Assessment of Therapeutic Potential of a Dual AAV Approach for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a yet incurable rare genetic disease that affects the skeletal and cardiac muscles, leading to progressive muscle wasting and premature death. DMD is caused by the lack of dystrophin, a muscle protein essential for the biochemical support and integrity of muscle fibers. Gene replacement strategies for Duchenne muscular dystrophy (DMD) employing the adeno-associated virus (AAV) face the challenge imposed by the limited packaging capacity of AAV, only allowing the accommodation of a short version of dystrophin (µDys) that is still far removed from correcting human disease. The need to develop strategies leading to the expression of a best performing dystrophin variant led to only few studies reporting on the use of dual vectors, but none reported on a method to assess in vivo transgene reconstitution efficiency, the degree of which directly affects the use of safe AAV dosing. We report here on the generation of a dual AAV vector approach for the expression of a larger dystrophin version (quasidystrophin) based on homologous recombination, and the development of a methodology employing a strategic droplet digital PCR design, to determine the recombination efficiency as well as the occurrence of unwanted concatemerization events or aberrant expression from the single vectors. We demonstrated that, upon systemic delivery in the dystrophic D2.B10-Dmd^mdx/J (DBA2mdx) mice, our dual AAV approach led to high transgene reconstitution efficiency and negligible Inverted Terminal Repeats (ITR)-dependent concatemerization, with consequent remarkable protein restoration in muscles and improvement of muscle pathology. This evidence supports the suitability of our system for gene therapy application and the potential of this methodology to assess and improve the feasibility for therapeutic translation of multiple vector approaches.

Co-Administration of Simvastatin Does Not Potentiate the Benefit of Gene Therapy in the mdx Mouse Model for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is the most common and cureless muscle pediatric genetic disease, which is caused by the lack or the drastically reduced expression of dystrophin. Experimental therapeutic approaches for DMD have been mainly focused in recent years on attempts to restore the expression of dystrophin. While significant progress was achieved, the therapeutic benefit of treated patients is still unsatisfactory. Efficiency in gene therapy for DMD is hampered not only by incompletely resolved technical issues, but likely also due to the progressive nature of DMD. It is indeed suspected that some of the secondary pathologies, which are evolving over time in DMD patients, are not fully corrected by the restoration of dystrophin expression. We recently identified perturbations of the mevalonate pathway and of cholesterol metabolism in DMD patients. Taking advantage of the mdx model for DMD, we then demonstrated that some of these perturbations are improved by treatment with the cholesterol-lowering drug, simvastatin. In the present investigation, we tested whether the combination of the restoration of dystrophin expression with simvastatin treatment could have an additive beneficial effect in the mdx model. We confirmed the positive effects of microdystrophin, and of simvastatin, when administrated separately, but detected no additive effect by their combination. Thus, the present study does not support an additive beneficial effect by combining dystrophin restoration with a metabolic normalization by simvastatin.

Titin M-line insertion sequence 7 is required for proper cardiac function in mice

Titin is a giant sarcomeric protein that is involved in a large number of functions, with a primary role in skeletal and cardiac sarcomere organization and stiffness. The titin gene (TTN) is subject to various alternative splicing events, but in the region that is present at the M-line, the only exon that can be spliced out is Mex5, which encodes for the insertion sequence 7 (is7). Interestingly, in the heart, the majority of titin isoforms are Mex5+, suggesting a cardiac role for is7. Here, we performed comprehensive functional, histological, transcriptomic, microscopic and molecular analyses of a mouse model lacking the Ttn Mex5 exon (ΔMex5), and revealed that the absence of the is7 is causative for dilated cardiomyopathy. ΔMex5 mice showed altered cardiac function accompanied by increased fibrosis and ultrastructural alterations. Abnormal expression of excitation-contraction coupling proteins was also observed. The results reported here confirm the importance of the C-terminal region of titin in cardiac function and are the first to suggest a possible relationship between the is7 and excitation-contraction coupling. Finally, these findings give important insights for the identification of new targets in the treatment of titinopathies.

