Deterministic Genes: These genes directly cause disease, and any individual who inherits the gene will develop a disorder. Worldwide, researchers have identified several rare genes that cause Alzheimer's in approximately a few hundred extended families. These genes are estimated to account for 1 percent or less of Alzheimer's cases and cause familial early-onset forms in which symptoms usually develop between a person's early 40s and mid-50s. On the other hand, the vast majority of individuals with AD have the late-onset disease, occurring at age 65 or later.
Although deterministic genes which can cause AD are rare, research on these genes has given us information on how these genes might cause AD. It has been shown that these genes affect the production and processing of beta-amyloid, the main protein fragment which clusters together, producing plaques that have a toxic effect on brain neurons. Since beta-amyloid is the prime suspect in the decline and death of brain cells, these genes may cause AD by affecting the production and processing of beta-amyloid. Several drugs are now in development that target beta-amyloid with the objective of stopping AD or significantly slowing its progression.
The following are the main deterministic genes found to cause AD:
- Amyloid precursor protein (APP): This gene was discovered in 1987 and is the first gene that, when mutated, was found to cause an inherited form of Alzheimer's.
- Presenilin-1 (PS-1): This gene identified in 1992 is the second gene in which mutations were found to cause inherited Alzheimer's. Moreover, variations in this gene are the most common cause of inherited Alzheimer's.
- Presenilin-2 (PS-2): This gene was discovered in 1993 and is the third gene with mutations found to cause inherited Alzheimer's.
Genetic tests are commercially available for the aforementioned rare genes that can directly cause AD. However, these tests are not currently recommended by healthcare professionals as routine tests to identify AD.
