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Background and Purpose: There is a need for effective anti‐COVID‐19 treatments, mainly for individuals at risk of severe disease such as the elderly and the immunosuppressed. Drug repositioning has proved effective in identifying drugs that can find a new application for the control of coronavirus disease, in particular COVID‐19. The purpose of the present study was to find synergistic antiviral combinations for COVID‐19 based on lethal mutagenesis. Experimental Approach: The effect of combinations of remdesivir and ribavirin on the infectivity of SARS‐CoV‐2 in cell culture has been tested. Viral populations were monitored by ultra‐deep sequencing, and the decrease of infectivity as a result of the treatment was measured. Key Results: Remdesivir and ribavirin exerted a synergistic inhibitory activity against SARS‐CoV‐2, quantified both by CompuSyn (Chou‐Talalay method) and Synergy Finder (ZIP‐score model). In serial passage experiments, virus extinction was readily achieved with remdesivir‐ribavirin combinations at concentrations well below their cytotoxic 50 value, but not with the drugs used individually. Deep sequencing of treated viral populations showed that remdesivir, ribavirin, and their combinations evoked significant increases of the number of viral mutations and haplotypes, as well as modification of diversity indices that characterize viral quasi‐species. Conclusion and Implications: SARS‐CoV‐2 extinction can be achieved by synergistic combination treatments based on lethal mutagenesis. In addition, the results offer prospects of triple drug treatments for effective SARS‐CoV‐2 suppression. [ABSTRACT FROM AUTHOR]

ABSTRACT Herpes simplex virus type 1 (HSV-1) is a highly prevalent human pathogen that causes a range of clinical manifestations, including oral and genital herpes, keratitis, encephalitis, and disseminated neonatal disease. Despite its significant health and economic burden, there is currently only a handful of approved antiviral drugs to treat HSV-1 infection. Acyclovir and its analogs are the first-line treatment, but resistance often arises during prolonged treatment periods, such as in immunocompromised patients. Therefore, there is a critical need to identify novel antiviral agents against HSV-1. Here, we performed a drug repurposing screen, testing the ability of 1,900 safe-in-human drugs to inhibit HSV-1 infection in vitro. The screen identified decitabine, a cytidine analog that is used to treat myelodysplastic syndromes and acute myeloid leukemia, as a potent anti-HSV-1 agent. We show that decitabine is effective in inhibiting HSV-1 infection in multiple cell types, including human keratinocytes, that it synergizes with acyclovir, and acyclovir-resistant HSV-1 is still sensitive to decitabine. We further show that decitabine causes G > C and C > G transversions across the viral genome, suggesting it exerts its antiviral activity by lethal mutagenesis, although a role for decitabine’s known targets, DNA methyl-transferases, has not been ruled out.IMPORTANCEHerpes simplex virus type 1 (HSV-1) is a prevalent human pathogen with a limited arsenal of antiviral agents, resistance to which can often develop during prolonged treatment, such as in the case of immunocompromised individuals. Development of novel antiviral agents is a costly and prolonged process, making new antivirals few and far between. Here, we employed an approach called drug repurposing to investigate the potential anti-HSV-1 activity of drugs that are known to be safe in humans, shortening the process of drug development considerably. We identified a nucleoside analog named decitabine as a potent anti-HSV-1 agent in cell culture and investigated its mechanism of action. Decitabine synergizes with the current anti herpetic acyclovir and increases the rate of mutations in the viral genome. Thus, decitabine is an attractive candidate for future studies in animal models to inform its possible application as a novel HSV-1 therapy.

Asexual populations are expected to accumulate deleterious mutations through a process known as Muller’s ratchet. Lynch and colleagues proposed that the ratchet eventually results in a vicious cycle of mutation accumulation and population decline that drives populations to extinction. They called this phenomenon mutational meltdown. Here, we analyze mutational meltdown using a multi-type branching process model where, in the presence of mutation, populations are doomed to extinction. We analyse the change in size and composition of the population and the time of extinction under this model. [ABSTRACT FROM AUTHOR]

