For the first time, gene therapy using a highly miniaturized dystrophin gene resulted in significant improvement in muscle structure and function in dogs with a disorder mimicking human Duchenne muscular dystrophy.
The MDA-supported findings may help advance the development of microdystrophin gene therapy for DMD and the related disorder Becker muscular dystrophy (BMD), both of which result from a deficiency of the dystrophin protein.
MDA research grantee Dongsheng Duan, a professor in the Department of Molecular Microbiology and Immunology at the University of Missouri in Columbia, coordinated the research team, whose findings were published online Jan. 15, 2013, in the journal Molecular Therapy.
Microdystrophin gene therapy has been in development for more than a decade but so far has shown better results in dystrophin-deficient mice than in dystrophin-deficient dogs or humans with DMD.
This is the first time that significant muscle-related benefits have been seen with microdystrophin gene therapy in a large animal model of DMD.
Although the results were encouraging, the researchers note that the response of the dogs to microdystrophin gene therapy was not as robust as the response of dystrophin-deficient mice in previous experiments. They say further studies are needed to create an optimal microdystrophin gene, AAV delivery system and regimen of immunosuppressive drugs.
MDA-supported investigators reported in late 2010 that four out of six boys with DMD who received microdystrophin gene therapy into a biceps muscle showed evidence that their immune systems rejected the newly synthesized dystrophin protein.
Previously reported studies of microdystrophin gene therapy in dystrophin-deficient dogs also have shown less than optimal results and some evidence of rejection of the therapy by the immune system.
It is widely believed that the dog model of DMD is more like the human disease than are mouse models. The canine immune system may also replicate the human immune system better than the mouse immune system does.
Six dystrophin-deficient dogs received injections into front leg muscles of microdystrophin genes, each encased in a delivery vehicle made from a modified type 9 adeno-associated virus (AAV9). Four of the dogs received a single gene therapy injection into one front leg muscle; two dogs received a single gene therapy injection into each front leg muscle.
All six dogs received temporary immunosuppression using two drugs, cyclosporine and myocophenolate mofetil (CellCept), intended to help them tolerate the gene transfer.
Two months after the gene injections, when treated muscle fibers were compared with untreated muscle fibers, the investigators saw:
The investigators saw no evidence of an immune response against either the newly made microdystrophin or the viral delivery vehicle. However, somewhat surprisingly, they saw an abundance of immune system T cells in the treated muscle fibers. The effects of the T cells and the reasons for their presence remain unclear.
Commenting on the relative success of the current gene therapy experiments in dystrophin-deficient dogs compared to other DMD dog gene transfer experiments, the investigators note that
"In summary," the researchers say, "our results have cleared uncertainty on microdystrophin therapy arisen from other dog studies. However, compared with what was reported in the mouse model, the improvement we saw in dystrophic dogs remained suboptimal."
For more about this study, see the Jan. 15, 2013, press release from the University of Missouri.
Work continues on optimizing dystrophin-based gene therapy, particularly with respect to overcoming unwanted immune responses to this strategy.