Versions of genes which protect against Alzheimer's have been found by British scientists in a breakthrough which lead to treatments to prevent the disease.

Previously scientists had found genes which raised the chance of developing Alzheimer's, and also mutations within those genes which switched off the effect, bringing the risk back to normal.

But it is the first time that a set of gene variants has been shown to protect in the general population.

Scientists at University College London (UCL) believe that in people over-60, carrying the gene variants would lower the risk of getting the disease from 10 per cent to roughly eight per cent.

Although it may not seem like a big difference, if a treatment could be created which mimics the effect, it would bring the number of people contracting the disease down from 10 in 100 to eight in 100, potentially sparing thousands of people.

“Finding DNA variants which modify the risk of Alzheimer's disease is useful as it may help us develop drugs which target the same proteins,” said Professor David Curtis, of the UCL Genetics Institute.

“Simultaneously, researchers at UCL and across the globe are finding ways to detect the earliest stages of Alzheimer's disease, before it causes any problems.

“As our understanding improves, there may be opportunities to intervene with treatments to prevent the disease from progressing.”

Around 850,000 people have dementia in Britain, and most people have Alzheimer’s disease. But despite numerous trials, no treatment to delay, prevent or reverse the condition currently exists.

The new research studied the DNA of 10,000 over-60s, half of whom had Alzheimer’s disease and half who were healthy.

They looked at variations in 15,000 genes, of which they found more than one million, and then worked out which were more common in people with or without Alzheimer’s.

The team discovered that a handful of gene variants reduced a type of protein that damages cell survival.

Previously, scientists had noticed that if they dialled down the same protein in mice and rats, the animals were protected against Alzheimer’s but they did not know that genes existed in humans that were having a similar effect.

The findings suggest that drugs which mirror the impact may also reduce the risk of Alzheimer’s in humans.

“These results are quite encouraging,” added Dr Curtis.

“Here’s a natural experiment in people that helps us understand how Alzheimer’s disease develops. As some people have these genetic variants and some don’t, we can see that the impact of having particular variants is a reduced likelihood of developing Alzheimer’s disease.”