Polycomb-Dependent Regulatory Contacts between Distant Hox Loci in Drosophila

PRC2 is high maintenance

As the process that silences gene expression ensues during development, the stage is set for the activity of Polycomb-repressive complex 2 (PRC2) to maintain these repressed gene profiles. PRC2 catalyzes a specific histone posttranslational modification (hPTM) that fosters chromatin compaction. PRC2 also facilitates the inheritance of this hPTM through its self-contained "write and read" activities, key to preserving cellular identity during cell division. As these changes in gene expression occur without changes in DNA sequence and are inherited, the process is epigenetic in scope. Mutants of mammalian PRC2 or of its histone substrate contribute to the cancer process and other diseases, and research into these aberrant pathways is yielding viable candidates for therapeutic targeting. The effectiveness of PRC2 hinges on its being recruited to the proper chromatin sites; however, resolving the determinants to this process in the mammalian case was not straightforward and thus piqued the interest of many in the field. Here, we chronicle the latest advances toward exposing mammalian PRC2 and its high maintenance.

Implications of CpG islands on chromosomal architectures and modes of global gene regulation

CpG islands (CGIs) have long been implicated in the regulation of vertebrate gene expression. However, the involvement of CGIs in chromosomal architectures and associated gene expression regulations has not yet been thoroughly explored. By combining large-scale integrative data analyses and experimental validations, we show that CGIs clearly reconcile two competing models explaining nuclear gene localizations. We first identify CGI-containing (CGI+) and CGI-less (CGI-) genes are non-randomly clustered within the genome, which reflects CGI-dependent spatial gene segregation in the nucleus and corresponding gene regulatory modes. Regardless of their transcriptional activities, CGI+ genes are mainly located at the nuclear center and encounter frequent long-range chromosomal interactions. Meanwhile, nuclear peripheral CGI- genes forming heterochromatin are activated and internalized into the nuclear center by local enhancer-promoter interactions. Our findings demonstrate the crucial implications of CGIs on chromosomal architectures and gene positioning, linking the critical importance of CGIs in determining distinct mechanisms of global gene regulation in three-dimensional space in the nucleus.

Epigenomics in 3D: importance of long-range spreading and specific interactions in epigenomic maintenance

Recent progresses of genome-wide chromatin conformation capture techniques have shown that the genome is segmented into hierarchically organized spatial compartments. However, whether this non-random 3D organization only reflects or indeed contributes—and how—to the regulation of genome function remain to be elucidated. The observation in many species that 3D domains correlate strongly with the 1D epigenomic information along the genome suggests a dynamic coupling between chromatin organization and epigenetic regulation. Here, we posit that chromosome folding may contribute to the maintenance of a robust epigenomic identity via the formation of spatial compartments like topologically-associating domains. Using a novel theoretical framework, the living chromatin model, we show that 3D compartmentalization leads to the spatial colocalization of epigenome regulators, thus increasing their local concentration and enhancing their ability to spread an epigenomic signal at long-range. Interestingly, we find that the presence of 1D insulator elements, like CTCF, may contribute greatly to the stable maintenance of adjacent antagonistic epigenomic domains. We discuss the generic implications of our findings in the light of various biological contexts from yeast to human. Our approach provides a modular framework to improve our understanding and to investigate in details the coupling between the structure and function of chromatin.

From insects to mammals: regulation of genome architecture in neural development

One of the hallmarks of the metazoan genome is that genes are non-randomly positioned within the cell nucleus; in fact, the entire genome is packaged in a highly organized manner to orchestrate proper gene expression for each cell type. This is an especially daunting task for the development of the brain, which consists of an incredibly diverse population of neural cells. How genome architecture is established, maintained, and regulated to promote diverse cell fates and functions are fascinating questions with important implications in development and disease. The explosion in various biochemical and imaging techniques to analyze chromatin is now making it possible to interrogate the genome at an unprecedented resolution. Here we will focus on current advances in understanding genome architecture and gene regulation in the context of neural development.

PRC1 collaborates with SMCHD1 to fold the X-chromosome and spread Xist RNA between chromosome compartments

X-chromosome inactivation triggers fusion of A/B compartments to inactive X (Xi)-specific structures known as S1 and S2 compartments. SMCHD1 then merges S1/S2s to form the Xi super-structure. Here, we ask how S1/S2 compartments form and reveal that Xist RNA drives their formation via recruitment of Polycomb repressive complex 1 (PRC1). Ablating Smchd1 in post-XCI cells unveils S1/S2 structures. Loss of SMCHD1 leads to trapping Xist in the S1 compartment, impairing RNA spreading into S2. On the other hand, depleting Xist, PRC1, or HNRNPK precludes re-emergence of S1/S2 structures, and loss of S1/S2 compartments paradoxically strengthens the partition between Xi megadomains. Finally, Xi-reactivation in post-XCI cells can be enhanced by depleting both SMCHD1 and DNA methylation. We conclude that Xist, PRC1, and SMCHD1 collaborate in an obligatory, sequential manner to partition, fuse, and direct self-association of Xi compartments required for proper spreading of Xist RNA.

