Flashback to the 1960s: utility of archived sera to explore the origin and evolution of Plasmodium falciparum chloroquine resistance in the Pacific.

ABSTRACT

The increasing frequencies of Plasmodium falciparum strains that are resistant to chloroquine (CQ) and other antimalarials are resulting in a global resurgence of malaria morbidity and mortality. CQ resistance (CQR) is associated with multiple mutations in the P. falciparum chloroquine resistance transporter (pfcrt) gene. The mode and tempo of the accumulation of substitutions leading to these complex CQR haplotypes remain speculative due to the dearth of samples temporally spanning the evolution of drug resistance. The origin and evolution of the CQR alleles of Papua New Guinea (PNG) is particularly ambiguous. It remains unclear whether the pfcrt haplotype in PNG resulted from an independent origin of a CQR haplotype identical in sequence to the South American haplotype, or if this haplotype originated in South America and recombined into a Southeast Asian-derived genome. We sequenced a segment of pfcrt exon 2 from 398 plasmid clones derived from archival human sera collected in the Pacific before and after the first reported cases of CQ treatment failure (n=251) and modern samples (n=147). None of the 251 pfcrt plasmid clones from nine archival samples displayed the C72S or the K76T mutations that are characteristic of CQR strains. In contrast, these two amino acid substitutions were present in all 147 pfcrt plasmid clones from five samples collected between 2001 and 2003; thus, the archival samples represent the baseline parasite genetic diversity before the evolution of CQR strains. We are currently expanding our analyses to include additional samples from the series described here and from series collected in the 1970s and the 1980s to evaluate the geographic origin of CQR strains in the Pacific and the validity of the sequential point mutation accumulation model of CQR evolution.