Dystrophin Threshold Level Necessary for Normalization of Neuronal Nitric Oxide Synthase, Inducible Nitric Oxide Synthase, and Ryanodine Receptor-Calcium Release Channel Type 1 Nitrosylation in Golden Retriever Muscular Dystrophy Dystrophinopathy

CRISPR-Based Gene Editing Techniques in Pediatric Neurological Disorders

The emergence of gene editing technologies offers a unique opportunity to develop mutation-specific treatments for pediatric neurological disorders. Gene editing systems can potentially alter disease trajectory by correcting dysfunctional mutations or therapeutically altering gene expression. Clustered regularly interspaced short palindromic repeats (CRISPR)-based approaches are attractive gene therapy platforms to personalize treatments because of their specificity, ease of design, versatility, and cost. However, many such approaches remain in the early stages of development, with ongoing efforts to optimize editing efficiency, minimize unintended off-target effects, and mitigate pathologic immune responses. Given the rapid evolution of CRISPR-based therapies, it is prudent for the clinically based child neurologist to have a conceptual understanding of what such therapies may entail, including both benefits and risks and how such therapies may be clinically applied. In this review, we describe the fundamentals of CRISPR-based therapies, discuss the opportunities and challenges that have arisen, and highlight preclinical work in several pediatric neurological diseases.

COVID-19 Mimics Pulmonary Dysfunction in Muscular Dystrophy as a Post-Acute Syndrome in Patients

Although progressive wasting and weakness of respiratory muscles are the prominent hallmarks of Duchenne muscular dystrophy (DMD) and long-COVID (also referred as the post-acute sequelae of COVID-19 syndrome); however, the underlying mechanism(s) leading to respiratory failure in both conditions remain unclear. We put together the latest relevant literature to further understand the plausible mechanism(s) behind diaphragm malfunctioning in COVID-19 and DMD conditions. Previously, we have shown the role of matrix metalloproteinase-9 (MMP9) in skeletal muscle fibrosis via a substantial increase in the levels of tumor necrosis factor-α (TNF-α) employing a DMD mouse model that was crossed-bred with MMP9-knockout (MMP9-KO or MMP9^-/-) strain. Interestingly, recent observations from clinical studies show a robust increase in neopterin (NPT) levels during COVID-19 which is often observed in patients having DMD. What seems to be common in both (DMD and COVID-19) is the involvement of neopterin (NPT). We know that NPT is generated by activated white blood cells (WBCs) especially the M1 macrophages in response to inducible nitric oxide synthase (iNOS), tetrahydrobiopterin (BH4), and tetrahydrofolate (FH4) pathways, i.e., folate one-carbon metabolism (FOCM) in conjunction with epigenetics underpinning as an immune surveillance protection. Studies from our laboratory, and others researching DMD and the genetically engineered humanized (hACE2) mice that were administered with the spike protein (SP) of SARS-CoV-2 revealed an increase in the levels of NPT, TNF-α, HDAC, IL-1β, CD147, and MMP9 in the lung tissue of the animals that were subsequently accompanied by fibrosis of the diaphragm depicting a decreased oscillation phenotype. Therefore, it is of interest to understand how regulatory processes such as epigenetics involvement affect DNMT, HDAC, MTHFS, and iNOS that help generate NPT in the long-COVID patients.

Genetic correction strategies for Duchenne Muscular Dystrophy and their impact on the heart

Background

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder with early childhood onset characterized by profound loss of muscle strength and associated cardiomyopathy. DMD affects is most often caused by deletions involving single or multiple exons that disrupt the open reading frame of the DMD gene. Mutations causing loss or premature truncation of dystrophin result in dystrophin protein deficiency, which renders the plasma membrane of skeletal myofibers and cardiomyocytes weakened.