Viral quasispecies are distinct but closely related mutants formed by the disparity in viral genomes due to recombination, mutations, competition, and selection pressure. Theoretical derivation for the origin of a quasispecies is owed to the error-prone replication by polymerase and mutants of RNA replicators. Here, we briefly addressed the theoretical and mathematical origin of quasispecies and their dynamics. The impact of quasispecies for major salient human pathogens is reviewed. In the current global scenario, rapid changes in geographical landscapes favor the origin and selection of mutants. It comes as no surprise that a cauldron of mutants poses a significant risk to public health, capable of causing pandemics. Mutation rates in RNA viruses are magnitudes higher than in DNA organisms, explaining their enhanced virulence and evolvability. RNA viruses cause the most devastating pandemics; for example, members of the Orthomyxoviridae family caused the great influenza pandemic (1918 flu or Spanish flu), the SARS (severe acute respiratory syndrome) and MERS (Middle East respiratory syndrome) outbreak, and the human immunodeficiency viruses (HIV), lentiviruses of the Retroviridae family, caused worldwide devastation. Rapidly evolving RNA virus populations are a daunting challenge for the designing of effective control measures like vaccines. Developing awareness of the evolutionary dispositions of RNA viral mutant spectra and what influences their adaptation and virulence will help curtail outbreaks of past and future pathogens. [ABSTRACT FROM AUTHOR]

Herpes simplex virus 1 (HSV-1) encodes a family B DNA polymerase (Pol) capable of exonucleolytic proofreading whose functions have been extensively studied in the past. Early studies on the in vitro activity of purified Pol protein found that the enzymatic functions of the holoenzyme are largely separate. Consequently, exonuclease activity can be reduced or abolished by certain point mutations within catalytically important regions, with no or only minor effects on polymerase activity. Despite unimpaired polymerase activity, the recovery of HSV-1 mutants with a catalytically inactive exonuclease has been so far unsuccessful. Hence, mutations such as D368A, which abolish exonuclease activity, are believed to be lethal. Here, we show that HSV-1 can be recovered in the absence of Pol intrinsic exonuclease activity and demonstrate that a lack of proofreading causes the rapid accumulation of likely detrimental mutations. Although mutations that abolish exonuclease activity do not appear to be lethal, the lack of proofreading yields viruses with a suicidal phenotype that cease to replicate within few passages following reconstitution. Hence, we conclude that high replication fidelity conferred by proofreading is essential to maintain HSV-1 genome integrity and that a lack of exonuclease activity produces an initially viable but rapidly suicidal phenotype. However, stably replicating viruses with reduced exonuclease activity and therefore elevated mutation rates can be generated by mutating a catalytically less important site located within a conserved exonuclease domain. IMPORTANCE Recovery of fully exonuclease-deficient herpes simplex virus 1 (HSV-1) DNA polymerase mutants has been so far unsuccessful. However, exonuclease activity is not known to be directly essential for virus replication, and the lethal phenotype of certain HSV-1 polymerase mutants is thus attributed to factors other than exonuclease activity. Here, we showed that the recovery of a variety of exonuclease-deficient HSV-1 polymerase mutants is possible and that these mutants are initially replication competent. We, however, observed a progressive loss of mutant viability upon cell culture passaging, which coincided with the rapid accumulation of mutations in exonuclease-deficient viruses. We thus concluded that a lack of DNA proofreading in exonuclease-deficient viruses causes an initially viable but rapidly suicidal hypermutator phenotype and, consequently, the extinction of mutant viruses within few generations following recovery. This would make the absence of exonuclease activity the primary reason for the long-reported difficulties in culturing exonuclease-deficient HSV-1 mutants. [ABSTRACT FROM AUTHOR]

Herpes simplex virus type 1 (HSV-1) is a highly prevalent human pathogen that causes a range of clinical manifestations, including oral and genital herpes, keratitis, encephalitis, and disseminated neonatal disease. Despite its significant health and economic burden, there is currently only a handful of approved antiviral drugs to treat HSV-1 infection. Acyclovir and its analogs are the first-line treatment, but resistance often arises during prolonged treatment periods, such as in immunocompromised patients. Therefore, there is a critical need to identify novel antiviral agents against HSV-1. Here, we performed a drug repurposing screen, testing the ability of 1,900 safe-in-human drugs to inhibit HSV-1 infection in vitro . The screen identified decitabine, a cytidine analog that is used to treat myelodysplastic syndromes and acute myeloid leukemia, as a potent anti-HSV-1 agent. We show that decitabine is effective in inhibiting HSV-1 infection in multiple cell types, including human keratinocytes, that it synergizes with acyclovir, and acyclovir-resistant HSV-1 is still sensitive to decitabine. We further show that decitabine causes G > C and C > G transversions across the viral genome, suggesting it exerts its antiviral activity by lethal mutagenesis, although a role for decitabine's known targets, DNA methyl-transferases, has not been ruled out., Importance: Herpes simplex virus type 1 (HSV-1) is a prevalent human pathogen with a limited arsenal of antiviral agents, resistance to which can often develop during prolonged treatment, such as in the case of immunocompromised individuals. Development of novel antiviral agents is a costly and prolonged process, making new antivirals few and far between. Here, we employed an approach called drug repurposing to investigate the potential anti-HSV-1 activity of drugs that are known to be safe in humans, shortening the process of drug development considerably. We identified a nucleoside analog named decitabine as a potent anti-HSV-1 agent in cell culture and investigated its mechanism of action. Decitabine synergizes with the current anti herpetic acyclovir and increases the rate of mutations in the viral genome. Thus, decitabine is an attractive candidate for future studies in animal models to inform its possible application as a novel HSV-1 therapy., Competing Interests: The authors declare no conflict of interest.

