Molecular impact of the M184V mutation in human immunodeficiency virus type 1 reverse transcriptase.

Abstract

The generation of drug-resistant variants of human immunodeficiency virus (HIV) type 1 (HIV-1) in vivo as well as in vitro is a consequence of the error proneness of the HIV-1 reverse transcriptase (RT) enzyme. During viral replication, RT copies the single-stranded RNA genome into double-stranded DNA. Due to the lack of 3′ exonuclease proofreading activity, the incorporation of missense nucleotides occurs at a relatively high frequency. The misincorporation rate of the RT enzyme ranges from 10−4 to 10−5, depending on the nature of the template and the source of the RT (4, 7, 31, 39, 54). During in vivo and in vitro selections, drug-resistant variants emerge; competition for replication then results in outgrowth of the fittest variant. The M184V mutation in HIV-1 RT is associated with high-level resistance to the antiviral drug 2′,3′-dideoxy-3′-thiacytidine (3TC) and has been extensively studied from the clinical, biological, and enzymatic perspectives. In fact, the first report of a change from a methionine to valine at residue 184 (M184V) in HIV was in regard to selection for resistance in tissue culture to didanosine (ddI) (23). Later, this mutation was shown to be responsible for both high-level resistance to 3TC (6, 17, 18, 22, 57, 63, 69) and low-level resistance to almost all the molecules that act as nucleoside analog RT inhibitors (NRTIs) (Table ​(Table1)1) (25, 53, 64). The appearance of the M184V substitution was found to be transiently preceded by another mutation, M184I, that also confers high-level resistance (about 1,000-fold) to 3TC (6) and occurs in patients treated with 3TC (58). Longitudinal sampling revealed a transient appearance of the M184I variant, which subsequently disappeared from the viral population due to the outgrowth of the M184V variant (3, 37). Eventual outgrowth of the M184V variant at the expense of M184I during therapy is consistent with superior RT polymerase function (3, 8, 11) and a higher viral replication rate of the M184V variant in primary cells (3). Inspection of the nucleotide sequences of both 3TC-resistant variants indicates that M184V (GTG) originates from wild-type (WT) Met (ATG) and not from the initial M184I variant (ATA) (34). Both variants are generated from the WT ATG sequence by transitional substitutions (G to A for 184I and A to G for 184V). The reason that M184I appears before M184V is that the G→A substitution is the type of mutation that most commonly occurs during HIV-1 replication (13, 24). TABLE 1. Resistance profile of the M184V mutation in RT