According to the National Institutes of Health, three-quarters of those who develop Alzheimer's disease (AD) have no family history of the
disease; but for the rest, researchers say several genes may contribute to their disorder.
AD is an irreversible and progressive condition.
It is characterized by inflammation and buildup of amyloid plaque and neurofibrillary tangles in the brain, leading to nerve cell damage.
There are two types of AD: early-onset and late-onset, both of which have genetic links.
According to the Alzheimer's Association, late-onset is the most common form of Alzheimer's disease and occurs in individuals 65 or older.
The most common gene linked to this type of AD is apolipoprotein E (APOE).
There are three APOE variations of the gene.
APOE e2 occurs least often and appears to reduce the risk of Alzheimer's.
APOE e3 is the most common variant and is not believed to affect a person's risk of getting the disease.
APOE e4, however, appears to contribute to Alzheimer's.
A person may carry two different types of the APOE gene, inheriting one from their mother and another from their father.
Therefore, a person can have two APOE e4 genes, which puts them at an even greater risk of developing Alzheimer's.
Other studies link late-onset Alzheimer's to the gene SORL1.
One study shows a variation of SORL1 is associated with a three times greater risk of developing AD in Dominican families.
Certain variations of the SORL1 increase the production of amyloid-beta, which contributes to the formation of amyloid plaques in the brain.
No specific mutation has been identified so far, but researchers say a potential treatment could be to target the gene and increase the production of the SORL1 receptor.
In June of 2008, researchers discovered another gene that increases the risk of late-onset Alzheimer's disease. Having one copy of calcium homeostasis modulator 1 (CALHM1) increases the chance of developing late-onset AD by 44 percent, while having two copies increases it by 77 percent.
According to Philippe Marambaud, Ph.D., study author and an assistant professor at The Feinstein Institute for Medical Research in Manhasset, N.Y., roughly a quarter of the population has one copy of the gene.
The CALHM1 gene regulates specific calcium channels in the brain, which permit electrically charged calcium molecules to enter cells and trigger production of beta amyloid proteins.
Drugs that target calcium channels currently exist; however, none are designed to target the channels the CALHM1 gene controls.
Less than 10 percent of AD sufferers have the early-onset variety, of which three genes have been linked to.
Inheriting just one of these mutated genes almost guarantees the disease will strike before you turn 65.
Amyloid precursor protein (APP), presenilin 1 (PS1) and presenilin 2
(PS2) all cause too much amyloid-beta peptide to be produced in the brain.
Roughly half of early-onset patients don't present any of these three genetic abnormalities, which means the aggressive version of AD may be caused by other genetic mutations that haven't been discovered.
For more information contact the Alzheimer's Association at (800) 272-3900.