Current and Future Therapeutic Strategies for Limb Girdle Muscular Dystrophy Type R1: Clinical and Experimental Approaches

Delivery of genetic medicines for muscular dystrophies

Muscular dystrophies are a group of heterogenic disorders characterized by progressive muscle weakness, the most common of them being Duchenne muscular dystrophy (DMD). Muscular dystrophies are caused by mutations in over 50 distinct genes, and many of them are caused by different genetic mechanisms. Currently, none of these diseases have a cure. However, in recent years, significant progress has been made to correct the underlying genetic cause. The clinical development of adeno-associated viral vector-based therapies has simultaneously produced excitement and disappointment in the research community due to the moderate effect, making it clear that new methods of muscle delivery have to be created. Herein, we review the main characteristics of major muscular dystrophies and outline various muscle-targeted delivery methods being explored for genetic medicines.

Capn3b-deficient zebrafish model reveals a key role of autoimmune response in LGMDR1

Mutations in calcium-dependent papain-like protease CALPAIN3 (CAPN3) cause Limb-Girdle Muscular Dystrophy Recessive Type 1 (LGMDR1), the most common limb-girdle muscular dystrophy in humans. In addition to progressive muscle weakness, persistent inflammatory infiltration is also a feature of LGMDR1. Despite the underlying mechanism remaining poorly understood, we consider that it may relate to the newly defined role of CAPN3/Capn3b in the nucleolus. Here, we report that the loss of function of zebrafish capn3b, the counterpart of human CAPN3, induces an autoimmune response akin to that in LGMDR1 patients. capn3b mutant larvae are more susceptible to Listeria monocytogenes injection, characterized by recruiting more macrophages. Under germ-free conditions, transcriptome analysis of the capn3b mutant muscle reveals a significant upregulation of the chemokine-production-related genes. Coincidently, more neutrophils are recruited to the injury site imposed by either muscle stabbing or tail fin amputation. Nucleolar proteomic analysis and enzymatic assays reveal NKAP, an activating factor of the NF-κB pathway, to be a target of CAPN3. We conclude that the accumulation of Nkap and other factors in the capn3b mutant may be involved in the over-activation of innate immunity. Our studies indicate that the zebrafish capn3b mutant is a powerful model for studying the immunity-related progression of human LGMDR1.

Identification and functional characterization of a novel heterozygous splice‑site mutation in the calpain 3 gene causes rare autosomal dominant limb‑girdle muscular dystrophy

Limb-girdle muscular dystrophies are a group of extremely heterogenous neuromuscular disorders that manifest with gradual and progressive weakness of both proximal and distal muscles. Autosomal dominant limb-girdle muscular dystrophy (LGMDD4) or calpainopathy is a very rare form of myopathy characterized by weakness and atrophy of both proximal and distal muscles with a variable age of onset. LGMDD4 is caused by germline heterozygous mutations of the calpain 3 (CAPN3) gene. Patients with LGMDD4 often show extreme phenotypic heterogeneity; however, most patients present with gait difficulties, increased levels of serum creatine kinase, myalgia and back pain. In the present study, a 16-year-old male patient, clinically diagnosed with LGMDD4, was investigated. The proband had been suffering from weakness and atrophy of both of their proximal and distal muscles, and had difficulty walking and standing independently. The serum creatine kinase levels (4,754 IU/l; normal, 35-232 IU/l) of the patient were markedly elevated. The younger sister and mother of the proband were also clinically diagnosed with LGMDD4, while the father was phenotypically normal. Whole exome sequencing identified a heterozygous novel splice-site (c.2440-1G>A) mutation in intron 23 of the CAPN3 gene in the proband. Sanger sequencing confirmed that this mutation was also present in both the younger sister and mother of the proband, but the father was not a carrier of this mutation. This splice-site (c.2440-1G>A) mutation causes aberrant splicing of CAPN3 mRNA, leading to the skipping of the last exon (exon 24) of CAPN3 mRNA and resulting in the removal of eight amino acids from the C-terminal of domain IV of the CAPN3 protein. Hence, this splice site mutation causes the formation of a truncated CAPN3 protein (p.Trp814*) of 813 amino acids instead of the wild-type CAPN3 protein that consists of 821 amino acids. This mutation causes partial loss of domain IV (PEF domain) in the CAPN3 protein, which is involved in calcium binding and homodimerization; therefore, this is a loss-of-function mutation. Relative expression of the mutated CAPN3 mRNA was reduced in comparison with the wild-type CAPN3 mRNA in the proband, and their younger sister and mother. This mutation was also not present in 100 normal healthy control individuals of the same ethnicity. The present study reported the first case of CAPN3 gene-associated LGMDD4 in the Chinese population.

Management of a 25-Year-Old Female Patient With Limb-Girdle Muscular Dystrophy With Physiotherapy: A Case Report

Limb-girdle muscular dystrophy (LGMD) is a collection of neuromuscular diseases that develop gradually and are rare, genetically, and clinically diverse. The weakness in muscles affecting the shoulder and pelvic girdles is a defining feature of LGMD. Calpainopathy is another name for limb-girdle muscular dystrophy type 2A (LGMD2A). Limb-girdle muscular dystrophy type 2A results from alterations in the calpain-3 (CAPN3) gene, which results in a CAPN3 protein shortage. Gower's sign is most commonly found in LGMD2A. The prevalence ranges from one person in every 14,500 to one in every 123,000. We present a case of a 25-year-old hypotensive female patient who complained of weakness in all four limbs and easy fatigue with a positive Gower's sign. For subsequent management, the neurologist referred the patient to the physical therapy department. The physical therapy goals included enhanced muscle strength, increased joint mobility, reduced fatigue, normalizing gait, and building dynamic balance and postural stability. Diagnosing LGMD clinical variability is important, emphasizing the importance of precise subtype identification and tailoring therapy. Tackling specific muscular deficits and functional restrictions emerges as a critical component in the holistic care of LGMD by physiotherapists. Continuous monitoring and evaluation using appropriate scales and measurements are essential for tracking performance and tailoring treatment strategies. Regular follow-up consultations with the physiotherapist are needed to identify changes in an individual's health and alter the treatment plan accordingly.

