Many red cell polymorphisms are a result of selective pressure by the malarial parasite. Here we add another red cell disease to the panoply of erythrocytic changes that give rise to resistance to malaria. Erythrocytes from individuals with erythropoietic protoporphyria (EPP) have low levels of the final enzyme in the heme biosynthetic pathway, ferrochelatase. Cells from these patients are resistant to the growth of the human malarial parasite, Plasmodium falciparum.
We first compared the growth and replication rates of P. falciparum cultured in red cells from EPP patients (n=4) and normal erythrocytes. There was a two to three-fold reduction in parasite growth in the patient cells. Next we sought to exclude the possible negative effects on the parasite due to elevated porphyrins and reduced cell hemoglobin. To do this we employed the EPP phenocopy, X-linked dominant protoporphyria (XLDPP), which has normal ferrochelatase activity. Cells from three individuals with XLDPP supported completely normal rates of parasite growth, proving that the EPP resistance phenomenon was due to the absence of ferrochelatase.
We also tested the requirement of host ferrochelatase during malarial infection by using mice with a hypomorphic mutation in the murine ferrochelatase gene and the rodent malarial species, P. chabaudi. Mice homozygous for the mutation, Fechm1Pas, have 5% residual ferrochelatase activity compared to wild-type littermates. Following infection, we observed an almost two fold reduction in peak parasitemia levels and two to three times greater rates of survival in the homozygous mice. Host ferrochelatase is therefore also necessary to sustain a normal malarial infection in mice. To determine the requirement of parasite-expressed ferrochelase, we produced a P. berghei parasite line carrying a complete deletion of the ferrochelatase gene. This strain grew normally in wild-type mouse erythrocytes, indicating that parasite ferrochelatase is dispensable during the erythrocytic stage of infection.
A complete absence of ferrochelatase in humans (and mice) is not compatible with life, and therefore testing parasite growth in complete knockout cells was not possible. Instead, we used a specific competitive inhibitor of the enzyme, N-methylprotoporphyrin (NMPP) to eliminate all ferrochelatase activity from the parasite and red cell. Treatment of P. falciparum cultures with NMPP resulted in potent cytocidal and growth inhibition effects against both antimalarial drug-sensitive and drug-resistant parasite lines. The activity of NMPP could be competitively removed by titration of the ferrochelatase substrate, protoporphyrin IX, proving that the effects of NMPP were due to specific enzyme inhibition and not off-target effects. Therefore ferrochelatase activity is also essential for the Plasmodium parasite.
We conclude that the refractoriness of ferrochelatse-deficient red cells to Plasmodium is due to the parasite's reliance on the host enzyme. Host ferrochelatase is probably utilized by the parasite for the biosynthesis of heme. In support of this hypothesis, others have observed that red cell ferrochelatase is imported by intraerythrocytic Plasmodium and enzymatic is retained. Finally, based on this collective data, we propose human ferrochelatase is a valid and novel "host-directed" target for an antimalarial therapy.