Canine X-linked muscular dystrophy studied with in vivo phosphorus magnetic resonance spectroscopy

Using MRI to quantify skeletal muscle pathology in Duchenne muscular dystrophy: A systematic mapping review

There is a great demand for accurate non‐invasive measures to better define the natural history of disease progression or treatment outcome in Duchenne muscular dystrophy (DMD) and to facilitate the inclusion of a large range of participants in DMD clinical trials. This review aims to investigate which MRI sequences and analysis methods have been used and to identify future needs. Medline, Embase, Scopus, Web of Science, Inspec, and Compendex databases were searched up to 2 November 2019, using keywords “magnetic resonance imaging” and “Duchenne muscular dystrophy.” The review showed the trend of using T1w and T2w MRI images for semi‐qualitative inspection of structural alterations of DMD muscle using a diversity of grading scales, with increasing use of T2map, Dixon, and MR spectroscopy (MRS). High‐field (>3T) MRI dominated the studies with animal models. The quantitative MRI techniques have allowed a more precise estimation of local or generalized disease severity. Longitudinal studies assessing the effect of an intervention have also become more prominent, in both clinical and animal model subjects. Quality assessment of the included longitudinal studies was performed using the Newcastle‐Ottawa Quality Assessment Scale adapted to comprise bias in selection, comparability, exposure, and outcome. Additional large clinical trials are needed to consolidate research using MRI as a biomarker in DMD and to validate findings against established gold standards. This future work should use a multiparametric and quantitative MRI acquisition protocol, assess the repeatability of measurements, and correlate findings to histologic parameters.

The Dog Model in the Spotlight: Legacy of a Trustful Cooperation

Dogs have long been used as a biomedical model system and in particular as a preclinical proof of concept for innovative therapies before translation to humans. A recent example of the utility of this animal model is the promising myotubularin gene delivery in boys affected by X-linked centronuclear myopathy after successful systemic, long-term efficient gene therapy in Labrador retrievers. Mostly, this is due to unique features that make dogs an optimal system. The continuous emergence of spontaneous inherited disorders enables the identification of reliable complementary molecular models for human neuromuscular disorders (NMDs). Dogs’ characteristics including size, lifespan and unprecedented medical care level allow a comprehensive longitudinal description of diseases. Moreover, the highly similar pathogenic mechanisms with human patients yield to translational robustness. Finally, interindividual phenotypic heterogeneity between dogs helps identifying modifiers and anticipates precision medicine issues.

This review article summarizes the present list of molecularly characterized dog models for NMDs and provides an exhaustive list of the clinical and paraclinical assays that have been developed. This toolbox offers scientists a sensitive and reliable system to thoroughly evaluate neuromuscular function, as well as efficiency and safety of innovative therapies targeting these NMDs. This review also contextualizes the model by highlighting its unique genetic value, shaped by the long-term coevolution of humans and domesticated dogs. Because the dog is one of the most protected research animal models, there is considerable opposition to include it in preclinical projects, posing a threat to the use of this model. We thus discuss ethical issues, emphasizing that unlike many other models, the dog also benefits from its contribution to comparative biomedical research with a drastic reduction in the prevalence of morbid alleles in the breeding stock and an improvement in medical care.

Animal models of Duchenne muscular dystrophy: from basic mechanisms to gene therapy

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disorder. It is caused by loss-of-function mutations in the dystrophin gene. Currently, there is no cure. A highly promising therapeutic strategy is to replace or repair the defective dystrophin gene by gene therapy. Numerous animal models of DMD have been developed over the last 30 years, ranging from invertebrate to large mammalian models. mdx mice are the most commonly employed models in DMD research and have been used to lay the groundwork for DMD gene therapy. After ~30 years of development, the field has reached the stage at which the results in mdx mice can be validated and scaled-up in symptomatic large animals. The canine DMD (cDMD) model will be excellent for these studies. In this article, we review the animal models for DMD, the pros and cons of each model system, and the history and progress of preclinical DMD gene therapy research in the animal models. We also discuss the current and emerging challenges in this field and ways to address these challenges using animal models, in particular cDMD dogs.

