Distinct Structures and Dynamics of Chromatosomes with Different Human Linker Histone Isoforms

Always on the Move: Overview on Chromatin Dynamics within Nuclear Processes

Our genome is organized into chromatin, a dynamic and modular structure made of nucleosomes. Chromatin organization controls access to the DNA sequence, playing a fundamental role in cell identity and function. How nucleosomes enable these processes is an active area of study. In this review, we provide an overview of chromatin dynamics, its properties, mechanisms, and functions. We highlight the diverse ways by which chromatin dynamics is controlled during transcription, DNA replication, and repair. Recent technological developments have promoted discoveries in this area, to which we provide an outlook on future research directions.

MagIC-Cryo-EM, structural determination on magnetic beads for scarce macromolecules in heterogeneous samples

Cryo-EM single-particle analyses typically require target macromolecule concentration at 0.05~5.0 mg/ml, which is often difficult to achieve. Here, we devise Magnetic Isolation and Concentration (MagIC)-cryo-EM, a technique enabling direct structural analysis of targets captured on magnetic beads, thereby reducing the targets' concentration requirement to <0.0005 mg/mL. Adapting MagIC-cryo-EM to a Chromatin Immunoprecipitation protocol, we characterized structural variations of the linker histone H1.8-associated nucleosomes that were isolated from interphase and metaphase chromosomes in Xenopus egg extract. Combining Duplicated Selection To Exclude Rubbish particles (DuSTER), a particle curation method that excludes low signal-to-noise ratio particles, we also resolved the 3D cryo-EM structures of nucleoplasmin NPM2 co-isolated with the linker histone H1.8 and revealed distinct open and closed structural variants. Our study demonstrates the utility of MagIC-cryo-EM for structural analysis of scarce macromolecules in heterogeneous samples and provides structural insights into the cell cycle-regulation of H1.8 association to nucleosomes.

DEK-nucleosome structure shows DEK modulates H3K27me3 and stem cell fate

DEK is a highly conserved chromatin-associated oncoprotein that has important roles in regulating chromatin dynamics and stem cell fate. Dysregulation of DEK is associated with stem cell dysfunction and cancers, including acute myeloid leukemia. Despite its importance in chromatin regulation, the structural mechanisms underlying DEK's interaction with chromatin and its influence on gene regulation remain poorly understood. Here we combined cryogenic electron microscopy (cryo-EM), biochemical and cellular approaches to investigate the molecular mechanisms and functional importance of DEK's interaction with chromatin. Our cryo-EM structures reveal the structural basis of the DEK-nucleosome interaction. Biochemical and cellular results demonstrate that this interaction is crucial for DEK deposition onto chromatin. Furthermore, our results reveal that DEK safeguards mouse embryonic stem cells from acquiring primitive endoderm fates by modulating the repressive histone mark H3K27me3. Together, our study provides crucial molecular insights into the structure and function of DEK, establishing a framework for understanding its roles in chromatin biology and cell fate determination.

The evolving role of histone H1 in shaping chromatin and epigenetic landscapes

Histone H1, long recognized for its fundamental role in stabilizing nucleosomes and compacting chromatin, is now emerging as a highly dynamic and versatile regulator essential for diverse nuclear processes. This review synthesizes recent advancements that move beyond H1's canonical structural functions, illuminating its intricate, often context-dependent, control over epigenetic modifications, gene expression, and 3D genome organization across eukaryotes. H1's ability to modulate chromatin accessibility and architecture, influenced by its local density, variant composition, and dynamic binding, dictates its species- and locus-specific impacts. H1 critically shapes DNA methylation landscapes and the deposition of key histone marks H3K27me3, often by affecting enzyme accessibility and inter-pathway dynamics. Its transcriptional impact transcends canonical transposable element silencing, extending to the selective fine-tuning of gene expression, with certain H1 variants even functioning as direct transcriptional activators. Regarding 3D genome organization, while H1's local density drives compartmentalization and influences epigenetic states in mammals, in Arabidopsis, it exhibits more complex, locus-specific roles including modulating telomere clustering, interstitial telomeric repeat insulation, and facilitating phase separation for heterochromatin foci assembly. Collectively, these findings establish histone H1 not merely as a structural backbone, but as a sophisticated regulator that intricately links chromatin's physical state to its functional outputs, profoundly impacting genome integrity, gene regulation, and cellular identity.

Nanoscale analysis of human G1 and metaphase chromatin in situ

The structure of chromatin at the nucleosome level inside cells is still incompletely understood. Here we present in situ electron cryotomography analyses of chromatin in both G1 and metaphase RPE-1 cells. G1 nucleosomes are concentrated in globular chromatin domains, and metaphase nucleosomes are concentrated in the chromatids. Classification analysis reveals that canonical mononucleosomes, and in some conditions ordered stacked dinucleosomes and mononucleosomes with a disordered gyre-proximal density, are abundant in both cell-cycle states. We do not detect class averages that have more than two stacked nucleosomes or side-by-side dinucleosomes, suggesting that groups of more than two nucleosomes are heterogeneous. Large multi-megadalton structures are abundant in G1 nucleoplasm, but not found in G1 chromatin domains and metaphase chromatin. The macromolecular phenotypes studied here represent a starting point for the comparative analysis of compaction in normal vs. unhealthy human cells, in other cell-cycle states, other organisms, and in vitro chromatin assemblies.

Role of charges in a dynamic disordered complex between an IDP and a folded domain

Protein complexes involving intrinsically disordered proteins (IDPs) cover a continuum from IDPs that fully fold upon binding to IDPs that remain fully disordered in the complex. Here we demonstrate a case of charge-driven interactions of a folded domain with an oppositely charged IDP that remains completely disordered in the complex. Using the negatively charged and fully disordered prothymosin α and the positively charged and folded globular domain of histone H1.0, we show that they form a low-micromolar-affinity complex without fixed relative orientations or persistent contacts between specific residues. Using 25 charge variants of the globular domain, we find that the binding affinity can be modulated both by net charge and charge clustering on the folded domain, indicating some selectivity in highly charged complexes. Our results highlight that a folded protein can provide a charged surface onto which an oppositely charged IDP can bind while retaining disorder. We expect that more such complexes exist.

Linker Histones Maintain Genome Stability and Drive the Process of Cellular Ageing

Ageing comprises a cascade of processes that are inherent in all living creatures. There are fourteen general hallmarks of cellular ageing, the majority of which occur at a molecular level. A significant disturbance in the regulation of genome activity is commonly observed during cellular ageing. Overall confusion and disruption in the proper functioning of the genome are also well-known prerogatives of cancerous cells, and it is believed that this genomic instability provides a direct link between aging and cancer. The spatial organization of nuclear DNA in chromatin is the foundation of the fine-tuning and refined regulation of gene activity, and it changes during ageing. Therefore, chromatin is the platform on which genes and the environment meet and interplay. Different protein factors, small molecules and metabolites affect this chromatin organization and, through it, drive cellular deterioration and, finally, ageing. Hence, studying chromatin structural organization and dynamics is crucial for understanding life, presumably the ageing process. The complex interplay among DNA and histone proteins folds, organizes, and adapts chromatin structure. Among histone proteins, the role of the family of linker histones comes to light. Recent data point out that linker histones play a unique role in higher-order chromatin organization, which, in turn, impacts ageing to a prominent degree. Here, we discuss emerging evidence that suggests linker histones have functions that extend beyond their traditional roles in chromatin architecture, highlighting their critical involvement in genome stability, cellular ageing, and cancer development, thereby establishing them as promising targets for therapeutic interventions.

