MONDAY, Sept. 29, 2003 (HealthDayNews) -- A large study of Canadian twins has confirmed much of what experts already knew about multiple sclerosis -- that genes and environment both contribute to the disease's onset.
The crucial question of exactly how both forces interplay still eludes experts, however.
MS is one of the most common neurological disorders among young adults. According to the National Multiple Sclerosis Society, about 400,000 Americans have MS and the disease may affect 2.5 million people worldwide. The diagnosis of MS is most often made when the person is between the ages of 20 and 50, and women are more likely to be affected than men.
The symptoms of MS are due to the destruction of myelin, fatty tissue that protects nerve fibers and helps them conduct electrical impulses. Those symptoms can vary from person to person and from time to time in the same person. For instance, one person may experience abnormal fatigue, while another might have severe vision problems. A person with MS could suffer from loss of balance and muscle coordination, making walking difficult; another person with the disease might have slurred speech, tremors, stiffness and bladder problems, the society says.
Studies of twins, considered the gold standard for trying to distinguish genetic influences from environmental ones, have looked at MS but have generally been small.
The main distinguishing feature of the new study is its size. Over the past 20 years, the researchers succeeded in identifying and analyzing more than 80 percent of all Canadian twins with MS. This amounted to a total of 370 pairs, 120 of them male and 250 female.
The study appears in this week's issue of the Proceedings of the National Academy of Sciences.
Most of the findings are in accordance with previous data. For instance, the researchers found that the identical twin of an MS patient was more likely to develop the disease than a non-identical twin. Slightly more than 25 percent of identical twins with MS had a twin with the disease. Among fraternal twins, that rate was only 5.4 percent; among regular siblings, it was even lower (2.9 percent).
"This is about what other studies have reported," says Stephen Reingold, vice president for research programs at the National Multiple Sclerosis Society.
This "concordance" (when both twins had MS) was mostly limited to females. Thirty-four percent of female identical twins with MS had a twin with the disease, which was about 10 times the rate among female fraternal twins (3.8 percent). For male identical twins, that rate was only 6.5 percent.
Again, this is in line with previous information. "They found that by far, the majority of the identical twins with MS are female and that the minority are male, and they indicate that the reason for this is likely the fact that the disease itself is much more prevalent among women," Reingold points out.
No one is sure why females are more susceptible to the disease. Dr. George Ebers, the study's lead author and professor and head of the department of clinical neurology at the University of Oxford in England, suspects that the risk may actually come from a gene the mother passes on.
Because the concordance rate is higher among fraternal twins than among regular siblings, Ebers also hypothesizes that differences in the uterine environment may be involved. "It's a hint that something early in life, maybe even in gestation, might be playing a role in risk," he says.
Finally, the authors determined that "twinning" itself did not have a relationship to MS. In other words, being a twin does not mean you're at a higher risk of developing the disease.
Whatever the genetic factors at play, they are clearly only part of the story.
"Even in identical twins, which are largely genetically the same, the concordance rate is only 25 to 30 percent," Reingold says. "That means other things are going on."
More information
For more on MS, visit the National Multiple Sclerosis Society or the National Institutes of Health.
