Dynamics of histone H3 deposition in vivo reveal a nucleosome gap-filling mechanism for H3.3 to maintain chromatin integrity

Acute multi-level response to defective de novo chromatin assembly in S-phase

Long-term perturbation of de novo chromatin assembly during DNA replication has profound effects on epigenome maintenance and cell fate. The early mechanistic origin of these defects is unknown. Here, we combine acute degradation of chromatin assembly factor 1 (CAF-1), a key player in de novo chromatin assembly, with single-cell genomics, quantitative proteomics, and live microscopy to uncover these initiating mechanisms in human cells. CAF-1 loss immediately slows down DNA replication speed and renders nascent DNA hyper-accessible. A rapid cellular response, distinct from canonical DNA damage signaling, is triggered and lowers histone mRNAs. In turn, histone variants' usage and their modifications are altered, limiting transcriptional fidelity and delaying chromatin maturation within a single S-phase. This multi-level response induces a p53-dependent cell-cycle arrest after mitosis. Our work reveals the immediate consequences of defective de novo chromatin assembly during DNA replication, indicating how at later times the epigenome and cell fate can be altered.

Local nuclear to cytoplasmic ratio regulates H3.3 incorporation via cell cycle state during zygotic genome activation

Early embryos often have unique chromatin states prior to zygotic genome activation (ZGA). In Drosophila, ZGA occurs after 13 reductive nuclear divisions during which the nuclear to cytoplasmic (N/C) ratio grows exponentially. Previous work found that histone H3 chromatin incorporation decreases while its variant H3.3 increases leading up to ZGA. In other cell types, H3.3 is associated with sites of active transcription and heterochromatin, suggesting a link between H3.3 and ZGA. Here, we test what factors regulate H3.3 incorporation at ZGA. We find that H3 nuclear availability falls more rapidly than H3.3 leading up to ZGA. We generate H3/H3.3 chimeric proteins at the endogenous H3.3A locus and observe that chaperone binding, but not gene structure, regulates H3.3 behavior. We identify the N/C ratio as a major determinant of H3.3 incorporation. To isolate how the N/C ratio regulates H3.3 incorporation we test the roles of genomic content, zygotic transcription, and cell cycle state. We determine that cell cycle regulation, but not H3 availability or transcription, controls H3.3 incorporation. Overall, we propose that local N/C ratios control histone variant usage via cell cycle state during ZGA.

The HUSH epigenetic repressor complex silences PML nuclear body-associated HSV-1 quiescent genomes

Herpes simplex virus 1 (HSV-1) latently infected neurons display diverse patterns in the distribution of the viral genomes within the nucleus. A key pattern involves quiescent HSV-1 genomes sequestered in promyelocytic leukemia nuclear bodies (PML NBs) forming viral DNA-containing PML-NBs (vDCP NBs). Using a cellular model that replicates vDCP NB formation, we previously demonstrated that these viral genomes are chromatinized with the H3.3 histone variant modified on its lysine 9 by trimethylation (H3.3K9me3), a mark associated with transcriptional repression. Here, we identify the HUSH complex and its effectors, SETDB1 and MORC2, as crucial for the acquisition of H3K9me3 on PML NB-associated HSV-1 and the maintenance of HSV-1 transcriptional repression. ChIP-seq analyses show H3K9me3 association with the entire viral genome. Inactivating the HUSH-SETDB1-MORC2 complex before infection significantly reduces H3K9me3 on the viral genome, with minimal impact on the cellular genome, aside from expected changes in LINE-1 retroelements. Depletion of HUSH, SETDB1, or MORC2 alleviates HSV-1 repression in infected primary human fibroblasts and human induced pluripotent stem cell-derived sensory neurons (hiPSDN). We found that the viral protein ICP0 induces MORC2 degradation via the proteasome machinery. This process is concurrent with ICP0 and MORC2 depletion capability to reactivate silenced HSV-1 in hiPSDN. Overall, our findings underscore the robust antiviral function of the HUSH-SETDB1-MORC2 repressor complex against a herpesvirus by modulating chromatin marks linked to repression, thus presenting promising avenues for anti-herpesvirus therapeutic strategies.

HIRA protects telomeres against R-loop-induced instability in ALT cancer cells

Inactivating mutations in chromatin modifiers, like the α-thalassemia/mental retardation, X-linked (ATRX)-death domain-associated protein (DAXX) chromatin remodeling/histone H3.3 deposition complex, drive the cancer-specific alternative lengthening of telomeres (ALT) pathway. Prior studies revealed that HIRA, another histone H3.3 chaperone, compensates for ATRX-DAXX loss at telomeres to sustain ALT cancer cell survival. How HIRA rescues telomeres from the consequences of ATRX-DAXX deficiency remains unclear. Here, using an assay for transposase-accessible chromatin using sequencing (ATAC-seq) and cleavage under targets and release using nuclease (CUT&RUN), we establish that HIRA-mediated deposition of new H3.3 maintains telomeric chromatin accessibility to prevent the detrimental accumulation of nucleosome-free single-stranded DNA (ssDNA) in ATRX-DAXX-deficient ALT cells. We show that the HIRA-UBN1/UBN2 complex deposits new H3.3 to prevent TERRA R-loop buildup and transcription-replication conflicts (TRCs) at telomeres. Furthermore, HIRA-mediated H3.3 incorporation into telomeric chromatin links productive ALT to the phosphorylation of serine 31, an H3.3-specific amino acid, by Chk1. Therefore, we identify a critical role for HIRA-mediated H3.3 deposition that ensures the survival of ATRX-DAXX-deficient ALT cancer cells.

Histone H3 mutations and their impact on genome stability maintenance

Histones are essential for maintaining chromatin structure and function. Histone mutations lead to changes in chromatin compaction, gene expression, and the recruitment of DNA repair proteins to the DNA lesion. These disruptions can impair critical DNA repair pathways, such as homologous recombination and non-homologous end joining, resulting in increased genomic instability, which promotes an environment favorable to tumor development and progression. Understanding these mechanisms underscores the potential of targeting DNA repair pathways in cancers harboring mutated histones, offering novel therapeutic strategies to exploit their inherent genomic instability for better treatment outcomes. Here, we examine how mutations in histone H3 disrupt normal chromatin function and DNA damage repair processes and how these mechanisms can be exploited for therapeutic interventions.

Mutant ATRX: pathogenesis of ATRX syndrome and cancer

The transcriptional regulator ATRX, a genetic factor, is associated with a range of disabilities, including intellectual, hematopoietic, skeletal, facial, and urogenital disabilities. ATRX mutations substantially contribute to the pathogenesis of ATRX syndrome and are frequently detected in gliomas and many other cancers. These mutations disrupt the organization, subcellular localization, and transcriptional activity of ATRX, leading to chromosomal instability and affecting interactions with key regulatory proteins such as DAXX, EZH2, and TERRA. ATRX also functions as a transcriptional regulator involved in the pathogenesis of neuronal disorders and various diseases. In conclusion, ATRX is a central protein whose abnormalities lead to multiple diseases.

