9th Conference on Retroviruses and Opportunistic Infections Seattle, Washington - February 24 -February 28, 2002

Conf Retroviruses Opportunistic Infect 2002 Feb 24-28;9:abstract no. L6
John Mellors
University of Pittsburgh Med. Ctr., PA

BACKGROUND: The genetic basis for HIV-1 resistance to many nucleoside analog inhibitors has been known for over a decade, but the biochemical and structural changes in reverse transcriptase (RT) that are responsible for the resistance phenotypes have only recently begun to be defined. The mechanisms by which mutations in HIV-1 RT cause resistance to nucleoside analogs can be categorized into 2 groups: those that increase the ability of HIV-1 RT to discriminate between the nucleoside analog and the normal substrate, leading to preferential incorporation of the normal substrate; and mutations that increase the ability of the enzyme to excise the chain terminating nucleoside analog that has been mis-incorporated into the viral DNA. To illustrate the latter, a longstanding conundrum was how AZT resistance mutations mediated their effects because HIV-1 RT encoding AZT resistance mutations incorporated AZT-monophosphate into viral cDNA almost as efficiently as the wild-type enzyme. It is now apparent that AZT resistance mutations enhance the ability of HIV-1 RT to excise AZT-monophosphate from the AZT-terminated cDNA, freeing up the 3'OH group and allowing DNA polymerization to proceed. This excision reaction is catalyzed by ATP (or pyrophosphate) and certain of the AZT resistance mutations (210W, 215Y/F) probably increase the binding affinity of ATP to the mutant enzyme through ring stack with the adenine base of ATP. This enhanced excision capability of the mutant enzyme is not specific for AZT-monophosphate terminated cDNA and this helps to explain the cross-resistance observed between AZT and some other nucleoside analogs (e.g., D4T, abacavir). Nucleoside analog excision is not the sole mechanism responsible for nucleoside resistance. Certain mutations in HIV-1 RT decrease the incorporation of the nucleoside analog monophosphate by reducing its binding or altering the position of its binding so it is unfavorable for incorporation. A prime example of this is the methionine to valine mutation at codon 184 of HIV-1 RT responsible for high-level resistance to 3TC and cross-resistance to other L-nucleosides. The beta-methyl group of 184V clashes sterically with the L-oxathiolane ring of 3TC resulting in malpositioning of 3TC-triphosphate and markedly reduced incorporation of 3TC-monophosphate and other L-nucleoside analogs into the viral cDNA. Other phenotypic effects of nucleoside (or pyrophosphate) analog resistance mutations include reversal of AZT resistance and hypersusceptibility to nonnucleoside RT inhibitors. The biochemical and structural mechanisms involved in these phenotypes are more complex and less well understood but are important for the clinical activity of nucleoside and nonnucleoside RT inhibitor.

CONCLUSIONS: Further insights into the mechanisms of resistance, cross-resistance, resistance reversal and hypersusceptibility will undoubtedly lead to the more rationale use and design of RT inhibitors.

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