Environmental and genetic factors are both important triggers in the development of late-onset Alzheimer's disease (AD), a new US study of twins confirms. Researchers at Duke University Medical Centre presented their research, carried out on more than 200 male twins who served during World War II, at the recent International Conference on Alzheimer's Disease and Related Disorders. They found that identical twins, who share the same genes, both develop AD by their late 70s about 40 per cent of the time; non-identical twins, who are no more genetically alike than ordinary brothers and sisters, both developed the disease only about 20 per cent of the time.
The researchers found that 28 of 69 pairs of identical twins both had AD, while 11 of 53 pairs of non-identical twins both had the condition. In both sets of twins, the first diagnosis occurred around the age of 70, but there was then an average of five years before the second twin was diagnosed. This figure is significant, since a previous study highlighted the fact that delaying the onset of AD by just five years could reduce the number of cases by half over the next 50 years. 'Data from these World War II veterans suggest that even barring any conscious effort to change the course of Alzheimer's, there are environmental factors at work that can affect the age of onset by that critical five-year margin', said study leader Brenda Plassman.
The study also found that twins who both developed AD were more likely to carry the APOE-e4 gene variant, which is linked to an increased risk of the disease. The researchers now plan to look at factors that could affect the age of onset, including head injury, medication and other medical conditions such as diabetes and high blood pressure.
Another study presented at the meeting found that children of people with AD did not become more anxious when tested and told of their genetic risk for the disease. The researchers gave one group a risk assessment based on age, sex and family history, while another group also received information about their APOE gene. 'This study is a first step toward showing that genetic risk information for Alzheimer's can be communicated safely and effectively', said study leader Robert Green. 'As treatments are developed that prevent or delay Alzheimer's, determining who is at higher risk will become more important', he added.