Resolving the 3D Landscape of Transcription-Linked Mammalian Chromatin Folding

CTCF orchestrates long-range cohesin-driven V(D)J recombinational scanning

The RAG endonuclease initiates Igh locus V(D)J recombination in progenitor (pro)-B cells1. Upon binding a recombination centre-based JH, RAG scans upstream chromatin via loop extrusion, potentially mediated by cohesin, to locate Ds and assemble a DJH-based recombination centre2. CTCF looping factor-bound elements (CBEs) within IGCR1 upstream of Ds impede RAG scanning3-5; however, their inactivation allows scanning to proximal VHs, where additional CBEs activate rearrangement and impede scanning any further upstream5. Distal VH utilization is thought to involve diffusional access to the recombination centre following large-scale Igh locus contraction6-8. Here we test the potential of linear RAG scanning to mediate distal VH usage in G1-arrested v-Abl pro-B cell lines9, which undergo robust D-to-JH but little VH-to-DJH rearrangements, presumably owing to lack of locus contraction2,5. Through an auxin-inducible approach10, we degraded the cohesin component RAD2110-12 or CTCF12,13 in these G1-arrested lines. Degradation of RAD21 eliminated all V(D)J recombination and interactions associated with RAG scanning, except for reecombination centre-located DQ52-to-JH joining, in which synapsis occurs by diffusion2. Remarkably, while degradation of CTCF suppressed most CBE-based chromatin interactions, it promoted robust recombination centre interactions with, and robust VH-to-DJH joining of, distal VHs, with patterns similar to those of 'locus-contracted' primary pro-B cells. Thus, downmodulation of CTCF-bound scanning-impediment activity promotes cohesin-driven RAG scanning across the 2.7-Mb Igh locus.

Alteration of genome folding via contact domain boundary insertion

Animal chromosomes are partitioned into contact domains. Pathogenic domain disruptions can result from chromosomal rearrangements or perturbation of architectural factors. However, such broad-scale alterations are insufficient to define the minimal requirements for domain formation. Moreover, to what extent domains can be engineered is just beginning to be explored. In an attempt to create contact domains, we inserted a 2-kb DNA sequence underlying a tissue-invariant domain boundary-containing a CTCF-binding site (CBS) and a transcription start site (TSS)-into 16 ectopic loci across 11 chromosomes, and characterized its architectural impact. Depending on local constraints, this fragment variably formed new domains, partitioned existing ones, altered compartmentalization and initiated contacts reflecting chromatin loop extrusion. Deletions of the CBS or the TSS individually or in combination within inserts revealed its distinct contributions to genome folding. Altogether, short DNA insertions can suffice to shape the spatial genome in a manner influenced by chromatin context.

Mustache: multi-scale detection of chromatin loops from Hi-C and Micro-C maps using scale-space representation

We present MUSTACHE, a new method for multi-scale detection of chromatin loops from Hi-C and Micro-C contact maps. MUSTACHE employs scale-space theory, a technical advance in computer vision, to detect blob-shaped objects in contact maps. MUSTACHE is scalable to kilobase-resolution maps and reports loops that are highly consistent between replicates and between Hi-C and Micro-C datasets. Compared to other loop callers, such as HiCCUPS and SIP, MUSTACHE recovers a higher number of published ChIA-PET and HiChIP loops as well as loops linking promoters to regulatory elements. Overall, MUSTACHE enables an efficient and comprehensive analysis of chromatin loops. Available at: https://github.com/ay-lab/mustache .

A saturating mutagenesis CRISPR-Cas9-mediated functional genomic screen identifies cis- and trans-regulatory elements of Oct4 in murine ESCs

