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Michael Dapp
Postdoctoral Fellow


Phone: (612) 624-5172 and (612) 625-7106


B.S., Pharmacology & Toxicology, University of Wisconsin-Madison

Ph.D. Pharmacology, University of Minnesota-Twin Cities

Research Interests

HIV-1 genetic variation, evolution, drug resistance; Viral quasispecies; Viral fitness

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 my research is 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 is being 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 provide 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 my research is 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 indicate 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 have been 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 provide 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 I have conducted to date 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) 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.

Selected Recent Publications


  • Dapp MJ, Bonnac, L, Patterson, SE, Mansky, LM. 2013. Discovery of novel ribonucleoside analogs with activity against human immunodeficiency virus type 1. Journal of Virology. Epub ahead of print.
  • Rawson, JM, Heineman, RH, Beach, LB, Martin, JL, Schnettler, EK, Dapp, MJ, Patterson, SE, Mansky, LM. 2013. 5,6-Dihydro-5-aza-2'-deoxycytidine potentiates the anti-HIV-1 activity of ribonucleotide reductase inhibitors. Bioorganic and Medicinal Chemistry. 21:7222-7228.
  • Dapp MJ, Patterson, SE, Mansky, LM. 2012. Back to the future: revisiting HIV-1 lethal mutagenesis. Trends in Microbiology. 21:56-62.
  • Dapp MJ, Heineman, RH, Mansky LM. 2012. Interrelationship between HIV-1 fitness and mutation rate. J Mol Biol. 425:41-53.
  • Dapp MJ, Holtz CM, Mansky LM. 2012. Concomitant lethal mutagenesis of human immunodeficiency virus type 1. J Mol Biol. 419:158-170.
  • Dapp, M.J., Clouser, C.L., Patterson, S.E. and L.M. Mansky. 2009. 5-Azacytidine can induce lethal mutagenesis in human immunodeficiency virus type 1. Journal of Virology 83:11950-58.


Featured News & Events

'Wisc-e-sota', a Joint UMN-UW Virology Training Grant Symposium was first held on Friday, Sepbember 20th, 2013 at the Uniiversity of Wisconsin-La Crosse, Cartwright Center. This was the inaugural collaborative symposium of the NIH T32-supported virology training programs at the University of Wisconsin-Madison and the University of Minnesota-Twin Cities. Talks and poster sessions were presented by students, postdocs and faculty. The second UMN-UW Virology Training Grant Symposium will be held in the Fall 2014. Details to follow.

The 2014 IMV Symposium will be held on May 12, 2014 and Mark Denison (Vanderbilt) and Bert Semler (UC-Irvine) will be the Keynote Speakers. Click on the link below to register and submit abstracts.

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