Histone H3K9 methylation is dispensable for Caenorhabditis elegans development but suppresses RNA:DNA hybrid-associated repeat instability

Synergistic lethality between BRCA1 and H3K9me2 loss reflects satellite derepression

Padeken et al. performed a genome-wide synthetic lethality screen and show that the BRCA1/BARD1 complex is necessary for germline viability in worms lacking MET-2 but not SET-25. The synthetic sterility upon BRCA1/BARD1 and H3K9me2 loss is directly linked to the DNA damage provoked by unscheduled satellite repeat transcription.

Caenorhabditis elegans has two histone H3 Lys9 methyltransferases, MET-2 (SETDB1 homolog) and SET-25 (G9a/SUV39H1 related). In worms, we found simple repeat sequences primarily marked by H3K9me2, while transposable elements and silent tissue-specific genes bear H3K9me3. RNA sequencing (RNA-seq) in histone methyltransferase (HMT) mutants shows that MET-2-mediated H3K9me2 is necessary for satellite repeat repression, while SET-25 silences a subset of transposable elements and tissue-specific genes through H3K9me3. A genome-wide synthetic lethality screen showed that RNA processing, nuclear RNA degradation, the BRCA1/BARD1 complex, and factors mediating replication stress survival are necessary for germline viability in worms lacking MET-2 but not SET-25. Unlike set-25 mutants, met-2-null worms accumulated satellite repeat transcripts, which form RNA:DNA hybrids on repetitive sequences, additively with the loss of BRCA1 or BARD1. BRCA1/BARD1-mediated H2A ubiquitination and MET-2 deposited H3K9me2 on satellite repeats are partially interdependent, suggesting both that the loss of silencing generates BRCA-recruiting DNA damage and that BRCA1 recruitment by damage helps silence repeats. The artificial induction of MSAT1 transcripts can itself trigger damage-induced germline lethality in a wild-type background, arguing that the synthetic sterility upon BRCA1/BARD1 and H3K9me2 loss is directly linked to the DNA damage provoked by unscheduled satellite repeat transcription.

A DNA repair protein and histone methyltransferase interact to promote genome stability in the Caenorhabditis elegans germ line

Histone modifications regulate gene expression and chromosomal events, yet how histone-modifying enzymes are targeted is poorly understood. Here we report that a conserved DNA repair protein, SMRC-1, associates with MET-2, the C. elegans histone methyltransferase responsible for H3K9me1 and me2 deposition. We used molecular, genetic, and biochemical methods to investigate the biological role of SMRC-1 and to explore its relationship with MET-2. SMRC-1, like its mammalian ortholog SMARCAL1, provides protection from DNA replication stress. SMRC-1 limits accumulation of DNA damage and promotes germline and embryonic viability. MET-2 and SMRC-1 localize to mitotic and meiotic germline nuclei, and SMRC-1 promotes an increase in MET-2 abundance in mitotic germline nuclei upon replication stress. In the absence of SMRC-1, germline H3K9me2 generally decreases after multiple generations at high culture temperature. Genetic data are consistent with MET-2 and SMRC-1 functioning together to limit replication stress in the germ line and in parallel to promote other germline processes. We hypothesize that loss of SMRC-1 activity causes chronic replication stress, in part because of insufficient recruitment of MET-2 to nuclei.

Heterochromatin suppresses gross chromosomal rearrangements at centromeres by repressing Tfs1/TFIIS-dependent transcription

Heterochromatin, characterized by histone H3 lysine 9 (H3K9) methylation, assembles on repetitive regions including centromeres. Although centromeric heterochromatin is important for correct segregation of chromosomes, its exact role in maintaining centromere integrity remains elusive. Here, we found in fission yeast that heterochromatin suppresses gross chromosomal rearrangements (GCRs) at centromeres. Mutations in Clr4/Suv39 methyltransferase increased the formation of isochromosomes, whose breakpoints were located in centromere repeats. H3K9A and H3K9R mutations also increased GCRs, suggesting that Clr4 suppresses centromeric GCRs via H3K9 methylation. HP1 homologs Swi6 and Chp2 and the RNAi component Chp1 were the chromodomain proteins essential for full suppression of GCRs. Remarkably, mutations in RNA polymerase II (RNAPII) or Tfs1/TFIIS, the transcription factor that facilitates restart of RNAPII after backtracking, specifically bypassed the requirement of Clr4 for suppressing GCRs. These results demonstrate that heterochromatin suppresses GCRs by repressing Tfs1-dependent transcription of centromere repeats.

Systematic evaluation of C. elegans lincRNAs with CRISPR knockout mutants

Background

Long intergenic RNAs (lincRNAs) play critical roles in eukaryotic cells, but systematic analyses of the lincRNAs of an animal for phenotypes are lacking. We generate CRISPR knockout strains for Caenorhabditis elegans lincRNAs and evaluate their phenotypes.

Results

C. elegans lincRNAs demonstrate global features such as shorter length and fewer exons than mRNAs. For the systematic evaluation of C. elegans lincRNAs, we produce CRISPR knockout strains for 155 of the total 170 C. elegans lincRNAs. Mutants of 23 lincRNAs show phenotypes in 6 analyzed traits. We investigate these lincRNAs by phenotype for their gene expression patterns and potential functional mechanisms. Some C. elegans lincRNAs play cis roles to modulate the expression of their neighboring genes, and several lincRNAs play trans roles as ceRNAs against microRNAs. We also examine the regulation of lincRNA expression by transcription factors, and we dissect the pathway by which two transcription factors, UNC-30 and UNC-55, together control the expression of linc-73. Furthermore, linc-73 possesses a cis function to modulate the expression of its neighboring kinesin gene unc-104 and thus plays roles in C. elegans locomotion.

