Cohesin and condensin extrude DNA loops in a cell cycle-dependent manner

Activity-driven chromatin organization during interphase: Compaction, segregation, and entanglement suppression

In mammalian cells, the cohesin protein complex is believed to translocate along chromatin during interphase to form dynamic loops through a process called active loop extrusion. Chromosome conformation capture and imaging experiments have suggested that chromatin adopts a compact structure with limited interpenetration between chromosomes and between chromosomal sections. We developed a theory demonstrating that active loop extrusion causes the apparent fractal dimension of chromatin to cross-over between two and four at contour lengths on the order of 30 kilo-base pairs. The anomalously high fractal dimension [Formula: see text] is due to the inability of extruded loops to fully relax during active extrusion. Compaction on longer contour length scales extends within topologically associated domains (TADs), facilitating gene regulation by distal elements. Extrusion-induced compaction segregates TADs such that overlaps between TADs are reduced to less than 35% and increases the entanglement strand of chromatin by up to a factor of 50 to several Mega-base pairs. Furthermore, active loop extrusion couples cohesin motion to chromatin conformations formed by previously extruding cohesins and causes the mean square displacement of chromatin loci during lag times ([Formula: see text]) longer than tens of minutes to be proportional to [Formula: see text]. We validate our results with hybrid molecular dynamics-Monte Carlo simulations and show that our theory is consistent with experimental data. This work provides a theoretical basis for the compact organization of interphase chromatin, explaining the physical reason for TAD segregation and suppression of chromatin entanglements which contribute to efficient gene regulation.

Mitotic chromosomes are self-entangled and disentangle through a topoisomerase-II-dependent two-stage exit from mitosis

The topological state of chromosomes determines their mechanical properties, dynamics, and function. Recent work indicated that interphase chromosomes are largely free of entanglements. Here, we use Hi-C, polymer simulations, and multi-contact 3C and find that, by contrast, mitotic chromosomes are self-entangled. We explore how a mitotic self-entangled state is converted into an unentangled interphase state during mitotic exit. Most mitotic entanglements are removed during anaphase/telophase, with remaining ones removed during early G1, in a topoisomerase-II-dependent process. Polymer models suggest a two-stage disentanglement pathway: first, decondensation of mitotic chromosomes with remaining condensin loops produces entropic forces that bias topoisomerase II activity toward decatenation. At the second stage, the loops are released, and the formation of new entanglements is prevented by lower topoisomerase II activity, allowing the establishment of unentangled and territorial G1 chromosomes. When mitotic entanglements are not removed in experiments and models, a normal interphase state cannot be acquired.

Cohesin-Dependent Loop Extrusion: Molecular Mechanics and Role in Cell Physiology

The most prominent representatives of multisubunit SMC complexes, cohesin and condensin, are best known as structural components of mitotic chromosomes. It turned out that these complexes, as well as their bacterial homologues, are molecular motors, the ATP-dependent movement of these complexes along DNA threads leads to the formation of DNA loops. In recent years, we have witnessed an avalanche-like accumulation of data on the process of SMC dependent DNA looping, also known as loop extrusion. This review briefly summarizes the current understanding of the place and role of cohesin-dependent extrusion in cell physiology and presents a number of models describing the potential molecular mechanism of extrusion in a most compelling way. We conclude the review with a discussion of how the capacity of cohesin to extrude DNA loops may be mechanistically linked to its involvement in sister chromatid cohesion.

Enhancer-promoter specificity in gene transcription: molecular mechanisms and disease associations

Although often located at a distance from their target gene promoters, enhancers are the primary genomic determinants of temporal and spatial transcriptional specificity in metazoans. Since the discovery of the first enhancer element in simian virus 40, there has been substantial interest in unraveling the mechanism(s) by which enhancers communicate with their partner promoters to ensure proper gene expression. These research efforts have benefited considerably from the application of increasingly sophisticated sequencing- and imaging-based approaches in conjunction with innovative (epi)genome-editing technologies; however, despite various proposed models, the principles of enhancer-promoter interaction have still not been fully elucidated. In this review, we provide an overview of recent progress in the eukaryotic gene transcription field pertaining to enhancer-promoter specificity. A better understanding of the mechanistic basis of lineage- and context-dependent enhancer-promoter engagement, along with the continued identification of functional enhancers, will provide key insights into the spatiotemporal control of gene expression that can reveal therapeutic opportunities for a range of enhancer-related diseases.

Footprints of loop extrusion in statistics of intra-chromosomal distances: An analytically solvable model

Active loop extrusion-the process of formation of dynamically growing chromatin loops due to the motor activity of DNA-binding protein complexes-is a firmly established mechanism responsible for chromatin spatial organization at different stages of a cell cycle in eukaryotes and bacteria. The theoretical insight into the effect of loop extrusion on the experimentally measured statistics of chromatin conformation can be gained with an appropriately chosen polymer model. Here, we consider the simplest analytically solvable model of an interphase chromosome, which is treated as an ideal chain with disorder of sufficiently sparse random loops whose conformations are sampled from the equilibrium ensemble. This framework allows us to arrive at the closed-form analytical expression for the mean-squared distance between pairs of genomic loci, which is valid beyond the one-loop approximation in diagrammatic representation. In addition, we analyze the loop-induced deviation of chain conformations from the Gaussian statistics by calculating kurtosis of probability density of the pairwise separation vector. The presented results suggest the possible ways of estimating the characteristics of the loop extrusion process based on the experimental data on the scale-dependent statistics of intra-chromosomal pair-wise distances.

Transcription-induced active forces suppress chromatin motion

The organization of interphase chromosomes in a number of species is starting to emerge thanks to advances in a variety of experimental techniques. However, much less is known about the dynamics, especially in the functional states of chromatin. Some experiments have shown that the motility of individual loci in human interphase chromosome decreases during transcription and increases upon inhibiting transcription. This is a counterintuitive finding because it is thought that the active mechanical force (F) on the order of ten piconewtons, generated by RNA polymerase II (RNAPII) that is presumably transmitted to the gene-rich region of the chromatin, would render it more open, thus enhancing the mobility. We developed a minimal active copolymer model for interphase chromosomes to investigate how F affects the dynamical properties of chromatin. The movements of the loci in the gene-rich region are suppressed in an intermediate range of F and are enhanced at small F values, which has also been observed in experiments. In the intermediate F, the bond length between consecutive loci increases, becoming commensurate with the distance at the minimum of the attractive interaction between nonbonded loci. This results in a transient disorder-to-order transition, leading to a decreased mobility during transcription. Strikingly, the F-dependent change in the locus dynamics preserves the organization of the chromosome at [Formula: see text]. Transient ordering of the loci, which is not found in the polymers with random epigenetic profiles, in the gene-rich region might be a plausible mechanism for nucleating a dynamic network involving transcription factors, RNAPII, and chromatin.

