TUCSON, Ariz. (Jan. 15, 2013) — The Muscular Dystrophy Association announced today that a microgene created to mimic the larger dystrophin gene missing in Duchenne muscular dystrophy (DMD) showed reduced inflammation and fibrosis, and also improved the muscle strength in a study conducted by University of Missouri scientists.
MDA contributed funding for this study and has provided more than $2 million for lead scientist Dongsheng Duan, the Margaret Proctor Mulligan Professor in Medical Research at the MU School of Medicine, since 2001. Duan and colleagues published their results online Jan. 15 in the journal Molecular Therapy.
Scientists treated dystrophic dogs with muscle injections of highly miniaturized dystrophin genes — a strategy known as microdystrophin gene therapy. This is the first time this type of therapy for DMD has been successful in large animals, although it has previously been successful in mice.
In contrast to previous studies of microdystrophin gene therapy in dogs with a DMD-like disorder and in humans with DMD, the researchers in this study did not detect immune system rejection of the treatment and saw improvements in muscle structure and function in the treated compared with untreated dog muscles.
DMD and the related disorder Becker muscular dystrophy (BMD) result from a complete (DMD) or partial (BMD) deficiency of the dystrophin protein in muscles.
“We are really excited by these results,” said MDA Vice President of Research Jane Larkindale. “MDA has been supporting gene therapy for neuromuscular diseases as a high priority since the 1980s, and we are delighted to see robust improvement in the DMD dog model with a gene therapy agent. This study provides the framework for a possible path forward for this kind of treatment.”
When the researchers compared muscle fibers in the dogs, they saw that the treated fibers showed robust production of dystrophin protein, much less scar tissue, less inflammation and a more normal fiber size. When the muscles were subjected to the stress of contractions, the treated fibers maintained their ability to generate force much better than the untreated fibers did.
Somewhat surprisingly, the investigators saw many more immune system cells known as T cells in the treated fibers than in the untreated fibers, even though they saw no evidence of a specific immune response against the newly made dystrophin protein or the viral delivery vehicle used to transport the new genes. They say this phenomenon needs further study.
Previous studies of microdystrophin gene therapy have shown great success in mouse models of DMD but not in dog models or in humans with the disease. Immune system rejection of the therapy is believed to be a major obstacle to this type of treatment.
Duan and colleagues say the relative success of their gene therapy study compared to others may be due to either the specific type of microdystrophin gene they used, the type of viral delivery vehicle they used, the particular combination of immunosuppressive drugs employed (cyclosporine and mycophenolate mofetil), and/or other factors.
“In contrast to the disappointing outcome in previous dog studies ... our results suggest that microdystrophins can at least partially protect skeletal muscle in large mammals,” the researchers say in their paper. They also note, however, that “compared with what was reported in the mouse model, the improvement we saw in dystrophic dogs remained suboptimal.”
They say further studies are needed to optimize this strategy.
To learn more, read DMD: Dystrophin-Deficient Dogs Benefit From Gene Therapy.
About DMD and BMD
Affecting boys almost exclusively, DMD is a degenerative muscle disease that involves progressive degeneration of voluntary and cardiac muscles, with resulting weakness and heart abnormalities. The disease manifests in early childhood, causing delayed motor milestones and, in some cases, cognitive, behavioral or language deficits. Loss of the ability to walk occurs in most children with DMD between the ages of 10 and 12 years; weakened cardiac and respiratory muscles severely limit life span.
The disease is caused by a number of different mutations in the X-chromosome gene that carries instructions for the muscle protein dystrophin. Without dystrophin, muscle fibers are abnormally fragile and break down under the stress of contractions.
Corticosteroid medications, such as prednisone and prednisolone, slow disease progression.
In general, Becker muscular dystrophy is a less severe disorder than DMD, although its effects on the heart can be life-threatening.
Besides gene therapy, MDA is funding research into a number of other strategies for DMD and BMD, including exon skipping, stem cells, utrophin upregulation and stop codon read-through. For more, visit mda.org/disease/duchenne-muscular-dystrophy/research.
MDA is the nonprofit health agency dedicated to finding treatments and cures for muscular dystrophy, ALS and related diseases by funding worldwide research. The Association also provides comprehensive health care and support services, advocacy and education.
In addition to funding some 300 research projects worldwide, MDA maintains a national network of 200 medical clinics; facilitates hundreds of support groups for families affected by neuromuscular diseases; and provides local summer camp opportunities for thousands of youngsters living with progressive muscle diseases.