Regulation of replication licensing by acetyltransferase Hbo1

HBO1, a MYSTerious KAT and its links to cancer

The histone acetyltransferase HBO1, also known as KAT7, is a major chromatin modifying enzyme responsible for H3 and H4 acetylation. It is found within two distinct tetrameric complexes, the JADE subunit-containing complex and BRPF subunit-containing complex. The HBO1-JADE complex acetylates lysine 5, 8 and 12 of histone H4, and the HBO1-BRPF complex acetylates lysine 14 of histone H3. HBO1 regulates gene transcription, DNA replication, DNA damage repair, and centromere function. It is involved in diverse signaling pathways and plays crucial roles in development and stem cell biology. Recent work has established a strong relationship of HBO1 with the histone methyltransferase MLL/KMT2A in acute myeloid leukemia. Here, we discuss functional and pathological links of HBO1 to cancer, highlighting the underlying mechanisms that may pave the way to the development of novel anti-cancer therapies.

Quantity and quality of minichromosome maintenance protein complexes couple replication licensing to genome integrity

Accurate and complete replication of genetic information is a fundamental process of every cell division. The replication licensing is the first essential step that lays the foundation for error-free genome duplication. During licensing, minichromosome maintenance protein complexes, the molecular motors of DNA replication, are loaded to genomic sites called replication origins. The correct quantity and functioning of licensed origins are necessary to prevent genome instability associated with severe diseases, including cancer. Here, we delve into recent discoveries that shed light on the novel functions of licensed origins, the pathways necessary for their proper maintenance, and their implications for cancer therapies.

KAT7 promotes radioresistance through upregulating PI3K/AKT signaling in breast cancer

Chromatin-modifying enzymes are commonly altered in cancers, but the molecular mechanism by which they regulate cancers remains poorly understood. Herein, we demonstrated that Lysine acetyltransferase 7 (KAT7) was upregulated in breast cancer. KAT7 expression negatively correlated with the survival of breast cancer patients, and KAT7 silencing suppressed breast cancer radioresistance in vitro. Mechanistically, KAT7 activated Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) transcription, leading to enhanced PI3K/AKT signaling and radioresistance. Overexpression of AKT or PIK3CA restored radioresistance suppression induced by KAT7 inhibition. Moreover, overexpression of KAT7, but not KAT7 acetyltransferase activity-deficient mutants promoted AKT phosphorylation at the Ser473 site, PIK3CA expression and radioresistance suppression due to KAT7 inhibition. In conclusion, KAT7 has huge prospects for clinical application as a new target for predicting radioresistance in breast cancer patients.

The CMG helicase and cancer: a tumor "engine" and weakness with missing mutations

The replicative Cdc45-MCM-GINS (CMG) helicase is a large protein complex that functions in the DNA melting and unwinding steps as a component of replisomes during DNA replication in mammalian cells. Although the CMG performs this important role in cell growth, the CMG is not a simple bystander in cell cycle events. Components of the CMG, specifically the MCM precursors, are also involved in maintaining genomic stability by regulating DNA replication fork speeds, facilitating recovery from replicative stresses, and preventing consequential DNA damage. Given these important functions, MCM/CMG complexes are highly regulated by growth factors such as TGF-ß1 and by signaling factors such as Myc, Cyclin E, and the retinoblastoma protein. Mismanagement of MCM/CMG complexes when these signaling mediators are deregulated, and in the absence of the tumor suppressor protein p53, leads to increased genomic instability and is a contributor to tumorigenic transformation and tumor heterogeneity. The goal of this review is to provide insight into the mechanisms and dynamics by which the CMG is regulated during its assembly and activation in mammalian genomes, and how errors in CMG regulation due to oncogenic changes promote tumorigenesis. Finally, and most importantly, we highlight the emerging understanding of the CMG helicase as an exploitable vulnerability and novel target for therapeutic intervention in cancer.

DNA replication: Mechanisms and therapeutic interventions for diseases

Accurate and integral cellular DNA replication is modulated by multiple replication-associated proteins, which is fundamental to preserve genome stability. Furthermore, replication proteins cooperate with multiple DNA damage factors to deal with replication stress through mechanisms beyond their role in replication. Cancer cells with chronic replication stress exhibit aberrant DNA replication and DNA damage response, providing an exploitable therapeutic target in tumors. Numerous evidence has indicated that posttranslational modifications (PTMs) of replication proteins present distinct functions in DNA replication and respond to replication stress. In addition, abundant replication proteins are involved in tumorigenesis and development, which act as diagnostic and prognostic biomarkers in some tumors, implying these proteins act as therapeutic targets in clinical. Replication-target cancer therapy emerges as the times require. In this context, we outline the current investigation of the DNA replication mechanism, and simultaneously enumerate the aberrant expression of replication proteins as hallmark for various diseases, revealing their therapeutic potential for target therapy. Meanwhile, we also discuss current observations that the novel PTM of replication proteins in response to replication stress, which seems to be a promising strategy to eliminate diseases.

CDT1 is the major functional regulatory subunit of the pre-replication complex in zygotes

Pre-replication complex (pre-RC) is critical for DNA replication initiation. CDT1 and MCM2 are the subunits of pre-RC, and proper regulation of CDT1 and MCM2 are necessary for DNA replication and cell proliferation. The present study aimed to explore the role of CDT1 and MCM2 in oocyte meiotic maturation and early embryonic development. The depletion and overexpression of Cdt1 and Mcm2 in oocyte and zygote were achieved by microinjecting specific siRNA and mRNA to explored their functions in oocyte meiotic maturation and embryonic development. Then, we examined the effect of CDT1 and MCM2 on other signal pathways by immunostaining the expression of related maker genes. We showed that neither depletion nor overexpression of Cdt1 affected oocyte meiotic progressions. The CDT1 was degraded in S phase and remained at a low level in G2 phase of zygote. Exogenous expression of Cdt1 in G2 phase led to embryo attest at zygote stage. Mechanistically, CDT1 overexpression induced DNA re-replication and thus DNA damage check-point activation. Protein abundance of MCM2 was stable throughout the cell cycle, and embryos with overexpressed MCM2 could develop to blastocysts normally. Overexpression or depletion of Mcm2 also had no effect on oocyte meiotic maturation. Our results indicate that pre-RC subunits CDT1 and MCM2 are not involved in oocyte meiotic maturation. In zygote, CDT1 but not MCM2 is the major regulator of DNA replication in a cell cycle dependent manner. Furthermore, its' degradation is essential for zygotes to prevent from DNA re-replication in G2 stage.

