Alzheimer's Disease

Description

Causes

Researchers are finding specific biologic factors involved with Alzheimer's disease. Various environmental and genetic players appear to contribute to or trigger the process by which these factors destroy nerve cells leading to this disease.

Biologic Factors in the Brain

Imaging techniques in patients with Alzheimer's disease have found significant loss of cells and volume in the regions of the brain devoted to memory and higher mental functioning. Important abnormalities have specifically been observed during biopsies:

• Twisted nerve cell fibers, known as neurofibrillary tangles.
• A sticky protein called beta amyloid.

Other factors also play a role.

The Effects of Neurofibrillary Tangles and Beta Amyloid in Alzheimer's Disease. These biologic factors appear to be involved in the development Alzheimer's disease in the following ways:

• Neurofibrillary tangles are the damaged remains of microtubules, the support structure that allows the flow of nutrients through the neurons (nerve cells). A key component in these tangled fibers is an abnormal form of the tau protein, which in its healthy version helps in the assembly of the microtubule structure. The defective tau, however, appears to block the actions of the normal version.
• Beta Amyloid (also called A beta) is the second significant finding. This insoluble protein accumulates and forms sticky patches called neuritic plaque, which are found surrounded by the debris of dying nerve cells in the brains of Alzheimer's victims.
• Amyloid precursor protein (APP) is a large nerve-protecting protein that is the source of beta amyloid. In Alzheimer's certain enzymes, particularly those called gamma-secretases, snip APP into beta amyloid pieces. This process is controlled by factors called presenilin proteins. (Genetic abnormalities that affect either APP or presenilin proteins occur in some inherited cases of early-onset Alzheimer's.)
• High levels of beta amyloid are associated with reduced levels of the neurotransmitter acetylcholine. (Neurotransmitters are chemical messengers in the brain.) Acetylcholine is part of the cholinergic system, which is essential for memory and learning and is progressively destroyed in Alzheimers patients.
• Beta amyloid may also disrupt channels that carry sodium, potassium, and calcium. These elements serve the brain as ions, producing electric charges that must fire regularly in order for signals to pass from one nerve cell to another. If the channels that carry ions are damaged, an imbalance can interfere with nerve function and signal transmission.

Click the icon to see an image of amyloidosis.

Other Proteins. Researchers have now identified other important proteins in the areas of the brain affected by Alzheimer's disease.

• ERAB (endoplasmic-reticulum associated binding protein) appears to combine with beta amyloid, which in turn attracts new beta amyloid from outside the cells. High amounts of ERAB may also enhance the nerve-destructive power of beta amyloid.
• AMY plaques resemble beta amyloid so closely that researchers were able to detect them only with the use of highly sophisticated techniques.
• Elevated levels of a protein called prostate apoptosis response-4 (Par-4) may cause nerve cells to self-destruct.

Oxidation and the Inflammatory Response

Researchers are also attempting to discover why beta amyloid is so toxic to nerve cells. Some researchers are focusing on two processes in the body that may be involved with Alzheimer's disease: oxidation and the inflammatory process. There is some evidence that such events can begin decades before Alzheimer's disease actually develops. One scenario for their role in Alzheimer's is as follows:

The Role of Oxidation.

• As beta amyloid breaks down it releases unstable chemicals called oxygen-free radicals. Once released, oxygen-free radicals bind to other molecules through a process called oxidation.
• Oxidation is the result of many common chemical processes in the body, but when oxidants are overproduced, they can cause severe damage in cells and tissue, including even affecting genetic material in cells (its DNA). Oxidation is known to play a role in many serious diseases, including coronary artery disease and cancers, and experts believe it may also contribute to Alzheimer's.

The Inflammatory Response.

• One result of oxidation is the marshaling of immune factors to repair the cellular injuries it produces. Overproduction of some of these factors, however, produces the so-called inflammatory response, in which the immune process itself can actually damage the body's own cells themselves.
• Principle immune cells in the brain are called macrophage/microglia (M phi). In the healthy brain, they play an important protective role against invading organisms. However, when they are activated by beta amyloid oxidation, they release toxic molecules called cytokines, which are known to cause harm. For example, significantly high levels of interleukin-6, a specific cytokine, have been detected in people with Alzheimer's.
• 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% and 20% for developing Alzheimers by age 85.
• In people with one copy of the gene, the risk is between 25% and 60%.
• In people with two copies, the risk ranges from 50% to 90%. (Only 2% of the population carry 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 Alzheimers disease do not carry the ApoE4 gene. Increasingly, researchers believe that many cases of late-onset Alzheimer's are a result of a collaboration of genetic factors that participate in the process of producing or degrading beta amyloid. Some under investigation are the following:

• Researchers are now targeting chromosome 10 as a possible location for genetic factors involved with Alzheimer's disease. (The ApoE4 gene is on chromosome 19.)
• 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.
• One 2000 study of an Arab community with a high incidence of Alzheimer's has found evidence for a recessive gene, which means that both parents must carry it in order for the disease to be passed on. (Surprisingly, the ApoE4 gene showed up in this population at the lowest levels on record.)

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 presenelin-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

Also of interest to researchers are the environmental factors (e.g., infections, metals, industrial or other toxins) that may trigger oxidation, inflammation, and the disease process, particularly in people with 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 Creuztfeldt-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. Studies that help support this theory are as follows:

• Evidence in one British research center has suggested that presence of herpesvirus (HSV) 1 increases the risk for Alzheimer's disease in individuals who carry ApoE4. (HSV1 is a form of herpes that can invade the central nervous system). In one study, the risk was normal in those with only one of these factors. Furthermore, research is finding that parts of the HSV1 protein strongly resemble beta amyloid and, in laboratory studies, even have been observed to kill brain cells and develop sticky plaques. Chlamydia pneumoniae is a common organism that causes respiratory infections. Researchers are finding that it may have very powerful inflammatory affects in blood vessels. And some studies (but not all) have found evidence of the organism in parts of the brain affected by late-onset Alzheimer's. More research is needed to determine the significance of these findings.

Click the icon to see an image of herpes.

Metals. Some laboratory studies have reported excessive amounts of metal ions such as zinc, copper in the brain of someone 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. Various studies offer different reasons for the association. Some suggest that magnetic fields may lower the concentration of calcium inside cells or reduce levels of melatonin, which are both believe to help protect nerve cells. In any event, the association between EMF and Alzheimer's is very weak.