Scientists at the University of Southern Florida Alzheimer’s Disease Research Center have determined that there is a common cause of both Down syndrome symptoms and Alzheimer’s disease symptoms, and that this disease mechanism may be related to other diseases including atherosclerosis and diabetes.
Down syndrome is caused by trisomy 21, having three copies of chromosome 21 in every cell instead of the normal two copies. Alzheimer’s patients have also been found to have some nerve cells containing three copies of chromosome 21.
The gene coding for amyloid protein is located on chromosome 21. Since people born with Down syndrome have an extra copy of this chromosome in all of their cells, more of the amyloid protein is produced and begins to accumulate over time. When they reach 30 or 40 years of age, individuals with Down syndrome develop amyloid plaques in their brains identical to those found in the brains of Alzheimer’s patients, leading to similar symptoms such as loss of nerve cells and dementia.
Within the cell there is a complex network of microtubules that play an important role in the cell’s internal structure and function. Microtubules are responsible for transporting proteins, chromosomes, and other cellular components around the cell, allowing the cell to carry out its normal activities. The presence of amyloid protein in the cell disrupts the microtubule network and prevents it from working properly.
One function of microtubules is to separate the new chromosomes when cells divide. If this is not done correctly, the new cells can receive the wrong number of chromosomes. This explains why nerve cells with three copies of chromosome 21 are found in Alzheimer’s patients. As the disease progresses, more trisomy 21 nerve cells are produced, which in turn manufacture more amyloid protein, causing increasing symptoms of brain pathology.
Alzheimer’s patients often suffer from other diseases such as atherosclerosis and diabetes. While a definite link has not been established, researchers suspect that amyloid protein may also be to blame for these other diseases. Damage to the network of microtubules within cells caused by amyloid protein can prevent cell receptors from reaching the surface of the cell in order to perform their functions, leading to pathological conditions.
If the receptors for LDL (bad) cholesterol are prevented from reaching the cell surface, cells will not be able to bind the LDL cholesterol, and it will remain circulating in the bloodstream and accumulate on the walls of arteries, leading to symptoms of cardiovascular disease. Insulin receptors and nerve cell signaling receptors similarly must reach the cell surface in order to do their jobs, or diabetes and memory problems can develop.
These findings may point researchers in the direction of promising new treatments for Alzheimer’s and other diseases.