People with the TREM2 gene variant lost brain tissue twice as fast as healthy elderly people, according to research published in the New England Journal of Medicine.
"This is the first study to use brain scans to show what this gene variant does, and it's very surprising," said co-author Paul Thompson of the University of Southern California.
"This gene speeds up brain loss at a terrific pace."
Thompson and colleagues did magnetic resonance imaging (MRI) scans on 478 adults, whose average age was 76, over the course of two years.
They found that mutation carriers lost 1.4 percent to 3.3 percent more of their brain tissue than non-carriers, and the deterioration happened twice as fast.
Brain tissue loss was concentrated in memory centers of the brain, including the temporal lobe and hippocampus.
The TREM2 variant was first described in January as rare mutation, existing in about one percent of the North American and European population, that could triple a person's lifetime risk of Alzheimer's disease.
Subsequent study has confirmed the mutation's link to Alzheimer's in blacks as well.
The genetic mutation has also been linked to an increased likelihood of Parkinson's disease and a rare form of early brain decline called Nasu-Hakola disease.