Malaria parasite gene targets identified

A Melbourne-Philadelphia research team has provided a graphic demonstration of the power of genomics and bioinformatics by identifying a host of prospective drug targets in the malaria parasite, Plasmodium falciparum.

The work builds on a 1995 discovery by Melbourne University's Prof Geoff McFadden that the malaria parasite evolved from a single-celled alga, and retains several metabolic pathways exclusive to plants.

McFadden's team at Melbourne University's Plant Cell Biology Centre, working with malaria researchers at Melbourne's Walter and Eliza Hall Institute of Medical Research and the University of Pennsylvania, has identified 466 nuclear genes in Plasmodium that 'export' protein and enzyme products into an internal structure called an apicoplast.

The parasite's apicoplast is actually a relict chloroplast -- the tiny, chlorophyll-filled structures in plants that capture sunlight for photosynthesis.

When the marine alga ancestor of Plasmodium moved over to the dark side, and became a mammalian blood parasite, it lost the genes for photosynthesis -- and hundreds of other genes involved in chloroplast function migrated to the nucleus. Because the proteins and enzymes they encode remain essential to metabolic processes in the apicoplast, they have to be imported.

Stuart Ralph, a PhD student in McFadden's group and one of the lead authors of this week's paper in Science, said three separate metabolic pathways had been identified in the apicoplast that could be disrupted by novel drugs -- the fatty-acid synthesis pathway, the haem synthesis pathway, and the isoprenoid pathway. Isoprenes are required for the normal activity of many proteins.

Some of the 466 genes identified in the new study code for enzymes with essential roles in these pathways; if these enzymes could be targeted with new drugs, the parasite would die.

Ralph said a German research team had recently demonstrated the potential of this approach, by curing patiens with chronic malaria with an experimental drug that disrupted an enzyme in the apicoplast's isoprenoid pathway.

The Melbourne-Philadelphia team identified the apicoplast-specific genes by training a neural network program to identify two peptide sequences shared by all 466 proteins.

The two peptide sequences function as address tags -- one guides the proteins through the internal membrane surrounding the apicoplast, the other directs them to their appropriate sites with within the apicoplast's metabolic machinery.

The Science paper described how the researchers were able to confirm the essential role of these peptide 'transit tags' in delivering the proteins, by experimentally mutating one apicoplast-targeted protein's tags.

By attaching a green fluorescent protein to the original protein, they were able to confirm that specific mutations in the 'address' peptides prevented the protein being delivered to the apicoplast's interior.

Ralph said the fact that the genes for the apicoplast's imported proteins had now been identified opens the way for chemists to create a novel class of combination therapies that would selectively disrupt key enzymes in the parasite's fatty acid, haem and isoprenoid-synthesis pathways.

These 'triple whammy' drugs would make it extremely difficult for the notoriously flexible parasite to mutate and develop drug resistance -- instead of mutating around a single drug, it would require simultaneous mutations to defeat all three drugs.