Two Parents With Alzheimer's Raises Child's Risk

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"A majority of adult children in these families haven't reached 70 yet, and that's when Alzheimer's really begins to become a problem," Bird noted.

Among the 240 children who have not developed Alzheimer's, 78.8 percent have not reached 70. This could mean that the estimate of 22.6 percent is really an underestimate of their true risk, Bird said.

If other family members also develop Alzheimer's disease, the children in the study were more likely to develop the disease earlier. If only the parents developed Alzheimer's, the typical age of onset among the children was 72. However, if one parent also had a family history of Alzheimer's, the typical age of disease onset in their child was much lower -- about age 60. And if both parents had other family members who developed Alzheimer's the typical age of onset for the children was 57, the researchers found.

But one expert said the study still hasn't teased out the role genes play in Alzheimer's disease.

"With early onset, Alzheimer's disease is typically clearly inherited, but the vast majority of the disease victims are older and the genetic factors involved are apparently weaker and evidently not powerful enough to cause early disease," said Greg M. Cole, associate director of the Alzheimer's Disease Research Center at the University of California, Los Angeles.

Since the majority of the children in the study still have not reached their 70s and 80s -- when Alzheimer's claims most of its victims -- it's still not clear how powerful the combined genetic effect will be, Cole said.

"The real value of this approach may be that additional and larger studies will allow us to find these weaker genetic risk factors as they act in concert to cause Alzheimer's and perhaps any environmental factors that are able to counteract them," Cole said. "Is there more than luck to the secret of the children who inherit risk of Alzheimer's disease from both parents and yet manage to escape the disease?" he wondered.

Another expert wasn't surprised by the findings.