THURSDAY, Feb. 2 (HealthDay News) -- People who develop Alzheimer's disease late in life may have the same gene mutations linked to the inherited, early onset form of the condition, according to a new study.

Researchers from Washington University School of Medicine in St. Louis suggested this could lead to changes in the way Alzheimer's disease is classified.

"It's always been somewhat arbitrary, figuring out where early onset ends and late-onset begins. So I no longer look at early- and late-onset disease as being different illnesses. I think of them as stages along a continuum," said the study's senior investigator, Alison Goate, in a university news release. "I think it's reasonable to assume that at least some cases among both early- and late-onset disease have the same causes."

People who develop Alzheimer's at a younger age probably have more risk factors and fewer protective factors for the brain disorder than those who develop it later in life, Goate noted. In both age groups, however, the disease mechanism may be identical, she explained.

In conducting their research, published online Feb. 1 in PLoS One, researchers studied the genetic information of 440 families that had at least four relatives diagnosed with Alzheimer's disease.

Specifically, they analyzed five genes associated with dementia. Mutations in three of the genes they examined are a known cause of early-onset Alzheimer's disease. The other two genes studied have been linked to another form of inherited dementia, called frontotemporal dementia.

The investigators found rare variants in Alzheimer's-related genes in 13 percent of the samples analyzed.

"Changes in these genes were more common in Alzheimer's cases with a family history of dementia, compared to normal individuals. This suggests that some of these gene variants are likely contributing to Alzheimer's disease risk," said Goate, who is also co-director of the Hope Center Program on Protein Aggregation and Neurodegeneration.

According to the study's first author, Carlos Cruchaga, an assistant professor of psychiatry, "of those rare gene variants, we think about 5 percent likely contribute to Alzheimer's disease. That may not seem like a lot, but so many people have the late-onset form of Alzheimer's that even a very small percentage of patients with changes in these genes could represent very large numbers of affected individuals," he explained in the news release. "

The study authors also pointed out that they discovered some of the gene mutations associated with frontotemporal dementia in Alzheimer's patients, suggesting these patients were actually misdiagnosed.

The researchers suggested doctors should screen for genetic mutations among their patients with late-onset Alzheimer's who have strong family histories of the disease.

More information

The U.S. National Library of Medicine has more about Alzheimer's disease.