Causes

Other inflammatory factors of specific interest in Alzheimer’s research are the enzyme cyclooxygenase (COX) and its products called prostaglandins. Excess amounts of these factors may increase levels of glutamate. Glutamate is an amino acid that excites nerves and, when overproduced, is a powerful nerve-cell killer.

The inflammatory process has also been associated with the release of soluble toxins called amyloid beta-derived diffusible ligands, which some investigators believe may prove to key players in the destructive process.

Genetic Factors

Major research targets in Alzheimer’s disease are the factors responsible for beta amyloid build-up and concentration in certain people and not in others. Genetic factors are believed to play a role in many cases. In 2003, the National Institute on Aging (NIA) launched the ambitious AD Genetics Initiative, a 3-year national project to bank genetic material from families who have at least two members with late-onset Alzheimer’s.

The ApoE Gene and Late-Onset Alzheimer’s. The major target in genetic research on late-onset Alzheimer’s disease (called LOAD) has been apolipoprotein E (ApoE), which plays a role in the movement and distribution of cholesterol for repairing nerve cells during development and after injury.

The gene for ApoE comes in three major types:

• ApoE4. Studies have reported the greatest deposits of beta amyloid in people with ApoE4, which is now believed to be a major risk factor for late-onset Alzheimer’s. Some evidence suggests that the ApoE protein removes beta amyloid but the ApoE4 variant does so less efficiently than other ApoE types. (ApoE4 has also been studied for years as a risk factor for heart disease.)
• ApoE3 and ApoE2. Fewer beta amyloid deposits have been observed in people with the ApoE3, and the fewest deposits have been observed in people with ApoE2, which may actually be protective.

People inherit a copy of one type from each parent, but Alzheimer’s disease is not inevitable even in people with two copies of the ApoE4 gene. Reports vary widely in estimating the extent of risk:

• People without ApoE4 have an estimated risk of between 9 - 20% for developing Alzheimer’s by age 85.
• In people with one copy of the gene, the risk is between 25 - 60%.
• In people with two copies, the risk ranges from 50 - 90%. (Only 2% of the population carries two copies of the ApoE4 gene.)

Some researchers suspect that some specific variation of the ApoE4 gene or combinations with other genes are critical for the disease, since many people who carry the ApoE4 exhibit no signs of Alzheimer’s. For example, evidence suggests that genetic factors play a role in a common subtype of late-onset Alzheimer’s disease that also includes psychosis. An important 2002 genetic study has identified certain genetic linkages associated with ApoE4 that appear to play a strong role in this subtype.

Other Genetic Factors in Late-Onset Alzheimer’s. Most people with late-onset Alzheimer’s disease do not carry the ApoE4 gene. Increasingly, researchers believe that many cases of late-onset Alzheimer’s result from a combination of genetic factors that participate in the process of producing or degrading beta amyloid. Some under investigation include:

• Researchers are targeting chromosomes 9, 10, and 12 as possible locations for genetic factors involved with Alzheimer’s disease. (The ApoE4 gene is on chromosome 19.) In 2005, researchers announced that mutations linked to the ubiquilin 1 (UBQLN1) gene, located on chromosome 9, might be associated with increased risk for late-onset Alzheimer’s disease.
• Researchers have detected mutations in the proteins amyloid precursor protein (APP) and ubiquitin-B (Ubi-B), which may account for some cases of late- and early-onset Alzheimer’s. Such mutations are not inherited, however, but appear to be genetic mistakes that occur during transcription, the coding process in which DNA establishes the pattern for the production of its proteins and other molecules.
• In 2007, researchers identified mutations in the SORL1 gene as a possible factor in late-onset Alzheimer’s disease. Researchers think that variations in this gene may contribute to amyloid plaque formation in Alzheimer’s disease.

Genetic Factors for Early-Onset Alzheimer’s. Scientists are coming closer to identifying defective genes responsible for early-onset Alzheimer’s, an uncommon, but extremely aggressive form of the disease.

• Mutations in genes known as presenilin-1 (PS1) and presenilin-2 (PS2) account for most cases of early-onset inherited Alzheimer’s disease. The defective genes appear to accelerate beta amyloid plaque formation and apoptosis, a natural process by which cells self-destruct.
• Genetic mutations in the genes that control amyloid precursor protein (APP) are also being targeted as causes of early-onset Alzheimer’s. The genetic disease Down syndrome, for example, overproduces beta-amyloid precursor protein (APP), the source of beta amyloid, and almost always leads to early Alzheimer’s. Other APP mutations are being identified.

Environmental Factors

Researchers are also investigating environmental factors (infections, metals, industrial and other toxins) that may trigger oxidation, inflammation, and the disease process, particularly in people with a genetic susceptibility to Alzheimer’s.

Infectious Organisms. Slow, infectious viruses cause a number of other degenerative neurologic diseases, such as kuru and Creutzfeldt-Jakob disease.

Click the icon to see an image of Creutzfeldt-Jakob disease.

Although no specific virus has been linked to Alzheimer’s, some researchers theorize that people with a genetic susceptibility to Alzheimer’s may be vulnerable to the actions of certain viruses, particularly under circumstances when the immune system may be weakened.

Metals. Some laboratory studies have reported excessive amounts of metal ions such as zinc, copper in the brain of people with Alzheimer’s disease. Such ions may possibly change the chemical architecture of normal beta amyloid, making it more harmful. A mildly acidic environment appears to be important in the process that binds these metals to beta amyloid. Experts observe that such conditions (acidic environment and higher levels of zinc and copper) commonly occur as part of the inflammatory response to local injury.

Electromagnetic Fields. Some studies on people exposed to intense electromagnetic fields (EMF) have reported a higher incidence of Alzheimer’s. However, the association between EMF and Alzheimer’s is very weak.