Minimal Consequences of CMAH and DBA/2 Backgrounds on a FKRP Deficient Model

BACKGROUND

Muscular dystrophies (MD) are a large group of genetic diseases characterized by a progressive loss of muscle. The Latent TGFβ Binding Protein 4 (LTBP4) in the DBA/2 background and the Cytidine Monophosphate-sialic Acid Hydroxylase (CMAH) proteins were previously identified as genetic modifiers in severe MD.

OBJECTIVE

We investigated whether these modifiers could also influence a mild phenotype such as the one observed in a mouse model of Limb-Girdle MD2I (LGMD2I).

METHODS

The FKRPL276I mouse model was backcrossed onto the DBA/2 background, and in separate experiments the Cmah gene was inactivated in FKRPL276I mice by crossing with a Cmah-/- mouse and selecting the double-mutants. The mdx mouse was used as control for these two genome modifications. Consequences at the histological level as well as quantification of expression level by RT-qPCR of genes relevant for muscular dystrophy were then performed.

RESULTS

We observed minimal to no effect of the DBA/2 background on the mild FKRPL276I mouse phenotype, while this same background was previously shown to increase inflammation and fibrosis in the mdx mouse. Similarly, the Cmah-/- deletion had no observable effect on the FKRPL276I mouse phenotype whereas it was seen to increase features of regeneration in mdx mice.

CONCLUSIONS

These modifiers were not observed to impact the severity of the presentation of the mild FKRPL276I model. An interesting association of the CMAH modifier with the regeneration process in the mdx model was seen and sheds new light on the influence of this protein on the dystrophic phenotype.

Titin splicing regulates cardiotoxicity associated with calpain 3 gene therapy for limb-girdle muscular dystrophy type 2A

Limb-girdle muscular dystrophy type 2A (LGMD2A or LGMDR1) is a neuromuscular disorder caused by mutations in the calpain 3 gene (CAPN3). Previous experiments using adeno-associated viral (AAV) vector-mediated calpain 3 gene transfer in mice indicated cardiac toxicity associated with the ectopic expression of the calpain 3 transgene. Here, we performed a preliminary dose study in a severe double-knockout mouse model deficient in calpain 3 and dysferlin. We evaluated safety and biodistribution of AAV9-desmin-hCAPN3 vector administration to nonhuman primates (NHPs) with a dose of 3 × 10¹³ viral genomes/kg. Vector administration did not lead to observable adverse effects or to detectable toxicity in NHP. Of note, the transgene expression did not produce any abnormal changes in cardiac morphology or function of injected animals while reaching therapeutic expression in skeletal muscle. Additional investigation on the underlying causes of cardiac toxicity observed after gene transfer in mice and the role of titin in this phenomenon suggest species-specific titin splicing. Mice have a reduced capacity for buffering calpain 3 activity compared to NHPs and humans. Our studies highlight a complex interplay between calpain 3 and titin binding sites and demonstrate an effective and safe profile for systemic calpain 3 vector delivery in NHP, providing critical support for the clinical potential of calpain 3 gene therapy in humans.

Single SERCA2a Therapy Ameliorated Dilated Cardiomyopathy for 18 Months in a Mouse Model of Duchenne Muscular Dystrophy

Loss of dystrophin leads to Duchenne muscular dystrophy (DMD). A pathogenic feature of DMD is the significant elevation of cytosolic calcium. Supraphysiological calcium triggers protein degradation, membrane damage, and eventually muscle death and dysfunction. Sarcoplasmic/endoplasmic reticulum (SR) calcium ATPase (SERCA) is a calcium pump that transports cytosolic calcium to the SR during excitation-contraction coupling. We hypothesize that a single systemic delivery of SERCA2a with adeno-associated virus (AAV) may improve calcium recycling and provide long-lasting benefits in DMD. To test this, we injected an AAV9 human SERCA2a vector (6 × 10¹² viral genome particles/mouse) intravenously to 3-month-old mdx mice, the most commonly used DMD model. Immunostaining and western blot showed robust human SERCA2a expression in the heart and skeletal muscle for 18 months. Concomitantly, SR calcium uptake was significantly improved in these tissues. SERCA2a therapy significantly enhanced grip force and treadmill performance, completely prevented myocardial fibrosis, and normalized electrocardiograms (ECGs). Cardiac catheterization showed normalization of multiple systolic and diastolic hemodynamic parameters in treated mice. Importantly, chamber dilation was completely prevented, and ejection fraction was restored to the wild-type level. Our results suggest that a single systemic AAV9 SERCA2a therapy has the potential to provide long-lasting benefits for DMD.

