Scientists have found the first genes linked with multiple sclerosis in more than two decades, in a study that raises the possibility of understanding the root causes of the debilitating nervous system disorder.

Two teams of researchers have each identified genetic variations in a pair of genes that substantially increase the chances of someone developing the disease, which is caused by the body's immune system attacking its own nerve cells.

Each variation appears to raise the probability of developing multiple sclerosis by between 20 and 30 per cent. Scientists believe the discovery is a major milestone which might lead to new forms of treatment. It is well known that there is a strong genetic basis for multiple sclerosis, and some genes have already been implicated, but the newly identified genes are directly linked with the immune system. This suggests that the discovery could lead to a better understanding of why the fatty sheaths that insulate the long fibres of nerve cells are attacked and destroyed by the immune defences of people with multiple sclerosis.

"The genetic factors that are already known to be associated with multiple sclerosis only explain less than half of the total genetic basis for the disease," said Simon Gregory, a molecular geneticist at Duke University in Durham, North Carolina.

"We have identified a gene that increases an individual's risk of multiple sclerosis by 30 per cent, and that this variant has an effect on the function of the gene. It is likely that variants of many genes are associated with the development of multiple sclerosis, so identifying a novel gene that is associated will be very helpful in understanding this complex disease," he said.

A second team studying the DNA of multiple sclerosis patients has identified variations in another gene which has also been linked to other autoimmune diseases - those where the body's immune system attacks its own tissues.

Both genes are involved in controlling the activity of a class of cells within the immune system known as regulatory T-cells. It is the first time that genetic changes to this group of genes have been associated with multiple sclerosis.

"Scientists are increasingly finding genetic links between autoimmune diseases that affect different tissues in the body, including type one diabetes and rheumatoid arthritis," said Professor David Hafler of Harvard Medical School in Cambridge, Massachusetts. "This study will likely spur further research into the connection between these seemingly separate conditions."

Both studies involved scanning the genomes of more than 20,000 healthy people and patients with multiple sclerosis in Britain, the rest of Europe and the US. It was made possible by the decoding of the human genome and with the development of sophisticated "gene chips" for scanning vast stretches of DNA for tiny variations.

The results are published in the journals Nature Genetics and the New England Journal of Medicine.