Dystrophin Gene-Editing Stability Is Dependent on Dystrophin Levels in Skeletal but Not Cardiac Muscles

Gene editing is often touted as a permanent method for correcting mutations, but its long-term benefits in Duchenne muscular dystrophy (DMD) may depend on sufficiently high editing efficiencies to halt muscle degeneration. Here, we explored the persistence of dystrophin expression following recombinant adeno-associated virus serotype 6 (rAAV6):CRISPR-Cas9-mediated multi-exon deletion/reframing in systemically injected 2- and 11-week-old dystrophic mice and show that induction of low dystrophin levels persists for several months in cardiomyocytes but not in skeletal muscles, where myofibers remain susceptible to necrosis and regeneration. Whereas gene-correction efficiency in both muscle types was enhanced with increased ratios of guide RNA (gRNA)-to-nuclease vectors, obtaining high dystrophin levels in skeletal muscles via multi-exon deletion remained challenging. In contrast, when AAV-microdystrophin was codelivered with editing components, long-term gene-edited dystrophins persisted in both muscle types. These results suggest that the high rate of necrosis and regeneration in skeletal muscles, compared with the relative stability of dystrophic cardiomyocytes, caused the rapid loss of edited genomes. Consequently, stable dystrophin expression in DMD skeletal muscles will require either highly efficient gene editing or the use of cotreatments that decrease skeletal muscle degeneration.

The clock regulator Bmal1 protects against muscular dystrophy

The muscle-intrinsic clock machinery is required for the maintenance of muscle growth, remodeling and function. Our previous studies demonstrated that the essential transcription activator of the molecular clock feed-back loop, Brain and Muscle Arnt-Like 1(Bmal1), plays a critical role in myogenic progenitor behavior to promote and regenerative myogenesis. Using genetic approaches targeting Bmal1 in the DMDmdx (mdx) dystrophic mouse model, here we report that the loss of Bmal1 function significantly accelerated dystrophic disease progression. In contrast to the mild dystrophic changes in mdx mice, the genetic loss-of-function of Bmal1 aggravated muscle damage in this dystrophic disease background, as indicated by persistently elevated creatine kinase levels, increased injury area and reduced muscle grip strength. Mechanistic studies revealed that markedly impaired myogenic progenitor proliferation and myogenic response underlie the defective new myofiber formation in the chronic dystrophic milieu. Taken together, our study identified the function of pro-myogenic clock gene Bmal1 in protecting against dystrophic damage, suggesting the potential for augmenting Bmal1 function to ameliorate dystrophic or degenerative muscle diseases.

PDE10A Inhibition Reduces the Manifestation of Pathology in DMD Zebrafish and Represses the Genetic Modifier PITPNA

Duchenne muscular dystrophy (DMD) is a severe genetic disorder caused by mutations in the DMD gene. Absence of dystrophin protein leads to progressive degradation of skeletal and cardiac function and leads to premature death. Over the years, zebrafish have been increasingly used for studying DMD and are a powerful tool for drug discovery and therapeutic development.

In our study, a birefringence screening assay led to identification of phosphodiesterase 10A (PDE10A) inhibitors that reduced the manifestation of dystrophic muscle phenotype in dystrophin-deficient sapje-like zebrafish larvae. PDE10A has been validated as a therapeutic target by pde10a morpholino-mediated reduction in muscle pathology and improvement in locomotion, muscle, and vascular function as well as long-term survival in sapje-like larvae. PDE10A inhibition in zebrafish and DMD patient-derived myoblasts were also associated with reduction of PITPNA expression that has been previously identified as a protective genetic modifier in two exceptional dystrophin-deficient golden retriever muscular dystrophy (GRMD) dogs that escaped the dystrophic phenotype. The combination of a phenotypic assay and relevant functional assessments in the sapje-like zebrafish enhances the potential for the prospective discovery of DMD therapeutics. Indeed, our results suggest a new application for a PDE10A inhibitor as a potential DMD therapeutic to be investigated in a mouse model of DMD.

