Nucleosome Turnover Regulates Histone Methylation Patterns over the Genome

Targeting DOT1L and EZH2 synergizes in breaking the germinal center identity of diffuse large B-cell lymphoma

ABSTRACT

Differentiation of antigen-activated B cells into proproliferative germinal center (GC) B cells depends on the activity of the transcription factors myelocytoma (MYC) and B-cell lymphoma 6 (BCL6), and the epigenetic writers disruptor of telomeric silencing 1-like (DOT1L) and enhancer of zeste homolog 2 (EZH2). GCB-like diffuse large B-cell lymphomas (GCB-DLBCLs) arise from GCB cells and closely resemble their cell of origin. Given the dependency of GCB cells on DOT1L and EZH2, we investigated the role of these epigenetic regulators in GCB-DLBCLs and observed that GCB-DLBCLs synergistically depend on the combined activity of DOT1L and EZH2. Mechanistically, inhibiting both enzymes led to enhanced derepression of polycomb repressive complex 2 target genes compared with EZH2 single treatment, along with the upregulation of BCL6 target genes and suppression of MYC target genes. The sum of all these alterations results in a "cell identity crisis," wherein GCB-DLBCLs lose their proproliferative GC identity and partially undergo plasma cell differentiation, a state associated with poor survival. In support of this model, combined epidrugging of DOT1L and EZH2 prohibited the outgrowth of human GCB-DLBCL xenografts in vivo. We conclude that the malignant behavior of GCB-DLBCLs strongly depends on DOT1L and EZH2 and that combined targeting of both epigenetic writers may provide an alternative differentiation-based treatment modality for GCB-DLBCL.

Metabolism-driven chromatin dynamics: Molecular principles and technological advances

Cells integrate metabolic information into core molecular processes such as transcription to adapt to environmental changes. Chromatin, the physiological template of the eukaryotic genome, has emerged as a sensor and rheostat for fluctuating intracellular metabolites. In this review, we highlight the growing list of chromatin-associated metabolites that are derived from diverse sources. We discuss recent advances in our understanding of the mechanisms by which metabolic enzyme activities shape the chromatin structure and modifications, how specificity may emerge from their seemingly broad effects, and technologies that facilitate the study of epigenome-metabolome interplay. The recognition that metabolites are immanent components of the chromatin regulatory network has significant implications for the evolution, function, and therapeutic targeting of the epigenome.

Combined small-molecule treatment accelerates maturation of human pluripotent stem cell-derived neurons

The maturation of human pluripotent stem cell (hPSC)-derived neurons mimics the protracted timing of human brain development, extending over months to years for reaching adult-like function. Prolonged in vitro maturation presents a major challenge to stem cell-based applications in modeling and treating neurological disease. Therefore, we designed a high-content imaging assay based on morphological and functional readouts in hPSC-derived cortical neurons which identified multiple compounds that drive neuronal maturation including inhibitors of lysine-specific demethylase 1 and disruptor of telomerase-like 1 and activators of calcium-dependent transcription. A cocktail of four factors, GSK2879552, EPZ-5676, N-methyl-D-aspartate and Bay K 8644, collectively termed GENtoniK, triggered maturation across all parameters tested, including synaptic density, electrophysiology and transcriptomics. Maturation effects were further validated in cortical organoids, spinal motoneurons and non-neural lineages including melanocytes and pancreatic β-cells. The effects on maturation observed across a broad range of hPSC-derived cell types indicate that some of the mechanisms controlling the timing of human maturation might be shared across lineages.

Nucleosome remodeler exclusion by histone deacetylation enforces heterochromatic silencing and epigenetic inheritance

Heterochromatin enforces transcriptional gene silencing and can be epigenetically inherited, but the underlying mechanisms remain unclear. Here, we show that histone deacetylation, a conserved feature of heterochromatin domains, blocks SWI/SNF subfamily remodelers involved in chromatin unraveling, thereby stabilizing modified nucleosomes that preserve gene silencing. Histone hyperacetylation, resulting from either the loss of histone deacetylase (HDAC) activity or the direct targeting of a histone acetyltransferase to heterochromatin, permits remodeler access, leading to silencing defects. The requirement for HDAC in heterochromatin silencing can be bypassed by impeding SWI/SNF activity. Highlighting the crucial role of remodelers, merely targeting SWI/SNF to heterochromatin, even in cells with functional HDAC, increases nucleosome turnover, causing defective gene silencing and compromised epigenetic inheritance. This study elucidates a fundamental mechanism whereby histone hypoacetylation, maintained by high HDAC levels in heterochromatic regions, ensures stable gene silencing and epigenetic inheritance, providing insights into genome regulatory mechanisms relevant to human diseases.

Precision epigenetic editing: Technological advances, enduring challenges, and therapeutic applications

The epigenome is a complex framework through which gene expression is precisely and flexibly modulated to incorporate heritable memory and responses to environmental stimuli. It governs diverse cellular processes, including cell fate, disease, and aging. The need to understand this system and precisely control gene expression outputs for therapeutic purposes has precipitated the development of a diverse set of epigenetic editing tools. Here, we review the existing toolbox for targeted epigenetic editing, technical considerations of the current technologies, and opportunities for future development. We describe applications of therapeutic epigenetic editing and their potential for treating disease, with a discussion of ongoing delivery challenges that impede certain clinical interventions, particularly in the brain. With simultaneous advancements in available engineering tools and appropriate delivery technologies, we predict that epigenetic editing will increasingly cement itself as a powerful approach for safely treating a wide range of disorders in all tissues of the body.

An emerging maestro of immune regulation: how DOT1L orchestrates the harmonies of the immune system

The immune system comprises a complex yet tightly regulated network of cells and molecules that play a critical role in protecting the body from infection and disease. The activity and development of each immune cell is regulated in a myriad of ways including through the cytokine milieu, the availability of key receptors, via tailored intracellular signalling cascades, dedicated transcription factors and even by directly modulating gene accessibility and expression; the latter is more commonly known as epigenetic regulation. In recent years, epigenetic regulators have begun to emerge as key players involved in modulating the immune system. Among these, the lysine methyltransferase DOT1L has gained significant attention for its involvement in orchestrating immune cell formation and function. In this review we provide an overview of the role of DOT1L across the immune system and the implications of this role on health and disease. We begin by elucidating the general mechanisms of DOT1L-mediated histone methylation and its impact on gene expression within immune cells. Subsequently, we provide a detailed and comprehensive overview of recent studies that identify DOT1L as a crucial regulator of immune cell development, differentiation, and activation. Next, we discuss the potential mechanisms of DOT1L-mediated regulation of immune cell function and shed light on how DOT1L might be contributing to immune cell homeostasis and dysfunction. We then provide food for thought by highlighting some of the current obstacles and technical limitations precluding a more in-depth elucidation of DOT1L's role. Finally, we explore the potential therapeutic implications of targeting DOT1L in the context of immune-related diseases and discuss ongoing research efforts to this end. Overall, this review consolidates the current paradigm regarding DOT1L's role across the immune network and emphasises its critical role in governing the healthy immune system and its potential as a novel therapeutic target for immune-related diseases. A deeper understanding of DOT1L's immunomodulatory functions could pave the way for innovative therapeutic approaches which fine-tune the immune response to enhance or restore human health.

