Fidelity Variants and RNA Quasispecies

Neutralization of African enterovirus A71 genogroups by antibodies to canonical genogroups

Enterovirus 71 (EV-A71) is a major public health problem, causing a range of illnesses from hand-foot-and-mouth disease to severe neurological manifestations. EV-A71 strains have been phylogenetically classified into eight genogroups (A to H), based on their capsid-coding genomic region. Genogroups B and C have caused large outbreaks worldwide and represent the two canonical circulating EV-A71 subtypes. Little is known about the antigenic diversity of new genogroups as compared to the canonical ones. Here, we compared the antigenic features of EV-A71 strains that belong to the canonical B and C genogroups and to genogroups E and F, which circulate in Africa. Analysis of the peptide sequences of EV-A71 strains belonging to different genogroups revealed a high level of conservation of the capsid residues involved in known linear and conformational neutralization antigenic sites. Using a published crystal structure of the EV-A71 capsid as a model, we found that most of the residues that are seemingly specific to some genogroups were mapped outside known antigenic sites or external loops. These observations suggest a cross-neutralization activity of anti-genogroup B or C antibodies against strains of genogroups E and F. Neutralization assays were performed with diverse rabbit and mouse anti-EV-A71 sera, anti-EV-A71 human standards and a monoclonal neutralizing antibody. All the batches of antibodies that were tested successfully neutralized all available isolates, indicating an overall broad cross-neutralization between the canonical genogroups B and C and genogroups E and F. A panel constituted of more than 80 individual human serum samples from Cambodia with neutralizing antibodies against EV-A71 subgenogroup C4 showed quite similar cross-neutralization activities between isolates of genogroups C4, E and F. Our results thus indicate that the genetic drift underlying the separation of EV-A71 strains into genogroups A, B, C, E and F does not correlate with the emergence of antigenically distinct variants.

Mutational analysis of catalytic site domain of CCHFV L RNA segment

INTRODUCTION

Crimean-Congo haemorrhagic fever virus (CCHFV) has tripartite RNA genome and is endemic in various countries of Asia, Africa and Europe.

METHOD

The present study is focused on mutation profiling of CCHFV L segment and phylogenetic clustering of protein dataset into six CCHFV genotypes.

RESULTS

Phylogenetic tree rooted with NCBI reference sequence (YP_325663.1) indicated less divergence from genotype III and the sequences belonging to same genotypes have shown less divergence among each other. Mutation frequency at 729 mutated positions was calculated and 563, 49, 33, 46 and 38 amino acid positions were found to be mutated at mutation frequency intervals of 0-0.2, 0.21-0.4, 0.41-0.6, 0.61-0.8 and 0.81-1.0 respectively. Thirty-eight highly frequent mutations (0.81-1.0 interval) were found in all genotypes and mapping in L segment (encoded for RdRp) revealed four mutations (V2074I, I2134T/A, V2148A and Q2695H/R) in catalytic site domain and no mutation in OTU domain. Molecular dynamic simulation and in silico analysis showed that catalytic site domain displayed large deviation and fluctuation upon introduction of these point mutations.

CONCLUSION

Overall study provides strong evidence that OTU domain is highly conserved and less prone to mutation whereas point mutations recorded in catalytic domain have affected the stability of protein and were found to be persistent in the large population.

Mechanisms of Coronavirus Genome Stability As Potential Targets for Antiviral Drugs

The COVID-19 pandemic has made it necessary to create antivirals active against the SARS-CoV-2 coronavirus. One of the widely used strategies to fight off viral infections is the use of modified nucleoside analogues that inhibit viral replication by incorporating DNA or RNA into the growing chain, thus stopping its synthesis. The difficulty of using this method of treatment in the case of SARS-CoV-2 is that coronaviruses have an effective mechanism for maintaining genome stability. Its central element is the nsp14 protein, which is characterized by exonuclease activity, due to which incorrectly included and noncanonical nucleotides are removed from the 3' end of the growing RNA chain. Inhibitors of nsp14 exonuclease and nucleoside analogues resistant to its action are viewed as potential targets for anticoronavirus therapy.

A Virus Is a Community: Diversity within Negative-Sense RNA Virus Populations

Negative-sense RNA virus populations are composed of diverse viral components that interact to form a community and shape the outcome of virus infections. At the genomic level, RNA virus populations consist not only of a homogeneous population of standard viral genomes but also of an extremely large number of genome variants, termed viral quasispecies, and nonstandard viral genomes, which include copy-back viral genomes, deletion viral genomes, mini viral RNAs, and hypermutated RNAs. At the particle level, RNA virus populations are composed of pleomorphic particles, particles missing or having additional genomes, and single particles or particle aggregates. As we continue discovering more about the components of negative-sense RNA virus populations and their crucial functions during virus infection, it will become more important to study RNA virus populations as a whole rather than their individual parts. In this review, we will discuss what is known about the components of negative-sense RNA virus communities, speculate how the components of the virus community interact, and summarize what vaccines and antiviral therapies are being currently developed to target or harness these components.

Rapid SARS-CoV-2 Adaptation to Available Cellular Proteases

Recent emergence of SARS-CoV-1 variants demonstrates the potential of this virus for targeted evolution, despite its overall genomic stability. Here we show the dynamics and the mechanisms behind the rapid adaptation of SARS-CoV-2 to growth in Vero E6 cells. The selective advantage for growth in Vero E6 cells is due to increased cleavage efficiency by cathepsins at the mutated S1/S2 site. S1/S2 site also constitutes a heparan sulfate (HS) binding motif that influenced virus growth in Vero E6 cells, but HS antagonist did not inhibit virus adaptation in these cells. The entry of Vero E6-adapted virus into human cells is defective because the mutated spike variants are poorly processed by furin or TMPRSS2. Minor subpopulation that lack the furin cleavage motif in the spike protein rapidly become dominant upon passaging through Vero E6 cells, but wild type sequences are maintained at low percentage in the virus swarm and mediate a rapid reverse adaptation if the virus is passaged again on TMPRSS2⁺ human cells. Our data show that the spike protein of SARS-CoV-2 can rapidly adapt itself to available proteases and argue for deep sequence surveillance to identify the emergence of novel variants.