Enhancer selectivity in space and time: from enhancer-promoter interactions to promoter activation

The primary regulators of metazoan gene expression are enhancers, originally functionally defined as DNA sequences that can activate transcription at promoters in an orientation-independent and distance-independent manner. Despite being crucial for gene regulation in animals, what mechanisms underlie enhancer selectivity for promoters, and more fundamentally, how enhancers interact with promoters and activate transcription, remain poorly understood. In this Review, we first discuss current models of enhancer-promoter interactions in space and time and how enhancers affect transcription activation. Next, we discuss different mechanisms that mediate enhancer selectivity, including repression, biochemical compatibility and regulation of 3D genome structure. Through 3D polymer simulations, we illustrate how the ability of 3D genome folding mechanisms to mediate enhancer selectivity strongly varies for different enhancer-promoter interaction mechanisms. Finally, we discuss how recent technical advances may provide new insights into mechanisms of enhancer-promoter interactions and how technical biases in methods such as Hi-C and Micro-C and imaging techniques may affect their interpretation.

Loop extrusion-mediated plasmid DNA cleavage by the bacterial SMC Wadjet complex

Structural maintenance of chromosomes (SMC) protein complexes play pivotal roles in genome organization and maintenance across all domains of life. In prokaryotes, SMC family Wadjet complexes structurally resemble the widespread MukBEF genome-organizing complexes but serve a defensive role by inhibiting plasmid transformation. We previously showed that Wadjet specifically cleaves circular DNA; however, the molecular mechanism underlying DNA substrate recognition remains unclear. Here, we use in vitro single-molecule imaging to directly visualize DNA loop extrusion and plasmid cleavage by Wadjet. We find that Wadjet is a symmetric DNA loop extruder that simultaneously reels in DNA from both sides of a growing loop and that this activity requires a dimeric JetABC supercomplex containing two dimers of the JetC motor subunit. On surface-anchored plasmid DNAs, Wadjet extrudes the full length of a 44 kilobase pair plasmid, stalls, and then cleaves DNA. Our findings reveal the role of loop extrusion in the specific recognition and elimination of plasmids by Wadjet, and establish loop extrusion as an evolutionarily conserved mechanism among SMC complexes across kingdoms of life.

PARP1-DNA co-condensation drives DNA repair site assembly to prevent disjunction of broken DNA ends

DNA double-strand breaks (DSBs) are repaired at DSB sites. How DSB sites assemble and how broken DNA is prevented from separating is not understood. Here we uncover that the synapsis of broken DNA is mediated by the DSB sensor protein poly(ADP-ribose) (PAR) polymerase 1 (PARP1). Using bottom-up biochemistry, we reconstitute functional DSB sites and show that DSB sites form through co-condensation of PARP1 multimers with DNA. The co-condensates exert mechanical forces to keep DNA ends together and become enzymatically active for PAR synthesis. PARylation promotes release of PARP1 from DNA ends and the recruitment of effectors, such as Fused in Sarcoma, which stabilizes broken DNA ends against separation, revealing a finely orchestrated order of events that primes broken DNA for repair. We provide a comprehensive model for the hierarchical assembly of DSB condensates to explain DNA end synapsis and the recruitment of effector proteins for DNA damage repair.

Probe-free optical chromatin deformation and measurement of differential mechanical properties in the nucleus

The nucleus is highly organized to facilitate coordinated gene transcription. Measuring the rheological properties of the nucleus and its sub-compartments will be crucial to understand the principles underlying nuclear organization. Here, we show that strongly localized temperature gradients (approaching 1°C/µm) can lead to substantial intra-nuclear chromatin displacements (>1 µm), while nuclear area and lamina shape remain unaffected. Using particle image velocimetry (PIV), intra-nuclear displacement fields can be calculated and converted into spatio-temporally resolved maps of various strain components. Using this approach, we show that chromatin displacements are highly reversible, indicating that elastic contributions are dominant in maintaining nuclear organization on the time scale of seconds. In genetically inverted nuclei, centrally compacted heterochromatin displays high resistance to deformation, giving a rigid, solid-like appearance. Correlating spatially resolved strain maps with fluorescent reporters in conventional interphase nuclei reveals that various nuclear compartments possess distinct mechanical identities. Surprisingly, both densely and loosely packed chromatin showed high resistance to deformation, compared to medium dense chromatin. Equally, nucleoli display particularly high resistance and strong local anchoring to heterochromatin. Our results establish how localized temperature gradients can be used to drive nuclear compartments out of mechanical equilibrium to obtain spatial maps of their material responses.

Single-Molecule Approaches to Study DNA Condensation

Proteins drive genome compartmentalization across different length scales. While the identities of these proteins have been well-studied, the physical mechanisms that drive genome organization have remained largely elusive. Studying these mechanisms is challenging owing to a lack of methodologies to parametrize physical models in cellular contexts. Furthermore, because of the complex, entangled, and dense nature of chromatin, conventional live imaging approaches often lack the spatial resolution to dissect these principles. In this chapter, we will describe how to image the interactions of λ-DNA with proteins under purified and cytoplasmic conditions. First, we will outline how to prepare biotinylated DNA, functionalize coverslips with biotin-conjugated poly-ethylene glycol (PEG), and assemble DNA microchannels compatible for the imaging of protein-DNA interactions using total internal fluorescence microscopy. Then we will describe experimental methods to image protein-DNA interactions in vitro and DNA loop extrusion using Xenopus laevis egg extracts.