Comprehensive genetic analysis of 961 unrelated Duchenne Muscular Dystrophy patients: Focus on diagnosis, prevention and therapeutic possibilities

Recently DNA sequencing analysis has played a vital role in the unambiguous diagnosis of clinically suspected patients with Duchenne Muscular Dystrophy (DMD). DMD is a monogenic, X-linked, recessive, degenerative pediatric neuromuscular disorder affecting males, invariably leading to fatal cardiopulmonary failure. Early and precise diagnosis of the disease is an essential part of an effective disease management strategy as care guidelines and prevention through counseling need to be initiated at the earliest particularly since therapies are now available for a subset of patients. In this manuscript we report the DMD gene mutational profiles of 961 clinically suspected male DMD patients, 99% of whom were unrelated. We utilized a molecular diagnostic approach which is cost-effective for most patients and follows a systematic process that sequentially involves identification of hotspot deletions using mPCR, large deletions and duplications using MLPA and small insertions/ deletions and point mutations using an NGS muscular dystrophy gene panel. Pathogenic DMD gene mutations were identified in 84% of patients. Our data compared well with the frequencies and distribution of deletions and duplications reported in the DMD gene in other published studies. We also describe a number of rare in-frame mutations, which appeared to be enriched in the 5’ proximal hotspot region of the DMD gene. Furthermore, we identified a family with a rare non-contiguous deletion mutation in the DMD gene where three males were affected and two females were deemed carriers. A subset of patients with mutations in the DMD gene who are likely to benefit therapeutically from new FDA and EMA approved drugs were found in our cohort. Given that the burden of care for DMD patients invariably falls on the mothers, particularly in rural India, effective genetic counseling followed by carrier screening is crucial for prevention of this disorder. We analyzed the carrier status of consented female relatives of 463 probands to gauge the percentage of patients with familial disease. Our analysis revealed 43.7% of mothers with DMD gene mutations. Our comprehensive efforts, involving complete genetic testing coupled with compassionate genetic counseling provided to DMD patients and their families, are intended to improve the quality of life of DMD patients and to empower carrier females to make informed reproductive choices to impede the propagation of this deadly disease.

Reduction of Oxidative Damage and Inflammatory Response in the Diaphragm Muscle of mdx Mice Using Iron Chelator Deferoxamine

Oxidative stress and inflammatory processes strongly contribute to pathogenesis in Duchenne muscular dystrophy (DMD). Based on evidence that excess iron may increase oxidative stress and contribute to the inflammatory response, we investigated whether deferoxamine (DFX), a potent iron chelating agent, reduces oxidative stress and inflammation in the diaphragm (DIA) muscle of mdx mice (an experimental model of DMD). Fourteen-day-old mdx mice received daily intraperitoneal injections of DFX at a dose of 150 mg/kg body weight, diluted in saline, for 14 days. C57BL/10 and control mdx mice received daily intraperitoneal injections of saline only, for 14 days. Grip strength was evaluated as a functional measure, and blood samples were collected for biochemical assessment of muscle fiber degeneration. In addition, the DIA muscle was removed and processed for histopathology and Western blotting analysis. In mdx mice, DFX reduced muscle damage and loss of muscle strength. DFX treatment also resulted in a significant reduction of dystrophic inflammatory processes, as indicated by decreases in the inflammatory area and in NF-κB levels. DFX significantly decreased oxidative damage, as shown by lower levels of 4-hydroxynonenal and a reduction in dihydroethidium staining in the DIA muscle of mdx mice. The results of the present study suggest that DFX may be useful in therapeutic strategies to ameliorate dystrophic muscle pathology, possibly via mechanisms involving oxidative and inflammatory pathways.

Long-term blocking of calcium channels in mdx mice results in differential effects on heart and skeletal muscle

The disease mechanisms underlying dystrophin-deficient muscular dystrophy are complex, involving not only muscle membrane fragility, but also dysregulated calcium homeostasis. Specifically, it has been proposed that calcium channels directly initiate a cascade of pathological events by allowing calcium ions to enter the cell. The objective of this study was to investigate the effect of chronically blocking calcium channels with the aminoglycoside antibiotic streptomycin from onset of disease in the mdx mouse model of Duchenne muscular dystrophy (DMD). Treatment in utero onwards delayed onset of dystrophic symptoms in the limb muscle of young mdx mice, but did not prevent degeneration and regeneration events occurring later in the disease course. Long-term treatment had a positive effect on limb muscle pathology, reduced fibrosis, increased sarcolemmal stability, and promoted muscle regeneration in older mice. However, streptomycin treatment did not show positive effects in diaphragm or heart muscle, and heart pathology was worsened. Thus, blocking calcium channels even before disease onset does not prevent dystrophy, making this an unlikely treatment for DMD. These findings highlight the importance of analyzing several time points throughout the life of the treated mice, as well as analyzing many tissues, to get a complete picture of treatment efficacy.