The inactive X (Xi)-specific S1/S2 chromosome compartments are merged by SMCHD1, but how the S1/S2 structure is constructed is unclear. The authors find that PRC1 drives the formation of S1/S2s and that the stepwise folding process of the Xi facilitates Xist RNA spreading between Xi compartments.

Chromosome conformation capture that detects novel cis- and trans-interactions in budding yeast

Chromosome Conformation Capture (3C) has emerged as a powerful approach for revealing the conformation and features of three-dimensional (3D) genomic organization. Yet attainment of higher resolution in organisms with compact genomes presents a challenge. Here, we describe modifications in the 3C technique that substantially enhance its resolution and sensitivity when applied to the 3D genome of budding yeast. Keys to our approach include use of a 4 bp cutter, Taq I, for cleaving the genome and quantitative PCR for measuring the frequency of ligation. Most importantly, we normalize the percent digestion at each restriction site to account for variation in accessibility of local chromatin structure under a given physiological condition. This strategy has led to the detection of physical interactions between regulatory elements and gene coding regions as well as intricate, stimulus-specific interchromosomal interactions between activated genes. We provide an algorithm that incorporates these and other modifications and allows quantitative determination of chromatin interaction frequencies in yeast under any physiological condition.

Polycomb Assemblies Multitask to Regulate Transcription

The Polycomb system is made of an evolutionary ancient group of proteins, present throughout plants and animals. Known initially from developmental studies with the fly Drosophila melanogaster, they were associated with stable sustainment of gene repression and maintenance of cell identity. Acting as multiprotein assemblies with an ability to modify chromatin, through chemical additions to histones and organization of topological domains, they have been involved subsequently in control of developmental transitions and in cell homeostasis. Recent work has unveiled an association of Polycomb components with transcriptionally active loci and the promotion of gene expression, in clear contrast with conventional recognition as repressors. Focusing on mammalian models, I review here advances concerning roles in transcriptional control. Among new findings highlighted is the regulation of their catalytic properties, recruiting to targets, and activities in chromatin organization and compartmentalization. The need for a more integrated approach to the study of the Polycomb system, given its fundamental complexity and its adaptation to cell context, is discussed.

Specific Contributions of Cohesin-SA1 and Cohesin-SA2 to TADs and Polycomb Domains in Embryonic Stem Cells

Cohesin exists in two variants carrying either STAG/SA1 or SA2. Here we have addressed their specific contributions to the unique spatial organization of the mouse embryonic stem cell genome, which ensures super-enhancer-dependent transcription of pluripotency factors and repression of lineage-specification genes within Polycomb domains. We find that cohesin-SA2 facilitates Polycomb domain compaction through Polycomb repressing complex 1 (PRC1) recruitment and promotes the establishment of long-range interaction networks between distant Polycomb-bound promoters that are important for gene repression. Cohesin-SA1, in contrast, disrupts these networks, while preserving topologically associating domain (TAD) borders. The diverse effects of both complexes on genome topology may reflect two modes of action of cohesin. One, likely involving loop extrusion, establishes overall genome arrangement in TADs together with CTCF and prevents excessive segregation of same-class compartment regions. The other is required for organization of local transcriptional hubs such as Polycomb domains and super-enhancers, which define cell identity.

The role of transcription in shaping the spatial organization of the genome

The spatial organization of the genome into compartments and topologically associated domains can have an important role in the regulation of gene expression. But could gene expression conversely regulate genome organization? Here, we review recent studies that assessed the requirement of transcription and/or the transcription machinery for the establishment or maintenance of genome topology. The results reveal different requirements at different genomic scales. Transcription is generally not required for higher-level genome compartmentalization, has only moderate effects on domain organization and is not sufficient to create new domain boundaries. However, on a finer scale, transcripts or transcription does seem to have a role in the formation of subcompartments and subdomains and in stabilizing enhancer-promoter interactions. Recent evidence suggests a dynamic, reciprocal interplay between fine-scale genome organization and transcription, in which each is able to modulate or reinforce the activity of the other.

Transcription factors and 3D genome conformation in cell-fate decisions

How cells adopt different identities has long fascinated biologists. Signal transduction in response to environmental cues results in the activation of transcription factors that determine the gene-expression program characteristic of each cell type. Technological advances in the study of 3D chromatin folding are bringing the role of genome conformation in transcriptional regulation to the fore. Characterizing this role of genome architecture has profound implications, not only for differentiation and development but also for diseases including developmental malformations and cancer. Here we review recent studies indicating that the interplay between transcription and genome conformation is a driving force for cell-fate decisions.

Large-scale 3D chromatin reconstruction from chromosomal contacts

BACKGROUND

Recent advances in genome analysis have established that chromatin has preferred 3D conformations, which bring distant loci into contact. Identifying these contacts is important for us to understand possible interactions between these loci. This has motivated the creation of the Hi-C technology, which detects long-range chromosomal interactions. Distance geometry-based algorithms, such as ChromSDE and ShRec3D, have been able to utilize Hi-C data to infer 3D chromosomal structures. However, these algorithms, being matrix-based, are space- and time-consuming on very large datasets. A human genome of 100 kilobase resolution would involve ∼30,000 loci, requiring gigabytes just in storing the matrices.

