miRNA independent hepacivirus variants suggest a strong evolutionary pressure to maintain miR-122 dependence

The emerging role of miRNA-122 in infectious diseases: Mechanisms and potential biomarkers

microRNAs (miRNAs) are small, non-coding RNA molecules that play crucial regulatory roles in numerous cellular processes. Recent investigations have highlighted the significant involvement of miRNA-122 (miR-122) in the pathogenesis of infectious diseases caused by diverse pathogens, encompassing viral, bacterial, and parasitic infections. In the context of viral infections, miR-122 exerts regulatory control over viral replication by binding to the viral genome and modulating the host's antiviral response. For instance, in hepatitis B virus (HBV) infection, miR-122 restricts viral replication, while HBV, in turn, suppresses miR-122 expression. Conversely, miR-122 interacts with the hepatitis C virus (HCV) genome, facilitating viral replication. Regarding bacterial infections, miR-122 has been found to regulate host immune responses by influencing inflammatory cytokine production and phagocytosis. In Vibrio anguillarum infections, there is a significant reduction in miR-122 expression, contributing to the pathophysiology of bacterial infections. Toll-like receptor 14 (TLR14) has been identified as a novel target gene of miR-122, affecting inflammatory and immune responses. In the context of parasitic infections, miR-122 plays a crucial role in regulating host lipid metabolism and immune responses. For example, during Leishmania infection, miR-122-containing extracellular vesicles from liver cells are unable to enter infected macrophages, leading to a suppression of the inflammatory response. Furthermore, miR-122 exhibits promise as a potential biomarker for various infectious diseases. Its expression level in body fluids, particularly in serum and plasma, correlates with disease severity and treatment response in patients affected by HCV, HBV, and tuberculosis. This paper also discusses the potential of miR-122 as a biomarker in infectious diseases. In summary, this review provides a comprehensive and insightful overview of the emerging role of miR-122 in infectious diseases, detailing its mechanism of action and potential implications for the development of novel therapeutic strategies.

The RNA Interference Effector Protein Argonaute 2 Functions as a Restriction Factor Against SARS-CoV-2

Argonaute 2 (Ago2) is a key component of the RNA interference (RNAi) pathway, a gene-regulatory system that is present in most eukaryotes. Ago2 uses microRNAs (miRNAs) and small interfering RNAs (siRNAs) for targeting to homologous mRNAs which are then degraded or translationally suppressed. In plants and invertebrates, the RNAi pathway has well-described roles in antiviral defense, but its function in limiting viral infections in mammalian cells is less well understood. Here, we examined the role of Ago2 in replication of the betacoronavirus SARS-CoV-2, the etiologic agent of COVID-19. Microscopic analyses of infected cells revealed that a pool of Ago2 closely associates with viral replication sites and gene ablation studies showed that loss of Ago2 resulted in over 1,000-fold increase in peak viral titers. Replication of the alphacoronavirus 229E was also significantly increased in cells lacking Ago2. The antiviral activity of Ago2 was dependent on both its ability to bind small RNAs and its endonuclease function. Interestingly, in cells lacking Dicer, an upstream component of the RNAi pathway, viral replication was the same as in parental cells. This suggests that the antiviral activity of Ago2 is independent of Dicer processed miRNAs. Deep sequencing of infected cells by other groups identified several SARS-CoV-2-derived small RNAs that bind to Ago2. A mutant virus lacking the most abundant ORF7A-derived viral miRNA was found to be significantly less sensitive to Ago2-mediated restriction. This combined with our findings that endonuclease and small RNA-binding functions of Ago2 are required for its antiviral function, suggests that Ago2-small viral RNA complexes target nascent viral RNA produced at replication sites for cleavage. Further studies are required to elucidate the processing mechanism of the viral small RNAs that are used by Ago2 to limit coronavirus replication.

