Three studies funded by the National Institute on Ageing in the USA are published this month and examine the tau gene.

They show that the gene, when it is defective, causes a tangle of protein which clogs up and eventually kills brain cells. Healthy forms of the gene help to carry chemical messages in and between brain cells.

Presenile dementia

One report, published in the Proceedings of the National Academy of Sciences, shows how the mutated gene can affect presenile dementia, a rare inherited disease whose symptoms include dementia, palsy and rigid muscles.

Scientists at Cambridge University say the research shows the tau gene has a big effect on dementia diseases which cause muscle spasms and shaking.

Train wreck

Dr Marcelle Morrison-Bogorad, of the National Institute of Ageing, said the stringy tangle of protein was like "a train wreck in the middle of a city, affecting all the streets coming into the centre".

In presenile dementia, the "wreck" is found in the front part of the brain, which controls conscious thought, while in Alzheimer's it is found in the hippocampus, which is linked to memory.

Around 650,000 people in the UK have Alzheimer's. Most are over 65. Two per cent of people aged between 65 and 75 have the progressive disease and 20% of over 75s develop it. There is no cure.