RESULTS

We propose a succinct representation of the distance matrices which tremendously reduces the space requirement. We give a complete solution, called SuperRec, for the inference of chromosomal structures from Hi-C data, through iterative solving the large-scale weighted multidimensional scaling problem.

CONCLUSIONS

SuperRec runs faster than earlier systems without compromising on result accuracy. The SuperRec package can be obtained from http://www.cs.cityu.edu.hk/~shuaicli/SuperRec .

Chromatin folding and nuclear architecture: PRC1 function in 3D

Embryonic development requires the intricate balance between the expansion and specialisation of defined cell types in time and space. The gene expression programmes that underpin this balance are regulated, in part, by modulating the chemical and structural state of chromatin. Polycomb repressive complexes (PRCs), a family of essential developmental regulators, operate at this level to stabilise or perpetuate a repressed but transcriptionally poised chromatin configuration. This dynamic state is required to control the timely initiation of productive gene transcription during embryonic development. The two major PRCs cooperate to target the genome, but it is PRC1 that appears to be the primary effector that controls gene expression. In this review I will discuss recent findings relating to how PRC1 alters chromatin accessibility, folding and global 3D nuclear organisation to control gene transcription.

Visualizing DNA folding and RNA in embryos at single-cell resolution

The establishment of cell types during development requires precise interactions between genes and distal regulatory sequences. We have a limited understanding of how these interactions look in three dimensions, vary across cell types in complex tissue, and relate to transcription. Here we describe optical reconstruction of chromatin architecture (ORCA), a method that can trace the DNA path in single cells with nanoscale accuracy and genomic resolution reaching two kilobases. We used ORCA to study a Hox gene cluster in cryosectioned Drosophila embryos and labelled around 30 RNA species in parallel. We identified cell-type-specific physical borders between active and Polycomb-repressed DNA, and unexpected Polycomb-independent borders. Deletion of Polycomb-independent borders led to ectopic enhancer-promoter contacts, aberrant gene expression, and developmental defects. Together, these results illustrate an approach for high-resolution, single-cell DNA domain analysis in vivo, identify domain structures that change with cell identity, and show that border elements contribute to the formation of physical domains in Drosophila.

Cell Fate and Developmental Regulation Dynamics by Polycomb Proteins and 3D Genome Architecture

Targeted transitions in chromatin states at thousands of genes are essential drivers of eukaryotic development. Therefore, understanding the in vivo dynamics of epigenetic regulators is crucial for deciphering the mechanisms underpinning cell fate decisions. This review illustrates how, in addition to its cell memory function, the Polycomb group of transcriptional regulators orchestrates temporal, cell and tissue-specific expression of master genes during development. These highly sophisticated developmental transitions are dependent on the context- and tissue-specific assembly of the different types of Polycomb Group (PcG) complexes, which regulates their targeting and/or activities on chromatin. Here, an overview is provided of how PcG complexes function at multiple scales to regulate transcription, local chromatin environment, and higher order structures that support normal differentiation and are perturbed in tumorigenesis.

Revealing Gene Function and Transcription Relationship by Reconstructing Gene-Level Chromatin Interaction

Chromatin is hierarchically organized in human interphase nuclei. Dynamic chromatin interactions are thought to influence gene transcription and cell fate determination. A consensus concept is that genes may form transcription factories within nucleus by spatially interaction. However, it is still not well known whether the function-related genes co-locate in three-dimensional (3D) space for co-transcription. Especially, there is a lack of visualization method that directly reflect the relationship between gene spatial interaction, gene function and co-transcription. In this study, we constructed three kinds of matrices based on gene ontology annotations, high-through chromosome conformation capture (Hi-C) data and RNA-seq data from twenty human tissues and cell lines. The comparative analysis for gene pairs revealed that 3D genome organization influences gene transcription predominantly at local scale. We found that the local genes within family clusters have similar transcription patterns. We also found that spatial reorganization of a histone gene cluster could control gene transcription. These observations suggest that function-related genes are close in space and activated or repressed together. Our work provided a framework for genome-wide studying the relationship between gene function, co-transcription and spatial interaction.

Three-dimensional organization and dynamics of the genome

Genome is a complex hierarchical structure, and its spatial organization plays an important role in its function. Chromatin loops and topological domains form the basic structural units of this multiscale organization and are essential to orchestrate complex regulatory networks and transcription mechanisms. They also form higher-order structures such as chromosomal compartments and chromosome territories. Each level of this intrinsic architecture is governed by principles and mechanisms that we only start to understand. In this review, we summarize the current view of the genome architecture on the scales ranging from chromatin loops to the whole genome. We describe cell-to-cell variability, links between genome reorganization and various genomic processes, such as chromosome X inactivation and cell differentiation, and the interplay between different experimental techniques.

