A discovery about the genetic causes of Alzheimer's could lead to new treatment.

Researchers who tested DNA samples from 6,000 volunteers found a gene that seems to be important in late-onset Alzheimer's, the form that affects 90 per cent of sufferers. Scientists hope that the gene, SORL1, will provide a new target for drugs aimed at treating the brain condition.

The discovery was described as "exciting" by the Alzheimer's Society.

A mutant gene called ApoE4, identified in 1993, is also known to be linked to late-onset Alzheimer's. However, as many as 60 per cent of sufferers do not carry it.

When it works properly, SORL1 recycles a brain substance called amyloid precursor protein, or APP. Defects in the gene allow APP to avoid recycling and generate toxic amyloid beta peptides. These protein-building blocks are thought to produce the amyloid deposits which accumulate in the brain and are associated with Alzheimer's.

A team of scientists found that mutant versions of SORL1 were more common in people with late-onset Alzheimer's than other individuals of the same age. The five-year study uncovered the link in four distinct ethnic groups - Carribean-Hispanics, north Europeans, African-Americans and Israeli-Arabs.

The research leader, Richard Mayeux, of Columbia University, New York, said: "The importance of the finding is that it opens new pathways to explore the cause as well as potential targets for treatment.

"This appears to be the fifth Alzheimer's disease gene, and there are likely to be other important genetic variants that need to be identified before the picture is complete."