There is a genetic element to multiple sclerosis, as it is more common in family members of people with the disease.
Some say MS is up to 80 times more common in first degree relatives like brothers, sisters or children of a person with MS.
Among identical twins, if one has MS, the other is 300 times more likely to get the disease than an unrelated person is. This risk increases with the number of relatives a person has with MS, and of course with environmental factors such as smoking, which doubles the risk.
A Canadian group has examined the family risks of getting MS. Initial studies published in 1995 showed that the increased risk of MS in a family was due to genetic factors, not factors in the shared environment.
Screening 15,000 people with MS, the researchers compared the risk of MS in genetically related versus adopted family members.
Adopted relatives proved to have no higher risk of getting MS than the general population did. So it is clear that people do not “catch” MS.
More recently, the same group evaluated MS risk in 687 step-siblings of 19,746 MS index cases to determine whether any transmissible factor in a family environment might enable one to 'catch' MS.
The risk of MS in these step-siblings was indistinguishable from that of the general population. Conclusion: environment influences MS risk at a population level, not at a family level.
No single gene
A study of the genes involved showed that MS is not inherited as a result of a single gene, the way cystic fibrosis or muscular dystrophy are.
Susceptibility to MS results from the interaction of several genes. At present, predicting MS in offspring is not possible.
Many wonder whether a brother or sister of someone with MS is at increased risk of getting the disease.
The Canadian group has shown that the risk is higher if:
- A parent also has or had MS
- The disease occurred at an earlier age
- The sibling for whom we are estimating the risk of MS is female
Depending on background risk in the general population (for instance, it is higher in Canada than in Australia), the combination of factors above, and such things as whether the person smokes and has a low vitamin D level, this risk may be as high as 1 in 5 or 1 in 10.
So, is MS hereditary? Well, yes to some extent, but the good news is that the risk may be very substantially reduced.
Ebers GC, Sadovnick AD, Risch NJ. A genetic basis for familial aggregation in multiple sclerosis. Canadian Collaborative Study Group. Nature 1995; 377:150-151.
Dyment DA, Yee IM, Ebers GC, et al. Multiple sclerosis in stepsiblings: recurrence risk and ascertainment. J Neurol Neurosurg Psychiatry 2006; 77:258-259
Ebers GC, Kukay K, Bulman DE, et al. A full genome search in multiple sclerosis. Nat Genet 1996; 13:472-476.
Sadovnick AD, Yee IM, Ebers GC, et al. Effect of age at onset and parental disease status on sibling risks for MS. Neurology 1998; 50:719-723.