Cohesin and CTCF do not assemble TADs in Xenopus sperm and male pronuclei

Paternal genomes are compacted during spermiogenesis and decompacted following fertilization. These processes are fundamental for inheritance but incompletely understood. We analyzed these processes in the frog Xenopus laevis, whose sperm can be assembled into functional pronuclei in egg extracts in vitro. In such extracts, cohesin extrudes DNA into loops, but in vivo cohesin only assembles topologically associating domains (TADs) at the mid-blastula transition (MBT). Why cohesin assembles TADs only at this stage is unknown. We first analyzed genome architecture in frog sperm and compared it to human and mouse. Our results indicate that sperm genome organization is conserved between frogs and humans and occurs without formation of TADs. TADs can be detected in mouse sperm samples, as reported, but these structures might originate from somatic chromatin contaminations. We therefore discuss the possibility that the absence of TADs might be a general feature of vertebrate sperm. To analyze sperm genome remodeling upon fertilization, we reconstituted male pronuclei in Xenopus egg extracts. In pronuclei, chromatin compartmentalization increases, but cohesin does not accumulate at CTCF sites and assemble TADs. However, if pronuclei are formed in the presence of exogenous CTCF, CTCF binds to its consensus sites, and cohesin accumulates at these and forms short-range chromatin loops, which are preferentially anchored at CTCF's N terminus. These results indicate that TADs are only assembled at MBT because before this stage CTCF sites are not occupied and cohesin only forms short-range chromatin loops.

High-speed AFM imaging reveals DNA capture and loop extrusion dynamics by cohesin-NIPBL

3D chromatin organization plays a critical role in regulating gene expression, DNA replication, recombination, and repair. While initially discovered for its role in sister chromatid cohesion, emerging evidence suggests that the cohesin complex (SMC1, SMC3, RAD21, and SA1/SA2), facilitated by NIPBL, mediates topologically associating domains and chromatin loops through DNA loop extrusion. However, information on how conformational changes of cohesin-NIPBL drive its loading onto DNA, initiation, and growth of DNA loops is still lacking. In this study, high-speed atomic force microscopy imaging reveals that cohesin-NIPBL captures DNA through arm extension, assisted by feet (shorter protrusions), and followed by transfer of DNA to its lower compartment (SMC heads, RAD21, SA1, and NIPBL). While binding at the lower compartment, arm extension leads to the capture of a second DNA segment and the initiation of a DNA loop that is independent of ATP hydrolysis. The feet are likely contributed by the C-terminal domains of SA1 and NIPBL and can transiently bind to DNA to facilitate the loading of the cohesin complex onto DNA. Furthermore, high-speed atomic force microscopy imaging reveals distinct forward and reverse DNA loop extrusion steps by cohesin-NIPBL. These results advance our understanding of cohesin by establishing direct experimental evidence for a multistep DNA-binding mechanism mediated by dynamic protein conformational changes.

Current working models of SMC-driven DNA-loop extrusion

Structural maintenance of chromosome (SMC) proteins play a key roles in the chromosome organization by condensing two meters of DNA into cell-sized structures considered as the SMC protein extrudes DNA loop. Recent sequencing-based high-throughput chromosome conformation capture technique (Hi-C) and single-molecule experiments have provided direct evidence of DNA-loop extrusion. However, the molecular mechanism by which SMCs extrude a DNA loop is still under debate. Here, we review DNA-loop extrusion studies with single-molecule assays and introduce recent structural studies of how the ATP-hydrolysis cycle is coupled to the conformational changes of SMCs for DNA-loop extrusion. In addition, we explain the conservation of the DNA-binding sites that are vital for dynamic DNA-loop extrusion by comparing Cryo-EM structures of SMC complexes. Based on this information, we compare and discuss four compelling working models that explain how the SMC complex extrudes a DNA loop.

Caenorhabditis elegans NSE3 homolog (MAGE-1) is involved in genome stability and acts in inter-sister recombination during meiosis

Melanoma antigen (MAGE) genes encode for a family of proteins that share a common MAGE homology domain. These genes are conserved in eukaryotes and have been linked to a variety of cellular and developmental processes including ubiquitination and oncogenesis in cancer. Current knowledge on the MAGE family of proteins mainly comes from the analysis of yeast and human cell lines, and their functions have not been reported at an organismal level in animals. Caenorhabditis elegans only encodes 1 known MAGE gene member, mage-1 (NSE3 in yeast), forming part of the SMC-5/6 complex. Here, we characterize the role of mage-1/nse-3 in mitosis and meiosis in C. elegans. mage-1/nse-3 has a role in inter-sister recombination repair during meiotic recombination and for preserving chromosomal integrity upon treatment with a variety of DNA-damaging agents. MAGE-1 directly interacts with NSE-1 and NSE-4. In contrast to smc-5, smc-6, and nse-4 mutants which cause the loss of NSE-1 nuclear localization and strong cytoplasmic accumulation, mage-1/nse-3 mutants have a reduced level of NSE-1::GFP, remnant NSE-1::GFP being partially nuclear but largely cytoplasmic. Our data suggest that MAGE-1 is essential for NSE-1 stability and the proper functioning of the SMC-5/6 complex.

Cohesin mediates DNA loop extrusion and sister chromatid cohesion by distinct mechanisms

Cohesin connects CTCF-binding sites and other genomic loci in cis to form chromatin loops and replicated DNA molecules in trans to mediate sister chromatid cohesion. Whether cohesin uses distinct or related mechanisms to perform these functions is unknown. Here, we describe a cohesin hinge mutant that can extrude DNA into loops but is unable to mediate cohesion in human cells. Our results suggest that the latter defect arises during cohesion establishment. The observation that cohesin's cohesion and loop extrusion activities can be partially separated indicates that cohesin uses distinct mechanisms to perform these two functions. Unexpectedly, the same hinge mutant can also not be stopped by CTCF boundaries as well as wild-type cohesin. This suggests that cohesion establishment and cohesin's interaction with CTCF boundaries depend on related mechanisms and raises the possibility that both require transient hinge opening to entrap DNA inside the cohesin ring.

