Studies on the basis of HIV-1 variation and its purposeful increase.

摘要

Unconventional measures are needed in an effort to outpace the HIV/AIDS pandemic. Even with access to the six classes of FDA-approved antiretrovirals, adverse drug effects and evolution of drug resistance still pose obstacles, and represent issues that will inevitably arise in the developing world. Along with the absence of a proven vaccine strategy, these shortcomings necessitate the continual search for new therapeutic targets. One such novel approach is to target HIV-1's low copying fidelity with a deliberate increase in viral mutational load using exogenous small molecule mutagens. However, inasmuch as this concept of viral lethal mutagenesis has gained notoriety, it is also essential to understand fundamental enzymatic components (i.e., reverse transcriptase) that contribute to variation. The goal of this dissertation was to advance basic models to better understand the causes and consequences of HIV-1 variation.;The components that influence variation within an HIV-1 population structure are critical to predict the emergence and direction of viral evolution (e.g., drug resistance). For these studies, the relationship between viral fitness and mutation rate was investigated. A panel of 10 reverse transcriptase mutants—most having drug resistance phenotypes—were analyzed for their effects based on these two biological properties. Mutation rate differences were measured using single-cycle vector assays, while fitness differences were identified using ex vivo head-to-head competition assays. As anticipated, viral mutants possessing either higher or lower mutation rate had a corresponding loss in fitness. These observations provided the first description of an interrelationship between HIV-1 fitness and mutation rate and support the conclusion that mutator and antimutator phenotypes correlate with reduced viral fitness.;A second focus of this dissertation was directed at studying novel mechanisms by which viral mutagens diminish HIV-1 infectivity. I have detailed the antiviral mechanisms of the ribonucleoside analog 5-azacytidine. It was demonstrated that the primary antiviral activity of 5-azacytidine can be attributed to its effect on the early phase of HIV-1 replication mediated by reverse transcriptase. Furthermore, the antiviral activity was associated with an increase in the frequency of viral mutants. Sequencing analysis showed enrichment in guanosine-to-cytidine (G-to-C) transversion mutations. These results indicated that 5-azacytidine was incorporated into viral DNA following its 2'-OH reduction to 5-aza-2'-deoxycytidine. Incorporation into the viral DNA led to an increase in mutant frequency, which is consistent with lethal mutagenesis.;Lastly, studies were directed at understanding concomitant exposure of two unrelated mutagenic agents. Because the APOBEC3 proteins' restrictive nature associated with cytosine deamination, I wanted to investigate its interplay with the small molecule cytosine analog, 5-azacytosine. Reduced viral infectivity and increased viral mutagenesis were observed with both the viral mutagen 5-azacytosine (i.e., G-to-C mutations) and the host restriction factor APOBEC3G (i.e., guanosine-to-adenosine (G-to-A) mutations); however, when combined, they had complex interactions. Nucleotide sequence analysis revealed that concomitant HIV-1 exposure to both 5-azacytosine and APOBEC3G resulted in an increase in G-to-A viral mutagenesis at the expense of G-to-C mutagenesis. Also, APOBEC3G catalytic activity was required for the diminution in G-to-C mutagenesis. These findings provided the first demonstration for potentiation of the mutagenic effect of a cytosine analog by APOBEC3G expression, resulting in concomitant HIV-1 lethal mutagenesis.;In summary, the studies conducted in this dissertation 1) provide the first direct experimental evidence of an interrelationship between HIV-1 fitness and mutation rate, 2) demonstrate that the primary antiviral mechanism of 5-azacytidine can be attributed to its ability to increase the HIV-1 mutation frequency through viral DNA incorporation during reverse transcription, and 3) provide the first demonstration for potentiation of the mutagenic effect of a cytosine analog (i.e., 5-azacytosine) by APOBEC3G expression, resulting in concomitant HIV-1 lethal mutagenesis.