Scientists have identified a genetic mutation in one of the 23,000 human genes that can double the risk of a stroke, which kills more than six million people worldwide each year and is the second top cause of death in developed countries.

A study of thousands of stroke patients in Britain and Germany found a link between the most common type of stroke – a blocked blood vessel leading to the brain – and a genetic variation in a gene known as HDAC9. Although strokes are known to run in families, this is one of the first studies to identify a precise DNA variant in the human genetic code that doubles a person's risk of developing a blocked artery supplying vital oxygen to the brain.

The HDAC9 gene was already known to be involved in the formation of muscle tissue and the development of the heart, but the latest research suggests it is also implicated in a particular kind of illness called large-artery ischaemic stroke.

Scientists said the DNA variation occurs on about 10 per cent of the chromosomes carrying the HDAC9 gene. People who inherit two copies of the variant, one from their mother and one from their father, face twice the risk of developing this type of stroke than people with no copies of the gene variant, they said. The study, published in Nature Genetics and funded by the Wellcome Trust charity, used a relatively new scientific technique of genome-wide association studies to compare the DNA of some 10,000 stroke patients with the DNA of 40,000 people who have not had a stroke.

Stroke, or cerebrovascular disease, is among the three most common causes of death and is a main cause of chronic disability and paralysis, especially in the elderly. Ischaemic stroke, caused by a blocked artery to the brain, can be caused by several different kinds of disease, the most common being a narrowing of blood vessels in the neck or head. About 80 per cent of strokes are ischaemic and one of the most common causes is a blockage in the large cerebral or carotid arteries.

Studies on identical twins and members of families with a history of stroke have established a clear genetic component but until now the nature of the precise mutations involved has eluded medical scientists. The hope is that by knowing the genetic faults that increase stroke risks, researchers can design better drugs.

"Our study shows that the different subtypes of stroke could involve quite different genetic mechanisms. This is really fascinating, and if it holds up more generally will move us closer to personalised medicine," Professor Peter Donnelly, of the University of Oxford, said.