Kick-starting the zygotic genome: licensors, specifiers, and beyond

Zygotic genome activation (ZGA), the first transcription event following fertilization, kickstarts the embryonic program that takes over the control of early development from the maternal products. How ZGA occurs, especially in mammals, is poorly understood due to the limited amount of research materials. With the rapid development of single-cell and low-input technologies, remarkable progress made in the past decade has unveiled dramatic transitions of the epigenomes, transcriptomes, proteomes, and metabolomes associated with ZGA. Moreover, functional investigations are yielding insights into the key regulators of ZGA, among which two major classes of players are emerging: licensors and specifiers. Licensors would control the permission of transcription and its timing during ZGA. Accumulating evidence suggests that such licensors of ZGA include regulators of the transcription apparatus and nuclear gatekeepers. Specifiers would instruct the activation of specific genes during ZGA. These specifiers include key transcription factors present at this stage, often facilitated by epigenetic regulators. Based on data primarily from mammals but also results from other species, we discuss in this review how recent research sheds light on the molecular regulation of ZGA and its executors, including the licensors and specifiers.

Protein UFMylation regulates early events during ribosomal DNA-damage response

The highly repetitive and transcriptionally active ribosomal DNA (rDNA) genes are exceedingly susceptible to genotoxic stress. Induction of DNA double-strand breaks (DSBs) in rDNA repeats is associated with ataxia-telangiectasia-mutated (ATM)-dependent rDNA silencing and nucleolar reorganization where rDNA is segregated into nucleolar caps. However, the regulatory events underlying this response remain elusive. Here, we identify protein UFMylation as essential for rDNA-damage response in human cells. We further show the only ubiquitin-fold modifier 1 (UFM1)-E3 ligase UFL1 and its binding partner DDRGK1 localize to nucleolar caps upon rDNA damage and that UFL1 loss impairs ATM activation and rDNA transcriptional silencing, leading to reduced rDNA segregation. Moreover, analysis of nuclear and nucleolar UFMylation targets in response to DSB induction further identifies key DNA-repair factors including ATM, in addition to chromatin and actin network regulators. Taken together, our data provide evidence of an essential role for UFMylation in orchestrating rDNA DSB repair.

Age-related histone H3.3 accumulation associates with a repressive chromatin in mouse tibialis anterior muscle

The present study aimed to investigate age-related changes in histone variant H3.3 and its role in the aging process of mouse tibialis anterior muscle. H3.3 level significantly increased with age and correlated with H3K27me3 level. Acute exercise successfully upregulated the target gene expression in 8-wk-old mice, whereas no upregulation was noted in 53-wk-old mice. H3K27me3 level was increased at these loci in response to acute exercise in 8-wk-old mice. However, in 53-wk-old mice, H3.3 and H3K27me3 levels were increased at rest and were not affected by acute exercise. Furthermore, forced H3.3 expression in the skeletal muscle of 8-wk-old mice led to a gradual improvement in motor function. The results suggest that age-related H3.3 accumulation induces the formation of repressive chromatin in the mouse tibialis anterior muscle. However, H3.3 accumulation also appears to play a positive role in enhancing skeletal muscle function.

Comprehensive posttranslational modifications in the testis-specific histone variant H3t protein validated in tagged knock-in mice

During the development of multicellular organisms and cell differentiation, the chromatin structure in the cell nucleus undergoes extensive changes, and the nucleosome structure is formed by a combination of various histone variants. Histone variants with diverse posttranslational modifications are known to play crucial roles in different regulatory functions. We have previously reported that H3t, a testis-specific histone variant, is essential for spermatogenesis. To elucidate the function of this chromatin molecule in vivo, we generated knock-in mice with a FLAG tag attached to the carboxyl terminus of H3t. In the present study, we evaluated the utility of the generated knock-in mice and comprehensively analyzed posttranslational modifications of canonical H3 and H3t using mass spectrometry. Herein, we found that H3t-FLAG was incorporated into spermatogonia and meiotic cells in the testes, as evidenced by immunostaining of testicular tissue. According to the mass spectrometry analysis, the overall pattern of H3t-FLAG posttranslational modification was comparable to that of the control H3, while the relative abundances of certain specific modifications differed between H3t-FLAG and the control bulk H3. The generated knock-in mice could be valuable for analyzing the function of histone variants in vivo.

Roles of Histone H2B, H3 and H4 Variants in Cancer Development and Prognosis

Histone variants are the paralogs of core histones (H2A, H2B, H3 and H4). They are stably expressed throughout the cell cycle in a replication-independent fashion and are capable of replacing canonical counterparts under different fundamental biological processes. Variants have been shown to take part in multiple processes, including DNA damage repair, transcriptional regulation and X chromosome inactivation, with some of them even specializing in lineage-specific roles like spermatogenesis. Several reports have recently identified some unprecedented variants from different histone families and exploited their prognostic value in distinct types of cancer. Among the four classes of canonical histones, the H2A family has the greatest number of variants known to date, followed by H2B, H3 and H4. In our prior review, we focused on summarizing all 19 mammalian histone H2A variants. Here in this review, we aim to complete the full summary of the roles of mammalian histone variants from the remaining histone H2B, H3, and H4 families, along with an overview of their roles in cancer biology and their prognostic value in a clinical context.

HIRA complex deposition of histone H3.3 is driven by histone tetramerization and histone-DNA binding

The HIRA histone chaperone complex is comprised of four protein subunits: HIRA, UBN1, CABIN1, and transiently associated ASF1a. All four subunits have been demonstrated to play a role in the deposition of the histone variant H3.3 onto areas of actively transcribed euchromatin in cells. The mechanism by which these subunits function together to drive histone deposition has remained poorly understood. Here we present biochemical and biophysical data supporting a model whereby ASF1a delivers histone H3.3/H4 dimers to the HIRA complex, H3.3/H4 tetramerization drives the association of two HIRA/UBN1 complexes, and the affinity of the histones for DNA drives release of ASF1a and subsequent histone deposition. These findings have implications for understanding how other histone chaperone complexes may mediate histone deposition.

Inhibition of CAF-1 histone chaperone complex triggers cytosolic DNA and dsRNA sensing pathways and induces intrinsic immunity of hepatocellular carcinoma

BACKGROUND AND AIMS

Chromatin assembly factor 1 (CAF-1) is a replication-dependent epigenetic regulator that controls cell cycle progression and chromatin dynamics. In this study, we aim to investigate the immunomodulatory role and therapeutic potential of the CAF-1 complex in HCC.