Function of HP1BP3 as a linker histone is regulated by linker histone chaperones, NPM1 and TAF-I

BACKGROUND

Linker histones constitute a class of proteins that are responsible for the formation of higher-order chromatin structures. Core histones are integral components of nucleosome core particles (NCPs), whereas linker histones bind to linker DNA between NCPs. Heterochromatin protein 1 binding protein 3 (HP1BP3) displays sequence similarity to linker histones, with the exception of the presence of three globular domains in its central region. However, the function of HP1BP3 as a linker histone has not been analyzed previously. The present study aimed to elucidate the function of full-length HP1BP3 as a linker histone variant.

RESULTS

The results of biochemical analyses demonstrate that HP1BP3 efficiently binds to NCPs with similar efficiency as linker histones, thereby forming a chromatosome. Notwithstanding the presence of three globular domains, the results suggest that a single HP1BP3 binds to a single NCP under our biochemical assay condition. Moreover, our findings revealed that the NCP binding activity of HP1BP3 is regulated by linker histone chaperones, nucleophosmin (NPM1) and template activating factor-I (TAF-I). The globular domains and the C-terminal disordered region of HP1BP3 are responsible for binding to histone chaperones. Chromatin immunoprecipitation-sequence analyses demonstrated that HP1BP3 exhibited weak preferences for the genomic loci where histone H3 active modification marks were enriched, whereas a linker histone variant, H1.2, showed weak preferences for the genomic loci where histone H3 inactive modification marks were enriched. It is noteworthy that the preferential binding tendencies of HP1BP3 and H1.2 to active and inactive genomic loci, respectively, are diminished upon the knockdown of either NPM1 or TAF-I.

CONCLUSIONS

Our findings indicate that HP1BP3 functions as a linker histone variant and that the chromatin binding preference of linker histones, including HP1BP3, is regulated by linker histone chaperones.

Linker histone H1.0 loads onto nucleosomes through multiple pathways that are facilitated by histone chaperones

Linker histone H1 is an essential chromatin architectural protein that compacts chromatin into transcriptionally silent regions by interacting with nucleosomal and linker DNA, while rapidly exchanging in vivo. How H1 targets nucleosomes while being dynamic remains unanswered. Using a single-molecule strategy, we investigated human H1.0 interactions with DNA and nucleosomes. H1.0 directly binds nucleosomes and naked DNA with a preference toward nucleosomes. DNA-bound H1.0 exhibited a range of bound lifetimes with both mobile and immobile states, where the mobile H1.0 did not load onto nucleosomes. The histone chaperone Nap1 facilitated H1.0-nucleosome loading by enabling H1.0 loading through DNA sliding, reducing DNA resident times without impacting nucleosome resident times, increasing mobility along DNA, and targeting H1.0 loading onto the nucleosome dyad. These findings reveal linker histones load onto nucleosomes through multiple distinct mechanisms that are facilitated by chaperones to regulate chromatin accessibility.

Structural insights into how DEK nucleosome binding facilitates H3K27 trimethylation in chromatin

Structural diversity of the nucleosome affects chromatin conformations and regulates eukaryotic genome functions. Here we identify DEK, whose function is unknown, as a nucleosome-binding protein. In embryonic neural progenitor cells, DEK colocalizes with H3 K27 trimethylation (H3K27me3), the facultative heterochromatin mark. DEK stimulates the methyltransferase activity of Polycomb repressive complex 2 (PRC2), which is responsible for H3K27me3 deposition in vitro. Cryo-electron microscopy structures of the DEK-nucleosome complexes reveal that DEK binds the nucleosome by its tripartite DNA-binding mode on the dyad and linker DNAs and interacts with the nucleosomal acidic patch by its newly identified histone-binding region. The DEK-nucleosome interaction mediates linker DNA reorientation and induces chromatin compaction, which may facilitate PRC2 activation. These findings provide mechanistic insights into chromatin structure-mediated gene regulation by DEK.

Cryo-EM structures of the BAF-Lamin A/C complex bound to nucleosomes

Barrier-to-autointegration factor (BAF) associates with mitotic chromosomes and promotes nuclear envelope assembly by recruiting proteins, such as Lamins, required for the reconstruction of the nuclear envelope and lamina. BAF also mediates chromatin anchoring to the nuclear lamina via Lamin A/C. However, the mechanism by which BAF and Lamin A/C bind chromatin and affect the chromatin organization remains elusive. Here we report the cryo-electron microscopy structures of BAF-Lamin A/C-nucleosome complexes. We find that the BAF dimer complexed with the Lamin A/C IgF domain occupies the nucleosomal dyad position, forming a tripartite nucleosomal DNA binding structure. We also show that the Lamin A/C Lys486 and His506 residues, which are reportedly mutated in lipodystrophy patients, directly contact the DNA at the nucleosomal dyad. Excess BAF-Lamin A/C complexes symmetrically bind other nucleosomal DNA sites and connect two BAF-Lamin A/C-nucleosome complexes. Although the linker histone H1 competes with BAF-Lamin A/C binding at the nucleosomal dyad region, the two BAF-Lamin A/C molecules still bridge two nucleosomes. These findings provide insights into the mechanism by which BAF, Lamin A/C, and/or histone H1 bind nucleosomes and influence chromatin organization within the nucleus.

Structural basis of SIRT7 nucleosome engagement and substrate specificity

Chromatin-modifying enzymes target distinct residues within histones to finetune gene expression profiles. SIRT7 is an NAD+-dependent deacylase often deregulated in cancer, which deacetylates either H3 lysine 36 (H3K36) or H3K18 with high specificity within nucleosomes. Here, we report structures of nucleosome-bound SIRT7, and uncover the structural basis of its specificity towards H3K36 and K18 deacylation, combining a mechanism-based cross-linking strategy, cryo-EM, and enzymatic and cellular assays. We show that the SIRT7 N-terminus represents a unique, extended nucleosome-binding domain, reaching across the nucleosomal surface to the acidic patch. The catalytic domain binds at the H3-tail exit site, engaging both DNA gyres of the nucleosome. Contacting H3K36 versus H3K18 requires a change in binding pose, and results in structural changes in both SIRT7 and the nucleosome. These structures reveal the basis of lysine specificity, allowing us to engineer SIRT7 towards enhanced H3K18ac selectivity, and provides a basis for small molecule modulator development.

Beyond the mono-nucleosome

Nucleosomes, the building block of chromatin, are responsible for regulating access to the DNA sequence. This control is critical for essential cellular processes, including transcription and DNA replication and repair. Studying chromatin can be challenging both in vitro and in vivo, leading many to use a mono-nucleosome system to answer fundamental questions relating to chromatin regulators and binding partners. However, the mono-nucleosome fails to capture essential features of the chromatin structure, such as higher-order chromatin folding, local nucleosome-nucleosome interactions, and linker DNA trajectory and flexibility. We briefly review significant discoveries enabled by the mono-nucleosome and emphasize the need to go beyond this model system in vitro. Di-, tri-, and tetra-nucleosome arrays can answer important questions about chromatin folding, function, and dynamics. These multi-nucleosome arrays have highlighted the effects of varying linker DNA lengths, binding partners, and histone post-translational modifications in a more chromatin-like environment. We identify various chromatin regulatory mechanisms yet to be explored with multi-nucleosome arrays. Combined with in-solution biophysical techniques, studies of minimal multi-nucleosome chromatin models are feasible.

