A Canadian study of twins published in the New England Journal of Medicine (December 1986) has provided the strongest evidence yet of a genetic link for multiple sclerosis (MS). The study found that in nearly 26 percent of identical twins studied both had MS if one did, while in only 2.3 percent of fraternal twins both had the disease if one did. This was a very significant finding, since identical twins come from a single egg and share all their genes, while fraternal twins come from separate eggs and have different genes. The study also showed that in some cases, the "healthy" identical twin had MS lesions, although the disease had never caused symptoms.
In the last ten years, researchers have found that the white blood cells of some people with MS have different human leukocyte antigens (HLA) than those of people without the disease. The HLA system is thought to be connected to the genes that control the immune system. The patterns of these antigens are inherited and can be traced much like blood types.
In the United States and northern Europe, three different HLA antigens are found in higher levels in people with MS than in those without the disease. MS populations in other parts of the world test higher for other HLA antigens. Obviously, the push is on to find if there is one H LA antigen found in the white blood cells of all people with MS and absent from all those without it. When and if this happens, the "susceptibility gene" will have been discovered.
While it is possible to inherit a genetic susceptibility to MS, it is not possible to inherit the disease. And even people who have all the necessary genes don't necessarily get MS. The disease, experts believe, must be triggered by environmental factors.
For MS to be considered a hereditary disease, the percentage of cases among all siblings, not just identical twins, would have to be 25 percent or more. Since it is not, in spite of the suspected genetic link, MS is not considered a hereditary disease.