Binding of PHF1 Tudor to H3K36me3 enhances nucleosome accessibility

The winged helix domain of MORF binds CpG islands and the TAZ2 domain of p300

Acetylation of histones by lysine acetyltransferases (KATs) provides a fundamental mechanism by which chromatin structure and transcriptional programs are regulated. Here, we describe a dual binding activity of the first winged helix domain of human MORF KAT (MORFWH1) that recognizes the TAZ2 domain of p300 KAT (p300TAZ2) and CpG rich DNA sequences. Structural and biochemical studies identified distinct DNA and p300TAZ2 binding sites, allowing MORFWH1 to independently engage either ligand. Genomic data show that MORF/MOZWH1 colocalizes with H3K18ac, a product of enzymatic activity of p300, on CpG rich promoters of target genes. Our findings suggest a functional cooperation of MORF and p300 KATs in transcriptional regulation.

Histone H4K16ac Binding Function of the Triple PHD Finger Cassette of MLL4

The human methyltransferase MLL4 plays a critical role in embryogenesis and development, and aberrant activity of MLL4 is linked to neurodegenerative and developmental disorders and cancer. MLL4 contains the catalytic SET domain that catalyzes mono methylation of lysine 4 of histone H3 (H3K4me1) and seven plant homeodomain (PHD) fingers, six of which have not been structurally and functionally characterized. Here, we demonstrate that the triple PHD finger cassette of MLL4, harboring its fourth, fifth and sixth PHD fingers (MLL4PHD456) forms an integrated module, maintains the binding selectivity of the PHD6 finger toward acetylated lysine 16 of histone H4 (H4K16ac), and is capable of binding to DNA. Our findings highlight functional correlation between H4K16ac and H3K4me1, two major histone modifications that are recognized and written, respectively, by MLL4.

Guiding the HBO1 complex function through the JADE subunit

JADE is a core subunit of the HBO1 acetyltransferase complex that regulates developmental and epigenetic programs and promotes gene transcription. Here we describe the mechanism by which JADE facilitates recruitment of the HBO1 complex to chromatin and mediates its enzymatic activity. Structural, genomic and complex assembly in vivo studies show that the PZP (PHD1-zinc-knuckle-PHD2) domain of JADE engages the nucleosome through binding to histone H3 and DNA and is necessary for the association with chromatin targets. Recognition of unmethylated H3K4 by PZP directs enzymatic activity of the complex toward histone H4 acetylation, whereas H3K4 hypermethylation alters histone substrate selectivity. We demonstrate that PZP contributes to leukemogenesis, augmenting transforming activity of the NUP98-JADE2 fusion. Our findings highlight biological consequences and the impact of the intact JADE subunit on genomic recruitment, enzymatic function and pathological activity of the HBO1 complex.

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.

Polycomb Repressive Complex 2 in Oncology

Dynamic regulation of the chromatin state by Polycomb Repressive Complex 2 (PRC2) provides an important mean for epigenetic gene control that can profoundly influence normal development and cell lineage specification. PRC2 and PRC2-induced methylation of histone H3 lysine 27 (H3K27) are critically involved in a wide range of DNA-templated processes, which at least include transcriptional repression and gene imprinting, organization of three-dimensional chromatin structure, DNA replication and DNA damage response and repair. PRC2-based genome regulation often goes wrong in diseases, notably cancer. This chapter discusses about different modes-of-action through which PRC2 and EZH2, a catalytic subunit of PRC2, mediate (epi)genomic and transcriptomic regulation. We will also discuss about how alteration or mutation of the PRC2 core or axillary component promotes oncogenesis, how post-translational modification regulates functionality of EZH2 and PRC2, and how PRC2 and other epigenetic pathways crosstalk. Lastly, we will briefly touch on advances in targeting EZH2 and PRC2 dependence as cancer therapeutics.

Histone Readers and Their Roles in Cancer

Histone proteins in eukaryotic cells are subjected to a wide variety of post-translational modifications, which are known to play an important role in the partitioning of the genome into distinctive compartments and domains. One of the major functions of histone modifications is to recruit reader proteins, which recognize the epigenetic marks and transduce the molecular signals in chromatin to downstream effects. Histone readers are defined protein domains with well-organized three-dimensional structures. In this Chapter, we will outline major histone readers, delineate their biochemical and structural features in histone recognition, and describe how dysregulation of histone readout leads to human cancer.

The role of histone H3K36me3 writers, readers and erasers in maintaining genome stability

Histone Post-Translational Modifications (PTMs) play fundamental roles in mediating DNA-related processes such as transcription, replication and repair. The histone mark H3K36me3 and its associated methyltransferase SETD2 (Set2 in yeast) are archetypical in this regard, performing critical roles in each of these DNA transactions. Here, we present an overview of H3K36me3 regulation and the roles of its writers, readers and erasers in maintaining genome stability through facilitating DNA double-strand break (DSB) repair, checkpoint signalling and replication stress responses. Further, we consider how loss of SETD2 and H3K36me3, frequently observed in a number of different cancer types, can be specifically targeted in the clinic through exploiting loss of particular genome stability functions.