Compromised 2-start zigzag chromatin folding in immature mouse retina cells driven by irregularly spaced nucleosomes with short DNA linkers

The formation of condensed heterochromatin is critical for establishing cell-specific transcriptional programs. To reveal structural transitions underlying heterochromatin formation in maturing mouse rod photoreceptors, we apply cryo-EM tomography, AI-assisted deep denoising, and molecular modeling. We find that chromatin isolated from immature retina cells contains many closely apposed nucleosomes with extremely short or absent nucleosome linkers, which are inconsistent with the typical two-start zigzag chromatin folding. In mature retina cells, the fraction of short-linker nucleosomes is much lower, supporting stronger chromatin compaction. By Cryo-EM-assisted nucleosome interaction capture we observe that chromatin in immature retina is enriched with i±1 interactions while chromatin in mature retina contains predominantly i±2 interactions typical of the two-start zigzag. By mesoscale modeling and computational simulation, we clarify that the unusually short linkers typical of immature retina are sufficient to inhibit the two-start zigzag and chromatin compaction by the interference of very short linkers with linker DNA stems. We propose that this short linker composition renders nucleosome arrays more open in immature retina and that, as the linker DNA length increases in mature retina, chromatin fibers become globally condensed via tight zigzag folding. This mechanism may be broadly utilized to introduce higher chromatin folding entropy for epigenomic plasticity.

MagIC-Cryo-EM: Structural determination on magnetic beads for scarce macromolecules in heterogeneous samples

Cryo-EM single-particle analyses typically require target macromolecule concentration at 0.05~5.0 mg/ml, which is often difficult to achieve. Here, we devise Magnetic Isolation and Concentration (MagIC)-cryo-EM, a technique enabling direct structural analysis of targets captured on magnetic beads, thereby reducing the targets' concentration requirement to < 0.0005 mg/ml. Adapting MagIC-cryo-EM to a Chromatin Immunoprecipitation protocol, we characterized structural variations of the linker histone H1.8-associated nucleosomes that were isolated from interphase and metaphase chromosomes in Xenopus egg extract. Combining Duplicated Selection To Exclude Rubbish particles (DuSTER), a particle curation method that excludes low signal-to-noise ratio particles, we also resolved the 3D cryo-EM structures of nucleoplasmin NPM2 co-isolated with the linker histone H1.8 and revealed distinct open and closed structural variants. Our study demonstrates the utility of MagIC-cryo-EM for structural analysis of scarce macromolecules in heterogeneous samples and provides structural insights into the cell cycle-regulation of H1.8 association to nucleosomes.

Read-write mechanisms of H2A ubiquitination by Polycomb repressive complex 1

Epigenetic inheritance of silent chromatin domains is fundamental to cellular memory during embryogenesis, but it must overcome the dilution of repressive histone modifications during DNA replication1. One such modification, histone H2A lysine 119 monoubiquitination (H2AK119Ub), needs to be re-established by the Polycomb repressive complex 1 (PRC1) E3 ligase to restore the silent Polycomb domain2,3. However, the exact mechanism behind this restoration remains unknown. Here, combining cryo-electron microscopy (cryo-EM) and functional approaches, we characterize the read-write mechanism of the non-canonical PRC1-containing RYBP (ncPRC1RYBP). This mechanism, which functions as a positive-feedback loop in epigenetic regulation4,5, emphasizes the pivotal role of ncPRC1RYBP in restoring H2AK119Ub. We observe an asymmetrical binding of ncPRC1RYBP to H2AK119Ub nucleosomes, guided in part by the N-terminal zinc-finger domain of RYBP binding to residual H2AK119Ub on nascent chromatin. This recognition positions the RING domains of RING1B and BMI1 on the unmodified nucleosome side, enabling recruitment of the E2 enzyme to ubiquitinate H2AK119 within the same nucleosome (intra-nucleosome read-write) or across nucleosomes (inter-nucleosome read-write). Collectively, our findings provide key structural and mechanistic insights into the dynamic interplay of epigenetic regulation, highlighting the significance of ncPRC1RYBP in H2AK119Ub restoration to sustain repressive chromatin domains.

Unveiling the guardians of the genome: The dynamic role of histones in DNA organization and disease

Histones are positively charged proteins found in the chromatin of eukaryotic cells. They regulate gene expression and are required for the organization and packaging of DNA within the nucleus. Histones are extremely conserved, allowing for transcription, replication, and repair. This review delves into their complex structure and function in DNA assembly, their role in nucleosome assembly, and the higher-order chromatin structures they generate. We look at the five different types of histone proteins: H1, H2A, H2B, H3, H4, and their variations. These histones bind with DNA to produce nucleosomes, the basic units of chromatin that are essential for compacting DNA and controlling its accessibility. Their dynamic control of chromatin accessibility has important implications for genomic stability and cellular activities. We elucidate regulatory mechanisms in both normal and pathological situations by investigating their structural features, diverse interaction mechanisms, and chromatin impact. In addition, we discuss the functions of histone post-translational modifications (PTMs) and their significance in various disorders. These alterations, which include methylation, acetylation, phosphorylation, and ubiquitination, are crucial in regulating histone function and chromatin dynamics. We specifically describe and explore the role of changed histones in the evolution of cancer, neurological disorders, sepsis, autoimmune illnesses, and inflammatory conditions. This comprehensive review emphasizes histone's critical role in genomic integrity and their potential as therapeutic targets in various diseases.

Fusion Dynamics and Size-Dependence of Droplet Microstructure in ssDNA-Mediated Protein Phase Separation

Biomolecular condensation involving proteins and nucleic acids has been recognized to play crucial roles in genome organization and transcriptional regulation. However, the biophysical mechanisms underlying the droplet fusion dynamics and microstructure evolution during the early stage of liquid-liquid phase separation (LLPS) remain elusive. In this work, we study the phase separation of linker histone H1, which is among the most abundant chromatin proteins, in the presence of single-stranded DNA (ssDNA) capable of forming a G-quadruplex by using molecular simulations and experimental characterization. We found that droplet fusion is a rather stochastic and kinetically controlled process. Productive fusion events are triggered by the formation of ssDNA-mediated electrostatic bridges within the droplet contacting zone. The droplet microstructure is size-dependent and evolves driven by maximizing the number of electrostatic contacts. We also showed that the folding of ssDNA to the G-quadruplex promotes LLPS by increasing the multivalency and strength of protein-DNA interactions. These findings provide deep mechanistic insights into the growth dynamics of biomolecular droplets and highlight the key role of kinetic control during the early stage of ssDNA-protein condensation.

