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Who Gets Alzheimer's? Genes Hold Key

But Genes Aren't Destiny, Study of Identical Twins Shows
By
WebMD Health News
Reviewed by Louise Chang, MD

Will you ever get Alzheimer's disease? Genetics may have the answer. The genes you've inherited carry most of the risk, an identical-twin study shows.

It's surprising news. While people clearly inherit Alzheimer's disease risk, most researchers have given equal blame to "environmental factors." These factors may include things we encounter, things we do or don't do, or diseases we develop.

But an international study of nearly 12,000 Swedish twin pairs -- a fourth of them identical twins -- now finds that some 80% of Alzheimer's risk is genetic. University of Southern California psychologist Margaret Gatz, PhD, and colleagues report the findings in the February issue of Archives of General Psychiatry.

"It appears that genetic influences outweigh environmental influences in relative importance for Alzheimer's risk," Gatz tells WebMD.

What this means is that close relatives of people with Alzheimer's disease are at much higher risk of getting the disease than people without such a relative. What it does not mean is that if you have such a relative, you're doomed to get Alzheimer's.

"'Genetic' does not mean cast in stone," Gatz tells WebMD. "In no way is having a relative with Alzheimer's disease -- even a genetically identical twin -- a guarantee that a person is going to get Alzheimer's disease."

When 1 Twin Has Alzheimer's

Gatz's team screened for dementia among twins over age 65 in the Swedish Twin Registry. They then tested each twin with dementia for Alzheimer's disease. Extensive medical and lifestyle information was available for each study participant.

The researchers found that genetic inheritance explained about 80% of Alzheimer's risk. Environmental factors not shared among twins explained the other 20%, as might be expected.

But even among identical twins -- who share the same genetic makeup -- Alzheimer's disease in one twin did not mean the other twin inevitably got the disease.

Among male identical twins, when one brother had Alzheimer's disease, the other developed the disease 45% of the time.

Among female identical twins, when one sister had Alzheimer's disease, the other developed the disease 60% of the time.

The difference between men and women, Gatz says, is simply that women live longer and thus have a better chance of surviving long enough to get Alzheimer's disease.

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