Development of Novel Micro-dystrophins with Enhanced Functionality

Split intein-mediated protein trans-splicing to express large dystrophins

Gene replacement using adeno-associated virus (AAV) vectors is a promising therapeutic approach for many diseases1,2. However, this therapeutic modality is challenged by the packaging capacity of AAVs (approximately 4.7 kilobases)3, limiting its application for disorders involving large coding sequences, such as Duchenne muscular dystrophy, with a 14 kilobase messenger RNA. Here we developed a new method for expressing large dystrophins by utilizing the protein trans-splicing mechanism mediated by split inteins. We identified several split intein pairs that efficiently join two or three fragments to generate a large midi-dystrophin or the full-length protein. We show that delivery of two or three AAVs into dystrophic mice results in robust expression of large dystrophins and significant physiological improvements compared with micro-dystrophins. Moreover, using the potent myotropic AAVMYO4, we demonstrate that low total doses (2 × 1013 viral genomes per kg) are sufficient to express large dystrophins in striated muscles body-wide with significant physiological corrections in dystrophic mice. Our data show a clear functional superiority of large dystrophins over micro-dystrophins that are being tested in clinical trials. This method could benefit many patients with Duchenne or Becker muscular dystrophy, regardless of genotype, and could be adapted to numerous other disorders caused by mutations in large genes that exceed the AAV capacity.

AAV-NRIP gene therapy ameliorates motor neuron degeneration and muscle atrophy in ALS model mice

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is characterized by progressive motor neuron (MN) degeneration, leading to neuromuscular junction (NMJ) dismantling and severe muscle atrophy. The nuclear receptor interaction protein (NRIP) functions as a multifunctional protein. It directly interacts with calmodulin or α-actinin 2, serving as a calcium sensor for muscle contraction and maintaining sarcomere integrity. Additionally, NRIP binds with the acetylcholine receptor (AChR) for NMJ stabilization. Loss of NRIP in muscles results in progressive motor neuron degeneration with abnormal NMJ architecture, resembling ALS phenotypes. Therefore, we hypothesize that NRIP could be a therapeutic factor for ALS.

METHODS

We used SOD1 G93A mice, expressing human SOD1 with the ALS-linked G93A mutation, as an ALS model. An adeno-associated virus vector encoding the human NRIP gene (AAV-NRIP) was generated and injected into the muscles of SOD1 G93A mice at 60 days of age, before disease onset. Pathological and behavioral changes were measured to evaluate the therapeutic effects of AAV-NRIP on the disease progression of SOD1 G93A mice.

RESULTS

SOD1 G93A mice exhibited lower NRIP expression than wild-type mice in both the spinal cord and skeletal muscle tissues. Forced NRIP expression through AAV-NRIP intramuscular injection was observed in skeletal muscles and retrogradely transduced into the spinal cord. AAV-NRIP gene therapy enhanced movement distance and rearing frequencies in SOD1 G93A mice. Moreover, AAV-NRIP increased myofiber size and slow myosin expression, ameliorated NMJ degeneration and axon terminal denervation at NMJ, and increased the number of α-motor neurons (α-MNs) and compound muscle action potential (CMAP) in SOD1 G93A mice.

CONCLUSIONS

AAV-NRIP gene therapy ameliorates muscle atrophy, motor neuron degeneration, and axon terminal denervation at NMJ, leading to increased NMJ transmission and improved motor functions in SOD1 G93A mice. Collectively, AAV-NRIP could be a potential therapeutic drug for ALS.

Systemic delivery of full-length dystrophin in Duchenne muscular dystrophy mice

Current gene therapy for Duchenne muscular dystrophy (DMD) utilizes adeno-associated virus (AAV) to deliver micro-dystrophin (µDys), which does not provide full protection for striated muscles as it lacks many important functional domains of full-length (FL) dystrophin. Here we develop a triple vector system to deliver FL-dystrophin into skeletal and cardiac muscles. We split FL-dystrophin into three fragments linked to two orthogonal pairs of split intein, allowing efficient assembly of FL-dystrophin. The three fragments packaged in myotropic AAV (MyoAAV4A) restore FL-dystrophin expression in both skeletal and cardiac muscles in male mdx4cv mice. Dystrophin-glycoprotein complex components are also restored at the sarcolemma of dystrophic muscles. MyoAAV4A-delivered FL-dystrophin significantly improves muscle histopathology, contractility, and overall strength comparable to µDys, but unlike µDys, it also restores defective cavin 4 localization and associated signaling in mdx4cv heart. Therefore, our data support the feasibility of a mutation-independent FL-dystrophin gene therapy for DMD, warranting further clinical development.

Systemic Treatment of Body-Wide Duchenne Muscular Dystrophy Symptoms

Duchenne muscular dystrophy (DMD) is a fatal X-linked disease that leads to premature death due to the loss of dystrophin. Current strategies predominantly focus on the therapeutic treatment of affected skeletal muscle tissue. However, certain results point to the fact that with successful treatment of skeletal muscle, DMD-exposed latent phenotypes in tissues, such as cardiac and smooth muscle, might lead to adverse effects and even death. Likewise, it is now clear that the absence of dystrophin affects the function of the nervous system, and that this phenotype is more pronounced when shorter dystrophins are absent, in addition to the full-length dystrophin that is present predominantly in the muscle. Here, I focus on the systemic aspects of DMD, highlighting the ubiquitous expression of the dystrophin gene in human tissues. Furthermore, I describe therapeutic strategies that have been tested in the clinic and point to unresolved questions regarding the function of distinct dystrophin isoforms, and the possibility of current therapeutic strategies to tackle phenotypes that relate to their absence.

Potential limitations of microdystrophin gene therapy for Duchenne muscular dystrophy

Clinical trials delivering high doses of adeno-associated viruses (AAVs) expressing truncated dystrophin molecules (microdystrophins) are underway for Duchenne muscular dystrophy (DMD). We examined the efficiency and efficacy of this strategy with 4 microdystrophin constructs (3 in clinical trials and a variant of the largest clinical construct), in a severe mouse model of DMD, using AAV doses comparable with those in clinical trials. We achieved high levels of microdystrophin expression in striated muscles with cardiac expression approximately 10-fold higher than that observed in skeletal muscle. Significant, albeit incomplete, correction of skeletal muscle disease was observed. Surprisingly, a lethal acceleration of cardiac disease occurred with 2 of the microdystrophins. The detrimental cardiac effect appears to be caused by variable competition (dependent on microdystrophin design and expression level) between microdystrophin and utrophin at the cardiomyocyte membrane. There may also be a contribution from an overloading of protein degradation. The significance of these observations for patients currently being treated with AAV-microdystrophin therapies is unclear since the levels of expression being achieved in the DMD hearts are unknown. However, these findings suggest that microdystrophin treatments need to avoid excessively high levels of expression in the heart and that cardiac function should be carefully monitored in these patients.

Systemic Delivery of Full-Length Dystrophin in DMD Mice

Current gene therapy for Duchenne muscular dystrophy (DMD) utilizes adeno-associated virus (AAV) to deliver miniaturized dystrophin (micro-dystrophin or µDys), which does not provide full protection for striated muscles as it lacks many important functional domains within full-length (FL) dystrophin. Here we develop a triple vector system to deliver FL-dystrophin into skeletal and cardiac muscles. We rationally split FL-dystrophin into three fragments (N, M, and C) linked to two orthogonal pairs of split intein, allowing efficient, unidirectional assembly of FL-dystrophin. The three fragments packaged in myotropic AAV (MyoAAV4A) restore FL-dystrophin expression in both skeletal and cardiac muscles in male mdx 4cv mice. Dystrophin-glycoprotein complex components are also restored in the sarcolemma of dystrophic muscles. MyoAAV4A-delivered FL-dystrophin significantly improves muscle histopathology, contractility, and overall strength comparable to µDys, but unlike µDys, it also restores defective ERK signaling in heart. The FL-dystrophin gene therapy therefore promises to offer superior protection for DMD.