Proline-directed yeast and human MAP kinases phosphorylate the Dot1p/DOT1L histone H3K79 methyltransferase

Disruptor of telomeric silencing 1 (Dot1p) is an exquisitely conserved histone methyltransferase and is the sole enzyme responsible for H3K79 methylation in the budding yeast Saccharomyces cerevisiae. It has been shown to be highly phosphorylated in vivo; however, the upstream kinases that act on Dot1p are almost entirely unknown in yeast and all other eukaryotes. Here, we used in vitro and in vivo kinase discovery approaches to show that mitogen-activated protein kinase HOG1 (Hog1p) is a bona fide kinase of the Dot1p methyltransferase. In vitro kinase assays showed that Hog1p phosphorylates Dot1p at multiple sites, including at several proline-adjacent sites that are consistent with known Hog1p substrate preferences. The activity of Hog1p was specifically enhanced at these proline-adjacent sites on Dot1p upon Hog1p activation by the osmostress-responsive MAP kinase kinase PBS2 (Pbs2p). Genomic deletion of HOG1 reduced phosphorylation at specific sites on Dot1p in vivo, providing further evidence for Hog1p kinase activity on Dot1p in budding yeast cells. Phenotypic analysis of knockout and phosphosite mutant yeast strains revealed the importance of Hog1p-catalysed phosphorylation of Dot1p for cellular responses to ultraviolet-induced DNA damage. In mammalian systems, this kinase-substrate relationship was found to be conserved: human DOT1L (the ortholog of yeast Dot1p) can be phosphorylated by the proline-directed kinase p38β (also known as MAPK11; the ortholog of yeast Hog1p) at multiple sites in vitro. Taken together, our findings establish Hog1p and p38β as newly identified upstream kinases of the Dot1p/DOT1L H3K79 methyltransferase enzymes in eukaryotes.

Context-specific functions of chromatin remodellers in development and disease

Chromatin remodellers were once thought to be highly redundant and nonspecific in their actions. However, recent human genetic studies demonstrate remarkable biological specificity and dosage sensitivity of the thirty-two adenosine triphosphate (ATP)-dependent chromatin remodellers encoded in the human genome. Mutations in remodellers produce many human developmental disorders and cancers, motivating efforts to investigate their distinct functions in biologically relevant settings. Exquisitely specific biological functions seem to be an emergent property in mammals, and in many cases are based on the combinatorial assembly of subunits and the generation of stable, composite surfaces. Critical interactions between remodelling complex subunits, the nucleosome and other transcriptional regulators are now being defined from structural and biochemical studies. In addition, in vivo analyses of remodellers at relevant genetic loci have provided minute-by-minute insights into their dynamics. These studies are proposing new models for the determinants of remodeller localization and function on chromatin.

Mono-methylated histones control PARP-1 in chromatin and transcription

PARP-1 is central to transcriptional regulation under both normal and stress conditions, with the governing mechanisms yet to be fully understood. Our biochemical and ChIP-seq-based analyses showed that PARP-1 binds specifically to active histone marks, particularly H4K20me1. We found that H4K20me1 plays a critical role in facilitating PARP-1 binding and the regulation of PARP-1-dependent loci during both development and heat shock stress. Here, we report that the sole H4K20 mono-methylase, pr-set7, and parp-1 Drosophila mutants undergo developmental arrest. RNA-seq analysis showed an absolute correlation between PR-SET7- and PARP-1-dependent loci expression, confirming co-regulation during developmental phases. PARP-1 and PR-SET7 are both essential for activating hsp70 and other heat shock genes during heat stress, with a notable increase of H4K20me1 at their gene body. Mutating pr-set7 disrupts monomethylation of H4K20 along heat shock loci and abolish PARP-1 binding there. These data strongly suggest that H4 monomethylation is a key triggering point in PARP-1 dependent processes in chromatin.

Development of a New Model System to Study Long-Distance Interactions Supported by Architectural Proteins

Chromatin architecture is critical for the temporal and tissue-specific activation of genes that determine eukaryotic development. The functional interaction between enhancers and promoters is controlled by insulators and tethering elements that support specific long-distance interactions. However, the mechanisms of the formation and maintenance of long-range interactions between genome regulatory elements remain poorly understood, primarily due to the lack of convenient model systems. Drosophila became the first model organism in which architectural proteins that determine the activity of insulators were described. In Drosophila, one of the best-studied DNA-binding architectural proteins, Su(Hw), forms a complex with Mod(mdg4)-67.2 and CP190 proteins. Using a combination of CRISPR/Cas9 genome editing and attP-dependent integration technologies, we created a model system in which the promoters and enhancers of two reporter genes are separated by 28 kb. In this case, enhancers effectively stimulate reporter gene promoters in cis and trans only in the presence of artificial Su(Hw) binding sites (SBS), in both constructs. The expression of the mutant Su(Hw) protein, which cannot interact with CP190, and the mutation inactivating Mod(mdg4)-67.2, lead to the complete loss or significant weakening of enhancer-promoter interactions, respectively. The results indicate that the new model system effectively identifies the role of individual subunits of architectural protein complexes in forming and maintaining specific long-distance interactions in the D. melanogaster model.

Two-way feedback between chromatin compaction and histone modification state explains Saccharomyces cerevisiae heterochromatin bistability

Compact chromatin is closely linked with gene silencing in part by sterically masking access to promoters, inhibiting transcription factor binding and preventing polymerase from efficiently transcribing a gene. However, a broader hypothesis suggests that chromatin compaction can be both a cause and a consequence of the locus histone modification state, with a tight bidirectional interaction underpinning bistable transcriptional states. To rigorously test this hypothesis, we developed a mathematical model for the dynamics of the HMR locus in Saccharomyces cerevisiae, that incorporates activating histone modifications, silencing proteins, and a dynamic, acetylation-dependent, three-dimensional locus size. Chromatin compaction enhances silencer protein binding, which in turn feeds back to remove activating histone modifications, leading to further compaction. The bistable output of the model was in good agreement with prior quantitative data, including switching rates from expressed to silent states (and vice versa), and protein binding/histone modification levels within the locus. We then tested the model by predicting changes in switching rates as the genetic length of the locus was increased, which were then experimentally verified. Such bidirectional feedback between chromatin compaction and the histone modification state may be a widespread and important regulatory mechanism given the hallmarks of many heterochromatic regions: physical chromatin compaction and dimerizing (or multivalent) silencing proteins.

Polycomb repression of Hox genes involves spatial feedback but not domain compaction or phase transition

Polycomb group proteins have a critical role in silencing transcription during development. It is commonly proposed that Polycomb-dependent changes in genome folding, which compact chromatin, contribute directly to repression by blocking the binding of activating complexes. Recently, it has also been argued that liquid-liquid demixing of Polycomb proteins facilitates this compaction and repression by phase-separating target genes into a membraneless compartment. To test these models, we used Optical Reconstruction of Chromatin Architecture to trace the Hoxa gene cluster, a canonical Polycomb target, in thousands of single cells. Across multiple cell types, we find that Polycomb-bound chromatin frequently explores decompact states and partial mixing with neighboring chromatin, while remaining uniformly repressed, challenging the repression-by-compaction or phase-separation models. Using polymer simulations, we show that these observed flexible ensembles can be explained by 'spatial feedback'-transient contacts that contribute to the propagation of the epigenetic state (epigenetic memory), without inducing a globular organization.

