Frequency of the Asn-108 and Thr-108 point mutations in the dihydrofolate reductase gene in Plasmodium falciparum from southwest Colombia.

Abstract

Several point mutations in the dihydrofolate reductase (DHFR) gene of Plasmodium falciparum have been correlated with in vitro anti-folate drug resistance of laboratory and field isolates. Furthermore, two different point mutations that generate amino acid substitutions at the same position of the enzyme have been observed in all the isolates studied to date. These point mutations change a serine (Ser-108) in the wild type to an asparagine (Asn-108 mutation) or to a threonine (Thr-108 mutation). Using the polymerase chain reaction (PCR), it is possible to identify isolates that present these mutations. We used a mutation-specific PCR to screen 71 samples from several geographic locations of Colombia for the Asn-108 mutation (pyrimethamine resistance). In this initial screening 53 of 71 yielded amplification product with the DHFR mutation-specific primers. We further analyzed the 18 samples that did not amplify using a mutation-specific nested PCR. Of those 18 samples, seven amplified with primers specific for the Thr-108 mutation (proguanil resistance), one with the wild type (Ser-108), and 10 did not amplify. Of these 10 samples, three were identified as P. falciparum using a species-specific diagnostic nested PCR base on sequences from the small ribosomal RNA subunit gene. Overall, 51.6% of the samples amplified for the Asn-108 mutation, 10.9% for the Thr-108 mutation, 35.9% with the wild type specific primer, and 4.8% did not amplify with any of the DHFR primers. We observed variability in the frequency of the mutation between the different geographic location. The frequency of the Asn-108 and Thr-108 mutations in the state of Narifio was 25% each, while in Valle del Cauca the frequencies were 59% and 11%, respectively. These results contrast with observations in Brazil in which the Asn-108 mutation was found in 90% of the blood samples screened.