APPROACH AND RESULTS

CAF-1 complex knockout cell lines were established using the CRISPR/Cas9 system. The effects of CAF-1 in HCC were studied in HCC cell lines, nude mice, and immunocompetent mice. RNA-sequencing, ChIP-Seq, and assay for transposase accessible chromatin with high-throughput sequencing (ATAC-Seq) were used to explore the changes in the epigenome and transcriptome. CAF-1 complex was significantly upregulated in human and mouse HCCs and was associated with poor prognosis in patients with HCC. Knockout of CAF-1 remarkably suppressed HCC growth in both in vitro and in vivo models. Mechanistically, depletion of CAF-1 induced replicative stress and chromatin instability, which eventually led to cytoplasmic DNA leakage as micronuclei. Also, chromatin immunoprecipitation sequencing analyses revealed a massive H3.3 histone variant replacement upon CAF-1 knockout. Enrichment of euchromatic H3.3 increased chromatin accessibility and activated the expression of endogenous retrovirus elements, a phenomenon known as viral mimicry. However, cytosolic micronuclei and endogenous retroviruses are recognized as ectopic elements by the stimulator of interferon genes and dsRNA viral sensing pathways, respectively. As a result, the knockout of CAF-1 activated inflammatory response and antitumor immune surveillance and thereby significantly enhanced the anticancer effect of immune checkpoint inhibitors in HCC.

CONCLUSIONS

Our findings suggest that CAF-1 is essential for HCC development; targeting CAF-1 may awaken the anticancer immune response and may work cooperatively with immune checkpoint inhibitor treatment in cancer therapy.

Replicating chromatin in the nucleus: A histone variant perspective

In eukaryotes, chromatin and DNA replication are intimately linked, whereby chromatin impacts DNA replication control while genome duplication involves recovery of chromatin organisation. Here, we review recent advances in this area using a histone variant lens. We highlight how nucleosomal features interplay with origin definition and how the order of origin firing links with chromatin states in early mammalian development. We next discuss histone recycling and de novo deposition at the fork to finally open on the post-replicative recovery of the chromatin landscape to promote maintenance of cell identity.

Structure of the Hir histone chaperone complex

The evolutionarily conserved HIRA/Hir histone chaperone complex and ASF1a/Asf1 co-chaperone cooperate to deposit histone (H3/H4)2 tetramers on DNA for replication-independent chromatin assembly. The molecular architecture of the HIRA/Hir complex and its mode of histone deposition have remained unknown. Here, we report the cryo-EM structure of the S. cerevisiae Hir complex with Asf1/H3/H4 at 2.9-6.8 Å resolution. We find that the Hir complex forms an arc-shaped dimer with a Hir1/Hir2/Hir3/Hpc2 stoichiometry of 2/4/2/4. The core of the complex containing two Hir1/Hir2/Hir2 trimers and N-terminal segments of Hir3 forms a central cavity containing two copies of Hpc2, with one engaged by Asf1/H3/H4, in a suitable position to accommodate a histone (H3/H4)2 tetramer, while the C-terminal segments of Hir3 harbor nucleic acid binding activity to wrap DNA around the Hpc2-assisted histone tetramer. The structure suggests a model for how the Hir/Asf1 complex promotes the formation of histone tetramers for their subsequent deposition onto DNA.

Mechanisms of Hepatitis B Virus cccDNA and Minichromosome Formation and HBV Gene Transcription

Hepatitis B virus (HBV) is the etiologic agent of chronic hepatitis B, which puts at least 300 million patients at risk of developing fibrosis, cirrhosis, and hepatocellular carcinoma. HBV is a partially double-stranded DNA virus of the Hepadnaviridae family. While HBV was discovered more than 50 years ago, many aspects of its replicative cycle remain incompletely understood. Central to HBV persistence is the formation of covalently closed circular DNA (cccDNA) from the incoming relaxed circular DNA (rcDNA) genome. cccDNA persists as a chromatinized minichromosome and is the major template for HBV gene transcription. Here, we review how cccDNA and the viral minichromosome are formed and how viral gene transcription is regulated and highlight open questions in this area of research.

Identification of ASF1A and HJURP by global H3-H4 histone chaperone analysis as a prognostic two-gene model in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a malignancy with poor prognosis. Abnormal expression of H3-H4 histone chaperones has been identified in many cancers and holds promise as a biomarker for diagnosis and prognosis. However, systemic analysis of H3-H4 histone chaperones in HCC is still lacking. Here, we investigated the expression of 19 known H3-H4 histone chaperones in HCC. Integrated analysis of multiple public databases indicated that these chaperones are highly expressed in HCC tumor tissues, which was further verified by immunohistochemistry (IHC) staining in offline samples. Additionally, survival analysis suggested that HCC patients with upregulated H3-H4 histone chaperones have poor prognosis. Using LASSO and Cox regression, we constructed a two-gene model (ASF1A, HJURP) that accurately predicts prognosis in ICGC-LIRI and GEO HCC data, which was further validated in HCC tissue microarrays with follow-up information. GSEA revealed that HCCs in the high-risk group were associated with enhanced cell cycle progression and DNA replication. Intriguingly, HCCs in the high-risk group exhibited increased immune infiltration and sensitivity to immune checkpoint therapy (ICT). In summary, H3-H4 histone chaperones play a critical role in HCC progression, and the two-gene (ASF1A, HJURP) risk model is effective for predicting survival outcomes and sensitivity to immunotherapy for HCC patients.

The histone chaperone SPT2 regulates chromatin structure and function in Metazoa

Histone chaperones control nucleosome density and chromatin structure. In yeast, the H3-H4 chaperone Spt2 controls histone deposition at active genes but its roles in metazoan chromatin structure and organismal physiology are not known. Here we identify the Caenorhabditis elegans ortholog of SPT2 (CeSPT-2) and show that its ability to bind histones H3-H4 is important for germline development and transgenerational epigenetic gene silencing, and that spt-2 null mutants display signatures of a global stress response. Genome-wide profiling showed that CeSPT-2 binds to a range of highly expressed genes, and we find that spt-2 mutants have increased chromatin accessibility at a subset of these loci. We also show that SPT2 influences chromatin structure and controls the levels of soluble and chromatin-bound H3.3 in human cells. Our work reveals roles for SPT2 in controlling chromatin structure and function in Metazoa.

Disordered regions and folded modules in CAF-1 promote histone deposition in Schizosaccharomyces pombe

Genome and epigenome integrity in eukaryotes depends on the proper coupling of histone deposition with DNA synthesis. This process relies on the evolutionary conserved histone chaperone CAF-1 for which the links between structure and functions are still a puzzle. While studies of the Saccharomyces cerevisiae CAF-1 complex enabled to propose a model for the histone deposition mechanism, we still lack a framework to demonstrate its generality and in particular, how its interaction with the polymerase accessory factor PCNA is operating. Here, we reconstituted a complete SpCAF-1 from fission yeast. We characterized its dynamic structure using NMR, SAXS and molecular modeling together with in vitro and in vivo functional studies on rationally designed interaction mutants. Importantly, we identify the unfolded nature of the acidic domain which folds up when binding to histones. We also show how the long KER helix mediates DNA binding and stimulates SpCAF-1 association with PCNA. Our study highlights how the organization of CAF-1 comprising both disordered regions and folded modules enables the dynamics of multiple interactions to promote synthesis-coupled histone deposition essential for its DNA replication, heterochromatin maintenance, and genome stability functions.

