Integrating case-control and TDT studies

Genetic-association studies are widely expected to unravel the genetic basis of complex diseases. The population-based case-control study, a commonly used approach for association studies, is subject to the problem of population admixture. Consequently, evidence of disease-marker associations obtained from such studies is ideally confirmed by alternative methods. The Transmission/Disequilibrium Test (TDT) is suitable to assess evidence of association obtained from case-control studies. Since data are increasingly available from both case-control and TDT studies of the same disease-marker association, it is useful to obtain a combined estimate of disease-marker association. The odds ratio is a commonly used measure of the magnitude of a disease-marker association that can be easily obtained in case-control studies. Here we show how an odds ratio estimate and its' associated standard error can be obtained from TDT results. Furthermore, we suggest a method for integrating results from case-control studies and the TDT to provide a combined estimate of disease-marker association. Such combined estimates can be used to contrast the results of the two studies and provides an overall picture of the effect size attributable to such polymorphism. An illustrative application is made to a published data set on type 2 diabetes.