The molecular basis of cell memory in mammals: The epigenetic cycle

Cell memory refers to the capacity of cells to maintain their gene expression program once the initiating environmental signal has ceased. This exceptional feature is key during the formation of mammalian organisms, and it is believed to be in part mediated by epigenetic factors that can endorse cells with the landmarks required to maintain transcriptional programs upon cell duplication. Here, we review current literature analyzing the molecular basis of epigenetic memory in mammals, with a focus on the mechanisms by which transcriptionally repressive chromatin modifications such as methylation of DNA and histone H3 are propagated through mitotic cell divisions. The emerging picture suggests that cellular memory is supported by an epigenetic cycle in which reversible activities carried out by epigenetic regulators in coordination with cell cycle transition create a multiphasic system that can accommodate both maintenance of cell identity and cell differentiation in proliferating stem cell populations.

Human neuronal maturation comes of age: cellular mechanisms and species differences

The delayed and prolonged postmitotic maturation of human neurons, compared with neurons from other species, may contribute to human-specific cognitive abilities and neurological disorders. Here we review the mechanisms of neuronal maturation, applying lessons from model systems to understand the specific features of protracted human cortical maturation and species differences. We cover cell-intrinsic features of neuronal maturation, including transcriptional, epigenetic and metabolic mechanisms, as well as cell-extrinsic features, including the roles of activity and synapses, the actions of glial cells and the contribution of the extracellular matrix. We discuss evidence for species differences in biochemical reaction rates, the proposed existence of an epigenetic maturation clock and the contributions of both general and modular mechanisms to species-specific maturation timing. Finally, we suggest approaches to measure, improve and accelerate the maturation of human neurons in culture, examine crosstalk and interactions among these different aspects of maturation and propose conceptual models to guide future studies.

Histone retention preserves epigenetic marks during heat stress-induced transcriptional memory in plants

Plants often experience recurrent stressful events, for example, during heat waves. They can be primed by heat stress (HS) to improve the survival of more severe heat stress conditions. At certain genes, sustained expression is induced for several days beyond the initial heat stress. This transcriptional memory is associated with hyper-methylation of histone H3 lysine 4 (H3K4me3), but it is unclear how this is maintained for extended periods. Here, we determined histone turnover by measuring the chromatin association of HS-induced histone H3.3. Genome-wide histone turnover was not homogenous; in particular, H3.3 was retained longer at heat stress memory genes compared to HS-induced non-memory genes during the memory phase. While low nucleosome turnover retained H3K4 methylation, methylation loss did not affect turnover, suggesting that low nucleosome turnover sustains H3K4 methylation, but not vice versa. Together, our results unveil the modulation of histone turnover as a mechanism to retain environmentally mediated epigenetic modifications.

DOT1L is a barrier to histone acetylation during reprogramming to pluripotency

Embryonic stem cells (ESCs) have transcriptionally permissive chromatin enriched for gene activation-associated histone modifications. A striking exception is DOT1L-mediated H3K79 dimethylation (H3K79me2) that is considered a positive regulator of transcription. We find that ESCs are depleted for H3K79me2 at shared locations of enrichment with somatic cells, which are highly and ubiquitously expressed housekeeping genes, and have lower RNA polymerase II (RNAPII) at the transcription start site (TSS) despite greater nascent transcription. Inhibiting DOT1L increases the efficiency of reprogramming of somatic to induced pluripotent stem cells, enables an ESC-like RNAPII pattern at the TSS, and functionally compensates for enforced RNAPII pausing. DOT1L inhibition increases H3K27 methylation and RNAPII elongation-enhancing histone acetylation without changing the expression of the causal histone-modifying enzymes. Only the maintenance of elevated histone acetylation is essential for enhanced reprogramming and occurs at loci that are depleted for H3K79me2. Thus, DOT1L inhibition promotes the hyperacetylation and hypertranscription pluripotent properties.

Nucleosome retention by histone chaperones and remodelers occludes pervasive DNA-protein binding

DNA packaging within chromatin depends on histone chaperones and remodelers that form and position nucleosomes. Cells express multiple such chromatin regulators with overlapping in-vitro activities. Defining specific in-vivo activities requires monitoring histone dynamics during regulator depletion, which has been technically challenging. We have recently generated histone-exchange sensors in Saccharomyces cerevisiae, which we now use to define the contributions of 15 regulators to histone dynamics genome-wide. While replication-independent exchange in unperturbed cells maps to promoters, regulator depletions primarily affected gene bodies. Depletion of Spt6, Spt16 or Chd1 sharply increased nucleosome replacement sequentially at the beginning, middle or end of highly expressed gene bodies. They further triggered re-localization of chaperones to affected gene body regions, which compensated for nucleosome loss during transcription complex passage, but concurred with extensive TF binding in gene bodies. We provide a unified quantitative screen highlighting regulator roles in retaining nucleosome binding during transcription and preserving genomic packaging.

DOT1L bridges transcription and heterochromatin formation at mammalian pericentromeres

Repetitive DNA elements are packaged in heterochromatin, but many require bursts of transcription to initiate and maintain long-term silencing. The mechanisms by which these heterochromatic genome features are transcribed remain largely unknown. Here, we show that DOT1L, a conserved histone methyltransferase that modifies lysine 79 of histone H3 (H3K79), has a specialized role in transcription of major satellite repeats to maintain pericentromeric heterochromatin and genome stability. We find that H3K79me3 is selectively enriched relative to H3K79me2 at repetitive elements in mouse embryonic stem cells (mESCs), that DOT1L loss compromises pericentromeric satellite transcription, and that this activity involves possible coordination between DOT1L and the chromatin remodeler SMARCA5. Stimulation of transcript production from pericentromeric repeats by DOT1L participates in stabilization of heterochromatin structures in mESCs and cleavage-stage embryos and is required for preimplantation viability. Our findings uncover an important role for DOT1L as a bridge between transcriptional activation of repeat elements and heterochromatin stability, advancing our understanding of how genome integrity is maintained and how chromatin state is set up during early development.

DOT1L activity affects neural stem cell division mode and reduces differentiation and ASNS expression

Cortical neurogenesis depends on the balance between self-renewal and differentiation of apical progenitors (APs). Here, we study the epigenetic control of AP's division mode by focusing on the enzymatic activity of the histone methyltransferase DOT1L. Combining lineage tracing with single-cell RNA sequencing of clonally related cells, we show at the cellular level that DOT1L inhibition increases neurogenesis driven by a shift of APs from asymmetric self-renewing to symmetric neurogenic consumptive divisions. At the molecular level, DOT1L activity prevents AP differentiation by promoting transcription of metabolic genes. Mechanistically, DOT1L inhibition reduces activity of an EZH2/PRC2 pathway, converging on increased expression of asparagine synthetase (ASNS), a microcephaly associated gene. Overexpression of ASNS in APs phenocopies DOT1L inhibition, and also increases neuronal differentiation of APs. Our data suggest that DOT1L activity/PRC2 crosstalk controls AP lineage progression by regulating asparagine metabolism.