HIV-1 Preintegration Complex Preferentially Integrates the Viral DNA into Nucleosomes Containing Trimethylated Histone 3-Lysine 36 Modification and Flanking Linker DNA

HIV-1 DNA is preferentially integrated into chromosomal hot spots by the preintegration complex (PIC). To understand the mechanism, we measured the DNA integration activity of PICs-extracted from infected cells-and intasomes, biochemically assembled PIC substructures using a number of relevant target substrates. We observed that PIC-mediated integration into human chromatin is preferred compared to genomic DNA. Surprisingly, nucleosomes lacking histone modifications were not preferred integration compared to the analogous naked DNA. Nucleosomes containing the trimethylated histone 3 lysine 36 (H3K36me3), an epigenetic mark linked to active transcription, significantly stimulated integration, but the levels remained lower than the naked DNA. Notably, H3K36me3-modified nucleosomes with linker DNA optimally supported integration mediated by the PIC but not by the intasome. Interestingly, optimal intasome-mediated integration required the cellular cofactor LEDGF. Unexpectedly, LEDGF minimally affected PIC-mediated integration into naked DNA but blocked integration into nucleosomes. The block for the PIC-mediated integration was significantly relieved by H3K36me3 modification. Mapping the integration sites in the preferred substrates revealed that specific features of the nucleosome-bound DNA are preferred for integration, whereas integration into naked DNA was random. Finally, biochemical and genetic studies demonstrate that DNA condensation by the H1 protein dramatically reduces integration, providing further evidence that features inherent to the open chromatin are preferred for HIV-1 integration. Collectively, these results identify the optimal target substrate for HIV-1 integration, report a mechanistic link between H3K36me3 and integration preference, and importantly, reveal distinct mechanisms utilized by the PIC for integration compared to the intasomes. IMPORTANCE HIV-1 infection is dependent on integration of the viral DNA into the host chromosomes. The preintegration complex (PIC) containing the viral DNA, the virally encoded integrase (IN) enzyme, and other viral/host factors carries out HIV-1 integration. HIV-1 integration is not dependent on the target DNA sequence, and yet the viral DNA is selectively inserted into specific "hot spots" of human chromosomes. A growing body of literature indicates that structural features of the human chromatin are important for integration targeting. However, the mechanisms that guide the PIC and enable insertion of the PIC-associated viral DNA into specific hot spots of the human chromosomes are not fully understood. In this study, we describe a biochemical mechanism for the preference of the HIV-1 DNA integration into open chromatin. Furthermore, our study defines a direct role for the histone epigenetic mark H3K36me3 in HIV-1 integration preference and identify an optimal substrate for HIV-1 PIC-mediated viral DNA integration.

Alternative Mechanisms for DNA Engagement by BET Bromodomain-Containing Proteins

Epigenetic reader domains regulate chromatin structure and modulate gene expression through the recognition of post-translational modifications on histones. Recently, reader domains have also been found to harbor double-stranded (ds) DNA-binding activity, which is as functionally critical as histone association. Here, we explore the dsDNA recognition of the N-terminal bromodomain of the bromodomain and extra-terminal (BET) protein, BRD4. Using protein-observed ¹⁹F NMR, ¹H-¹⁵N HSQC NMR, electrophoretic mobility shift assays (EMSA), and competitive-inhibition assays, we establish the binding surface of dsDNA and find it to be largely overlapping with the acetylated histone (KAc)-binding site. Rather than engaging in electrostatic contacts, we find dsDNA to interact competitively within the KAc-binding pocket. These interactions are distinct from the highly homologous BET bromodomain, BRDT. Nine additional bromodomains have also been characterized for interacting with dsDNA, including tandem BET bromodomains. Together, these studies help establish a binding model for dsDNA interactions with BRD4 bromodomains and elucidate the chromatin-recognition mechanisms of the BRD4 protein for regulating gene expression.

The Chromatin Landscape Channels DNA Double-Strand Breaks to Distinct Repair Pathways

DNA double-strand breaks (DSBs), the most deleterious DNA lesions, are primarily repaired by two pathways, namely homologous recombination (HR) and non-homologous end joining (NHEJ), the choice of which is largely dependent on cell cycle phase and the local chromatin landscape. Recent studies have revealed that post-translational modifications on histones play pivotal roles in regulating DSB repair pathways including repair pathway choice. In this review, we present our current understanding of how these DSB repair pathways are employed in various chromatin landscapes to safeguard genomic integrity. We place an emphasis on the impact of different histone post-translational modifications, characteristic of euchromatin or heterochromatin regions, on DSB repair pathway choice. We discuss the potential roles of damage-induced chromatin modifications in the maintenance of genome and epigenome integrity. Finally, we discuss how RNA transcripts from the vicinity of DSBs at actively transcribed regions also regulate DSB repair pathway choice.

Advances in Single-Cell Multi-Omics and Application in Cardiovascular Research

With the development of ever more powerful and versatile high-throughput sequencing techniques and innovative ways to capture single cells, mapping the multicellular tissues at the single-cell level is becoming routine practice. However, it is still challenging to depict the epigenetic landscape of a single cell, especially the genome-wide chromatin accessibility, histone modifications, and DNA methylation. We summarize the most recent methodologies to profile these epigenetic marks at the single-cell level. We also discuss the development and advancement of several multi-omics sequencing technologies from individual cells. Advantages and limitations of various methods to compare and integrate datasets obtained from different sources are also included with specific practical notes. Understanding the heart tissue at single-cell resolution and multi-modal levels will help to elucidate the cell types and states involved in physiological and pathological events during heart development and disease. The rich information produced from single-cell multi-omics studies will also promote the research of heart regeneration and precision medicine on heart diseases.

DNA binding by polycomb-group proteins: searching for the link to CpG islands

Polycomb group proteins predominantly exist in polycomb repressive complexes (PRCs) that cooperate to maintain the repressed state of thousands of cell-type-specific genes. Targeting PRCs to the correct sites in chromatin is essential for their function. However, the mechanisms by which PRCs are recruited to their target genes in mammals are multifactorial and complex. Here we review DNA binding by polycomb group proteins. There is strong evidence that the DNA-binding subunits of PRCs and their DNA-binding activities are required for chromatin binding and CpG targeting in cells. In vitro, CpG-specific binding was observed for truncated proteins externally to the context of their PRCs. Yet, the mere DNA sequence cannot fully explain the subset of CpG islands that are targeted by PRCs in any given cell type. At this time we find very little structural and biophysical evidence to support a model where sequence-specific DNA-binding activity is required or sufficient for the targeting of CpG-dinucleotide sequences by polycomb group proteins while they are within the context of their respective PRCs, either PRC1 or PRC2. We discuss the current knowledge and open questions on how the DNA-binding activities of polycomb group proteins facilitate the targeting of PRCs to chromatin.

