R-Loops Enhance Polycomb Repression at a Subset of Developmental Regulator Genes

Global coupling of R-loop dynamics with RNA polymerase II modulates gene expression and early development of Drosophila

R-loops are involved in many biological processes in cells, yet the regulatory principles for R-loops in vivo and their impact on development remain to be explored. Here, we modified the CUT&Tag strategy to profile R-loops in Drosophila at multiple developmental stages. While high GC content promotes R-loop formation in mammalian cells, it is not required in Drosophila. In contrast, RNAPII abundance appears to be a universal inducing factor for R-loop formation, including active promoters and enhancers, and H3K27me3 decorated repressive regions and intergenic repeat sequences. Importantly, such a regulatory relationship is dynamically maintained throughout development, and development-related transcription factors may regulate RNAPII activation and R-loop dynamics. By ablating Spt6, we further showed the global R-loop induction coupled with RNAPII pausing. Importantly, depending on the gene length, genes underwent up- or down-regulation, both of which were largely reversed by rnh1 overexpression, suggesting that R-loops play a significant role in the divergent regulation of transcription by Spt6 ablation. DNA damage, defects in survival, and cuticle development were similarly alleviated by rnh1 overexpression. Altogether, our findings indicate that dynamic R-loop regulation is dictated by RNAPII pausing and transcription activity, and plays a feedback role in gene regulation, genome stability maintenance, and Drosophila development.

EZH2-Mediated H3K27 Trimethylation in the Liver of Mice Is an Early Epigenetic Event Induced by High-Fat Diet Exposure

BACKGROUND/OBJECTIVES

Methyltransferase EZH2-mediated H3K27me3 is involved in liver inflammation and fibrosis, but its role in hepatic metabolic derangements is not yet clearly defined. We investigated if a high-fat diet (HFD) induced early changes in EZH2 expression and H3K27 me3 in the liver of mice.

METHODS

Five-week-old mice were fed an HFD or a low-fat diet (Control) for 2 weeks (2 W) or 8 weeks (8 W). Body weight was recorded weekly. Glycemia and oral glucose tolerance were assessed at baseline and after 2 W-8 W. Finally, livers were collected for further analysis.

RESULTS

As expected, mice that received 8 W HFD showed an increase in body weight, glycemia, and liver steatosis and an impairment in glucose tolerance; no alterations were observed in 2 W HFD mice. Eight weeks of HFD caused hepatic EZH2 nuclear localization and increased H3 K27me3; surprisingly, the same alterations occurred in 2 W HFD mice livers, even before overweight onset. We demonstrated that selective EZH2 inhibition reduced H3K27me3 and counteracted lipid accumulation in HUH-7 cells upon palmitic acid treatment.

CONCLUSIONS

In conclusion, we point to EZH2/H3K27me3 as an early epigenetic event occurring in fatty-acid-challenged livers both in vivo and in vitro, thus establishing EZH2 as a potential pharmacological target for metabolic derangements.

A Compendium of G-Flipon Biological Functions That Have Experimental Validation

As with all new fields of discovery, work on the biological role of G-quadruplexes (GQs) has produced a number of results that at first glance are quite baffling, sometimes because they do not fit well together, but mostly because they are different from commonly held expectations. Like other classes of flipons, those that form G-quadruplexes have a repeat sequence motif that enables the fold. The canonical DNA motif (G3N1-7)3G3, where N is any nucleotide and G is guanine, is a feature that is under active selection in avian and mammalian genomes. The involvement of G-flipons in genome maintenance traces back to the invertebrate Caenorhabditis elegans and to ancient DNA repair pathways. The role of GQs in transcription is supported by the observation that yeast Rap1 protein binds both B-DNA, in a sequence-specific manner, and GQs, in a structure-specific manner, through the same helix. Other sequence-specific transcription factors (TFs) also engage both conformations to actuate cellular transactions. Noncoding RNAs can also modulate GQ formation in a sequence-specific manner and engage the same cellular machinery as localized by TFs, linking the ancient RNA world with the modern protein world. The coevolution of noncoding RNAs and sequence-specific proteins is supported by studies of early embryonic development, where the transient formation of G-quadruplexes coordinates the epigenetic specification of cell fate.

HPV induced R-loop formation represses innate immune gene expression while activating DNA damage repair pathways

R-loops are trimeric nucleic acid structures that form when an RNA molecule hybridizes with its complementary DNA strand, displacing the opposite strand. These structures regulate transcription as well as replication, but aberrant R-loops can form, leading to DNA breaks and genomic instability if unresolved. R-loop levels are elevated in many cancers as well as cells that maintain high-risk human papillomaviruses. We investigated how the distribution as well as function of R-loops changed between normal keratinocytes and HPV positive cells derived from a precancerous lesion of the cervix (CIN I). The levels of R-loops associated with cellular genes were found to be up to 10-fold higher in HPV positive cells than in normal keratinocytes while increases at ALU1 elements increased by up to 500-fold. The presence of enhanced R-loops resulted in altered levels of gene transcription, with equal numbers increased as decreased. While no uniform global effects on transcription due to the enhanced levels of R-loops were detected, genes in several pathways were coordinately increased or decreased in expression only in the HPV positive cells. This included the downregulation of genes in the innate immune pathway, such as DDX58, IL-6, STAT1, IFN-β, and NLRP3. All differentially expressed innate immune genes dependent on R-loops were also associated with H3K36me3 modified histones. Genes that were upregulated by the presence of R-loops in HPV positive cells included those in the DNA damage repair such as ATM, ATRX, and members of the Fanconi Anemia pathway. These genes exhibited a linkage between R-loops and H3K36me3 as well as γH2AX histone marks only in HPV positive cells. These studies identify a potential link in HPV positive cells between DNA damage repair as well as innate immune regulatory pathways with R-loops and γH2AX/H3K36me3 histone marks that may contribute to regulating important functions for HPV pathogenesis.

PRC2-AgeIndex as a universal biomarker of aging and rejuvenation

DNA methylation (DNAm) is one of the most reliable biomarkers of aging across mammalian tissues. While the age-dependent global loss of DNAm has been well characterized, DNAm gain is less characterized. Studies have demonstrated that CpGs which gain methylation with age are enriched in Polycomb Repressive Complex 2 (PRC2) targets. However, whole-genome examination of all PRC2 targets as well as determination of the pan-tissue or tissue-specific nature of these associations is lacking. Here, we show that low-methylated regions (LMRs) which are highly bound by PRC2 in embryonic stem cells (PRC2 LMRs) gain methylation with age in all examined somatic mitotic cells. We estimated that this epigenetic change represents around 90% of the age-dependent DNAm gain genome-wide. Therefore, we propose the "PRC2-AgeIndex," defined as the average DNAm in PRC2 LMRs, as a universal biomarker of cellular aging in somatic cells which can distinguish the effect of different anti-aging interventions.