Regulation of replicative histone RNA metabolism by the histone chaperone ASF1

In S phase, duplicating and assembling the whole genome into chromatin requires upregulation of replicative histone gene expression. Here, we explored how histone chaperones control histone production in human cells to ensure a proper link with chromatin assembly. Depletion of the ASF1 chaperone specifically decreases the pool of replicative histones both at the protein and RNA levels. The decrease in their overall expression, revealed by total RNA sequencing (RNA-seq), contrasted with the increase in nascent/newly synthesized RNAs observed by 4sU-labeled RNA-seq. Further inspection of replicative histone RNAs showed a 3' end processing defect with an increase of pre-mRNAs/unprocessed transcripts likely targeted to degradation. Collectively, these data argue for a production defect of replicative histone RNAs in ASF1-depleted cells. We discuss how this regulation of replicative histone RNA metabolism by ASF1 as a "chaperone checkpoint" fine-tunes the histone dosage to avoid unbalanced situations deleterious for cell survival.

DAXX promotes centromeric stability independently of ATRX by preventing the accumulation of R-loop-induced DNA double-stranded breaks

Maintaining chromatin integrity at the repetitive non-coding DNA sequences underlying centromeres is crucial to prevent replicative stress, DNA breaks and genomic instability. The concerted action of transcriptional repressors, chromatin remodelling complexes and epigenetic factors controls transcription and chromatin structure in these regions. The histone chaperone complex ATRX/DAXX is involved in the establishment and maintenance of centromeric chromatin through the deposition of the histone variant H3.3. ATRX and DAXX have also evolved mutually-independent functions in transcription and chromatin dynamics. Here, using paediatric glioma and pancreatic neuroendocrine tumor cell lines, we identify a novel ATRX-independent function for DAXX in promoting genome stability by preventing transcription-associated R-loop accumulation and DNA double-strand break formation at centromeres. This function of DAXX required its interaction with histone H3.3 but was independent of H3.3 deposition and did not reflect a role in the repression of centromeric transcription. DAXX depletion mobilized BRCA1 at centromeres, in line with BRCA1 role in counteracting centromeric R-loop accumulation. Our results provide novel insights into the mechanisms protecting the human genome from chromosomal instability, as well as potential perspectives in the treatment of cancers with DAXX alterations.

HIRA vs. DAXX: the two axes shaping the histone H3.3 landscape

H3.3, the most common replacement variant for histone H3, has emerged as an important player in chromatin dynamics for controlling gene expression and genome integrity. While replicative variants H3.1 and H3.2 are primarily incorporated into nucleosomes during DNA synthesis, H3.3 is under the control of H3.3-specific histone chaperones for spatiotemporal incorporation throughout the cell cycle. Over the years, there has been progress in understanding the mechanisms by which H3.3 affects domain structure and function. Furthermore, H3.3 distribution and relative abundance profoundly impact cellular identity and plasticity during normal development and pathogenesis. Recurrent mutations in H3.3 and its chaperones have been identified in neoplastic transformation and developmental disorders, providing new insights into chromatin biology and disease. Here, we review recent findings emphasizing how two distinct histone chaperones, HIRA and DAXX, take part in the spatial and temporal distribution of H3.3 in different chromatin domains and ultimately achieve dynamic control of chromatin organization and function. Elucidating the H3.3 deposition pathways from the available histone pool will open new avenues for understanding the mechanisms by which H3.3 epigenetically regulates gene expression and its impact on cellular integrity and pathogenesis.

Independent and Complementary Functions of Caf1b and Hir1 for Chromatin Assembly in Tetrahymena thermophila

Histones and DNA associate to form the nucleosomes of eukaryotic chromatin. Chromatin assembly factor 1 (CAF-1) complex and histone regulatory protein A (HIRA) complex mediate replication-couple (RC) and replication-independent (RI) nucleosome assembly, respectively. CHAF1B and HIRA share a similar domain but play different roles in nucleosome assembly by binding to the different interactors. At present, there is limited understanding for the similarities and differences in their respective functions. Tetrahymena thermophila contains transcriptionally active polyploid macronuclei (MAC) and transcriptionally silent diploid micronuclei (MIC). Here, the distribution patterns of Caf1b and Hir1 exhibited both similarities and distinctions. Both proteins localized to the MAC and MIC during growth, and to the MIC during conjugation. However, Hir1 exhibited additional signaling on parental MAC and new MAC during sexual reproduction and displayed a punctate signal on developing anlagen. Caf1b and Hir1 only co-localized in the MIC with Pcna1 during conjugation. Knockdown of CAF1B impeded cellular growth and arrested sexual reproductive development. Loss of HIR1 led to MIC chromosome defects and aborted sexual development. Co-interference of CAF1B and HIR1 led to a more severe phenotype. Moreover, CAF1B knockdown led to the up-regulation of HIR1 expression, while knockdown of HIR1 also led to an increase in CAF1B expression. Furthermore, Caf1b and Hir1 interacted with different interactors. These results showed that CAF-1 and Hir1 have independent and complementary functions for chromatin assembly in T. thermophila.

Activation-induced cytidine deaminase an antibody diversification enzyme interacts with chromatin modifier UBN1 in B-cells

Activation-induced cytidine deaminase (AID) is the key mediator of antibody diversification in activated B-cells by the process of somatic hypermutation (SHM) and class switch recombination (CSR). Targeting AID to the Ig genes requires transcription (initiation and elongation), enhancers, and its interaction with numerous factors. Furthermore, the HIRA chaperon complex, a regulator of chromatin architecture, is indispensable for SHM. The HIRA chaperon complex consists of UBN1, ASF1a, HIRA, and CABIN1 that deposit H3.3 onto the DNA, the SHM hallmark. We explored whether UBN1 interacts with AID using computational and in-vitro experiments. Interestingly, our in-silico studies, such as molecular docking and molecular dynamics simulation results, predict that AID interacts with UBN1. Subsequently, co-immunoprecipitation and pull-down experiments established interactions between UBN1 and AID inside B-cells. Additionally, a double immunofluorescence assay confirmed that AID and UBN1 were co-localized in the human and chicken B-cell lines. Moreover, proximity ligation assay studies validated that AID interacts with UBN1. Ours is the first report on the interaction of genome mutator enzyme AID with UBN1. Nevertheless, the fate of interaction between UBN1 and AID is yet to be explored in the context of SHM or CSR.

Mitotic inheritance of genetic and epigenetic information via the histone H3.1 variant

The replication-dependent histone H3.1 variant, ubiquitous in multicellular eukaryotes, has been proposed to play key roles during chromatin replication due to its unique expression pattern restricted to the S phase of the cell cycle. Here, we describe recent discoveries in plants regarding molecular mechanisms and cellular pathways involving H3.1 that contribute to the maintenance of genomic and epigenomic information. First, we highlight new advances concerning the contribution of the histone chaperone CAF-1 and the TSK-H3.1 DNA repair pathway in preventing genomic instability during replication. We then summarize the evidence connecting H3.1 to specific roles required for the mitotic inheritance of epigenetic states. Finally, we discuss the recent identification of a specific interaction between H3.1 and DNA polymerase epsilon and its functional implications.