In Vivo Identification of H3K9me2/H3K79me3 as an Epigenetic Barrier to Carcinogenesis

The highly dynamic nature of chromatin's structure, due to the epigenetic alterations of histones and DNA, controls cellular plasticity and allows the rewiring of the epigenetic landscape required for either cell differentiation or cell (re)programming. To dissect the epigenetic switch enabling the programming of a cancer cell, we carried out wide genome analysis of Histone 3 (H3) modifications during osteogenic differentiation of SH-SY5Y neuroblastoma cells. The most significant modifications concerned H3K27me2/3, H3K9me2, H3K79me1/2, and H3K4me1 that specify the process of healthy adult stem cell differentiation. Next, we translated these findings in vivo, assessing H3K27, H3K9, and H3K79 methylation states in biopsies derived from patients affected by basalioma, head and neck carcinoma, and bladder tumors. Interestingly, we found a drastic decrease in H3K9me2 and H3K79me3 in cancer specimens with respect to their healthy counterparts and also a positive correlation between these two epigenetic flags in all three tumors. Therefore, we suggest that elevated global levels of H3K9me2 and H3K79me3, present in normal differentiated cells but lost in malignancy, may reflect an important epigenetic barrier to tumorigenesis. This suggestion is further corroborated, at least in part, by the deranged expression of the most relevant H3 modifier enzymes, as revealed by bioinformatic analysis. Overall, our study indicates that the simultaneous occurrence of H3K9me2 and H3K79me3 is fundamental to ensure the integrity of differentiated tissues and, thus, their combined evaluation may represent a novel diagnostic marker and potential therapeutic target.

Histone exchange sensors reveal variant specific dynamics in mouse embryonic stem cells

Eviction of histones from nucleosomes and their exchange with newly synthesized or alternative variants is a central epigenetic determinant. Here, we define the genome-wide occupancy and exchange pattern of canonical and non-canonical histone variants in mouse embryonic stem cells by genetically encoded exchange sensors. While exchange of all measured variants scales with transcription, we describe variant-specific associations with transcription elongation and Polycomb binding. We found considerable exchange of H3.1 and H2B variants in heterochromatin and repeat elements, contrasting the occupancy and little exchange of H3.3 in these regions. This unexpected association between H3.3 occupancy and exchange of canonical variants is also evident in active promoters and enhancers, and further validated by reduced H3.1 dynamics following depletion of H3.3-specific chaperone, HIRA. Finally, analyzing transgenic mice harboring H3.1 or H3.3 sensors demonstrates the vast potential of this system for studying histone exchange and its impact on gene expression regulation in vivo.

Structure-Based Discovery of Inhibitors of the SARS-CoV-2 Nsp14 N7-Methyltransferase

An under-explored target for SARS-CoV-2 is the S-adenosyl methionine (SAM)-dependent methyltransferase Nsp14, which methylates the N7-guanosine of viral RNA at the 5'-end, allowing the virus to evade host immune response. We sought new Nsp14 inhibitors with three large library docking strategies. First, up to 1.1 billion lead-like molecules were docked against the enzyme's SAM site, leading to three inhibitors with IC50 values from 6 to 50 μM. Second, docking a library of 16 million fragments revealed 9 new inhibitors with IC50 values from 12 to 341 μM. Third, docking a library of 25 million electrophiles to covalently modify Cys387 revealed 7 inhibitors with IC50 values from 3.5 to 39 μM. Overall, 32 inhibitors encompassing 11 chemotypes had IC50 values < 50 μM and 5 inhibitors in 4 chemotypes had IC50 values < 10 μM. These molecules are among the first non-SAM-like inhibitors of Nsp14, providing starting points for future optimization.

The molecular basis of heterochromatin assembly and epigenetic inheritance

Heterochromatin plays a fundamental role in gene regulation, genome integrity, and silencing of repetitive DNA elements. Histone modifications are essential for the establishment of heterochromatin domains, which is initiated by the recruitment of histone-modifying enzymes to nucleation sites. This leads to the deposition of histone H3 lysine-9 methylation (H3K9me), which provides the foundation for building high-concentration territories of heterochromatin proteins and the spread of heterochromatin across extended domains. Moreover, heterochromatin can be epigenetically inherited during cell division in a self-templating manner. This involves a "read-write" mechanism where pre-existing modified histones, such as tri-methylated H3K9 (H3K9me3), support chromatin association of the histone methyltransferase to promote further deposition of H3K9me. Recent studies suggest that a critical density of H3K9me3 and its associated factors is necessary for the propagation of heterochromatin domains across multiple generations. In this review, I discuss the key experiments that have highlighted the importance of modified histones for epigenetic inheritance.

The H3K79 methylase DOT1, unreported in photosynthetic plants, exists in Alexandrium pacificum and participates in its growth regulation

Alexandrium pacificum is one of the typical toxic dinoflagellate species leading to harmful algal blooms (HABs). Histone modifications play key roles in many cellular events, but little is known about the mechanism of regulating A. pacificum growth. In this study, a total of 30 proteins containing the DOT1 domain were identified and analyzed. Some ApDOT1 gene expression levels were significantly influenced by light intensity and nitrogen by expression analysis and RT-qPCR validation. The enrichment of H3K79 methylation also showed a similar trend. In addition, ApDOT1.9 protein was proved to have the function of catalyzing the methylation of H3K79 by homology analysis and in vitro methylation. The results suggested that ApDOT1 proteins and H3K79 methylation were involved in responding to harmful algal blooms-inducing conditions (high light intensity, and high nitrogen), which provided basic information for further exploration of the regulatory mechanism of histone methylation in A. pacificum rapid growth.

Bidirectional regulation of postmitotic H3K27me3 distributions underlie cerebellar granule neuron maturation dynamics

The functional maturation of neurons is a prolonged process that extends past the mitotic exit and is mediated by the chromatin-dependent orchestration of gene transcription programs. We find that expression of this maturation gene program in mouse cerebellar granule neurons (CGNs) requires dynamic changes in the genomic distribution of histone H3 lysine 27 trimethylation (H3K27me3), demonstrating a function for this chromatin modification beyond its role in cell fate specification. The developmental loss of H3K27me3 at promoters of genes activated as CGNs mature is facilitated by the lysine demethylase and ASD-risk gene, Kdm6b. Interestingly, inhibition of the H3K27 methyltransferase EZH2 in newborn CGNs not only blocks the repression of progenitor genes but also impairs the induction of mature CGN genes, showing the importance of bidirectional H3K27me3 regulation across the genome. These data demonstrate that H3K27me3 turnover in developing postmitotic neurons regulates the temporal coordination of gene expression programs that underlie functional neuronal maturation.

Methylation of histone H3 lysine 4 is required for maintenance of beta cell function in adult mice

AIMS/HYPOTHESIS

Beta cells control glucose homeostasis via regulated production and secretion of insulin. This function arises from a highly specialised gene expression programme that is established during development and then sustained, with limited flexibility, in terminally differentiated cells. Dysregulation of this programme is seen in type 2 diabetes but mechanisms that preserve gene expression or underlie its dysregulation in mature cells are not well resolved. This study investigated whether methylation of histone H3 lysine 4 (H3K4), a marker of gene promoters with unresolved functional importance, is necessary for the maintenance of mature beta cell function.

