Histone PTMs Research Articles

A Live-Cell Epigenome Manipulation by Photo-Stimuli-Responsive Histone Methyltransferase Inhibitor.

Post-translational modifications on the histone H3 tail regulate chromatin structure, impact epigenetics, and hence the gene expressions. Current chemical modulation tools, such as unnatural amino acid incorporation, protein splicing, and sortase-based editing, have allowed for the modification of histones with various PTMs in cellular contexts, but are not applicable for editing native chromatin. The use of small organic molecules to manipulate histone-modifying enzymes alters endogenous histone PTMs but lacks precise temporal and spatial control. To date, there has been no achievement in modulating histone methylation in living cells with spatiotemporal resolution. In this study, a new method is presented for temporally manipulating histone dimethylation H3K9me2 using a photo-responsive inhibitor that specifically targets the methyltransferase G9a on demand. The photo-caged molecule is stable under physiological conditions and cellular environments, but rapidly activated upon exposure to light, releasing the bioactive component that can immediately inhibit the catalytic ability of the G9a in vitro. Besides, this masked compound could also efficiently reactivate the inhibition of methyltransferase activity in living cells, subsequently suppress H3K9me2, a mark that regulates various chromatin functions. Therefore, the chemical system will be a valuable tool for manipulating the epigenome for therapeutic purposes and furthering the understanding of epigenetic mechanisms.

Single cell proteomics by mass spectrometry reveals deep epigenetic insight into the actions of an orphan histone deacetylase inhibitor.

Epigenetic programming has been shown to play a role in nearly every human system and disease where anyone has thought to look. However, the levels of heterogeneity at which epigenetic or epiproteomic modifications occur at single cell resolution across a population remains elusive. While recent advances in sequencing technology have allowed between 1 and 3 histone post-translational modifications to be analyzed in each single cell, over twenty separate chemical PTMs are known to exist, allowing thousands of possible combinations. Single cell proteomics by mass spectrometry (SCP) is an emerging technology in which hundreds or thousands of proteins can be directly quantified in typical human cells. As the proteins detected and quantified by SCP are heavily biased toward proteins of highest abundance, chromatin proteins are an attractive target for analysis. To this end, I applied SCP to the analysis of cancer cells treated with mocetinostat, a class specific histone deacetylase inhibitor. I find that 16 PTMs can be confidently identified and localized with high site specificity in single cells. In addition, the high abundance of histone proteins allows higher throughput methods to be utilized for SCP than ever described. While quantitative accuracy suffers when analyzing more than 700 cells per day, 9 histone proteins can be measured in single cells analyzed at even 3,500 cells per day, a throughput 10-fold greater than any previous report. In addition, the unbiased global approach utilized herein identifies a previously uncharacterized response to this drug through the S100-A8/S100-A9 protein complex partners. This response is observed in nearly every cell of the over 1,000 analyzed in this study, regardless of the relative throughput of the method utilized. While limitations exist in the methods described herein, current technologies can easily improve upon the results presented here to allow comprehensive analysis of histone PTMs to be performed in any mass spectrometry lab. All raw and processed data described in this study has been made publicly available through the ProteomeXchange/MASSIVE repository system as MSV000093434.

MDB-08. MYC AND TGFΒ PROMOTE GROUP 3 MEDULLOBLASTOMA TUMOR RESISTANCE THROUGH DEREGULATION OF POLYCOMB TARGETS

Abstract Medulloblastoma (MB) is one of the most prevalent malignant brain tumors in children, with tremendous cognitive and neuroendocrine disability among survivors. Group 3 (G3) MBs have poor overall survival at <50%, higher frequency of metastasis, and no targeted therapies. Amplification of MYC and activation of TGFβ signaling are frequent in G3MB. Remarkably, some tumors have no reported mutations, suggesting roles for epigenetic mechanisms in driving disease. We have generated new humanized models for G3MB by transforming neuroepithelial stem cells (NESC) derived from human induced pluripotent stem cells with TGFβ effectors alone and/or in combination with MYC, prioritizing combinations observed in patients. Excitingly, both MYC and TGFβ effectors drove tumor formation in vivo with the combination of TGFβ effectors and MYC leading to more aggressive tumors. Moreover, NESC-derived tumors clustered with human G3/G4MB samples, indicating our models recapitulates human disease. We next found that NESCs expressing MYC with either TGFβR1 or TGFβ1 showed resistance to clinical TGFβR1 inhibitors, compared to cells driven by either TGFβR1 or TGFβ1 alone. To decipher mechanisms of resistance, we integrated CUT&RUN to probe for MYC genomic localization and relevant histone PTMs with RNA-seq analysis and discovered a subset of genes upregulated in MYC and TGFβ-driven lines that are targets of the histone demethylase KDM2B and Polycomb Repressive Complex 2 (PRC2) member SUZ12. Moreover, we found that MYC is bound to regions normally regulated by PRC interactors. We postulate that chromatin remodeling via MYC and recruitment of other MYC-interacting cofactors to PRC targets culminates in transcriptional changes that lead to aggressive disease. Work from others suggests that PRC-mediated chromatin regulation is critical in many adult and pediatric brain tumors, underscoring its significance in MB. Our studies provide important insight on identifying new therapeutic avenues with a focus on the epigenetic landscape for patients with MYC and TGFβ driven G3MB.

