Inheritance of Alzheimer's disease: epidemiologic evidence.
The available evidence suggests that there may be two subtypes of AD-- inherited and noninherited. Inherited AD may have certain characteristics, e.g. younger age at onset and some clinical signs or symptoms, which distinguish them from noninherited cases. It is possible that the noninherited type of AD may also have a similar genetic defect as the inherited kind, the only difference being that in one case the defect is inherited from the parents and in the other the defect arises de novo during embryogenesis. An environmental factor operating during embryogenesis may be responsible for causing a genetic defect which later manifests as noninherited AD. Currently, however, there is no precise way to separate these two subtypes. The exact proportion of all cases of AD which are inherited on a genetic basis is not known, but it must be small. The mode of inheritance of genetic AD is not known. It must be emphasized that the subtypes of inherited and noninherited AD are not synonymous with the older categorization of AD into presenile and senile AD.