Histone modification and histone modification-targeted anti-cancer drugs in breast cancer: Fundamentals and beyond

Dysregulated epigenetic enzymes and resultant abnormal epigenetic modifications (EMs) have been suggested to be closely related to tumor occurrence and progression. Histone modifications (HMs) can assist in maintaining genome stability, DNA repair, transcription, and chromatin modulation within breast cancer (BC) cells. In addition, HMs are reversible, dynamic processes involving the associations of different enzymes with molecular compounds. Abnormal HMs (e.g. histone methylation and histone acetylation) have been identified to be tightly related to BC occurrence and development, even though their underlying mechanisms remain largely unclear. EMs are reversible, and as a result, epigenetic enzymes have aroused wide attention as anti-tumor therapeutic targets. At present, treatments to restore aberrant EMs within BC cells have entered preclinical or clinical trials. In addition, no existing studies have comprehensively analyzed aberrant HMs within BC cells; in addition, HM-targeting BC treatments remain to be further investigated. Histone and non-histone protein methylation is becoming an attractive anti-tumor epigenetic therapeutic target; such methylation-related enzyme inhibitors are under development at present. Consequently, the present work focuses on summarizing relevant studies on HMs related to BC and the possible mechanisms associated with abnormal HMs. Additionally, we also aim to analyze existing therapeutic agents together with those drugs approved and tested through pre-clinical and clinical trials, to assess their roles in HMs. Moreover, epi-drugs that target HMT inhibitors and HDAC inhibitors should be tested in preclinical and clinical studies for the treatment of BC. Epi-drugs that target histone methylation (HMT inhibitors) and histone acetylation (HDAC inhibitors) have now entered clinical trials or are approved by the US Food and Drug Administration (FDA). Therefore, the review covers the difficulties in applying HM-targeting treatments in clinics and proposes feasible approaches for overcoming such difficulties and promoting their use in treating BC cases.

BRPF1-KAT6A/KAT6B Complex: Molecular Structure, Biological Function and Human Disease

The bromodomain and PHD finger-containing protein1 (BRPF1) is a member of family IV of the bromodomain-containing proteins that participate in the post-translational modification of histones. It functions in the form of a tetrameric complex with a monocytic leukemia zinc finger protein (MOZ or KAT6A), MOZ-related factor (MORF or KAT6B) or HAT bound to ORC1 (HBO1 or KAT7) and two small non-catalytic proteins, the inhibitor of growth 5 (ING5) or the paralog ING4 and MYST/Esa1-associated factor 6 (MEAF6). Mounting studies have demonstrated that all the four core subunits play crucial roles in different biological processes across diverse species, such as embryonic development, forebrain development, skeletal patterning and hematopoiesis. BRPF1, KAT6A and KAT6B mutations were identified as the cause of neurodevelopmental disorders, leukemia, medulloblastoma and other types of cancer, with germline mutations associated with neurodevelopmental disorders displaying intellectual disability, and somatic variants associated with leukemia, medulloblastoma and other cancers. In this paper, we depict the molecular structures and biological functions of the BRPF1-KAT6A/KAT6B complex, summarize the variants of the complex related to neurodevelopmental disorders and cancers and discuss future research directions and therapeutic potentials.

NOGOB receptor deficiency increases cerebrovascular permeability and hemorrhage via impairing histone acetylation-mediated CCM1/2 expression

The loss function of cerebral cavernous malformation (CCM) genes leads to most CCM lesions characterized by enlarged leaking vascular lesions in the brain. Although we previously showed that NOGOB receptor (NGBR) knockout in endothelial cells (ECs) results in cerebrovascular lesions in the mouse embryo, the molecular mechanism by which NGBR regulates CCM1/2 expression has not been elucidated. Here, we show that genetic depletion of Ngbr in ECs at both postnatal and adult stages results in CCM1/2 expression deficiency and cerebrovascular lesions such as enlarged vessels, blood-brain-barrier hyperpermeability, and cerebral hemorrhage. To reveal the molecular mechanism, we used RNA-sequencing analysis to examine changes in the transcriptome. Surprisingly, we found that the acetyltransferase HBO1 and histone acetylation were downregulated in NGBR-deficient ECs. The mechanistic studies elucidated that NGBR is required for maintaining the expression of CCM1/2 in ECs via HBO1-mediated histone acetylation. ChIP-qPCR data further demonstrated that loss of NGBR impairs the binding of HBO1 and acetylated histone H4K5 and H4K12 on the promotor of the CCM1 and CCM2 genes. Our findings on epigenetic regulation of CCM1 and CCM2 that is modulated by NGBR and HBO1-mediated histone H4 acetylation provide a perspective on the pathogenesis of sporadic CCMs.

Now open: Evolving insights to the roles of lysine acetylation in chromatin organization and function

Protein acetylation is conserved across phylogeny and has been recognized as one of the most prominent post-translational modifications since its discovery nearly 60 years ago. Histone acetylation is an active mark characteristic of open chromatin, but acetylation on specific lysine residues and histone variants occurs in different biological contexts and can confer various outcomes. The significance of acetylation events is indicated by the associations of lysine acetyltransferases, deacetylases, and acetyl-lysine readers with developmental disorders and pathologies. Recent advances have uncovered new roles of acetylation regulators in chromatin-centric events, which emphasize the complexity of these functional networks. In this review, we discuss mechanisms and dynamics of acetylation in chromatin organization and DNA-templated processes, including gene transcription and DNA repair and replication.

HBO1 is a versatile histone acyltransferase critical for promoter histone acylations

Recent studies demonstrate that histones are subjected to a series of short-chain fatty acid modifications that is known as histone acylations. However, the enzymes responsible for histone acylations in vivo are not well characterized. Here, we report that HBO1 is a versatile histone acyltransferase that catalyzes not only histone acetylation but also propionylation, butyrylation and crotonylation both in vivo and in vitro and does so in a JADE or BRPF family scaffold protein-dependent manner. We show that the minimal HBO1/BRPF2 complex can accommodate acetyl-CoA, propionyl-CoA, butyryl-CoA and crotonyl-CoA. Comparison of CBP and HBO1 reveals that they catalyze histone acylations at overlapping as well as distinct sites, with HBO1 being the key enzyme for H3K14 acylations. Genome-wide chromatin immunoprecipitation assay demonstrates that HBO1 is highly enriched at and contributes to bulk histone acylations on the transcriptional start sites of active transcribed genes. HBO1 promoter intensity highly correlates with the level of promoter histone acylation, but has no significant correlation with level of transcription. We also show that HBO1 is associated with a subset of DNA replication origins. Collectively our study establishes HBO1 as a versatile histone acyltransferase that links histone acylations to promoter acylations and selection of DNA replication origins.