Regulatory elements (REs) consist of enhancers and promoters that occupy a significant portion of the noncoding genome and control gene expression programs either in cis or in trans Putative REs have been identified largely based on their regulatory features (co-occupancy of ESC-specific transcription factors, enhancer histone marks, and DNase hypersensitivity) in mouse embryonic stem cells (mESCs). However, less has been established regarding their regulatory functions in their native context. We deployed cis- and trans-regulatory elements scanning through saturating mutagenesis and sequencing (ctSCAN-SMS) to target elements within the ∼12-kb cis-region (cis-REs; CREs) of the Oct4 gene locus, as well as genome-wide 2,613 high-confidence trans-REs (TREs), in mESCs. ctSCAN-SMS identified 10 CREs and 12 TREs as novel candidate REs of the Oct4 gene in mESCs. Furthermore, deletions of these candidate REs confirmed that the majority of the REs are functionally active, and CREs are more active than TREs in controlling Oct4 gene expression. A subset of active CREs and TREs physically interact with the Oct4 promoter to varying degrees; specifically, a greater number of active CREs, compared with active TREs, physically interact with the Oct4 promoter. Moreover, comparative genomics analysis reveals that a greater number of active CREs than active TREs are evolutionarily conserved between mice and primates, including humans. Taken together, our study demonstrates the reliability and robustness of ctSCAN-SMS screening to identify critical REs and investigate their roles in the regulation of transcriptional output of a target gene (in this case Oct4) in their native context.

A modified protocol of Capture-C allows affordable and flexible high-resolution promoter interactome analysis

Large-scale epigenomic projects have mapped hundreds of thousands of potential regulatory sites in the human genome, but only a small proportion of these elements are proximal to transcription start sites. It is believed that the majority of these sequences are remote promoter-activating genomic sites scattered within several hundreds of kilobases from their cognate promoters and referred to as enhancers. It is still unclear what principles, aside from relative closeness in the linear genome, determine which promoter(s) is controlled by a given enhancer; however, this understanding is of great fundamental and clinical relevance. In recent years, C-methods (chromosome conformation capture-based methods) have become a powerful tool for the identification of enhancer-promoter spatial contacts that, in most cases, reflect their functional link. Here, we describe a new hybridisation-based promoter Capture-C protocol that makes use of biotinylated dsDNA probes generated by PCR from a custom pool of long oligonucleotides. The described protocol allows high-resolution promoter interactome description, providing a flexible and cost-effective alternative to the existing promoter Capture-C modifications. Based on the obtained data, we propose several tips on probe design that could potentially improve the results of future experiments.

Genome folding and refolding in differentiation and cellular senescence

The spatial conformation of chromatin within the confines of eukaryotic cell nuclei is now acknowledged as a decisive epigenetic mechanism for the modulation of such cellular functions as gene expression regulation, DNA replication or DNA damage repair. Of course, these processes are tightly regulated during organismal development and markedly affected by cellular ageing. Thus, the question that arises is to what extent does folding or refolding of the genome in three-dimensional space underlie the progression of development or ageing? Herein, we discuss recent experimental and modelling evidence to address this question and revisit how these seemingly different processed might represent two sides of the same coin.

G1/S transcription factors assemble in increasing numbers of discrete clusters through G1 phase

In budding yeast, the transcription factors SBF and MBF activate a large program of gene expression in late G1 phase that underlies commitment to cell division, termed Start. SBF/MBF are limiting with respect to target promoters in small G1 phase cells and accumulate as cells grow, raising the questions of how SBF/MBF are dynamically distributed across the G1/S regulon and how this impacts the Start transition. Super-resolution Photo-Activatable Localization Microscopy (PALM) mapping of the static positions of SBF/MBF subunits in fixed cells revealed each transcription factor was organized into discrete clusters containing approximately eight copies regardless of cell size and that the total number of clusters increased as cells grew through G1 phase. Stochastic modeling using reasonable biophysical parameters recapitulated growth-dependent SBF/MBF clustering and predicted TF dynamics that were confirmed in live cell PALM experiments. This spatio-temporal organization of SBF/MBF may help coordinate activation of G1/S regulon and the Start transition.

Direct DNA crosslinking with CAP-C uncovers transcription-dependent chromatin organization at high resolution

Determining the spatial organization of chromatin in cells mainly relies on crosslinking-based chromosome conformation capture techniques, but resolution and signal-to-noise ratio of these approaches is limited by interference from DNA-bound proteins. Here we introduce chemical-crosslinking assisted proximity capture (CAP-C), a method that uses multifunctional chemical crosslinkers with defined sizes to capture chromatin contacts. CAP-C generates chromatin contact maps at subkilobase (sub-kb) resolution with low background noise. We applied CAP-C to formaldehyde prefixed mouse embryonic stem cells (mESCs) and investigated loop domains (median size of 200 kb) and nonloop domains (median size of 9 kb). Transcription inhibition caused a greater loss of contacts in nonloop domains than loop domains. We uncovered conserved, transcription-state-dependent chromatin compartmentalization at high resolution that is shared from Drosophila to human, and a transcription-initiation-dependent nuclear subcompartment that brings multiple nonloop domains in close proximity. We also showed that CAP-C could be used to detect native chromatin conformation without formaldehyde prefixing.

