H2A.Z landscapes and dual modifications in pluripotent and multipotent stem cells underlie complex genome regulatory functions

Incorporation of the histone variant H2A.Z counteracts gene silencing mediated by H3K27 trimethylation in Fusarium fujikuroi

BACKGROUND

Fusarium fujikuroi is a pathogen of rice causing diverse disease symptoms such as 'bakanae' or stunting, most likely due to the production of various natural products (NPs) during infection. Fusaria have the genetic potential to synthesize a plethora of these compounds with often diverse bioactivity. The capability to synthesize NPs exceeds the number of those being produced by far, implying a gene regulatory network decisive to induce production. One such regulatory layer is the chromatin structure and chromatin-based modifications associated with it. One prominent example is the exchange of histones against histone variants such as the H2A variant H2A.Z. Though H2A.Z already is well studied in several model organisms, its regulatory functions are not well understood. Here, we used F. fujikuroi as a model to explore the role of the prominent histone variant FfH2A.Z in gene expression within euchromatin and facultative heterochromatin.

RESULTS

Through the combination of diverse '-omics' methods, we show the global distribution of FfH2A.Z and analyze putative crosstalks between the histone variant and two prominent histone marks, i.e., H3K4me3 and H3K27me3, important for active gene transcription and silencing, respectively. We demonstrate that, if FfH2A.Z is positioned at the + 1-nucleosome, it poises chromatin for gene transcription, also within facultative heterochromatin. Lastly, functional characterization of FfH2A.Z overexpression and depletion mutants revealed that FfH2A.Z is important for wild type-like fungal development and secondary metabolism.

CONCLUSION

In this study, we show that the histone variant FfH2A.Z is a mark of positive gene transcription and acts independently of the chromatin state most likely through the stabilization of the + 1-nucleosome. Furthermore, we demonstrate that FfH2A.Z depletion does not influence the establishment of both H3K27me3 and H3K4me3, thus indicating no crosstalk between FfH2A.Z and both histone marks. These results highlight the manifold functions of the histone variant FfH2A.Z in the phytopathogen F. fujikuroi, which are distinct regarding gene transcription and crosstalk with the two prominent histone marks H3K27me3 and H3K4me3, as proposed for other model organisms.

An iron rheostat controls hematopoietic stem cell fate

Mechanisms governing the maintenance of blood-producing hematopoietic stem and multipotent progenitor cells (HSPCs) are incompletely understood, particularly those regulating fate, ensuring long-term maintenance, and preventing aging-associated stem cell dysfunction. We uncovered a role for transitory free cytoplasmic iron as a rheostat for adult stem cell fate control. We found that HSPCs harbor comparatively small amounts of free iron and show the activation of a conserved molecular response to limited iron-particularly during mitosis. To study the functional and molecular consequences of iron restriction, we developed models allowing for transient iron bioavailability limitation and combined single-molecule RNA quantification, metabolomics, and single-cell transcriptomic analyses with functional studies. Our data reveal that the activation of the limited iron response triggers coordinated metabolic and epigenetic events, establishing stemness-conferring gene regulation. Notably, we find that aging-associated cytoplasmic iron loading reversibly attenuates iron-dependent cell fate control, explicating intervention strategies for dysfunctional aged stem cells.

Identification and validation of an H2AZ1-based index model: a novel prognostic tool for hepatocellular carcinoma

The H2A.Z variant histone 1 (H2AZ1) is aberrantly expressed in various tumors, correlating with an unfavorable prognosis. However, its role in hepatocellular carcinoma (HCC) remains unclear. We aimed to elucidate the pathways affected by H2AZ1 and identify promising therapeutic targets for HCC. Following bioinformatic analysis of gene expression and clinical data from The Cancer Genome Atlas and Gene Expression Omnibus database, we found 6,344 dysregulated genes related to H2AZ1 overexpression in HCC tissues (P < 0.05). We performed weighted gene co-expression network analysis to identify the gene module most related to H2AZ1. The H2AZ1-based index was further developed using Cox regression analysis, which revealed that the poor prognosis in the high H2AZ1-based index group could be attributed to elevated tumor stemness (P < 0.05). Moreover, the clinical model showed good prognostic potential (AUC > 0.7). We found that H2AZ1 knockdown led to reduced superoxide dismutase (SOD) activity, elevated malondialdehyde (MDA) levels, and increased apoptosis rate in tumor cells (P < 0.001). Thus, we developed an H2AZ1-based index model with the potential to predict the prognosis of patients with HCC. Our findings provide initial evidence that H2AZ1 overexpression plays a pivotal role in HCC initiation and progression.

PARP-1 is a transcriptional rheostat of metabolic and bivalent genes during development

PARP-1 participates in various cellular processes, including gene regulation. In Drosophila, PARP-1 mutants undergo developmental arrest during larval-to-pupal transition. In this study, we investigated PARP-1 binding and its transcriptional regulatory role at this stage. Our findings revealed that PARP-1 binds and represses active metabolic genes, including glycolytic genes, whereas activating low-expression developmental genes, including a subset of "bivalent" genes in third-instar larvae. These bivalent promoters, characterized by dual enrichment of low H3K4me3 and high H3K27me3, a unimodal H3K4me1 enrichment at the transcription start site (conserved in C. elegans and zebrafish), H2Av depletion, and high accessibility, may persist throughout development. In PARP-1 mutant third-instar larvae, metabolic genes typically down-regulated during the larval-to-pupal transition in response to reduced energy needs were repressed by PARP-1. Simultaneously, developmental and bivalent genes typically active at this stage were activated by PARP-1. In addition, glucose and ATP levels were significantly reduced in PARP-1 mutants, suggesting an imbalance in metabolic regulation. We propose that PARP-1 is essential for maintaining the delicate balance between metabolic and developmental gene expression programs to ensure proper developmental progression.

Spatiotemporal transcriptome atlas reveals the regional specification of the developing human brain

Different functional regions of brain are fundamental for basic neurophysiological activities. However, the regional specification remains largely unexplored during human brain development. Here, by combining spatial transcriptomics (scStereo-seq) and scRNA-seq, we built a spatiotemporal developmental atlas of multiple human brain regions from 6-23 gestational weeks (GWs). We discovered that, around GW8, radial glia (RG) cells have displayed regional heterogeneity and specific spatial distribution. Interestingly, we found that the regional heterogeneity of RG subtypes contributed to the subsequent neuronal specification. Specifically, two diencephalon-specific subtypes gave rise to glutamatergic and GABAergic neurons, whereas subtypes in ventral midbrain were associated with the dopaminergic neurons. Similar GABAergic neuronal subtypes were shared between neocortex and diencephalon. Additionally, we revealed that cell-cell interactions between oligodendrocyte precursor cells and GABAergic neurons influenced and promoted neuronal development coupled with regional specification. Altogether, this study provides comprehensive insights into the regional specification in the developing human brain.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Competition for H2A.Z underlies the developmental impacts of repetitive element de-repression

The histone variant H2A.Z is central to early embryonic development, determining transcriptional competency through chromatin regulation of gene promoters and enhancers. In addition to genic loci, we find that H2A.Z resides at a subset of evolutionarily young repetitive elements, including DNA transposons, long interspersed nuclear elements and long terminal repeats, during early zebrafish development. Moreover, increases in H2A.Z occur when repetitive elements become transcriptionally active. Acquisition of H2A.Z corresponds with a reduction in the levels of the repressive histone modification H3K9me3 and a moderate increase in chromatin accessibility. Notably, however, de-repression of repetitive elements also leads to a significant reduction in H2A.Z over non-repetitive genic loci. Genic loss of H2A.Z is accompanied by transcriptional silencing at adjacent coding sequences, but remarkably, these impacts are mitigated by augmentation of total H2A.Z protein via transgenic overexpression. Our study reveals that levels of H2A.Z protein determine embryonic sensitivity to de-repression of repetitive elements, that repetitive elements can function as a nuclear sink for epigenetic factors and that competition for H2A.Z greatly influences overall transcriptional output during development. These findings uncover general mechanisms in which counteractive biological processes underlie phenotypic outcomes.

