Original article published in Alzheimers.gov
Among sets of twins in which one has dementia, the other may have an increased risk of a shortened lifespan, even if that person does not have the disease, according to an NIA-funded study. The findings, published in Alzheimer’s & Dementia, suggest the higher risk of death following a dementia diagnosis may be explained in part by genetic and environmental factors shared by siblings in the same family.
Previous research revealed reduced life expectancy among individuals with dementia. However, the complex genetic and environmental mechanisms underlying dementia and mortality are not fully understood. Twins offer the opportunity to explore how genetic and environmental factors contribute to dementia because identical twins share all their genetic material, and fraternal twins (as well as full siblings) share on average approximately 50%. Additionally, twins raised in the same household generally share environmental influences such as education, physical activity, diet, and pollution exposure. In this study, a team of scientists from the University of Southern California and the Karolinska Institute in Sweden examined data from participants of the Swedish Twin Registry, a cohort study of more than 45,000 Swedish twins. They identified pairs of twins in which one had dementia and one did not, including 90 pairs of identical twins and 288 pairs of fraternal twins.
The researchers analyzed the data in two ways: first by comparing twins with dementia to unrelated individuals without dementia, and next by comparing twins with dementia to their twin siblings without the diagnosis. They found that individuals with dementia had a shortened life expectancy compared to those without dementia, which aligns with previous research. However, when looking at discordant pairs (one twin with dementia and the other without), they found that undiagnosed twins had an elevated risk of death. In identical twins, both siblings had a similar survival time. For these pairs, a dementia diagnosis in only one twin may be due to nonshared environmental factors (e.g. differences in lifestyle, education, or occupation). In fraternal twins, the one who had not developed dementia had a slightly shortened life expectancy, compared to those who did not have a twin sibling with dementia.
After completing these analyses, the scientists compared mortality risk between undiagnosed twin siblings of those with dementia to twins from pairs where neither had dementia. They discovered that twins who had a co-twin with dementia had increased mortality risk compared to twin pairs in which neither had a dementia diagnosis, even after adjusting for age, education, and self-reported health status, further indicating a genetic influence on lifespan.
Overall, the study’s findings indicate that receiving a dementia diagnosis is associated with a shortened lifespan for both diagnosed and undiagnosed twins, and this association is influenced by both genetic and environmental factors shared by siblings. Limitations include the study’s focus on Swedish twins, which, while nationally representative and large, may limit generalization to other populations, and the inability to focus on specific types of dementia due to sample size restraints. Nonetheless, this study provides valuable insights into the relationship between dementia and mortality that may aid in care planning and decision-making for individuals, families, health care providers, and policymakers.
This research was supported in part by NIA grants R01-AG060470-05, R01-AG060470-04, and T32-AG000037-45.
