Transmitted human immunodeficiency virus type 1 carrying the D67N or K219Q/E mutation evolves rapidly to zidovudine resistance in vitro and shows a high replicative fitness in the presence of zidovudine

A Case of in utero Transmission of Drug-resistant HIV in the United States

Thanks to the development of antiretroviral drugs and the implementation of routine perinatal prophylaxis, primarily containing zidovudine, modern-day rates of perinatal transmission of HIV are very low in developed countries. We present a case of perinatal transmission of HIV with extensive nucleoside reverse transcriptase inhibitor resistance as a reminder that perinatal transmission of resistance mutations can occur. This case calls for further investigation into the utility of using genotype to determine neonatal prophylaxis in the setting of maternal HIV drug resistance.

An MHV-68 Mutator Phenotype Mutant Virus, Confirmed by CRISPR/Cas9-Mediated Gene Editing of the Viral DNA Polymerase Gene, Shows Reduced Viral Fitness

Drug resistance studies on human γ-herpesviruses are hampered by the absence of an in vitro system that allows efficient lytic viral replication. Therefore, we employed murine γ-herpesvirus-68 (MHV-68) that efficiently replicates in vitro as a model to study the antiviral resistance of γ-herpesviruses. In this study, we investigated the mechanism of resistance to nucleoside (ganciclovir (GCV)), nucleotide (cidofovir (CDV), HPMP-5azaC, HPMPO-DAPy) and pyrophosphate (foscarnet (PFA)) analogues and the impact of these drug resistance mutations on viral fitness. Viral fitness was determined by dual infection competition assays, where MHV-68 drug-resistant viral clones competed with the wild-type virus in the absence and presence of antivirals. Using next-generation sequencing, the composition of the viral populations was determined at the time of infection and after 5 days of growth. Antiviral drug resistance selection resulted in clones harboring mutations in the viral DNA polymerase (DP), denoted Y383S^GCV, Q827R^HPMP-5azaC, G302W^PFA, K442T^PFA, G302W+K442T^PFA, C297W^HPMPO-DAPy and C981Y^CDV. Without antiviral pressure, viral clones Q827R^HPMP-5azaC, G302W^PFA, K442T^PFA and G302W+K442T^PFA grew equal to the wild-type virus. However, in the presence of antivirals, these mutants had a growth advantage over the wild-type virus that was moderately to very strongly correlated with antiviral resistance. The Y383S^GCV mutant was more fit than the wild-type virus with and without antivirals, except in the presence of brivudin. The C297W and C981Y changes were associated with a mutator phenotype and had a severely impaired viral fitness in the absence and presence of antivirals. The mutator phenotype caused by C297W in MHV-68 DP was validated by using a CRISPR/Cas9 genome editing approach.

Avoiding Drug Resistance in HIV Reverse Transcriptase

HIV reverse transcriptase (RT) is an enzyme that plays a major role in the replication cycle of HIV and has been a key target of anti-HIV drug development efforts. Because of the high genetic diversity of the virus, mutations in RT can impart resistance to various RT inhibitors. As the prevalence of drug resistance mutations is on the rise, it is necessary to design strategies that will lead to drugs less susceptible to resistance. Here we provide an in-depth review of HIV reverse transcriptase, current RT inhibitors, novel RT inhibitors, and mechanisms of drug resistance. We also present novel strategies that can be useful to overcome RT's ability to escape therapies through drug resistance. While resistance may not be completely avoidable, designing drugs based on the strategies and principles discussed in this review could decrease the prevalence of drug resistance.

Sexual intermingling of Arab and Jewish MSM in Israel: results of a molecular epidemiology study

OBJECTIVES

MSM comprise ∼30% of new HIV infections in Israel, a country with mixed Jewish and Arab populations. We molecularly characterized HIV-1 in the Arab and Jewish MSM (AMSM, JMSM) populations to reveal possible interethnical connections.

DESIGN

Cross-sectional study.

METHODS

All Israeli-born, HIV-1-infected MSM diagnosed between 2005 and 2016 (n = 1143) were cross-matched with the National Civil Registry to identify religion (Jews/Muslims/Christians). Transmitted drug-resistance mutations (TDRM) and HIV-1 subtypes were determined on the first partial protease and reverse transcriptase sequences from treatment-naive patients and phylogenetic trees were constructed.

RESULTS

Among MSM, 6.4% (73/1143) were Arabs and 93.6% (1070/1143) were Jews. Interestingly, a higher proportion of Arabs was identified among non-MSM (19%, 46/247 versus 6.4%, 73/1143, P < 0.01). Subtype analysis of 62 HIV-1 AMSM and 440 randomly selected HIV-1 JMSM sequences revealed 80.6, 8.1, 4.8 and 6.5% of AMSM and 82.3, 9.5, 4.1 and 4.1% of JMSM had B, A, C and non-A/B/C, respectively. Overall, 13.1% (66/502) had TDRM; reverse transcriptase-K103N/S, M184 V, T215S and protease-L90M were the most common. TDRM prevalence was not significantly higher in JMSM compared to AMSM (P = 0.1) and no temporal changes were observed in their frequency. Phylogenetic analysis demonstrated AMSM and JMSM clusters including L90M, K103N/S or T215S TDRM.

CONCLUSION

Intermingling of AMSM and JMSM HIV-1 in clusters of HIV-1 sequences suggest interethnical sexual contacts among these MSM. Interventions aiming to prevent HIV-transmission in MSM should similarly address both populations groups. The high TDRM frequency requires continuation of resistance testing.

Pretreatment integrase strand transfer inhibitor resistance in North Carolina from 2010-2016

OBJECTIVE

We sought to define the prevalence of pretreatment integrase strand transfer inhibitor (INSTI) resistance and assess the transmission networks of those with pretreatment INSTI resistance.

DESIGN

A retrospective cohort study of HIV-positive patients with genotypic resistance testing sent to a single referral laboratory in North Carolina between 2010 and 2016.

METHODS

We linked genotype and public health data for in-care HIV-positive individuals to determine the prevalence of INSTI resistance among treatment-naive (defined as those with a first genotype ≤3 months after diagnosis) and treatment-experienced (defined as those with a first genotype >3 months after diagnosis) patients. We performed molecular and phylogenetic analyses to assess whether pretreatment INSTI resistance mutations represented clustered HIV transmission.