Enhanced Virus Translation Enables miR-122-Independent Hepatitis C Virus Propagation

The 5' untranslated region (UTR) of the hepatitis C virus (HCV) genome forms RNA structures that regulate virus replication and translation. The region contains an internal ribosomal entry site (IRES) and a 5'-terminal region. Binding of the liver-specific microRNA (miRNA) miR-122 to two binding sites in the 5'-terminal region regulates viral replication, translation, and genome stability and is essential for efficient virus replication, but its precise mechanism of action is still unresolved. A current hypothesis is that miR-122 binding stimulates viral translation by facilitating the viral 5' UTR to form the translationally active HCV IRES RNA structure. While miR-122 is essential for detectable replication of wild-type HCV genomes in cell culture, several viral variants with 5' UTR mutations exhibit low-level replication in the absence of miR-122. We show that HCV mutants capable of replicating independently of miR-122 display an enhanced translation phenotype that correlates with their ability to replicate independently of miR-122. Further, we provide evidence that translation regulation is the major role for miR-122 and show that miR-122-independent HCV replication can be rescued to miR-122-dependent levels by the combined impacts of 5' UTR mutations that stimulate translation and by stabilizing the viral genome by knockdown of host exonucleases and phosphatases that degrade the genome. Finally, we show that HCV mutants capable of replicating independently of miR-122 also replicate independently of other microRNAs generated by the canonical miRNA synthesis pathway. Thus, we provide a model suggesting that translation stimulation and genome stabilization are the primary roles for miR-122 in promoting HCV. IMPORTANCE The unusual and essential role of miR-122 in promoting HCV propagation is incompletely understood. To better understand its role, we have analyzed HCV mutants capable of replicating independently of miR-122. Our data show that the ability of viruses to replicate independently of miR-122 correlates with enhanced virus translation but that genome stabilization is required to restore efficient HCV replication. This suggests that viruses must gain both abilities to escape the need for miR-122 and impacts the possibility that HCV can evolve to replicate outside the liver.

Dose-Dependent Hepacivirus Infection Reveals Linkage between Infectious Dose and Immune Response

More than 70 million people worldwide are still infected with the hepatitis C virus 30 years after its discovery, underscoring the need for a vaccine. To develop an effective prophylactic vaccine, detailed knowledge of the correlates of protection and an immunocompetent surrogate model are needed. In this study, we describe the minimum dose required for robust equine hepacivirus (EqHV) infection in equids and examined how this relates to duration of infection, seroconversion, and transcriptomic responses. To investigate mechanisms of hepaciviral persistence, immune response, and immune-mediated pathology, we inoculated eight EqHV naive horses with doses ranging from 1-2 copies to 1.3 × 106 RNA copies per inoculation. We characterized infection kinetics, pathology, and transcriptomic responses via next generation sequencing. The minimal infectious dose of EqHV in horses was estimated at 13 RNA copies, whereas 6 to 7 copies were insufficient to cause infection. Peak viremia did not correlate with infectious dose, while seroconversion and duration of infection appeared to be affected. Notably, seroconversion was undetectable in the low-dose infections within the surveillance period (40 to 50 days). In addition, transcriptomic analysis revealed a nearly dose-dependent effect, with greater immune activation and inflammatory response observed in high-dose infections than in low-dose infections. Interestingly, inoculation with 6-7 copies of RNA that did not result in productive infection, but was associated with a strong immune response, similar to that observed in the high-dose infections. IMPORTANCE We demonstrate that the EqHV dose of infection plays an important role for inducing immune responses, possibly linked to early clearance in high-dose and prolonged viremia in low-dose infections. In particular, pathways associated with innate and adaptive immune responses, as well as inflammatory responses, were more strongly upregulated in high-dose infections than in lower doses. Hence, inoculation with low doses may enable EqHV to evade strong immune responses in the early phase and therefore promote robust, long-lasting infection.

Equine Hepacivirus: A Systematic Review and a Meta-Analysis of Serological and Biomolecular Prevalence and a Phylogenetic Update

Viral hepatitis has recently assumed relevance for equine veterinary medicine since a variety of new viruses have been discovered. Equine Hepacivirus (EqHV) is an RNA virus belonging to the Flaviviridae family that can cause subclinical hepatitis in horses, occasionally evolving into a chronic disease. EqHV, to date, is considered the closest known relative of human HCV. EqHV has been reported worldwide therefore assessing its features is relevant, considering both the wide use of blood products and transfusions in veterinary therapies and its similitude to HCV. The present review resumes the actual knowledge on EqHV epidemiology, risk factors and immunology, together with potential diagnostics and good practices for prevention. Moreover, adhering to PRISMA guidelines for systematic reviews a meta-analysis of serological and biomolecular prevalence and an updated phylogenetic description is presented as a benchmark for further studies.

