Scientists may be closer to understanding the cause of Alzheimer's disease with the identification of a gene called Sorl 1 that seems to play a role in the processes leading up to onset of the disease.

Several genes already are known to be associated with Alzheimer's. Some are rare, and their presence almost certainly indicates a person will develop the disease. Others that are relatively more common have an association that is less strong.

Alzheimer's disease, a form of dementia, initially involves parts of the brain that control thought, memory, and language. It's cause is unknown, and there is no cure.

An estimated 4.5 million Americans suffer from Alzheimer's, according to the National Institute on Aging. Its onset usually occurs after age 60, and risk increases with age. It is much less common, although not unknown, in younger individuals. About 5 percent of people 65 to 74, and as many as half of those age 85 and older may have Alzheimer's.

The devastating impact of the disease is likely to intensify as the population ages.

In the latest research, Dr. Peter St. George Hyslop of the University of Toronto and colleagues conducted genetic tests on more than 6,800 people, of whom about half were Alzheimer's patients. They found a gene they're calling "SORL 1" that seems to explain part of the puzzle of Alzheimer's.

The presence of SORL 1 may affect how the body processes APP, a brain protein that triggers the metabolic changes the precede Alzheimer's, Hyslop conjectured.

In the presence of the protein made by the normal SORL 1 gene, APP protein is less active; essentially, the SORL 1 protein binds to APP and prevents it from being degraded.

When there is less SORL 1 protein, which may occur when individuals carry a variant of the gene, the APP protein can go into parts of a cell where it becomes fragmented, producing a toxic substance called "amyloid," which might trigger the development of Alzheimer's.

The discovery could lead to new treatments for Alzheimer's, the researchers suggested.