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Antiviral drug discovery aims to identify novel agents that inhibit viral replication or reduce expression of key pathogenic viral proteins while minimizing adverse effects. Target-based screening has been employed extensively to identify compounds targeting both viral and host proteins. While phenotypic screening has successfully identified antiviral compounds, determining each compound's target is challenging. Approximately 240 million individuals worldwide are chronically infected with hepatitis B virus (HBV), and more than 650 000 people die per year from HBV-associated liver diseases. The hallmarks of chronic HBV infection are high viral load (HBV DNA) and higher levels of non-infectious particles containing the tolerogenic viral S antigen (HBsAg). The current standard of care effectively reduces viremia, but rarely results in a functional cure, defined as sustained HBsAg loss. Recently, a novel, potent, small-molecule inhibitor of HBV gene expression (RG7834) was discovered from a phenotypic screen focused on identifying HBsAg production inhibitors. Target identification efforts led to the discovery of the non-canonical poly(A) RNA polymerases PAPD5 and PAPD7, which are required for HBV RNA stabilization, and are effectively antagonized by RG7834. Thus, we highlight the powerful approach of phenotypic screening for identifying novel antiviral drugs and unraveling key biological mechanisms important for the viral lifecycle.

GRAPHICAL ABSTRACTSUMMARYA hallmark of chronic hepatitis B virus (CHB) infection is the presence of high circulating levels of non-infectious small lipid HBV surface antigen (HBsAg) vesicles. Although rare, sustained HBsAg loss is the idealized endpoint of any CHB therapy. A novel small molecule RG7834 has been previously reported to inhibit HBsAg expression by targeting terminal nucleotidyltransferase protein 4A and 4B (TENT4A and TENT4B). In this study, we describe a genome-wide CRISPR screen to identify other potential novel host factors required for HBsAg expression and to gain further insights into the mechanism of RG7834. We report more than 60 genes involved in regulating HBsAg and identified novel factors involved in RG7834 activity, including a zinc finger CCHC-type containing 14 (ZCCHC14) protein. We show that ZCCHC14, together with TENT4A/B, stabilizes HBsAg expression through HBV RNA tailing, providing a potential new therapeutic target to achieve functional cure in CHB patients.

Hepatitis E virus (HEV) is a 7.2 kb positive-sense, single-stranded RNA virus containing three partially overlapping reading frames, ORF 1-3. All non-structural proteins required for viral replication are encoded by ORF1 and are transcribed as a single transcript. Computational analysis of the complete ORF1 polyprotein identified a previously uncharacterized region of predicted secondary structure bordered by two disordered regions coinciding partially with a region predicted as a putative cysteine protease. Following successful cloning, expression and purification of this region, the crystal structure of the identified protein was determined and identified to have considerable structural homology to a fatty acid binding domain. Further analysis of the structure revealed a metal binding site, shown unambiguously to specifically bind zinc via a non-classical, potentially catalytic zinc-binding motif. We present analysis for the first time of this identified non-structural protein, expanding the knowledge and understanding of the complex mechanisms of HEV biology.ImportanceHepatitis E virus (HEV) is an emerging virus found predominately in developing countries causing an estimated 20 million infections, which result in approximately 57,000 deaths a year. Although it is known that the non-structural proteins of the HEV ORF1 are expressed as a single transcript, there is debate as to whether ORF1 functions as a single polyprotein or if it is processed into separate domains via a viral or endogenous cellular protease. In the following paper, we present the first structural and biophysical characterization of a HEV non-structural protein using a construct that has partially overlapping boundaries with the predicted putative cysteine protease. Based on the structural homology of the HEV protein with known structures, along with the presence of a catalytic zinc-binding motif, it is possible that the identified protein corresponds to the HEV protease, which could require activation or repression through the binding of a fatty acid. This represents a significant step forward in the characterization and the understanding of the molecular mechanisms of the HEV genome.

