Are Transporter Genes Other than the Chloroquine Resistance Locus ( pfcrt ) and Multidrug Resistance Gene ( pfmdr ) Associated with Antimalarial Drug Resistance?

Abstract

We replicated and extended this work by examining polymorphisms in these genes and in vitro resistance to eight drugs in parasites collected from the Thailand-Burma border. To minimize problems of multiple testing, we used a two-phase study design, while to minimize problems caused by population structure, we analyzed parasite isolates collected from a single clinic. We first examined associations between genotype and drug response in 108 unique single-clone parasite isolates. We found strong associations between single nucleotide polymorphisms in pfmdr and mefloquine (MFQ), artesunate (AS), and lumefantrine (LUM) response. We also observed associations between an ABC transporter (G7) and response to QN and AS and between another ABC transporter (G49) and response to dihydro-artemisinin (DHA). We reexamined significant associations in an independent sample of 199 unique single-clone infections from the same location. The significant associations with pfmdr-1042 detected in the first survey remained. However, with the exception of the G7-artesunate association, all other associations observed with the nine new candidate transporters disappeared. We also examined linkage disequilibrium (LD) between markers and phenotypic correlations between drug responses. We found minimal LD between genes. Furthermore, we found no correlation between chloroquine and quinine responses, although we did find expected strong correlations between MFQ, QN, AS, DHA, and LUM. To conclude, we found no evidence for an association between 8/9 candidate genes and response to eight different antimalarial drugs. However, the consistent association observed between a 3-bp indel in G7 and AS response merits further investigation.Transporter loci are attractive candidates for antimalarial drug resistance genes. Two of four known drug resistance loci in Plasmodium falciparum are transporters, and transporters play major roles in drug resistance in both bacteria and cancer cells (1, 6, 13). A previous study (14) located 49 genes encoding transporters in the P. falciparum genome and sequenced these genes in a panel of 97 isolates from worldwide locations. Polymorphisms in 11/49 genes showed significant association with in vitro drug resistance to chloroquine (CQ) and/or quinine (QN). Two of these genes, pfcrt and pfmdr, are already known to play a role in resistance (9,12,26,28), while the remaining nine candidates are new. Of these nine candidates, six were associated with either a CQ response or a QN response in a southeast Asian population sample. There was also strong linkage disequilibrium (LD) between loci on different chromosomes, consistent with coselection or epistasis (8), and strong correlations between 50% inhibitory concentrations (IC 50 ) with CQ and QN, suggesting that common genes may underlie resistance to both drugs.While these results are exciting, some caution is needed in interpreting such candidate gene studies. There are two particular problems. First, when multiple tests are carried out, some significant results may occur by...

Disruption of a Plasmodium falciparum Multidrug Resistance-associated Protein (PfMRP) Alters Its Fitness and Transport of Antimalarial Drugs and Glutathione

“…Although the parasite sample size was small in the study, a pattern can be seen in QN responses for parasites from South America when parasite responses to the drug were repeatedly tested in vitro. Two subsequent studies of in vitro drug assays on parasites collected directly from patients, however, did not support the associations (31,32). To resolve the differences, more association studies using parasites from the same location and accurate drug assays on culture-adapted parasites are necessary.…”

“…PfMRP is encoded by a single exon of 5469 bp with 11 predicted transmembrane ␣-helix segments and is expressed from early trophozoite to late schizont, according to microarray analyses (28 -30). Two mutations in the PfMRP were associated with higher levels of IC 50 to CQ and QN in P. falciparum field isolates, with good correlation of the mutations in the gene and higher IC 50 (24); however, whether PfMRP contributes to parasite drug responses remains controversial (31,32) and requires additional functional studies.…”

Malaria Chemotherapeutics Part I: History of Antimalarial Drug Development, Currently Used Therapeutics, and Drugs in Clinical Development

“…[129] Resistance against quinine (1), mefloquine (29), halofantrine (30), and lumefantrine (31), as well as decreased sensitivity against the structurally unrelated dihydroartemisinin (39) is associated with an increased copy number of the wild-type pfmdr1 gene. [49,[130][131][132][133][134] This leads to an increase of the median IC 50 value for mefloquine from 50 to 170 nm, for quinine from 900 to 1717 nm, for halofantrine from 6.4 to 26.4 nm, and for artesunate (42) from 3.5 to 6.2 nm. [112] The numbers of pfmdr1 gene copies correlates positively with the risk of failure of mefloquine and mefloquine-artesunate therapy.…”

“…[48] In addition to PfCRT, the involvement of other transport proteins in chloroquine resistance is discussed. [49,50] A mutation (N86Y) in the pfmdr1 (multidrug resistance) gene that codes for another membrane transporter (PfMDR1) is significantly related to the pfcrt K76T mutation. Recent results suggest that www.chemmedchem.org this mutation plays a compensatory role in chloroquine-resistant isolates under CQ pressure and may also have some finetuning effects on the degree of chloroquine resistance.…”

“…This effect has been observed in laboratory strains with IC 50 values between 0.56 and 4.53 nm. [298] In different studies with field isolates, mean IC 50 values of 1.1, [172] 3.56, [299] and 6.2 nm [49] were obtained. The mammalian cytochrome complex is up to 1000-fold less sensitive against atovaquone (88).…”