Conclusions

By using CRISPR/cas9 technology, we generate knockout strains of 155 C. elegans lincRNAs as valuable resources for studies in noncoding RNAs, and we provide biological insights for 23 lincRNAs with the phenotypes identified in this study.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1619-6) contains supplementary material, which is available to authorized users.

Inverted repeat structures are associated with essential and highly expressed genes on C. elegans autosome distal arms

Complementary sequences in cellular transcripts base-pair to form double-stranded RNA (dsRNA) structures. Because transposon-derived repeats often give rise to self-complementary sequences, dsRNA structures are prevalent in eukaryotic genomes, typically occurring in gene introns and untranslated regions (UTRs). However, the regulatory impact of double-stranded structures within genes is not fully understood. We used three independent methods to define loci in Caenorhabditis elegans predicted to form dsRNA and correlated these structures with patterns of gene expression, gene essentiality, and genome organization. As previously observed, dsRNA loci are enriched on distal arms of C. elegans autosomes, where genes typically show less conservation and lower overall expression. In contrast, we find that dsRNAs are associated with essential genes on autosome arms, and dsRNA-associated genes exhibit higher-than-expected expression and histone modification patterns associated with transcriptional elongation. Genes with significant repetitive sequence content are also highly expressed, and, thus, observed gene expression trends may relate either to dsRNA structures or to repeat content. Our results raise the possibility that as-yet-undescribed mechanisms promote expression of loci that produce dsRNAs, despite their well-characterized roles in gene silencing.

Multiple Histone Methyl-Lysine Readers Ensure Robust Development and Germline Immortality in Caenorhabditis elegans

Chromatin modifications, including methylation of histone H3 at lysine 27 (H3K27me) by the Polycomb group proteins, play a broadly conserved role in the maintenance of cell fate. Diverse chromatin organization modifier (chromo) domain proteins act as "readers" of histone methylation states. However, understanding the functional relationships among chromo domains and their roles in the inheritance of gene expression patterns remains challenging. Here, we identify two chromo-domain proteins, CEC-1 and CEC-6, as potential readers of H3K27me in Caenorhabditis elegans, where they have divergent expression patterns and contribute to distinct phenotypes. Both cec-1 and cec-6 genetically interact with another chromo-domain gene, cec-3, a reader of H3K9 methylation. Combined loss of cec-1 and cec-3 leads to developmental defects in the adult that result in decreased fitness. Furthermore, loss of cec-6 and cec-3 surprisingly leads to a progressive loss of fertility across generations, a "mortal germline" phenotype. Our results provide evidence of functional compensation between H3K27me and H3K9me heterochromatin pathways, and show that histone methylation readers contribute to both somatic development and transgenerational fitness.

Heterochromatin: Guardian of the Genome

Constitutive heterochromatin is a major component of the eukaryotic nucleus and is essential for the maintenance of genome stability. Highly concentrated at pericentromeric and telomeric domains, heterochromatin is riddled with repetitive sequences and has evolved specific ways to compartmentalize, silence, and repair repeats. The delicate balance between heterochromatin epigenetic maintenance and cellular processes such as mitosis and DNA repair and replication reveals a highly dynamic and plastic chromatin domain that can be perturbed by multiple mechanisms, with far-reaching consequences for genome integrity. Indeed, heterochromatin dysfunction provokes genetic turmoil by inducing aberrant repeat repair, chromosome segregation errors, transposon activation, and replication stress and is strongly implicated in aging and tumorigenesis. Here, we summarize the general principles of heterochromatin structure and function, discuss the importance of its maintenance for genome integrity, and propose that more comprehensive analyses of heterochromatin roles in tumorigenesis will be integral to future innovations in cancer treatment.

Principles of Transgenerational Small RNA Inheritance in Caenorhabditis elegans

Examples of transgenerational inheritance of environmental responses are rapidly accumulating. In Caenorhabditis elegans nematodes, such heritable information transmits across generations in the form of RNA-dependent RNA polymerase-amplified small RNAs. Regulatory small RNAs enable sequence-specific gene regulation, and unlike chromatin modifications, can move between tissues, and escape from immediate germline reprogramming. In this review, we discuss the path that small RNAs take from the soma to the germline, and elaborate on the mechanisms that maintain or erase parental small RNA responses after a specific number of generations. We focus on the intricate interactions between heritable small RNAs and histone modifications, deposited on specific loci. A trace of heritable chromatin marks, in particular trimethylation of histone H3 lysine 9, is deposited on RNAi-targeted loci. However, how these modifications regulate RNAi or small RNA inheritance was until recently unclear. Integrating the very latest literature, we suggest that changes to histone marks may instigate transgenerational gene regulation indirectly, by affecting the biogenesis of heritable small RNAs. Inheritance of small RNAs could spread adaptive ancestral responses.

Heterochromatin-Encoded Satellite RNAs Induce Breast Cancer

Heterochromatic repetitive satellite RNAs are extensively transcribed in a variety of human cancers, including BRCA1 mutant breast cancer. Aberrant expression of satellite RNAs in cultured cells induces the DNA damage response, activates cell cycle checkpoints, and causes defects in chromosome segregation. However, the mechanism by which satellite RNA expression leads to genomic instability is not well understood. Here we provide evidence that increased levels of satellite RNAs in mammary glands induce tumor formation in mice. Using mass spectrometry, we further show that genomic instability induced by satellite RNAs occurs through interactions with BRCA1-associated protein networks required for the stabilization of DNA replication forks. Additionally, de-stabilized replication forks likely promote the formation of RNA-DNA hybrids in cells expressing satellite RNAs. These studies lay the foundation for developing novel therapeutic strategies that block the effects of non-coding satellite RNAs in cancer cells.

