Host genetic factors in susceptibility to malaria and tuberculosis
Plasmodium falciparum and Mycobacterium tuberculosis infections collectively cause as many as five million deaths world-wide each year. In the most afflicted populations, currently available drugs and vaccines appear inadequate. By offering insight into the pathophysiology of diseases, genetic studies provide options for new therapeutic approaches to major health problems. The results of case-control studies of genetic factors associated with disease outcomes in malaria and tuberculosis in an African setting are presented in this thesis. Glucose-6-Phosphate dehydrogenase (G6PD) deficiency, the commonest enzymopathy of humans, affects over 400 million people. The geographical correlation of its distribution with the historical endemicity of malaria suggests that this disorder has risen in frequency through natural selection by malaria. However, attempts to confirm that G6PD deficiency is protective in case-control studies of malaria have yielded conflicting results. Hence, for this X-linked disorder, it is unclear whether both male hemizygotes and female heterozygotes are protected or, as frequently suggested, only females. Furthermore, how much protection may be afforded is unknown. In two large case-control studies of over 2000 African children, I found that the common African form of G6PD deficiency (G6PD A-) is associated with a 46-58% reduction in risk of severe malaria for both female heterozygotes and male hemizygotes. A mathematical model incorporating the measured selective advantage against malaria suggests that a counterbalancing selective disadvantage, associated with this enzyme deficiency, has retarded its rise in frequency in malaria-endemic regions. There is some evidence that two T helper cell subsets, Thl and Th2, regulate the immune response and thus influence the course of infections in mammalian hosts. These T cell subsets are reciprocal and associated with distinct cytokine profiles. Th2 T cell differentiation is promoted mainly by interleukin-4. Analysis of an IL-4 promoter polymorphism indicates that homozygosity for a putatively upregulatory IL-4 promoter variant is associated with a signficantly increased risk for severe malaria whilst heterozygotes are protected against this condition. Epidemiological evidence implicates host genetic factors as major determinants of variable susceptibility to tuberculosis. Most attempts to define the genetic factor(s) have focused on the HLA genes but only one result, an association of HLA-DR2 with increased susceptibility to disease in Asian populations, has been reported with any consistency. The genetic component in tuberculosis is likely to be determined by multiple genes and, therefore, in this study, the role of both HLA and non-HLA candidate genes was investigated. No association was found with variants of the macrophage gene, NRAMP1, the homologue of which has been implicated in the regulation of genetic resistance in the mouse model. Examination of certain class I and class II HLA alleles as well as the -590 interleukin-4 promoter polymorphism also did not show any association with disease. However, heterozygotes for a promoter polymorphism at position -238 of the tumour necrosis factor gene and homozygotes for dysfunctional variants of the gene encoding the collectin, mannose binding protein, were both at increased risk of developing pulmonary tuberculosis.