Rescue of developmental defects by blastocyst stem cell injection: towards elucidation of neomorphic corrective pathways

Stem cell-based therapy is an exciting area of high potential for regenerative medicine. To study disease prevention, we inject mouse embryonic stem cells (ESCs) into a variety of mouse blastocysts, most of which harbor mutations. Mice derived from these mutant blastocysts develop human-like diseases, either at developmental stages or in the adult, but blastocyst injection of ESCs prevents disease from occurring. Rather than entirely repopulating the affected organs, with just 20% of chimerism, the ESCs replenish protein levels that are absent in mutant mice, and induce novel or "neomorphic" signals that help circumvent the requirements for the mutations. We also show data indicating that the "neomorphic" mechanisms arise as a result of blastocyst injection of ESCs, regardless of the nature of the host blastocyst (mutant or wild-type). Thus, blastocyst injection of ESCs not only allows the study of disease prevention, but also unveils novel pathways whose activation may aid in the correction of congenital or acquired disease.

Green tea extract decreases muscle pathology and NF-kappaB immunostaining in regenerating muscle fibers of mdx mice

BACKGROUND & AIMS

Duchenne muscular dystrophy is a debilitating genetic disorder characterized by severe muscle wasting and early death in afflicted boys. The primary cause of this disease is mutations in the dystrophin gene resulting in massive muscle degeneration and inflammation. The purpose of this study was to determine if dystrophic muscle pathology and inflammation were decreased by pre-natal and early dietary intervention with green tea extract.

METHODS

Mdx breeder mice and pups were fed diets containing 0.25% or 0.5% green tea extract and compared to untreated mdx and C57BL/6J mice. Serum creatine kinase was assessed as a systemic indicator of muscle damage. Quantitative histopathological and immunohistochemical techniques were used to determine muscle pathology, macrophage infiltration, and NF-kappaB localization.

RESULTS

Early treatment of mdx mice with green tea extract significantly decreased serum creatine kinase by approximately 85% at age 42 days (P< or =0.05). In these mice, the area of normal fiber morphology was increased by as much as approximately 32% (P< or =0.05). The primary histopathological change was a approximately 21% decrease in the area of regenerating fibers (P< or =0.05). NF-kappaB staining in regenerating muscle fibers was also significantly decreased in green tea extract-treated mdx mice when compared to untreated mdx mice (P< or =0.05).

CONCLUSION

Early treatment with green tea extract decreases dystrophic muscle pathology potentially by regulating NF-kappaB activity in regenerating muscle fibers.

Perindopril preventive treatment on mortality in Duchenne muscular dystrophy: 10 years' follow-up

BACKGROUND

Duchenne muscular dystrophy (DMD), an X-linked disorder due to lack of dystrophin, is associated with muscle weakness and myocardial dysfunction. Although preliminary data support the efficacy of angiotensin-converting enzyme inhibitors on left ventricular (LV) function, our aim was to examine the long-term impact of a preventive treatment with perindopril on mortality in children with DMD.

METHODS

Patients with DMD between the ages of 9.5 and 13 years presenting with normal LV ejection fraction were included in this prospective study. They were randomly assigned for 3 years to perindopril, 2 to 4 mg (group 1), or placebo (group 2) in a double-blind protocol, followed by open-label treatment with perindopril for up to 10 years. Survival rate at 10 years in each group is reported.

RESULTS

There were 28 patients assigned to group 1 and 29 to group 2. Baseline characteristics were similar in both groups. At the end of the 10 years' follow-up period, survival status was available for all included patients: 26 (92.9%) of 28 patients in group 1 were alive at 10 years versus 19 (65.5%) of 29 in group 2 (P = .02). Kaplan-Meier cumulative survival was significantly lower in group 2 than in group 1 (P = .013).

CONCLUSION

Early initiation of treatment with perindopril is associated with a lower mortality in patients with DMD with normal LV ejection fraction at study entry.