Perturbed autophagy and DNA repair converge to promote neurodegeneration in amyotrophic lateral sclerosis and dementia

Maintaining genomic stability constitutes a major challenge facing cells. DNA breaks can arise from direct oxidative damage to the DNA backbone, the inappropriate activities of endogenous enzymes such as DNA topoisomerases, or due to transcriptionally-derived RNA/DNA hybrids (R-loops). The progressive accumulation of DNA breaks has been linked to several neurological disorders. Recently, however, several independent studies have implicated nuclear and mitochondrial genomic instability, perturbed co-transcriptional processing, and impaired cellular clearance pathways as causal and intertwined mechanisms underpinning neurodegeneration. Here, we discuss this emerging paradigm in the context of amyotrophic lateral sclerosis and frontotemporal dementia, and outline how this knowledge paves the way to novel therapeutic interventions.

Ribosomal DNA copy loss and repeat instability in ATRX-mutated cancers

ATRX (alpha thalassemia/mental retardation X-linked) complexes with DAXX to deposit histone variant H3.3 into repetitive heterochromatin. Recent genome sequencing studies in cancers have revealed mutations in ATRX and their association with ALT (alternative lengthening of telomeres) activation. Here we report depletion of ATRX in mouse ES cells leads to selective loss in ribosomal RNA gene (rDNA) copy number. Supporting this, ATRX-mutated human ALT-positive tumors also show a substantially lower rDNA copy than ALT-negative tumors. Further investigation shows that the rDNA copy loss and repeat instability are caused by a disruption in H3.3 deposition and thus a failure in heterochromatin formation at rDNA repeats in the absence of ATRX. We also find that ATRX-depleted cells are reduced in ribosomal RNA transcription output and show increased sensitivity to RNA polymerase I (Pol I) transcription inhibitor CX5461. In addition, human ALT-positive cancer cell lines are also more sensitive to CX5461 treatment. Our study provides insights into the contribution of ATRX loss of function to tumorigenesis through the loss of rDNA stability and suggests the therapeutic potential of targeting Pol I transcription in ALT cancers.

Deletion of HP1γ in cardiac myocytes affects H4K20me3 levels but does not impact cardiac growth

BACKGROUND

Heterochromatin, which is formed when tri-methyl lysine 9 of histone H3 (H3K9me3) is bound by heterochromatin 1 proteins (HP1s), plays an important role in differentiation and senescence by silencing cell cycle genes. Cardiac myocytes (CMs) accumulate heterochromatin during differentiation and demethylation of H3K9me3 inhibits cell cycle gene silencing and cell cycle exit in CMs; however, it is unclear if this process is mediated by HP1s. In this study, we created a conditional CM-specific HP1 gamma (HP1γ) knockout (KO) mouse model and tested whether HP1γ is required for cell cycle gene silencing and cardiac growth.

RESULTS

HP1γ KO mice were generated by crossing HP1γ floxed mice (fl) with mice expressing Cre recombinase driven by the Nkx2.5 (cardiac progenitor gene) promoter (Cre). We confirmed that deletion of critical exons of HP1γ led to undetectable levels of HP1γ protein in HP1γ KO (Cre;fl/fl) CMs. Analysis of cardiac size and function by echo revealed no significant differences between HP1γ KO and control (WT, Cre, fl/fl) mice. No significant difference in expression of cell cycle genes or cardiac-specific genes was observed. Global transcriptome analysis demonstrated a very moderate effect of HP1γ deletion on global gene expression, with only 51 genes differentially expressed in HP1γ KO CMs. We found that HP1β protein, but not HP1α, was significantly upregulated and that subnuclear localization of HP1β to perinuclear heterochromatin was increased in HP1γ KO CMs. Although HP1γ KO had no effect on H3K9me3 levels, we found a significant reduction in another major heterochromatin mark, tri-methylated lysine 20 of histone H4 (H4K20me3).

CONCLUSIONS

These data indicate that HP1γ is dispensable for cell cycle exit and normal cardiac growth but has a significant role in maintaining H4K20me3 and regulating a limited number of genes in CMs.

Mechanisms of Oncogene-Induced Replication Stress: Jigsaw Falling into Place

Oncogene activation disturbs cellular processes and accommodates a complex landscape of changes in the genome that contribute to genomic instability, which accelerates mutation rates and promotes tumorigenesis. Part of this cellular turmoil involves deregulation of physiologic DNA replication, widely described as replication stress. Oncogene-induced replication stress is an early driver of genomic instability and is attributed to a plethora of factors, most notably aberrant origin firing, replication-transcription collisions, reactive oxygen species, and defective nucleotide metabolism.Significance: Replication stress is a fundamental step and an early driver of tumorigenesis and has been associated with many activated oncogenes. Deciphering the mechanisms that contribute to the replication stress response may provide new avenues for targeted cancer treatment. In this review, we discuss the latest findings on the DNA replication stress response and examine the various mechanisms through which activated oncogenes induce replication stress. Cancer Discov; 8(5); 537-55. ©2018 AACR.

