From VOA Learning English, this is the Health & Lifestyle report.

　　An estimated two million people around the world have multiple sclerosis, a disease commonly called MS.

　　The disease can affect a person's brain, spinal cord, and optic nerves in the eyes. It can cause problems with eyesight, balance, muscle control, and other body functions. The severity often differs from one MS patient to the next.

　　The most severe form of the disease is called "primary progressive." Scientists have found that a single change in a person's genetic material can cause primary progressive MS. They say this finding could lead to a cure for a disease that cripples millions of people.

　　Researchers have long suspected that multiple sclerosis resulted from some kind of interaction among genes.

　　Individuals with that genetic interaction, they thought, were more likely than others to get sick. They also believed that something in the environment, such as a viral infection or lack of vitamins in the diet, could activate the disease.

　　But Canadian researchers have found that is not the cause of the most severe form of multiple sclerosis. With primary progressive MS, a single mutated gene can greatly increase the risk for developing the disease.

　　This single gene is called NR1H3.

　　The Canadian researchers are with the University of British Columbia in Vancouver. They reported the findings in the journal Neuron.

　　The researchers examined medical records for 2,000 Canadian families with family members who had MS. The investigation led them to two families that carried a mutation, a structural change, in the NR1H3 gene. Members of those families suffered from the most severe form of multiple sclerosis.

　　Medical geneticist Carles Vilariño-Güell was the lead writer of the report. He noted that 15 to 20 percent of MS cases were thought to be hereditary, but that has never been proven.

　　Vilariño-Güell says other studies have linked other genes to the disease. But those genes increased the risk of getting MS by, at most, three percent.

　　"With this mutation, your risk is 60- to 70-percent. So, that's a 600 to 700 (percent) increase in comparison to the general population. And that's what really makes this completely different from other genetic findings."

　　In other experiments, the researchers examined the NR1H3 gene in mice. Animals with a normal copy of the gene developed neurological problems when the gene was removed. The problems included a decrease in the production of myelin.

　　Myelin protects protein on the outside of neurons. It speeds the electrical messages that control movement. A common sign of MS is muscle weakness and a loss of coordination.

　　Many research teams are studying how to repair myelin in MS patients. Currently, drugs to treat it only slow down progression of the disease. They do not cure it.

　　Discovery of a single genetic defect as the cause of MS, Vilariño-Güell says, could mean it may one day be possible to repair the gene. This means curing the crippling disorder.

　　"You know, the more understanding we have of the mechanism of disease for all these families allows us to develop different treatments for different forms of the disease."

　　Researchers say they will continue their search for a single genetic cause for the most common form of multiple sclerosis, called "relapsing remitting MS."

　　They say their hope is that a cure could be discovered soon.

　　I'm Anna Matteo.

　　Jessica Berman reported this story for VOANews. Anna Matteo adapted it for Learning English. George Grow was the editor.