Drosophila TET acts with PRC1 to activate gene expression independently of its catalytic activity

Enzymes of the ten-eleven translocation (TET) family play a key role in the regulation of gene expression by oxidizing 5-methylcytosine (5mC), a prominent epigenetic mark in many species. Yet, TET proteins also have less characterized noncanonical modes of action, notably in Drosophila, whose genome is devoid of 5mC. Here, we show that Drosophila TET activates the expression of genes required for larval central nervous system (CNS) development mainly in a catalytic-independent manner. Genome-wide profiling shows that TET is recruited to enhancer and promoter regions bound by Polycomb group complex (PcG) proteins. We found that TET interacts and colocalizes on chromatin preferentially with Polycomb repressor complex 1 (PRC1) rather than PRC2. Furthermore, PRC1 but not PRC2 is required for the activation of TET target genes. Last, our results suggest that TET and PRC1 binding to activated genes is interdependent. These data highlight the importance of TET noncatalytic function and the role of PRC1 for gene activation in the Drosophila larval CNS.

Approaches for Mapping and Analysis of R-loops

R-loops are nucleic acid structures composed of a DNA:RNA hybrid with a displaced non-template single-stranded DNA. Current approaches to identify and map R-loop formation across the genome employ either an antibody targeted against R-loops (S9.6) or a catalytically inactivated form of RNase H1 (dRNH1), a nuclease that can bind and resolve DNA:RNA hybrids via RNA exonuclease activity. This overview article outlines several ways to map R-loops using either methodology, explaining the differences and similarities among the approaches. Bioinformatic analysis of R-loops involves several layers of quality control and processing before visualizing the data. This article provides resources and tools that can be used to accurately process R-loop mapping data and explains the advantages and disadvantages of the resources as compared to one another. © 2024 Wiley Periodicals LLC.

ZFP281 controls transcriptional and epigenetic changes promoting mouse pluripotent state transitions via DNMT3 and TET1

The progression from naive through formative to primed in vitro pluripotent stem cell states recapitulates epiblast development in vivo during the peri-implantation period of mouse embryo development. Activation of the de novo DNA methyltransferases and reorganization of transcriptional and epigenetic landscapes are key events that occur during these pluripotent state transitions. However, the upstream regulators that coordinate these events are relatively underexplored. Here, using Zfp281 knockout mouse and degron knockin cell models, we identify the direct transcriptional activation of Dnmt3a/3b by ZFP281 in pluripotent stem cells. Chromatin co-occupancy of ZFP281 and DNA hydroxylase TET1, which is dependent on the formation of R-loops in ZFP281-targeted gene promoters, undergoes a "high-low-high" bimodal pattern regulating dynamic DNA methylation and gene expression during the naive-formative-primed transitions. ZFP281 also safeguards DNA methylation in maintaining primed pluripotency. Our study demonstrates a previously unappreciated role for ZFP281 in coordinating DNMT3A/3B and TET1 functions to promote pluripotent state transitions.

Epigenetic regulatory layers in the 3D nucleus

Nearly 7 decades have elapsed since Francis Crick introduced the central dogma of molecular biology, as part of his ideas on protein synthesis, setting the fundamental rules of sequence information transfer from DNA to RNAs and proteins. We have since learned that gene expression is finely tuned in time and space, due to the activities of RNAs and proteins on regulatory DNA elements, and through cell-type-specific three-dimensional conformations of the genome. Here, we review major advances in genome biology and discuss a set of ideas on gene regulation and highlight how various biomolecular assemblies lead to the formation of structural and regulatory features within the nucleus, with roles in transcriptional control. We conclude by suggesting further developments that will help capture the complex, dynamic, and often spatially restricted events that govern gene expression in mammalian cells.

PSIP1/LEDGF reduces R-loops at transcription sites to maintain genome integrity

R-loops that accumulate at transcription sites pose a persistent threat to genome integrity. PSIP1 is a chromatin protein associated with transcriptional elongation complex, possesses histone chaperone activity, and is implicated in recruiting RNA processing and DNA repair factors to transcription sites. Here, we show that PSIP1 interacts with R-loops and other proteins involved in R-loop homeostasis, including PARP1. Genome-wide mapping of PSIP1, R-loops and γ-H2AX in PSIP1-depleted human and mouse cell lines revealed an accumulation of R-loops and DNA damage at gene promoters in the absence of PSIP1. R-loop accumulation causes local transcriptional arrest and transcription-replication conflict, leading to DNA damage. PSIP1 depletion increases 53BP1 foci and reduces RAD51 foci, suggesting altered DNA repair choice. Furthermore, PSIP1 depletion increases the sensitivity of cancer cells to PARP1 inhibitors and DNA-damaging agents that induce R-loop-induced DNA damage. These findings provide insights into the mechanism through which PSIP1 maintains genome integrity at the site of transcription.

Arabidopsis thaliana: a powerful model organism to explore histone modifications and their upstream regulations

Histones are subjected to extensive covalent modifications that affect inter-nucleosomal interactions as well as alter chromatin structure and DNA accessibility. Through switching the corresponding histone modifications, the level of transcription and diverse downstream biological processes can be regulated. Although animal systems are widely used in studying histone modifications, the signalling processes that occur outside the nucleus prior to histone modifications have not been well understood due to the limitations including non viable mutants, partial lethality, and infertility of survivors. Here, we review the benefits of using Arabidopsis thaliana as the model organism to study histone modifications and their upstream regulations. Similarities among histones and key histone modifiers such as the Polycomb group (PcG) and Trithorax group (TrxG) in Drosophila, Human, and Arabidopsis are examined. Furthermore, prolonged cold-induced vernalization system has been well-studied and revealed the relationship between the controllable environment input (duration of vernalization), its chromatin modifications of FLOWERING LOCUS C (FLC), following gene expression, and the corresponding phenotypes. Such evidence suggests that research on Arabidopsis can bring insights into incomplete signalling pathways outside of the histone box, which can be achieved through viable reverse genetic screenings based on the phenotypes instead of direct monitoring of histone modifications among individual mutants. The potential upstream regulators in Arabidopsis can provide cues or directions for animal research based on the similarities between them.