A review on mechanistic insights into structure and function of dystrophin protein in pathophysiology and therapeutic targeting of Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder characterized by progressive and severe muscle weakening and degeneration. Among the various forms of muscular dystrophy, it stands out as one of the most common and impactful, predominantly affecting boys. The condition arises due to mutations in the dystrophin gene, a key player in maintaining the structure and function of muscle fibers. The manuscript explores the structural features of dystrophin protein and their pivotal roles in DMD. We present an in-depth analysis of promising therapeutic approaches targeting dystrophin and their implications for the therapeutic management of DMD. Several therapies aiming to restore dystrophin protein or address secondary pathology have obtained regulatory approval, and many others are ongoing clinical development. Notably, recent advancements in genetic approaches have demonstrated the potential to restore partially functional dystrophin forms. The review also provides a comprehensive overview of the status of clinical trials for major therapeutic genetic approaches for DMD. In addition, we have summarized the ongoing therapeutic approaches and advanced mechanisms of action for dystrophin restoration and the challenges associated with DMD therapeutics.

Therapeutic strategies for aberrant splicing in cancer and genetic disorders

Accurate pre-mRNA splicing is essential for proper protein translation; however, aberrant splicing is commonly observed in the context of cancer and genetic disorders. Notably, in genetic diseases, these splicing abnormalities often play a pivotal role. Substantial challenges persist in accurately identifying and classifying disease-induced aberrant splicing, as well as in development of targeted therapeutic strategies. In this review, we examine prevalent forms of aberrant splicing and explore potential therapeutic approaches aimed at addressing these splicing-related diseases. This summary contributes to a deeper understanding of the complexities about aberrant splicing and provide a foundation for the development of effective therapeutic interventions in the field of genetic disorders and cancer.

Gene therapy delivered micro-dystrophins co-localize with transgenic utrophin in dystrophic skeletal muscle fibers

Duchenne muscular dystrophy (DMD) is a devastating muscle disease caused by the absence of functional dystrophin. There are multiple ongoing clinical trials for DMD that are testing gene therapy treatments consisting of adeno-associated viral (AAV) vectors carrying miniaturized versions of dystrophin optimized for function, termed micro-dystrophins (μDys). Utrophin, the fetal homolog of dystrophin, has repeatedly been reported to be upregulated in human DMD muscle as a compensatory mechanism, but whether µDys displaces full-length utrophin is unknown. In this study, dystrophin/utrophin-deficient mice with transgenic overexpression of full-length utrophin in skeletal muscles were systemically administered low doses of either AAV6-CK8e-Hinge3-µDys (μDysH3) or AAV6-CK8e-μDys5 (μDys5). We used immunofluorescence to qualitatively assess the localization of μDys with transgenic utrophin and neuronal nitric oxide synthase (nNOS) in quadriceps muscles. μDys protein resulting from both gene therapies co-localized at myofiber membranes with transgenic utrophin. We also confirmed the sarcolemmal co-localization of nNOS with μDys5, but not with transgenic utrophin expression or μDysH3. Transgenic utrophin expression and μDys proteins produced from both therapies stabilize the dystrophin-glycoprotein complex as observed by sarcolemmal localization of β-dystroglycan. This study suggests that µDys gene therapy will likely not inhibit any endogenous compensation by utrophin in DMD muscle.

Targeted CRISPR activation is functional in engineered human pluripotent stem cells but undergoes silencing after differentiation into cardiomyocytes and endothelium

Human induced pluripotent stem cells (hiPSCs) offer opportunities to study human biology where primary cell types are limited. CRISPR technology allows forward genetic screens using engineered Cas9-expressing cells. Here, we sought to generate a CRISPR activation (CRISPRa) hiPSC line to activate endogenous genes during pluripotency and differentiation. We first targeted catalytically inactive Cas9 fused to VP64, p65 and Rta activators (dCas9-VPR) regulated by the constitutive CAG promoter to the AAVS1 safe harbor site. These CRISPRa hiPSC lines effectively activate target genes in pluripotency, however the dCas9-VPR transgene expression is silenced after differentiation into cardiomyocytes and endothelial cells. To understand this silencing, we systematically tested different safe harbor sites and different promoters. Targeting to safe harbor sites hROSA26 and CLYBL loci also yielded hiPSCs that expressed dCas9-VPR in pluripotency but silenced during differentiation. Muscle-specific regulatory cassettes, derived from cardiac troponin T or muscle creatine kinase promoters, were also silent after differentiation when dCas9-VPR was introduced. In contrast, in cell lines where the dCas9-VPR sequence was replaced with cDNAs encoding fluorescent proteins, expression persisted during differentiation in all loci and with all promoters. Promoter DNA was hypermethylated in CRISPRa-engineered lines, and demethylation with 5-azacytidine enhanced dCas9-VPR gene expression. In summary, the dCas9-VPR cDNA is readily expressed from multiple loci during pluripotency but induces silencing in a locus- and promoter-independent manner during differentiation to mesoderm derivatives. Researchers intending to use this CRISPRa strategy during stem cell differentiation should pilot their system to ensure it remains active in their population of interest.

Dystrophin- and Utrophin-Based Therapeutic Approaches for Treatment of Duchenne Muscular Dystrophy: A Comparative Review

Duchenne muscular dystrophy is a devastating disease that leads to progressive muscle loss and premature death. While medical management focuses mostly on symptomatic treatment, decades of research have resulted in first therapeutics able to restore the affected reading frame of dystrophin transcripts or induce synthesis of a truncated dystrophin protein from a vector, with other strategies based on gene therapy and cell signaling in preclinical or clinical development. Nevertheless, recent reports show that potentially therapeutic dystrophins can be immunogenic in patients. This raises the question of whether a dystrophin paralog, utrophin, could be a more suitable therapeutic protein. Here, we compare dystrophin and utrophin amino acid sequences and structures, combining published data with our extended in silico analyses. We then discuss these results in the context of therapeutic approaches for Duchenne muscular dystrophy. Specifically, we focus on strategies based on delivery of micro-dystrophin and micro-utrophin genes with recombinant adeno-associated viral vectors, exon skipping of the mutated dystrophin pre-mRNAs, reading through termination codons with small molecules that mask premature stop codons, dystrophin gene repair by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated genetic engineering, and increasing utrophin levels. Our analyses highlight the importance of various dystrophin and utrophin domains in Duchenne muscular dystrophy treatment, providing insights into designing novel therapeutic compounds with improved efficacy and decreased immunoreactivity. While the necessary actin and β-dystroglycan binding sites are present in both proteins, important functional distinctions can be identified in these domains and some other parts of truncated dystrophins might need redesigning due to their potentially immunogenic qualities. Alternatively, therapies based on utrophins might provide a safer and more effective approach.

