The profile of repeat-associated histone lysine methylation states in the mouse epigenome

Setdb1 and Atf7IP form a hetero-trimeric complex that blocks Setdb1 nuclear export

Histone H3K9 methylation (H3K9me) by Setdb1 silences retrotransposons (rTE) by sequestering them in heterochromatin. Atf7IP is a constitutive binding partner of Setdb1 and is responsible for Setdb1 nuclear localization, activation and chromatin recruitment. However, structural details of the Setdb1/Atf7IP interaction have not been elucidated. We used Alphafold2 predictions and biochemical reconstitutions to show that one copy of Setdb1 and two copies of Atf7IP form a hetero-trimeric complex in vitro and in cells. We also find that Atf7IP self-associates, forming multimeric complexes that are resolved upon Setdb1 binding. Setdb1 binds to Atf7IP through coiled coil interactions that include both Setdb1 nuclear export signals (NES). Atf7IP directly competes with CRM1 to bind the Setdb1 NES motifs, explaining how Atf7IP prevents CRM1-mediated nuclear export of Setdb1. Setdb1 also forms hetero-trimeric complexes with the Atf7IP paralog Atf7IP2 and we show that Setdb1 can form mixed heterotrimers comprising one copy of each Setdb1, Atf7IP and Atf7IP2. Atf7IP and Atf7IP2 are co-expressed in many tissues suggesting that heterotrimers with different compositions of Atf7IP and Atf7IP2 may differentially regulate H3K9me by fine-tuning Setdb1 localization and activity.

Transposable elements as instructors of the immune system

Transposable elements (TEs) are mobile repetitive nucleic acid sequences that have been incorporated into the genome through spontaneous integration, accounting for almost 50% of human DNA. Even though most TEs are no longer mobile today, studies have demonstrated that they have important roles in different biological processes, such as ageing, embryonic development, and cancer. TEs influence these processes through various mechanisms, including active transposition of TEs contributing to ongoing evolution, transposon transcription generating RNA or protein, and by influencing gene regulation as enhancers. However, how TEs interact with the immune system remains a largely unexplored field. In this Perspective, we describe how TEs might influence different aspects of the immune system, such as innate immune responses, T cell activation and differentiation, and tissue adaptation. Furthermore, TEs can serve as a source of neoantigens for T cells in antitumour immunity. We suggest that TE biology is an important emerging field of immunology and discuss the potential to harness the TE network therapeutically, for example, to improve immunotherapies for cancer and autoimmune and inflammatory diseases.

Reactivation of retrotransposable elements is associated with environmental stress and ageing

Retrotransposable elements (RTEs) are interspersed repetitive sequences that represent a large portion of eukaryotic genomes. Ancestral expansions of RTEs directly contributed to the shaping of these genomes and to the evolution of different species, particularly mammals. RTE activity is tightly regulated by different epigenetic mechanisms but this control becomes compromised as cells age and RTEs are reactivated. This dysregulation of RTEs leads to perturbation of cell function and organ and organismal homeostasis, which drives ageing and age-related disease. Environmental stress is associated with both ageing-related characteristics and the epigenetic mechanisms that control RTE activity, with accumulating evidence indicating that RTE reactivation mediates the effects of environmental stressors on ageing onset and progression. A better understanding of how RTEs are reactivated and their subsequent biological roles may help the development of therapies against ageing-related phenotypes and diseases.

20 years of stemness: From stem cells to hypertranscription and back

Transcriptional profiling of stem cells came of age at the beginning of the century with the use of microarrays to analyze cell populations in bulk. Since then, stem cell transcriptomics has become increasingly sophisticated, notably with the recent widespread use of single-cell RNA sequencing. Here, we provide a perspective on how an early signature of genes upregulated in embryonic and adult stem cells, identified using microarrays over 20 years ago, serendipitously led to the recent discovery that stem/progenitor cells across organs are in a state of hypertranscription, a global elevation of the transcriptome. Looking back, we find that the 2002 stemness signature is a robust marker of stem cell hypertranscription, even though it was developed well before it was known what hypertranscription meant or how to detect it. We anticipate that studies of stem cell hypertranscription will be rich in novel insights in physiological and disease contexts for years to come.

NSD3 protein methylation and stabilization transforms human ES cells into variant state

Cultured human embryonic stem cells (hESCs) can develop genetic anomalies that increase their susceptibility to transformation. In this study, we characterized a variant hESC (vhESC) line and investigated the molecular mechanisms leading to the drift towards a transformed state. Our findings revealed that vhESCs up-regulate EMT-specific markers, accelerate wound healing, exhibit compromised lineage differentiation, and retain pluripotency gene expression in teratomas. Furthermore, we discovered an altered epigenomic landscape and overexpression of the lysine methyltransferases EHMT1, EHMT2, and NSD group of proteins in vhESCs. Remarkably, depleting NSD3 oncogene reversed the molecular and phenotypic changes in vhESCs. We identified a detailed mechanism where EHMT2 interacts and methylates NSD3 at lysine 477, stabilizing its protein levels in vhESCs. In addition, we showed that NSD3 levels are regulated by protein degradation in hESCs, and its stabilization leads to the emergence of the variant state. Overall, our study identify that misregulation of NSD3 in pluripotent stem cells, through methylation-mediated abrogation of its protein degradation, drives hESCs towards oncogenic transformation.

KMT5C leverages disorder to optimize cooperation with HP1 for heterochromatin retention

A defining feature of constitutive heterochromatin compartments is the heterochromatin protein-1 (HP1) family, whose members display fast internal mobility and rapid exchange with the surrounding nucleoplasm. Here, we describe a paradoxical state for the lysine methyltransferase KMT5C characterized by rapid internal diffusion but minimal nucleoplasmic exchange. This retentive behavior is conferred by sparse sequence features that constitute two modules tethered by an intrinsically disordered linker. While both modules harbor variant HP1 interaction motifs, the first comprises adjacent sequences that increase affinity using avidity. The second motif increases HP1 effective concentration to further enhance affinity in a context-dependent manner, which is evident using distinct heterochromatin recruitment strategies and heterologous linkers with defined conformational ensembles. Despite the linker sequence being highly divergent, it is under evolutionary constraint for functional length, suggesting conformational buffering can support cooperativity between modules across distant orthologs. Overall, we show that KMT5C has evolved a robust tethering strategy that uses minimal sequence determinants to harness highly dynamic HP1 proteins for retention within heterochromatin compartments.

Setdb1 and Atf7IP form a hetero-trimeric complex that blocks Setdb1 nuclear export

Histone H3K9 methylation (H3K9me) by Setdb1 silences retrotransposons (rTE) by sequestering them in constitutive heterochromatin. Atf7IP is a constitutive binding partner of Setdb1 and is responsible for Setdb1 nuclear localization, activation and chromatin recruitment. However, structural details of the Setdb1/Atf7IP interaction have not been evaluated. We used Alphafold2 predictions and biochemical reconstitutions to show that one copy of Setdb1 and two copies of Atf7IP form a hetero-trimeric complex in vitro and in cells. We also find that Atf7IP self-associates, forming multimeric complexes that are resolved upon Setdb1 binding. Setdb1 binds to Atf7IP through coiled coil interactions that include both Setdb1 nuclear export signals (NES). Atf7IP directly competes with CRM1 to bind the Setdb1 NES motifs, explaining how Atf7IP prevents CRM1-mediated nuclear export of Setdb1. Setdb1 also forms hetero-trimeric complexes with the Atf7IP paralog Atf7IP2 and we show that Setdb1 can form mixed heterotrimers comprising one copy of each Setdb1, Atf7IP and Atf7IP2. Atf7IP and Atf7IP2 are co-expressed in many tissues suggesting that heterotrimers with different compositions of Atf7IP and Atf7IP2 may differentially regulate H3K9me by fine-tuning Setdb1 localization and activity.

