Project Summary
Malaria caused by infection with Plasmodium vivax is an enormous public health burden throughout the world,
and the cause of significant morbidity. The antimalarial drug chloroquine is the first line of drug treatment for P.
vivax in most countries, and has proven highly efficacious. However, chloroquine resistant (CQR) P. vivax
infections have been widely reported, seriously hampering case management, malaria control efforts, and
elimination programs. Most work on resistance mechanisms has focused on Plasmodium falciparum, while P.
vivax remains poorly studied; nevertheless, P. vivax poses a major impediment to eradication. The molecular
determinants of CQR and resistance to other antimalarials in P. vivax remain unclear, largely due to the lack of
in vitro culture, precluding reverse genetics and robust drug assays. Several candidate drug transporter genes
have been identified in P. vivax with polymorphism in sequence and expression level that could be associated
with CQR and/or resistance to other antimalarials. These genes include pvmdr1, pvcrt-o and pvmrp1, orthologs
of the Plasmodium falciparum pfmdr1, pfcrt and pfmrp1 genes, respectively, that are key determinants of
antimalarial susceptibility in P. falciparum. Ex vivo P. vivax susceptibility to chloroquine is associated with specific
P. vivax drug transporter polymorphisms in some but not all studies. It is not clear whether these discrepancies
represent regional differences in parasite diversity and history of exposure to drugs; or result from technical
differences, as no genetic validation has hitherto been possible. Here, we propose a comprehensive analysis of
drug-resistance polymorphisms of P. vivax, utilizing two novel approaches leveraging the zoonotic macaque
parasite Plasmodium knowlesi, which is much closer to P. vivax phylogenetically than is P. falciparum; possesses
superior in vitro genetics with higher transfection efficiencies; and importantly, permits robust determinations of
antimalarial drug susceptibility. We aim to utilize both cutting-edge evolutionary genomic and experimental
forward genetic screening to identify putative determinants of P. vivax drug-resistance. We will subsequently use
in vitro reverse genetic methods in P. knowlesi to functionally assess the importance of prioritized P. vivax genetic
polymorphisms in mediating antimalarial drug-resistance. We hypothesize that these polymorphisms, either
singly or in combination, have been selected in specific regions of the world by antimalarial use against P. vivax.
The identification of specific polymorphisms that mediate different levels of susceptibility to chloroquine, and
other antimalarial compounds in current clinical use, including artemisinin and its partner drugs, will be key to
their use in surveillance, molecular epidemiological studies, and the design of strategies to prolong the
usefulness of these drugs.