BRD4 regulates Nanog expression in mouse embryonic stem cells and preimplantation embryos

BET activity plays an essential role in control of stem cell attributes in Xenopus

Neural crest cells are a stem cell population unique to vertebrate embryos that retains broad multi-germ layer developmental potential through neurulation. Much remains to be learned about the genetic and epigenetic mechanisms that control the potency of neural crest cells. Here, we examine the role that epigenetic readers of the BET (bromodomain and extra terminal) family play in controlling the potential of pluripotent blastula and neural crest cells. We find that inhibiting BET activity leads to loss of pluripotency at blastula stages and a loss of neural crest at neurula stages. We compare the effects of HDAC (an eraser of acetylation marks) and BET (a reader of acetylation) inhibition and find that they lead to similar cellular outcomes through distinct effects on the transcriptome. Interestingly, loss of BET activity in cells undergoing lineage restriction is coupled to increased expression of genes linked to pluripotency and prolongs the competence of initially pluripotent cells to transit to a neural progenitor state. Together these findings advance our understanding of the epigenetic control of pluripotency and the formation of the vertebrate neural crest.

Dux activates metabolism-lactylation-MET network during early iPSC reprogramming with Brg1 as the histone lactylation reader

The process of induced pluripotent stem cells (iPSCs) reprogramming involves several crucial events, including the mesenchymal-epithelial transition (MET), activation of pluripotent genes, metabolic reprogramming, and epigenetic rewiring. Although these events intricately interact and influence each other, the specific element that regulates the reprogramming network remains unclear. Dux, a factor known to promote totipotency during the transition from embryonic stem cells (ESC) to 2C-like ESC (2CLC), has not been extensively studied in the context of iPSC reprogramming. In this study, we demonstrate that the modification of H3K18la induced by Dux overexpression controls the metabolism-H3K18la-MET network, enhancing the efficiency of iPSC reprogramming through a metabolic switch and the recruitment of p300 via its C-terminal domain. Furthermore, our proteomic analysis of H3K18la immunoprecipitation experiment uncovers the specific recruitment of Brg1 during reprogramming, with both H3K18la and Brg1 being enriched on the promoters of genes associated with pluripotency and epithelial junction. In summary, our study has demonstrated the significant role of Dux-induced H3K18la in the early reprogramming process, highlighting its function as a potent trigger. Additionally, our research has revealed, for the first time, the binding of Brg1 to H3K18la, indicating its role as a reader of histone lactylation.

Znf687 recruits Brd4-Smrt complex to regulate gfi1aa during neutrophil development

Neutrophils are key component of the innate immune system in vertebrates. Diverse transcription factors and cofactors act in a well-coordinated manner to ensure proper neutrophil development. Dysregulation of the transcriptional program triggering neutrophil differentiation is associated with various human hematologic disorders such as neutropenia, neutrophilia, and leukemia. In the current study we show the zinc finger protein Znf687 is a lineage-preferential transcription factor, whose deficiency leads to an impaired neutrophil development in zebrafish. Mechanistically, Znf687 functions as a negative regulator of gfi1aa, a pivotal modulator in terminal granulopoiesis, to regulate neutrophil maturation. Moreover, we found BRD4, an important epigenetic regulator, directly interacts with ZNF687 in neutrophils. Deficiency of brd4 results in similar defective neutrophil development as observed in znf687 mutant zebrafish. Biochemical and genetic analyses further reveal that instead of serving as a canonical transcriptional coactivator, Brd4 directly interacts and bridges Znf687 and Smrt nuclear corepressor on gfi1aa gene's promoter to exert transcription repression. In addition, the ZNF687-BRD4-SMRT-GFI1 transcriptional regulatory network is evolutionary conserved in higher vertebrate. Overall, our work indicates Znf687 and Brd4 are two novel master regulators in promoting terminal granulopoiesis.

Histone deacetylases maintain expression of the pluripotent gene network via recruitment of RNA polymerase II to coding and noncoding loci

Histone acetylation is a dynamic modification regulated by the opposing actions of histone acetyltransferases (HATs) and histone deacetylases (HDACs). Deacetylation of histone tails results in chromatin tightening, and therefore, HDACs are generally regarded as transcriptional repressors. Counterintuitively, simultaneous deletion of Hdac1 and Hdac2 in embryonic stem cells (ESCs) reduces expression of the pluripotency-associated transcription factors Pou5f1, Sox2, and Nanog (PSN). By shaping global histone acetylation patterns, HDACs indirectly regulate the activity of acetyl-lysine readers, such as the transcriptional activator BRD4. Here, we use inhibitors of HDACs and BRD4 (LBH589 and JQ1, respectively) in combination with precision nuclear run-on and sequencing (PRO-seq) to examine their roles in defining the ESC transcriptome. Both LBH589 and JQ1 cause a marked reduction in the pluripotent gene network. However, although JQ1 treatment induces widespread transcriptional pausing, HDAC inhibition causes a reduction in both paused and elongating polymerase, suggesting an overall reduction in polymerase recruitment. Using enhancer RNA (eRNA) expression to measure enhancer activity, we find that LBH589-sensitive eRNAs are preferentially associated with superenhancers and PSN binding sites. These findings suggest that HDAC activity is required to maintain pluripotency by regulating the PSN enhancer network via the recruitment of RNA polymerase II.

Targeting the epigenetic reader "BET" as a therapeutic strategy for cancer

Bromodomain and extraterminal (BET) proteins have the ability to bind to acetylated lysine residues present in both histones and non-histone proteins. This binding is facilitated by the presence of tandem bromodomains. The regulatory role of BET proteins extends to chromatin dynamics, cellular processes, and disease progression. The BET family comprises of BRD 2, 3, 4 and BRDT. The BET proteins are a class of epigenetic readers that regulate the transcriptional activity of a multitude of genes that are involved in the pathogenesis of cancer. Thus, targeting BET proteins has been identified as a potentially efficacious approach for the treatment of cancer. BET inhibitors (BETis) are known to interfere with the binding of BET proteins to acetylated lysine residues of chromatin, thereby leading to the suppression of transcription of several genes, including oncogenic transcription factors. Here in this review, we focus on role of Bromodomain and extra C-terminal (BET) proteins in cancer progression. Furthermore, numerous small-molecule inhibitors with pan-BET activity have been documented, with certain compounds currently undergoing clinical assessment. However, it is apparent that the clinical effectiveness of the present BET inhibitors is restricted, prompting the exploration of novel technologies to enhance their clinical outcomes and mitigate undesired adverse effects. Thus, strategies like development of selective BET-BD1, & BD2 inhibitors, dual and acting BET are also presented in this review and attempts to cover the chemistry needed for proper establishment of designed molecules into BRD have been made. Moreover, the review attempts to summarize the details of research till date and proposes a space for future development of BET inhibitor with diminished side effects. It can be concluded that discovery of isoform selective BET inhibitors can be a way forward in order to develop BET inhibitors with negligible side effects.

Limitations of X:autosome ratio as a measurement of X-chromosome upregulation

Lentini and Reinius address issues in interpreting non-allelic gene expression measurements of dosage compensation during murine embryonic development.