Recent Advances in Investigating Functional Dynamics of Chromatin

Dynamics spanning the picosecond-minute time domain and the atomic-subcellular spatial window have been observed for chromatin in vitro and in vivo. The condensed organization of chromatin in eukaryotic cells prevents regulatory factors from accessing genomic DNA, which requires dynamic stabilization and destabilization of structure to initiate downstream DNA activities. Those processes are achieved through altering conformational and dynamic properties of nucleosomes and nucleosome–protein complexes, of which delineating the atomistic pictures is essential to understand the mechanisms of chromatin regulation. In this review, we summarize recent progress in determining chromatin dynamics and their modulations by a number of factors including post-translational modifications (PTMs), incorporation of histone variants, and binding of effector proteins. We focus on experimental observations obtained using high-resolution techniques, primarily including nuclear magnetic resonance (NMR) spectroscopy, Förster (or fluorescence) resonance energy transfer (FRET) microscopy, and molecular dynamics (MD) simulations, and discuss the elucidated dynamics in the context of functional response and relevance.

Visualizing Conformational Ensembles of the Nucleosome by NMR

The formation of chromatin not only compacts the eukaryotic genome into the nucleus but also provides a mechanism for the regulation of all DNA templated processes. Spatial and temporal modulation of the chromatin structure is critical in such regulation and involves fine-tuned functioning of the basic subunit of chromatin, the nucleosome. It has become apparent that the nucleosome is an inherently dynamic system, but characterization of these dynamics at the atomic level has remained challenging. NMR spectroscopy is a powerful tool for investigating the conformational ensemble and dynamics of proteins and protein complexes, and recent advances have made the study of large systems possible. Here, we review recent studies which utilize NMR spectroscopy to uncover the atomic level conformation and dynamics of the nucleosome and provide a better understanding of the importance of these dynamics in key regulatory events.

Quantifying epigenetic modulation of nucleosome breathing by high-throughput AFM imaging

Nucleosomes are the basic units of chromatin and critical for storage and expression of eukaryotic genomes. Chromatin accessibility and gene readout are heavily regulated by epigenetic marks, in which post-translational modifications of histones play a key role. However, the mode of action and the structural implications at the single-molecule level of nucleosomes is still poorly understood. Here we apply a high-throughput atomic force microscopy imaging and analysis pipeline to investigate the conformational landscape of the nucleosome variants three additional methyl groups at lysine 36 of histone H3 (H3K36me3), phosphorylation of H3 histones at serine 10 (H3S10phos), and acetylation of H4 histones at lysines 5, 8, 12, and 16 (H4K5/8/12/16ac). Our data set of more than 25,000 nucleosomes reveals nucleosomal unwrapping steps corresponding to 5-bp DNA. We find that H3K36me3 nucleosomes unwrap significantly more than wild-type nucleosomes and additionally unwrap stochastically from both sides, similar to centromere protein A (CENP-A) nucleosomes and in contrast to the highly anticooperative unwrapping of wild-type nucleosomes. Nucleosomes with H3S10phos or H4K5/8/12/16ac modifications show unwrapping populations similar to wild-type nucleosomes and also retain the same level of anticooperativity. Our findings help to put the mode of action of these modifications into context. Although H3K36me3 likely acts partially by directly affecting nucleosome structure on the single-molecule level, H3S10phos and H4K5/8/12/16ac must predominantly act through higher-order processes. Our analysis pipeline is readily applicable to other nucleosome variants and will facilitate future high-resolution studies of the conformational landscape of nucleoprotein complexes.

Evolutionary adaptation of the Polycomb repressive complex 2

The Polycomb repressive complex 2 (PRC2) is an essential chromatin regulatory complex involved in repressing the transcription of diverse developmental genes. PRC2 consists of a core complex; possessing H3K27 methyltransferase activity and various associated factors that are important to modulate its function. During evolution, the composition of PRC2 and the functionality of PRC2 components have changed considerably. Here, we compare the PRC2 complex members of Drosophila and mammals and describe their adaptation to altered biological needs. We also highlight how the PRC2.1 subcomplex has gained multiple novel functions and discuss the implications of these changes for the function of PRC2 in chromatin regulation.

Not just a writer: PRC2 as a chromatin reader

PRC2 deposits the H3K27me3 repressive mark, which facilitates transcription repression of developmental genes. The decision of whether a particular gene is silenced at a given point during development is heavily dependent on the chromatin context. More than just a simple epigenetic writer, PRC2 employs several distinct chromatin reading capabilities to sense the local chromatin environment and modulate the H3K27me3 writer activity in a context-dependent manner. Here we discuss the complex interplay of PRC2 with the hallmarks of active and repressive chromatin, how it affects H3K27me3 deposition and how it guides transcriptional activity.

Mechanistic similarities in recognition of histone tails and DNA by epigenetic readers

The past two decades have witnessed rapid advances in the identification and characterization of epigenetic readers, capable of recognizing or reading post-translational modifications in histones. More recently, a new set of readers with the ability to interact with the nucleosome through concomitant binding to histones and DNA has emerged. In this review, we discuss mechanistic insights underlying bivalent histone and DNA recognition by newly characterized readers and highlight the importance of binding to DNA for their association with chromatin.