Effective repetitive dystrophin gene transfer into skeletal muscle of adult mdx mice using a helper-dependent adenovirus vector expressing the coxsackievirus and adenovirus receptor (CAR) and dystrophin

BACKGROUND

The helper-dependent adenovirus (HDAd) vector is less immunogenic and has a larger cloning capacity of up to 37 kb enough to carry the full-length dystrophin cDNA. However, high and long-term expression of dystrophin transduced to mature muscle still remains difficult. One of the main reasons for this is that the expression of the coxsackievirus and adenovirus receptor (CAR) is very low in mature muscle.

METHODS

We have constructed two different HDAd vectors. One contains the LacZ and the murine full-length dystrophin expression cassette (HDAdLacZ-dys), and the other is a new, improved vector containing the CAR and the dystrophin expression cassette (HDAdCAR-dys).

RESULTS

We initially demonstrated high dystrophin expression and prevention of the dystrophic pathology in mdx muscle injected during the neonatal phase with HDAdLacZ-dys. Furthermore, we demonstrated that repeated injections of HDAdCAR-dys into mature muscle led to approximately nine times greater dystrophin-positive fibers in number than a single injection, thereby recovering the expression of dystrophin-associated proteins. This data has also shown that HDAdCAR-dys enabled administration of adenovirus (Ad) vector to the host with pre-existing immunity to the same serotype of Ad.

CONCLUSIONS

Repetitive injections of the HDAd vector containing the CAR and the dystrophin expression cassette could improve the efficiency of subsequent dystrophin gene transfer to mature mdx muscle. This result suggests that our new HDAd vector will provide a novel gene therapy strategy for Duchenne muscular dystrophy, raising the prospects for gene therapy of other hereditary myopathies.

[First case of successful pregnancy after preimplantation genetic diagnosis]

Preimplantation genetic diagnosis is introduced for prevention of genetic disorders. The combination of in vitro fertilization technique and single cell molecular genetic diagnosis allows only unaffected embryos to be selected for embryotransfer, providing a healthy pregnancy and so also avoiding the need for its possible termination.

CASE REPORT

The authors report the first successful case of the clinical application of preimplantation genetic diagnosis in Hungary, resulting the birth of an unaffected baby. Preembryonal biopsy and subsequent fluorescent-PCR analysis of the blastomeres taken from the preembryos of a woman who is a carrier for Duchenne muscular dystrophy was performed. Sexing of the preembryos by X and Y chromosome specific primers were performed and three female preembryos were found.

RESULT

All three preembryos were transferred into the uterus, which resulted a singleton pregnancy, resulting the birth of a healthy female baby. The authors are offering preimplantation genetic diagnosis with sexing for couples, who are at risk of having a child with X-linked diseases, or are carriers for the delta-F508 mutation of cystic fibrosis.

CONCLUSIONS

The application of this diagnostic tool is also planned for other monogenic disorders.

Prevention of pathology in mdx mice by expression of utrophin: analysis using an inducible transgenic expression system

Duchenne muscular dystrophy results from the absence of dystrophin, a cytoskeletal protein. Previously, we have shown in a transgenic mouse model of the disease (mdx) that high levels of expression of the dystrophin-related protein, utrophin can prevent pathology. We developed a new transgenic mouse model where muscle specific utrophin expression was conditioned by addition of tetracycline in water. Transgene expression was turned on at different time points: in utero, at birth, 10 and 30 days after birth. We obtained moderate levels of expression, variable from fibre to fibre (mosaicism) but sufficient to induce a correct localization of the dystro-sarcoglycan complex. Histology revealed a reduction of necrotic foci and of the percentage of centronucleated fibres, which remained still largely above the normal level. Isometric force was not improved but the resistance to eccentric contractions was significantly stronger. When utrophin expression was activated 30 days after birth, improvements were marginal, suggesting that the age at which utrophin therapy is initiated could be an important factor. Our results also provide an unexpected insight into the pathogenesis of the dystrophinopathies. We observed a complete normalization of the characteristics of the mechano-sensitive/voltage-independent Ca(2+) channels (occurrence, open probabilities and Ca(2+) currents), while the classical markers of dystrophy were still abnormal. These observations question the role of increased Ca(2+) channel activity in initiating the dystrophic process. The new model shows that utrophin therapy, initiated after birth, can be effective, but the extent of correction of the various symptoms of dystrophinopathy critically depends on the amount of utrophin expressed.