Transcriptional silencing in Saccharomyces cerevisiae: known unknowns

Transcriptional silencing in Saccharomyces cerevisiae is a persistent and highly stable form of gene repression. It involves DNA silencers and repressor proteins that bind nucleosomes. The silenced state is influenced by numerous factors including the concentration of repressors, nature of activators, architecture of regulatory elements, modifying enzymes and the dynamics of chromatin.Silencers function to increase the residence time of repressor Sir proteins at silenced domains while clustering of silenced domains enables increased concentrations of repressors and helps facilitate long-range interactions. The presence of an accessible NDR at the regulatory regions of silenced genes, the cycling of chromatin configurations at regulatory sites, the mobility of Sir proteins, and the non-uniform distribution of the Sir proteins across the silenced domain, all result in silenced chromatin that only stably silences weak promoters and enhancers via changes in transcription burst duration and frequency.These data collectively suggest that silencing is probabilistic and the robustness of silencing is achieved through sub-optimization of many different nodes of action such that a stable expression state is generated and maintained even though individual constituents are in constant flux.

Cryo-EM structure and biochemical analyses of the nucleosome containing the cancer-associated histone H3 mutation E97K

The Lys mutation of the canonical histone H3.1 Glu97 residue (H3E97K) is found in cancer cells. Previous biochemical analyses revealed that the nucleosome containing the H3E97K mutation is extremely unstable as compared to the wild-type nucleosome. However, the mechanism by which the H3E97K mutation causes nucleosome instability has not been clarified yet. In the present study, the cryo-electron microscopy structure of the nucleosome containing the H3E97K mutation revealed that the entry/exit DNA regions of the H3E97K nucleosome are disordered, probably by detachment of the nucleosomal DNA from the H3 N-terminal regions. This may change the intra-molecular amino acid interactions with the replaced H3 Lys97 residue, inducing structural distortion around the mutated position in the nucleosome. Consistent with the nucleosomal DNA end flexibility and the nucleosome instability, the H3E97K mutation exhibited reduced binding of linker histone H1 to the nucleosome, defective activation of PRC2 (the essential methyltransferase for facultative heterochromatin formation) with a poly-nucleosome, and enhanced nucleosome transcription by RNA polymerase II.

Interpretable deep residual network uncovers nucleosome positioning and associated features

Nucleosomes represent elementary building units of eukaryotic chromosomes and consist of DNA wrapped around a histone octamer flanked by linker DNA segments. Nucleosomes are central in epigenetic pathways and their genomic positioning is associated with regulation of gene expression, DNA replication, DNA methylation and DNA repair, among other functions. Building on prior discoveries that DNA sequences noticeably affect nucleosome positioning, our objective is to identify nucleosome positions and related features across entire genome. Here, we introduce an interpretable framework based on the concepts of deep residual networks (NuPoSe). Trained on high-coverage human experimental MNase-seq data, NuPoSe is able to learn sequence and structural patterns associated with nucleosome organization in human genome. NuPoSe can be also applied to unseen data from different organisms and cell types. Our findings point to 43 informative features, most of them constitute tri-nucleotides, di-nucleotides and one tetra-nucleotide. Most features are significantly associated with the nucleosomal structural characteristics, namely, periodicity of nucleosomal DNA and its location with respect to a histone octamer. Importantly, we show that features derived from the 27 bp linker DNA flanking nucleosomes contribute up to 10% to the quality of the prediction model. This, along with the comprehensive training sets, deep-learning architecture, and feature selection method, may contribute to the NuPoSe's 80-89% classification accuracy on different independent datasets.

Structural insights into the cooperative nucleosome recognition and chromatin opening by FOXA1 and GATA4

Mouse FOXA1 and GATA4 are prototypes of pioneer factors, initiating liver cell development by binding to the N1 nucleosome in the enhancer of the ALB1 gene. Using cryoelectron microscopy (cryo-EM), we determined the structures of the free N1 nucleosome and its complexes with FOXA1 and GATA4, both individually and in combination. We found that the DNA-binding domains of FOXA1 and GATA4 mainly recognize the linker DNA and an internal site in the nucleosome, respectively, whereas their intrinsically disordered regions interact with the acidic patch on histone H2A-H2B. FOXA1 efficiently enhances GATA4 binding by repositioning the N1 nucleosome. In vivo DNA editing and bioinformatics analyses suggest that the co-binding mode of FOXA1 and GATA4 plays important roles in regulating genes involved in liver cell functions. Our results reveal the mechanism whereby FOXA1 and GATA4 cooperatively bind to the nucleosome through nucleosome repositioning, opening chromatin by bending linker DNA and obstructing nucleosome packing.

A DNA condensation code for linker histones

Linker histones play an essential role in chromatin packaging by facilitating compaction of the 11-nm fiber of nucleosomal "beads on a string." The result is a heterogeneous condensed state with local properties that range from dynamic, irregular, and liquid-like to stable and regular structures (the 30-nm fiber), which in turn impact chromatin-dependent activities at a fundamental level. The properties of the condensed state depend on the type of linker histone, particularly on the highly disordered C-terminal tail, which is the most variable region of the protein, both between species, and within the various subtypes and cell-type specific variants of a given organism. We have developed an in vitro model system comprising linker histone tail and linker DNA, which although very minimal, displays surprisingly complex behavior, and is sufficient to model the known states of linker histone-condensed chromatin: disordered "fuzzy" complexes ("open" chromatin), dense liquid-like assemblies (dynamic condensates), and higher-order structures (organized 30-nm fibers). A crucial advantage of such a simple model is that it allows the study of the various condensed states by NMR, circular dichroism, and scattering methods. Moreover, it allows capture of the thermodynamics underpinning the transitions between states through calorimetry. We have leveraged this to rationalize the distinct condensing properties of linker histone subtypes and variants across species that are encoded by the amino acid content of their C-terminal tails. Three properties emerge as key to defining the condensed state: charge density, lysine/arginine ratio, and proline-free regions, and we evaluate each separately using a strategic mutagenesis approach.

Mechanism of heterochromatin remodeling revealed by the DDM1 bound nucleosome structures

The SWI/SNF2 chromatin remodeling factor decreased DNA methylation 1 (DDM1) is essential for the silencing of transposable elements (TEs) in both euchromatic and heterochromatic regions. Here, we determined the cryo-EM structures of DDM1-nucleosomeH2A and DDM1-nucleosomeH2A.W complexes at near-atomic resolution in the presence of the ATP analog ADP-BeFx. The structures show that nucleosomal DNA is unwrapped more on the surface of the histone octamer containing histone H2A than that containing histone H2A.W. DDM1 embraces one DNA gyre of the nucleosome and interacts with the N-terminal tails of histone H4. Although we did not observe DDM1-H2A.W interactions in our structures, the results of the pull-down experiments suggest a direct interaction between DDM1 and the core region of histone H2A.W. Our work provides mechanistic insights into the heterochromatin remodeling process driven by DDM1 in plants.

Beyond the Usual Suspects: Examining the Role of Understudied Histone Variants in Breast Cancer

The incorporation of histone variants has structural ramifications on nucleosome dynamics and stability. Due to their unique sequences, histone variants can alter histone-histone or histone-DNA interactions, impacting the folding of DNA around the histone octamer and the overall higher-order structure of chromatin fibers. These structural modifications alter chromatin compaction and accessibility of DNA by transcription factors and other regulatory proteins to influence gene regulatory processes such as DNA damage and repair, as well as transcriptional activation or repression. Histone variants can also generate a unique interactome composed of histone chaperones and chromatin remodeling complexes. Any of these perturbations can contribute to cellular plasticity and the progression of human diseases. Here, we focus on a frequently overlooked group of histone variants lying within the four human histone gene clusters and their contribution to breast cancer.

