2298 The Journal of Clinical Investigation http://www.jci.org Volume 123 Number 5 May 2013
Assessment of disease activity in muscular
dystrophies by noninvasive imaging
Katie K. Maguire,1 Leland Lim,2 Sedona Speedy,1 and Thomas A. Rando1,2,3
1Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California, USA.
2Neurology Service and RR&D Center of Excellence, Veterans Affairs Palo Alto Health Care System, Palo Alto, California, USA.
3Glenn Laboratories for the Biology of Aging, Stanford University School of Medicine, Stanford, California, USA.
Muscular dystrophies are a class of disorders that cause progressive muscle wasting. A major hurdle for dis-
covering treatments for the muscular dystrophies is a lack of reliable assays to monitor disease progression
in animal models. We have developed a novel mouse model to assess disease activity noninvasively in mice
with muscular dystrophies. These mice express an inducible luciferase reporter gene in muscle stem cells. In
dystrophic mice, muscle stem cells activate and proliferate in response to muscle degeneration, resulting in
an increase in the level of luciferase expression, which can be monitored by noninvasive, bioluminescence
imaging. We applied this noninvasive imaging to assess disease activity in a mouse model of the human disease
limb girdle muscular dystrophy 2B (LGMD2B), caused by a mutation in the dysferlin gene. We monitored the
natural history and disease progression in these dysferlin-deficient mice up to 18 months of age and were able
to detect disease activity prior to the appearance of any overt disease manifestation by histopathological anal-
yses. Disease activity was reflected by changes in luciferase activity over time, and disease burden was reflected
by cumulative luciferase activity, which paralleled disease progression as determined by histopathological
analysis. The ability to monitor disease activity noninvasively in mouse models of muscular dystrophy will be
invaluable for the assessment of disease progression and the effectiveness of therapeutic interventions.
Muscular dystrophies are a class of inherited muscle disorders
that are characterized by progressive muscle weakness and wast-
ing. These diseases often result from mutations of genes that
are critical for muscle cell structure or function (1). Therapeu-
tic strategies to treat muscular dystrophies, including gene ther-
apies and small molecule therapies, are being investigated, but
currently there are few treatments available and none that sub-
stantially alter disease progression (2). Many dystrophic animal
models exist and provide valuable resources for understanding
the disease pathogenesis and for testing therapeutic interven-
tions (3). One of the major limitations to the study of therapeutic
agents for the treatment of muscular dystrophies is the absence of
reliable assays of disease activity in living animals.
The gold standard for monitoring disease progression or the
response to treatments in animals is the analysis of muscle histo-
pathology. This approach is labor intensive, difficult to quantify,
and usually terminal for the experimental animal. As such, inves-
tigators have sought methods to assess disease activity or progres-
sion using noninvasive or minimally invasive methods. In animal
models, levels of serum biomarkers, strength measurements, and
MRI evaluations have been used to assess disease activity and pro-
gressive deterioration of dystrophic muscle (4–6). However, these
techniques are either highly variable (especially serum biomark-
ers), nonspecific (especially strength measurements), expensive
(especially MRI), or some combination of the three. As such, there
remains a critical need for a method to provide quantitative and
reliable assessment of ongoing and cumulative disease activity
that closely reflects the histopathological changes occurring in
the muscle in dystrophic animal models.
In this report, we describe a novel mouse model in which muscle
regeneration, reflecting the response to degeneration that occurs
in the muscular dystrophies, can be measured noninvasively and
quantitatively in living mice over time. This mouse expresses an
estrogen-responsive Cre-recombinase under the control of the
Pax7 locus and a luciferase reporter gene that is Cre dependent.
Following tamoxifen treatment, luciferase is expressed only in
muscle satellite cells, since these are the only cells in the adult
(other than cells in small regions in the brain) that express Pax7
(7, 8). Therefore, each time the muscle undergoes degeneration
and regeneration, luciferase-expressing satellite cells give rise to
progeny that also express the reporter gene as they proliferate and
differentiate to repair the muscle, and that luciferase activity can
be measured noninvasively in a highly quantitative manner (9–12).
We applied this model to the study of a mouse model of a form of
limb girdle muscular dystrophy (LGMD) and found a remarkable
correlation between the results of noninvasive imaging and dis-
ease activity and progression as determined histopathologically
over the course of 18 months. This technology, which is applica-
ble to all murine models of muscular dystrophy, will dramatically
improve characterizations of the natural history and progression
of muscle diseases and will be an invaluable tool for measuring the
effectiveness of experimental therapeutics.
Characterization of the “regeneration reporter” strain. With the goal
of developing a mouse model to monitor muscle regeneration
as a surrogate for ongoing disease activity in mice with muscu-
lar dystrophies, we used Pax7CreER/LuSEAP mice in which an
estrogen-responsive Cre-recombinase is induced to permanently
activate a luciferase gene in muscle satellite cells (10). To charac-
terize this “regeneration reporter” strain, mice were first imaged
prior to administration of tamoxifen in order to determine the
Conflict of interest: The authors have declared that no conflict of interest exists.
Citation for this article: J Clin Invest. 2013;123(5):2298–2305. doi:10.1172/JCI68458.
Related Commentary, page 1931
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