Aim of Review

Genetic correction is in use to treat DMD, since several drugs have been already approved which partially restore dystrophin production through the use of antisense oligonucleotides. There are multiple ongoing clinical trials to evaluate the efficacy of treating DMD with micro-dystrophins delivered by adeno-associated viruses. Future approaches entail gene editing to target the single copy of the DMD gene on the X-chromosome. The primary, near-term goal is restoration of skeletal muscle dystrophin, and for some of these treatments, the efficacy in the heart is not fully known. Here, we discuss the anticipated cardiac outcomes of dystrophin-targeted therapies, and how this information informs genomic medicine for cardiomyopathies, especially in childhood.

Key Scientific Concepts of Review

Many genetic treatment strategies are being implemented to treat DMD. Since most preclinical testing has focused on skeletal muscle, there is a gap in knowledge about the expected effects of these approaches on cardiac genetic correction and cardiomyopathy progression in DMD. Additional study is needed.

Toward the correction of muscular dystrophy by gene editing

Recent advances in gene editing technologies are enabling the potential correction of devastating monogenic disorders through elimination of underlying genetic mutations. Duchenne muscular dystrophy (DMD) is an especially severe genetic disorder caused by mutations in the gene encoding dystrophin, a membrane-associated protein required for maintenance of muscle structure and function. Patients with DMD succumb to loss of mobility early in life, culminating in premature death from cardiac and respiratory failure. The disease has thus far defied all curative strategies. CRISPR gene editing has provided new opportunities to ameliorate the disease by eliminating DMD mutations and thereby restore dystrophin expression throughout skeletal and cardiac muscle. Proof-of-concept studies in rodents, large mammals, and human cells have validated the potential of this approach, but numerous challenges remain to be addressed, including optimization of gene editing, delivery of gene editing components throughout the musculature, and mitigation of possible immune responses. This paper provides an overview of recent work from our laboratory and others toward the genetic correction of DMD and considers the opportunities and challenges in the path to clinical translation. Lessons learned from these studies will undoubtedly enable further applications of gene editing to numerous other diseases of muscle and other tissues.

The lncRNA 44s2 Study Applicability to the Design of 45-55 Exon Skipping Therapeutic Strategy for DMD

In skeletal muscle, long noncoding RNAs (lncRNAs) are involved in dystrophin protein stabilization but also in the regulation of myocytes proliferation and differentiation. Hence, they could represent promising therapeutic targets and/or biomarkers for Duchenne and Becker muscular dystrophy (DMD/BMD). DMD and BMD are X-linked myopathies characterized by a progressive muscular dystrophy with or without dilatative cardiomyopathy. Two-thirds of DMD gene mutations are represented by deletions, and 63% of patients carrying DMD deletions are eligible for 45 to 55 multi-exons skipping (MES), becoming BMD patients (BMDΔ45-55). We analyzed the genomic lncRNA presence in 38 BMDΔ45-55 patients and characterized the lncRNA localized in introns 44 and 55 of the DMD gene. We highlighted that all four lncRNA are differentially expressed during myogenesis in immortalized and primary human myoblasts. In addition, the lncRNA44s2 was pointed out as a possible accelerator of differentiation. Interestingly, lncRNA44s expression was associated with a favorable clinical phenotype. These findings suggest that lncRNA44s2 could be involved in muscle differentiation process and become a potential disease progression biomarker. Based on these results, we support MES45-55 therapy and propose that the design of the CRISPR/Cas9 MES45-55 assay consider the lncRNA sequences bordering the exonic 45 to 55 deletion.