A comparison of AAV strategies distinguishes overlapping vectors for efficient systemic delivery of the 6.2 kb Dysferlin coding sequence

Recombinant adeno-associated virus (rAAV) is currently the best vector for gene delivery into the skeletal muscle. However, the 5-kb packaging size of this virus is a major obstacle for large gene transfer. This past decade, many different strategies were developed to circumvent this issue (concatemerization-splicing, overlapping vectors, hybrid dual or fragmented AAV). Loss of function mutations in the DYSF gene whose coding sequence is 6.2kb lead to progressive muscular dystrophies (LGMD2B: OMIM_253601; MM: OMIM_254130; DMAT: OMIM_606768). In this study, we compared large gene transfer techniques to deliver the DYSF gene into the skeletal muscle. After rAAV8s intramuscular injection into dysferlin deficient mice, we showed that the overlap strategy is the most effective approach to reconstitute a full-length messenger. After systemic administration, the level of dysferlin obtained on different muscles corresponded to 0.5- to 2-fold compared to the normal level. We further demonstrated that the overlapping vector set was efficient to correct the histopathology, resistance to eccentric contractions and whole body force in the dysferlin deficient mice. Altogether, these data indicate that using overlapping vectors could be a promising approach for a potential clinical treatment of dysferlinopathies.

Full-length dystrophin reconstitution with adeno-associated viral vectors

Duchenne muscular dystrophy (DMD) is the most common lethal muscle disorder in children. It is caused by mutations of the dystrophin gene. Adeno-associated virus (AAV)-mediated gene replacement therapy has been actively pursued to treat DMD. However, this promising therapeutic modality has been challenged by the small packaging capacity of the AAV vector. The size of the full-length dystrophin cDNA is >11 kb, while an AAV virus can carry only a 5 kb genome. Innovative high-capacity AAV vectors may offer an opportunity to express the full-length dystrophin coding sequence. Here we describe several sets of tri-AAV vectors for full-length human dystrophin delivery. In each set, the full-length human dystrophin cDNA was split into three fragments and independently packaged into separate recombinant AAV vectors. Each vector was engineered with unique recombination signals for directional recombination. Tri-AAV vectors were coinjected into the tibialis anterior muscle of dystrophin-deficient mdx4cv mice. Thirty-five days after injection, dystrophin expression was examined by immunofluorescence staining. Despite low reconstitution efficiency, full-length human dystrophin was successfully expressed from the tri-AAV vectors. Our results suggest that AAV can be engineered to express an extra-large (up to 15 kb) gene that is approximately three times the size of the wild-type AAV genome. Further optimization of the trivector strategy may expand the utility of AAV for human gene therapy.

Long-term robust myocardial transduction of the dog heart from a peripheral vein by adeno-associated virus serotype-8

Molecular intervention using noninvasive myocardial gene transfer holds great promise for treating heart diseases. Robust cardiac transduction from peripheral vein injection has been achieved in rodents using adeno-associated virus (AAV) serotype-9 (AAV-9). However, a similar approach has failed to transduce the heart in dogs, a commonly used large animal model for heart diseases. To develop an effective noninvasive method to deliver exogenous genes to the dog heart, we employed an AAV-8 vector that expresses human placental alkaline phosphatase reporter gene under the transcriptional regulation of the Rous sarcoma virus promoter. Vectors were delivered to three neonatal dogs at the doses of 1.35×10(14), 7.14×10(14), and 9.06×10(14) viral genome particles/kg body weight via the jugular vein. Transduction efficiency and overall safety were evaluated at 1.5, 2.5, and 12 months postinjection. AAV delivery was well tolerated and dog growth was normal. Blood chemistry and internal organ histology were unremarkable. Widespread skeletal muscle transduction was observed in all dogs without T-cell infiltration. Encouragingly, whole heart myocardial transduction was achieved in two dogs that received higher doses and cardiac expression lasted for at least 1 year. In summary, peripheral vein AAV-8 injection may represent a simple heart gene transfer method in large mammals. Further optimization of this gene delivery strategy may open the door for a readily applicable gene therapy method to treat many heart diseases.