Transcription factors: building hubs in the 3D space

The hierarchical three-dimensional folding of the mammalian genome constitutes an important regulatory layer of gene expression and cell fate control during processes such as development and tumorigenesis. Accumulating evidence supports the existence of complex topological assemblies in which multiple genes and regulatory elements are frequently interacting with each other in the 3D nucleus. Here, we will discuss the nature, organizational principles, and potential function of such assemblies, including the recently reported enhancer “hubs,” “cliques,” and FIREs (frequently interacting regions) as well as multi-contact hubs. We will also review recent studies that investigate the role of transcription factors (TFs) in driving the topological genome reorganization and hub formation in the context of cell fate transitions and cancer. Finally, we will highlight technological advances that enabled these studies, current limitations, and future directions necessary to advance our understating in the field.

Epigenetic regulation of intestinal stem cell differentiation

To fulfill the lifelong need to supply diverse epithelial cells, intestinal stem cells (ISCs) rely on executing accurate transcriptional programs. This review addresses the mechanisms that control those programs. Genes that define cell behaviors and identities are regulated principally through thousands of dispersed enhancers, each individually <1 kb long and positioned from a few to hundreds of kilobases away from transcription start sites, upstream or downstream from coding genes or within introns. Wnt, Notch, and other epithelial control signals feed into these cis-regulatory DNA elements, which are also common loci of polymorphisms and mutations that confer disease risk. Cell-specific gene activity requires promoters to interact with the correct combination of signal-responsive enhancers. We review the current state of knowledge in ISCs regarding active enhancers, the nucleosome modifications that may enable appropriate and hinder inappropriate enhancer-promoter contacts, and the roles of lineage-restricted transcription factors.

The Role of Noncoding Variants in Heritable Disease

The genetic basis of disease has largely focused on coding regions. However, it has become clear that a large proportion of the noncoding genome is functional and harbors genetic variants that contribute to disease etiology. Here, we review recent examples of inherited noncoding alterations that are responsible for Mendelian disorders or act to influence complex traits. We explore both rare and common genetic variants and discuss the wide range of mechanisms by which they affect gene regulation to promote disease. We also debate the challenges and progress associated with identifying and interpreting the functional and clinical significance of genetic variation in the context of the noncoding regulatory landscape.

Nucleosome positioning and chromatin organization

In our cells, DNA is folded and packed with the help of many proteins into chromatin whose basic unit is a nucleosome-DNA wrapped around octamer of histone proteins. The chain of nucleosomes is further folded and arranged into many layers and has a dynamic organization. How does the complex chromatin organization emerge from interactions among DNA, histones, and non-histone proteins have been a question of great interest. Here we review recent literature that investigated how nucleosome positioning and nucleosome-mediated interactions drive chromatin organization. Unlike our earlier understanding, chromatin is organized into 3D domains of various sizes having irregularly organized nucleosomes. These domains emerge due to heterogeneous nucleosome positioning and diverse inter-nucleosome interactions that vary in space and time.

Genome-wide analyses of chromatin interactions after the loss of Pol I, Pol II, and Pol III

BACKGROUND

The relationship between transcription and the 3D chromatin structure is debated. Multiple studies have shown that transcription affects global Cohesin binding and 3D genome structures. However, several other studies have indicated that inhibited transcription does not alter chromatin conformations.