BRPF3-HUWE1-mediated regulation of MYST2 is required for differentiation and cell-cycle progression in embryonic stem cells

Brpf-histone acetyltransferase (HAT) complexes have important roles in embryonic development and regulating differentiation in ESCs. Among Brpf family, Brpf3 is a scaffold protein of Myst2 histone acetyltransferase complex that plays crucial roles in gene regulation, DNA replication, development as well as maintaining pluripotency in embryonic stem cells (ESCs). However, its biological functions in ESCs are not elucidated. In this study, we find out that Brpf3 protein level is critical for Myst2 stability and E3 ligase Huwe1 functions as a novel negative regulator of Myst2 via ubiquitin-mediated degradation. Importantly, Brpf3 plays an antagonistic role in Huwe1-mediated degradation of Myst2, suggesting that protein-protein interaction between Brpf3 and Myst2 is required for retaining Myst2 stability. Further, Brpf3 overexpression causes the aberrant upregulation of Myst2 protein levels which in turn induces the dysregulated cell-cycle progression and also delay of early embryonic development processes such as embryoid-body formation and lineage commitment of mouse ESCs. The Brpf3 overexpression-induced phenotypes can be reverted by Huwe1 overexpression. Together, these results may provide novel insights into understanding the functions of Brpf3 in proper differentiation as well as cell-cycle progression of ESCs via regulation of Myst2 stability by obstructing Huwe1-mediated ubiquitination. In addition, we suggest that this is a useful report which sheds light on the function of an unknown gene in ESC field.

Role of histone acetyltransferase inhibitors in cancer therapy

The development of cancer is a complex phenomenon driven by various extrinsic as well as intrinsic risk factors including epigenetic modifications. These post-translational modifications are encountered in diverse cancer cells and appear for a relatively short span of time. These changes can significantly affect various oncogenic genes and proteins involved in cancer initiation and progression. Histone lysine acetylation and deacetylation processes are controlled by two opposing classes of enzymes that modulate gene regulation either by adding an acetyl moiety on a histone lysine residue by histone lysine acetyltransferases (KATs) or via removing it by histone deacetylases (KDACs). Deregulated KAT activity has been implicated in the development of several diseases including cancer and can be targeted for the development of anti-neoplastic drugs. Here, we describe the predominant epigenetic changes that can affect key KAT superfamily members during carcinogenesis and briefly highlight the pharmacological potential of employing lysine acetyltransferase inhibitors (KATi) for cancer therapy.

Origin of Genome Instability and Determinants of Mutational Landscape in Cancer Cells

Genome instability is a crucial and early event associated with an increased predisposition to tumor formation. In the absence of any exogenous agent, a single human cell is subjected to about 70,000 DNA lesions each day. It has now been shown that physiological cellular processes including DNA transactions during DNA replication and transcription contribute to DNA damage and induce DNA damage responses in the cell. These processes are also influenced by the three dimensional-chromatin architecture and epigenetic regulation which are altered during the malignant transformation of cells. In this review, we have discussed recent insights about how replication stress, oncogene activation, chromatin dynamics, and the illegitimate recombination of cell-free chromatin particles deregulate cellular processes in cancer cells and contribute to their evolution. The characterization of such endogenous sources of genome instability in cancer cells can be exploited for the development of new biomarkers and more effective therapies for cancer treatment.

Protein kinase D1 phosphorylation of KAT7 enhances its protein stability and promotes replication licensing and cell proliferation

The histone acetyltransferase (HAT) KAT7/HBO1/MYST2 plays a crucial role in the pre-replication complex (pre-RC) formation, DNA replication and cell proliferation via acetylation of histone H4 and H3. In a search for protein kinase D1 (PKD1)-interacting proteins, we have identified KAT7 as a potential PKD1 substrate. We show that PKD1 directly interacts and phosphorylates KAT7 at Thr97 and Thr331 in vitro and in vivo. PKD1-mediated phosphorylation of KAT7 enhances its expression levels and stability by reducing its ubiquitination-mediated degradation. Significantly, the phospho-defective mutant KAT7-Thr97/331A attenuates histone H4 acetylation levels, MCM2/6 loading on the chromatin, DNA replication and cell proliferation. Similarly, PKD1 knockdown decreases, whereas the constitutive active mutant PKD1-CA increases histone H4 acetylation levels and MCM2/6 loading on the chromatin. Overall, these results suggest that PKD1-mediated phosphorylation of KAT7 may be required for pre-RC formation and DNA replication.

Histone Modifications and Other Facets of Epigenetic Regulation in Trypanosomatids: Leaving Their Mark

Histone posttranslational modifications (PTMs) modulate several eukaryotic cellular processes, including transcription, replication, and repair. Vast arrays of modifications have been identified in conventional eukaryotes over the last 20 to 25 years. While initial studies uncovered these primarily on histone tails, multiple modifications were subsequently found on the central globular domains as well. Histones are evolutionarily conserved across eukaryotes, and a large number of their PTMs and the functional relevance of these PTMs are largely conserved.

Histone posttranslational modifications (PTMs) modulate several eukaryotic cellular processes, including transcription, replication, and repair. Vast arrays of modifications have been identified in conventional eukaryotes over the last 20 to 25 years. While initial studies uncovered these primarily on histone tails, multiple modifications were subsequently found on the central globular domains as well. Histones are evolutionarily conserved across eukaryotes, and a large number of their PTMs and the functional relevance of these PTMs are largely conserved. Trypanosomatids, however, are early diverging eukaryotes. Although possessing all four canonical histones as well as several variants, their sequences diverge from those of other eukaryotes, particularly in the tails. Consequently, the modifications they carry also vary. Initial analyses almost 15 years ago suggested that trypanosomatids possessed a smaller collection of histone modifications. However, exhaustive high resolution mass spectrometry analyses in the last few years have overturned this belief, and it is now evident that the “histone code” proposed by Allis and coworkers in the early years of this century is as complex in these organisms as in other eukaryotes. Trypanosomatids cause several diseases, and the members of this group of organisms have varied lifestyles, evolving diverse mechanisms to evade the host immune system, some of which have been found to be principally controlled by epigenetic mechanisms. This minireview aims to acquaint the reader with the impact of histone PTMs on trypanosomatid cellular processes, as well as other facets of trypanosomatid epigenetic regulation, including the influence of three-dimensional (3D) genome architecture, and discusses avenues for future investigations.