R-Loop Defines Neural Stem/Progenitor Cells During Mouse Neurodevelopment

Neural stem/progenitor cells (NSPCs) are present in the mammalian brain throughout life and are involved in neurodevelopment and central nervous system repair. Although typical epigenetic signatures, including DNA methylation, histone modifications, and microRNAs, play a pivotal role in regulation of NSPCs, several of the epigenetic regulatory mechanisms of NSPCs remain unclear. Thus, defining a novel epigenetic feature of NSPCs is crucial for developing stem cell therapy to address neurologic disorders caused by injury. In this study, we aimed to define the R-loop, a three-stranded nucleic acid structure, as an epigenetic characteristic of NSPCs during neurodevelopment. Our results demonstrated that R-loop levels change dynamically throughout neurodevelopment. Cells with high levels of R-loops consistently decreased and were enriched in the area of neurogenesis. Additionally, these cells costained with SOX2 during neurodevelopment. Furthermore, these cells with high R-loop levels expressed Ki-67 and exhibited a high degree of overlap with the transcriptional activation markers, H3K4me3, ser5, and H3K27ac. These findings suggest that R-loops may serve as an epigenetic feature for transcriptional activation in NSPCs, indicating their role in gene expression regulation and neurogenesis.

R-LOOPs on Short Tandem Repeat Expansion Disorders in Neurodegenerative Diseases

Expansions of short tandem repeats (STRs) have been found to be present in more than 50 diseases and have a close connection with neurodegenerative diseases. Transcriptional silencing and R-LOOP formation, RNA-mediated sequestration of RNA-binding proteins (RBPs), gain-of-function (GOF) proteins containing expanded repeats, and repeat-associated non-AUG (RAN) translation of toxic repeat peptides are some potential molecular mechanisms underlying STR expansion disorders. R-LOOP, a byproduct of transcription, is a three-stranded nucleic acid structure with abnormal accumulation that participates in the pathogenesis of STR expansion disorders by inducing DNA damage and genome instability. R-LOOPs can engender a series of DNA damage, such as DNA double-strand breaks (DSBs), single-strand breaks (SSBs), DNA recombination, or mutations in the DNA replication, transcription, or repair processes. In this review, we provide an in-depth discussion of recent advancements in R-LOOP and systematically elaborate on its genetic destabilizing effects in several neurodegenerative diseases. These molecular mechanisms will provide novel targets for drug design and therapeutic upgrading of these devastating diseases.

A DNA adenine demethylase impairs PRC2-mediated repression of genes marked by a specific chromatin signature

BACKGROUND

The Fe (II)- and α-ketoglutarate-dependent AlkB family dioxygenases are implicated in nucleotide demethylation. AlkB homolog1 (ALKBH1) is shown to demethylate DNA adenine methylation (6mA) preferentially from single-stranded or unpaired DNA, while its demethylase activity and function in the chromatin context are unclear.

RESULTS

Here, we find that loss-of-function of the rice ALKBH1 gene leads to increased 6mA in the R-loop regions of the genome but has a limited effect on the overall 6mA level. However, in the context of mixed tissues, rather than on individual loci, the ALKBH1 mutation or overexpression mainly affects the expression of genes with a specific combination of chromatin modifications in the body region marked with H3K4me3 and H3K27me3 but depleted of DNA CG methylation. In the similar context of mixed tissues, further analysis reveals that the ALKBH1 protein preferentially binds to genes marked by the chromatin signature and has a function to maintain a high H3K4me3/H3K27me3 ratio by impairing the binding of Polycomb repressive complex 2 (PRC2) to the targets, which is required for both the basal and stress-induced expression of the genes.

CONCLUSION

Our findings unravel a function of ALKBH1 to control the balance between the antagonistic histone methylations for gene activity and provide insight into the regulatory mechanism of PRC2-mediated H3K27me3 deposition within the gene body region.

R-loop landscapes in the developing human brain are linked to neural differentiation and cell-type specific transcription

Here, we construct genome-scale maps for R-loops, three-stranded nucleic acid structures comprised of a DNA/RNA hybrid and a displaced single strand of DNA, in the proliferative and differentiated zones of the human prenatal brain. We show that R-loops are abundant in the progenitor-rich germinal matrix, with preferential formation at promoters slated for upregulated expression at later stages of differentiation, including numerous neurodevelopmental risk genes. RNase H1-mediated contraction of the genomic R-loop space in neural progenitors shifted differentiation toward the neuronal lineage and was associated with transcriptomic alterations and defective functional and structural neuronal connectivity in vivo and in vitro. Therefore, R-loops are important for fine-tuning differentiation-sensitive gene expression programs of neural progenitor cells.

Cohesin-independent STAG proteins interact with RNA and R-loops and promote complex loading

Most studies of cohesin function consider the Stromalin Antigen (STAG/SA) proteins as core complex members given their ubiquitous interaction with the cohesin ring. Here, we provide functional data to support the notion that the SA subunit is not a mere passenger in this structure, but instead plays a key role in the localization of cohesin to diverse biological processes and promotes loading of the complex at these sites. We show that in cells acutely depleted for RAD21, SA proteins remain bound to chromatin, cluster in 3D and interact with CTCF, as well as with a wide range of RNA binding proteins involved in multiple RNA processing mechanisms. Accordingly, SA proteins interact with RNA, and R-loops, even in the absence of cohesin. Our results place SA1 on chromatin upstream of the cohesin ring and reveal a role for SA1 in cohesin loading which is independent of NIPBL, the canonical cohesin loader. We propose that SA1 takes advantage of structural R-loop platforms to link cohesin loading and chromatin structure with diverse functions. Since SA proteins are pan-cancer targets, and R-loops play an increasingly prevalent role in cancer biology, our results have important implications for the mechanistic understanding of SA proteins in cancer and disease.

Unscheduled epigenetic modifications cause genome instability and sterility through aberrant R-loops following starvation

During starvation, organisms modify both gene expression and metabolism to adjust to the energy stress. We previously reported that Caenorhabditis elegans lacing AMP-activated protein kinase (AMPK) exhibit transgenerational reproductive defects associated with abnormally elevated trimethylated histone H3 at lysine 4 (H3K4me3) levels in the germ line following recovery from acute starvation. Here, we show that these H3K4me3 marks are significantly increased at promoters, driving aberrant transcription elongation resulting in the accumulation of R-loops in starved AMPK mutants. DNA-RNA immunoprecipitation followed by high-throughput sequencing (DRIP-seq) analysis demonstrated that a significant proportion of the genome was affected by R-loop formation. This was most pronounced in the promoter-transcription start site regions of genes, in which the chromatin was modified by H3K4me3. Like H3K4me3, the R-loops were also found to be heritable, likely contributing to the transgenerational reproductive defects typical of these mutants following starvation. Strikingly, AMPK mutant germ lines show considerably more RAD-51 (the RecA recombinase) foci at sites of R-loop formation, potentially sequestering them from their roles at meiotic breaks or at sites of induced DNA damage. Our study reveals a previously unforeseen role of AMPK in maintaining genome stability following starvation. The downstream effects of R-loops on DNA damage sensitivity and germline stem cell integrity may account for inappropriate epigenetic modification that occurs in numerous human disorders, including various cancers.

