| Despite the fact that malaria has been described for centuries, it remains one of the most important global health crises of the 21st century. Each year, nearly half of the world's population is at risk for contracting malaria and 300-500 million clinical cases are identified, resulting in 1 to 3 million deaths, most of which are in African children younger than 5 years old. Resistance to most antimalarials is widespread throughout the world, yet the mechanisms that initiate drug resistance are not well understood. The fact that resistance consistently and predictably emerges from certain parts of the world raises some interesting questions about the acquisition of drug resistance. Parasites from South East Asia consistently acquire resistance to new and unrelated antimalarials three to four orders of magnitude more frequently than parasites from other parts of the world. This ability, called the Accelerated Resistance to Multiple Drugs (ARMD) phenotype, suggests that parasites from South East Asia may harbor genetic traits that greatly facilitate their acquisition of drug resistance. Here, we describe two complimentary methods which each provide clues as to the mechanism of the accelerated acquisition of resistance in ARMD parasites. In the first method, by assessing the ease with which progeny from an HB3xDd2 genetic cross acquire resistance to the antifolate, 1843U89, we show that the ARMD phenotype is a complex, multigenic trait, with some progeny exhibiting intermediate phenotypes. Our analysis identified at least four regions on chromosomes 4, 5, 7, and 13, acting in an additive or synergistic manner to confer the ARMD phenotype. In the second method, we used a small chemical probe, DSM1, to explore the early events associated with the acquisition of resistance in ARMD parasites. We selected for 3 to 5-fold, stable resistance to the novel DHODH inhibitor DSM1 in ARMD Dd2 parasites. While direct sequencing revealed no mutations in the DHODH gene, comparative genomic hybridization showed a 34-95kb amplification event at the DHODH locus in four independent DSM1-resistant clones. Based on the results from our two studies, we propose a general model for the accelerated acquisition of drug resistance in ARMD parasites. |