RESULTS

Of 8825 individuals who contributed sequences for protease, reverse transcriptase, or INSTI genotypic resistance testing during the study period, 2784 (31%) contributed at least one sequence for INSTI resistance testing. Of these, 840 were treatment-naive individuals and 20 [2.4%, 95% confidence interval (CI): 1.5, 3.6%] had INSTI mutations; only two (0.2%, 95% CI: 0.02, 0.9%) had major mutations. Of 1944 treatment-experienced individuals, 9.6% (95% CI: 8.3, 11.0%) had any INSTI mutation and 7.0% (95% CI: 5.9, 8.3%) had major mutations; the prevalence of INSTI mutations among treatment-experienced patients decreased overtime (P < 0.001). In total 12 of 20 individuals with pretreatment INSTI mutations were part of 10 molecular transmission clusters; only one cluster shared identical minor mutations.

CONCLUSION

The prevalence of major pretreatment INSTI resistance is very low. Pretreatment INSTI mutations do not appear to represent clustered HIV transmission.

Determination of Phenotypic Resistance Cutoffs From Routine Clinical Data

Supplemental Digital Content is Available in the Text.

Background:

HIV-1 drug resistance can be measured with phenotypic drug-resistance tests. However, the output of these tests, the resistance factor (RF), requires interpretation with respect to the in vivo activity of the tested variant. Specifically, the dynamic range of the RF for each drug has to be divided into a suitable number of clinically meaningful intervals.

Methods:

We calculated a susceptible-to-intermediate and an intermediate-to-resistant cutoff per drug for RFs predicted by geno2pheno_[resistance]. Probability densities for therapeutic success and failure were estimated from 10,444 treatment episodes. The density estimation procedure corrects for the activity of the backbone drug compounds and for therapy failure without drug resistance. For estimating the probability of therapeutic success given an RF, we fit a sigmoid function. The cutoffs are given by the roots of the third derivative of the sigmoid function.

Results:

For performance assessment, we used geno2pheno_[resistance] RF predictions and the cutoffs for predicting therapeutic success in 2 independent sets of therapy episodes. HIVdb was used for performance comparison. On one test set (n = 807), our cutoffs and HIVdb performed equally well receiver operating characteristic curve [(ROC)–area under the curve (AUC): 0.68]. On the other test set (n = 917), our cutoffs (ROC–AUC: 0.63) and HIVdb (ROC–AUC: 0.65) performed comparatively well.

Conclusions:

Our method can be used for calculating clinically relevant cutoffs for (predicted) RFs. The method corrects for the activity of the backbone drug compounds and for therapy failure without drug resistance. Our method's performance is comparable with that of HIVdb. RF cutoffs for the latest version of geno2pheno_[resistance] have been estimated with this method.

Using drug exposure for predicting drug resistance - A data-driven genotypic interpretation tool

Antiretroviral treatment history and past HIV-1 genotypes have been shown to be useful predictors for the success of antiretroviral therapy. However, this information may be unavailable or inaccurate, particularly for patients with multiple treatment lines often attending different clinics. We trained statistical models for predicting drug exposure from current HIV-1 genotype. These models were trained on 63,742 HIV-1 nucleotide sequences derived from patients with known therapeutic history, and on 6,836 genotype-phenotype pairs (GPPs). The mean performance regarding prediction of drug exposure on two test sets was 0.78 and 0.76 (ROC-AUC), respectively. The mean correlation to phenotypic resistance in GPPs was 0.51 (PhenoSense) and 0.46 (Antivirogram). Performance on prediction of therapy-success on two test sets based on genetic susceptibility scores was 0.71 and 0.63 (ROC-AUC), respectively. Compared to geno2pheno_[resistance], our novel models display a similar or superior performance. Our models are freely available on the internet via www.geno2pheno.org. They can be used for inferring which drug compounds have previously been used by an HIV-1-infected patient, for predicting drug resistance, and for selecting an optimal antiretroviral therapy. Our data-driven models can be periodically retrained without expert intervention as clinical HIV-1 databases are updated and therefore reduce our dependency on hard-to-obtain GPPs.

High Prevalence of HIV Low Abundance Drug-Resistant Variants in a Treatment-Naive Population in North Rift Kenya

The advent of antiretroviral treatment (ART) has resulted in a dramatic reduction in AIDS-related morbidity and mortality. However, the emergence and spread of antiretroviral drug resistance (DR) threaten to negatively impact treatment regimens and compromise efforts to control the epidemic. It is recommended that surveillance of drug resistance occur in conjunction with scale-up efforts to ensure that appropriate first-line therapy is offered relative to the resistance that exists. However, standard resistance testing methods used in Sub-Saharan Africa rely on techniques that do not include low abundance DR variants (LADRVs) that have been documented to contribute to treatment failure. The use of next generation sequencing (NGS) has been shown to be more sensitive to LADRVS. We have carried out a preliminary investigation using NGS to determine the prevalence of LDRVS among a drug-naive population in North Rift Kenya. Antiretroviral-naive patients attending a care clinic in North Rift Kenya were requested to provide and with consent provided blood samples for DR analysis. DNA was extracted and amplified and nested PCR was conducted on the pol RT region using primers tagged with multiplex identifiers (MID). Resulting PCR amplicons were purified, quantified, and pyrosequenced using a GS FLX Titanium PicoTiterPlate (Roche). Valid pyrosequencing reads were aligned with HXB-2 and the frequency and distribution of nucleotide and amino acid changes were determined using an in-house Perl script. DR mutations were identified using the IAS-USA HIV DR mutation database. Sixty samples were successfully sequenced of which 26 were subtype A, 9 were subtype D, 2 were subtype C, and the remaining were recombinants. Forty-six (76.6%) had at least one drug resistance mutation, with 25 (41.6%) indicated as major and the remaining 21 (35%) indicated as minor. The most prevalent mutation was NRTI position K219Q/R (11/46, 24%) followed by NRTI M184V (5/46, 11%) and NNRTI K103N (4/46, 9%). Our use of NGS technology revealed a high prevalence of LADRVs among drug-naive populations in Kenya, a region with predominantly non-B subtypes. The impact of these mutations on the clinical outcome of ART can be ascertained only through long-term follow-up.

Pairwise growth competition assay for determining the replication fitness of human immunodeficiency viruses

In vitro fitness assays are essential tools for determining viral replication fitness for viruses such as HIV-1. Various measurements have been used to extrapolate viral replication fitness, ranging from the number of viral particles per infectious unit, growth rate in cell culture, and relative fitness derived from multiple-cycle growth competition assays. Growth competition assays provide a particularly sensitive measurement of fitness since the viruses are competing for cellular targets under identical growth conditions. There are several experimental factors to consider when conducting growth competition assays, including the multiplicity of infection (MOI), sampling times, and viral detection and fitness calculation methods. Each factor can affect the end result and hence must be considered carefully during the experimental design. The protocol presented here includes steps from constructing a new recombinant HIV-1 clone to performing growth competition assays and analyzing the experimental results. This protocol utilizes experimental parameter values previously shown to yield consistent and robust results. Alternatives are discussed, as some parameters need to be adjusted according to the cell type and viruses being studied. The protocol contains two alternative viral detection methods to provide flexibility as the availability of instruments, reagents and expertise varies between laboratories.