Histone Deacetylases (HDACs) maintain expression of the pluripotent gene network via recruitment of RNA polymerase II to coding and non-coding loci

Histone acetylation is a dynamic modification regulated by the opposing actions of histone acetyltransferases (HATs) and histone deacetylases (HDACs). Deacetylation of histone tails results in chromatin tightening and therefore HDACs are generally regarded as transcriptional repressors. Counterintuitively, simultaneous deletion of Hdac1 and Hdac2 in embryonic stem cells (ESC) reduced expression of pluripotent transcription factors, Oct4, Sox2 and Nanog (OSN). By shaping global histone acetylation patterns, HDACs indirectly regulate the activity of acetyl-lysine readers, such as the transcriptional activator, BRD4. We used inhibitors of HDACs and BRD4 (LBH589 and JQ1 respectively) in combination with precision nuclear run-on and sequencing (PRO-seq) to examine their roles in defining the ESC transcriptome. Both LBH589 and JQ1 caused a marked reduction in the pluripotent network. However, while JQ1 treatment induced widespread transcriptional pausing, HDAC inhibition caused a reduction in both paused and elongating polymerase, suggesting an overall reduction in polymerase recruitment. Using enhancer RNA (eRNA) expression to measure enhancer activity we found that LBH589-sensitive eRNAs were preferentially associated with super-enhancers and OSN binding sites. These findings suggest that HDAC activity is required to maintain pluripotency by regulating the OSN enhancer network via the recruitment of RNA polymerase II.

Bromodomain (BrD) Family Members as Regulators of Cancer Stemness-A Comprehensive Review

Epigenetic mechanisms involving DNA methylation and chromatin modifications have emerged as critical facilitators of cancer heterogeneity, substantially affecting cancer development and progression, modulating cell phenotypes, and enhancing or inhibiting cancer cell malignant properties. Not surprisingly, considering the importance of epigenetic regulators in normal stem cell maintenance, many chromatin-related proteins are essential to maintaining the cancer stem cell (CSC)-like state. With increased tumor-initiating capacities and self-renewal potential, CSCs promote tumor growth, provide therapy resistance, spread tumors, and facilitate tumor relapse after treatment. In this review, we characterized the epigenetic mechanisms that regulate the acquisition and maintenance of cancer stemness concerning selected epigenetic factors belonging to the Bromodomain (BrD) family of proteins. An increasing number of BrD proteins reinforce cancer stemness, supporting the maintenance of the cancer stem cell population in vitro and in vivo via the utilization of distinct mechanisms. As bromodomain possesses high druggable potential, specific BrD proteins might become novel therapeutic targets in cancers exhibiting de-differentiated tumor characteristics.

Emerging roles of BET proteins in transcription and co-transcriptional RNA processing

Transcription by RNA polymerase II (Pol II) gives rise to all nuclear protein-coding and a large set of non-coding RNAs, and is strictly regulated and coordinated with RNA processing. Bromodomain and extraterminal (BET) family proteins including BRD2, BRD3, and BRD4 have been implicated in the regulation of Pol II transcription in mammalian cells. However, only recent technological advances have allowed the analysis of direct functions of individual BET proteins with high precision in cells. These studies shed new light on the molecular mechanisms of transcription control by BET proteins challenging previous longstanding views. The most studied BET protein, BRD4, emerges as a master regulator of transcription elongation with roles also in coupling nascent transcription with RNA processing. In contrast, BRD2 is globally required for the formation of transcriptional boundaries to restrict enhancer activity to nearby genes. Although these recent findings suggest non-redundant functions of BRD4 and BRD2 in Pol II transcription, more research is needed for further clarification. Little is known about the roles of BRD3. Here, we illuminate experimental work that has initially linked BET proteins to Pol II transcription in mammalian cells, outline main methodological breakthroughs that have strongly advanced the understanding of BET protein functions, and discuss emerging roles of individual BET proteins in transcription and transcription-coupled RNA processing. Finally, we propose an updated model for the function of BRD4 in transcription and co-transcriptional RNA maturation. This article is categorized under: RNA Processing > 3' End Processing RNA Processing > Splicing Regulation/Alternative Splicing.

The Beneficial Effects of Static Magnetic Field and Iron Oxide Nanoparticles on the Vitrification of Mature Mice Oocytes

This study was conducted to evaluate the effects of static magnetic field (SMF) and nanoparticles (NPs) on the vitrification of cumulus-oocyte-complex (COC). To this end, the non-vitrified (nVit) and vitrified groups (Vit) that contain NPs, with or without SMF were labeled nVit_NPs, nVit_NPs_SMF, Vit_NPs, and Vit_NPs_SMF, respectively. The non-toxic dosages of NPs were first determined to be 0.008% w/v. The survival, apoptosis, and necrosis, mitochondrial activity, fertilization rate, subsequent-derived embryo development, and gene expressions were examined. The viability rates obtained by trypan blue and Anx-PI staining were meaningfully smaller in the Vit groups, compared to the nVit groups. The JC1 red/green signal ratios were reduced considerably in the Vit group, compared to the nVit. Transmission electron microscopy (TEM) was performed to assess the entry of the NPs into the oocytes. TEM images showed that NPs were present in nVit_NPs, and Vit_NPs. Thereafter, the effects of NPs and SMF on in vitro fertilization (IVF) were examined. The difference in blastocyst rates between nVit and Vit_NPs_SMF groups was significant. Finally, Nanog, Cdx2, Oct4, and Sox2 genes were evaluated. There were substantial differences in Cdx2 gene expressions between the Vit_NPs and nVit groups. The expression of Nanog in Vit was significantly higher than those of the Vit_NPs, Vit_NPs_SMF, and nVit groups. The data presented here provide deeper insight into the application of iron oxide nanoparticles in COC vitrification. It appears that using SMF and supplemented CPA by NPs inhibits cryoinjury and promote the embryo development capacity of vitrified-warmed COCs.

Lipid Nanoparticles Delivering Constitutively Active STING mRNA to Stimulate Antitumor Immunity

Treating immunosuppressive tumors represents a major challenge in cancer therapies. Activation of STING signaling has shown remarkable potential to invigorate the immunologically "cold" tumor microenvironment (TME). However, we have shown that STING is silenced in many human cancers, including pancreatic ductal adenocarcinoma (PDAC) and Merkel cell carcinoma (MCC). In this study, we demonstrated that mRNA-lipid nanoparticle (LNP) technology could be used to efficiently deliver naturally occurring constitutively active STING mutant STING^R284S into these cancer cells to reactivate STING antitumor immunity and trigger robust killing of tumor cells. STING agonists are being actively pursued as cancer immunotherapies. However, traditional STING agonists can induce T cell cytotoxicity, counteracting the desired antitumor immune response. In addition, the antitumor efficacy of traditional STING agonists obligatorily depends on STING expression and does not work in STING-silenced cancers. Importantly, we found that STING^R284S mRNA-LNP does not introduce T cell cytotoxicity. Our studies demonstrated that mRNA-LNP delivery of STING^R284S can reactivate the antitumor response without introducing antiproliferative effects in lymphocytic immune cells, overcoming the toxicity and limitations of conventional STING agonists. Our work therefore identifies a novel therapeutic tool for reactivating antitumor immunity in an array of STING-silenced immunologically "cold" tumors that are refractory to current therapies.

Impact of Histone Modifications and Their Therapeutic Targeting in Hematological Malignancies

Hematologic malignancies are a large and heterogeneous group of neoplasms characterized by complex pathogenetic mechanisms. The abnormal regulation of epigenetic mechanisms and specifically, histone modifications, has been demonstrated to play a central role in hematological cancer pathogenesis and progression. A variety of epigenetic enzymes that affect the state of histones have been detected as deregulated, being either over- or underexpressed, which induces changes in chromatin compaction and, subsequently, affects gene expression. Recent advances in the field of epigenetics have revealed novel therapeutic targets, with many epigenetic drugs being investigated in clinical trials. The present review focuses on the biological impact of histone modifications in the pathogenesis of hematologic malignancies, describing a wide range of therapeutic agents that have been discovered to target these alterations and are currently under investigation in clinical trials.

