Mutation rates in the dihydrofolate reductase gene of Plasmodium falciparum.
A new method has been established to define the limits on a spontaneous mutation rate for a gene in Plasmodium falciparum. The method combines mathematical modelling and large-scale in vitro culturing and calculates the difference in mutant frequencies at 2 separate time-points. We measured the mutation rate at 2 positions in the dihydrofolate reductase (DHFR) gene of 3D7, a pyrimethamine-sensitive line of P. falciparum. This line was re-cloned and an effectively large population was treated with a selective pyrimethamine concentration of 40 nM. We detected point mutations at codon-46 (TTA to TCA) and codon-108 (AGC to AAC), resulting in serine replacing leucine and asparagine replacing serine respectively in the corresponding gene product. The substitutions caused a decrease in pyrimethamine sensitivity. By mathematical modelling we determined that the mutation rate at a given position in DHFR was low and occurred at less than 2.5 x 10(-9) mutations/DHFR gene/replication. This result has important implications for Plasmodium genetic diversity and antimalarial drug therapy by demonstrating that even with low mutation rates anti-malarial resistance will inevitably arise when mutant alleles are selected under drug pressure.
Paget-McNicol
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The University of Queensland and the Malaria and Arbovirus Unit, Queensland Institute of Medical Research, Australia. sarahP@qimr.edu.au
Saul
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A
D019943:Amino Acid Substitution; D000818:Animals; D003062:Codon; D007930:Leucine / Q000235:genetics; D008962:Models, Theoretical; D010641:Phenotype; D010963:Plasmodium falciparum / Q000187:drug effects / Q000201:enzymology / Q000235:genetics*; D017354:Point Mutation; D011739:Pyrimethamine / Q000494:pharmacology; D012694:Serine / Q000235:genetics; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics* / Q000378:metabolism
D003062:Codon; D012694:Serine; D013762:Tetrahydrofolate Dehydrogenase; D007930:Leucine; D011739:Pyrimethamine