Malaria is a disease caused by infection with Plasmodium parasites, of which P. falciparum is the deadliest. Half of the world's population is at risk of infection. Increased access to artemisinin-based combination therapies (ACTs) and mosquito vector control initiatives have saved an estimated 3.3 million lives since the year 2000. Despite this substantial progress, malaria remains a major burden to human health with approximately 627,000 deaths in 2012. There is no effective vaccine. Alarmingly, artemisinin resistance has emerged in Southeast Asia. It is imperative that we understand the mechanism and extent of resistance to help prevent a regional crisis from becoming a global disaster. Genomic and transcriptomic analyses of artemisinin-resistant parasites have recently implicated mutations in the K13 gene, with secondary contributions from genes involved in iron transport and ubiquitin-mediated protein degradation. We used zinc-finger nucleases (ZFNs), a transformative genetic editing tool, to show that removing K13 mutations from artemisinin-resistant Cambodian parasites ablates resistance. Inserting K13 mutations into artemisinin-sensitive parasite lines conferred variable degrees of resistance, indicating the influence of a permissive genetic background. We believe that our combination of ongoing evaluation of parasites from Southeast Asia and the ability to swiftly add or remove candidate mutations from isogenic, well-characterized parasites will allow a comprehensive dissection of the resistance mechanism. We will use ZFNs to study the function of K13 and the influence of genetic background on the emergence and spread of artemisinin resistance. Emergence is tied to the mechanism of resistance, whereas spread is tied to transmission. We will use ZFNs to add or remove potential resistance mutations into isogenic parasite lines alone or in combination. To study emergence, we will use both the ring-stage survival assay (RSA0-3h) and a modified [3H]-hypoxanthine incorporation assay to determine artemisinin sensitivity, and explore K13 function through identifying changes in the parasite redox metabolome, ubiquitin-mediated proteasomal degradation, the definition of interacting partners, and K13 subcellular localization. Given current data, we hypothesize that K13 functions as a negative regulator of oxidative stress responses that respond to iron-mediated redox damage. We posit that resistance-associated mutations in K13 reduce function, leading to increased antioxidant capability. To study transmission, we will perform pairwise fitness comparisons of parasites in both asexual blood stages and sexual mosquito stages. An improved understanding of artemisinin resistance will help guide the targeted geographical use of non-artemisinin-based second-line or novel therapies to reduce the burden of malaria.
A global drive to eliminate and eradicate malaria has reduced malaria-related deaths from one million per year to 627,000 per year in the last decade, but emerging resistance to our current gold-standard Artemisinin-based Combination Therapies (ACTs) threatens to erase and even reverse this progress. This proposal uses innovative genetic tools and malaria parasites isolated from patients who have failed treatment to dissect the mechanism of resistance to ACTs, with a focus on the primary determinant K13. Understanding the mechanism of resistance will help develop novel strategies to treat drug-resistant malaria.
| Ross, Leila S; Dhingra, Satish K; Mok, Sachel et al. (2018) Emerging Southeast Asian PfCRT mutations confer Plasmodium falciparum resistance to the first-line antimalarial piperaquine. Nat Commun 9:3314 |
| Witkowski, Benoit; Duru, Valentine; Khim, Nimol et al. (2017) A surrogate marker of piperaquine-resistant Plasmodium falciparum malaria: a phenotype-genotype association study. Lancet Infect Dis 17:174-183 |