Deciphering structure, function and mechanism of lysine acetyltransferase HBO1 in protein acetylation, transcription regulation, DNA replication and its oncogenic properties in cancer

HBO1 complexes are major acetyltransferase responsible for histone H4 acetylation in vivo, which belongs to the MYST family. As the core catalytic subunit, HBO1 consists of an N-terminal domain and a C-terminal MYST domain that are in charge of acetyl-CoA binding and acetylation reaction. HBO1 complexes are multimeric and normally consist of two native subunits MEAF6, ING4 or ING5 and two kinds of cofactors as chromatin reader: Jade-1/2/3 and BRPF1/2/3. The choices of subunits to form the HBO1 complexes provide a regulatory switch to potentiate its activity between histone H4 and H3 tails. Thus, HBO1 complexes present multiple functions in histone acetylation, gene transcription, DNA replication, protein ubiquitination, and immune regulation, etc. HBO1 is a co-activator for CDT1 to facilitate chromatin loading of MCM complexes and promotes DNA replication licensing. This process is regulated by mitotic kinases such as CDK1 and PLK1 by phosphorylating HBO1 and modulating its acetyltransferase activity, therefore, connecting histone acetylation to regulations of cell cycle and DNA replication. In addition, both gene amplification and protein overexpression of HBO1 confirmed its oncogenic role in cancers. In this paper, we review the recent advances and discuss our understanding of the multiple functions, activity regulation, and disease relationship of HBO1.

HBO1 (KAT7) Does Not Have an Essential Role in Cell Proliferation, DNA Replication, or Histone 4 Acetylation in Human Cells

HBO1 (MYST2/KAT7) is essential for histone 3 lysine 14 acetylation (H3K14ac) but is dispensable for H4 acetylation and DNA replication in mouse tissues. In contrast, previous studies using small interfering RNA (siRNA) knockdown in human cell lines have suggested that HBO1 is essential for DNA replication. To determine if HBO1 has distinctly different roles in immortalized human cell lines and normal mouse cells, we performed siRNA knockdown of HBO1. In addition, we used CRISPR/Cas9 to generate 293T, MCF7, and HeLa cell lines lacking HBO1. Using both techniques, we show that HBO1 is essential for all H3K14ac in human cells and is unlikely to have a direct effect on H4 acetylation and only has minor effects on cell proliferation. Surprisingly, the loss of HBO1 and H3K14ac in HeLa cells led to the secondary loss of almost all H4 acetylation after 4 weeks. Thus, HBO1 is dispensable for DNA replication and cell proliferation in immortalized human cells. However, while cell proliferation proceeded without HBO1 and H3K14ac, HBO1 gene deletion led to profound changes in cell adhesion, particularly in 293T cells. Consistent with this phenotype, the loss of HBO1 in both 293T and HeLa principally affected genes mediating cell adhesion, with comparatively minor effects on other cellular processes.

UHRF1-KAT7-mediated regulation of TUSC3 expression via histone methylation/acetylation is critical for the proliferation of colon cancer cells

The epigenetic factor UHRF1 regulates transcription by modulating DNA methylation and histone modification, and plays critical roles in proliferation, development, and tumorigenesis. Here, we show that Wnt/c-Myc signaling upregulates UHRF1, which in turn downregulates TUSC3, a candidate tumor suppressor gene that is frequently deleted or downregulated in several cancers. We also show that UHRF1-mediated downregulation of TUSC3 is required for the proliferation of colon cancer cells. Furthermore, we demonstrate that UHRF1 suppresses TUSC3 expression by interacting with methylated H3K14 and thereby suppressing the acetylation of H3K14 by the histone acetyltransferase KAT7. Our study provides evidence for the significance of UHRF1-KAT7-mediated regulation of histone methylation/acetylation in the proliferation of tumor cells and in a diverse set of biological processes controlled by Wnt/c-Myc signaling.

High-Expression HBO1 Predicts Poor Prognosis in Gastric Cancer

OBJECTIVES

Our goal was to assess the expression of histone acetyltransferase binding to origin recognition complex 1 (HBO1) in gastric cancer and the effect on prognosis for the patients.

METHODS

We used quantitative reverse transcription polymerase chain reaction, Western blot, and tissue microarray immunohistochemistry to investigate the expressions of HBO1 messenger RNA (mRNA) and protein in gastric cancer tissues. Online resources, including Oncomine and Kaplan-Meier Plotter, were used to further assess the correlation between HBO1 expression and the prognosis of the patients with gastric cancer.

RESULTS

HBO1 mRNA and protein expressions in gastric cancer tissues were both significantly higher than those in normal tissues. The correlations between high HBO1 expression and differentiation, invasive depth (T), lymph node metastasis (N), distant metastasis (M), TNM staging, and serum carcinoembryonic antigen levels were positive. High HBO1 expression was negatively correlated with survival time in patients with gastric cancer.

CONCLUSIONS

HBO1 might be a valuable biomarker to evaluate the prognosis of patients with gastric cancer.

5-hydroxymethylcytosine Marks Mammalian Origins Acting as a Barrier to Replication

In most mammalian cells, DNA replication occurs once, and only once between cell divisions. Replication initiation is a highly regulated process with redundant mechanisms that prevent errant initiation events. In lower eukaryotes, replication is initiated from a defined consensus sequence, whereas a consensus sequence delineating mammalian origin of replication has not been identified. Here we show that 5-hydroxymethylcytosine (5hmC) is present at mammalian replication origins. Our data support the hypothesis that 5hmC has a role in cell cycle regulation. We show that 5hmC level is inversely proportional to proliferation; indeed, 5hmC negatively influences cell division by increasing the time a cell resides in G1. Our data suggest that 5hmC recruits replication-licensing factors, then is removed prior to or during origin firing. Later we propose that TET2, the enzyme catalyzing 5mC to 5hmC conversion, acts as barrier to rereplication. In a broader context, our results significantly advance the understating of 5hmC involvement in cell proliferation and disease states.

Origin Firing Regulations to Control Genome Replication Timing

Complete genome duplication is essential for genetic homeostasis over successive cell generations. Higher eukaryotes possess a complex genome replication program that involves replicating the genome in units of individual chromatin domains with a reproducible order or timing. Two types of replication origin firing regulations ensure complete and well-timed domain-wise genome replication: (1) the timing of origin firing within a domain must be determined and (2) enough origins must fire with appropriate positioning in a short time window to avoid inter-origin gaps too large to be fully copied. Fundamental principles of eukaryotic origin firing are known. We here discuss advances in understanding the regulation of origin firing to control firing time. Work with yeasts suggests that eukaryotes utilise distinct molecular pathways to determine firing time of distinct sets of origins, depending on the specific requirements of the genomic regions to be replicated. Although the exact nature of the timing control processes varies between eukaryotes, conserved aspects exist: (1) the first step of origin firing, pre-initiation complex (pre-IC formation), is the regulated step, (2) many regulation pathways control the firing kinase Dbf4-dependent kinase, (3) Rif1 is a conserved mediator of late origin firing and (4) competition between origins for limiting firing factors contributes to firing timing. Characterization of the molecular timing control pathways will enable us to manipulate them to address the biological role of replication timing, for example, in cell differentiation and genome instability.

