Discussion2

BCMP 207: Study Questions for Discussion of February 24, 2000
The two papers for this discussion take genetic approaches. The first paper uses a model organism, C. elegans, to examine potential targets for a famous, but poorly understooddrug, fluoxetine (a.k.a Prozac). Aside from raising the question -- Do worms get depressed? --it illustrates this approach and its potential advantages and disadvantages. The second paperdeals with the issue of drug resistance in HIV and its impact on drug therapy. It proposes thatthe efficacy of the AZT-3TC combination derives from the suppression of AZT-resistance by3TC-resistance mutations. In other words, 3TC-resistance may be clinically useful! This paperillustrates the kinds of interesting and important inferences that can be drawn from clinicalstudies, but please consider also potential pitfalls of such studies. Please be prepared not onlyto discuss the merits of the papers, but to take stands and propose studies that could test yourideas.
Choy and Thomas. Mol. Cell 4:143-152 (1999). This paper reports the isolation andcharacterization of nematodes that are resistant to Prozac (fluoxetine).
1. What is the primary model for how antidepressants work and why has it been difficult tovalidate this model? 2. What is the evidence that some effects of fluoxetine on C. elegans are mediated by effectson serotonin and others are not? What is the evidence that the Nrf mutants are specific forfluoxetine? How convincing is this evidence? 3. How did the authors show that mutations in the genes identified in Fig. 4 were responsiblefor the Nrf phenotypes? 4. What is the evidence that nrf-6 and ndg-4 function in the same pathway? How convincingis this evidence? 5. Have the authors discovered the basis for fluoxetine's anti-depressant effects? Was this agood approach? What would you do next? Larder et al. Science 269:696-699 (1995). This paper reports what were, at the time, fairlyimpressive virological differences between AZT-3TC combination chemotherapy versus AZTalone, despite the rapid emergence of 3TC resistance. These results combined with clinicalperformance (e.g. patients remaining healthy) is why the AZT-3TC combination was thestarting point to which other drugs (e.g. protease inhibitors) were added.
1. How do the authors show that HIV pol mutations confer resistance to AZT or 3TC? 2. What is the evidence that 3TC-resistance mutations suppress AZT-resistance? What is therationale for this? 3. Does the 3TC-resistance mutation combined with the AZT-resistance mutations result invirus that is as susceptible to AZT as wild type HIV? 4. Wainberg et al. [Science 271:1282-1285 (1996)] have proposed an alternate explanation forthe benefit of 3TC in combination with AZT. They presented evidence that the M184V 3TC-resistance mutation makes the HIV RT more faithful (i.e. lower the mutation rate) so that thevirus is less able to generate mutations to other drugs (e.g. AZT).
What other explanations can you propose to explain the results?What clinical studies could you devise to test these alternate explanations vs. that put forth by Larder et al. (i.e. make some testable predictions)?

Source: http://www.jiaowu.buct.edu.cn/Courseware/Harvard/BCMP207/pdf/disc_quest2.pdf