Regulation of chromatin architecture by protein binding: insights from molecular modeling

Histone and non-histone proteins play key roles in the activation and repression of genes. In addition to experimental studies of their regulation of gene expression, molecular modeling at the nucleosome, chromatin, and chromosome levels can contribute insights into the molecular mechanisms involved. In this review, we provide an overview for protein-bound chromatin modeling, and describe how our group has integrated protein binding into genome systems across the scales, from all-atom to coarse-grained models, using explicit to implicit descriptions. We describe the associated applications to protein binding effects and biological mechanisms of genome folding and gene regulation. We end by illustrating the application of machine learning tools like AlphaFold2 to proteins relevant to chromatin systems.

Epigenomic insights into common human disease pathology

The epigenome-the chemical modifications and chromatin-related packaging of the genome-enables the same genetic template to be activated or repressed in different cellular settings. This multi-layered mechanism facilitates cell-type specific function by setting the local sequence and 3D interactive activity level. Gene transcription is further modulated through the interplay with transcription factors and co-regulators. The human body requires this epigenomic apparatus to be precisely installed throughout development and then adequately maintained during the lifespan. The causal role of the epigenome in human pathology, beyond imprinting disorders and specific tumour suppressor genes, was further brought into the spotlight by large-scale sequencing projects identifying that mutations in epigenomic machinery genes could be critical drivers in both cancer and developmental disorders. Abrogation of this cellular mechanism is providing new molecular insights into pathogenesis. However, deciphering the full breadth and implications of these epigenomic changes remains challenging. Knowledge is accruing regarding disease mechanisms and clinical biomarkers, through pathogenically relevant and surrogate tissue analyses, respectively. Advances include consortia generated cell-type specific reference epigenomes, high-throughput DNA methylome association studies, as well as insights into ageing-related diseases from biological 'clocks' constructed by machine learning algorithms. Also, 3rd-generation sequencing is beginning to disentangle the complexity of genetic and DNA modification haplotypes. Cell-free DNA methylation as a cancer biomarker has clear clinical utility and further potential to assess organ damage across many disorders. Finally, molecular understanding of disease aetiology brings with it the opportunity for exact therapeutic alteration of the epigenome through CRISPR-activation or inhibition.

Structural mechanism of synergistic targeting of the CX3CR1 nucleosome by PU.1 and C/EBPα

Pioneer transcription factors are vital for cell fate changes. PU.1 and C/EBPα work together to regulate hematopoietic stem cell differentiation. However, how they recognize in vivo nucleosomal DNA targets remains elusive. Here we report the structures of the nucleosome containing the mouse genomic CX3CR1 enhancer DNA and its complexes with PU.1 alone and with both PU.1 and the C/EBPα DNA binding domain. Our structures reveal that PU.1 binds the DNA motif at the exit linker, shifting 17 bp of DNA into the core region through interactions with H2A, unwrapping ~20 bp of nucleosomal DNA. C/EBPα binding, aided by PU.1's repositioning, unwraps ~25 bp of entry DNA. The PU.1 Q218H mutation, linked to acute myeloid leukemia, disrupts PU.1-H2A interactions. PU.1 and C/EBPα jointly displace linker histone H1 and open the H1-condensed nucleosome array. Our study unveils how two pioneer factors can work cooperatively to open closed chromatin by altering DNA positioning in the nucleosome.

Mechanism of phase condensation for chromosome architecture and function

Chromosomal phase separation is involved in a broad spectrum of chromosome organization and functional processes. Nonetheless, the intricacy of this process has left its molecular mechanism unclear. Here, we introduce the principles governing phase separation and its connections to physiological roles in this context. Our primary focus is contrasting two phase separation mechanisms: self-association-induced phase separation (SIPS) and bridging-induced phase separation (BIPS). We provide a comprehensive discussion of the distinct features characterizing these mechanisms and offer illustrative examples that suggest their broad applicability. With a detailed understanding of these mechanisms, we explore their associations with nucleosomes and chromosomal biological functions. This comprehensive review contributes to the exploration of uncharted territory in the intricate interplay between chromosome architecture and function.

Explicit ion modeling predicts physicochemical interactions for chromatin organization

Molecular mechanisms that dictate chromatin organization in vivo are under active investigation, and the extent to which intrinsic interactions contribute to this process remains debatable. A central quantity for evaluating their contribution is the strength of nucleosome-nucleosome binding, which previous experiments have estimated to range from 2 to 14 kBT. We introduce an explicit ion model to dramatically enhance the accuracy of residue-level coarse-grained modeling approaches across a wide range of ionic concentrations. This model allows for de novo predictions of chromatin organization and remains computationally efficient, enabling large-scale conformational sampling for free energy calculations. It reproduces the energetics of protein-DNA binding and unwinding of single nucleosomal DNA, and resolves the differential impact of mono- and divalent ions on chromatin conformations. Moreover, we showed that the model can reconcile various experiments on quantifying nucleosomal interactions, providing an explanation for the large discrepancy between existing estimations. We predict the interaction strength at physiological conditions to be 9 kBT, a value that is nonetheless sensitive to DNA linker length and the presence of linker histones. Our study strongly supports the contribution of physicochemical interactions to the phase behavior of chromatin aggregates and chromatin organization inside the nucleus.

Interactome of intact chromatosome variants with site-specifically ubiquitylated and acetylated linker histone H1.2

Post-translational modifications (PTMs) of histones have fundamental effects on chromatin structure and function. While the impact of PTMs on the function of core histones are increasingly well understood, this is much less the case for modifications of linker histone H1, which is at least in part due to a lack of proper tools. In this work, we establish the assembly of intact chromatosomes containing site-specifically ubiquitylated and acetylated linker histone H1.2 variants obtained by a combination of chemical biology approaches. We then use these complexes in a tailored affinity enrichment mass spectrometry workflow to identify and comprehensively characterize chromatosome-specific cellular interactomes and the impact of site-specific linker histone modifications on a proteome-wide scale. We validate and benchmark our approach by western-blotting and by confirming the involvement of chromatin-bound H1.2 in the recruitment of proteins involved in DNA double-strand break repair using an in vitro ligation assay. We relate our data to previous work and in particular compare it to data on modification-specific interaction partners of free H1. Taken together, our data supports the role of chromatin-bound H1 as a regulatory protein with distinct functions beyond DNA compaction and constitutes an important resource for future investigations of histone epigenetic modifications.

Contributions of histone tail clipping and acetylation in nucleosome transcription by RNA polymerase II

The N-terminal tails of histones protrude from the nucleosome core and are target sites for histone modifications, such as acetylation and methylation. Histone acetylation is considered to enhance transcription in chromatin. However, the contribution of the histone N-terminal tail to the nucleosome transcription by RNA polymerase II (RNAPII) has not been clarified. In the present study, we reconstituted nucleosomes lacking the N-terminal tail of each histone, H2A, H2B, H3 or H4, and performed RNAPII transcription assays. We found that the N-terminal tail of H3, but not H2A, H2B and H4, functions in RNAPII pausing at the SHL(-5) position of the nucleosome. Consistently, the RNAPII transcription assay also revealed that the nucleosome containing N-terminally acetylated H3 drastically alleviates RNAPII pausing at the SHL(-5) position. In addition, the H3 acetylated nucleosome produced increased amounts of the run-off transcript. These results provide important evidence that the H3 N-terminal tail plays a role in RNAPII pausing at the SHL(-5) position of the nucleosome, and its acetylation directly alleviates this nucleosome barrier.

