For people with a common genetic variation, researchers have discovered signs of the possible onset of Alzheimer's before a patient would be clinically diagnosed by a doctor.
In people with the ApoE4 gene variation, one previously implicated as affecting the likelihood of Alzheimer's, researchers have been able to pinpoint some signs of memory loss beginning in the person's mid- to late-50s -- without the patient having full-blown Alzheimer's disease or dementia.
"[One could argue] we really captured for the first time the onset of Alzheimer's disease," explained Dr. Richard Caselli, a neurologist at the Mayo Clinic in Scottsdale, Ariz.
"What's passing as normal aging itself correlates with the most common genetic risk factor for Alzheimer's disease," he said, adding that the symptoms are noticeable in a clinical setting, but not in everyday life.
"It's not the sort of thing that you can look at somebody or they can look at themselves and know."
Researchers caution that when in interpreting the findings, one should keep in mind that people who had shown some memory loss were still functioning normally and having the gene did not impair anyone at an earlier age.
For the study, researchers followed and looked at data for 815 subjects, 317 of whom had the ApoE4 variant. They administered a variety of neuropsychological tests to the patients and found that memory was affected in some patients with the ApoE4 gene as they reached their late 50s and into their 60s.
The study results are published in the most recent issue of the New England Journal of Medicine.
"You can start to see cognitive impairment, memory decline, in ApoE4 carriers early, although they're not clinically diagnosed with dementia or mild cognitive impairment," said Rudy Tanzi, director of the genetics and aging unit at Massachusetts General Hospital. "I think it's important that this study has elegantly confirmed what many of us have been dancing around for years."
But researchers caution that ApoE4 is not a gene that by itself determines the fate of a person's brain.
"ApoE4 is not sufficient to give you the disease. It works together with not only your lifestyle and environmental factors…but it works together with other genetic factors, some of which confirm risk…but some of which confirm protection," said Tanzi.
Dr. Richard Lipton, professor and vice chair of neurology at the Albert Einstein College of Medicine concurred.
"Not everyone who carries the E4 gene develops Alzheimer's disease," he wrote in an e-mail to ABC News. "We have 100-year-olds in one of our studies who carry the gene and have no evidence of memory decline. And not everyone who develops Alzheimer's disease has an E4 gene; only about half of people who develop Alzheimer's disease carry an E4 gene."
While another Alzheimer's study published in the most recent New England Journal issue found patients who underwent testing for the ApoE4 gene and received counseling handled it well, researchers generally advised against getting such a genetic test.
"I do not recommend that people go out and have this test," said Caselli. "I think it's an important tool for research right now, but as of this moment there isn't any routine benefit people would get from this information.