Covalently closed circular DNA (cccDNA) forms the basis for replication and persistence of hepatitis B virus (HBV) in the chronically infected liver. We have previously shown that viral transcription is subject to regulation by posttranslational modifications (PTMs) of histone proteins bound to cccDNA through analysis of de novo HBV-infected cell lines. We now report the successful adaptation of this chromatin immunoprecipitation sequencing (ChIPseq) approach for analysis of fine-needle patient liver biopsy specimens to investigate the role of histone PTMs in chronically HBV-infected patients. Using 18 specimens from patients in different stages of chronic HBV infection, our work shows that the profile of histone PTMs in chronic infection is more nuanced than previously observed in in vitro models of acute infection. In line with our previous findings, we find that the majority of HBV-derived sequences are associated with the activating histone PTM H3K4me3. However, we show a striking interpatient variability of its deposition in this patient cohort correlated with viral transcription and patient HBV early antigen (HBeAg) status. Unexpectedly, we detected deposition of the classical inhibitory histone PTM H3K9me3 on HBV-DNA in around half of the patient biopsy specimens, which could not be linked to reduced levels of viral transcripts. Our results show that current in vitro models are unable to fully recapitulate the complex epigenetic landscape of chronic HBV infection observed in vivo and demonstrate that fine-needle liver biopsy specimens can provide sufficient material to further investigate the interaction of viral and host proteins on HBV-DNA. IMPORTANCE Hepatitis B virus (HBV) is a major global health concern, chronically infecting millions of patients and contributing to a rising burden of liver disease. The viral genome forms the basis for chronic infection and has been shown to be subject to regulation by epigenetic mechanisms, such as posttranslational modification of histone proteins. Here, we confirm and expand on previous results by adapting a high-resolution technique for analysis of histone modifications for use with patient-derived fine-needle liver biopsy specimens. Our work highlights that the situation in vivo is more complex than predicted by current in vitro models, for example, by suggesting a novel, noncanonical role of the histone modification H3K9me3 in the HBV life cycle. Importantly, enabling the use of fine-needle liver biopsy specimens for such high-resolution analyses may facilitate further research into the epigenetic regulation of the HBV genome.

Summary: A hallmark of chronic hepatitis B (CHB) virus infection is the presence of high circulating levels of non-infectious small lipid HBV surface antigen (HBsAg) vesicles. Although rare, sustained HBsAg loss is the idealized endpoint of any CHB therapy. A small molecule, RG7834, has been previously reported to inhibit HBsAg expression by targeting terminal nucleotidyltransferase proteins 4A and 4B (TENT4A and TENT4B). In this study, we describe a genome-wide CRISPR screen to identify other potential host factors required for HBsAg expression and to gain further insights into the mechanism of RG7834. We report more than 60 genes involved in regulating HBsAg and identify additional factors involved in RG7834 activity, including a zinc finger CCHC-type containing 14 (ZCCHC14) protein. We show that ZCCHC14, together with TENT4A/B, stabilizes HBsAg expression through HBV RNA tailing, providing a potential new therapeutic target to achieve functional cure in CHB patients. : Hyrina et al. employ a non-biased functional CRISPR screening approach to identify host factors regulating HBsAg expression as well as those targeted by RG7834, a HBsAg inhibitor. The screen highlighted over 60 genes and identified a mechanism by which ZCCHC14, together with TENT4A/B, stabilizes HBsAg expression through HBV RNA tailing. Keywords: HBV, HBsAg, CRISPR, genome-wide screen, RG7834, ZCCHC14, TENT4B, RNA tailing

Latency-associated nuclear antigen (LANA), a multifunctional protein expressed by the Kaposi sarcoma-associated herpesvirus (KSHV) in latently infected cells, is required for stable maintenance of the viral episome. This is mediated by two interactions: LANA binds to specific sequences (LBS1 and LBS2) on viral DNA and also engages host histones, tethering the viral genome to host chromosomes in mitosis. LANA has also been suggested to affect host gene expression, but both the mechanism(s) and role of this dysregulation in KSHV biology remain unclear. Here, we have examined LANA interactions with host chromatin on a genome-wide scale using chromatin immunoprecipitation with high-throughput sequencing (ChIP-seq) and show that LANA predominantly targets human genes near their transcriptional start sites (TSSs). These host LANA-binding sites are generally found within transcriptionally active promoters and display striking overrepresentation of a consensus DNA sequence virtually identical to the LANA-binding site 1 (LBS1) motif in KSHV DNA. Comparison of the ChIP-seq profile with whole-transcriptome (high-throughput sequencing of RNA transcripts [RNA-seq]) data reveals that few of the genes that are differentially regulated in latent infection are occupied by LANA at their promoters. This suggests that direct LANA binding to promoters is not the prime determinant of altered host transcription in KSHV-infected cells. Most surprisingly, the association of LANA to both host and viral DNA is strongly disrupted during the lytic cycle of KSHV. This disruption can be prevented by the inhibition of viral DNA synthesis, suggesting the existence of novel and potent regulatory mechanisms linked to either viral DNA replication or late gene expression. IMPORTANCE Here, we employ complementary genome-wide analyses to evaluate the distribution of the highly abundant latency-associated nuclear antigen, LANA, on the host genome and its impact on host gene expression during KSHV latent infection. Combined, ChIP-seq and RNA-seq reveal that LANA accumulates at active gene promoters that harbor specific short DNA sequences that are highly reminiscent of its cognate binding sites in the virus genome. Unexpectedly, we found that such association does not lead to remodeling of global host transcription during latency. We also report for the first time that LANA's ability to bind host and viral chromatin is highly dynamic and is disrupted in cells undergoing an extensive lytic reactivation. This therefore suggests that the association of LANA to chromatin during a productive infection cycle is controlled by a new regulatory mechanism.

