Genes and Environment Affect Alzheimer's Risk

July 21, 2004 (Philadelphia) -- A new study shows that there is much more to Alzheimer's disease than just genetics.

Speaking at a meeting of Alzheimer's experts, Brenda L. Plassman, PhD, says that her study shows that oftentimes identical twins -- exact genetic copies of each other -- don't both develop Alzheimer's disease.

She says that even when both twins do develop Alzheimer's disease, they don't do so at the same time. Plassman is associate research professor at the department of psychiatry at Duke University Medical Center in Durham, N.C.

"It is not uncommon for a twin to develop Alzheimer's disease five years after his brother was diagnosed," she says. Even more intriguing, "in some twin pairs -- even identical twins -- one twin develops Alzheimer's disease and the other never develops the disease."

Plassman says scientists know that genetics account for only a part of the 4.5 million Alzheimer's disease patients in the U.S., but they are unsure about the real impact of factors such as education, diet, and socioeconomic status.

"Twins, especially identical twins, offer an excellent opportunity to study those environmental factors that may play a role in the risk of Alzheimer's disease," she says. "From conception through much of their early life, identical twins 'share' the same environment."

Since identical twins have the same genes, any difference in Alzheimer's disease risk would likely be caused by some outside, environmental factor. Therefore, uncovering these factors -- likely the reason for the five-year gap between diagnoses of Alzheimer's disease in twins -- could help develop a treatment aimed at slowing or preventing the disease in the general population.

Advances Could Delay Onset by 5 years

Moreover, she says that a strategy that could delay Alzheimer's disease onset by five years could significantly reduce the Alzheimer's disease burden both nationally in terms of treatment costs and on an individual level in terms of "maintaining the brain."

William Thies, PhD, vice president of medical and scientific affairs for the Alzheimer's Association, tells WebMD that identifying a factor that could delay Alzheimer's disease onset by five years could cut the number of cases in half. "The impact of five years is huge. Overnight, we could go from 4.5 million cases to 2.25 million cases," he says. Likewise, it would mean reducing Alzheimer's-associated costs which now are estimated at nearly $100 million a year to half that amount.

In this latest study, Plassman reports on 69 pairs of identical twins and 53 pairs of fraternal twins (two separate eggs fertilized at the same time and nonidentical). Fraternal twins are no more genetically similar than ordinary siblings. She says that in 28 of the identical pairs and 11 of fraternal pairs both twins have Alzheimer's, but there was a five-year gap between diagnosis of first and second twin. In both identical and fraternal twins the first Alzheimer's diagnosis occurred at about age 70.