Abstract 4293: Chromosomal instability causes epigenetic aberrations in cancer

Abstract Chromosomal instability and changes in the epigenome are defining features in most metastatic cancers. Our research links chromosomal missegregation, their containment in micronuclei, and subsequent rupture of the micronuclear envelope to histone post-translational modification aberrations. This relationship was across all tested human and murine cancer cell lines, as well as non-transformed cells. Furthermore, we discovered that the observed changes in histone PTMs were caused by either micronuclear rupture or the persistence of histone PTM status during cell division. Moreover, we revealed substantial chromatin accessibility differences in micronuclei. Notably, there is a distinct bias in chromatin accessibility between promoter regions compared to distal and intergenic regions of the genome, which aligned well with observed changes in histone modifications. The induction of chromosomal instability leads to widespread, heterogeneous, and heritable abnormalities in the epigenetic landscape of cancer cells. Therefore, on top of genomic copy number changes, chromosomal instability causes epigenetic reprogramming, which adds another layer to cancer heterogeneity. Citation Format: Albert S. Agustinus, Duaa Al-Rawi, Bhargavi Dameracharla, Ramya Raviram, Bailey S. Jones, Stephanie Stransky, Lorenzo Scipioni, Jens Luebeck, Melody Di Bona, Danguole Norkunaite, Robert M. Myers, Mercedes Duran, Seongmin Choi, Britta Weigelt, Shira Yomtoubian, Andrew McPherson, Eleonore Toufektchan, Kristina Keuper, Paul S. Mischel, Vivek Mittal, Sohrab P. Shah, John Maciejowski, Zuzana Storchova, Enrico Gratton, Peter Ly, Dan Landau, Mathieu F. Bakhoum, Richard P. Koche, Simone Sidoli, Vineet Bafna, Yael David, Samuel F. Bakhoum. Chromosomal instability causes epigenetic aberrations in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4293.

Abstract 2848: MYC and TGFβ promote group 3 medulloblastoma tumor resistance through deregulation of Polycomb targets

Abstract Medulloblastoma (MB) is one of the most prevalent malignant brain tumors in children, with tremendous cognitive and neuroendocrine disability among survivors. Group 3 (G3) MBs have poor overall survival at <50%, few recurrent mutations, higher frequency of metastasis, and no targeted therapies. Amplification of MYC and activation of TGFβ signaling are frequent in G3MB. Remarkably, some MB tumors have no reported mutations, suggesting roles for epigenetic mechanisms in driving disease. We hypothesize that the TGFβ pathway and MYC contribute to the intrinsic resistance of G3MB through deregulation of key genes and pathways. We previously established humanized models for SHHMB by introducing MYCN or PTCH1 deletions into neuroepithelial stem cells (NESC) derived from normal human induced pluripotent stem cells (hIPSCs). In this study, we transduced NESCs with TGFb effectors activated in G3MB alone and/or in combination with MYC, prioritizing combinations observed in patients. Excitingly, both MYC and TGFβ effectors drove tumor formation in vivo with the combination of TGFβ effectors with MYC leading to more aggressive tumors. Moreover, they clustered with human G3/G4MB samples, indicating they phenocopy the relevant tumor of interest. We next found that NESCs expressing MYC with either TGFβR1 or TGFβ1 showed resistance to clinical TGFβR1 inhibitors, compared to cells driven by either TGFβR1 or TGFβ1 alone. To decipher mechanisms of resistance, we integrated CUT&RUN to probe for MYC genomic localization and relevant histone PTMs with RNA-seq analysis and discovered a subset of genes upregulated in MYC and TGFb-driven lines that are targets of the histone demethylase KDM2B. Loss of function mutations in KDM2B occur in G3MB patients, indicating that KDM2B deregulated genes are critical in G3MB. We postulate that epigenetic remodeling via MYC and recruitment of other MYC-interacting cofactors to PRC targets culminates in transcriptional changes that lead to aggressive disease. Work from others suggests that PRC-mediated chromatin regulation is critical in many adult and pediatric brain tumors. Overall, our studies provide important insight on identifying new therapeutic avenues for patients with MYC and TGFβ driven G3MB. Citation Format: Zulekha A. Qadeer, Samanta Westelman, Mackenzie O. Johnson, Elise Hou, Shane Grele, Kyle Smith, Liam Hendrikse, Linyu Wang, Sarah Husain, Mary Clare Beytagh, Christin Schmidt, Miller Huang, Michael D. Taylor, Paul Northcott, William A. Weiss. MYC and TGFβ promote group 3 medulloblastoma tumor resistance through deregulation of Polycomb targets [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2848.

