SCIENTISTS CORRECT GENETIC DEFECT IN DOG WITH DUCHENNE MUSCULAR DYSTROPHY
TUCSON, Ariz., June 1, 2000 - Researchers from three American and one British university say they've used a new approach to achieve long-term repair of a genetic defect that causes Duchenne muscular dystrophy, the Muscular Dystrophy Association (MDA) announced today.
The strategy used by the scientific team, whose results are published in the June issue of Nature Biotechnology and presented today at the American Society of Gene Therapy Meeting in Denver attended by some 4,000 scientists from around the globe, differs from previous gene therapy experiments. It focuses on repairing an existing gene, rather than inserting a new gene. Inserting new genes into muscles generally requires the use of viruses, can provoke unwanted immune responses and, so far, hasn't shown long-lasting benefits in animal studies in muscular dystrophy.
"This pioneering work represents a new era of hope, a promising area of investigation that ultimately could lead to treatments for hundreds of genetic diseases," said Leon I. Charash, chairman of the MDA Medical Advisory Committee. "Much more needs to be learned about this innovative approach that's now been used to repair individual muscles in dog and mouse models for muscular dystrophy. But the idea of stimulating genetic repair without causing an immune response is provocative, indeed."
Duchenne muscular dystrophy is the most common childhood form of muscular dystrophy. It affects males almost exclusively and results from a mutation in the gene for the muscle protein dystrophin. Approximately one in 3,500 male babies is born with mutations in this gene, which was first identified by MDA-funded scientists in 1986. The disease causes progressive loss of muscle function during childhood and adolescence, and usually results in death by the 20s from respiratory and cardiac muscle degeneration.
The new technique, according to Joe Kornegay, an MDA-supported veterinary neurologist and pathologist at the University of Missouri in Columbia, "relies upon an innate system that the body has to correct genetic lesions [mutations]."
Kornegay was part of a team that also included scientists from the University of Miami, Ohio State University in Columbus and the North East Wales Institute (in Great Britain), where research team member Glenn Morris also does neuromuscular disease research with MDA support.
When asked how long the genetic correction could be expected to last, Kornegay said, "Theoretically, it would be permanent." The body, he said, doesn't appear to mount an immune response to genes corrected this way, and, once the gene is corrected in a cell, that cell's progeny will inherit the correction. (When a gene is added, the new gene can be diluted out when cells divide.)
Team leader Richard Bartlett, a molecular biologist affiliated with the University of Missouri's College of Veterinary Medicine and the National Institutes of Health, described the technique as "genetic surgery" to correct an existing gene mutation using a synthetic oligonucleotide [a short strand of nucleic acid].
"The oligonucleotide targets the mutation in the dystrophin gene and pairs with the chromosome, creating a hybrid molecule that is recognized by DNA repair enzymes which correct the mutation based on the sequence defined by the oligonucleotide," Bartlett said.
No other gene therapy has lasted this long, he explained.
"This is permanent. The other thing is that these oligonucleotides are not immunogenic. If we chronically treat, keep putting more and more in, it could have an additive effect and you could get more and more repair." Bartlett recently joined the National Institute of Arthritis and Musculoskeletal and Skin Diseases at the National Institutes of Health in Bethesda, Md.
Bartlett added that the strategy is also superior to adding a new gene because there's no chance the protein made from the new gene will have harmful interactions with the nonfunctional protein made from the old gene. "Everyone else is trying to put a complementary gene in to complement the missing [or defective] protein," he said. "In some cases, they may very well have problems. If you put a new protein in when you already have a defective protein, the interaction between the two may create new problems."
The researchers injected the oligonucleotide "patch kit," a paper-clip-shaped molecule called a chimeric oligonucleotide, directly into a shin muscle of a 6-week-old golden retriever with a genetic defect that leads to Duchenne muscular dystrophy in dogs. Eleven months later, the injected muscle continued to show a significant amount of normal dystrophin, the protein missing or seriously flawed in Duchenne muscular dystrophy.
"It potentially avoids some of the complications of gene therapy that we've all recently seen with the adenovirus [virus used to deliver large genes like dystrophin to cells]," Kornegay said.
Kornegay also noted, "The greatest challenge is that we're still only at that individual muscle level. It's not correcting the genetic defect in a generalized sense." For that, he said, "a systemic method of delivery will probably have to be found."
Both scientists said the genetic mutation in the dog is a type called a point mutation, a genetic "typo" that doesn't involve missing DNA. This type of defect, they say, represents a small percentage of the mutations that affect humans with the disease.
However, the strategy, once perfected, might be expanded to help in treating patients with diseases caused by genetic deletions (where a piece of DNA is missing from a gene). Bartlett underscored this point by explaining that the dogs in his study have muscular dystrophy because a portion of the normal messenger RNA for dystrophin is omitted, not unlike the omission typically found in humans affected by Duchenne muscular dystrophy caused by genetic deletions.
"There are hundreds of genetic disorders, including hemophilia, cystic fibrosis, glycogen storage diseases and sickle cell anemia, for which this repair strategy could potentially be applicable," said MDA grantee Thomas Rando, assistant professor of neurology and neurological sciences at Stanford University in Palo Alto, Calif. Rando recently corrected the dystrophin gene in a muscle in a mouse model of Duchenne dystrophy using a chimeric oligonucleotide. The results are in the May 9 issue of Proceedings of the National Academy of Sciences. Rando, while emphasizing his commitment to studying the gene repair strategy, added, "I can't urge enough caution in thinking about this as anything other than a first step."
MDA is the nonprofit health agency dedicated to curing muscular dystrophy, ALS and related diseases by funding worldwide research. The Association also provides comprehensive health care and support services, advocacy and education. Recognized by the American Medical Association with a Lifetime Achievement Award "for significant and lasting contributions to the health and welfare of humanity," MDA maintains 230 hospital-affiliated clinics that offer families the best in care for progressive neuromuscular diseases.
MDA annually funds some 400 scientific teams worldwide. These investigators have made significant advances toward cures for several muscle-wasting diseases. They also have pioneered breakthroughs that may well lead to therapies for heart disease, cancer, AIDS, Alzheimer's, Huntington's, Parkinson's and cystic fibrosis.
In addition to that of MDA, funding for the study reported in Nature Biotechnology was provided by the Association Francaise Contre les Myopathies, the Muscular Dystrophy Group of Great Britain and Northern Ireland, the Diabetes Research Institute Foundation, the Parent Project for Muscular Dystrophy and Kimeragen Pharmaceuticals (Newtown, Pa.).
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