Structures of chromatin modulators in complex with nucleosome

The chromatin structure is dynamically regulated by many different modulators that post-translationally modify histones, replace canonical histones with histone variants, and unwind nucleosomal DNA, thereby modulating the accessibility of nucleosomal DNA and facilitating downstream DNA-templated nuclear processes. To understand how these modulators change the chromatin structure, it is essential to determine the 3D structures of chromatin modulators in complex with nucleosome. Here, we review the very recent progress in structural studies of some selected chromatin modulators in complex with nucleosome, including those of histone demethylases LSD1/2, some pioneer transcription factors, and the PWWP domain-containing protein LEDGF.

Histone Tail Conformations: A Fuzzy Affair with DNA

The core histone tails are critical in chromatin structure and signaling. Studies over the past several decades have provided a wealth of information on the histone tails and their interaction with chromatin factors. However, the conformation of the histone tails in a chromatin relevant context has remained elusive. Only recently has enough evidence emerged to start to build a structural model of the tails in the context of nucleosomes and nucleosome arrays. Here, we review these studies and propose that the histone tails adopt a high-affinity fuzzy complex with DNA, characterized by robust but dynamic association. Furthermore, we discuss how these DNA-bound conformational ensembles promote distinct chromatin structure and signaling, and that their fuzzy nature is important in transitioning between functional states.

Of numbers and movement - understanding transcription factor pathogenesis by advanced microscopy

Transcription factors (TFs) are life-sustaining and, therefore, the subject of intensive research. By regulating gene expression, TFs control a plethora of developmental and physiological processes, and their abnormal function commonly leads to various developmental defects and diseases in humans. Normal TF function often depends on gene dosage, which can be altered by copy-number variation or loss-of-function mutations. This explains why TF haploinsufficiency (HI) can lead to disease. Since aberrant TF numbers frequently result in pathogenic abnormalities of gene expression, quantitative analyses of TFs are a priority in the field. In vitro single-molecule methodologies have significantly aided the identification of links between TF gene dosage and transcriptional outcomes. Additionally, advances in quantitative microscopy have contributed mechanistic insights into normal and aberrant TF function. However, to understand TF biology, TF-chromatin interactions must be characterised in vivo, in a tissue-specific manner and in the context of both normal and altered TF numbers. Here, we summarise the advanced microscopy methodologies most frequently used to link TF abundance to function and dissect the molecular mechanisms underlying TF HIs. Increased application of advanced single-molecule and super-resolution microscopy modalities will improve our understanding of how TF HIs drive disease.

Structural basis for PRC2 decoding of active histone methylation marks H3K36me2/3

Repression of genes by Polycomb requires that PRC2 modifies their chromatin by trimethylating lysine 27 on histone H3 (H3K27me3). At transcriptionally active genes, di- and tri-methylated H3K36 inhibit PRC2. Here, the cryo-EM structure of PRC2 on dinucleosomes reveals how binding of its catalytic subunit EZH2 to nucleosomal DNA orients the H3 N-terminus via an extended network of interactions to place H3K27 into the active site. Unmodified H3K36 occupies a critical position in the EZH2-DNA interface. Mutation of H3K36 to arginine or alanine inhibits H3K27 methylation by PRC2 on nucleosomes in vitro. Accordingly, Drosophila H3K36A and H3K36R mutants show reduced levels of H3K27me3 and defective Polycomb repression of HOX genes. The relay of interactions between EZH2, the nucleosomal DNA and the H3 N-terminus therefore creates the geometry that permits allosteric inhibition of PRC2 by methylated H3K36 in transcriptionally active chromatin.

Mimicking the Nucleosomal Context in Peptide-Based Binders of a H3K36me Reader Increases Binding Affinity While Altering the Binding Mode

Targeting of proteins in the histone modification machinery has emerged as a promising new direction to fight disease. The search for compounds that inhibit proteins that readout histone modification has led to several new epigenetic drugs, mostly for proteins involved in recognition of acetylated lysines. However, this approach proved to be a challenging task for methyllysine readers, which typically feature shallow binding pockets. Moreover, reader proteins of trimethyllysine K36 on the histone H3 (H3K36me3) not only bind the methyllysine but also the nucleosomal DNA. Here, we sought to find peptide-based binders of H3K36me3 reader PSIP1, which relies on DNA interactions to tightly bind H3K36me3 modified nucleosomes. We designed several peptides that mimic the nucleosomal context of H3K36me3 recognition by including negatively charged Glu-rich regions. Using a detailed NMR analysis, we find that addition of negative charges boosts binding affinity up to 50-fold while decreasing binding to the trimethyllysine binding pocket. Since screening and selection of compounds for reader domains is typically based solely on affinity measurements due to their lack of enzymatic activity, our case highlights the need to carefully control for the binding mode, in particular for the challenging case of H3K36me3 readers.

PHF1 fusions cause distinct gene expression and chromatin accessibility profiles in ossifying fibromyxoid tumors and mesenchymal cells

Ossifying fibromyxoid tumor (OFMT) is a soft tissue tumor frequently displaying gene fusions, most of which affect the PHF1 gene. PHF1 encodes plant homeodomain finger protein 1, which is involved in various processes regulating gene transcription, including those orchestrated by the polycomb repressor complex 2. Here, a series of 37 OFMTs, including 18 typical, 9 atypical, and 10 malignant variants, was analyzed with regard to transcriptomic features, gene fusion and copy number status, and/or single-nucleotide variants. The effects on gene expression and chromatin accessibility of three detected fusions (EP400-PHF1, MEAF6-PHF1, and PHF1-TFE3) were further evaluated in fibroblasts. Genomic imbalances showed a progression-related pattern, with more extensive copy number changes among atypical/malignant lesions than among typical OFMTs; loss of the RB1 gene was restricted to atypical/malignant OFMTs, occurring in one-third of the cases. RNA sequencing identified fusion transcripts in >80% of the cases analyzed, including a novel CSMD1-MEAF6. The gene-expression profile of OFMT was distinct from that of other soft tissue tumors, with extensive transcriptional upregulation of genes in OFMT. These findings were largely recapitulated in gene fusion-expressing fibroblast lines, suggesting that genes involved in, e.g., Wnt signaling and/or being regulated through trimethylation of lysine 27 in histone 3 (H3K27me3) are pivotal for OFMT development. The genes showing differentially higher expression in fusion-expressing cells paralleled increased chromatin accessibility, as revealed by ATAC sequencing. Thus, the present study suggests that OFMT develops through gene fusions that have extensive epigenetic consequences.

