The same genetic influence operates for both men and women, reported Margaret Gatz, Ph.D., of the University of Southern California here, who headed a collaboration of U.S. and Swedish researchers. The study included male and female monozygotic twins, male and female dizygotic twins, and unlike-sex twins.
Environment and lifestyle also play important roles in Alzheimer's development, particularly perhaps for timing of disease onset, Dr. Gatz and colleagues reported in the February issue of the Archives of General Psychiatry. Investigators at the Karolinska Institute in Stockholm also participated in the research.
Their study screened all twins in the Swedish Twin Registry ages 65 and older for dementia -- nearly 12,000 twin pairs in all. The researchers then focused statistical analysis on 392 pairs in whom one or both members had Alzheimer's. This sample is four times larger than any other ever reported, the researches said.
Overall, genetics appeared to play a role in 58% of cases (95% confidence interval=19%-87%). However, when considering only twins who did not share environmental influences, genetics appeared to account for 79% of cases (95% CI=67%-88%). That figure is not 100% because lifestyle factors also influence development of Alzheimer's, the researchers said.
When the researchers considered only unlike-sex pairs of twins, the genetic influence held, "providing no evidence that different sets of genes are important for men and women," the authors said.
In addition, within pairs concordant for Alzheimer's, intrapair difference in age at onset was significantly greater in dizygotic than in monozygotic pairs, suggesting genetic influences on timing of the disease.
The research bolsters results of recent studies that yielded similarly high estimates of the inheritability of Alzheimer's, they added.
"The results of the present study also provide important evidence for the extent to which environment influences disease occurrence," the authors concluded. "Concordance is not perfect; there is important influence due to nonshared environment, and age at onset in concordant pairs varies, all suggesting that there are nongenetic lifestyle factors that can affect the risk and timing of Alzheimer's disease."
- Reviewed by Zalman S. Agus, MD Emeritus Professor at the University of Pennsylvania School of Medicine
Archives of General PsychiatrySource Reference: Gatz M et al. Role of genes and environments for explaining Alzheimer disease. Archives of General Psychiatry. 2006; 63:168-174.