Function of Junk: Pericentromeric Satellite DNA in Chromosome Maintenance

Satellite DNAs are simple tandem repeats that exist at centromeric and pericentromeric regions on eukaryotic chromosomes. Unlike the centromeric satellite DNA that comprises the vast majority of natural centromeres, function(s) for the much more abundant pericentromeric satellite repeats are poorly understood. In fact, the lack of coding potential allied with rapid divergence of repeat sequences across eukaryotes has led to their dismissal as "junk DNA" or "selfish parasites." Although implicated in various biological processes, a conserved function for pericentromeric satellite DNA remains unidentified. We have addressed the role of satellite DNA through studying chromocenters, a cytological aggregation of pericentromeric satellite DNA from multiple chromosomes into DNA-dense nuclear foci. We have shown that multivalent satellite DNA-binding proteins cross-link pericentromeric satellite DNA on chromosomes into chromocenters. Disruption of chromocenters results in the formation of micronuclei, which arise by budding off the nucleus during interphase. We propose a model that satellite DNAs are critical chromosome elements that are recognized by satellite DNA-binding proteins and incorporated into chromocenters. We suggest that chromocenters function to preserve the entire chromosomal complement in a single nucleus, a fundamental and unquestioned feature of eukaryotic genomes. We speculate that the rapid divergence of satellite DNA sequences between closely related species results in discordant chromocenter function and may underlie speciation and hybrid incompatibility.

Ten principles of heterochromatin formation and function

Heterochromatin is a key architectural feature of eukaryotic chromosomes, which endows particular genomic domains with specific functional properties. The capacity of heterochromatin to restrain the activity of mobile elements, isolate DNA repair in repetitive regions and ensure accurate chromosome segregation is crucial for maintaining genomic stability. Nucleosomes at heterochromatin regions display histone post-translational modifications that contribute to developmental regulation by restricting lineage-specific gene expression. The mechanisms of heterochromatin establishment and of heterochromatin maintenance are separable and involve the ability of sequence-specific factors bound to nascent transcripts to recruit chromatin-modifying enzymes. Heterochromatin can spread along the chromatin from nucleation sites. The propensity of heterochromatin to promote its own spreading and inheritance is counteracted by inhibitory factors. Because of its importance for chromosome function, heterochromatin has key roles in the pathogenesis of various human diseases. In this Review, we discuss conserved principles of heterochromatin formation and function using selected examples from studies of a range of eukaryotes, from yeast to human, with an emphasis on insights obtained from unicellular model organisms.

A conserved function for pericentromeric satellite DNA

A universal and unquestioned characteristic of eukaryotic cells is that the genome is divided into multiple chromosomes and encapsulated in a single nucleus. However, the underlying mechanism to ensure such a configuration is unknown. Here, we provide evidence that pericentromeric satellite DNA, which is often regarded as junk, is a critical constituent of the chromosome, allowing the packaging of all chromosomes into a single nucleus. We show that the multi-AT-hook satellite DNA-binding proteins, Drosophila melanogaster D1 and mouse HMGA1, play an evolutionarily conserved role in bundling pericentromeric satellite DNA from heterologous chromosomes into ‘chromocenters’, a cytological association of pericentromeric heterochromatin. Defective chromocenter formation leads to micronuclei formation due to budding from the interphase nucleus, DNA damage and cell death. We propose that chromocenter and satellite DNA serve a fundamental role in encapsulating the full complement of the genome within a single nucleus, the universal characteristic of eukaryotic cells.

On Earth, life is divided into three domains. The smallest of these domains includes all the creatures, from sunflowers to yeasts to humans, that have the genetic information within their cells encased in a structure known as the nucleus. The genomes of these organisms are formed of long pieces of DNA, called chromosomes, which are packaged tightly and then unpackaged every time the cell divides. When a cell is not dividing, the chromosomes in the nucleus are loosely bundled up together.

It is well known that DNA is the blueprint for the building blocks of life, but actually most of the genetic information in a cell codes for nothing, and has unknown roles. An example of such ‘junk DNA’ is pericentrometric satellite DNA, small repetitive sequences found on all chromosomes.

However, new experiments by Jagannathan et al. show that, in the nucleus of animal cells, certain DNA binding proteins make chromosomes huddle together by attaching to multiple pericentrometric satellite DNA sequences on different chromosomes. In fact, if these proteins are removed from mice and fruit flies cells grown in the laboratory, the chromosomes cannot be clustered together. Instead, they ‘float away’, and the membranes of the nucleus get damaged, possibly buckling under the pressure of the unorganized DNA.

These events damage the genetic information, which can lead to the cell dying or forming tumors. ‘Junk DNA’ therefore appears to participate in fundamental cellular processes across species, a result that opens up several new lines of research.

JMJD-1.2 controls multiple histone post-translational modifications in germ cells and protects the genome from replication stress

Post-translational modifications of histones, constitutive components of chromatin, regulate chromatin compaction and control all DNA-based cellular processes. C. elegans JMJD-1.2, a member of the KDM7 family, is a demethylase active towards several lysine residues on Histone 3 (H3), but its contribution in regulating histone methylation in germ cells has not been fully investigated. Here, we show that jmjd-1.2 is expressed abundantly in the germline where it controls the level of histone 3 lysine 9, lysine 23 and lysine 27 di-methylation (H3K9/K23/K27me2) both in mitotic and meiotic cells. Loss of jmjd-1.2 is not associated with major defects in the germ cells in animals grown under normal conditions or after DNA damage induced by UV or ionizing irradiation. However, jmjd-1.2 mutants are more sensitive to replication stress and the progeny of mutant animals exposed to hydroxyurea show increased embryonic lethality and mutational rate, compared to wild-type. Thus, our results suggest a role for jmjd-1.2 in the maintenance of genome integrity after replication stress and emphasize the relevance of the regulation of histone methylation in genomic stability.