The P. falciparum alternative histones Pf H2A.Z and Pf H2B.Z are dynamically acetylated and antagonized by PfSir2 histone deacetylases at heterochromatin boundaries

The Plasmodium falciparum alternative histones Pf H2A.Z and Pf H2B.Z are enriched in the same nucleosomes in intergenic euchromatin but depleted from heterochromatin. They occupy most promoters but are only dynamically associated with expression at var genes. In other organisms, acetylation of H2A.Z is important for its functions in gene expression and chromatin structure. Here, we show that acetylated Pf H2A.Z and Pf H2B.Z are dynamically associated with gene expression at promoters. In addition, acetylated Pf H2A.Z and Pf H2B.Z are antagonized by the sirtuin class III histone deacetylases (HDAC) PfSir2A and B at heterochromatin boundaries and encroach upon heterochromatin in parasites lacking PfSir2A or B. However, the majority of acetylated Pf H2A.Z and Pf H2B.Z are deacetylated by class I or II HDACs. Acetylated Pf H2A.Z and Pf H2B.Z are also dynamically associated with promoter activity of both canonical upstream var gene promoters and var gene introns. These findings suggest that both acetylated Pf H2A.Z and Pf H2B.Z play critical roles in gene expression and contribute to maintenance of chromatin structure at the boundaries of subtelomeric, facultative heterochromatin, critical for the variegated expression of genes that enable rapid adaptation to altered host environments.IMPORTANCEThe malaria parasite Plasmodium falciparum relies on variant expression of members of multi-gene families as a strategy for environmental adaptation to promote parasite survival and pathogenesis. These genes are located in transcriptionally silenced DNA regions. A limited number of these genes escape gene silencing, and switching between them confers variant fitness on parasite progeny. Here, we show that PfSir2 histone deacetylases antagonize DNA-interacting acetylated alternative histones at the boundaries between active and silent DNA. This finding implicates acetylated alternative histones in the mechanism regulating P. falciparum variant gene silencing and thus malaria pathogenesis. This work also revealed that acetylation of alternative histones at promoters is dynamically associated with promoter activity across the genome, implicating acetylation of alternative histones in gene regulation genome wide. Understanding mechanisms of gene regulation in P. falciparum may aid in the development of new therapeutic strategies for malaria, which killed 619,000 people in 2021.

Deep multiple-instance learning accurately predicts gene haploinsufficiency and deletion pathogenicity

Copy number losses (deletions) are a major contributor to the etiology of severe genetic disorders. Although haploinsufficient genes play a critical role in deletion pathogenicity, current methods for deletion pathogenicity prediction fail to integrate multiple lines of evidence for haploinsufficiency at the gene level, limiting their power to pinpoint deleterious deletions associated with genetic disorders. Here we introduce DosaCNV, a deep multiple-instance learning framework that, for the first time, models deletion pathogenicity jointly with gene haploinsufficiency. By integrating over 30 gene-level features potentially predictive of haploinsufficiency, DosaCNV shows unmatched performance in prioritizing pathogenic deletions associated with a broad spectrum of genetic disorders. Furthermore, DosaCNV outperforms existing methods in predicting gene haploinsufficiency even though it is not trained on known haploinsufficient genes. Finally, DosaCNV leverages a state-of-the-art technique to quantify the contributions of individual gene-level features to haploinsufficiency, allowing for human-understandable explanations of model predictions. Altogether, DosaCNV is a powerful computational tool for both fundamental and translational research.

An unsupervised deep learning framework for predicting human essential genes from population and functional genomic data

BACKGROUND

The ability to accurately predict essential genes intolerant to loss-of-function (LOF) mutations can dramatically improve the identification of disease-associated genes. Recently, there have been numerous computational methods developed to predict human essential genes from population genomic data. While the existing methods are highly predictive of essential genes of long length, they have limited power in pinpointing short essential genes due to the sparsity of polymorphisms in the human genome.

RESULTS

Motivated by the premise that population and functional genomic data may provide complementary evidence for gene essentiality, here we present an evolution-based deep learning model, DeepLOF, to predict essential genes in an unsupervised manner. Unlike previous population genetic methods, DeepLOF utilizes a novel deep learning framework to integrate both population and functional genomic data, allowing us to pinpoint short essential genes that can hardly be predicted from population genomic data alone. Compared with previous methods, DeepLOF shows unmatched performance in predicting ClinGen haploinsufficient genes, mouse essential genes, and essential genes in human cell lines. Notably, at a false positive rate of 5%, DeepLOF detects 50% more ClinGen haploinsufficient genes than previous methods. Furthermore, DeepLOF discovers 109 novel essential genes that are too short to be identified by previous methods.

CONCLUSION

The predictive power of DeepLOF shows that it is a compelling computational method to aid in the discovery of essential genes.

Epigenomic signatures reveal mechanistic clues and predictive markers for autism spectrum disorder

Autism spectrum disorder (ASD) comprises a heterogeneous group of neurodevelopmental outcomes in children with a commonality in deficits in social communication and language combined with repetitive behaviors and interests. The etiology of ASD is heterogeneous, as several hundred genes have been implicated as well as multiple in utero environmental exposures. Over the past two decades, epigenetic investigations, including DNA methylation, have emerged as a novel way to capture the complex interface of multivariate ASD etiologies. More recently, epigenome-wide association studies using human brain and surrogate accessible tissues have revealed some convergent genes that are epigenetically altered in ASD, many of which overlap with known genetic risk factors. Unlike transcriptomes, epigenomic signatures defined by DNA methylation from surrogate tissues such as placenta and cord blood can reflect past differences in fetal brain gene transcription, transcription factor binding, and chromatin. For example, the discovery of NHIP (neuronal hypoxia inducible, placenta associated) through an epigenome-wide association in placenta, identified a common genetic risk for ASD that was modified by prenatal vitamin use. While epigenomic signatures are distinct between different genetic syndromic causes of ASD, bivalent chromatin and some convergent gene pathways are consistently epigenetically altered in both syndromic and idiopathic ASD, as well as some environmental exposures. Together, these epigenomic signatures hold promising clues towards improved early prediction and prevention of ASD as well genes and gene pathways to target for pharmacological interventions. Future advancements in single cell and multi-omic technologies, machine learning, as well as non-invasive screening of epigenomic signatures during pregnancy or newborn periods are expected to continue to impact the translatability of the recent discoveries in epigenomics to precision public health.

