Chicago - Swiss researchers said on Monday that they have pinpointed a second gene that, when defective, increases the risk of Alzheimer's disease.
The gene, known as CYP46, produces a protein that helps the brain process and break down excess cholesterol, which plays a role in the accumulation of proteins (beta-amyolids) found to clog the brains of Alzheimer's victims.
People with mutated forms of both the CYP46 and APOE4 genes have 10 times the risk of developing the disease than those without the mutations. Defective forms of APOE4, which also has a role in processing cholesterol, have previously been shown to increase Alzheimer's risk.
"These observations underscore a possible relationship between cholesterol and brain amyloid formation," study author Andreas Papassotiropoulos of the University of Zurich wrote.
The study, published in the Archives of Neurology journal, was based on examinations of hundreds of living and dead patients with and without the disease.
An estimated four million Americans are believed to suffer from Alzheimer's. The risk increases after age 60. Roughly three percent of men and women aged between 65 and 74 have Alzheimer's, with nearly half of those over age 85 afflicted.
The exact cause is unknown, but autopsies on the brains of victims find abnormal clumps and tangled bundles of fibres as well as high levels of beta-amyolids. Some researchers believe the accumulation of beta-amyolids may interfere with electric signals between brain cells, triggering many cells to die.
An accompanying editorial, written by Benjamin Wolozin of Loyola University Medical Centre in Maywood, Illinois, said the latest findings point to the body's inability to metabolise cholesterol as a probable culprit for Alzheimer's disease.
Cholesterol is already a risk factor for heart disease, Wolozin said, and the global epidemic of obesity may pose an added risk factor for the brain and the body's nervous system.