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

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

Interplay between PML NBs and HIRA for H3.3 dynamics following type I interferon stimulus

Promyelocytic leukemia Nuclear Bodies (PML NBs) are nuclear membrane-less organelles physically associated with chromatin underscoring their crucial role in genome function. The H3.3 histone chaperone complex HIRA accumulates in PML NBs upon senescence, viral infection or IFN-I treatment in primary cells. Yet, the molecular mechanisms of this partitioning and its function in regulating histone dynamics have remained elusive. By using specific approaches, we identify intermolecular SUMO-SIM interactions as an essential mechanism for HIRA recruitment in PML NBs. Hence, we describe a role of PML NBs as nuclear depot centers to regulate HIRA distribution in the nucleus, dependent both on SP100 and DAXX/H3.3 levels. Upon IFN-I stimulation, PML is required for interferon-stimulated genes (ISGs) transcription and PML NBs become juxtaposed to ISGs loci at late time points of IFN-I treatment. HIRA and PML are necessary for the prolonged H3.3 deposition at the transcriptional end sites of ISGs, well beyond the peak of transcription. Though, HIRA accumulation in PML NBs is dispensable for H3.3 deposition on ISGs. We thus uncover a dual function for PML/PML NBs, as buffering centers modulating the nuclear distribution of HIRA, and as chromosomal hubs regulating ISGs transcription and thus HIRA-mediated H3.3 deposition at ISGs upon inflammatory response.

CAF-1 deposits newly synthesized histones during DNA replication using distinct mechanisms on the leading and lagging strands

During every cell cycle, both the genome and the associated chromatin must be accurately replicated. Chromatin Assembly Factor-1 (CAF-1) is a key regulator of chromatin replication, but how CAF-1 functions in relation to the DNA replication machinery is unknown. Here, we reveal that this crosstalk differs between the leading and lagging strand at replication forks. Using biochemical reconstitutions, we show that DNA and histones promote CAF-1 recruitment to its binding partner PCNA and reveal that two CAF-1 complexes are required for efficient nucleosome assembly under these conditions. Remarkably, in the context of the replisome, CAF-1 competes with the leading strand DNA polymerase epsilon (Polϵ) for PCNA binding. However, CAF-1 does not affect the activity of the lagging strand DNA polymerase Delta (Polδ). Yet, in cells, CAF-1 deposits newly synthesized histones equally on both daughter strands. Thus, on the leading strand, chromatin assembly by CAF-1 cannot occur simultaneously to DNA synthesis, while on the lagging strand these processes may be coupled. We propose that these differences may facilitate distinct parental histone recycling mechanisms and accommodate the inherent asymmetry of DNA replication.

NADK-mediated de novo NADP(H) synthesis is a metabolic adaptation essential for breast cancer metastasis

Metabolic reprogramming and metabolic plasticity allow cancer cells to fine-tune their metabolism and adapt to the ever-changing environments of the metastatic cascade, for which lipid metabolism and oxidative stress are of particular importance. NADPH is a central co-factor for both lipid and redox homeostasis, suggesting that cancer cells may require larger pools of NADPH to efficiently metastasize. NADPH is recycled through reduction of NADP⁺ by several enzymatic systems in cells; however, de novo NADP⁺ is synthesized only through one known enzymatic reaction, catalyzed by NAD⁺ kinase (NADK). Here, we show that NADK is upregulated in metastatic breast cancer cells enabling de novo production of NADP(H) and the expansion of the NADP(H) pools thereby increasing the ability of these cells to adapt to the challenges of the metastatic cascade and efficiently metastasize. Mechanistically, we found that metastatic signals lead to a histone H3.3 variant-mediated epigenetic regulation of the NADK promoter, resulting in increased NADK levels in cells with metastatic ability. Together, our work presents a previously uncharacterized role for NADK and de novo NADP(H) production as a contributor to breast cancer progression and suggests that NADK constitutes an important and much needed therapeutic target for metastatic breast cancers.

Developmentally programmed histone H3 expression regulates cellular plasticity at the parental-to-early embryo transition

During metazoan development, the marked change in developmental potential from the parental germline to the embryo raises an important question regarding how the next life cycle is reset. As the basic unit of chromatin, histones are essential for regulating chromatin structure and function and, accordingly, transcription. However, the genome-wide dynamics of the canonical, replication-coupled (RC) histones during gametogenesis and embryogenesis remain unknown. In this study, we use CRISPR-Cas9-mediated gene editing in Caenorhabditis elegans to investigate the expression pattern and role of individual RC histone H3 genes and compare them to the histone variant, H3.3. We report a tightly regulated epigenome landscape change from the germline to embryos that are regulated through differential expression of distinct histone gene clusters. Together, this study reveals that a change from a H3.3- to H3-enriched epigenome during embryogenesis restricts developmental plasticity and uncovers distinct roles for individual H3 genes in regulating germline chromatin.

Coordinated histone variant H2A.Z eviction and H3.3 deposition control plant thermomorphogenesis

Plants can sense temperature changes and adjust their development and morphology accordingly in a process called thermomorphogenesis. This phenotypic plasticity implies complex mechanisms regulating gene expression reprogramming in response to environmental alteration. Histone variants often associate with specific chromatin states; yet, how their deposition/eviction modulates transcriptional changes induced by environmental cues remains elusive. In Arabidopsis thaliana, temperature elevation-induced transcriptional activation at thermo-responsive genes entails the chromatin eviction of a histone variant H2A.Z by INO80, which is recruited to these loci via interacting with a key thermomorphogenesis regulator PIF4. Here, we show that both INO80 and the deposition chaperones of another histone variant H3.3 associate with ELF7, a critical component of the transcription elongator PAF1 complex. H3.3 promotes thermomorphogenesis and the high temperature-enhanced RNA Pol II transcription at PIF4 targets, and it is broadly required for the H2A.Z removal-induced gene activation. Reciprocally, INO80 and ELF7 regulate H3.3 deposition, and are necessary for the high temperature-induced H3.3 enrichment at PIF4 targets. Our findings demonstrate close coordination between H2A.Z eviction and H3.3 deposition in gene activation induced by high temperature, and pinpoint the importance of histone variants dynamics in transcriptional regulation.

ATRX, a guardian of chromatin

ATRX (alpha-thalassemia mental retardation X-linked) is one of the most frequently mutated tumor suppressor genes in human cancers, especially in glioma, and recent findings indicate roles for ATRX in key molecular pathways, such as the regulation of chromatin state, gene expression, and DNA damage repair, placing ATRX as a central player in the maintenance of genome stability and function. This has led to new perspectives about the functional role of ATRX and its relationship with cancer. Here, we provide an overview of ATRX interactions and molecular functions and discuss the consequences of its impairment, including alternative lengthening of telomeres and therapeutic vulnerabilities that may be exploited in cancer cells.