IMPORTANCE Recently emerging SARS-CoV-2 variants B.1.1.7 (alpha variant), B.1.617.2 (delta variant), and B.1.1.529 (omicron variant) harbor spike mutations and have been linked to increased virus pathogenesis. The emergence of these novel variants highlights coronavirus adaptation and evolution potential, despite the stable consensus genotype of clinical isolates. We show that subdominant variants maintained in the virus population enable the virus to rapidly adapt to selection pressure. Although these adaptations lead to genotype change, the change is not absolute and genomes with original genotype are maintained in the virus swarm. Thus, our results imply that the relative stability of SARS-CoV-2 in numerous independent clinical isolates belies its potential for rapid adaptation to new conditions.

Current view on novel vaccine technologies to combat human infectious diseases

Inactivated and live attenuated vaccines have improved human life and significantly reduced morbidity and mortality of several human infectious diseases. However, these vaccines have faults, such as reactivity or suboptimal efficacy and expensive and time-consuming development and production. Additionally, despite the enormous efforts to develop vaccines against some infectious diseases, the traditional technologies have not been successful in achieving this. At the same time, the concerns about emerging and re-emerging diseases urge the need to develop technologies that can be rapidly applied to combat the new challenges. Within the last two decades, the research of vaccine technologies has taken several directions to achieve safe, efficient, and economic platforms or technologies for novel vaccines. This review will give a brief overview of the current state of the novel vaccine technologies, new vaccine candidates in clinical trial phases 1-3 (listed by European Medicines Agency (EMA) and Food and Drug Administration (FDA)), and vaccines based on the novel technologies which have already been commercially available (approved by EMA and FDA) with the special reference to pandemic COVID-19 vaccines. KEY POINTS: • Vaccines of the new generation follow the minimalist strategy. • Some infectious diseases remain a challenge for the vaccine development. • The number of new vaccine candidates in the late phase clinical trials remains low.

Rescuing eGFP-Tagged Canine Distemper Virus for 40 Serial Passages Separately in Ribavirin- and Non-Treated Cells: Comparative Analysis of Viral Mutation Profiles

Due to lacking a proofreading mechanism in their RNA-dependent RNA polymerases (RdRp), RNA viruses generally possess high mutation frequencies, making them evolve rapidly to form viral quasispecies during serial passages in cells, especially treated with mutagens, like ribavirin. Canine distemper virus (CDV) belongs to the genus Morbillivirus. Its L protein functions as an RdRp during viral replication. In this study, a recombinant enhanced green fluorescence protein-tagged CDV (rCDV-eGFP) was rescued from its cDNA clone, followed by viral identification and characterization at passage-7 (P7). This recombinant was independently subjected to extra 40 serial passages (P8 to 47) in ribavirin- and non-treated cells. Two viral progenies, undergoing passages in ribavirin- and non-treated VDS cells, were named rCDV-eGFP-R and -N, respectively. Both progenies were simultaneously subjected to next-generation sequencing (NGS) at P47 for comparing their quasispecies diversities with each other. The rCDV-eGFP-R and -N showed 62 and 23 single-nucleotide mutations (SNMs) in individual antigenomes, respectively, suggesting that the ribavirin conferred a mutagenic effect on the rCDV-eGFP-R. The spectrum of 62 SNMs contained 26 missense and 36 silent mutations, and that of 23 SNMs was composed of 17 missense and 6 silent mutations. Neither the rCDV-eGFP-R nor -N exhibited nonsense mutation in individual antigenomes. We speculate that the rCDV-eGFP-R may contain at least one P47 sub-progeny characterized by high-fidelity replication in cells. If such a sub-progeny can be purified from the mutant swarm, its L protein would elucidate a molecular mechanism of CDV high-fidelity replication.

Mutation Rates, Mutation Frequencies, and Proofreading-Repair Activities in RNA Virus Genetics

The error rate displayed during template copying to produce viral RNA progeny is a biologically relevant parameter of the replication complexes of viruses. It has consequences for virus-host interactions, and it represents the first step in the diversification of viruses in nature. Measurements during infections and with purified viral polymerases indicate that mutation rates for RNA viruses are in the range of 10-3 to 10-6 copying errors per nucleotide incorporated into the nascent RNA product. Although viruses are thought to exploit high error rates for adaptation to changing environments, some of them possess misincorporation correcting activities. One of them is a proofreading-repair 3' to 5' exonuclease present in coronaviruses that may decrease the error rate during replication. Here we review experimental evidence and models of information maintenance that explain why elevated mutation rates have been preserved during the evolution of RNA (and some DNA) viruses. The models also offer an interpretation of why error correction mechanisms have evolved to maintain the stability of genetic information carried out by large viral RNA genomes such as the coronaviruses.

Tradeoffs for a viral mutant with enhanced replication speed

RNA viruses exist as genetically heterogeneous populations due to high mutation rates, and many of these mutations reduce fitness and/or replication speed. However, it is unknown whether mutations can increase replication speed of a virus already well adapted to replication in cultured cells. By sequentially passaging coxsackievirus B3 in cultured cells and collecting the very earliest progeny, we selected for increased replication speed. We found that a single mutation in a viral capsid protein, VP1-F106L, was sufficient for the fast-replication phenotype. Characterization of this mutant revealed quicker genome release during entry compared to wild-type virus, highlighting a previously unappreciated infection barrier. However, this mutation also reduced capsid stability in vitro and reduced replication and pathogenesis in mice. These results reveal a tradeoff between overall replication speed and fitness. Importantly, this approach-selecting for the earliest viral progeny-could be applied to a variety of viral systems and has the potential to reveal unanticipated inefficiencies in viral replication cycles.

Mutation Profiles of eGFP-Tagged Small Ruminant Morbillivirus During 45 Serial Passages in Ribavirin-Treated Cells

Small ruminant morbillivirus (SRMV), formerly known as peste-des-petits-ruminants virus, classified into the genus Morbillivirus in the family Paramyxoviridae. Its L protein functions as the RNA-dependent RNA polymerases (RdRp) during viral replication. Due to the absence of efficient proofreading activity in their RdRps, various RNA viruses reveal high mutation frequencies, making them evolve rapidly during serial passages in cells, especially treated with a certain mutagen, like ribavirin. We have previously rescued a recombinant enhanced green fluorescence protein-tagged SRMV (rSRMV-eGFP) using reverse genetics. In this study, the rSRMV-eGFP was subjected to serial passages in ribavirin-treated cells. Due to the ribavirin-exerted selective pressure, it was speculated that viral progenies would form quasispecies after dozens of passages. Viral progenies at passage-10, -20, -30, -40, and -50 were separately subjected to next-generation sequencing (NGS), consequently revealing a total of 34 single-nucleotide variations, including five synonymous, 21 missense, and one non-sense mutations. The L sequence was found to harbor eight missense mutations during serial passaging. It was speculated that at least one high-fidelity variant was present in viral quasispecies at passage-50. If purified from the population of viral progenies, this putative variant would contribute to clarifying a molecular mechanism in viral high-fidelity replication in vitro.