H3K36me3, message from chromatin to DNA damage repair

Histone marks control many cellular processes including DNA damage repair. This review will focus primarily on the active histone mark H3K36me3 in the regulation of DNA damage repair and the maintenance of genomic stability after DNA damage. There are diverse clues showing H3K36me3 participates in DNA damage response by directly recruiting DNA repair machinery to set the chromatin at a “ready” status, leading to a quick response upon damage. Reduced H3K36me3 is associated with low DNA repair efficiency. This review will also place a main emphasis on the H3K36me3-mediated DNA damage repair in the tumorigenesis of the newly found oncohistone mutant tumors. Gaining an understanding of different aspects of H3K36me3 in DNA damage repair, especially in cancers, would share the knowledge of chromatin and DNA repair to serve to the drug discovery and patient care.

Structure of H3K36-methylated nucleosome-PWWP complex reveals multivalent cross-gyre binding

Recognition of histone-modified nucleosomes by specific reader domains underlies the regulation of chromatin-associated processes. Whereas structural studies revealed how reader domains bind modified histone peptides, it is unclear how reader domains interact with modified nucleosomes. Here we report the cryo-electron microscopy (cryo-EM) structure of the PWWP reader domain of human transcriptional coactivator LEDGF in complex with a H3K36-methylated nucleosome at 3.2 Å resolution. The structure reveals multivalent binding of the reader domain to the methylated histone tail and to both gyres of nucleosomal DNA, explaining the known cooperative interactions. The observed cross-gyre binding may contribute to nucleosome integrity during transcription. The structure also explains how human PWWP domain-containing proteins are recruited to H3K36-methylated regions of the genome for transcription, histone acetylation and methylation, and for DNA methylation and repair.

DNA sequence influences hexasome orientation to regulate DNA accessibility

Nucleosomes, the fundamental organizing units of eukaryotic genomes, contain ∼146 base pairs of DNA wrapped around a histone H3–H4 tetramer and two histone H2A–H2B dimers. Converting nucleosomes into hexasomes by removal of a H2A–H2B dimer is an important regulatory event, but its regulation and functional consequences are not well-understood. To investigate the influence of hexasomes on DNA accessibility, we used the property of the Widom-601 Nucleosome Positioning Sequence (NPS) to form homogeneously oriented hexasomes in vitro. We find that DNA accessibility to transcription factors (TF) on the hexasome H2A–H2B distal side is identical to naked DNA, while the accessibility on the H2A–H2B proximal side is reduced by 2-fold, which is due to a 2-fold reduction in hexasome unwrapping probability. We then determined that a 23 bp region of the Widom-601 NPS is responsible for forming homogeneously oriented hexasomes. Analysis of published ChIP-exo data of hexasome containing genes identified two DNA sequence motifs that correlate with hexasome orientation in vivo, while ExoIII mapping studies of these sequences revealed they generate homogeneously oriented hexasomes in vitro. These results indicate that hexasome orientation, which is influenced by the underlying DNA sequence in vivo, is important for modulating DNA accessibility to regulate transcription.

Crystal structure of TbEsa1 presumed Tudor domain from Trypanosoma brucei

The essential SAS2-related acetyltransferase 1 (Esa1), as a acetyltransferase of MYST family, is indispensable for the cell cycle and transcriptional regulation. The Tudor domain consists of 60 amino acids and belongs to the Royal family, which serves as a module interacting with methylated histone and/or DNA. Although Tudor domain has been widely studied in higher eukaryotes, its structure and function remain unclear in Trypanosoma brucei (T. brucei), a protozoan unicellular parasite causing sleeping sickness in human and nagana in cattle in sub-Saharan Africa. Here, we determined a high-resolution structure of TbEsa1 presumed Tudor domain from T. brucei by X-ray crystallography. TbEsa1 Tudor domain adopts a conserved Tudor-like fold, which is comprised of a five-stranded β-barrel surrounded by two short α-helices. Furthermore, we revealed a non-specific DNA binding pattern of TbEsa1 Tudor domain. However, TbEsa1 Tudor domain showed no methyl-histone binding ability, due to the absence of key aromatic residues forming a conserved aromatic cage.

Molecular Basis for the PZP Domain of BRPF1 Association with Chromatin

The assembly of human histone acetyltransferase MOZ/MORF complexes relies on the scaffolding bromodomain plant homeodomain (PHD) finger 1 (BRPF1) subunit. The PHD-zinc-knuckle-PHD module of BRPF1 (BRPF1_PZP) has been shown to associate with the histone H3 tail and DNA; however, the molecular mechanism underlying recognition of H3 and the relationship between the histone and DNA-binding activities remain unclear. In this study, we report the crystal structure of BRPF1_PZP bound to the H3 tail and characterize the role of the bipartite interaction in the engagement of BRPF1_PZP with the nucleosome core particle (NCP). We find that although both interactions of BRPF1_PZP with the H3 tail and DNA are required for tight binding to NCP and for acetyltransferase function of the BRPF1-MORF-ING5-MEAF6 complex, binding to extranucleosomal DNA dominates. Our findings suggest that functionally active BRPF1_PZP might be important in stabilization of the MOZ/MORF complexes at chromatin with accessible DNA.

