Babesia rodhainiandPlasmodium berghei

Complementation of Plasmodium berghei TRAP knockout parasites using human dihydrofolate reductase gene as a selectable marker

Previously we have used the Plasmodium dihydrofolate reductase thymidylate synthase (DHFR-TS) selectable marker to generate Plasmodium berghei TRAP null mutant parasites. These TRAP null mutants do not glide and they showed a great reduction in their ability to infect mosquito salivary glands and the hepatocytes of the vertebrate host. Thus far, complementation of these knockout parasites was not possible due to the lack of additional selectable markers. Recently, a new selectable marker, based on the human dihydrofolate reductase (hDHFR) gene, has been developed which confers resistance to the antifolate drug WR99210. This drug has been found to be highly active against pyrimethamine-sensitive and -resistant strains of P. berghei. In this study, we have used the hDHFR gene as a second selectable marker for the complementation of P. berghei TRAP null mutant parasites. Restoration of the TRAP null mutant parasites to the wild-type phenotype was achieved in this study via autonomously replicating episomes bearing a wild-type copy of the TRAP gene. This is the first report of complementation of a mutant phenotype in malaria parasites.

The selectable marker human dihydrofolate reductase enables sequential genetic manipulation of the Plasmodium berghei genome

Genetic transformation of malaria parasites has been limited by the number of selectable markers available. For the rodent malaria parasite, Plasmodium berghei, only a single selection marker has been at hand, utilising the dihydrofolate reductase-thymidylate synthase gene from either P. berghei or Toxoplasma gondii to confer resistance to the anti-malarial drug pyrimethamine. Here we report the use of the human dihydrofolate reductase (hDHFR) gene as a new selectable marker, which confers resistance to the antifolate inhibitor WR99210 upon both pyrimethamine sensitive and resistant isolates of P. berghei. Transfection with circular constructs containing the hDHFR gene resulted in the generation of highly resistant parasites containing multiple copies of episomally-maintained plasmids. These parasites showed around a 1000-fold increase in resistance to WR99210 compared to the parental parasites. We were also able to generate and select transgenic parasites harbouring only a single copy of hDHFR targeted into their genome, despite the fact that these parasites showed only a fivefold increase in resistance to WR99210 compared to the parental parasites. Importantly, and for the first time with malaria parasites, the hDHFR gene could be used in conjunction with the existing pyrimethamine selectable markers. This was demonstrated by reintroducing the circumsporozoite (CS) gene into transgenic CS-knockout mutant parasites that contained the P. berghei DHFR-TS selectable marker. The development of hDHFR as a second selectable marker will greatly expand the use of transformation technology in Plasmodium, enabling more extensive genetic manipulation and thus facilitating more comprehensive studies on the biology of the malaria parasite.

Transformation with human dihydrofolate reductase renders malaria parasites insensitive to WR99210 but does not affect the intrinsic activity of proguanil

Increasing resistance of Plasmodium falciparum malaria parasites to chloroquine and the dihydrofolate reductase (DHFR) inhibitors pyrimethamine and cycloguanil have sparked renewed interest in the antimalarial drugs WR99210 and proguanil, the cycloguanil precursor. To investigate suggestions that WR99210 and proguanil act against a target other than the reductase moiety of the P. falciparum bifunctional DHFR-thymidylate synthase enzyme, we have transformed P. falciparum with a variant form of human DHFR selectable by methotrexate. Human DHFR was found to fully negate the antiparasitic effect of WR99210, thus demonstrating that the only significant action of WR99210 is against parasite DHFR. Although the human enzyme also resulted in greater resistance to cycloguanil, no decrease was found in the level of susceptibility of transformed parasites to proguanil, thus providing evidence of intrinsic activity of this parent compound against a target other than DHFR. The transformation system described here has the advantage that P. falciparum drug-resistant lines are uniformly sensitive to methotrexate and will complement transformation with existing pyrimethamine-resistance markers in functional studies of P. falciparum genes. This system also provides an approach for screening and identifying novel DHFR inhibitors that will be important in combined chemotherapeutic formulations against malaria.

Analogues of N-benzyloxydihydrotriazines: in vitro antimalarial activity against Plasmodium falciparum

The antimalarial activities of a series of chlorophenyloxyalkoxy and chlorophenalkoxy N-substituted diamino-dihydrotriazines were determined in vitro against three strains of Plasmodium falciparum (Malayan Camp, Vietnam Smith, FCB) with diverse levels of resistance to chloroquine, pyrimethamine, and cycloguanil. Parasite viability was assayed by the inhibition of the uptake of radiolabelled hypoxanthine. Most of the ID-50S of these compounds were less than 1.0 ng ml-1. Consistent differences in sensitivities to these compounds were observed and appeared to be strain related. The Malayan Camp was the most sensitive and Vietnam Smith was the least sensitive. These differences appeared to be related primarily to an inherent sensitivity of a particular strain to the series of analogues examined rather than to a pattern of cross-resistance to chloroquine, pyrimethamine, or cycloguanil.