Histone H2A deubiquitinases in the transcriptional programs of development and hematopoiesis: a consolidated analysis

Monoubiquitinated lysine 119 of histone H2A (H2AK119ub) is a highly abundant epigenetic mark, associated with gene repression and deposited on chromatin by the polycomb repressor complex 1 (PRC1), which is an essential regulator of diverse transcriptional programs in mammalian development and tissue homeostasis. While multiple deubiquitinases (DUBs) with catalytic activity for H2AK119ub (H2A-DUBs) have been identified, we lack systematic analyses of their roles and cross-talk in transcriptional regulation. Here, we address H2A-DUB functions in epigenetic regulation of mammalian development and tissue maintenance by conducting a meta-analysis of 248 genomics datasets from 32 independent studies, focusing on the mouse model and covering embryonic stem cells (ESCs), hematopoietic, and immune cell lineages. This covers all the publicly available datasets that map genomic H2A-DUB binding and H2AK119ub distributions (ChIP-Seq), and all datasets assessing dysregulation in gene expression in the relevant H2A-DUB knockout models (RNA-Seq). Many accessory datasets for PRC1-2 and DUB-interacting proteins are also analyzed and interpreted, as well as further data assessing chromatin accessibility (ATAC-Seq) and transcriptional activity (RNA-seq). We report co-localization in the binding of H2A-DUBs BAP1, USP16, and to a lesser extent others that is conserved across different cell-types, and also the enrichment of antagonistic PRC1-2 protein complexes at the same genomic locations. Such conserved sites enriched for the H2A-DUBs and PRC1-2 are proximal to transcriptionally active genes that engage in housekeeping cellular functions. Nevertheless, they exhibit H2AK119ub levels significantly above the genomic average that can undergo further increase with H2A-DUB knockout. This indicates a cooperation between H2A-DUBs and PRC1-2 in the modulation of housekeeping transcriptional programs, conserved across many cell types, likely operating through their antagonistic effects on H2AK119ub and the regulation of local H2AK119ub turnover. Our study further highlights existing knowledge gaps and discusses important directions for future work.

Actin-related protein 6 facilitates proneural protein-induced gene activation for rapid neural differentiation

Neurogenesis is initiated by basic helix-loop-helix proneural proteins. Here, we show that Actin-related protein 6 (Arp6), a core component of the H2A.Z exchange complex SWR1, interacts with proneural proteins and is crucial for efficient onset of proneural protein target gene expression. Arp6 mutants exhibit reduced transcription in sensory organ precursors (SOPs) downstream of the proneural protein patterning event. This leads to retarded differentiation and division of SOPs and smaller sensory organs. These phenotypes are also observed in proneural gene hypomorphic mutants. Proneural protein expression is not reduced in Arp6 mutants. Enhanced proneural gene expression fails to rescue retarded differentiation in Arp6 mutants, suggesting that Arp6 acts downstream of or in parallel with proneural proteins. H2A.Z mutants display Arp6-like retardation in SOPs. Transcriptomic analyses demonstrate that loss of Arp6 and H2A.Z preferentially decreases expression of proneural protein-activated genes. H2A.Z enrichment in nucleosomes around the transcription start site before neurogenesis correlates highly with greater activation of proneural protein target genes by H2A.Z. We propose that upon proneural protein binding to E-box sites, H2A.Z incorporation around the transcription start site allows rapid and efficient activation of target genes, promoting rapid neural differentiation.

Contribution of the histone variant H2A.Z to expression of responsive genes in plants

The histone variant H2A.Z plays a critical role in chromatin-based processes such as transcription, replication, and repair in eukaryotes. Although many H2A.Z-associated processes and features are conserved in plants and animals, a distinguishing feature of plant chromatin is the enrichment of H2A.Z in the bodies of genes that exhibit dynamic expression, particularly in response to differentiation and the environment. Recent work sheds new light on the plant machinery that enables dynamic changes in H2A.Z enrichment and identifies additional chromatin-based pathways that contribute to transcriptional properties of H2A.Z-enriched chromatin. In particular, analysis of a variety of responsive loci reveals a repressive role for H2A.Z in expression of responsive genes and identifies roles for SWR1 and INO80 chromatin remodelers in enabling dynamic regulation of H2A.Z levels and transcription. These studies lay the groundwork for understanding how this ancient histone variant is harnessed by plants to enable responsive and dynamic gene expression (Graphical Abstract).

Tip60-mediated H2A.Z acetylation promotes neuronal fate specification and bivalent gene activation

Cell lineage specification is accomplished by a concerted action of chromatin remodeling and tissue-specific transcription factors. However, the mechanisms that induce and maintain appropriate lineage-specific gene expression remain elusive. Here, we used an unbiased proteomics approach to characterize chromatin regulators that mediate the induction of neuronal cell fate. We found that Tip60 acetyltransferase is essential to establish neuronal cell identity partly via acetylation of the histone variant H2A.Z. Despite its tight correlation with gene expression and active chromatin, loss of H2A.Z acetylation had little effect on chromatin accessibility or transcription. Instead, loss of Tip60 and acetyl-H2A.Z interfered with H3K4me3 deposition and activation of a unique subset of silent, lineage-restricted genes characterized by a bivalent chromatin configuration at their promoters. Altogether, our results illuminate the mechanisms underlying bivalent chromatin activation and reveal that H2A.Z acetylation regulates neuronal fate specification by establishing epigenetic competence for bivalent gene activation and cell lineage transition.

RINGs, DUBs and Abnormal Brain Growth-Histone H2A Ubiquitination in Brain Development and Disease

During mammalian neurodevelopment, signaling pathways converge upon transcription factors (TFs) to establish appropriate gene expression programmes leading to the production of distinct neural and glial cell types. This process is partially regulated by the dynamic modulation of chromatin states by epigenetic systems, including the polycomb group (PcG) family of co-repressors. PcG proteins form multi-subunit assemblies that sub-divide into distinct, yet functionally related families. Polycomb repressive complexes 1 and 2 (PRC1 and 2) modify the chemical properties of chromatin by covalently modifying histone tails via H2A ubiquitination (H2AK119ub1) and H3 methylation, respectively. In contrast to the PRCs, the Polycomb repressive deubiquitinase (PR-DUB) complex removes H2AK119ub1 from chromatin through the action of the C-terminal hydrolase BAP1. Genetic screening has identified several PcG mutations that are causally associated with a range of congenital neuropathologies associated with both localised and/or systemic growth abnormalities. As PRC1 and PR-DUB hold opposing functions to control H2AK119ub1 levels across the genome, it is plausible that such neurodevelopmental disorders arise through a common mechanism. In this review, we will focus on advancements regarding the composition and opposing molecular functions of mammalian PRC1 and PR-DUB, and explore how their dysfunction contributes to the emergence of neurodevelopmental disorders.

Interplay Between the Histone Variant H2A.Z and the Epigenome in Pancreatic Cancer

BACKGROUND

The oncogenic process is orchestrated by a complex network of chromatin remodeling elements that shape the cancer epigenome. Histone variant H2A.Z regulates DNA control elements such as promoters and enhancers in different types of cancer; however, the interplay between H2A.Z and the pancreatic cancer epigenome is unknown.

OBJECTIVE

This study analyzed the role of H2A.Z in different DNA regulatory elements.

METHODS

We performed Chromatin Immunoprecipitation Sequencing assays (ChiP-seq) with total H2A.Z and acetylated H2A.Z (acH2A.Z) antibodies and analyzed published data from ChIP-seq, RNA-seq, bromouridine labeling-UV and sequencing (BruUV-seq), Hi-C and ATAC-seq (Assay for Transposase-Accessible Chromatin using sequencing) in the pancreatic cancer cell line PANC-1.

RESULTS

The results indicate that total H2A.Z facilitates the recruitment of RNA polymerase II and transcription factors at promoters and enhancers allowing the expression of pro-oncogenic genes. Interestingly, we demonstrated that H2A.Z is enriched in super-enhancers (SEs) contributing to the transcriptional activation of key genes implicated in tumor development. Importantly, we established that H2A.Z contributes to the three-dimensional (3D) genome organization of pancreatic cancer and that it is a component of the Topological Associated Domains (TADs) boundaries in PANC-1 and that total H2A.Z and acH2A.Z are associated with A and B compartments, respectively.

CONCLUSIONS

H2A.Z participates in the biology and development of pancreatic cancer by generating a pro-oncogenic transcriptome through its posttranslational modifications, interactions with different partners, and regulatory elements, contributing to the oncogenic 3D genome organization. These data allow us to understand the molecular mechanisms that promote an oncogenic transcriptome in pancreatic cancer mediated by H2A.Z.