Genome Evolution of Two Genetically Homogeneous Infectious Bursal Disease Virus Strains During Passages in vitro and ex vivo in the Presence of a Mutagenic Nucleoside Analog

The avibirnavirus infectious bursal disease virus (IBDV) is responsible for a highly contagious and sometimes lethal disease of chickens (Gallus gallus). IBDV genetic variation is well-described for both field and live-attenuated vaccine strains, however, the dynamics and selection pressures behind this genetic evolution remain poorly documented. Here, genetically homogeneous virus stocks were generated using reverse genetics for a very virulent strain, rvv, and a vaccine-related strain, rCu-1. These viruses were serially passaged at controlled multiplicities of infection in several biological systems, including primary chickens B cells, the main cell type targeted by IBDV in vivo. Passages were also performed in the absence or presence of a strong selective pressure using the antiviral nucleoside analog 7-deaza-2'-C-methyladenosine (7DMA). Next Generation Sequencing (NGS) of viral genomes after the last passage in each biological system revealed that (i) a higher viral diversity was generated in segment A than in segment B, regardless 7DMA treatment and viral strain, (ii) diversity in segment B was increased by 7DMA treatment in both viruses, (iii) passaging of IBDV in primary chicken B cells, regardless of 7DMA treatment, did not select cell-culture adapted variants of rvv, preserving its capsid protein (VP2) properties, (iv) mutations in coding and non-coding regions of rCu-1 segment A could potentially associate to higher viral fitness, and (v) a specific selection, upon 7DMA addition, of a Thr329Ala substitution occurred in the viral polymerase VP1. The latter change, together with Ala270Thr change in VP2, proved to be associated with viral attenuation in vivo. These results identify genome sequences that are important for IBDV evolution in response to selection pressures. Such information will help tailor better strategies for controlling IBDV infection in chickens.

Genomic diversity contributes to the neuroinvasiveness of the Yellow fever French neurotropic vaccine

Mass vaccination with the live attenuated vaccine YF-17D is the current way to prevent infection with Yellow fever virus (YFV). However, 0.000012-0.00002% of vaccinated patients develop post-vaccination neurological syndrome (YEL-AND). Understanding the factors responsible for neuroinvasion, neurotropism, and neurovirulence of the vaccine is critical for improving its biosafety. The YF-FNV vaccine strain, known to be associated with a higher frequency of YEL-AND (0.3-0.4%) than YF-17D, is an excellent model to study vaccine neuroinvasiveness. We determined that neuroinvasiveness of YF-FNV occured both via infection and passage through human brain endothelial cells. Plaque purification and next generation sequencing (NGS) identified several neuroinvasive variants. Their neuroinvasiveness was not higher than that of YF-FNV. However, rebuilding the YF-FNV population diversity from a set of isolated YF-FNV-N variants restored the original neuroinvasive phenotype of YF-FNV. Therefore, we conclude that viral population diversity is a critical factor for YFV vaccine neuroinvasiveness.

Increased RNA virus population diversity improves adaptability

The replication machinery of most RNA viruses lacks proofreading mechanisms. As a result, RNA virus populations harbor a large amount of genetic diversity that confers them the ability to rapidly adapt to changes in their environment. In this work, we investigate whether further increasing the initial population diversity of a model RNA virus can improve adaptation to a single selection pressure, thermal inactivation. For this, we experimentally increased the diversity of coxsackievirus B3 (CVB3) populations across the capsid region. We then compared the ability of these high diversity CVB3 populations to achieve resistance to thermal inactivation relative to standard CVB3 populations in an experimental evolution setting. We find that viral populations with high diversity are better able to achieve resistance to thermal inactivation at both the temperature employed during experimental evolution as well as at a more extreme temperature. Moreover, we identify mutations in the CVB3 capsid that confer resistance to thermal inactivation, finding significant mutational epistasis. Our results indicate that even naturally diverse RNA virus populations can benefit from experimental augmentation of population diversity for optimal adaptation and support the use of such viral populations in directed evolution efforts that aim to select viruses with desired characteristics.

A 5-Year Review of Senecavirus A in China since Its Emergence in 2015

Senecavirus A (SVA), previously known as Seneca Valley virus, is classified into the genus Senecavirus in the family Picornaviridae. This virus can cause vesicular disease and epidemic transient neonatal losses in swine. Typical clinical signs include vesicular and/or ulcerative lesions on the snout, oral mucosa, coronary bands and hooves. SVA emerged in Guangdong Province of China in 2015, and thereafter gradually spread into other provinces, autonomous regions and municipalities (P.A.M.s). Nowadays more than half of the P.A.M.s have been affected by SVA, and asymptomatic infection has occurred in some areas. The phylogenetic analysis shows that China isolates are clustered into five genetic branches, implying a fast evolutionary speed since SVA emergence in 2015. This review presented current knowledge concerning SVA infection in China, including its history, epidemiology, evolutionary characteristics, diagnostics and vaccines.