LPS promotes HBO1 stability via USP25 to modulate inflammatory gene transcription in THP-1 cells

The histone acetyltransferase HBO1 (Histone acetyltransferase binding to origin recognition complex 1, Myst2/Kat7) participates in a range of life processes including DNA replication and tumorigenesis. Recent studies revealed that HBO1 is involved in gene transcriptional activation. However, the molecular behavior of HBO1 in inflammation is yet to be studied. Here we report that endotoxin lipopolysaccharide (LPS) elevates HBO1 protein level via up-regulating UPS25 (ubiquitin specific peptidase 25) and alters inflammatory gene transcription in THP-1 monocytes and in human primary macrophages. LPS protects HBO1 from ubiquitin proteasomal degradation without significantly altering its transcription. By immunoprecipitation, we identified that HBO1 associates with a deubiquitinating enzyme USP25 in THP-1 cells. LPS increases protein level of USP25 resulting in accumulation of HBO1 by suppression of HBO1 ubiquitination. Stabilized-HBO1 modulates inflammatory gene transcription in THP-1 cells. These findings indicate that USP25 promotes stability of HBO1 in bacterial infection thereby enhances HBO1-mediated inflammatory gene transcription.

Conservation and Variation in Strategies for DNA Replication of Kinetoplastid Nuclear Genomes

Introduction:

Understanding how the nuclear genome of kinetoplastid parasites is replicated received experimental stimulus from sequencing of the Leishmania major, Trypanosoma brucei and Trypanosoma cruzi genomes around 10 years ago. Gene annotations suggested key players in DNA replication initiation could not be found in these organisms, despite considerable conservation amongst characterised eukaryotes. Initial studies that indicated trypanosomatids might possess an archaeal-like Origin Recognition Complex (ORC), composed of only a single factor termed ORC1/CDC6, have been supplanted by the more recent identification of an ORC in T. brucei. However, the constituent subunits of T. brucei ORC are highly diverged relative to other eukaryotic ORCs and the activity of the complex appears subject to novel, positive regulation. The availability of whole genome sequences has also allowed the deployment of genome-wide strategies to map DNA replication dynamics, to date in T. brucei and Leishmania. ORC1/CDC6 binding and function in T. brucei displays pronounced overlap with the unconventional organisation of gene expression in the genome. Moreover, mapping of sites of replication initiation suggests pronounced differences in replication dynamics in Leishmania relative to T. brucei.

Conclusion:

Here we discuss what implications these emerging data may have for parasite and eukaryotic biology of DNA replication.

Structural and mechanistic insights into regulation of HBO1 histone acetyltransferase activity by BRPF2

HBO1, a member of the MYST family of histone acetyltransferases (HATs), is required for global acetylation of histone H3K14 and embryonic development. It functions as a catalytic subunit in multisubunit complexes comprising a BRPF1/2/3 or JADE1/2/3 scaffold protein, and two accessory proteins. BRPF2 has been shown to be important for the HAT activity of HBO1 toward H3K14. Here we demonstrated that BRPF2 can regulate the HAT activity of HBO1 toward free H3 and H4, and nucleosomal H3. Particularly, a short N-terminal region of BRPF2 is sufficient for binding to HBO1 and can potentiate its activity toward H3K14. The crystal structure of the HBO1 MYST domain in complex with this segment of BRPF2 together with the biochemical and cell biological data revealed the key residues responsible for the HBO1–BRPF2 interaction. Our structural and functional data together indicate that the N-terminal region of BRPF2 plays an important role in the binding of HBO1 and a minor role in the binding of nucleosomes, which provide new mechanistic insights into the regulation of the HAT activity of HBO1 by BRPF2.

Myst2/Kat7 histone acetyltransferase interaction proteomics reveals tumour-suppressor Niam as a novel binding partner in embryonic stem cells

MYST histone acetyltransferases have crucial functions in transcription, replication and DNA repair and are hence implicated in development and cancer. Here we characterise Myst2/Kat7/Hbo1 protein interactions in mouse embryonic stem cells by affinity purification coupled to mass spectrometry. This study confirms that in embryonic stem cells Myst2 is part of H3 and H4 histone acetylation complexes similar to those described in somatic cells. We identify a novel Myst2-associated protein, the tumour suppressor protein Niam (Nuclear Interactor of ARF and Mdm2). Human NIAM is involved in chromosome segregation, p53 regulation and cell proliferation in somatic cells, but its role in embryonic stem cells is unknown. We describe the first Niam embryonic stem cell interactome, which includes proteins with roles in DNA replication and repair, transcription, splicing and ribosome biogenesis. Many of Myst2 and Niam binding partners are required for correct embryonic development, implicating Myst2 and Niam in the cooperative regulation of this process and suggesting a novel role for Niam in embryonic biology. The data provides a useful resource for exploring Myst2 and Niam essential cellular functions and should contribute to deeper understanding of organism early development and survival as well as cancer. Data are available via ProteomeXchange with identifier PXD005987.

Phosphorylated SIRT1 associates with replication origins to prevent excess replication initiation and preserve genomic stability

Chromatin structure affects DNA replication patterns, but the role of specific chromatin modifiers in regulating the replication process is yet unclear. We report that phosphorylation of the human SIRT1 deacetylase on Threonine 530 (T530-pSIRT1) modulates DNA synthesis. T530-pSIRT1 associates with replication origins and inhibits replication from a group of 'dormant' potential replication origins, which initiate replication only when cells are subject to replication stress. Although both active and dormant origins bind T530-pSIRT1, active origins are distinguished from dormant origins by their unique association with an open chromatin mark, histone H3 methylated on lysine 4. SIRT1 phosphorylation also facilitates replication fork elongation. SIRT1 T530 phosphorylation is essential to prevent DNA breakage upon replication stress and cells harboring SIRT1 that cannot be phosphorylated exhibit a high prevalence of extrachromosomal elements, hallmarks of perturbed replication. These observations suggest that SIRT1 phosphorylation modulates the distribution of replication initiation events to insure genomic stability.

DNA replication origins-where do we begin?

In this review, Prioleau and MacAlpine summarize recent advances in our understanding of how primary sequence, chromatin environment, and nuclear architecture contribute to the dynamic selection and activation of replication origins across diverse cell types and developmental stages.

For more than three decades, investigators have sought to identify the precise locations where DNA replication initiates in mammalian genomes. The development of molecular and biochemical approaches to identify start sites of DNA replication (origins) based on the presence of defining and characteristic replication intermediates at specific loci led to the identification of only a handful of mammalian replication origins. The limited number of identified origins prevented a comprehensive and exhaustive search for conserved genomic features that were capable of specifying origins of DNA replication. More recently, the adaptation of origin-mapping assays to genome-wide approaches has led to the identification of tens of thousands of replication origins throughout mammalian genomes, providing an unprecedented opportunity to identify both genetic and epigenetic features that define and regulate their distribution and utilization. Here we summarize recent advances in our understanding of how primary sequence, chromatin environment, and nuclear architecture contribute to the dynamic selection and activation of replication origins across diverse cell types and developmental stages.