OR16-02 Histone ADP-Ribosylation Site Mutations As Oncogenic Drivers In Breast And Ovarian Cancers

Abstract Disclosure: D. Huang: None. C.V. Camacho: None. A. Nagari: None. W.L. Kraus: Other; Self; - Founder and stockholder for Ribon Therapeutics, Inc. - Founder, stockholder, scientific advisory board member, member of the board of directors for ARase Therapeutics, Inc. - Coholder of U.S. Patent. Recent studies have identified oncohistones, which are encoded by histone genes that harbor missense mutations, as a new class of cancer drivers. We found that many of the oncohistone mutations identified with the highest frequencies in human cancers occur on glutamate (Glu, E) and aspartate (Asp, D) residues, which are potential sites of ADP-ribosylation (ADPRylation). ADPRylation is a post-translational modification, which is catalyzed by the PARP family of ADP-ribosyl transferases and results in the covalent attachment of ADP-ribose on protein substrates. Unlike other well characterized histone modifications, the functions of histone ADPRylation are poorly characterized. Mass spectrometry analyses confirmed that 20 Glu or Asp residues subjected to oncohistone mutation are bona fide sites of ADPRylation by PARP-1. Ectopic expression of each of six different oncohistone mutants from three different core histones (H2B, H3, and H4) altered the proliferation of at least 2 out of 3 cancer cell lines, including MCF-7 and MDA-MB-231 breast cancer cells, as well as OVCAR-3 ovarian cancer cells. We focused on H2B-D51, whose mutation resulted in enhanced cell proliferation in both of the breast cancer cell lines for further characterization of its potential transcription regulatory effects. Proteomic (histone PTMs) and genomic (ATAC-Seq, ChIP-Seq and RNA-Seq) analyses revealed that loss of ADPRylation on H2B-D51 by mutation to alanine (Ala, A) or asparagine (Asn, N) dramatically enhanced p300-mediated acetylation of H2B, altered chromatin accessibility, and changed the pattern of gene expression in MDA-MB-231 cells. To determine the effects of H2B-D51 ADPRylation on cancer cell growth in vivo and their responses to the FDA-approved PARP inhibitor Niraparib, we performed xenograft tumor growth assays in immunocompromised mice using MDA-MB-231 cells ectopically expressing wild-type or D51A mutant histone H2B. Ectopic expression of the H2B-D51A mutant led to a significant increase in tumor growth versus wild-type H2B. Interestingly, these effects were inhibited by treatment with Niraparib, suggesting that the H2B-D51A mutant alone does not confer resistance to PARP inhibitor. Taken together, this study demonstrates that functional sites of Glu/Asp ADPRylation on histones are mutated in cancers, allowing cancer cells to escape the growth-regulating effects of ADPRylation. (This work was supported by grants from the NIH/NIDDK and CPRIT, and funds from the Cecil H. and Ida Green Center for Reproductive Biology Sciences Endowment to W.L.K.) Presentation: Friday, June 16, 2023