PHF2-mediated H3K9me balance orchestrates heterochromatin stability and neural progenitor proliferation

Heterochromatin stability is crucial for progenitor proliferation during early neurogenesis. It relays on the maintenance of local hubs of H3K9me. However, understanding the formation of efficient localized levels of H3K9me remains limited. To address this question, we used neural stem cells to analyze the function of the H3K9me2 demethylase PHF2, which is crucial for progenitor proliferation. Through mass-spectroscopy and genome-wide assays, we show that PHF2 interacts with heterochromatin components and is enriched at pericentromeric heterochromatin (PcH) boundaries where it maintains transcriptional activity. This binding is essential for silencing the satellite repeats, preventing DNA damage and genome instability. PHF2's depletion increases the transcription of heterochromatic repeats, accompanied by a decrease in H3K9me3 levels and alterations in PcH organization. We further show that PHF2's PHD and catalytic domains are crucial for maintaining PcH stability, thereby safeguarding genome integrity. These results highlight the multifaceted nature of PHF2's functions in maintaining heterochromatin stability and regulating gene expression during neural development. Our study unravels the intricate relationship between heterochromatin stability and progenitor proliferation during mammalian neurogenesis.

Species-Specific Transcription Factors Associated with Long Terminal Repeat Promoters of Endogenous Retroviruses: A Comprehensive Review

Endogenous retroviruses (ERVs) became a part of the eukaryotic genome through endogenization millions of years ago. Moreover, they have lost their innate capability of virulence or replication. Nevertheless, in eukaryotic cells, they actively engage in various activities that may be advantageous or disadvantageous to the cells. The mechanisms by which transcription is triggered and implicated in cellular processes are complex. Owing to the diversity in the expression of transcription factors (TFs) in cells and the TF-binding motifs of viruses, the comprehensibility of ERV initiation and its impact on cellular functions are unclear. Currently, several factors are known to be related to their initiation. TFs that bind to the viral long-terminal repeat (LTR) are critical initiators. This review discusses the TFs shown to actively associate with ERV stimulation across species such as humans, mice, pigs, monkeys, zebrafish, Drosophila, and yeast. A comprehensive summary of the expression of previously reported TFs may aid in identifying similarities between animal species and endogenous viruses. Moreover, an in-depth understanding of ERV expression will assist in elucidating their physiological roles in eukaryotic cell development and in clarifying their relationship with endogenous retrovirus-associated diseases.

Diversity and functional specialization of H3K9-specific histone methyltransferases

Histone modifications play a critical role in the control over activities of the eukaryotic genome; among these chemical alterations, the methylation of lysine K9 in histone H3 (H3K9) is one of the most extensively studied. The number of enzymes capable of methylating H3K9 varies greatly across different organisms: in fission yeast, only one such methyltransferase is present, whereas in mammals, 10 are known. If there are several such enzymes, each of them must have some specific function, and they can interact with one another. Thus arises a complex system of interchangeability, "division of labor," and contacts with each other and with diverse proteins. Histone methyltransferases specialize in the number of methyl groups that they attach and have different intracellular localizations as well as different distributions on chromosomes. Each also shows distinct binding to different types of sequences and has a specific set of nonhistone substrates.

The Proteomic Composition and Organization of Constitutive Heterochromatin in Mouse Tissues

Pericentric heterochromatin (PCH) forms spatio-temporarily distinct compartments and affects chromosome organization and stability. Albeit some of its components are known, an elucidation of its proteome and how it differs between tissues in vivo is lacking. Here, we find that PCH compartments are dynamically organized in a tissue-specific manner, possibly reflecting compositional differences. As the mouse brain and liver exhibit very different PCH architecture, we isolated native PCH fractions from these tissues, analyzed their protein compositions using quantitative mass spectrometry, and compared them to identify common and tissue-specific PCH proteins. In addition to heterochromatin-enriched proteins, the PCH proteome includes RNA/transcription and membrane-related proteins, which showed lower abundance than PCH-enriched proteins. Thus, we applied a cut-off of PCH-unspecific candidates based on their abundance and validated PCH-enriched proteins. Amongst the hits, MeCP2 was classified into brain PCH-enriched proteins, while linker histone H1 was not. We found that H1 and MeCP2 compete to bind to PCH and regulate PCH organization in opposite ways. Altogether, our workflow of unbiased PCH isolation, quantitative mass spectrometry, and validation-based analysis allowed the identification of proteins that are common and tissue-specifically enriched at PCH. Further investigation of selected hits revealed their opposing role in heterochromatin higher-order architecture in vivo.

Cell-type differential targeting of SETDB1 prevents aberrant CTCF binding, chromatin looping, and cis-regulatory interactions

SETDB1 is an essential histone methyltransferase that deposits histone H3 lysine 9 trimethylation (H3K9me3) to transcriptionally repress genes and repetitive elements. The function of differential H3K9me3 enrichment between cell-types remains unclear. Here, we demonstrate mutual exclusivity of H3K9me3 and CTCF across mouse tissues from different developmental timepoints. We analyze SETDB1 depleted cells and discover that H3K9me3 prevents aberrant CTCF binding independently of DNA methylation and H3K9me2. Such sites are enriched with SINE B2 retrotransposons. Moreover, analysis of higher-order genome architecture reveals that large chromatin structures including topologically associated domains and subnuclear compartments, remain intact in SETDB1 depleted cells. However, chromatin loops and local 3D interactions are disrupted, leading to transcriptional changes by modifying pre-existing chromatin landscapes. Specific genes with altered expression show differential interactions with dysregulated cis-regulatory elements. Collectively, we find that cell-type specific targets of SETDB1 maintain cellular identities by modulating CTCF binding, which shape nuclear architecture and transcriptomic networks.

HIV-1 Gag co-localizes with euchromatin histone marks at the nuclear periphery

IMPORTANCE

The traditional view of retrovirus assembly posits that packaging of gRNA by HIV-1 Gag occurs in the cytoplasm or at the plasma membrane. However, our previous studies showing that HIV-1 Gag enters the nucleus and binds to USvRNA at transcription sites suggest that gRNA selection may occur in the nucleus. In the present study, we observed that HIV-1 Gag trafficked to the nucleus and co-localized with USvRNA within 8 hours of expression. In infected T cells (J-Lat 10.6) reactivated from latency and in a HeLa cell line stably expressing an inducible Rev-dependent HIV-1 construct, we found that Gag preferentially localized with euchromatin histone marks associated with enhancer and promoter regions near the nuclear periphery, which is the favored site HIV-1 integration. These observations support the innovative hypothesis that HIV-1 Gag associates with euchromatin-associated histones to localize to active transcription sites, promoting capture of newly synthesized gRNA for packaging.

EZH2 inhibition stimulates repetitive element expression and viral mimicry in resting splenic B cells

Mammalian cells repress expression of repetitive genomic sequences by forming heterochromatin. However, the consequences of ectopic repeat expression remain unclear. Here we demonstrate that inhibitors of EZH2, the catalytic subunit of the Polycomb repressive complex 2 (PRC2), stimulate repeat misexpression and cell death in resting splenic B cells. B cells are uniquely sensitive to these agents because they exhibit high levels of histone H3 lysine 27 trimethylation (H3K27me3) and correspondingly low DNA methylation at repeat elements. We generated a pattern recognition receptor loss-of-function mouse model, called RIC, with mutations in Rigi (encoding for RIG-I), Ifih1 (MDA5), and Cgas. In both wildtype and RIC mutant B cells, EZH2 inhibition caused loss of H3K27me3 at repetitive elements and upregulated their expression. However, NF-κB-dependent expression of inflammatory chemokines and subsequent cell death was suppressed by the RIC mutations. We further show that inhibition of EZH2 in cancer cells requires the same pattern recognition receptors to activate an interferon response. Together, the results reveal chemokine expression induced by EZH2 inhibitors in B cells as a novel inflammatory response to genomic repeat expression. Given the overlap of genes induced by EZH2 inhibitors and Epstein-Barr virus infection, this response can be described as a form of viral mimicry.