METHODS

Beta cell function, gene expression and chromatin modifications were analysed in conditional Dpy30 knockout mice, in which H3K4 methyltransferase activity is impaired, and in a mouse model of diabetes.

RESULTS

H3K4 methylation maintains expression of genes that are important for insulin biosynthesis and glucose responsiveness. Deficient methylation of H3K4 leads to a less active and more repressed epigenome profile that locally correlates with gene expression deficits but does not globally reduce gene expression. Instead, developmentally regulated genes and genes in weakly active or suppressed states particularly rely on H3K4 methylation. We further show that H3K4 trimethylation (H3K4me3) is reorganised in islets from the Lepr^db/db mouse model of diabetes in favour of weakly active and disallowed genes at the expense of terminal beta cell markers with broad H3K4me3 peaks.

CONCLUSIONS/INTERPRETATION

Sustained methylation of H3K4 is critical for the maintenance of beta cell function. Redistribution of H3K4me3 is linked to gene expression changes that are implicated in diabetes pathology.

Antecedent chromatin organization determines cGAS recruitment to ruptured micronuclei

Micronuclei (MN) are cytosolic bodies that sequester acentric fragments or mis-segregated chromosomes from the primary nucleus. Spontaneous rupture of the MN envelope allows recognition by the viral receptor cyclic GMP-AMP synthase (cGAS), initiating interferon signaling downstream of DNA damage. Here, we demonstrate that MN rupture is permissive but not sufficient for cGAS localization. Chromatin characteristics such as histone 3, lysine 79 dimethylation (H3K79me2) are present in the nucleus before DNA damage, retained in ruptured MN, and regulate cGAS recruitment. cGAS is further responsive to dynamic intra-MN processes occurring prior to rupture, including transcription. MN chromatin tethering via the nucleosome acidic patch is necessary for cGAS-dependent interferon signaling. Our data suggest that both damage-antecedent nuclear chromatin status and MN-contained chromatin organizational changes dictate cGAS recruitment and the magnitude of the cGAS-driven interferon cascade. Our work defines MN as integrative signaling hubs for the cellular response to genotoxic stress.

Neuronal Dot1l Activity Acts as a Mitochondrial Gene-Repressor Associated with Human Brain Aging via H3K79 Hypermethylation

Methylation of histone 3 at lysine 79 (H3K79) and its catalyst, a disrupter of telomeric silencing (DOT1l), have been coupled to multiple forms of stress, such as bioenergetic and ER challenges. However, studies on H3K79 methylation and Dot1l in the (aging) brain and neurons are limited. This, together with the increasing evidence of a dynamic neuroepigenome, made us wonder if H3K79 methylation and its activator Dot1l could play important roles in brain aging and associated disorders. In aged humans, we found strong and consistent global hypermethylation of H3K79 in neurons. Specific in dopaminergic neurons, we found a strong increase in H3K79 methylation in lipofucsin positive neurons, which are linked to pathology. In animals, where we conditionally removed Dot1l, we found a rapid loss of H3K79 methylation. As a consequence, we found some decrease in specific dopaminergic genes, and surprisingly, a clear up-regulation of almost all genes belonging to the family of the respiratory chain. These data, in relation to the observed increase in global H3K79 methylation, suggest that there is an inverse relationship between H3K79 methylation and the capacity of energy metabolism in neuronal systems.

DOT1L inhibition does not modify the sensitivity of cutaneous T cell lymphoma to pan-HDAC inhibitors in vitro

Cutaneous T-cell lymphomas (CTCLs) are a subset of T-cell malignancies presenting in the skin. The treatment options for CTCL, in particular in advanced stages, are limited. One of the emerging therapies for CTCL is treatment with histone deacetylase (HDAC) inhibitors. We recently discovered an evolutionarily conserved crosstalk between HDAC1, one of the targets of HDAC inhibitors, and the histone methyltransferase DOT1L. HDAC1 negatively regulates DOT1L activity in yeast, mouse thymocytes, and mouse thymic lymphoma. Here we studied the functional relationship between HDAC inhibitors and DOT1L in two human CTCL cell lines, specifically addressing the question whether the crosstalk between DOT1L and HDAC1 observed in mouse T cells plays a role in the therapeutic effect of clinically relevant broad-acting HDAC inhibitors in the treatment of human CTCL. We confirmed that human CTCL cell lines were sensitive to treatment with pan-HDAC inhibitors. In contrast, the cell lines were not sensitive to DOT1L inhibitors. Combining both types of inhibitors did neither enhance nor suppress the inhibitory effect of HDAC inhibitors on CTCL cells. Thus our in vitro studies suggest that the effect of commonly used pan-HDAC inhibitors in CTCL cells relies on downstream effects other than DOT1L misregulation.

Current understanding of epigenetics role in melanoma treatment and resistance

Melanoma is the most aggressive form of skin cancer resulting from genetic mutations in melanocytes. Several factors have been considered to be involved in melanoma progression, including genetic alteration, processes of damaged DNA repair, and changes in mechanisms of cell growth and proliferation. Epigenetics is the other factor with a crucial role in melanoma development. Epigenetic changes have become novel targets for treating patients suffering from melanoma. These changes can alter the expression of microRNAs and their interaction with target genes, which involves cell growth, differentiation, or even death. Given these circumstances, we conducted the present review to discuss the melanoma risk factors and represent the current knowledge about the factors related to its etiopathogenesis. Moreover, various epigenetic pathways, which are involved in melanoma progression, treatment, and chemo-resistance, as well as employed epigenetic factors as a solution to the problems, will be discussed in detail.

Histone deacetylation primes self-propagation of heterochromatin domains to promote epigenetic inheritance

Heterochromatin assembly, involving histone H3 lysine-9 methylation (H3K9me), is nucleated at specific genomic sites but can self-propagate across extended domains and, indeed, generations. Self-propagation requires Clr4/Suv39h methyltransferase recruitment by pre-existing H3K9 tri-methylation (H3K9me3) to perpetuate H3K9me deposition and is dramatically affected by chromatin context. However, the mechanism priming self-propagation of heterochromatin remains undefined. We show that robust chromatin association of fission yeast class II histone deacetylase Clr3 is necessary and sufficient to support heterochromatin propagation in different chromosomal contexts. Efficient targeting of Clr3, which suppresses histone turnover and maintains H3K9me3, enables self-propagation of an ectopic heterochromatin domain via the Clr4/Suv39h read-write mechanism requiring methylated histones. The deacetylase activity of Clr3 is necessary and, when inactivated, heterochromatin propagation can be recapitulated by removing two major histone acetyltransferases. Our results show that histone deacetylation, a conserved heterochromatin feature, preserves H3K9me3 that transmits epigenetic memory for stable propagation of silenced chromatin domains through multiple generations.