Silent mutations at codons 65 and 66 in reverse transcriptase alleviate indel formation and restore fitness in subtype B HIV-1 containing D67N and K70R drug resistance mutations

Resistance to combined antiretroviral therapy (cART) in HIV-1-infected individuals is typically due to nonsynonymous mutations that change the protein sequence; however, the selection of synonymous or ‘silent’ mutations in the HIV-1 genome with cART has been reported. These silent K65K and K66K mutations in the HIV-1 reverse transcriptase (RT) occur in over 35% of drug-experienced individuals and are highly associated with the thymidine analog mutations D67N and K70R, which confer decreased susceptibility to most nucleoside and nucleotide RT inhibitors. However, the basis for selection of these silent mutations under selective drug pressure is unknown. Using Illumina next-generation sequencing, we demonstrate that the D67N/K70R substitutions in HIV-1 RT increase indel frequency by 100-fold at RT codons 65–67, consequently impairing viral fitness. Introduction of either K65K or K66K into HIV-1 containing D67N/K70R reversed the error-prone DNA synthesis at codons 65–67 in RT and improved viral replication fitness, but did not impact RT inhibitor drug susceptibility. These data provide new mechanistic insights into the role of silent mutations selected during antiretroviral therapy and have broader implications for the relevance of silent mutations in the evolution and fitness of RNA viruses.

Impact of human immunodeficiency virus type-1 sequence diversity on antiretroviral therapy outcomes

Worldwide circulating HIV-1 genomes show extensive variation represented by different subtypes, polymorphisms and drug-resistant strains. Reports on the impact of sequence variation on antiretroviral therapy (ART) outcomes are mixed. In this review, we summarize relevant published data from both resource-rich and resource-limited countries in the last 10 years on the impact of HIV-1 sequence diversity on treatment outcomes. The prevalence of transmission of drug resistant mutations (DRMs) varies considerably, ranging from 0% to 27% worldwide. Factors such as geographic location, access and availability to ART, duration since inception of treatment programs, quality of care, risk-taking behaviors, mode of transmission, and viral subtype all dictate the prevalence in a particular geographical region. Although HIV-1 subtype may not be a good predictor of treatment outcome, review of emerging evidence supports the fact that HIV-1 genome sequence-resulting from natural polymorphisms or drug-associated mutations-matters when it comes to treatment outcomes. Therefore, continued surveillance of drug resistant variants in both treatment-naïve and treatment-experienced populations is needed to reduce the transmission of DRMs and to optimize the efficacy of the current ART armamentarium.

Trends in prevalence of HIV-1 drug resistance in Thailand 2009-2010

BACKGROUND

Treatment failure of antiretroviral therapy in HIV-1 infection is increasing due to development of viral resistance. Trends of resistance-associated mutation lead to the ineffective treatment in HIV-infected individuals.

METHODS

Extracted viral RNA from HIV-infected subjects in 2009 to 2010 was performed. The genotypic resistance testing was investigated for HIV-1 drug resistance in RT and PR genes. Frequencies of mutation were compared by a Fischer's exact test.

RESULTS

Three hundred and sixty-nine samples (147 in 2009 and 222 in 2010) were genotyped. At least one mutation was found in 90.8% (335/369) in PR gene and 87.0% (321/369) in RT gene. Three sequences in PR gene, M36I, H69K, and L90M, were decreased significantly in 2010 when compared to 2009. Mutations associated with resistance to nucleoside analogue reverse transcriptase inhibitors (NRTI's) were found in 61.0% and 64.2% in nonnucleoside analogue reverse transcriptase inhibitors (NNRTI's). A total of 49.6% was found in combined NRTI and NNRTI. In 2010, M41L was increased significantly from 7.5% to 14.9%. However, there was a decrease in the frequency of the mutations at position 67, 70, and 184 between 2009 and 2010.

CONCLUSIONS

In 2010, three mutations in PR gene, M36I, H69K, and L90M, were decreased significantly. However, only one mutation in RT gene, M41L was significantly increased.

Local phylogenetic analysis identifies distinct trends in transmitted HIV drug resistance: implications for public health interventions

BACKGROUND

HIV transmitted drug resistance (TDR) surveillance is usually conducted by sampling from a large population. However, overall TDR prevalence results may be inaccurate for many individual clinical setting. We analyzed HIV genotypes at a tertiary care setting in Ottawa, Ontario in order to evaluate local TDR patterns among sub-populations.

METHOD

Genotyping reports were digitized from ART naïve patients followed at the Immunodeficiency Clinic at the Ottawa Hospital, between 2008 and 2010. Quality controlled, digitized sequence data were assessed for TDR using the Stanford HIV Database. Patient characteristics were analyzed according to TDR patterns. Finally, a phylogenetic tree was constructed to elucidate the observed pattern of HIV TDR.

RESULTS

Among the 155 clinic patients there was no statistically significantly difference in demographics as compared to the Ontario provincial HIV population. The clinic prevalence of TDR was 12.3%; however, in contrast to the data from Ontario, TDR patterns were inverted with a 21% prevalence among MSM and 5.5% among IDU. Furthermore, nearly 80% of the observed TDR was a D67N/K219Q pattern with 87% of these infections arising from a distinct phylogenetic cluster.

CONCLUSIONS

Local patterns of TDR were distinct to what had been observed provincially. Phylogenetic analysis uncovered a cluster of related infections among MSM that appeared more likely to be recent infections. Results support a paradigm of routine local TDR surveillance to identify the sub-populations under care. Furthermore, the routine application of phylogenetic analysis in the TDR surveillance context provides insights into how best to target prevention strategies; and how to correctly measure outcomes.

HIV-1 reverse transcriptase and antiviral drug resistance. Part 2

Structures of RT and its complexes combined with biochemical and clinical data help in illuminating the molecular mechanisms of different drug-resistance mutations. The NRTI drugs that are used in combinations have different primary mutation sites. RT mutations that confer resistance to one drug can be hypersensitive to another RT drug. Structure of an RT-DNA-nevirapine complex revealed how NNRTI binding forbids RT from forming a polymerase competent complex. Collective knowledge about various mechanisms of drug resistance by RT has broader implications for understanding and targeting drug resistance in general. In Part 1, we discussed the role of RT in developing HIV-1 drug resistance, structural and functional states of RT, and the nucleoside/nucleotide analog (NRTI) and non-nucleoside (NNRTI) drugs used in treating HIV-1 infections. In this part, we discuss structural understanding of various mechanisms by which RT confers antiviral drug resistance.