Expression of Dp260 in muscle tethers the actin cytoskeleton to the dystrophin-glycoprotein complex and partially prevents dystrophy

Dystrophin forms a mechanical link between the actin cytoskeleton and the extracellular matrix in muscle that helps maintain sarcolemmal integrity. Two regions of dystrophin have been shown to bind actin: the N-terminal domain and rod domain repeats 11-17. To better understand the roles of these two domains and whether the rod domain actin-binding domain alone can support a mechanically functional link with actin, we constructed transgenic mice expressing Dp260 in skeletal muscle. Dp260, the retinal isoform of dystrophin, lacks the N-terminal domain and a significant portion of the rod domain, but retains the rod domain actin-binding domain. Our results indicate that Dp260 expression restores a stable association between costameric actin and the sarcolemma, assembles the dystrophin-glycoprotein complex, and significantly slows the progression of the dystrophy in the dystrophin-deficient mdx mouse. We assessed the functional integrity of the mechanical link in Dp260 transgenic mdx mice and found that Dp260 muscles showed normal resistance to contraction-induced injury, but dramatic reductions in force generation similar to those found with mdx muscles. Morphologically, Dp260 muscles displayed reduced amounts of inflammation and fibrosis, but still showed a significant, albeit reduced, amount of degeneration/regeneration. These data demonstrate that protection from contraction-induced injury can dramatically ameliorate, but not completely halt, the dystrophic process. We suggest that a non-mechanical defect, attributed to the loss of the N terminus of dystrophin, is likely responsible for the residual dystrophy observed.

Green tea extract decreases muscle necrosis in mdx mice and protects against reactive oxygen species

BACKGROUND

Duchenne muscular dystrophy is a severe X-linked congenital disorder characterized by lethal muscle wasting caused by the absence of the structural protein dystrophin.

OBJECTIVE

Because generation of reactive oxygen species appears to play an important role in the pathogenesis of this disease, we tested whether antioxidant green tea extract could diminish muscle necrosis in the mdx mouse dystrophy model.

DESIGN

A diet supplemented with 0.01% or 0.05% green tea extract was fed to dams and neonates for 4 wk beginning on the day of birth. Muscle necrosis and regeneration were determined in stained cryosections of soleus and elongator digitorum longus muscles. Radical scavenging by green tea extract was determined in differentiated cultured C2C12 cells treated with tert-butylhydroperoxide, with the use of 2',7'-dichlorofluorescin diacetate as a radical detector.

RESULTS

This feeding regimen significantly and dose-dependently reduced necrosis in the fast-twitch muscle elongator digitorum longus but at the doses tested had no effect on the slow-twitch soleus muscle. Green tea extract concentration-dependently decreased oxidative stress induced by tert-butylhydroperoxide treatment of cultured mouse C2C12 myotubes. The lower effective dose tested in mdx mice corresponds to approximately equal to 1.4 L (7 cups) green tea/d in humans.

CONCLUSION

Green tea extract may improve muscle health by reducing or delaying necrosis in mdx mice by an antioxidant mechanism.

Prevention of the dystrophic phenotype in dystrophin/utrophin-deficient muscle following adenovirus-mediated transfer of a utrophin minigene

Duchenne muscular dystrophy (DMD) is a progressive muscle wasting disorder caused by the lack of a subsarcolemmal protein, dystrophin. We have previously shown that the dystrophin-related protein, utrophin is able to compensate for the lack of dystrophin in the mdx mouse, the mouse model for DMD. Here, we explore whether utrophin delivered to the limb muscle of dystrophin/utrophin-deficient double knockout (dko) neonatal mice can protect the muscle from subsequent dystrophic damage. Utrophin delivery may avoid the potential problems of an immune response associated with the delivery of dystrophin to a previously dystrophin-deficient host. Dko muscle (tibialis anterior) was injected with a first generation recombinant adenovirus containing a utrophin minigene. Up to 95% of the fibres continued expressing the minigene 30 days after injection. Expression of utrophin caused a marked reduction from 80% centrally nucleated fibres (CNFs) in the uninjected dko TA to 12% in the injected dko TA. Within the region of the TA expressing the utrophin minigene, a significant decrease in the prevelance of necrosis was noted. These results demonstrate that the utrophin minigene delivered using an adenoviral vector is able to afford protection to the dystrophin/utrophin-deficient muscle of the dko mouse. Gene Therapy (2000) 7, 201-204.