Novel Point Mutations in the Dihydrofolate Reductase Gene of Plasmodium vivax : Evidence for Sequential Selection by Drug Pressure

Abstract

Mutations in the dihydrofolate reductase (dhfr) genes of Plasmodium falciparum and P. vivax are associated with resistance to the antifolate antimalarial drugs. P. vivax dhfr sequences were obtained from 55 P. vivax isolates (isolates Belem and Sal 1, which are established lines originating from Latin America, and isolates from patient samples from Thailand [n ‫؍‬ 44], India [n ‫؍‬ 5], Iran [n ‫؍‬ 2], and Madagascar [n ‫؍‬ 2]) by direct sequencing of both strands of the purified PCR product and were compared to the P. vivax dhfr sequence from a P. vivax parasite isolated in Pakistan (isolate ARI/Pakistan), considered to represent the wild-type sequence. In total, 144 P. vivax dhfr mutations were found at only 12 positions, of which 4 have not been described previously. An F3L mutation at residue 57 had been observed previously, but a novel codon (TTA) resulted in a mutation in seven of the nine mutated variant sequences. A new mutation at residue 117 resulted in S3T (S3N has been described previously). These two variants are the same as those observed in the P. falciparum dhfr gene at residue 108, where they are associated with different levels of antifolate resistance. Two novel mutations, I3L at residue 13 and T3M at residue 61, appear to be unique to P. vivax. The clinical, epidemiological, and sequence data suggest a sequential pathway for the acquisition of the P. vivax dhfr mutations. Mutations at residues 117 and 58 arise first when drug pressure is applied. Highly mutated genes carry the S3T rather than the S3N mutation at residue 117. Mutations at residues 57 and 61 then occur, followed by a fifth mutation at residue 13.With the global spread of chloroquine resistance in Plasmodium falciparum, the combination of sulfadoxine and pyrimethamine (S-P) is now used as first-line antimalarial treatment in an increasing number of countries. This is usually followed rapidly by the emergence of resistance. For example, in Thailand, high-level resistance to S-P arose within 5 years of its deployment in the late 1970s (14, 21). Similar patterns have been observed in other Asian countries, South America, and southern Africa. The combination of antifolate and sulfonamide drugs targets the folate metabolism of the malaria parasite. Molecular clinical and epidemiological studies have clearly shown that resistance to pyrimethamine and sulfadoxine results from specific point mutations in the parasite genes dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps), respectively. These cause alterations in crucial residues in the active sites of these enzymes, resulting in reduced drug affinity (5,11,13,(17)(18)(19)(20). Detection of these mutations in isolates in blood samples collected in the field has proved very valuable in the mapping of resistance and the monitoring of malaria control measures.In areas of malaria endemicity outside Africa, the prevalence of P. vivax equals and often exceeds that of P. falciparum. Chloroquine remains the first-line treatment for P. vivax infection, although in most areas wher...

Different Allele Prevalence in the Dihydrofolate Reductase and Dihydropteroate Synthase Genes in Plasmodium vivax Populations from China

“…17 Consistent with this proposal, mutation S117N was the most prevalent point mutation in pvdhfr in the two P. vivax populations in China. In regions of extensive SP use, two mutant genotypes at residue 117 (S117T and S117N) have been observed; the former has been associated with highly mutated pvdhfr and may be a key mutation for subsequent acquisition of additional mutations and development of high resistance to SP.…”

“…19,22 In Thailand, for example, the pvdhfr T61M mutation, which has reached a high level of prevalence (> 60%), is mostly linked with the S117T mutation. 17,27 Consistent with the scarcity of the S117T mutation in China, highly mutated pvdhfr (triple or quadruple mutations) was also rare. However, the 61M mutation was relatively frequent (20%), especially in the temperate zone P. vivax population.…”

“…17,28,36 Size polymorphism in this region is resulted from variation in number of copies of the 18-basepair repeat, which is not essential for substrate binding. 21 Most (77.0%) of the isolates sequenced contain three copies of the repeat, and 23.0% contained two 18-basepair repeats.…”

“…17,27 This finding has been thought to be caused by the intrinsic difference in mutation rates associated with these two genes, which reflects the relative fitness of the resulting enzymes in the absence of the drug pressure.…”

“…16 To date, some 20 mutations have been detected in pvdhfr ; among them mutations at codons 57, 58, 61, 117, and 173, corresponding to positions 50, 51, 59, 108 and 164 in pfdhfr , respectively, are associated with pyrimethamine resistance. [17][18][19][20] A structural study has confirmed that the most common mutations (S58R and S117N) cause structural changes in P . vivax DHFR and lead to decreased binding to pyrimethamine.…”