N348I in the connection domain of HIV-1 reverse transcriptase confers zidovudine and nevirapine resistance

A survey of resistance mutations to reverse transcriptase inhibitors (RTIs) among HIV-1 patients in northeast of Iran

The use of a combination of three-drug regimen has improved HIV-1 infected patients' life span and quality; however the emergence of drug-resistant strains remains a main problem. Reverse transcriptase inhibitors (RTIs) consist of a main part of highly active anti-retroviral therapy (HAART) regimen. The present study aimed to investigate resistant mutations to RTI drugs in both treatment naïve and under treatment HIV patients in Mashhad city, north-eastern Iran. RNA was extracted from sera of 22 treatment naïve and 22 under treatment patients. The mean age of under treated and treatment naive groups were 38.5±6.7 and 40.8±7.9 respectively. cDNA was synthesized and amplified with Nested PCR assay targeting specific sequences of RT gene. The PCR products were sent for sequencing. Bidirectional sequencing results were analysed using HIV drug resistance database supplied by Stanford University (HIV Drug Resistance Database, https://hivdb.stanford.edu). Among under treatment patients 10 out of 22 (45%) had at least one high-level resistance mutation which was higher than high level resistance mutation rate among treatment naive cases (P<0.01). Detected resistance mutations were as follows: K101E, K103N, K103E, V106M, V108I, E138A, V179T, Y181C, M184V, Y188L, Y188H, Y188F, G190A, L210W, T215F, T215Y, K219Q, and P225H. A high level of resistance mutations to RT inhibitors was observed that causes drug resistance especially against lamivudine (3TC). Such mutations should be considered as probable responsible for therapeutic failure. Serial surveillance studies of circulating drug resistance mutations are recommended.

Targeting HIV-1 Reverse Transcriptase Using a Fragment-Based Approach

Human immunodeficiency virus type I (HIV-1) is a retrovirus that infects cells of the host's immune system leading to acquired immunodeficiency syndrome and potentially death. Although treatments are available to prevent its progression, HIV-1 remains a major burden on health resources worldwide. Continued emergence of drug-resistance mutations drives the need for novel drugs that can inhibit HIV-1 replication through new pathways. The viral protein reverse transcriptase (RT) plays a fundamental role in the HIV-1 replication cycle, and multiple approved medications target this enzyme. In this study, fragment-based drug discovery was used to optimize a previously identified hit fragment (compound B-1), which bound RT at a novel site. Three series of compounds were synthesized and evaluated for their HIV-1 RT binding and inhibition. These series were designed to investigate different vectors around the initial hit in an attempt to improve inhibitory activity against RT. Our results show that the 4-position of the core scaffold is important for binding of the fragment to RT, and a lead compound with a cyclopropyl substitution was selected and further investigated. Requirements for binding to the NNRTI-binding pocket (NNIBP) and a novel adjacent site were investigated, with lead compound 27-a minimal but efficient NNRTI-offering a starting site for the development of novel dual NNIBP-Adjacent site inhibitors.

New antiretroviral inhibitors and HIV-1 drug resistance: more focus on 90% HIV-1 isolates?

Combined HIV antiretroviral therapy (cART) has been effective except if drug resistance emerges. As cART has been rolled out in low-income countries, drug resistance has emerged at higher rates than observed in high income countries due to factors including initial use of these less tolerated cART regimens, intermittent disruptions in drug supply, and insufficient treatment monitoring. These socioeconomic factors impacting drug resistance are compounded by viral mechanistic differences by divergent HIV-1 non-B subtypes compared to HIV-1 subtype B that largely infects the high-income countries (just 10% of 37 million infected). This review compares the inhibition and resistance of diverse HIV-1 subtypes and strains to the various approved drugs as well as novel inhibitors in clinical trials. Initial sequence variations and differences in replicative fitness between HIV-1 subtypes pushes strains through different fitness landscapes to escape from drug selective pressure. The discussions here provide insight to patient care givers and policy makers on how best to use currently approved ART options and reduce the emergence of drug resistance in ∼33 million individuals infected with HIV-1 subtype A, C, D, G, and recombinants forms. Unfortunately, over 98% of the literature on cART resistance relates to HIV-1 subtype B.

Evaluation of a novel in-house HIV-1 genotype drug resistance assay using clinical samples in China

Background

HIV drug resistance poses a major challenge for anti-retroviral treatment (ART) and the prevention and control of HIV epidemic.

Objective

The study aims to establish a novel in-house assay with high efficiency, named AP in- house method, that would be suitable for HIV-1 drug resistance detection in China.

Methods

An in-house HIV-1 genotyping method was used to sequence the partial pol gene from 60 clinical plasma samples; the results of our test were compared with a commercial ViroSeq HIV-1 genotyping system.

Results

Among sixty samples, 58(96.7%) were successfully amplified by AP in-house method, five of them harbored viral load below 1,000 copies/ml. The genotype distribution was 43.1% CRF07_BC (25/58), 39.7% CRF01_AE (23/58), 6.9% CRF55_01B (4/58), 5.2% subtype B (3/58) and 5.2% CRF08_BC (3/58). Compared with that of the ViroSeq system, the consistent rate of these nucleotides and amino acids obtained by AP in-house method was up to 99.5 ± 0.4% and 99.5 ± 0.4%, respectively. A total of 290 HIV-1 drug resistance mutations were identified by two methods, including 126 nucleoside reverse transcriptase inhibitors (NRTIs), 145 non-nucleoside reverse transcriptase inhibitors (NNRTIs) and 19 protease inhibitors (PIs) resistance mutations. Out of them, 94.1% (273/290) were completely concordant between the AP in-house method and the ViroSeq system.

Conclusion

Overall, the evaluation of AP in-house method provided comparable results to those of the ViroSeq system on diversified HIV-1 subtypes in China.

Analysis and Molecular Determinants of HIV RNase H Cleavage Specificity at the PPT/U3 Junction

HIV reverse transcriptases (RTs) convert viral genomic RNA into double-stranded DNA. During reverse transcription, polypurine tracts (PPTs) resilient to RNase H cleavage are used as primers for plus-strand DNA synthesis. Nonnucleoside RT inhibitors (NNRTIs) can interfere with the initiation of plus-strand DNA synthesis by enhancing PPT removal, while HIV RT connection subdomain mutations N348I and N348I/T369I mitigate this effect by altering RNase H cleavage specificity. Now, we demonstrate that among approved nonnucleoside RT inhibitors (NNRTIs), nevirapine and doravirine show the largest effects. The combination N348I/T369I in HIV-1_BH10 RT has a dominant effect on the RNase H cleavage specificity at the PPT/U3 site. Biochemical studies showed that wild-type HIV-1 and HIV-2 RTs were able to process efficiently and accurately all tested HIV PPT sequences. However, the cleavage accuracy at the PPT/U3 junction shown by the HIV-2_EHO RT was further improved after substituting the sequence YQEPFKNLKT of HIV-1_BH10 RT (positions 342–351) for the equivalent residues of the HIV-2 enzyme (HQGDKILKV). Our results highlight the role of β-sheets 17 and 18 and their connecting loop (residues 342–350) in the connection subdomain of the large subunit, in determining the RNase H cleavage window of HIV RTs.

Large Multidomain Protein NMR: HIV-1 Reverse Transcriptase Precursor in Solution

NMR studies of large proteins, over 100 kDa, in solution are technically challenging and, therefore, of considerable interest in the biophysics field. The challenge arises because the molecular tumbling of a protein in solution considerably slows as molecular mass increases, reducing the ability to detect resonances. In fact, the typical ¹H-¹³C or ¹H-¹⁵N correlation spectrum of a large protein, using a ¹³C- or ¹⁵N-uniformly labeled protein, shows severe line-broadening and signal overlap. Selective isotope labeling of methyl groups is a useful strategy to reduce these issues, however, the reduction in the number of signals that goes hand-in-hand with such a strategy is, in turn, disadvantageous for characterizing the overall features of the protein. When domain motion exists in large proteins, the domain motion differently affects backbone amide signals and methyl groups. Thus, the use of multiple NMR probes, such as ¹H, ¹⁹F, ¹³C, and ¹⁵N, is ideal to gain overall structural or dynamical information for large proteins. We discuss the utility of observing different NMR nuclei when characterizing a large protein, namely, the 66 kDa multi-domain HIV-1 reverse transcriptase that forms a homodimer in solution. Importantly, we present a biophysical approach, complemented by biochemical assays, to understand not only the homodimer, p66/p66, but also the conformational changes that contribute to its maturation to a heterodimer, p66/p51, upon HIV-1 protease cleavage.