Histone deacetylase inhibition modulates histone acetylation at gene promoter regions and affects genome-wide gene transcription in Schistosoma mansoni

BACKGROUND

Schistosomiasis is a parasitic disease infecting hundreds of millions of people worldwide. Treatment depends on a single drug, praziquantel, which kills the Schistosoma spp. parasite only at the adult stage. HDAC inhibitors (HDACi) such as Trichostatin A (TSA) induce parasite mortality in vitro (schistosomula and adult worms), however the downstream effects of histone hyperacetylation on the parasite are not known.

METHODOLOGY/PRINCIPAL FINDINGS

TSA treatment of adult worms in vitro increased histone acetylation at H3K9ac and H3K14ac, which are transcription activation marks, not affecting the unrelated transcription repression mark H3K27me3. We investigated the effect of TSA HDACi on schistosomula gene expression at three different time points, finding a marked genome-wide change in the transcriptome profile. Gene transcription activity was correlated with changes on the chromatin acetylation mark at gene promoter regions. Moreover, combining expression data with ChIP-Seq public data for schistosomula, we found that differentially expressed genes having the H3K4me3 mark at their promoter region in general showed transcription activation upon HDACi treatment, compared with those without the mark, which showed transcription down-regulation. Affected genes are enriched for DNA replication processes, most of them being up-regulated. Twenty out of 22 genes encoding proteins involved in reducing reactive oxygen species accumulation were down-regulated. Dozens of genes encoding proteins with histone reader motifs were changed, including SmEED from the PRC2 complex. We targeted SmEZH2 methyltransferase PRC2 component with a new EZH2 inhibitor (GSK343) and showed a synergistic effect with TSA, significantly increasing schistosomula mortality.

CONCLUSIONS/SIGNIFICANCE

Genome-wide gene expression analyses have identified important pathways and cellular functions that were affected and may explain the schistosomicidal effect of TSA HDACi. The change in expression of dozens of histone reader genes involved in regulation of the epigenetic program in S. mansoni can be used as a starting point to look for possible novel schistosomicidal targets.

Order from clutter: selective interactions at mammalian replication origins

Mammalian chromosome duplication progresses in a precise order and is subject to constraints that are often relaxed in developmental disorders and malignancies. Molecular information about the regulation of DNA replication at the chromatin level is lacking because protein complexes that initiate replication seem to bind chromatin indiscriminately. High-throughput sequencing and mathematical modelling have yielded detailed genome-wide replication initiation maps. Combining these maps and models with functional genetic analyses suggests that distinct DNA-protein interactions at subgroups of replication initiation sites (replication origins) modulate the ubiquitous replication machinery and supports an emerging model that delineates how indiscriminate DNA-binding patterns translate into a consistent, organized replication programme.

The gastrula transition reorganizes replication-origin selection in Caenorhabditis elegans

Although some features underlying replication-origin activation in metazoan cells have been determined, little is known about their regulation during metazoan development. Using the nascent-strand purification method, here we identified replication origins throughout Caenorhabditis elegans embryonic development and found that the origin repertoire is thoroughly reorganized after gastrulation onset. During the pluripotent embryonic stages (pregastrula), potential cruciform structures and open chromatin are determining factors that establish replication origins. The observed enrichment of replication origins in transcription factor-binding sites and their presence in promoters of highly transcribed genes, particularly operons, suggest that transcriptional activity contributes to replication initiation before gastrulation. After the gastrula transition, when embryonic differentiation programs are set, new origins are selected at enhancers, close to CpG-island-like sequences, and at noncoding genes. Our findings suggest that origin selection coordinates replication initiation with transcriptional programs during metazoan development.

Intrinsic ubiquitin E3 ligase activity of histone acetyltransferase Hbo1 for estrogen receptor α

Estrogen receptors (ER) are important transcription factors to relay signals from estrogen and to regulate proliferation of some of breast cancers. The cycling of estrogen-induced DNA binding and ubiquitin-linked proteolysis of ER potentiates ER-mediated transcription. Indeed, several transcriptional coactivators for ER-dependent transcription ubiquitinate ER. Histone acetyltransferase (HAT) Hbo1/KAT7/MYST2, involved in global histone acetylation, DNA replication, transcription, and cellular proliferation, promotes proteasome-dependent degradation of ERα through ubiquitination. However, molecular mechanism for ubiquitination of ERα by Hbo1 is unknown. Here we report the intrinsic ubiquitin E3 ligase activity of Hbo1 toward the ERα. The ligand, estradiol-17β, inhibited E3 ligase activity of Hbo1 for ERα in vitro, whereas hyperactive ERα mutants from metastatic breast cancers resistant to hormonal therapy, were better substrates for ERα ubiquitination by Hbo1. Hbo1 knock-down caused increase in ERα expression. Hbo1 is another ERα coactivator that ubiquitinates ERα.

Regulation of Replication Origins

In eukaryotes, genome duplication starts concomitantly at many replication initiation sites termed replication origins. The replication initiation program is spatially and temporally coordinated to ensure accurate, efficient DNA synthesis that duplicates the entire genome while maintaining other chromatin-dependent functions. Unlike in prokaryotes, not all potential replication origins in eukaryotes are needed for complete genome duplication during each cell cycle. Instead, eukaryotic cells vary the use of initiation sites so that only a fraction of potential replication origins initiate replication each cell cycle. Flexibility in origin choice allows each eukaryotic cell type to utilize different initiation sites, corresponding to unique nuclear DNA packaging patterns. These patterns coordinate replication with gene expression and chromatin condensation. Budding yeast replication origins share a consensus sequence that marks potential initiation sites. Metazoan origins, on the other hand, lack a consensus sequence. Rather, they are associated with a collection of structural features, chromatin packaging features, histone modifications, transcription, and DNA-DNA/DNA-protein interactions. These features confer cell type-specific replication and expression and play an essential role in maintaining genomic stability.

Human T-cell leukemia virus type-1-encoded protein HBZ represses p53 function by inhibiting the acetyltransferase activity of p300/CBP and HBO1

Adult T-cell leukemia (ATL) is an often fatal malignancy caused by infection with the complex retrovirus, human T-cell Leukemia Virus, type 1 (HTLV-1). In ATL patient samples, the tumor suppressor, p53, is infrequently mutated; however, it has been shown to be inactivated by the viral protein, Tax. Here, we show that another HTLV-1 protein, HBZ, represses p53 activity. In HCT116 p53+/+ cells treated with the DNA-damaging agent, etoposide, HBZ reduced p53-mediated activation of p21/CDKN1A and GADD45A expression, which was associated with a delay in G2 phase-arrest. These effects were attributed to direct inhibition of the histone acetyltransferase (HAT) activity of p300/CBP by HBZ, causing a reduction in p53 acetylation, which has be linked to decreased p53 activity. In addition, HBZ bound to, and inhibited the HAT activity of HBO1. Although HBO1 did not acetylate p53, it acted as a coactivator for p53 at the p21/CDKN1A promoter. Therefore, through interactions with two separate HAT proteins, HBZ impairs the ability of p53 to activate transcription. This mechanism may explain how p53 activity is restricted in ATL cells that do not express Tax due to modifications of the HTLV-1 provirus, which accounts for a majority of patient samples.

