Therapy insight: cardiovascular complications associated with muscular dystrophies

Genetic burden linked to founder effects in Saguenay-Lac-Saint-Jean illustrates the importance of genetic screening test availability

The Saguenay–Lac-Saint-Jean (SLSJ) region located in the province of Quebec was settled in the 19th century by pioneers issued from successive migration waves starting in France in the 17th century and continuing within Quebec until the beginning of the 20th century. The genetic structure of the SLSJ population is considered to be the product of a triple founder effect and is characterised by a higher prevalence of some rare genetic diseases. Several studies were performed to elucidate the historical, demographic and genetic background of current SLSJ inhabitants to assess the origins of these rare disorders and their distribution in the population. Thanks to the development of new sequencing technologies, the genes and the variants responsible for the most prevalent conditions were identified. Combined with other resources such as the BALSAC population database, identifying the causal genes and the pathogenic variants allowed to assess the impacts of some of these founder mutations on the population health and to design precision medicine public health strategies based on carrier testing. Furthermore, it stimulated the establishment of many public programmes.

We report here a review and an update of a subset of inherited disorders and founder mutations in the SLSJ region. Data were collected from published scientific sources. This work expands the knowledge about the current frequencies of these rare disorders, the frequencies of other rare genetic diseases in this population, the relevance of the carrier tests offered to the population, as well as the current available treatments and research about future therapeutic avenues for these inherited disorders.

Next-generation muscle-directed gene therapy by in silico vector design

There is an urgent need to develop the next-generation vectors for gene therapy of muscle disorders, given the relatively modest advances in clinical trials. These vectors should express substantially higher levels of the therapeutic transgene, enabling the use of lower and safer vector doses. In the current study, we identify potent muscle-specific transcriptional cis-regulatory modules (CRMs), containing clusters of transcription factor binding sites, using a genome-wide data-mining strategy. These novel muscle-specific CRMs result in a substantial increase in muscle-specific gene transcription (up to 400-fold) when delivered using adeno-associated viral vectors in mice. Significantly higher and sustained human micro-dystrophin and follistatin expression levels are attained than when conventional promoters are used. This results in robust phenotypic correction in dystrophic mice, without triggering apoptosis or evoking an immune response. This multidisciplinary approach has potentially broad implications for augmenting the efficacy and safety of muscle-directed gene therapy.

Adeno-associated viral vectors (AAV) are being developed for gene therapy of skeletal muscle, but it is a challenge to achieve robust gene expression. Here, the authors identify muscle-specific cisregulatory elements that lead to a substantial increase in micro-dystrophin and follistatin expression, resulting in a safe and sustainable rescue of the dystrophic phenotype in mouse models.

A Systematic Review of Continuum Modeling of Skeletal Muscles: Current Trends, Limitations, and Recommendations

Finite elasticity theory has been commonly used to model skeletal muscle. A very large range of heterogeneous constitutive laws has been proposed. In this review, the most widely used continuum models of skeletal muscles were synthetized and discussed. Trends and limitations of these laws were highlighted to propose new recommendations for future researches. A systematic review process was performed using two reliable search engines as PubMed and ScienceDirect. 40 representative studies (13 passive muscle materials and 27 active muscle materials) were included into this review. Note that exclusion criteria include tendon models, analytical models, 1D geometrical models, supplement papers, and indexed conference papers. Trends of current skeletal muscle modeling relate to 3D accurate muscle representation, parameter identification in passive muscle modeling, and the integration of coupled biophysical phenomena. Parameter identification for active materials, assumed fiber distribution, data assumption, and model validation are current drawbacks. New recommendations deal with the incorporation of multimodal data derived from medical imaging, the integration of more biophysical phenomena, and model reproducibility. Accounting for data uncertainty in skeletal muscle modeling will be also a challenging issue. This review provides, for the first time, a holistic view of current continuum models of skeletal muscles to identify potential gaps of current models according to the physiology of skeletal muscle. This opens new avenues for improving skeletal muscle modeling in the framework of in silico medicine.

Voltage-Dependent Sarcolemmal Ion Channel Abnormalities in the Dystrophin-Deficient Heart

Mutations in the gene encoding for the intracellular protein dystrophin cause severe forms of muscular dystrophy. These so-called dystrophinopathies are characterized by skeletal muscle weakness and degeneration. Dystrophin deficiency also gives rise to considerable complications in the heart, including cardiomyopathy development and arrhythmias. The current understanding of the pathomechanisms in the dystrophic heart is limited, but there is growing evidence that dysfunctional voltage-dependent ion channels in dystrophin-deficient cardiomyocytes play a significant role. Herein, we summarize the current knowledge about abnormalities in voltage-dependent sarcolemmal ion channel properties in the dystrophic heart, and discuss the potentially underlying mechanisms, as well as their pathophysiological relevance.

