A gene mutation linked to the accumulation of the peptide beta-amyloid in the brain - thought to cause Alzheimer's - has been shown to trigger the disease when individuals carry two copies and help protect against the disease when they carry only one, according to a study published in the journal Science last week. This is the first time that a recessive mutation - meaning that two copies of the gene variant are required to cause the illness - has been linked to Alzheimer's.

The researchers discovered the gene variant by examining the DNA of a family in which a 44 year old man carrying two copies of the gene, known as A673V, developed severe Alzheimer's, while his 88 year old aunt and several other relatives, carrying only one copy of the gene, remained healthy. Scientists have previously linked a small proportion of Alzheimer's cases to three other mutations in a gene called APP, which produces amyloid precursor protein; however these only required only one copy of the gene to cause the disease. Fabrizio Tagliavini and colleagues at the Carlo Besta National Neurological Institute in Milan, Italy, found that mixing the mutant and normal versions of the amyloid precursor protein together in a test tube resulted in far less beta-amyloid formation than either the mutant or normal versions of the protein alone, explaining why individuals with one copy of each seem less likely to develop Alzheimer's disease.

'What makes it interesting is that you can have this apparently relatively innocuous change and end up with some individuals having full expression of the disease, and in others, protection,' commented Steve Snyder, an Alzheimer's researcher at the National Institute on Aging, in Bethesda, Maryland, US, who was not involved in the study. 'It calls upon the entire Alzheimer's disease genetics community to take a fresh look at the number of genes involved,' he told Bloomberg Press.

If scientists can develop a means of delivering a copy of the normal version of the protein into the cells of patients with two copies of the defective gene, it could help to prevent the onset of Alzheimer's, however extensive research will be needed before a therapy can be developed for humans, stress the researchers.