Negative regulation of biofilm development by the CUG-Ser1 clade-specific histone H3 variant is dependent on the canonical histone chaperone CAF-1 complex in Candida albicans

The CUG-Ser1 clade-specific histone H3 variant (H3V^CTG ) has been reported to be a negative regulator of planktonic to biofilm growth transition in Candida albicans. The preferential binding of H3V^CTG at the biofilm gene promoters makes chromatin repressive for the biofilm mode of growth. The two evolutionarily conserved chaperone complexes involved in incorporating histone H3 are CAF-1 and HIRA. In this study, we sought to identify the chaperone complex(es) involved in loading H3V^CTG . We demonstrate that C. albicans cells lacking either Cac1 or Cac2 subunit of the CAF-1 chaperone complex, exhibit a hyper-filamentation phenotype on solid surfaces and form more robust biofilms than wild-type cells, thereby mimicking the phenotype of the H3V^CTG null mutant. None of the subunits of the HIRA chaperone complex shows any significant difference in biofilm growth as compared to the wild type. The occupancy of H3V^CTG is found to be significantly reduced at the promoters of biofilm genes in the absence of CAF-1 subunits. Hence, we provide evidence that CAF-1, a chaperone known to load canonical histone H3 in mammalian cells, is involved in chaperoning of variant histone H3V^CTG at the biofilm gene promoters in C. albicans. Our findings also illustrate the acquisition of an unconventional role of the CAF-1 chaperone complex in morphogenesis in C. albicans.

H3.3 contributes to chromatin accessibility and transcription factor binding at promoter-proximal regulatory elements in embryonic stem cells

BACKGROUND

The histone variant H3.3 is enriched at active regulatory elements such as promoters and enhancers in mammalian genomes. These regions are highly accessible, creating an environment that is permissive to transcription factor binding and the recruitment of transcriptional coactivators that establish a unique chromatin post-translational landscape. How H3.3 contributes to the establishment and function of chromatin states at these regions is poorly understood.

RESULTS

We perform genomic analyses of features associated with active promoter chromatin in mouse embryonic stem cells (ESCs) and find evidence of subtle yet widespread promoter dysregulation in the absence of H3.3. Loss of H3.3 results in reduced chromatin accessibility and transcription factor (TF) binding at promoters of expressed genes in ESCs. Likewise, enrichment of the transcriptional coactivator p300 and downstream histone H3 acetylation at lysine 27 (H3K27ac) is reduced at promoters in the absence of H3.3, along with reduced enrichment of the acetyl lysine reader BRD4. Despite the observed chromatin dysregulation, H3.3 KO ESCs maintain transcription from ESC-specific genes. However, upon undirected differentiation, H3.3 KO cells retain footprinting of ESC-specific TF motifs and fail to generate footprints of lineage-specific TF motifs, in line with their diminished capacity to differentiate.

CONCLUSIONS

H3.3 facilitates DNA accessibility, transcription factor binding, and histone post-translational modification at active promoters. While H3.3 is not required for maintaining transcription in ESCs, it does promote de novo transcription factor binding which may contribute to the dysregulation of cellular differentiation in the absence of H3.3.

Human Adenovirus Gene Expression and Replication Is Regulated through Dynamic Changes in Nucleoprotein Structure throughout Infection

Human adenovirus (HAdV) is extremely common and can rapidly spread in confined populations such as daycare centers, hospitals, and retirement homes. Although HAdV usually causes only minor illness in otherwise healthy patients, HAdV can cause significant morbidity and mortality in certain populations, such as the very young, very old, or immunocompromised individuals. During infection, the viral DNA undergoes dramatic changes in nucleoprotein structure that promote the rapid expression of viral genes, replication of the DNA, and generation of thousands of new infectious virions-each process requiring a distinct complement of virus and host-encoded proteins. In this review, we summarize our current understanding of the nucleoprotein structure of HAdV DNA during the various phases of infection, the cellular proteins implicated in mediating these changes, and the role of epigenetics in HAdV gene expression and replication.

Nuclear envelope, chromatin organizers, histones, and DNA: The many achilles heels exploited across cancers

In eukaryotic cells, the genome is organized in the form of chromatin composed of DNA and histones that organize and regulate gene expression. The dysregulation of chromatin remodeling, including the aberrant incorporation of histone variants and their consequent post-translational modifications, is prevalent across cancers. Additionally, nuclear envelope proteins are often deregulated in cancers, which impacts the 3D organization of the genome. Altered nuclear morphology, genome organization, and gene expression are defining features of cancers. With advances in single-cell sequencing, imaging technologies, and high-end data mining approaches, we are now at the forefront of designing appropriate small molecules to selectively inhibit the growth and proliferation of cancer cells in a genome- and epigenome-specific manner. Here, we review recent advances and the emerging significance of aberrations in nuclear envelope proteins, histone variants, and oncohistones in deregulating chromatin organization and gene expression in oncogenesis.

Mapping histone variant genomic distribution: Exploiting SNAP-tag labeling to follow the dynamics of incorporation of H3 variants

In the eukaryotic cell nucleus, in addition to the genomic information, chromatin organization provides an additional set of information which is more versatile and associates with distinct cell identities. In particular, the marking of the nucleosomes by a choice of specific histone variants can potentially confer distinct functional properties critical for genome function and stability. To understand how this unique marking operates we need to access to the genomic distribution of each variant. A general approach based on ChIP-Seq, relies on the specific isolation of DNA bound to the variant of interest, usually using cross-linked material and specific antibodies. The availability of reliable specific antibodies recognizing with high affinity crosslinked antigen represents a limitation. Here, we describe an experimental approach exploiting a tag fused to the protein of interest. The chose protein is a histone variant and we use native conditions for the selective capture of the histone variant in a nucleosome. Most importantly, we describe how to use a particular labeling system, with a SNAP tag enabling to specifically capture nucleosomes comprising newly synthesized histones. This method allows to follow the newly deposited histone variant at various times thereby offering a unique opportunity to evaluate the dynamics of histone deposition genome wide. We describe the method here for H3 variant, but it can be adapted to any histone variant with the appropriate fused tag to address genome wide a turn-over associated to the biological context of interest.

Lagging strand gap suppression connects BRCA-mediated fork protection to nucleosome assembly through PCNA-dependent CAF-1 recycling

The inability to protect stalled replication forks from nucleolytic degradation drives genome instability and underlies chemosensitivity in BRCA-deficient tumors. An emerging hallmark of BRCA-deficiency is the inability to suppress replication-associated single-stranded DNA (ssDNA) gaps. Here, we report that lagging strand ssDNA gaps interfere with the ASF1-CAF-1 nucleosome assembly pathway, and drive fork degradation in BRCA-deficient cells. We show that CAF-1 function at replication forks is lost in BRCA-deficient cells, due to defects in its recycling during replication stress. This CAF-1 recycling defect is caused by lagging strand gaps which preclude PCNA unloading, causing sequestration of PCNA-CAF-1 complexes on chromatin. Importantly, correcting PCNA unloading defects in BRCA-deficient cells restores CAF-1-dependent fork stability. We further show that the activation of a HIRA-dependent compensatory histone deposition pathway restores fork stability to BRCA-deficient cells. We thus define lagging strand gap suppression and nucleosome assembly as critical enablers of BRCA-mediated fork stability.