Exacerbation of dystrophic cardiomyopathy by phospholamban deficiency mediated chronically increased cardiac Ca2+ cycling in vivo

Cardiomyopathy is a significant contributor to morbidity and mortality in Duchenne muscular dystrophy (DMD). Membrane instability, leading to intracellular Ca²⁺ mishandling and overload, causes myocyte death and subsequent fibrosis in DMD cardiomyopathy. On a cellular level, cardiac myocytes from mdx mice have dysregulated Ca²⁺ handling, including increased resting Ca²⁺ and slow Ca²⁺ decay, especially evident under stress conditions. Sarco(endo)plasmic reticulum Ca²⁺ ATPase and its regulatory protein phospholamban (PLN) are potential therapeutic targets for DMD cardiomyopathy owing to their key role in regulating intracellular Ca²⁺ cycling. We tested the hypothesis that enhanced cardiac Ca²⁺ cycling would remediate cardiomyopathy caused by dystrophin deficiency. We used a genetic complementation model approach by crossing dystrophin-deficient mdx mice with PLN knockout (PLNKO) mice [termed double-knockout (DKO) mice]. As expected, adult cardiac myocytes isolated from DKO mice exhibited increased contractility and faster relaxation associated with increased Ca²⁺ transient peak height and faster Ca²⁺ decay rate compared with control mice. However, compared with wild-type, mdx, and PLNKO mice, DKO mice unexpectedly had reduced in vivo systolic and diastolic function as measured by echocardiography. Furthermore, Evans blue dye uptake was increased in DKO hearts compared with control, mdx, and PLNKO hearts, demonstrating increased membrane damage, which subsequently led to increased fibrosis in the DKO myocardium in vivo. In conclusion, despite enhanced intracellular Ca²⁺ handling at the myocyte level, DMD cardiomyopathy was exacerbated owing to unregulated chronic increases in Ca²⁺ cycling in DKO mice in vivo. These findings have potentially important implications for ongoing therapeutic strategies for the dystrophic heart. NEW & NOTEWORTHY This study examined the effects of phospholamban ablation on the pathophysiology of cardiomyopathy in dystrophin-deficient mice. In this setting, contractility and Ca²⁺ cycling were enhanced in isolated myocytes; however, in vivo heart function was diminished. Additionally, sarcolemmal integrity was compromised and fibrosis was increased. This is the first study, to our knowledge, examining unregulated Ca²⁺ cycling in the dystrophin-deficient heart. Results from this study have implications for potential therapies targeting Ca²⁺ handling in dystrophic cardiomyopathy. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/unregulated-ca2-cycling-exacerbates-dmd-cardiomyopathy/ .

Mechano-chemo-transduction in cardiac myocytes

The heart has the ability to adjust to changing mechanical loads. The Frank-Starling law and the Anrep effect describe exquisite intrinsic mechanisms the heart has for autoregulating the force of contraction to maintain cardiac output under changes of preload and afterload. Although these mechanisms have been known for more than a century, their cellular and molecular underpinnings are still debated. How does the cardiac myocyte sense changes in preload or afterload? How does the myocyte adjust its response to compensate for such changes? In cardiac myocytes Ca²⁺ is a crucial regulator of contractile force and in this review we compare and contrast recent studies from different labs that address these two important questions. The 'dimensionality' of the mechanical milieu under which experiments are carried out provide important clues to the location of the mechanosensors and the kinds of mechanical forces they can sense and respond to. As a first approximation, sensors inside the myocyte appear to modulate reactive oxygen species while sensors on the cell surface appear to also modulate nitric oxide signalling; both signalling pathways affect Ca²⁺ handling. Undoubtedly, further studies will add layers to this simplified picture. Clarifying the intimate links from cellular mechanics to reactive oxygen species and nitric oxide signalling and to Ca²⁺ handling will deepen our understanding of the Frank-Starling law and the Anrep effect, and also provide a unified view on how arrhythmias may arise in seemingly disparate diseases that have in common altered myocyte mechanics.