Common dysmorphic oocytes and embryos in assisted reproductive technology laboratory in association with gene alternations

Amorphic or defected oocytes and embryos are commonly observed in assisted reproductive technology (ART) laboratories. It is believed that a proper gene expression at each stage of embryo development contributes to the possibility of a decent-quality embryo leading to successful implantation. Many studies reported that several defects in embryo morphology are associated with gene expressions during in vitro fertilization (IVF) treatment. There is lacking literature review on summarizing common morphological defects about gene alternations. In this review, we summarized the current literature. We selected 64 genes that have been reported to be involved in embryo morphological abnormalities in animals and humans, 30 of which were identified in humans and might be the causes of embryonic changes. Five papers focusing on associations of multiple gene expressions and embryo abnormalities using RNA transcriptomes were also included during the search. We have also reviewed our time-lapse image database with over 3000 oocytes/embryos to show morphological defects possibly related to gene alternations reported previously in the literature. This holistic review can better understand the associations between gene alternations and morphological changes. It is also beneficial to select important biomarkers with strong evidence in IVF practice and reveal their potential application in embryo selection. Also, identifying genes may help patients with genetic disorders avoid unnecessary treatments by providing preimplantation genetic testing for monogenic/single gene defects (PGT-M), reduce embryo replacements by less potential, and help scientists develop new methods for oocyte/embryo research in the near future.

A small proportion of X-linked genes contribute to X chromosome upregulation in early embryos via BRD4-mediated transcriptional activation

Females have two X chromosomes and males have only one in most mammals. X chromosome inactivation (XCI) occurs in females to equalize X-dosage between sexes. Besides, mammals also balance the dosage between X chromosomes and autosomes via X chromosome upregulation (XCU) to fine-tune X-linked expression and thus maintain genomic homeostasis. Despite some studies highlighting the importance of XCU in somatic cells, little is known about how XCU is achieved and its developmental role during early embryogenesis. Herein, using mouse preimplantation embryos as the model, we reported that XCU initially occurs upon major zygotic genome activation and co-regulates X-linked expression in cooperation with imprinted XCI during preimplantation development. An in-depth analysis further indicated, unexpectedly, only a small proportion of, but not X chromosome-wide, X-linked genes contribute greatly to XCU. Furthermore, we identified that bromodomain containing 4 (BRD4) plays a key role in the transcription activation of XCU during preimplantation development. BRD4 deficiency or inhibition caused an impaired XCU, thus leading to reduced developmental potential and mitochondrial dysfunctions of blastocysts. Our finding was also supported by the tight association of BRD4 dysregulation and XCU disruption in the pathology of cholangiocarcinoma. Thus, our results not only advanced the current knowledge of X-dosage compensation and provided a mechanism for understanding XCU initiation but also presented an important clue for understanding the developmental and pathological role of XCU.

An interplay between BRD4 and G9a regulates skeletal myogenesis

Histone acetylation and methylation are epigenetic modifications that are dynamically regulated by chromatin modifiers to precisely regulate gene expression. However, the interplay by which histone modifications are synchronized to coordinate cellular differentiation is not fully understood. In this study, we demonstrate a relationship between BRD4, a reader of acetylation marks, and G9a, a writer of methylation marks in the regulation of myogenic differentiation. Using loss- and gain-of-function studies, as well as a pharmacological inhibition of its activity, we examined the mechanism by which BRD4 regulates myogenesis. Transcriptomic analysis using RNA sequencing revealed that a number of myogenic differentiation genes are downregulated in Brd4-depleted cells. Interestingly, some of these genes were upregulated upon G9a knockdown, indicating that BRD4 and G9a play opposing roles in the control of myogenic gene expression. Remarkably, the differentiation defect caused by Brd4 knockdown was rescued by inhibition of G9a methyltransferase activity. These findings demonstrate that the absence of BRD4 results in the upregulation of G9a activity and consequently impaired myogenic differentiation. Collectively, our study identifies an interdependence between BRD4 and G9a for the precise control of transcriptional outputs to regulate myogenesis.

BRD4 promotes resection and homology-directed repair of DNA double-strand breaks

Double-strand breaks (DSBs) are one of the most toxic forms of DNA damage and represent a major source of genomic instability. Members of the bromodomain and extra-terminal (BET) protein family are characterized as epigenetic readers that regulate gene expression. However, evidence suggests that BET proteins also play a more direct role in DNA repair. Here, we establish a cell-free system using Xenopus egg extracts to elucidate the gene expression-independent functions of BET proteins in DSB repair. We identify the BET protein BRD4 as a critical regulator of homologous recombination and describe its role in stimulating DNA processing through interactions with the SWI/SNF chromatin remodeling complex and resection machinery. These results establish BRD4 as a multifunctional regulator of chromatin binding that links transcriptional activity and homology-directed repair.

BRD4 is a multifunctional regulator of chromatin binding that plays a direct role in DNA double-strand break repair. BRD4 interacts with the SWI/SNF chromatin remodeling complex and resection machinery to promote homologous recombination.

BET proteins are essential for the specification and maintenance of the epiblast lineage in mouse preimplantation embryos

Background

During mammalian preimplantation development, as the fertilized egg develops and differentiates, three cell lineages become specified: trophectoderm (TE), epiblast, and primitive endoderm (PrE). Through two steps of cell fate decisions, 16-cell blastomeres develop into TE and an inner cell mass (ICM), and thereafter, the latter differentiates into pluripotent epiblast and PrE. Although bromodomain and extra-terminal domain (BET) proteins, such as BRD4, are necessary for the transcriptional activation of genes involved in the maintenance of mouse embryonic stem cells by occupying their enhancers, their roles in the development of mouse preimplantation are unknown.

Results

To evaluate the effect of BET protein deficiency on cell lineage formation, we cultured preimplantation embryos in the presence of JQ1, which blocks the binding of BET bromodomains to acetylated-histones. We found BET inhibition blocked the transcriptional activation of genes, such as Nanog, Otx2, and Sox2, important for the formation of the epiblast lineage in blastocysts. Expression studies with lineage-specific markers in morulae and blastocysts revealed BET proteins were essential for the specification and maintenance of the epiblast lineage but were dispensable for the formation of primarily extraembryonic TE and PrE lineages. Additional Ingenuity Pathway Analysis and expression studies with a transcriptionally active form of signal transducer and activator of the transcription 3 (STAT3) suggested BET-dependent activation was partly associated with the STAT3-dependent pathway to maintain the epiblast lineage. To identify BET proteins involved in the formation of the epiblast lineage, we analyzed mutant embryos deficient in Brd4, Brd2, and double mutants. Abolishment of NANOG-positive epiblast cells was only evident in Brd4/Brd2 double-deficient morulae. Thus, the phenotype of JQ1-treated embryos is reproduced not by a Brd4- or Brd2-single deficiency, but only Brd4/Brd2-double deficiency, demonstrating the redundant roles of BRD2 and BRD4 in the specification of the epiblast lineage.

Conclusions

BET proteins are essential to the specification and maintenance of the epiblast lineage by activating lineage-specific core transcription factors during mouse preimplantation development. Among BET proteins, BRD4 plays a central role and BRD2 a complementary role in the specification and maintenance of epiblast lineages. Additionally, BET-dependent maintenance of the epiblast lineage may be partly associated with the STAT3-dependent pathway.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01251-0.

