Discordant patterns of genetic variation at two chloroquine resistance loci in worldwide populations of the malaria parasite Plasmodium falciparum.

Mutations in the chloroquine resistance (CQR) transporter gene of Plasmodium falciparum (Pfcrt; chromosome 7) play a key role in CQR, while mutations in the multidrug resistance gene (Pfmdr1; chromosome 5) play a significant role in the parasite's resistance to a variety of antimalarials and also modulate CQR. To compare patterns of genetic variation at Pfcrt and Pfmdr1 loci, we investigated 460 blood samples from P. falciparum-infected patients from four Asian, three African, and three South American countries, analyzing microsatellite (MS) loci flanking Pfcrt (five loci [approximately 40 kb]) and Pfmdr1 (either two loci [approximately 5 kb] or four loci [approximately 10 kb]). CQR Pfmdr1 allele-associated MS haplotypes showed considerably higher genetic diversity and higher levels of subdivision than CQR Pfcrt allele-associated MS haplotypes in both Asian and African parasite populations. However, both Pfcrt and Pfmdr1 MS haplotypes showed similar levels of low diversity in South American parasite populations. Median-joining network analyses showed that the Pfcrt MS haplotypes correlated well with geography and CQR Pfcrt alleles, whereas there was no distinct Pfmdr1 MS haplotype that correlated with geography and/or CQR Pfmdr1 alleles. Furthermore, multiple independent origins of CQR Pfmdr1 alleles in Asia and Africa were inferred. These results suggest that variation at Pfcrt and Pfmdr1 loci in both Asian and African parasite populations is generated and/or maintained via substantially different mechanisms. Since Pfmdr1 mutations may be associated with resistance to artemisinin combination therapies that are replacing CQ, particularly in Africa, it is important to determine if, and how, the genetic characteristics of this locus change over time.

Insight into the early spread of chloroquine-resistant Plasmodium falciparum infections in Papua New Guinea.

The first report of Plasmodium falciparum chloroquine (CQ) resistance (CQR) in Papua New Guinea (PNG) appeared in 1974. Although the current prevalence of CQR-associated parasite gene polymorphisms has been documented for some regions, the spatial and temporal relationships that characterize CQ-resistant parasites in PNG are unknown. Insight into the evolution of CQ-resistant parasites could be provided by evaluating genetic markers in parasite populations. We compared pfcrt and pfmdr1 polymorphisms and flanking microsatellite (MS) polymorphisms between P. falciparum-infected placental tissue (early 1980s) and blood (late 1990s) samples collected throughout PNG. Consistent with the results of recent studies, pfcrt-SVMNT and pfmdr1-86Y were the only CQR-associated alleles observed in the placental tissue samples, and they were observed together in 79% of the samples. Results of analysis of MS flanking pfcrt (approximately 40 kb) suggested that there was less diversity in the samples collected during the 1980s than in those collected during the 1990s and that the 1990s parasites were significantly differentiated from the 1980s parasites. On the other hand, for MS flanking pfmdr1 (approximately 5 kb) and for 1 putatively neutral locus, diversity levels were similar, and the 2 parasite populations were not significantly differentiated. These results suggest that selection for CQR was operating on the pfcrt-SVMNT allele during the early 1980s. Thus, archival samples can provide novel insight into the dynamics of CQR.