Nucleocapsid Protein Precursors NCp9 and NCp15 Suppress ATP-Mediated Rescue of AZT-Terminated Primers by HIV-1 Reverse Transcriptase

In HIV-1, development of resistance to AZT (3'-azido-3'-deoxythymidine) is mediated by the acquisition of thymidine analogue resistance mutations (TAMs) (i.e., M41L, D67N, K70R, L210W, T215F/Y, and K219E/Q) in the viral reverse transcriptase (RT). Clinically relevant combinations of TAMs, such as M41L/T215Y or D67N/K70R/T215F/K219Q, enhance the ATP-mediated excision of AZT monophosphate (AZTMP) from the 3' end of the primer, allowing DNA synthesis to continue. Additionally, during HIV-1 maturation, the Gag polyprotein is cleaved to release a mature nucleocapsid protein (NCp7) and two intermediate precursors (NCp9 and NCp15). NC proteins interact with the viral genome and facilitate the reverse transcription process. Using wild-type and TAM-containing RTs, we showed that both NCp9 and NCp15 inhibited ATP-mediated rescue of AZTMP-terminated primers annealed to RNA templates but not DNA templates, while NCp7 had no effect on rescue activity. RNase H inactivation by introducing the active-site mutation E478Q led to the loss of the inhibitory effect shown by NCp9. NCp15 had a stimulatory effect on the RT's RNase H activity not observed with NCp7 and NCp9. However, analysis of RNase H cleavage patterns revealed that in the presence of NCp9, RNA/DNA complexes containing duplexes of 12 bp had reduced stability in comparison with those obtained in the absence of NC or with NCp7 or NCp15. These effects are expected to have a strong influence on the inhibitory action of NCp9 and NCp15 by affecting the efficiency of RNA-dependent DNA polymerization after unblocking DNA primers terminated with AZTMP and other nucleotide analogues.

The HIV-1 reverse transcriptase E138A natural polymorphism decreases the genetic barrier to resistance to etravirine in vitro

OBJECTIVES

The HIV-1 reverse transcriptase (RT) natural polymorphism E138A is included among the mutations with a minor impact on response to etravirine. However, the interpretation of E138A on etravirine susceptibility is not consistent across different genotypic resistance algorithms. The aim of the study was to investigate the effect of E138A on the genetic barrier to resistance to etravirine in vitro.

METHODS

A panel of 20 clinically derived recombinant viruses (10 with WT 138E and 10 with 138A, all without any other resistance mutation) were cultured in the presence of increasing etravirine concentrations and analysed for genotypic changes at virus breakthrough. Parallel experiments were conducted with 138E/A/G/K/Q NL4-3-based clones.

RESULTS

In the NL4-3 background, codon 138 changes increased etravirine resistance in the following order: Q > K > A > G > E. The 138A viruses were less susceptible to etravirine compared with the 138E viruses [median (IQR) fold change, 1.8 (1.5-2.8) versus 1.3 (0.8-1.8); P = 0.026], overcame etravirine pressure earlier [HR (95% CI) for viral outgrowth with 138A, 5.48 (2.95-28.24); P < 0.001] and grew at higher drug concentrations [median (IQR), 1350 (1350-1350) versus 0 (0-1350) nM; P = 0.005]. A variety of etravirine resistance-related mutations and changes in the RT connection and RNase H domains accumulated without any consistent pattern depending on baseline codon 138.

CONCLUSIONS

E138A can contribute to reduced response to etravirine through a decreased genetic barrier to resistance. In vitro drug resistance selection is a valuable complement to define the full potential of low-level resistance mutations.

A 30 kDa polyethylene glycol-enfuvirtide complex enhances the exposure of enfuvirtide in lymphatic viral reservoirs in rats

HIV therapy with anti-retroviral drugs is limited by the poor exposure of viral reservoirs, such as lymphoid tissue, to these small molecule drugs. We therefore investigated the effect of PEGylation on the anti-retroviral activity and subcutaneous lymphatic pharmacokinetics of the peptide-based fusion inhibitor enfuvirtide in thoracic lymph duct cannulated rats. Both the peptide and the PEG were quantified in plasma and lymph via ELISA. Conjugation to a single 5 kDa linear PEG decreased anti-HIV activity three-fold compared to enfuvirtide. Whilst plasma and lymphatic exposure to peptide mass was moderately increased, the loss of anti-viral activity led to an overall decrease in exposure to enfuvirtide activity. A 20 kDa 4-arm branched PEG conjugated with an average of two enfuvirtide peptides decreased peptide activity by six-fold. Plasma and lymph exposure to enfuvirtide, however, increased significantly such that anti-viral activity was increased two- and six-fold respectively. The results suggest that a multi-enfuvirtide-PEG complex may optimally enhance the anti-retroviral activity of the peptide in plasma and lymph.

Molecular dissection of an inhibitor targeting the HIV integrase dependent preintegration complex nuclear import

Human immunodeficiency virus (HIV) continues to be a major contributor to morbidity and mortality worldwide, particularly in developing nations where high cost and logistical issues severely limit the use of current HIV therapeutics. This, combined HIV's high propensity to develop resistance, means that new antiviral agents against novel targets are still urgently required. We previously identified novel anti-HIV agents directed against the nuclear import of the HIV integrase (IN) protein, which plays critical roles in the HIV lifecycle inside the cell nucleus, as well as in transporting the HIV preintegration complex (PIC) into the nucleus. Here we investigate the structure activity relationship of a series of these compounds for the first time, including a newly identified anti-IN compound, budesonide, showing that the extent of binding to the IN core domain correlates directly with the ability of the compound to inhibit IN nuclear transport in a permeabilised cell system. Importantly, compounds that inhibited the nuclear transport of IN were found to significantly decrease HIV viral replication, even in a dividing cell system. Significantly, budesonide or its analogue flunisolide, were able to effect a significant reduction in the presence of specific nuclear forms of the HIV DNA (2-LTR circles), suggesting that the inhibitors work though blocking IN, and potentially PIC, nuclear import. The work presented here represents a platform for further development of these specific inhibitors of HIV replication with therapeutic and prophylactic potential.

Anti-HIV-1 Activity of Lactic Acid in Human Cervicovaginal Fluid

Women of reproductive age with a Lactobacillus-dominated vaginal microbiota have a reduced risk of acquiring and transmitting HIV and a vaginal pH of ~4 due to the presence of ~1% (wt/vol) lactic acid. While lactic acid has potent HIV virucidal activity in vitro, whether lactic acid present in the vaginal lumen inactivates HIV has not been investigated. Here we evaluated the anti-HIV-1 activity of native, minimally diluted cervicovaginal fluid obtained from women of reproductive age (n = 20) with vaginal microbiota dominated by Lactobacillus spp. Inhibition of HIVBa-L was significantly associated with the protonated form of lactic acid in cervicovaginal fluid. The HIVBa-L inhibitory activity observed in the <3-kDa acidic filtrate was similar to that of the corresponding untreated native cervicovaginal fluid as well as that of clarified neat cervicovaginal fluid subjected to protease digestion. These ex vivo studies indicate that protonated lactic acid is a major anti-HIV-1 metabolite present in acidic cervicovaginal fluid, suggesting a potential role in reducing HIV transmission by inactivating virus introduced or shed into the cervicovaginal lumen.IMPORTANCE The Lactobacillus-dominated vaginal microbiota is associated with a reduced risk of acquiring and transmitting HIV and other sexually transmitted infections (STIs). Lactic acid is a major organic acid metabolite produced by lactobacilli that acidifies the vagina and has been reported to have inhibitory activity in vitro against bacterial, protozoan, and viral STIs, including HIV infections. However, the anti-HIV properties of lactic acid in native vaginal lumen fluids of women colonized with Lactobacillus spp. have not yet been established. Our study, using native cervicovaginal fluid from women, found that potent and irreversible anti-HIV-1 activity is significantly associated with the concentration of the protonated (acidic, uncharged) form of lactic acid. This work advances our understanding of the mechanisms by which vaginal microbiota modulate HIV susceptibility and could lead to novel strategies to prevent women from acquiring HIV or transmitting the virus during vaginal intercourse and vaginal birth.

Rapid accumulation of HIV-1 thymidine analogue mutations and phenotypic impact following prolonged viral failure on zidovudine-based first-line ART in sub-Saharan Africa

Background: Lack of viral load monitoring of ART is known to be associated with slower switch from a failing regimen and thereby higher prevalence of MDR HIV-1. Many countries have continued to use thymidine analogue drugs despite recommendations to use tenofovir in combination with a cytosine analogue and NNRTI as first-line ART. The effect of accumulated thymidine analogue mutations (TAMs) on phenotypic resistance over time has been poorly characterized in the African setting.

