HERITABILITY ANALYSIS OF INTERMEDIATE PHENOTYPES OF TUBERCULOSIS: AIDS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Tuberculosis (TB) is a growing global public health problem. Several studies suggest a role for host genetics in disease susceptibility, but studies to date have been inconsistent and a comprehensive genetic model has not emerged. A limitation of previous genetic studies is that they only analyzed the binary trait TB, which does not reflect disease heterogeneity. Furthermore, these studies have not accounted for the influence of shared environment within households on TB risk, which may spuriously inflate estimates of heritability. We conducted a household contact study in a TB-endemic community in Uganda. Previously, we estimated the heritability of three cytokines as endophenotypes for TB: interferon-gamma, tumor necrosis factor-alpha, and transforming growth factor-beta. In that analysis, tumor necrosis factor-alpha demonstrated high (68%) heritability, and we followed it up with path and segregation analysis. Path analysis, conducted to assess the effect of shared environment, suggested that TNFalpha is heritable (narrow sense heritability = 34-66%); the effect of shared environment is minimal (1-14%), but gene-environment interaction may be involved. Segregation analysis of TNFalpha suggested a major gene model that explained one-third of the phenotypic variance, and provided putative evidence of natural selection acting on this phenotype. Our data further support TNFalpha as an endophenotype for TB, as it may increase power to detect disease-predisposing loci. We are currently conducting linkage and association analysis of candidate genes, and a genome scan is also underway.