Mutagenic drugs are promising candidates for the treatment of various RNA virus infections. Increasing the mutation rate of the virus leads to rapid accumulation of deleterious mutation load, which is proposed to ultimately result in extinction as described by the theoretical concepts of mutational meltdown and lethal mutagenesis. However, the conditions and potential mechanisms of viral escape from the effects of mutagenic drugs have not been conceptually explored. Here we apply a computational approach to quantify the population dynamics and genetics of a population under high mutation rates and discuss the likelihood of adaptation to a mutagenic drug by means of three proposed mechanisms: (1) a proportion of "traditional" beneficial mutations that increase growth/fitness, (2) a mutation rate modifier (i.e., evolution of resistance to the mutagenic drug) that reduces the mutation rate, and (3) a modifier of the distribution of fitness effects, which either decreases or increases deleterious effects of mutations (i.e., evolution of tolerance to the mutagenic drug). We track the population dynamics and genetics of evolving populations and find that successful adaptations have to appear early to override the increasing mutational load and rescue the population from its imminent extinction. We highlight that the observed stochasticity of adaptation, especially by means of modifiers of the distribution of fitness effects, is difficult to capture in experimental trials, which may leave potential dangers of the use of mutagenic treatments unexposed. [ABSTRACT FROM AUTHOR]

We report a quantification of the decrease of effectiveness of antiviral agents directed to hepatitis C virus, when the agents are added during an ongoing infection in cell culture vs. when they are added at the beginning of the infection. Major determinants of the decrease of inhibitory activity are the time post-infection of inhibitor administration and viral replicative fitness. The efficacy decrease has been documented with antiviral assays involving the combination of the direct-acting antiviral agents, daclatasvir and sofosbuvir, and with the combination of the lethal mutagens, favipiravir and ribavirin. The results suggest that strict antiviral effectiveness assays in preclinical trials may involve the use of high fitness viral populations and the delayed administration of the agents, relative to infection onset. [ABSTRACT FROM AUTHOR]

Mutational meltdown describes an eco‐evolutionary process in which the accumulation of deleterious mutations causes a fitness decline that eventually leads to the extinction of a population. Possible applications of this concept include medical treatment of RNA virus infections based on mutagenic drugs that increase the mutation rate of the pathogen. To determine the usefulness and expected success of such an antiviral treatment, estimates of the expected time to mutational meltdown are necessary. Here, we compute the extinction time of a population under high mutation rates, using both analytical approaches and stochastic simulations. Extinction is the result of three consecutive processes: (a) initial accumulation of deleterious mutations due to the increased mutation pressure; (b) consecutive loss of the fittest haplotype due to Muller's ratchet; (c) rapid population decline toward extinction. We find accurate analytical results for the mean extinction time, which show that the deleterious mutation rate has the strongest effect on the extinction time. We confirm that intermediate‐sized deleterious selection coefficients minimize the extinction time. Finally, our simulations show that the variation in extinction time, given a set of parameters, is surprisingly small. [ABSTRACT FROM AUTHOR]

Mutagenic drugs are promising candidates for the treatment of various RNA virus infections. Increasing the mutation rate of the virus leads to rapid accumulation of deleterious mutation load, which is proposed to ultimately result in extinction as described by the theoretical concepts of mutational meltdown and lethal mutagenesis. However, the conditions and potential mechanisms of viral escape from the effects of mutagenic drugs have not been conceptually explored. Here we apply a computational approach to quantify the population dynamics and genetics of a population under high mutation rates and discuss the likelihood of adaptation to a mutagenic drug by means of three proposed mechanisms: (1) a proportion of “traditional” beneficial mutations that increase growth/fitness, (2) a mutation rate modifier (i.e., evolution of resistance to the mutagenic drug) that reduces the mutation rate, and (3) a modifier of the distribution of fitness effects, which either decreases or increases deleterious effects of mutations (i.e., evolution of tolerance to the mutagenic drug). We track the population dynamics and genetics of evolving populations and find that successful adaptations have to appear early to override the increasing mutational load and rescue the population from its imminent extinction. We highlight that the observed stochasticity of adaptation, especially by means of modifiers of the distribution of fitness effects, is difficult to capture in experimental trials, which may leave potential dangers of the use of mutagenic treatments unexposed.