Patients and methods: A retrospective analysis of individuals with ongoing viral failure between weeks 48 and 96 in the NORA (Nevirapine OR Abacavir) study was conducted. We analysed 36 genotype pairs from weeks 48 and 96 of first-line ART (14 treated with zidovudine/lamivudine/nevirapine and 22 treated with zidovudine/lamivudine/abacavir). Phenotypic drug resistance was assessed using the Antivirogram assay (v. 2.5.01, Janssen Diagnostics).

Results: At 96 weeks, extensive TAMs (≥3 mutations) were present in 50% and 73% of nevirapine- and abacavir-treated patients, respectively. The mean (SE) number of TAMs accumulating between week 48 and week 96 was 1.50 (0.37) in nevirapine-treated participants and 1.82 (0.26) in abacavir-treated participants. Overall, zidovudine susceptibility of viruses was reduced between week 48 [geometric mean fold change (FC) 1.3] and week 96 (3.4, P = 0.01). There was a small reduction in tenofovir susceptibility (FC 0.7 and 1.0, respectively, P = 0.18).

Conclusions: Ongoing viral failure with zidovudine-containing first-line ART is associated with rapidly increasing drug resistance that could be mitigated with effective viral load monitoring.

HIV-1 drug resistance before initiation or re-initiation of first-line antiretroviral therapy in low-income and middle-income countries: a systematic review and meta-regression analysis

Background

Pretreatment drug resistance in people initiating or re-initiating antiretroviral therapy (ART) containing non-nucleoside reverse transcriptase inhibitors (NNRTIs) might compromise HIV control in low-income and middle-income countries (LMICs). We aimed to assess the scale of this problem and whether it is associated with the intiation or re-initiation of ART in people who have had previous exposure to antiretroviral drugs.

Methods

This study was a systematic review and meta-regression analysis. We assessed regional prevalence of pretreatment drug resistance and risk of pretreatment drug resistance in people initiating ART who reported previous ART exposure. We systematically screened publications and unpublished datasets for pretreatment drug-resistance data in individuals in LMICs initiating or re-initiating first-line ART from LMICs. We searched for studies in PubMed and Embase and conference abstracts and presentations from the Conference on Retroviruses and Opportunistic Infections, the International AIDS Society Conference, and the International Drug Resistance Workshop for the period Jan 1, 2001, to Dec 31, 2016. To assess the prevalence of drug resistance within a specified region at any specific timepoint, we extracted study level data and pooled prevalence estimates within the region using an empty logistic regression model with a random effect at the study level. We used random effects meta-regression to relate sampling year to prevalence of pretreatment drug resistance within geographical regions.

Findings

We identified 358 datasets that contributed data to our analyses, representing 56 044 adults in 63 countries. Prevalence estimates of pretreatment NNRTI resistance in 2016 were 11·0% (7·5–15·9) in southern Africa, 10·1% (5·1–19·4) in eastern Africa, 7·2% (2·9–16·5) in western and central Africa, and 9·4% (6·6–13·2) in Latin America and the Caribbean. There were substantial increases in pretreatment NNRTI resistance per year in all regions. The yearly increases in the odds of pretreatment drug resistance were 23% (95% CI 16–29) in southern Africa, 17% (5–30) in eastern Africa, 17% (6–29) in western and central Africa, 11% (5–18) in Latin America and the Caribbean, and 11% (2–20) in Asia. Estimated increases in the absolute prevalence of pretreatment drug resistance between 2015 and 2016 ranged from 0·3% in Asia to 1·8% in southern Africa.

Interpretation

Pretreatment drug resistance is increasing at substantial rate in LMICs, especially in sub-Saharan Africa. In 2016, the prevalence of pretreatment NNRTI resistance was near WHO's 10% threshold for changing first-line ART in southern and eastern Africa and Latin America, underscoring the need for routine national HIV drug-resistance surveillance and review of national policies for first-line ART regimen composition.

Funding

Bill & Melinda Gates Foundation and World Health Organization.

Identification of Novel Resistance-Related Polymorphisms in HIV-1 Subtype C RT Connection and RNase H Domains from Patients Under Virological Failure in Brazil

Mutations in the connection and RNase H C-terminal reverse transcriptase (RT) domains of HIV-1 have been shown to impact drug resistance to RT inhibitors. However, their impact in the context of non-B subtypes has been poorly assessed. This study aimed to characterize resistance-related mutations in the C-terminal portions of RT in treatment-failing patients from southern Brazil, a region with endemic HIV-1 subtype C (HIV-1C). Viral RNA was isolated and reverse transcribed from 280 infected subjects, and genomic regions were analyzed by polymerase chain reaction, DNA sequencing, and phylogenetic analysis. Two novel mutations, M357R and E529D, were evidenced in Brazilian HIV-1C strains from treatment-failing patients. In global viral isolates of subjects on treatment, M357R was selected in HIV-1C and CRF01_AE and E529D was selected in HIV-1 subtype B (HIV-1B). While most C-terminal RT mutations described for HIV-1B also occur in HIV-1C, this work pinpointed novel mutations that display subtype-specific predominance or occurrence.

Drug resistance-related mutations T369V/I in the connection subdomain of HIV-1 reverse transcriptase severely impair viral fitness

Fitness is a key parameter in the measurement of transmission capacity of individual drug-resistant HIV. Drug-resistance related mutations (DRMs) T369V/I and A371V in the connection subdomain (CN) of reverse transcriptase (RT) occur at higher frequencies in the individuals experiencing antiretroviral therapy failure. Here, we evaluated the effects of T369V/I and A371V on viral fitness, in the presence or in the absence of thymidine analogue resistance-associated mutations (TAMs) and assessed the effect of potential RT structure-related mechanism on change in viral fitness. Mutations T369V/I, A371V, alone or in combination with TAMs were introduced into a modified HIV-1 infectious clone AT1 by site-directed mutagenesis. Then, experiments on mutant and wild-type virus AT2 were performed separately using a growth-competition assay, and then the relative fitness was calculated. Structural analysis of RT was conducted using Pymol software. Results showed that T369V/I severely impaired the relative virus fitness, and A371V compensated for the viral fitness reduction caused by TAMs. Structural modeling of RT suggests that T369V/I substitutions disrupt powerful hydrogen bonds formed by T369 and V365 in p51 and p66. This study indicates that the secondary DRMs within CN might efficiently damage viral fitness, and provides valuable information for clinical surveillance and prevention of HIV-1 strains carrying these DRMs.

Structural biology in antiviral drug discovery

Structural biology has emerged during the last thirty years as a powerful tool for rational drug discovery. Crystal structures of biological targets alone and in complex with ligands and inhibitors provide essential insights into the mechanisms of actions of enzymes, their conformational changes upon ligand binding, the architectures and interactions of binding pockets. Structure-based methods such as crystallographic fragment screening represent nowadays invaluable instruments for the identification of new biologically active compounds. In this context, three-dimensional protein structures have played essential roles for the understanding of the activity and for the design of novel antiviral agents against several different viruses. In this review, the evolution in the resolution of viral structures is analysed, along with the role of crystal structures in the discovery and optimisation of new antivirals.

Resistance to reverse transcriptase inhibitors used in the treatment and prevention of HIV-1 infection

Inhibitors that target the retroviral enzyme reverse transcriptase (RT) have played an indispensable role in the treatment and prevention of HIV-1 infection. They can be grouped into two distinct therapeutic groups, namely the nucleoside and nucleotide RT inhibitors (NRTIs), and the non-nucleoside RT inhibitors (NNRTIs). NRTIs form the backbones of most first- and second-line antiretroviral therapy (ART) regimens formulated for the treatment of HIV-1 infection. They are also used to prevent mother-to-child transmission, and as pre-exposure prophylaxis in individuals at risk of HIV-1 infection. The NNRTIs nevirapine (NVP), efavirenz and rilpivirine also used to form part of first-line ART regimens, although this is no longer recommended, while etravirine can be used in salvage ART regimens. A single-dose of NVP administered to both mother and child has routinely been used in resource-limited settings to reduce the rate of HIV-1 transmission. Unfortunately, the development of HIV-1 resistance to RT inhibitors can compromise the efficacy of these antiviral drugs in both the treatment and prevention arenas. Here, we provide an up-to-date review on drug-resistance mutations in HIV-1 RT, and discuss their cross-resistance profiles, molecular mechanisms and clinical significance.