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

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

R-loop: The new genome regulatory element in plants

An R-loop is a three-stranded chromatin structure that consists of a displaced single strand of DNA and an RNA:DNA hybrid duplex, which was thought to be a rare by-product of transcription. However, recent genome-wide data have shown that R-loops are widespread and pervasive in a variety of genomes, and a growing body of experimental evidence indicates that R-loops have both beneficial and harmful effects on an organism. To maximize benefit and avoid harm, organisms have evolved several means by which they tightly regulate R-loop levels. Here, we summarize our current understanding of the biogenesis and effects of R-loops, the mechanisms that regulate them, and methods of R-loop profiling, reviewing recent research advances on R-loops in plants. Furthermore, we provide perspectives on future research directions for R-loop biology in plants, which might lead to a more comprehensive understanding of R-loop functions in plant genome regulation and contribute to future agricultural improvements.

Replication collisions induced by de-repressed S-phase transcription are connected with malignant transformation of adult stem cells

Transcription replication collisions (TRCs) constitute a major intrinsic source of genome instability but conclusive evidence for a causal role of TRCs in tumor initiation is missing. We discover that lack of the H4K20-dimethyltransferase KMT5B (also known as SUV4-20H1) in muscle stem cells de-represses S-phase transcription by increasing H4K20me1 levels, which induces TRCs and aberrant R-loops in oncogenic genes. The resulting replication stress and aberrant mitosis activate ATR-RPA32-P53 signaling, promoting cellular senescence, which turns into rapid rhabdomyosarcoma formation when p53 is absent. Inhibition of S-phase transcription ameliorates TRCs and formation of R-loops in Kmt5b-deficient MuSCs, validating the crucial role of H4K20me1-dependent, tightly controlled S-phase transcription for preventing collision errors. Low KMT5B expression is prevalent in human sarcomas and associated with tumor recurrence, suggesting a common function of KMT5B in sarcoma formation. The study uncovers decisive functions of KMT5B for maintaining genome stability by repressing S-phase transcription via control of H4K20me1 levels.

RAD51AP1 regulates ALT-HDR through chromatin-directed homeostasis of TERRA

Alternative lengthening of telomeres (ALT) is a homology-directed repair (HDR) mechanism of telomere elongation that controls proliferation in subsets of aggressive cancer. Recent studies have revealed that telomere repeat-containing RNA (TERRA) promotes ALT-associated HDR (ALT-HDR). Here, we report that RAD51AP1, a crucial ALT factor, interacts with TERRA and utilizes it to generate D- and R-loop HR intermediates. We also show that RAD51AP1 binds to and might stabilize TERRA-containing R-loops as RAD51AP1 depletion reduces R-loop formation at telomere DNA breaks. Proteomic analyses uncover a role for RAD51AP1-mediated TERRA R-loop homeostasis in a mechanism of chromatin-directed suppression of TERRA and prevention of transcription-replication collisions (TRCs) during ALT-HDR. Intriguingly, we find that both TERRA binding and this non-canonical function of RAD51AP1 require its intrinsic SUMO-SIM regulatory axis. These findings provide insights into the multi-contextual functions of RAD51AP1 within the ALT mechanism and regulation of TERRA.

Non-canonical DNA/RNA structures associated with the pathogenesis of Fragile X-associated tremor/ataxia syndrome and Fragile X syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X syndrome (FXS) are primary examples of fragile X-related disorders (FXDs) caused by abnormal expansion of CGG repeats above a certain threshold in the 5'-untranslated region of the fragile X mental retardation (FMR1) gene. Both diseases have distinct clinical manifestations and molecular pathogenesis. FXTAS is a late-adult-onset neurodegenerative disorder caused by a premutation (PM) allele (CGG expansion of 55-200 repeats), resulting in FMR1 gene hyperexpression. On the other hand, FXS is a neurodevelopmental disorder that results from a full mutation (FM) allele (CGG expansions of ≥200 repeats) leading to heterochromatization and transcriptional silencing of the FMR1 gene. The main challenge is to determine how CGG repeat expansion affects the fundamentally distinct nature of FMR1 expression in FM and PM ranges. Abnormal CGG repeat expansions form a variety of non-canonical DNA and RNA structures that can disrupt various cellular processes and cause distinct effects in PM and FM alleles. Here, we review these structures and how they are related to underlying mutations and disease pathology in FXS and FXTAS. Finally, as new CGG expansions within the genome have been identified, it will be interesting to determine their implications in disease pathology and treatment.

The Arabidopsis APOLO and human UPAT sequence-unrelated long noncoding RNAs can modulate DNA and histone methylation machineries in plants

BACKGROUND

RNA-DNA hybrid (R-loop)-associated long noncoding RNAs (lncRNAs), including the Arabidopsis lncRNA AUXIN-REGULATED PROMOTER LOOP (APOLO), are emerging as important regulators of three-dimensional chromatin conformation and gene transcriptional activity.

RESULTS

Here, we show that in addition to the PRC1-component LIKE HETEROCHROMATIN PROTEIN 1 (LHP1), APOLO interacts with the methylcytosine-binding protein VARIANT IN METHYLATION 1 (VIM1), a conserved homolog of the mammalian DNA methylation regulator UBIQUITIN-LIKE CONTAINING PHD AND RING FINGER DOMAINS 1 (UHRF1). The APOLO-VIM1-LHP1 complex directly regulates the transcription of the auxin biosynthesis gene YUCCA2 by dynamically determining DNA methylation and H3K27me3 deposition over its promoter during the plant thermomorphogenic response. Strikingly, we demonstrate that the lncRNA UHRF1 Protein Associated Transcript (UPAT), a direct interactor of UHRF1 in humans, can be recognized by VIM1 and LHP1 in plant cells, despite the lack of sequence homology between UPAT and APOLO. In addition, we show that increased levels of APOLO or UPAT hamper VIM1 and LHP1 binding to YUCCA2 promoter and globally alter the Arabidopsis transcriptome in a similar manner.

CONCLUSIONS

Collectively, our results uncover a new mechanism in which a plant lncRNA coordinates Polycomb action and DNA methylation through the interaction with VIM1, and indicates that evolutionary unrelated lncRNAs with potentially conserved structures may exert similar functions by interacting with homolog partners.

