Alzheimer's disease in twins.

Besides familial Alzheimer's disease (AD), the genetic susceptibility has also been found in sporadic cases of AD, mostly related to the apolipoprotein E polymorphism. The penetrance of AD is determined by age and probably by environmental exposure. Gene-environment interaction of a disease can be examined through studies of twins. The relative roles of genetic and environmental influences can be estimated by comparing the concordance rates between monozygotic (MZ) and dizygotic (DZ) twins. Genetic models can be used to specify contributions both from genetic as well as shared and unique environmental effects. The role of environmental factors can be investigated in the co-twin control study, either by comparing environmental exposure in MZ twins discordant for a disease or by comparing MZ twins discordant for an exposure suspected of causing a particular disease. The sampling of twin pairs AD can be carried out using voluntary recruitment, linkage of twin and hospital discharge registries or screening of twin registry population. Potential sources of biases in sampling are discussed. The majority of the published twin studies on AD are case reports or based on selected materials. In MZ pairs, the concordance rates for AD have varied between 31% and 83%. Only one co-twin control study in twins discordant for AD has been published. Published twin studies on AD are briefly reviewed.