Histone H3 Tail Modifications Alter Structure and Dynamics of the H1 C-Terminal Domain Within Nucleosomes

The highly positively charged and intrinsically disordered H1 C-terminal domain (CTD) undergoes extensive condensation upon binding to nucleosomes, and stabilizes nucleosomes and higher-order chromatin structures but its interactions in chromatin are not well defined. Using single-molecule FRET we found that about half of the H1 CTDs in H1-nucleosome complexes exhibit well-defined FRET values indicative of distinct, static conformations, while the remainder of the population exhibits exchange between multiple defined FRET structures. Moreover, crosslinking studies indicate that the first 30 residues of the H1 CTD participate in relatively localized contacts with the first ∼25 bp of linker DNA, and that two separate regions in the CTD contribute to H1-dependent organization of linker DNA. Finally, we show that acetylation mimetics within the histone H3 tail markedly reduce the overall extent of H1 CTD condensation and significantly increase the fraction of H1 CTDs undergoing dynamic exchange between FRET states. Our results indicate the nucleosome-bound H1 CTD adopts loosely defined structures that exhibit significantly enhanced dynamics and decondensation upon epigenetic acetylation within the H3 tail.

Structural basis of nucleosome deacetylation and DNA linker tightening by Rpd3S histone deacetylase complex

In Saccharomyces cerevisiae, cryptic transcription at the coding region is prevented by the activity of Sin3 histone deacetylase (HDAC) complex Rpd3S, which is carried by the transcribing RNA polymerase II (RNAPII) to deacetylate and stabilize chromatin. Despite its fundamental importance, the mechanisms by which Rpd3S deacetylates nucleosomes and regulates chromatin dynamics remain elusive. Here, we determined several cryo-EM structures of Rpd3S in complex with nucleosome core particles (NCPs), including the H3/H4 deacetylation states, the alternative deacetylation state, the linker tightening state, and a state in which Rpd3S co-exists with the Hho1 linker histone on NCP. These structures suggest that Rpd3S utilizes a conserved Sin3 basic surface to navigate through the nucleosomal DNA, guided by its interactions with H3K36 methylation and the extra-nucleosomal DNA linkers, to target acetylated H3K9 and sample other histone tails. Furthermore, our structures illustrate that Rpd3S reconfigures the DNA linkers and acts in concert with Hho1 to engage the NCP, potentially unraveling how Rpd3S and Hho1 work in tandem for gene silencing.

Epigallocatechin Gallate Affects the Structure of Chromatosomes, Nucleosomes and Their Complexes with PARP1

The natural flavonoid epigallocatechin gallate has a wide range of biological activities, including being capable of binding to nucleic acids; however, the mechanisms of the interactions of epigallocatechin gallate with DNA organized in chromatin have not been systematically studied. In this work, the interactions of epigallocatechin gallate with chromatin in cells and with nucleosomes and chromatosomes in vitro were studied using fluorescent microscopy and single-particle Förster resonance energy transfer approaches, respectively. Epigallocatechin gallate effectively penetrates into the nuclei of living cells and binds to DNA there. The interaction of epigallocatechin gallate with nucleosomes in vitro induces a large-scale, reversible uncoiling of nucleosomal DNA that occurs without the dissociation of DNA or core histones at sub- and low-micromolar concentrations of epigallocatechin gallate. Epigallocatechin gallate does not reduce the catalytic activity of poly(ADP-ribose) polymerase 1, but causes the modulation of the structure of the enzyme-nucleosome complex. Epigallocatechin gallate significantly changes the structure of chromatosomes, but does not cause the dissociation of the linker histone. The reorganization of nucleosomes and chromatosomes through the use of epigallocatechin gallate could facilitate access to protein factors involved in DNA repair, replication and transcription to DNA and, thus, might contribute to the modulation of gene expression through the use of epigallocatechin gallate, which was reported earlier.

Structural mechanism of synergistic targeting of the CX3CR1 nucleosome by PU.1 and C/EBPα

Pioneer transcription factors are vital for cell fate changes. PU.1 and C/EBPα work together to regulate hematopoietic stem cell differentiation. However, how they recognize in vivo nucleosomal DNA targets remain elusive. Here we report the structures of the nucleosome containing the mouse genomic CX3CR1 enhancer DNA and its complexes with PU.1 alone and with both PU.1 and the C/EBPα DNA binding domain. Our structures reveal that PU.1 binds the DNA motif at the exit linker, shifting 17 bp of DNA into the core region through interactions with H2A, unwrapping ~20 bp of nucleosomal DNA. C/EBPα binding, aided by PU.1's repositioning, unwraps ~25 bp entry DNA. The PU.1 Q218H mutation, linked to acute myeloid leukemia, disrupts PU.1-H2A interactions. PU.1 and C/EBPα jointly displace linker histone H1 and open the H1-condensed nucleosome array. Our study unveils how two pioneer factors can work cooperatively to open closed chromatin by altering DNA positioning in the nucleosome.

Structural basis of histone H2A lysine 119 deubiquitination by Polycomb repressive deubiquitinase BAP1/ASXL1

Histone H2A lysine 119 (H2AK119Ub) is monoubiquitinated by Polycomb repressive complex 1 and deubiquitinated by Polycomb repressive deubiquitinase complex (PR-DUB). PR-DUB cleaves H2AK119Ub to restrict focal H2AK119Ub at Polycomb target sites and to protect active genes from aberrant silencing. The PR-DUB subunits (BAP1 and ASXL1) are among the most frequently mutated epigenetic factors in human cancers. How PR-DUB establishes specificity for H2AK119Ub over other nucleosomal ubiquitination sites and how disease-associated mutations of the enzyme affect activity are unclear. Here, we determine a cryo-EM structure of human BAP1 and the ASXL1 DEUBAD in complex with a H2AK119Ub nucleosome. Our structural, biochemical, and cellular data reveal the molecular interactions of BAP1 and ASXL1 with histones and DNA that are critical for restructuring the nucleosome and thus establishing specificity for H2AK119Ub. These results further provide a molecular explanation for how >50 mutations in BAP1 and ASXL1 found in cancer can dysregulate H2AK119Ub deubiquitination, providing insight into understanding cancer etiology.

Combining molecular dynamics simulations and scoring method to computationally model ubiquitylated linker histones in chromatosomes

The chromatin in eukaryotic cells plays a fundamental role in all processes during a cell's life cycle. This nucleoprotein is normally tightly packed but needs to be unpacked for expression and division. The linker histones are critical for such packaging processes and while most experimental and simulation works recognize their crucial importance, the focus is nearly always set on the nucleosome as the basic chromatin building block. Linker histones can undergo several modifications, but only few studies on their ubiquitylation have been conducted. Mono-ubiquitylated linker histones (HUb), while poorly understood, are expected to influence DNA compaction. The size of ubiquitin and the globular domain of the linker histone are comparable and one would expect an increased disorder upon ubiquitylation of the linker histone. However, the formation of higher order chromatin is not hindered and ubiquitylation of the linker histone may even promote gene expression. Structural data on chromatosomes is rare and HUb has never been modeled in a chromatosome so far. Descriptions of the chromatin complex with HUb would greatly benefit from computational structural data. In this study we generate molecular dynamics simulation data for six differently linked HUb variants with the help of a sampling scheme tailored to drive the exploration of phase space. We identify conformational sub-states of the six HUb variants using the sketch-map algorithm for dimensionality reduction and iterative HDBSCAN for clustering on the excessively sampled, shallow free energy landscapes. We present a highly efficient geometric scoring method to identify sub-states of HUb that fit into the nucleosome. We predict HUb conformations inside a nucleosome using on-dyad and off-dyad chromatosome structures as reference and show that unbiased simulations of HUb produce significantly more fitting than non-fitting HUb conformations. A tetranucleosome array is used to show that ubiquitylation can even occur in chromatin without too much steric clashes.