Genome control by SMC complexes

Many cellular processes require large-scale rearrangements of chromatin structure. Structural maintenance of chromosomes (SMC) protein complexes are molecular machines that can provide structure to chromatin. These complexes can connect DNA elements in cis, walk along DNA, build and processively enlarge DNA loops and connect DNA molecules in trans to hold together the sister chromatids. These DNA-shaping abilities place SMC complexes at the heart of many DNA-based processes, including chromosome segregation in mitosis, transcription control and DNA replication, repair and recombination. In this Review, we discuss the latest insights into how SMC complexes such as cohesin, condensin and the SMC5-SMC6 complex shape DNA to direct these fundamental chromosomal processes. We also consider how SMC complexes, by building chromatin loops, can counteract the natural tendency of alike chromatin regions to cluster. SMC complexes thus control nuclear organization by participating in a molecular tug of war that determines the architecture of our genome.

Cohesin-mediated DNA loop extrusion resolves sister chromatids in G2 phase

Genetic information is stored in linear DNA molecules, which are highly folded inside cells. DNA replication along the folded template path yields two sister chromatids that initially occupy the same nuclear region in an intertwined arrangement. Dividing cells must disentangle and condense the sister chromatids into separate bodies such that a microtubule-based spindle can move them to opposite poles. While the spindle-mediated transport of sister chromatids has been studied in detail, the chromosome-intrinsic mechanics presegregating sister chromatids have remained elusive. Here, we show that human sister chromatids resolve extensively already during interphase, in a process dependent on the loop-extruding activity of cohesin, but not that of condensins. Increasing cohesin's looping capability increases sister DNA resolution in interphase nuclei to an extent normally seen only during mitosis, despite the presence of abundant arm cohesion. That cohesin can resolve sister chromatids so extensively in the absence of mitosis-specific activities indicates that DNA loop extrusion is a generic mechanism for segregating replicated genomes, shared across different Structural Maintenance of Chromosomes (SMC) protein complexes in all kingdoms of life.

The material properties of mitotic chromosomes

Chromosomes transform during the cell cycle, allowing transcription and replication during interphase and chromosome segregation during mitosis. Morphological changes are thought to be driven by the combined effects of DNA loop extrusion and a chromatin solubility phase transition. By extruding the chromatin fibre into loops, condensins enrich at an axial core and provide resistance to spindle pulling forces. Mitotic chromosomes are further compacted by deacetylation of histone tails, rendering chromatin insoluble and resistant to penetration by microtubules. Regulation of surface properties by Ki-67 allows independent chromosome movement in early mitosis and clustering during mitotic exit. Recent progress has provided insight into how the extraordinary material properties of chromatin emerge from these activities, and how these properties facilitate faithful chromosome segregation.

Single cohesin molecules generate force by two distinct mechanisms

Spatial organization of DNA is facilitated by cohesin protein complexes that move on DNA and extrude DNA loops. How cohesin works mechanistically as a molecular machine is poorly understood. Here, we measure mechanical forces generated by conformational changes in single cohesin molecules. We show that bending of SMC coiled coils is driven by random thermal fluctuations leading to a ~32 nm head-hinge displacement that resists forces up to 1 pN; ATPase head engagement occurs in a single step of ~10 nm and is driven by an ATP dependent head-head movement, resisting forces up to 15 pN. Our molecular dynamic simulations show that the energy of head engagement can be stored in a mechanically strained conformation of NIPBL and released during disengagement. These findings reveal how single cohesin molecules generate force by two distinct mechanisms. We present a model, which proposes how this ability may power different aspects of cohesin-DNA interaction.

Loops and the activity of loop extrusion factors constrain chromatin dynamics

The chromosomes-DNA polymers and their binding proteins-are compacted into a spatially organized, yet dynamic, three-dimensional structure. Recent genome-wide chromatin conformation capture experiments reveal a hierarchical organization of the DNA structure that is imposed, at least in part, by looping interactions arising from the activity of loop extrusion factors. The dynamics of chromatin reflects the response of the polymer to a combination of thermal fluctuations and active processes. However, how chromosome structure and enzymes acting on chromatin together define its dynamics remains poorly understood. To gain insight into the structure-dynamics relationship of chromatin, we combine high-precision microscopy in living Schizosaccharomyces pombe cells with systematic genetic perturbations and Rouse model polymer simulations. We first investigated how the activity of two loop extrusion factors, the cohesin and condensin complexes, influences chromatin dynamics. We observed that deactivating cohesin, or to a lesser extent condensin, increased chromatin mobility, suggesting that loop extrusion constrains rather than agitates chromatin motion. Our corresponding simulations reveal that the introduction of loops is sufficient to explain the constraining activity of loop extrusion factors, highlighting that the conformation adopted by the polymer plays a key role in defining its dynamics. Moreover, we find that the number of loops or residence times of loop extrusion factors influence the dynamic behavior of the chromatin polymer. Last, we observe that the activity of the INO80 chromatin remodeler, but not the SWI/SNF or RSC complexes, is critical for ATP-dependent chromatin mobility in fission yeast. Taking the data together, we suggest that thermal and INO80-dependent activities exert forces that drive chromatin fluctuations, which are constrained by the organization of the chromosome into loops.

Integrative approaches to study enhancer-promoter communication

The spatiotemporal control of gene expression in complex multicellular organisms relies on noncoding regulatory sequences such as enhancers, which activate transcription of target genes often over large genomic distances. Despite the advances in the identification and characterization of enhancers, the principles and mechanisms by which enhancers select and control their target genes remain largely unknown. Here, we review recent interdisciplinary and quantitative approaches based on emerging techniques that aim to address open questions in the field, notably how regulatory information is encoded in the DNA sequence, how this information is transferred from enhancers to promoters, and how these processes are regulated in time.

Mesoscale, long-time mixing of chromosomes and its connection to polymer dynamics

Chromosomes are arranged in distinct territories within the nucleus of animal cells. Recent experiments have shown that these territories overlap at their edges, suggesting partial mixing during interphase. Experiments that knock-down of condensin II proteins during interphase indicate increased chromosome mixing, which demonstrates control of the mixing. In this study, we use a generic polymer simulation to quantify the dynamics of chromosome mixing over time. We introduce the chromosome mixing index, which quantifies the mixing of distinct chromosomes in the nucleus. We find that the chromosome mixing index in a small confinement volume (as a model of the nucleus), increases as a power-law of the time, with the scaling exponent varying non-monotonically with self-interaction and volume fraction. By comparing the chromosome mixing index with both monomer subdiffusion due to (non-topological) intermingling of chromosomes as well as even slower reptation, we show that for relatively large volume fractions, the scaling exponent of the chromosome mixing index is related to Rouse dynamics for relatively weak chromosome attractions and to reptation for strong attractions. In addition, we extend our model to more realistically account for the situation of the Drosophila chromosome by including the heterogeneity of the polymers and their lengths to account for microphase separation of euchromatin and heterochromatin and their interactions with the nuclear lamina. We find that the interaction with the lamina further impedes chromosome mixing.