Quantitative NMRI and NMRS identify augmented disease progression after loss of ambulation in forearms of boys with Duchenne muscular dystrophy

Quantitative NMRI and (31)P NMRS indices are reported in the forearms of 24 patients with Duchenne muscular dystrophy (DMD) (6-18 years, 14 non-ambulant) amenable to exon 53 skipping therapy and in 12 age-matched male controls (CONT). Examinations carried out at 3 T comprised multi-slice 17-echo measurements of muscle water T2 and heterogeneity, three-point Dixon imaging of fat fraction in flexor and extensor muscles (FLEX, EXT), and non-localised spectroscopy of phosphate metabolites. We studied four imaging indices, eight metabolic ratios combining ATP, phosphocreatine, phosphomonoesters and phosphodiesters, the cytosolic inorganic phosphate (Pia ) and an alkaline (Pib) pool present in dystrophic muscle, and average pH. All indices differed between DMD and CONT, except for muscle water T2 . Measurements were outside the 95th percentile of age-matched CONT values in over 65% of cases for percentage fat signal (%F), and in 78-100% of cases for all spectroscopic indices. T2 was elevated in one-third of FLEX measurements, whereas %pixels > 39 ms and T2 heterogeneity were abnormal in one-half of the examinations. The FLEX muscles had higher fat infiltration and T2 than EXT muscle groups. All indices, except pH, correlated with patient age, although the correlation was negative for T2 . However, in non-ambulant patients, the correlation with years since loss of ambulation was stronger than the correlation with age, and the slope of evolution per year was steeper after loss of ambulation. All indices except Pi/gATP differed between ambulant and non-ambulant patients; however, T2 and %pixels > 39 ms were highest in ambulant patients, possibly owing to the greater extent of inflammatory processes earlier in the disease. All other indices were worse in non-ambulant subjects. Quantitative measurements obtained from patients at different disease stages covered a broad range of abnormalities that evolved with the disease, and metabolic indices were up to 10-fold above normal from the onset, thus establishing a variety of potential markers for future therapy.

Splitting of Pi and other ³¹P NMR anomalies of skeletal muscle metabolites in canine muscular dystrophy

Many anomalies exist in the resting (31) P muscle spectra of boys with Duchenne muscular dystrophy (DMD) but few have been reported in Golden Retriever muscular dystrophy (GRMD), the closest existing animal model for DMD. Because GRMD is recommended for preclinical evaluation of therapies and quantitative outcome measures are needed, we investigated anomalies of (31) P NMRS in tibial cranial and biceps femoris muscles from 14 GRMD compared to 9 control (CONT) dogs. Alterations observed in DMD children - low phosphocreatine and high phospho-monoesters and -diesters - were all found in GRMD but increased pH was not. More surprisingly, inorganic phosphate (Pi) appeared to present a prominent splitting with an enhanced Pi(b) resonance at 0.3 ppm downfield of Pi(a) . Assuming that both resonances are Pi, the pH for Pi(a) in GRMD corresponded to a physiological intracellular pH(a) (6.97 ± 0.05), while pH(b) approached the extracellular range (7.27 ± 0.10) and correlated with pH(a) in GRMD (R(2)  = 0.65). Both Pi(a) and Pi(b) were elevated compared to CONT and Pi(a) increased with age for GRMD (R(2)  = 0.48, p < 0.001). Magnetisation transfer experiments between γATP and Pi were conducted to better characterise Pi pools. Equal T1 relaxation times for Pi(b) and Pi(a) did not support a mitochondrial origin of Pi(b) . We suggest that Pi(b) could originate from degenerating hypercontracted cells that have a leaky membrane and inadequate cell homeostasis and pH regulation. Pi(b) showed minimal chemical exchange in all dogs, while the exchange rate of Pi(a) was reduced in GRMD and might extraneously reflect low glycolytic activity in DMD. Taken together, the ensemble of (31) P NMRS alterations identifies muscle dysfunction and could provide useful biomarkers of therapeutic efficacy. Furthermore, among these, two might relate more specifically to dystrophic processes and merit further investigation: one is the existence of the enhanced alkaline Pi(b) pool; the other, mechanisms by which membrane disruption might increase phosphodiesters in dystrophy.