Broad and Dynamic Diversification of Infectious Hepatitis C Virus in a Cell Culture Environment

Previous studies documented that long-term hepatitis C virus (HCV) replication in human hepatoma Huh-7.5 cells resulted in viral fitness gain, expansion of the mutant spectrum, and several phenotypic alterations. In the present work, we show that mutational waves (changes in frequency of individual mutations) occurred continuously and became more prominent as the virus gained fitness. They were accompanied by an increasing proportion of heterogeneous genomic sites that affected 1 position in the initial HCV population and 19 and 69 positions at passages 100 and 200, respectively. Analysis of biological clones of HCV showed that these dynamic events affected infectious genomes, since part of the fluctuating mutations became incorporated into viable genomes. While 17 mutations were scored in 3 biological clones isolated from the initial population, the number reached 72 in 3 biological clones from the population at passage 200. Biological clones differed in their responses to antiviral inhibitors, indicating a phenotypic impact of viral dynamics. Thus, HCV adaptation to a specific constant environment (cell culture without external influences) broadens the mutant repertoire and does not focus the population toward a limited number of dominant genomes. A retrospective examination of mutant spectra of foot-and-mouth disease virus passaged in cell cultures suggests a parallel behavior here described for HCV. We propose that virus diversification in a constant environment has its basis in the availability of multiple alternative mutational pathways for fitness gain. This mechanism of broad diversification should also apply to other replicative systems characterized by high mutation rates and large population sizes.IMPORTANCE The study shows that extensive replication of an RNA virus in a constant biological environment does not limit exploration of sequence space and adaptive options. There was no convergence toward a restricted set of adapted genomes. Mutational waves and mutant spectrum broadening affected infectious genomes. Therefore, profound modifications of mutant spectrum composition and consensus sequence diversification are not exclusively dependent on environmental alterations or the intervention of population bottlenecks.

Molecular basis of genetic variation of viruses

Genetic variation is a necessity of all biological systems. Viruses use all known mechanisms of variation; mutation, several forms of recombination, and segment reassortment in the case of viruses with a segmented genome. These processes are intimately connected with the replicative machineries of viruses, as well as with fundamental physical-chemical properties of nucleotides when acting as template or substrate residues. Recombination has been viewed as a means to rescue viable genomes from unfit parents or to produce large modifications for the exploration of phenotypic novelty. All types of genetic variation can act conjointly as blind processes to provide the raw materials for adaptation to the changing environments in which viruses must replicate. A distinction is made between mechanistically unavoidable and evolutionarily relevant mutation and recombination.

Tyr82 Amino Acid Mutation in PB1 Polymerase Induces an Influenza Virus Mutator Phenotype

In various positive-sense single-stranded RNA viruses, a low-fidelity viral RNA-dependent RNA polymerase (RdRp) confers attenuated phenotypes by increasing the mutation frequency. We report a negative-sense single-stranded RNA virus RdRp mutant strain with a mutator phenotype. Based on structural data of RdRp, rational targeting of key residues, and screening of fidelity variants, we isolated a novel low-fidelity mutator strain of influenza virus that harbors a Tyr82-to-Cys (Y82C) single-amino-acid substitution in the PB1 polymerase subunit. The purified PB1-Y82C polymerase indeed showed an increased frequency of misincorporation compared with the wild-type PB1 in an in vitro biochemical assay. To further investigate the effects of position 82 on PB1 polymerase fidelity, we substituted various amino acids at this position. As a result, we isolated various novel mutators other than PB1-Y82C with higher mutation frequencies. The structural model of influenza virus polymerase complex suggested that the Tyr82 residue, which is located at the nucleoside triphosphate entrance tunnel, may influence a fidelity checkpoint. Interestingly, although the PB1-Y82C variant replicated with wild-type PB1-like kinetics in tissue culture, the 50% lethal dose of the PB1-Y82C mutant was 10 times lower than that of wild-type PB1 in embryonated chicken eggs. In conclusion, our data indicate that the Tyr82 residue of PB1 has a crucial role in regulating polymerase fidelity of influenza virus and is closely related to attenuated pathogenic phenotypes in vivo IMPORTANCE Influenza A virus rapidly acquires antigenic changes and antiviral drug resistance, which limit the effectiveness of vaccines and drug treatments, primarily owing to its high rate of evolution. Virus populations formed by quasispecies can contain resistance mutations even before a selective pressure is applied. To study the effects of the viral mutation spectrum and quasispecies, high- and low-fidelity variants have been isolated for several RNA viruses. Here, we report the discovery of a low-fidelity RdRp variant of influenza A virus that contains a substitution at Tyr82 in PB1. Viruses containing the PB1-Y82C substitution showed growth kinetics and viral RNA synthesis levels similar to those of the wild-type virus in cell culture; however, they had significantly attenuated phenotypes in a chicken egg infection experiment. These data demonstrated that decreased RdRp fidelity attenuates influenza A virus in vivo, which is a desirable feature for the development of safer live attenuated vaccine candidates.

Viral quasispecies

Viral quasispecies refers to a population structure that consists of extremely large numbers of variant genomes, termed mutant spectra, mutant swarms or mutant clouds. Fueled by high mutation rates, mutants arise continually, and they change in relative frequency as viral replication proceeds. The term quasispecies was adopted from a theory of the origin of life in which primitive replicons) consisted of mutant distributions, as found experimentally with present day RNA viruses. The theory provided a new definition of wild type, and a conceptual framework for the interpretation of the adaptive potential of RNA viruses that contrasted with classical studies based on consensus sequences. Standard clonal analyses and deep sequencing methodologies have confirmed the presence of myriads of mutant genomes in viral populations, and their participation in adaptive processes. The quasispecies concept applies to any biological entity, but its impact is more evident when the genome size is limited and the mutation rate is high. This is the case of the RNA viruses, ubiquitous in our biosphere, and that comprise many important pathogens. In virology, quasispecies are defined as complex distributions of closely related variant genomes subjected to genetic variation, competition and selection, and that may act as a unit of selection. Despite being an integral part of their replication, high mutation rates have an upper limit compatible with inheritable information. Crossing such a limit leads to RNA virus extinction, a transition that is the basis of an antiviral design termed lethal mutagenesis.

Directed evolution as a tool for the selection of oncolytic RNA viruses with desired phenotypes

Viruses have some characteristics in common with cell-based life. They can evolve and adapt to environmental conditions. Directed evolution can be used by researchers to produce viral strains with desirable phenotypes. Through bioselection, improved strains of oncolytic viruses can be obtained that have better safety profiles, increased specificity for malignant cells, and more efficient spread among tumor cells. It is also possible to select strains capable of killing a broader spectrum of cancer cell variants, so as to achieve a higher frequency of therapeutic responses. This review describes and analyses virus adaptation studies performed with members of four RNA virus families that are used for viral oncolysis: reoviruses, paramyxoviruses, enteroviruses, and rhabdoviruses.

