Removal of heterologous sequences from Plasmodium falciparum mutants using FLPe-recombinase.

Genetically-modified mutants are now indispensable Plasmodium gene-function reagents, which are also being pursued as genetically attenuated parasite vaccines. Currently, the generation of transgenic malaria-parasites requires the use of drug-resistance markers. Here we present the development of an FRT/FLP-recombinase system that enables the generation of transgenic parasites free of resistance genes. We demonstrate in the human malaria parasite, P. falciparum, the complete and efficient removal of the introduced resistance gene. We targeted two neighbouring genes, p52 and p36, using a construct that has a selectable marker cassette flanked by FRT-sequences. This permitted the subsequent removal of the selectable marker cassette by transient transfection of a plasmid that expressed a 37°C thermostable and enhanced FLP-recombinase. This method of removing heterologous DNA sequences from the genome opens up new possibilities in Plasmodium research to sequentially target multiple genes and for using genetically-modified parasites as live, attenuated malaria vaccines.