PRC2 is high maintenance

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

Understanding histone H3 lysine 36 methylation and its deregulation in disease

Methylation of histone H3 lysine 36 (H3K36) plays crucial roles in the partitioning of chromatin to distinctive domains and the regulation of a wide range of biological processes. Trimethylation of H3K36 (H3K36me3) demarcates body regions of the actively transcribed genes, providing signals for modulating transcription fidelity, mRNA splicing and DNA damage repair; and di-methylation of H3K36 (H3K36me2) spreads out within large intragenic regions, regulating distribution of histone H3 lysine 27 trimethylation (H3K27me3) and possibly DNA methylation. These H3K36 methylation-mediated events are biologically crucial and controlled by different classes of proteins responsible for either 'writing', 'reading' or 'erasing' of H3K36 methylation marks. Deregulation of H3K36 methylation and related regulatory factors leads to pathogenesis of disease such as developmental syndrome and cancer. Additionally, recurrent mutations of H3K36 and surrounding histone residues are detected in human tumors, further highlighting the importance of H3K36 in biology and medicine. This review will elaborate on current advances in understanding H3K36 methylation and related molecular players during various chromatin-templated cellular processes, their crosstalks with other chromatin factors, as well as their deregulations in the diseased contexts.

Strategies for Generating Modified Nucleosomes: Applications within Structural Biology Studies

Post-translational modifications on histone proteins play critical roles in the regulation of chromatin structure and all DNA-templated processes. Accumulating evidence suggests that these covalent modifications can directly alter chromatin structure, or they can modulate activities of chromatin-modifying and -remodeling factors. Studying these modifications in the context of the nucleosome, the basic subunit of chromatin, is thus of great interest; however, the generation of specifically modified nucleosomes remains challenging. This is especially problematic for most structural biology approaches in which a large amount of material is often needed. Here we discuss the strategies currently available for generation of these substrates. We in particular focus on novel ideas and discuss challenges in the application to structural biology studies.

Structural basis of specific H2A K13/K15 ubiquitination by RNF168

Ubiquitination of chromatin by modification of histone H2A is a critical step in both regulation of DNA repair and regulation of cell fate. These very different outcomes depend on the selective modification of distinct lysine residues in H2A, each by a specific E3 ligase. While polycomb PRC1 complexes modify K119, resulting in gene silencing, the E3 ligase RNF168 modifies K13/15, which is a key event in the response to DNA double-strand breaks. The molecular origin of ubiquitination site specificity by these related E3 enzymes is one of the open questions in the field. Using a combination of NMR spectroscopy, crosslinking mass-spectrometry, mutagenesis and data-driven modelling, here we show that RNF168 binds the acidic patch on the nucleosome surface, directing the E2 to the target lysine. The structural model highlights the role of E3 and nucleosome in promoting ubiquitination and provides a basis for understanding and engineering of chromatin ubiquitination specificity.

Unspinning chromatin: Revealing the dynamic nucleosome landscape by NMR

NMR is an essential technique for obtaining information at atomic resolution on the structure, motions and interactions of biomolecules. Here, we review the contribution of NMR to our understanding of the fundamental unit of chromatin: the nucleosome. Nucleosomes compact the genome by wrapping the DNA around a protein core, the histone octamer, thereby protecting genomic integrity. Crucially, the imposed barrier also allows strict regulation of gene expression, DNA replication and DNA repair processes through an intricate system of histone and DNA modifications and a wide range of interactions between nucleosomes and chromatin factors. In this review, we describe how NMR has contributed to deciphering the molecular basis of nucleosome function. Starting from pioneering studies in the 1960s using natural abundance NMR studies, we focus on the progress in sample preparation and NMR methodology that has allowed high-resolution studies on the nucleosome and its subunits. We summarize the results and approaches of state-of-the-art NMR studies on nucleosomal DNA, histone complexes, nucleosomes and nucleosomal arrays. These studies highlight the particular strength of NMR in studying nucleosome dynamics and nucleosome-protein interactions. Finally, we look ahead to exciting new possibilities that will be afforded by on-going developments in solution and solid-state NMR. By increasing both the depth and breadth of nucleosome NMR studies, it will be possible to offer a unique perspective on the dynamic landscape of nucleosomes and its interacting proteins.

PHF1 is required for chromosome alignment and asymmetric division during mouse meiotic oocyte maturation

In recent years, the etiological study of oocyte maturation failure and other mechanisms of early embryonic development has gradually advanced. However, while some achievements have been made in this field, the intrinsic mechanisms underlying disordered oocyte maturation remain unclear. Polycomb group proteins (PcG) are a family of proteins that are involved in the epigenetic silencing of genes. Many members of this family are reportedly involved in mammalian oocyte maturation and early embryonic development. PHD finger protein 1 (PHF1) is a core member of the polycomblike group of proteins, although its role in oocyte maturation and early embryonic development are unknown. A previous study by our group using single cell transcriptome analysis and high-throughput technology revealed that PHF1 mRNA was elevated in the human oocyte and the early preimplantation embryo. This suggests that PHF1 may play an important role in oocyte maturation and early embryonic development. In the present study, we aimed to reveal the biological function of PHF1 in mouse oocyte maturation and illuminate its regulatory mechanisms. We report here, for the first time, that PHF1 is necessary for the accurate alignment of chromosomes and oocyte euploidy, as well for the regulation of the asymmetric division of oocytes in mouse. The results of the present study may have the potential to provide a new research direction of human oocyte maturation disorder and early embryonic development block. These results may also provide new diagnosis or treatment strategies for clinical patients.

