Poster Presentation First Malaria World Congress 2018

A genetic resistance trap in malaria parasites: exploiting the differential metabolism of the parasite between its two hosts (#245)

Hayley D Buchanan 1 , Christopher D Goodman 1 , Geoffrey I McFadden 1
  1. University of Melbourne, Parkville, VIC, Australia

Drug resistance in Plasmodium massively impedes global malaria control and eradication efforts. Recent emergence of resistance to frontline antimalarial treatments means new strategies are needed to outmanoeuvre the parasite's remarkable evolutionary agility. Malaria parasites undergo immense changes in metabolic activity between their mammalian and mosquito life cycle stages. Consequently, drug selection for resistance during the vertebrate stage can have dire consequences for parasite fitness when it transitions to insect stages. We are pursuing drugs whose targets are under markedly different selection pressures between the two hosts to identify resistance mutations that will drive parasite failure in the mosquito, thereby blocking transmission. This works with the mitochondrion-encoded cytochrome b inhibitor atovaquone, where resistance is trapped in the host. We hypothesised that the same trap would hold true for drug targets encoded by the apicoplast, a parasite compartment homologous to the plastids of plants and algae, and also under differential selection between mammalian and insect life cycle stages. The macrolide antibiotic azithromycin is a safe and effective antimalarial. Azithromycin inhibits apicoplast protein synthesis by blocking the peptide exit tunnel of the large ribosomal subunit. We generated two independent azithromycin resistant lines in P. berghei (PbAZMR), each harbouring a different point mutation in Rpl4, an apicoplast-encoded protein adjacent the exit tunnel in ribosomes. Rpl4 mutations have been shown to confer azithromycin resistance in P. falciparum. PbAZMR parasites produced viable gametes and were able to infect mosquitoes. However, PbAZMR parasites produce fewer midgut oocysts, vastly reduced numbers of salivary gland sporozoites compared to WT and are unable to infect naïve mice. PbAZMR parasites are therefore unable to transmit and major defects in oocyst apicoplast development apparently underlie this transmission block. Our results suggest that the resistance trap does indeed work for other organelle-encoded targets and may be exploited to optimise antimalarial combination treatments.