Therapeutic Strategies for Duchenne Muscular Dystrophy: An Update

Neuromuscular disorders encompass a heterogeneous group of conditions that impair the function of muscles, motor neurons, peripheral nerves, and neuromuscular junctions. Being the most common and most severe type of muscular dystrophy, Duchenne muscular dystrophy (DMD), is caused by mutations in the X-linked dystrophin gene. Loss of dystrophin protein leads to recurrent myofiber damage, chronic inflammation, progressive fibrosis, and dysfunction of muscle stem cells. Over the last few years, there has been considerable development of diagnosis and therapeutics for DMD, but current treatments do not cure the disease. Here, we review the current status of DMD pathogenesis and therapy, focusing on mutational spectrum, diagnosis tools, clinical trials, and therapeutic approaches including dystrophin restoration, gene therapy, and myogenic cell transplantation. Furthermore, we present the clinical potential of advanced strategies combining gene editing, cell-based therapy with tissue engineering for the treatment of muscular dystrophy.

iNOS is not responsible for RyR1 S-nitrosylation in mdx mice with truncated dystrophin

Background

Previous research indicated that nitric oxide synthase (NOS) is the key molecule for S-nitrosylation of ryanodine receptor 1 (RyR1) in DMD model mice (mdx mice) and that both neuronal NOS (nNOS) and inducible NOS (iNOS) might contribute to the reaction because nNOS is mislocalized in the cytoplasm and iNOS expression is higher in mdx mice. We investigated the effect of iNOS on RyR1 S-nitrosylation in mdx mice and whether transgenic expression of truncated dystrophin reduced iNOS expression in mdx mice or not.

Methods

Three- to 4-month-old C57BL/6 J, mdx, and transgenic mdx mice expressing exon 45–55-deleted human dystrophin (Tg/mdx mice) were used. We also generated two double mutant mice, mdx iNOS KO and Tg/mdx iNOS KO to reveal the iNOS contribution to RyR1 S-nitrosylation. nNOS and iNOS expression levels in skeletal muscle of these mice were assessed by immunohistochemistry (IHC), qRT-PCR, and Western blotting. Total NOS activity was measured by a citrulline assay. A biotin-switch method was used for detection of RyR1 S-nitrosylation. Statistical differences were assessed by one-way ANOVA with Tukey-Kramer post-hoc analysis.

Results

mdx and mdx iNOS KO mice showed the same level of RyR1 S-nitrosylation. Total NOS activity was not changed in mdx iNOS KO mice compared with mdx mice. iNOS expression was undetectable in Tg/mdx mice expressing exon 45–55-deleted human dystrophin, but the level of RyR1 S-nitrosylation was the same in mdx and Tg/mdx mice.

Conclusion

Similar levels of RyR1 S-nitrosylation and total NOS activity in mdx and mdx iNOS KO demonstrated that the proportion of iNOS in total NOS activity was low, even in mdx mice. Exon 45–55-deleted dystrophin reduced the expression level of iNOS, but it did not correct the RyR1 S-nitrosylation. These results indicate that iNOS was not involved in RyR1 S-nitrosylation in mdx and Tg/mdx mice muscles.

Combined Treatment with Peptide-Conjugated Phosphorodiamidate Morpholino Oligomer-PPMO and AAV-U7 Rescues the Severe DMD Phenotype in Mice

Duchenne muscular dystrophy (DMD) is a devastating neuromuscular disease caused by an absence of the dystrophin protein, which is essential for muscle fiber integrity. Among the developed therapeutic strategies for DMD, the exon-skipping approach corrects the frameshift and partially restores dystrophin expression. It could be achieved through the use of antisense sequences, such as peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO) or the small nuclear RNA-U7 carried by an adeno-associated virus (AAV) vector. AAV-based gene therapy approaches have potential for use in DMD treatment but are subject to a major limitation: loss of the AAV genome, necessitating readministration of the vector, which is not currently possible, due to the immunogenicity of the capsid. The PPMO approach requires repeated administrations and results in only weak cardiac dystrophin expression. Here, we evaluated a combination of PPMO- and AAV-based therapy in a mouse model of severe DMD. Striking benefits of this combined therapy were observed in striated muscles, with marked improvements in heart and diaphragm structure and function, with unrivalled extent of survival, opening novel therapeutic perspectives for patients.