Expression and function of four AAV-based constructs for dystrophin restoration in the mdx mouse model of Duchenne muscular dystrophy

Robust expression of shortened, functional dystrophin provided impetus to develop adeno-associated virus (AAV)-based constructs for clinical application. Because several cassettes are being tested in clinical trials, this study compared the efficacies of four shortened dystrophin-promoter combinations with implications for outcomes in clinical trials: MHCK7 or MCK promoter with a shortened dystrophin transgene containing the N-terminus and spectrin repeats R1, R2, R3 and R24 (rAAVrh74.MHCK7.micro-dystrophin and rAAVrh74.MCK.micro-dystrophin, respectively); shortened dystrophin construct containing the neuronal nitric oxide (nNOS) binding site (rAAVrh74.MHCK7.DV.mini-dystrophin); and shortened dystrophin containing the C-terminus (rAAVrh74.MHCK7.micro-dystrophin.Cterm). Functional and histological benefit were examined at 4 weeks following intramuscular delivery in mdx mice. rAAVrh74.MHCK7.micro-dystrophin provided the most robust transgene expression and significantly increased specific force output in the tibialis anterior muscle. Muscle environment was normalized (i.e. reductions in central nucleation), indicating functional and histological advantages of rAAVrh74.MHCK7.micro-dystrophin. Thus, promoter choice and transgene design are critical for optimal dystrophin expression/distribution for maximal functional improvement.

Evaluation of an AAV9-mini-dystrophin gene therapy candidate in a rat model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by loss-of-function mutations in the dystrophin gene and is characterized by muscle wasting and early mortality. Adeno-associated virus-mediated gene therapy is being investigated as a treatment for DMD. In the nonclinical study documented here, we determined the effective dose of fordadistrogene movaparvovec, a clinical candidate adeno-associated virus serotype 9 vector carrying a human mini-dystrophin transgene, after single intravenous injection in a dystrophin-deficient (DMDmdx) rat model of DMD. Overall, we found that transduction efficiency, number of muscle fibers expressing the human mini-dystrophin polypeptide, improvement of the skeletal and cardiac muscle tissue architecture, correction of muscle strength and fatigability, and improvement of diastolic and systolic cardiac function were directly correlated with the amount of vector administered. The effective dose was then tested in older DMDmdx rats with a more dystrophic phenotype similar to the pathology observed in older patients with DMD. Except for a less complete rescue of muscle function in the oldest cohort, fordadistrogene movaparvovec was also found to be therapeutically effective in older DMDmdx rats, suggesting that this product may be appropriate for evaluation in patients with DMD at all stages of disease.

Innate and adaptive AAV-mediated immune responses in a mouse model of Duchenne muscular dystrophy

High systemic doses of adeno-associated viruses (AAVs) have been associated with immune-related serious adverse events (SAEs). Although AAV was well tolerated in preclinical models, SAEs were observed in clinical trials, indicating the need for improved preclinical models to understand AAV-induced immune responses. Here, we show that mice dual-dosed with AAV9 at 4-week intervals better recapitulate aspects of human immunity to AAV. In the model, anti-AAV9 immunoglobulin G (IgGs) increased in a linear fashion between the first and second AAV administrations. Complement activation was only observed in the presence of high levels of both AAV and anti-AAV IgG. Myeloid-derived pro-inflammatory cytokines were significantly induced in the same pattern as complement activation, suggesting that myeloid cell activation to AAV may rely on the presence of both AAV and anti-AAV IgG complexes. Single-cell RNA sequencing of peripheral blood mononuclear cells confirmed that activated monocytes were a primary source of pro-inflammatory cytokines and chemokines, which were significantly increased after a second AAV9 exposure. The same activated monocyte clusters expressed both Fcγ and complement receptors, suggesting that anti-AAV-mediated activation of myeloid cells through Fcγ receptors and/or complement receptors is one mechanism by which anti-AAV antigen complexes may prime antigen-presenting cells and amplify downstream immunity.

Extracellular Matrix Proteomics: The mdx-4cv Mouse Diaphragm as a Surrogate for Studying Myofibrosis in Dystrophinopathy

The progressive degeneration of the skeletal musculature in Duchenne muscular dystrophy is accompanied by reactive myofibrosis, fat substitution, and chronic inflammation. Fibrotic changes and reduced tissue elasticity correlate with the loss in motor function in this X-chromosomal disorder. Thus, although dystrophinopathies are due to primary abnormalities in the DMD gene causing the almost-complete absence of the cytoskeletal Dp427-M isoform of dystrophin in voluntary muscles, the excessive accumulation of extracellular matrix proteins presents a key histopathological hallmark of muscular dystrophy. Animal model research has been instrumental in the characterization of dystrophic muscles and has contributed to a better understanding of the complex pathogenesis of dystrophinopathies, the discovery of new disease biomarkers, and the testing of novel therapeutic strategies. In this article, we review how mass-spectrometry-based proteomics can be used to study changes in key components of the endomysium, perimysium, and epimysium, such as collagens, proteoglycans, matricellular proteins, and adhesion receptors. The mdx-4cv mouse diaphragm displays severe myofibrosis, making it an ideal model system for large-scale surveys of systematic alterations in the matrisome of dystrophic fibers. Novel biomarkers of myofibrosis can now be tested for their appropriateness in the preclinical and clinical setting as diagnostic, pharmacodynamic, prognostic, and/or therapeutic monitoring indicators.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

Prolonged voluntary wheel running reveals unique adaptations in mdx mice treated with microdystrophin constructs ± the nNOS-binding site

We tested the effects of prolonged voluntary wheel running on the muscle function of mdx mice treated with one of two different microdystrophin constructs. At 7 weeks of age mdx mice were injected with a single dose of AAV9-CK8-microdystrophin with (gene therapy 1, GT1) or without (gene therapy 2, GT2) the nNOS-binding domain and were assigned to one of four gene therapy treated groups: mdxRGT1 (run, GT1), mdxGT1 (no run, GT1), or mdxRGT2 (run,GT2), mdxGT2 (no run, GT2). There were two mdx untreated groups injected with excipient: mdxR (run, no gene therapy) and mdx (no run, no gene therapy). A third no treatment group, Wildtype (WT) received no injection and did not run. mdxRGT1, mdxRGT2 and mdxR performed voluntary wheel running for 52 weeks; WT and remaining mdx groups were cage active. Robust expression of microdystrophin occurred in diaphragm, quadriceps, and heart muscles of all treated mice. Dystrophic muscle pathology was high in diaphragms of non-treated mdx and mdxR mice and improved in all treated groups. Endurance capacity was rescued by both voluntary wheel running and gene therapy alone, but their combination was most beneficial. All treated groups increased in vivo plantarflexor torque over both mdx and mdxR mice. mdx and mdxR mice displayed ∼3-fold lower diaphragm force and power compared to WT values. Treated groups demonstrated partial improvements in diaphragm force and power, with mdxRGT2 mice experiencing the greatest improvement at ∼60% of WT values. Evaluation of oxidative red quadriceps fibers revealed the greatest improvements in mitochondrial respiration in mdxRGT1 mice, reaching WT levels. Interestingly, mdxGT2 mice displayed diaphragm mitochondrial respiration values similar to WT but mdxRGT2 animals showed relative decreases compared to the no run group. Collectively, these data demonstrate that either microdystrophin construct combined with voluntary wheel running increased in vivo maximal muscle strength, power, and endurance. However, these data also highlighted important differences between the two microdystrophin constructs. GT1, with the nNOS-binding site, improved more markers of exercise-driven adaptations in metabolic enzyme activity of limb muscles, while GT2, without the nNOS-binding site, demonstrated greater protection of diaphragm strength after chronic voluntary endurance exercise but decreased mitochondrial respiration in the context of running.

