A Rare Case of Severe Congenital RYR1-Associated Myopathy

A novel, patient-derived RyR1 mutation impairs muscle function and calcium homeostasis in mice

RYR1 is the most commonly mutated gene associated with congenital myopathies, a group of early-onset neuromuscular conditions of variable severity. The functional effects of a number of dominant RYR1 mutations have been established; however, for recessive mutations, these effects may depend on multiple factors, such as the formation of a hypomorphic allele, or on whether they are homozygous or compound heterozygous. Here, we functionally characterize a new transgenic mouse model knocked-in for mutations identified in a severely affected child born preterm and presenting limited limb movement. The child carried the homozygous c.14928C>G RYR1 mutation, resulting in the p.F4976L substitution. In vivo and ex vivo assays revealed that homozygous mice fatigued sooner and their muscles generated significantly less force compared with their WT or heterozygous littermates. Electron microscopy, biochemical, and physiological analyses showed that muscles from RyR1 p.F4976L homozygous mice have the following properties: (1) contain fewer calcium release units and show areas of myofibrillar degeneration, (2) contain less RyR1 protein, (3) fibers show smaller electrically evoked calcium transients, and (4) their SR has smaller calcium stores. In addition, single-channel recordings indicate that RyR1 p.F4976L exhibits higher Po in the presence of 100 μM [Ca2+]. Our mouse model partly recapitulates the clinical picture of the homozygous human patient and provides significant insight into the functional impact of this mutation. These results will help understand the pathology of patients with similar RYR1 mutations.

Bone Quality in Patients with a Congenital Myopathy: A Scoping Review

BACKGROUND

Congenital myopathies are rare neuromuscular disorders presenting with a wide spectrum of clinical features, including long bone fractures (LBFs) that negatively influence functional prognosis, quality of life and survival. Systematic research on bone quality in these patients is lacking.

OBJECTIVE

This scoping review aims to summarize all evidence on bone quality and to deduce recommendations for bone quality management in congenital myopathies.

METHODS

Five electronic databases (Pubmed, Embase, Cochrane, Web of Science, CINAHL) were searched. All studies on bone quality in congenital myopathies were included. Decreased bone quality was defined as low bone mineral density and/or (fragility) LBFs. Study selection and data extraction were performed by three independent reviewers.

RESULTS

We included 244 single cases (mean: 4.1±7.6 years; median: 0 years) diagnosed with a congenital myopathy from 35 articles. Bone quality was decreased in 93 patients (37%) (mean: 2.6±6.8 years; median: 0 years). Low bone mineral density was reported in 11 patients (4.5%) (mean: 10.9±9.7; median: 11 years). Congenital LBFs were reported in 64 patients (26%). (Fragility) LBFs later at life were described in 24 patients (9.8%) (mean: 14.9±11.0; median: 14 years). Four cases (1.6%) were reported to receive vitamin D and/or calcium supplementation or diphosphonate administration.

CONCLUSION

LBFs are thus frequently reported in congenital myopathies. We therefore recommend optimal bone quality management through bone mineral density assessment, vitamin D and calcium suppletion, and referral to internal medicine or pediatrics for consideration of additional therapies in order to prevent complications of low bone mineral density.

Early Findings in Neonatal Cases of RYR1-Related Congenital Myopathies

Ryanodine receptor type 1-related congenital myopathies are the most represented subgroup among congenital myopathies (CMs), typically presenting a central core or multiminicore muscle histopathology and high clinical heterogeneity. We evaluated a cohort of patients affected with Ryanodine receptor type 1-related congenital myopathy (RYR1-RCM), focusing on four patients who showed a severe congenital phenotype and underwent a comprehensive characterization at few months of life. To date there are few reports on precocious instrumental assessment. In two out of the four patients, a muscle biopsy was performed in the first days of life (day 5 and 37, respectively) and electron microscopy was carried out in two patients detecting typical features of congenital myopathy. Two patients underwent brain MRI in the first months of life (15 days and 2 months, respectively), one also a fetal brain MRI. In three children electromyography was performed in the first week of life and neurogenic signs were excluded. Muscle MRI obtained within the first years of life showed a typical pattern of RYR1-CM. The diagnosis was confirmed through genetic analysis in three out of four cases using Next Generation Sequencing (NGS) panels. The development of a correct and rapid diagnosis is a priority and may lead to prompt medical management and helps optimize inclusion in future clinical trials.