MicroRNA-122 Regulation of HCV Infections: Insights from Studies of miR-122-Independent Replication

Despite the advancement in antiviral therapy, Hepatitis C remains a global health challenge and one of the leading causes of hepatitis related deaths worldwide. Hepatitis C virus, the causative agent, is a positive strand RNA virus that requires a liver specific microRNA called miR-122 for its replication. Unconventional to the canonical role of miRNAs in translation suppression by binding to 3'Untranslated Region (UTR) of messenger RNAs, miR-122 binds to two sites on the 5'UTR of viral genome and promotes viral propagation. In this review, we describe the unique relationship between the liver specific microRNA and HCV, the current knowledge on the mechanisms by which the virus uses miR-122 to promote the virus life cycle, and how miR-122 impacts viral tropism and pathogenesis. We will also discuss the use of anti-miR-122 therapy and its impact on viral evolution of miR-122-independent replication. This review further provides insight into how viruses manipulate host factors at the initial stage of infection to establish a successful infection.

Intra-host analysis of hepaciviral glycoprotein evolution reveals signatures associated with viral persistence and clearance

Even 30 years after the discovery of the hepatitis C virus (HCV) in humans there is still no vaccine available. Reasons for this include the high mutation rate of HCV, which allows the virus to escape immune recognition and the absence of an immunocompetent animal model for vaccine development. Phylogenetically distinct hepaciviruses (genus Hepacivirus, family Flaviviridae) have been isolated from diverse species, each with a narrow host range: the equine hepacivirus (EqHV) is the closest known relative of HCV. In this study, we used amplicon-based deep-sequencing to investigate the viral intra-host population composition of the genomic regions encoding the surface glycoproteins E1 and E2. Patterns of E1E2 substitutional evolution were compared in longitudinally sampled EqHV-positive sera of naturally and experimentally infected horses and HCV-positive patients. Intra-host virus diversity was higher in chronically than in acutely infected horses, a pattern which was similar in the HCV-infected patients. However, overall glycoprotein variability was higher in HCV compared to EqHV. Additionally, selection pressure in HCV populations was higher, especially within the N-terminal region of E2, corresponding to the hypervariable region 1 (HVR1) in HCV. An alignment of glycoprotein sequences from diverse hepaciviruses identified the HVR1 as a unique characteristic of HCV: hepaciviruses from non-human species lack this region. Together, these data indicate that EqHV infection of horses could represent a powerful surrogate animal model to gain insights into hepaciviral evolution and HCVs HVR1-mediated immune evasion strategy.

Viral Causality of Human Cancer and Potential Roles of Human Endogenous Retroviruses in the Multi-Omics Era: An Evolutionary Epidemiology Review

Being responsible for almost 12% of cancers worldwide, viruses are among the oldest known and most prevalent oncogenic agents. The quality of the evidence for the in vivo tumorigenic potential of microorganisms varies, thus accordingly, viruses were classified in 4 evidence-based categories by the International Agency for Research on Cancer in 2009. Since then, our understanding of the role of viruses in cancer has significantly improved, firstly due to the emergence of high throughput sequencing technologies that allowed the “brute-force” recovery of unknown viral genomes. At the same time, multi-omics approaches unravelled novel virus-host interactions in stem-cell biology. We now know that viral elements, either exogenous or endogenous, have multiple sometimes conflicting roles in human pathophysiology and the development of cancer. Here we integrate emerging evidence on viral causality in human cancer from basic mechanisms to clinical studies. We analyze viral tumorigenesis under the scope of deep-in-time human-virus evolutionary relationships and critically comment on the evidence through the eyes of clinical epidemiology, firstly by reviewing recognized oncoviruses and their mechanisms of inducing tumorigenesis, and then by examining the potential role of integrated viruses in our genome in the process of carcinogenesis.

