Researchers say they have pinpointed the gene that causes the most common adult form of muscular dystrophy.
The disease, called myotonic dystrophy, affects about one in every 7,000 to 8,000 people worldwide. The discovery should lead to early diagnosis and perhaps eventual treatment and already provides more clues about why this inherited disease gets worse as it is passed from generation to generation.
Three research teams with members in the United States, Canada and Europe discovered the gene almost simultaneously. One of their reports will be published in today's issue of the journal Cell. The other two will appear March 6 in the journal Science.
"The new results will permit very precise methods for diagnosing the disease prenatally and before symptoms appear and identifying carriers" of the gene, said Dr. Henry Epstein of Baylor College of Medicine in Houston, a member of one of the research teams.
Experts caution however that the discovery does not immediately suggest any obvious ways of stopping or preventing the disease.
Myotonic dystrophy causes progressive weakness of muscles in the arms and legs, and sufferers may develop cataracts and diabetes.
"Treatment is not just around the corner," said Dr. David Brook of Massachusetts Institute of Technology, another co-discoverer of the gene.
However, the findings at least help scientists begin a systematic search for therapies.
"Knowing the gene permits one to work toward that goal," Epstein said. "It doesn't necessarily tell us we could design a specific drug right now, but it may offer that possibility."
Just two weeks ago, many of the same researchers reported the discovery of an abnormal section of gene that appears to be the critical flaw in this form of muscular dystrophy. Now scientists have isolated virtually the whole gene and provided more details about how it goes bad.
The discovery should allow them to figure out what role this gene normally plays in healthy people. Then scientists will be able to decipher the exact irregularities that occur when it is defective.
Doctors theorize that the gene contains the code for the body to manufacture an enzyme.
This substance, in turn, regulates the working of other crucial proteins that are found in cell membranes.
The researchers are uncertain whether the muscular dystrophy gene makes too much of this enzyme, too little or a defective form of it.
However, their work shows the abnormal form of the gene is active in the brain, heart and muscles, all parts of the body that are affected by the disease.
"It's an amazing, baffling situation," said Dr. Robert G. Korneluk of the University of Ottawa, who headed another of the teams of gene hunters.
The genetic defect involves the explosive copying of one tiny fragment of genetic code in this gene. The researchers found that the more times this segment is repeated, the more severe the disease.
Normally this section, known in genetic shorthand as CTG, is repeated five to 27 times in the cells of people who don't have myotonic dystrophy. Those with symptoms of the disorder have at least 50 copies. And those who are severely affected may have several thousand copies.
The number of repeated sequences increases as the gene is passed from parent to child. Brook said a grandparent with 50 copies of the gene may have few outward symptoms of the disease. He may produce a son with 200 or 300 copies and disease that requires he use a wheelchair by age 60. The son, in turn, may have a child with 2,000 copies and symptoms that emerge while a teen-ager.
The most common childhood muscle disease and best-known muscular dystrophy _ Duchenne muscular dystrophy _ is caused by defects in an entirely different gene. It usually shows up in children ages 2 to 6. It causes difficulty in walking; later, patients fall frequently and cannot run.
_ Information from World Book Encyclopedia was used in this report.