Efficient DNA replication is crucial for genome stability. Here, Thakar et al. report that accumulation of lagging strand ssDNA gaps during replication interferes with nucleosome assembly and drives replication fork degradation in BRCA-deficient cells.

The Role of the TSK/TONSL-H3.1 Pathway in Maintaining Genome Stability in Multicellular Eukaryotes

Replication-dependent histone H3.1 and replication-independent histone H3.3 are nearly identical proteins in most multicellular eukaryotes. The N-terminal tails of these H3 variants, where the majority of histone post-translational modifications are made, typically differ by only one amino acid. Despite extensive sequence similarity with H3.3, the H3.1 variant has been hypothesized to play unique roles in cells, as it is specifically expressed and inserted into chromatin during DNA replication. However, identifying a function that is unique to H3.1 during replication has remained elusive. In this review, we discuss recent findings regarding the involvement of the H3.1 variant in regulating the TSK/TONSL-mediated resolution of stalled or broken replication forks. Uncovering this new function for the H3.1 variant has been made possible by the identification of the first proteins containing domains that can selectively bind or modify the H3.1 variant. The functional characterization of H3-variant-specific readers and writers reveals another layer of chromatin-based information regulating transcription, DNA replication, and DNA repair.

Managing the Steady State Chromatin Landscape by Nucleosome Dynamics

Gene regulation arises out of dynamic competition between nucleosomes, transcription factors, and other chromatin proteins for the opportunity to bind genomic DNA. The timescales of nucleosome assembly and binding of factors to DNA determine the outcomes of this competition at any given locus. Here, we review how these properties of chromatin proteins and the interplay between the dynamics of different factors are critical for gene regulation. We discuss how molecular structures of large chromatin-associated complexes, kinetic measurements, and high resolution mapping of protein-DNA complexes in vivo set the boundary conditions for chromatin dynamics, leading to models of how the steady state behaviors of regulatory elements arise.

Human testis-specific Y-encoded protein-like protein 5 is a histone H3/H4-specific chaperone that facilitates histone deposition in vitro

DNA and core histones are hierarchically packaged into a complex organization called chromatin. The nucleosome assembly protein (NAP) family of histone chaperones is involved in the deposition of histone complexes H2A/H2B and H3/H4 onto DNA and prevents nonspecific aggregation of histones. Testis-specific Y-encoded protein (TSPY)–like protein 5 (TSPYL5) is a member of the TSPY-like protein family, which has been previously reported to interact with ubiquitin-specific protease USP7 and regulate cell proliferation and is thus implicated in various cancers, but its interaction with chromatin has not been investigated. In this study, we characterized the chromatin association of TSPYL5 and found that it preferentially binds histone H3/H4 via its C-terminal NAP-like domain both in vitro and ex vivo. We identified the critical residues involved in the TSPYL5–H3/H4 interaction and further quantified the binding affinity of TSPYL5 toward H3/H4 using biolayer interferometry. We then determined the binding stoichiometry of the TSPYL5–H3/H4 complex in vitro using a chemical cross-linking assay and size-exclusion chromatography coupled with multiangle laser light scattering. Our results indicate that a TSPYL5 dimer binds to either two histone H3/H4 dimers or a single tetramer. We further demonstrated that TSPYL5 has a specific affinity toward longer DNA fragments and that the same histone-binding residues are also critically involved in its DNA binding. Finally, employing histone deposition and supercoiling assays, we confirmed that TSPYL5 is a histone chaperone responsible for histone H3/H4 deposition and nucleosome assembly. We conclude that TSPYL5 is likely a new member of the NAP histone chaperone family.

HIRA Supports Hepatitis B Virus Minichromosome Establishment and Transcriptional Activity in Infected Hepatocytes

BACKGROUND & AIMS

Upon hepatitis B virus (HBV) infection, partially double-stranded viral DNA converts into a covalently closed circular chromatinized episomal structure (cccDNA). This form represents the long-lived genomic reservoir responsible for viral persistence in the infected liver. Although the involvement of host cell DNA damage response in cccDNA formation has been established, this work investigated the yet-to-be-identified histone dynamics on cccDNA during early phases of infection in human hepatocytes.

METHODS

Detailed studies of host chromatin-associated factors were performed in cell culture models of natural infection (ie, Na⁺-taurocholate cotransporting polypeptide (NTCP)-overexpressing HepG2 cells, HepG2^hNTCP) and primary human hepatocytes infected with HBV, by cccDNA-specific chromatin immunoprecipitation and loss-of-function experiments during early kinetics of viral minichromosome establishment and onset of viral transcription.

RESULTS

Our results show that cccDNA formation requires the deposition of the histone variant H3.3 via the histone regulator A (HIRA)-dependent pathway. This occurs simultaneously with repair of the cccDNA precursor and independently from de novo viral protein expression. Moreover, H3.3 in its S31 phosphorylated form appears to be the preferential H3 variant found on transcriptionally active cccDNA in infected cultured cells and human livers. HIRA depletion after cccDNA pool establishment showed that HIRA recruitment is required for viral transcription and RNA production.

CONCLUSIONS

Altogether, we show a crucial role for HIRA in the interplay between HBV genome and host cellular machinery to ensure the formation and active transcription of the viral minichromosome in infected hepatocytes.

Transcription-coupled H3.3 recycling: A link with chromatin states

Histone variant H3.3 is incorporated into chromatin throughout the cell cycle and even in non-cycling cells. This histone variant marks actively transcribed chromatin regions with high nucleosome turnover, as well as silent pericentric and telomeric repetitive regions. In the past few years, significant progress has been made in our understanding of mechanisms involved in the transcription-coupled deposition of H3.3. Here we review how, during transcription, new H3.3 deposition intermingles with the fate of the old H3.3 variant and its recycling. First, we describe pathways enabling the incorporation of newly synthesized vs old H3.3 histones in the context of transcription. We then review the current knowledge concerning differences between these two H3.3 populations, focusing on their PTMs composition. Finally, we discuss the implications of H3.3 recycling for the maintenance of the transcriptional state and underline the emerging importance of H3.3 as a potent epigenetic regulator for both maintaining and switching a transcriptional state.

Stable inheritance of H3.3-containing nucleosomes during mitotic cell divisions

Newly synthesized H3.1 and H3.3 histones are assembled into nucleosomes by different histone chaperones in replication-coupled and replication-independent pathways, respectively. However, it is not clear how parental H3.3 molecules are transferred following DNA replication, especially when compared to H3.1. Here, by monitoring parental H3.1- and H3.3-SNAP signals, we show that parental H3.3, like H3.1, are stably transferred into daughter cells. Moreover, Mcm2-Pola1 and Pole3-Pole4, two pathways involved in parental histone transfer based upon the analysis of modifications on parental histones, participate in the transfer of both H3.1 and H3.3 following DNA replication. Lastly, we found that Mcm2, Pole3 and Pole4 mutants defective in parental histone transfer show defects in chromosome segregation. These results indicate that in contrast to deposition of newly synthesized H3.1 and H3.3, transfer of parental H3.1 and H3.3 is mediated by these shared mechanisms, which contributes to epigenetic memory of gene expression and maintenance of genome stability.