TcoFBase: a comprehensive database for decoding the regulatory transcription co-factors in human and mouse

Transcription co-factors (TcoFs) play crucial roles in gene expression regulation by communicating regulatory cues from enhancers to promoters. With the rapid accumulation of TcoF associated chromatin immunoprecipitation sequencing (ChIP-seq) data, the comprehensive collection and integrative analyses of these data are urgently required. Here, we developed the TcoFBase database (http://tcof.liclab.net/TcoFbase), which aimed to document a large number of available resources for mammalian TcoFs and provided annotations and enrichment analyses of TcoFs. TcoFBase curated 2322 TcoFs and 6759 TcoFs associated ChIP-seq data from over 500 tissues/cell types in human and mouse. Importantly, TcoFBase provided detailed and abundant (epi) genetic annotations of ChIP-seq based TcoF binding regions. Furthermore, TcoFBase supported regulatory annotation information and various functional annotations for TcoFs. Meanwhile, TcoFBase embedded five types of TcoF regulatory analyses for users, including TcoF gene set enrichment, TcoF binding genomic region annotation, TcoF regulatory network analysis, TcoF-TF co-occupancy analysis and TcoF regulatory axis analysis. TcoFBase was designed to be a useful resource that will help reveal the potential biological effects of TcoFs and elucidate TcoF-related regulatory mechanisms.

PRC2 loss of function confers a targetable vulnerability to BET proteins in T-ALL

T-cell acute lymphoblastic leukemia (T-ALL) is a group of aggressive hematological cancers with dismal outcomes that are in need of new therapeutic options. Polycomb repressor complex 2 (PRC2) loss-of-function alterations were reported in pediatric T-ALL, yet their clinical relevance and functional consequences remain elusive. Here, we extensively analyzed PRC2 alterations in a large series of 218 adult T-ALL patients. We found that PRC2 genetic lesions are frequent events in T-ALL and are not restricted to early thymic precursor ALL. PRC2 loss of function associates with activating mutations of the IL7R/JAK/STAT pathway. PRC2-altered T-ALL patients respond poorly to prednisone and have low bone marrow blast clearance and persistent minimal residual disease. Furthermore, we identified that PRC2 loss of function profoundly reshapes the genetic and epigenetic landscapes, leading to the reactivation of stem cell programs that cooperate with bromodomain and extraterminal (BET) proteins to sustain T-ALL. This study identifies BET proteins as key mediators of the PRC2 loss of function-induced remodeling. Our data have uncovered a targetable vulnerability to BET inhibition that can be exploited to treat PRC2-altered T-ALL patients.

ARID1A Mutation in Metastatic Breast Cancer: A Potential Therapeutic Target

Distant metastasis is the principal cause of mortality for breast cancer patients. Targeting specific mutations that have been acquired during the evolution process of advanced breast cancer is a potential means of enhancing the clinical efficacy of treatment strategies. In metastatic breast cancer, ARID1A is the most prevalent mutation of the SWI/SNF complex, which regulates DNA repair, recombination, and gene transcription. The low expression of ARID1A is associated with poor disease-free survival and overall survival of patients with luminal A or HER2-rich breast cancer. In addition, ARID1A plays a prominent role in maintaining luminal characteristics and has an advantage for identifying responses to treatment, including endocrine therapies, HDAC inhibitors and CDK4/6 inhibitors. The therapeutic vulnerabilities initiated by ARID1A alterations encourage us to explore new approaches to cope with ARID1A mutant-related drug resistance or metastasis. In this review, we describe the mutation profiles of ARID1A in metastatic breast cancer and the structure and function of ARID1A and the SWI/SNF complex as well as discuss the potential mechanisms of ARID1A-mediated endocrine resistance and therapeutic potential.

BRD4 in physiology and pathology: ''BET'' on its partners

Bromodomain-containing 4 (BRD4), a member of Bromo and Extra-Terminal (BET) family, recognizes acetylated histones and is of importance in transcription, replication, and DNA repair. It also binds non-histone proteins, DNA and RNA, contributing to development, tissue growth, and various physiological processes. Additionally, BRD4 has been implicated in driving diverse diseases, ranging from cancer, viral infection, inflammation to neurological disorders. Inhibiting its functions with BET inhibitors (BETis) suppresses the progression of several types of cancer, creating an impetus for translating these chemicals to the clinic. The diverse roles of BRD4 are largely dependent on its interaction partners in different contexts. In this review we discuss the molecular mechanisms of BRD4 with its interacting partners in physiology and pathology. Current development of BETis is also summarized. Further understanding the functions of BRD4 and its partners will facilitate resolving the liabilities of present BETis and accelerate their clinical translation.

BRD4 REGULATES TRANSCRIPTION FACTOR ∆Np63αTO DRIVE A CANCER STEM CELL PHENOTYPE IN SQUAMOUS CELL CARCINOMAS

Bromodomain containing protein 4 (BRD4) plays a critical role in controlling the expression of genes involved in development and cancer. Inactivation of BRD4 inhibits cancer growth, making it a promising anticancer drug target. The cancer stem cell population is a key driver of recurrence and metastasis in cancer patients. Here we show that cancer stem-like cells can be enriched from squamous cell carcinomas, and that these cells display an aggressive phenotype with enhanced stem cell marker expression, migration, invasion, and tumor growth. BRD4 was highly elevated in this aggressive subpopulation of cells, and its function is critical for these cancer stem cell-like properties. Moreover, BRD4 regulated ∆Np63α, a key transcription factor that is essential for epithelial stem cell function that is often overexpressed in cancers. BRD4 regulated an EZH2/STAT3 complex that led to increased ∆Np63α-mediated transcription. Targeting BRD4 in human squamous cell carcinoma reduces ∆Np63α, leading to inhibition of spheroid formation, migration, invasion and tumor growth. These studies identify a novel BRD4-regulated signaling network in a subpopulation of cancer stem-like cells elucidating a possible avenue for effective therapeutic intervention.

Emerging roles of bromodomain protein 4 in regulation of stem cell identity

Understanding the mechanism of fate decision and lineage commitment is the key step for developing novel stem cell applications in therapeutics. This process is coordinately regulated through systematic epigenetic reprogramming and concomitant changes in the transcriptional landscape of the stem cells. One of the bromo- and extra-terminal domain (BET) family member proteins, bromodomain protein 4 (BRD4), performs the role of epigenetic reader and modulates gene expression by recruiting other transcription factors and directly regulating RNA polymerase II elongation. Controlled gene regulation is the critical step in maintenance of stem cell potency and dysregulation may lead to tumor formation. As a key transcriptional factor and epigenetic regulator, BRD4 contributes to stem cell maintenance in several ways. Being a druggable target, BRD4 is an attractive candidate for exploiting its potential in stem cell therapeutics. Therefore, it is crucial to elucidate how BRD4, through its interplay with pluripotency transcriptional regulators, control lineage commitment in stem cells. Here, we systemically review the role of BRD4 in complex gene regulatory network during three specific states of stem cell transitions: cell differentiation, cell reprogramming and transdifferentiation. A thorough understanding of BRD4 mediated epigenetic regulation in the maintenance of stem cell potency will be helpful to strategically control stem cell fates in regenerative medicine.

Bromodomain and extraterminal domain protein bromodomain inhibitor based cancer therapeutics

PURPOSE OF REVIEW

Bromodomain and extraterminal domain (BET) proteins are evolutionarily conserved, multifunctional super-regulators that specifically recognize acetyl-lysine on histones and other proteins controlling gene transcription. Several studies show that small molecules targeting these regulators preferentially suppress the transcription of cancer-promoting genes. Consequently, several BET inhibitors reached clinical trials and are in various stages for different kind of malignancies. In this review, we provide a concise summary of the molecular basis and preliminary clinical outcomes of BET inhibitors as anticancer therapeutics.

RECENT FINDINGS

Results from early clinical trials with BET inhibitors confirmed their antitumor potential in both hematologic and solid tumours, but the evidence does not support the application of BET inhibitors as a monotherapy for cancer treatment. Treatment-emergent toxicities such as thrombocytopenia and gastrointestinal disorders are also reported. Preclinical data suggest that BET inhibitors may have a promising future in combination with other anticancer agents.