Hierarchical Accumulation of Histone Variant H2A.Z Regulates Transcriptional States and Histone Modifications in Early Mammalian Embryos

Early embryos undergo extensive epigenetic reprogramming to achieve gamete-to-embryo transition, which involves the loading and removal of histone variant H2A.Z on chromatin. However, how does H2A.Z regulate gene expression and histone modifications during preimplantation development remains unrevealed. Here, by using ultra-low-input native chromatin immunoprecipitation and sequencing, the genome-wide distribution of H2A.Z is delineated in mouse oocytes and early embryos. These landscapes indicate that paternal H2A.Z is removed upon fertilization, followed by unbiased accumulation on parental genomes during zygotic genome activation (ZGA). Remarkably, H2A.Z exhibits hierarchical accumulation as different peak types at promoters: promoters with double H2A.Z peaks are colocalized with H3K4me3 and indicate transcriptional activation; promoters with a single H2A.Z peak are more likely to occupy bivalent marks (H3K4me3+H3K27me3) and indicate development gene suppression; promoters with no H2A.Z accumulation exhibit persisting gene silencing in early embryos. Moreover, H2A.Z depletion changes the enrichment of histone modifications and RNA polymerase II binding at promoters, resulting in abnormal gene expression and developmental arrest during lineage commitment. Furthermore, similar transcription and accumulation patterns between mouse and porcine embryos indicate that a dual role of H2A.Z in regulating the epigenome required for proper gene expression is conserved during mammalian preimplantation development.

The Role of the Histone Variant H2A.Z in Metazoan Development

During the emergence and radiation of complex multicellular eukaryotes from unicellular ancestors, transcriptional systems evolved by becoming more complex to provide the basis for this morphological diversity. The way eukaryotic genomes are packaged into a highly complex structure, known as chromatin, underpins this evolution of transcriptional regulation. Chromatin structure is controlled by a variety of different epigenetic mechanisms, including the major mechanism for altering the biochemical makeup of the nucleosome by replacing core histones with their variant forms. The histone H2A variant H2A.Z is particularly important in early metazoan development because, without it, embryos cease to develop and die. However, H2A.Z is also required for many differentiation steps beyond the stage that H2A.Z-knockout embryos die. H2A.Z can facilitate the activation and repression of genes that are important for pluripotency and differentiation, and acts through a variety of different molecular mechanisms that depend upon its modification status, its interaction with histone and nonhistone partners, and where it is deposited within the genome. In this review, we discuss the current knowledge about the different mechanisms by which H2A.Z regulates chromatin function at various developmental stages and the chromatin remodeling complexes that determine when and where H2A.Z is deposited.

Integrated bioinformatics analysis reveals that EZH2-rich domains promote transcriptional repression in cervical cancer

Cervical cancer is the third female cancer most common worldwide. The carcinogenic process involves an alteration of the mechanisms associated with transcription. Several studies have reported an oncogenic role of the polycomb complex subunit, EZH2. However, the role of EZH2 in cervical cancer is unknown. Hence, the objective of this study was to determine the role of EZH2 in transcriptional regulation in cervical cancer. The EZH2 expression and the methylation status of its promoter were analyzed in The Cancer Genome Atlas. The EZH2 enrichment profile was analyzed using chromatin immunoprecipitation with massively parallel DNA sequencing data provided by ENCODE project. The chromatin compartments were identified in the 4D Nucleome Data Portal. The functional annotation was examined in Enrichr. We report that EZH2 expression is increased in cervical cancer which is associated with hypomethylation of its promoter. EZH2 is enriched at promoter and distal intergenic regions. We identified that EZH2 defines chromatin domains enriched with H3K27me3 within repressive compartments in the HeLa-S3 cell line. Additionally, high EZH2 expression is associated with the repression of the senescent phenotype in cervical cancer patients. Our results suggest the participation of EZH2 in the generation of domains with a silencer function in cervical cancer, which regulate the expression of genes associated with cellular senescence.

H2A.Z's 'social' network: functional partners of an enigmatic histone variant

The histone variant H2A.Z has been extensively studied to understand its manifold DNA-based functions. In the past years, researchers identified its specific binding partners, the 'H2A.Z interactome', that convey H2A.Z-dependent chromatin changes. Here, we summarize the latest findings regarding vertebrate H2A.Z-associated factors and focus on their roles in gene activation and repression, cell cycle regulation, (neuro)development, and tumorigenesis. Additionally, we demonstrate how protein-protein interactions and post-translational histone modifications can fine-tune the complex interplay of H2A.Z-regulated gene expression. Last, we review the most recent results on interactors of the two isoforms H2A.Z.1 and H2A.Z.2.1, which differ in only three amino acids, and focus on cancer-associated mutations of H2A and H2A.Z, which reveal fascinating insights into the functional importance of such minuscule changes.

Disordered regions tune order in chromatin organization and function

Intrinsically disordered proteins or hybrid proteins with ordered domains and disordered regions (both collectively designated as IDP(R)s) defy the well-established structure-function paradigm due to their ability to perform multiple biological functions even in the absence of a well-defined 3D structure. IDP(R)s have a unique ability to exist as a functional heterogeneous ensemble, where they adopt multiple thermodynamically stable conformations with low energy barriers between states. The resultant structural plasticity or conformational adaptability provides them with a high functional diversity and ease of regulation. Hence, IDP(R)s are highly efficient biological machinery to mediate intricate cellular functions such as signaling, gene expression, and assembly of complex structures. One such structure is the nucleoprotein complex known as Chromatin. Interestingly, the proteins involved in shaping up the structure and function of chromatin are abundant in disordered regions, which serve more than just as mere flexible linkers. The disordered regions are involved in crucial processes such as gene expression regulation, chromatin architecture maintenance, and liquid-liquid phase separation initiation. This review is an attempt to explore the advantages and the functional and regulatory roles of intrinsic disorder in several Chromatin Associated Proteins from a mechanistic standpoint.

Epigenomic Modifications in Modern and Ancient Genomes

Epigenetic changes have been identified as a major driver of fundamental metabolic pathways. More specifically, the importance of epigenetic regulatory mechanisms for biological processes like speciation and embryogenesis has been well documented and revealed the direct link between epigenetic modifications and various diseases. In this review, we focus on epigenetic changes in animals with special attention on human DNA methylation utilizing ancient and modern genomes. Acknowledging the latest developments in ancient DNA research, we further discuss paleoepigenomic approaches as the only means to infer epigenetic changes in the past. Investigating genome-wide methylation patterns of ancient humans may ultimately yield in a more comprehensive understanding of how our ancestors have adapted to the changing environment, and modified their lifestyles accordingly. We discuss the difficulties of working with ancient DNA in particular utilizing paleoepigenomic approaches, and assess new paleoepigenomic data, which might be helpful in future studies.

INO80 requires a polycomb subunit to regulate the establishment of poised chromatin in murine spermatocytes

INO80 is the catalytic subunit of the INO80-chromatin remodeling complex that is involved in DNA replication, repair and transcription regulation. Ino80 deficiency in murine spermatocytes (Ino80cKO) results in pachytene arrest of spermatocytes due to incomplete synapsis and aberrant DNA double-strand break repair, which leads to apoptosis. RNA-seq on Ino80cKO spermatocytes revealed major changes in transcription, indicating that an aberrant transcription program arises upon INO80 depletion. In Ino80WT spermatocytes, genome-wide analysis showed that INO80-binding sites were mostly promoter proximal and necessary for the regulation of spermatogenic gene expression, primarily of premeiotic and meiotic genes. Furthermore, most of the genes poised for activity, as well as those genes that are active, shared INO80 binding. In Ino80cKO spermatocytes, most poised genes demonstrated de-repression due to reduced H3K27me3 enrichment and, in turn, showed increased expression levels. INO80 interacts with the core PRC2 complex member SUZ12 and promotes its recruitment. Furthermore, INO80 mediates H2A.Z incorporation at the poised promoters, which was reduced in Ino80cKO spermatocytes. Taken together, INO80 is emerging as a major regulator of the meiotic transcription program by mediating poised chromatin establishment through SUZ12 binding.