Subtype-independent near full-length HIV-1 genome sequencing and assembly to be used in large molecular epidemiological studies and clinical management

Introduction

HIV-1 near full-length genome (HIV-NFLG) sequencing from plasma is an attractive multidimensional tool to apply in large-scale population-based molecular epidemiological studies. It also enables genotypic resistance testing (GRT) for all drug target sites allowing effective intervention strategies for control and prevention in high-risk population groups. Thus, the main objective of this study was to develop a simplified subtype-independent, cost- and labour-efficient HIV-NFLG protocol that can be used in clinical management as well as in molecular epidemiological studies.

Methods

Plasma samples (n=30) were obtained from HIV-1B (n=10), HIV-1C (n=10), CRF01_AE (n=5) and CRF01_AG (n=5) infected individuals with minimum viral load >1120 copies/ml. The amplification was performed with two large amplicons of 5.5 kb and 3.7 kb, sequenced with 17 primers to obtain HIV-NFLG. GRT was validated against ViroSeq™ HIV-1 Genotyping System.

Results

After excluding four plasma samples with low-quality RNA, a total of 26 samples were attempted. Among them, NFLG was obtained from 24 (92%) samples with the lowest viral load being 3000 copies/ml. High (>99%) concordance was observed between HIV-NFLG and ViroSeq™ when determining the drug resistance mutations (DRMs). The N384I connection mutation was additionally detected by NFLG in two samples.

Conclusions

Our high efficiency subtype-independent HIV-NFLG is a simple and promising approach to be used in large-scale molecular epidemiological studies. It will facilitate the understanding of the HIV-1 pandemic population dynamics and outline effective intervention strategies. Furthermore, it can potentially be applicable in clinical management of drug resistance by evaluating DRMs against all available antiretrovirals in a single assay.

Identification of mechanistically distinct inhibitors of HIV-1 reverse transcriptase through fragment screening

Fragment-based screening methods can be used to discover novel active site or allosteric inhibitors for therapeutic intervention. Using saturation transfer difference (STD) NMR and in vitro activity assays, we have identified fragment-sized inhibitors of HIV-1 reverse transcriptase (RT) with distinct chemical scaffolds and mechanisms compared to nonnucleoside RT inhibitors (NNRTIs) and nucleoside/nucleotide RT inhibitors (NRTIs). Three compounds were found to inhibit RNA- and DNA-dependent DNA polymerase activity of HIV-1 RT in the micromolar range while retaining potency against RT variants carrying one of three major NNRTI resistance mutations: K103N, Y181C, or G190A. These compounds also inhibit Moloney murine leukemia virus RT but not the Klenow fragment of Escherichia coli DNA polymerase I. Steady-state kinetic analyses demonstrate that one of these fragments is a competitive inhibitor of HIV-1 RT with respect to deoxyribonucleoside triphosphate (dNTP) substrate, whereas a second compound is a competitive inhibitor of RT polymerase activity with respect to the DNA template/primer (T/P), and consequently also inhibits RNase H activity. The dNTP competing RT inhibitor retains activity against the NRTI-resistant mutants K65R and M184V, demonstrating a drug resistance profile distinct from the nucleotide competing RT inhibitors indolopyridone-1 (INDOPY-1) and 4-dimethylamino-6-vinylpyrimidine-1 (DAVP-1). In antiviral assays, the T/P competing compound inhibits HIV-1 replication at a step consistent with an RT inhibitor. Screening of additional structurally related compounds to the three fragments led to the discovery of molecules with improved potency against HIV-1 RT. These fragment inhibitors represent previously unidentified scaffolds for development of novel drugs for HIV-1 prevention or treatment.

Viral envelope is a major determinant of enhanced fitness of a multidrug-resistant HIV-1 variant

Multidrug-resistant (MDR) HIV-1 viruses are thought to be less pathogenic than wild-type viruses because of the fitness costs of drug-resistance mutations. However, we identified an individual infected with MDR virus associated with rapid disease progression referred to as MDR-1. To study the contribution of virologic factors to rapid disease progression, we constructed molecular clones that demonstrated high replication fitness and cytopathicity. To dissect determinants of enhanced fitness of a cytopathic clone, pMDR-1c, we divided its genome into 2 parts: the envelope (gp160) and the remaining backbone genome, and constructed mutual chimeric viruses with a reference, wild-type virus clone, pNL4-3. The growth competition assay indicated that pMDR-1c has high fitness (1.62), although its envelope confers remarkably enhanced fitness (2.29) and its backbone confers reduced fitness (0.56) as compared with pNL4-3. We also performed a similar study with a less cytopathic pMDR-5a, a molecular clone derived from another subject MDR-5, infected with MDR HIV-1, and associated with slower clinical progression. The results indicated that pMDR-5a has reduced fitness (0.82), although its envelope confers enhanced fitness (1.64) and its backbone confers reduced fitness (0.49), a fitness pattern compatible with envelope-mediated fitness compensation. These results suggest that the viral envelope may be a major determinant of the enhanced fitness of the MDR HIV-1 variant isolated from a patient with rapid disease progression. Furthermore, we speculate that compensation conferred by envelope may be a mechanism by which MDR HIV-1 maintains overall fitness despite the presence of changes in pol, which reduce replication capacity.

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.

Effects of HIV-1 reverse transcriptase connection subdomain mutations on polypurine tract removal and initiation of (+)-strand DNA synthesis

HIV-1 reverse transcriptase (RT) connection subdomain mutations at positions 348, 369 and 376 have been associated with resistance to non-nucleoside RT inhibitors (NNRTIs). N348I may interfere with the initiation of (+)-strand DNA synthesis by reducing polypurine tract (PPT) removal in the presence of nevirapine. The effect of NNRTIs on the RNase H-mediated cleavage of PPT-containing template-primers has been studied with wild-type HIV-1 RT and mutants N348I, T369I, T369V, T376S and N348I/T369I. In the presence of NNRTIs, all RTs were able to stimulate PPT cleavage after primer elongation. The enhancing effects of nevirapine and efavirenz were reduced in RTs carrying mutation N348I, and specially N348I/T369I. However, those mutations had no effect on rilpivirine-mediated cleavage. Prior to elongation, the PPT remains resilient to cleavage, although efavirenz and rilpivirine facilitate RNase H-mediated trimming of its 3′-end. The integrity of the 3′-end is essential for the initiation of (+)-strand DNA synthesis. In the presence of dNTPs, rilpivirine was the most effective inhibitor of (+)-strand DNA synthesis blocking nucleotide incorporation and preventing usage of available PPT primers. The N348I/T369I RT showed reduced ability to generate short RNA products revealing a cleavage window defect. Its lower RNase H activity could be attributed to enhanced rigidity compared to the wild-type enzyme.

In vitro selection of HIV-1 CRF08_BC variants resistant to reverse transcriptase inhibitors

Human immunodeficiency virus type 1 (HIV-1) circulating recombinant form 08_BC (CRF08_BC), carrying the recombinant reverse transcriptase (RT) gene from subtypes B and C, has recently become highly prevalent in Southern China. As the number of patients increases, it is important to characterize the drug resistance mutations of CRF08_BC, especially against widely used antiretrovirals. In this study, clinically isolated virus (2007CNGX-HK), confirmed to be CRF08_BC with its sequence deposited in GenBank (KF312642), was propagated in human peripheral blood mononuclear cells (PBMCs) with increasing concentrations of nevirapine (NVP), efavirenz (EFV), or lamivudine (3TC). Three different resistance patterns led by initial mutations of Y181C, E138G, and Y188C were detected after the selection with NVP. Initial mutations, in combination with other previously reported substitutions (K20R, D67N, V90I, K101R/E, V106I/A, V108I, F116L, E138R, A139V, V189I, G190A, D218E, E203K, H221Y, F227L, N348I, and T369I) or novel mutations (V8I, S134N, C162Y, L228I, Y232H, E396G, and D404N), developed during NVP selection. EFV-associated variations contained two initial mutations (L100I and Y188C) and three other mutations (V106L, F116Y, and A139V). Phenotypic analyses showed that E138R, Y181C, and G190A contributed high-level resistance to NVP, while L100I and V106L significantly reduced virus susceptibility to EFV. Y188C was 20-fold less sensitive to both NVP and EFV. As expected, M184I alone, or with V90I or D67N, decreased 3TC susceptibility by over 1,000-fold. Although the mutation profile obtained in culture may be different from the patients, these results may still provide useful information to monitor and optimize the antiretroviral regimens.

A novel mutation, D404N, in the connection subdomain of reverse transcriptase of HIV-1 CRF08_BC subtype confers cross-resistance to NNRTIs

OBJECTIVES

Growing evidence suggests that mutations in the connection domain of the HIV-1 reverse transcriptase (RT) can contribute to viral resistance to RT inhibitors. This work was designed to determine the effects of a novel mutation, D404N, in the connection subdomain of RT of HIV-1 CRF08_BC subtype on drug resistance, viral replication capacity (RC) and RT activity.