SUMMARY

Despite of various challenges, BET inhibitors have high potential in combinatorial therapy and the future development of next-generation inhibitors could be promising. Further studies are needed to determine the predictive biomarkers for therapeutic response, which would translate into the long-term success of BET inhibitors as personalized medicines in cancer treatment.

Merkel Cell Polyomavirus Infection Induces an Antiviral Innate Immune Response in Human Dermal Fibroblasts

Merkel cell polyomavirus (MCPyV) infects most of the human population asymptomatically, but in rare cases it leads to a highly aggressive skin cancer called Merkel cell carcinoma (MCC). MCC incidence is much higher in aging and immunocompromised populations. The epidemiology of MCC suggests that dysbiosis between the host immune response and the MCPyV infectious cycle could contribute to the development of MCPyV-associated MCC. Insufficient restriction of MCPyV by normal cellular processes, for example, could promote the incidental oncogenic MCPyV integration events and/or entry into the original cell of MCC. Progress toward understanding MCPyV biology has been hindered by its narrow cellular tropism. Our discovery that primary human dermal fibroblasts (HDFs) support MCPyV infection has made it possible to closely model cellular responses to different stages of the infectious cycle. The present study reveals that the onset of MCPyV replication and early gene expression induces an inflammatory cytokine and interferon-stimulated gene (ISG) response. The cGAS-STING pathway, in coordination with NF-κB, mediates induction of this innate immune gene expression program. Further, silencing of cGAS or NF-κB pathway factors led to elevated MCPyV replication. We also discovered that the PYHIN protein IFI16 localizes to MCPyV replication centers but does not contribute to the induction of ISGs. Instead, IFI16 upregulates inflammatory cytokines in response to MCPyV infection by an alternative mechanism. The work described herein establishes a foundation for exploring how changes to the skin microenvironment induced by aging or immunodeficiency might alter the fate of MCPyV and its host cell to encourage carcinogenesis. IMPORTANCE MCC has a high rate of mortality and an increasing incidence. Immune-checkpoint therapies have improved the prognosis of patients with metastatic MCC. Still, a significant proportion of the patients fail to respond to immune-checkpoint therapies or have a medical need for iatrogenic immune-suppression. A greater understanding of MCPyV biology could inform targeted therapies for MCPyV-associated MCC. Moreover, cellular events preceding MCC oncogenesis remain largely unknown. The present study aims to explore how MCPyV interfaces with innate immunity during its infectious cycle. We describe how MCPyV replication and/or transcription elicit an innate immune response via cGAS-STING, NF-κB, and IFI16. We also explore the effects of this response on MCPyV replication. Our findings illustrate how healthy cellular conditions may allow low-level infection that evades immune destruction until highly active replication is restricted by host responses. Conversely, pathological conditions could result in unbridled MCPyV replication that licenses MCC tumorigenesis.

Low complexity domains, condensates, and stem cell pluripotency

Biological reactions require self-assembly of factors in the complex cellular milieu. Recent evidence indicates that intrinsically disordered, low-complexity sequence domains (LCDs) found in regulatory factors mediate diverse cellular processes from gene expression to DNA repair to signal transduction, by enriching specific biomolecules in membraneless compartments or hubs that may undergo liquid-liquid phase separation (LLPS). In this review, we discuss how embryonic stem cells take advantage of LCD-driven interactions to promote cell-specific transcription, DNA damage response, and DNA repair. We propose that LCD-mediated interactions play key roles in stem cell maintenance and safeguarding genome integrity.

Activation of STING Signaling Pathway Effectively Blocks Human Coronavirus Infection

The COVID-19 pandemic poses a serious global health threat. The rapid global spread of SARS-CoV-2 highlights an urgent need to develop effective therapeutics for blocking SARS-CoV-2 infection and spread. Stimulator of Interferon Genes (STING) is a chief element in host antiviral defense pathways. In this study, we examined the impact of the STING signaling pathway on coronavirus infection using the human coronavirus OC43 (HCoV-OC43) model. We found that HCoV-OC43 infection did not stimulate the STING signaling pathway, but the activation of STING signaling effectively inhibits HCoV-OC43 infection to a much greater extent than that of type I interferons (IFNs). We also discovered that IRF3, the key STING downstream innate immune effector, is essential for this anticoronavirus activity. In addition, we found that the amidobenzimidazole (ABZI)-based human STING agonist diABZI robustly blocks the infection of not only HCoV-OC43 but also SARS-CoV-2. Therefore, our study identifies the STING signaling pathway as a potential therapeutic target that could be exploited for developing broad-spectrum antiviral therapeutics against multiple coronavirus strains in order to face the challenge of future coronavirus outbreaks.IMPORTANCE The highly infectious and lethal SARS-CoV-2 is posing an unprecedented threat to public health. Other coronaviruses are likely to jump from a nonhuman animal to humans in the future. Novel broad-spectrum antiviral therapeutics are therefore needed to control known pathogenic coronaviruses such as SARS-CoV-2 and its newly mutated variants, as well as future coronavirus outbreaks. STING signaling is a well-established host defense pathway, but its role in coronavirus infection remains unclear. In the present study, we found that activation of the STING signaling pathway robustly inhibits infection of HCoV-OC43 and SARS-CoV-2. These results identified the STING pathway as a novel target for controlling the spread of known pathogenic coronaviruses, as well as emerging coronavirus outbreaks.

NR5A2 transcriptional activation by BRD4 promotes pancreatic cancer progression by upregulating GDF15

NR5A2 is a transcription factor regulating the expression of various oncogenes. However, the role of NR5A2 and the specific regulatory mechanism of NR5A2 in pancreatic ductal adenocarcinoma (PDAC) are not thoroughly studied. In our study, Western blotting, real-time PCR, and immunohistochemistry were conducted to assess the expression levels of different molecules. Wound-healing, MTS, colony formation, and transwell assays were employed to evaluate the malignant potential of pancreatic cancer cells. We demonstrated that NR5A2 acted as a negative prognostic biomarker in PDAC. NR5A2 silencing inhibited the proliferation and migration abilities of pancreatic cancer cells in vitro and in vivo. While NR5A2 overexpression markedly promoted both events in vitro. We further identified that NR5A2 was transcriptionally upregulated by BRD4 in pancreatic cancer cells and this was confirmed by Chromatin immunoprecipitation (ChIP) and ChIP-qPCR. Besides, transcriptome RNA sequencing (RNA-Seq) was performed to explore the cancer-promoting effects of NR5A2, we found that GDF15 is a component of multiple down-regulated tumor-promoting gene sets after NR5A2 was silenced. Next, we showed that NR5A2 enhanced the malignancy of pancreatic cancer cells by inducing the transcription of GDF15. Collectively, our findings suggest that NR5A2 expression is induced by BRD4. In turn, NR5A2 activates the transcription of GDF15, promoting pancreatic cancer progression. Therefore, NR5A2 and GDF15 could be promising therapeutic targets in pancreatic cancer.

Differential transcriptional regulation of the NANOG gene in chicken primordial germ cells and embryonic stem cells

Background

NANOG is a core transcription factor (TF) in embryonic stem cells (ESCs) and primordial germ cells (PGCs). Regulation of the NANOG gene by TFs, epigenetic factors, and autoregulatory factors is well characterized in ESCs, and transcriptional regulation of NANOG is well established in these cells. Although NANOG plays a key role in germ cells, the molecular mechanism underlying its transcriptional regulation in PGCs has not been studied. Therefore, we investigated the mechanism that regulates transcription of the chicken NANOG (cNANOG) gene in PGCs and ESCs.