In the era of rapid mRNA-based vaccines: Why is there no effective hepatitis C virus vaccine yet?

Hepatitis C virus (HCV) is responsible for no less than 71 million people chronically infected and is one of the most frequent indications for liver transplantation worldwide. Despite direct-acting antiviral therapies fuel optimism in controlling HCV infections, there are several obstacles regarding treatment accessibility and reinfection continues to remain a possibility. Indeed, the majority of new HCV infections in developed countries occur in people who inject drugs and are more plausible to get reinfected. To achieve global epidemic control of this virus the development of an effective prophylactic or therapeutic vaccine becomes a must. The coronavirus disease 19 (COVID-19) pandemic led to auspicious vaccine development against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) virus, which has renewed interest on fighting HCV epidemic with vaccination. The aim of this review is to highlight the current situation of HCV vaccine candidates designed to prevent and/or to reduce HCV infectious cases and their complications. We will emphasize on some of the crossroads encountered during vaccine development against this insidious virus, together with some key aspects of HCV immunology which have, so far, hampered the progress in this area. The main focus will be on nucleic acid-based as well as recombinant viral vector-based vaccine candidates as the most novel vaccine approaches, some of which have been recently and successfully employed for SARS-CoV-2 vaccines. Finally, some ideas will be presented on which methods to explore for the design of live-attenuated vaccines against HCV.

Pathogenesis, MicroRNA-122 Gene-Regulation, and Protective Immune Responses After Acute Equine Hepacivirus Infection

BACKGROUND AND AIMS

Equine hepacivirus (EqHV) is phylogenetically the closest relative of HCV and shares genome organization, hepatotropism, transient or persistent infection outcome, and the ability to cause hepatitis. Thus, EqHV studies are important to understand equine liver disease and further as an outbred surrogate animal model for HCV pathogenesis and protective immune responses. Here, we aimed to characterize the course of EqHV infection and associated protective immune responses.

APPROACH AND RESULTS

Seven horses were experimentally inoculated with EqHV, monitored for 6 months, and rechallenged with the same and, subsequently, a heterologous EqHV. Clearance was the primary outcome (6 of 7) and was associated with subclinical hepatitis characterized by lymphocytic infiltrate and individual hepatocyte necrosis. Seroconversion was delayed and antibody titers waned slowly. Clearance of primary infection conferred nonsterilizing immunity, resulting in shortened duration of viremia after rechallenge. Peripheral blood mononuclear cell responses in horses were minimal, although EqHV-specific T cells were identified. Additionally, an interferon-stimulated gene signature was detected in the liver during EqHV infection, similar to acute HCV in humans. EqHV, as HCV, is stimulated by direct binding of the liver-specific microRNA (miR), miR-122. Interestingly, we found that EqHV infection sequesters enough miR-122 to functionally affect gene regulation in the liver. This RNA-based mechanism thus could have consequences for pathology.

CONCLUSIONS

EqHV infection in horses typically has an acute resolving course, and the protective immune response lasts for at least a year and broadly attenuates subsequent infections. This could have important implications to achieve the primary goal of an HCV vaccine; to prevent chronicity while accepting acute resolving infection after virus exposure.

Evolutionary selection of pestivirus variants with altered or no microRNA dependency

Host microRNA (miRNA) dependency is a hallmark of the human pathogen hepatitis C virus (HCV) and was also described for the related pestiviruses, which are important livestock pathogens. The liver-specific miR-122 binds within the HCV 5′ untranslated region (UTR), whereas the broadly expressed let-7 and miR-17 families bind two sites (S1 and S2, respectively) in the pestiviral 3′ UTR. Here, we dissected the mechanism of miRNA dependency of the pestivirus bovine viral diarrhea virus (BVDV). Argonaute 2 (AGO2) and miR-17 binding were essential for viral replication, whereas let-7 binding was mainly required for full translational efficiency. Furthermore, using seed site randomized genomes and evolutionary selection experiments, we found that tropism could be redirected to different miRNAs. AGO cross-linking and immunoprecipitation (CLIP) experiments and miRNA antagonism demonstrated that these alternative variants bound and depended on the corresponding miRNAs. Interestingly, we also identified miRNA-independent variants that were obtained through acquisition of compensatory mutations near the genomic 3′ terminus. Rescue experiments demonstrated that miRNA binding and 3′ mutagenesis contribute to replication through mutually exclusive mechanisms. Altogether, our findings suggest that pestiviruses, although capable of miRNA-independent replication, took advantage of miRNAs as essential host factors, suggesting a favorable path during evolutionary adaptation.