Characterization of dystrophic muscle in golden retriever muscular dystrophy dogs by nuclear magnetic resonance imaging

The Golden Retriever Muscular Dystrophy dog lacks dystrophin. Disease progression in this model shares many similarities with the Duchenne muscular dystrophy, both from anatomico pathological and clinical standpoints. The model is increasingly used in pre-clinical trials but needs to be further investigated, particularly with reference to the evaluation of therapies. The aim of this study was to identify quantitative indices that would help characterize the dystrophic dog non-invasively using NMR imaging. Two-month-old dystrophic dogs and healthy control animals were scanned at 4T. Standard T2- and T1-weighted images, fat-saturated T1-weighted images pre- and post-gadolinium chelate injection were acquired and kinetics of muscle enhancement were studied over a 2-h period. Several indices were found to be abnormally high in dystrophic dogs: the T2-weighted/T1-weighted signal ratio, T2-weighted image heterogeneity and maximal signal enhancement post-gadolinium. These may be proposed to evaluate muscle structural alterations non-invasively in this disease.

Adaptations to exercise training and contraction-induced muscle injury in animal models of muscular dystrophy

This article reviews the current status of exercise training and contraction-induced muscle-injury investigations in animal models of muscular dystrophy. Most exercise-training studies have compared the adaptations of normal and dystrophic muscles with exercise. Adaptation of diseased muscle to exercise occurs at many levels, starting with the extracellular matrix, but also involves cytoskeletal architecture, muscle contractility, repair mechanisms, and gene regulation. The majority of exercise-injury investigations have attempted to determine the susceptibility of dystrophin-deficient muscles to contraction-induced injury. There is some evidence in animal models that diseased muscle can adapt and respond to mechanical stress. However, exercise-injury studies show that dystrophic muscles have an increased susceptibility to high mechanical forces. Most of the studies involving exercise training have shown that muscle adaptations in dystrophic animals were qualitatively similar to the adaptations observed in control muscle. Deleterious effects of the dystrophy usually occur only in older animals with advanced muscle fiber degeneration or after high-resistive eccentric training. The main limitations in applying these conclusions to humans are the differences in phenotypic expression between humans and genetically homologous animal models and in the significant biomechanical differences between humans and these animal models.

Molecular pathophysiology of myofiber injury in deficiencies of the dystrophin-glycoprotein complex

Duchenne muscular dystrophy is caused by mutations in the gene encoding dystrophin, a 427 kd protein normally found at the cytoplasmic face of the sarcolemma. In normal muscle, dystrophin is associated with a multimolecular glycoprotein complex. Primary mutations in the genes encoding members of this glycoprotein complex are also associated with muscular dystrophy. The dystrophin-glycoprotein complex provides a physical linkage between the internal cytoskeleton of myofibers and the extracellular matrix, but the precise functions of the dystrophin-glycoprotein complex remain uncertain. In this review, five potential pathogenetic mechanisms implicated in the initiation of myofiber injury in dystrophin-glycoprotein complex deficiencies are discussed: (1) mechanical weakening of the sarcolemma, (2) inappropriate calcium influx, (3) aberrant cell signaling, (4) increased oxidative stress, and (5) recurrent muscle ischemia. Particular emphasis is placed on the multifunctional nature of the dystrophin-glycoprotein complex and the fact that the above mechanisms are in no way mutually exclusive and may interact with one another to a significant degree.

Response to high-intensity eccentric muscle contractions in persons with myopathic disease

Although the response to intense eccentric muscle contractions is well described in normal subjects, concern exists about possible untoward effects in persons with myopathic diseases. We investigated 14 subjects with slowly progressive muscular dystrophies including myotonic muscular dystrophy (n = 9), facioscapulohumeral dystrophy (n = 2), limb-girdle syndrome (n = 2), and Becker muscular dystrophy (n = 1). Control subjects consisted of 18 able-bodied persons. Subjects performed two sets of eight maximal-effort eccentric repetitions of the elbow flexors, with measurement of maximal concentric strength, serum creatine kinase, resting and flexed arm angle, arm circumference, and soreness at days 0, 3, and 7. Although the myopathic group had less initial strength, both groups demonstrated a similar response to the protocol over 7 days. Both groups had a significant rise in serum creatine kinase, which was still elevated at 7 days (P < 0.05). The control group demonstrated a slightly greater injury response in terms of soreness, resting and flexed arm angles, and arm swelling. Both groups of subjects appeared to respond similarly to an acute bout of eccentric contractions. However, the potential long-term effects of this type of exercise in persons with myopathic diseases remains unknown.

