In a new study, researchers found that people with dementia – whose parents also had dementia – develop symptoms an average of six years earlier than their parents.
Factors such as education, blood pressure and carrying the genetic variant APOE4, which increases the risk of dementia, accounted for less than a third of the variation in the age at the onset – meaning that more than two-thirds remains to be explained.
The research was conducted by a team from the Washington University School of Medicine in St. Louis.
Alzheimer’s disease is the most common cause of dementia, affecting an estimated 5.8 million people in the United States.
Between 10% and 15% of the children of Alzheimer’s patients go on to develop symptoms of the disease themselves.
A person’s chance of developing dementia is influenced by family history, variations in certain genes, and medical conditions such as cardiovascular disease and diabetes.
But less is known about the factors that affect when the first symptoms of forgetfulness and confusion will arise.
In the study, the team studied people with dementia who were participating in research studies.
They identified 164 people with dementia who had at least one parent who had been diagnosed with dementia.
Using medical records and interviews with participants and knowledgeable friends or family members, the researchers determined the age at onset of dementia for each participant and his or her parent or parents.
People with one parent with dementia developed symptoms an average of 6.1 years earlier than the parent had.
If both parents had dementia, the age at onset was 13 years earlier than the average of the parents’ ages at diagnosis.
As part of this study, the researchers analyzed a large set of known risk factors for Alzheimer’s disease.
They studied heritable factors such as ethnicity, race, genetic variants and which parent had the disease.
They also looked at education, body mass index, diabetes, cardiovascular disease, blood pressure, blood cholesterol level, depression, tobacco use, excessive alcohol use, and histories of traumatic brain injury.
All of the factors together only accounted for 29% of the variability, meaning that most of what influences the age of dementia onset remains to be identified.
Intriguingly, the researchers found that people who were diagnosed with Alzheimer’s disease at unexpectedly younger or older ages than their parents were more likely than people diagnosed at the expected age to have certain mutations in Alzheimer’s genes – although it wasn’t clear what effect these mutations have.
The team says by learning more about the effect of these genes on Alzheimer’s disease, we may be able to develop novel treatments.
The lead author of the study is Gregory Day, MD, an assistant professor of neurology.
The study is published in JAMA Network Open.
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