Climate Change and New Challenges for Rural Communities: Particulate Matter Matters

Climate change presents multiple challenges to rural communities. Here, we investigated the toxicological potential of the six types of particulate matter most common to rural Arkansas: soil, road, and agricultural dusts, pollen, traffic exhaust, and particles from biomass burning in human small airway epithelial cells (SAECs). Biomass burning and agricultural dust demonstrated the most potent toxicological responses, exhibited as significant (p < 0.05) up-regulation of HMOX1 (oxidative stress) and TNFα (inflammatory response) genes as well as epigenetic alterations (altered expression of DNA methyltransferases DNMT1, DNMT3A, and DNMT3B, enzymatic activity, and DNA methylation of alpha satellite elements) that were evident at both 24 h and 72 h of exposure. We further demonstrate evidence of aridification in the state of Arkansas and the presence of winds capable of transporting agricultural dust- and biomass burning-associated particles far beyond their origination. Partnerships in the form of citizen science projects may provide important solutions to prevent and mitigate the negative effects of the rapidly evolving climate and improve the well-being of rural communities. Furthermore, the identification of the most toxic types of particulate matter could inform local policies related to agriculture, biomass burning, and dust control.

Selective control of transposable element expression during T cell exhaustion and anti-PD-1 treatment

In chronic infections and cancer, T cells are exposed to prolonged antigen stimulation, resulting in loss of function (or exhaustion) and impairment of effective immunological protection. Exhausted T cells are heterogeneous and include early progenitors (Tpex) and terminally exhausted cells (Tex). Here, we used bulk and single-cell transcriptomics to analyze expression of transposable elements (TEs) in subpopulations of mouse and human CD8+ tumor-infiltrating T lymphocytes (TILs). We show that in mice, members of the virus-like murine VL30 TE family (mostly intact, evolutionary young ERV1s) are strongly repressed in terminally exhausted CD8+ T cells in both tumor and viral models of exhaustion. Tpex expression of these VL30s, which are mainly intergenic and transcribed independently of their closest gene neighbors, was driven by Fli1, a transcription factor involved in progression from Tpex to Tex. Immune checkpoint blockade (ICB) in both mice and patients with cancer increased TE expression (including VL30 in mice), demonstrating that TEs may be applicable as ICB response biomarkers. We conclude that expression of TEs is tightly regulated in TILs during establishment of exhaustion and reprogramming by ICB. Analyses of TE expression on single cells and bulk populations open opportunities for understanding immune cell identity and heterogeneity, as well as for defining cellular gene expression signatures and disease biomarkers.

DAXX safeguards heterochromatin formation in embryonic stem cells

Genomes comprise a large fraction of repetitive sequences folded into constitutive heterochromatin, which protect genome integrity and cell identity. De novo formation of heterochromatin during preimplantation development is an essential step for preserving the ground-state of pluripotency and the self-renewal capacity of embryonic stem cells (ESCs). However, the molecular mechanisms responsible for the remodeling of constitutive heterochromatin are largely unknown. Here, we identify that DAXX, an H3.3 chaperone essential for the maintenance of mouse ESCs in the ground state, accumulates in pericentromeric regions independently of DNA methylation. DAXX recruits PML and SETDB1 to promote the formation of heterochromatin, forming foci that are hallmarks of ground-state ESCs. In the absence of DAXX or PML, the three-dimensional (3D) architecture and physical properties of pericentric and peripheral heterochromatin are disrupted, resulting in de-repression of major satellite DNA, transposable elements and genes associated with the nuclear lamina. Using epigenome editing tools, we observe that H3.3, and specifically H3.3K9 modification, directly contribute to maintaining pericentromeric chromatin conformation. Altogether, our data reveal that DAXX is crucial for the maintenance and 3D organization of the heterochromatin compartment and protects ESC viability.

Transposable elements as essential elements in the control of gene expression

Interspersed repetitions called transposable elements (TEs), commonly referred to as mobile elements, make up a significant portion of the genomes of higher animals. TEs contribute in controlling the expression of genes locally and even far away at the transcriptional and post-transcriptional levels, which is one of their significant functional effects on gene function and genome evolution. There are different mechanisms through which TEs control the expression of genes. First, TEs offer cis-regulatory regions in the genome with their inherent regulatory features for their own expression, making them potential factors for controlling the expression of the host genes. Promoter and enhancer elements contain cis-regulatory sites generated from TE, which function as binding sites for a variety of trans-acting factors. Second, a significant portion of miRNAs and long non-coding RNAs (lncRNAs) have been shown to have TEs that encode for regulatory RNAs, revealing the TE origin of these RNAs. Furthermore, it was shown that TE sequences are essential for these RNAs' regulatory actions, which include binding to the target mRNA. By being a member of cis-regulatory and regulatory RNA sequences, TEs therefore play essential regulatory roles. Additionally, it has been suggested that TE-derived regulatory RNAs and cis-regulatory regions both contribute to the evolutionary novelty of gene regulation. Additionally, these regulatory systems arising from TE frequently have tissue-specific functions. The objective of this review is to discuss TE-mediated gene regulation, with a particular emphasis on the processes, contributions of various TE types, differential roles of various tissue types, based mostly on recent studies on humans.

Fluorescent protein lifetimes report densities and phases of nuclear condensates during embryonic stem-cell differentiation

Fluorescent proteins (FP) are frequently used for studying proteins inside cells. In advanced fluorescence microscopy, FPs can report on additional intracellular variables. One variable is the local density near FPs, which can be useful in studying densities within cellular bio-condensates. Here, we show that a reduction in fluorescence lifetimes of common monomeric FPs reports increased levels of local densities. We demonstrate the use of this fluorescence-based variable to report the distribution of local densities within heterochromatin protein 1α (HP1α) in mouse embryonic stem cells (ESCs), before and after early differentiation. We find that local densities within HP1α condensates in pluripotent ESCs are heterogeneous and cannot be explained by a single liquid phase. Early differentiation, however, induces a change towards a more homogeneous distribution of local densities, which can be explained as a liquid-like phase. In conclusion, we provide a fluorescence-based method to report increased local densities and apply it to distinguish between homogeneous and heterogeneous local densities within bio-condensates.

Two RNA binding proteins, ADAD2 and RNF17, interact to form a heterogeneous population of novel meiotic germ cell granules with developmentally dependent organelle association

Mammalian male germ cell differentiation relies on complex RNA biogenesis events, many of which occur in non-membrane bound organelles termed RNA germ cell granules that are rich in RNA binding proteins (RBPs). Though known to be required for male germ cell differentiation, we understand little of the relationships between the numerous granule subtypes. ADAD2, a testis specific RBP, is required for normal male fertility and forms a poorly characterized granule in meiotic germ cells. This work aimed to understand the role of ADAD2 granules in male germ cell differentiation by clearly defining their molecular composition and relationship to other granules. Biochemical analyses identified RNF17, a testis specific RBP that forms meiotic male germ cell granules, as an ADAD2-interacting protein. Phenotypic analysis of Adad2 and Rnf17 mutants identified a rare post-meiotic chromatin defect, suggesting shared biological roles. ADAD2 and RNF17 were found to be dependent on one another for granularization and together form a previously unstudied set of germ cell granules. Based on co-localization studies with well-characterized granule RBPs and organelle-specific markers, a subset of the ADAD2-RNF17 granules are found to be associated with the intermitochondrial cement and piRNA biogenesis. In contrast, a second, morphologically distinct population of ADAD2-RNF17 granules co-localized with the translation regulators NANOS1 and PUM1, along with the molecular chaperone PDI. These large granules form a unique funnel-shaped structure that displays distinct protein subdomains and is tightly associated with the endoplasmic reticulum. Developmental studies suggest the different granule populations represent different phases of a granule maturation process. Lastly, a double Adad2-Rnf17 mutant model suggests the interaction between ADAD2 and RNF17, as opposed to loss of either, is the likely driver of the Adad2 and Rnf17 mutant phenotypes. These findings shed light on the relationship between germ cell granule pools and define new genetic approaches to their study.

