Spread and distribution of drug resistance and compensatory mutations in Plasmodium falciparum

Abstract

In 2010 there were an estimated 216 million cases of malaria worldwide. In Honduras there were ~9000 cases of which 88% were due to Plasmodium vivax mono-infection. Chloroquine (CQ) resistant Plasmodium falciparum have spread throughout the world curtailing its use. The only exception appears to be north of Panama where CQ reportedly remains efficacious and the drug of choice for treating both P. falciparum and P. vivax. Resistance to antimalarials is associated with specific genetic polymorphisms and recently a putative H+ pump (pfvp2) has been suggested to be linked to CQ resistant P. falciparum. The aim of this thesis was to identify resistance associated genetic polymorphisms in P. falciparum and P.vivax from Honduras and to describe the worldwide distribution of pfvp2 polymorphisms and their correlation to CQ resistance.

Resistance associated genetic polymorphisms in P. falciparum and P. vivax multidrug resistance gene (pfmdr1 and pvmdr1), dihydrofolate reductase (pfdhfr and pvdhfr), P. falciparum chloroquine resistance transporter (pfcrt), dihydropteroate synthase (pfdhps) and V-type H+ pyrophosphatase (pfvp2) were identified in field samples using PCR based methods. From Honduras, 37 P. falciparum and 64 P. vivax samples, collected from symptomatic patients were used. In addition, 50 samples from each of Colombia, Liberia, Guinea-Bissau, Tanzania, Iran, Thailand and Vanuatu were used. The samples represented a time period from 1978 to 2009 and areas with different prevalence of CQ resistant P. falciparum.

In samples from Honduras no genetic polymorphisms associated with CQ or sulphadoxinepyrimethamine (SP) resistance were found in P. falciparum. In P. vivax, the CQ resistance associated pvmdr1 976F allele was found in 7/37 samples and the SP resistance associated pvdhfr 57L+58R alleles were found in 2/57 samples. When analysing the worldwide collection of samples, the pfvp2 405V, 582K and 711P haplotype was associated with the for CQ resistance essential allele, pfcrt 76T (P=0.007). Samples with pfvp2 405I and/or 582R and/or 711S were significantly more common in Liberia in 1978-1980 (P=0.01), all African countries (P=0.004) and all African countries + Honduras (P=0.01) compared to the rest of the world.

Our results suggest that P. falciparum and P. vivax in Honduras are sensitive to CQ and SP. However, small numbers of P. vivax had genetic polymorphisms suggesting a degree of tolerance to CQ and SP. The association between pfcrt 76T and the pfvp2 405V, 582K and 711P haplotype suggest that this haplotype is associated with CQ resistance. This is in line with previous research that has described increased expression of pfvp2 during CQ exposure. The higher frequency of pfvp2 405I and/or 582R and/or 711S in CQ sensitive settings in Africa and Honduras suggests a larger variation in the pfvp2 genome prior to the spread of CQ resistance further supporting the association between pfvp2 and CQ resistance.