METHODS

Mutation D404N, alone or together with the other reported mutations, was introduced into an HIV-1 CRF08_BC subtype infectious clone by site-directed mutagenesis. Viral susceptibility to nine RT inhibitors, viral RC and the DNA polymerase activity of viral RT of the constructed virus mutants were investigated. A modelling study using the server SWISS-MODEL was conducted to explore the possible structure-related drug resistance mechanism of the mutation D404N.

RESULTS

Single mutations D404N and H221Y conferred low-level resistance to nevirapine, efavirenz, rilpivirine and zidovudine. Double mutations Y181C/D404N and Y181C/H221Y significantly reduced susceptibility to NNRTIs. The most pronounced resistance to NNRTIs was observed with the triple mutation Y181C/D404N/H221Y. Virus containing D404N as the only mutation displayed ∼50% RC compared with the WT virus. The modelling study suggested that the D404N mutation might abolish the hydrogen bonds between residues 404 and K30 in p51 or K431 in p66, leading to impaired RT subunit structure and enhanced drug resistance.

CONCLUSIONS

These results indicate that D404N is a novel NNRTI-associated mutation in the HIV-1 subtype CRF08_BC and provides information valuable for the monitoring of clinical RTI resistance.

Impact of drug resistance-associated amino acid changes in HIV-1 subtype C on susceptibility to newer nonnucleoside reverse transcriptase inhibitors

The objective of this study was to assess the phenotypic susceptibility of HIV-1 subtype C isolates, with nonnucleoside reverse transcriptase inhibitor (NNRTI) resistance-associated amino acid changes, to newer NNRTIs. A panel of 52 site-directed mutants and 38 clinically derived HIV-1 subtype C clones was created, and the isolates were assessed for phenotypic susceptibility to etravirine (ETR), rilpivirine (RPV), efavirenz (EFV), and nevirapine (NVP) in an in vitro single-cycle phenotypic assay. The amino acid substitutions E138Q/R, Y181I/V, and M230L conferred high-level resistance to ETR, while K101P and Y181I/V conferred high-level resistance to RPV. Y181C, a major NNRTI resistance-associated amino acid substitution, caused decreased susceptibility to ETR and, to a lesser extent, RPV when combined with other mutations. These included N348I and T369I, amino acid changes in the connection domain that are not generally assessed during resistance testing. However, the prevalence of these genotypes among subtype C sequences was, in most cases, <1%. The more common EFV/NVP resistance-associated substitutions, such as K103N, V106M, and G190A, had no major impact on ETR or RPV susceptibility. The low-level resistance to RPV and ETR conferred by E138K was not significantly enhanced in the presence of M184V/I, unlike for EFV and NVP. Among patient samples, 97% were resistant to EFV and/or NVP, while only 24% and 16% were resistant to ETR and RPV, respectively. Overall, only a few, relatively rare NNRTI resistance-associated amino acid substitutions caused resistance to ETR and/or RPV in an HIV-1 subtype C background, suggesting that these newer NNRTIs would be effective in NVP/EFV-experienced HIV-1 subtype C-infected patients.

Drug resistance in non-B subtype HIV-1: impact of HIV-1 reverse transcriptase inhibitors

Human immunodeficiency virus (HIV) causes approximately 2.5 million new infections every year, and nearly 1.6 million patients succumb to HIV each year. Several factors, including cross-species transmission and error-prone replication have resulted in extraordinary genetic diversity of HIV groups. One of these groups, known as group M (main) contains nine subtypes (A-D, F-H and J-K) and causes ~95% of all HIV infections. Most reported data on susceptibility and resistance to anti-HIV therapies are from subtype B HIV infections, which are prevalent in developed countries but account for only ~12% of all global HIV infections, whereas non-B subtype HIV infections that account for ~88% of all HIV infections are prevalent primarily in low and middle-income countries. Although the treatments for subtype B infections are generally effective against non-B subtype infections, there are differences in response to therapies. Here, we review how polymorphisms, transmission efficiency of drug-resistant strains, and differences in genetic barrier for drug resistance can differentially alter the response to reverse transcriptase-targeting therapies in various subtypes.

The emerging profile of cross-resistance among the nonnucleoside HIV-1 reverse transcriptase inhibitors

Nonnucleoside reverse transcriptase inhibitors (NNRTIs) are widely used to treat HIV-1-infected individuals; indeed most first-line antiretroviral therapies typically include one NNRTI in combination with two nucleoside analogs. In 2008, the next-generation NNRTI etravirine was approved for the treatment of HIV-infected antiretroviral therapy-experienced individuals, including those with prior NNRTI exposure. NNRTIs are also increasingly being included in strategies to prevent HIV-1 infection. For example: (1) nevirapine is used to prevent mother-to-child transmission; (2) the ASPIRE (MTN 020) study will test whether a vaginal ring containing dapivirine can prevent HIV-1 infection in women; (3) a microbicide gel formulation containing the urea-PETT derivative MIV-150 is in a phase I study to evaluate safety, pharmacokinetics, pharmacodynamics and acceptability; and (4) a long acting rilpivirine formulation is under-development for pre-exposure prophylaxis. Given their widespread use, particularly in resource-limited settings, as well as their low genetic barriers to resistance, there are concerns about overlapping resistance between the different NNRTIs. Consequently, a better understanding of the resistance and cross-resistance profiles among the NNRTI class is important for predicting response to treatment, and surveillance of transmitted drug-resistance.

Molecular dynamics study of HIV-1 RT-DNA-nevirapine complexes explains NNRTI inhibition and resistance by connection mutations

HIV-1 reverse transcriptase (RT) is a multifunctional enzyme that is targeted by nucleoside analogs (NRTIs) and non-nucleoside RT inhibitors (NNRTIs). NNRTIs are allosteric inhibitors of RT, and constitute an integral part of several highly active antiretroviral therapy regimens. Under selective pressure, HIV-1 acquires resistance against NNRTIs primarily by selecting mutations around the NNRTI pocket. Complete RT sequencing of clinical isolates revealed that spatially distal mutations arising in connection and the RNase H domain also confer NNRTI resistance and contribute to NRTI resistance. However, the precise structural mechanism by which the connection domain mutations confer NNRTI resistance is poorly understood. We performed 50-ns molecular dynamics (MD) simulations, followed by essential dynamics, free-energy landscape analyses, and network analyses of RT-DNA, RT-DNA-nevirapine (NVP), and N348I/T369I mutant RT-DNA-NVP complexes. MD simulation studies revealed altered global motions and restricted conformational landscape of RT upon NVP binding. Analysis of protein structure network parameters demonstrated a dissortative hub pattern in the RT-DNA complex and an assortative hub pattern in the RT-DNA-NVP complex suggesting enhanced rigidity of RT upon NVP binding. The connection subdomain mutations N348I/T369I did not induce any significant structural change; rather, these mutations modulate the conformational dynamics and alter the long-range allosteric communication network between the connection subdomain and NNRTI pocket. Insights from the present study provide a structural basis for the biochemical and clinical findings on drug resistance caused by the connection and RNase H mutations.

Molecular basis of the association of H208Y and thymidine analogue resistance mutations M41L, L210W and T215Y in the HIV-1 reverse transcriptase of treated patients

Thymidine analogue resistance mutations (TAMs) in HIV-1 reverse transcriptase (RT) associate in two clusters: (i) TAM1 (M41L, L210W and T215Y) and TAM2 (D67N, K70R, K219E/Q, and sometimes T215F). The amino acid substitution H208Y shows increased prevalence in patients treated with nucleoside analogues and is frequently associated with TAM1 mutations. We studied the molecular mechanism favoring the selection of H208Y in the presence of zidovudine, tenofovir and other nucleoside RT inhibitors (NRTIs). NRTI susceptibility was not affected by the addition of H208Y in phenotypic assays carried out in MT-4 cells using recombinant HIV-1 containing wild-type (subtype B, BH10), H208Y, M41L/L210W/T215Y or M41L/H208Y/L210W/T215Y RTs. However, enzymatic studies carried out with purified RTs revealed that in the presence of M41L/L210W/T215Y, H208Y increases the RT's ability to unblock and extend primers terminated with zidovudine, tenofovir and in a lesser extent, stavudine. These effects were observed with DNA/DNA complexes (but not with RNA/DNA) and resulted from the higher ATP-dependent excision activity of the M41L/H208Y/L210W/T215Y RT compared with the M41L/L210W/T215Y mutant. The increased rescue efficiency of the M41L/H208Y/L210W/T215Y RT was observed in the presence of ATP but not with GTP or ITP. Molecular dynamics studies predict an alteration of the stacking interactions between Tyr(215) and the adenine ring of ATP due to long-distance effects caused by tighter packaging of Tyr(208) and Trp(212). These studies provide a mechanistic explanation for the association of TAM-1 and H208Y mutations in viral isolates from patients treated with NRTIs.