MDB-07. MYC AND TGFΒ PROMOTE GROUP 3 MEDULLOBLASTOMA TUMOR RESISTANCE THROUGH DEREGULATION OF KDM2B TARGETS

Abstract Medulloblastoma (MB) is one of the most prevalent malignant brain tumors in children, with tremendous cognitive and neuroendocrine disability among survivors. Group 3 (G3) MBs have poor overall survival at <50%, few recurrent mutations, higher frequency of metastasis, and no targeted therapies. Amplification of MYC and activation of TGFβ signaling are frequent in G3MB. Remarkably, some MB tumors have no reported mutations, suggesting roles for epigenetic mechanisms in driving disease. We hypothesize that the TGFβ pathway and MYC contribute to the intrinsic resistance of G3MB through deregulation of key genes and pathways. We previously established humanized models for SHHMB by introducing MYCN or PTCH1 deletions into neuroepithelial stem cells (NESC) derived from normal human induced pluripotent stem cells (hIPSCs). In this study, we transduced NESCs with TGFb effectors activated in G3MB alone and/or in combination with MYC, prioritizing combinations observed in patients. Excitingly, both MYC and TGFβ effectors drove tumor formation in vivo with the combination of TGFβ effectors with MYC leading to more aggressive tumors. We next found that NESCs expressing MYC with either TGFβR1 or TGFβ1 showed resistance to clinical TGFβR1 inhibitors, compared to cells driven by either TGFβR1 or TGFβ1 alone. To decipher mechanisms of resistance, we integrated CUT&RUN to probe for MYC genomic localization and relevant histone PTMs with RNA-seq analysis and discovered a subset of genes upregulated in MYC and TGFb-driven lines that are targets of the histone demethylase KDM2B. Loss of function mutations in KDM2B occur in G3MB patients, indicating that KDM2B deregulated genes are critical in G3MB. We postulate that epigenetic remodeling via MYC and recruitment of other MYC-interacting cofactors to KDM2B targets culminates in transcriptional changes that lead to aggressive disease. Overall, our studies provide important insight on identifying new therapeutic avenues for patients with MYC and TGFβ driven G3MB.

Abstract PR002: Insights into the histone code recognition by the ATAD2B bromodomain

Abstract The ATPase family, AAA domain-containing protein 2 (ATAD2/ANCCA), is an AAA nuclear co-regulator protein containing two ATPase domains and a bromodomain. Its closely related paralog, ATAD2B (KIAA1240), also contains two ATPase domains and a bromodomain. The AAA ATPase domains are broadly associated with ATP-driven molecular remodeling reactions while the bromodomains are evolutionarily conserved, chromatin ‘reader’ domains known to regulate gene expression by binding to acetyllysine on histones. Extensive studies on ATAD2 discuss its upregulation and correlation with poor prognosis in various cancers. Despite sharing high structural and sequence similarity with ATAD2, ATAD2B remains a poorly understood paralog. While previous immunostaining studies detected high expression levels of ATAD2B in breast carcinoma, there is minimal information on its role in oncogenesis. Previously, we have shown that the ATAD2B BRD can recognize mono- and di-acetyllysine modifications on N-terminal tails of histones H4 and H2A. However, since initiation and progression of cancer are characterized by multivalent histone marks, our goal is to understand how crosstalk between combinatorial histone modifications affects the acetyllysine recognition activity of the ATAD2B BRD. We hypothesize that the bromodomain recognizes the acetyllysine mark on multivalent histones and interacts with the chromatin and the ATPase domain. We expect that our results will explain the function of the ATAD2B bromodomain in the context of the overall role of ATAD2B in normal biological and cellular processes. Since bromodomains are excellent drug targets, blocking their activity will also suggest how to disrupt their possible role in oncogenesis. In this study, we have combined various innovative and interdisciplinary approaches ranging from biochemistry and structural biology to functional genomics and cellular biology to gain insights into how crosstalk between different histone H4 PTMs can modulate the chromatin ‘reader’ activity of the ATAD2B BRD. Binding affinities obtained from ITC indicate that the ATAD2B BRD can recognize the acetyllysine mark amongst multiple histone H4 PTMs. Most of these adjacent PTMs are permissible and show high affinities, while some combinations weaken the acetyllysine interaction. Our novel X-ray structures demonstrate a unique binding mode of ATAD2B BRD compared to ATAD2. While the difference in ligand selectivity could suggest that ATAD2B may regulate different biological functions, our cellular assays, including western blot and functional genomic studies, indicate that ATAD2B is conserved in its ability to bind the chromatin. Finally, we discuss how known ‘onco’ mutations within the histone H4 tail region, occurring at or around sites of PTMs, can affect the ATAD2B BRD - acetyllysine coordination. Overall, our study highlights how crosstalk between various histone PTMs regulates the ‘reader’ domain of ATAD2B and affects its recruitment to the chromatin, thus providing new insights for developing therapeutic interventions for treating various diseases. Citation Format: Margaret Phillips, Kathleen Quinn, Cameron Montgomery, Samuel P. Boyson, Sunsik Chang, Jay C. Nix, Seth E. Frietze, Karen C. Glass. Insights into the histone code recognition by the ATAD2B bromodomain. [abstract]. In: Proceedings of the AACR Special Conference: Cancer Epigenomics; 2022 Oct 6-8; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_2):Abstract nr PR002.