A sensitive real-time PCR based assay to estimate the impact of amino acid substitutions on the competitive replication fitness of human immunodeficiency virus type 1 in cell culture

Fixation of mutations in human immunodeficiency virus type 1 (HIV-1), such as those conferring drug resistance and immune escape, can result in a change in replication fitness. To assess these changes, a real-time TaqMan PCR detection assay and statistical methods for data analysis were developed to estimate sensitively relative viral fitness in competitive viral replication experiments in cell culture. Chimeric viruses with the gene of interest in an HIV-1NL4-3 backbone were constructed in two forms, vifA (native vif gene in NL4-3) and vifB (vif gene with six synonymous nucleotide differences from vifA). Subsequently, mutations of interest were introduced into the chimeric viruses in NL4-3VifA backbones, and the mutants were competed against the chimera with the isogenic viral sequence in the NL4-3VifB backbone in cell culture. In order to assess subtle fitness differences, culture supernatants were sampled longitudinally, and the viruses differentially quantified using vifA- and vifB-specific primers in real-time PCR assays. Based on an exponential net growth model, the growth rate of each virus was determined and the fitness cost of the mutation(s) distinguishing the two viruses represented as the net growth rate difference between the mutant and the native variants. Using this assay, the fitness impact of eight amino acid substitutions was quantitated at highly conserved sites in HIV-1 Gag and Env.

Universal amplification, next-generation sequencing, and assembly of HIV-1 genomes

Whole HIV-1 genome sequences are pivotal for large-scale studies of inter- and intrahost evolution, including the acquisition of drug resistance mutations. The ability to rapidly and cost-effectively generate large numbers of HIV-1 genome sequences from different populations and geographical locations and determine the effect of minority genetic variants is, however, a limiting factor. Next-generation sequencing promises to bridge this gap but is hindered by the lack of methods for the enrichment of virus genomes across the phylogenetic breadth of HIV-1 and methods for the robust assembly of the virus genomes from short-read data. Here we report a method for the amplification, next-generation sequencing, and unbiased de novo assembly of HIV-1 genomes of groups M, N, and O, as well as recombinants, that does not require prior knowledge of the sequence or subtype. A sensitivity of at least 3,000 copies/ml was determined by using plasma virus samples of known copy numbers. We applied our novel method to compare the genome diversities of HIV-1 groups, subtypes, and genes. The highest level of diversity was found in the env, nef, vpr, tat, and rev genes and parts of the gag gene. Furthermore, we used our method to investigate mutations associated with HIV-1 drug resistance in clinical samples at the level of the complete genome. Drug resistance mutations were detected as both major variant and minor species. In conclusion, we demonstrate the feasibility of our method for large-scale HIV-1 genome sequencing. This will enable the phylogenetic and phylodynamic resolution of the ongoing pandemic and efficient monitoring of complex HIV-1 drug resistance genotypes.

Mechanism of HIV antiretroviral drugs progress toward drug resistance

The rapid replication rate of HIV-1 RNA and its inherent genetic variation have led to the production of many HIV-1 variants with decreased drug susceptibility. The capacity of HIV to develop drug resistance mutations is a major obstacle to long-term effective anti-HIV therapy. Incomplete suppression of viral replication with an initial drug regimen diminishes the clinical benefit to the patient and may promote the development of broader drug resistance that may cause subsequent treatment regimens to be ineffective. The increased clinical use of combination antiretroviral treatment for HIV-1 infection has led to the selection of viral strains resistant to multiple drugs, including strains resistant to all licensed nucleoside analog RT inhibitors and protease inhibitors. Therefore, it is important to understand the influence of such mutations on viral properties such as replicative fitness, fidelity, and mutation rates. Although research continues to improve our understanding of resistance, leading to refined treatment strategies and, in some cases, improved outcome, resistance to antiretroviral therapy remains a major cause of treatment failure among patients living with HIV-1.

2011 update of the drug resistance mutations in HIV-1

This November 2011 edition of the IAS-USA drug resistance mutations list updates the figures last published in December 2010 (Johnson VA et al, Top HIV Med, 2010;18:156-163).

Transmitted drug resistance in the CFAR network of integrated clinical systems cohort: prevalence and effects on pre-therapy CD4 and viral load

Human immunodeficiency virus type 1 (HIV-1) genomes often carry one or more mutations associated with drug resistance upon transmission into a therapy-naïve individual. We assessed the prevalence and clinical significance of transmitted drug resistance (TDR) in chronically-infected therapy-naïve patients enrolled in a multi-center cohort in North America. Pre-therapy clinical significance was quantified by plasma viral load (pVL) and CD4+ cell count (CD4) at baseline. Naïve bulk sequences of HIV-1 protease and reverse transcriptase (RT) were screened for resistance mutations as defined by the World Health Organization surveillance list. The overall prevalence of TDR was 14.2%. We used a Bayesian network to identify co-transmission of TDR mutations in clusters associated with specific drugs or drug classes. Aggregate effects of mutations by drug class were estimated by fitting linear models of pVL and CD4 on weighted sums over TDR mutations according to the Stanford HIV Database algorithm. Transmitted resistance to both classes of reverse transcriptase inhibitors was significantly associated with lower CD4, but had opposing effects on pVL. In contrast, position-specific analyses of TDR mutations revealed substantial effects on CD4 and pVL at several residue positions that were being masked in the aggregate analyses, and significant interaction effects as well. Residue positions in RT with predominant effects on CD4 or pVL (D67 and M184) were re-evaluated in causal models using an inverse probability-weighting scheme to address the problem of confounding by other mutations and demographic or risk factors. We found that causal effect estimates of mutations M184V/I (-1.7 log₁₀pVL) and D67N/G (-2.1[³√CD4] and 0.4 log₁₀pVL) were compensated by K103N/S and K219Q/E/N/R. As TDR becomes an increasing dilemma in this modern era of highly-active antiretroviral therapy, these results have immediate significance for the clinical management of HIV-1 infections and our understanding of the ongoing adaptation of HIV-1 to human populations.

Prevalence of transmitted drug resistance and impact of transmitted resistance on treatment success in the German HIV-1 Seroconverter Cohort

BACKGROUND

The aim of this study is to analyse the prevalence of transmitted drug resistance, TDR, and the impact of TDR on treatment success in the German HIV-1 Seroconverter Cohort.

