The M184V Mutation Reduces the Selective Excision of Zidovudine 5′-Monophosphate (AZTMP) by the Reverse Transcriptase of Human Immunodeficiency Virus Type 1

Abstract

The M184V mutation in human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) causes resistance to lamivudine, but it also increases the sensitivity of the virus to zidovudine (3-azido-3-deoxythymidine; AZT). This sensitization to AZT is seen both in the presence and the absence of the mutations that confer resistance to AZT. AZT resistance is due to enhanced excision of AZT 5-monophosphate (AZTMP) from the end of the primer by the RT of the resistant virus. Published data suggest that the excision reaction involves pyrophosphorolysis but that the likely in vivo pyrophosphate donor is not pyrophosphate but ATP. The mutations that lead to AZT resistance enhance ATP binding and, in so doing, enhance pyrophosphorolysis. The excision reaction is specific for AZT because HIV-1 RT, which can form a closed complex with a dideoxyterminated primer and an incoming deoxynucleoside triphosphate (dNTP), does not form the closed complex with an AZTMP-terminated primer and an incoming dNTP. This means that an AZTMP-terminated primer has better access to the site where it can be excised. The M184V mutation alters the polymerase active site in a fashion that specifically interferes with ATP-mediated excision of AZTMP from the end of the primer strand. The M184V mutation does not affect the incorporation of AZT 5-triphosphate (AZTTP), either in the presence or the absence of mutations that enhance AZTMP excision. However, in the presence of ATP, the M184V mutation does decrease the ability of HIV-1 RT to carry out AZTMP excision. Based on these results, and on the results of other excision experiments, we present a model to explain how the M184V mutation affects AZTMP excision.Although combination therapy has been useful in the treatment of human immunodeficiency virus type 1 (HIV-1) infections, the development of drug resistance is a major problem. The drugs that have been approved for HIV-1 therapy target two viral enzymes, reverse transcriptase (RT) and protease (PR). There are two types of drugs that block reverse transcription, nucleoside analogs and nonnucleoside RT inhibitors. Because there are just three classes of approved anti-HIV-1 drugs, there is considerable cross-resistance within each of the three drug classes. This exacerbates the problem of drug resistance and places a premium on therapies that involve combinations of drugs that do not induce significant cross-resistance.Nucleoside analogs lack the 3Ј OH that is present in normal deoxynucleoside triphosphates (dNTPs). If a nucleoside analog is stably incorporated into viral DNA by HIV-1 RT, it acts as a chain terminator and blocks viral DNA synthesis. Viruses that are resistant to nucleoside analogs have specific mutations in RT. Because these drug-resistant RTs must be able to incorporate normal dNTPs reasonably efficiently, resistance must involve an increase in the discrimination between normal dNTPs and the analogs. This increased discrimination can arise either as a specific block to the incorporation of the nucleoside analog or by selective e...

The HBV Drug Entecavir — Effects on HIV-1 Replication and Resistance

“…The reference strain HIV carrying the M184V mutation was resistant to entecavir ( Figure 2C). In control experiments ( Figure 2C), we showed that the same M184V mutant virus had markedly decreased susceptibility to lamivudine and the expected hypersusceptiblity to zidovudine (16,17,19,20). A post-entecavir isolate from patient #1 containing the M184V mutation behaved similarly ( Figure 2D).…”

Thymidine Analogue Resistance Suppression by V75I of HIV-1 Reverse Transcriptase

“…Thus, A62V, an accessory mutation of the Q151M complex, was reported to increase the ATP-dependent phosphorolytic activity on thymidine analogue-terminated primers of RTs bearing dipeptide insertions (16), whereas the excision activity promoted by the presence of TAMs can be modulated by mutations that influence nucleotide discrimination (e.g. K65R, L74V, and M184V) (17)(18)(19)(20)(21).…”

Mutations in the connection domain of HIV-1 reverse transcriptase increase 3′-azido-3′-deoxythymidine resistance

“…2). The pol domain of patient T-6 contained the M184V and L74V substitutions in addition to the TAMs, both of which have been shown to increase AZT sensitivity by decreasing the efficiency of nucleotide excision (12)(13)(14)(15). This result suggested that the cn domain mutations were likely to have been selected during the treatment of patient T-6 and the AZT-sensitizing mutations M184V and L74V were selected later, perhaps in response to treatment with 2Ј,3Ј-dideoxy-3Јthiacytidine (3TC), abacavir and/or dideoxyinosine 2Ј,3Ј-dideoxyinosine (ddI) (SI Table 3).…”