Early-onset Alzheimer’s disease occurs in people age 30 to 60. It is rare, representing less than 5 percent of all people who have Alzheimer’s. Some cases of early-onset Alzheimer’s have no known cause, but most cases are inherited, a type known as familial Alzheimer’s disease (FAD).
Familial Alzheimer’s disease (FAD) is caused by any one of a number of different single-gene mutations on chromosomes 21, 14, and 1. Each of these mutations causes abnormal proteins to be formed. Mutations on chromosome 21 cause the formation of abnormal amyloid precursor protein (APP). A mutation on chromosome 14 causes abnormal presenilin 1 to be made, and a mutation on chromosome 1 leads to abnormal presenilin 2.
Scientists know that each of these mutations plays a role in the breakdown of APP, a protein whose precise function is not yet known. This breakdown is part of a process that generates harmful forms of amyloid plaques, a hallmark of the disease. A child whose mother or father carries a genetic mutation for FAD has a 50/50 chance of inheriting that mutation. If the mutation is in fact inherited, the child almost surely will develop FAD.
Critical research findings about early-onset Alzheimer’s have helped identify key steps in the formation of brain abnormalities typical of Alzheimer’s disease. They have also led to the development of imaging tests that show the accumulation of amyloid in the living brain. In addition, the study of Alzheimer’s genetics has helped explain some of the variation in the age at which the disease develops.
Scientists are continuing research through the Dominantly Inherited Alzheimer Network (DIAN), an international partnership to study families with a genetic mutation that causes early-onset Alzheimer’s disease.
By observing the biological changes that occur in these families long before symptoms appear, scientists hope to gain insight into how and why the disease develops in both its early- and late-onset forms. In addition, scientists are attempting to develop tests that will enable diagnosis of Alzheimer’s before clinical signs and symptoms appear, as it is likely that early treatment will be critical as therapies become available.