Dystrophin-Deficient Dogs Benefit From Gene Therapy
University of Missouri.
Jan 15 2013.
MDA-supported investigators found that
intramuscular injections of microdystrophin genes
improved muscle health in
dystrophin-deficient dogs, a response not previously seen in large
animals or
humans
 |
| MDA research grantee Dongsheng Duan and colleagues have found that dogs with a DMD-like disease can be successfully treated with microdystrophin gene therapy |
Article Highlight:
- Researchers supported in part by MDA used highly miniaturized dystrophin (microdystrophin) genes encased in AAV9 delivery vehicles to treat six dystrophin-deficient dogs that had a disease mimicking human Duchenne muscular dystrophy (DMD).
- The gene injections were made directly into the dogs’ front leg muscles.
- Muscle
fibers that received the gene therapy showed good dystrophin protein
production, improvements in muscle appearance, and partial protection from
the weakness that occurs in dystrophin-deficient muscles after repeated
contractions.
- No specific immune responses were detected against the newly made microdystrophin protein or the vehicle used to deliver the new genes, although immune system T cells were seen in the treated muscle fibers.
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.
Immune response has been a challenge
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.
Improvements seen in muscle fiber structure
and function
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:
- robust production of dystrophin
from the microdystrophin genes (microdystrophin protein);
- restoration of a cluster of
proteins at the muscle-fiber membrane that is disrupted when dystrophin is
absent;
- much less calcification of muscle;
- substantially less scar tissue (fibrosis);
- less invasion of muscle tissue by
inflammatory cells;
- more normal muscle fiber size; and
- significant protection of muscle
fibers against the weakness that occurs with repeated contractions in
dystrophin-deficient muscles.
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.
What makes these experiments different
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
- the specific microdystrophin
construct used in these experiments was different from that used in other
experiments;
- the experimental design was
different from that used in other studies;
- the gene delivery strategy was
different; and
- the age of the dogs was different
(older).
"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."