Abstract Mutational meltdown describes an eco‐evolutionary process in which the accumulation of deleterious mutations causes a fitness decline that eventually leads to the extinction of a population. Possible applications of this concept include medical treatment of RNA virus infections based on mutagenic drugs that increase the mutation rate of the pathogen. To determine the usefulness and expected success of such an antiviral treatment, estimates of the expected time to mutational meltdown are necessary. Here, we compute the extinction time of a population under high mutation rates, using both analytical approaches and stochastic simulations. Extinction is the result of three consecutive processes: (a) initial accumulation of deleterious mutations due to the increased mutation pressure; (b) consecutive loss of the fittest haplotype due to Muller's ratchet; (c) rapid population decline toward extinction. We find accurate analytical results for the mean extinction time, which show that the deleterious mutation rate has the strongest effect on the extinction time. We confirm that intermediate‐sized deleterious selection coefficients minimize the extinction time. Finally, our simulations show that the variation in extinction time, given a set of parameters, is surprisingly small.

In RNA viruses, a small increase in their mutation rates can be sufficient to exceed their threshold of viability. Lethal mutagenesis is a therapeutic strategy based on the use of mutagens, driving viral populations to extinction. Extinction catastrophe can be experimentally induced by promutagenic nucleosides in cell culture models. The loss of HIV infectivity has been observed after passage in 5-hydroxydeoxycytidine or 5,6-dihydro-5-aza-2′-deoxycytidine while producing a two-fold increase in the viral mutation frequency. Among approved nucleoside analogs, experiments with polioviruses and other RNA viruses suggested that ribavirin can be mutagenic, although its mechanism of action is not clear. Favipiravir and molnupiravir exert an antiviral effect through lethal mutagenesis. Both drugs are broad-spectrum antiviral agents active against RNA viruses. Favipiravir incorporates into viral RNA, affecting the G→A and C→U transition rates. Molnupiravir (a prodrug of β-d-N4-hydroxycytidine) has been recently approved for the treatment of SARS-CoV-2 infection. Its triphosphate derivative can be incorporated into viral RNA and extended by the coronavirus RNA polymerase. Incorrect base pairing and inefficient extension by the polymerase promote mutagenesis by increasing the G→A and C→U transition frequencies. Despite having remarkable antiviral action and resilience to drug resistance, carcinogenic risks and genotoxicity are important concerns limiting their extended use in antiviral therapy. [ABSTRACT FROM AUTHOR]

Over the past two and a half years the world has seen a desperate scramble to find a treatment for SARS-CoV-2 and COVID. In that regard, nucleosides have long served as the cornerstone to antiviral treatments due to their resemblance to the naturally occurring nucleosides that are involved in numerous biological processes. Unlike other viruses however, it was found early on during the search for drugs to treat SARS-1 and later MERS, that the coronaviruses possess a unique repair enzyme, an exonuclease (ExoN)[3] which rendered nucleoside analogues useless, thus negating their use.[4] During the current outbreak however, as both well-known and new nucleoside analogues were investigated or reinvestigated as a possible cure for SARS-CoV-2, several novel and/or lesser-known mechanisms of action were uncovered. This review briefly describes these mechanisms.

Viral quasispecies are distinct but closely related mutants formed by the disparity in viral genomes due to recombination, mutations, competition, and selection pressure. Theoretical derivation for the origin of a quasispecies is owed to the error-prone replication by polymerase and mutants of RNA replicators. Here, we briefly addressed the theoretical and mathematical origin of quasispecies and their dynamics. The impact of quasispecies for major salient human pathogens is reviewed. In the current global scenario, rapid changes in geographical landscapes favor the origin and selection of mutants. It comes as no surprise that a cauldron of mutants poses a significant risk to public health, capable of causing pandemics. Mutation rates in RNA viruses are magnitudes higher than in DNA organisms, explaining their enhanced virulence and evolvability. RNA viruses cause the most devastating pandemics; for example, members of the Orthomyxoviridae family caused the great influenza pandemic (1918 flu or Spanish flu), the SARS (severe acute respiratory syndrome) and MERS (Middle East respiratory syndrome) outbreak, and the human immunodeficiency viruses (HIV), lentiviruses of the Retroviridae family, caused worldwide devastation. Rapidly evolving RNA virus populations are a daunting challenge for the designing of effective control measures like vaccines. Developing awareness of the evolutionary dispositions of RNA viral mutant spectra and what influences their adaptation and virulence will help curtail outbreaks of past and future pathogens.