Cohesin: an emerging master regulator at the heart of cardiac development

Cohesins are ATPase complexes that play central roles in cellular processes such as chromosome division, DNA repair, and gene expression. Cohesinopathies arise from mutations in cohesin proteins or cohesin complex regulators and encompass a family of related developmental disorders that present with a range of severe birth defects, affect many different physiological systems, and often lead to embryonic fatality. Treatments for cohesinopathies are limited, in large part due to the lack of understanding of cohesin biology. Thus, characterizing the signaling networks that lie upstream and downstream of cohesin-dependent pathways remains clinically relevant. Here, we highlight alterations in cohesins and cohesin regulators that result in cohesinopathies, with a focus on cardiac defects. In addition, we suggest a novel and more unifying view regarding the mechanisms through which cohesinopathy-based heart defects may arise.

Structural changes in chromosomes driven by multiple condensin motors during mitosis

We create a computational framework that utilizes loop extrusion (LE) by multiple condensin I/II motors to predict changes in chromosome organization during mitosis. The theory accurately reproduces the experimental contact probability profiles for the mitotic chromosomes in HeLa and DT40 cells. The LE rate is smaller at the start of mitosis and increases as the cells approach metaphase. Condensin II-mediated mean loop size is about six times larger than loops because of condensin I. The loops, which overlap each other, are stapled to a central dynamically changing helical scaffold formed by the motors during the LE process. A polymer physics-based data-driven method that uses the Hi-C contact map as the only input shows that the helix is characterized as random helix perversions (RHPs) in which the handedness changes randomly along the scaffold. The theoretical predictions, which are testable using imaging experiments, do not contain any parameters.

Super-enhancer landscape rewiring in cancer: The epigenetic control at distal sites

Super-enhancers evolve as elements at the top of the hierarchical control of gene expression. They are important end-gatherers of signaling pathways that control stemness, differentiation or adaptive responses. Many epigenetic regulations focus on these regions, and not surprisingly, during the process of tumorigenesis, various alterations can account for their dysfunction. Super-enhancers are emerging as key drivers of the aberrant gene expression landscape that sustain the aggressiveness of cancer cells. In this review, we will describe and discuss about the structure of super-enhancers, their epigenetic regulation, and the major changes affecting their functionality in cancer.

DNA double-strand break end synapsis by DNA loop extrusion

DNA double-strand breaks (DSBs) occur every cell cycle and must be efficiently repaired. Non-homologous end joining (NHEJ) is the dominant pathway for DSB repair in G1-phase. The first step of NHEJ is to bring the two DSB ends back into proximity (synapsis). Although synapsis is generally assumed to occur through passive diffusion, we show that passive diffusion is unlikely to produce the synapsis speed observed in cells. Instead, we hypothesize that DNA loop extrusion facilitates synapsis. By combining experimentally constrained simulations and theory, we show that a simple loop extrusion model constrained by previous live-cell imaging data only modestly accelerates synapsis. Instead, an expanded loop extrusion model with targeted loading of loop extruding factors (LEFs), a small portion of long-lived LEFs, and LEF stabilization by boundary elements and DSB ends achieves fast synapsis with near 100% efficiency. We propose that loop extrusion contributes to DSB repair by mediating fast synapsis.

Full circle: a brief history of cohesin and the regulation of gene expression

The cohesin complex has a range of crucial functions in the cell. Cohesin is essential for mediating chromatid cohesion during mitosis, for repair of double-strand DNA breaks, and for control of gene transcription. This last function has been the subject of intense research ever since the discovery of cohesin's role in the long-range regulation of the cut gene in Drosophila. Subsequent research showed that the expression of some genes is exquisitely sensitive to cohesin depletion, while others remain relatively unperturbed. Sensitivity to cohesin depletion is also remarkably cell type- and/or condition-specific. The relatively recent discovery that cohesin is integral to forming chromatin loops via loop extrusion should explain much of cohesin's gene regulatory properties, but surprisingly, loop extrusion has failed to identify a 'one size fits all' mechanism for how cohesin controls gene expression. This review will illustrate how early examples of cohesin-dependent gene expression integrate with later work on cohesin's role in genome organization to explain mechanisms by which cohesin regulates gene expression.

CTCF is a DNA-tension-dependent barrier to cohesin-mediated loop extrusion

In eukaryotes, genomic DNA is extruded into loops by cohesin1. By restraining this process, the DNA-binding protein CCCTC-binding factor (CTCF) generates topologically associating domains (TADs)2,3 that have important roles in gene regulation and recombination during development and disease1,4-7. How CTCF establishes TAD boundaries and to what extent these are permeable to cohesin is unclear8. Here, to address these questions, we visualize interactions of single CTCF and cohesin molecules on DNA in vitro. We show that CTCF is sufficient to block diffusing cohesin, possibly reflecting how cohesive cohesin accumulates at TAD boundaries, and is also sufficient to block loop-extruding cohesin, reflecting how CTCF establishes TAD boundaries. CTCF functions asymmetrically, as predicted; however, CTCF is dependent on DNA tension. Moreover, CTCF regulates cohesin's loop-extrusion activity by changing its direction and by inducing loop shrinkage. Our data indicate that CTCF is not, as previously assumed, simply a barrier to cohesin-mediated loop extrusion but is an active regulator of this process, whereby the permeability of TAD boundaries can be modulated by DNA tension. These results reveal mechanistic principles of how CTCF controls loop extrusion and genome architecture.