The Role of the Liver-Specific microRNA, miRNA-122 in the HCV Replication Cycle

Hepatitis C virus (HCV) replication requires annealing of a liver specific microRNA, miR-122 to 2 sites on 5' untranslated region (UTR). While, microRNAs downregulate gene expression by binding to the 3' untranslated region of the target mRNA, in this case, the microRNA anneals to the 5'UTR of the viral genomes and upregulates the viral lifecycle. In this review, we explore the current understandings of the mechanisms by which miR-122 promotes the HCV lifecycle, and its contributions to pathogenesis. Annealing of miR-122 has been reported to (a) stimulate virus translation by promoting the formation of translationally active internal ribosome entry site (IRES) RNA structure, (b) stabilize the genome, and (c) induce viral genomic RNA replication. MiR-122 modulates lipid metabolism and suppresses tumor formation, and sequestration by HCV may influence virus pathogenesis. We also discuss the possible use of miR-122 as a biomarker for chronic hepatitis and as a therapeutic target. Finally, we discuss roles for miR-122 and other microRNAs in promoting other viruses.

Animal Models of Hepatitis C Virus Infection

Hepatitis C virus (HCV) is an important and underreported infectious disease, causing chronic infection in ∼71 million people worldwide. The limited host range of HCV, which robustly infects only humans and chimpanzees, has made studying this virus in vivo challenging and hampered the development of a desperately needed vaccine. The restrictions and ethical concerns surrounding biomedical research in chimpanzees has made the search for an animal model all the more important. In this review, we discuss different approaches that are being pursued toward creating small animal models for HCV infection. Although efforts to use a nonhuman primate species besides chimpanzees have proven challenging, important advances have been achieved in a variety of humanized mouse models. However, such models still fall short of the overarching goal to have an immunocompetent, inheritably susceptible in vivo platform in which the immunopathology of HCV could be studied and putative vaccines development. Alternatives to overcome this include virus adaptation, such as murine-tropic HCV strains, or the use of related hepaciviruses, of which many have been recently identified. Of the latter, the rodent/rat hepacivirus from Rattus norvegicus species-1 (RHV-rn1) holds promise as a surrogate virus in fully immunocompetent rats that can inform our understanding of the interaction between the immune response and viral outcomes (i.e., clearance vs. persistence). However, further characterization of these animal models is necessary before their use for gaining new insights into the immunopathogenesis of HCV and for conceptualizing HCV vaccines.

Structure-function analysis of the equine hepacivirus 5' untranslated region highlights the conservation of translational mechanisms across the hepaciviruses

Equine hepacivirus (EHcV) (now also classified as hepacivirus A) is the closest genetic relative to hepatitis C virus (HCV) and is proposed to have diverged from HCV within the last 1000 years. The 5' untranslated regions (UTRs) of both HCV and EHcV exhibit internal ribosome entry site (IRES) activity, allowing cap-independent translational initiation, yet only the HCV 5'UTR has been systematically analysed. Here, we report a detailed structural and functional analysis of the EHcV 5'UTR. The secondary structure was determined using selective 2' hydroxyl acylation analysed by primer extension (SHAPE), revealing four stem-loops, termed SLI, SLIA, SLII and SLIII, by analogy to HCV. This guided a mutational analysis of the EHcV 5'UTR, allowing us to investigate the roles of the stem-loops in IRES function. This approach revealed that SLI was not required for EHcV IRES-mediated translation. Conversely, SLIII was essential, specifically SLIIIb, SLIIId and a GGG motif that is conserved across the Hepaciviridae. Further SHAPE analysis provided evidence that this GGG motif mediated interaction with the 40S ribosomal subunit, whilst a CUU sequence in the apical loop of SLIIIb mediated an interaction with eIF3. In addition, we showed that a microRNA122 target sequence located between SLIA and SLII mediated an enhancement of translation in the context of a subgenomic replicon. Taken together, these results highlight the conservation of hepaciviral translation mechanisms, despite divergent primary sequences.