We investigated the mutation profiles of severe acute respiratory syndrome coronavirus 2 in samples collected from a molnupiravir and nirmatrelvir/ritonavir combination therapy in macaques. We found that molnupiravir induced several nirmatrelvir resistance mutations at low abundance that were not further selected in combination therapy. Coadministration of nirmatrelvir/ritonavir lowered the magnitude of the mutagenetic effect of molnupiravir., Competing Interests: Potential conflicts of interest. The University of North Carolina is pursuing intellectual property protection for Primer ID and R. S. has received nominal royalties. All other authors report no potential conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed., (© The Author(s) 2024. Published by Oxford University Press on behalf of Infectious Diseases Society of America. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

This study mathematically and computationally analyzes the effects of a combination of a chemical mutagen and an immune checkpoint inhibitor to develop an alternative anti-retroviral therapy. First, we addressed how the condition for HIV cure depends the efficacy of these two agents. We concluded that HIV cure only depends on the efficacy of the chemical mutagen, not on the immune checkpoint inhibitor. In other words, the combinatorial use of the two agents did not affect the necessary condition for the HIV "cured state". On the other hand, we confirmed the effectiveness of the combinatorial treatment with the two agents on the eradication of productively infected cells. Computer simulations clarified that combination treatment with the two agents enables eradication of productively infected cells faster than does treatment with the mutagen alone. [ABSTRACT FROM AUTHOR]

In RNA viruses, a small increase in their mutation rates can be sufficient to exceed their threshold of viability. Lethal mutagenesis is a therapeutic strategy based on the use of mutagens, driving viral populations to extinction. Extinction catastrophe can be experimentally induced by promutagenic nucleosides in cell culture models. The loss of HIV infectivity has been observed after passage in 5-hydroxydeoxycytidine or 5,6-dihydro-5-aza-2′-deoxycytidine while producing a two-fold increase in the viral mutation frequency. Among approved nucleoside analogs, experiments with polioviruses and other RNA viruses suggested that ribavirin can be mutagenic, although its mechanism of action is not clear. Favipiravir and molnupiravir exert an antiviral effect through lethal mutagenesis. Both drugs are broad-spectrum antiviral agents active against RNA viruses. Favipiravir incorporates into viral RNA, affecting the G→A and C→U transition rates. Molnupiravir (a prodrug of β-d-N4-hydroxycytidine) has been recently approved for the treatment of SARS-CoV-2 infection. Its triphosphate derivative can be incorporated into viral RNA and extended by the coronavirus RNA polymerase. Incorrect base pairing and inefficient extension by the polymerase promote mutagenesis by increasing the G→A and C→U transition frequencies. Despite having remarkable antiviral action and resilience to drug resistance, carcinogenic risks and genotoxicity are important concerns limiting their extended use in antiviral therapy.

Following widespread infections of the most recent coronavirus known to infect humans, SARS‐CoV‐2, attention has turned to potential therapeutic options. With no drug or vaccine yet approved, one focal point of research is to evaluate the potential value of repurposing existing antiviral treatments, with the logical strategy being to identify at least a short‐term intervention to prevent within‐patient progression, while long‐term vaccine strategies unfold. Here, we offer an evolutionary/population‐genetic perspective on one approach that may overwhelm the capacity for pathogen defense (i.e., adaptation) – induced mutational meltdown – providing an overview of key concepts, review of previous theoretical and experimental work of relevance, and guidance for future research. Applied with appropriate care, including target specificity, induced mutational meltdown may provide a general, rapidly implemented approach for the within‐patient eradication of a wide range of pathogens or other undesirable microorganisms. [ABSTRACT FROM AUTHOR]

This study tried to mathematically unveil the difficulties associated with an anti-HIV therapy based on the lethal mutagenesis method; two difficulties were identified. The first difficulty of this method was that the efficacy of a mutagen must be over a certain threshold to treat HIV-1 infection, which could explain the apparent failure of the KP-1461 trial. Furthermore, we investigated the influence of the latency of HIV-1 on the treatment period and clarified that the latency increases the treatment period in a manner almost independent of the efficacy of a mutagen. This is the second difficulty associated with the lethal mutagenesis method. [ABSTRACT FROM AUTHOR]

The establishment of a population into a new empty habitat outside of its initial niche is a phenomenon akin to evolutionary rescue in the presence of immigration. It underlies a wide range of processes, such as biological invasions by alien organisms, host shifts in pathogens, or the emergence of resistance to pesticides or antibiotics from untreated areas. We derive an analytically tractable framework to describe the evolutionary and demographic dynamics of asexual populations in a source‐sink system. We analyze the influence of several factors on the establishment success in the sink, and on the time until establishment. To this aim, we use a classic phenotype‐fitness landscape (Fisher's geometrical model in n dimensions) where the source and sink habitats have different phenotypic optima. In case of successful establishment, the mean fitness in the sink follows a typical four‐phases trajectory. The waiting time to establishment is independent of the immigration rate and has a "U‐shaped" dependence on the mutation rate, until some threshold where lethal mutagenesis impedes establishment and the sink population remains so. We use these results to get some insight into possible effects of several management strategies. [ABSTRACT FROM AUTHOR]