Nucleosome occupancy as a novel chromatin parameter for replication origin functions

Eukaryotic DNA replication initiates from multiple discrete sites in the genome, termed origins of replication (origins). Prior to S phase, multiple origins are poised to initiate replication by recruitment of the pre-replicative complex (pre-RC). For proper replication to occur, origin activation must be tightly regulated. At the population level, each origin has a distinct firing time and frequency of activation within S phase. Many studies have shown that chromatin can strongly influence initiation of DNA replication. However, the chromatin parameters that affect properties of origins have not been thoroughly established. We found that nucleosome occupancy in G1 varies greatly around origins across the S. cerevisiae genome, and nucleosome occupancy around origins significantly correlates with the activation time and efficiency of origins, as well as pre-RC formation. We further demonstrate that nucleosome occupancy around origins in G1 is established during transition from G2/M to G1 in a pre-RC-dependent manner. Importantly, the diminished cell-cycle changes in nucleosome occupancy around origins in the orc1-161 mutant are associated with an abnormal global origin usage profile, suggesting that proper establishment of nucleosome occupancy around origins is a critical step for regulation of global origin activities. Our work thus establishes nucleosome occupancy as a novel and key chromatin parameter for proper origin regulation.

Essential role for the histone acetyltransferase KAT7 in T cell development, fitness, and survival

Histone acetylation has an important role in gene regulation, DNA replication, and repair. Because these processes are central to the development of the immune system, we investigated the role of a previously unstudied histone acetyltransferase named KAT7 (also known as Myst2 or HBO1) in the regulation of thymopoiesis and observed a critical role in the regulation of conventional and innate-like T cell development. We found that KAT7-deficient thymocytes displayed normal, positive selection and development into mature single-positive αβ thymocytes; however, we observed few peripheral CD4⁺ or CD8⁺ T cells. The observed effects did not appear to arise from alterations to DNA replication, the TCR repertoire, or a block in thymocyte maturation and, more likely, was linked to survival defects related to gene deregulation because KAT7 deficiency led to an almost complete and specific loss of global histone-H3 lysine 14 acetylation (H3K14ac). Overall, we demonstrated a nonredundant role for KAT7 in the maintenance of H3K14ac, which is intimately linked with the ability to develop a normal immune system.

Nucleosome assembly and disassembly activity of GRWD1, a novel Cdt1-binding protein that promotes pre-replication complex formation

GRWD1 was previously identified as a novel Cdt1-binding protein that possesses histone-binding and nucleosome assembly activities and promotes MCM loading, probably by maintaining chromatin openness at replication origins. However, the molecular mechanisms underlying these activities remain unknown. We prepared reconstituted mononucleosomes from recombinant histones and a DNA fragment containing a nucleosome positioning sequence, and investigated the effects of GRWD1 on them. GRWD1 could disassemble these preformed mononucleosomes in vitro in an ATP-independent manner. Thus, our data suggest that GRWD1 facilitates removal of H2A-H2B dimers from nucleosomes, resulting in formation of hexasomes. The activity was compromised by deletion of the acidic domain, which is required for efficient histone binding. In contrast, nucleosome assembly activity of GRWD1 was not affected by deletion of the acidic domain. In HeLa cells, the acidic domain of GRWD1 was necessary to maintain chromatin openness and promote MCM loading at replication origins. Taken together, our results suggest that GRWD1 promotes chromatin fluidity by influencing nucleosome structures, e.g., by transient eviction of H2A-H2B, and thereby promotes efficient MCM loading at replication origins.

Histone Deacetylases with Antagonistic Roles in Saccharomyces cerevisiae Heterochromatin Formation

As the only catalytic member of the Sir-protein gene-silencing complex, Sir2's catalytic activity is necessary for silencing. The only known role for Sir2's catalytic activity in Saccharomyces cerevisiae silencing is to deacetylate N-terminal tails of histones H3 and H4, creating high-affinity binding sites for the Sir-protein complex, resulting in association of Sir proteins across the silenced domain. This histone deacetylation model makes the simple prediction that preemptively removing Sir2's H3 and H4 acetyl substrates, by mutating these lysines to unacetylatable arginines, or removing the acetyl transferase responsible for their acetylation, should restore silencing in the Sir2 catalytic mutant. However, this was not the case. We conducted a genetic screen to explore what aspect of Sir2's catalytic activity has not been accounted for in silencing. Mutation of a nonsirtuin histone deacetylase, Rpd3, restored Sir-protein-based silencing in the absence of Sir2's catalytic activity. Moreover, this antagonism could be mediated by either the large or the small Rpd3-containing complex. Interestingly, this restoration of silencing appeared independent of any known histone H3 or H4 substrates of Rpd3 Investigation of Sir-protein association in the Rpd3 mutant revealed that the restoration of silencing was correlated with an increased association of Sir proteins at the silencers, suggesting that Rpd3 was an antagonist of Sir2's function in nucleation of Sir proteins to the silencer. Additionally, restoration of silencing by Rpd3 was dependent on another sirtuin family member, Hst3, indicating multiple antagonistic roles for deacetylases in S. cerevisiae silencing.

Structure, function and regulation of jade family PHD finger 1 (JADE1)

The family of JADE proteins includes three paralogues encoded by individual genes and designated PHF17 (JADE1), PHF16 (JADE2), and PHF15 (JADE3). All three JADE proteins bear in tandem two Plant Homeo-domains (PHD) which are zinc finger domains. This review focuses on one member of the JADE family, JADE1. Studies addressing the biochemical, cellular and biological role of JADE1 are discussed. Recent discoveries of JADE1 function in the regulation of the epithelial cell cycle with potential relevance to disease are presented. Unresolved questions and future directions are formulated.

Replication origins: determinants or consequences of nuclear organization?

Chromosome replication, gene expression and chromatin assembly all occur on the same template, necessitating a tight spatial and temporal coordination to maintain genomic stability. The distribution of replication initiation events is responsive to local and global changes in chromatin structure and is affected by transcriptional activity. Concomitantly, replication origin sequences, which determine the locations of replication initiation events, can affect chromatin structure and modulate transcriptional efficiency. The flexibility observed in the replication initiation landscape might help achieve complete and accurate genome duplication while coordinating the DNA replication program with transcription and other nuclear processes in a cell-type specific manner. This review discusses the relationships among replication origin distribution, local and global chromatin structures and concomitant nuclear metabolic processes.