Distinct transcription kinetics of pluripotent cell states

Mouse embryonic stem cells (mESCs) can adopt naïve, ground, and paused pluripotent states that give rise to unique transcriptomes. Here, we use transient transcriptome sequencing (TT-seq) to define both coding and non-coding transcription units (TUs) in these three pluripotent states and combine TT-seq with RNA polymerase II occupancy profiling to unravel the kinetics of RNA metabolism genome-wide. Compared to the naïve state (serum), RNA synthesis and turnover rates are globally reduced in the ground state (2i) and the paused state (mTORi). The global reduction in RNA synthesis goes along with a genome-wide decrease of polymerase elongation velocity, which is related to epigenomic features and alterations in the Pol II termination window. Our data suggest that transcription activity is the main determinant of steady state mRNA levels in the naïve state and that genome-wide changes in transcription kinetics invoke ground and paused pluripotent states.

Histone variant H2A.Z regulates zygotic genome activation

During embryogenesis, the genome shifts from transcriptionally quiescent to extensively active in a process known as Zygotic Genome Activation (ZGA). In Drosophila, the pioneer factor Zelda is known to be essential for the progression of development; still, it regulates the activation of only a small subset of genes at ZGA. However, thousands of genes do not require Zelda, suggesting that other mechanisms exist. By conducting GRO-seq, HiC and ChIP-seq in Drosophila embryos, we demonstrate that up to 65% of zygotically activated genes are enriched for the histone variant H2A.Z. H2A.Z enrichment precedes ZGA and RNA Polymerase II loading onto chromatin. In vivo knockdown of maternally contributed Domino, a histone chaperone and ATPase, reduces H2A.Z deposition at transcription start sites, causes global downregulation of housekeeping genes at ZGA, and compromises the establishment of the 3D chromatin structure. We infer that H2A.Z is essential for the de novo establishment of transcriptional programs during ZGA via chromatin reorganization.

During embryogenesis, the genome becomes transcriptionally active in a process known as zygotic genome activation (ZGA); how ZGA is initiated is still an open question. Here the authors show histone variant H2A.Z deposition precedes RNA polymerase II binding on chromatin, before ZGA. H2A.Z loss causes transcriptional downregulation of ZGA genes and leads to changes in the 3D genome organization.

The H2A.Z-nuclesome code in mammals: emerging functions

H2A.Z is a histone variant that provides specific structural and docking-side properties to the nucleosome, resulting in diverse and specialised molecular and cellular functions. In this review, we discuss the latest studies uncovering new functional aspects of mammalian H2A.Z in gene transcription, including pausing and elongation of RNA polymerase II (RNAPII) and enhancer activity; DNA repair; DNA replication; and 3D chromatin structure. We also review the recently described role of H2A.Z in embryonic development, cell differentiation, neurodevelopment, and brain function. In conclusion, our cumulative knowledge of H2A.Z over the past 40 years, in combination with the implementation of novel molecular technologies, is unravelling an unexpected and complex role of histone variants in gene regulation and disease.

Identification of chromatin states during zebrafish gastrulation using CUT&RUN and CUT&Tag

BACKGROUND

Cell fate decisions are governed by interactions between sequence-specific transcription factors and a dynamic chromatin landscape. Zebrafish offer a powerful system for probing the mechanisms that drive these cell fate choices, especially in the context of early embryogenesis. However, technical challenges associated with conventional methods for chromatin profiling have slowed progress toward understanding the exact relationships between chromatin changes, transcription factor binding, and cellular differentiation during zebrafish embryogenesis.

RESULTS

To overcome these challenges, we adapted the chromatin profiling methods Cleavage Under Targets and Release Using Nuclease (CUT&RUN) and CUT&Tag for use in zebrafish and applied these methods to generate high-resolution enrichment maps for H3K4me3, H3K27me3, H3K9me3, RNA polymerase II, and the histone variant H2A.Z using tissue isolated from whole, mid-gastrula stage embryos. Using this data, we identify a subset of genes that may be bivalently regulated during both zebrafish and mouse gastrulation, provide evidence for an evolving H2A.Z landscape during embryo development, and demonstrate the effectiveness of CUT&RUN for detecting H3K9me3 enrichment at repetitive sequences.

CONCLUSIONS

Our results demonstrate the power of combining CUT&RUN and CUT&Tag methods with the strengths of the zebrafish system to define emerging chromatin landscapes in the context of vertebrate embryogenesis.

Tip60 activates Hoxa9 and Meis1 expression through acetylation of H2A.Z, promoting MLL-AF10 and MLL-ENL acute myeloid leukemia

Chromosome translocations involving the MLL gene are common rearrangements in leukemia. Such translocations fuse the MLL 5'-region to partner genes in frame, producing MLL-fusions that cause MLL-related leukemia. MLL-fusions activate transcription of target genes such as HoxA cluster and Meis1, but the underlying mechanisms remain to be fully elucidated. In this study, we discovered that Tip60, a MYST-type histone acetyltransferase, was required for the expression of HoxA cluster and Meis1 genes and the development of MLL-fusion leukemia. Tip60 was recruited by MLL-AF10 and MLL-ENL fusions to the Hoxa9 locus, where it acetylated H2A.Z, thereby promoting Hoxa9 gene expression. Conditional deletion of Tip60 prevented the development of MLL-AF10 and MLL-ENL leukemia, indicating that Tip60 is indispensable for the leukemogenic activity of the MLL-AF10 and MLL-ENL-fusions. Our findings provide novel insight about epigenetic regulation in the development of MLL-AF10 and MLL-ENL-fusion leukemia.

Deficient H2A.Z deposition is associated with genesis of uterine leiomyoma

One in four women suffers from uterine leiomyomas (ULs)-benign tumours of the uterine wall, also known as uterine fibroids-at some point in premenopausal life. ULs can cause excessive bleeding, pain and infertility¹, and are a common cause of hysterectomy². They emerge through at least three distinct genetic drivers: mutations in MED12 or FH, or genomic rearrangement of HMGA2³. Here we created genome-wide datasets, using DNA, RNA, assay for transposase-accessible chromatin (ATAC), chromatin immunoprecipitation (ChIP) and HiC chromatin immunoprecipitation (HiChIP) sequencing of primary tissues to profoundly understand the genesis of UL. We identified somatic mutations in genes encoding six members of the SRCAP histone-loading complex⁴, and found that germline mutations in the SRCAP members YEATS4 and ZNHIT1 predispose women to UL. Tumours bearing these mutations showed defective deposition of the histone variant H2A.Z. In ULs, H2A.Z occupancy correlated positively with chromatin accessibility and gene expression, and negatively with DNA methylation, but these correlations were weak in tumours bearing SRCAP complex mutations. In these tumours, open chromatin emerged at transcription start sites where H2A.Z was lost, which was associated with upregulation of genes. Furthermore, YEATS4 defects were associated with abnormal upregulation of bivalent embryonic stem cell genes, as previously shown in mice⁵. Our work describes a potential mechanism of tumorigenesis-epigenetic instability caused by deficient H2A.Z deposition-and suggests that ULs arise through an aberrant differentiation program driven by deranged chromatin, emanating from a small number of mutually exclusive driver mutations.

