Major twin study: Genes matter, but don’t rule out environment
Alzheimer’s disease is strongly influenced by genes but other factors also play a major role, concludes a study of nearly 12,000 pairs of Swedish twins age 65 and older appearing in the February Archives of General Psychiatry.
When one twin developed Alzheimer’s, the researchers found the other twin’s risk of developing the disease ranged from 58 to 79 percent. The risk tended to run higher for identical twins, who are born with virtually the same genes, and lower for fraternal twins who share, on average, about 50 percent of their genetic material.
The risk was similar for male, female and mixed-sex twins, suggesting the same genes contribute to risk in men and women.
“Scientists love to look to twins for help in understanding how much genes matter,” says William H. Thies, Ph.D., Alzheimer’s Association vice president, medical and scientific relations. “In general, the more likely both twins are to develop a disorder, the stronger the genetic factors. While this study falls into line with others suggesting a significant role for genes in Alzheimer’s, we have to recognize some special issues involved because Alzheimer’s is so closely tied to advancing age. With nearly 50 percent of all people over age 85 affected, you’re going to find growing percentages of both twins diagnosed over time just because your twins are getting older.”
The timing of Alzheimer’s also differed for identical and fraternal twins. On average, identical pairs tended to be closer to one another in age of onset than fraternal twins, but the range varied significantly for both groups.
This study did not shed any light on additional genes that might be linked to Alzheimer’s or on specific environmental or lifestyle factors that might influence risk. The National Institute on Aging (NIA) has launched a late-onset genetics initiative to help clarify the role of genes, and the Alzheimer’s Association Maintain Your Brain® public health and education campaign offers information on strategies to support brain health and possibly help reduce the risk of dementia.