Dynamical modeling of the H3K27 epigenetic landscape in mouse embryonic stem cells

The Polycomb system via the methylation of the lysine 27 of histone H3 (H3K27) plays central roles in the silencing of many lineage-specific genes during development. Recent experimental evidence suggested that the recruitment of histone modifying enzymes like the Polycomb repressive complex 2 (PRC2) at specific sites and their spreading capacities from these sites are key to the establishment and maintenance of a proper epigenomic landscape around Polycomb-target genes. Here, to test whether such mechanisms, as a minimal set of qualitative rules, are quantitatively compatible with data, we developed a mathematical model that can predict the locus-specific distributions of H3K27 modifications based on previous biochemical knowledge. Within the biological context of mouse embryonic stem cells, our model showed quantitative agreement with experimental profiles of H3K27 acetylation and methylation around Polycomb-target genes in wild-type and mutants. In particular, we demonstrated the key role of the reader-writer module of PRC2 and of the competition between the binding of activating and repressing enzymes in shaping the H3K27 landscape around transcriptional start sites. The predicted dynamics of establishment and maintenance of the repressive trimethylated H3K27 state suggest a slow accumulation, in perfect agreement with experiments. Our approach represents a first step towards a quantitative description of PcG regulation in various cellular contexts and provides a generic framework to better characterize epigenetic regulation in normal or disease situations.

Painters in chromatin: a unified quantitative framework to systematically characterize epigenome regulation and memory

In eukaryotes, many stable and heritable phenotypes arise from the same DNA sequence, owing to epigenetic regulatory mechanisms relying on the molecular cooperativity of 'reader-writer' enzymes. In this work, we focus on the fundamental, generic mechanisms behind the epigenome memory encoded by post-translational modifications of histone tails. Based on experimental knowledge, we introduce a unified modeling framework, the painter model, describing the mechanistic interplay between sequence-specific recruitment of chromatin regulators, chromatin-state-specific reader-writer processes and long-range spreading mechanisms. A systematic analysis of the model building blocks highlights the crucial impact of tridimensional chromatin organization and state-specific recruitment of enzymes on the stability of epigenomic domains and on gene expression. In particular, we show that enhanced 3D compaction of the genome and enzyme limitation facilitate the formation of ultra-stable, confined chromatin domains. The model also captures how chromatin state dynamics impact the intrinsic transcriptional properties of the region, slower kinetics leading to noisier expression. We finally apply our framework to analyze experimental data, from the propagation of γH2AX around DNA breaks in human cells to the maintenance of heterochromatin in fission yeast, illustrating how the painter model can be used to extract quantitative information on epigenomic molecular processes.

Postmitotic accumulation of histone variant H3.3 in new cortical neurons establishes neuronal chromatin, transcriptome, and identity

Histone variants, which can be expressed outside of S-phase and deposited DNA synthesis-independently, provide long-term histone replacement in postmitotic cells, including neurons. Beyond replenishment, histone variants also play active roles in gene regulation by modulating chromatin states or enabling nucleosome turnover. Here, we uncover crucial roles for the histone H3 variant H3.3 in neuronal development. We find that newborn cortical excitatory neurons, which have only just completed replication-coupled deposition of canonical H3.1 and H3.2, substantially accumulate H3.3 immediately postmitosis. Codeletion of H3.3-encoding genes H3f3a and H3f3b from newly postmitotic neurons abrogates H3.3 accumulation, markedly alters the histone posttranslational modification landscape, and causes widespread disruptions to the establishment of the neuronal transcriptome. These changes coincide with developmental phenotypes in neuronal identities and axon projections. Thus, preexisting, replication-dependent histones are insufficient for establishing neuronal chromatin and transcriptome; de novo H3.3 is required. Stage-dependent deletion of H3f3a and H3f3b from 1) cycling neural progenitor cells, 2) neurons immediately postmitosis, or 3) several days later, reveals the first postmitotic days to be a critical window for de novo H3.3. After H3.3 accumulation within this developmental window, codeletion of H3f3a and H3f3b does not lead to immediate H3.3 loss, but causes progressive H3.3 depletion over several months without widespread transcriptional disruptions or cellular phenotypes. Our study thus uncovers key developmental roles for de novo H3.3 in establishing neuronal chromatin, transcriptome, identity, and connectivity immediately postmitosis that are distinct from its role in maintaining total histone H3 levels over the neuronal lifespan.

A Review of Mathematical and Computational Methods in Cancer Dynamics

Cancers are complex adaptive diseases regulated by the nonlinear feedback systems between genetic instabilities, environmental signals, cellular protein flows, and gene regulatory networks. Understanding the cybernetics of cancer requires the integration of information dynamics across multidimensional spatiotemporal scales, including genetic, transcriptional, metabolic, proteomic, epigenetic, and multi-cellular networks. However, the time-series analysis of these complex networks remains vastly absent in cancer research. With longitudinal screening and time-series analysis of cellular dynamics, universally observed causal patterns pertaining to dynamical systems, may self-organize in the signaling or gene expression state-space of cancer triggering processes. A class of these patterns, strange attractors, may be mathematical biomarkers of cancer progression. The emergence of intracellular chaos and chaotic cell population dynamics remains a new paradigm in systems medicine. As such, chaotic and complex dynamics are discussed as mathematical hallmarks of cancer cell fate dynamics herein. Given the assumption that time-resolved single-cell datasets are made available, a survey of interdisciplinary tools and algorithms from complexity theory, are hereby reviewed to investigate critical phenomena and chaotic dynamics in cancer ecosystems. To conclude, the perspective cultivates an intuition for computational systems oncology in terms of nonlinear dynamics, information theory, inverse problems, and complexity. We highlight the limitations we see in the area of statistical machine learning but the opportunity at combining it with the symbolic computational power offered by the mathematical tools explored.

Connecting the DOTs on Cell Identity

DOT1-Like (DOT1L) is the sole methyltransferase of histone H3K79, a modification enriched mainly on the bodies of actively transcribing genes. DOT1L has been extensively studied in leukemia were some of the most frequent onco-fusion proteins contain portions of DOT1L associated factors that mislocalize H3K79 methylation and drive oncogenesis. However, the role of DOT1L in non-transformed, developmental contexts is less clear. Here we assess the known functional roles of DOT1L both in vitro cell culture and in vivo models of mammalian development. DOT1L is evicted during the 2-cell stage when cells are totipotent and massive epigenetic and transcriptional alterations occur. Embryonic stem cell lines that are derived from the blastocyst tolerate the loss of DOT1L, while the reduction of DOT1L protein levels or its catalytic activity greatly enhances somatic cell reprogramming to induced pluripotent stem cells. DOT1L knockout mice are embryonically lethal when organogenesis commences. We catalog the rapidly increasing studies of total and lineage specific knockout model systems that show that DOT1L is broadly required for differentiation. Reduced DOT1L activity is concomitant with increased developmental potential. Contrary to what would be expected of a modification that is associated with active transcription, loss of DOT1L activity results in more upregulated than downregulated genes. DOT1L also participates in various epigenetic networks that are both cell type and developmental stage specific. Taken together, the functions of DOT1L during development are pleiotropic and involve gene regulation at the locus specific and global levels.

