Scientists say they have found a gene that causes a rare but inherited form of multiple sclerosis.

It affects about one in every thousand MS patients and, according to the Canadian researchers, is proof that the disease is passed down generations.

Experts have long suspected there's a genetic element to MS, but had thought there would be lots of genes involved, as well as environmental factors.

The finding offers hope of targeted screening and therapy, Neuron reports.

The University of British Columbia studied the DNA of hundreds of families affected by MS to hunt for a culprit gene.

They found it in two sets of families containing several members with a rapidly progressive type of MS.

In these families, 70% of the people with the mutation developed the disease.

Although other factors may still be important and necessary to trigger the disease process, the gene itself is a substantial causative risk factor that is passed down from parents to their children, say the researchers.

The mutation is in a gene called NR1H3, which makes a protein that acts as a switch controlling inflammation.

In MS the body's immune system mistakenly attacks the protective layer of myelin that surrounds nerve fibres in the brain and spinal cord, leading to muscle weakness and other symptoms.

Studies in mice show that knocking out the function of the same gene leads to neurological problems and decreased myelin production.

The researchers believe stopping the inflammation early might prevent or delay the disease. They already have drugs in mind that might do this by targeting the NR1H3 gene pathway.

These drugs are in development for other diseases, including cardiovascular disease and diabetes.

Researcher Dr Carles Vilarino-Guell said: "These are still early days and there is a lot to test, but if we are able to repurpose some of these experimental drugs, it could shorten the time it takes to develop targeted MS treatments."

He said the same treatments might help other patients with progressive MS - even if they don't have exactly the same gene mutation.

Dr Sorrel Bickley from the MS Society said understanding how genes influence a person's risk of developing MS is a really important area of research.

"Whilst the gene variant identified was associated with rapidly progressing forms of MS in the two families studied, the variant itself is rare and most people with MS won't have it. This research does however give us an insight into how progressive forms of MS develop, which could help to inform the development of new treatments in the future."