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Table 2) [12,54]. For example, none of the five mutations, in HIV-1 RT Met41Leu, Asp67Asn,
Lys70Arg, Thr215Tyr, and Lys219Gln [55] (Table 2) consistently associated with resistance to AZT,
are at locations close to the dNTP-binding site. However, most (but not all, c.f. [56]) biochemical studies
have failed to show that recombinant HIV-1 RT enzymes containing these mutations are more resistant
to inhibition by AZT triphosphate than the wild-type HIV-1 RT [27,57,58]. Other mutations that confer
resistance to NRTIs have been identified at positions 50, 65, 69, 74, 75, 89, 115, 135, 151, and 184 of
HIV-1 RT (Figure 5 and Table 2). Most of these mutations do not lie close to the dNTP-binding site
(Met184Val/Ile are the exception), but instead are located at positions where they could interact with the
nucleic acid template-primer [12,59,60]. Biochemical data have shown that only when the 5'-template
extension length is greater than three nucleotides does the wild-type RT begin to incorporate
dideoxynucleotides effectively [54]. If the template extension is less than three nucleotides in length,
wild-type HIV-1 RT is resistant to dideoxynucleotides. On the other hand, HIV-1 RT variants containing
the mutations Leu74Val or Glu89Gly did not readily incorporate dideoxynucleotides either with short or
long template extensions [54]. Based on both structural and biochemical data, it was proposed that
mutations that cause HIV-1 RT to have reduced sensitivity to NRTIs exert their effects via interactions
with the nucleic acid template-primer, which consequently alter the geometry of the polymerase active
site [54]. It has been suggested that mutations that confer resistance to foscarnet might use a similar
mechanism [61]. One possible exception to this mechanism might be the mutations of Met184Val and
Met184Ile (see review [27]). Part of the highly conserved YMDD motif, Met184 is adjacent to residues
Asp185 and Asp186, which are two of the three catalytically essential aspartic acid residues at the
polymerase active site. In addition, Met184 appears to interact with the ribose moiety of the 3'-terminal
nucleotide of the primer strand [12,38,53] (Figure 4). Therefore, mutations at this position could affect
interactions with the incoming dNTP directly and/or alter the positioning of the nucleic acid. These
mechanisms are not mutually exclusive and which mechanism is responsible for resistance has not yet
been resolved [62]. There are two recent reports suggesting that the Met184Val mutant HIV-1 RT has
approximately three-fold higher fidelity than the wild-type enzyme [63,64]. Based on these data, it was
suggested that this increase in fidelity might reduce the overall rate of generation of viral variants in
patients treated with 3TC or other dideoxynucleosides [64]. However, owing to both theoretical and
technical problems with these analyses, these conclusions are controversial. Determination of crystal
structures of both wild-type and mutant HIV-1 RT complexed with individual NRTIs in the presence of
a variety of template-primers and/or dNTP substrates should provide a better understanding of the
mechanisms of dNTP selection and drug resistance.ed at Arial for catalysis [41].
IV. Mechanism of NRTI-Resistance Mutations
Development of resistance to NRTIs has been
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