The emergence or re-emergence of viruses with epidemic and/or pandemic potential, such as Ebola, Zika, Middle East Respiratory Syndrome (MERS-CoV), Severe Acute Respiratory Syndrome Coronavirus 1 and 2 (SARS and SARS-CoV-2) viruses, or new strains of influenza represents significant human health threats due to the absence of available treatments. Vaccines represent a key answer to control these viruses. However, in the case of a public health emergency, vaccine development, safety, and partial efficacy concerns may hinder their prompt deployment. Thus, developing broad-spectrum antiviral molecules for a fast response is essential to face an outbreak crisis as well as for bioweapon countermeasures. So far, broad-spectrum antivirals include two main categories: the family of drugs targeting the host-cell machinery essential for virus infection and replication, and the family of drugs directly targeting viruses. Among the molecules directly targeting viruses, nucleoside analogues form an essential class of broad-spectrum antiviral drugs. In this review, we will discuss the interest for broad-spectrum antiviral strategies and their limitations, with an emphasis on virus-targeted, broad-spectrum, antiviral nucleoside analogues and their mechanisms of action.

Populations may genetically adapt to severe stress that would otherwise cause their extirpation. Recent theoretical work, combining stochastic demography with Fisher's geometric model of adaptation, has shown how evolutionary rescue becomes unlikely beyond some critical intensity of stress. Increasing mutation rates may however allow adaptation to more intense stress, raising concerns about the effectiveness of treatments against pathogens. This previous work assumes that populations are rescued by the rise of a single resistance mutation. However, even in asexual organisms, rescue can also stem from the accumulation of multiple mutations in a single genome. Here, we extend previous work to study the rescue process in an asexual population where the mutation rate is sufficiently high so that such events may be common. We predict both the ultimate extinction probability of the population and the distribution of extinction times. We compare the accuracy of different approximations covering a large range of mutation rates. Moderate increase in mutation rates favors evolutionary rescue. However, larger increase leads to extinction by the accumulation of a large mutation load, a process called lethal mutagenesis. We discuss how these results could help design "evolution‐proof" antipathogen treatments that even highly mutable strains could not overcome. [ABSTRACT FROM AUTHOR]

RNA viruses comprise vast populations of closely related, but highly genetically diverse, entities known as quasispecies. Understanding the mechanisms by which this extreme diversity is generated and maintained is fundamental when approaching viral persistence and pathobiology in infected hosts. In this paper, we access quasispecies theory through a mathematical model based on the theory of multitype branching processes, to better understand the roles of mechanisms resulting in viral diversity, persistence and extinction. We accomplish this understanding by a combination of computational simulations and the theoretical analysis of the model. In order to perform the simulations, we have implemented the mathematical model into a computational platform capable of running simulations and presenting the results in a graphical format in real time. Among other things, we show that the establishment of virus populations may display four distinct regimes from its introduction into new hosts until achieving equilibrium or undergoing extinction. Also, we were able to simulate different fitness distributions representing distinct environments within a host which could either be favorable or hostile to the viral success. We addressed the most used mechanisms for explaining the extinction of RNA virus populations called lethal mutagenesis and mutational meltdown. We were able to demonstrate a correspondence between these two mechanisms implying the existence of a unifying principle leading to the extinction of RNA viruses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Viral agents have been continuing to cause life threatening chronic infections, deaths and economic losses in humans and animals. Recently a limited number of antiviral drug is available to treat human, animal and zoonotic virus infections. The emergence of resistance to these antivirals has created an obstacle for the treatment of patients who infected with viruses. To struggle with viral infections, there is need to prevent the emergence of antiviral drug resistance by developing the new strategies or tactics. Recently fidelity variants and lethal mutagenesis is considered to be an effective strategy for the prevention of drug resistance. This review summarises the new strategies and antiviral mechanisms being tried to develop in order to fight viral infections. [ABSTRACT FROM AUTHOR]