Repressive Chromatin in Caenorhabditis elegans: Establishment, Composition, and Function

Chromatin is organized and compacted in the nucleus through the association of histones and other proteins, which together control genomic activity. Two broad types of chromatin can be distinguished: euchromatin, which is generally transcriptionally active, and heterochromatin, which is repressed. Here we examine the current state of our understanding of repressed chromatin in Caenorhabditis elegans, focusing on roles of histone modifications associated with repression, such as methylation of histone H3 lysine 9 (H3K9me2/3) or the Polycomb Repressive Complex 2 (MES-2/3/6)-deposited modification H3K27me3, and on proteins that recognize these modifications. Proteins involved in chromatin repression are important for development, and have demonstrated roles in nuclear organization, repetitive element silencing, genome integrity, and the regulation of euchromatin. Additionally, chromatin factors participate in repression with small RNA pathways. Recent findings shed light on heterochromatin function and regulation in C. elegans, and should inform our understanding of repressed chromatin in other animals.

Epigenetic mechanisms in health and disease: BCEC 2017

The Barcelona Conference on Epigenetics and Cancer (BCEC) entitled "Epigenetic Mechanisms in Health and Disease" was held in Barcelona, October 26-26, 2017. The 2017 BCEC was the fifth and last edition of a series of annual conferences organized as a joint effort of five leading Barcelona research institutes together with B-Debate. This edition was organized by Albert Jordan from the Molecular Biology Institute of Barcelona (IBMB-CSIC) and Marcus Bushbeck from the Josep Carreras Leukaemia Research Institute (IJC). Jordi Bernués, Marian Martínez-Balbás, and Ferran Azorín were also part of the scientific committee. In 22 talks and 51 posters, researchers presented their latest results in the fields of histone variants, epigenetic regulation, and chromatin 3D organization to an audience of around 250 participants from 16 countries. This year, a broad number of talks focused on the epigenetic causes and possible related treatments of complex diseases such as cancer. Participants at the 2017 BCEC elegantly closed the series, discussing progress made in the field of epigenetics and highlighting its role in human health and disease.

Functional Redundancy of Variant and Canonical Histone H3 Lysine 9 Modification in Drosophila

Histone post-translational modifications (PTMs) and differential incorporation of variant and canonical histones into chromatin are central modes of epigenetic regulation. Despite similar protein sequences, histone variants are enriched for different suites of PTMs compared to their canonical counterparts. For example, variant histone H3.3 occurs primarily in transcribed regions and is enriched for "active" histone PTMs like Lys9 acetylation (H3.3K9ac), whereas the canonical histone H3 is enriched for Lys9 methylation (H3K9me), which is found in transcriptionally silent heterochromatin. To determine the functions of K9 modification on variant vs. canonical H3, we compared the phenotypes caused by engineering H3.3K9R and H3K9R mutant genotypes in Drosophila melanogaster Whereas most H3.3K9R , and a small number of H3K9R , mutant animals are capable of completing development and do not have substantially altered protein-coding transcriptomes, all H3.3K9R H3K9R combined mutants die soon after embryogenesis and display decreased expression of genes enriched for K9ac. These data suggest that the role of K9ac in gene activation during development can be provided by either H3 or H3.3. Conversely, we found that H3.3K9 is methylated at telomeric transposons and that this mark contributes to repressive chromatin architecture, supporting a role for H3.3 in heterochromatin that is distinct from that of H3. Thus, our genetic and molecular analyses demonstrate that K9 modification of variant and canonical H3 have overlapping roles in development and transcriptional regulation, though to differing extents in euchromatin and heterochromatin.

Regulation of mitotic recombination between DNA repeats in centromeres

Centromeres that are essential for faithful segregation of chromosomes consist of unique DNA repeats in many eukaryotes. Although recombination is under-represented around centromeres during meiosis, little is known about recombination between centromere repeats in mitotic cells. Here, we compared spontaneous recombination that occurs between ade6B/ade6X inverted repeats integrated at centromere 1 (cen1) or at a non-centromeric ura4 locus in fission yeast. Remarkably, distinct mechanisms of homologous recombination (HR) were observed in centromere and non-centromere regions. Rad51-dependent HR that requires Rad51, Rad54 and Rad52 was predominant in the centromere, whereas Rad51-independent HR that requires Rad52 also occurred in the arm region. Crossovers between inverted repeats (i.e. inversions) were under-represented in the centromere as compared to the arm region. While heterochromatin was dispensable, Mhf1/CENP–S, Mhf2/CENP–X histone-fold proteins and Fml1/FANCM helicase were required to suppress crossovers. Furthermore, Mhf1 and Fml1 were found to prevent gross chromosomal rearrangements mediated by centromere repeats. These data uncovered the regulation of mitotic recombination between DNA repeats in centromeres and its physiological role in maintaining genome integrity.

The Importance of Satellite Sequence Repression for Genome Stability

Up to two-thirds of eukaryotic genomes consist of repetitive sequences, which include both transposable elements and tandemly arranged simple or satellite repeats. Whereas extensive progress has been made toward understanding the danger of and control over transposon expression, only recently has it been recognized that DNA damage can arise from satellite sequence transcription. Although the structural role of satellite repeats in kinetochore function and end protection has long been appreciated, it has now become clear that it is not only these functions that are compromised by elevated levels of transcription. RNA from simple repeat sequences can compromise replication fork stability and genome integrity, thus compromising germline viability. Here we summarize recent discoveries on how cells control the transcription of repeat sequence and the dangers that arise from their expression. We propose that the link between the DNA damage response and the transcriptional silencing machinery may help a cell or organism recognize foreign DNA insertions into an evolving genome.