METHODS

Genotypic resistance analysis was performed in treatment-naïve study patients whose sample was available 1,312/1,564 (83.9% October 2008). A genotypic resistance result was obtained for 1,276/1,312 (97.3%). The resistance associated mutations were identified according to the surveillance drug resistance mutations list recommended for drug-naïve patients. Treatment success was determined as viral suppression below 500 copies/ml.

RESULTS

Prevalence of TDR was stable at a high level between 1996 and 2007 in the German HIV-1 Seroconverter Cohort (N = 158/1,276; 12.4%; CI(wilson) 10.7-14.3; p(for trend) = 0.25). NRTI resistance was predominant (7.5%) but decreased significantly over time (CI(Wilson): 6.2-9.1, p(for trend) = 0.02). NNRTI resistance tended to increase over time (NNRTI: 3.5%; CI(Wilson): 2.6-4.6; p(for trend)= 0.07), whereas PI resistance remained stable (PI: 3.0%; CI(Wilson): 2.1-4.0; p(for trend) = 0.24). Resistance to all drug classes was frequently caused by singleton resistance mutations (NRTI 55.6%, PI 68.4%, NNRTI 99.1%). The majority of NRTI-resistant strains (79.8%) carried resistance-associated mutations selected by the thymidine analogues zidovudine and stavudine. Preferably 2NRTI/1PIr combinations were prescribed as first line regimen in patients with resistant HIV as well as in patients with susceptible strains (susceptible 45.3%; 173/382 vs. resistant 65.5%; 40/61). The majority of patients in both groups were treated successfully within the first year after ART-initiation (susceptible: 89.9%; 62/69; resistant: 7/9; 77.8%).

CONCLUSION

Overall prevalence of TDR remained stable at a high level but trends of resistance against drug classes differed over time. The significant decrease of NRTI-resistance in patients newly infected with HIV might be related to the introduction of novel antiretroviral drugs and a wider use of genotypic resistance analysis prior to treatment initiation.

Viral fitness: relation to drug resistance mutations and mechanisms involved: nucleoside reverse transcriptase inhibitor mutations

PURPOSE OF REVIEW

Nucleoside and nucleotide reverse transcriptase inhibitors constitute the backbone of highly active antiretroviral therapy in the treatment of HIV-1 infection. One of the major obstacles in achieving the long-term efficacy of anti-HIV-1 therapy is the development of resistance. The advent of resistance mutations is usually accompanied by a change in viral replicative fitness. This review focuses on the most common nucleoside reverse transcriptase inhibitor-associated mutations and their effects on HIV-1 replicative fitness.

RECENT FINDINGS

Recent studies have explained the two main mechanisms of resistance to nucleoside reverse transcriptase inhibitors and their role in HIV-1 replicative fitness. The first involves mutations directly interfering with binding or incorporation and seems to impact replicative fitness more adversely than the second mechanism, which involves enhanced excision of the newly incorporated analogue. Further studies have helped explain the antagonistic effects between amino acid substitutions, K65R, L74V, M184V, and thymidine analogue mutations, showing how viral replicative fitness influences the evolution of thymidine analogue resistance pathways.

SUMMARY

Nucleoside reverse transcriptase inhibitor resistance mutations impact HIV-1 replicative fitness to a lesser extent than protease resistance mutations. The monitoring of viral replicative fitness may help in the management of HIV-1 infection in highly antiretroviral-experienced individuals.

Impact of novel human immunodeficiency virus type 1 reverse transcriptase mutations P119S and T165A on 4'-ethynylthymidine analog resistance profile

2',3'-Didehydro-3'-deoxy-4'-ethynylthymidine (4'-Ed4T), a derivative of stavudine (d4T), has potent activity against human immunodeficiency virus and is much less inhibitory to mitochondrial DNA synthesis and cell growth than its progenitor, d4T. 4'-Ed4T triphosphate was a better reverse transcriptase (RT) inhibitor than d4T triphosphate, due to the additional binding of the 4'-ethynyl group at a presumed hydrophobic pocket in the RT active site. Previous in vitro selection for 4'-Ed4T-resistant viral strains revealed M184V and P119S/T165A/M184V mutations on days 26 and 81, respectively; M184V and P119S/T165A/M184V conferred 3- and 130-fold resistance to 4'-Ed4T, respectively. We investigated the relative contributions of these mutations, engineered into the strain NL4-3 background, to drug resistance, RT activity, and viral growth. Viral variants with single RT mutations (P119S or T165A) did not show resistance to 4'-Ed4T; however, M184V and P119S/T165A/M184V conferred three- and fivefold resistance, respectively, compared with that of the wild-type virus. The P119S/M184V and T165A/M184V variants showed about fourfold resistance to 4'-Ed4T. The differences in the growth kinetics of the variants were not more than threefold. The purified RT of mutants with the P119S/M184V and T165A/M184V mutations were inhibited by 4'-Ed4TTP with 8- to 13-fold less efficiency than wild-type RT. M184V may be the primary resistance-associated mutation of 4'-Ed4T, and P119S and T165A are secondary mutations. On the basis of our findings and the results of structural modeling, a virus with a high degree of resistance to 4'-Ed4T (e.g., more than 50-fold resistance) will be difficult to develop. The previously observed 130-fold resistance of the virus with P119S/T165A/M184V to 4'-Ed4T may be partly due to mutations both in the RT sequence and outside the RT sequence.

Antiviral resistance and impact on viral replication capacity: evolution of viruses under antiviral pressure occurs in three phases

Resistance development is a major obstacle to antiviral therapy, and all active antiviral agents have shown to select for resistance mutations. Aspects of antiviral resistance development are discussed for specific compounds or drug classes in the previous chapters, while this chapter provides an overview regarding the evolution of different viruses (HIV, HBV, HCV, and Influenza) under pressure of antiviral therapy. Virus replication is an error prone process resulting in a large number of variants (quasispecies) in patients. Resistance evolution under suboptimal therapy can be schematically distinguished into three phases. (1) preexisting variants less sensitive to the respective drug are selected from the quasispecies population, (2) outgrowing variants acquire additional mutations increasing their resistance, and (3) compensatory mutations accumulate to overcome the generally reduced replicative capacity of resistant variants. Successful therapy should be aimed at suppression of all existing viral variants, thus preventing selection of minority species and their subsequent evolution. This implies that the amount of mutations required for first escape to the viral regimen (genetic barrier) should be larger than the expected number of mutations present in viruses in the quasispecies. Accordingly, combination therapy can achieve complete inhibition of replication for most HIV, HBV, and Influenza infected patients without resistance development. However, resistant viruses can become selected under circumstances of suboptimal antiviral therapy and these resistant viruses can be transmitted. Proper use of drugs and worldwide monitoring for the presence and spread of drug resistant viruses are therefore of utmost importance.