Severe dystrophic cardiomyopathy caused by the enteroviral protease 2A-mediated C-terminal dystrophin cleavage fragment

Enterovirus infection can cause severe cardiomyopathy in humans. The virus-encoded 2A protease is known to cleave the cytoskeletal protein dystrophin. It is unclear, however, whether cardiomyopathy results from the loss of dystrophin or is due to the emergence of a dominant-negative dystrophin cleavage product. We show for the first time that the 2A protease-mediated carboxyl-terminal dystrophin cleavage fragment (CtermDys) is sufficient to cause marked dystrophic cardiomyopathy. The sarcolemma-localized CtermDys fragment caused myocardial fibrosis, heightened susceptibility to myocardial ischemic injury, and increased mortality during cardiac stress testing in vivo. CtermDys cardiomyopathy was more severe than in hearts completely lacking dystrophin. In vivo titration of CtermDys peptide content revealed an inverse relationship between the decay of membrane-bound CtermDys and the restoration of full-length dystrophin at the sarcolemma, in support of a physiologically relevant loss of dystrophin function in this model. CtermDys gene titration and dystrophin replacement studies further established a target threshold of 50% membrane-bound intact dystrophin necessary to prevent mice from CtermDys cardiomyopathy. Conversely, the NtermDys fragment did not compete with dystrophin and had no pathological effect. Thus, CtermDys must be localized to the sarcolemma, with intact dystrophin <50% of normal levels, to exert dominant-negative peptide-dependent cardiomyopathy. These data support a two-hit dominant-negative disease mechanism where membrane-associated CtermDys severs the link to cortical actin and inhibits both full-length dystrophin and compensatory utrophin from binding at the membrane. Therefore, membrane-bound CtermDys is a new potential translational target for virus-mediated cardiomyopathy.

Cardiomyopathy of Duchenne muscular dystrophy: pathogenesis and prospect of membrane sealants as a new therapeutic approach

Duchenne muscular dystrophy (DMD) is a devastating progressive disease of striated muscle deterioration. This fatal X-linked disorder results from the loss of the protein dystrophin, which in turn causes striated muscle membrane instability. Cardiac dysfunction is a growing problem in patients with DMD, but relatively little is known about the pathophysiology of the dystrophic heart. At present, there is no effective treatment for DMD and the current clinical approaches are primarily supportive in nature. This review will discuss the pathogenesis of DMD in the heart and discuss how these pathogenic processes have led to a new class of agents directed specifically at restoring membrane integrity to dystrophic myocardium. The tri-block poloxamers, specifically poloxamer 188 (P188), are able to stabilize the membranes of dystrophic myocardium in animal models and may offer a new therapeutic approach for cardiac disease in DMD.

Voltage-gated ion channel dysfunction precedes cardiomyopathy development in the dystrophic heart

Background

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is associated with severe cardiac complications including cardiomyopathy and cardiac arrhythmias. Recent research suggests that impaired voltage-gated ion channels in dystrophic cardiomyocytes accompany cardiac pathology. It is, however, unknown if the ion channel defects are primary effects of dystrophic gene mutations, or secondary effects of the developing cardiac pathology.

Methodology/Principal Findings

To address this question, we first investigated sodium channel impairments in cardiomyocytes derived from dystrophic neonatal mice prior to cardiomyopahty development, by using the whole cell patch clamp technique. Besides the most common model for DMD, the dystrophin-deficient mdx mouse, we also used mice additionally carrying an utrophin mutation. In neonatal cardiomyocytes, dystrophin-deficiency generated a 25% reduction in sodium current density. In addition, extra utrophin-deficiency significantly altered sodium channel gating parameters. Moreover, also calcium channel inactivation was considerably reduced in dystrophic neonatal cardiomyocytes, suggesting that ion channel abnormalities are universal primary effects of dystrophic gene mutations. To assess developmental changes, we also studied sodium channel impairments in cardiomyocytes derived from dystrophic adult mice, and compared them with the respective abnormalities in dystrophic neonatal cells. Here, we found a much stronger sodium current reduction in adult cardiomyocytes. The described sodium channel impairments slowed the upstroke of the action potential in adult cardiomyocytes, and only in dystrophic adult mice, the QRS interval of the electrocardiogram was prolonged.

Conclusions/Significance

Ion channel impairments precede pathology development in the dystrophic heart, and may thus be considered potential cardiomyopathy triggers.

Phenotyping cardiac gene therapy in mice

Heart disease is the leading health problem of industrialized countries. The development of gene therapies tailored towards the heart has grown exponentially over the past decade. Murine models of heart diseases have played a pivotal role in testing novel cardiac gene therapy approaches. Unfortunately, the small body size and rapid heart rate of mice present a great challenge to heart function evaluation. Here we outline the commonly used cardiac phenotyping methods of treadmill exercise regimen, full 12-lead electrocardiographic assay and left ventricular catheterization hemodynamic assay. Application of these protocols will allow critical testing of gene therapy efficacy in mouse models of heart diseases.