PRION+] States Are Associated with Specific Histone H3 Post-Translational Modification Changes.

Prions are proteins able to take on alternative conformations and propagate them in a self-templating process. In Saccharomyces cerevisiae, prions enable heritable responses to environmental conditions through bet-hedging mechanisms. Hence, [PRION+] states may serve as an atypical form of epigenetic control, producing heritable phenotypic change via protein folding. However, the connections between prion states and the epigenome remain unknown. Do [PRION+] states link to canonical epigenetic channels, such as histone post-translational modifications? Here, we map out the histone H3 modification landscape in the context of the [SWI+] and [PIN+] prion states. [SWI+] is propagated by Swi1, a subunit of the SWI/SNF chromatin remodeling complex, while [PIN+] is propagated by Rnq1, a protein of unknown function. We find [SWI+] yeast display decreases in the levels of H3K36me2 and H3K56ac compared to [swi-] yeast. In contrast, decreases in H3K4me3, H3K36me2, H3K36me3 and H3K79me3 are connected to the [PIN+] state. Curing of the prion state by treatment with guanidine hydrochloride restored histone PTM to [prion-] state levels. We find histone PTMs in the [PRION+] state do not match those in loss-of-function models. Our findings shed light into the link between prion states and histone modifications, revealing novel insight into prion function in yeast.

De novo methylation of histone H3K23 by the methyltransferases EHMT1/GLP and EHMT2/G9a

Epigenetic modifications to histone proteins serve an important role in regulating permissive and repressive chromatin states, but despite the identification of many histone PTMs and their perceived role, the epigenetic writers responsible for generating these chromatin signatures are not fully characterized. Here, we report that the canonical histone H3K9 methyltransferases EHMT1/GLP and EHMT2/G9a are capable of catalyzing methylation of histone H3 lysine 23 (H3K23). Our data show that while both enzymes can mono- and di-methylate H3K23, only EHMT1/GLP can tri-methylate H3K23. We also show that pharmacologic inhibition or genetic ablation of EHMT1/GLP and/or EHMT2/G9a leads to decreased H3K23 methylation in mammalian cells. Taken together, this work identifies H3K23 as a new direct methylation target of EHMT1/GLP and EHMT2/G9a, and highlights the differential activity of these enzymes on H3K23 as a substrate.

DDDR-33. TARGETING TGFΒ PATHWAY DEPENDENCIES IN GROUP 3 MEDULLOBLASTOMA

Abstract Medulloblastoma (MB) is one of the most prevalent malignant brain tumors in children, with tremendous cognitive and neuroendocrine disability among survivors. Group 3 (G3) MBs have poor overall survival at < 50%, few recurrent mutations, higher frequency of metastasis, and no targeted therapies. Amplification of MYC (c-myc) and activation of TGFβ signaling are frequent in G3 MB. We hypothesize that the TGFβ pathway and MYC contribute to the intrinsic resistance of G3 MB through deregulation of key genes and pathways. We previously established humanized models for SHH MB by introducing MYCN or PTCH1 deletions into neuroepithelial stem (NES) cells derived from normal human induced pluripotent stem cells (hIPSCs). In this study, we transduced NES cells with TGFb effectors activated in G3 MB (ACVR2A, TGFbR1, TGFb1, TGFb3, and SMAD5) alone and/or in combination with MYC, prioritizing combinations observed in patients. Excitingly, both MYC and TGFβ effectors drove tumor formation in vivo with the combination of TGFβ effectors with MYC leading to more aggressive tumors. We thus describe six new humanized isogenic models for both non-MYC and MYC driven G3 MB. We next found that NES cells expressing MYC with either TGFβR1 or TGFβ1 showed resistance to clinical TGFβR1 inhibitors, compared to cells driven by either TGFβR1 or TGFβ1 alone. To decipher mechanisms of resistance, we integrated CUT & RUN to probe for MYC genomic localization and relevant histone PTMs with RNA-seq analysis of MYC and TGFβ pathway driven NES cells. We discovered a subset of genes upregulated in MYC and TGFb-driven lines that are targets of the histone demethylase KDM2B. We postulate that epigenetic remodeling via MYC and recruitment of other MYC-interacting cofactors culminates in transcriptional changes that lead to aggressive disease. Overall, our studies provide insights on identifying new therapeutic avenues for patients with MYC and TGFβ driven G3 MB.