Comparison of algorithms that interpret genotypic HIV-1 drug resistance to determine the prevalence of transmitted drug resistance

OBJECTIVE

We compared eight genotypic interpretation methods to determine whether the method used would affect the rates of reported transmitted drug resistance.

DESIGN

Retrospective cohort study.

METHODS

For the International AIDS Society-USA method we classified a mutation as resistant if it was a 'major' resistance-associated mutation. For the Stanford algorithm, we classified a mutation as resistant if the score was at least 60 (Stanford 60), and alternatively, if the score was at least 30 (Stanford 30). For Agence Nationale de Recherches sur le SIDA and Rega, we interpreted resistance as either 'intermediate resistance' or 'resistance' (ANRS 1 and Rega 1), and 'resistance' only (ANRS 2 and Rega 2). We also used the calibrated population resistance algorithm. We then determined the rates of transmitted drug resistance within the Acute Infection Early Disease Research Program cohort (n = 1311) enrolled between March 1995 and August 2006 using each method; agreement was assessed using kappa coefficients.

RESULTS

Differences in estimated rates of transmitted drug resistance using International AIDS Society-USA, calibrated population resistance, Stanford 30, ANRS 1, Rega 1 and Rega 2 methods were mostly minor for resistance to protease and non-nucleoside reverse transcriptase inhibitors (1% range) and more pronounced for nucleoside reverse transcriptase inhibitors (5% range). For these methods kappa agreement was substantial or almost perfect across all drug classes. The Stanford 60 was most conservative.

CONCLUSIONS

The persistent high rates of transmitted drug resistance support the need for continued genotypic surveillance. The currently available interpretation algorithms can be used for this purpose.

HIV-1 reverse transcriptase inhibitor resistance mutations and fitness: a view from the clinic and ex vivo

Genetic diversity plays a key role in human immunodeficiency virus (HIV) adaptation, providing a mechanism to escape host immune responses and develop resistance to antiretroviral drugs. This process is driven by the high-mutation rate during DNA synthesis by reverse transcriptase (RT), by the large viral populations, by rapid viral turnover, and by the high-recombination rate. Drugs targeting HIV RT are included in all regimens of highly active antiretroviral therapy (HAART), which helps to reduce the morbidity and mortality of HIV-infected patients. However, the emergence of resistant viruses is a significant obstacle to effective long-term management of HIV infection and AIDS. The increasing complexity of antiretroviral regimens has favored selection of HIV variants harboring multiple drug resistance mutations. Evolution of drug resistance is characterized by severe fitness losses when the drug is not present, which can be partially overcome by compensatory mutations or other adaptive changes that restore replication capacity. Here, we review the impact of mutations conferring resistance to nucleoside and nonnucleoside RT inhibitors on in vitro and in vivo fitness, their involvement in pathogenesis, persistence upon withdrawal of treatment, and transmission. We describe the techniques used to estimate viral fitness, the molecular mechanisms that help to improve the viral fitness of drug-resistant variants, and the clinical implications of viral fitness data, by exploring the potential relationship between plasma viral load, drug resistance, and disease progression.

Viral Quasispecies: Dynamics, Interactions, and Pathogenesis*

Quasispecies theory is providing a solid, evolving conceptual framework for insights into virus population dynamics, adaptive potential, and response to lethal mutagenesis. The complexity of mutant spectra can influence disease progression and viral pathogenesis, as demonstrated using virus variants selected for increased replicative fidelity. Complementation and interference exerted among components of a viral quasispecies can either reinforce or limit the replicative capacity and disease potential of the ensemble. In particular, a progressive enrichment of a replicating mutant spectrum with interfering mutant genomes prompted by enhanced mutagenesis may be a key event in the sharp transition of virus populations into error catastrophe that leads to virus extinction. Fitness variations are influenced by the passage regimes to which viral populations are subjected, notably average fitness decreases upon repeated bottleneck events and fitness gains upon competitive optimization of large viral populations. Evolving viral quasispecies respond to selective constraints by replication of subpopulations of variant genomes that display higher fitness than the parental population in the presence of the selective constraint. This has been profusely documented with fitness effects of mutations associated with resistance of pathogenic viruses to antiviral agents. In particular, selection of HIV-1 mutants resistant to one or multiple antiretroviral inhibitors, and the compensatory effect of mutations in the same genome, offers a compendium of the molecular intricacies that a virus can exploit for its survival. This chapter reviews the basic principles of quasispecies dynamics as they can serve to explain the behavior of viruses.

Clinical significance of human immunodeficiency virus type 1 replication fitness

The relative fitness of a variant, according to population genetics theory, is that variant's relative contribution to successive generations. Most drug-resistant human immunodeficiency virus type 1 (HIV-1) variants have reduced replication fitness, but at least some of these deficits can be compensated for by the accumulation of second-site mutations. HIV-1 replication fitness also appears to influence the likelihood of a drug-resistant mutant emerging during treatment failure and is postulated to influence clinical outcomes. A variety of assays are available to measure HIV-1 replication fitness in cell culture; however, there is no agreement regarding which assays best correlate with clinical outcomes. A major limitation is that there is no high-throughput assay that incorporates an internal reference strain as a control and utilizes intact virus isolates. Some retrospective studies have demonstrated statistically significant correlations between HIV-1 replication fitness and clinical outcomes in some patient populations. However, different studies disagree as to which clinical outcomes are most closely associated with fitness. This may be in part due to assay design, sample size limitations, and differences in patient populations. In addition, the strength of the correlations between fitness and clinical outcomes is modest, suggesting that, at present, it would be difficult to utilize these assays for clinical management.

Transmitted drug-resistant HIV-1 in primary HIV-1 infection; incidence, evolution and impact on response to antiretroviral therapy

OBJECTIVES

The aim of the study was to determine the incidence and persistence of transmitted drug-resistant HIV-1 in an incident cohort between 2000 and 2004, and to investigate the impact of transmitted drug-resistant HIV-1 on the response to antiretroviral therapy (ART).