CRISPR for Neuromuscular Disorders: Gene Editing and Beyond

This is a review describing advances in CRISPR/Cas-mediated therapies for neuromuscular disorders (NMDs). We explore both CRISPR-mediated editing and dead Cas approaches as potential therapeutic strategies for multiple NMDs. Last, therapeutic considerations, including delivery and off-target effects, are also discussed.

What is the level of dystrophin expression required for effective therapy of Duchenne muscular dystrophy?

Duchenne muscular dystrophy (DMD) is a fatal X-linked muscle wasting disease. The disease is due to mutations in the DMD gene that encodes for a large intracellular protein called dystrophin. Dystrophin plays a critical role in linking the internal cytoskeleton of the striated muscle cell with the extracellular matrix as well as having cell signalling functions. In its absence muscle contraction is associated with cycles of damage, repair, inflammation and fibrosis with eventual loss of muscle and replacement with fat. Experiments in animal models of DMD have generated a number of different approaches to the induction of dystrophin including viral vector mediated delivery of a recombinant dystrophin gene, antisense oligonucleotide mediated exon-skipping to restore the open reading frame in the dystrophin mRNA, read-through of premature stop mutations, genome modification using CRISPR-Cas9 or cell based transfer of a functional dystrophin gene. In all cases, it will be important to understand how much dystrophin expression is required for a clinically effective therapy and this review examines the data from humans and animal models to estimate the percentage of endogenous dystrophin that is likely to have significant clinical benefit. While there are a number of important caveats to consider, including the appropriate outcome measures, this review suggests that approximately 20% of endogenous levels uniformly distributed within the skeletal muscles and the heart may be sufficient to largely prevent disease progression.

miR-708-5p and miR-34c-5p are involved in nNOS regulation in dystrophic context

Background

Duchenne (DMD) and Becker (BMD) muscular dystrophies are caused by mutations in the DMD gene coding for dystrophin, a protein being part of a large sarcolemmal protein scaffold that includes the neuronal nitric oxide synthase (nNOS). The nNOS was shown to play critical roles in a variety of muscle functions and alterations of its expression and location in dystrophic muscle fiber leads to an increase of the muscle fatigability. We previously revealed a decrease of nNOS expression in BMD patients all presenting a deletion of exons 45 to 55 in the DMD gene (BMDd45-55), impacting the nNOS binding site of dystrophin. Since several studies showed deregulation of microRNAs (miRNAs) in dystrophinopathies, we focused on miRNAs that could target nNOS in dystrophic context.

Methods

By a screening of 617 miRNAs in BMDd45-55 muscular biopsies using TLDA and an in silico study to determine which one could target nNOS, we selected four miRNAs. In order to select those that targeted a sequence of 3′UTR of NOS1, we performed luciferase gene reporter assay in HEK393T cells. Finally, expression of candidate miRNAs was modulated in control and DMD human myoblasts (DMDd45-52) to study their ability to target nNOS.

Results

TLDA assay and the in silico study allowed us to select four miRNAs overexpressed in muscle biopsies of BMDd45-55 compared to controls. Among them, only the overexpression of miR-31, miR-708, and miR-34c led to a decrease of luciferase activity in an NOS1-3′UTR-luciferase assay, confirming their interaction with the NOS1-3′UTR. The effect of these three miRNAs was investigated on control and DMDd45-52 myoblasts. First, we showed a decrease of nNOS expression when miR-708 or miR-34c were overexpressed in control myoblasts. We then confirmed that DMDd45-52 cells displayed an endogenous increased of miR-31, miR-708, and miR-34c and a decreased of nNOS expression, the same characteristics observed in BMDd45-55 biopsies. In DMDd45-52 cells, we demonstrated that the inhibition of miR-708 and miR-34c increased nNOS expression, confirming that both miRNAs can modulate nNOS expression in human myoblasts.

Conclusion

These results strongly suggest that miR-708 and miR-34c, overexpressed in dystrophic context, are new actors involved in the regulation of nNOS expression in dystrophic muscle.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0161-2) contains supplementary material, which is available to authorized users.