Single-swap editing for the correction of common Duchenne muscular dystrophy mutations

Duchenne muscular dystrophy (DMD) is a fatal X-linked recessive disease of progressive muscle weakness and wasting caused by the absence of dystrophin protein. Current gene therapy approaches using antisense oligonucleotides require lifelong dosing and have limited efficacy in restoring dystrophin production. A gene editing approach could permanently correct the genome and restore dystrophin protein expression. Here, we describe single-swap editing, in which an adenine base editor edits a single base pair at a splice donor site or splice acceptor site to enable exon skipping or reframing. In human induced pluripotent stem cell-derived cardiomyocytes, we demonstrate that single-swap editing can enable beneficial exon skipping or reframing for the three most therapeutically relevant exons-DMD exons 45, 51, and 53-which could be beneficial for 30% of all DMD patients. Furthermore, an adeno-associated virus delivery method for base editing components can efficiently restore dystrophin production locally and systemically in skeletal and cardiac muscles of a DMD mouse model containing a deletion of Dmd exon 44. Our studies demonstrate single-swap editing as a potential gene editing therapy for common DMD mutations.

Enveloped viruses pseudotyped with mammalian myogenic cell fusogens target skeletal muscle for gene delivery

Entry of enveloped viruses into cells is mediated by viral fusogenic proteins that drive membrane rearrangements needed for fusion between viral and target membranes. Skeletal muscle development also requires membrane fusion events between progenitor cells to form multinucleated myofibers. Myomaker and Myomerger are muscle-specific cell fusogens but do not structurally or functionally resemble classical viral fusogens. We asked whether the muscle fusogens could functionally substitute for viral fusogens, despite their structural distinctiveness, and fuse viruses to cells. We report that engineering of Myomaker and Myomerger on the membrane of enveloped viruses leads to specific transduction of skeletal muscle. We also demonstrate that locally and systemically injected virions pseudotyped with the muscle fusogens can deliver μDystrophin to skeletal muscle of a mouse model of Duchenne muscular dystrophy and alleviate pathology. Through harnessing the intrinsic properties of myogenic membranes, we establish a platform for delivery of therapeutic material to skeletal muscle.

The Implication of Hinge 1 and Hinge 4 in Micro-Dystrophin Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a fatal muscle disease caused by dystrophin deficiency. Dystrophin consists of the amino terminus, central rod domain with 24 spectrin-like repeats and four hinges (H), cysteine-rich domain, and carboxyl terminus. Several highly abbreviated micro-dystrophins (μDys) are currently in clinical trials. They all carry H1 and H4. In this study, we investigated whether these two hinges are essential for μDy function in murine DMD models. Three otherwise identical μDys were engineered to contain H1 and/or H4 and were named H1/H4 (with both H1 and H4), ΔH1 (without H1), and ΔH4 (without H4). These constructs were packaged in adeno-associated virus serotype-9 and delivered to the tibialis anterior muscle of 3-month-old male mdx4cv mice (1E12 vector genome particles/muscle). Three months later, we detected equivalent μDys expression in total muscle lysate. However, only H1/H4 and ΔH1 showed correct sarcolemmal localization. ΔH4 mainly existed as sarcoplasmic aggregates. H1/H4 and ΔH1, but not ΔH4, fully restored the dystrophin-associated protein complex and significantly improved the specific muscle force. Eccentric contraction-induced force decline was best protected by H1/H4, followed by ΔH1, but not by ΔH4. Next, we compared H1/H4 and ΔH1 in 6-week-old male mdx mice by intravenous injection (1E13 vector genome particles/mouse). Four months postinjection, H1/H4 significantly outperformed ΔH1 in extensor digitorum longus muscle force measurements but two constructs yielded comparable electrocardiography improvements. We conclude that H4 is essential for μDys function and H1 facilitates force production. Our findings will help develop next-generation μDys gene therapy.

Microdystrophin Expression as a Surrogate Endpoint for Duchenne Muscular Dystrophy Clinical Trials

Duchenne muscular dystrophy (DMD) is a serious, rare genetic disease, affecting primarily boys. It is caused by mutations in the DMD gene and is characterized by progressive muscle degeneration that results in loss of function and early death due to respiratory and/or cardiac failure. Although limited treatment options are available, some for only small subsets of the patient population, DMD remains a disease with large unmet medical needs. The adeno-associated virus (AAV) vector is the leading gene delivery system for addressing genetic neuromuscular diseases. Since the gene encoding the full-length dystrophin protein exceeds the packaging capacity of a single AAV vector, gene replacement therapy based on AAV-delivery of shortened, yet, functional microdystrophin genes has emerged as a promising treatment. This article seeks to explain the rationale for use of the accelerated approval pathway to advance AAV microdystrophin gene therapy for DMD. Specifically, we provide support for the use of microdystrophin expression as a surrogate endpoint that could be used in clinical trials to support accelerated approval.

Lifelong Outcomes of Systemic Adeno-Associated Virus Micro-Dystrophin Gene Therapy in a Murine Duchenne Muscular Dystrophy Model

Adeno-associated virus (AAV)-mediated systemic micro-dystrophin (μDys) therapy is currently in clinical trials. The hope is to permanently improve the life quality of Duchenne muscular dystrophy (DMD) patients. Numerous preclinical studies have been conducted to support these trials. However, none examined whether a single therapy at a young age can lead to lifelong disease amelioration. To address this critical question, we injected 1 × 1013 vg particles/mouse of an AAV serotype-9 μDys vector to 3-month-old mdx mice through the tail vein. Therapeutic outcomes were evaluated at the age of 11 months (adulthood, 8 months postinjection) and 21 months (terminal age, 18 months postinjection). Immunostaining and Western blot showed saturated supraphysiological levels of μDys expression in skeletal muscle and heart till the end of the study. Treatment significantly improved grip force and treadmill running, and significantly reduced the serum creatine kinase level at both time points. Since cardiac death is a major threat in late-stage patients, we evaluated cardiac electrophysiology and hemodynamics by ECG and the closed-chest cardiac catheter assay, respectively. Significant improvements were observed in these assays. Importantly, many ECG and hemodynamic parameters (heart rate, PR interval, QRS duration, QTc interval, end-diastolic/systolic volume, dP/dt max and min, max pressure, and ejection fraction) were completely normalized at 21 months of age. Our results have provided direct evidence that a single systemic AAV μDys therapy has the potential to provide lifelong benefits in the murine DMD model.

Cell based dATP delivery as a therapy for chronic heart failure

Transplanted human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) improve ventricular performance when delivered acutely post-myocardial infarction but are ineffective in chronic myocardial infarction/heart failure. 2'-deoxy-ATP (dATP) activates cardiac myosin and potently increases contractility. Here we engineered hPSC-CMs to overexpress ribonucleotide reductase, the enzyme controlling dATP production. In vivo, dATP-producing CMs formed new myocardium that transferred dATP to host cardiomyocytes via gap junctions, increasing their dATP levels. Strikingly, when transplanted into chronically infarcted hearts, dATP-producing grafts increased left ventricular function, whereas heart failure worsened with wild-type grafts or vehicle injections. dATP-donor cells recipients had greater voluntary exercise, improved cardiac metabolism, reduced pulmonary congestion and pathological cardiac hypertrophy, and improved survival. This combination of remuscularization plus enhanced host contractility offers a novel approach to treating the chronically failing heart.