Challenges and guidelines toward 4D nucleome data and model standards

Due to recent advances in experimental and theoretical approaches, the dynamic three-dimensional organization (3D) of the nucleus has become a very active area of research in life sciences. We now understand that the linear genome is folded in ways that may modulate how genes are expressed during the basic functioning of cells. Importantly, it is now possible to build 3D models of how the genome folds within the nucleus and changes over time (4D). Because genome folding influences its function, this opens exciting new possibilities to broaden our understanding of the mechanisms that determine cell fate. However, the rapid evolution of methods and the increasing complexity of data can result in ambiguity and reproducibility challenges, which may hamper the progress of this field. Here, we describe such challenges ahead and provide guidelines to think about strategies for shared standardized validation of experimental 4D nucleome data sets and models.

Promoter Capture Hi-C: High-resolution, Genome-wide Profiling of Promoter Interactions

The three-dimensional organization of the genome is linked to its function. For example, regulatory elements such as transcriptional enhancers control the spatio-temporal expression of their target genes through physical contact, often bridging considerable (in some cases hundreds of kilobases) genomic distances and bypassing nearby genes. The human genome harbors an estimated one million enhancers, the vast majority of which have unknown gene targets. Assigning distal regulatory regions to their target genes is thus crucial to understand gene expression control. We developed Promoter Capture Hi-C (PCHi-C) to enable the genome-wide detection of distal promoter-interacting regions (PIRs), for all promoters in a single experiment. In PCHi-C, highly complex Hi-C libraries are specifically enriched for promoter sequences through in-solution hybrid selection with thousands of biotinylated RNA baits complementary to the ends of all promoter-containing restriction fragments. The aim is to then pull-down promoter sequences and their frequent interaction partners such as enhancers and other potential regulatory elements. After high-throughput paired-end sequencing, a statistical test is applied to each promoter-ligated restriction fragment to identify significant PIRs at the restriction fragment level. We have used PCHi-C to generate an atlas of long-range promoter interactions in dozens of human and mouse cell types. These promoter interactome maps have contributed to a greater understanding of mammalian gene expression control by assigning putative regulatory regions to their target genes and revealing preferential spatial promoter-promoter interaction networks. This information also has high relevance to understanding human genetic disease and the identification of potential disease genes, by linking non-coding disease-associated sequence variants in or near control sequences to their target genes.

How epigenome drives chromatin folding and dynamics, insights from efficient coarse-grained models of chromosomes

The 3D organization of chromosomes is crucial for regulating gene expression and cell function. Many experimental and polymer modeling efforts are dedicated to deciphering the mechanistic principles behind chromosome folding. Chromosomes are long and densely packed-topologically constrained-polymers. The main challenges are therefore to develop adequate models and simulation methods to investigate properly the multi spatio-temporal scales of such macromolecules. Here, we proposed a generic strategy to develop efficient coarse-grained models for self-avoiding polymers on a lattice. Accounting accurately for the polymer entanglement length and the volumic density, we show that our simulation scheme not only captures the steady-state structural and dynamical properties of the system but also tracks the same dynamics at different coarse-graining. This strategy allows a strong power-law gain in numerical efficiency and offers a systematic way to define reliable coarse-grained null models for chromosomes and to go beyond the current limitations by studying long chromosomes during an extended time period with good statistics. We use our formalism to investigate in details the time evolution of the 3D organization of chromosome 3R (20 Mbp) in drosophila during one cell cycle (20 hours). We show that a combination of our coarse-graining strategy with a one-parameter block copolymer model integrating epigenomic-driven interactions quantitatively reproduce experimental data at the chromosome-scale and predict that chromatin motion is very dynamic during the cell cycle.

Dosage Compensation of the X Chromosome: A Complex Epigenetic Assignment Involving Chromatin Regulators and Long Noncoding RNAs

X chromosome regulation represents a prime example of an epigenetic phenomenon where coordinated regulation of a whole chromosome is required. In flies, this is achieved by transcriptional upregulation of X chromosomal genes in males to equalize the gene dosage differences in females. Chromatin-bound proteins and long noncoding RNAs (lncRNAs) constituting a ribonucleoprotein complex known as the male-specific lethal (MSL) complex or the dosage compensation complex mediate this process. MSL complex members decorate the male X chromosome, and their absence leads to male lethality. The male X chromosome is also enriched with histone H4 lysine 16 acetylation (H4K16ac), indicating that the chromatin compaction status of the X chromosome also plays an important role in transcriptional activation. How the X chromosome is specifically targeted and how dosage compensation is mechanistically achieved are central questions for the field. Here, we review recent advances, which reveal a complex interplay among lncRNAs, the chromatin landscape, transcription, and chromosome conformation that fine-tune X chromosome gene expression.

Polycomb group (PcG) proteins and Pax6 cooperate to inhibit in vivo reprogramming of the developing Drosophila eye

How different cells and tissues commit to and determine their fates has been a central question in developmental biology since the seminal embryological experiments conducted by Wilhelm Roux and Hans Driesch in sea urchins and frogs. Here, we demonstrate that Polycomb group (PcG) proteins maintain Drosophila eye specification by suppressing the activation of alternative fate choices. The loss of PcG in the developing eye results in a cellular reprogramming event in which the eye is redirected to a wing fate. This fate transformation occurs with either the individual loss of Polycomb proteins or the simultaneous reduction of the Pleiohomeotic repressive complex and Pax6. Interestingly, the requirement for retinal selector genes is limited to Pax6, as the removal of more downstream members does not lead to the eye-wing transformation. We also show that distinct PcG complexes are required during different developmental windows throughout eye formation. These findings build on earlier observations that the eye can be reprogrammed to initiate head epidermis, antennal and leg development.