miR-122, small RNA annealing and sequence mutations alter the predicted structure of the Hepatitis C virus 5' UTR RNA to stabilize and promote viral RNA accumulation

Annealing of the liver-specific microRNA, miR-122, to the Hepatitis C virus (HCV) 5′ UTR is required for efficient virus replication. By using siRNAs to pressure escape mutations, 30 replication-competent HCV genomes having nucleotide changes in the conserved 5′ untranslated region (UTR) were identified. In silico analysis predicted that miR-122 annealing induces canonical HCV genomic 5′ UTR RNA folding, and mutant 5′ UTR sequences that promoted miR-122-independent HCV replication favored the formation of the canonical RNA structure, even in the absence of miR-122. Additionally, some mutant viruses adapted to use the siRNA as a miR-122-mimic. We further demonstrate that small RNAs that anneal with perfect complementarity to the 5′ UTR stabilize and promote HCV genome accumulation. Thus, HCV genome stabilization and life-cycle promotion does not require the specific annealing pattern demonstrated for miR-122 nor 5′ end annealing or 3′ overhanging nucleotides. Replication promotion by perfect-match siRNAs was observed in Ago2 knockout cells revealing that other Ago isoforms can support HCV replication. At last, we present a model for miR-122 promotion of the HCV life cycle in which miRNA annealing to the 5′ UTR, in conjunction with any Ago isoform, modifies the 5′ UTR structure to stabilize the viral genome and promote HCV RNA accumulation.

TNRC6 proteins modulate hepatitis C virus replication by spatially regulating the binding of miR-122/Ago2 complexes to viral RNA

The liver-specific microRNA, miR-122, is an essential host factor for replication of the hepatitis C virus (HCV). miR-122 stabilizes the positive-strand HCV RNA genome and promotes its synthesis by binding two sites (S1 and S2) near its 5' end in association with Ago2. Ago2 is essential for both host factor activities, but whether other host proteins are involved is unknown. Using an unbiased quantitative proteomics screen, we identified the TNRC6 protein paralogs, TNRC6B and TNRC6C, as functionally important but redundant components of the miR-122/Ago2 host factor complex. Doubly depleting TNRC6B and TNRC6C proteins reduced HCV replication in human hepatoma cells, dampening miR-122 stimulation of viral RNA synthesis without reducing the stability or translational activity of the viral RNA. TNRC6B/C were required for optimal miR-122 host factor activity only when S1 was able to bind miR-122, and restricted replication when S1 was mutated and only S2 bound by miR-122. TNRC6B/C preferentially associated with S1, and TNRC6B/C depletion enhanced Ago2 association at S2. Collectively, these data suggest a model in which TNRC6B/C regulate the assembly of miR-122/Ago complexes on HCV RNA, preferentially directing miR-122/Ago2 to S1 while restricting its association with S2, thereby fine-tuning the spatial organization of miR-122/Ago2 complexes on the viral genome.

Evolutionary biology of human hepatitis viruses

Hepatitis viruses are major threats to human health. During the last decade, highly diverse viruses related to human hepatitis viruses were found in animals other than primates. Herein, we describe both surprising conservation and striking differences of the unique biological properties and infection patterns of human hepatitis viruses and their animal homologues, including transmission routes, liver tropism, oncogenesis, chronicity, pathogenesis and envelopment. We discuss the potential for translation of newly discovered hepatitis viruses into preclinical animal models for drug testing, studies on pathogenesis and vaccine development. Finally, we re-evaluate the evolutionary origins of human hepatitis viruses and discuss the past and present zoonotic potential of their animal homologues.