INO80 promotes H2A.Z occupancy to regulate cell fate transition in pluripotent stem cells

The INO80 chromatin remodeler is involved in many chromatin-dependent cellular functions. However, its role in pluripotency and cell fate transition is not fully defined. We examined the impact of Ino80 deletion in the naïve and primed pluripotent stem cells. We found that Ino80 deletion had minimal effect on self-renewal and gene expression in the naïve state, but led to cellular differentiation and de-repression of developmental genes in the transition toward and maintenance of the primed state. In the naïve state, INO80 pre-marked gene promoters that would adopt bivalent histone modifications by H3K4me3 and H3K27me3 upon transition into the primed state. In the primed state, in contrast to its known role in H2A.Z exchange, INO80 promoted H2A.Z occupancy at these bivalent promoters and facilitated H3K27me3 installation and maintenance as well as downstream gene repression. Together, our results identified an unexpected function of INO80 in H2A.Z deposition and gene regulation. We showed that INO80-dependent H2A.Z occupancy is a critical licensing step for the bivalent domains, and thereby uncovered an epigenetic mechanism by which chromatin remodeling, histone variant deposition and histone modification coordinately control cell fate.

H2A.Z acetylation by lincZNF337-AS1 via KAT5 implicated in the transcriptional misregulation in cancer signaling pathway in hepatocellular carcinoma

In eukaryotes, histones and their variants are essential for chromatin structure and function; both play important roles in the regulation of gene transcription, as well as the development of tumors. We aimed to explore the genomics data of hepatocellular carcinoma (HCC), combined with literature analysis, in terms of the histone variant H2A.Z. Cell phenotype assay confirmed the effect of H2A.Z on the proliferation, metastasis, apoptosis, and cell cycle of HCC cells. H2A.Z was shown to function via the tumor dysregulation signaling pathway, with BCL6 as its interacting protein. In addition, the acetylation level of H2A.Z was higher in HCC and was related to tumor formation. We found the acetylation of H2A.Z to be related to and regulated by lincZNF337-AS1. LincZNF337-AS1 was found to bind to H2A.Z and KAT5 at different sites, promoting the acetylation of H2A.Z through KAT5. We concluded that, in HCC, H2A.Z is an oncogene, whose acetylation promotes the transcription of downstream genes, and is regulated by lincZNF331-AS1.

Crosstalk between H2A variant-specific modifications impacts vital cell functions

Selection of C-terminal motifs participated in evolution of distinct histone H2A variants. Hybrid types of variants combining motifs from distinct H2A classes are extremely rare. This suggests that the proximity between the motif cases interferes with their function. We studied this question in flowering plants that evolved sporadically a hybrid H2A variant combining the SQ motif of H2A.X that participates in the DNA damage response with the KSPK motif of H2A.W that stabilizes heterochromatin. Our inventory of PTMs of H2A.W variants showed that in vivo the cell cycle-dependent kinase CDKA phosphorylates the KSPK motif of H2A.W but only in absence of an SQ motif. Phosphomimicry of KSPK prevented DNA damage response by the SQ motif of the hybrid H2A.W/X variant. In a synthetic yeast expressing the hybrid H2A.W/X variant, phosphorylation of KSPK prevented binding of the BRCT-domain protein Mdb1 to phosphorylated SQ and impaired response to DNA damage. Our findings illustrate that PTMs mediate interference between the function of H2A variant specific C-terminal motifs. Such interference could explain the mutual exclusion of motifs that led to evolution of H2A variants.

Control of Genome through Variative Nature of Histone-Modifying Ubiquitin Ligases

Covalent attachment of ubiquitin residue is not only the proteasomal degradation signal, but also a widespread posttranslational modification of cellular proteins in eukaryotes. One of the most important targets of the regulatory ubiquitination are histones. Localization of ubiquitin residue in different regions of the nucleosome attracts a strictly determined set of cellular factors with varied functionality. Depending on the type of histone and the particular lysine residue undergoing modification, histone ubiquitination can lead both to transcription activation and to gene repression, as well as contribute to DNA repair via different mechanisms. An extremely interesting feature of the family of RING E3 ubiquitin ligases catalyzing histone ubiquitination is the striking structural diversity of the domains providing high specificity of modification very similar initial targets. It is obvious that further elucidation of peculiarities of the ubiquitination system involved in histone modification, as well as understanding of physiological role of this process in the maintenance of homeostasis of both single cells and the entire organism, will substantially expand the possibilities of treating a number of socially significant diseases.

A dual role for H2A.Z.1 in modulating the dynamics of RNA polymerase II initiation and elongation

RNA polymerase II (RNAPII) pausing immediately downstream of the transcription start site is a critical rate-limiting step for the expression of most metazoan genes. During pause release, RNAPII encounters a highly conserved +1 H2A.Z nucleosome, yet how this histone variant contributes to transcription is poorly understood. Here, using an inducible protein degron system combined with genomic approaches and live cell super-resolution microscopy, we show that H2A.Z.1 modulates RNAPII dynamics across most genes in murine embryonic stem cells. Our quantitative analysis shows that H2A.Z.1 slows the rate of RNAPII pause release and consequently impacts negative elongation factor dynamics as well as nascent transcription. Consequently, H2A.Z.1 also impacts re-loading of the pre-initiation complex components TFIIB and TBP. Altogether, this work provides a critical mechanistic link between H2A.Z.1 and the proper induction of mammalian gene expression programs through the regulation of RNAPII dynamics and pause release.

Solid tumours hijack the histone variant network

Cancer is a complex disease characterized by loss of cellular homeostasis through genetic and epigenetic alterations. Emerging evidence highlights a role for histone variants and their dedicated chaperones in cancer initiation and progression. Histone variants are involved in processes as diverse as maintenance of genome integrity, nuclear architecture and cell identity. On a molecular level, histone variants add a layer of complexity to the dynamic regulation of transcription, DNA replication and repair, and mitotic chromosome segregation. Because these functions are critical to ensure normal proliferation and maintenance of cellular fate, cancer cells are defined by their capacity to subvert them. Hijacking histone variants and their chaperones is emerging as a common means to disrupt homeostasis across a wide range of cancers, particularly solid tumours. Here we discuss histone variants and histone chaperones as tumour-promoting or tumour-suppressive players in the pathogenesis of cancer.

An interpretable bimodal neural network characterizes the sequence and preexisting chromatin predictors of induced transcription factor binding

BACKGROUND

Transcription factor (TF) binding specificity is determined via a complex interplay between the transcription factor's DNA binding preference and cell type-specific chromatin environments. The chromatin features that correlate with transcription factor binding in a given cell type have been well characterized. For instance, the binding sites for a majority of transcription factors display concurrent chromatin accessibility. However, concurrent chromatin features reflect the binding activities of the transcription factor itself and thus provide limited insight into how genome-wide TF-DNA binding patterns became established in the first place. To understand the determinants of transcription factor binding specificity, we therefore need to examine how newly activated transcription factors interact with sequence and preexisting chromatin landscapes.

RESULTS

Here, we investigate the sequence and preexisting chromatin predictors of TF-DNA binding by examining the genome-wide occupancy of transcription factors that have been induced in well-characterized chromatin environments. We develop Bichrom, a bimodal neural network that jointly models sequence and preexisting chromatin data to interpret the genome-wide binding patterns of induced transcription factors. We find that the preexisting chromatin landscape is a differential global predictor of TF-DNA binding; incorporating preexisting chromatin features improves our ability to explain the binding specificity of some transcription factors substantially, but not others. Furthermore, by analyzing site-level predictors, we show that transcription factor binding in previously inaccessible chromatin tends to correspond to the presence of more favorable cognate DNA sequences.

CONCLUSIONS

Bichrom thus provides a framework for modeling, interpreting, and visualizing the joint sequence and chromatin landscapes that determine TF-DNA binding dynamics.

