April 11, 2008
If both of your parents have been diagnosed with Alzheimer's, you are at increased risk of developing the disease as well. Those are the findings of a new study from the University of Washington in Seattle that looked at parents with dementia and their offspring.
"Because Alzheimer's disease is so common in the general population, it is not uncommon for both spouses to develop the disease," wrote the authors of the report, which appeared in the Archives of Neurology, from the American Medical Association.
The authors note that various genes have been linked to Alzheimer's, so it is not surprising that the disease might run in families. "Offspring of two such affected individuals would presumably carry a higher burden of these Alzheimer's disease-associated genes," they said.
Three genes have been identified that cause the early-onset form of the disease, which strikes people at a young age, often in their 40s or 50s. If you carry one of these genes -- called amyloid precursor protein, presenilin 1, or presenilin 2 -- you are almost certain to develop Alzheimer's. However, only 1 percent to 3 percent of all Alzheimer's cases represent the early-onset, inherited form of the disease.
Another gene, called APO-E4, has been linked to the far more common late-onset form of Alzheimer's, which strikes seniors later in life, typically in their 60s, 70s or older. While inheriting the APO-E4 gene increases your risk of developing Alzheimer's late in life, it is by no means a guarantee that the disease will develop. Indeed, about half of people with Alzheimer's disease do not carry the APO-E4 gene. Scientists are studying additional genes and other factors that may predispose to Alzheimer's as well.
In the current study, University of Washington researchers studied the frequency of Alzheimer's disease in adult children of 111 families in which both parents had been diagnosed with the disease. The age at which symptoms of dementia appeared were also recorded.
Of the 297 offspring surviving to adulthood, just over 22 percent developed Alzheimer's disease. That compared with an estimated 6 percent to 13 percent of the general population who would be expected to be diagnosed with Alzheimer's disease after age 65.
The average age at onset for children of couples with the illness was 66. The risk of developing the disease increased with age, with 31 percent of those older than age 60 affected and almost 42 percent of those older than age 70 affected.
"Of the 240 unaffected individuals, 189 (78.8 percent) had not yet reached age 70 years, suggesting that the incidence of Alzheimer's disease (22.6 percent) is an underestimation of the final incidence rate of Alzheimer's disease in this population," the authors wrote.
Having additional family members with Alzheimer's disease did not increase the risk of developing the disease, but was associated with a younger age at onset for those who did develop the illness. Children with no family history of the disease beyond the parents had an older age at onset (72 years) compared with those who had one parent and additional family members with the disease (60 years) or both parents with family history of the illness (57 years).
"The role of family history and the specific genes involved in this phenomenon require a better definition," the authors conclude. "Following these families as the offspring continue to age will provide increasingly informative data."
Jayadev S, Steinbart EJ, Chi Y-Y, et al.: "Conjugal Alzheimer Disease: Risk in Children When Both Parents Have Alzheimer Disease." Archives of Neurology 2008; Volume 65: pages 373-378.