Role of PfATP6 and pfMRP1 in Plasmodium falciparum resistance to antimalarial drugs
Sammanfattning: Half of the world s population live at risk for malaria and nearly one million people die from the disease every year. The malaria burden is greatest in children and pregnant women in sub-Saharan Africa. As effective treatment is crucial for malaria control, the spread of antimalarial drug resistance has contributed significantly to malaria attributed morbidity and mortality. The current cornerstones in malaria treatment are artemisininbased combination therapy (ACT) for treatment of uncomplicated Plasmodium falciparum malaria and sulfadoxine-pyrimethamine (SP) for intermittent preventive treatment of pregnant women. While resistance to SP is already established, recent advances have provided the first evidence of decreased susceptibility or resistance to the ACT components; artemisinins (ART) and the key ACT partner drugs lumefantrine, amodiaquine and mefloquine. Development of resistance to ACTs and aggravation of SP resistance could be devastating, as there are presently no further well established treatment options. This stresses the importance to understand the molecular mechanism of resistance in order to potentially prevent its emergence and spread. The sarco/endoplasmic reticulum Ca2+-ATPase orthologue of P. falciparum (PfATP6) has been suggested to be a target for ART. Multidrug resistance proteins (MRPs) are known to be related with multidrug resistance in many organisms. The P. falciparum Multidrug Resistance Protein 1 (pfMRP1) has been suggested to have a role in the parasite response to several antimalarial drugs through drug efflux The aim of this thesis is to understand the role of PfATP6 and pfMRP1 in decreased susceptibility/resistance to ACTs and SP in vivo and in vitro. In two comprehensive studies the global biodiversity of PfATP6 and pfmrp1 was determined, resulting in the identification of a large number of SNPs suggesting that both genes harbor significant diversity. The contribution of the identified pfmrp1 polymorphisms in the parasite drug response in vivo was studied in P. falciparum infected patients from clinical drug trials. We observed a selection of parasites harboring pfmrp1 I876 in patients with recurrent infections after treatment with artemether-lumefantrine and of pfmrp1 K1466 in recrudescent infections after SP treatment, providing the first indications ever that pfMRP1 may have a role in P. falciparum drug response in vivo. In vitro associations between pfmrp1 SNPs and decreased susceptibility to a large number of structurally unrelated antimalarial drugs, including artemisinin, lumefantrine, amodiaquine and mefloquine, were observed in P. falciparum fresh isolates. However, we could not detect any association with PfATP6 SNPs. In conclusion, in this work no further evidence was found supporting the hypothesis of PfATP6 as a target of ART. Nevertheless it was demonstrated that PfATP6 harbors considerable sequence biodiversity which can be the basis for following studies investigating the association of PfATP6 polymorphisms with artemisinin resistant or tolerant phenotypes, as they emerge. The association of pfmrp1 polymorphism with P. falciparum response to a number of structurally unrelated drugs suggests that pfMRP1 may be a true multidrug resistance factor. Potential pfMRP1-based cross-resistance between ART and the partner drugs may have implications for development of resistance to ACTs.
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