De Novo Polycomb Recruitment and Repressive Domain Formation

Every cell of an organism shares the same genome; even so, each cellular lineage owns a different transcriptome and proteome. The Polycomb group proteins (PcG) are essential regulators of gene repression patterning during development and homeostasis. However, it is unknown how the repressive complexes, PRC1 and PRC2, identify their targets and elicit new Polycomb domains during cell differentiation. Classical recruitment models consider the pre-existence of repressive histone marks; still, de novo target binding overcomes the absence of both H3K27me3 and H2AK119ub. The CpG islands (CGIs), non-core proteins, and RNA molecules are involved in Polycomb recruitment. Nonetheless, it is unclear how de novo targets are identified depending on the physiological context and developmental stage and which are the leading players stabilizing Polycomb complexes at domain nucleation sites. Here, we examine the features of de novo sites and the accessory elements bridging its recruitment and discuss the first steps of Polycomb domain formation and transcriptional regulation, comprehended by the experimental reconstruction of the repressive domains through time-resolved genomic analyses in mammals.

Quality-controlled R-loop meta-analysis reveals the characteristics of R-loop consensus regions

R-loops are three-stranded nucleic acid structures formed from the hybridization of RNA and DNA. While the pathological consequences of R-loops have been well-studied to date, the locations, classes, and dynamics of physiological R-loops remain poorly understood. R-loop mapping studies provide insight into R-loop dynamics, but their findings are challenging to generalize. This is due to the narrow biological scope of individual studies, the limitations of each mapping modality, and, in some cases, poor data quality. In this study, we reprocessed 810 R-loop mapping datasets from a wide array of biological conditions and mapping modalities. From this data resource, we developed an accurate R-loop data quality control method, and we reveal the extent of poor-quality data within previously published studies. We then identified a set of high-confidence R-loop mapping samples and used them to define consensus R-loop sites called 'R-loop regions' (RL regions). In the process, we identified a stark divergence between RL regions detected by S9.6 and dRNH-based mapping methods, particularly with respect to R-loop size, location, and colocalization with RNA binding factors. Taken together, this work provides a much-needed method to assess R-loop data quality and offers novel context regarding the differences between dRNH- and S9.6-based R-loop mapping approaches.

Nuclear Coregulatory Complexes in Tregs as Targets to Promote Anticancer Immune Responses

T-regulatory (Treg) cells display considerable heterogeneity in their responses to various cancers. The functional differences among this cell type are heavily influenced by multiprotein nuclear complexes that control their gene expression. Many such complexes act mechanistically by altering epigenetic profiles of genes important to Treg function, including the forkhead P3 (Foxp3) transcription factor. Complexes that form with certain members of the histone/protein deacetylase (HDAC) class of enzymes, like HDACs 1, 2, and 3, along with histone methyltransferase complexes, are important in the induction and stabilization of Foxp3 and Treg identity. The functional behavior of both circulating and intratumoral Tregs greatly impacts the antitumor immune response and can be predictive of patient outcome. Thus, targeting these regulatory complexes within Tregs may have therapeutic potential, especially in personalized immunotherapies.

ZFP281-BRCA2 prevents R-loop accumulation during DNA replication

R-loops are prevalent in mammalian genomes and involved in many fundamental cellular processes. Depletion of BRCA2 leads to aberrant R-loop accumulation, contributing to genome instability. Here, we show that ZFP281 cooperates with BRCA2 in preventing R-loop accumulation to facilitate DNA replication in embryonic stem cells. ZFP281 depletion reduces PCNA levels on chromatin and impairs DNA replication. Mechanistically, we demonstrate that ZFP281 can interact with BRCA2, and that BRCA2 is enriched at G/C-rich promoters and requires both ZFP281 and PRC2 for its proper recruitment to the bivalent chromatin at the genome-wide scale. Furthermore, depletion of ZFP281 or BRCA2 leads to accumulation of R-loops over the bivalent regions, and compromises activation of the developmental genes by retinoic acid during stem cell differentiation. In summary, our results reveal that ZFP281 recruits BRCA2 to the bivalent chromatin regions to ensure proper progression of DNA replication through preventing persistent R-loops.

R-loops are prevalent in mammalian genomes and involved in many fundamental cellular processes. Here, Wang et al. report that ZFP281 cooperates with BRCA2 in preventing R-loop accumulation to facilitate DNA replication in embryonic stem cells.

Sex-specific variation in R-loop formation in Drosophila melanogaster

R-loops are three-stranded nucleotide structures consisting of a DNA:RNA hybrid and a displaced ssDNA non-template strand. Previous work suggests that R-loop formation is primarily determined by the thermodynamics of DNA:RNA binding, which are governed by base composition (e.g., GC skew) and transcription-induced DNA superhelicity. However, R-loops have been described at genomic locations that lack these properties, suggesting that they may serve other context-specific roles. To better understand the genetic determinants of R-loop formation, we have characterized the Drosophila melanogaster R-loop landscape across strains and between sexes using DNA:RNA immunoprecipitation followed by high-throughput sequencing (DRIP-seq). We find that R-loops are associated with sequence motifs that are G-rich or exhibit G/C skew, as well as highly expressed genes, tRNAs, and small nuclear RNAs, consistent with a role for DNA sequence and torsion in R-loop specification. However, we also find motifs associated with R-loops that are A/T-rich and lack G/C skew as well as a subset of R-loops that are enriched in polycomb-repressed chromatin. Differential enrichment analysis reveals a small number of sex-biased R-loops: while non-differentially enriched and male-enriched R-loops form at similar genetic features and chromatin states and contain similar sequence motifs, female-enriched R-loops form at unique genetic features, chromatin states, and sequence motifs and are associated with genes that show ovary-biased expression. Male-enriched R-loops are most abundant on the dosage-compensated X chromosome, where R-loops appear stronger compared to autosomal R-loops. R-loop-containing genes on the X chromosome are dosage-compensated yet show lower MOF binding and reduced H4K16ac compared to R-loop-absent genes, suggesting that H4K16ac or MOF may attenuate R-loop formation. Collectively, these results suggest that R-loop formation in vivo is not fully explained by DNA sequence and topology and raise the possibility that a distinct subset of these hybrid structures plays an important role in the establishment and maintenance of epigenetic differences between sexes.

