Gene Studied as Possible Cause of Alzheimer’s : Health: British researchers isolate a mutation that alters a protein found in large quantities in brains of persons with the disease.

British researchers said Friday they had pinpointed for the first time a possible genetic cause of Alzheimer’s disease.

By studying two families prone to developing the degenerative brain disorder at an early age, the researchers said they were able to isolate a genetic mutation that alters a protein found in abnormally large quantities in the brains of people with the disease.

Alzheimer’s disease is the fourth leading cause of death for adult Americans, and medical experts estimate that as many as one in 10 adults over the age of 65 suffer from it.

The British research team, from St. Mary’s Hospital Medical School in London, said that further investigation of other families would be needed to prove the gene was the cause. They cautioned that the mutation apparently causes only a small proportion of Alzheimer’s disease cases.

However, they said it was the first cause of the disease to be identified. Alzheimer’s destroys brain cells, causing its victims to sink into progressive senility, leading to complete physical disability and death.

There is no known cure and the disease cannot even be accurately diagnosed until death, when an autopsy of the brain reveals the damaged cells.

Between 10% and 30% of all victims suffer an inherited form of the disease called familial Alzheimer’s disease, which can strike as early as the age of 30.

In 1987 a team from Harvard Medical School narrowed the search for the mutated gene that causes the inherited form to a region of the body’s 21st chromosome.

The British team, working with doctors in Connecticut and North Carolina, said they found a mutated gene in that region that alters the amyloid protein, a substance always found in excessive amounts in the brains of Alzheimer’s disease victims.

Researchers first speculated three years ago that defects in the gene that produces the amyloid protein might be responsible for the familial cases, but experiments at that time seemed to rule it out.

The St. Mary’s team said advances in molecular biology techniques allowed them to analyze the structure of amyloid in more detail than was previously possible.

They said the gene mutation caused a slight change in the amyloid structure, which might make it more difficult for the brain cells to break down the protein.