Depending on age of acquisition, hepatitis B virus (HBV) can induce a cell-mediated immune response that results in either cure or progressive liver injury. In adult-acquired infection, HBV antigens are usually cleared, whereas in infancy-acquired infection, they persist. Individuals infected during infancy therefore represent the majority of patients chronically infected with HBV (CHB). A therapy that can promote viral antigen clearance in most CHB patients has not been developed and would represent a major health care advance and cost mitigator. Using an age-dependent mouse model of HBV clearance and persistence in conjunction with human blood and liver tissue, we studied mechanisms of viral clearance to identify new therapeutic targets. We demonstrate that age-dependent expression of the costimulatory molecule OX40 ligand (OX40L) by hepatic innate immune cells is pivotal in determining HBV immunity, and that treatment with OX40 agonists leads to improved HBV antigen clearance in young mice, as well as increased strength of T cell responses in young mice and adult mice that were exposed to HBV when they were young and developed a CHB serological profile. Similarly, in humans, we show that hepatic OX40L transcript expression is age-dependent and that increased OX40 expression on peripheral CD4+ T cells in adults is associated with HBV clearance. These findings provide new mechanistic understanding of the immune pathways and cells necessary for HBV immunity and identify potential therapeutic targets for resolving CHB.

Mutations in human (Homo sapiens) ETHYLMALONIC ENCEPHALOPATHY PROTEIN1 (ETHE1) result in the complex metabolic disease ethylmalonic encephalopathy, which is characterized in part by brain lesions, lactic acidemia, excretion of ethylmalonic acid, and ultimately death. ETHE1-like genes are found in a wide range of organisms; however, the biochemical and physiological role(s) of ETHE1 have not been examined outside the context of ethylmalonic encephalopathy. In this study we characterized Arabidopsis (Arabidopsis thaliana) ETHE1 and determined the effect of an ETHE1 loss-of-function mutation to investigate the role(s) of ETHE1 in plants. Arabidopsis ETHE1 is localized in the mitochondrion and exhibits sulfur dioxygenase activity. Seeds homozygous for a DNA insertion in ETHE1 exhibit alterations in endosperm development that are accompanied by a delay in embryo development followed by embryo arrest by early heart stage. Strong ETHE1 labeling was observed in the peripheral and chalazal endosperm of wild-type seeds prior to cellularization. Therefore, ETHE1 appears to play an essential role in regulating sulfide levels in seeds.

ETHE1 (ethylmalonic encephalopathy protein 1) is a beta-lactamase fold-containing protein that is essential for the survival of a range of organisms. In spite of the apparent importance of this enzyme, very little is known about its function or biochemical properties. In this study Arabidopsis ETHE1 was over-expressed and purified and shown to bind tightly to 1.2+/-0.2 equivalents of iron. (1)H NMR and EPR studies demonstrate that the predominant oxidation state of Fe in ETHE1 is Fe(II), and NMR studies confirm that two histidines are bound to Fe(II). EPR studies show that there is no antiferromagnetically coupled Fe(III)Fe(II) center in ETHE1. Gel filtration studies reveal that ETHE1 is a dimer in solution, which is consistent with previous crystallographic studies. Although very similar in terms of amino acid sequence to glyoxalase II, ETHE1 exhibits no thioester hydrolase activity, and activity screening assays reveal that ETHE1 exhibits low level esterase activity. Taken together, ETHE1 is a novel, mononuclear Fe(II)-containing member of the beta-lactamase fold superfamily.