The double-stranded RNA binding protein RDE-4 can act cell autonomously during feeding RNAi in C. elegans

Long double-stranded RNA (dsRNA) can silence genes of matching sequence upon ingestion in many invertebrates and is therefore being developed as a pesticide. Such feeding RNA interference (RNAi) is best understood in the worm Caenorhabditis elegans, where the dsRNA-binding protein RDE-4 initiates silencing by recruiting an endonuclease to process long dsRNA into short dsRNA. These short dsRNAs are thought to move between cells because muscle-specific rescue of rde-4 using repetitive transgenes enables silencing in other tissues. Here, we extend this observation using additional promoters, report an inhibitory effect of repetitive transgenes, and discover conditions for cell-autonomous silencing in animals with tissue-specific rescue of rde-4. While expression of rde-4(+) in intestine, hypodermis, or neurons using a repetitive transgene can enable silencing also in unrescued tissues, silencing can be inhibited wihin tissues that express a repetitive transgene. Single-copy transgenes that express rde-4(+) in body-wall muscles or hypodermis, however, enable silencing selectively in the rescued tissue but not in other tissues. These results suggest that silencing by the movement of short dsRNA between cells is not an obligatory feature of feeding RNAi in C. elegans. We speculate that similar control of dsRNA movement could modulate tissue-specific silencing by feeding RNAi in other invertebrates.

Broad Chromatin Domains: An Important Facet of Genome Regulation

Chromatin composition differs across the genome, with distinct compositions characterizing regions associated with different properties and functions. Whereas many histone modifications show local enrichment over genes or regulatory elements, marking can also span large genomic intervals defining broad chromatin domains. Here we highlight structural and functional features of chromatin domains marked by histone modifications, with a particular emphasis on the potential roles of H3K27 methylation domains in the organization and regulation of genome activity in metazoans.

MORC-1 Integrates Nuclear RNAi and Transgenerational Chromatin Architecture to Promote Germline Immortality

Germline-expressed endogenous small interfering RNAs (endo-siRNAs) transmit multigenerational epigenetic information to ensure fertility in subsequent generations. In Caenorhabditis elegans, nuclear RNAi ensures robust inheritance of endo-siRNAs and deposition of repressive H3K9me3 marks at target loci. How target silencing is maintained in subsequent generations is poorly understood. We discovered that morc-1 is essential for transgenerational fertility and acts as an effector of endo-siRNAs. Unexpectedly, morc-1 is dispensable for siRNA inheritance but is required for target silencing and maintenance of siRNA-dependent chromatin organization. A forward genetic screen identified mutations in met-1, which encodes an H3K36 methyltransferase, as potent suppressors of morc-1(-) and nuclear RNAi mutant phenotypes. Further analysis of nuclear RNAi and morc-1(-) mutants revealed a progressive, met-1-dependent enrichment of H3K36me3, suggesting that robust fertility requires repression of MET-1 activity at nuclear RNAi targets. Without MORC-1 and nuclear RNAi, MET-1-mediated encroachment of euchromatin leads to detrimental decondensation of germline chromatin and germline mortality.

Silencing of retrotransposons by SETDB1 inhibits the interferon response in acute myeloid leukemia

Cancer cells can rewire genetic and epigenetic regulatory networks to promote cell proliferation and evade the immune system. Using a focused CRISPR/Cas9 genetic screen, Cuellar et al. identify a novel role for the SETDB1 histone methyltransferase in regulating the antiviral response in AML cells via the suppression of transposable elements.

A propensity for rewiring genetic and epigenetic regulatory networks, thus enabling sustained cell proliferation, suppression of apoptosis, and the ability to evade the immune system, is vital to cancer cell propagation. An increased understanding of how this is achieved is critical for identifying or improving therapeutic interventions. In this study, using acute myeloid leukemia (AML) human cell lines and a custom CRISPR/Cas9 screening platform, we identify the H3K9 methyltransferase SETDB1 as a novel, negative regulator of innate immunity. SETDB1 is overexpressed in many cancers, and loss of this gene in AML cells triggers desilencing of retrotransposable elements that leads to the production of double-stranded RNAs (dsRNAs). This is coincident with induction of a type I interferon response and apoptosis through the dsRNA-sensing pathway. Collectively, our findings establish a unique gene regulatory axis that cancer cells can exploit to circumvent the immune system.

Nucleolar expansion and elevated protein translation in premature aging

Premature aging disorders provide an opportunity to study the mechanisms that drive aging. In Hutchinson-Gilford progeria syndrome (HGPS), a mutant form of the nuclear scaffold protein lamin A distorts nuclei and sequesters nuclear proteins. We sought to investigate protein homeostasis in this disease. Here, we report a widespread increase in protein turnover in HGPS-derived cells compared to normal cells. We determine that global protein synthesis is elevated as a consequence of activated nucleoli and enhanced ribosome biogenesis in HGPS-derived fibroblasts. Depleting normal lamin A or inducing mutant lamin A expression are each sufficient to drive nucleolar expansion. We further show that nucleolar size correlates with donor age in primary fibroblasts derived from healthy individuals and that ribosomal RNA production increases with age, indicating that nucleolar size and activity can serve as aging biomarkers. While limiting ribosome biogenesis extends lifespan in several systems, we show that increased ribosome biogenesis and activity are a hallmark of premature aging.

HGPS is a premature aging disease caused by mutations in the nuclear protein lamin A. Here, the authors show that cells from patients with HGPS have expanded nucleoli and increased protein synthesis, and report that nucleoli also expand as aging progresses in cells derived from healthy individuals.