Characterization of Mutational Tolerance of a Viral RNA-Protein Interaction

Replication of RNA viruses is generally markedly error-prone. Nevertheless, these viruses usually retain their identity under more or less constant conditions due to different mechanisms of mutation tolerance. However, there exists only limited information on quantitative aspects of the mutational tolerance of distinct viral functions. To address this problem, we used here as a model the interaction between a replicative cis-acting RNA element (oriL) of poliovirus and its ligand (viral protein 3CD). The mutational tolerance of a conserved tripeptide of 3CD, directly involved in this interaction, was investigated. Randomization of the relevant codons and reverse genetics were used to define the space of viability-compatible sequences. Surprisingly, at least 11 different amino acid substitutions in this tripeptide were not lethal. Several altered viruses exhibited wild-type-like phenotypes, whereas debilitated (but viable) genomes could increase their fitness by the acquisition of reversions or compensatory mutations. Together with our study on the tolerance of oriL (Prostova et al., 2015), the results demonstrate that at least 42 out of 51 possible nucleotide replacements within the two relevant genomic regions are viability-compatible. These results provide new insights into structural aspects of an important viral function as well as into the general problems of viral mutational robustness and evolution.

Stronger together: Multi-genome transmission of measles virus

Measles virus (MeV) is an immunosuppressive, extremely contagious RNA virus that remains a leading cause of death among children. MeV is dual-tropic: it replicates first in lymphatic tissue, causing immunosuppression, and then in epithelial cells of the upper airways, accounting for extremely efficient contagion. Efficient contagion is counter-intuitive because the enveloped MeV particles are large and relatively unstable. However, MeV particles can contain multiple genomes, which can code for proteins with different functional characteristics. These proteins can cooperate to promote virus spread in tissue culture, prompting the question of whether multi-genome MeV transmission may promote efficient MeV spread also in vivo. Consistent with this hypothesis, in well-differentiated primary human airway epithelia large genome populations spread rapidly through intercellular pores. In another line of research, it was shown that distinct lymphocytic-adapted and epithelial-adapted genome populations exist; cyclical adaptation studies indicate that suboptimal variants in one environment may constitute a low frequency reservoir for adaptation to the other environment. Altogether, these observations suggest that, in humans, MeV spread relies on en bloc genome transmission, and that genomic diversity is instrumental for rapid MeV dissemination within hosts.

Expanding Repertoire of Plant Positive-Strand RNA Virus Proteases

Many plant viruses express their proteins through a polyprotein strategy, requiring the acquisition of protease domains to regulate the release of functional mature proteins and/or intermediate polyproteins. Positive-strand RNA viruses constitute the vast majority of plant viruses and they are diverse in their genomic organization and protein expression strategies. Until recently, proteases encoded by positive-strand RNA viruses were described as belonging to two categories: (1) chymotrypsin-like cysteine and serine proteases and (2) papain-like cysteine protease. However, the functional characterization of plant virus cysteine and serine proteases has highlighted their diversity in terms of biological activities, cleavage site specificities, regulatory mechanisms, and three-dimensional structures. The recent discovery of a plant picorna-like virus glutamic protease with possible structural similarities with fungal and bacterial glutamic proteases also revealed new unexpected sources of protease domains. We discuss the variety of plant positive-strand RNA virus protease domains. We also highlight possible evolution scenarios of these viral proteases, including evidence for the exchange of protease domains amongst unrelated viruses.

Structural and Nonstructural Genes Contribute to the Genetic Diversity of RNA Viruses

With the advent of advanced sequencing technology, studies of RNA viruses have shown that genetic diversity can contribute to both attenuation and virulence and the paradigm is that this is controlled by the error-prone RNA-dependent RNA polymerase (RdRp). Since wild-type yellow fever virus (YFV) strain Asibi has genetic diversity typical of a wild-type RNA virus, while 17D virus vaccine has limited diversity, it provides a unique opportunity to investigate RNA population theory in the context of a well-characterized live attenuated vaccine. Utilizing infectious clone-derived viruses, we show that genetic diversity of RNA viruses is complex and that multiple genes, including structural genes and NS2B and NS4B genes also contribute to genetic diversity. We suggest that the replication complex as a whole, rather than only RdRp, drives genetic diversity, at least for YFV.

One paradigm to explain the complexity of viral RNA populations is that the low fidelity of the RNA-dependent RNA polymerase (RdRp) drives high mutation rates and consequently genetic diversity. Like most RNA viruses, wild-type yellow fever virus (YFV) replication is error-prone due to the lack of proofreading by the virus-encoded RdRp. However, there is evidence that replication of the live attenuated YF vaccine virus 17D, derived from wild-type strain Asibi, is less error-prone than wild-type RNA viruses. Recent studies comparing the genetic diversity of wild-type Asibi and 17D vaccine virus found that wild-type Asibi has the typical heterogeneous population of an RNA virus, while there is limited intra- and interpopulation variability of 17D vaccine virus. Utilizing chimeric and mutant infectious clone-derived viruses, we show that high and low genetic diversity profiles of wild-type Asibi virus and vaccine virus 17D, respectively, are multigenic. Introduction of either structural (pre-membrane and envelope) genes or NS2B or NS4B substitutions into the Asibi and 17D backbone resulted in altered variant population, nucleotide diversity, and mutation frequency compared to the parental viruses. Additionally, changes in genetic diversity of the chimeric and mutant viruses correlated with the phenotype of multiplication kinetics in human alveolar A549 cells. Overall, the paradigm that only the error-prone RdRp controls genetic diversity needs to be expanded to address the role of other genes in genetic diversity, and we hypothesize that it is the replication complex as a whole and not the RdRp alone that controls genetic diversity.

Contribution of a Multifunctional Polymerase Region of Foot-and-Mouth Disease Virus to Lethal Mutagenesis