Efficacy of N-163 beta-glucan in beneficially improving biomarkers of relevance to muscle function in patients with muscular dystrophies in a pilot clinical study

Background

Muscular dystrophies other than Duchenne muscular dystrophy (DMD) are genetic diseases characterized by increasing muscle weakness, loss of ambulation, and ultimately cardiac and respiratory failure. There are currently no effective therapeutics available. Having demonstrated the efficacy of a N-163 strain of Aureobasidium Pullulans (Neu-REFIX) produced B-1, 3-1,6-Glucan in pre-clinical and clinical studies of Duchenne muscular dystrophy (DMD) earlier, we assessed the effectiveness of this novel Beta glucan in the other muscular dystrophies in the present study.

Methods

In this 60-day study, six patients with muscular dystrophies other than DMD consumed one 8g gel of Neu-REFIX beta-glucan along with their usual standard of care treatment regimen, and their biomarkers of relevance to muscle function such as serum calcium (SC), creatine phosphokinase (CPK), and alkaline phosphatase (ALP) levels along with functional improvement criteria, which is, Medical research council (MRC) scale and North Star Ambulatory assessment (NSAA), assessed at baseline and following the intervention.

Results

After the intervention, the SC levels significantly decreased from a mean baseline value of 9.28 mg/dL to 8.31 mg/dL (p-value = 0.02). With a p-value of 0.29, the mean CPK value dropped from 2192.33 IU/L to 1567.5 IU/L. Following the intervention, the ALP levels dropped from 200.33 to 75.5 U/L (p-value = 0.15). MRC scale improved in three out of six patients. NSAA remained stable. There were no adverse effects.

Conclusion

This study has proven the safety of Neu REFIX beta-glucan food supplement and its efficacy in improving both plasma biomarkers and functional parameters of muscle in a short duration of 2 months. Further validation by evaluation of muscle function for a longer duration is recommended to confirm the efficacy of Neu-REFIX food supplement as a potential adjuvant DMT in muscular dystrophies.

A retrospective study on the clinical and molecular outcomes of calpainopathy in a Turkish patient cohort

Background and aim

Calpainopathy, also known as limb-girdle muscular dystrophy recessive type 1, is a progressive muscle disorder that impacts the muscles around the hips and shoulders. The disease is caused by defects in the CAPN3 gene and can be inherited in both recessive and dominant forms. In this retrospective study, we aimed to evaluate the clinical and molecular results of our patients with calpainopathy and to examine the CAPN3 variants in Turkish and global populations.

Materials and methods

Molecular analyses were performed using the next-generation sequencing (NGS) method. CAPN3 variants were identified through the examination of various databases.

Results

In this retrospective study, the cohort consisted of seven patients exhibiting the CAPN3 (NM_000070.3) mutation and a phenotype compatible with calpainopathy at a single center in Türkiye. All patients displayed high CK levels and muscle weakness. We report a novel missense c.2437G>A variant that causes the autosomal dominant form of calpainopathy. Interestingly, the muscle biopsy report for the patient with the novel mutation indicated sarcoglycan deficiency. Molecular findings for the remaining individuals in the cohort included a compound heterozygous variant (frameshift and missense), one homozygous nonsense, one homozygous intronic deletion, and three homozygous missense variants. The most common variant in the Turkish population was c.550del. In both populations, pathogenic variants were most frequently located in exon 21, according to exon length. Variants were stochastically distributed based on consequences in CAPN3 domains.

Conclusion

Therefore, the NGS method proves highly effective in diagnosing rare diseases characterized by clinical heterogeneity. Assessing variants based on ethnicity holds significance in the development of precise therapies.

Loss of calpain3b in Zebrafish, a Model of Limb-Girdle Muscular Dystrophy, Increases Susceptibility to Muscle Defects Due to Elevated Muscle Activity

Limb-Girdle Muscular Dystrophy Type R1 (LGMDR1; formerly LGMD2A), characterized by progressive hip and shoulder muscle weakness, is caused by mutations in CAPN3. In zebrafish, capn3b mediates Def-dependent degradation of p53 in the liver and intestines. We show that capn3b is expressed in the muscle. To model LGMDR1 in zebrafish, we generated three deletion mutants in capn3b and a positive-control dmd mutant (Duchenne muscular dystrophy). Two partial deletion mutants showed transcript-level reduction, whereas the RNA-less mutant lacked capn3b mRNA. All capn3b homozygous mutants were developmentally-normal adult-viable animals. Mutants in dmd were homozygous-lethal. Bathing wild-type and capn3b mutants in 0.8% methylcellulose (MC) for 3 days beginning 2 days post-fertilization resulted in significantly pronounced (20-30%) birefringence-detectable muscle abnormalities in capn3b mutant embryos. Evans Blue staining for sarcolemma integrity loss was strongly positive in dmd homozygotes, negative in wild-type embryos, and negative in MC-treated capn3b mutants, suggesting membrane instability is not a primary muscle pathology determinant. Increased birefringence-detected muscle abnormalities in capn3b mutants compared to wild-type animals were observed following induced hypertonia by exposure to cholinesterase inhibitor, azinphos-methyl, reinforcing the MC results. These mutant fish represent a novel tractable model for studying the mechanisms underlying muscle repair and remodeling, and as a preclinical tool for whole-animal therapeutics and behavioral screening in LGMDR1.