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.
Although a number of useful drugs have been developed to treat HIV-1 infections, the virus can become resistant to all of the drugs. Resistance involves the acquisition of mutations in the part of the virus that is the drug target. AZT resistance involves mutations in the viral enzyme reverse transcriptase (RT); RT copies the viral genetic information from RNA into DNA. HIV-1 can develop AZT resistance in two ways; each resistance pathway is characterized by distinct RT mutations. One of the pathways is used by the vast majority of AZT-resistant viruses isolated from patients. However, the closely related virus HIV-2 appears to use the other pathway more frequently than HIV-1. The authors compared the structures and biochemical properties of HIV-1 and HIV-2 RT to try to understand why these two related viruses acquire different AZT-resistance mutations. Although the two RTs are biochemically and structurally similar, there are differences in the respective wild-type RTs, and each can be considered to be part way along the two different resistance pathways. For this reason, it is easier for each virus to acquire the mutations that allow it to use the resistance pathway that extends the properties of the respective wild-type RTs.