Epigenetic regulation of HIV-1 latency: focus on polycomb group (PcG) proteins

HIV-1 latency allows the virus to persist until reactivation, in a transcriptionally silent form in its cellular reservoirs despite the presence of effective cART. Such viral persistence represents a major barrier to HIV eradication since treatment interruption leads to rebound plasma viremia. Polycomb group (PcG) proteins have recently got a considerable attention in regulating HIV-1 post-integration latency as they are involved in the repression of proviral gene expression through the methylation of histones. This epigenetic regulation plays an important role in the establishment and maintenance of HIV-1 latency. In fact, PcG proteins act in complexes and modulate the epigenetic signatures of integrated HIV-1 promoter. Key role played by PcG proteins in the molecular control of HIV-1 latency has led to hypothesize that PcG proteins may represent a valuable target for future HIV-1 therapy in purging HIV-1 reservoirs. In this regard, various small molecules have been synthesized or explored to specifically block the epigenetic activity of PcG. In this review, we will highlight the possible therapeutic approaches to achieve either a functional or sterilizing cure of HIV-1 infection with special focus on histone methylation by PcG proteins together with current and novel pharmacological approaches to reactivate HIV-1 from latency that could ultimately lead towards a better clearance of viral latent reservoirs.

Subnuclear distribution of proteins: Links with genome architecture

Metazoan genomes have a hierarchal 3-dimensional (3D) organization scaling from nucleosomes, loops, topologically associating domains (TADs), compartments, to chromosome territories. The 3D organization of genome has been linked with development, differentiation and disease. However, the principles governing the 3D chromatin architecture are just beginning to get unraveled. The nucleus has very high concentration of proteins and these proteins are either diffusely distributed throughout the nucleus, or aggregated in the form of foci/bodies/clusters/speckles or in combination of both. Several evidences suggest that the distribution of proteins within the nuclear space is linked to the organization and function of genome. Here, we describe advances made in understanding the relationship between subnuclear distribution of proteins and genome architecture.

Network analysis identifies chromosome intermingling regions as regulatory hotspots for transcription

We develop a network analysis approach for identifying clusters of interactions between chromosomes, which we validate experimentally. Our method integrates 1D features of the genome, such as epigenetic marks, with 3D interactions, allowing us to study spatially colocalized regions between chromosomes that are functionally relevant. We observe that clusters of interchromosomal regions fall into active and inactive categories. We find that active clusters share transcription factors and are enriched for transcriptional machinery, suggesting that chromosome intermingling regions play a key role in genome regulation. Our method provides a unique quantitative framework that can be broadly applied to study the principles of genome organization and regulation during processes such as cell differentiation and reprogramming.

The 3D structure of the genome plays a key role in regulatory control of the cell. Experimental methods such as high-throughput chromosome conformation capture (Hi-C) have been developed to probe the 3D structure of the genome. However, it remains a challenge to deduce from these data chromosome regions that are colocalized and coregulated. Here, we present an integrative approach that leverages 1D functional genomic features (e.g., epigenetic marks) with 3D interactions from Hi-C data to identify functional interchromosomal interactions. We construct a weighted network with 250-kb genomic regions as nodes and Hi-C interactions as edges, where the edge weights are given by the correlation between 1D genomic features. Individual interacting clusters are determined using weighted correlation clustering on the network. We show that intermingling regions generally fall into either active or inactive clusters based on the enrichment for RNA polymerase II (RNAPII) and H3K9me3, respectively. We show that active clusters are hotspots for transcription factor binding sites. We also validate our predictions experimentally by 3D fluorescence in situ hybridization (FISH) experiments and show that active RNAPII is enriched in predicted active clusters. Our method provides a general quantitative framework that couples 1D genomic features with 3D interactions from Hi-C to probe the guiding principles that link the spatial organization of the genome with regulatory control.

Soft X-Ray Tomography Reveals Gradual Chromatin Compaction and Reorganization during Neurogenesis In Vivo

The realization that nuclear distribution of DNA, RNA, and proteins differs between cell types and developmental stages suggests that nuclear organization serves regulatory functions. Understanding the logic of nuclear architecture and how it contributes to differentiation and cell fate commitment remains challenging. Here, we use soft X-ray tomography (SXT) to image chromatin organization, distribution, and biophysical properties during neurogenesis in vivo. Our analyses reveal that chromatin with similar biophysical properties forms an elaborate connected network throughout the entire nucleus. Although this interconnectivity is present in every developmental stage, differentiation proceeds with concomitant increase in chromatin compaction and re-distribution of condensed chromatin toward the nuclear core. HP1β, but not nucleosome spacing or phasing, regulates chromatin rearrangements because it governs both the compaction of chromatin and its interactions with the nuclear envelope. Our experiments introduce SXT as a powerful imaging technology for nuclear architecture.

