Scientists don’t yet fully understand what causes Alzheimer's disease, but it has become increasingly clear that it develops because of a complex series of events that take place in the brain over a long period of time. It is likely that the causes include some mix of genetic, environmental, and lifestyle factors. Because people differ in their genetic make-up and lifestyle, the importance of any one of these factors in increasing or decreasing the risk of developing Alzheimer's differs from person to person.
One of the great mysteries of Alzheimer’s disease is why it largely strikes older adults. Research on how the brain changes normally with age is shedding light on this question. For example, scientists are learning how age-related changes in the brain may harm neurons and contribute to Alzheimer’s damage.
The more researchers learn about Alzheimer's disease, the more they realize that genes play an important role in its development.
Early-onset Alzheimer’s is a rare form of the disease. It occurs in people age 30 to 60 and represents less than 5 percent of all people who have Alzheimer’s disease. Most cases of early-onset Alzheimer’s are familial Alzheimer’s disease, caused by changes in one of three known genes inherited from a parent.
For more information about early-onset Alzheimer’s, see NIA's resource list. You can also view this webinar video, which discusses research in early-onset families in Colombia, South America. The webinar was hosted by the NIH Fogarty Center as part of its “Brain Disorders in the Developing World” program.
Most people with Alzheimer’s disease have “late-onset” Alzheimer’s, which usually develops after age 60. Many studies have linked the apolipoprotein E (APOE) gene to late-onset Alzheimer’s. This gene has several forms. One of them, APOE ε4, seems to increase a person’s risk of getting the disease. However, carrying the APOE ε4 form of the gene does not necessarily mean that a person will develop Alzheimer’s disease, and people carrying no APOE ε4 can also develop the disease.
Most experts believe that additional genes may influence the development of late-onset Alzheimer’s. Scientists around the world are searching for these genes, and have identified a number of common genes in addition to APOE ε4 that may increase a person’s risk for late-onset Alzheimer’s.
For more about this area of research, see the Alzheimer’s Disease Genetics Fact Sheet.
Research also suggests that a host of factors beyond basic genetics may play a role in the development and course of Alzheimer’s disease. There is a great deal of interest, for example, in associations between cognitive decline and vascular and metabolic conditions such as heart disease, stroke, high blood pressure, diabetes, and obesity. Understanding these relationships and testing them in clinical trials will help us understand whether reducing risk factors for these conditions may help with Alzheimer’s as well.
For a more extensive discussion of the causes of Alzheimer's disease, read the section from Alzheimer's Disease: Unraveling the Mystery, "Looking for the Causes of AD"
As Alzheimer's disease genetics research has intensified, it has become clear that scientists need many genetic samples to make further progress. NIA supports several major genetics research programs, including the Alzheimer's Disease Genetics Consortium (ADGC). The ADGC is a collaborative effort of geneticists to collect and conduct genome-wide association studies (GWAS) with more than 10,000 samples from thousands of families around the world with members who do and do not have late-onset Alzheimer's. Read more about ADGC research findings »