Results

We first identified the transcription start site of cNANOG by 5′-rapid amplification of cDNA ends PCR analysis. Then, we measured the promoter activity of various 5′ flanking regions of cNANOG in chicken PGCs and ESCs using the luciferase reporter assay. cNANOG expression required transcriptional regulatory elements, which were positively regulated by POU5F3 (OCT4) and SOX2 and negatively regulated by TP53 in PGCs. The proximal region of the cNANOG promoter contains a positive transcriptional regulatory element (CCAAT/enhancer-binding protein (CEBP)-binding site) in ESCs. Furthermore, small interfering RNA-mediated knockdown demonstrated that POU5F3, SOX2, and CEBP played a role in cell type-specific transcription of cNANOG.

Conclusions

We show for the first time that different trans-regulatory elements control transcription of cNANOG in a cell type-specific manner. This finding might help to elucidate the mechanism that regulates cNANOG expression in PGCs and ESCs.

Chromatin and Epigenetic Rearrangements in Embryonic Stem Cell Fate Transitions

A major event in embryonic development is the rearrangement of epigenetic information as the somatic genome is reprogrammed for a new round of organismal development. Epigenetic data are held in chemical modifications on DNA and histones, and there are dramatic and dynamic changes in these marks during embryogenesis. However, the mechanisms behind this intricate process and how it is regulating and responding to embryonic development remain unclear. As embryos develop from totipotency to pluripotency, they pass through several distinct stages that can be captured permanently or transiently in vitro. Pluripotent naïve cells resemble the early epiblast, primed cells resemble the late epiblast, and blastomere-like cells have been isolated, although fully totipotent cells remain elusive. Experiments using these in vitro model systems have led to insights into chromatin changes in embryonic development, which has informed exploration of pre-implantation embryos. Intriguingly, human and mouse cells rely on different signaling and epigenetic pathways, and it remains a mystery why this variation exists. In this review, we will summarize the chromatin rearrangements in early embryonic development, drawing from genomic data from in vitro cell lines, and human and mouse embryos.

Bromodomain and BET family proteins as epigenetic targets in cancer therapy: their degradation, present drugs, and possible PROTACs

Alteration in the pattern of epigenetic marking leads to cancer, neurological disorders, inflammatory problems etc. These changes are due to aberration in histone modification enzymes that function as readers, writers and erasers. Bromodomains (BDs) and BET proteins that recognize acetylation of chromatin regulate gene expression. To block the function of any of these BrDs and/or BET protein can be a controlling agent in disorders such as cancer. BrDs and BET proteins are now emerging as targets for new therapeutic development. Traditional drugs like enzyme inhibitors and protein–protein inhibitors have many limitations. Recently Proteolysis-Targeting Chimeras (PROTACs) have become an advanced tool in therapeutic intervention as they remove disease causing proteins. This review provides an overview of the development and mechanisms of PROTACs for BRD and BET protein regulation in cancer and advanced possibilities of genetic technologies in therapeutics.

Alteration in the pattern of epigenetic marking leads to cancer, neurological disorders, inflammatory problems etc.

BRD4 regulates self-renewal ability and tumorigenicity of glioma-initiating cells by enrichment in the Notch1 promoter region

Bromodomain and extraterminal domain (BET) family proteins are considered to be epigenetic readers that regulate gene expression by recognizing acetyl lysine residues on histones and nonhistone chromatin factors and have been classified as curative targets for a variety of cancers. Glioma-initiating cells (GICs), which commit self-renewal, perpetual proliferation, multidirectional differentiation, and vigorous tumorigenicity, sustain the peculiar genetic and epigenetic diversification in the GBM patients, thus, GICs result in tumor recurrence. Abundant evidence demonstrates that BET proteins regulate differentiation of stem cells. However, it endures ambiguous how individual BET proteins take part in GIC advancement, and how do small molecule inhibitors like I-BET151 target functional autonomous BET proteins. Here, we validated that BRD4, not BRD2 or BRD3, has value in targeted glioma therapy. We announce a signaling pathway concerning BRD4 and Notch1 that sustains the self-renewal of GICs. Moreover, in-depth mechanistic research showed that BRD4 was concentrated at the promoter region of Notch1 and may be involved in the process of tumor metabolism. Furthermore, in intracranial models, I-BET151 eliminated U87 GICs' tumorigenicity. The outcomes of this research could be conducive to design clinical trials for treatment of glioma based on BRD4.

Inhibition of Brd4 by JQ1 Promotes Functional Recovery From Spinal Cord Injury by Activating Autophagy

Spinal cord injury (SCI) is a destructive neurological disorder that is characterized by impaired sensory and motor function. Inhibition of bromodomain protein 4 (Brd4) has been shown to promote the maintenance of cell homeostasis by activating autophagy. However, the role of Brd4 inhibition in SCI and the underlying mechanisms are poorly understood. Thus, the goal of the present study was to evaluate the effects of sustained Brd4 inhibition using the bromodomain and extraterminal domain (BET) inhibitor JQ1 on the regulation of apoptosis, oxidative stress and autophagy in a mouse model of SCI. First, we observed that Brd4 expression at the lesion sites of mouse spinal cords increased after SCI. Treatment with JQ1 significantly decreased the expression of Brd4 and improved functional recovery for up to 28 day after SCI. In addition, JQ1-mediated inhibition of Brd4 reduced oxidative stress and inhibited the expression of apoptotic proteins to promote neural survival. Our results also revealed that JQ1 treatment activated autophagy and restored autophagic flux, while the positive effects of JQ1 were abrogated by autophagy inhibitor 3-MA intervention, indicating that autophagy plays a crucial role in therapeutic effects Brd4 induced by inhibition of the functional recovery SCI. In the mechanistic analysis, we observed that modulation of the AMPK-mTOR-ULK1 pathway is involved in the activation of autophagy mediated by Brd4 inhibition. Taken together, the results of our investigation provides compelling evidence that Brd4 inhibition by JQ1 promotes functional recovery after SCI and that Brd4 may serve as a potential target for SCI treatment.

Domain-selective targeting of BET proteins in cancer and immunological diseases

Cancer and inflammation are strongly interconnected processes. Chronic inflammatory pathologies can be at the heart of tumor development; similarly, tumor-elicited inflammation is a consequence of many cancers. The mechanistic interdependence between cancer and inflammatory pathologies points toward common protein effectors which represent potential shared targets for pharmacological intervention. Epigenetic mechanisms often drive resistance to cancer therapy and immunomodulatory strategies. The bromodomain and extraterminal domain (BET) proteins are epigenetic adapters which play a major role in controlling cell proliferation and the production of inflammatory mediators. A plethora of small molecules aimed at inhibiting BET protein function to treat cancer and inflammatory diseases have populated academic and industry efforts in the last 10 years. In this review, we will discuss recent pharmacological approaches aimed at targeting a single or a subset of the eight bromodomains within the BET family which have the potential to tease apart clinical efficacy and safety signals of BET inhibitors.

β-Catenin safeguards the ground state of mousepluripotency by strengthening the robustness of the transcriptional apparatus

Mouse embryonic stem cells cultured with MEK (mitogen-activated protein kinase kinase) and GSK3 (glycogen synthase kinase 3) inhibitors (2i) more closely resemble the inner cell mass of preimplantation blastocysts than those cultured with SL [serum/leukemia inhibitory factor (LIF)]. The transcriptional mechanisms governing this pluripotent ground state are unresolved. Release of promoter-proximal paused RNA polymerase II (Pol2) is a multistep process necessary for pluripotency and cell cycle gene transcription in SL. We show that β-catenin, stabilized by GSK3 inhibition in medium with 2i, supplies transcriptional coregulators at pluripotency loci. This selectively strengthens pluripotency loci and renders them addicted to transcription initiation for productive gene body elongation in detriment to Pol2 pause release. By contrast, cell cycle genes are not bound by β-catenin, and proliferation/self-renewal remains tightly controlled by Pol2 pause release under 2i conditions. Our findings explain how pluripotency is reinforced in the ground state and also provide a general model for transcriptional resilience/adaptation upon network perturbation in other contexts.