Leveraging Gene Redundancy to Find New Histone Drivers in Cancer

Histones play a critical role in chromatin function but are susceptible to mutagenesis. In fact, numerous mutations have been observed in several cancer types, and a few of them have been associated with carcinogenesis. Histones are peculiar, as they are encoded by a large number of genes, and the majority of them are clustered in three regions of the human genome. In addition, their replication and expression are tightly regulated in a cell. Understanding the etiology of cancer mutations in histone genes is impeded by their functional and sequence redundancy, their unusual genomic organization, and the necessity to be rapidly produced during cell division. Here, we collected a large data set of histone gene mutations in cancer and used it to investigate their distribution over 96 human histone genes and 68 different cancer types. This analysis allowed us to delineate the factors influencing the probability of mutation accumulation in histone genes and to detect new histone gene drivers. Although no significant difference in observed mutation rates between different histone types was detected for the majority of cancer types, several cancers demonstrated an excess or depletion of mutations in histone genes. As a consequence, we identified seven new histone genes as potential cancer-specific drivers. Interestingly, mutations were found to be distributed unevenly in several histone genes encoding the same protein, pointing to different factors at play, which are specific to histone function and genomic organization. Our study also elucidated mutational processes operating in genomic regions harboring histone genes, highlighting POLE as a factor of potential interest.

A large expert-curated cryo-EM image dataset for machine learning protein particle picking

Cryo-electron microscopy (cryo-EM) is a powerful technique for determining the structures of biological macromolecular complexes. Picking single-protein particles from cryo-EM micrographs is a crucial step in reconstructing protein structures. However, the widely used template-based particle picking process is labor-intensive and time-consuming. Though machine learning and artificial intelligence (AI) based particle picking can potentially automate the process, its development is hindered by lack of large, high-quality labelled training data. To address this bottleneck, we present CryoPPP, a large, diverse, expert-curated cryo-EM image dataset for protein particle picking and analysis. It consists of labelled cryo-EM micrographs (images) of 34 representative protein datasets selected from the Electron Microscopy Public Image Archive (EMPIAR). The dataset is 2.6 terabytes and includes 9,893 high-resolution micrographs with labelled protein particle coordinates. The labelling process was rigorously validated through 2D particle class validation and 3D density map validation with the gold standard. The dataset is expected to greatly facilitate the development of both AI and classical methods for automated cryo-EM protein particle picking.

Structural mechanism of LIN28B nucleosome targeting by OCT4

Pioneer transcription factors are essential for cell fate changes by targeting closed chromatin. OCT4 is a crucial pioneer factor that can induce cell reprogramming. However, the structural basis of how pioneer factors recognize the in vivo nucleosomal DNA targets is unknown. Here, we determine the high-resolution structures of the nucleosome containing human LIN28B DNA and its complexes with the OCT4 DNA binding region. Three OCT4s bind the pre-positioned nucleosome by recognizing non-canonical DNA sequences. Two use their POUS domains while the other uses the POUS-loop-POUHD region; POUHD serves as a wedge to unwrap ∼25 base pair DNA. Our analysis of previous genomic data and determination of the ESRRB-nucleosome-OCT4 structure confirmed the generality of these structural features. Moreover, biochemical studies suggest that multiple OCT4s cooperatively open the H1-condensed nucleosome array containing the LIN28B nucleosome. Thus, our study suggests a mechanism of how OCT4 can target the nucleosome and open closed chromatin.

Histone H3 tail modifications regulate structure and dynamics of the H1 C-terminal domain within nucleosomes

Despite their importance, how linker histone H1s interact in chromatin and especially how the highly positively charged and intrinsically disordered H1 C-terminal domain (CTD) binds and stabilizes nucleosomes and higher-order chromatin structures remains unclear. Using single-molecule FRET we found that about half of the H1 CTDs in H1-nucleosome complexes exhibit well-defined FRET values indicative of distinct, static conformations, while the remainder of the population exhibits dynamically changing values, similar to that observed for H1 in the absence of nucleosomes. We also find that the first 30 residues of the CTD participate in relatively localized interactions with the first ∼20 bp of linker DNA, and that two separate regions in the CTD contribute to H1-dependent organization of linker DNA, consistent with some non-random CTD-linker DNA interactions. Finally, our data show that acetylation mimetics within the histone H3 tail induce decondensation and enhanced dynamics of the nucleosome-bound H1 CTD. (148 words).

Basis of the H2AK119 specificity of the Polycomb repressive deubiquitinase

Repression of gene expression by protein complexes of the Polycomb group is a fundamental mechanism that governs embryonic development and cell-type specification1-3. The Polycomb repressive deubiquitinase (PR-DUB) complex removes the ubiquitin moiety from monoubiquitinated histone H2A K119 (H2AK119ub1) on the nucleosome4, counteracting the ubiquitin E3 ligase activity of Polycomb repressive complex 1 (PRC1)5 to facilitate the correct silencing of genes by Polycomb proteins and safeguard active genes from inadvertent silencing by PRC1 (refs. 6-9). The intricate biological function of PR-DUB requires accurate targeting of H2AK119ub1, but PR-DUB can deubiquitinate monoubiquitinated free histones and peptide substrates indiscriminately; the basis for its exquisite nucleosome-dependent substrate specificity therefore remains unclear. Here we report the cryo-electron microscopy structure of human PR-DUB, composed of BAP1 and ASXL1, in complex with the chromatosome. We find that ASXL1 directs the binding of the positively charged C-terminal extension of BAP1 to nucleosomal DNA and histones H3-H4 near the dyad, an addition to its role in forming the ubiquitin-binding cleft. Furthermore, a conserved loop segment of the catalytic domain of BAP1 is situated near the H2A-H2B acidic patch. This distinct nucleosome-binding mode displaces the C-terminal tail of H2A from the nucleosome surface, and endows PR-DUB with the specificity for H2AK119ub1.

H2A Ubiquitination Alters H3-tail Dynamics on Linker-DNA to Enhance H3K27 Methylation

Polycomb repressive complex 1 (PRC1) and PRC2 are responsible for epigenetic gene regulation. PRC1 ubiquitinates histone H2A (H2Aub), which subsequently promotes PRC2 to introduce the H3 lysine 27 tri-methyl (H3K27me3) repressive chromatin mark. Although this mechanism provides a link between the two key transcriptional repressors, PRC1 and PRC2, it is unknown how histone-tail dynamics contribute to this process. Here, we have examined the effect of H2A ubiquitination and linker-DNA on H3-tail dynamics and H3K27 methylation by PRC2. In naïve nucleosomes, the H3-tail dynamically contacts linker DNA in addition to core DNA, and the linker-DNA is as important for H3K27 methylation as H2A ubiquitination. H2A ubiquitination alters contacts between the H3-tail and DNA to improve the methyltransferase activity of the PRC2-AEBP2-JARID2 complex. Collectively, our data support a model in which H2A ubiquitination by PRC1 synergizes with linker-DNA to hold H3 histone tails poised for their methylation by PRC2-AEBP2-JARID2.