Inhibitor of growth-4 is a potential target for cancer therapy

The inhibitor of growth-4 (ING-4) belongs to the inhibitor of growth (ING) family that is a type II tumor suppressor gene including five members (ING1-5). As a tumor suppressor, ING4 inhibits tumor growth, invasion, and metastasis by multiple signaling pathways. In addition to that, ING4 can facilitate cancer cell sensitivity to chemotherapy and radiotherapy. Although ING4 loss is observed for many types of cancers, increasing evidences show that ING4 can be used for gene therapy. In this review, the recent progress of ING4 regulating tumorigenesis is discussed.

Aberrant lysine acetylation in tumorigenesis: Implications in the development of therapeutics

The 'language' of covalent histone modifications translates environmental and cellular cues into gene expression. This vast array of post-translational modifications on histones are more than just covalent moieties added onto a protein, as they also form a platform on which crucial cellular signals are relayed. The reversible lysine acetylation has emerged as an important post-translational modification of both histone and non-histone proteins, dictating numerous epigenetic programs within a cell. Thus, understanding the complex biology of lysine acetylation and its regulators is essential for the development of epigenetic therapeutics. In this review, we will attempt to address the complexities of lysine acetylation in the context of tumorigenesis, their role in cancer progression and emphasize on the modalities developed to target lysine acetyltransferases towards cancer treatment.

Orc5 induces large-scale chromatin decondensation in a GCN5-dependent manner

In eukaryotes, origin recognition complex (ORC) proteins establish the pre-replicative complex (preRC) at the origins, and this is essential for the initiation of DNA replication. Open chromatin structures regulate the efficiency of preRC formation and replication initiation. However, the molecular mechanisms that control chromatin structure, and how the preRC components establish themselves on the chromatin remain to be understood. In human cells, the ORC is a highly dynamic complex with many separate functions attributed to sub-complexes or individual subunits of the ORC, including heterochromatin organization, telomere and centromere function, centrosome duplication and cytokinesis. We demonstrate that human Orc5, unlike other ORC subunits, when ectopically tethered to a chromatin locus, induces large-scale chromatin decondensation, predominantly during G1 phase of the cell cycle. Orc5 associates with the H3 histone acetyl transferase GCN5 (also known as KAT2A), and this association enhances the chromatin-opening function of Orc5. In the absence of Orc5, histone H3 acetylation is decreased at the origins. We propose that the ability of Orc5 to induce chromatin unfolding during G1 allows the establishment of the preRC at the origins.

BRPF3-HBO1 regulates replication origin activation and histone H3K14 acetylation

During DNA replication, thousands of replication origins are activated across the genome. Chromatin architecture contributes to origin specification and usage, yet it remains unclear which chromatin features impact on DNA replication. Here, we perform a RNAi screen for chromatin regulators implicated in replication control by measuring RPA accumulation upon replication stress. We identify six factors required for normal rates of DNA replication and characterize a function of the bromodomain and PHD finger-containing protein 3 (BRPF3) in replication initiation. BRPF3 forms a complex with HBO1 that specifically acetylates histone H3K14, and genomewide analysis shows high enrichment of BRPF3, HBO1 and H3K14ac at ORC1-binding sites and replication origins found in the vicinity of TSSs. Consistent with this, BRPF3 is necessary for H3K14ac at selected origins and efficient origin activation. CDC45 recruitment, but not MCM2-7 loading, is impaired in BRPF3-depleted cells, identifying a BRPF3-dependent function of HBO1 in origin activation that is complementary to its role in licencing. We thus propose that BRPF3-HBO1 acetylation of histone H3K14 around TSS facilitates efficient activation of nearby replication origins.

DNA replication origin activation in space and time

DNA replication begins with the assembly of pre-replication complexes (pre-RCs) at thousands of DNA replication origins during the G1 phase of the cell cycle. At the G1-S-phase transition, pre-RCs are converted into pre-initiation complexes, in which the replicative helicase is activated, leading to DNA unwinding and initiation of DNA synthesis. However, only a subset of origins are activated during any S phase. Recent insights into the mechanisms underlying this choice reveal how flexibility in origin usage and temporal activation are linked to chromosome structure and organization, cell growth and differentiation, and replication stress.

The Effects of Simazine, a Chlorotriazine Herbicide, on the Expression of Genes in Developing Male Xenopus laevis

Simazine was investigated for gene expression concurrent with simazine-induced phenotype changes during development of male Xenopus laevis. X. laevis tadpoles (Nieuwkoop-Faber stage 46) were exposed to 0.1, 1.2, 11.0 and 100.9 μg/L simazine for 100 days. The results showed that an increased mortality of X. laevis, decreased gonad weight and altered gonadosomatic index of males significantly (p<0.05) when exposed to simazine at 11.0 and 100.9 µg/L. Significant degeneration in testicular tissues was observed when tadpoles were exposed to simazine at 100.9 µg/L. To investigate the molecular mechanisms behind the testicular degeneration by simazine, we evaluated gene expression in animals treated with 100.9 µg/L simazine and found that 1,315 genes were significantly altered (454 upregulated, 861 downregulated). Genes involved in the cell cycle control, and amino acid metabolism pathways were significantly downregulated. These results indicate that simazine affects the related gene expressions which may be helpful for the understanding of the reason for the reproductive toxicity of simazine on male X. laevis.

DNA sequence templates adjacent nucleosome and ORC sites at gene amplification origins in Drosophila

Eukaryotic origins of DNA replication are bound by the origin recognition complex (ORC), which scaffolds assembly of a pre-replicative complex (pre-RC) that is then activated to initiate replication. Both pre-RC assembly and activation are strongly influenced by developmental changes to the epigenome, but molecular mechanisms remain incompletely defined. We have been examining the activation of origins responsible for developmental gene amplification in Drosophila. At a specific time in oogenesis, somatic follicle cells transition from genomic replication to a locus-specific replication from six amplicon origins. Previous evidence indicated that these amplicon origins are activated by nucleosome acetylation, but how this affects origin chromatin is unknown. Here, we examine nucleosome position in follicle cells using micrococcal nuclease digestion with Ilumina sequencing. The results indicate that ORC binding sites and other essential origin sequences are nucleosome-depleted regions (NDRs). Nucleosome position at the amplicons was highly similar among developmental stages during which ORC is or is not bound, indicating that being an NDR is not sufficient to specify ORC binding. Importantly, the data suggest that nucleosomes and ORC have opposite preferences for DNA sequence and structure. We propose that nucleosome hyperacetylation promotes pre-RC assembly onto adjacent DNA sequences that are disfavored by nucleosomes but favored by ORC.