METHODS

A prospective, nonrandomized study was carried out on 140 individuals identified with primary HIV-1 infection (PHI). PHI was defined as an HIV-positive antibody test with an HIV antibody-negative result in the prior 6 months (n = 69); positive HIV DNA in the absence of antibody (n = 30); an evolving titre positive HIV antibody test (n = 23), or an incident 'detuned' assay (B clade viruses only) (n = 18). Genotypic resistance testing was performed at baseline, following ART and annually over a 4-year period.

RESULTS

The prevalence of transmitted drug-resistant HIV-1 infection between January 2000 and June 2004 was nine in 140 (6.0%) and the annual incidence was stable. Seven of these nine patients had a single point mutation conferring single-class drug resistance and the other two patients had multiple mutations conferring multiclass drug resistance (MDR). In eight of the nine cases, mutations conferring drug resistance persisted for more than 12 months off therapy. In contrast to transmitted MDR HIV-1, the virological response to initial ART and CD4 decline were comparable in those with wild-type virus, virus with 'polymorphisms' (secondary mutations) and virus with single drug-resistance mutations.

CONCLUSIONS

The incidence of transmitted drug-resistant HIV remained stable and low over a 4-year period. Although MDR remains rare, its presence significantly affects the response to first-line ART, predisposes towards the accumulation of new resistance mutations and is associated with a more rapid CD4 decline.

Prevalence and determinants of transmitted antiretroviral drug resistance in HIV-1 infection

Transmission of drug-resistant HIV-1 variants from antiretroviral treatment-experienced persons has been documented to occur through multiple routes, including sexual intercourse, intravenous drug use and vertically from mother to child. Newly infected persons with transmitted drug resistance (TDR) also act as a source for the onward transmission of resistant variants. Rates of virological suppression and behavioural patterns of treated populations and the relative fitness of drug-resistant variants are important determinants of the prevalence of TDR. Current estimates indicate that the prevalence is highest in regions and populations with long-established use of antiretroviral therapy. Limited data suggest that the incidence of TDR is rising in developing countries where access to therapy is increasing. There are methodological variations between studies, however, including those relative to the selection of the study population and the resistance interpretation system, which can skew prevalence estimates. TDR has important implications for the successful management of antiretroviral therapy. Routine resistance testing of drug-naive persons has been widely adopted in affluent countries and shown to effectively guide the selection of first-line regimens. Genotypic resistance tests offer a practical approach for detecting TDR. However, routine methods can only detect resistant mutants within the dominant quasi-species and fail to detect low-frequency resistant variants, which may become important once selective drug pressure is introduced. More sensitive testing methods are being evaluated but remain research tools at present. In addition, factors such as superinfection and possible differences in resistance patterns between plasma and cellular reservoirs and between anatomical compartments should be considered when evaluating TDR.

Prevalence and predictors of antiretroviral drug resistance in newly diagnosed HIV-1 infection

OBJECTIVES

To determine prevalence and predictors of antiretroviral drug resistance in newly diagnosed individuals with HIV-1 infection, using a systematic approach to avoid selection bias.

METHODS

Plasma samples from all persons diagnosed HIV-1 seropositive at a large London centre between April 2004 and February 2006 underwent sequencing of HIV-1 reverse transcriptase (RT) and protease genes. Subtype was assigned by phylogenetic analysis. Resistance was scored according to the IAS-USA list (2005) modified to include T215revertants and exclude isolated E44D or V118I and minor protease mutations. Recent seroconversion was identified by HIV antibody avidity testing.

RESULTS

The cohort of 239 included 169 (70.7%) males, 126 (52.7%) homosexuals, 118 (49.5%) persons of white ethnicity and 144 (60.0%) persons born outside the UK. Subtypes included B 134 (56.1%), C 46 (19.2%), A 17 (7.1%), other non-B 42 (17.6%). The prevalence of resistance mutations was 17/239 (7.1%; 95% confidence interval 4.5-11.1%), comprising 10/239 (4.2%) nucleoside/nucleotide RT inhibitor (NRTI); 4/239 (1.7%) non-nucleoside RT inhibitor (NNRTI) and 4/239 (1.7%) protease inhibitor (PI) associated mutations. Dual-class (NRTI + PI) resistance mutations were detected in 1/239 (0.4%) person. The prevalence of resistance mutations was 7/85 (8.2%) and 10/154 (6.5%) in persons with recent and established infection, respectively. In multivariate analysis, having been born in the UK and high CD4 count, but not gender, age, risk group, ethnicity or subtype, were independent predictors of resistance.

CONCLUSIONS

In an unselected UK cohort, subtypes other than B accounted for 43.9% of new HIV-1 diagnoses. The prevalence of resistance mutations was 7.1% and highest in those born in the UK.

The fitness cost of mutations associated with human immunodeficiency virus type 1 drug resistance is modulated by mutational interactions

It is generally accepted that the fitness cost of resistance mutations plays a role in the persistence of transmitted drug-resistant human immunodeficiency virus type 1 and that mutations that confer a high fitness cost are less able to persist in the absence of drug pressure. Here, we show that the fitness cost of reverse transcriptase (RT) mutations can vary within a 72-fold range. We also demonstrate that the fitness cost of M184V and K70R can be decreased or enhanced by other resistance mutations such as D67N and K219Q. We conclude that the persistence of transmitted RT mutants might range widely on the basis of fitness and that the modulation of fitness cost by mutational interactions will be a critical determinant of persistence.

Precise identification of a human immunodeficiency virus type 1 antigen processing mutant

Human immunodeficiency virus type 1 (HIV-1) evokes a strong immune response, but the virus persists. Polymorphisms within known antigenic sites result in loss of immune recognition and can be positively selected. Amino acid variation outside known HLA class I restricted epitopes can also enable immune escape by interfering with the processing of the optimal peptide antigen. However, the lack of precise rules dictating epitope generation and the enormous genetic diversity of HIV make prediction of processing mutants very difficult. Polymorphism E169D in HIV-1 reverse transcriptase (RT) is significantly associated with HLA-B*0702 in HIV-1-infected individuals. This polymorphism does not map within a known HLA-B*0702 epitope; instead, it is located five residues downstream of a HLA-B*0702-restricted epitope SPAIFQSSM (SM9). Here we investigate the association between E169D and HLA-B*0702 for immune escape via the SM9 epitope. We show that this single amino acid variation prevents the immune recognition of the flanked SM9 epitope by cytotoxic T cells through lack of generation of the epitope, which is a result of aberrant proteasomal cleavage. The E169D polymorphism also maps within and abrogates the recognition of an HLA-A*03-restricted RT epitope MR9. This study highlights the potential for using known statistical associations as indicators for viral escape but also the complexity involved in interpreting the immunological consequences of amino acid changes in HIV sequences.

