Scientists have identified a new gene variant, which seems to strongly raise the risk for Alzheimer’s disease.

While the problem gene is not common (less than 1 percent of people are thought to have it), it roughly triples the chances of developing the disorder compared to people with the normal version of the gene. It also seems to harm memory and thinking in older people without dementia.

The main reason scientists are excited by the discovery is what this gene does, and how it might reveal what causes Alzheimer’s and ways to prevent it. The gene helps the immune system control inflammation in the brain and clear junk, such as sticky deposits that are the hallmark of the disease. Mutations in the gene may impair these tasks, so treatments to restore the gene’s function may help.

Until now, only one gene — ApoE — has been found to have a big impact on Alzheimer's risk. About 17 percent of the population has at least one copy of the problem version of this gene but nearly half of all people with Alzheimer's do. Other genes that have been tied to the disease raise risk only a little, or cause the less common type of Alzheimer's that develops earlier in life, before age 60.

Further tests on 3,550 Alzheimer's patients and more than 110,000 people without dementia in several countries, including the United States, found that the gene variant was more common in Alzheimer's patients.

"It's a very strong effect," raising the risk of Alzheimer's by three to four times — about the same amount as the problem version of the ApoE gene does," said Dr. Allan Levey, director of an Alzheimer's program at Emory University, one of the academic centers participating in the research.

Researchers also tested more than 1,200 people over age 85 who did not have Alzheimer's disease and found that those with the variant TREM2 gene had lower mental function scores than those without it. This adds evidence the gene variant is important in cognition, even short of causing Alzheimer's.

"It's another piece in the puzzle. It suggests the immune system is important in Alzheimer's disease," said Andrew Singleton, a geneticist with the National Institute on Aging, which helped pay for the study.

The gene is described in a study by an international group published online Wednesday by the New England Journal of Medicine.

About 35 million people worldwide have dementia, and Alzheimer’s is the most common type. In the U.S., about 5 million have Alzheimer’s. Medicines, such as Aricept and Namenda just temporarily ease symptoms. There is no known cure.

Based on reporting by the Associated Press.

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