Genome-wide chromatin accessibility is restricted by ANP32E

Genome-wide chromatin state underlies gene expression potential and cellular function. Epigenetic features and nucleosome positioning contribute to the accessibility of DNA, but widespread regulators of chromatin state are largely unknown. Our study investigates how coordination of ANP32E and H2A.Z contributes to genome-wide chromatin state in mouse fibroblasts. We define H2A.Z as a universal chromatin accessibility factor, and demonstrate that ANP32E antagonizes H2A.Z accumulation to restrict chromatin accessibility genome-wide. In the absence of ANP32E, H2A.Z accumulates at promoters in a hierarchical manner. H2A.Z initially localizes downstream of the transcription start site, and if H2A.Z is already present downstream, additional H2A.Z accumulates upstream. This hierarchical H2A.Z accumulation coincides with improved nucleosome positioning, heightened transcription factor binding, and increased expression of neighboring genes. Thus, ANP32E dramatically influences genome-wide chromatin accessibility through subtle refinement of H2A.Z patterns, providing a means to reprogram chromatin state and to hone gene expression levels.

Chromatin state underlies cellular function, and transcription factor binding patterns along with epigenetic marks define chromatin state. Here the authors show that the histone chaperone ANP32E functions through regulation of H2A.Z to restrict genome-wide chromatin accessibility and to inhibit gene transcriptional activation.

Lysine acetyltransferase Tip60 is required for hematopoietic stem cell maintenance

Hematopoietic stem cells (HSCs) have the potential to replenish the blood system for the lifetime of the organism. Their 2 defining properties, self-renewal and differentiation, are tightly regulated by the epigenetic machineries. Using conditional gene-knockout models, we demonstrated a critical requirement of lysine acetyltransferase 5 (Kat5, also known as Tip60) for murine HSC maintenance in both the embryonic and adult stages, which depends on its acetyltransferase activity. Genome-wide chromatin and transcriptome profiling in murine hematopoietic stem and progenitor cells revealed that Tip60 colocalizes with c-Myc and that Tip60 deletion suppress the expression of Myc target genes, which are associated with critical biological processes for HSC maintenance, cell cycling, and DNA repair. Notably, acetylated H2A.Z (acH2A.Z) was enriched at the Tip60-bound active chromatin, and Tip60 deletion induced a robust reduction in the acH2A.Z/H2A.Z ratio. These results uncover a critical epigenetic regulatory layer for HSC maintenance, at least in part through Tip60-dependent H2A.Z acetylation to activate Myc target genes.

Effect of H2A.Z deletion is rescued by compensatory mutations in Fusarium graminearum

Fusarium head blight is a destructive disease of grains resulting in reduced yields and contamination of grains with mycotoxins worldwide; Fusarium graminearum is its major causal agent. Chromatin structure changes play key roles in regulating mycotoxin biosynthesis in filamentous fungi. Using a split-marker approach in three F. graminearum strains INRA156, INRA349 and INRA812 (PH-1), we knocked out the gene encoding H2A.Z, a ubiquitous histone variant reported to be involved in a diverse range of biological processes in yeast, plants and animals, but rarely studied in filamentous fungi. All ΔH2A.Z mutants exhibit defects in development including radial growth, sporulation, germination and sexual reproduction, but with varying degrees of severity between them. Heterogeneity of osmotic and oxidative stress response as well as mycotoxin production was observed in ΔH2A.Z strains. Adding-back wild-type H2A.Z in INRA349ΔH2A.Z could not rescue the phenotypes. Whole genome sequencing revealed that, although H2A.Z has been removed from the genome and the deletion cassette is inserted at H2A.Z locus only, mutations occur at other loci in each mutant regardless of the genetic background. Genes affected by these mutations encode proteins involved in chromatin remodeling, such as the helicase Swr1p or an essential subunit of the histone deacetylase Rpd3S, and one protein of unknown function. These observations suggest that H2A.Z and the genes affected by such mutations are part or the same genetic interaction network. Our results underline the genetic plasticity of F. graminearum facing detrimental gene perturbation. These findings suggest that intergenic suppressions rescue deleterious phenotypes in ΔH2A.Z strains, and that H2A.Z may be essential in F. graminearum. This assumption is further supported by the fact that H2A.Z deletion failed in another Fusarium spp., i.e., the rice pathogen Fusarium fujikuroi.

Establishment and function of chromatin modification at enhancers

How a single genome can give rise to distinct cell types remains a fundamental question in biology. Mammals are able to specify and maintain hundreds of cell fates by selectively activating unique subsets of their genome. This is achieved, in part, by enhancers-genetic elements that can increase transcription of both nearby and distal genes. Enhancers can be identified by their unique chromatin signature, including transcription factor binding and the enrichment of specific histone post-translational modifications, histone variants, and chromatin-associated cofactors. How each of these chromatin features contributes to enhancer function remains an area of intense study. In this review, we provide an overview of enhancer-associated chromatin states, and the proteins and enzymes involved in their establishment. We discuss recent insights into the effects of the enhancer chromatin state on ongoing transcription versus their role in the establishment of new transcription programmes, such as those that occur developmentally. Finally, we highlight the role of enhancer chromatin in new conceptual advances in gene regulation such as condensate formation.

Normal Patterns of Histone H3K27 Methylation Require the Histone Variant H2A.Z in Neurospora crassa

Neurospora crassa contains a minimal Polycomb repression system, which provides rich opportunities to explore Polycomb-mediated repression across eukaryotes and enables genetic studies that can be difficult in plant and animal systems. Polycomb Repressive Complex 2 is a multi-subunit complex that deposits mono-, di-, and trimethyl groups on lysine 27 of histone H3, and trimethyl H3K27 is a molecular marker of transcriptionally repressed facultative heterochromatin. In mouse embryonic stem cells and multiple plant species, H2A.Z has been found to be colocalized with H3K27 methylation. H2A.Z is required for normal H3K27 methylation in these experimental systems, though the regulatory mechanisms are not well understood. We report here that Neurospora crassa mutants lacking H2A.Z or SWR-1, the ATP-dependent histone variant exchanger, exhibit a striking reduction in levels of H3K27 methylation. RNA-sequencing revealed downregulation of eed, encoding a subunit of PRC2, in an hH2Az mutant compared to wild type, and overexpression of EED in a ΔhH2Az;Δeed background restored most H3K27 methylation. Reduced eed expression leads to region-specific losses of H3K27 methylation, suggesting that differential dependence on EED concentration is critical for normal H3K27 methylation at certain regions in the genome.

The roles of histone variants in fine-tuning chromatin organization and function

Histones serve to both package and organize DNA within the nucleus. In addition to histone post-translational modification and chromatin remodelling complexes, histone variants contribute to the complexity of epigenetic regulation of the genome. Histone variants are characterized by a distinct protein sequence and a selection of designated chaperone systems and chromatin remodelling complexes that regulate their localization in the genome. In addition, histone variants can be enriched with specific post-translational modifications, which in turn can provide a scaffold for recruitment of variant-specific interacting proteins to chromatin. Thus, through these properties, histone variants have the capacity to endow specific regions of chromatin with unique character and function in a regulated manner. In this Review, we provide an overview of recent advances in our understanding of the contribution of histone variants to chromatin function in mammalian systems. First, we discuss new molecular insights into chaperone-mediated histone variant deposition. Next, we discuss mechanisms by which histone variants influence chromatin properties such as nucleosome stability and the local chromatin environment both through histone variant sequence-specific effects and through their role in recruiting different chromatin-associated complexes. Finally, we focus on histone variant function in the context of both embryonic development and human disease, specifically developmental syndromes and cancer.