Linker histone H1 prevents R-loop accumulation and genome instability in heterochromatin

Linker histone H1 is an important structural component of chromatin that stabilizes the nucleosome and compacts the nucleofilament into higher-order structures. The biology of histone H1 remains, however, poorly understood. Here we show that Drosophila histone H1 (dH1) prevents genome instability as indicated by the increased γH2Av (H2AvS137P) content and the high incidence of DNA breaks and sister-chromatid exchanges observed in dH1-depleted cells. Increased γH2Av occurs preferentially at heterochromatic elements, which are upregulated upon dH1 depletion, and is due to the abnormal accumulation of DNA:RNA hybrids (R-loops). R-loops accumulation is readily detectable in G1-phase, whereas γH2Av increases mainly during DNA replication. These defects induce JNK-mediated apoptosis and are specific of dH1 depletion since they are not observed when heterochromatin silencing is relieved by HP1a depletion. Altogether, our results suggest that histone H1 prevents R-loops-induced DNA damage in heterochromatin and unveil its essential contribution to maintenance of genome stability.While structural importance of linker histone H1 in packaging eukaryotic genome into chromatin is well known, its biological function remains poorly understood. Here the authors reveal that Drosophila linker histone H1 prevents DNA:RNA hybrids accumulation and genome instability in heterochromatin.

The Helicase Aquarius/EMB-4 Is Required to Overcome Intronic Barriers to Allow Nuclear RNAi Pathways to Heritably Silence Transcription

Small RNAs play a crucial role in genome defense against transposable elements and guide Argonaute proteins to nascent RNA transcripts to induce co-transcriptional gene silencing. However, the molecular basis of this process remains unknown. Here, we identify the conserved RNA helicase Aquarius/EMB-4 as a direct and essential link between small RNA pathways and the transcriptional machinery in Caenorhabditis elegans. Aquarius physically interacts with the germline Argonaute HRDE-1. Aquarius is required to initiate small-RNA-induced heritable gene silencing. HRDE-1 and Aquarius silence overlapping sets of genes and transposable elements. Surprisingly, removal of introns from a target gene abolishes the requirement for Aquarius, but not HRDE-1, for small RNA-dependent gene silencing. We conclude that Aquarius allows small RNA pathways to compete for access to nascent transcripts undergoing co-transcriptional splicing in order to detect and silence transposable elements. Thus, Aquarius and HRDE-1 act as gatekeepers coordinating gene expression and genome defense.

The Role of Replication-Associated Repair Factors on R-Loops

The nascent RNA can reinvade the DNA double helix to form a structure termed the R-loop, where a single-stranded DNA (ssDNA) is accompanied by a DNA-RNA hybrid. Unresolved R-loops can impede transcription and replication processes and lead to genomic instability by a mechanism still not fully understood. In this sense, a connection between R-loops and certain chromatin markers has been reported that might play a key role in R-loop homeostasis and genome instability. To counteract the potential harmful effect of R-loops, different conserved messenger ribonucleoprotein (mRNP) biogenesis and nuclear export factors prevent R-loop formation, while ubiquitously-expressed specific ribonucleases and DNA-RNA helicases resolve DNA-RNA hybrids. However, the molecular events associated with R-loop sensing and processing are not yet known. Given that R-loops hinder replication progression, it is plausible that some DNA replication-associated factors contribute to dissolve R-loops or prevent R-loop mediated genome instability. In support of this, R-loops accumulate in cells depleted of the BRCA1, BRCA2 or the Fanconi anemia (FA) DNA repair factors, indicating that they play an active role in R-loop dissolution. In light of these results, we review our current view of the role of replication-associated DNA repair pathways in preventing the harmful consequences of R-loops.

RNA-DNA hybrid (R-loop) immunoprecipitation mapping: an analytical workflow to evaluate inherent biases

The impact of R-loops on the physiology and pathology of chromosomes has been demonstrated extensively by chromatin biology research. The progress in this field has been driven by technological advancement of R-loop mapping methods that largely relied on a single approach, DNA-RNA immunoprecipitation (DRIP). Most of the DRIP protocols use the experimental design that was developed by a few laboratories, without paying attention to the potential caveats that might affect the outcome of RNA-DNA hybrid mapping. To assess the accuracy and utility of this technology, we pursued an analytical approach to estimate inherent biases and errors in the DRIP protocol. By performing DRIP-sequencing, qPCR, and receiver operator characteristic (ROC) analysis, we tested the effect of formaldehyde fixation, cell lysis temperature, mode of genome fragmentation, and removal of free RNA on the efficacy of RNA-DNA hybrid detection and implemented workflows that were able to distinguish complex and weak DRIP signals in a noisy background with high confidence. We also show that some of the workflows perform poorly and generate random answers. Furthermore, we found that the most commonly used genome fragmentation method (restriction enzyme digestion) led to the overrepresentation of lengthy DRIP fragments over coding ORFs, and this bias was enhanced at the first exons. Biased genome sampling severely compromised mapping resolution and prevented the assignment of precise biological function to a significant fraction of R-loops. The revised workflow presented herein is established and optimized using objective ROC analyses and provides reproducible and highly specific RNA-DNA hybrid detection.

Coordinated control of terminal differentiation and restriction of cellular plasticity

The acquisition of a specific cellular identity is usually paralleled by a restriction of cellular plasticity. Whether and how these two processes are coordinated is poorly understood. Transcription factors called terminal selectors activate identity-specific effector genes during neuronal differentiation to define the structural and functional properties of a neuron. To study restriction of plasticity, we ectopically expressed C. elegans CHE-1, a terminal selector of ASE sensory neuron identity. In undifferentiated cells, ectopic expression of CHE-1 results in activation of ASE neuron type-specific effector genes. Once cells differentiate, their plasticity is restricted and ectopic expression of CHE-1 no longer results in activation of ASE effector genes. In striking contrast, removal of the respective terminal selectors of other sensory, inter-, or motor neuron types now enables ectopically expressed CHE-1 to activate its ASE-specific effector genes, indicating that terminal selectors not only activate effector gene batteries but also control the restriction of cellular plasticity. Terminal selectors mediate this restriction at least partially by organizing chromatin. The chromatin structure of a CHE-1 target locus is less compact in neurons that lack their resident terminal selector and genetic epistasis studies with H3K9 methyltransferases suggest that this chromatin modification acts downstream of a terminal selector to restrict plasticity. Taken together, terminal selectors activate identity-specific genes and make non-identity-defining genes less accessible, thereby serving as a checkpoint to coordinate identity specification with restriction of cellular plasticity.