Lack of correlation between outcomes of membrane repair assay and correction of dystrophic changes in experimental therapeutic strategy in dysferlinopathy

Mutations in the dysferlin gene are the cause of Limb-girdle Muscular Dystrophy type 2B and Miyoshi Myopathy. The dysferlin protein has been implicated in sarcolemmal resealing, leading to the idea that the pathophysiology of dysferlin deficiencies is due to a deficit in membrane repair. Here, we show using two different approaches that fullfiling membrane repair as asseyed by laser wounding assay is not sufficient for alleviating the dysferlin deficient pathology. First, we generated a transgenic mouse overexpressing myoferlin to test the hypothesis that myoferlin, which is homologous to dysferlin, can compensate for the absence of dysferlin. The myoferlin overexpressors show no skeletal muscle abnormalities, and crossing them with a dysferlin-deficient model rescues the membrane fusion defect present in dysferlin-deficient mice in vitro. However, myoferlin overexpression does not correct muscle histology in vivo. Second, we report that AAV-mediated transfer of a minidysferlin, previously shown to correct the membrane repair deficit in vitro, also fails to improve muscle histology. Furthermore, neither myoferlin nor the minidysferlin prevented myofiber degeneration following eccentric exercise. Our data suggest that the pathogenicity of dysferlin deficiency is not solely related to impairment in sarcolemmal repair and highlight the care needed in selecting assays to assess potential therapies for dysferlinopathies.

A naturally occurring human minidysferlin protein repairs sarcolemmal lesions in a mouse model of dysferlinopathy

Dysferlinopathies are autosomal recessive, progressive muscle dystrophies caused by mutations in DYSF, leading to a loss or a severe reduction of dysferlin, a key protein in sarcolemmal repair. Currently, no etiological treatment is available for patients affected with dysferlinopathy. As for other muscular dystrophies, gene therapy approaches based on recombinant adeno-associated virus (rAAV) vectors are promising options. However, because dysferlin messenger RNA is far above the natural packaging size of rAAV, full-length dysferlin gene transfer would be problematic. In a patient presenting with a late-onset moderate dysferlinopathy, we identified a large homozygous deletion, leading to the production of a natural "minidysferlin" protein. Using rAAV-mediated gene transfer into muscle, we demonstrated targeting of the minidysferlin to the muscle membrane and efficient repair of sarcolemmal lesions in a mouse model of dysferlinopathy. Thus, as previously demonstrated in the case of dystrophin, a deletion mutant of the dysferlin gene is also functional, suggesting that dysferlin's structure is modular. This minidysferlin protein could be used as part of a therapeutic strategy for patients affected with dysferlinopathies.

Efficient recovery of dysferlin deficiency by dual adeno-associated vector-mediated gene transfer

Deficiency of the dysferlin protein presents as two major clinical phenotypes: limb-girdle muscular dystrophy type 2B and Miyoshi myopathy. Dysferlin is known to participate in membrane repair, providing a potential hypothesis to the underlying pathophysiology of these diseases. The size of the dysferlin cDNA prevents its direct incorporation into an adeno-associated virus (AAV) vector for therapeutic gene transfer into muscle. To bypass this limitation, we split the dysferlin cDNA at the exon 28/29 junction and cloned it into two independent AAV vectors carrying the appropriate splicing sequences. Intramuscular injection of the corresponding vectors into a dysferlin-deficient mouse model led to the expression of full-length dysferlin for at least 1 year. Importantly, systemic injection in the tail vein of the two vectors led to a widespread although weak expression of the full-length protein. Injections were associated with an improvement of the histological aspect of the muscle, a reduction in the number of necrotic fibers, restoration of membrane repair capacity and a global improvement in locomotor activity. Altogether, these data support the use of such a strategy for the treatment of dysferlin deficiency.