Live Cell Synthetic Histone Acetylation by Chemical Catalyst.

Posttranslational modifications (PTMs) of histones, such as lysine acetylation and ubiquitination, regulate chromatin structure and gene expression. In living organisms, histone PTMs are catalyzed by histone-modifying enzymes. Here, we describe an entirely chemical method to introduce histone modifications in living cells without genetic manipulation. The chemical catalyst PEG-LANA-DSSMe activates a thioester acetyl donor, N,S-diacetylcysteamine (NAC-Ac), and promotes regioselective, synthetic histone acetylation at H2BK120 in living cells.

Evaluating timsTOF Pro 2 for the analysis of histone PTMs measured in Global Chromatin Profiling using dia-PASEF and prm-PASEF

Evaluating timsTOF Pro 2 for the analysis of histone PTMs measured in Global Chromatin Profiling using dia-PASEF and prm-PASEF

Post-Translational Modifications of Histones Are Versatile Regulators of Fungal Development and Secondary Metabolism.

Chromatin structure is a major regulator of DNA-associated processes, such as transcription, DNA repair, and replication. Histone post-translational modifications, or PTMs, play a key role on chromatin dynamics. PTMs are involved in a wide range of biological processes in eukaryotes, including fungal species. Their deposition/removal and their underlying functions have been extensively investigated in yeasts but much less in other fungi. Nonetheless, the major role of histone PTMs in regulating primary and secondary metabolisms of filamentous fungi, including human and plant pathogens, has been pinpointed. In this review, an overview of major identified PTMs and their respective functions in fungi is provided, with a focus on filamentous fungi when knowledge is available. To date, most of these studies investigated histone acetylations and methylations, but the development of new methodologies and technologies increasingly allows the wider exploration of other PTMs, such as phosphorylation, ubiquitylation, sumoylation, and acylation. Considering the increasing number of known PTMs and the full range of their possible interactions, investigations of the subsequent Histone Code, i.e., the biological consequence of the combinatorial language of all histone PTMs, from a functional point of view, are exponentially complex. Better knowledge about histone PTMs would make it possible to efficiently fight plant or human contamination, avoid the production of toxic secondary metabolites, or optimize the industrial biosynthesis of certain beneficial compounds.

Chromatin landscape associated with sexual differentiation in a UV sex determination system.

In many eukaryotes, such as dioicous mosses and many algae, sex is determined by UV sex chromosomes and is expressed during the haploid phase of the life cycle. In these species, the male and female developmental programs are initiated by the presence of the U- or V-specific regions of the sex chromosomes but, as in XY and ZW systems, sexual differentiation is largely driven by autosomal sex-biased gene expression. The mechanisms underlying the regulation of sex-biased expression of genes during sexual differentiation remain elusive. Here, we investigated the extent and nature of epigenomic changes associated with UV sexual differentiation in the brown alga Ectocarpus, a model UV system. Six histone modifications were quantified in near-isogenic lines, leading to the identification of 16 chromatin signatures across the genome. Chromatin signatures correlated with levels of gene expression and histone PTMs changes in males versus females occurred preferentially at genes involved in sex-specific pathways. Despite the absence of chromosome scale dosage compensation and the fact that UV sex chromosomes recombine across most of their length, the chromatin landscape of these chromosomes was remarkably different to that of autosomes. Hotspots of evolutionary young genes in the pseudoautosomal regions appear to drive the exceptional chromatin features of UV sex chromosomes.

Single-cell-resolved dynamics of chromatin architecture delineate cell and regulatory states in zebrafish embryos

DNA accessibility of cis-regulatory elements (CREs) dictates transcriptional activity and drives cell differentiation during development. While many genes regulating embryonic development have been identified, the underlying CRE dynamics controlling their expression remain largely uncharacterized. To address this, we produced a multimodal resource and genomic regulatory map for the zebrafish community, which integrates single-cell combinatorial indexing assay for transposase-accessible chromatin with high-throughput sequencing (sci-ATAC-seq) with bulk histone PTMs and Hi-C data to achieve a genome-wide classification of the regulatory architecture determining transcriptional activity in the 24-h post-fertilization (hpf) embryo. We characterized the genome-wide chromatin architecture at bulk and single-cell resolution, applying sci-ATAC-seq on whole 24-hpf stage zebrafish embryos, generating accessibility profiles for ∼23,000 single nuclei. We developed a genome segmentation method, ScregSeg (single-cell regulatory landscape segmentation), for defining regulatory programs, and candidate CREs, specific to one or more cell types. We integrated the ScregSeg output with bulk measurements for histone post-translational modifications and 3D genome organization and identified new regulatory principles between chromatin modalities prevalent during zebrafish development. Sci-ATAC-seq profiling of npas4l/cloche mutant embryos identified novel cellular roles for this hematovascular transcriptional master regulator and suggests an intricate mechanism regulating its expression. Our work defines regulatory architecture and principles in the zebrafish embryo and establishes a resource of cell-type-specific genome-wide regulatory annotations and candidate CREs, providing a valuable open resource for genomics, developmental, molecular, and computational biology.

