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Cohesin and CTCF are master regulators of genome topology. How these ubiquitous proteins contribute to cell-type specific genome structure is poorly understood. Here, we explore quantitative aspects of topologically associated domains (TAD) between pluripotent embryonic stem cells (ESC) and lineage-committed cells. ESCs exhibit permissive topological configurations which manifest themselves as increased inter- TAD interactions, weaker intra-TAD interactions, and a unique intra-TAD connectivity whereby one border makes pervasive interactions throughout the domain. Such ‘stripe’ domains are associated with both poised and active chromatin landscapes and transcription is not a key determinant of their structure. By tracking the developmental dynamics of stripe domains, we show that stripe formation is linked to the functional state of the cell through cohesin loading at lineage-specific enhancers and developmental control of CTCF binding site occupancy. We propose that the unique topological configuration of stripe domains represents a permissive landscape facilitating both productive and opportunistic gene regulation and is important for cellular identity. Eukaryotic genomes fold into topologically associated domains (TAD). Here the authors characterise a TAD regulatory architecture underlying lineage-specific gene regulation, finding that stripe TADs are associated with poised and active chromatin landscapes and linked to the cells functional state.

Structural variants (SVs) can contribute to oncogenesis through a variety of mechanisms. Despite their importance, the identification of SVs in cancer genomes remains challenging. Here, we present a framework that integrates optical mapping, high-throughput chromosome conformation capture (Hi-C), and whole-genome sequencing to systematically detect SVs in a variety of normal or cancer samples and cell lines. We identify the unique strengths of each method and demonstrate that only integrative approaches can comprehensively identify SVs in the genome. By combining Hi-C and optical mapping, we resolve complex SVs and phase multiple SV events to a single haplotype. Furthermore, we observe widespread structural variation events affecting the functions of noncoding sequences, including the deletion of distal regulatory sequences, alteration of DNA replication timing, and the creation of novel three-dimensional chromatin structural domains. Our results indicate that noncoding SVs may be underappreciated mutational drivers in cancer genomes. The authors present an integrative framework for identifying structural variants (SVs) in cancer that applies optical mapping, Hi-C, and whole-genome sequencing. They find SVs affecting distal regulatory sequences, DNA replication, and three-dimensional chromatin structure.

To ensure proper gene regulation within constrained nuclear space, chromosomes facilitate access to transcribed regions, while compactly packaging all other information. Recent studies revealed that chromosomes are organized into megabase‐scale domains that demarcate active and inactive genetic elements, suggesting that compartmentalization is important for genome function. Here, we show that very specific long‐range interactions are anchored by cohesin/CTCF sites, but not cohesin‐only or CTCF‐only sites, to form a hierarchy of chromosomal loops. These loops demarcate topological domains and form intricate internal structures within them. Post‐mitotic nuclei deficient for functional cohesin exhibit global architectural changes associated with loss of cohesin/CTCF contacts and relaxation of topological domains. Transcriptional analysis shows that this cohesin‐dependent perturbation of domain organization leads to widespread gene deregulation of both cohesin‐bound and non‐bound genes. Our data thereby support a role for cohesin in the global organization of domain structure and suggest that domains function to stabilize the transcriptional programmes within them. Chromosomal compartmentalization has been recognized as important for genome function. High‐resolution techniques such as Hi‐C, ChIP‐ and 4C‐seq offer novel insights into cohesin's dynamic role in shaping the nuclear architecture.

