Tuesday March 11 2008
The study highlights the heritability of Alzheimer’s
“Alzheimer's risk doubles ‘if parents suffer’”, reads the headline in The Daily Telegraph. It says a study of 111 families where both parents had Alzheimer’s disease found that the children “had a 22.6% chance of developing the condition compared with the estimated 6-13% chance in the population as a whole”.
This large study has shown that the offspring of parents who both have Alzheimer’s have a higher risk of developing the condition compared to the general population. Of the people who were involved in this study, 23% developed it themselves, and this risk increased with age to 31% in those over 60 and 42% in those aged over 70. The full extent of the risk is not clear, as, at the time of the study, most of these people were yonger than 70. It is important to note that the people in this study may have been particularly prone to Alzheimer’s, as they had been referred to a specialist centre, and therefore may have been more likely to have had a strong family history of Alzheimer’s.
This study highlights the heritability of Alzheimer’s, a factor that can be used in further research towards understanding the causes behind this complex disease.
Where did the story come from?
Dr Suman Jayadev and colleagues from the Universities of Washington and Florida, and the VA Puget Sound Health Care System carried out the research. The study was funded by the National Institute on Aging, and the National Institutes of Health and Veterans Affairs. The study was published in the peer-reviewed medical journal: Archives of Neurology.
What kind of scientific study was this?
This was a retrospective cohort (group) study designed to investigate the risk of Alzheimer’s disease in adults whose parents both suffered from the condition.
The researchers identified 111 families in which both parents had probable or definite Alzheimer’s disease from the University of Washington Alzheimer’s Disease Research Center registry. The researchers then obtained all of the medical records for these families, including any brain imaging studies, autopsy results, and family histories. If the parents were still alive, they were assessed and interviewed. Relatives were also interviewed to obtain further family history details, including whether other people in the family had the disease.
The researchers then investigated if these parents’ children had Alzheimer’s disease. They identified 297 children who had been diagnosed with the disease, and documented when they had first shown signs of the disease (memory loss or behaviour changes). This information was confirmed with relatives.
Parents and children with DNA available were tested to see if they had the ε4 form of the APOE gene, as this form of the gene is associated with an increased risk of Alzheimer’s.
What were the results of the study?
The researchers found that about 23% of people whose parents both had Alzheimer’s disease had developed it themselves. The risk of Alzheimer’s increased with age with 31% of offspring over the age of 60 developing Alzheimer’s. This increased again to 42% among those aged over 70.
Most of the offspring (almost 80%) had not yet reached the age of 70, so the overall risk of Alzheimer’s is likely to be higher than 23%. People with other relatives who had Alzheimer’s were not at a greater risk of developing it than those who did not, but they did develop the disease earlier on average.
This study found a higher cumulative risk for developing Alzheimer’s than that seen in a previous study for groups of offspring with either one or no parents with the condition. There were only 17 offspring with DNA available and therefore too few to perform reliable statistical analyses.
What interpretations did the researchers draw from these results?
The researchers concluded that people with two parents who have Alzheimer’s disease are at a greater risk of developing the disease in adulthood than the general population.
What does the NHS Knowledge Service make of this study?
This relatively large study gives an idea of the risk of developing Alzheimer ’s disease in people whose parents both have the disease.
- The authors note that the people referred to the Alzheimer’s research centre may have been more likely to have strong family histories of the disease, and might be particularly prone to developing it. For this reason, these results may not apply to all people in the general population with two parents with Alzheimer’s.
- Diagnosing Alzheimer’s disease is a process of exclusion, which means that if a person has the right symptoms, all other potential causes have to be ruled out first before a diagnosis is made. The diagnosis can only be confirmed when brain tissue is examined on autopsy. Only about 22% of the parents and 10% of offspring had autopsy confirmed-diagnoses. It is possible that some cases would not be confirmed at autopsy, which would affect the risk levels.
- This study did not look at people with only one parent or no parents with Alzheimer’s. It is difficult to compare the risk of Alzheimer’s in the offspring in this study with those in similar studies, as they may have differed in ways other than whether their parents had Alzheimer’s. Despite this, it does seem that offspring in this study do have a considerably increased risk of developing the disease compared to the estimated risk for the general population.
Sir Muir Gray adds...
I don’t think this adds much to what we know, namely that genetic factors play a part in determining who will be affected by Alzheimer’s.