Characterization of cytochrome b from Toxoplasma gondii and Q(o) domain mutations as a mechanism of atovaquone-resistance

An rRNA mutation identifies the apicoplast as the target for clindamycin in Toxoplasma gondii

Toxoplasma gondii is a protozoan sensitive to several inhibitors of prokaryotic translation (e.g. clindamycin, macrolides and tetracyclines). A priori, two prokaryotic-like organelles, the 'apicoplast' (a non-photosynthetic plastid) and the mitochondrion, are likely targets for these drugs. Without using overt mutagenesis, we selected two independent clones (ClnR-4 and ClnR-21) with strong and stable clindamycin resistance. Several lines with substantial but lower levels of resistance were also isolated with (XR-46) or without (ClnR-23) overt mutagenesis. The ClnR-4 and ClnR-21 mutants uniquely possess a G-->U point mutation at position 1857 of the apicoplast large-subunit rRNA, whereas no mutation was identified in this region for ClnR-23 or XR-46. Position 1857 corresponds to position 2061 in Escherichia coli where it is predicted to bind clindamycin. The mutation is present in all the apicoplast rDNA copies (an estimated 12 per organelle), indicative of a strong selective advantage in the presence of clindamycin. In the absence of drug, however, such a mutation is unlikely to be neutral, as the G is a critical contributor to the transpeptidation reaction and absolutely conserved in all kingdoms. This may explain why ClnR-4 shows a slight growth defect in vitro. These mutants provide direct genetic evidence that apicoplast translation is the target for clindamycin in Toxoplasma. Further, their sensitivity profiles to other antibiotics specific for the large ribosomal subunit (macrolides and chloramphenicol) and, intriguingly, the small subunit (doxycycline) argue that these drugs also target the apicoplast ribosome.

Are cytochrome b gene mutations the only cause of atovaquone resistance in Pneumocystis?

There is evidence that exposure of the opportunistic pathogen Pneumocystis to atovaquone enhances the development of resistance to the drug. Atovaquone is a structural analog of ubiquinone, which binds to the mitochondrial cytochrome bc(1) complex and inhibits electron transport. Like the parasites Plasmodium and Toxoplasma, atovaquone resistance can result from mutations in the cytochrome b gene of Pneumocystis. However, atovaquone resistance cannot be explained by cytochrome b gene mutations in all cases. The discovery that atovaquone also inhibits biosynthesis of ubiquinone in P. carinii may unfold other mechanisms by which drug resistance develops.

Resistance as a tool in the study of old and new drug targets in Toxoplasma

Drug resistance generated in vitro in the protozoan parasite Toxoplasma gondii is described. We focus on drugs that are in use in patients, that show some promise for such use, or that represent lead compounds for further development. No instance has yet been reported where resistance to any of these drugs has arisen in a patient or in the field although different strains do show varying degrees of sensitivity. For many of these drugs, however, resistant lines have been generated in the laboratory and these have proven very useful for elucidating a given drug's target. These targets range from metabolic pathways in the cytosol to organellar functions encoded in the mitochondrion or plastid. Such information makes predictions about how fast resistance will arise in the field but more importantly, it helps identify targets that are crucial to the parasite and predicts which combinations of drugs should act synergistically.

Atovaquone resistance in malaria parasites

Atovaquone is a broad-spectrum antiparasitic agent active against malaria, Pneumocystis carinii pneumonia, toxoplasmosis and babesiosis. When used as a single agent, resistance to atovaquone arose rapidly in falciparum malaria, requiring the development of a new antimalarial drug combination of atovaquone and proguanil. Recent laboratory investigations have provided insights into the mode of atovaquone action, and identified the molecular basis for the resistance development. Mutations within a catalytic domain of the cytochrome bc(1)complex present within the parasite mitochondrial inner membrane were shown to be responsible for atovaquone resistance. Here, we review these studies and propose a mechanism by which atovaquone resistance may arise quickly in malaria parasites. Copyright 2000 Harcourt Publishers Ltd.