Viral RNA-dependent RNA polymerases (RdRps) are major determinants of high mutation rates and generation of mutant spectra that mediate RNA virus adaptability. The RdRp of the picornavirus foot-and-mouth disease virus (FMDV), termed 3D, is a multifunctional protein that includes a nuclear localization signal (NLS) in its N-terminal region. Previous studies documented that some amino acid substitutions within the NLS altered nucleotide recognition and enhanced the incorporation of the mutagenic purine analogue ribavirin in viral RNA, but the mutants tested were not viable and their response to lethal mutagenesis could not be studied. Here we demonstrate that NLS amino acid substitution M16A of FMDV serotype C does not affect infectious virus production but accelerates ribavirin-mediated virus extinction. The mutant 3D displays polymerase activity, RNA binding, and copying processivity that are similar to those of the wild-type enzyme but shows increased ribavirin-triphosphate incorporation. Crystal structures of the mutant 3D in the apo and RNA-bound forms reveal an expansion of the template entry channel due to the replacement of the bulky Met by Ala. This is a major difference with other 3D mutants with altered nucleotide analogue recognition. Remarkably, two distinct loop β9-α11 conformations distinguish 3Ds that exhibit higher or lower ribavirin incorporation than the wild-type enzyme. This difference identifies a specific molecular determinant of ribavirin sensitivity of FMDV. Comparison of several polymerase mutants indicates that different domains of the molecule can modify nucleotide recognition and response to lethal mutagenesis. The connection of this observation with current views on quasispecies adaptability is discussed.IMPORTANCE The nuclear localization signal (NLS) of the foot-and-mouth disease virus (FMDV) polymerase includes residues that modulate the sensitivity to mutagenic agents. Here we have described a viable NLS mutant with an amino acid replacement that facilitates virus extinction by ribavirin. The corresponding polymerase shows increased incorporation of ribavirin triphosphate and local structural modifications that implicate the template entry channel. Specifically, comparison of the structures of ribavirin-sensitive and ribavirin-resistant FMDV polymerases has identified loop β9-α11 conformation as a determinant of sensitivity to ribavirin mutagenesis.

Rare haplotype load as marker for lethal mutagenesis

RNA viruses replicate with a template-copying fidelity, which lies close to an extinction threshold. Increases of mutation rate by nucleotide analogues can drive viruses towards extinction. This transition is the basis of an antiviral strategy termed lethal mutagenesis. We have introduced a new diversity index, the rare haplotype load (RHL), to describe NS5B (polymerase) mutant spectra of hepatitis C virus (HCV) populations passaged in absence or presence of the mutagenic agents favipiravir or ribavirin. The increase in RHL is more prominent in mutant spectra whose expansions were due to nucleotide analogues than to multiple passages in absence of mutagens. Statistical tests for paired mutagenized versus non-mutagenized samples with 14 diversity indices show that RHL provides consistently the highest standardized effect of mutagenic treatment difference for ribavirin and favipiravir. The results indicate that the enrichment of viral quasispecies in very low frequency minority genomes can serve as a robust marker for lethal mutagenesis. The diagnostic value of RHL from deep sequencing data is relevant to experimental studies on enhanced mutagenesis of viruses, and to pharmacological evaluations of inhibitors suspected to have a mutagenic activity.

Foot-and-mouth disease virus type O specific mutations determine RNA-dependent RNA polymerase fidelity and virus attenuation

Previous studies have shown that the FMDV Asia1/YS/CHA/05 high-fidelity mutagen-resistant variants are attenuated (Zeng et al., 2014). Here, we introduced the same single or multiple-amino-acid substitutions responsible for increased 3D^pol fidelity of type Asia1 FMDV into the type O FMDV O/YS/CHA/05 infectious clone. The rescued viruses O-DA and O-DAMM are lower replication fidelity mutants and showed an attenuated phenotype. These results demonstrated that the same amino acid substitution of 3D^pol in different serotypes of FMDV strains had different effects on viral fidelity. In addition, nucleoside analogues were used to select high-fidelity mutagen-resistant type O FMDV variants. The rescued mutagen-resistant type O FMDV high-fidelity variants exhibited significantly attenuated fitness and a reduced virulence phenotype. These results have important implications for understanding the molecular mechanism of FMDV evolution and pathogenicity, especially in developing a safer modified live-attenuated vaccine against FMDV.

Quasispecies and virus

Quasispecies theory has been instrumental in the understanding of RNA virus population dynamics because it considered for the first time mutation as an integral part of the replication process. The key influences of quasispecies theory on experimental virology have been: (1) to disclose the mutant spectrum nature of viral populations and to evaluate its consequences; (2) to unveil collective properties of genome ensembles that can render a mutant spectrum a unit of selection; and (3) to identify new vulnerability points of pathogenic RNA viruses on three fronts: the need to apply multiple selective constraints (in the form of drug combinations) to minimize selection of treatment-escape variants, to translate the error threshold concept into antiviral designs, and to construct attenuated vaccine viruses through alterations of viral polymerase copying fidelity or through displacements of viral genomes towards unfavorable regions of sequence space. These three major influences on the understanding of viral pathogens preceded extensions of quasispecies to non-viral systems such as bacterial and tumor cell collectivities and prions. These developments are summarized here.

Pressure for Pattern-Specific Intertypic Recombination between Sabin Polioviruses: Evolutionary Implications

Complete genomic sequences of a non-redundant set of 70 recombinants between three serotypes of attenuated Sabin polioviruses as well as location (based on partial sequencing) of crossover sites of 28 additional recombinants were determined and compared with the previously published data. It is demonstrated that the genomes of Sabin viruses contain distinct strain-specific segments that are eliminated by recombination. The presumed low fitness of these segments could be linked to mutations acquired upon derivation of the vaccine strains and/or may have been present in wild-type parents of Sabin viruses. These “weak” segments contribute to the propensity of these viruses to recombine with each other and with other enteroviruses as well as determine the choice of crossover sites. The knowledge of location of such segments opens additional possibilities for the design of more genetically stable and/or more attenuated variants, i.e., candidates for new oral polio vaccines. The results also suggest that the genome of wild polioviruses, and, by generalization, of other RNA viruses, may harbor hidden low-fitness segments that can be readily eliminated only by recombination.

Extinction of West Nile Virus by Favipiravir through Lethal Mutagenesis

Favipiravir is an antiviral agent effective against several RNA viruses. The drug has been shown to protect mice against experimental infection with a lethal dose of West Nile virus (WNV), a mosquito-borne flavivirus responsible for outbreaks of meningitis and encephalitis for which no antiviral therapy has been licensed; however, the mechanism of action of the drug is still not well understood. Here, we describe the potent in vitro antiviral activity of favipiravir against WNV, showing that it decreases virus-specific infectivity and drives the virus to extinction. Two passages of WNV in the presence of 1 mM favipiravir-a concentration that is more than 10-fold lower than its 50% cytotoxic concentration (CC₅₀)-resulted in a significant increase in mutation frequency in the mutant spectrum and in a bias toward A→G and G→A transitions relative to the population passaged in the absence of the drug. These data, together with the fact that the drug is already licensed in Japan against influenza virus and in a clinical trial against Ebola virus, point to favipiravir as a promising antiviral agent to fight medically relevant flaviviral infections, such as that caused by WNV.