Functional Interplay between RNA Viruses and Non-Coding RNA in Mammals

Exploring virus–host interactions is key to understand mechanisms regulating the viral replicative cycle and any pathological outcomes associated with infection. Whereas interactions at the protein level are well explored, RNA interactions are less so. Novel sequencing methodologies have helped uncover the importance of RNA–protein and RNA–RNA interactions during infection. In addition to messenger RNAs (mRNAs), mammalian cells express a great number of regulatory non-coding RNAs, some of which are crucial for regulation of the immune system whereas others are utilized by viruses. It is thus becoming increasingly clear that RNA interactions play important roles for both sides in the arms race between virus and host. With the emerging field of RNA therapeutics, such interactions are promising antiviral targets. In this review, we discuss direct and indirect RNA interactions occurring between RNA viruses or retroviruses and host non-coding transcripts upon infection. In addition, we review RNA virus derived non-coding RNAs affecting immunological and metabolic pathways of the host cell typically to provide an advantage to the virus. The relatively few known examples of virus–host RNA interactions suggest that many more await discovery.

Virus discovery reveals frequent infection by diverse novel members of the Flaviviridae in wild lemurs

Lemurs are highly endangered mammals inhabiting the forests of Madagascar. In this study, we performed virus discovery on serum samples collected from 84 wild lemurs and identified viral sequence fragments from 4 novel viruses within the family Flaviviridae, including members of the genera Hepacivirus and Pegivirus. The sifaka hepacivirus (SifHV, two genotypes) and pegivirus (SifPgV, two genotypes) were discovered in the diademed sifaka (Propithecus diadema), while other pegiviral fragments were detected in samples from the indri (Indri indri, IndPgV) and the weasel sportive lemur (Lepilemur mustelinus, LepPgV). Although data are preliminary, each viral species appeared host species-specific and frequent infection was detected (18 of 84 individuals were positive for at least one virus). The complete coding sequence and partial 5' and 3' untranslated regions (UTRs) were obtained for SifHV and its genomic organization was consistent with that of other hepaciviruses, with one unique polyprotein and highly structured UTRs. Phylogenetic analyses showed the SifHV belonged to a clade that includes several viral species identified in rodents from Asia and North America, while SifPgV and IndPgV were more closely related to pegiviral species A and C, that include viruses found in humans as well as New- and Old-World monkeys. Our results support the current proposed model of virus-host co-divergence with frequent occurrence of cross-species transmission for these genera and highlight how the discovery of more members of the Flaviviridae can help clarify the ecology and evolutionary history of these viruses. Furthermore, this knowledge is important for conservation and captive management of lemurs.

Variation in the Untranslated Genome and Susceptibility to Infections

The clinical outcomes of infections are highly variable among individuals and are determined by complex host-pathogen interactions. Genome-wide association studies (GWAS) are powerful tools to unravel common genetic variations that are associated with disease risk and clinical outcomes. However, GWAS has only rarely revealed information on the exact genetic elements and their effects underlying an association because the majority of the hits are within non-coding regions. Some of the variants or the linked polymorphisms are now being discovered to have functional significance, such as regulatory elements in the promoter and enhancer regions or the microRNA binding sites in the 3′untranslated region of the protein-coding genes, which influence transcription, RNA stability, and translation of the protein-coding genes. However, only 3% of the entire transcriptome is protein-coding, signifying that non-coding RNAs represent most of the transcripts. Thus, a large portion of previously identified intergenic GWAS single nucleotide polymorphisms (SNPs) is in the non-coding RNAs. The non-coding RNAs form a large-scale regulatory network across the transcriptome, greatly expanding the complexity of gene regulation. Accumulating evidence also suggests that the “non-coding” genome regions actively regulate the highly dynamic three dimensional (3D) chromatin structures, which are critical for genome function. Epigenetic modulation like DNA methylation and histone modifications further affect chromatin accessibility and gene expression adding another layer of complexity to the functional interpretation of genetic variation associated with disease outcomes. We provide an overview of the current information on the influence of variation in these “untranslated” regions of the human genome on infectious diseases. The focus of this review is infectious disease-associated polymorphisms and gene regulatory mechanisms of pathophysiological relevance.

