Environmental factors, rather than genetics, may play a key role in multiple sclerosis (MS), according to new research published in Nature. The US scientists sequenced the whole genome of one pair of identical twins, only one of the pair who had MS, but failed to find any notable differences, discovering little about the origin of the disease.

Scientists also found no differences in the levels of gene activity when they compared the immune cells of these twins and two other sets of similarly-affected twins.

Researchers had previously identified a handful of gene variants linked to a higher risk of getting MS, and all the twins in the study had at least some of them. 'Both twins came into the world with the same set of high risks for developing MS', said Dr Stephen Kingsmore, a lead geneticist on the study at the National Centre for Genome Resources, US, '....but those genetic factors seem to have been insufficient to cause disease on their own'. The finding 'points to some novel environmental trigger that must be very important to the disease'.

Exactly what environmental factors contribute to causing MS is still unknown. The leading theory is that a virus triggers the immune system reaction that leads to the disease. Smoking and Vitamin D deficiency have also been suggested as possible environmental contributors.

The findings do not mean that genetics plays no role in the disease: the new study is small - it examines only three pairs of twins and one type of immune cell known to be involved in MS. Also, some scientists feel that the analysis was too broad to catch what is likely a 'needle in the haystack' genetic cause of a very complicated disease.

Genes are also known to have some function in the disease - when one identical twin has MS, there is 30 per cent increased risk that the identical sibling will develop it. That compares with an increased risk of only five per cent for a non-identical twin.