Alzheimer’s disease seems to be highly heritable, and genetic factors may also influence the timing of the disease, concludes a large study of twins published this week.
The study involved 11 884 pairs of twins aged 65 years or older in the Swedish twin registry, including 392 pairs in which one or both members had Alzheimer’s disease (Arch Gen Psychiatry 2006;63:168&;74). All individuals were screened for cognitive dysfunction. Participants with suspected dementia and their twins then underwent clinical diagnostic evaluations for Alzheimer’s disease.
The results indicated that Alzheimer’s disease was associated with genetic factors in 58&; to 79&; of cases. The variation was explained by non&;shared environmental influences. After adjusting for age, the researchers found no significant differences between men and women in prevalence or heritability of Alzheimer’s disease. The rate of concordance was higher in women, reflecting their greater longevity.
The average age at onset of the disease was 78.1 years in identical (monozygotic) and in fraternal (dizygotic) pairs of twins who both developed the disease. The age at onset was more similar in monozygotic pairs of twins than in dizygotic pairs. The average difference between twins in age at onset of disease was 3.7 years in the 25 sets of identical twins and 8.1 years in the 20 sets of fraternal twins. This led the researchers to conclude that genes had a role in the timing of the disease as well as in causing it.
Margaret Gatz, professor of psychology at the University of Southern California, Los Angeles, and lead author of the study, said: “These results from the largest population based twin study of dementia to date, including like and unlike sex pairs, have confirmed that heritability for Alzheimer’s disease is high and that the same genetic factors are influential for both men and women.”
However, Dr Gatz added: “Non&;genetic risk factors also play an important role and might be the focus for interventions to reduce disease risk or delay disease onset. There is important influence due to non&;shared environment, and age at onset in concordant pairs varies, all suggesting that there are non&;genetic lifestyle factors that can affect the risk and timing of Alzheimer’s disease.”
