Plasmodium falciparum pfmdrl amplification, mefloquine resistance, and parasite fitness

Abstract

Mefloquine is widely used in combination with artemisinin derivatives for the treatment of falciparum malaria. Mefloquine resistance in Plasmodium falciparum has been related to increased copy numbers of multidrug-resistant gene 1 (pfmdrl). We studied the ex vivo dynamics of pfmdrl gene amplification in culture-adapted P. falciparum in relation to mefloquine resistance and parasite fitness. A Thai P. falciparum isolate (isolate TM036) was assessed by the use of multiple genetic markers as a single genotype. Resistance was selected by exposure to stepwise increasing concentrations of mefloquine up to 30 ng/ml in continuous culture. The pfmdrl gene copy numbers increased as susceptibility to mefloquine declined (P = 0.03). No codon mutations at positions 86,184,1034,1042, and 1246 in the pfmdrl gene were detected. Two subclones of selected parasites (average copy numbers, 2.3 and 3.1, respectively) showed a fitness disadvantage when they were grown together with the original parasites containing a single pfmdrl gene copy in the absence of mefloquine; the multiplication rates were 6.3% and 8.7% lower, respectively (P < 0.01). Modeling of the dynamics of the pfmdrl copy numbers over time in relation to the relative fitness of the parasites suggested that net pfmdrl gene amplification from one to two copies occurs once in every 10s parasites and that amplification from two to three copies occurs once in every 103 parasites, pfmdrl gene amplification in P. falciparum is a frequent event and confers mefloquine resistance. Parasites with multiple copies of the pfmdrl gene have decreased survival fitness in the absence of drug pressure. Copyright © 2009, American Society for Microbiology. All Rights Reserved.