The Diverse Roles of microRNAs at the Host⁻Virus Interface

MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression at the post-transcriptional level. Through this activity, they are implicated in almost every cellular process investigated to date. Hence, it is not surprising that miRNAs play diverse roles in regulation of viral infections and antiviral responses. Diverse families of DNA and RNA viruses have been shown to take advantage of cellular miRNAs or produce virally encoded miRNAs that alter host or viral gene expression. MiRNA-mediated changes in gene expression have been demonstrated to modulate viral replication, antiviral immune responses, viral latency, and pathogenesis. Interestingly, viruses mediate both canonical and non-canonical interactions with miRNAs to downregulate specific targets or to promote viral genome stability, translation, and/or RNA accumulation. In this review, we focus on recent findings elucidating several key mechanisms employed by diverse virus families, with a focus on miRNAs at the host–virus interface during herpesvirus, polyomavirus, retroviruses, pestivirus, and hepacivirus infections.

RNA triphosphatase DUSP11 enables exonuclease XRN-mediated restriction of hepatitis C virus

Seventy percent of people infected with hepatitis C virus (HCV) will suffer chronic infection, putting them at risk for liver disease, including hepatocellular carcinoma. The full range of mechanisms that render some people more susceptible to chronic infection and liver disease is still being elucidated. XRN exonucleases can restrict HCV replication and may help to resolve HCV infections. However, it is unknown how 5' triphosphorylated HCV transcripts, primary products of the viral polymerase, become susceptible to attack by 5' monophosphate-specific XRNs. Here, we show that the 5' RNA triphosphatase DUSP11 acts on HCV transcripts, rendering them susceptible to XRN-mediated attack. Cells lacking DUSP11 show substantially enhanced HCV replication, and this effect is diminished when XRN expression is reduced. MicroRNA-122 (miR-122), a target of current phase II anti-HCV drugs, is known to protect HCV transcripts against XRNs. We show that HCV replication is less dependent on miR-122 in cells lacking DUSP11. Combined, these results implicate DUSP11 as an important component of XRN-mediated restriction of HCV.

Roles of the 5' Untranslated Region of Nonprimate Hepacivirus in Translation Initiation and Viral Replication

The 5' untranslated region (UTR) of hepatitis C virus (HCV), which is composed of four domains (I, II, III, and IV) and a pseudoknot, is essential for translation and viral replication. Equine nonprimate hepacivirus (EHcV) harbors a 5' UTR consisting of a large 5'-terminal domain (I); three additional domains (I', II, and III), which are homologous to domains I, II, and III, respectively, of HCV; and a pseudoknot, in the order listed. In this study, we investigated the roles of the EHcV 5' UTR in translation and viral replication. The internal ribosome entry site (IRES) activity of the EHcV 5' UTR was lower than that of the HCV 5' UTR in several cell lines due to structural differences in domain III. Domains I and III of EHcV were functional in the HCV 5' UTR in terms of IRES activity and the replication of the subgenomic replicon (SGR), although domain II was not exchangeable between EHcV and HCV for SGR replication. Furthermore, the region spanning domains I and I' of EHcV (the 5'-proximal EHcV-specific region) improved RNA stability and provided the HCV SGR with microRNA 122 (miR-122)-independent replication capability, while EHcV domain I alone improved SGR replication and RNA stability irrespective of miR-122. These data suggest that the region spanning EHcV domains I and I' improves RNA stability and viral replication regardless of miR-122 expression. The 5'-proximal EHcV-specific region may represent an inherent mechanism to facilitate viral replication in nonhepatic tissues.IMPORTANCE EHcV is the closest viral homolog to HCV among other hepaciviruses. HCV exhibits a narrow host range and liver-specific tropism, while epidemiological reports suggest that EHcV infects the liver and respiratory organs in horses, donkeys, and dogs. However, the mechanism explaining the differences in host or organ tropism between HCV and EHcV is unknown. In this study, our data suggest that the 5' untranslated region (UTR) of EHcV is composed of an internal ribosome entry site (IRES) element that is functionally exchangeable with HCV IRES elements. Furthermore, the 5'-proximal EHcV-specific region enhances viral replication and RNA stability in a miR-122-independent manner. Our data suggest that the region upstream of domain II in the EHcV 5' UTR contributes to the differences in tissue tropism observed between these hepaciviruses.