Transcription shapes 3D chromatin organization by interacting with loop extrusion

Cohesin folds mammalian interphase chromosomes by extruding the chromatin fiber into numerous loops. "Loop extrusion" can be impeded by chromatin-bound factors, such as CTCF, which generates characteristic and functional chromatin organization patterns. It has been proposed that transcription relocalizes or interferes with cohesin and that active promoters are cohesin loading sites. However, the effects of transcription on cohesin have not been reconciled with observations of active extrusion by cohesin. To determine how transcription modulates extrusion, we studied mouse cells in which we could alter cohesin abundance, dynamics, and localization by genetic "knockouts" of the cohesin regulators CTCF and Wapl. Through Hi-C experiments, we discovered intricate, cohesin-dependent contact patterns near active genes. Chromatin organization around active genes exhibited hallmarks of interactions between transcribing RNA polymerases (RNAPs) and extruding cohesins. These observations could be reproduced by polymer simulations in which RNAPs were moving barriers to extrusion that obstructed, slowed, and pushed cohesins. The simulations predicted that preferential loading of cohesin at promoters is inconsistent with our experimental data. Additional ChIP-seq experiments showed that the putative cohesin loader Nipbl is not predominantly enriched at promoters. Therefore, we propose that cohesin is not preferentially loaded at promoters and that the barrier function of RNAP accounts for cohesin accumulation at active promoters. Altogether, we find that RNAP is an extrusion barrier that is not stationary, but rather, translocates and relocalizes cohesin. Loop extrusion and transcription might interact to dynamically generate and maintain gene interactions with regulatory elements and shape functional genomic organization.

Widespread allele-specific topological domains in the human genome are not confined to imprinted gene clusters

BACKGROUND

There is widespread interest in the three-dimensional chromatin conformation of the genome and its impact on gene expression. However, these studies frequently do not consider parent-of-origin differences, such as genomic imprinting, which result in monoallelic expression. In addition, genome-wide allele-specific chromatin conformation associations have not been extensively explored. There are few accessible bioinformatic workflows for investigating allelic conformation differences and these require pre-phased haplotypes which are not widely available.

RESULTS

We developed a bioinformatic pipeline, "HiCFlow," that performs haplotype assembly and visualization of parental chromatin architecture. We benchmarked the pipeline using prototype haplotype phased Hi-C data from GM12878 cells at three disease-associated imprinted gene clusters. Using Region Capture Hi-C and Hi-C data from human cell lines (1-7HB2, IMR-90, and H1-hESCs), we can robustly identify the known stable allele-specific interactions at the IGF2-H19 locus. Other imprinted loci (DLK1 and SNRPN) are more variable and there is no "canonical imprinted 3D structure," but we could detect allele-specific differences in A/B compartmentalization. Genome-wide, when topologically associating domains (TADs) are unbiasedly ranked according to their allele-specific contact frequencies, a set of allele-specific TADs could be defined. These occur in genomic regions of high sequence variation. In addition to imprinted genes, allele-specific TADs are also enriched for allele-specific expressed genes. We find loci that have not previously been identified as allele-specific expressed genes such as the bitter taste receptors (TAS2Rs).

CONCLUSIONS

This study highlights the widespread differences in chromatin conformation between heterozygous loci and provides a new framework for understanding allele-specific expressed genes.

Condensin pinches a short negatively supercoiled DNA loop during each round of ATP usage

Condensin, an SMC (structural maintenance of chromosomes) protein complex, extrudes DNA loops using an ATP-dependent mechanism that remains to be elucidated. Here, we show how condensin activity alters the topology of the interacting DNA. High condensin concentrations restrain positive DNA supercoils. However, in experimental conditions of DNA loop extrusion, condensin restrains negative supercoils. Namely, following ATP-mediated loading onto DNA, each condensin complex constrains a DNA linking number difference (∆Lk) of -0.4. This ∆Lk increases to -0.8 during ATP binding and resets to -0.4 upon ATP hydrolysis. These changes in DNA topology do not involve DNA unwinding, do not spread outside the condensin-DNA complex and can occur in the absence of the condensin subunit Ycg1. These findings indicate that during ATP binding, a short DNA domain delimited by condensin is pinched into a negatively supercoiled loop. We propose that this loop is the feeding segment of DNA that is subsequently merged to enlarge an extruding loop. Such a "pinch and merge" mechanism implies that two DNA-binding sites produce the feeding loop, while a third site, plausibly involving Ycg1, might anchor the extruding loop.

Cohesin and CTCF complexes mediate contacts in chromatin loops depending on nucleosome positions

The spatial organization of the eukaryotic genome plays an important role in regulating transcriptional activity. In the nucleus, chromatin forms loops that assemble into fundamental units called topologically associating domains that facilitate or inhibit long-range contacts. These loops are formed and held together by the ring-shaped cohesin protein complex, and this can involve binding of CCCTC-binding factor (CTCF). High-resolution conformation capture experiments provide the frequency at which two DNA fragments physically associate in three-dimensional space. However, technical limitations of this approach, such as low throughput, low resolution, or noise in contact maps, make data interpretation and identification of chromatin intraloop contacts, e.g., between distal regulatory elements and their target genes, challenging. Herein, an existing coarse-grained model of chromatin at single-nucleosome resolution was extended by integrating potentials describing CTCF and cohesin. We performed replica-exchange Monte Carlo simulations with regularly spaced nucleosomes and experimentally determined nucleosome positions in the presence of cohesin-CTCF, as well as depleted systems as controls. In fully extruded loops caused by the presence of cohesin and CTCF, the number of contacts within the formed loops was increased. The number and types of these contacts were impacted by the nucleosome distribution and loop size. Microloops were observed within cohesin-mediated loops due to thermal fluctuations without additional influence of other factors, and the number, size, and shape of microloops were determined by nucleosome distribution and loop size. Nucleosome positions directly affect the spatial structure and contact probability within a loop, with presumed consequences for transcriptional activity.

Extracting and characterizing protein-free megabase-pair DNA for in vitro experiments

Chromosome structure and function is studied using various cell-based methods as well as with a range of in vitro single-molecule techniques on short DNA substrates. Here, we present a method to obtain megabase-pair-length deproteinated DNA for in vitro studies. We isolated chromosomes from bacterial cells and enzymatically digested the native proteins. Mass spectrometry indicated that 97%-100% of DNA-binding proteins are removed from the sample. Fluorescence microscopy analysis showed an increase in the radius of gyration of the DNA polymers, while the DNA length remained megabase-pair sized. In proof-of-concept experiments using these deproteinated long DNA molecules, we observed DNA compaction upon adding the DNA-binding protein Fis or PEG crowding agents and showed that it is possible to track the motion of a fluorescently labeled DNA locus. These results indicate the practical feasibility of a "genome-in-a-box" approach to study chromosome organization from the bottom up.