Venezuelan equine encephalitis virus (VEEV) is a representative member of the New World alphaviruses. It is transmitted by mosquito vectors and causes highly debilitating disease in humans, equids, and other vertebrate hosts. Despite a continuous public health threat, very few compounds with anti-VEEV activity in cell culture and in mouse models have been identified to date, and rapid development of virus resistance to some of them has been recorded. In this study, we investigated the possibility of using a modified nucleoside analog, + -D-N4-hydroxycytidine (NHC), as an anti-VEEV agent and defined the mechanism of its anti-VEEV activity. The results demonstrate that NHC is a very potent antiviral agent. It affects both the release of genome RNA-containing VEE virions and their infectivity. Both of these antiviral activities are determined by the NHC-induced accumulation of mutations in virus-specific RNAs. The antiviral effect is most prominent when NHC is applied early in the infectious process, during the amplification of negative- and positive-strand RNAs in infected cells. Most importantly, only a low-level resistance of VEEV to NHC can be developed, and it requires acquisition and cooperative function of more than one mutation in nsP4. These adaptive mutations are closely located in the same segment of nsP4. Our data suggest that NHC is more potent than ribavirin as an anti- VEEV agent and likely can be used to treat other alphavirus infections. IMPORTANCE Venezuelan equine encephalitis virus (VEEV) can cause widespread epidemics among humans and domestic animals. VEEV infections result in severe meningoencephalitis and long-term sequelae. No approved therapeutics exist for treatment of VEEV infections. Our study demonstrates that + -D-N4-hydroxycytidine (NHC) is a very potent anti-VEEV compound, with the 50% effective concentration being below 1 μM. The mechanism of NHC antiviral activity is based on induction of high mutation rates in the viral genome. Accordingly, NHC treatment affects both the rates of particle release and the particle infectivity. Most importantly, in contrast to most of the anti-alphavirus drugs that are under development, resistance of VEEV to NHC develops very inefficiently. Even low levels of resistance require acquisition of multiple mutations in the gene of the VEEV-specific RNA-dependent RNA polymerase nsP4. [ABSTRACT FROM AUTHOR]

Current advancements in antiretroviral therapy (ART) have turned HIV-1 infection into a chronic and manageable disease. However, treatment is only effective until HIV-1 develops resistance against the administered drugs. The most recent antiretroviral drugs have become superior at delaying the evolution of acquired drug resistance. In this review, the viral fitness and its correlation to HIV-1 mutation rates and drug resistance are discussed while emphasizing the concept of lethal mutagenesis as an alternative therapy. The development of resistance to the different classes of approved drugs and the importance of monitoring antiretroviral drug resistance are also summarized briefly.

Chronic viral disease constitutes a major global health problem, with several hundred million people affected and an associated elevated number of deaths. An increasing number of disorders caused by human flaviviruses are related to their capacity to establish a persistent infection. Here we show that Usutu virus (USUV), an emerging zoonotic flavivirus linked to sporadic neurologic disease in humans, can establish a persistent infection in cell culture. Two independent lineages of Vero cells surviving USUV lytic infection were cultured over 82 days (41 cell transfers) without any apparent cytopathology crisis associated. We found elevated titers in the supernatant of these cells, with modest fluctuations during passages but no overall tendency towards increased or decreased infectivity. In addition to full-length genomes, viral RNA isolated from these cells at passage 40 revealed the presence of defective genomes, containing different deletions at the 5’ end. These truncated transcripts were all predicted to encode shorter polyprotein products lacking membrane and envelope structural proteins, and most of non-structural protein 1. Treatment with different broad-range antiviral nucleosides revealed that USUV is sensitive to these compounds in the context of a persistent infection, in agreement with previous observations during lytic infections. The exposure of infected cells to prolonged treatment (10 days) with favipiravir and/or ribavirin resulted in the complete clearance of infectivity in the cellular supernatants (decrease of ~5 log10 in virus titers and RNA levels), although modest changes in intracellular viral RNA levels were recorded (10 decrease). Drug withdrawal after treatment day 10 resulted in a relapse in virus titers. These results encourage the use of persistently-infected cultures as a surrogate system in the identification of improved antivirals against flaviviral chronic disease.

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 themutagen 5-fluorouracil (FU) through a single polymerase substitution that prevents an excess of A to Gand U to C transitions evoked by FU on thewild-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 associatedwith 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 lethalmutagenesis-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. [ABSTRACT FROM AUTHOR]

RNA viruses replicate with very high error rates, which makes them more sensitive to additional increases in this parameter. This fact has inspired an antiviral strategy named lethal mutagenesis, which is based on the artificial increase of the error rate above a threshold incompatible with virus infectivity. A relevant issue concerning lethal mutagenesis is whether incomplete treatments might enhance the adaptive possibilities of viruses. We have addressed this question by subjecting an RNA virus, the bacteriophage Qβ, to different transmission regimes in the presence or the absence of sublethal concentrations of the mutagenic nucleoside analogue 5-azacytidine (AZC). Populations obtained were subsequently exposed to a non-optimal temperature and analyzed to determine their consensus sequences. Our results show that previously mutagenized populations rapidly fixed a specific set of mutations upon propagation at the new temperature, suggesting that the expansion of the mutant spectrum caused by AZC has an influence on later evolutionary behavior. [ABSTRACT FROM AUTHOR]