Muscular dystrophy: centronucleation may reflect a compensatory activation of defective myonuclei

Muscular dystrophy has long been believed to be characterized by degeneration and abortive regeneration of muscle fibers (the muscle degeneration theory), but unfortunately its pathogenesis is still unclear and an effective treatment has yet to be developed. As a challenge to the theory, we have proposed an alternative muscle-defective-growth theory and a further bone muscle growth imbalance hypothesis supposing possible defects in bone-growth-dependent muscle growth based on our findings in hereditary dystrophic dy mice (C57BL/6J dy/dy). This review presents some new insights into the pathogenesis of the disease along with our hypothesis, focusing on the physiological meaning of centronucleation, one of the major pathological changes commonly observed in dystrophic muscles of man and experimental animals.

Sarcoglycans in muscular dystrophy

Muscular dystrophy is a heterogeneous genetic disease that affects skeletal and cardiac muscle. The genetic defects associated with muscular dystrophy include mutations in dystrophin and its associated glycoproteins, the sarcoglycans. Furthermore, defects in dystrophin have been shown to cause a disruption of the normal expression and localization of the sarcoglycan complex. Thus, abnormalities of sarcoglycan are a common molecular feature in a number of dystrophies. By combining biochemistry, molecular cell biology, and human and mouse genetics, a growing understanding of the sarcoglycan complex is emerging. Sarcoglycan appears to be an important, independent mediator of dystrophic pathology in both skeletal muscle and heart. The absence of sarcoglycan leads to alterations of membrane permeability and apoptosis, two shared features of a number of dystrophies. beta-sarcoglycan and delta-sarcoglycan may form the core of the sarcoglycan subcomplex with alpha- and gamma-sarcoglycan less tightly associated to this core. The relationship of epsilon-sarcoglycan to the dystrophin-glycoprotein complex remains unclear. Animals lacking alpha-, gamma- and delta-sarcoglycan have been described and provide excellent opportunities for further investigation of the function of sarcoglycan. Dystrophin with dystroglycan and laminin may be a mechanical link between the actin cytoskeleton and the extracellular matrix. By positioning itself in close proximity to dystrophin and dystroglycan, sarcoglycan may function to couple mechanical and chemical signals in striated muscle. Sarcoglycan may be an independent signaling or regulatory module whose position in the membrane is determined by dystrophin but whose function is carried out independent of the dystrophin-dystroglycan-laminin axis.

Muscle degeneration without mechanical injury in sarcoglycan deficiency

In humans, mutations in the genes encoding components of the dystrophin-glycoprotein complex cause muscular dystrophy. Specifically, primary mutations in the genes encoding alpha-, beta-, gamma-, and delta-sarcoglycan have been identified in humans with limb-girdle muscular dystrophy. Mice lacking gamma-sarcoglycan develop progressive muscular dystrophy similar to human muscular dystrophy. Without gamma-sarcoglycan, beta- and delta-sarcoglycan are unstable at the muscle membrane and alpha-sarcoglycan is severely reduced. The expression and localization of dystrophin, dystroglycan, and laminin-alpha2, a mechanical link between the actin cytoskeleton and the extracellular matrix, appears unaffected by the loss of sarcoglycan. We assessed the functional integrity of this mechanical link and found that isolated muscles lacking gamma-sarcoglycan showed normal resistance to mechanical strain induced by eccentric muscle contraction. Sarcoglycan-deficient muscles also showed normal peak isometric and tetanic force generation. Furthermore, there was no evidence for contraction-induced injury in mice lacking gamma-sarcoglycan that were subjected to an extended, rigorous exercise regimen. These data demonstrate that mechanical weakness and contraction-induced muscle injury are not required for muscle degeneration and the dystrophic process. Thus, a nonmechanical mechanism, perhaps involving some unknown signaling function, likely is responsible for muscular dystrophy where sarcoglycan is deficient.