Molecular and Functional Bases of Selection against a Mutation Bias in an RNA Virus

The selective pressures acting on viruses that replicate under enhanced mutation rates are largely unknown. Here, we describe resistance of foot-and-mouth disease virus to the mutagen 5-fluorouracil (FU) through a single polymerase substitution that prevents an excess of A to G and U to C transitions evoked by FU on the wild-type foot-and-mouth disease virus, while maintaining the same level of mutant spectrum complexity. The polymerase substitution inflicts upon the virus a fitness loss during replication in absence of FU but confers a fitness gain in presence of FU. The compensation of mutational bias was documented by in vitro nucleotide incorporation assays, and it was associated with structural modifications at the N-terminal region and motif B of the viral polymerase. Predictions of the effect of mutations that increase the frequency of G and C in the viral genome and encoded polymerase suggest multiple points in the virus life cycle where the mutational bias in favor of G and C may be detrimental. Application of predictive algorithms suggests adverse effects of the FU-directed mutational bias on protein stability. The results reinforce modulation of nucleotide incorporation as a lethal mutagenesis-escape mechanism (that permits eluding virus extinction despite replication in the presence of a mutagenic agent) and suggest that mutational bias can be a target of selection during virus replication.

Attenuation of Foot-and-Mouth Disease Virus by Engineered Viral Polymerase Fidelity

Foot-and-mouth disease virus (FMDV) RNA-dependent RNA polymerase (RdRp) (3Dpol) catalyzes viral RNA synthesis. Its characteristic low fidelity and absence of proofreading activity allow FMDV to rapidly mutate and adapt to dynamic environments. In this study, we used the structure of FMDV 3Dpol in combination with previously reported results from similar picornaviral polymerases to design point mutations that would alter replication fidelity. In particular, we targeted Trp237 within conserved polymerase motif A because of the low reversion potential inherent in the single UGG codon. Using biochemical and genetic tools, we show that the replacement of tryptophan 237 with phenylalanine imparts higher fidelity, but replacements with isoleucine and leucine resulted in lower-fidelity phenotypes. Viruses containing these W237 substitutions show in vitro growth kinetics and plaque morphologies similar to those of the wild-type (WT) A24 Cruzeiro strain in BHK cells, and both high- and low-fidelity variants retained fitness during coinfection with the wild-type virus. The higher-fidelity W237F (W237FHF) mutant virus was more resistant to the mutagenic nucleoside analogs ribavirin and 5-fluorouracil than the WT virus, whereas the lower-fidelity W237I (W237ILF) and W237LLF mutant viruses exhibited lower ribavirin resistance. Interestingly, the variant viruses showed heterogeneous and slightly delayed growth kinetics in primary porcine kidney cells, and they were significantly attenuated in mouse infection experiments. These data demonstrate, for a single virus, that either increased or decreased RdRp fidelity attenuates virus growth in animals, which is a desirable feature for the development of safer and genetically more stable vaccine candidates.IMPORTANCE Foot-and-mouth disease (FMD) is the most devastating disease affecting livestock worldwide. Here, using structural and biochemical analyses, we have identified FMDV 3Dpol mutations that affect polymerase fidelity. Recombinant FMDVs containing substitutions at 3Dpol tryptophan residue 237 were genetically stable and displayed plaque phenotypes and growth kinetics similar to those of the wild-type virus in cell culture. We further demonstrate that viruses harboring either a W237FHF substitution or W237ILF and W237LLF mutations were highly attenuated in animals. Our study shows that obtaining 3Dpol fidelity variants by protein engineering based on polymerase structure and function could be exploited for the development of attenuated FMDV vaccine candidates that are safer and more stable than strains obtained by selective pressure via mutagenic nucleotides or adaptation approaches.

Attenuation of RNA viruses by redirecting their evolution in sequence space

RNA viruses pose serious threats to human health. Their success relies on their capacity to generate genetic variability and, consequently, on their adaptive potential. We describe a strategy to attenuate RNA viruses by altering their evolutionary potential. We rationally altered the genomes of Coxsackie B3 and influenza A viruses to redirect their evolutionary trajectories towards detrimental regions in sequence space. Specifically, viral genomes were engineered to harbour more serine and leucine codons with nonsense mutation targets: codons that could generate Stop mutations after a single nucleotide substitution. Indeed, these viruses generated more Stop mutations both in vitro and in vivo, accompanied by significant losses in viral fitness. In vivo, the viruses were attenuated, generated high levels of neutralizing antibodies and protected against lethal challenge. Our study demonstrates that cornering viruses in ‘risky’ areas of sequence space may be implemented as a broad-spectrum vaccine strategy against RNA viruses.

Virus attenuation is used to obtain vaccine strains. Here, the rapid evolution of RNA viruses is exploited by engineering their genomes to encode sites that are a mutation away from a stop codon, a clever method to generate attenuated viruses.

Serotype-specific interactions among functional domains of dengue virus 2 nonstructural proteins (NS) 5 and NS3 are crucial for viral RNA replication

Four serotypes of mosquito-borne dengue virus (DENV), evolved from a common ancestor, are human pathogens of global significance for which there is no vaccine or antiviral drug available. The N-terminal domain of DENV NS5 has guanylyltransferase and methyltransferase (MTase), and the C-terminal region has the polymerase (POL), all of which are important for 5'-capping and RNA replication. The crystal structure of NS5 shows it as a dimer, but the functional evidence for NS5 dimer is lacking. Our studies showed that the substitution of DENV2 NS5 MTase or POL for DENV4 NS5 within DENV2 RNA resulted in a severe attenuation of replication in the transfected BHK-21 cells. A replication-competent species was evolved with the acquired mutations in the DENV2 and DENV4 NS5 MTase or POL domain or in the DENV2 NS3 helicase domain in the DENV2 chimera RNAs by repeated passaging of infected BHK-21 or mosquito cells. The linker region of seven residues in NS5, rich in serotype-specific residues, is important for the recovery of replication fitness in the chimera RNA. Our results, taken together, provide genetic evidence for a serotype-specific interaction between NS3 and NS5 as well as specific interdomain interaction within NS5 required for RNA replication. Genome-wide RNAseq analysis revealed the distribution of adaptive mutations in RNA quasispecies. Those within NS3 and NS5 are located at the surface and/or within the NS5 dimer interface, providing a functional significance to the crystal structure NS5 dimer.