Multiscale 3D Genome Rewiring during Mouse Neural Development

Chromosome conformation capture technologies have revealed important insights into genome folding. Yet, how spatial genome architecture is related to gene expression and cell fate remains unclear. We comprehensively mapped 3D chromatin organization during mouse neural differentiation in vitro and in vivo, generating the highest-resolution Hi-C maps available to date. We found that transcription is correlated with chromatin insulation and long-range interactions, but dCas9-mediated activation is insufficient for creating TAD boundaries de novo. Additionally, we discovered long-range contacts between gene bodies of exon-rich, active genes in all cell types. During neural differentiation, contacts between active TADs become less pronounced while inactive TADs interact more strongly. An extensive Polycomb network in stem cells is disrupted, while dynamic interactions between neural transcription factors appear in vivo. Finally, cell type-specific enhancer-promoter contacts are established concomitant to gene expression. This work shows that multiple factors influence the dynamics of chromatin interactions in development.

Upregulation of long noncoding RNA Xist promotes proliferation of osteosarcoma by epigenetic silencing of P21

Recent studies show that lncRNAs involve in the initiation and progression of various cancers including osteosarcoma (OS). IncRNA Xist has been verified as an oncogene in several human cancers, and its abnormal expression was closely associated with tumor initiation and progression. Nevertheless, the role of Xist in OS remains unclear. Here, we revealed the Xist expression level was up-regulated in OS tissues and discovered that Xist knockdown significantly repressed OS cell proliferation. Additionally, mechanistic analysis revealed that Xist can repress P21 expression to regulate OS cell cycle and proliferation by binding to EZH2. Taking all into account, Xist may function in promoting OS cell proliferation and may potentially serve as a novel biomarker and therapeutic target for OS.

Polycomb Repressive Complex 1 Generates Discrete Compacted Domains that Change during Differentiation

Master regulatory genes require stable silencing by the polycomb group (PcG) to prevent misexpression during differentiation and development. Some PcG proteins covalently modify histones, which contributes to heritable repression. The role for other effects on chromatin structure is less understood. We characterized the organization of PcG target genes in ESCs and neural progenitors using 5C and super-resolution microscopy. The genomic loci of repressed PcG targets formed discrete, small (20-140 Kb) domains of tight interaction that corresponded to locations bound by canonical polycomb repressive complex 1 (PRC1). These domains changed during differentiation as PRC1 binding changed. Their formation depended upon the Polyhomeotic component of canonical PRC1 and occurred independently of PRC1-catalyzed ubiquitylation. PRC1 domains differ from topologically associating domains in size and boundary characteristics. These domains have the potential to play a key role in transmitting epigenetic silencing of PcG targets by linking PRC1 to formation of a repressive higher-order structure.

Altered chromatin compaction and histone methylation drive non-additive gene expression in an interspecific Arabidopsis hybrid

BACKGROUND

The merging of two diverged genomes can result in hybrid offspring that phenotypically differ greatly from both parents. In plants, interspecific hybridization plays important roles in evolution and speciation. In addition, many agricultural and horticultural species are derived from interspecific hybridization. However, the detailed mechanisms responsible for non-additive phenotypic novelty in hybrids remain elusive.

RESULTS

In an interspecific hybrid between Arabidopsis thaliana and A. lyrata, the vast majority of genes that become upregulated or downregulated relative to the parents originate from A. thaliana. Among all differentially expressed A. thaliana genes, the majority is downregulated in the hybrid. To understand why parental origin affects gene expression in this system, we compare chromatin packing patterns and epigenomic landscapes in the hybrid and parents. We find that the chromatin of A. thaliana, but not that of A. lyrata, becomes more compact in the hybrid. Parental patterns of DNA methylation and H3K27me3 deposition are mostly unaltered in the hybrid, with the exception of higher CHH DNA methylation in transposon-rich regions. However, A. thaliana genes enriched for the H3K27me3 mark are particularly likely to differ in expression between the hybrid and parent.

CONCLUSIONS

It has long been suspected that genome-scale properties cause the differential responses of genes from one or the other parent to hybridization. Our work links global chromatin compactness and H3K27me3 histone modification to global differences in gene expression in an interspecific Arabidopsis hybrid.

Polycomb-mediated chromatin loops revealed by a subkilobase-resolution chromatin interaction map

The locations of chromatin loops in Drosophila were determined by Hi-C (chemical cross-linking, restriction digestion, ligation, and high-throughput DNA sequencing). Whereas most loop boundaries or "anchors" are associated with CTCF protein in mammals, loop anchors in Drosophila were found most often in association with the polycomb group (PcG) protein Polycomb (Pc), a subunit of polycomb repressive complex 1 (PRC1). Loops were frequently located within domains of PcG-repressed chromatin. Promoters located at PRC1 loop anchors regulate some of the most important developmental genes and are less likely to be expressed than those not at PRC1 loop anchors. Although DNA looping has most commonly been associated with enhancer-promoter communication, our results indicate that loops are also associated with gene repression.

