The genetics of Alzheimer disease.

Alzheimer disease (AD) is the most common cause of dementia in late life. AD has been described as early onset, occurring at or younger than age 50, or late onset, occurring at or older than age 70. Although AD is a genetically heterogeneous disorder, it is classified as familial or sporadic. Cases in which a clear pattern of inheritance within a family is established are termed familial Alzheimer disease (FAD). Sporadic, however, is problematic when studying a late onset disease such as AD. It is likely that people who may carry a mutation die of another cause before reaching the age of risk. This article describes the known genetic mutations resulting in an autosomal dominant pattern of inheritance in early onset AD and a polymorphism associated with the more common late onset disorder.