1st Workshop of the Canadian Society for Virology

The 1st Workshop of the Canadian Society for Virology (CSV2016) was a Special Workshop of the 35th Annual Meeting for the American Society for Virology, held on 18 June 2016 on the beautiful Virginia Tech campus in Blacksburg, Virginia. The workshop provided a forum for discussion of recent advances in the field, in an informal setting conducive to interaction with colleagues. CSV2016 featured two internationally-renowned Canadian keynote speakers who discussed translational virology research; American Society for Virology President Grant McFadden (then from University of Florida, now relocated to Arizona State University) who presented his studies of oncolytic poxviruses, while Matthew Miller (McMaster University) reviewed the prospects for a universal influenza vaccine. The workshop also featured a variety of trainee oral and poster presentations, and a panel discussion on the topic of the future of the CSV and virus research in Canada.

Mechanisms of viral mutation

The remarkable capacity of some viruses to adapt to new hosts and environments is highly dependent on their ability to generate de novo diversity in a short period of time. Rates of spontaneous mutation vary amply among viruses. RNA viruses mutate faster than DNA viruses, single-stranded viruses mutate faster than double-strand virus, and genome size appears to correlate negatively with mutation rate. Viral mutation rates are modulated at different levels, including polymerase fidelity, sequence context, template secondary structure, cellular microenvironment, replication mechanisms, proofreading, and access to post-replicative repair. Additionally, massive numbers of mutations can be introduced by some virus-encoded diversity-generating elements, as well as by host-encoded cytidine/adenine deaminases. Our current knowledge of viral mutation rates indicates that viral genetic diversity is determined by multiple virus- and host-dependent processes, and that viral mutation rates can evolve in response to specific selective pressures.

Norovirus Polymerase Fidelity Contributes to Viral Transmission In Vivo

Virus replication fidelity and hence the intrahost genetic diversity of viral populations are known to be intricately linked to viral pathogenesis and tropism as well as to immune and antiviral escape during infection. In this study, we investigated whether changes in replication fidelity can impact the ability of a virus to transmit between susceptible hosts by the use of a mouse model for norovirus. We show that a variant encoding a high-fidelity polymerase is transmitted less efficiently between mice than the wild-type strain. This constitutes the first experimental demonstration that the polymerase fidelity of viruses can impact transmission of infection in their natural hosts. These results provide further insight into potential reasons for the global emergence of pandemic human noroviruses that display alterations in the replication fidelity of their polymerases compared to nonpandemic strains.

Intrahost genetic diversity and replication error rates are intricately linked to RNA virus pathogenesis, with alterations in viral polymerase fidelity typically leading to attenuation during infections in vivo. We have previously shown that norovirus intrahost genetic diversity also influences viral pathogenesis using the murine norovirus model, as increasing viral mutation frequency using a mutagenic nucleoside resulted in clearance of a persistent infection in mice. Given the role of replication fidelity and genetic diversity in pathogenesis, we have now investigated whether polymerase fidelity can also impact virus transmission between susceptible hosts. We have identified a high-fidelity norovirus RNA-dependent RNA polymerase mutant (I391L) which displays delayed replication kinetics in vivo but not in cell culture. The I391L polymerase mutant also exhibited lower transmission rates between susceptible hosts than the wild-type virus and, most notably, another replication defective mutant that has wild-type levels of polymerase fidelity. These results provide the first experimental evidence that norovirus polymerase fidelity contributes to virus transmission between hosts and that maintaining diversity is important for the establishment of infection. This work supports the hypothesis that the reduced polymerase fidelity of the pandemic GII.4 human norovirus isolates may contribute to their global dominance.

IMPORTANCE Virus replication fidelity and hence the intrahost genetic diversity of viral populations are known to be intricately linked to viral pathogenesis and tropism as well as to immune and antiviral escape during infection. In this study, we investigated whether changes in replication fidelity can impact the ability of a virus to transmit between susceptible hosts by the use of a mouse model for norovirus. We show that a variant encoding a high-fidelity polymerase is transmitted less efficiently between mice than the wild-type strain. This constitutes the first experimental demonstration that the polymerase fidelity of viruses can impact transmission of infection in their natural hosts. These results provide further insight into potential reasons for the global emergence of pandemic human noroviruses that display alterations in the replication fidelity of their polymerases compared to nonpandemic strains.

Involvement of a joker mutation in a polymerase-independent lethal mutagenesis escape mechanism

We previously characterized a foot-and-mouth disease virus (FMDV) with three amino acid replacements in its polymerase (3D) that conferred resistance to the mutagenic nucleoside analogue ribavirin. Here we show that passage of this mutant in the presence of high ribavirin concentrations resulted in selection of viruses with the additional replacement I248T in 2C. This 2C substitution alone (even in the absence of replacements in 3D) increased FMDV fitness mainly in the presence of ribavirin, prevented an incorporation bias in favor of A and U associated with ribavirin mutagenesis, and conferred the ATPase activity of 2C decreased sensitivity to ribavirin-triphosphate. Since in previous studies we described that 2C with I248T was selected under different selective pressures, this replacement qualifies as a joker substitution in FMDV evolution. The results have identified a role of 2C in nucleotide incorporation, and have unveiled a new polymerase-independent mechanism of virus escape to lethal mutagenesis.

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A replacement in FMDV protein 2C confers reduced sensitivity to the mutagen ribavirin.

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The effect of the replacement is to prevent a mutational bias evoked by ribavirin.

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2C has an effect in nucleotide incorporation by the FMDV polymerase.

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We describe a new molecular mechanism of escape to ribavirin-mediated extinction.

Resistance of Hepatitis C Virus to Inhibitors: Complexity and Clinical Implications

Selection of inhibitor-resistant viral mutants is universal for viruses that display quasi-species dynamics, and hepatitis C virus (HCV) is no exception. Here we review recent results on drug resistance in HCV, with emphasis on resistance to the newly-developed, directly-acting antiviral agents, as they are increasingly employed in the clinic. We put the experimental observations in the context of quasi-species dynamics, in particular what the genetic and phenotypic barriers to resistance mean in terms of exploration of sequence space while HCV replicates in the liver of infected patients or in cell culture. Strategies to diminish the probability of viral breakthrough during treatment are briefly outlined.