Repetitive elements in aging and neurodegeneration

Repetitive elements (REs), such as transposable elements (TEs) and satellites, comprise much of the genome. Here, we review how TEs and (peri)centromeric satellite DNA may contribute to aging and neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). Alterations in RE expression, retrotransposition, and chromatin microenvironment may shorten lifespan, elicit neurodegeneration, and impair memory and movement. REs may cause these phenotypes via DNA damage, protein sequestration, insertional mutagenesis, and inflammation. We discuss several TE families, including gypsy, HERV-K, and HERV-W, and how TEs interact with various factors, including transactive response (TAR) DNA-binding protein 43 kDa (TDP-43) and the siRNA and piwi-interacting (pi)RNA systems. Studies of TEs in neurodegeneration have focused on Drosophila and, thus, further examination in mammals is needed. We suggest that therapeutic silencing of REs could help mitigate neurodegenerative disorders.

Transposable elements and their role in aging

Transposable elements (TEs) are an important part of eukaryotic genomes. The role of somatic transposition in aging, carcinogenesis, and other age-related diseases has been determined. This review discusses the fundamental properties of TEs and their complex interactions with cellular processes, which are crucial for understanding the diverse effects of their activity on the genetics and epigenetics of the organism. The interactions of TEs with recombination, replication, repair, and chromosomal regulation; the ability of TEs to maintain a balance between their own activity and repression, the involvement of TEs in the creation of new or alternative genes, the expression of coding/non-coding RNA, and the role in DNA damage and modification of regulatory networks are reviewed. The contribution of the derepressed TEs to age-dependent effects in individual cells/tissues in different organisms was assessed. Conflicting information about TE activity under stress as well as theories of aging mechanisms related to TEs is discussed. On the one hand, transposition activity in response to stressors can lead to organisms acquiring adaptive innovations of great importance for evolution at the population level. On the other hand, the TE expression can cause decreased longevity and stress tolerance at the individual level. The specific features of TE effects on aging processes in germline and soma and the ways of their regulation in cells are highlighted. Recent results considering somatic mutations in normal human and animal tissues are indicated, with the emphasis on their possible functional consequences. In the context of aging, the correlation between somatic TE activation and age-related changes in the number of proteins required for heterochromatin maintenance and longevity regulation was analyzed. One of the original features of this review is a discussion of not only effects based on the TEs insertions and the associated consequences for the germline cell dynamics and somatic genome, but also the differences between transposon- and retrotransposon-mediated structural genome changes and possible phenotypic characteristics associated with aging and various age-related pathologies. Based on the analysis of published data, a hypothesis about the influence of the species-specific features of number, composition, and distribution of TEs on aging dynamics of different animal genomes was formulated.

RNA-mediated heterochromatin formation at repetitive elements in mammals

The failure to repress transcription of repetitive genomic elements can lead to catastrophic genome instability and is associated with various human diseases. As such, multiple parallel mechanisms cooperate to ensure repression and heterochromatinization of these elements, especially during germline development and early embryogenesis. A vital question in the field is how specificity in establishing heterochromatin at repetitive elements is achieved. Apart from trans-acting protein factors, recent evidence points to a role of different RNA species in targeting repressive histone marks and DNA methylation to these sites in mammals. Here, we review recent discoveries on this topic and predominantly focus on the role of RNA methylation, piRNAs, and other localized satellite RNAs.

Remodeling of H3K9me3 during the pluripotent to totipotent-like state transition

Multiple chromatin modifiers associated with H3K9me3 play important roles in the transition from embryonic stem cells to 2-cell (2C)-like cells. However, it remains elusive how H3K9me3 is remodeled and its association with totipotency. Here, we integrated transcriptome and H3K9me3 profiles to conduct a detailed comparison of 2C embryos and 2C-like cells. Globally, H3K9me3 is highly preserved and H3K9me3 dynamics within the gene locus is not associated with gene expression change during 2C-like transition. Promoter-deposited H3K9me3 plays non-repressive roles in the activation of genes during 2C-like transition. In contrast, transposable elements, residing in the nearby regions of up-regulated genes, undergo extensive elimination of H3K9me3 and are tended to be induced in 2C-like transitions. Furthermore, a large fraction of trophoblast stem cell-specific enhancers undergo loss of H3K9me3 exclusively in MERVL⁺/Zscan4⁺ cells. Our study therefore reveals the unique H3K9me3 profiles of 2C-like cells, facilitating the further exploration of totipotency.

Loss of SUV420H2-Dependent Chromatin Compaction Drives Right-Sided Colon Cancer Progression

BACKGROUND & AIMS

Epigenetic processes regulating gene expression contribute markedly to epithelial cell plasticity in colorectal carcinogenesis. The lysine methyltransferase SUV420H2 comprises an important regulator of epithelial plasticity and is primarily responsible for trimethylation of H4K20 (H4K20me3). Loss of H4K20me3 has been suggested as a hallmark of human cancer due to its interaction with DNMT1. However, the role of Suv4-20h2 in colorectal cancer is unknown.

METHODS

We examined the alterations in histone modifications in patient-derived colorectal cancer organoids. Patient-derived colorectal cancer organoids and mouse intestinal organoids were genetically manipulated for functional studies in patient-derived xenograft and orthotopic transplantation. Gene expression profiling, micrococcal nuclease assay, and chromatin immunoprecipitation were performed to understand epigenetic regulation of chromatin states and gene expression in patient-derived and mouse intestinal organoids.

RESULTS

We found that reduced H4K20me3 levels occurred predominantly in right-sided patient-derived colorectal cancer organoids, which were associated with increased chromatin accessibility. Re-compaction of chromatin by methylstat, a histone demethylase inhibitor, resulted in reduced growth selectively in subcutaneously grown tumors derived from right-sided cancers. Using mouse intestinal organoids, we confirmed that Suv4-20h2-mediated H4K20me3 is required for maintaining heterochromatin compaction and to prevent R-loop formation. Cross-species comparison of Suv4-20h2-depleted murine organoids with right-sided colorectal cancer organoids revealed a large overlap of gene signatures involved in chromatin silencing, DNA methylation, and stemness/Wnt signaling.

CONCLUSIONS

Loss of Suv4-20h2-mediated H4K20me3 drives right-sided colorectal tumorigenesis through an epigenetically controlled mechanism of chromatin compaction. Our findings unravel a conceptually novel approach for subtype-specific therapy of this aggressive form of colorectal cancer.

Targeted DNA Methylation in Mouse Early Embryos

In germ cell lines, including early preimplantation embryos, centromeres and pericentromeres are known to show a marked hypomethylation pattern compared to somatic cells. Elucidation of the biological function of this region-specific DNA hypomethylation state, region-specific epigenomic manipulation is essential as an analytical method. We have applied genome editing to show that region-specific DNA methylation can be effectively introduced by a fusion protein, TALE, which recognizes pericentromeres, and SssI, a bacterial CpG methyltransferase. This makes it possible to increase the DNA methylation state of the pericentromeres, which is normally about 20%, to about 60-75%, enabling comparative analysis of the developmental processes of normal embryos with hypomethylated pericentromeres and embryos that have been epigenetically edited to be hypermethylated. In this chapter, we describe a method for introducing DNA methylation into pericentromeres of early mouse embryos by expressing TALE-SssI fusion protein and a method for detecting DNA methylation.

