramming.

LOGIQA: a database dedicated to long-range genome interactions quality assessment

Background

Proximity ligation-mediated methods are essential to study the impact of three-dimensional chromatin organization on gene programming. Albeit significant progress has been made in the development of computational tools that assess long-range chromatin interactions, next to nothing is known about the quality of the generated datasets.

Method

We have developed LOGIQA (www.ngs-qc.org/logiqa), a database hosting quality scores for long-range genome interaction assays, accessible through a user-friendly web-based environment.

Results

Currently, LOGIQA harbors QC scores for >900 datasets, which provides a global view of their relative quality and reveals the impact of genome size, coverage and other technical aspects. LOGIQA provides a user-friendly dataset query panel and a genome viewer to assess local genome-interaction maps at different resolution and quality-assessment conditions.

Conclusions

LOGIQA is the first database hosting quality scores dedicated to long-range chromatin interaction assays, which in addition provides a platform for visualizing genome interactions made available by the scientific community.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2642-1) contains supplementary material, which is available to authorized users.

Genome maintenance in the context of 4D chromatin condensation

The eukaryotic genome is packaged in the three-dimensional nuclear space by forming loops, domains, and compartments in a hierarchical manner. However, when duplicated genomes prepare for segregation, mitotic cells eliminate topologically associating domains and abandon the compartmentalized structure. Alongside chromatin architecture reorganization during the transition from interphase to mitosis, cells halt most DNA-templated processes such as transcription and repair. The intrinsically condensed chromatin serves as a sophisticated signaling module subjected to selective relaxation for programmed genomic activities. To understand the elaborate genome-epigenome interplay during cell cycle progression, the steady three-dimensional genome requires a time scale to form a dynamic four-dimensional and a more comprehensive portrait. In this review, we will dissect the functions of critical chromatin architectural components in constructing and maintaining an orderly packaged chromatin environment. We will also highlight the importance of the spatially and temporally conscious orchestration of chromatin remodeling to ensure high-fidelity genetic transmission.

CRISPR Double Cutting through the Labyrinthine Architecture of 3D Genomes

The genomes are organized into ordered and hierarchical topological structures in interphase nuclei. Within discrete territories of each chromosome, topologically associated domains (TADs) play important roles in various nuclear processes such as gene regulation. Inside TADs separated by relatively constitutive boundaries, distal elements regulate their gene targets through specific chromatin-looping contacts such as long-distance enhancer-promoter interactions. High-throughput sequencing studies have revealed millions of potential regulatory DNA elements, which are much more abundant than the mere ∼20,000 genes they control. The recently emerged CRISPR-Cas9 genome editing technologies have enabled efficient and precise genetic and epigenetic manipulations of genomes. The multiplexed and high-throughput CRISPR capabilities facilitate the discovery and dissection of gene regulatory elements. Here, we describe the applications of CRISPR for genome, epigenome, and 3D genome editing, focusing on CRISPR DNA-fragment editing with Cas9 and a pair of sgRNAs to investigate topological folding of chromatin TADs and developmental gene regulation.

Snaps and mends: DNA breaks and chromosomal translocations

Integrity in entirety is the preferred state of any organism. The temporal and spatial integrity of the genome ensures continued survival of a cell. DNA breakage is the first step towards creation of chromosomal translocations. In this review, we highlight the factors contributing towards the breakage of chromosomal DNA. It has been well-established that the structure and sequence of DNA play a critical role in selective fragility of the genome. Several non-B-DNA structures such as Z-DNA, cruciform DNA, G-quadruplexes, R loops and triplexes have been implicated in generation of genomic fragility leading to translocations. Similarly, specific sequences targeted by proteins such as Recombination Activating Genes and Activation Induced Cytidine Deaminase are involved in translocations. Processes that ensure the integrity of the genome through repair may lead to persistence of breakage and eventually translocations if their actions are anomalous. An insufficient supply of nucleotides and chromatin architecture may also play a critical role. This review focuses on a range of events with the potential to threaten the genomic integrity of a cell, leading to cancer.

Synthesis and evaluation of novel cyclic Peptide inhibitors of lysine-specific demethylase 1

Lysine specific demethylase 1 (LSD1) selectively removes methyl groups from mono- and dimethylated histone 3 lysine 4 (H3K4), resulting in gene silencing. LSD1 is overexpressed in many human cancers, resulting in aberrant silencing of tumor suppressor genes. Thus, LSD1 is a validated target for the discovery of antitumor agents. Using a ligand-based approach, we designed and synthesized a series of cyclic and linear peptides that are effective inhibitors of LSD1. Linear peptide 7 and cyclic peptide 9 inhibited LSD1 in vitro by 91 and 94%, respectively, at a concentration of 10 μM. Compound 9 was a potent LSD1 inhibitor (IC50 2.1 μM; K i 385 nM) and had moderate antitumor activity in the MCF-7 and Calu-6 cell lines in vitro. Importantly, 9 is significantly more stable to hydrolysis in rat plasma than the linear analogue 7. The cyclic peptides described herein represent important lead structures in the search for inhibitors of flavin-dependent histone demethylases.

Spatial localization of genes determined by intranuclear DNA fragmentation with the fusion proteins lamin KRED and histone KRED und visible light

The highly organized DNA architecture inside of the nuclei of cells is accepted in the scientific world. In the human genome about 3 billion nucleotides are organized as chromatin in the cell nucleus. In general, they are involved in gene regulation and transcription by histone modification. Small chromosomes are localized in a central nuclear position whereas the large chromosomes are peripherally positioned. In our experiments we inserted fusion proteins consisting of a component of the nuclear lamina (lamin B1) and also histone H2A, both combined with the light inducible fluorescence protein KillerRed (KRED). After activation, KRED generates reactive oxygen species (ROS) producing toxic effects and may cause cell death. We analyzed the spatial damage distribution in the chromatin after illumination of the cells with visible light. The extent of DNA damage was strongly dependent on its localization inside of nuclei. The ROS activity allowed to gain information about the location of genes and their functions via sequencing and data base analysis of the double strand breaks of the isolated DNA. A connection between the damaged gene sequences and some diseases was found.