Host candidate gene polymorphisms and clearance of drug-resistant Plasmodium falciparum parasites.

BACKGROUND: Resistance to anti-malarial drugs is a widespread problem for control programmes for this devastating disease. Molecular tests are available for many anti-malarial drugs and are useful tools for the surveillance of drug resistance. However, the correlation of treatment outcome and molecular tests with particular parasite markers is not perfect, due in part to individuals who are able to clear genotypically drug-resistant parasites. This study aimed to identify molecular markers in the human genome that correlate with the clearance of malaria parasites after drug treatment, despite the drug resistance profile of the protozoan as predicted by molecular approaches. METHODS: 3721 samples from five African countries, which were known to contain genotypically drug resistant parasites, were analysed. These parasites were collected from patients who subsequently failed to clear their infection following drug treatment, as expected, but also from patients who successfully cleared their infections with drug-resistant parasites. 67 human polymorphisms (SNPs) on 17 chromosomes were analysed using Sequenom's mass spectrometry iPLEX gold platform, to identify regions of the human genome, which contribute to enhanced clearance of drug resistant parasites. RESULTS: An analysis of all data from the five countries revealed significant associations between the phenotype of ability to clear drug-resistant Plasmodium falciparum infection and human immune response loci common to all populations. Overall, three SNPs showed a significant association with clearance of drug-resistant parasites with odds ratios of 0.76 for SNP rs2706384 (95% CI 0.71-0.92, P = 0.005), 0.66 for SNP rs1805015 (95% CI 0.45-0.97, P = 0.03), and 0.67 for SNP rs1128127 (95% CI 0.45-0.99, P = 0.05), after adjustment for possible confounding factors. The first two SNPs (rs2706384 and rs1805015) are within loci involved in pro-inflammatory (interferon-gamma) and anti-inflammatory (IL-4) cytokine responses. The third locus encodes a protein involved in the degradation of misfolded proteins within the endoplasmic reticulum, and its role, if any, in the clearance phenotype is unclear. CONCLUSIONS: The study showed significant association of three loci in the human genome with the ability of parasite to clear drug-resistant P. falciparum in samples taken from five countries distributed across sub-Saharan Africa. Both SNP rs2706384 and SNP1805015 have previously been reported to be associated with risk of malaria infection in African populations. The loci are involved in the Th1/Th2 balance, and the association of SNPs within these genes suggests a key role for antibody in the clearance of drug-resistant parasites. It is possible that patients able to clear drug-resistant infections have an enhanced ability to control parasite growth.

High frequency of Plasmodium falciparum PfCRT K76T and PfpghN86Y in patients clearing infection after chloroquine treatment in the Sudan.

The clinical response following treatment with chloroquine, and the prevalence of two Plasmodium falciparum DNA polymorphisms known to associate with drug resistance, namely PfCRT K76T and Pfpgh N86Y were investigated in two sites in central and eastern Sudan. Patient's sensitivity to chloroquine was determined according to the standard in vivo test as recommended by the WHO protocol in days 0, 3, 7 and 14, respectively. Clinical un-responsiveness was 75.9% in Gadaref in eastern Sudan and 32.1% in Haj Yousif of the Khartoum state. Difference between the two sites in treatment outcome is not tantamount to allele frequency and genotype distribution of neither Pfcrt K76T nor PfpghN86Y. All post treatment samples in the two areas were carrying the mutant allele of Pfcrt K76T. The higher frequency of PfpghN86Y in Haj Yousif (0.86) than Gadaref (0.72), where chloroquine resistance is higher suggests a minor role, if any, for PfpghN86Y in resistance to chloroquine. Age effect on the clearance of parasitemia was evident in both areas, more significantly though in Gedaref (P<0.000) than Haj Yousif (P=0.043) These results add to reports in the literature, pointing to the complexity of factors that may contribute to a clinical outcome following chloroquine treatment, particularly, in this case are elements of the host immunity that are yet to be identified.