DOI:http://dx.doi.org/10.7554/eLife.24100.001

Cross-Regulation between Transposable Elements and Host DNA Replication

Transposable elements subvert host cellular functions to ensure their survival. Their interaction with the host DNA replication machinery indicates that selective pressures lead them to develop ancestral and convergent evolutionary adaptations aimed at conserved features of this fundamental process. These interactions can shape the co-evolution of the transposons and their hosts.

SNF2 Family Protein Fft3 Suppresses Nucleosome Turnover to Promote Epigenetic Inheritance and Proper Replication

Heterochromatin can be epigenetically inherited in cis, leading to stable gene silencing. However, the mechanisms underlying heterochromatin inheritance remain unclear. Here, we identify Fft3, a fission yeast homolog of the mammalian SMARCAD1 SNF2 chromatin remodeler, as a factor uniquely required for heterochromatin inheritance, rather than for de novo assembly. Importantly, we find that Fft3 suppresses turnover of histones at heterochromatic loci to facilitate epigenetic transmission of heterochromatin in cycling cells. Moreover, Fft3 also precludes nucleosome turnover at several euchromatic loci to prevent R-loop formation, ensuring proper replication progression. Our analyses show that overexpression of Clr4/Suv39h, which is also required for efficient replication through these loci, suppresses phenotypes associated with the loss of Fft3. This work uncovers a conserved factor critical for epigenetic inheritance of heterochromatin and describes a mechanism in which suppression of nucleosome turnover prevents formation of structural barriers that impede replication at fragile regions in the genome.

A team of heterochromatin factors collaborates with small RNA pathways to combat repetitive elements and germline stress

Repetitive sequences derived from transposons make up a large fraction of eukaryotic genomes and must be silenced to protect genome integrity. Repetitive elements are often found in heterochromatin; however, the roles and interactions of heterochromatin proteins in repeat regulation are poorly understood. Here we show that a diverse set of C. elegans heterochromatin proteins act together with the piRNA and nuclear RNAi pathways to silence repetitive elements and prevent genotoxic stress in the germ line. Mutants in genes encoding HPL-2/HP1, LIN-13, LIN-61, LET-418/Mi-2, and H3K9me2 histone methyltransferase MET-2/SETDB1 also show functionally redundant sterility, increased germline apoptosis, DNA repair defects, and interactions with small RNA pathways. Remarkably, fertility of heterochromatin mutants could be partially restored by inhibiting cep-1/p53, endogenous meiotic double strand breaks, or the expression of MIRAGE1 DNA transposons. Functional redundancy among factors and pathways underlies the importance of safeguarding the genome through multiple means.

Decoupling the downstream effects of germline nuclear RNAi reveals that H3K9me3 is dispensable for heritable RNAi and the maintenance of endogenous siRNA-mediated transcriptional silencing in Caenorhabditis elegans

BACKGROUND

Germline nuclear RNAi in C. elegans is a transgenerational gene-silencing pathway that leads to H3K9 trimethylation (H3K9me3) and transcriptional silencing at the target genes. H3K9me3 induced by either exogenous double-stranded RNA (dsRNA) or endogenous siRNA (endo-siRNA) is highly specific to the target loci and transgenerationally heritable. Despite these features, the role of H3K9me3 in siRNA-mediated transcriptional silencing and inheritance of the silencing state at native target genes is unclear. In this study, we took combined genetic and whole-genome approaches to address this question.

RESULTS

Here we demonstrate that siRNA-mediated H3K9me3 requires combined activities of three H3K9 histone methyltransferases: MET-2, SET-25, and SET-32. set-32 single, met-2 set-25 double, and met-2 set-25;set-32 triple mutant adult animals all exhibit prominent reductions in H3K9me3 throughout the genome, with met-2 set-25;set-32 mutant worms losing all detectable H3K9me3 signals. Surprisingly, loss of high-magnitude H3K9me3 at the native nuclear RNAi targets has no effect on the transcriptional silencing state. In addition, the exogenous dsRNA-induced transcriptional silencing and heritable RNAi at oma-1, a well-established nuclear RNAi reporter gene, are completely resistant to the loss of H3K9me3.

CONCLUSIONS

Nuclear RNAi-mediated H3K9me3 in C. elegans requires multiple histone methyltransferases, including MET-2, SET-25, and SET-32. H3K9me3 is not essential for dsRNA-induced heritable RNAi or the maintenance of endo-siRNA-mediated transcriptional silencing in C. elegans. We propose that siRNA-mediated transcriptional silencing in C. elegans can be maintained by an H3K9me3-independent mechanism.

Dangerous R loops form in the absence of H3K9 methylation

Methylation of histone H3 on lysine 9 (H3K9) is a hallmark of transcriptionally inactive heterochromatin that is deregulated in pathological conditions. A new study shows that complete loss of H3K9 methylation in Caenorhabditis elegans leads to derepression of repetitive elements and formation of DNA:RNA hybrids (R loops), resulting in increased rates of repeat-specific mutation.