Structural basis of histone H2A lysine 119 deubiquitination by Polycomb Repressive Deubiquitinase BAP1/ASXL1

The maintenance of gene expression patterns during metazoan development is achieved by the actions of Polycomb group (PcG) complexes. An essential modification marking silenced genes is monoubiquitination of histone H2A lysine 119 (H2AK119Ub) deposited by the E3 ubiquitin ligase activity of the non-canonical Polycomb Repressive Complex 1. The Polycomb Repressive Deubiquitinase (PR-DUB) complex cleaves monoubiquitin from histone H2A lysine 119 (H2AK119Ub) to restrict focal H2AK119Ub at Polycomb target sites and to protect active genes from aberrant silencing. BAP1 and ASXL1, subunits that form active PR-DUB, are among the most frequently mutated epigenetic factors in human cancers, underscoring their biological importance. How PR-DUB achieves specificity for H2AK119Ub to regulate Polycomb silencing is unknown, and the mechanisms of most of the mutations in BAP1 and ASXL1 found in cancer have not been established. Here we determine a cryo-EM structure of human BAP1 bound to the ASXL1 DEUBAD domain in complex with a H2AK119Ub nucleosome. Our structural, biochemical, and cellular data reveal the molecular interactions of BAP1 and ASXL1 with histones and DNA that are critical for remodeling the nucleosome and thus establishing specificity for H2AK119Ub. These results further provide a molecular explanation for how >50 mutations in BAP1 and ASXL1 found in cancer can dysregulate H2AK119Ub deubiquitination, providing new insight into understanding cancer etiology.

One Sentence Summary

We reveal the molecular mechanism of nucleosomal H2AK119Ub deubiquitination by human BAP1/ASXL1.

MORF and MOZ acetyltransferases target unmethylated CpG islands through the winged helix domain

Human acetyltransferases MOZ and MORF are implicated in chromosomal translocations associated with aggressive leukemias. Oncogenic translocations involve the far amino terminus of MOZ/MORF, the function of which remains unclear. Here, we identified and characterized two structured winged helix (WH) domains, WH1 and WH2, in MORF and MOZ. WHs bind DNA in a cooperative manner, with WH1 specifically recognizing unmethylated CpG sequences. Structural and genomic analyses show that the DNA binding function of WHs targets MORF/MOZ to gene promoters, stimulating transcription and H3K23 acetylation, and WH1 recruits oncogenic fusions to HOXA genes that trigger leukemogenesis. Cryo-EM, NMR, mass spectrometry and mutagenesis studies provide mechanistic insight into the DNA-binding mechanism, which includes the association of WH1 with the CpG-containing linker DNA and binding of WH2 to the dyad of the nucleosome. The discovery of WHs in MORF and MOZ and their DNA binding functions could open an avenue in developing therapeutics to treat diseases associated with aberrant MOZ/MORF acetyltransferase activities.

Nucleosome dyad determines the H1 C-terminus collapse on distinct DNA arms

Nucleosomes are symmetric structures. However, binding of linker histones generates an inherently asymmetric H1-nucleosome complex, and whether this asymmetry is transmitted to the overall nucleosome structure, and therefore also to chromatin, is unclear. Efforts to investigate potential asymmetry due to H1s have been hampered by the DNA sequence, which naturally differs in each gyre. To overcome this issue, we designed and analyzed by cryo-EM a nucleosome reconstituted with a palindromic (601L) 197-bp DNA. As in the non-palindromic 601 sequence, H1 restricts linker DNA flexibility but reveals partial asymmetrical unwrapping. However, in contrast to the non-palindromic nucleosome, in the palindromic nucleosome H1 CTD collapses to the proximal linker. Molecular dynamics simulations show that this could be dictated by a slightly tilted orientation of the globular domain (GD) of H1, which could be linked to the DNA sequence of the nucleosome dyad.

Genome modeling: From chromatin fibers to genes

The intricacies of the 3D hierarchical organization of the genome have been approached by many creative modeling studies. The specific model/simulation technique combination defines and restricts the system and phenomena that can be investigated. We present the latest modeling developments and studies of the genome, involving models ranging from nucleosome systems and small polynucleosome arrays to chromatin fibers in the kb-range, chromosomes, and whole genomes, while emphasizing gene folding from first principles. Clever combinations allow the exploration of many interesting phenomena involved in gene regulation, such as nucleosome structure and dynamics, nucleosome-nucleosome stacking, polynucleosome array folding, protein regulation of chromatin architecture, mechanisms of gene folding, loop formation, compartmentalization, and structural transitions at the chromosome and genome levels. Gene-level modeling with full details on nucleosome positions, epigenetic factors, and protein binding, in particular, can in principle be scaled up to model chromosomes and cells to study fundamental biological regulation.

Structural mechanism of LIN28B nucleosome targeting by OCT4 for pluripotency

Pioneer transcription factors are essential for cell fate changes by targeting closed chromatin. OCT4 is a crucial pioneer factor that can induce cell reprogramming. However, the structural basis of how pioneer factors recognize the in vivo nucleosomal DNA targets is unknown. Here, we determine the high-resolution structures of the nucleosome containing human LIN28B DNA and its complexes with the OCT4 DNA binding region. Three OCT4s bind the pre-positioned nucleosome by recognizing non-canonical DNA motifs. Two use their POUS domains by forming extensive hydrogen bonds. The other uses the POUS-loop-POUHD region; POUHD serves as a wedge to unwrap ∼25 base pair DNA. Biochemical studies suggest that multiple OCT4s cooperatively open the H1-condensed nucleosome array containing the LIN28B nucleosome. Our study suggests a mechanism whereby OCT4s target the LIN28B nucleosome by forming multivalent interactions with nucleosomal motifs, unwrapping nucleosomal DNA, evicting H1, and cooperatively open closed chromatin to initiate cell reprogramming.

Patient-facing digital tools for delivering genetic services: a systematic review

This study systematically reviewed the literature on the impact of digital genetics tools on patient care and system efficiencies. MEDLINE and Embase were searched for articles published between January 2010 and March 2021. Studies evaluating the use of patient-facing digital tools in the context of genetic service delivery were included. Two reviewers screened and extracted patient-reported and system-focused outcomes from each study. Data were synthesised using a descriptive approach. Of 3226 unique studies identified, 87 were included. A total of 70 unique digital tools were identified. As a result of using digital tools, 84% of studies reported a positive outcome in at least one of the following patient outcomes: knowledge, psychosocial well-being, behavioural/management changes, family communication, decision-making or level of engagement. Digital tools improved workflow and efficiency for providers and reduced the amount of time they needed to spend with patients. However, we identified a misalignment between study purpose and patient-reported outcomes measured and a lack of tools that encompass the entire genetic counselling and testing trajectory. Given increased demand for genetic services and the shift towards virtual care, this review provides evidence that digital tools can be used to efficiently deliver patient-centred care. Future research should prioritise development, evaluation and implementation of digital tools that can support the entire patient trajectory across a range of clinical settings. PROSPERO registration numberCRD42020202862.