R-loop mapping and characterization during Drosophila embryogenesis reveals developmental plasticity in R-loop signatures

R-loops are involved in transcriptional regulation, DNA and histone post-translational modifications, genome replication and genome stability. To what extent R-loop abundance and genome-wide localization is actively regulated during metazoan embryogenesis is unknown. Drosophila embryogenesis provides a powerful system to address these questions due to its well-characterized developmental program, the sudden onset of zygotic transcription and available genome-wide data sets. Here, we measure the overall abundance and genome localization of R-loops in early and late-stage embryos relative to Drosophila cultured cells. We demonstrate that absolute R-loop levels change during embryogenesis and that RNaseH1 catalytic activity is critical for embryonic development. R-loop mapping by strand-specific DRIP-seq reveals that R-loop localization is plastic across development, both in the genes which form R-loops and where they localize relative to gene bodies. Importantly, these changes are not driven by changes in the transcriptional program. Negative GC skew and absolute changes in AT skew are associated with R-loop formation in Drosophila. Furthermore, we demonstrate that while some chromatin binding proteins and histone modifications such as H3K27me3 are associated with R-loops throughout development, other chromatin factors associated with R-loops in a developmental specific manner. Our findings highlight the importance and developmental plasticity of R-loops during Drosophila embryogenesis.

Role of Nuclear Non-Canonical Nucleic Acid Structures in Organismal Development and Adaptation to Stress Conditions

Over the last decades, numerous examples have involved nuclear non-coding RNAs (ncRNAs) in the regulation of gene expression. ncRNAs can interact with the genome by forming non-canonical nucleic acid structures such as R-loops or DNA:RNA triplexes. They bind chromatin and DNA modifiers and transcription factors and favor or prevent their targeting to specific DNA sequences and regulate gene expression of diverse genes. We review the function of these non-canonical nucleic acid structures in regulating gene expression of multicellular organisms during development and in response to different stress conditions and DNA damage using examples described in several organisms, from plants to humans. We also overview recent techniques developed to study where R-loops or DNA:RNA triplexes are formed in the genome and their interaction with proteins.

Induction of senescence-associated secretory phenotype underlies the therapeutic efficacy of PRC2 inhibition in cancer

The methyltransferase Polycomb Repressive Complex 2 (PRC2), composed of EZH2, SUZ12, and EED subunits, is associated with transcriptional repression via tri-methylation of histone H3 on lysine 27 residue (H3K27me3). PRC2 is a valid drug target, as the EZH2 gain-of-function mutations identified in patient samples drive tumorigenesis. PRC2 inhibitors have been discovered and demonstrated anti-cancer efficacy in clinic. However, their pharmacological mechanisms are poorly understood. MAK683 is a potent EED inhibitor in clinical development. Focusing on MAK683-sensitive tumors with SMARCB1 or ARID1A loss, we identified a group of PRC2 target genes with high H3K27me3 signal through epigenomic and transcriptomic analysis. Multiple senescence-associated secretory phenotype (SASP) genes, such as GATA4, MMP2/10, ITGA2 and GBP1, are in this group besides previously identified CDKN2A/p16. Upon PRC2 inhibition, the de-repression of SASP genes is detected in multiple sensitive models and contributes to decreased Ki67+, extracellular matrix (ECM) reorganization, senescence associated inflammation and tumor regression even in CDKN2A/p16 knockout tumor. And the combination of PRC2 inhibitor and CDK4/6 inhibitor leads to better effect. The genes potential regulated by PRC2 in neuroblastoma samples exhibited significant enrichment of ECM and senescence associated inflammation, supporting the clinical relevance of our results. Altogether, our results unravel the pharmacological mechanism of PRC2 inhibitors and propose a combination strategy for MAK683 and other PRC2 drugs.

Polycomb Repressive Complex 2 in Eukaryotes-An Evolutionary Perspective

Polycomb repressive complex 2 (PRC2) represents a group of evolutionarily conserved multi-subunit complexes that repress gene transcription by introducing trimethylation of lysine 27 on histone 3 (H3K27me3). PRC2 activity is of key importance for cell identity specification and developmental phase transitions in animals and plants. The composition, biochemistry, and developmental function of PRC2 in animal and flowering plant model species are relatively well described. Recent evidence demonstrates the presence of PRC2 complexes in various eukaryotic supergroups, suggesting conservation of the complex and its function. Here, we provide an overview of the current understanding of PRC2-mediated repression in different representatives of eukaryotic supergroups with a focus on the green lineage. By comparison of PRC2 in different eukaryotes, we highlight the possible common and diverged features suggesting evolutionary implications and outline emerging questions and directions for future research of polycomb repression and its evolution.

RNase H1, the Gold Standard for R-Loop Detection

RNase H1 has become an essential tool to uncover the physiological and pathological roles of R-loops, three-stranded structures consisting of and RNA-DNA hybrid opposite to a single DNA strand (ssDNA). RNase H1 degrades the RNA portion of the R-loops returning the two DNA strands to double-stranded form (dsDNA). Overexpression of RNase H1 in different systems has helped to address the questions of where R-loops are located, their abundance, and mechanisms of formation, stability, and degradation. In this chapter we review multiple studies that used RNase H1 as an instrument to investigate R-loops multiple functions and their relevance in health and diseases.

Detection of R-Loop Structures by Immunofluorescence Using the S9.6 Monoclonal Antibody

This protocol describes a method of detection of R-loop structures by Immunofluorescence using the S9.6 antibody. R-loops are three-stranded nucleic acid structures that comprise the nascent RNA hybridized with the DNA template strand (RNA-DNA hybrid) leaving the nontemplate DNA strand single-stranded (ssDNA). R-loops are dynamic structures that have been linked to transcription-associated DNA damage and genomic instability in certain contexts but they also possess critical regulatory functions. They are direct products of transcription and they have been associated with transcriptional activation, repression and termination in cases of both protein-coding and noncoding genes. Visualizing and mapping R-loops has been a sought-after task over the last years. Next-generation sequencing of RNA-DNA hybrids, which are components of R-loops, using the S9.6 antibody, aims to detect R-loops genome-wide, whereas Immunofluorescence is performed to visualize R-loops in single cells. While mapping R-loops genome-wide is very important for identifying and studying their location-specific role, microscopy offers the advantage of spatial information and the ability to quantify them on a single cell level. In this chapter, I will describe the protocol I have used to image RNA-DNA hybrids in the nucleus of mammalian cells.

The role of chromatin at transcription-replication conflicts as a genome safeguard

DNA replication ensures the correct copying of the genome and the faithful transfer of the genetic information to the offspring. However, obstacles to replication fork (RF) progression cause RF stalling and compromise efficient genome duplication. Since replication uses the same DNA template as transcription, both transcription and replication must be coordinated to prevent Transcription-Replication Conflicts (TRCs) that could stall RF progression. Several factors contribute to limit the occurrence of such conflicts and their harmful impact on genome integrity. Increasing evidence indicates that chromatin homeostasis plays a key role in the cellular response to TRCs as well as in the preservation of genome integrity. Indeed, chromatin regulating enzymes are frequently mutated in cancer cells, a common characteristic of which is genome instability. Therefore, understanding the role of chromatin in TRC occurrence and resolution may help identify the molecular mechanism by which chromatin protects genome integrity, and the causes and physiological relevance of the high mutation rates of chromatin regulating factors in cancer. Here we review the current knowledge in the field, as well as the perspectives and future applications.

