Sequence Variations of the Plasmodium vivax Dihydrofolate Reductase Gene from Thai Isolates

idespread drug resistance is one of the most serious problems in malaria control in Thailand. Mutation in the gene encoding DHFR in Plasmodium falciparum has been associated with antifolate resistance. Mutation at homologous position has also been observed in P. vivax DHFR gene. To determine sequence variations in the human malaria parasite, P. vivax, the gene encoding dihydrofolate reductase was isolated by polymerase chain reaction from the genomic DNA of 20 clinical isolates from various geographic areas in Thailand. Analysis of the nucleotide sequence between different parasite isolates showed several point mutations in codons 57: Phe>>Leu, Ile 58: Ser>>Arg 61: Thr>>Met and 117: Ser>>Thr, Asn, including short tandem repeats which generated length polymorphism in the PvDHFR gene. Sequences were compared with those previously reported. The results revealed double mutations in seven isolates (35%) and multiple mutations in 13 isolates (65%). Three GGDN repeat units were present in 12 isolates and two GGDN repeat units were present in eight isolates. This study was undertaken to provide information on the distribution of the genetic diversity and the genetic basis of drug resistance of P. vivax in Thai isolates in the DHFR gene. However, further study is needed to determine the relation between point mutations and in vivo and in vitro pyrimethamine resistance in P. vivax.