Causes of Alzheimer's Disease

Genetic Factors

Major research targets in Alzheimer's disease are the factors responsible for beta amyloid build-up and concentration in certain people and not in others. Genetic factors are believed to play a role in many cases. In 2003, the National Institute on Aging (NIA) launched the ambitious AD Genetics Initiative, a 3-year national project to bank genetic material from families who have at least two members with late-onset Alzheimer's.

The ApoE Gene and Late-Onset Alzheimer's. The major target in genetic research on late-onset Alzheimer's disease (called LOAD) has been apolipoprotein E (ApoE), which plays a role in the movement and distribution of cholesterol for repairing nerve cells during development and after injury.

The gene for ApoE comes in three major types:

• ApoE4. Studies have reported the greatest deposits of beta amyloid in people with ApoE4, which is now believed to be a major risk factor for late-onset Alzheimer's. Some evidence suggests that the ApoE protein removes beta amyloid but the ApoE4 variant does so less efficiently than other ApoE types. (ApoE4 has also been studied for years as a risk factor for heart disease.)
• ApoE3 and ApoE2. Fewer beta amyloid deposits have been observed in people with the ApoE3, and the fewest deposits have been observed in people with ApoE2, which may actually be protective.

People inherit a copy of one type from each parent, but Alzheimer's disease is not inevitable even in people with two copies of the ApoE4 gene. Reports vary widely in estimating the extent of risk:

• People without ApoE4 have an estimated risk of between 9 - 20% for developing Alzheimer's by age 85.
• In people with one copy of the gene, the risk is between 25 - 60%.
• In people with two copies, the risk ranges from 50 - 90%. (Only 2% of the population carries two copies of the ApoE4 gene.)

Some researchers suspect that some specific variation of the ApoE4 gene or combinations with other genes are critical for the disease, since many people who carry the ApoE4 exhibit no signs of Alzheimer's. For example, evidence suggests that genetic factors play a role in a common subtype of late-onset Alzheimer's disease that also includes psychosis. An important 2002 genetic study has identified certain genetic linkages associated with ApoE4 that appear to play a strong role in this subtype.