HP1 proteins regulate nucleolar structure and function by secluding pericentromeric constitutive heterochromatin

Nucleoli are nuclear compartments regulating ribosome biogenesis and cell growth. In embryonic stem cells (ESCs), nucleoli containing transcriptionally active ribosomal genes are spatially separated from pericentromeric satellite repeat sequences packaged in largely repressed constitutive heterochromatin (PCH). To date, mechanisms underlying such nuclear partitioning and the physiological relevance thereof are unknown. Here we show that repressive chromatin at PCH ensures structural integrity and function of nucleoli during cell cycle progression. Loss of heterochromatin proteins HP1α and HP1β causes deformation of PCH, with reduced H3K9 trimethylation (H3K9me3) and HP1γ levels, absence of H4K20me3 and upregulated major satellites expression. Spatially, derepressed PCH aberrantly associates with nucleoli accumulating severe morphological defects during S/G2 cell cycle progression. Hp1α/β deficiency reduces cell proliferation, ribosomal RNA biosynthesis and mobility of Nucleophosmin, a major nucleolar component. Nucleolar integrity and function require HP1α/β proteins to be recruited to H3K9me3-marked PCH and their ability to dimerize. Correspondingly, ESCs deficient for both Suv39h1/2 H3K9 HMTs display similar nucleolar defects. In contrast, Suv4-20h1/2 mutant ESCs lacking H4K20me3 at PCH do not. Suv39h1/2 and Hp1α/β deficiency-induced nucleolar defects are reminiscent of those defining human ribosomopathy disorders. Our results reveal a novel role for SUV39H/HP1-marked repressive constitutive heterochromatin in regulating integrity, function and physiology of nucleoli.

Targets of histone H3 lysine 9 methyltransferases

Histone H3 lysine 9 di- and trimethylation are well-established marks of constitutively silenced heterochromatin domains found at repetitive DNA elements including pericentromeres, telomeres, and transposons. Loss of heterochromatin at these sites causes genomic instability in the form of aberrant DNA repair, chromosome segregation defects, replication stress, and transposition. H3K9 di- and trimethylation also regulate cell type-specific gene expression during development and form a barrier to cellular reprogramming. However, the role of H3K9 methyltransferases extends beyond histone methylation. There is a growing list of non-histone targets of H3K9 methyltransferases including transcription factors, steroid hormone receptors, histone modifying enzymes, and other chromatin regulatory proteins. Additionally, two classes of H3K9 methyltransferases modulate their own function through automethylation. Here we summarize the structure and function of mammalian H3K9 methyltransferases, their roles in genome regulation and constitutive heterochromatin, as well as the current repertoire of non-histone methylation targets including cases of automethylation.

ATF7-dependent epigenetic changes induced by high temperature during early porcine embryonic development

BACKGROUND

Activating transcription factor 7 (ATF7) is a member of the ATF/cAMP response element (CRE) B superfamily. ATF2, ATF7, and CRE-BPa are present in vertebrates. Drosophila and fission yeast have only one homologue: dATF2 and Atf1, respectively. Under normal conditions, ATF7 promotes heterochromatin formation by recruiting histone H3K9 di- and tri-methyltransferases. Once the situation changes, all members are phosphorylated by the stress-activated kinase P38 in response to various stressors. However, the role of ATF7 in early porcine embryonic development remains unclear.

RESULTS

In this study, we found that ATF7 gradually accumulated in the nucleus and then localized on the pericentric heterochromatin after the late 4-cell stage, while being co-localized with heterochromatin protein 1 (HP1). Knockdown of ATF7 resulted in decreases in the blastocyst rate and blastocyst cell number. ATF7 depletion resulted in downregulation of HP1 and histone 3 lysine 9 dimethylation (H3K9me2) expression. These effects were alleviated when P38 activity was inhibited. High temperatures increased the expression level of pP38, while reducing the quality of porcine embryos, and led to ATF7 phosphorylation. The expression level of H3K9me2 and HP1 was decreased and regulated by P38 activity.

CONCLUSION

Stress-induced ATF7-dependent epigenetic changes play important roles in early porcine embryonic development.

H3K9me1/2 methylation limits the lifespan of daf-2 mutants in C. elegans

Histone methylation plays crucial roles in the development, gene regulation, and maintenance of stem cell pluripotency in mammals. Recent work shows that histone methylation is associated with aging, yet the underlying mechanism remains unclear. In this work, we identified a class of putative histone 3 lysine 9 mono/dimethyltransferase genes (met-2, set-6, set-19, set-20, set-21, set-32, and set-33), mutations in which induce synergistic lifespan extension in the long-lived DAF-2 (insulin growth factor 1 [IGF-1] receptor) mutant in Caenorhabditis elegans. These putative histone methyltransferase plus daf-2 double mutants not only exhibited an average lifespan nearly three times that of wild-type animals and a maximal lifespan of approximately 100 days, but also significantly increased resistance to oxidative and heat stress. Synergistic lifespan extension depends on the transcription factor DAF-16 (FOXO). mRNA-seq experiments revealed that the mRNA levels of DAF-16 Class I genes, which are activated by DAF-16, were further elevated in the daf-2;set double mutants. Among these genes, tts-1, F35E8.7, ins-35, nhr-62, sod-3, asm-2, and Y39G8B.7 are required for the lifespan extension of the daf-2;set-21 double mutant. In addition, treating daf-2 animals with the H3K9me1/2 methyltransferase G9a inhibitor also extends lifespan and increases stress resistance. Therefore, investigation of DAF-2 and H3K9me1/2 deficiency-mediated synergistic longevity will contribute to a better understanding of the molecular mechanisms of aging and therapeutic applications.

IFI16 Partners with KAP1 to Maintain Epstein-Barr Virus Latency

Herpesviruses establish latency to ensure permanent residence in their hosts. Upon entry into a cell, these viruses are rapidly silenced by the host, thereby limiting the destructive viral lytic phase while allowing the virus to hide from the immune system. Notably, although the establishment of latency by the oncogenic herpesvirus Epstein-Barr virus (EBV) requires the expression of viral latency genes, latency can be maintained with a negligible expression of viral genes. Indeed, in several herpesviruses, the host DNA sensor IFI16 facilitated latency via H3K9me3 heterochromatinization. This silencing mark is typically imposed by the constitutive heterochromatin machinery (HCM). The HCM, in an antiviral role, also silences the lytic phase of EBV and other herpes viruses. We investigated if IFI16 restricted EBV lytic activation by partnering with the HCM and found that IFI16 interacted with core components of the HCM, including the KRAB-associated protein 1 (KAP1) and the site-specific DNA binding KRAB-ZFP SZF1. This partnership silenced the EBV lytic switch protein ZEBRA, encoded by the BZLF1 gene, thereby favoring viral latency. Indeed, IFI16 contributed to H3K9 trimethylation at lytic genes of all kinetic classes. In defining topology, we found that IFI16 coenriched with KAP1 at the BZLF1 promoter, and while IFI16 and SZF1 were each adjacent to KAP1 in latent cells, IFI16 and SZF1 were not. Importantly, we also found that disruption of latency involved rapid downregulation of IFI16 transcription. These findings revealed a previously unknown partnership between IFI16 and the core HCM that supports EBV latency via antiviral heterochromatic silencing. IMPORTANCE The interferon-gamma inducible protein 16 (IFI16) is a nuclear DNA sensor that mediates antiviral responses by activating the inflammasome, triggering an interferon response, and silencing lytic genes of herpesviruses. The last, which helps maintain latency of the oncoherpesvirus Epstein-Barr virus (EBV), is accomplished via H3K9me3 heterochromatinization through unknown mechanisms. Here, we report that IFI16 physically partners with the core constitutive heterochromatin machinery to silence the key EBV lytic switch protein, thereby ensuring continued viral latency in B lymphocytes. We also find that disruption of latency involves rapid transcriptional downregulation of IFI16. These findings point to hitherto unknown physical and functional partnerships between a well-known antiviral mechanism and the core components of the constitutive heterochromatin machinery.

