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Structural Basis for HIV-1 Reverse Transcriptase Drug Resistance to Zidovudine and Tenofovir
Kalyan Das*1, X Tu1, R Bandwar1, S Sarafianos1, S Tuske1, P Boyer2, K White3, M Miller3, S Hughes2, and E Arnold1
1Ctr for Advanced Biotech and Med, Rutgers Univ, Piscataway, NJ, US; 2HIV Drug Resistance Prgm, NCI, Frederick, MD, US; and 3Gilead Sci, Foster City, CA, US
Background: NRTI are used in almost all ARV regimens treating
HIV-1 infection. Resistance of HIV-1 reverse transcriptase (RT) to zidovudine (AZT)
involves an ATP-mediated excision reaction in which AZTMP is removed from the
terminated primer, forming a dinucleoside tetraphosphate product, AZTppppA. The
mutation K65R emerges in patients treated with tenofovir (TDF) and K65R has
intricate relationships with other NRTI-resistance mutations.
Methods: We have used X-ray crystallography to study the
structural basis for the mechanisms of resistance to AZT and TDF. A series of
crystal structures of AZT-resistant HIV-1 RT, including ternary complexes with
a template-primer and the excision product AZTppppA, were solved. Also, crystal
structures of K65R RT in ternary complexes with TDF and dATP were determined.
Results: The structures delineate the roles played by the
major AZT-resistance mutations in the excision mechanism of AZT resistance. TAM
like K70R and T215Y help in binding the ATP molecule to the mutant RT, and
therefore can be functionally classified as excision-enhancing mutations. In
K65R mutant structures, the stacking of the guanidinium groups of K65R and R72
creates a “molecular platform” that reduces dNTP incorporation. Distinct
conformations of K65R and R72 in dATP- vs TDF-bound structures allow the mutant
RT to discriminate the drug from the normal substrate, dATP. The structural
results also explain complex relationships of K65R mutation with other
NRTI-resistance mutations including the antagonistic relationship with excision-enhancing
mutations/TAM.
Conclusions: The crystal structures help explain the observed
effects of the mutations on RT polymerization, excision, and zidovudine (AZT)
and TDF resistance, the effects that K65R mutation has when it is present in
combination with other NRTI-resistance mutations (TAM/excision-enhancing
mutations, L74V, and M184V), and the clinical benefit of administering TDF with
AZT, emtricitabine (FTC), or lamivudine (3TC) even though all of these
antiretroviral drugs are NRTI DNA chain terminators.
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