If both parents have Alzheimer’s disease, their children face an increased risk of developing the condition, a new study suggests.

Overall, 42 percent of offspring whose parents both had Alzheimer’s went on to develop the disease themselves by age 70, the researchers found.

The risk is also greater of developing the disease early if additional relatives had Alzheimer’s disease, researchers say.

Most experts agree that genetics plays a role in Alzheimer’s disease, but the degree to which genetics is responsible for the disease is still unclear.

“There probably is an increased risk of Alzheimer’s disease in the children of spouses that both have the disease,” said lead researcher Dr. Thomas D. Bird, a professor of neurology at the University of Washington School of Medicine in Seattle. “The exact magnitude of the risk, we don’t know yet.”

Bird’s group is trying to determine the genetic factors at play in Alzheimer’s disease. So far, only one genetic factor has been documented, Bird said. “The hope is, there will be others and they will be found,” he said. “Presumably, these children would have a higher concentration of those factors. So, that’s what ought to be looked for.”

The report appears in the March issue of the Archives of Neurology.

In the study, Bird’s team collected data on the grown children in 111 families where both parents had Alzheimer’s disease.

“There were 98 children who had gotten to age 70, and of that group 41 had developed Alzheimer’s disease. That’s about 42 percent,” Bird said. “We felt that’s pretty important.”

The researchers found that for the total group of 297 children, 22.6 percent had developed Alzheimer’s disease. That compares with an expected 6 percent to 13 percent of people in the general population who would be expected to develop the disease.

Alzheimer’s typically started at about age 66 in children with two affected parents, the study found, and the odds of developing Alzheimer’s rose as the children got older. In fact, 31 percent of those older than 60 developed Alzheimer’s, as did 41.8 percent of those older than 70.

“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.

“This new paper documents that the children of two affected parents do indeed have risks higher than the general population, as expected,” said Dr. Sam Gandy, chairman of the National Medical and Scientific Advisory Council of the Alzheimer’s Association.

“Plus, the paper goes on to provide direct support for the existence of an ‘additivity’ of the risk of each of the two parents. For children of two parents with Alzheimer’s, these data provide direct evidence that their risk of developing dementia is nearly one in two,” Gandy said.

(Sources: Thomas D. Bird, M.D., professor, neurology, University of Washington School of Medicine, Seattle; Sam Gandy, M.D., Ph.D., chairman, National Medical and Scientific Advisory Council, Alzheimer’s Association; Greg M. Cole, Ph.D., neuroscientist, Greater Los Angeles VA Healthcare System, and associate director, Alzheimer’s Disease Research Center, University of California, Los Angeles, David Geffen School of Medicine; March 2008 Archives of Neurology)