Bromodomain-Containing Protein BRD4 Is Hyperphosphorylated in Mitosis

The epigenetic reader BRD4 binds acetylated histones and plays a central role in controlling cellular gene transcription and proliferation. Dysregulation of BRD4's activity has been implicated in the pathogenesis of a wide variety of cancers. While blocking BRD4 interaction with acetylated histones using BET inhibitors (BETis) has been tested in clinical trials, many cancers have acquired BETi resistance. However, the underlying mechanisms are poorly understood and BETi resistance remains a pressing clinical problem. We previously showed that BRD4 phosphorylation supports stronger chromatin binding and target oncogene expression. In this study, we discovered that BRD4 is hyperphosphorylated by CDK1 during mitosis and determined the major CDK1 phosphorylation sites in BRD4. Using CRISPR/Cas9 gene editing, we replaced endogenous BRD4 with a non-phosphorylatable mutant and demonstrated that CDK1-mediated BRD4 phosphorylation contributes to BETi resistance. CDK1 over-activation frequently observed in cancers has the potential to cause aberrant BRD4 hyperphosphorylation persisting outside of mitosis to strengthen its target gene binding and confer BETi resistance. We found that dual CDK1 and BET inhibition generates a synergistic effect in killing BETi-resistant cancer cells. Our study therefore suggests that CDK1 inhibition can be employed to overcome tumor BETi resistance and improve treatments for BRD4-associated cancers.

Kethoxal-assisted single-stranded DNA sequencing captures global transcription dynamics and enhancer activity in situ

Transcription is a highly dynamic process that generates single-stranded DNA (ssDNA) in the genome as ‘transcription bubbles’. Here we describe a kethoxal-assisted single-stranded DNA sequencing (KAS-seq) approach, based on the fast and specific reaction between N₃-kethoxal and guanines in ssDNA in live cells and mouse tissues. KAS-seq enables rapid (within 5 min), sensitive, and genome-wide capture and mapping of ssDNA produced by transcriptionally active RNA polymerases or other processes in situ by using as few as 1,000 cells. KAS-seq defines a group of enhancers that are single-stranded, which enrich unique sequence motifs and are associated with specific transcription factor binding and more enhancer-promotor interactions. Under protein condensation inhibition conditions, KAS-seq uncovers a rapid release of RNA polymerase II (Pol II) from a group of promotors. KAS-seq thus facilitates fast, comprehensive, and accurate analysis of transcription dynamics and enhancer activities simultaneously in a low input and high-throughput manner.

Bromodomain and extra-terminal domain (BET) proteins regulate melanocyte differentiation

BACKGROUND

Pharmacologic inhibition of bromodomain and extra-terminal (BET) proteins is currently being explored as a new therapeutic approach in cancer. Some studies have also implicated BET proteins as regulators of cell identity and differentiation through their interactions with lineage-specific factors. However, the role of BET proteins has not yet been investigated in melanocyte differentiation. Melanocyte inducing transcription factor (MITF) is the master regulator of melanocyte differentiation, essential for pigmentation and melanocyte survival. In this study, we tested the hypothesis that BET proteins regulate melanocyte differentiation through interactions with MITF.

RESULTS

Here we show that chemical inhibition of BET proteins prevents differentiation of unpigmented melanoblasts into pigmented melanocytes and results in de-pigmentation of differentiated melanocytes. BET inhibition also slowed cell growth, without causing cell death, increasing the number of cells in G1. Transcriptional profiling revealed that BET inhibition resulted in decreased expression of pigment-specific genes, including many MITF targets. The expression of pigment-specific genes was also down-regulated in melanoma cells, but to a lesser extent. We found that RNAi depletion of the BET family members, bromodomain-containing protein 4 (BRD4) and bromodomain-containing protein 2 (BRD2) inhibited expression of two melanin synthesis enzymes, TYR and TYRP1. Both BRD4 and BRD2 were detected on melanocyte promoters surrounding MITF-binding sites, were associated with open chromatin structure, and promoted MITF binding to these sites. Furthermore, BRD4 and BRD2 physically interacted with MITF.

CONCLUSION

These findings indicate a requirement for BET proteins in the regulation of pigmentation and melanocyte differentiation. We identified changes in pigmentation specific gene expression that occur upon BET inhibition in melanoblasts, melanocytes, and melanoma cells.

Brd4's Bromodomains Mediate Histone H3 Acetylation and Chromatin Remodeling in Pluripotent Cells through P300 and Brg1

The pluripotent state of embryonic stem cells (ESCs) is defined by its transcriptome and epigenome. The chromatin reader Brd4 determines ESC identity. Although Brd4 regulation in gene transcription has been well described, its contribution to the chromatin landscape is less known. Here, we show that Brd4's bromodomains partner with the histone acetyltransferase P300, increasing its enzymatic activities. Augmenting histone acetylation by Brd4-P300 interaction recruits the chromatin remodeler Brg1 altering chromatin structure. This pathway is important for maintaining the expression and chromatin patterns of pluripotency-associated genes, such as Oct4, Nanog, and the X chromosome regulatory long noncoding RNAs Tsix and Xite. Furthermore, we show that the Brd4-P300 interaction regulates the de novo formation of chromatin marks during ESC differentiation, as exemplified by controlling the master regulators of mesoderm formation. Collectively, we delineate the function of Brd4 in organizing the chromatin structure that contributes to gene transcriptional regulation and cell fate determination.

Interference with the bromodomain epigenome readers drives p21 expression and tumor senescence

Head and neck cancer (HNSCC) are one of the most common solid malignancies of the world, being responsible for over 350,000 deaths every year. Much of the complications in managing and treating HNSCC advent from the complex genetic and epigenetic landscape of the disease. Emerging information has shown promising results in targeting BRD4, an epigenetic regulator bromodomain that functions as a scaffold for transcription factors at promoters and super-enhancers. Here we show that by disrupting the interaction between BRD4 and histones using the bromodomain inhibitor JQ1, HNSCC cells undergo cell growth arrest followed by cellular senescence. Mechanistically, JQ1 negatively impacted the phosphorylation levels of SIRT1 along with the acetylation levels of mutant p53 (active). In vivo administration of JQ1 resulted in disruption of HNSCC growth along with the activation of cellular senescence, observed by the accumulation of DNA double-strand breaks, p16^ink4, accumulation of senescence-associated beta-galactosidase, and loss of phosphorylated Sirt1^ser47. Furthermore, we also demonstrate that JQ1 was efficient in reducing the population of cancer stem cells from HNSCC xenografts.

Merkel Cell Polyomavirus Infection and Detection

Merkel cell polyomavirus (MCPyV) infection can lead to Merkel cell carcinoma (MCC), a highly aggressive form of skin cancer. Mechanistic studies to fully investigate MCPyV molecular biology and oncogenic mechanisms have been hampered by a lack of adequate cell culture models. Here, we describe a set of protocols for performing and detecting MCPyV infection of primary human skin cells. The protocols describe the isolation of human dermal fibroblasts, preparation of recombinant MCPyV virions, and detection of virus infection by both immunofluorescent (IF) staining and in situ DNA-hybridization chain reaction (HCR), which is a highly sensitive fluorescence in situ hybridization (FISH) approach. The protocols herein can be adapted by interested researchers to identify other cell types or cell lines that support MCPyV infection. The described FISH approach could also be adapted for detecting low levels of viral DNAs present in the infected human skin.

