BAR HARBOR, MAINE, JULY 24 -- Alzheimer's disease may be a natural part of the aging process that occurs when a specific normal gene wears out as people get older, a Duke University researcher reported here today.

"Just as people get wrinkles and gray hair at different ages, the rate of Alzheimer's cases increases with age," Allen Roses said at a genetics conference at the Jackson Laboratory. "But if we live long enough, we will all get Alzheimer's disease."

Alzheimer's, which affects an estimated 4 million Americans, is marked by progressive memory loss associated with dead and dying brain cells. Late-onset Alzheimer's usually starts in the sixties and by the age of 90, one out of three people is affected. By age 95, one out of two people is affected and at 120 years; 100 percent of the people have it, Roses said.

"Alzheimer's used to be considered rare several decades ago, but now it is thought to be an epidemic," he said. "There hasn't been an increase in defective genes but only an increase in lifespan, which is making Alzheimer's more visible."

Roses said his study of 32 families in which Alzheimer's is prevalent indicates that the gene responsible for brain cell death is located on chromosome 19, he said. A person is born with 23 pairs of chromosomes which contain the genes necessary for growth and development. The exact gene has not been found.

"We think that this is a housekeeping gene that is supposed to keep cells tidy and operational but it gets burned out with age," he said.

When the function of the gene declines, metabolic waste backs up in brain cells and kills them, leaving the scarred brain tissue that is characteristic of this condition, Roses said.

Earlier reports suggested that the faulty gene may be located on chromosome 21, but Roses said that his and other research has failed to support this. A rare form of Alzheimer's that occurs in young people, may, however, be linked to 21, he said.