Gains Made Against Muscular Dystrophy
Gene therapy can cure muscular dystrophy in mice, a finding that suggests it should also be safe and effective in humans, researchers reported Wednesday.
The results appear to clear one of the major roadblocks in pursuing new genetic technology as a cure for Duchenne muscular dystrophy, which affects 1 in every 3,500 boys born in the United States and for which there currently is no treatment.
“This finding removes all doubt that gene therapy is the avenue of choice for treating Duchenne muscular dystrophy,” said Don Wood, director of science technology at the Muscular Dystrophy Assn.
The disease, characterized by progressive muscle weakening and death in the patient’s early 20s, is caused by defects in a protein called dystrophin. Researchers had feared that the therapy was not possible because the gene for a functional dystrophin is far too large to be manipulated for gene therapy.
But scientists from three universities said that they had developed a “mini-gene,” less than 1% the size of the dystrophin gene, that is completely effective in animals.
The report, presented at a news conference in Las Vegas and published today in the journal Nature, “is a significant step forward,” said Dr. Helen M. Blau of the Stanford University School of Medicine. “Gene replacement . . . is the strategy for the near future.”
Researchers cautioned that the approach to gene therapy used in the mice involves treating a fertilized egg and for that reason would not be applicable to humans. New techniques will have to be developed for inserting the gene into human cells, they said.
But the demonstration that the mini-gene works should substantially accelerate the development of such vectors, said geneticist Jeffrey Chamberlain of the University of Michigan, the primary author of the report.
The defective gene that causes Duchenne muscular dystrophy was discovered in 1986. The next year, researchers identified dystrophin as the protein for which the gene is a blueprint. Researchers have subsequently found that a related, less common disorder called Becker muscular dystrophy is caused by a less severe mutation in the same gene, and experts believe that the new findings will apply to this disease as well.
The dystrophin gene is located on the X, or female, chromosome, one of the two sex chromosomes. The gene is carried and transmitted by women, but usually develops only in males.
Scientists were astonished to discover that the dystrophin gene accounts for 1.5% of the X chromosome, making it “by far the largest gene ever identified--much too large to manipulate in laboratories,” Chamberlain said. The actual part of the gene that serves as a blueprint for dystrophin is much smaller, amounting to about 0.6% of the total gene. But the blueprint is in segments that are interspersed throughout the remainder of the gene.
Chamberlain’s team, which includes researchers at the universities of Washington and Iowa, isolated these segments and pieced them together to form the mini-gene. They attached another genetic fragment from a different gene so that the dystrophin would be produced only in muscle cells, where it is needed.
They tested the new gene in a mouse strain called mdx , which also has a defect in its dystrophin gene and develops a disease very much like human muscular dystrophy. Employing commonly used techniques, they removed newly fertilized eggs from mdx mice, placed them in a petri dish and inserted several copies of the new gene into each with a slender needle.
The eggs were placed back in the mothers and allowed to grow to maturity. Healthy dystrophin was produced in many of the mice, rendering them completely healthy and showing none of the symptoms of muscular dystrophy.
In particular, the team studied the effects of the treatment on muscles of the diaphragm, which control breathing and is most severely affected in the mouse model. They found that the diaphragm muscles in the treated mice were as strong as those in mice from a strain that does not develop muscular dystrophy. This is important, Blau said, because failure of the diaphragm and lungs is the most common cause of death in Duchenne muscular dystrophy.
Before this study, researchers were also concerned that the treated cells might make too much dystrophin, thereby producing deleterious side effects. The team found, in fact, that many of the treated cells produced as much as 50 times the normal quantity of dystrophin, but that this extra protein had no adverse effects. That was “a major surprise,” Blau said.
Chamberlain cautioned: “This is not a method we would apply to humans. It is too crude and can cause damage to embryos.” Not only is it extremely difficult to identify and retrieve a fertilized egg before it starts dividing, he said, but it is possible for the needle to damage the nucleus of the egg.
The most likely method of gene delivery in humans would be in a virus. Altered cold viruses, for example, can be used to deliver the gene to lung cells, while other viruses can get it to muscles in the limbs and heart.