In vivo determination of altered hemoglobin saturation in dogs with M-type phosphofructokinase deficiency

Muscle-type phosphofructokinase (M-PFK) deficiency causes an exertional myopathy and chronic hemolysis in affected humans and dogs, the only animal model available. Deficient individuals have impaired glycolytic metabolism, impaired oxidative metabolism, and increased hemoglobin-oxygen (HbO2) affinity as a result of low 2,3-diphosphoglycerate (2,3-DPG) levels. The purpose of this study was to determine if PFK-deficient muscle has abnormal oxygen saturation during exercise. Oxygen saturation of hemoglobin/myoglobin was measured noninvasively in skeletal muscle during progressive muscle activation using near-infrared spectroscopy (NIRS). Muscle metabolites were also measured using magnetic resonance spectroscopy (MRS). PFK-deficient and normal dogs were anesthetized and the cranial tibial muscles stimulated for 6 min at each of four different rates (1, 2, 4, and 8 Hz). With increasing stimulation, muscles from normal dogs showed progressive decrease in hemoglobin saturation. In contrast, PFK-deficient dogs exhibited either an increase in hemoglobin saturation or an initial decrease with no further change. PFK-deficient muscles accumulated 11.1 +/- 3.5 mmol/L of sugar phosphate which was not seen in normal muscle and had higher calculated [ADP] levels at each stimulation level, indicating impaired oxidative metabolism. These findings are consistent with the hypothesis that these animals have impaired oxidative metabolism and impaired muscle O2 extraction from hemoglobin due to increased HbO2 affinity. NIRS appears to be a useful noninvasive method of monitoring tissue oxygen saturation in normal or disease conditions.

Corticosteroid therapy does not alter the threshold for contraction-induced injury in dystrophic (mdx) mouse diaphragm

The effects of methylprednisolone therapy on the susceptibility of dystrophin-deficient myofibers to contraction-induced injury were evaluated in the mdx mouse diaphragm model of Duchenne dystrophy. Mdx myofibers were abnormally vulnerable to injury induced by high-stress eccentric contractions. However, methylprednisolone therapy did not significantly alter the degree of contraction-induced injury. These data suggest that beneficial effects of corticosteroid therapy in Duchenne dystrophy are unlikely to be related to a change in the threshold for contraction-induced myofiber damage.

Dystrophin deficiency, altered cell signalling and fibre hypertrophy

Dystrophin is a subsarcolemmal protein which is defective in Duchenne and Becker muscular dystrophy (DMD/BMD), and in three animal models. Clinical manifestations of dystrophin deficiency in humans range from a mild calf muscle hypertrophy with cramps to the classical progressive degenerative hypertrophic myopathy of Duchenne. A common feature in the clinical presentation of dystrophin deficiency in humans and in the three documented animal models is the presence of muscle fibre hypertrophy. This paper explores the hypothesis that membrane-bound signalling processes are disrupted in the absence of dystrophin, and suggests that these abnormalities may contribute to both the hypertrophic and degenerative changes of dystrophin deficiency.

The membrane hypothesis of Duchenne muscular dystrophy: quest for functional evidence

(1) The location of dystrophin in normal muscle, its molecular structure and associations, characterize it as a component of the submembrane cytoskeleton. When dystrophin is missing the cytoskeleton will therefore be defective, and it has been supposed that this renders the muscle membrane more vulnerable to mechanical damage. With the discovery of animal strains lacking in dystrophin, this hypothesis has been put to experimental tests. Contradictory results have been obtained by workers using different exercise regimens and different indices of fibre damage. (2) Direct measurements of the tensile strength of the membrane have been made on patches of cultured myotubes or isolated muscle fibres, and on sarcolemmal vesicles by pipette aspiration. Neither method has revealed a difference in the tensile strength between normal and dystrophic membrane. The most plausible explanation is that the tensile strength of the membrane is a property more of the lipid bilayer than of the cytoskeleton. (3) In another experimental approach tensile membrane stress has been produced by exposing isolated muscle fibres and myotubes in culture to hypotonic solutions. In such experiments fibres and myotubes lacking dystrophin have been found to lyse more readily than do normal ones. This difference does not conflict with the similarity in tensile strength of normal and dystrophic fibre membranes noted above. Rather, the predisposition to osmotic lysis of dystrophic fibres and myotubes may signify a lower ratio of membrane surface to cell volume, perhaps as a result of loss of some of the spare membrane normally possessed by skeletal muscle fibres and myotubes. (4) In red blood cells the membrane cytoskeleton functions to maintain membrane deformability and stability. Deficiency in spectrin, the main cytoskeletal component, predisposes red cells to cytoskeletal rupture and membrane loss when they experience shear stress. Skeletal muscle fibres, especially long fibres contracting eccentrically, are susceptible to shear stress as a result of uneven contraction along their length. In that event, fibres lacking dystrophin may similarly shed membrane more readily.