Genetic Epidemiology Strategies in Psychiatric Diseases

Summary

In affective disorders and schizophrenia, as in most psychiatric diseases, a genetic component has been shown to exist, but information provided by the segregation of the disease in families is not sufficient to determine the way in which genetic susceptibility is transmitted. To study such diseases with a complex and most likely heterogeneous etiology, genetic epidemiology now focuses on new strategies using genetic markers which can be applied at different levels:

• 1. At a population level, using “association studies,” which compare the marker allele distribution in a sample of unrelated affected individuals with the distribution in a sample of unrelated control individuals.

• 2. At a family level, using “linkage studies”, which test the independence of segregation of the disease and the marker in a family sample.

Results of association and linkage studies must be interpreted with caution, in particular concerning the significance levels attributed and the possible inferences about the etiology of the disease. It is also important to define the population of patients affected by the conclusion. Replication studies are often necessary to obtain good significance levels. The strategy for such replication, as well as possible inferences about the etiology of disease, will be discussed, and an example illustrating the usefulness of genetic markers in the study of complex disorders will be provided, type 1 diabetes.