The three-dimensional genome organization of Drosophila melanogaster through data integration

BACKGROUND

Genome structures are dynamic and non-randomly organized in the nucleus of higher eukaryotes. To maximize the accuracy and coverage of three-dimensional genome structural models, it is important to integrate all available sources of experimental information about a genome's organization. It remains a major challenge to integrate such data from various complementary experimental methods. Here, we present an approach for data integration to determine a population of complete three-dimensional genome structures that are statistically consistent with data from both genome-wide chromosome conformation capture (Hi-C) and lamina-DamID experiments.

RESULTS

Our structures resolve the genome at the resolution of topological domains, and reproduce simultaneously both sets of experimental data. Importantly, this data deconvolution framework allows for structural heterogeneity between cells, and hence accounts for the expected plasticity of genome structures. As a case study we choose Drosophila melanogaster embryonic cells, for which both data types are available. Our three-dimensional genome structures have strong predictive power for structural features not directly visible in the initial data sets, and reproduce experimental hallmarks of the D. melanogaster genome organization from independent and our own imaging experiments. Also they reveal a number of new insights about genome organization and its functional relevance, including the preferred locations of heterochromatic satellites of different chromosomes, and observations about homologous pairing that cannot be directly observed in the original Hi-C or lamina-DamID data.

CONCLUSIONS

Our approach allows systematic integration of Hi-C and lamina-DamID data for complete three-dimensional genome structure calculation, while also explicitly considering genome structural variability.

The SERTAD protein Taranis plays a role in Polycomb-mediated gene repression

The Polycomb group (PcG) proteins have been implicated in epigenetic transcriptional repression in development, stem cell maintenance and in cancer. The chromodomain protein Polycomb (Pc) is a key member of the PcG. Pc binds to the histone mark, trimethylated histone 3 lysine 27 (H3K27me3), to initiate transcriptional repression. How PcG proteins are recruited to target loci is not fully understood. Here we show that the Drosophila SERTA domain protein Taranis (Tara) is involved in transcriptional regulation of Pc target genes. Embryos lacking Tara exhibit a partial homeotic transformation of cuticular the segments, a phenotype associated with the loss of Pc function. Moreover, Drosophila embryos homozygous for a tara hypomorphic allele also misexpress engrailed, a Pc-regulated gene, and this phenotype is associated with the loss of Pc binding to the cis response element in the engrailed enhancer. In relation to that, Pc recruitment is reduced on the salivary gland polytene chromosomes and specifically at the engrailed locus. These results suggest that Tara might be required for positioning Pc to a subset of its target genes.

Three-Dimensional Genome Organization and Function in Drosophila

Understanding how the metazoan genome is used during development and cell differentiation is one of the major challenges in the postgenomic era. Early studies in Drosophila suggested that three-dimensional (3D) chromosome organization plays important regulatory roles in this process and recent technological advances started to reveal connections at the molecular level. Here we will consider general features of the architectural organization of the Drosophila genome, providing historical perspective and insights from recent work. We will compare the linear and spatial segmentation of the fly genome and focus on the two key regulators of genome architecture: insulator components and Polycomb group proteins. With its unique set of genetic tools and a compact, well annotated genome, Drosophila is poised to remain a model system of choice for rapid progress in understanding principles of genome organization and to serve as a proving ground for development of 3D genome-engineering techniques.

Stable Polycomb-dependent transgenerational inheritance of chromatin states in Drosophila

Transgenerational Epigenetic Inheritance (TEI) studies the transmission of alternative functional states through multiple generations in the presence of the same genomic DNA sequence. Very little is known on the principles and the molecular mechanisms governing this type of inheritance. Here, by transiently enhancing 3D chromatin interactions, we established stable and isogenic Drosophila epilines that carry alternative epialleles, defined by differential levels of the Polycomb-dependent H3K27me3 mark. Once established, epialleles can be dominantly transmitted to naïve flies and induce paramutation. Importantly, epilines can be reset to a naïve state by disrupting chromatin interactions. Finally, we show that environmental changes can modulate the expressivity of the epialleles and we extend our paradigm to naturally occurring phenotypes. Our work sheds light on how nuclear organization and Polycomb group proteins contribute to epigenetically inheritable phenotypic variability.

Chromosome conformation capture-based studies reveal novel features of plant nuclear architecture

Nuclear genome organization has recently received increasing attention due to its manifold functions in basic nuclear processes, such as replication, transcription, and the maintenance of genome integrity. Using technologies based on chromosome conformation capture, such as Hi-C, we now have the possibility to study the three-dimensional organization of the genome at unprecedented resolution, shedding light onto a previously unexplored level of nuclear architecture. In plants, research in this field is still in its infancy but a number of publications provided first insights into basic principles of nuclear genome organization and the factors that influence it. Apart from general aspects, newly discovered three-dimensional conformations, such as the KNOT, raise special interest on how nuclear organization may influence the function of the genome in previously unexpected ways.

