Why Do HIV-1 and HIV-2 Use Different Pathways to Develop AZT Resistance?

Paul L Boyer,Stephen H Hughes,Eddy Arnold,Patrick K Clark,Stefan G Sarafianos,Susan Ross

doi:10.1371/journal.ppat.0020010

Abstract

The human immunodeficiency virus type 1 (HIV-1) develops resistance to all available drugs, including the nucleoside analog reverse transcriptase inhibitors (NRTIs) such as AZT. ATP-mediated excision underlies the most common form of HIV-1 resistance to AZT. However, clinical data suggest that when HIV-2 is challenged with AZT, it usually accumulates resistance mutations that cause AZT resistance by reduced incorporation of AZTTP rather than selective excision of AZTMP. We compared the properties of HIV-1 and HIV-2 reverse transcriptase (RT) in vitro. Although both RTs have similar levels of polymerase activity, HIV-1 RT more readily incorporates, and is more susceptible to, inhibition by AZTTP than is HIV-2 RT. Differences in the region around the polymerase active site could explain why HIV-2 RT incorporates AZTTP less efficiently than HIV-1 RT. HIV-1 RT is markedly more efficient at carrying out the excision reaction with ATP as the pyrophosphate donor than is HIV-2 RT. This suggests that HIV-1 RT has a better nascent ATP binding site than HIV-2 RT, making it easier for HIV-1 RT to develop a more effective ATP binding site by mutation. A comparison of HIV-1 and HIV-2 RT shows that there are numerous differences in the putative ATP binding sites that could explain why HIV-1 RT binds ATP more effectively. HIV-1 RT incorporates AZTTP more efficiently than does HIV-2 RT. However, HIV-1 RT is more efficient at ATP-mediated excision of AZTMP than is HIV-2 RT. Mutations in HIV-1 RT conferring AZT resistance tend to increase the efficiency of the ATP-mediated excision pathway, while mutations in HIV-2 RT conferring AZT resistance tend to increase the level of AZTTP exclusion from the polymerase active site. Thus, each RT usually chooses the pathway best suited to extend the properties of the respective wild-type enzymes.

Highlights

Considerable progress has been made in developing successful anti-human immunodeficiency virus type 1 (HIV-1) drugs and drug therapies, there are serious problems with drug toxicity and the development of drug-resistant viral strains
We found that the two enzymes have similar activities in standard assays; it appears that HIV2 reverse transcriptase (RT) is slightly less active in a simple polymerase assay (;75% of HIV-1 RT) and slightly less processive than HIV-1 RT on a single-stranded M13 DNA template (Figure 1)
This is in agreement with a recent study suggesting that HIV-1 viral replication is more sensitive to AZT than is HIV-2 [16]

Summary

Introduction

Considerable progress has been made in developing successful anti-human immunodeficiency virus type 1 (HIV-1) drugs and drug therapies, there are serious problems with drug toxicity and the development of drug-resistant viral strains. HIV-1 can develop resistance to all 21 of the currently approved drugs to treat it. Of these 21 drugs, 13 inhibit the virally encoded reverse transcriptase (RT). The available RT inhibitors can be divided into two classes: nucleoside reverse transcriptase inhibitors (NRTIs) and nonnucleoside inhibitors (NNRTIs). Both classes of inhibitors block the polymerase activity of RT. NNRTIs bind in a hydrophobic pocket in HIV-1 RT near the polymerase active site. Most NRTIs are given to patients in an unphosphorylated state (the exception is tenofovir, a nucleotide analog that is given as a pro-drug). NRTIs must be taken up by cells and phosphorylated by cellular enzymes before they can be incorporated by RT [1] and references in [1]

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Conclusion