Premature Activation of Immune Transcription Programs in Autoimmune-Predisposed Mouse Embryonic Stem Cells and Blastocysts

Autoimmune diabetes is a complex multifactorial disease with genetic and environmental factors playing pivotal roles. While many genes associated with the risk of diabetes have been identified to date, the mechanisms by which external triggers contribute to the genetic predisposition remain unclear. Here, we derived embryonic stem (ES) cell lines from diabetes-prone non-obese diabetic (NOD) and healthy C57BL/6 (B6) mice. While overall pluripotency markers were indistinguishable between newly derived NOD and B6 ES cells, we discovered several differentially expressed genes that normally are not expressed in ES cells. Several genes that reside in previously identified insulin-dependent diabetics (Idd) genomic regions were up-regulated in NOD ES cells. Gene set enrichment analysis showed that different groups of genes associated with immune functions are differentially expressed in NOD. Transcriptomic analysis of NOD blastocysts validated several differentially overexpressed Idd genes compared to B6. Genome-wide mapping of active histone modifications using ChIP-Seq supports active expression as the promoters and enhancers of activated genes are also marked by active histone modifications. We have also found that NOD ES cells secrete more inflammatory cytokines. Our data suggest that the known genetic predisposition of NOD to autoimmune diabetes leads to epigenetic instability of several Idd regions.

Epigenetic priming by Dppa2 and 4 in pluripotency facilitates multi-lineage commitment

How the epigenetic landscape is established in development is still being elucidated. Here, we uncover developmental pluripotency associated 2 and 4 (DPPA2/4) as epigenetic priming factors that establish a permissive epigenetic landscape at a subset of developmentally important bivalent promoters characterized by low expression and poised RNA-polymerase. Differentiation assays reveal that Dppa2/4 double knockout mouse embryonic stem cells fail to exit pluripotency and differentiate efficiently. DPPA2/4 bind both H3K4me3-marked and bivalent gene promoters and associate with COMPASS- and Polycomb-bound chromatin. Comparing knockout and inducible knockdown systems, we find that acute depletion of DPPA2/4 results in rapid loss of H3K4me3 from key bivalent genes, while H3K27me3 is initially more stable but lost following extended culture. Consequently, upon DPPA2/4 depletion, these promoters gain DNA methylation and are unable to be activated upon differentiation. Our findings uncover a novel epigenetic priming mechanism at developmental promoters, poising them for future lineage-specific activation.

RUVBL1 is an amplified epigenetic factor promoting proliferation and inhibiting differentiation program in head and neck squamous cancers

Mutations in histone modifying enzymes and histone variants were identified in multiple cancers in The Cancer Genome Atlas (TCGA) studies. However, very little progress and understanding has been made in identifying the contribution of epigenetic factors in head and neck squamous cell carcinoma (HNSCC). Here, we report the identification of RUVBL1 (TIP49a), a component of the TIP60 histone modifying complex as being amplified and overexpressed in HNSCC. RUVBL1 plays a key role in incorporating histone variant H2AZ in chromatin thereby regulating transcription of key genes involved in differentiation, cancer cell proliferation and invasion. H2AZ is also overexpressed in HNSCC tumors thereby regulating RUVBL1/H2AZ dependent transcriptional programs. Patient data analysis of multiple cohorts including TCGA and single cell HNSCC data indicated RUVBL1 overexpression as a poor prognostic marker and predicts poor survival. In vitro experiments indicate a pro-proliferative role for RUVBL1/H2AZ in HNSCC cells. RUVBL1 inversely correlates with differentiation program and positively correlates with oncogenic programs, making it a key contributor to tumorigenesis and a vulnerable therapeutic target in HNSCC patients.

Specialized RSC: Substrate Specificities for a Conserved Chromatin Remodeler

The remodel the structure of chromatin (RSC) nucleosome remodeling complex is a conserved chromatin regulator with roles in chromatin organization, especially over nucleosome depleted regions therefore functioning in gene expression. Recent reports in Saccharomyces cerevisiae have identified specificities in RSC activity toward certain types of nucleosomes. RSC has now been shown to preferentially evict nucleosomes containing the histone variant H2A.Z in vitro. Furthermore, biochemical activities of distinct RSC complexes has been found to differ when their nucleosome substrate is partially unraveled. Mammalian BAF complexes, the homologs of yeast RSC and SWI/SNF complexes, are also linked to nucleosomes with H2A.Z, but this relationship may be complex and extent of conservation remains to be determined. The interplay of remodelers with specific nucleosome substrates and regulation of remodeler outcomes by nucleosome composition are tantalizing questions given the wave of structural data emerging for RSC and other SWI/SNF family remodelers.

H2A.Z and chromatin remodelling complexes: a focus on fungi

Chromatin is a highly dynamic structure that closely relates with gene expression in eukaryotes. ATP-dependent chromatin remodelling, histone post-translational modification and DNA methylation are the main ways that mediate such plasticity. The histone variant H2A.Z is frequently encountered in eukaryotes, and can be deposited or removed from nucleosomes by chromatin remodelling complex SWR1 or INO80, respectively, leading to altered chromatin state. H2A.Z has been found to be involved in a diverse range of biological processes, including genome stability, DNA repair and transcriptional regulation. Due to their formidable production of secondary metabolites, filamentous fungi play outstanding roles in pharmaceutical production, food safety and agriculture. During the last few years, chromatin structural changes were proven to be a key factor associated with secondary metabolism in fungi. However, studies on the function of H2A.Z are scarce. Here, we summarize current knowledge of H2A.Z functions with a focus on filamentous fungi. We propose that H2A.Z is a potential target involved in the regulation of secondary metabolite biosynthesis by fungi.

Prediabetes Induced by a Single Autoimmune B Cell Clone

While B cells play a significant role in the onset of type-1 diabetes (T1D), little is know about their role in those early stages. Thus, to gain new insights into the role of B cells in T1D, we converted a physiological early pancreas-infiltrating B cell into a novel BCR mouse model using Somatic Cell Nuclear Transfer (SCNT). Strikingly, SCNT-derived B1411 model displayed neither developmental block nor anergy. Instead, B1411 underwent spontaneous germinal center reactions. Without T cell help, B1411-Rag1^−/− was capable of forming peri-/intra-pancreatic lymph nodes, and undergoing class-switching. RNA-Seq analysis identified 93 differentially expressed genes in B1411 compared to WT B cells, including Irf7, Usp18, and Mda5 that had been linked to a potential viral etiology of T1D. We also found various members of the oligoadenylate synthase (OAS) family to be enriched in B1411, such as Oas1, which had recently also been linked to T1D. Strikingly, when challenged with glucose B1411-Rag1^−/− mice displayed impaired glucose tolerance.

Single Amino Acid Change Underlies Distinct Roles of H2A.Z Subtypes in Human Syndrome

The developmental disorder Floating-Harbor syndrome (FHS) is caused by heterozygous truncating mutations in SRCAP, a gene encoding a chromatin remodeler mediating incorporation of histone variant H2A.Z. Here, we demonstrate that FHS-associated mutations result in loss of SRCAP nuclear localization, alter neural crest gene programs in human in vitro models and Xenopus embryos, and cause craniofacial defects. These defects are mediated by one of two H2A.Z subtypes, H2A.Z.2, whose knockdown mimics and whose overexpression rescues the FHS phenotype. Selective rescue by H2A.Z.2 is conferred by one of the three amino acid differences between the H2A.Z subtypes, S38/T38. We further show that H2A.Z.1 and H2A.Z.2 genomic occupancy patterns are qualitatively similar, but quantitatively distinct, and H2A.Z.2 incorporation at AT-rich enhancers and expression of their associated genes are both sensitized to SRCAP truncations. Altogether, our results illuminate the mechanism underlying a human syndrome and uncover selective functions of H2A.Z subtypes during development.