Multi-omic profiling of histone variant H3.3 lysine 27 methylation reveals a distinct role from canonical H3 in stem cell differentiation

Histone variants, such as histone H3.3, replace canonical histones within the nucleosome to alter chromatin accessibility and gene expression. Although the biological roles of selected histone post-translational modifications (PTMs) have been extensively characterized, the potential differences in the function of a given PTM on different histone variants is almost always elusive. By applying proteomics and genomics techniques, we investigate the role of lysine 27 tri-methylation specifically on the histone variant H3.3 (H3.3K27me3) in the context of mouse embryonic stem cell pluripotency and differentiation as a model system for development. We demonstrate that while the steady state overall levels of methylation on both H3K27 and H3.3K27 decrease during differentiation, methylation dynamics studies indicate that methylation on H3.3K27 is maintained more than on H3K27. Using a custom-made antibody, we identify a unique enrichment of H3.3K27me3 at lineage-specific genes, such as olfactory receptor genes, and at binding motifs for the transcription factors FOXJ2/3. REST, a predicted FOXJ2/3 target that acts as a transcriptional repressor of terminal neuronal genes, was identified with H3.3K27me3 at its promoter region. H3.3K27A mutant cells confirmed an upregulation of FOXJ2/3 targets upon the loss of methylation at H3.3K27. Thus, while canonical H3K27me3 has been characterized to regulate the expression of transcription factors that play a general role in differentiation, our work suggests H3.3K27me3 is essential for regulating distinct terminal differentiation genes. This work highlights the importance of understanding the effects of PTMs not only on canonical histones but also on specific histone variants, as they may exhibit distinct roles.

Dynamic Opposition of Histone Modifications

Gene transcription does not only require writers of active histone modifications; on-site opposition by erasers is essential for many genes. Here, we propose the concept of dynamic opposition of histone modifications to explain this conundrum. We highlight the requirement of HDACs for acetylation balance at superenhancers, and the requirement of KDM5A for H4K3me3 recycling at highly active gene promoters. We propose that histone post-translational modifications regulate charge balance for biomolecular condensate formation and nucleosome turnover and form a short-term memory that informs lock-and-step checkpoints for chromatin engagement by RNA polymerase II.

Chaperoning heterochromatin: new roles of FACT in chromatin silencing

The multitasking histone chaperone FACT (FAcilitates Chromatin Transcription) contributes to actively transcribed euchromatin and repressed heterochromatin. However, its precise role in gene silencing has remained obscure. Here, we discuss new insights into the silent chromatin functions and recruitment mechanisms of FACT, and their possible implications in cell identity and cancer.

The histone chaperone FACT facilitates heterochromatin spreading by regulating histone turnover and H3K9 methylation states

Heterochromatin formation requires three distinct steps: nucleation, self-propagation (spreading) along the chromosome, and faithful maintenance after each replication cycle. Impeding any of those steps induces heterochromatin defects and improper gene expression. The essential histone chaperone FACT (facilitates chromatin transcription) has been implicated in heterochromatin silencing, but the mechanisms by which FACT engages in this process remain opaque. Here, we pinpoint its function to the heterochromatin spreading process in fission yeast. FACT impairment reduces nucleation-distal H3K9me3 and HP1/Swi6 accumulation at subtelomeres and derepresses genes in the vicinity of heterochromatin boundaries. FACT promotes spreading by repressing heterochromatic histone turnover, which is crucial for the H3K9me2 to me3 transition that enables spreading. FACT mutant spreading defects are suppressed by removal of the H3K9 methylation antagonist Epe1. Together, our study identifies FACT as a histone chaperone that promotes heterochromatin spreading and lends support to the model that regulated histone turnover controls the propagation of repressive methylation marks.

Heterochromatin establishment requires distinct nucleation and spreading steps. Murawska et al. show that the conserved and essential histone chaperone FACT facilitates the heterochromatin spreading process by maintaining low heterochromatic histone turnover, which enables a productive H3K9 trimethylation step by the methyltransferase Clr4 in fission yeast.

PRC2 activity, recruitment, and silencing: a comparative perspective

Polycomb repressive complex (PRC)-mediated gene silencing is vital for cell identity and development in both the plant and the animal kingdoms. It also modulates responses to stress. Two major protein complexes, PRC1 and PRC2, execute conserved nuclear functions in metazoans and plants through covalent modification of histones and by compacting chromatin. While a general requirement for Polycomb complexes in mitotically heritable gene repression in the context of chromatin is well established, recent studies have brought new insights into the regulation of Polycomb complex activity and recruitment. Here, we discuss these recent advances with emphasis on PRC2.

Hybrid protein assembly-histone modification mechanism for PRC2-based epigenetic switching and memory

The histone modification H3K27me3 plays a central role in Polycomb-mediated epigenetic silencing. H3K27me3 recruits and allosterically activates Polycomb Repressive Complex 2 (PRC2), which adds this modification to nearby histones, providing a read/write mechanism for inheritance through DNA replication. However, for some PRC2 targets, a purely histone-based system for epigenetic inheritance may be insufficient. We address this issue at the Polycomb target FLOWERING LOCUS C (FLC) in Arabidopsis thaliana, as a narrow nucleation region of only ~three nucleosomes within FLC mediates epigenetic state switching and subsequent memory over many cell cycles. To explain the memory's unexpected persistence, we introduce a mathematical model incorporating extra protein memory storage elements with positive feedback that persist at the locus through DNA replication, in addition to histone modifications. Our hybrid model explains many features of epigenetic switching/memory at FLC and encapsulates generic mechanisms that may be widely applicable.

Synthesis and Biological Activity of a Cytostatic Inhibitor of MLLr Leukemia Targeting the DOT1L Protein

Histone methyltransferase DOT1L catalyzes mono-, di- and trimethylation of histone 3 at lysine residue 79 (H3K79) and hypermethylation of H3K79 has been linked to the development of acute leukemias characterized by the MLL (mixed-lineage leukemia) rearrangements (MLLr cells). The inhibition of H3K79 methylation inhibits MLLr cells proliferation, and an inhibitor specific for DOT1L, pinometostat, was in clinical trials (Phase Ib/II). However, the compound showed poor pharmacological properties. Thus, there is a need to find new potent inhibitors of DOT1L for the treatment of rearranged leukemias. Here we present the design, synthesis, and biological evaluation of a small molecule that inhibits in the nM level the enzymatic activity of hDOT1L, H3K79 methylation in MLLr cells with comparable potency to pinometostat, associated with improved metabolic stability and a characteristic cytostatic effect.

Malignant melanoma: Underlying epigenetic mechanisms

Although malignant melanoma is not the most common type of skin cancer, it is the most aggressive and fatal type as it can spread out and metastasize progressively. Early diagnosis and interventions lead to improved patient survival. The incidence rate of melanoma is dramatically increasing, with a few newer therapeutic options available. Therefore, establishing a reliable genetic or epigenetic-based diagnostic and prognostic tool is really important. In this review, we highlight the underlying epigenetic mechanisms involved in melanoma. Furthermore, the epigenetic-based therapeutic options will be also discussed. One of the key areas of discussion will be microRNA which is a small, single-stranded RNA molecule that serves as a regulatory element and found to regulate nearly a third of human genes. MicroRNAs play a role in a wide range of diseases including cancer. In malignant cells, it regulates cell proliferation, invasion, and metastasis.