Enveloped viruses pseudotyped with mammalian myogenic cell fusogens target skeletal muscle for gene delivery

Entry of enveloped viruses into cells is mediated by fusogenic proteins that form a complex between membranes to drive rearrangements needed for fusion. Skeletal muscle development also requires membrane fusion events between progenitor cells to form multinucleated myofibers. Myomaker and Myomerger are muscle-specific cell fusogens, but do not structurally or functionally resemble classical viral fusogens. We asked if the muscle fusogens could functionally substitute for viral fusogens, despite their structural distinctiveness, and fuse viruses to cells. We report that engineering of Myomaker and Myomerger on the membrane of enveloped viruses leads to specific transduction of skeletal muscle. We also demonstrate that locally and systemically injected virions pseudotyped with the muscle fusogens can deliver micro-Dystrophin (μDys) to skeletal muscle of a mouse model of Duchenne muscular dystrophy. Through harnessing the intrinsic properties of myogenic membranes, we establish a platform for delivery of therapeutic material to skeletal muscle.

Micro-dystrophin gene therapy demonstrates long-term cardiac efficacy in a severe Duchenne muscular dystrophy model

Micro-dystrophin gene replacement therapies for Duchenne muscular dystrophy (DMD) are currently in clinical trials, but have not been thoroughly investigated for their efficacy on cardiomyopathy progression to heart failure. We previously validated Fiona/dystrophin-utrophin-deficient (dko) mice as a DMD cardiomyopathy model that progresses to reduced ejection fraction indicative of heart failure. Adeno-associated viral (AAV) vector delivery of an early generation micro-dystrophin prevented cardiac pathology and functional decline through 1 year of age in this new model. We now show that gene therapy using a micro-dystrophin optimized for skeletal muscle efficacy (AAV-μDys5), and which is currently in a clinical trial, is able to fully prevent cardiac pathology and cardiac strain abnormalities and maintain normal (>45%) ejection fraction through 18 months of age in Fiona/dko mice. Early treatment with AAV-μDys5 prevents inflammation and fibrosis in Fiona/dko hearts. Collagen in cardiac fibrotic scars becomes more tightly packed from 12 to 18 months in Fiona/dko mice, but the area of fibrosis containing tenascin C does not change. Increased tight collagen correlates with unexpected improvements in Fiona/dko whole-heart function that maintain impaired cardiac strain and strain rate. This study supports micro-dystrophin gene therapy as a promising intervention for preventing DMD cardiomyopathy progression.

PCYT2-regulated lipid biosynthesis is critical to muscle health and ageing

Muscle degeneration is the most prevalent cause for frailty and dependency in inherited diseases and ageing. Elucidation of pathophysiological mechanisms, as well as effective treatments for muscle diseases, represents an important goal in improving human health. Here, we show that the lipid synthesis enzyme phosphatidylethanolamine cytidyltransferase (PCYT2/ECT) is critical to muscle health. Human deficiency in PCYT2 causes a severe disease with failure to thrive and progressive weakness. pcyt2-mutant zebrafish and muscle-specific Pcyt2-knockout mice recapitulate the participant phenotypes, with failure to thrive, progressive muscle weakness and accelerated ageing. Mechanistically, muscle Pcyt2 deficiency affects cellular bioenergetics and membrane lipid bilayer structure and stability. PCYT2 activity declines in ageing muscles of mice and humans, and adeno-associated virus-based delivery of PCYT2 ameliorates muscle weakness in Pcyt2-knockout and old mice, offering a therapy for individuals with a rare disease and muscle ageing. Thus, PCYT2 plays a fundamental and conserved role in vertebrate muscle health, linking PCYT2 and PCYT2-synthesized lipids to severe muscle dystrophy and ageing.

Meeting Report: 2022 Muscular Dystrophy Association Summit on 'Safety and Challenges in Gene Transfer Therapy'

Muscular Dystrophy Association (MDA) has invested over $125M in the development of gene therapy for neuromuscular diseases (NMDs) over the past 20 years. As a lead initiator of progress in this important field of medicine and to help ensure continued progress towards therapies for patients, MDA organized a dedicated summit in January 2022 to address emerging challenges in safely delivering AAV gene therapies with a focus on their application in NMD. In this meeting, chaired by Carsten Bönnemann (NINDS, NIH) and Barry Byrne (University of Florida), academic and industry experts and stakeholders convened to openly discuss adverse events linked to clinical trials, as well as other challenges emerging in preclinical studies associated with difficulties in the translation of AAV gene therapies.

Assessment of systemic AAV-microdystrophin gene therapy in the GRMD model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive muscle wasting disease caused by the absence of dystrophin, a membrane-stabilizing protein encoded by the DMD gene. Although mouse models of DMD provide insight into the potential of a corrective therapy, data from genetically homologous large animals, such as the dystrophin-deficient golden retriever muscular dystrophy (GRMD) model, may more readily translate to humans. To evaluate the clinical translatability of an adeno-associated virus serotype 9 vector (AAV9)-microdystrophin (μDys5) construct, we performed a blinded, placebo-controlled study in which 12 GRMD dogs were divided among four dose groups [control, 1 × 1013 vector genomes per kilogram (vg/kg), 1 × 1014 vg/kg, and 2 × 1014 vg/kg; n = 3 each], treated intravenously at 3 months of age with a canine codon-optimized microdystrophin construct, rAAV9-CK8e-c-μDys5, and followed for 90 days after dosing. All dogs received prednisone (1 milligram/kilogram) for a total of 5 weeks from day -7 through day 28. We observed dose-dependent increases in tissue vector genome copy numbers; μDys5 protein in multiple appendicular muscles, the diaphragm, and heart; limb and respiratory muscle functional improvement; and reduction of histopathologic lesions. As expected, given that a truncated dystrophin protein was generated, phenotypic test results and histopathologic lesions did not fully normalize. All administrations were well tolerated, and adverse events were not seen. These data suggest that systemically administered AAV-microdystrophin may be dosed safely and could provide therapeutic benefit for patients with DMD.

Clinical potential of microdystrophin as a surrogate endpoint

Accelerated approval based on a likely surrogate endpoint can be life-changing for patients suffering from a rare progressive disease with unmet medical need, as it substantially hastens access to potentially lifesaving therapies. In one such example, antisense morpholinos were approved to treat Duchenne muscular dystrophy (DMD) based on measurement of shortened dystrophin in skeletal muscle biopsies as a surrogate biomarker. New, promising therapeutics for DMD include AAV gene therapy to restore another form of dystrophin termed mini- or microdystrophin. AAV-microdystrophins are currently in clinical trials but have yet to be accepted by regulatory agencies as reasonably likely surrogate endpoints. To evaluate microdystrophin expression as a reasonably likely surrogate endpoint for DMD, this review highlights dystrophin biology in the context of functional and clinical benefit to support the argument that microdystrophin proteins have a high probability of providing clinical benefit based on their rational design. Unlike exon-skipping based strategies, the approach of rational design allows for functional capabilities (i.e. quality) of the protein to be maximized with every patient receiving the same optimized microdystrophin. Therefore, the presence of rationally designed microdystrophin in a muscle biopsy is likely to predict clinical benefit and is consequently a strong candidate for a surrogate endpoint analysis to support accelerated approval.

