A truncation in the RYR1 gene associated with central core lesions in skeletal muscle fibres

Sorcin is an early marker of neurodegeneration, Ca2+ dysregulation and endoplasmic reticulum stress associated to neurodegenerative diseases

Dysregulation of calcium signaling is emerging as a key feature in the pathogenesis of neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD), and targeting this process may be therapeutically beneficial. Under this perspective, it is important to study proteins that regulate calcium homeostasis in the cell. Sorcin is one of the most expressed calcium-binding proteins in the human brain; its overexpression increases endoplasmic reticulum (ER) calcium concentration and decreases ER stress in the heart and in other cellular types. Sorcin has been hypothesized to be involved in neurodegenerative diseases, since it may counteract the increased cytosolic calcium levels associated with neurodegeneration. In the present work, we show that Sorcin expression levels are strongly increased in cellular, animal, and human models of AD, PD, and HD, vs. normal cells. Sorcin partially colocalizes with RyRs in neurons and microglia cells; functional experiments with microsomes containing high amounts of RyR2 and RyR3, respectively, show that Sorcin is able to regulate these ER calcium channels. The molecular basis of the interaction of Sorcin with RyR2 and RyR3 is demonstrated by SPR. Sorcin also interacts with other ER proteins as SERCA2 and Sigma-1 receptor in a calcium-dependent fashion. We also show that Sorcin regulates ER calcium transients: Sorcin increases the velocity of ER calcium uptake (increasing SERCA activity). The data presented here demonstrate that Sorcin may represent both a novel early marker of neurodegenerative diseases and a response to cellular stress dependent on neurodegeneration.

Preclinical model systems of ryanodine receptor 1-related myopathies and malignant hyperthermia: a comprehensive scoping review of works published 1990-2019

BACKGROUND

Pathogenic variations in the gene encoding the skeletal muscle ryanodine receptor (RyR1) are associated with malignant hyperthermia (MH) susceptibility, a life-threatening hypermetabolic condition and RYR1-related myopathies (RYR1-RM), a spectrum of rare neuromuscular disorders. In RYR1-RM, intracellular calcium dysregulation, post-translational modifications, and decreased protein expression lead to a heterogenous clinical presentation including proximal muscle weakness, contractures, scoliosis, respiratory insufficiency, and ophthalmoplegia. Preclinical model systems of RYR1-RM and MH have been developed to better understand underlying pathomechanisms and test potential therapeutics.

METHODS

We conducted a comprehensive scoping review of scientific literature pertaining to RYR1-RM and MH preclinical model systems in accordance with the PRISMA Scoping Reviews Checklist and the framework proposed by Arksey and O'Malley. Two major electronic databases (PubMed and EMBASE) were searched without language restriction for articles and abstracts published between January 1, 1990 and July 3, 2019.

RESULTS

Our search yielded 5049 publications from which 262 were included in this review. A majority of variants tested in RYR1 preclinical models were localized to established MH/central core disease (MH/CCD) hot spots. A total of 250 unique RYR1 variations were reported in human/rodent/porcine models with 95% being missense substitutions. The most frequently reported RYR1 variant was R614C/R615C (human/porcine total n = 39), followed by Y523S/Y524S (rabbit/mouse total n = 30), I4898T/I4897T/I4895T (human/rabbit/mouse total n = 20), and R163C/R165C (human/mouse total n = 18). The dyspedic mouse was utilized by 47% of publications in the rodent category and its RyR1-null (1B5) myotubes were transfected in 23% of publications in the cellular model category. In studies of transfected HEK-293 cells, 57% of RYR1 variations affected the RyR1 channel and activation core domain. A total of 15 RYR1 mutant mouse strains were identified of which ten were heterozygous, three were compound heterozygous, and a further two were knockout. Porcine, avian, zebrafish, C. elegans, canine, equine, and drosophila model systems were also reported.

CONCLUSIONS

Over the past 30 years, there were 262 publications on MH and RYR1-RM preclinical model systems featuring more than 200 unique RYR1 variations tested in a broad range of species. Findings from these studies have set the foundation for therapeutic development for MH and RYR1-RM.

Congenital myopathies in the adult neuromuscular clinic: Diagnostic challenges and pitfalls

Objective

To investigate the spectrum of undiagnosed congenital myopathies (CMs) in adults presenting to our neuromuscular clinic and to identify the pitfalls responsible for diagnostic delays.

Methods

We conducted a retrospective review of patients diagnosed with CM in adulthood in our neuromuscular clinic between 2008 and 2018. Patients with an established diagnosis of CM before age 18 years were excluded.

Results

We identified 26 patients with adult-onset CM and 18 patients with pediatric-onset CM who were only diagnosed in adulthood. Among patients with adult onset, the median age at onset was 47 years, and the causative genes were RYR1 (11 families), MYH7 (3 families) and ACTA1 (2 families), and SELENON, MYH2, DNM2, and CACNA1S (1 family each). Of 33 patients who underwent muscle biopsy, only 18 demonstrated histologic abnormalities characteristic of CM. Before their diagnosis of CM, 23 patients had received other diagnoses, most commonly non-neurologic disorders. The main causes of diagnostic delays were mildness of the symptoms delaying neurologic evaluation and attribution of the symptoms to coexisting comorbidities, particularly among pediatric-onset patients.