Structural insights into the π-π-π stacking mechanism and DNA-binding activity of the YEATS domain

The YEATS domain has been identified as a reader of histone acylation and more recently emerged as a promising anti-cancer therapeutic target. Here, we detail the structural mechanisms for π-π-π stacking involving the YEATS domains of yeast Taf14 and human AF9 and acylated histone H3 peptides and explore DNA-binding activities of these domains. Taf14-YEATS selects for crotonyllysine, forming π stacking with both the crotonyl amide and the alkene moiety, whereas AF9-YEATS exhibits comparable affinities to saturated and unsaturated acyllysines, engaging them through π stacking with the acyl amide. Importantly, AF9-YEATS is capable of binding to DNA, whereas Taf14-YEATS is not. Using a structure-guided approach, we engineered a mutant of Taf14-YEATS that engages crotonyllysine through the aromatic-aliphatic-aromatic π stacking and shows high selectivity for the crotonyl H3K9 modification. Our findings shed light on the molecular principles underlying recognition of acyllysine marks and reveal a previously unidentified DNA-binding activity of AF9-YEATS.

YEATS domains are histone acylation readers that recognize crotonyllysine and acetyllysine. Here the authors provide structural insights into how YEATS domains recognize acetyllysines and further show that the human AF9 YEATS domain also binds DNA.

Reading More than Histones: The Prevalence of Nucleic Acid Binding among Reader Domains

The eukaryotic genome is packaged into the cell nucleus in the form of chromatin, a complex of genomic DNA and histone proteins. Chromatin structure regulation is critical for all DNA templated processes and involves, among many things, extensive post-translational modification of the histone proteins. These modifications can be “read out” by histone binding subdomains known as histone reader domains. A large number of reader domains have been identified and found to selectively recognize an array of histone post-translational modifications in order to target, retain, or regulate chromatin-modifying and remodeling complexes at their substrates. Interestingly, an increasing number of these histone reader domains are being identified as also harboring nucleic acid binding activity. In this review, we present a summary of the histone reader domains currently known to bind nucleic acids, with a focus on the molecular mechanisms of binding and the interplay between DNA and histone recognition. Additionally, we highlight the functional implications of nucleic acid binding in chromatin association and regulation. We propose that nucleic acid binding is as functionally important as histone binding, and that a significant portion of the as yet untested reader domains will emerge to have nucleic acid binding capabilities.

Recognition of cancer mutations in histone H3K36 by epigenetic writers and readers

Histone posttranslational modifications control the organization and function of chromatin. In particular, methylation of lysine 36 in histone H3 (H3K36me) has been shown to mediate gene transcription, DNA repair, cell cycle regulation, and pre-mRNA splicing. Notably, mutations at or near this residue have been causally linked to the development of several human cancers. These observations have helped to illuminate the role of histones themselves in disease and to clarify the mechanisms by which they acquire oncogenic properties. This perspective focuses on recent advances in discovery and characterization of histone H3 mutations that impact H3K36 methylation. We also highlight findings that the common cancer-related substitution of H3K36 to methionine (H3K36M) disturbs functions of not only H3K36me-writing enzymes but also H3K36me-specific readers. The latter case suggests that the oncogenic effects could also be linked to the inability of readers to engage H3K36M.

A Comprehensive Roadmap of Murine Spermatogenesis Defined by Single-Cell RNA-Seq

Spermatogenesis requires intricate interactions between the germline and somatic cells. Within a given cross section of a seminiferous tubule, multiple germ and somatic cell types co-occur. This cellular heterogeneity has made it difficult to profile distinct cell types at different stages of development. To address this challenge, we collected single-cell RNA sequencing data from ∼35,000 cells from the adult mouse testis and identified all known germ and somatic cells, as well as two unexpected somatic cell types. Our analysis revealed a continuous developmental trajectory of germ cells from spermatogonia to spermatids and identified candidate transcriptional regulators at several transition points during differentiation. Focused analyses delineated four subtypes of spermatogonia and nine subtypes of Sertoli cells; the latter linked to histologically defined developmental stages over the seminiferous epithelial cycle. Overall, this high-resolution cellular atlas represents a community resource and foundation of knowledge to study germ cell development and in vivo gametogenesis.

Covalent Modifications of Histone H3K9 Promote Binding of CHD3

Chromatin remodeling is required for genome function and is facilitated by ATP-dependent complexes, such as nucleosome remodeling and deacetylase (NuRD). Among its core components is the chromodomain helicase DNA binding protein 3 (CHD3) whose functional significance is not well established. Here, we show that CHD3 co-localizes with the other NuRD subunits, including HDAC1, near the H3K9ac-enriched promoters of the NuRD target genes. The tandem PHD fingers of CHD3 bind histone H3 tails and posttranslational modifications that increase hydrophobicity of H3K9-methylation or acetylation (H3K9me3 or H3K9ac)-enhance this interaction. Binding of CHD3 PHDs promotes H3K9_Cme3-nucleosome unwrapping in vitro and perturbs the pericentric heterochromatin structure in vivo. Methylation or acetylation of H3K9 uniquely alleviates the intra-nucleosomal interaction of histone H3 tails, increasing H3K9 accessibility. Collectively, our data suggest that the targeting of covalently modified H3K9 by CHD3 might be essential in diverse functions of NuRD.

The conformation of the histone H3 tail inhibits association of the BPTF PHD finger with the nucleosome

Histone tails harbor a plethora of post-translational modifications that direct the function of chromatin regulators, which recognize them through effector domains. Effector domain/histone interactions have been broadly studied, but largely using peptide fragments of histone tails. Here, we extend these studies into the nucleosome context and find that the conformation adopted by the histone H3 tails is inhibitory to BPTF PHD finger binding. Using NMR spectroscopy and MD simulations, we show that the H3 tails interact robustly but dynamically with nucleosomal DNA, substantially reducing PHD finger association. Altering the electrostatics of the H3 tail via modification or mutation increases accessibility to the PHD finger, indicating that PTM crosstalk can regulate effector domain binding by altering nucleosome conformation. Together, our results demonstrate that the nucleosome context has a dramatic impact on signaling events at the histone tails, and highlights the importance of studying histone binding in the context of the nucleosome.