Childhood muscular dystrophies

Infancy- and childhood-onset muscular dystrophies are associated with a characteristic distribution and progression of motor dysfunction. The underlying causes of progressive childhood muscular dystrophies are heterogeneous involving diverse genetic pathways and genes that encode proteins of the plasma membrane, extracellular matrix, sarcomere, and nuclear membrane components. The prototypical clinicopathological features in an affected child may be adequate to fully distinguish it from other likely diagnoses based on four common features: (1) weakness and wasting of pelvic-femoral and scapular muscles with involvement of heart muscle; (2) elevation of serum muscle enzymes in particular serum creatine kinase; (3) necrosis and regeneration of myofibers; and (4) molecular neurogenetic assessment particularly utilizing next-generation sequencing of the genome of the likeliest candidates genes in an index case or family proband. A number of different animal models of therapeutic strategies have been developed for gene transfer therapy, but so far these techniques have not yet entered clinical practice. Treatment remains for the most part symptomatic with the goal of ameliorating locomotor and cardiorespiratory manifestations of the disease.

Dystrophin Restoration after Adeno-Associated Virus U7-Mediated Dmd Exon Skipping Is Modulated by Muscular Exercise in the Severe D2-Mdx Duchenne Muscular Dystrophy Murine Model

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by Dmd mutations, resulting in the absence of dystrophin in skeletal muscle, and a greater susceptibility to damage during contraction (exercise). The current study evaluated whether voluntary exercise impacts a Dmd exon skipping and muscle physiology in a severe DMD murine model. D2-mdx mice were intramuscularly injected with an adeno-associated virus (AAV) U7 snRNA to correct Dmd reading frame, and allowed to voluntary run on a wheel for 1 month. Voluntary running did not induce muscle fiber regeneration, as indicated by the percentage of centronucleated fibers, Myh3 and Myh4 expression, and maximal force production, and thus possibly did not compromise the gene therapy approach. Voluntary running did not impact the number of viral genomes and the expression of U7 and Dmd 1 month after injection of AAV-U7 injected just before exercise initiation, but reduced the amount of dystrophin in dystrophin-expressing fibers from 80% to 65% of the muscle cross-sectional area. In conclusion, voluntary running did not induce muscle damage and had no drastic detrimental effect on the AAV gene therapy exon skipping approach in a severe murine DMD model. Moreover, these results suggest considering exercise as an additional element in the design and conception of future therapeutic approaches for DMD.

Isolation and Characterization of Primary DMD Pig Muscle Cells as an In Vitro Model for Preclinical Research on Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is the most frequent genetic myopathy in childhood and leads to progressive muscle atrophy, weakness, and premature death. So far, there is no curative treatment available. Therapeutic development from bench to bedside takes time, and promising therapies need to be tested in suitable preclinical animal models prior to clinical trials in DMD patients. Existing mouse and dog models are limited with regard to the comparability of the clinical phenotype and the underlying mutation. Therefore, our group established a tailored large animal model of DMD, the DMD pig, mirroring the human size, anatomy, and physiology. For testing novel approaches, we developed a corresponding in vitro model, facilitating preclinical testing for toxicity, dosing, and efficacy, which we describe here. We first extracted primary muscle cells from wild-type and DMD pigs of different age groups and characterized those cells, then improved their differentiation process for identification of dystrophin and utrophin in myotubes. Our porcine in vitro model represents an important step for the development of novel therapeutic approaches, which should be validated further to minimize the need for living animals for bioassays, and thereby support the '3R' (replace, reduce, refine) principle, as fewer animals have to be raised and treated for preclinical trials.

The skeletal muscle phenotype of the DE50-MD dog model of Duchenne muscular dystrophy

Background: Animal models of Duchenne muscular dystrophy (DMD) are essential to study disease progression and assess efficacy of therapeutic intervention, however dystrophic mice fail to display a clinically relevant phenotype, limiting translational utility. Dystrophin-deficient dogs exhibit disease similar to humans, making them increasingly important for late-stage preclinical evaluation of candidate therapeutics. The DE50-MD canine model of DMD carries a mutation within a human 'hotspot' region of the dystrophin gene, amenable to exon-skipping and gene editing strategies. As part of a large natural history study of disease progression, we have characterised the DE50-MD skeletal muscle phenotype to identify parameters that could serve as efficacy biomarkers in future preclinical trials. Methods: Vastus lateralis muscles were biopsied from a large cohort of DE50-MD dogs and healthy male littermates at 3-monthly intervals (3-18 months) for longitudinal analysis, with multiple muscles collected post-mortem to evaluate body-wide changes. Pathology was characterised quantitatively using histology and measurement of gene expression to determine statistical power and sample sizes appropriate for future work. Results: DE50-MD skeletal muscle exhibits widespread degeneration/regeneration, fibrosis, atrophy and inflammation. Degenerative/inflammatory changes peak during the first year of life, while fibrotic remodelling appears more gradual. Pathology is similar in most skeletal muscles, but in the diaphragm, fibrosis is more prominent, associated with fibre splitting and pathological hypertrophy. Picrosirius red and acid phosphatase staining represent useful quantitative histological biomarkers for fibrosis and inflammation respectively, while qPCR can be used to measure regeneration ( MYH3, MYH8), fibrosis ( COL1A1), inflammation ( SPP1), and stability of DE50-MD dp427 transcripts. Conclusion: The DE50-MD dog is a valuable model of DMD, with pathological features similar to young, ambulant human patients. Sample size and power calculations show that our panel of muscle biomarkers are of strong pre-clinical value, able to detect therapeutic improvements of even 25%, using trials with only six animals per group.

Mineralocorticoid Receptor Signaling in the Inflammatory Skeletal Muscle Microenvironments of Muscular Dystrophy and Acute Injury

Duchenne muscular dystrophy (DMD) is a striated muscle degenerative disease due to loss of functional dystrophin protein. Loss of dystrophin results in susceptibility of muscle membranes to damage, leading to muscle degeneration and continuous inflammation and fibrosis that further exacerbate pathology. Long-term glucocorticoid receptor (GR) agonist treatment, the current standard-of-care for DMD, modestly improves prognosis but has serious side effects. The mineralocorticoid receptor (MR), a ligand-activated transcription factor present in many cell types, has been implicated as a therapeutic target for DMD. MR antagonists (MRAs) have fewer side effects than GR agonists and are used clinically for heart failure. MRA efficacy has recently been demonstrated for DMD cardiomyopathy and in preclinical studies, MRAs also alleviate dystrophic skeletal muscle pathology. MRAs lead to improvements in muscle force and membrane stability and reductions in degeneration, inflammation, and fibrosis in dystrophic muscles. Myofiber-specific MR knockout leads to most of these improvements, supporting an MR-dependent mechanism of action, but MRAs additionally stabilize myofiber membranes in an MR-independent manner. Immune cell MR signaling in dystrophic and acutely injured normal muscle contributes to wound healing, and myeloid-specific MR knockout is detrimental. More research is needed to fully elucidate MR signaling in striated muscle microenvironments. Direct comparisons of genomic and non-genomic effects of glucocorticoids and MRAs on skeletal muscles and heart will contribute to optimal temporal use of these drugs, since they compete for binding conserved receptors. Despite the advent of genetic medicines, therapies targeting inflammation and fibrosis will be necessary to achieve optimal patient outcomes.