Conclusions

CMs in adulthood represent a diagnostic challenge, as they may lack the clinical and myopathologic features classically associated with CM. Our findings underscore the need for a revision of the terminology and current classification of these disorders.

Severe congenital RYR1-associated myopathy: the expanding clinicopathologic and genetic spectrum

OBJECTIVE

To report a series of 11 patients on the severe end of the spectrum of ryanodine receptor 1 (RYR1) gene-related myopathy, in order to expand the clinical, histologic, and genetic heterogeneity associated with this group of patients.

METHODS

Eleven patients evaluated in the neonatal period with severe neonatal-onset RYR1-associated myopathy confirmed by genetic testing were ascertained. Clinical features, molecular testing results, muscle imaging, and muscle histology are reviewed.

RESULTS

Clinical features associated with the severe neonatal presentation of RYR1-associated myopathy included decreased fetal movement, hypotonia, poor feeding, respiratory involvement, arthrogryposis, and ophthalmoplegia in 3 patients, and femur fractures or hip dislocation at birth. Four patients had dominant RYR1 mutations, and 7 had recessive RYR1 mutations. One patient had a cleft palate, and another a congenital rigid spine phenotype-findings not previously described in the literature in patients with early-onset RYR1 mutations. Six patients who underwent muscle ultrasound showed relative sparing of the rectus femoris muscle. Histologically, all patients with dominant mutations had classic central cores on muscle biopsy. Patients with recessive mutations showed great histologic heterogeneity, including fibrosis, variation in fiber size, skewed fiber typing, very small fibers, and nuclear internalization with or without ill-defined cores.

CONCLUSIONS

This series confirms and expands the clinical and histologic variability associated with severe congenital RYR1-associated myopathy. Both dominant and recessive mutations of the RYR1 gene can result in a severe neonatal-onset phenotype, but more clinical and histologic heterogeneity has been seen in those with recessive RYR1 gene mutations. Central cores are not obligatory histologic features in recessive RYR1 mutations. Sparing of the rectus femoris muscle on imaging should prompt evaluation for RYR1-associated myopathy in the appropriate clinical context.

Clinical features and ryanodine receptor type 1 gene mutation analysis in a Chinese family with central core disease

Central core disease is a rare inherited neuromuscular disorder caused by mutations in ryanodine receptor type 1 gene. The clinical phenotype of the disease is highly variable. We report a Chinese pedigree with central core disease confirmed by the gene sequencing. All 3 patients in the family presented with mild proximal limb weakness. The serum level of creatine kinase was normal, and electromyography suggested myogenic changes. The histologic analysis of muscle biopsy showed identical central core lesions in almost all of the muscle fibers in the index case. Exon 90-106 in the C-terminal domain of the ryanodine receptor type 1 gene was amplified using polymerase chain reaction. One heterozygous missense mutation G14678A (Arg4893Gln) in exon 102 was identified in all 3 patients. This is the first report of a familial case of central core disease confirmed by molecular study in mainland China.

Malignant hyperthermia

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle that presents as a hypermetabolic response to potent volatile anesthetic gases such as halothane, sevoflurane, desflurane and the depolarizing muscle relaxant succinylcholine, and rarely, in humans, to stresses such as vigorous exercise and heat. The incidence of MH reactions ranges from 1:5,000 to 1:50,000-100,000 anesthesias. However, the prevalence of the genetic abnormalities may be as great as one in 3,000 individuals. MH affects humans, certain pig breeds, dogs, horses, and probably other animals. The classic signs of MH include hyperthermia to marked degree, tachycardia, tachypnea, increased carbon dioxide production, increased oxygen consumption, acidosis, muscle rigidity, and rhabdomyolysis, all related to a hypermetabolic response. The syndrome is likely to be fatal if untreated. Early recognition of the signs of MH, specifically elevation of end-expired carbon dioxide, provides the clinical diagnostic clues. In humans the syndrome is inherited in autosomal dominant pattern, while in pigs in autosomal recessive. The pathophysiologic changes of MH are due to uncontrolled rise of myoplasmic calcium, which activates biochemical processes related to muscle activation. Due to ATP depletion, the muscle membrane integrity is compromised leading to hyperkalemia and rhabdomyolysis. In most cases, the syndrome is caused by a defect in the ryanodine receptor. Over 90 mutations have been identified in the RYR-1 gene located on chromosome 19q13.1, and at least 25 are causal for MH. Diagnostic testing relies on assessing the in vitro contracture response of biopsied muscle to halothane, caffeine, and other drugs. Elucidation of the genetic changes has led to the introduction, on a limited basis so far, of genetic testing for susceptibility to MH. As the sensitivity of genetic testing increases, molecular genetics will be used for identifying those at risk with greater frequency. Dantrolene sodium is a specific antagonist of the pathophysiologic changes of MH and should be available wherever general anesthesia is administered. Thanks to the dramatic progress in understanding the clinical manifestation and pathophysiology of the syndrome, the mortality from MH has dropped from over 80% thirty years ago to less than 5%.