The Drosophila speciation factor HMR localizes to genomic insulator sites

Hybrid incompatibility between Drosophila melanogaster and D. simulans is caused by a lethal interaction of the proteins encoded by the Hmr and Lhr genes. In D. melanogaster the loss of HMR results in mitotic defects, an increase in transcription of transposable elements and a deregulation of heterochromatic genes. To better understand the molecular mechanisms that mediate HMR’s function, we measured genome-wide localization of HMR in D. melanogaster tissue culture cells by chromatin immunoprecipitation. Interestingly, we find HMR localizing to genomic insulator sites that can be classified into two groups. One group belongs to gypsy insulators and another one borders HP1a bound regions at active genes. The transcription of the latter group genes is strongly affected in larvae and ovaries of Hmr mutant flies. Our data suggest a novel link between HMR and insulator proteins, a finding that implicates a potential role for genome organization in the formation of species.

Perspectives: using polymer modeling to understand the formation and function of nuclear compartments

Compartmentalization is a ubiquitous feature of cellular function. In the nucleus, early observations revealed a non-random spatial organization of the genome with a large-scale segregation between transcriptionally active—euchromatin—and silenced—heterochromatin—parts of the genome. Recent advances in genome-wide mapping and imaging techniques have strikingly improved the resolution at which nuclear genome folding can be analyzed and have revealed a multiscale spatial compartmentalization with increasing evidences that such compartment may indeed result from and participate to genome function. Understanding the underlying mechanisms of genome folding and in particular the link to gene regulation requires a cross-disciplinary approach that combines the new high-resolution techniques with computational modeling of chromatin and chromosomes. In this perspective article, we first present how the copolymer theoretical framework can account for the genome compartmentalization. We then suggest, in a second part, that compartments may act as a “nanoreactor,” increasing the robustness of either activation or repression by enhancing the local concentration of regulators. We conclude with the need to develop a new framework, namely the “living chromatin” model that will allow to explicitly investigate the coupling between spatial compartmentalization and gene regulation.

Coupling 1D modifications and 3D nuclear organization: data, models and function

Over the past decade, advances in molecular methods have strikingly improved the resolution at which nuclear genome folding can be analyzed. This revealed a wealth of conserved features organizing the one dimensional DNA molecule into tridimensional nuclear domains. In this review, we briefly summarize the main findings and highlight how models based on polymer physics shed light on the principles underlying the formation of these domains. Finally, we discuss the mechanistic similarities allowing self-organization of these structures and the functional importance of these in the maintenance of transcriptional programs.

Regulation of disease-associated gene expression in the 3D genome

Genetic variation associated with disease often appears in non-coding parts of the genome. Understanding the mechanisms by which this phenomenon leads to disease is necessary to translate results from genetic association studies to the clinic. Assigning function to this type of variation is notoriously difficult because the human genome harbours a complex regulatory landscape with a dizzying array of transcriptional regulatory sequences, such as enhancers that have unpredictable, promiscuous and context-dependent behaviour. In this Review, we discuss how technological advances have provided increasingly detailed information on genome folding; for example, genome folding forms loops that bring enhancers and target genes into close proximity. We also now know that enhancers function within topologically associated domains, which are structural and functional units of chromosomes. Studying disease-associated mutations and chromosomal rearrangements in the context of the 3D genome will enable the identification of dysregulated target genes and aid the progression from descriptive genetic association results to discovering molecular mechanisms underlying disease.

The epigenetic basis of hematopoietic stem cell aging

Highly proliferative tissues such as the gut, skin, and bone marrow lose millions of cells each day to normal attrition and challenge from different biological adversities. To achieve a lifespan beyond the longevity of individual cell types, tissue-specific stem cells sustain these tissues throughout the life of a human. For example, the lifespan of erythrocytes is about 100 days and adults make about two million new erythrocytes every second. A small pool of hematopoietic stem cells (HSCs) in the bone marrow is responsible for the lifetime maintenance of these populations. However, there are changes that occur within the HSC pool during aging. Biologically, these changes manifest as blunted immune responses, decreased bone marrow cellularity, and increased risk of myeloid diseases. Understanding the molecular mechanisms underlying dysfunction of aging HSCs is an important focus of biomedical research. With advances in modern health care, the average age of the general population is ever increasing. If molecular or pharmacological interventions could be discovered that rejuvenate aging HSCs, it could reduce the burden of age related immune system compromise as well as open up new opportunities for treatment of hematological disorders with regenerative therapy.

Transferable model for chromosome architecture

In vivo, the human genome folds into a characteristic ensemble of 3D structures. The mechanism driving the folding process remains unknown. We report a theoretical model for chromatin (Minimal Chromatin Model) that explains the folding of interphase chromosomes and generates chromosome conformations consistent with experimental data. The energy landscape of the model was derived by using the maximum entropy principle and relies on two experimentally derived inputs: a classification of loci into chromatin types and a catalog of the positions of chromatin loops. First, we trained our energy function using the Hi-C contact map of chromosome 10 from human GM12878 lymphoblastoid cells. Then, we used the model to perform molecular dynamics simulations producing an ensemble of 3D structures for all GM12878 autosomes. Finally, we used these 3D structures to generate contact maps. We found that simulated contact maps closely agree with experimental results for all GM12878 autosomes. The ensemble of structures resulting from these simulations exhibited unknotted chromosomes, phase separation of chromatin types, and a tendency for open chromatin to lie at the periphery of chromosome territories.