It must be noted that a family history of Alzheimer’s is not necessary for an individual to develop the disease. On the other hand, clinical research has demonstrated that individuals who have a parent or sibling with AD are more likely to develop the disease. Moreover, individuals who have more than one first-degree relative with AD are even at a greater risk for the disease.
Several genes possibly related to Alzheimer’s have been identified and fall into two main categories; (1) risk genes and (2) deterministic genes.
Risk genes: These genes increase the risk of developing a disease but are not a direct cause of the disease. Researchers have identified several genes that increase the risk of AD. Foremost among these genes is the apolipoprotein E gene (APOE). One form of the gene, the APOE-e4 risk gene, was the first Alzheimer’s risk gene identified and remains the gene with the largest risk for AD. The APOE-e4 gene was discovered in 1993 and ongoing research studies have suggested that 40-65 percent of individuals diagnosed with AD have the APOE-e4 gene. There are several other risk genes for AD but they have limited significance in the general population mainly because they are rare and only slightly increase the risk of AD.
The APOE gene provides the code for a protein that transports cholesterol in the blood. There are three forms of the APOE gene (e2, e3, and e4), with all individuals inheriting one of the gene forms from each parent. The e3 form is the most common, with about 60 percent of the U.S. population inheriting e3 from both parents. The e2 and e4 forms are much less common. An estimated 20 to 30 percent of individuals in the United States will have one copy of the e4 form and 2 percent will have two copies of e4. Approximately 10 to 20 percent will have one or two copies of e2.
Individuals having the APOE e3 form are believed to neither increase nor decrease the risk of Alzheimer’s, while having the e2 form may decrease the risk. However, having the APOE e4 form is believed to increase the risk of developing AD and of developing it at a younger age. In addition, individuals who inherit two e4 genes are believed to be at a higher risk than those with one e4 gene. Researchers estimate that between 40 and 65 percent of people diagnosed with Alzheimer’s have one or two copies of the APOE-e4 gene.
Since the APOE e4 gene is considered a risk gene, inheriting this form of the gene does not guarantee that an individual will develop AD but only that there is an increased risk of developing AD.
APOE-e4 Genetic Test: Genetic testing for all forms of the APOE gene, including the APOE-e4 form in both blood and CSF, is available from several commercial diagnostic companies. Testing is typically performed using Liquid Chromatography/Tandem Mass Spectrometry (LC/MS/MS) methodology to identify the specific forms of the APOE gene and produce an algorithm that stratifies patients into various risk categories for AD. Although testing for the APOE-e4 gene may identify individuals at higher risk for AD, the test does not indicate whether an individual has AD or will develop AD and should not be used to diagnose AD. At present, the APOE-e4 genetic test may be useful for clinical trial or research studies to help identify those at higher AD risk.
