People born with a particular gene may have an increased risk of developing Alzheimer's disease later in life, according to researchers.
The brown amyloid plaques are linked to Alzheimer's. Pic: UCLA
Canadian and US scientists linked the illness with the SORL1 gene.
The gene produces a protein that may play a role in getting rid of the build-up of chemicals that can damage the brain.
The study, in Nature Genetics, suggested people with Alzheimer's have less of this protein in their blood.
There are estimated to be more than 750,000 people in the UK suffering from some form of dementia, and Alzheimer's disease is the most common form of the illness.
The most common symptoms are memory loss, confusion and problems with speech and understanding.
Most cases of Alzheimer's are defined as "late onset", meaning that symptoms start to show at 65 or older.
Investigations into the causes have centred on the formation of so-called "amyloid plaques" - clumps of a protein called A-beta peptide found in areas of damaged brain tissue.
One theory is that the process of controlling the production of A-beta peptide in the brain has gone wrong, as tissue taken from Alzheimer's patients has shown lower levels of natural chemicals thought to play a role.
The researchers searched for a genetic reason for the lower levels of these chemicals by examining the genes involved in their manufacture.
Several different versions of one of these genes, SORL1, were found, and, more importantly, in patients with Alzheimer's, a particular variant was more likely to be found.
Patients with Alzheimer's had lower levels of the SORL1 protein in their blood, and in the laboratory, the researchers showed that reducing the level of the protein in cells appeared to increase the production of the toxic A-beta peptide.
They say that all of this points to a people with this version of the SORL1 gene carrying a greater risk of developing Alzheimer's in old age.
Drugs and treatments
However, their finding does not offer a simple single gene that could be targeted to prevent or treat Alzheimer's - if verified, this would be the second gene found so far that appears to play a role in the development of the illness.
Clive Ballard, the director of research at the Alzheimer's Society in the UK, said that the finding was "exciting".
He said: "The study identifies a novel link between the plaques that develop in the brain of people with Alzheimer's disease and the SORL1 gene.
"Amyloid protein is at the core of the plaques that develop with Alzheimer's disease and is the popular target for research into Alzheimer's disease.
"This latest study is exciting because it suggests the SORL1 gene plays a significant role in the recycling and disposal of amyloid protein and late-onset Alzheimer's diseases.
"The results put the spotlight on an important new area for the development of drugs and treatment targets to tackle Alzheimer's disease."