Comparison of dystrophin expression following gene editing and gene replacement in an aged preclinical DMD animal model

Gene editing has shown promise for correcting or bypassing dystrophin mutations in Duchenne muscular dystrophy (DMD). However, preclinical studies have focused on young animals with limited muscle fibrosis and wasting, thereby favoring muscle transduction, myonuclear editing, and prevention of disease progression. Here, we explore muscle-specific dystrophin gene editing following intramuscular delivery of AAV6:CK8e-CRISPR/SaCas9 in 3- and 8-year-old dystrophic CXMD dogs and provide a qualitative comparison to AAV6:CK8e-micro-dystrophin gene replacement at 6 weeks post-treatment. Gene editing restored the dystrophin reading frame in ∼1.3% of genomes and in up to 4.0% of dystrophin transcripts following excision of a 105-kb mutation containing region spanning exons 6-8. However, resulting dystrophin expression levels and effects on muscle pathology were greater with the use of micro-dystrophin gene transfer. This study demonstrates that our muscle-specific multi-exon deletion strategy can correct a frequently mutated region of the dystrophin gene in an aged large animal DMD model, but underscores that further enhancements are required to reach efficiencies comparable to AAV micro-dystrophin. Our observations also indicate that treatment efficacy and state of muscle pathology at the time of intervention are linked, suggesting the need for additional methodological optimizations related to age and disease progression to achieve relevant clinical translation of CRISPR-based therapies to all DMD patients.

SERUM EXTRACELLULAR VESICLES FOR DELIVERY OF CRISPR-CAS9 RIBONUCLEOPROTEINS TO MODIFY THE DYSTROPHIN GENE

Extracellular vesicles (EVs) mediate intercellular biomolecule exchanges in the body, making them promising delivery vehicles for therapeutic cargo. Genetic engineering by CRISPR system is an interesting therapeutic avenue for genetic diseases such as Duchenne Muscular Dystrophy (DMD). We developed a simple method for loading EVs with CRISPR ribonucleoproteins (RNPs) consisting of SpCas9 proteins and guide RNAs (gRNAs). EVs were first purified from human or mouse serum using ultrafiltration and size-exclusion chromatography. Using protein transfectant to load RNPs into serum EVs, we showed that EVs are good carriers of RNPs in vitro and restored the expression of the tdTomato fluorescent protein in muscle fibers of Ai9 mice. EVs carrying RNPs targeting introns 22 and 24 of the DMD gene were also injected into muscles of mdx mice having a non-sense mutation in exon 23. Up to 19% of the cDNA extracted from treated mdx mice had the intended deletion of exons 23 and 24, allowing dystrophin expression in muscle fibers. RNPs alone, without EVs, were inefficient in generating detectable deletions in mouse muscles. This method opens new opportunities for rapid and safe delivery of CRISPR components to treat DMD.

Therapeutic Application of Extracellular Vesicles-Capsulated Adeno-Associated Virus Vector via nSMase2/Smpd3, Satellite, and Immune Cells in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by loss-of-function mutations in the dystrophin gene on chromosome Xp21. Disruption of the dystrophin-glycoprotein complex (DGC) on the cell membrane causes cytosolic Ca2+ influx, resulting in protease activation, mitochondrial dysfunction, and progressive myofiber degeneration, leading to muscle wasting and fragility. In addition to the function of dystrophin in the structural integrity of myofibers, a novel function of asymmetric cell division in muscular stem cells (satellite cells) has been reported. Therefore, it has been suggested that myofiber instability may not be the only cause of dystrophic degeneration, but rather that the phenotype might be caused by multiple factors, including stem cell and myofiber functions. Furthermore, it has been focused functional regulation of satellite cells by intracellular communication of extracellular vesicles (EVs) in DMD pathology. Recently, a novel molecular mechanism of DMD pathogenesis-circulating RNA molecules-has been revealed through the study of target pathways modulated by the Neutral sphingomyelinase2/Neutral sphingomyelinase3 (nSMase2/Smpd3) protein. In addition, adeno-associated virus (AAV) has been clinically applied for DMD therapy owing to the safety and long-term expression of transduction genes. Furthermore, the EV-capsulated AAV vector (EV-AAV) has been shown to be a useful tool for the intervention of DMD, because of the high efficacy of the transgene and avoidance of neutralizing antibodies. Thus, we review application of AAV and EV-AAV vectors for DMD as novel therapeutic strategy.

Therapy development for spinal muscular atrophy: perspectives for muscular dystrophies and neurodegenerative disorders

BACKGROUND

Major efforts have been made in the last decade to develop and improve therapies for proximal spinal muscular atrophy (SMA). The introduction of Nusinersen/Spinraza™ as an antisense oligonucleotide therapy, Onasemnogene abeparvovec/Zolgensma™ as an AAV9-based gene therapy and Risdiplam/Evrysdi™ as a small molecule modifier of pre-mRNA splicing have set new standards for interference with neurodegeneration.

MAIN BODY

Therapies for SMA are designed to interfere with the cellular basis of the disease by modifying pre-mRNA splicing and enhancing expression of the Survival Motor Neuron (SMN) protein, which is only expressed at low levels in this disorder. The corresponding strategies also can be applied to other disease mechanisms caused by loss of function or toxic gain of function mutations. The development of therapies for SMA was based on the use of cell culture systems and mouse models, as well as innovative clinical trials that included readouts that had originally been introduced and optimized in preclinical studies. This is summarized in the first part of this review. The second part discusses current developments and perspectives for amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease, as well as the obstacles that need to be overcome to introduce RNA-based therapies and gene therapies for these disorders.

CONCLUSION

RNA-based therapies offer chances for therapy development of complex neurodegenerative disorders such as amyotrophic lateral sclerosis, muscular dystrophies, Parkinson's and Alzheimer's disease. The experiences made with these new drugs for SMA, and also the experiences in AAV gene therapies could help to broaden the spectrum of current approaches to interfere with pathophysiological mechanisms in neurodegeneration.

Cas9-specific immune responses compromise local and systemic AAV CRISPR therapy in multiple dystrophic canine models

Adeno-associated virus (AAV)-mediated CRISPR-Cas9 editing holds promise to treat many diseases. The immune response to bacterial-derived Cas9 has been speculated as a hurdle for AAV-CRISPR therapy. However, immunological consequences of AAV-mediated Cas9 expression have thus far not been thoroughly investigated in large mammals. We evaluate Cas9-specific immune responses in canine models of Duchenne muscular dystrophy (DMD) following intramuscular and intravenous AAV-CRISPR therapy. Treatment results initially in robust dystrophin restoration in affected dogs but also induces muscle inflammation, and Cas9-specific humoral and cytotoxic T-lymphocyte (CTL) responses that are not prevented by the muscle-specific promoter and transient prednisolone immune suppression. In normal dogs, AAV-mediated Cas9 expression induces similar, though milder, immune responses. In contrast, other therapeutic (micro-dystrophin and SERCA2a) and reporter (alkaline phosphatase, AP) vectors result in persistent expression without inducing muscle inflammation. Our results suggest Cas9 immunity may represent a critical barrier for AAV-CRISPR therapy in large mammals.

Cardiac Myoediting Attenuates Cardiac Abnormalities in Human and Mouse Models of Duchenne Muscular Dystrophy

[Figure: see text].