Phenotypic and genotypic analyses to guide selection of reverse transcriptase inhibitors in second-line HIV therapy following extended virological failure in Uganda

Objectives

We investigated phenotypic and genotypic resistance after 2 years of first-line therapy with two HIV treatment regimens in the absence of virological monitoring.

Methods

NORA [Nevirapine OR Abacavir study, a sub-study of the Development of AntiRetroviral Therapy in Africa (DART) trial] randomized 600 symptomatic HIV-infected Ugandan adults (CD4 cell count <200 cells/mm³) to receive zidovudine/lamivudine plus abacavir (cABC arm) or nevirapine (cNVP arm). All virological tests were performed retrospectively, including resistance tests on week 96 plasma samples with HIV RNA levels ≥1000 copies/mL. Phenotypic resistance was expressed as fold-change in IC₅₀ (FC) relative to wild-type virus.

Results

HIV-1 RNA viral load ≥1000 copies/mL at week 96 was seen in 58/204 (28.4%) cABC participants and 21/159 (13.2%) cNVP participants. Resistance results were available in 35 cABC and 17 cNVP participants; 31 (89%) cABC and 16 (94%) cNVP isolates had a week 96 FC below the biological cut-off for tenofovir (2.2). In the cNVP arm, 16/17 participants had resistance mutations synonymous with high-level resistance to nevirapine and efavirenz; FC values for etravirine were above the biological cut-off in 9 (53%) isolates. In multivariate regression models, K65R, Y115F and the presence of thymidine analogue-associated mutations were associated with increased susceptibility to etravirine in the cABC arm.

Conclusions

Our data support the use of tenofovir following failure of a first-line zidovudine-containing regimen and shed further light on non-nucleoside reverse transcriptase inhibitor hypersusceptibility.

Quantifying susceptibility of CD4+ stem memory T-cells to infection by laboratory adapted and clinical HIV-1 strains

CD4⁺ T cells are principal targets for human immunodeficiency virus type 1 (HIV-1) infection. CD4⁺ T cell subsets are heterogeneous cell populations, divided by functional and phenotypic differences into naïve and memory T cells. The memory CD4⁺ T cells are further segregated into central, effector and transitional memory cell subsets by functional, phenotypic and homeostatic characteristics. Defining the distribution of HIV-1 infection in different T cell subsets is important, as this can play a role in determining the size and composition of the viral reservoir. Both central memory and transitional memory CD4⁺ T cells have been described as long-lived viral reservoirs for HIV. Recently, the newly described stem memory T cell subset has also been implicated as a long-lived HIV reservoir. Using green fluorescent protein (GFP) reporter strains of HIV-1 and multi parameter flow cytometry, we developed an assay to simultaneously quantify the susceptibility of stem memory (TSCM), central memory, effector memory, transitional memory and naïve CD4⁺ T cell subsets, to HIV-1 infection in vitro. We show that TSCM are susceptible to infection with laboratory adapted and clinical HIV-1 strains. Our system facilitates the quantitation of HIV-1 infection in alternative T cell subsets by CCR5- and CXCR4-using viruses across different HIV-1 subtypes, and will be useful for studies of HIV-1 pathogenesis and viral reservoirs.

Transient kinetic analyses of the ribonuclease H cleavage activity of HIV-1 reverse transcriptase in complex with efavirenz and/or a β-thujaplicinol analogue

EFV (efavirenz) and β-thujaplicinol [2,7-dihydroxy-4-1(methylethyl)-2,4,6-cycloheptatrien-1-one] have contrasting effects on the RNase H activity of HIV-1 RT (reverse transcriptase). EFV binds in the non-nucleoside inhibitor-binding pocket and accelerates this activity, whereas β-thujaplicinol binds in the RNase H active site and inhibits it. We have used pre-steady-state kinetic analyses to gain an insight into the mechanism by which EFV and a β-thujaplicinol analogue [19616 (2,7-dihydroxy-2,4,6-cyclo-heptatrien-1-one)] modulate RT RNase H activity. Our data show that EFV and 19616 have no effect on polymerase-dependent RNase H cleavages. However, both compounds significantly affected the rates of polymerase-independent RNase H cleavages. In regard to the latter, we found no evidence that the bound RNA/DNA template/primer substrate restricted 19616 from interacting with RT. In light of these data, we propose a model in which 19616 binds to the RNase H active site of RT after the primary polymerase-dependent RNase H cleavage has occurred and stabilizes the 3'-end of the DNA primer in the polymerase active site thus blocking the enzyme's ability to carry out the polymerase-independent cleavages. By contrast, EFV destabilizes the 3'-end of the DNA primer in the DNA polymerase active site and promotes RT-mediated polymerase-independent cleavages. Consistent with this model, we show antagonism between EFV and 19616.

The connection domain mutation N348I in HIV-1 reverse transcriptase enhances resistance to etravirine and rilpivirine but restricts the emergence of the E138K resistance mutation by diminishing viral replication capacity

Clinical resistance to rilpivirine (RPV), a novel nonnucleoside reverse transcriptase (RT) inhibitor (NNRTI), is associated an E-to-K mutation at position 138 (E138K) in RT together with an M184I/V mutation that confers resistance against emtricitabine (FTC), a nucleoside RT inhibitor (NRTI) that is given together with RPV in therapy. These two mutations can compensate for each other in regard to fitness deficits conferred by each mutation alone, raising the question of why E138K did not arise spontaneously in the clinic following lamivudine (3TC) use, which also selects for the M184I/V mutations. In this context, we have investigated the role of a N348I connection domain mutation that is prevalent in treatment-experienced patients. N348I confers resistance to both the NRTI zidovudine (ZDV) and the NNRTI nevirapine (NVP) and was also found to be associated with M184V and to compensate for deficits associated with the latter mutation. Now, we show that both N348I alone and N348I/M184V can prevent or delay the emergence of E138K under pressure with RPV or a related NNRTI, termed etravirine (ETR). N348I also enhanced levels of resistance conferred by E138K against RPV and ETR by 2.2- and 2.3-fold, respectively. The presence of the N348I or M184V/N348I mutation decreased the replication capacity of E138K virus, and biochemical assays confirmed that N348I, in a background of E138K, impaired RT catalytic efficiency and RNase H activity. These findings help to explain the low viral replication capacity of viruses containing the E138K/N348I mutations and how N348I delayed or prevented the emergence of E138K in patients with M184V-containing viruses.

Molecular mechanism of HIV-1 resistance to 3'-azido-2',3'-dideoxyguanosine

We reported that 3'-azido-2',3'-dideoxyguanosine (3'-azido-ddG) selected for the L74V, F77L, and L214F mutations in the polymerase domain and K476N and V518I mutations in the RNase H domain of HIV-1 reverse transcriptase (RT). In this study, we have defined the molecular mechanisms of 3'-azido-ddG resistance by performing in-depth biochemical analyses of HIV-1 RT containing mutations L74V, F77L, V106I, L214F, R277K, and K476N (SGS3). The SGS3 HIV-1 RT was from a single-genome-derived full-length RT sequence obtained from 3'-azido-ddG resistant HIV-1 selected in vitro. We also analyzed two additional constructs that either lacked the L74V mutation (SGS3-L74V) or the K476N mutation (SGS3-K476N). Pre-steady-state kinetic experiments revealed that the L74V mutation allows RT to effectively discriminate between the natural nucleotide (dGTP) and 3'-azido-ddG-triphosphate (3'-azido-ddGTP). 3'-azido-ddGTP discrimination was primarily driven by a decrease in 3'-azido-ddGTP binding affinity (Kd) and not by a decreased rate of incorporation (kpol). The L74V mutation was found to severely impair RT's ability to excise the chain-terminating 3'-azido-ddG-monophosphate (3'-azido-ddGMP) moiety. However, the K476N mutation partially restored the enzyme's ability to excise 3'-azido-ddGMP on an RNA/DNA, but not on a DNA/DNA, template/primer by selectively decreasing the frequency of secondary RNase H cleavage events. Collectively, these data provide strong additional evidence that the nucleoside base structure is major determinant of HIV-1 resistance to the 3'-azido-2',3'-dideoxynucleosides.

