Drug resistance genomics of the antimalarial drug artemisinin

Abstract

Across the globe, over 200 million annual malaria infections result in up to 660,000 deaths, 77% of which occur in children under the age of five years. Although prevention is important, malaria deaths are typically prevented by using antimalarial drugs that eliminate symptoms and clear parasites from the blood. Artemisinins are one of the few remaining compound classes that can be used to cure multidrug-resistant Plasmodium falciparum infections. Unfortunately, clinical trials from Southeast Asia are showing that artemisinin-based treatments are beginning to lose their effectiveness, adding renewed urgency to the search for the genetic determinants of parasite resistance to this important drug class. We review the genetic and genomic approaches that have led to an improved understanding of artemisinin resistance, including the identification of resistance-conferring mutations in the P. falciparum kelch13 gene.

Figure 1

The lifecycle of Plasmodium that begins with the bite of a female Anopheles mosquito which releases infective sporozoites into the blood of the host. The sporozoites travel to the liver and invade liver cells. Within the liver the sporozoites mature into schizonts, which subsequently grow and produce haploid forms called merozoites. In P. vivax, these liver forms can remain dormant for years as hypnozoites and cause relapse of infection. Merozoites re-enter the bloodstream and invade red blood cells and undergo a cycle of asexual multiplication (A); however, some of the merozoites develop into sexual forms called gametocytes, which circulate in the bloodstream, and can be ingested by a mosquito, thus continuing the malaria lifecycle (B). Drug efficacy can be monitored by collecting blood samples in a treated patient and counting the number of infected erythrocytes using microscopy, or the parasites can be taken into long-term or short-term tissue culture, and these cultured parasites can be mixed with a drug at different concentrations and their in vitro survival or growth monitored [51]. The grey arrows depict the progression of the Plasmodium lifecycle and the black arrows indicate lifecycle forms.

Figure 2

Chemical structures of commonly used antimalarials, including artemisinin, artesunate, artemether and OZ439. The chemistry of artemisinins is described in detail in Box 2.

Figure 3

Kelch structure, the position of mutations in the pfKelch13 gene, and a theoretical functional model. (a) Homology model of PfKelch13p (amino acids 444 through 723) generated using SWISS-MODEL and the human Kelch-like protein 12 crystral structure (2vpj.1.A) as a template. Two rotation views are shown. (b) Predicted domains and locations of mutations in pfkelch13 (PF3D7_1343700) identified either in vitro [35,36] or in vivo [36,56]. Mutations in pink are in the Kelch domain modeled above while green ones are in the predicted regions with more ambiguous function. Mutations mentioned in the text are specifically indicated. In addition to Kelch domains, PfKelch13p contains a BTB domain, typically involved in dimerization. (c) Theoretical model of PfKelch function in artemisinin resistance. See Box 3 for details. TF, transcription factor.