Genetic testing is available for both risk genes and deterministic genes for the assessment of AD. Based upon the current knowledge of the genetics of Alzheimer’s and the availability of specific genetic tests for the disease, the following conclusions can be drawn:
- Genetic mutations that can directly cause AD occur in a small percentage, estimated as 1% or less of Alzheimer’s cases. The genes with these specific mutations are called “deterministic genes.” At present there are three such genes (amyloid precursor protein, presenilin-1 and presenilin-2) and the inheritance of one of these genes virtually assures an individual will develop AD. In these individuals, AD tends to be early-onset with disease symptoms developing before age 65 and sometimes as early as age 30.
- A large majority of those individuals with AD have late-onset disease occurring at age 65 or later. Genes identified as risk genes have been discovered for AD. These risk genes increase the risk of developing the disease but do not directly cause the disease. Foremost among these risk genes is the apolipoprotein E gene (APOE) with the most common form being the APOE-e4 gene. Studies suggest that approximately 40-65% of individuals diagnosed with AD have the APOE-e4 gene.
- Although genetic testing for both risk genes and deterministic genes are available, these tests are not routinely used in clinical settings to diagnose or predict the risk of developing AD or related dementia. Because there is no current cure or preventive treatment for AD, the results of genetic testing typically do not have a practical impact on medical treatment decisions. Testing for the APOE-e4 gene has a special challenge in that many individuals who have the gene may never experience Alzheimer’s symptoms, while many who do develop the disease may not have any copies of the APOE-e4 gene. Therefore, there may be false hope in those individuals without the APOE-e4 gene because they will assume that they will never develop AD.
- On rare occasions, medical specialists may order a genetic test, especially if a person develops early-onset AD at an early age or there is a strong family history of AD. Typically, genetic counseling is accompanied with the genetic test to discuss the risks, benefits and limitations of the test results. In addition, genetic tests for AD assessment may be performed in the context of clinical trials and research studies to help identify patients at risk, explain disease progression and assess the effects of a treatment.
- Currently, the Alzheimer’s Association does not recommend genetic testing for Alzheimer’s disease for the general population.