The molecular principles of gene regulation by Polycomb repressive complexes

Precise control of gene expression is fundamental to cell function and development. Although ultimately gene expression relies on DNA-binding transcription factors to guide the activity of the transcription machinery to genes, it has also become clear that chromatin and histone post-translational modification have fundamental roles in gene regulation. Polycomb repressive complexes represent a paradigm of chromatin-based gene regulation in animals. The Polycomb repressive system comprises two central protein complexes, Polycomb repressive complex 1 (PRC1) and PRC2, which are essential for normal gene regulation and development. Our early understanding of Polycomb function relied on studies in simple model organisms, but more recently it has become apparent that this system has expanded and diverged in mammals. Detailed studies are now uncovering the molecular mechanisms that enable mammalian PRC1 and PRC2 to identify their target sites in the genome, communicate through feedback mechanisms to create Polycomb chromatin domains and control transcription to regulate gene expression. In this Review, we discuss and contextualize the emerging principles that define how this fascinating chromatin-based system regulates gene expression in mammals.

R-loop and its functions at the regulatory interfaces between transcription and (epi)genome

R-loop represents a prevalent and specialized chromatin structure critically involved in a wide range of biological processes. In particular, co-transcriptional R-loops, produced often due to RNA polymerase pausing or RNA biogenesis malfunction, can initiate molecular events to context-dependently regulate local gene transcription and crosstalk with chromatin modifications. Cellular "readers" of R-loops are identified, exerting crucial impacts on R-loop homeostasis and gene regulation. Mounting evidence also supports R-loop deregulation as a frequent, sometimes initiating, event during the development of human pathologies, notably cancer and neurological disorder. The purpose of this review is to cover recent advances in understanding the fundamentals of R-loop biology, which have started to unveil complex interplays of R-loops with factors involved in various biological processes such as transcription, RNA processing and epitranscriptomic modification (such as N6-methyladenosine), DNA damage sensing and repair, and epigenetic regulation.

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.

Toward an understanding of the detection and function of R-loops in plants

Although lagging behind studies in humans and other mammals, studies of R-loops in plants have recently entered an exciting stage in which the roles of R-loops in gene expression, genome stability, epigenomic signatures, and plant development and stress responses are being elucidated. Here, we review the strengths and weaknesses of existing methodologies, which were largely developed for R-loop studies in mammals, and then discuss the potential challenges of applying these methodologies to R-loop studies in plants. We then focus on recent advances in the functional characterization of R-loops in Arabidopsis thaliana and rice. Recent studies in plants indicate that there are coordinated relationships between R-loops and gene expression, and between R-loops and epigenomic signatures that depend, in part, on the types of R-loops involved. Finally, we discuss the emerging roles of R-loops in plants and directions for future research.

The Polycomb protein RING1B enables estrogen-mediated gene expression by promoting enhancer-promoter interaction and R-loop formation

Polycomb complexes have traditionally been prescribed roles as transcriptional repressors, though increasing evidence demonstrate they can also activate gene expression. However, the mechanisms underlying positive gene regulation mediated by Polycomb proteins are poorly understood. Here, we show that RING1B, a core component of Polycomb Repressive Complex 1, regulates enhancer-promoter interaction of the bona fide estrogen-activated GREB1 gene. Systematic characterization of RNA:DNA hybrid formation (R-loops), nascent transcription and RNA Pol II activity upon estrogen administration revealed a key role of RING1B in gene activation by regulating R-loop formation and RNA Pol II elongation. We also found that the estrogen receptor alpha (ERα) and RNA are both necessary for full RING1B recruitment to estrogen-activated genes. Notably, RING1B recruitment was mostly unaffected upon RNA Pol II depletion. Our findings delineate the functional interplay between RING1B, RNA and ERα to safeguard chromatin architecture perturbations required for estrogen-mediated gene regulation and highlight the crosstalk between steroid hormones and Polycomb proteins to regulate oncogenic programs.

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.

Characterization of R-Loop-Interacting Proteins in Embryonic Stem Cells Reveals Roles in rRNA Processing and Gene Expression

Chromatin-associated RNAs have diverse roles in the nucleus. However, their mechanisms of action are poorly understood, in part because of the inability to identify proteins that specifically associate with chromatin-bound RNAs. Here, we address this problem for a subset of chromatin-associated RNAs that form R-loops-RNA-DNA hybrid structures that include a displaced strand of ssDNA. R-loops generally form cotranscriptionally and have important roles in regulation of gene expression, immunoglobulin class switching, and other processes. However, unresolved R-loops can lead to DNA damage and chromosome instability. To identify factors that may bind and regulate R-loop accumulation or mediate R-loop-dependent functions, we used a comparative immunoprecipitation/MS approach, with and without RNA-protein crosslinking, to identify a stringent set of R-loop-binding proteins in mouse embryonic stem cells. We identified 364 R-loop-interacting proteins, which were highly enriched for proteins with predicted RNA-binding functions. We characterized several R-loop-interacting proteins of the DEAD-box family of RNA helicases and found that these proteins localize to the nucleolus and, to a lesser degree, the nucleus. Consistent with their localization patterns, we found that these helicases are required for rRNA processing and regulation of gene expression. Surprisingly, depletion of these helicases resulted in misregulation of highly overlapping sets of protein-coding genes, including many genes that function in differentiation and development. We conclude that R-loop-interacting DEAD-box helicases have nonredundant roles that are critical for maintaining the normal embryonic stem cell transcriptome.

(Dys)function Follows Form: Nucleic Acid Structure, Repeat Expansion, and Disease Pathology in FMR1 Disorders

Fragile X-related disorders (FXDs), also known as FMR1 disorders, are examples of repeat expansion diseases (REDs), clinical conditions that arise from an increase in the number of repeats in a disease-specific microsatellite. In the case of FXDs, the repeat unit is CGG/CCG and the repeat tract is located in the 5' UTR of the X-linked FMR1 gene. Expansion can result in neurodegeneration, ovarian dysfunction, or intellectual disability depending on the number of repeats in the expanded allele. A growing body of evidence suggests that the mutational mechanisms responsible for many REDs share several common features. It is also increasingly apparent that in some of these diseases the pathologic consequences of expansion may arise in similar ways. It has long been known that many of the disease-associated repeats form unusual DNA and RNA structures. This review will focus on what is known about these structures, the proteins with which they interact, and how they may be related to the causative mutation and disease pathology in the FMR1 disorders.

