1 Molecular, Cellular and Developmental Biology Graduate Program, Ohio State University, Columbus, OH 43210; 2 Ohio State Biochemistry Graduate Program, Ohio State University, Columbus, OH 43210; 3 Integrated Biomedical Science Graduate Program, Ohio State University, Columbus, OH 43210; 4 Department of Molecular Virology, Immunology, and Medical Genetics, Center for Retrovirus Research, and Comprehensive Cancer Center Ohio State University Medical Center, Columbus, OH 43210, USA
Keywords: NRTIs, resistance, evolution, mutagenesis, retrovirus
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Introduction |
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In this leading article, we discuss literature that shows a correlation between the evolution of drug resistance and increased HIV and other pathogen mutation rates. In the case of HIV, NRTIs and NRTI drug resistance can increase HIV mutation rates, and can act together to further increase these rates. These increased mutation rates predict that drug failure during initial treatment could increase the probability of subsequent drug therapy failures due to the selection of mutator strains. However, increased mutagenesis of HIV by NRTIs could be viewed as an advantage in therapies directed at extinguishing virus infectivity by lethal mutagenesis.
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NRTIs, drug resistance and HIV mutagenesis |
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Recent studies with other NRTIs [didanosine (ddI), stavudine (d4T) and abacavir (ABC); Figure 1] indicate that NRTI drug treatment may generally lead to increased virus mutant frequencies during HIV-1 replication (R. Chen and L. M. Mansky, unpublished observations).3 The way in which NRTIs increase HIV-1 mutant frequencies may involve a similar mechanism, since it has been observed that virus mutant frequencies increase in an additive manner during virus replication in the presence of two NRTIs (e.g. AZT and 3TC, AZT and ddI, and 3TC and ddI).3 Hypotheses proposed to explain how NRTIs influence mutation rates include: (i) NRTIs alter nucleotide pools; (ii) NTRIs are incorporated into plus-strand DNA and may result in discontinuous DNA synthesis of viral DNAs with proper ends that integrate with subsequent error-prone repair by the host cell; and (iii) NRTIs may bind non-catalytically to RT and cause a conformational change that influences enzyme fidelity.4
These observations suggest that when virus replication occurs in the presence of suboptimal concentrations of drug, drug-resistant virus is selected for, and that replication of drug-resistant virus in the presence of drug could further increase the virus mutation rate. To test this hypothesis, the combined effects of drug and drug-resistant virus were analysed.4 It was found that the replication of AZT-resistant HIV-1 in the presence of AZT led to a multiplicative 24-fold increase in the virus mutant frequency compared with that observed with wild-type virus in the absence of drug (Table 1). This indicates that when drug failure occurs due to the evolution of drug resistance, replication of the drug-resistant virus in the presence of AZT significantly increases HIV-1 mutagenesis. In addition, it was observed that replication of an AZT/3TC dual-resistant virus in the presence of AZT and 3TC also led to a multiplicative 22.5-fold increase in mutant frequencies (Table 1). Thus, each of these drugs tested acted together with drug-resistant RT and increased virus mutant frequencies.
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Salvage therapy and increased HIV mutagenesis |
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Interestingly, different drugs used in conjunction with the AZT-resistant virus led to a similar multiplicative increase in virus mutant frequencies.4 This indicates that when new drugs are added in drug therapy regimens, they could also act with the drug-resistant virus to further increase virus mutant frequencies, even though the drug-resistance phenotype is associated with another drug. For example, 3TC increased mutant frequencies of AZT-resistant virus to 13.6-fold compared with that of wild-type virus in the absence of drug (Table 1). Hydroxyurea, a well-documented drug used in HIV-1 treatment, is known to alter intracellular dNTP pools by inhibiting ribonucleotide reductase, and results in a depletion of all dNTPs. AZT-resistant HIV-1 replication in the presence of hydroxyurea resulted in a 21.8-fold increase in mutant frequencies compared with that observed in the absence of drug (Table 1). Like hydroxyurea, thymidine has also been shown to alter intracellular dNTP pools and in addition has been shown to increase retrovirus mutation rates. AZT-resistant HIV-1 replication in the presence of thymidine increased mutant frequencies by 16.7-fold (Table 1). Thioguanine (an anti-leukaemic agent that has been reported to inhibit RNaseH activity) has been shown to increase HIV-1 mutant frequencies by four-fold and to significantly alter mutant frequencies during virus replication with RTs containing mutations not associated with the drug.2 These data suggest that subsequent therapies could lead to increased HIV-1 mutagenesis even though the drug-resistant phenotype is not directed against the new drug(s) used in the drug therapy regimen.
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Therapeutic application of increased HIV-1 mutagenesis by NRTIs |
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An intentional increase in mutation rate has been speculated as a rational approach for antiviral treatment of RNA virus infections.5 RNA viruses have high mutation rates and are particularly vulnerable to increases in mutation rate that could extinguish virus replication by error catastrophe. There are few nucleoside analogues that are clinically effective in blocking RNA virus replication. Ribavirin (1-ß-D-ribofuranosyl-1,2,4-triazole-3-carboxamide), a ribonucleoside analogue, has antiviral activity that can be mutagenic in some cases,6 but not in others.7
Promutagenic nucleoside analogues, which are incorporated into the viral genome during nucleic acid replication and result in a progressive accumulation of mutations that would ultimately lead to a drastic reduction in virus replication and fitness, have been used to extinguish HIV-1 replication.8 Given that the majority of mutations are deleterious, selection against such variants would reduce virus yield within a single cycle of replication and allow the maintenance of some significant level of virus fitness within the population. Based upon the analysis of HIV-1 mutation rates, an 30-fold increase in mutation rate would be necessary to extinguish infectious virus replication.4 The success of eliminating HIV-1 replication by this approach (called lethal mutagenesis, or error catastrophe), has yet to be tested clinically. Systematic analysis of NRTIs could identify NRTI combinations that increase HIV mutagenesis and enhance the likelihood of lethal mutagenesis.
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Antimicrobial drug resistance and increased pathogen mutation rates |
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Conclusions |
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Acknowledgements |
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Footnotes |
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References |
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2
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Mansky, L. M., Le Rouzic, E., Benichou, S. et al. (2003). Influence of reverse transcriptase variants, drugs, and Vpr on human immunodeficiency virus type 1 mutant frequencies. Journal of Virology 77, 207180.
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4
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Mansky, L. M., Pearl, D. K. & Gajary, L. C. (2002). Combination of drugs and drug-resistant reverse transcriptase results in a multiplicative increase of human immunodeficiency virus type 1 mutant frequencies. Journal of Virology 76, 92539.
5
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Drake, J. W. & Holland, J. J. (1999). Mutation rates among RNA viruses. Proceedings of the National Academy of Sciences, USA 96, 139103.
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7 . Ruiz-Jarabo, C. M., Ly, C., Domingo, E. et al. (2003). Lethal mutagenesis of the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV). Virology 308, 3747.[CrossRef][ISI][Medline]
8
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Loeb, L. A., Essigmann, J. M., Kazazi, F. et al. (1999). Lethal mutagenesis of HIV with mutagenic nucleoside analogs. Proceedings of the National Academy of Sciences, USA 96, 14927.
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