Most studies of cohesin function consider the Stromalin Antigen (STAG/SA) proteins as core complex members given their ubiquitous interaction with the cohesin ring. Here, we provide functional data to support the notion that the SA subunit is not a mere passenger in this structure, but instead plays a key role in the localization of cohesin to diverse biological processes and promotes loading of the complex at these sites. We show that in cells acutely depleted for RAD21, SA proteins remain bound to chromatin, cluster in 3D and interact with CTCF, as well as with a wide range of RNA binding proteins involved in multiple RNA processing mechanisms. Accordingly, SA proteins interact with RNA, and R-loops, even in the absence of cohesin. Our results place SA1 on chromatin upstream of the cohesin ring and reveal a role for SA1 in cohesin loading which is independent of NIPBL, the canonical cohesin loader. We propose that SA1 takes advantage of structural R-loop platforms to link cohesin loading and chromatin structure with diverse functions. Since SA proteins are pan-cancer targets, and R-loops play an increasingly prevalent role in cancer biology, our results have important implications for the mechanistic understanding of SA proteins in cancer and disease.

Cohesin branches out As well as its role in sister chromatid cohesion, the cohesin protein complex is thought be involved in the control of gene expression. Hadjur et al . show here that cohesin is involved topologically and mechanistically in long-range interactions at the cytokine locus IFNG . As well as its role in sister chromatid cohesion, cohesin is thought to have a role in the control of gene expression. Here, cohesin is shown to form the topological and mechanistic basis for cell-type-specific long-range chromosomal interactions at the developmentally regulated cytokine locus IFNG . Cohesin-mediated sister chromatid cohesion is essential for chromosome segregation and post-replicative DNA repair 1 , 2 . In addition, evidence from model organisms 3 , 4 , 5 , 6 and from human genetics 7 suggests that cohesin is involved in the control of gene expression 8 , 9 . This non-canonical role has recently been rationalized by the findings that mammalian cohesin complexes are recruited to a subset of DNase I hypersensitive sites and to conserved noncoding sequences by the DNA-binding protein CTCF 10 , 11 , 12 , 13 . CTCF functions at insulators (which control interactions between enhancers and promoters) and at boundary elements (which demarcate regions of distinct chromatin structure) 14 , and cohesin contributes to its enhancer-blocking activity 10 , 11 . The underlying mechanisms remain unknown, and the full spectrum of cohesin functions remains to be determined. Here we show that cohesin forms the topological and mechanistic basis for cell-type-specific long-range chromosomal interactions in cis at the developmentally regulated cytokine locus IFNG . Hence, the ability of cohesin to constrain chromosome topology is used not only for the purpose of sister chromatid cohesion 1 , 2 , but also to dynamically define the spatial conformation of specific loci. This new aspect of cohesin function is probably important for normal development 3 , 4 , 5 , 6 and disease 7 .

Extensive research has revealed that cohesin acts as a topological device, trapping chromosomal DNA within a large tripartite ring. In so doing, cohesin contributes to the formation of compact and organized genomes. How exactly the cohesin subunits interact, how it opens, closes, and translocates on chromatin, and how it actually tethers DNA strands together are still being elucidated. A comprehensive understanding of these questions will shed light on how cohesin performs its many functions, including its recently proposed role as a chromatid loop extruder. Here, we discuss this possibility in light of our understanding of the molecular properties of cohesin complexes.

Background Type II DNA topoisomerases (TOP2) regulate DNA topology by generating transient double stranded breaks during replication and transcription. Topoisomerase II beta (TOP2B) facilitates rapid gene expression and functions at the later stages of development and differentiation. To gain new insight into the genome biology of TOP2B, we used proteomics (BioID), chromatin immunoprecipitation, and high-throughput chromosome conformation capture (Hi-C) to identify novel proximal TOP2B protein interactions and characterize the genomic landscape of TOP2B binding at base pair resolution. Results Our human TOP2B proximal protein interaction network included members of the cohesin complex and nucleolar proteins associated with rDNA biology. TOP2B associates with DNase I hypersensitivity sites, allele-specific transcription factor (TF) binding, and evolutionarily conserved TF binding sites on the mouse genome. Approximately half of all CTCF/cohesion-bound regions coincided with TOP2B binding. Base pair resolution ChIP-exo mapping of TOP2B, CTCF, and cohesin sites revealed a striking structural ordering of these proteins along the genome relative to the CTCF motif. These ordered TOP2B-CTCF-cohesin sites flank the boundaries of topologically associating domains (TADs) with TOP2B positioned externally and cohesin internally to the domain loop. Conclusions TOP2B is positioned to solve topological problems at diverse cis-regulatory elements and its occupancy is a highly ordered and prevalent feature of CTCF/cohesin binding sites that flank TADs.