Chronic hepatitis B virus (HBV) infection affects 240 million people worldwide and is a major risk factor for liver failure and hepatocellular carcinoma. Current antiviral therapy inhibits cytoplasmic HBV genomic replication, but is not curative because it does not directly affect nuclear HBV closed circular DNA (cccDNA), the genomic form that templates viral transcription and sustains viral persistence. Novel approaches that directly target cccDNA regulation would therefore be highly desirable. cccDNA is assembled with cellular histone proteins into chromatin, but little is known about the regulation of HBV chromatin by histone posttranslational modifications (PTMs). Here, using a new cccDNA ChIP-Seq approach, we report, to our knowledge, the first genome-wide maps of PTMs in cccDNA-containing chromatin from de novo infected HepG2 cells, primary human hepatocytes, and from HBV-infected liver tissue. We find high levels of PTMs associated with active transcription enriched at specific sites within the HBV genome and, surprisingly, very low levels of PTMs linked to transcriptional repression even at silent HBV promoters. We show that transcription and active PTMs in HBV chromatin are reduced by the activation of an innate immunity pathway, and that this effect can be recapitulated with a small molecule epigenetic modifying agent, opening the possibility that chromatin-based regulation of cccDNA transcription could be a new therapeutic approach to chronic HBV infection.

Productive herpesvirus infection requires a profound, time-controlled remodeling of the viral transcriptome and proteome. To gain insights into the genomic architecture and gene expression control in Kaposi's sarcoma-associated herpesvirus (KSHV), we performed a systematic genome-wide survey of viral transcriptional and translational activity throughout the lytic cycle. Using mRNA-sequencing and ribosome profiling, we found that transcripts encoding lytic genes are promptly bound by ribosomes upon lytic reactivation, suggesting their regulation is mainly transcriptional. Our approach also uncovered new genomic features such as ribosome occupancy of viral non-coding RNAs, numerous upstream and small open reading frames (ORFs), and unusual strategies to expand the virus coding repertoire that include alternative splicing, dynamic viral mRNA editing, and the use of alternative translation initiation codons. Furthermore, we provide a refined and expanded annotation of transcription start sites, polyadenylation sites, splice junctions, and initiation/termination codons of known and new viral features in the KSHV genomic space which we have termed KSHV 2.0. Our results represent a comprehensive genome-scale image of gene regulation during lytic KSHV infection that substantially expands our understanding of the genomic architecture and coding capacity of the virus., Author Summary Kaposi's sarcoma-associated herpesvirus (KSHV) is a cancer-causing agent in immunocompromised patients that establishes long-lasting infections in its hosts. Initially described in 1994 and extensively studied ever since, KSHV molecular biology is understood in broad outline, but many detailed questions are still to be resolved. After almost two decades, specific aspects pertaining to the organization of the KSHV genome as well as the fate of the viral transcripts during the productive stages of infection remain unexplored. Here we use a systematic genome-wide approach to investigate changes in gene and protein expression during the productive stage of infection known as the lytic cycle. We found that the viral genome has a large coding capacity, capable of generating at least 45% more products than initially anticipated by bioinformatic analyses alone, and that it uses multiple strategies to expand its coding capacity well beyond what is determined solely by the DNA sequence of its genome. We also provide an expanded and highly detailed annotation of known and new genomic features in KSHV. We have termed this new architectural and functional annotation KSHV 2.0. Our results indicate that viral genomes are more complex than anticipated, and that they are subject to tight mechanisms of regulation to ensure correct gene expression.

The human CCCTC-binding factor, CTCF, regulates transcription of the double-stranded DNA genomes of herpesviruses. The architectural complex cohesin and RNA Polymerase II also contribute to this organization. We profiled the occupancy of CTCF, cohesin, and RNA Polymerase II on the episomal genome of the Epstein–Barr virus in a cell culture model of latent infection. CTCF colocalizes with cohesin but not RNA Polymerase II. CTCF and cohesin bind specific sequences throughout the genome that are found not just proximal to the regulatory elements of latent genes, but also near lytic genes. In addition to tracking with known transcripts, RNA Polymerase II appears at two unannotated positions, one of which lies within the latent origin of replication. The widespread occupancy profile of each protein reveals binding near or at a myriad of regulatory elements and suggests context-dependent functions.

The protein product of gene At1g53580 from Arabidopsis thaliana possesses 54% sequence identity to a human enzyme that has been implicated in the rare disorder ethylmalonic encephalopathy. The structure of the At1g53580 protein has been solved to a nominal resolution of 1.48 Angstrom. This structure reveals tertiary structure differences between the ETHE1-like enzyme and glyoxalase II enzymes that are likely to account for differences in reaction chemistry and multimeric state between the two types of enzymes. In addition, the Arabidopsis ETHE1 protein is used as a model to explain the significance of several mutations in the human enzyme that have been observed in patients with ethylmalonic encephalopathy.