A very small percentage of people who develop Alzheimer's disease have the early-onset type, which is classified as beginning before age 65.
Scientists have identified three genes in which mutations cause early-onset Alzheimer's disease. If you inherit one of these mutated genes from either parent, you'll likely experience Alzheimer's symptoms before age 65. The genes involved are:
- Amyloid precursor protein (APP)
- Presenilin 1 (PSEN1)
- Presenilin 2 (PSEN2)
Mutations of these genes cause the production of excessive amounts of a toxic protein fragment called amyloid-beta peptide. As these fragments stick together and collect in the brain as amyloid plaques, the tau protein malfunctions. As the tau protein particles stick together and form neurofibrillary tangles, the brain cells die and the signs and symptoms of Alzheimer's disease develop.
However, some people who have early-onset Alzheimer's don't have mutations in these three genes. That suggests that this early-onset form of Alzheimer's disease is linked to other genetic mutations that haven't been identified yet.
Most experts don't recommend genetic testing for late-onset Alzheimer's. In some instances of early-onset Alzheimer's, however, genetic testing may be appropriate.
In the case of APOE, knowing whether you have the e4 variety doesn't tell you whether you'll develop Alzheimer's. Although many people with APOE e4 develop Alzheimer's, many don't. Conversely, some people with no APOE e4 genes develop Alzheimer's. Most clinicians discourage testing for the APOE genotype because the results are difficult to interpret.
Testing for the mutant genes that have been linked to early-onset Alzheimer's — APP, PSEN1 and PSEN2 — may provide more certain results and have implications for current and future therapeutic drug trials.
Before being tested, it's important to weigh the emotional consequences of having that information. The results may affect your eligibility for certain forms of insurance, such as disability, long-term care and life insurance.
Doctors often can accurately diagnose Alzheimer's disease without genetic testing.
Researchers and genes
Researchers suspect that many more genes that haven't been identified yet affect Alzheimer's disease risk. Such information may prove vital in the development of new ways to treat, or even prevent, Alzheimer's disease in the future.
The Alzheimer's Disease Genetics Study, sponsored by the National Institute on Aging, is examining genetic information from families that have at least two family members who have developed Alzheimer's after age 65. If your family is interested in participating in this study, visit the website for the National Cell Repository for Alzheimer's Disease.
Several other studies evaluate genetics of people with Alzheimer's disease and their family members.
Feb. 09, 2013
See more In-depth
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