Using computational modelling to reveal mechanisms of epigenetic Polycomb control

The Polycomb system is essential for stable gene silencing in many organisms. This regulation is achieved in part through addition of the histone modifications H3K27me2/me3 by Polycomb Repressive Complex 2 (PRC2). These modifications are believed to be the causative epigenetic memory elements of PRC2-mediated silencing. As these marks are stored locally in the chromatin, PRC2-based memory is a cis-acting system. A key feature of stable epigenetic memory in cis is PRC2-mediated, self-reinforcing feedback from K27-methylated histones onto nearby histones in a read-write paradigm. However, it was not clear under what conditions such feedback can lead to stable memory, able, for example, to survive the perturbation of histone dilution at DNA replication. In this context, computational modelling has allowed a rigorous exploration of possible underlying memory mechanisms and has also greatly accelerated our understanding of switching between active and silenced states. Specifically, modelling has predicted that switching and memory at Polycomb loci is digital, with a locus being either active or inactive, rather than possessing intermediate, smoothly varying levels of activation. Here, we review recent advances in models of Polycomb control, focusing on models of epigenetic switching through nucleation and spreading of H3K27me2/me3. We also examine models that incorporate transcriptional feedback antagonism and those including bivalent chromatin states. With more quantitative experimental data on histone modification kinetics, as well as single-cell resolution data on transcription and protein levels for PRC2 targets, we anticipate an expanded need for modelling to help dissect increasingly interconnected and complex memory mechanisms.

Histone methyltransferase DOT1L controls state-specific identity during B cell differentiation

Differentiation of naïve peripheral B cells into terminally differentiated plasma cells is characterized by epigenetic alterations, yet the epigenetic mechanisms that control B‐cell fate remain unclear. Here, we identified a role for the histone H3K79 methyltransferase DOT1L in controlling B‐cell differentiation. Mouse B cells lacking Dot1L failed to establish germinal centers (GC) and normal humoral immune responses in vivo. In vitro, activated B cells in which Dot1L was deleted showed aberrant differentiation and prematurely acquired plasma cell characteristics. Similar results were obtained when DOT1L was chemically inhibited in mature B cells in vitro. Mechanistically, combined epigenomics and transcriptomics analysis revealed that DOT1L promotes expression of a pro‐proliferative, pro‐GC program. In addition, DOT1L indirectly supports the repression of an anti‐proliferative plasma cell differentiation program by maintaining the repression of Polycomb Repressor Complex 2 (PRC2) targets. Our findings show that DOT1L is a key modulator of the core transcriptional and epigenetic landscape in B cells, establishing an epigenetic barrier that warrants B‐cell naivety and GC B‐cell differentiation.

DOT1L-mediated murine neuronal differentiation associates with H3K79me2 accumulation and preserves SOX2-enhancer accessibility

During neuronal differentiation, the transcriptional profile and the epigenetic context of neural committed cells is subject to significant rearrangements, but a systematic quantification of global histone modification changes is still missing. Here, we show that H3K79me2 increases and H3K27ac decreases globally during in-vitro neuronal differentiation of murine embryonic stem cells. DOT1L mediates all three degrees of methylation of H3K79 and its enzymatic activity is critical to modulate cellular differentiation and reprogramming. In this context, we find that inhibition of DOT1L in neural progenitor cells biases the transcriptional state towards neuronal differentiation, resulting in transcriptional upregulation of genes marked with H3K27me3 on the promoter region. We further show that DOT1L inhibition affects accessibility of SOX2-bound enhancers and impairs SOX2 binding in neural progenitors. Our work provides evidence that DOT1L activity gates differentiation of progenitors by allowing SOX2-dependent transcription of stemness programs.

Epigenetic Regulations of AhR in the Aspect of Immunomodulation

Environmental factors contribute to autoimmune disease manifestation, and as regarded today, AhR has become an important factor in studies of immunomodulation. Besides immunological aspects, AhR also plays a role in pharmacological, toxicological and many other physiological processes such as adaptive metabolism. In recent years, epigenetic mechanisms have provided new insight into gene regulation and reveal a new contribution to autoimmune disease pathogenesis. DNA methylation, histone modifications, chromatin alterations, microRNA and consequently non-genetic changes in phenotypes connect with environmental factors. Increasing data reveals AhR cross-roads with the most significant in immunology pathways. Although study on epigenetic modulations in autoimmune diseases is still not well understood, therefore future research will help us understand their pathophysiology and help to find new therapeutic strategies. Present literature review sheds the light on the common ground between remodeling chromatin compounds and autoimmune antibodies used in diagnostics. In the proposed review we summarize recent findings that describe epigenetic factors which regulate AhR activity and impact diverse immunological responses and pathological changes.

The histone methyltransferase DOT1L prevents antigen-independent differentiation and safeguards epigenetic identity of CD8+ T cells

Cytotoxic T cell differentiation is guided by epigenome adaptations, but how epigenetic mechanisms control lymphocyte development has not been well defined. Here we show that the histone methyltransferase DOT1L, which marks the nucleosome core on active genes, safeguards normal differentiation of CD8+ T cells. T cell-specific ablation of Dot1L resulted in loss of naïve CD8+ T cells and premature differentiation toward a memory-like state, independent of antigen exposure and in a cell-intrinsic manner. Mechanistically, DOT1L controlled CD8+ T cell differentiation by ensuring normal T cell receptor density and signaling. DOT1L also maintained epigenetic identity, in part by indirectly supporting the repression of developmentally regulated genes. Finally, deletion of Dot1L in T cells resulted in an impaired immune response. Through our study, DOT1L is emerging as a central player in physiology of CD8+ T cells, acting as a barrier to prevent premature differentiation and controlling epigenetic integrity.

Histone methyltransferase DOT1L coordinates AR and MYC stability in prostate cancer

The histone methyltransferase DOT1L methylates lysine 79 (K79) on histone H3 and is involved in Mixed Lineage Leukemia (MLL) fusion leukemogenesis; however, its role in prostate cancer (PCa) is undefined. Here we show that DOT1L is overexpressed in PCa and is associated with poor outcome. Genetic and chemical inhibition of DOT1L selectively impaired the viability of androgen receptor (AR)-positive PCa cells and organoids, including castration-resistant and enzalutamide-resistant cells. The sensitivity of AR-positive cells is due to a distal K79 methylation-marked enhancer in the MYC gene bound by AR and DOT1L not present in AR-negative cells. DOT1L inhibition leads to reduced MYC expression and upregulation of MYC-regulated E3 ubiquitin ligases HECTD4 and MYCBP2, which promote AR and MYC degradation. This leads to further repression of MYC in a negative feed forward manner. Thus DOT1L selectively regulates the tumorigenicity of AR-positive prostate cancer cells and is a promising therapeutic target for PCa.

Histone methyltransferase, DOTL1 is implicated in the pathogenesis of MLL-rearranged leukemia, however, not much is known of its role in prostate cancer (PCa). Here, the authors report that DOTL1 inhibition suppresses both androgen receptor and MYC pathways in a negative feed forward manner to reduce growth of AR-positive PCa.