Causes of Alzheimer’s disease (AD)
Scientists do not yet fully understand what causes AD, but it is 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 genetic, environmental, and lifestyle factors. Because people differ in their genetic make-up and lifestyle, the importance of these factors for preventing or delaying AD differs from person to person.
Genetics play a role in some people with AD. A rare type of AD, called early-onset AD, affects people ages 30 to 60. Some cases of early-onset AD, called familial AD, are inherited. Familial AD is caused by mutations (permanent changes) in three genes. Offspring in the same generation have a 50-50 chance of developing familial AD if one of their parents had it.
Most cases of AD are late-onset AD, which develops after age 60. Although a specific gene has not been identified as the cause of late-onset AD, genetic factors do appear to increase a person’s risk of developing the disease. This increased risk is related to the apoliprotein E (APOE) gene. The APOE gene has several forms. One of them, APOE e4, occurs in about 40 percent of all people who develop late-onset AD. However, at least one-third of people with AD do not have this form of the gene.
Four to seven other AD risk-factor genes may exist as well. One of them, SORL1, was discovered in 2007. Large-scale genetic research studies are looking for other risk-factor genes.
Research suggests that certain lifestyle factors, such as a nutritious diet, exercise, social engagement, and mentally stimulating pursuits, might help to reduce the risk of cognitive decline and AD. Scientists are investigating associations between cognitive decline and heart disease, 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 diseases may help with AD as well.