Thomas W. North
Professor

email: twnorth@ucdavis.edu

Specialty: Antiviral drugs, drug-resist

Undergraduate Education: BS, Lewis and Clark College, Portland, OR

Other School: PhD, Biochemistry, University of Arizona

A major barrier to successful treatment of the acquired immune deficiency syndrome (AIDS) with antiviral drugs is the emergence of drug-resistant mutants of the human immunodeficiency virus type-1 (HIV-1). The long range objective of my research is to understand the mechanisms by which lentiviruses become resistant to antiviral drugs and to utilize this information to help develop improved strategies for chemotherapy of AIDS. We have developed model systems using feline immunodeficiency virus (FIV) and simian immunodeficiency virus (SIV) for studies of resistance to AIDS chemotherapy. The in vitro systems we have developed permit selection and characterization of mutants more rapidly and over a broader range of selection conditions than has been possible with HIV systems. Furthermore, these models offer the potential to answer important questions about drug-resistance with in vivo experiments.

Although the FIV and SIV models have been very well suited for studies of nucleoside analogs, they cannot be used for studies with the full spectrum of the drugs employed in combinations of Highly Active AntiRetroviral Therapy (HAART) that are used for human AIDS. In particular, FIV and SIV are not susceptible to non-nucleoside inhibitors of reverse transcriptase (NNRTI), and little work has been done with protease inhibitors. A major focus of my research is to develop chimeric viruses consisting of SIV in which specific region are replaced with the HIV-1 counterparts (SHIVs). We are currently working with a RT-SHIV, which has the HIV-1 reverse transcriptase, and are developing SHIVs that also contain other HIV-1 targets. The long range goal of this work is to develop a model that will enable studies of modern HAART in SHIV-infected rhesus macaques so that we can perform detailed studies of mechanisms by which the virus can evade AIDS therapy. This will enable us to study emergence of multi-drug-resistant mutants. In addition, we will attempt to more fully define sites of residual replication during HAART, and reservoirs of latent virus.

The remainder of my research is focused on strategies to combat viral resistance to AIDS therapy. We are taking two approaches. The first is an attempt to identify pathways of drug-resistance that attenuate the virus. We are investigating SIV mutants resistant to (-)-ß-2',3'-dideoxy-3'-thiacytidine (3TC) in most detail because they are phenotypically and genotypically similar to important mutants of HIV-1 that occur clinically, and because these mutations may alter the pathogenicity of the virus in ways that will be beneficial for long-term therapy. We have demonstrated that these SIV mutants have reduced fitness in vivo, and we are investigating additional mutations that will further reduce virus replication and pathgenicity.

The other approach I am taking to combat resistance is a combinatorial approach. Combinatorial libraries will enable rapid re-design of inhibitors in response to emergence of resistance. We hypothesize that mixtures of inhibitors targeted against the wild-type enzyme and the major drug-resistant variants will greatly reduce, or eliminate, emergence of drug-resistant mutants. We have used an oligonucleotide-based approach, the Systematic Evolution of Ligands by Exponential Enrichment (SELEX), and have developed potent inhibitors of RT. To be useful for AIDS therapy, genes that express these oligonucleotides will have to be delivered with gene therapy vectors. The other combinatorial approach we have recently initiated utilizes peptide combinatorial libraries directed against viral components. We focusing on inhibitors of the virion gp41 because these inhibitors will inactivate the virus and are active against extracellular virus.

Selected Publications: Chen, H., McBroom, D. G., Zhu, Y.-Q., Gold, L and North, T. W. Inhibitory RNA ligand to feline immunodeficiency virus reverse transcriptase. Biochemistry 35: 6923-6930 (1996).

Zhu. Y.-Q., Remington, K. M. and North, T. W. Mutants of Feline Immunodeficiency Virus Resistant to 2',3'-Dideoxy-2',3'-didehydrothymidine. Antimicrob. Agents Chemother. 40: 1983-1987 (1996).

Arts, E. J., Stetor, S. R., Li, X., Rausch, J. W., Howard, K. J., Ehresmann, B., North, T.W., Wöhrl, B. M., Goody, R. S., Wainberg, M. A. and Le Grice, S. F. J. Initiation of (-) strand DNA synthesis from tRNA Lys,3 on lentiviral RNAs: implications of specific HIV-1 RNA-tRNALys,3 interactions inhibiting primer utilization by retroviral reverse transcriptases. Proc. Nat. Acad. Sci. USA 93: 10063-10068 (1996).

Smith, R. A., Remington, K. M., Lloyd, R. J., Jr., Schinazi, R. F. and North, T. W. A novel Met to Thr mutation in the conserved YMDD motif of reverse transcriptase from feline immunodeficiency virus confers resistance to oxathiolane nucleosides. J. Virol. 71: 2357-2362 (1997).

Smith, R. A., Remington, K. M., Schinazi, R. F. and North, T. W. A novel point mutation inreverse transcriptase from feline immunodeficiency virus confers resistance to the combination of 3'-azido-3'-deoxythymidine and (-)-ß-L-2',3'-dideoxy-3'-thiacytidine. J. Virol. 72: 2335-2340 (1998).

Smith, R. A., Klarmann, G. J., Stray, K. M., von Schwedler, U. K., Schinazi, R. F., Preston, B. D. and North, T. W. A new point mutation (P157S) in the reverse transcriptase of human immunodeficiency virus type 1 confers low-level resistance to (-)-ß-2',3'-dideoxy-3'-thiacytidine. Antimicrob. Agents Chemother. 43: 2077-2080 (1999).

Klarman. G. J., Smith, R. A., Schinazi, R. F., North, T. W. and Preston, B. D. Site-specific incorporation of nucleoside analogs by HIV-1 reverse transcriptase and the template grip mutant P157S. Template interactions influence substrate recognition at the polymerase active site. J. Biol. Chem. 275: 359-366 (2000).

Leutenegger, C. M., Higgins, J., Matthews, T. B., Tarantal, A. F., Luciw, P. A., Pedersen, N. C.and North, T. W. Real-time PCR as a specific and more sensitive alternative to the branched-chain DNA assay for quantitation of simian immunodeficiency virus RNA. AIDS Res. Human Retroviruses 17: 243-251 (2001).

Auwerx, J., North, T. W., Preston, B. D., Klarrman, G. J., DeClercq, E. and Balzarini, J. Chimeric human immunodeficiency virus type 1 and feline immunodeficiency virus reverse transcriptases: role of the subunits in resistance/sensitivity to non-nucleoside reverse transcriptase inhibitors. Mol. Pharmacol. 61: 400-406 (2002).

Van Rompay, K. K. A., Matthews, T. B., Higgins, J., Canfield, D. R., Tarara, R. P., Wainberg, M. A., Schinazi, R. F., Pedersen, N. C. and North, T. W. Virulence and reduced fitness of simian immunodeficiency virus with the M184V mutation in reverse transcriptase. J. Virol. 76: 6083-6092 (2002).