Chemical biology approaches to study histone interactors.

Protein-protein interactions (PPIs) in the nucleus play key roles in transcriptional regulation and ensure genomic stability. Critical to this are histone-mediated PPI networks, which are further fine-tuned through dynamic post-translational modification. Perturbation to these networks leads to genomic instability and disease, presenting epigenetic proteins as key therapeutic targets. This mini-review will describe progress in mapping the combinatorial histone PTM landscape, and recent chemical biology approaches to map histone interactors. Recent advances in mapping direct interactors of histone PTMs as well as local chromatin interactomes will be highlighted, with a focus on mass-spectrometry based workflows that continue to illuminate histone-mediated PPIs in unprecedented detail.

Integrative Chemical Biology Approaches to Deciphering the Histone Code: A Problem-Driven Journey.

The hereditary blueprint of a eukaryotic cell is encoded in its genomic DNA that is tightly compacted into chromatin together with histone proteins. The basic repeating units of chromatin fibers are nucleosomes, in which approximately 1.7 turns of DNA wrap around a proteinaceous octamer consisting of two copies of histones H2A, H2B, H3, and H4. Histones are extensively decorated by a variety of posttranslational modifications (PTMs, e.g., methylation, acetylation, ubiquitylation, phosphorylation, etc.), serving as one of the cellular mechanisms that regulates DNA-templated processes, including but not limited to gene transcription, DNA replication, and DNA damage repair. Most of the histone PTMs exist in dynamic fluctuations, and their on and off states are exquisitely regulated by enzymes known as "writers" and "erasers", respectively. When installed at certain sites, histone PTMs can change the local physicochemical environment and thereby directly influence the nucleosome and chromatin structures. Alternatively, histone PTMs can recruit effectors (or "readers") to signal the downstream events. A "histone code" hypothesis has been proposed in which the combinatory actions of different histone PTMs orchestrate the epigenetic landscape of cells, modulating the activity of the underlying DNA and maintaining the genome stability between generations. Accumulating evidence also suggests that malfunctions of histone PTMs are associated with the pathogenesis of human diseases, such as cancer. It is therefore important to fully decipher the histone code, namely, to dissect the regulatory mechanisms and biological functions of histone PTMs.Owing to the advances in state-of-the-art mass spectrometry, dozens of novel histone modifications have been archived during the past decade. However, most of these newly identified histone PTMs remain poorly explored. To unravel the roles played by these PTMs in histone code, key questions that have driven our study are (i) how to detect the novel histone PTMs; (ii) how to identify the enzymes that catalyze the addition (writers) and removal (erasers) of the histone PTMs along with the regulating mechanisms; (iii) what is the biological significance of the histone PTMs and how do they function, by affecting the nucleosome and chromatin dynamics or by recruiting readers; and (iv) how to develop chemical probes to interrogate the histone PTMs or even serve as potential leads for the drug discovery campaigns to treat diseases caused by abnormalities in the regulation of histone PTMs.This Account focuses on our efforts in developing and applying chemical tools and methods to answer the above questions. Specifically, we review the detection of negatively charged histone acylations by developing and applying chemical reporters; preparing homogeneous nucleosomes carrying negatively charged acylations by protein chemistry approaches and the in vitro biophysical analyses of the effects of the acylations on nucleosome structures; investigating the negatively charged acylations' influence on chromatin dynamics in vivo using yeast genetic approaches; identifying and characterizing protein-protein interactions (PPIs) mediated by histone PTMs in different biological contexts (i.e., to identify the readers and erasers) by establishing a chemical proteomics platform that is enabled by photo-cross-linking chemistry and quantitative proteomics strategies; and manipulating PTM-mediated PPIs by the structure-guided design of inhibitors. We also discuss possible future directions in our journey to fully decipher the histone code.

