Exploring the Genetic Basis of Fitness in Drug Resistant Malaria Parasites

Plasmodium falciparum malaria remains one of the world’s most serious infectious diseases, but the use of artemisinin (ART) in combination with other antimalarials has been important in reducing deaths and disease. However, the recent rise in resistance in the Greater Mekong subregion of Southeast Asia threatens the efficacy of ART and raises the concern that malaria cases will rise and resistance will spread. The spread of resistant strains to sub-Saharan Africa would be devastating in an area with the highest malaria mortality rates in the world. To combat the spread of ART resistance, there is a critical need to understand what drives its emergence and spread.

The rate of emergence and spread of resistance is closely related to the fitness of resistant parasites. The fitness cost of resistance mutations and the compensatory mutations that accompany resistance mutations play a role in determining whether resistance will spread in a population. Knowledge of the molecular basis of the cost of resistance is important for understanding how resistance mutations arise and stabilize in populations.

The costs of resistance can be studied in vitro by using fitness assays, including individual growth assays, competitive growth assays, and bulk segregant analysis. Individual growth assays, such as parasite growth rate, determine which parasites have high fitness. Competitive growth assays measure the relative fitness between co-infecting parasites and reveal fitness disparities between the parasites. Bulk segregant analysis measures the change in allele frequencies due to parasite growth over time to determine loci related to fitness. These assays highlight the phenotypes necessary to have a “fit” parasite and as quantitative measures of fitness, they can be used for quantitative trait locus (QTL) mapping to identify genetic loci that correlate with that fitness phenotype. By phenotyping parasites isolated from patients in Southeast Asia and Africa and using these parasites as parents in genetic crosses, we can study a variety of drug resistance and fitness phenotypes. Improved understanding of the in vitro fitness costs of different parasites and the role different resistance and compensatory mutations play will contribute to an understanding of the potential for specific mutations to spread in populations.