Transcription-coupled DNA double-strand break repair

The genomic DNA is constantly under attack by cellular and/or environmental factors. Fortunately, the cell is armed to safeguard its genome by various mechanisms such as nucleotide excision, base excision, mismatch and DNA double-strand break repairs. While these processes maintain the integrity of the genome throughout, DNA repair occurs preferentially faster at the transcriptionally active genes. Such transcription-coupled repair phenomenon plays important roles to maintain active genome integrity, failure of which would interfere with transcription, leading to an altered gene expression (and hence cellular pathologies/diseases). Among the various DNA damages, DNA double-strand breaks are quite toxic to the cells. If DNA double-strand break occurs at the active gene, it would interfere with transcription/gene expression, thus threatening cellular viability. Such DNA double-strand breaks are found to be repaired faster at the active gene in comparison to its inactive state or the inactive gene, thus supporting the existence of a new phenomenon of transcription-coupled DNA double-strand break repair. Here, we describe the advances of this repair process.

An EZH2-dependent transcriptional complex promotes aberrant epithelial remodelling after injury

Unveiling the molecular mechanisms of tissue remodelling following injury is imperative to elucidate its regenerative capacity and aberrant repair in disease. Using different omics approaches, we identified enhancer of zester homolog 2 (EZH2) as a key regulator of fibrosis in injured lung epithelium. Epithelial injury drives an enrichment of nuclear transforming growth factor-β-activated kinase 1 (TAK1) that mediates EZH2 phosphorylation to facilitate its liberation from polycomb repressive complex 2 (PRC2). This process results in the establishment of a transcriptional complex of EZH2, RNA-polymerase II (POL2) and nuclear actin, which orchestrates aberrant epithelial repair programmes. The liberation of EZH2 from PRC2 is accompanied by an EZH2-EZH1 switch to preserve H3K27me3 deposition at non-target genes. Loss of epithelial TAK1, EZH2 or blocking nuclear actin influx attenuates the fibrotic cascade and restores respiratory homeostasis. Accordingly, EZH2 inhibition significantly improves outcomes in a pulmonary fibrosis mouse model. Our results reveal an important non-canonical function of EZH2, paving the way for new therapeutic interventions in fibrotic lung diseases.

Polycomb Gene Silencing Mechanisms: PRC2 Chromatin Targeting, H3K27me3 'Readout', and Phase Separation-Based Compaction

Modulation of chromatin structure and/or modification by Polycomb repressive complexes (PRCs) provides an important means to partition the genome into functionally distinct subdomains and to regulate the activity of the underlying genes. Both the enzymatic activity of PRC2 and its chromatin recruitment, spreading, and eviction are exquisitely regulated via interactions with cofactors and DNA elements (such as unmethylated CpG islands), histones, RNA (nascent mRNA and long noncoding RNA), and R-loops. PRC2-catalyzed histone H3 lysine 27 trimethylation (H3K27me3) is recognized by distinct classes of effectors such as canonical PRC1 and BAH module-containing proteins (notably BAHCC1 in human). These effectors mediate gene silencing by different mechanisms including phase separation-related chromatin compaction and histone deacetylation. We discuss recent advances in understanding the structural architecture of PRC2, the regulation of its activity and chromatin recruitment, and the molecular mechanisms underlying Polycomb-mediated gene silencing. Because PRC deregulation is intimately associated with the development of diseases, a better appreciation of Polycomb-based (epi)genomic regulation will have far-reaching implications in biology and medicine.

A functional LSD1 coregulator screen reveals a novel transcriptional regulatory cascade connecting R-loop homeostasis with epigenetic regulation

The lysine specific demethylase 1 (LSD1) plays a pivotal role in cellular differentiation by regulating the expression of key developmental genes in concert with different coregulatory proteins. This process is impaired in different cancer types and incompletely understood. To comprehensively identify functional coregulators of LSD1, we established a novel tractable fluorescent reporter system to monitor LSD1 activity in living cells. Combining this reporter system with a state-of-the-art multiplexed RNAi screen, we identify the DEAD-box helicase 19A (DDX19A) as a novel coregulator and demonstrate that suppression of Ddx19a results in an increase of R-loops and reduced LSD1-mediated gene silencing. We further show that DDX19A binds to tri-methylated lysine 27 of histone 3 (H3K27me3) and it regulates gene expression through the removal of transcription promoting R-loops. Our results uncover a novel transcriptional regulatory cascade where the downregulation of genes is dependent on the LSD1 mediated demethylation of histone H3 lysine 4 (H3K4). This allows the polycomb repressive complex 2 (PRC2) to methylate H3K27, which serves as a binding site for DDX19A. Finally, the binding of DDX19A leads to the efficient removal of R-loops at active promoters, which further de-represses LSD1 and PRC2, establishing a positive feedback loop leading to a robust repression of the target gene.

Genome-wide R-loop Landscapes during Cell Differentiation and Reprogramming

DNA:RNA hybrids play key roles in both physiological and disease states by regulating chromatin and genome organization. Their homeostasis during cell differentiation and cell plasticity remains elusive. Using an isogenic human stem cell platform, we systematically characterize R-loops, DNA methylation, histone modifications, and chromatin accessibility in pluripotent cells and their lineage-differentiated derivatives. We confirm that a portion of R-loops formed co-transcriptionally at pluripotency genes in pluripotent stem cells and at lineage-controlling genes in differentiated lineages. Notably, a subset of R-loops maintained after differentiation are associated with repressive chromatin marks on silent pluripotency genes and undesired lineage genes. Moreover, in reprogrammed pluripotent cells, cell-of-origin-specific R-loops are initially present but are resolved with serial passaging. Our analysis suggests a multifaceted role of R-loops in cell fate determination that may serve as an additional layer of modulation on cell fate memory and cell plasticity.

RNA Epigenetics: Fine-Tuning Chromatin Plasticity and Transcriptional Regulation, and the Implications in Human Diseases

Chromatin structure plays an essential role in eukaryotic gene expression and cell identity. Traditionally, DNA and histone modifications have been the focus of chromatin regulation; however, recent molecular and imaging studies have revealed an intimate connection between RNA epigenetics and chromatin structure. Accumulating evidence suggests that RNA serves as the interplay between chromatin and the transcription and splicing machineries within the cell. Additionally, epigenetic modifications of nascent RNAs fine-tune these interactions to regulate gene expression at the co- and post-transcriptional levels in normal cell development and human diseases. This review will provide an overview of recent advances in the emerging field of RNA epigenetics, specifically the role of RNA modifications and RNA modifying proteins in chromatin remodeling, transcription activation and RNA processing, as well as translational implications in human diseases.