RESULTS

We provide the most comprehensive evidence to date to demonstrate that transcription plays a relatively modest role in organizing the local, small-scale chromatin structures in mammalian cells. We show degraded Pol I, Pol II, and Pol III proteins in mESCs cause few or no changes in large-scale 3D chromatin structures, selected RNA polymerases with a high abundance of binding sites or active promoter-associated interactions appear to be relatively more affected after the degradation, transcription inhibition alters local, small loop domains, as indicated by high-resolution chromatin interaction maps, and loops with bound Pol II but without Cohesin or CTCF are identified and found to be largely unchanged after transcription inhibition. Interestingly, Pol II depletion for a longer time significantly affects the chromatin accessibility and Cohesin occupancy, suggesting that RNA polymerases are capable of affecting the 3D genome indirectly. These direct and indirect effects explain the previous inconsistent findings on the influence of transcription inhibition on the 3D genome.

CONCLUSIONS

We conclude that Pol I, Pol II, and Pol III loss alters local, small-scale chromatin interactions in mammalian cells, suggesting that the 3D chromatin structures are pre-established and relatively stable.

Of Dots and Stripes: The Morse Code of Micro-C Reveals the Ultrastructure of Transcriptional and Architectural Mammalian 3D Genome Organization

Using Micro-C, Hsieh et al. (2020) and Krietenstein et al. (2020) investigated 3D chromatin folding in human and mouse cells at unprecedented resolution to uncover ultra-fine-scale chromatin structures that provide direct links between genome architecture, gene expression, and gene regulation.

Coolpup.py: versatile pile-up analysis of Hi-C data

MOTIVATION

Hi-C is currently the method of choice to investigate the global 3D organization of the genome. A major limitation of Hi-C is the sequencing depth required to robustly detect loops in the data. A popular approach used to mitigate this issue, even in single-cell Hi-C data, is genome-wide averaging (piling-up) of peaks, or other features, annotated in high-resolution datasets, to measure their prominence in less deeply sequenced data. However, current tools do not provide a computationally efficient and versatile implementation of this approach.

RESULTS

Here, we describe coolpup.py-a versatile tool to perform pile-up analysis on Hi-C data. We demonstrate its utility by replicating previously published findings regarding the role of cohesin and CTCF in 3D genome organization, as well as discovering novel details of Polycomb-driven interactions. We also present a novel variation of the pile-up approach that can aid the statistical analysis of looping interactions. We anticipate that coolpup.py will aid in Hi-C data analysis by allowing easy to use, versatile and efficient generation of pile-ups.

AVAILABILITY AND IMPLEMENTATION

Coolpup.py is cross-platform, open-source and free (MIT licensed) software. Source code is available from https://github.com/Phlya/coolpuppy and it can be installed from the Python Packaging Index.

Reorganized 3D Genome Structures Support Transcriptional Regulation in Mouse Spermatogenesis

Three-dimensional chromatin structures undergo dynamic reorganization during mammalian spermatogenesis; however, their impacts on gene regulation remain unclear. Here, we focused on understanding the structure-function regulation of meiotic chromosomes by Hi-C and other omics techniques in mouse spermatogenesis across five stages. Beyond confirming recent reports regarding changes in compartmentalization and reorganization of topologically associating domains (TADs), we further demonstrated that chromatin loops are present prior to and after, but not at, the pachytene stage. By integrating Hi-C and RNA-seq data, we showed that the switching of A/B compartments between spermatogenic stages is tightly associated with meiosis-specific mRNAs and piRNAs expression. Moreover, our ATAC-seq data indicated that chromatin accessibility per se is not responsible for the TAD and loop diminishment at pachytene. Additionally, our ChIP-seq data demonstrated that CTCF and cohesin remain bound at TAD boundary regions throughout meiosis, suggesting that dynamic reorganization of TADs does not require CTCF and cohesin clearance.

Chromosome Conformation Capture and Beyond: Toward an Integrative View of Chromosome Structure and Function

Rapidly developing technologies have recently fueled an exciting era of discovery in the field of chromosome structure and nuclear organization. In addition to chromosome conformation capture (3C) methods, new alternative techniques have emerged to study genome architecture and biological processes in the nucleus, often in single or living cells. This sets an unprecedented stage for exploring the mechanisms that link chromosome structure and biological function. Here we review popular as well as emerging approaches to study chromosome organization, focusing on the contribution of complementary methodologies to our understanding of structures revealed by 3C methods and their biological implications, and discuss the next technical and conceptual frontiers.