Inherited Neuromuscular Disorders: Which Role for Serum Biomarkers?

Inherited neuromuscular disorders (INMD) are a heterogeneous group of rare diseases that involve muscles, motor neurons, peripheral nerves or the neuromuscular junction. Several different lab abnormalities have been linked to INMD: sometimes they are typical of the disorder, but they usually appear to be less specific. Sometimes serum biomarkers can point out abnormalities in presymtomatic or otherwise asymptomatic patients (e.g., carriers). More often a biomarker of INMD is evaluated by multiple clinicians other than expert in NMD before the diagnosis, because of the multisystemic involvement in INMD. The authors performed a literature search on biomarkers in inherited neuromuscular disorders to provide a practical approach to the diagnosis and the correct management of INMD. A considerable number of biomarkers have been reported that support the diagnosis of INMD, but the role of an expert clinician is crucial. Hence, the complete knowledge of such abnormalities can accelerate the diagnostic workup supporting the referral to specialists in neuromuscular disorders.

iPSC-derived progenitor stromal cells provide new insights into aberrant musculoskeletal development and resistance to cancer in down syndrome

Down syndrome (DS) is a congenital disorder caused by trisomy 21 (T21). It is associated with cognitive impairment, muscle hypotonia, heart defects, and other clinical anomalies. At the same time, individuals with Down syndrome have lower prevalence of solid tumor formation. To gain new insights into aberrant DS development during early stages of mesoderm formation and its possible connection to lower solid tumor prevalence, we developed the first model of two types of DS iPSC-derived stromal cells. Utilizing bioinformatic and functional analyses, we identified over 100 genes with coordinated expression among mesodermal and endothelial cell types. The most significantly down-regulated processes in DS mesodermal progenitors were associated with decreased stromal progenitor performance related to connective tissue organization as well as muscle development and functionality. The differentially expressed genes included cytoskeleton-related genes (actin and myosin), ECM genes (Collagens, Galectin-1, Fibronectin, Heparan Sulfate, LOX, FAK1), cell cycle genes (USP16, S1P complexes), and DNA damage repair genes. For DS endothelial cells, our analysis revealed most down-regulated genes associated with cellular response to external stimuli, cell migration, and immune response (inflammation-based). Together with functional assays, these results suggest an impairment in mesodermal development capacity during early stages, which likely translates into connective tissue impairment in DS patients. We further determined that, despite differences in functional processes and characteristics, a significant number of differentially regulated genes involved in tumorigenesis were expressed in a highly coordinated manner across endothelial and mesodermal cells. These findings strongly suggest that microRNAs (miR-24-4, miR-21), cytoskeleton remodeling, response to stimuli, and inflammation can impact resistance to tumorigenesis in DS patients. Furthermore, we also show that endothelial cell functionality is impaired, and when combined with angiogenic inhibition, it can provide another mechanism for decreased solid tumor development. We propose that the same processes, which specify the basis of connective tissue impairment observed in DS patients, potentially impart a resistance to cancer by hindering tumor progression and metastasis. We further establish that cancer-related genes on Chromosome 21 are up-regulated, while genome-wide cancer-related genes are down-regulated. These results suggest that trisomy 21 induces a modified regulation and compensation of many biochemical pathways across the genome. Such downstream interactions may contribute toward promoting tumor resistant mechanisms.

Severe Neonatal RYR1 Myopathy With Pathological Features of Congenital Muscular Dystrophy

The phenotypes associated with pathogenic variants in the ryanodine receptor 1 gene (RYR1, OMIM# 180901) have greatly expanded over the last few decades as genetic testing for RYR1 variants has become more common. Initially described in association with malignant hyperthermia, pathogenic variants in RYR1 are typically associated with core pathology in muscle biopsies (central core disease or multiminicore disease) and symptomatic myopathies with symptoms ranging from mild weakness to perinatal lethality. We describe a 2-week-old male patient with multiple congenital dysmorphisms, severe perinatal weakness, and subsequent demise, whose histopathology on autopsy was consistent with congenital muscular dystrophy. Immunohistochemical analysis of dystrophy-associated proteins was normal. Rapid exome sequencing revealed a novel heterozygous nonsense variant (p.Trp661Ter) in RYR1, as well as a previously described RYR1 pathogenic variant associated with congenital myopathy (p.Phe4976Leu). This highlights the potential for RYR1 pathogenic variants to produce pathological findings most consistent with congenital muscular dystrophy.