Biophysical Insights into the Inhibitory Mechanism of Non-Nucleoside HIV-1 Reverse Transcriptase Inhibitors

HIV-1 reverse transcriptase (RT) plays a central role in HIV infection. Current United States Federal Drug Administration (USFDA)-approved antiretroviral therapies can include one of five approved non-nucleoside RT inhibitors (NNRTIs), which are potent inhibitors of RT activity. Despite their crucial clinical role in treating and preventing HIV-1 infection, their mechanism of action remains elusive. In this review, we introduce RT and highlight major advances from experimental and computational biophysical experiments toward an understanding of RT function and the inhibitory mechanism(s) of NNRTIs.

A polymorphism at position 400 in the connection subdomain of HIV-1 reverse transcriptase affects sensitivity to NNRTIs and RNaseH activity

Reverse transcriptase (RT) plays an essential role in HIV-1 replication, and inhibition of this enzyme is a key component of HIV-treatment. However, the use of RT inhibitors can lead to the emergence of drug-resistant variants. Until recently, most clinically relevant resistance mutations were found in the polymerase domain of RT. Lately, an increasing number of resistance mutations has been identified in the connection and RNaseH domain. To further explore the role of these domains we analyzed the complete RT sequence of HIV-1 subtype B patients failing therapy. Position A/T400 in the connection subdomain is polymorphic, but the proportion of T400 increases from 41% in naïve patients to 72% in patients failing therapy. Previous studies suggested a role for threonine in conferring resistance to nucleoside RT inhibitors. Here we report that T400 also mediates resistance to non-nucleoside RT inhibitors. The susceptibility to NVP and EFV was reduced 5-fold and 2-fold, respectively, in the wild-type subtype B NL4.3 background. We show that substitution A400T reduces the RNaseH activity. The changes in enzyme activity are remarkable given the distance to both the polymerase and RNaseH active sites. Molecular dynamics simulations were performed, which provide a novel atomistic mechanism for the reduction in RNaseH activity induced by T400. Substitution A400T was found to change the conformation of the RNaseH primer grip region. Formation of an additional hydrogen bond between residue T400 and E396 may play a role in this structural change. The slower degradation of the viral RNA genome may provide more time for dissociation of the bound NNRTI from the stalled RT-template/primer complex, after which reverse transcription can resume.

Evolving uses of oral reverse transcriptase inhibitors in the HIV-1 epidemic: from treatment to prevention

The HIV epidemic continues unabated, with no highly effective vaccine and no cure. Each new infection has significant economic, social and human costs and prevention efforts are now as great a priority as global antiretroviral therapy (ART) scale up. Reverse transcriptase inhibitors, the first licensed class of ART, have been at the forefront of treatment and prevention of mother to child transmission over the past two decades. Now, their use in adult prevention is being extensively investigated. We describe two approaches: treatment as prevention (TasP) - the use of combination ART (2NRTI and 1NNRTI) following HIV diagnosis to limit transmission and pre-exposure prophylaxis (PrEP) –the use of single or dual oral agents prior to sexual exposure. Prevention of mother-to-child transmission using NRTI has been highly successful, though does not involve sustained use of NRTI to limit transmission. Despite theoretical and preliminary support for TasP and PrEP, data thus far indicate that adherence, retention in care and late diagnosis are the major barriers to their successful, sustained implementation. Future advances in drug technologies will be needed to overcome the issue of drug adherence, through development of drugs that involve both less frequent dosing as well as reduced toxicity, possibly through specific targeting of infected cells.

Resistance mutations outside the integrase coding region have an effect on human immunodeficiency virus replicative fitness but do not affect its susceptibility to integrase strand transfer inhibitors

Most studies describing phenotypic resistance to integrase strand transfer inhibitors have analyzed viruses carrying only patient-derived HIV-1 integrase genes (INT-recombinant viruses). However, to date, many of the patients on INSTI-based treatment regimes, such as raltegravir (RAL), elvitegravir (EVG), and dolutegravir (DTG) are infected with multidrug-resistant HIV-1 strains. Here we analyzed the effect of drug resistance mutations in Gag (p2/NCp7/p1/p6), protease (PR), reverse transcriptase (RT), and integrase (IN) coding regions on susceptibility to INSTIs and viral replicative fitness using a novel HIV-1 phenotyping assay. Initial characterization based on site-directed mutant INSTI-resistant viruses confirmed the effect of a series of INSTI mutations on reduced susceptibility to EVG and RAL and viral replicative fitness (0.6% to 99% relative to the HIV-1_NL4-3 control). Two sets of recombinant viruses containing a 3,428-bp gag-p2/NCp7/p1/p6/pol-PR/RT/IN (p2-INT) or a 1,088 bp integrase (INT) patient-derived fragment were constructed from plasma samples obtained from 27 virologic failure patients participating in a 48-week dose-ranging study of elvitegravir, GS-US-183-0105. A strong correlation was observed when susceptibility to EVG and RAL was assayed using p2-INT- vs. INT-recombinant viruses (Pearson coefficient correlation 0.869 and 0.918, P<0.0001 for EVG and RAL, respectively), demonstrating that mutations in the protease and RT have limited effect on susceptibility to these INSTIs. On the other hand, the replicative fitness of viruses harboring drug resistance mutations in PR, RT, and IN was generally impaired compared to viruses carrying only INSTI-resistance mutations. Thus, in the absence of drug pressure, drug resistance mutations in the PR and RT contribute to decrease the replicative fitness of the virus already impaired by mutations in the integrase. The use of recombinant viruses containing most or all HIV-1 regions targeted by antiretroviral drugs might be essential to understand the collective effect of epistatic interactions in multidrug-resistant viruses.

Phenotypic susceptibility to antiretrovirals among clades C, F, and B/F recombinant antiretroviral-naive HIV type 1 strains

To evaluate antiretroviral phenotypic susceptibility of wild-type HIV-1 strains circulating in Brazil, samples from antiretroviral-naive individuals infected with subtypes C (n=16), F (n=9), or B/F (n=7), where reverse transcriptase is B and protease is F, were phenotyped using the Antivirogram Assay (Virco, Mechelen, Belgium). Reduced susceptibility to protease inhibitors (PIs) was observed in one C and three F isolates. None of these samples had any known PI resistance mutations. The phenotypic fold change to one PI was above the biological cut-off in three of 96 (3.1%) clade F phenotypic determinations and in one of 96 (1.0%) clade C. Phenotypic resistance to at least one nucleoside reverse transcriptase inhibitor (NRTI) was found for two B/F, four C, and three F isolates. The phenotypic fold change in susceptibility to NRTIs was above the cut-off value in nine of 111 (8.1%) clade C determinations, as compared to three of 63 (4.8%) for clade F and two of 49 (4.1%) for clade B. The phenotypic fold change to non-NRTI (NNRTI) was above the cut-off in seven of 32 (21.9%) of C isolates determinations, whereas none of the F isolates had a decrease of susceptibility. Only two of the 16 C samples had a known NNRTI resistance mutation. The NNRTI fold change was above the cut-off value in three of 14 (21.4%) phenotypic determinations of Brazilian B/F recombinants, representing clade B reverse transcriptase. NNRTI susceptibility should be better investigated in clade C and B/F recombinants.

Vaginal concentrations of lactic acid potently inactivate HIV

OBJECTIVES

When Lactobacillus spp. dominate the vaginal microbiota of women of reproductive age they acidify the vagina to pH <4.0 by producing ∼1% lactic acid in a nearly racemic mixture of d- and l-isomers. We determined the HIV virucidal activity of racemic lactic acid, and its d- and l-isomers, compared with acetic acid and acidity alone (by the addition of HCl).

METHODS

HIV-1 and HIV-2 were transiently treated with acids in the absence or presence of human genital secretions at 37°C for different time intervals, then immediately neutralized and residual infectivity determined in the TZM-bl reporter cell line.

RESULTS

l-lactic acid at 0.3% (w/w) was 17-fold more potent than d-lactic acid in inactivating HIVBa-L. Complete inactivation of different HIV-1 subtypes and HIV-2 was achieved with ≥0.4% (w/w) l-lactic acid. At a typical vaginal pH of 3.8, l-lactic acid at 1% (w/w) more potently and rapidly inactivated HIVBa-L and HIV-1 transmitter/founder strains compared with 1% (w/w) acetic acid and with acidity alone, all adjusted to pH 3.8. A final concentration of 1% (w/w) l-lactic acid maximally inactivated HIVBa-L in the presence of cervicovaginal secretions and seminal plasma. The anti-HIV activity of l-lactic acid was pH dependent, being abrogated at neutral pH, indicating that its virucidal activity is mediated by protonated lactic acid and not the lactate anion.