Although many compounds have been approved for the treatment of human immunodeficiency type-1 (HIV-1) infection, additional anti-HIV-1 drugs (particularly those belonging to new drug classes) are still needed due to issues such as long-term drug-associated toxicities, transmission of drug-resistant variants, and development of multi-class resistance. Lethal mutagenesis represents an antiviral strategy that has not yet been clinically translated for HIV-1 and is based on the use of small molecules to induce excessive levels of deleterious mutations within the viral genome. Here, we show that 5-azacytidine (5-aza-C), a ribonucleoside analog that induces the lethal mutagenesis of HIV-1, and multiple inhibitors of the enzyme ribonucleotide reductase (RNR) interact in a synergistic fashion to more effectively reduce the infectivity of HIV-1. In these drug combinations, RNR inhibitors failed to significantly inhibit the conversion of 5-aza-C to 5-aza-2′-deoxycytidine, suggesting that 5-aza-C acts primarily as a deoxyribonucleoside even in the presence of RNR inhibitors. The mechanism of antiviral synergy was further investigated for the combination of 5-aza-C and one specific RNR inhibitor, resveratrol, as this combination improved the selectivity index of 5-aza-C to the greatest extent. Antiviral synergy was found to be primarily due to the reduced accumulation of reverse transcription products rather than the enhancement of viral mutagenesis. To our knowledge, these observations represent the first demonstration of antiretroviral synergy between a ribonucleoside analog and RNR inhibitors, and encourage the development of additional ribonucleoside analogs and RNR inhibitors with improved antiretroviral activity. [ABSTRACT FROM AUTHOR]

Human noroviruses are a leading cause of gastroenteritis worldwide, yet there are no licensed antivirals. There is an urgent need for norovirus therapeutics, particularly for chronic infections in immunocompromised individuals, but also a potential need for prophylactic use in epidemics. Continued research has led to the identification of compounds that inhibit norovirus replication in vitro and, at least in some cases, are also effective in vivo against murine norovirus. Progress has included classical approaches targeting viral proteins and harnessing the antiviral action of interferon, strategies targeting essential host cell factors, and novel strategies exploiting the high mutation rate of noroviruses. [ABSTRACT FROM AUTHOR]

Lethal mutagenesis has emerged as a novel potential therapeutic approach to treat viral infections. Several studies have demonstrated that increases in the high mutation rates inherent to RNA viruses lead to viral extinction in cell culture, but evidence during infections in vivo is limited. In this study, we show that the broad-range antiviral nucleoside favipiravir reduces viral load in vivo by exerting antiviral mutagenesis in a mouse model for norovirus infection. Increased mutation frequencies were observed in samples from treated mice and were accompanied with lower or in some cases undetectable levels of infectious virus in faeces and tissues. Viral RNA isolated from treated animals showed reduced infectivity, a feature of populations approaching extinction during antiviral mutagenesis. These results suggest that favipiravir can induce norovirus mutagenesis in vivo, which in some cases leads to virus extinction, providing a proof-of-principle for the use of favipiravir derivatives or mutagenic nucleosides in the clinical treatment of noroviruses.

We report that ribavirin exerts an inhibitory and mutagenic activity on SARS-CoV-2-infecting Vero cells, with a therapeutic index higher than 10. Deep sequencing analysis of the mutant spectrum of SARS-CoV-2 replicating in the absence or presence of ribavirin indicated an increase in the number of mutations, but not in deletions, and modification of diversity indices, expected from a mutagenic activity. Notably, the major mutation types enhanced by replication in the presence of ribavirin were A→G and U→C transitions, a pattern which is opposite to the dominance of G→A and C→U transitions previously described for most RNA viruses. Implications of the inhibitory activity of ribavirin, and the atypical mutational bias produced on SARS-CoV-2, for the search for synergistic anti-COVID-19 lethal mutagen combinations are discussed.

Current advancements in antiretroviral therapy (ART) have turned HIV-1 infection into a chronic and manageable disease. However, treatment is only effective until HIV-1 develops resistance against the administered drugs. The most recent antiretroviral drugs have become superior at delaying the evolution of acquired drug resistance. In this review, the viral fitness and its correlation to HIV-1 mutation rates and drug resistance are discussed while emphasizing the concept of lethal mutagenesis as an alternative therapy. The development of resistance to the different classes of approved drugs and the importance of monitoring antiretroviral drug resistance are also summarized briefly. [ABSTRACT FROM AUTHOR]