H3K27me3 at pericentromeric heterochromatin is a defining feature of the early mouse blastocyst

Early mouse development is characterized by structural and epigenetic changes while cells progress towards differentiation. At blastocyst stage, the segregation of the three primordial lineages is accompanied by establishment of differential patterns of DNA methylation and post-translational modifications of histones, such as H3K27me3. Here, we analysed the dynamics of H3K27me3 at pericentromeric heterochromatin (PCH) during early development. We also followed the localization of EZH2 and BEND3, previously shown in ESCs to drive PRC2 to hypomethylated PCH. We show that the location of H3K27me3 at PCH, in addition to H3K9me3, is a defining feature of embryonic cells in vivo. Moreover, it may play an important role in structuring PCH and preserving genomic integrity at a time of globally relaxed chromatin. At peri-implantation stages, while DNA methylation is still low, EZH2 and then H3K27me3, leave PCH in epiblast progenitors at the time of their spatial segregation from primitive endoderm cells, while BEND3 remains there up to implantation. The comparison with stem cells (ESCs and TSCs) reveals that the epigenetic marks (i.e. H3K9me3 and H3K27me3) of PCH are reset during in vitro derivation and only partially restored thereafter. This highlights possible divergences between in vitro and "in embryo" epigenetic regulation regarding constitutive heterochromatin.

Role of Transposable Elements in Genome Stability: Implications for Health and Disease

Most living organisms have in their genome a sizable proportion of DNA sequences capable of mobilization; these sequences are commonly referred to as transposons, transposable elements (TEs), or jumping genes. Although long thought to have no biological significance, advances in DNA sequencing and analytical technologies have enabled precise characterization of TEs and confirmed their ubiquitous presence across all forms of life. These findings have ignited intense debates over their biological significance. The available evidence now supports the notion that TEs exert major influence over many biological aspects of organismal life. Transposable elements contribute significantly to the evolution of the genome by giving rise to genetic variations in both active and passive modes. Due to their intrinsic nature of mobility within the genome, TEs primarily cause gene disruption and large-scale genomic alterations including inversions, deletions, and duplications. Besides genomic instability, growing evidence also points to many physiologically important functions of TEs, such as gene regulation through cis-acting control elements and modulation of the transcriptome through epigenetic control. In this review, we discuss the latest evidence demonstrating the impact of TEs on genome stability and the underling mechanisms, including those developed to mitigate the deleterious impact of TEs on genomic stability and human health. We have also highlighted the potential therapeutic application of TEs.

Genome-wide identification of functional enhancers and their potential roles in pig breeding

Background

The pig is an economically important livestock species and is a widely applied large animal model in medical research. Enhancers are critical regulatory elements that have fundamental functions in evolution, development and disease. Genome-wide quantification of functional enhancers in the pig is needed.

Results

We performed self-transcribing active regulatory region sequencing (STARR-seq) in the porcine kidney epithelial PK15 and testicular ST cell lines, and reliably identified 2576 functional enhancers. Most of these enhancers were located in repetitive sequences and were enriched within silent and lowly expressed genes. Enhancers poorly overlapped with chromatin accessibility regions and were highly enriched in chromatin with the repressive histone modification H3K9me3, which is different from predicted pig enhancers detected using ChIP-seq for H3K27ac or/and H3K4me1 modified histones. This suggests that most pig enhancers identified with STARR-seq are endogenously repressed at the chromatin level and may function during cell type-specific development or at specific developmental stages. Additionally, the PPP3CA gene is associated with the loin muscle area trait and the QKI gene is associated with alkaline phosphatase activity that may be regulated by distal functional enhancers.

Conclusions

In summary, we generated the first functional enhancer map in PK15 and ST cells for the pig genome and highlight its potential roles in pig breeding.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-022-00726-y.

Satellite repeat transcripts modulate heterochromatin condensates and safeguard chromosome stability in mouse embryonic stem cells

Heterochromatin maintains genome integrity and function, and is organised into distinct nuclear domains. Some of these domains are proposed to form by phase separation through the accumulation of HP1ɑ. Mouse heterochromatin contains noncoding major satellite repeats (MSR), which are highly transcribed in mouse embryonic stem cells (ESCs). Here, we report that MSR transcripts can drive the formation of HP1ɑ droplets in vitro, and modulate heterochromatin into dynamic condensates in ESCs, contributing to the formation of large nuclear domains that are characteristic of pluripotent cells. Depleting MSR transcripts causes heterochromatin to transition into a more compact and static state. Unexpectedly, changing heterochromatin's biophysical properties has severe consequences for ESCs, including chromosome instability and mitotic defects. These findings uncover an essential role for MSR transcripts in modulating the organisation and properties of heterochromatin to preserve genome stability. They also provide insights into the processes that could regulate phase separation and the functional consequences of disrupting the properties of heterochromatin condensates.

Repeat expression is linked to patient survival and exhibits single nucleotide variation in pancreatic cancer revealing LTR70:r.879A>G

Despite an overwhelming number of cancer literature reporting the links between patient survival and the expression levels of genes or mutations/single nucleotide variations (SNVs) on them, there is only limited information on repeat elements, which make at least half the human genome. Here, we analysed RNA-seq data obtained from primary pancreatic cancer tissues of 51 patients and revealed that two transposons, HERVI-int and X6A_LINE, showed an upregulation trend in the patients who lived shorter, along with 56 other potential repeats which were linked to survival. We also detected expressed single nucleotide variations (SNVs) on repeats, among which LTR70:r.879A>G stands out with the effect of its presence on this particular repeat's expression levels and a significant link to overall patient survival. Interestingly, the expression of LTR70:r.879A>G correlated with different cancer genes in comparison to its reference version highlighting the involvement of BRAF and Fumerate Hydratase with this expressed SNV. This is one of the first studies revealing possible links between repeat expression and survival in cancer and it warrants further research in this avenue.

Establishment of H3K9-methylated heterochromatin and its functions in tissue differentiation and maintenance

Heterochromatin is characterized by dimethylated or trimethylated histone H3 Lys9 (H3K9me2 or H3K9me3, respectively) and is found at transposable elements, satellite repeats and genes, where it ensures their transcriptional silencing. The histone methyltransferases (HMTs) that methylate H3K9 — in mammals Suppressor of variegation 3–9 homologue 1 (SUV39H1), SUV39H2, SET domain bifurcated 1 (SETDB1), SETDB2, G9A and G9A-like protein (GLP) — and the ‘readers’ of H3K9me2 or H3K9me3 are highly conserved and show considerable redundancy. Despite their redundancy, genetic ablation or mistargeting of an individual H3K9 methyltransferase can correlate with impaired cell differentiation, loss of tissue identity, premature aging and/or cancer. In this Review, we discuss recent advances in understanding the roles of the known H3K9-specific HMTs in ensuring transcriptional homeostasis during tissue differentiation in mammals. We examine the effects of H3K9-methylation-dependent gene repression in haematopoiesis, muscle differentiation and neurogenesis in mammals, and compare them with mechanistic insights obtained from the study of model organisms, notably Caenorhabditis elegans and Drosophila melanogaster. In all these organisms, H3K9-specific HMTs have both unique and redundant roles that ensure the maintenance of tissue integrity by restricting the binding of transcription factors to lineage-specific promoters and enhancer elements.

Histone H3 Lys9 (H3K9)-methylated heterochromatin ensures transcriptional silencing of repetitive elements and genes, and its deregulation leads to impaired cell and tissue identity, premature aging and cancer. Recent studies in mammals clarified the roles H3K9-specific histone methyltransferases in ensuring transcriptional homeostasis during tissue differentiation.