CONCLUSIONS

l-lactic acid at physiological concentrations demonstrates potent HIV virucidal activity distinct from acidity alone and greater than acetic acid, suggesting a protective role in the sexual transmission of HIV.

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

HIV-1 reverse transcriptase (RT) contributes to the development of resistance to all anti-AIDS drugs by introducing mutations into the viral genome. At the molecular level, mutations in RT result in resistance to RT inhibitors. Eight nucleoside/nucleotide analogs (NRTIs) and five non-nucleoside inhibitors (NNRTIs) are approved HIV-1 drugs. Structures of RT have been determined in complexes with substrates and/or inhibitors, and the structures have illuminated different conformational and functional states of the enzyme. Understanding the molecular mechanisms of resistance to NRTIs and NNRTIs, and their complex relationships, may help in designing new drugs that are periodically required to overcome existing as well as emerging trends of drug resistance.

The magnitude of HIV-1 resistance to the CCR5 antagonist maraviroc may impart a differential alteration in HIV-1 tropism for macrophages and T-cell subsets

Human immunodeficiency virus type 1 (HIV-1) resistance to CCR5 antagonists, including maraviroc (MVC), results from alterations in the HIV-1 envelope glycoproteins (Env) enabling recognition of antagonist-bound CCR5. Here, we characterized tropism alterations for CD4+ T-cell subsets and macrophages by Envs from two subjects who developed MVC resistance in vivo, which displayed either relatively efficient or inefficient recognition of MVC-bound CCR5. We show that MVC-resistant Env with efficient recognition of drug-bound CCR5 displays a tropism shift for CD4+ T-cell subsets associated with increased infection of central memory T-cells and reduced infection of effector memory and transitional memory T-cells, and no change in macrophage infectivity. In contrast, MVC-resistant Env with inefficient recognition of drug-bound CCR5 displays no change in tropism for CD4+ T-cell subsets, but exhibits a significant reduction in macrophage infectivity. The pattern of HIV-1 tropism alterations for susceptible cells may therefore be variable in subjects with MVC resistance.

Examining the role of the HIV-1 reverse transcriptase p51 subunit in positioning and hydrolysis of RNA/DNA hybrids

Recent crystallographic analysis of p66/p51 human immunodeficiency virus (HIV) type 1 reverse transcriptase (RT) complexed with a non-polypurine tract RNA/DNA hybrid has illuminated novel and important contacts between structural elements at the C terminus of the noncatalytic p51 subunit and the nucleic acid duplex in the vicinity of the ribonuclease H (RNase H) active site. In particular, a short peptide spanning residues Phe-416-Pro-421 was shown to interact with the DNA strand, cross the minor groove of the helix, and then form Van der Waals contacts with the RNA strand adjacent to the scissile phosphate. At the base of the adjoining α-helix M', Tyr-427 forms a hydrogen bond with Asn-348, the latter of which, when mutated to Ile, is implicated in resistance to both nucleoside and non-nucleoside RT inhibitors. Based on our structural data, we analyzed the role of the p51 C terminus by evaluating selectively mutated p66/p51 heterodimers carrying (i) p51 truncations that encroach on α-M', (ii) alterations that interrupt the Asn-348-Tyr-427 interaction, and (iii) alanine substitutions throughout the region Phe-416-Pro-421. Collectively, our data support the notion that the p51 C terminus makes an important contribution toward hybrid binding and orienting the RNA strand for catalysis at the RNase H active site.

Molecular basis of human immunodeficiency virus type 1 drug resistance: overview and recent developments

The introduction of potent combination therapies in the mid-90s had a tremendous effect on AIDS mortality. However, drug resistance has been a major factor contributing to antiretroviral therapy failure. Currently, there are 26 drugs approved for treating human immunodeficiency virus (HIV) infections, although some of them are no longer prescribed. Most of the available antiretroviral drugs target HIV genome replication (i.e. reverse transcriptase inhibitors) and viral maturation (i.e. viral protease inhibitors). Other drugs in clinical use include a viral coreceptor antagonist (maraviroc), a fusion inhibitor (enfuvirtide) and two viral integrase inhibitors (raltegravir and elvitegravir). Elvitegravir and the nonnucleoside reverse transcriptase inhibitor rilpivirine have been the most recent additions to the antiretroviral drug armamentarium. An overview of the molecular mechanisms involved in antiretroviral drug resistance and the role of drug resistance-associated mutations was previously presented (Menéndez-Arias, L., 2010. Molecular basis of human immunodeficiency virus drug resistance: an update. Antiviral Res. 85, 210-231). This article provides now an updated review that covers currently approved drugs, new experimental agents (e.g. neutralizing antibodies) and selected drugs in preclinical or early clinical development (e.g. experimental integrase inhibitors). Special attention is dedicated to recent research on resistance to reverse transcriptase and integrase inhibitors. In addition, recently discovered interactions between HIV and host proteins and novel strategies to block HIV assembly or viral entry emerge as promising alternatives for the development of effective antiretroviral treatments.

Subtype-specific differences in the development of accessory mutations associated with high-level resistance to HIV-1 nucleoside reverse transcriptase inhibitors

OBJECTIVES

To identify accessory mutations associated with high-level resistance to reverse transcriptase (RT) inhibitors in HIV-1 subtypes B and C.

METHODS

Changes relative to the wild-type for codons 1-400 of RT were analysed from treatment-experienced patients infected with subtypes B (5464 patients) and C (1920 patients). Positions associated with the accumulation of mutations conferring resistance to thymidine analogues and to non-nucleoside RT inhibitors (NNRTIs) were identified. A subtype-specific single-replication cycle drug susceptibility assay was used to determine whether some of the mutations affected drug susceptibility or viral infectivity.

RESULTS

In subtype B, mutations at 31 and 26 positions were associated with the accumulation of thymidine analogue mutations (TAMs) and NNRTI mutations, respectively; in subtype C, 18 and 13 positions were identified, respectively. Amino acid changes at the following positions were differentially associated with (i) the accumulation of 0-4+ TAMs in subtypes B and C (away from consensus): 43 (27.0% B versus 2.5% C); 118 (36.4% B versus 16.2% C); 135 (12.5% B versus 28.0% C); and 326 (2.6% towards consensus in B versus 7.6% away in C) and (ii) the accumulation of 0-3+ NNRTI mutations (away from consensus): 43 (10.2% B versus 0.5% C); and 68 (5.2% B versus 10.3% C). Codon changes K43E, E44D and V118I were found to have no effect on susceptibility to three NRTIs with or without TAMs in either subtype; however, some accessory mutations had subtype-specific effects on viral infectivity.

CONCLUSIONS

Differences between subtypes B and C were observed in the development and effect of accessory mutations associated with high-level resistance to RT inhibitors.

Connection subdomain mutations in HIV-1 subtype-C treatment-experienced patients enhance NRTI and NNRTI drug resistance

Mutations in the connection subdomain (CN) and RNase H domain (RH) of HIV-1 reverse transcriptase (RT) from subtype B-infected patients enhance nucleoside and nonnucleoside RT inhibitor (NRTI and NNRTI) resistance by affecting the balance between polymerization and RNase H activity. To determine whether CN mutations in subtype C influence drug sensitivity, single genome sequencing was performed on Brazilian subtype C-infected patients failing RTI therapy. CN mutations identified were similar to subtype B, including A376S, A400T, Q334D, G335D, N348I, and A371V, and increased AZT resistance in the presence of thymidine analog mutations. CN mutations also enhanced NNRTI resistance in the presence of classical NNRTI mutations: etravirine resistance was enhanced 6- to 11-fold in the presence of L100I/K103N/Y181C. These results indicate that selection of CN mutations in treatment-experienced patients also occurs in subtype-C-infected patients and are likely to provide valuable information in predicting clinical RTI resistance.

Complexes of HIV-1 RT, NNRTI and RNA/DNA hybrid reveal a structure compatible with RNA degradation

Structures of type-1 human immunodeficiency virus (HIV-1) reverse transcriptase (RT) have been determined in several forms, but only one contains an RNA/DNA hybrid. Here we report three structures of HIV-1 RT complexed with a non-nucleotide RT inhibitor (NNRTI) and an RNA/DNA hybrid. In the presence of an NNRTI, the RNA/DNA structure differs from all prior nucleic acid bound to RT including the RNA/DNA hybrid. The enzyme structure also differs from all previous RT–DNA complexes. As a result, the hybrid has ready access to the RNase H active site. These observations indicate that an RT–nucleic acid complex may adopt two structural states, one competent for DNA polymerization and the other for RNA degradation. RT mutations that confer drug resistance but are distant from the inhibitor-binding sites often map to the unique RT–hybrid interface that undergoes conformational changes between two catalytic states.