As treatment pathways for Alzheimer’s disease become more clear, the importance of early diagnosis is greater than ever and patients are asking: is there a blood test for Alzheimer’s genes? Researchers have identified several genes that increase the risk of developing Alzheimer’s disease (risk genes) as well as genetic mutations within specific genes which guarantee the development of Alzheimer’s disease (deterministic genes).
Genetic testing for genes associated with Alzheimer’s disease remains rather controversial. Genetic testing companies, such as 23andMe, can test for the carriage of the Apolipoprotein E (APOE) gene while utilizing a direct-to-consumer model, selling individuals their raw data to do with what they please. This leaves many medical professionals uneasy, questioning the ethicality of providing such information to individuals without counseling. Nevertheless, the growing evidence linking the APOE ε4 allele to Alzheimer’s disease has driven research into novel treatments aiming to target this gene.
Let’s delve a little deeper into the specifics of using a blood test for Alzheimer’s disease.
While those who develop Alzheimer’s disease are likely to have similar symptoms, the two main types of Alzheimer’s are categorized as early-onset, or young-onset, Alzheimer’s and late-onset Alzheimer’s. Early-onset Alzheimer’s disease is less common, accounting for less than 10% of those with Alzheimer’s, and refers to individuals under 65 who develop Alzheimer’s. Late-onset refers to individuals 65 and older who develop the disease. Accordingly, researchers have found that there are specific genes or genetic mutations associated with each type of Alzheimer’s disease. Aside from the two main categorizations of Alzheimer’s, Familial Alzheimer’s disease is known with certainty to be linked to genetics—however, it is quite rare, accounting for 1% or less of Alzheimer’s cases.
Genetic tests for Alzheimer’s disease are typically completed in the form of a blood test or saliva test. Below we detail common genes or genetic mutations that are tested for relating to late-onset and early-onset Alzheimer’s disease.
The most common and widely researched gene associated with late-onset Alzheimer’s disease is APOE, which has been identified as a risk gene. APOE is responsible for creating a protein that helps carry cholesterol and other fats in the bloodstream.
Genetic variants on the APOE gene on chromosome 19 are known to increase the risk of developing Alzheimer’s. APOE comes in several forms, or alleles—APOE ε2, APOE ε3, and APOE ε4. Individuals inherit two APOE alleles, one from each biological parent.
|APOE Allele||Genetic Significance|
|APOE ε2||APOE ε2 is relatively rare and may provide individuals with some level of protection against Alzheimer’s disease. Typically, if an individual with this allele develops Alzheimer’s, the development of the disease occurs later in life compared to an individual with the APOE ε4 allele.|
|APOE ε3||APOE ε3 is the most common APOE allele and is currently believed to play no role in increasing or decreasing the risk of developing Alzheimer’s.|
|APOE ε4||APOE ε4 is believed to increase the risk of developing Alzheimer’s disease and is associated with an earlier onset of the disease. Approximately 25% of people carry a single copy of APOE ε4 and 2-3% of people carry two copies. Individuals with one copy have an increased risk of developing Alzheimer’s. The presence of two copies is a stronger indicator that an individual may develop the disease.|
Although the presence of one or two copies of APOE ε4 can indicate a higher risk of developing late-onset Alzheimer’s disease, neither case can definitively predict whether or not an individual will develop Alzheimer’s, as genetics are not the only factor. Some individuals who develop Alzheimer’s do not carry the APOE ε4 allele.
Other genes being studied which may be associated with late-onset Alzheimer’s disease include ABCA7, CLU, CR1, PICALM, PLD3, TREM2, and SORL1.
Although early-onset Alzheimer’s disease is rare in comparison to late-onset Alzheimer’s, it is more strongly linked to genetics. Researchers and scientists have discovered that inherited genetic mutations on three particular genes can cause early-onset Alzheimer’s disease. The involved genes include:
When mutations on the above genes are present, the result is the production of excessive amounts of beta-amyloid peptide caused by the breakdown of APP. This production leads to a buildup of amyloid protein plaques in the brain, a neuropathological hallmark of Alzheimer’s disease. It should be noted that some individuals who have early-onset Alzheimer’s disease do not have mutations in these three genes, indicating that some forms of this type of Alzheimer’s may be linked to other factors that have not yet been identified.
Testing for brain health is becoming more important to patients every day, and while there is a blood test for Alzheimer’s genes, there are still unknown factors that prevent it from definitively diagnosing Alzheimer’s or other forms of dementia. However, technologies for assessing and diagnosing neurological diseases are rapidly growing and improving.
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