UC Study Links Spinal Defect to Abnormality
UC Irvine researchers report that they have identified a genetic abnormality they believe causes spinal muscular atrophy, the most common genetic cause of death in infants.
“This is a long-anticipated finding that should quickly improve accuracy in diagnosis of the forms of SMA,” said Donald S. Wood, director of science technology for the Muscular Dystrophy Assn. “Most important, it will accelerate our continuing search for a form of treatment for SMA.”
No treatment for the disease now exists.
The disorder, which in its most severe form is uniformly fatal before the age of 2, affects an estimated 20,000 Americans, making it the second most common lethal genetic disorder after cystic fibrosis. Although the disease affects one in 10,000 births, it is estimated that one in 40 adults carries the defective gene.
The identification of the gene is reported by a team led by UC Irvine molecular biologist John J. Wasmuth in the January issue of the journal Nature Genetics, released today.
Wasmuth’s team believes that the disorder is caused by the loss of segments of a gene called XS2G3 on chromosome 5, one of the 23 pairs of chromosomes that make up the human genetic blueprint.
They studied DNA from 29 patients with the most severe form of the disease, called Type I SMA, and compared it to DNA from 235 healthy people. They found that 17 of the 29 patients were missing fragments of XS2G3 in both copies of the gene (one from each parent). In contrast, only two of the 235 healthy people--both of them known carriers of the disorder--were missing fragments of the gene, and only in one copy.
The Muscular Dystrophy Assn.’s Wood speculated that fragments are missing in all 29 patients but that researchers were unable to detect them in 12 cases because of the extreme complexity of that area of chromosome 5. He added that several other teams are expected to report soon on research that may verify the UC Irvine team’s findings.
There are four distinct forms of SMA, varying in severity. Type I, known as Werdnig-Hoffman disease, is characterized by generalized muscle weakness, trouble swallowing and sucking, breathing distress and paralysis of legs and arms within three months after birth. It is usually fatal by age 2. Type II strikes in the first six months to three years and is usually fatal during childhood.
Types III and IV strike later in life. Although they are crippling, they do not affect life span. The researchers speculate that the severity of the disease is controlled by the amount of DNA missing from the gene.
Other members of the team included molecular biologists Arthur H.M. Burghes of Ohio State University and Louise R. Simard of the Hopital Sainte-Justine in Montreal.