Histone H3.3 phosphorylation promotes heterochromatin formation by inhibiting H3K9/K36 histone demethylase

Histone H3.3 is an H3 variant which differs from the canonical H3.1/2 at four residues, including a serine residue at position 31 which is evolutionarily conserved. The H3.3 S31 residue is phosphorylated (H3.3 S31Ph) at heterochromatin regions including telomeres and pericentric repeats. However, the role of H3.3 S31Ph in these regions remains unknown. In this study, we find that H3.3 S31Ph regulates heterochromatin accessibility at telomeres during replication through regulation of H3K9/K36 histone demethylase KDM4B. In mouse embryonic stem (ES) cells, substitution of S31 with an alanine residue (H3.3 A31 -phosphorylation null mutant) results in increased KDM4B activity that removes H3K9me3 from telomeres. In contrast, substitution with a glutamic acid (H3.3 E31, mimics S31 phosphorylation) inhibits KDM4B, leading to increased H3K9me3 and DNA damage at telomeres. H3.3 E31 expression also increases damage at other heterochromatin regions including the pericentric heterochromatin and Y chromosome-specific satellite DNA repeats. We propose that H3.3 S31Ph regulation of KDM4B is required to control heterochromatin accessibility of repetitive DNA and preserve chromatin integrity.

Histone chaperone HIRA complex regulates retrotransposons in embryonic stem cells

BACKGROUND

Histone cell cycle regulator (HIRA) complex is an important histone chaperone that mediates the deposition of the H3.3 histone variant onto chromatin independently from DNA synthesis. However, it is still unknown whether it participates in the expression control of retrotransposons and cell fate determination.

METHODS

We screened the role of HIRA complex members in repressing the expression of retrotransposons by shRNA depletion in embryonic stem cells (ESCs) followed by RT-qPCR. RNA-seq was used to study the expression profiles after depletion of individual HIRA member. RT-qPCR and western blot were used to determine overexpression of HIRA complex members. Chromatin immunoprecipitation (ChIP)-qPCR was used to find the binding of H3.3, HIRA members to chromatin. Co-immunoprecipitation was used to identify the interaction between Hira mutant and Ubn2. ChIP-qPCR was used to identify H3.3 deposition change and western blot of chromatin extract was used to validate the epigenetic change. Bioinformatics analysis was applied for the analysis of available ChIP-seq data.

RESULTS

We revealed that Hira, Ubn2, and Ubn1 were the main repressors of 2-cell marker retrotransposon MERVL among HIRA complex members. Surprisingly, Ubn2 and Hira targeted different groups of retrotransposons and retrotransposon-derived long noncoding RNAs (lncRNAs), despite that they partially shared target genes. Furthermore, Ubn2 prevented ESCs to gain a 2-cell like state or activate trophectodermal genes upon differentiation. Mechanistically, Ubn2 and Hira suppressed retrotransposons by regulating the deposition of histone H3.3. Decreased H3.3 deposition, that was associated with the loss of Ubn2 or Hira, caused the reduction of H3K9me2 and H3K9me3, which are known repressive marks of retrotransposons.

CONCLUSIONS

Overall, our findings shed light on the distinct roles of HIRA complex members in controlling retrotransposons and cell fate conversion in ESCs.

HIRA-dependent boundaries between H3 variants shape early replication in mammals

The lack of a consensus DNA sequence defining replication origins in mammals has led researchers to consider chromatin as a means to specify these regions. However, to date, there is no mechanistic understanding of how this could be achieved and maintained given that nucleosome disruption occurs with each fork passage and with transcription. Here, by genome-wide mapping of the de novo deposition of the histone variants H3.1 and H3.3 in human cells during S phase, we identified how their dual deposition mode ensures a stable marking with H3.3 flanked on both sides by H3.1. These H3.1/H3.3 boundaries correspond to the initiation zones of early origins. Loss of the H3.3 chaperone HIRA leads to the concomitant disruption of H3.1/H3.3 boundaries and initiation zones. We propose that the HIRA-dependent deposition of H3.3 preserves H3.1/H3.3 boundaries by protecting them from H3.1 invasion linked to fork progression, contributing to a chromatin-based definition of early replication zones.

Dynamic Activity of Histone H3-Specific Chaperone Complexes in Oncogenesis

Canonical histone H3.1 and variant H3.3 deposit at different sites of the chromatin via distinct histone chaperones. Histone H3.1 relies on chaperone CAF-1 to mediate replication-dependent nucleosome assembly during S-phase, while H3.3 variant is regulated and incorporated into the chromatin in a replication-independent manner through HIRA and DAXX/ATRX. Current literature suggests that dysregulated expression of histone chaperones may be implicated in tumor progression. Notably, ectopic expression of CAF-1 can promote a switch between canonical H3.1 and H3 variants in the chromatin, impair the chromatic state, lead to chromosome instability, and impact gene transcription, potentially contributing to carcinogenesis. This review focuses on the chaperone proteins of H3.1 and H3.3, including structure, regulation, as well as their oncogenic and tumor suppressive functions in tumorigenesis.

CHAF1b, chromatin assembly factor-1 subunit b, is essential for mouse preimplantation embryos

Chromatin assembly factor-1, subunit b (CHAF1b), the p60 subunit of the chromatin-assembly factor-1 (CAF-1) complex, is an evolutionarily conserved protein that has been implicated in various biological processes. Although a variety of functions have been attributed to CHAF1b, its function in preimplantation embryos remains obscure. In this study, we showed that CHAF1b knockdown did not affect the blastocyst rate, but resulted in a low blastocyst hatching rate, outgrowth failure in vitro, and embryonic lethality after implantation in vivo. Notably, CHAF1b depletion increased apoptosis and caused down-regulated expression of key regulators of cell fate specification, including Oct4, Cdx2, Sox2, and Nanog. Further analysis revealed that CHAF1b mediated the replacement of H3.3 with H3.1/3.2, which was associated with decreased repressive histone marks (H3K9me2/3 and H3K27me2/3) and increased active histone marks (H3K4me2/3). Moreover, RNA-sequencing analysis revealed that CHAF1b depletion resulted in the differential expression of 1508 genes, including epigenetic modifications genes, multiple lineage-specific genes, and several genes encoding apoptosis proteins. In addition, assay for transposase-accessible chromatin-sequencing analysis demonstrated that silencing CHAF1b altered the chromatin accessibility of lineage-specific genes and epigenetic modifications genes. Taken together, these data imply that CHAF1b plays significant roles in preimplantation embryos, probably by regulating epigenetic modifications and lineage specification.