Translational Lessons Learned from a Canine Model of Duchenne Muscular Dystrophy
Joe N. Kornegay, DVM; 1 James F. Howard, Jr, MD; 2 and H. Roger Brown, DVM3
Departments of Pathology and Laboratory Medicine1 and Neurology,2 School of Medicine, University of
North Carolina at Chapel Hill, Chapel Hill, NC 27599 and Department of Toxicogenomics,3
GlaxoSmithKline, Inc, Research Triangle Park, North Carolina, 27709
This presentation will focus on the role of a particular animal model (golden retriever muscular
dystrophy - GRMD) in drug development for Duchenne muscular dystrophy (DMD) of humans. We will
focus specifically on the potential for myostatin gene/protein inhibition to promote more effective
muscle regeneration/differentiation. While the presentation will emphasize GRMD/DMD, there will be
application to other muscle wasting disorders. Dr. Kornegay will start with background on DMD/GRMD
and the rationale for myostatin inhibition as a treatment, together with some preliminary data on cross
breeding dystrophic and myostatin-deficient dogs; followed by Dr. Howard, who will provide perspective
on the clinical implications for myostatin inhibition in people with neuromuscular disease; and Dr.
Brown will conclude with an overview of the pharmaceutical industry’s interest in myostatin inhibition
as a therapeutic option. Participants in the course should gain an appreciation for each of these
components of drug development.
Abstract
Duchenne muscular dystrophy (DMD) is an X-linked, recessive disorder that affects 1 in 3,600-6,000
newborn human males in whom absence of the protein dystrophin causes progressive degeneration of
skeletal and cardiac muscle. Potential treatments for DMD may be broadly categorized as molecular,
cellular, or pharmacologic. Although several therapies have shown promise, questions must be
addressed in relevant animal models before moving to human clinical trials. Spontaneous forms of Xlinked muscular dystrophy due to dystrophin deficiency have been identified in mice, multiple dog
breeds, and cats. Unlike the mdx mouse, which remains relatively normal clinically, affected dogs
develop progressive, fatal disease. Accordingly, studies in canine models such as golden retriever
muscular dystrophy (GRMD) may be more likely to predict efficacy of treatments proposed for DMD.
Beyond the incalculable emotional trauma for DMD patients and their families, society pays a huge
price. Yearly health care costs exceed $30,000/patient, 10-20 times those of other hospitalized groups.
There are even greater costs for personal care and lost productivity. Australia, a country with ~ 1/15th
the US population, estimates that overall annual costs are ~ $1.5 billion for DMD and $6 billion for all
neuromuscular diseases. These figures offer a window into the broader societal impact of more
common degenerative muscle disorders such as sarcopenia and cancer cachexia. Sarcopenia incidence
increases over life, with ~ 15% of women and 30% of men affected at age 80. Direct healthcare costs
attributable to sarcopenia in the US approached $20 billion in 2000.
The collective incidence and cost of health care for the muscular dystrophies and other muscle wasting
disorders is a major driver for the pharmaceutical industry. Considerable interest has focused recently
on the potential to inhibit the myostatin gene/protein, which normally serves as a negative regulator of
muscle growth. In principle, such disinhibition of the effects of myostatin could lead to more
successful/complete muscle regeneration/differentiation. Se-Jin Lee and his colleagues originally
characterized myostatin and have done much of the work to define its role in muscle. With regard to
the clinical effects, studies have shown that humans, cattle, sheep, and dogs with myostatin mutations
have dramatic muscle hypertrophy. Dystrophin-deficient mdx mice in which myostatin is knocked out or
inhibited postnatally also have a less severe phenotype. These findings have provided impetus to
further explore therapeutic benefits of myostatin inhibition. However, the effects of myostatin
inhibition in other murine muscle disease models have varied, with some mice showing increased
morbidity. There are also questions about potential senescence of myostatin-deficient cells that have
undergone multiple divisions. Moreover, abnormalities have been identified in muscle tendons from
myostatin-null mice.
While murine models have been powerful tools in defining disease mechanisms, results do not
necessarily extrapolate to humans, presumably due to differences between murine and human size and
physiology. These shortcomings are partially obviated with large animal (mostly dogs and pigs) models.
With mice, the role of specific genes and their interactions can be studied using knockout and transgenic
models. Unfortunately, transgenic technology in dogs is very cumbersome, essentially precluding use of
this approach. Thus, it is not possible to conduct myostatin knock out studies in dystrophic dogs.
Fortuitously, a spontaneous deletion has been identified in the myostatin gene of the Whippet breed.
Dogs that are homozygous for the myostatin-null allele (so called bully whippets) have gross
enlargement of muscle, while those that are heterozygous have intermediate muscle mass and are
significantly faster than their myostatin-normal counterparts. To provide further justification for use of
myostatin blockade as a treatment for DMD, we have initiated a set of experiments to define the
phenotype of dogs produced by breeding dogs carrying both dystrophin and myostatin gene mutations.
Our initial hypothesis was that myostatin-null heterozygote or homozygote dystrophic dogs would have
a less severe phenotype. But, in our initial litter produced from mating a GRMD carrier with a
myostatin-null homozygote Whippet, the single myostatin null heterozygote GRMD dog actually
developed a more severe phenotype in association with joint contractures, seemingly due to an
imbalance between selectively hypertrophied flexor muscles and weakened extensors. We have seen
similar results in a second litter produced from mating a GRMD male and a double myostatin/dystrophin
heterozygote null female. Clearly, data derived from animals in which myostatin has been downregulated from conception do not fully extrapolate to a clinical situation in which treatment is applied
postnatally as the disease progresses. Nonetheless, our results from these dystrophic, myostatin null
heterozygote dogs suggest that down-regulation of myostatin can have deleterious effects.