October 1, 2003
Research Programs Department
The National Multiple Sclerosis Society
A large, long-term study involving 370 Canadian twin pairs in which one or both have MS confirmed the risk of a twin developing MS when his or her co-twin has MS:
The most comprehensive study of its kind has confirmed the probability faced by the identical twin of a person with multiple sclerosis that they, too, will develop the disease. The study, involving 370 Canadian twin pairs in which one or both have MS, showed that in identical twins, the overall risk is about 25%, or one in four, and was highest in females identical twins, at 34%, or about one in three.
This study, spanning two decades, is an important confirmation of both the genetic underpinnings of MS and of the fact that a person must encounter some other, non-hereditary factor to develop the disease. The study was published in the Proceedings of the National Academy of Sciences by Drs. C.J. Willer, George C. Ebers and colleagues from the Wellcome Trust Centre for Human Genetics at the University of Oxford (United Kingdom), along with collaborators at Stanford University, the University of British Columbia and the Canadian Collaborative Study Group. (Published the week of September 29, 2003 in the journal’s online Early Edition.)
Multiple sclerosis is an unpredictable neurological disease of unknown cause, which is thought to occur in persons with a genetically determined susceptibility to the disease. MS strikes women more often than men, usually in early adulthood, and involves destructive immune attacks against the central nervous system. While the disease tends to occur more often than expected in families, the genetic basis of susceptibility is unknown. Studies of twins and siblings are an important source of information to help define the genetic underpinnings of MS.
This large twins study, supported by the Multiple Sclerosis Society of Canada’s Scientific Research Foundation, tracked the disease concordance (meaning that both have the disease) between identical twins, non-identical twins, and between non-twin siblings. The general risk of MS in a person who does not have a sibling or parent with MS in Canada is about one-tenth of 1%, or 1 in 1000. In this study, the risk of MS in the non-identical twin of a person diagnosed with MS was found to be 5.4%, or about 1 in 20. (This risk is slightly higher than the known risk of 2.8% in non-twin siblings of a person with MS, but the difference was not statistically significant.)
The risk of MS jumped to 34% when identical twins were female. This is at least in part because MS is known to occur in two to three times as many women as men. Female co-identical twins were far more likely to both have MS than female non-identical twins, but in males, the risk of MS in identical twins was reported to be the same as in non-identical twins. This finding was unexpected, but requires confirmation: The small numbers of male twins in the study and the lower prevalence of MS in males in general might be confounding factors.
This important, long-term study of a unique population of individuals at risk for developing MS contributes to our growing understanding of this complex disease. As this and previous studies have shown, even in identical twins (who possess the same arrangement of genes), other factors are at play in helping to determine whether any individual will develop MS. Continuing research is helping to uncover the mechanisms underlying multiple sclerosis, how genes contribute to susceptibility, and how to better treat it.
Studies of genetics and of gender differences in MS are the focus of
major research initiatives supported by the National Multiple Sclerosis
Society. See the Society’s Web site at the following link: http://www.nationalmssociety.org/Research-Targeted.asp.)
Copyright © 2003, The National Multiple Sclerosis Society