BRD4 and Cancer: going beyond transcriptional regulation

BRD4, member of the Bromodomain and Extraterminal (BET) protein family, is largely acknowledged in cancer for its role in super-enhancers (SEs) organization and oncogenes expression regulation. Inhibition of BRD4 shortcuts the communication between SEs and target promoters with a subsequent cell-specific repression of oncogenes to which cancer cells are addicted and cell death. To date, this is the most credited mechanism of action of BET inhibitors, a class of small molecules targeting BET proteins which are currently in clinical trials in several cancer settings.

However, recent evidence indicates that BRD4 relevance in cancer goes beyond its role in transcription regulation and identifies this protein as a keeper of genome stability.

Indeed, a non-transcriptional role of BRD4 in controlling DNA damage checkpoint activation and repair as well as telomere maintenance has been proposed, throwing new lights into the multiple functions of this protein and opening new perspectives on the use of BETi in cancer. Here we discuss the current available information on non-canonical, non-transcriptional functions of BRD4 and on their implications in cancer biology. Integrating this information with the already known BRD4 role in gene expression regulation, we propose a “common” model to explain BRD4 genomic function. Furthermore, in light of the transversal function of BRD4, we provide new interpretation for the cytotoxic activity of BETi and we discuss new possibilities for a wide and focused employment of these drugs in clinical settings.

Inhibition of BET selectively eliminates undifferentiated pluripotent stem cells

Embryonic stem cells (ESCs) maintain their cellular identity through the systematic regulation of master transcription factors and chromatin remodeling complexes. Recent work has shown that the unusually large-scale enhancers-namely super-enhancers (SEs), on which BRD4, a member of the bromodomain and extraterminal domain (BET) family is highly enriched-could regulate pluripotency-related transcription factors. Moreover, inhibition of BRD4 binding on SEs has been shown to induce the differentiation of ESCs. However, the underlying mechanism of BRD4 inhibition-mediated stem cell differentiation remains elusive. Here we show that both mouse and human ESCs lose their capacity for self-renewal upon treatment with JQ1, a selective inhibitor of BET family including BRD4, with rapid suppression of pluripotency-associated genes. Notably, a high concentration of JQ1 could selectively eliminate ESCs via apoptosis, without affecting the functionality of differentiated somatic cells from ESCs, suggesting that inhibition of BET may have a beneficial effect on the development of pluripotent stem cell-based cell therapy.

Lyar-Mediated Recruitment of Brd2 to the Chromatin Attenuates Nanog Downregulation Following Induction of Differentiation

During development, cellular differentiation programs need tight regulation for proper display of the activity of multiple factors in time and space. Chromatin adaptors of the BET family (Brd2, Brd3, Brd4 and Brdt in vertebrates) are transcription co-regulators tightly associated with the progression of the cell cycle. A key question regarding their function is whether they work as part of the general transcription machinery or, on the contrary, they are precisely recruited to the chromatin through specific transcription factors. Here, we report the selective recruitment of Brd2 to the chromatin by the transcription factor Lyar. We show that Lyar downregulation results in Brd2 dissociation from a number of promoters studied. On the contrary, dissociation of BET proteins from the chromatin has no effect on Lyar occupancy. Under differentiation conditions, the absence of Lyar leads to impaired downregulation of the pluripotency gene Nanog, with concomitant reduction in the upregulation of differentiation markers. Interestingly, following the induction of differentiation, Brd2 depletion exhibits the same effects as expressing a truncated Lyar molecule lacking the Brd2 interacting domain. Both approaches result in stronger Nanog repression, indicating that Lyar-mediated recruitment of Brd2 moderates Nanog downregulation when differentiation is triggered. Moreover, expression of truncated Lyar leads to impaired differentiation and increased apoptosis. Thus, Lyar-mediated recruitment of Brd2 would participate in preserving a proper timing for Nanog silencing ensuring the appropriate establishment of the differentiation program.

Targeting bromodomain and extraterminal proteins in breast cancer

Breast cancer is a collection of distinct tumor subtypes that are driven by unique gene expression profiles. These transcriptomes are controlled by various epigenetic marks that dictate which genes are expressed and suppressed. During carcinogenesis, extensive restructuring of the epigenome occurs, including aberrant acetylation, alteration of methylation patterns, and accumulation of epigenetic readers at oncogenes. As epigenetic alterations are reversible, epigenome-modulating drugs could provide a mechanism to silence numerous oncogenes simultaneously. Here, we review the impact of inhibitors of the Bromodomain and Extraterminal (BET) family of epigenetic readers in breast cancer. These agents, including the prototypical BET inhibitor JQ1, have been shown to suppress a variety of oncogenic pathways while inducing minimal, if any, toxicity in models of several subtypes of breast cancer. BET inhibitors also synergize with multiple approved anti-cancer drugs, providing a greater response in breast cancer cell lines and mouse models than either single agent. The combined findings of the studies discussed here provide an excellent rationale for the continued investigation of the utility of BET inhibitors in breast cancer.

Merkel Cell Polyomavirus Infection of Animal Dermal Fibroblasts

Merkel cell polyomavirus (MCPyV) is the first polyomavirus to be associated with human cancer. Mechanistic studies attempting to fully elucidate MCPyV's oncogenic mechanisms have been hampered by the lack of animal models for MCPyV infection. In this study, we examined the ability of MCPyV-GFP pseudovirus (containing a green fluorescent protein [GFP] reporter construct), MCPyV recombinant virions, and several MCPyV chimeric viruses to infect dermal fibroblasts isolated from various model animals, including mouse (Mus musculus), rabbit (Oryctolagus cuniculus), rat (Rattus norvegicus), chimpanzee (Pan troglodytes), rhesus macaque (Macaca mulatta), patas monkey (Erythrocebus patas), common woolly monkey (Lagothrix lagotricha), red-chested mustached tamarin (Saguinus labiatus), and tree shrew (Tupaia belangeri). We found that MCPyV-GFP pseudovirus was able to enter the dermal fibroblasts of all species tested. Chimpanzee dermal fibroblasts were the only type that supported vigorous MCPyV gene expression and viral replication, and they did so to a level beyond that of human dermal fibroblasts. We further demonstrated that both human and chimpanzee dermal fibroblasts produce infectious MCPyV virions that can successfully infect new cells. In addition, rat dermal fibroblasts supported robust MCPyV large T antigen expression after infection with an MCPyV chimeric virus in which the entire enhancer region of the MCPyV early promoter has been replaced with the simian virus 40 (SV40) analog. Our results suggest that viral transcription and/or replication events represent the major hurdle for MCPyV cross-species transmission. The capacity of rat dermal fibroblasts to support MCPyV early gene expression suggests that the rat is a candidate model organism for studying viral oncogene function during Merkel cell carcinoma (MCC) oncogenic progression.IMPORTANCE MCPyV plays an important role in the development of a highly aggressive form of skin cancer, Merkel cell carcinoma (MCC). With the increasing number of MCC diagnoses, there is a need to better understand the virus and its oncogenic potential. However, studies attempting to fully elucidate MCPyV's oncogenic mechanisms have been hampered by the lack of animal models for MCPyV infection. To pinpoint the best candidate for developing an MCPyV infection animal model, we examined MCPyV's ability to infect dermal fibroblasts isolated from various established model animals. Of the animal cell types we tested, chimpanzee dermal fibroblasts were the only isolates that supported the full MCPyV infectious cycle. To overcome the infection blockade in the other model animals, we constructed chimeric viruses that achieved robust MCPyV entry and oncogene expression in rat fibroblasts. Our results suggest that the rat may serve as an in vivo model to study MCV oncogenesis.