HIV-1 Transmission Cluster with M41L ‘Singleton’ Mutation and Decreased Transmission of Resistance in Newly Diagnosed Swedish Homosexual Men

Objective

To investigate the prevalence of HIV-1 with major drug resistance-associated mutations among 261 men who have sex with men (MSM) who were newly diagnosed as HIV-1-infected at Venhälsan, Stockholm, between 1992–2002.

Methods

Major resistance-associated mutations were identified using an in-house method on stored plasma samples collected within 6 months of diagnosis. Additional samples were investigated from selected patients. Phylogenetic tree analyses were used to study evolutionary relationships between the viruses. Epidemiological data were retrieved from the partner notification investigations and the medical records.

Results

Informed consent as well as results from the resistance test were available for 201 out of 261 patients (77%) diagnosed during 1992–2002. Viruses from 28 of these 201 patients (14%) displayed major resistance-associated mutations; 27 of these viruses displayed only zidovudine/stavudine resistance-associated mutations. None of the patients displayed resistance mutations to protease inhibitors. The prevalence of resistance-associated mutations decreased over time; 20% in 1992–1996 versus 9% in 1997–2002 ( P=0.04). A transmission cluster involving six patients with a singleton M41L mutation was identified. These viruses were phenotypically sensitive to zidovudine and stavudine. The M41L mutation, as well as most other resistance mutations, was stable for many years after transmission and may have been fixated by other putative compensatory mutations.

Conclusions

In this Swedish population of MSM with newly diagnosed HIV-1 infection, the prevalence of resistance-associated mutations decreased over time. Reversion of resistance-associated mutations following transmission was slow and incomplete. A large transmission cluster with an interesting M41L singleton mutation was also observed.

Rapid reversion of sequence polymorphisms dominates early human immunodeficiency virus type 1 evolution

The error-prone replication of human immunodeficiency virus type 1 (HIV-1) enables it to continuously evade host CD8+ T-cell responses. The observed transmission, and potential accumulation, of CD8+ T-cell escape mutations in the population may suggest a gradual adaptation of HIV-1 to immune pressures. Recent reports, however, have highlighted the propensity of some escape mutations to revert upon transmission to a new host in order to restore efficient replication capacity. To more specifically address the role of reversions in early HIV-1 evolution, we examined sequence polymorphisms arising across the HIV-1 genome in seven subjects followed longitudinally 1 year from primary infection. As expected, numerous nonsynonymous mutations were associated with described CD8+ T-cell epitopes, supporting a prominent role for cellular immune responses in driving early HIV-1 evolution. Strikingly, however, a substantial proportion of substitutions (42%) reverted toward the clade B consensus sequence, with nearly one-quarter of them located within defined CD8 epitopes not restricted by the contemporary host's HLA. More importantly, these reversions arose significantly faster than forward mutations, with the most rapidly reverting mutations preferentially arising within structurally conserved residues. These data suggest that many transmitted mutations likely incur a fitness cost that is recovered through retrieval of an optimal, or ancestral, form of the virus. The propensity of mutations to revert may limit the accumulation of immune pressure-driven mutations in the population, thus preserving critical CD8+ T-cell epitopes as vaccine targets, and argue against an unremitting adaptation of HIV-1 to host immune pressures.

The Effect of HIV-1 Resistance Mutations after First-Line Virological Failure on the Possibility to Sequence Antiretroviral Drugs in Second-Line Regimens

Background

One of the more vigorous debates in the field of highly active antiretroviral therapy (HAART) is how to start it and what the optimal drug sequence is.

Methods

A retrospective cohort analysis was performed. The aim was to evaluate which variables could influence the virological response to second-line genotypic-based HAART in patients with virological documented first-line HAART failure. A positive response was defined as a confirmed HIV RNA level <50 copies/ml.

Results

Two hundred and eight patients were included. Demographic characteristics, risk factors for HIV acquisition, and drugs included in the initial treatment did not significantly influence the considered outcome. According to a multiple logistic model, the presence of thymidine analogue mutations (TAMs) had a negative association with the virological outcome ( P=0.006), whereas the use of a boosted protease inhibitor (PI) in second-line HAART was positively associated with the endpoint ( P=0.001). Patients receiving a genotypic-based second-line HAART containing a boosted PI achieved a viral load <50 copies/ml in a 74.2% of cases compared with 52.2% of those whose therapy did not contain a boosted PI. This difference was statistically significant ( P=0.002) with an odds ratio (OR) of 2.63 and a 95% confidence interval (CI) ranging from 1.46 to 4.76. This last variable positively influenced the outcome even when the analysis was restricted to patients harbouring a virus presenting TAMs. In this case, second-line HAART was successful in 66.7% of cases with an OR of 3.25 and a 95% CI ranging from 1.28 to 8.25 ( P=0.014).

Conclusions

the wider range of available therapeutic options has made resistance and drug-sequencing considerations a crucial point in selecting first-line HAART. Our data indicate that, by limiting the risk of selecting or accumulating TAMs, it could be possible to save further therapeutic options. In second-line regimens, the higher antiviral effect and genetic barrier of boosted PIs may overcome the limits of the use of NRTI backbones, which retain only a partial effectiveness.

Virus fitness: concept, quantification, and application to HIV population dynamics

Viral fitness has been broadly studied during the past three decades, mainly to test evolutionary models and population theories difficult to analyze and interpret with more complex organisms. More recent studies, however, are focused in the role of fitness on viral transmission, pathogenesis, and drug resistance. Here, we used human immunodeficiency virus (HIV) as one of the most relevant models to evaluate the importance of viral quasispecies and fitness in HIV evolution, population dynamics, disease progression, and potential clinical implications.

Diversity of thymidine analogue resistance genotypes among newly diagnosed HIV-1-infected persons

The introduction of highly active antiretroviral therapy (HAART) has resulted in a significant decrease in HIV and AIDS-related mortality and morbidity. However, these treatments can select for drug-resistant viruses which are associated with poor virological responses to the antiretroviral therapy and possible loss of clinical benefit. Drug-resistant viruses can also be transmitted between individuals. In the absence of drug pressure, transmitted drug-resistant viruses gradually lose resistance mutations that confer a selective disadvantage as they evolve to more fit viruses. As a result, unusual resistance-related genotypes not commonly seen in treated patients may arise in the population. Viruses with unique patterns of thymidine analogue-associated mutations (TAMs) have now been identified in a substantial proportion of treatment-naive recently diagnosed persons. In this leading article, I discuss these findings and the potential impact of these unique reverse transcriptase (RT) genotypes on evolution of resistance and treatment responses.