Myocardial membrane injury in pediatric cardiac surgery: An animal model

OBJECTIVE

Reduced myocardial performance invariably follows pediatric cardiac surgery and is manifested by a low cardiac output state in its severest form. The role of myocardial membrane proteins in this setting is unknown. Dystrophin and dysferlin are involved in membrane integrity, whereas aquaporins selectively transport water. These proteins were examined in a model of pediatric cardiac surgery, together with a trial of poloxamer 188, which may reduce membrane injury.

METHODS

Eight lambs were randomized to saline with or without poloxamer 188. Lambs underwent 2 hours of cardiopulmonary bypass and aortic crossclamping. After a further 9 hours of monitoring, the hearts were assessed for water content, capillary leak, and protein expression.

RESULTS

Dystrophin expression was unaffected by ischemia/reperfusion, but dysferlin expression was reduced. Aquaporin 1 protein increased after ischemia/reperfusion. Poloxamer 188 administration was associated with supranormal levels of dystrophin, preservation of dysferlin expression, and normalization of aquaporin 1 expression. Poloxamer 188 was associated with less capillary leak, maintained colloid osmotic pressure, and less hemodilution. Poloxamer 188 was associated with an improved hemodynamic profile (higher blood pressure, higher venous saturation, and lower lactate), although the heart rate tended to be higher.

CONCLUSIONS

Changes in protein expression within the myocardial membrane were found in a clinically relevant model of pediatric cardiac surgery. Indicators of reduced performance, such as lower blood pressure and lower oxygen delivery, were lessened in association with the administration of the membrane protecting poloxamer 188. Poloxamer 188 was also associated with potentially beneficial changes in membrane protein expression, reduced capillary leakage, and less hemodilution.

Towards developing standard operating procedures for pre-clinical testing in the mdx mouse model of Duchenne muscular dystrophy

This review discusses various issues to consider when developing standard operating procedures for pre-clinical studies in the mdx mouse model of Duchenne muscular dystrophy (DMD). The review describes and evaluates a wide range of techniques used to measure parameters of muscle pathology in mdx mice and identifies some basic techniques that might comprise standardised approaches for evaluation. While the central aim is to provide a basis for the development of standardised procedures to evaluate efficacy of a drug or a therapeutic strategy, a further aim is to gain insight into pathophysiological mechanisms in order to identify other therapeutic targets. The desired outcome is to enable easier and more rigorous comparison of pre-clinical data from different laboratories around the world, in order to accelerate identification of the best pre-clinical therapies in the mdx mouse that will fast-track translation into effective clinical treatments for DMD.

Phenotypic Correction of α-Sarcoglycan Deficiency by Intra-arterial Injection of a Muscle-specific Serotype 1 rAAV Vector

alpha-Sarcoglycanopathy (limb-girdle muscular dystrophy type 2D, LGMD2D) is a recessive muscular disorder caused by deficiency in alpha-sarcoglycan, a transmembrane protein part of the dystrophin-associated complex. To date, no treatment exists for this disease. We constructed recombinant pseudotype-1 adeno-associated virus (rAAV) vectors expressing the human alpha-sarcoglycan cDNA from a ubiquitous or a muscle-specific promoter. Evidence of specific immune response leading to disappearance of the vector was observed with the ubiquitous promoter. In contrast, efficient and sustained transgene expression with correct sarcolemmal localization and without evident toxicity was obtained with the muscle-specific promoter after intra-arterial injection into the limbs of an LGMD2D murine model. Transgene expression resulted in restoration of the sarcoglycan complex, histological improvement, membrane stabilization, and correction of pseudohypertrophy. More importantly, alpha-sarcoglycan transfer produced full rescue of the contractile force deficits and stretch sensibility and led to an increase of the global activity of the animals when both posterior limbs are injected. Our results establish the feasibility for AAV-mediated alpha-sarcoglycan gene transfer as a therapeutic approach.

Muscle stem cells in development, regeneration, and disease

Somatic stem cell populations participate in the development and regeneration of their host tissues. Skeletal muscle is capable of complete regeneration due to stem cells that reside in skeletal muscle and nonmuscle stem cell populations. However, in severe myopathic diseases such as Duchenne Muscular Dystrophy, this regenerative capacity is exhausted. In the present review, studies will be examined that focus on the origin, gene expression, and coordinated regulation of stem cell populations to highlight the regenerative capacity of skeletal muscle and emphasize the challenges for this field. Intense interest has focused on cell-based therapies for chronic, debilitating myopathic diseases. Future studies that enhance our understanding of stem cell biology and repair mechanisms will provide a platform for therapeutic applications directed toward these chronic, life-threatening diseases.