StripeDiff: Model-based algorithm for differential analysis of chromatin stripe

Multiple recent studies revealed stripes as an architectural feature of three-dimensional chromatin and found stripes connected to epigenetic regulation of transcription. Whereas a couple of tools are available to define stripes in a single sample, there is yet no reported method to quantitatively measure the dynamic change of each stripe between samples. Here, we developed StripeDiff, a bioinformatics tool that delivers a set of statistical methods to detect differential stripes between samples. StripeDiff showed optimal performance in both simulation data analysis and real Hi-C data analysis. Applying StripeDiff to 12 sets of Hi-C data revealed new insights into the connection between change of chromatin stripe and change of chromatin modification, transcriptional regulation, and cell differentiation. StripeDiff will be a robust tool for the community to facilitate understanding of stripes and their function in numerous biological models.

MoDLE: high-performance stochastic modeling of DNA loop extrusion interactions

DNA loop extrusion emerges as a key process establishing genome structure and function. We introduce MoDLE, a computational tool for fast, stochastic modeling of molecular contacts from DNA loop extrusion capable of simulating realistic contact patterns genome wide in a few minutes. MoDLE accurately simulates contact maps in concordance with existing molecular dynamics approaches and with Micro-C data and does so orders of magnitude faster than existing approaches. MoDLE runs efficiently on machines ranging from laptops to high performance computing clusters and opens up for exploratory and predictive modeling of 3D genome structure in a wide range of settings.

Topoisomerases I and II facilitate condensin DC translocation to organize and repress X chromosomes in C. elegans

Condensins are evolutionarily conserved molecular motors that translocate along DNA and form loops. To address how DNA topology affects condensin translocation, we applied auxin-inducible degradation of topoisomerases I and II and analyzed the binding and function of an interphase condensin that mediates X chromosome dosage compensation in C. elegans. TOP-2 depletion reduced long-range spreading of condensin-DC (dosage compensation) from its recruitment sites and shortened 3D DNA contacts measured by Hi-C. TOP-1 depletion did not affect long-range spreading but resulted in condensin-DC accumulation within expressed gene bodies. Both TOP-1 and TOP-2 depletion resulted in X chromosome derepression, indicating that condensin-DC translocation at both scales is required for its function. Together, the distinct effects of TOP-1 and TOP-2 suggest two distinct modes of condensin-DC association with chromatin: long-range DNA loop extrusion that requires decatenation/unknotting of DNA and short-range translocation across genes that requires resolution of transcription-induced supercoiling.

3DPolyS-LE: an accessible simulation framework to model the interplay between chromatin and loop extrusion

SUMMARY

Recent studies suggest that the loop extrusion activity of Structural Maintenance of Chromosomes complexes is central to proper organization of genomes in vivo. Polymer physics-based modeling of chromosome structure has been instrumental to assess which structures such extrusion can create. Only few laboratories however have the technical and computational expertise to create in silico models combining dynamic features of chromatin and loop extruders. Here, we present 3DPolyS-LE, a self-contained, easy to use modeling and simulation framework allowing non-specialists to ask how specific properties of loop extruders and boundary elements impact on 3D chromosome structure. 3DPolyS-LE also provides algorithms to compare predictions with experimental Hi-C data.

AVAILABILITY AND IMPLEMENTATION

Software available at https://gitlab.com/togop/3DPolyS-LE; implemented in Python and Fortran 2003 and supported on any Unix-based operating system (Linux and Mac OS).

SUPPLEMENTARY INFORMATION

Supplementary information are available at Bioinformatics online.

Regulation of the mitotic chromosome folding machines

Over the last several years enormous progress has been made in identifying the molecular machines, including condensins and topoisomerases that fold mitotic chromosomes. The discovery that condensins generate chromatin loops through loop extrusion has revolutionized, and energized, the field of chromosome folding. To understand how these machines fold chromosomes with the appropriate dimensions, while disentangling sister chromatids, it needs to be determined how they are regulated and deployed. Here, we outline the current understanding of how these machines and factors are regulated through cell cycle dependent expression, chromatin localization, activation and inactivation through post-translational modifications, and through associations with each other, with other factors and with the chromatin template itself. There are still many open questions about how condensins and topoisomerases are regulated but given the pace of progress in the chromosome folding field, it seems likely that many of these will be answered in the years ahead.

Chromatin jets define the properties of cohesin-driven in vivo loop extrusion

Complex genomes show intricate organization in three-dimensional (3D) nuclear space. Current models posit that cohesin extrudes loops to form self-interacting domains delimited by the DNA binding protein CTCF. Here, we describe and quantitatively characterize cohesin-propelled, jet-like chromatin contacts as landmarks of loop extrusion in quiescent mammalian lymphocytes. Experimental observations and polymer simulations indicate that narrow origins of loop extrusion favor jet formation. Unless constrained by CTCF, jets propagate symmetrically for 1-2 Mb, providing an estimate for the range of in vivo loop extrusion. Asymmetric CTCF binding deflects the angle of jet propagation as experimental evidence that cohesin-mediated loop extrusion can switch from bi- to unidirectional and is controlled independently in both directions. These data offer new insights into the physiological behavior of in vivo cohesin-mediated loop extrusion and further our understanding of the principles that underlie genome organization.

It's all in the numbers: Cohesin stoichiometry

Cohesin, a structural maintenance of chromosome (SMC) complex, organizes chromatin into three-dimensional structures by threading chromatin into loops and stabilizing long-range chromatin interactions. Four subunits in a 1:1:1:1 ratio compose the cohesin core, which is regulated by auxiliary factors that interact with or modify the core subunits. An ongoing debate about cohesin's mechanism of action regards its stoichiometry. Namely, is cohesin activity mediated by a single complex or cooperation between several complexes that organize into dimers or oligomers? Several investigations that used various experimental approaches have tried to resolve this dispute. Some have convincingly demonstrated that the cohesin monomer is the active unit. However, others have revealed the formation of cohesin dimers and higher-order clusters on and off chromosomes. Elucidating the biological function of cohesin clusters and determining what regulates their formation are just two of the many new questions raised by these findings. We briefly review the history of the argument about cohesin stoichiometry and the central evidence for cohesin activity as a monomer vs. an oligomer. Finally, we discuss the possible biological significance of cohesin oligomerization and present open questions that remain to be answered.

