OHSU

Genes and Alzheimer's Disease

Genetic risk of developing Alzheimer's disease (AD)

The greatest risk factor associated with AD is increasing age, though a family history of AD is also a risk factor. People of the same age who have a parent or sibling with AD are two to three times more likely to develop the disease than people who do not.

There are two forms of Alzheimer's disease, sporadic AD and familial AD. In sporadic AD, the most common form:

  • Genes alone do not cause the disease, but may influence the risk of developing it.
  • It occurs less predictably and fewer family members are affected.
  • Most often occurs after age 60, so it is often called late-onset Alzheimer's.

In familial AD, which is very rare:

  • Genes are the direct cause.
  • Many people in one family and in multiple generations are affected.
  • Most cases occur before the age of 60, so it is often called early-onset, or younger-onset Alzheimer's. Familial AD is very rare.

Genetic testing

Testing is available for the mutations (changes) in the three genes that cause familial/early-onset AD, but having these tests must be considered very carefully.

  • These mutations are rare: only about 200 families in the world are known to carry the mutations.
  • Test results will have no practical effect on treatment decisions because no preventive treatment is available.
  • Positive results (knowing you have the mutation) may affect your mental health and ability to buy long-term care insurance.
  • You should talk about these issues with a trained genetic counselor before deciding on a test or after you get results.

If you do not have symptoms of dementia, genetic testing for sporadic (late-onset) AD is not recommended. A positive test for the APOE4 gene does not provide useful information because:

  • If you have the APOE4 gene, you may or may not develop AD.
  • If you have no APOE4 genes, you may still develop AD.
  • There are medical, legal, social and ethical issues involved.
  • Testing for the APOE4 gene may cause employment, insurance and psychological (mental and emotional health) problems. 

If you do have dementia symptoms, APOE gene testing has limited value. In a thorough evaluation, without APOE testing, doctors can diagnose Alzheimer's disease with approximately 90 percent accuracy.

Testing for research

Genetic testing can be very important for AD research. If you are in an AD research study, test results can be kept private if you or your family don't want to know them.

For more information:

Genetic Testing: Ethical Issues in Alzheimer's Disease
A statement by the Alzheimer's Association Ethics Advisory Panel.

The Alzheimer's Disease Education and Referral Center (ADEAR)

Alzheimer's Disease: What Causes It?
An interactive information page on AD symptoms, causes, treatment and testing.