Fitness costs of antimalarial drug resistance
Drug targets are molecules that are essential for a pathogen’s physiology. Drugs against the malaria parasite Plasmodium falciparum target metabolic enzymes or transmembrane transporters. Point mutations in those parasite genes were found to be associated with drug resistance. In a haploid organism, such as P. falciparum, mutations affecting metabolic and transport processes are expected to be exposed to strong natural selection. Mutations deviating from the evolutionary optimal biochemical structure are likely to be disadvantageous in the absence of drug, and natural selection acts against the changes induced by mutation. Thus, fitness of a mutant parasite is reduced relative to the non-mutated parasite. This reduction is termed ‘fitness costs of drug resistance’. Defining the costs of antimalarial drug resistance will provide a variable that is urgently needed for modelling the evolution and spread of drug resistance. Such models are currently developed and increasingly applied to direct public health measures and policy making.
The costs incurred by mutations are difficult to measure in vivo because fitness, measured as increase or decrease in offspring numbers, eludes direct experimental quantification in a natural parasite population due to technical and ethical obstacles. To estimate transmission success to new hosts, a few surrogate markers have been used, such as asexual parasite densities or gametocyte counts. Our approach is to estimate the probability of transmission via a model that describes the relationship between asexual parasite densities of P. falciparum and the infectivity to mosquitoes. For each genotype the relative transmission probability will be predicted. The selection coefficient as a measure of the extent to which natural selection reduces the relative contribution of a genotype to the next generation is derived directly from transmission probabilities.
This first attempt ever to measure fitness costs of drug resistance mutations directly in the human host in a malaria endemic area comprises detection by microarray technology of 36 single point mutations in 5 genes associated with malaria drug resistance. The duration of infection of individual parasite clones is determined by genotyping consecutive blood samples from 300 children from Papua New Guinea followed over 16 months. Fitness of mutant versus wild type parasite clones is determined by estimating the transmission probability using asexual density and survival of individual clones in the host.