The role of chromatin loop extrusion in antibody diversification

Cohesin mediates chromatin loop formation across the genome by extruding chromatin between convergently oriented CTCF-binding elements. Recent studies indicate that cohesin-mediated loop extrusion in developing B cells presents immunoglobulin heavy chain (Igh) variable (V), diversity (D) and joining (J) gene segments to RAG endonuclease through a process referred to as RAG chromatin scanning. RAG initiates V(D)J recombinational joining of these gene segments to generate the large number of different Igh variable region exons that are required for immune responses to diverse pathogens. Antigen-activated mature B cells also use chromatin loop extrusion to mediate the synapsis, breakage and end joining of switch regions flanking Igh constant region exons during class-switch recombination, which allows for the expression of different antibody constant region isotypes that optimize the functions of antigen-specific antibodies to eliminate pathogens. Here, we review recent advances in our understanding of chromatin loop extrusion during V(D)J recombination and class-switch recombination at the Igh locus.

Causality in transcription and genome folding: Insights from X inactivation

The spatial organization of genomes is becoming increasingly understood. In mammals, where it is most investigated, this organization ties in with transcription, so an important research objective is to understand whether gene activity is a cause or a consequence of genome folding in space. In this regard, the phenomena of X-chromosome inactivation and reactivation open a unique window of investigation because of the singularities of the inactive X chromosome. Here we focus on the cause-consequence nexus between genome conformation and transcription and explain how recent results about the structural changes associated with inactivation and reactivation of the X chromosome shed light on this problem.

Turning coldspots into hotspots: targeted recruitment of axis protein Hop1 stimulates meiotic recombination in Saccharomyces cerevisiae

The DNA double strand breaks (DSBs) that initiate meiotic recombination are formed in the context of the meiotic chromosome axis, which in Saccharomyces cerevisiae contains a meiosis-specific cohesin isoform and the meiosis-specific proteins Hop1 and Red1. Hop1 and Red1 are important for DSB formation; DSB levels are reduced in their absence and their levels, which vary along the lengths of chromosomes, are positively correlated with DSB levels. How axis protein levels influence DSB formation and recombination remains unclear. To address this question, we developed a novel approach that uses a bacterial ParB-parS partition system to recruit axis proteins at high levels to inserts at recombination coldspots where Hop1 and Red1 levels are normally low. Recruiting Hop1 markedly increased DSBs and homologous recombination at target loci, to levels equivalent to those observed at endogenous recombination hotspots. This local increase in DSBs did not require Red1 or the meiosis-specific cohesin component Rec8, indicating that, of the axis proteins, Hop1 is sufficient to promote DSB formation. However, while most crossovers at endogenous recombination hotspots are formed by the meiosis-specific MutLγ resolvase, crossovers that formed at an insert locus were only modestly reduced in the absence of MutLγ, regardless of whether or not Hop1 was recruited to that locus. Thus, while local Hop1 levels determine local DSB levels, the recombination pathways that repair these breaks can be determined by other factors, raising the intriguing possibility that different recombination pathways operate in different parts of the genome.

Making Mitotic Chromosomes in a Test Tube

Mitotic chromosome assembly is an essential preparatory step for accurate transmission of the genome during cell division. During the past decades, biochemical approaches have uncovered the molecular basis of mitotic chromosomes. For example, by using cell-free assays of frog egg extracts, the condensin I complex central for the chromosome assembly process was first identified, and its functions have been intensively studied. A list of chromosome-associated proteins has been almost completed, and it is now possible to reconstitute structures resembling mitotic chromosomes with a limited number of purified factors. In this review, I introduce how far we have come in understanding the mechanism of chromosome assembly using cell-free assays and reconstitution assays, and I discuss their potential applications to solve open questions.

Domain Model of Eukaryotic Genome Organization: From DNA Loops Fixed on the Nuclear Matrix to TADs

The article reviews the development of ideas on the domain organization of eukaryotic genome, with special attention on the studies of DNA loops anchored to the nuclear matrix and their role in the emergence of the modern model of eukaryotic genome spatial organization. Critical analysis of results demonstrating that topologically associated chromatin domains are structural-functional blocks of the genome supports the notion that these blocks are fundamentally different from domains whose existence was proposed by the domain hypothesis of eukaryotic genome organization formulated in the 1980s. Based on the discussed evidence, it is concluded that the model postulating that eukaryotic genome is built from uniformly organized structural-functional blocks has proven to be untenable.

Reeling it in: how DNA topology drives loop extrusion by condensin

Structural maintenance of chromosomes (SMC) complexes such as condensin regulate chromosome organization by extruding loops. A new study uses single-molecule imaging of condensin on supercoiled DNA to understand how condensins navigate the under- and overwound DNA states common throughout the genome.

Condensin-driven loop extrusion on supercoiled DNA

Condensin, a structural maintenance of chromosomes (SMC) complex, has been shown to be a molecular motor protein that organizes chromosomes by extruding loops of DNA. In cells, such loop extrusion is challenged by many potential conflicts, for example, the torsional stresses that are generated by other DNA-processing enzymes. It has so far remained unclear how DNA supercoiling affects loop extrusion. Here, we use time-lapse single-molecule imaging to study condensin-driven DNA loop extrusion on supercoiled DNA. We find that condensin binding and DNA looping are stimulated by positively supercoiled DNA, and condensin preferentially binds near the tips of supercoiled plectonemes. Upon loop extrusion, condensin collects nearby plectonemes into a single supercoiled loop that is highly stable. Atomic force microscopy imaging shows that condensin generates supercoils in the presence of ATP. Our findings provide insight into the topology-regulated loading and formation of supercoiled loops by SMC complexes and clarify the interplay of loop extrusion and supercoiling.