Lrwd1 impacts cell proliferation and the silencing of repetitive DNA elements

LRWD1, also known as ORCA, is a nuclear protein functioning in multiple biological processes. Using its WD40 domain LRWD1 interacts with repressive histone marks and maintains the silencing of heterochromatin regions in mammalian cells. ORCA also associates with the origin recognition complex (ORC) and facilitates prereplication complex formation at late‐replicating origins. However, whether LRWD1 plays a role during development and the functional significance of LRWD1 in vivo remains largely unknown. Using gene‐trap approach we generated Lrwd1 knockout mice and examined the expression of Lrwd1 during embryonic development. We found that Lrwd1 is ubiquitously expressed in the majority of the developing mouse embryo. Depletion of LRWD1 did not affect embryonic development but the postnatal growth of the homozygous mutants is retarded. In vitro cultured mouse embryonic fibroblasts (MEFs) depleted of LRWD1 displayed a reduced proliferation compared to wild type cells. We also showed that the knockout of Lrwd1 in MEFs increased the expression of the epigenetically silenced repetitive elements but with minimal effect on the expression of protein coding genes. Together, these results suggest that LRWD1 plays an important, but not essential, role in postnatal development by regulating cell proliferation likely through modulating DNA replication.

Transposable Elements: Major Players in Shaping Genomic and Evolutionary Patterns

Transposable elements (TEs) are ubiquitous genetic elements, able to jump from one location of the genome to another, in all organisms. For this reason, on the one hand, TEs can induce deleterious mutations, causing dysfunction, disease and even lethality in individuals. On the other hand, TEs can increase genetic variability, making populations better equipped to respond adaptively to environmental change. To counteract the deleterious effects of TEs, organisms have evolved strategies to avoid their activation. However, their mobilization does occur. Usually, TEs are maintained silent through several mechanisms, but they can be reactivated during certain developmental windows. Moreover, TEs can become de-repressed because of drastic changes in the external environment. Here, we describe the ‘double life’ of TEs, being both ‘parasites’ and ‘symbionts’ of the genome. We also argue that the transposition of TEs contributes to two important evolutionary processes: the temporal dynamic of evolution and the induction of genetic variability. Finally, we discuss how the interplay between two TE-dependent phenomena, insertional mutagenesis and epigenetic plasticity, plays a role in the process of evolution.

HP1 maintains protein stability of H3K9 methyltransferases and demethylases

Di- or tri-methylated H3K9 (H3K9me2/3) is an epigenetic mark of heterochromatin. Heterochromatin protein 1 (HP1) specifically recognizes H3K9me2/3, contributing to transcriptional suppression and spread of H3K9me2/3. Here, we demonstrate another role of HP1 in heterochromatin organization: regulation of protein stability of H3K9 methyltransferases (H3K9 MTs) and demethylases (H3K9 DMs). We show that HP1 interaction-defective mutants of H3K9 MTs, Suv39h1 and Setdb1, undergo protein degradation. We further establish mouse embryonic stem cell lines lacking all three HP1 paralogs. In the HP1-deficient cells, Suv39h1, Suv39h2, Setdb1, and G9a/GLP complex decrease at the protein level, and the enzymes are released from chromatin. HP1 mutants that cannot recognize H3K9me2/3 or form dimers cannot stabilize these enzymes, indicating that the tethering of H3K9 MTs to chromatin is critical for their protein stability. We show that HP1 also stabilizes H3K9 DMs, Jmjd1a and Jmjd1b. Our study indicates that mammalian HP1 forms a heterochromatin hub that governs protein stability of H3K9 MTs and H3K9 DMs.

Histone Modifications in Mouse Pronuclei and Consequences for Embryo Development

Epigenetic marks, such as DNA methylation and posttranslational modifications of core histones, are the key regulators of gene expression. In the mouse, many of these marks are erased during gamete formation and must be introduced de novo after fertilization. Some of them appear synchronously, but the others are deposited asynchronously and/or remain differently distributed on maternal and paternal chromatin. Although the mechanisms regulating these processes are not entirely understandable, it is commonly accepted that epigenetic reprogramming occurring during the first cell cycle of a mouse embryo is crucial for its further development. This chapter focuses on selected epigenetic modifications, such as DNA methylation, the introduction of histone variants, histones acetylation, phosphorylation, and methylation. Properly depositing these marks on maternal and paternal chromatin is crucial for normal embryonic development.

Global Gene Expression Regulation Mediated by TGFβ Through H3K9me3 Mark

Background:

Epigenetic alterations play an important part in carcinogenesis. Different biological responses, including cell proliferation, migration, apoptosis, invasion, and senescence, are affected by epigenetic alterations in cancer. In addition, growth factors, such as transforming growth factor beta (TGFβ) are important regulators of tumorigenesis. Our understanding of the interplay between the epigenetic bases of tumorigenesis and growth factor signaling in tumorigenesis is rudimentary. Some studies suggest a link between TGFβ signaling and the heterochromatinizing histone mark H3K9me3. There is evidence for signal-dependent interactions between R-Smads and histone methyltransferases. However, the effects of TGFβ signaling on genome wide H3K9me3 landscape remains unknown. Our research examines TGFβ -induced genome-wide H3K9me3 in prostate cancer.

Method:

Chromatin-Immunoprecipitation followed by sequencing was performed to analyze genome-wide association of H3K9me3 epigenetic mark. DAVID Functional annotation tool was utilized to understand the involvement of different Biological Processes and Molecular Function. MEME-ChIP tool was also used to analyze known and novel DNA-binding motifs.

Results:

H3K9me3 occupancy appears to increase at intronic regions after short-term (6 hours) TGFβ stimulation and at distal intergenic regions during long-term stimulation (24 hours). We also found evidence for a possible association of SLC transporters with H3K9me3 mark in presence of TGFβ during tumorigenesis. No direct correlation was found between the occupancy of H3K9me3 mark and the expression of various genes. The epigenetic mechanisms-mediated regulation of gene expression by TGFβ was concentrated at promoters rich in SRY and FOXJ3 binding sites.

Conclusion:

Our results point toward a positive association of oncogenic function of TGFβ and the H3K9me3 mark and provide a context to the role of H3K9me3 in TGFβ-induced cell migration and cell adhesion. Interestingly, these functions of TGFβ through H3K9me3 mark regulation seem to depend on transcriptional activation in contrast to the conventionally known repressive nature of H3K9me3.

CUT&Tag-BS for simultaneous profiling of histone modification and DNA methylation with high efficiency and low cost

It remains a challenge to decipher the complex relationship between DNA methylation, histone modification, and the underlying DNA sequence with limited input material. Here, we developed an efficient, low-input, and low-cost method for the simultaneous profiling of genomic localization of histone modification and methylation status of the underlying DNA at single-base resolution from the same cells in a single experiment by integrating cleavage under targets and tagmentation (CUT&Tag) with tagmentation-based bisulfite sequencing (CUT&Tag-BS). We demonstrated the validity of our method using representative histone modifications of euchromatin and constitutive and facultative heterochromatin (H3K4me1, H3K9me3, and H3K27me3, respectively). Similar histone modification enrichment patterns were observed in CUT&Tag-BS compared with non-bisulfite-treated control, and H3K4me1-marked regions were found to mostly be CpG poor, lack methylation concordance, and exhibit prevalent DNA methylation heterogeneity among mouse embryonic stem cells (mESCs). We anticipate that CUT&Tag-BS will be widely applied to directly address the genomic relationship between DNA methylation and histone modification, especially in low-input scenarios with precious biological samples.

Foxd3 controls heterochromatin-mediated repression of repeat elements and 2-cell state transcription

Repeat element transcription plays a vital role in early embryonic development. The expression of repeats such as MERVL characterises mouse embryos at the 2‐cell stage and defines a 2‐cell‐like cell (2CLC) population in a mouse embryonic stem cell culture. Repeat element sequences contain binding sites for numerous transcription factors. We identify the forkhead domain transcription factor FOXD3 as a regulator of major satellite repeats and MERVL transcription in mouse embryonic stem cells. FOXD3 binds to and recruits the histone methyltransferase SUV39H1 to MERVL and major satellite repeats, consequentially repressing the transcription of these repeats by the establishment of the H3K9me3 heterochromatin modification. Notably, depletion of FOXD3 leads to the de‐repression of MERVL and major satellite repeats as well as a subset of genes expressed in the 2‐cell state, shifting the balance between the stem cell and 2‐cell‐like population in culture. Thus, FOXD3 acts as a negative regulator of repeat transcription, ascribing a novel function to this transcription factor.

