Studies of Thai blood group and platelet polymorphism. Implications for malaria susceptibility
Abstract: The objectives of the thesis were to seek a better understanding of polymorphic RBC surface molecules in the Thai, and to investigate the role of certain blood groups in malarial invasion. MALDI-TOF MS was employed in Study I as an efficient method for genotyping to establish the common blood group and platelet antigen genotypes of 396 blood donors. Investigation of outliers identified variation among these samples, both expected and novel. The expected high prevalence of the Mi(a+) phenotype was observed, and identified that 2.3% of samples carried FY c.265T, a polymorphism carried on FY'01 instead of FY'02 as found in the Caucasians. Of potential clinical relevance in a region where transfusion-dependent thalassemia is common, we identified two RHCE'02 alleles known to encode an e-variant antigen. Allelic discrimination assays for extended testing of platelet polymorphism predicted the expected antigen distribution for Southeast Asia. In Study II, homozygosity for a novel nonsense mutation (c.420T>G) in B3GALNT1 (GLOB'01N.13) was identified in two Thai sisters suffering from recurrent spontaneous abortions due to the Pk phenotype, adding to the twelve null alleles already known in the GLOB system. Screening of the blood donor cohort revealed another individual carrying this allele, suggesting regional occurrence of this rare allele. In Study III, DNA sequencing not only confirmed that GYP'Mur is the most frequent allele of the variant glycophorin genes in Thai blood donors, but also identified a GYP'Bun-like allele (designated GYP'Thai) that was shown to be relatively common (MAF = 0.01) in this group. These hybrid alleles have a previously uncharacterized effect on s antigen expression on RBCs, qualitatively and quantitatively, that has implications for reagent selection in transfusion medicine. Study IV turned to platelets, which showed an inhibitory effect on RBC invasion by P. falciparum under physiological platelet:RBC ratios (approx. 1:10 - 1:40) in a dose-dependent manner and that was still effective at levels considered to be thrombocytopenic. At higher platelet concentrations, the trend was reversed so that platelets did not increase the inhibitory effect on RBC invasion further. However, platelet-dependent killing, as witnessed by increased extracellular parasites, remained effective. Despite all we know about blood group polymorphism, much remains to learn. This thesis has explored allelic variation in a region of the world where malaria has long been endemic and the observed blood cell traits highlight our intriguing ability to adapt.
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