STAG proteins are key regulators of the cohesin complex and are often linked to alterations in cell identity and disease. Among the mammalian STAG paralogs, STAG3 has been less extensively studied beyond its known roles in meiosis. In this work, we demonstrate that STAG3 is expressed in mouse embryonic stem cells (mESCs) and primordial germ cell-like cells (PGCLCs), where it is required for cell fate decisions. Distinct from the other STAG proteins, STAG3 mediates its effects in the cytoplasm, facilitating the post-transcriptional regulation of gene expression. Furthermore, STAG3 localises to the centrosome independently of cohesin and interacts with proteins involved in mRNA localisation and stability. The knockdown of STAG3 in mESCs using siRNAs results in the destabilisation of the centrosome and the key P-body RNA-induced silencing complex (RISC) component TNRC6C, leading to the derepression of P-body localised mRNAs, such as DPPA3. Our results propose a model in which STAG3 collaborates with RNA-binding proteins (RBPs) and specific target mRNAs to control post-transcriptional gene expression and facilitate the transition from pluripotency in mESCs. Given that STAG3 is upregulated in various cancers, our results provide a novel perspective on how STAG proteins might contribute to cell identity and disease.

Brain derived neurotrophic factor (BDNF) is a critical secreted peptide that promotes neuronal differentiation and survival, and its downregulation is implicated in many neurological disorders. Here, we investigated the regulation of the mouse Bdnf gene in cortical neurons and identified a novel enhancer that promotes the expression of many Bdnf transcript variants during differentiation, increasing total Bdnf mRNA levels. Enhancer activity contributes to Bdnf-mediated effects on neuronal clustering and activity-dependent dendritogenesis. During Bdnf activation, enhancer-promoter contacts increase, and the region moves away from the repressive nuclear periphery. Our findings suggest that changes in nuclear structure may contribute to the expression of essential growth factors during neuronal development.

Several studies have shown a role for Stag proteins in cell identity. Our understanding of how Stag proteins contribute to cell identity have largely been focused on its roles in chromosome topology as part of the cohesin complex and the impact on protein-coding gene expression. Furthermore, several Stag paralogs exist in mammalian cells with non-reciprocal chromosome structure and cohesion functions. Why cells have so many Stag proteins and what specific functions each Stag protein performs to support a given cell state are poorly understood. Here we reveal that Stag1 is the dominant paralog in mouse embryonic stem cells (mESC) and is required for pluripotency. Through the discovery of diverse, naturally occurring Stag1 isoforms in mESCs, we shed new light not only on the unique ends of Stag1 but also the critical role that their levels play in stem cell identity. Furthermore, we revel a new role for Stag1, and specifically its unique N-terminal end, in regulating nucleolar integrity and safeguarding mESCs from totipotency. Stag1 is localised to repressive perinucleolar regions, bound at repeats and interacts with Nucleolin and TRIM28. Loss of the Stag1 N-terminus, leads to decreased LINE-1 and rRNA expression and disruption of nucleolar structure and function which consequently leads to activation of the two-cell-like (2C-LC)-specific transcription factor DUX and conversion of pluripotent mESCs to totipotent 2C-LCs. Our results move beyond protein coding gene regulation via chromatin loops into a new role for Stag1 in repeat regulation and nucleolar structure, and offer fresh perspectives on how Stag proteins contribute to cell identity and disease.Competing Interest StatementThe authors have declared no competing interest.Footnotes* includes more detailed information around nucleolar structure in Stag1-deficient cells.