CHD4 Conceals Aberrant CTCF-Binding Sites at TAD Interiors by Regulating Chromatin Accessibility in Mouse Embryonic Stem Cells

CCCTC-binding factor (CTCF) critically contributes to 3D chromatin organization by determining topologically associated domain (TAD) borders. Although CTCF primarily binds at TAD borders, there also exist putative CTCF-binding sites within TADs, which are spread throughout the genome by retrotransposition. However, the detailed mechanism responsible for masking the putative CTCF-binding sites remains largely elusive. Here, we show that the ATP-dependent chromatin remodeler, chromodomain helicase DNA-binding 4 (CHD4), regulates chromatin accessibility to conceal aberrant CTCF-binding sites embedded in H3K9me3-enriched heterochromatic B2 short interspersed nuclear elements (SINEs) in mouse embryonic stem cells (mESCs). Upon CHD4 depletion, these aberrant CTCF-binding sites become accessible and aberrant CTCF recruitment occurs within TADs, resulting in disorganization of local TADs. RNA-binding intrinsically disordered domains (IDRs) of CHD4 are required to prevent this aberrant CTCF binding, and CHD4 is critical for the repression of B2 SINE transcripts. These results collectively reveal that a CHD4-mediated mechanism ensures appropriate CTCF binding and associated TAD organization in mESCs.

Is There a Histone Code for Cellular Quiescence?

Many of the cells in our bodies are quiescent, that is, temporarily not dividing. Under certain physiological conditions such as during tissue repair and maintenance, quiescent cells receive the appropriate stimulus and are induced to enter the cell cycle. The ability of cells to successfully transition into and out of a quiescent state is crucial for many biological processes including wound healing, stem cell maintenance, and immunological responses. Across species and tissues, transcriptional, epigenetic, and chromosomal changes associated with the transition between proliferation and quiescence have been analyzed, and some consistent changes associated with quiescence have been identified. Histone modifications have been shown to play a role in chromatin packing and accessibility, nucleosome mobility, gene expression, and chromosome arrangement. In this review, we critically evaluate the role of different histone marks in these processes during quiescence entry and exit. We consider different model systems for quiescence, each of the most frequently monitored candidate histone marks, and the role of their writers, erasers and readers. We highlight data that support these marks contributing to the changes observed with quiescence. We specifically ask whether there is a quiescence histone “code,” a mechanism whereby the language encoded by specific combinations of histone marks is read and relayed downstream to modulate cell state and function. We conclude by highlighting emerging technologies that can be applied to gain greater insight into the role of a histone code for quiescence.

Differential enrichment of H3K9me3 at annotated satellite DNA repeats in human cell lines and during fetal development in mouse

BACKGROUND

Trimethylation of histone H3 on lysine 9 (H3K9me3) at satellite DNA sequences has been primarily studied at (peri)centromeric regions, where its level shows differences associated with various processes such as development and malignant transformation. However, the dynamics of H3K9me3 at distal satellite DNA repeats has not been thoroughly investigated.

RESULTS

We exploit the sets of publicly available data derived from chromatin immunoprecipitation combined with massively parallel DNA sequencing (ChIP-Seq), produced by the The Encyclopedia of DNA Elements (ENCODE) project, to analyze H3K9me3 at assembled satellite DNA repeats in genomes of human cell lines and during mouse fetal development. We show that annotated satellite elements are generally enriched for H3K9me3, but its level in cancer cell lines is on average lower than in normal cell lines. We find 407 satellite DNA instances with differential H3K9me3 enrichment between cancer and normal cells including a large 115-kb cluster of GSATII elements on chromosome 12. Differentially enriched regions are not limited to satellite DNA instances, but instead encompass a wider region of flanking sequences. We found no correlation between the levels of H3K9me3 and noncoding RNA at corresponding satellite DNA loci. The analysis of data derived from multiple tissues identified 864 instances of satellite DNA sequences in the mouse reference genome that are differentially enriched between fetal developmental stages.

CONCLUSIONS

Our study reveals significant differences in H3K9me3 level at a subset of satellite repeats between biological states and as such contributes to understanding of the role of satellite DNA repeats in epigenetic regulation during development and carcinogenesis.

COVID-19 Is a Multi-Organ Aggressor: Epigenetic and Clinical Marks

The progression of coronavirus disease 2019 (COVID-19), resulting from a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, may be influenced by both genetic and environmental factors. Several viruses hijack the host genome machinery for their own advantage and survival, and similar phenomena might occur upon SARS-CoV-2 infection. Severe cases of COVID-19 may be driven by metabolic and epigenetic driven mechanisms, including DNA methylation and histone/chromatin alterations. These epigenetic phenomena may respond to enhanced viral replication and mediate persistent long-term infection and clinical phenotypes associated with severe COVID-19 cases and fatalities. Understanding the epigenetic events involved, and their clinical significance, may provide novel insights valuable for the therapeutic control and management of the COVID-19 pandemic. This review highlights different epigenetic marks potentially associated with COVID-19 development, clinical manifestation, and progression.

Role of long interspersed nuclear element-1 in the regulation of chromatin landscapes and genome dynamics

LINE-1 retrotransposon, the most active mobile element of the human genome, is subject to tight regulatory control. Stressful environments and disease modify the recruitment of regulatory proteins leading to unregulated activation of LINE-1. The activation of LINE-1 influences genome dynamics through altered chromatin landscapes, insertion mutations, deletions, and modulation of cellular plasticity. To date, LINE-1 retrotransposition has been linked to various cancer types and may in fact underwrite the genetic basis of various other forms of chronic human illness. The occurrence of LINE-1 polymorphisms in the human population may define inter-individual differences in susceptibility to disease. This review is written in honor of Dr Peter Stambrook, a friend and colleague who carried out highly impactful cancer research over many years of professional practice. Dr Stambrook devoted considerable energy to helping others live up to their full potential and to navigate the complexities of professional life. He was an inspirational leader, a strong advocate, a kind mentor, a vocal supporter and cheerleader, and yes, a hard critic and tough friend when needed. His passionate stand on issues, his witty sense of humor, and his love for humanity have left a huge mark in our lives. We hope that that the knowledge summarized here will advance our understanding of the role of LINE-1 in cancer biology and expedite the development of innovative cancer diagnostics and treatments in the ways that Dr Stambrook himself had so passionately envisioned.

More than causing (epi)genomic instability: emerging physiological implications of transposable element modulation

Transposable elements (TEs) initially attracted attention because they comprise a major portion of the genomic sequences in plants and animals. TEs may jump around the genome and disrupt both coding genes as well as regulatory sequences to cause disease. Host cells have therefore evolved various epigenetic and functional RNA-mediated mechanisms to mitigate the disruption of genomic integrity by TEs. TE associated sequences therefore acquire the tendencies of attracting various epigenetic modifiers to induce epigenetic alterations that may spread to the neighboring genes. In addition to posting threats for (epi)genome integrity, emerging evidence suggested the physiological importance of endogenous TEs either as cis-acting control elements for controlling gene regulation or as TE-containing functional transcripts that modulate the transcriptome of the host cells. Recent advances in long-reads sequence analysis technologies, bioinformatics and genetic editing tools have enabled the profiling, precise annotation and functional characterization of TEs despite their challenging repetitive nature. The importance of specific TEs in preimplantation embryonic development, germ cell differentiation and meiosis, cell fate determination and in driving species specific differences in mammals will be discussed.