Evaluating the potential of novel genetic approaches for the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked progressive muscle-wasting disorder that is caused by a lack of functional dystrophin, a cytoplasmic protein necessary for the structural integrity of muscle. As variants in the dystrophin gene lead to a disruption of the reading frame, pharmacological treatments have only limited efficacy; there is currently no effective therapy and consequently, a significant unmet clinical need for DMD. Recently, novel genetic approaches have shown real promise in treating DMD, with advancements in the efficacy and tropism of exon skipping and surrogate gene therapy. CRISPR-Cas9 has the potential to be a 'one-hit' curative treatment in the coming decade. The current limitations of gene editing, such as off-target effects and immunogenicity, are in fact partly constraints of the delivery method itself, and thus research focus has shifted to improving the viral vector. In order to halt the loss of ambulation, early diagnosis and treatment will be pivotal. In an era where genetic sequencing is increasingly utilised in the clinic, genetic therapies will play a progressively central role in DMD therapy. This review delineates the relative merits of cutting-edge genetic approaches, as well as the challenges that still need to be overcome before they become clinically viable.

Current Pharmacological Strategies for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal, X-linked neuromuscular disorder caused by the absence of dystrophin protein, which is essential for muscle fiber integrity. Loss of dystrophin protein leads to recurrent myofiber damage, chronic inflammation, progressive fibrosis, and dysfunction of muscle stem cells. There is still no cure for DMD so far and the standard of care is principally limited to symptom relief through glucocorticoids treatments. Current therapeutic strategies could be divided into two lines. Dystrophin-targeted therapeutic strategies that aim at restoring the expression and/or function of dystrophin, including gene-based, cell-based and protein replacement therapies. The other line of therapeutic strategies aims to improve muscle function and quality by targeting the downstream pathological changes, including inflammation, fibrosis, and muscle atrophy. This review introduces the important developments in these two lines of strategies, especially those that have entered the clinical phase and/or have great potential for clinical translation. The rationale and efficacy of each agent in pre-clinical or clinical studies are presented. Furthermore, a meta-analysis of gene profiling in DMD patients has been performed to understand the molecular mechanisms of DMD.

Female Outperformance in Voluntary Running Persists in Dystrophin-Null and Klotho-Overexpressing Mice

BACKGROUND

Duchenne muscular dystrophy is a degenerative muscle disease that results from impairment of the dystrophin gene. The disease causes progressive loss in muscle mass and function.

OBJECTIVE

The anti-aging protein, α-klotho, has been implicated in the regulation of muscle regeneration. We previously discovered that mice harboring reduced α-klotho levels exhibited a decline in muscle strength and running endurance.

METHOD

To investigate the ability of α-klotho to improve overall endurance in a dystrophin null murine model, we examined the voluntary wheel running performance of dystrophin-null, mdx4cv mice overexpressing an α-klotho transgene.

RESULTS

As expected, compared to wild type, both male and female dystrophic mice exhibited reduced running ability that was characterized by shorter running duration and longer periods of rest between cycles of activity. While our results did not detect an improvement in running performance with α-klotho overexpression, we identified distinct differences in the running patterns between females and males from all mouse strains analyzed (i.e., mdx4cv, mdx4cv overexpressing α-klotho, α-klotho overexpressing, α-klotho hypomorph, and wild type). For all strains, male mice displayed significantly reduced voluntary running ability compared to females. Further analysis of the mdx4cv strains demonstrated that male mice ran for shorter lengths of time and took longer breaks. However, we did not identify gender-associated differences in the actual speed at which mdx4cv mice ran.

CONCLUSION

Our data suggest key differences in the running capabilities of female and male mice, which are of particular relevance to studies of dystrophin-null mice.

Long-term effect of human mini-dystrophin in transgenic mdx mice improves muscle physiological function

Duchenne muscular dystrophy (DMD) is a lethal genetic muscle disorder caused by recessive mutations in dystrophin gene, affecting 1/3000 males. Gene therapy has been proven to ameliorate dystrophic pathology. To investigate therapeutic benefits from long-term effect of human mini-dystrophin and functional outcomes, transgenic mdx mice (Tg-mdx) containing a single copy of human mini-dystrophin (∆hDys3849) gene, five rods (Rods1-2, Rods22-24), and two hinges (H1 and H4) driven by a truncated creatine-kinase promoter (dMCK) in a recombinant adeno-associated viral vector (rAAV) backbone, were generated and used to determine gene expression and improvement of muscle function. Human mini-dystrophin gene expression was found in a majority of the skeletal muscles, but no expression in cardiac muscle. Dystrophin-associated glycoproteins (DAGs) such as sarcoglycans and nNOS were restored at the sarcolemma and coincided with human mini-dystrophin gene expression at the ages of 6, 10, and 20 months; Morphology of dystrophic muscle expressing the human mini-dystrophin gene was improved and central nuclei were reduced. Myofiber membrane integrity was improved by Evans blue dye test. Improvement in treadmill running and grip force was observed in transgenic mice at 6 months. Tetanic force and specific force of tibialis anterior (TA) muscle were significantly increased at the ages of 6, 10, and 20 months. Pseudohypertrophy was not found in TA muscle at 10 and 20 months when compared with wild-type C57 (WT) group. This study demonstrated that the long-term effects of human mini-dystrophin effectively ameliorated pathology and improved the functions of the dystrophic muscles in the transgenic DMD mouse model.

Dose-Dependent Microdystrophin Expression Enhancement in Cardiac Muscle by a Cardiac-Specific Regulatory Element

Duchenne muscular dystrophy (DMD) is an X-linked recessive disease that affects 1:5,000 live male births and is characterized by muscle wasting. By the age of 13 years, affected individuals are often wheelchair bound and suffer from respiratory and cardiac failure, which results in premature death. Although the administration of corticosteroids and ventilation can relieve the symptoms and extend the patients' lifespan, currently no cure exists for DMD. Among the different approaches under preclinical and clinical testing, gene therapy, using adeno-associated viral (AAV) vectors, is one of the most promising. In this study, we delivered intravenously AAV9 vectors expressing the microdystrophin MD1 (ΔR4-R23/ΔCT) under control of the synthetic muscle-specific promoter Spc5-12 and assessed the effect of adding a cardiac-specific cis-regulatory module (designated as CS-CRM4) on its expression profile in skeletal and cardiac muscles. Results show that Spc5-12 promoter, in combination with an AAV serotype that has high tropism for the heart, drives high MD1 expression levels in cardiac muscle in mdx mice. The additional regulatory element CS-CRM4 can further improve MD1 expression in cardiac muscles, but its effect is dose dependent and enhancement becomes evident only at lower vector doses.

Micro-dystrophin gene therapy prevents heart failure in an improved Duchenne muscular dystrophy cardiomyopathy mouse model

Gene replacement for Duchenne muscular dystrophy (DMD) with micro-dystrophins has entered clinical trials, but efficacy in preventing heart failure is unknown. Although most patients with DMD die from heart failure, cardiomyopathy is undetectable until the teens, so efficacy from trials in young boys will be unknown for a decade. Available DMD animal models were sufficient to demonstrate micro-dystrophin efficacy on earlier onset skeletal muscle pathology underlying loss of ambulation and respiratory insufficiency in patients. However, no mouse models progressed into heart failure, and dog models showed highly variable progression insufficient to evaluate efficacy of micro-dystrophin or other therapies on DMD heart failure. To overcome this barrier, we have generated the first DMD mouse model to our knowledge that reproducibly progresses into heart failure. This model shows cardiac inflammation and fibrosis occur prior to reduced function. Fibrosis does not continue to accumulate, but inflammation persists after function declines. We used this model to test micro-dystrophin gene therapy efficacy on heart failure prevention for the first time. Micro-dystrophin prevented declines in cardiac function and prohibited onset of inflammation and fibrosis. This model will allow identification of committed pathogenic steps to heart failure and testing of genetic and nongenetic therapies to optimize cardiac care for patients with DMD.