Genome-wide occupancy reveals the localization of H1T2 (H1fnt) to repeat regions and a subset of transcriptionally active chromatin domains in rat spermatids

BackgroundH1T2/H1FNT is a germ cell-specific linker histone variant expressed during spermiogenesis specifically in round and elongating spermatids. Infertile phenotype of homozygous H1T2 mutant male mice revealed the essential function of H1T2 for the DNA condensation and histone-to-protamine replacement in spermiogenesis. However, the mechanism by which H1T2 imparts the inherent polarity within spermatid nucleus including the additional protein partners and the genomic domains occupied by this linker histone are unknown.ResultsSequence analysis revealed the presence of Walker motif, SR domains and putative coiled-coil domains in the C-terminal domain of rat H1T2 protein. Genome-wide occupancy analysis using highly specific antibody against the CTD of H1T2 demonstrated the binding of H1T2 to the LINE L1 repeat elements and to a significant percentage of the genic regions (promoter-TSS, exons and introns) of the rat spermatid genome. Immunoprecipitation followed by mass spectrometry analysis revealed the open chromatin architecture of H1T2 occupied chromatin encompassing the H4 acetylation and other histone PTMs characteristic of transcriptionally active chromatin. In addition, the present study has identified the interacting protein partners of H1T2-associated chromatin mainly as nucleo-skeleton components, RNA-binding proteins and chaperones.ConclusionsLinker histone H1T2 possesses unique domain architecture which can account for the specific functions associated with chromatin remodeling events facilitating the initiation of histone to transition proteins/protamine transition in the polar apical spermatid genome. Our results directly establish the unique function of H1T2 in nuclear shaping associated with spermiogenesis by mediating the interaction between chromatin and nucleo-skeleton, positioning the epigenetically specialized chromatin domains involved in transcription coupled histone replacement initiation towards the apical pole of round/elongating spermatids.

Linker histone epitopes are hidden by in situ higher-order chromatin structure

BackgroundHistone H1 is the most mobile histone in the cell nucleus. Defining the positions of H1 on chromatin in situ, therefore, represents a challenge. Immunoprecipitation of formaldehyde-fixed and sonicated chromatin, followed by DNA sequencing (xChIP-seq), is traditionally the method for mapping histones onto DNA elements. But since sonication fragmentation precedes ChIP, there is a consequent loss of information about chromatin higher-order structure. Here, we present a new method, xxChIP-seq, employing antibody binding to fixed intact in situ chromatin, followed by extensive washing, a second fixation, sonication and immunoprecipitation. The second fixation is intended to prevent the loss of specifically bound antibody during washing and subsequent sonication and to prevent antibody shifting to epitopes revealed by the sonication process. In many respects, xxChIP-seq is comparable to immunostaining microscopy, which also involves interaction of the primary antibody with fixed and permeabilized intact cells. The only epitopes displayed after immunostaining are the “exposed” epitopes, not “hidden” by the fixation of chromatin higher-order structure. Comparison of immunoprecipitated fragments between xChIP-seq versus xxChIP-seq should indicate which epitopes become inaccessible with fixation and identify their associated DNA elements.ResultsWe determined the genomic distribution of histone variants H1.2 and H1.5 in human myeloid leukemia cells HL-60/S4 and compared their epitope exposure by both xChIP-seq and xxChIP-seq, as well as high-resolution microscopy, illustrating the influences of preserved chromatin higher-order structure in situ. We found that xChIP and xxChIP H1 signals are in general negatively correlated, with differences being more pronounced near active regulatory regions. Among the intriguing observations, we find that transcription-related regions and histone PTMs (i.e., enhancers, promoters, CpG islands, H3K4me1, H3K4me3, H3K9ac, H3K27ac and H3K36me3) exhibit significant deficiencies (depletions) in H1.2 and H1.5 xxChIP-seq reads, compared to xChIP-seq. These observations suggest the existence of in situ transcription-related chromatin higher-order structures stabilized by formaldehyde.ConclusionComparison of H1 xxChIP-seq to H1 xChIP-seq allows the development of hypotheses on the chromosomal localization of (stabilized) higher-order structure, indicated by the generation of “hidden” H1 epitopes following formaldehyde crosslinking. Changes in H1 epitope exposure surrounding averaged chromosomal binding sites or epigenetic modifications can also indicate whether these sites have chromatin higher-order structure. For example, comparison between averaged active or inactive promoter regions suggests that both regions can acquire stabilized higher-order structure with hidden H1 epitopes. However, the H1 xChIP-seq comparison cannot define their differences. Application of the xxChIP-seq versus H1 xChIP-seq method is particularly relevant to chromatin-associated proteins, such as linker histones, that play dynamic roles in establishing chromatin higher-order structure.