Accessibility of the histone H3 tail in the nucleosome for binding of paired readers

Combinatorial polyvalent contacts of histone-binding domains or readers commonly mediate localization and activities of chromatin-associated proteins. A pair of readers, the PHD fingers of the protein CHD4, has been shown to bivalently recognize histone H3 tails. Here we describe a mechanism by which these linked but independent readers bind to the intact nucleosome core particle (NCP). Comprehensive NMR, chemical reactivity, molecular dynamics, and fluorescence analyses point to the critical roles of intra-nucleosomal histone-DNA interactions that reduce the accessibility of H3 tails in NCP, the nucleosomal DNA, and the linker between readers in modulating nucleosome- and/or histone-binding activities of the readers. We show that the second PHD finger of CHD4 initiates recruitment to the nucleosome, however both PHDs are required to alter the NCP dynamics. Our findings reveal a distinctive regulatory mechanism for the association of paired readers with the nucleosome that provides an intricate balance between cooperative and individual activities of the readers.

The chromatin remodeller CHD4 contains two PHD finger reader domains that have been shown to bivalently recognize H3 histone tails. Here, the authors describe a mechanism by which the PHD fingers bind to the intact nucleosome core particle, revealing both cooperative and individual interactions.

PHF1 Tudor and N-terminal domains synergistically target partially unwrapped nucleosomes to increase DNA accessibility

The Tudor domain of human PHF1 recognizes trimethylated lysine 36 on histone H3 (H3K36me3). PHF1 relies on this interaction to regulate PRC2 methyltransferase activity, localize to DNA double strand breaks and mediate nucleosome accessibility. Here, we investigate the impact of the PHF1 N-terminal domain (NTD) on the Tudor domain interaction with the nucleosome. We show that the NTD is partially ordered when it is natively attached to the Tudor domain. Through a combination of FRET and single molecule studies, we find that the increase of DNA accessibility within the H3K36me3-containing nucleosome, instigated by the Tudor binding to H3K36me3, is dramatically enhanced by the NTD. We demonstrate that this nearly order of magnitude increase is due to preferential binding of PHF1 to partially unwrapped nucleosomes, and that PHF1 alters DNA–protein binding within the nucleosome by decreasing dissociation rates. These results highlight the potency of a PTM-binding protein to regulate DNA accessibility and underscores the role of the novel mechanism by which nucleosomes control DNA–protein binding through increasing protein dissociation rates.

Methods for Investigating DNA Accessibility with Single Nucleosomes

Nucleosomes are the fundamental organizing unit of all eukaryotic genomes. Understanding how proteins gain access to DNA-binding sites located within nucleosomes is important for understanding DNA processing including transcription, replication, and repair. Single-molecule total internal reflection fluorescence (smTIRF) microscopy measurements can provide key insight into how proteins gain and maintain access to DNA sites within nucleosomes. Here, we describe methods for smTIRF experiments including the preparation of fluorophore-labeled nucleosomes, the smTIRF system, data acquisition, analysis, and controls. These methods are presented for investigating transcription factor binding within nucleosomes. However, they are applicable for investigating the binding of any site-specific DNA-binding protein within nucleosomes.

DNA binding drives the association of BRG1/hBRM bromodomains with nucleosomes

BRG1 and BRM, central components of the BAF (mSWI/SNF) chromatin remodelling complex, are critical in chromatin structure regulation. Here, we show that the human BRM (hBRM) bromodomain (BRD) has moderate specificity for H3K14ac. Surprisingly, we also find that both BRG1 and hBRM BRDs have DNA-binding activity. We demonstrate that the BRDs associate with DNA through a surface basic patch and that the BRD and an adjacent AT-hook make multivalent contacts with DNA, leading to robust affinity and moderate specificity for AT-rich elements. Although we show that the BRDs can bind to both DNA and H3K14ac simultaneously, the histone-binding activity does not contribute substantially to nucleosome targeting in vitro. In addition, we find that neither BRD histone nor DNA binding contribute to the global chromatin affinity of BRG1 in mouse embryonic stem cells. Together, our results suggest that association of the BRG1/hBRM BRD with nucleosomes plays a regulatory rather than targeting role in BAF activity.

Nucleosome Histone Tail Conformation and Dynamics: Impacts of Lysine Acetylation and a Nearby Minor Groove Benzo[a]pyrene-Derived Lesion

Histone tails in nucleosomes play critical roles in regulation of many biological processes, including chromatin compaction, transcription, and DNA repair. Moreover, post-translational modifications, notably lysine acetylation, are crucial to these functions. While the tails have been intensively studied, how the structures and dynamics of tails are impacted by the presence of a nearby bulky DNA lesion is a frontier research area, and how these properties are impacted by tail lysine acetylation remains unexplored. To obtain molecular insight, we have utilized all atom 3 μs molecular dynamics simulations of nucleosome core particles (NCPs) to determine the impact of a nearby DNA lesion, 10S (+)-trans-anti-B[a]P-N²-dG-the major adduct derived from the procarcinogen benzo[a]pyrene-on H2B tail behavior in unacetylated and acetylated states. We similarly studied lesion-free NCPs to investigate the normal properties of the H2B tail in both states. In the lesion-free NCPs, charge neutralization upon lysine acetylation causes release of the tail from the DNA. When the lesion is present, it stably engulfs part of the nearby tail, impairing the interactions between DNA and tail. With the tail in an acetylated state, the lesion still interacts with part of it, although unstably. The lesion's partial entrapment of the tail should hinder the tail from interacting with other nucleosomes, and other proteins such as acetylases, deacetylases, and acetyl-lysine binding proteins, and thus disrupt critical tail-governed processes. Hence, the lesion would impede tail functions modulated by acetylation or deacetylation, causing aberrant chromatin structures and impaired biological transactions such as transcription and DNA repair.