4-Chloro-m-Cresol Cannot Detect Malignant Hyperthermia Equivocal Cells in an Alternative Minimally Invasive Diagnostic Test of Malignant Hyperthermia Susceptibility

Effects of Remimazolam on Intracellular Calcium Dynamics in Myotubes Derived from Patients with Malignant Hyperthermia and Functional Analysis of Type 1 Ryanodine Receptor Gene Variants

Remimazolam is a novel general anesthetic and its safety in patients with malignant hyperthermia (MH) is unknown. We used myotubes derived from the skeletal muscle of patients with MH to examine the response to ryanodine receptor 1 (RYR1) agonist and remimazolam in MH-susceptible patients. Patients underwent muscle biopsy for the Ca2+-induced Ca2+ release (CICR) rate test, a diagnostic tool for MH in Japan. Ten patients had myotubes obtained from skeletal muscle cultures, and the genes associated with malignant hyperthermia in these patients were analyzed. The EC50 of caffeine, cresol, and remimazolam to induce intracellular calcium concentration change were compared between myotubes from CICR-negative genetic test patients and myotubes from other patients. Eight of the ten were CICR-positive, five of whom had RYR1 causative gene mutations or variants. Two patients had CICR-negative genetic tests, and as expected had the highest EC50 (the concentration of a drug that gives a half-maximal response) in response to caffeine, 4CmC and remimazolam. Three patients had a positive CICR but no known variants in RYR1 or CACNA1S (voltage-gated calcium channel subunit alpha1S). Myotubes in these patients had significantly lower EC50s for all agents than myotubes in CICR-negative patients. When myotubes from a patient who was CICR-negative and had no gene variant were used as a control, myotubes from CICR-positive patients were more hyper-responsive than controls to all stimulants used. The EC50 for remimazolam was lowest for myotubes from CICR-positive, RYR1-mutant patients, at 206 µM (corresponding to 123 µg/mL). The concentration was more than 80-times higher than the clinical concentration. RYR1 gene variants in R4645Q and W5020G were shown to be causative gene mutations for MH. Intracellular calcium in myotubes from MH patients are elevated at high concentrations of remimazolam but not at clinically used concentrations of remimazolam. Remimazolam appears to be safe to use in patients with MH.

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.

Analysis of human cultured myotubes responses mediated by ryanodine receptor 1

Malignant hyperthermia is a life-threatening condition caused by autosomal dominant mutations in the ryanodine receptor type 1 gene. Identifying patients predisposed to malignant hyperthermia is done through the Ca-induced Ca release test in Japan. We examined the intracellular calcium concentration in human cultured muscle cells and compared the sensitivity of myotubes to ryanodine receptor type 1 activators based on the Ca-induced Ca release rate. We assessed the utility of this method as an identifying test for predisposition to malignant hyperthermia. Muscle specimens were obtained from 34 individuals undergoing the Ca-induced Ca release test. We cultured myotubes from residual material and monitored changes in intracellular calcium concentration after exposure to the ryanodine receptor type 1 activators caffeine, halothane and 4-chloro-m-cresol by measuring fura-2 fluorescence. We determined the half maximal effective concentrations (EC50) for the test compounds in each myotube and calculated cut-off points using receiver operating characteristic curves. Seventeen patients each were classified into the accelerated and non-accelerated groups based on their Ca-induced Ca release rate. The EC50 values for caffeine, halothane and 4-chloro-m-cresol of the accelerated group were significant lower than those of the non-accelerated group (P < 0.001, P < 0.001 and P < 0.001, respectively). The calculated cut-off points of EC50 values for caffeine, halothane and 4-CmC were 3.62 mM, 2.28 mM and 197 microM, respectively. An increased sensitivity to ryanodine receptor type 1 activators was seen in myotubes in the accelerated group. This functional test on human cultured myotubes indicates that the alteration of their intracellular Ca2+ homeostasis may identify the predisposition to malignant hyperthermia.

Functional analysis of ryanodine receptor type 1 p.R2508C mutation in exon 47

PURPOSE

Malignant hyperthermia (MH) is a pharmacogenetic disorder of intracellular calcium homeostasis with an autosomal dominant inheritance. Most of the reported mutations in exon 47 were identified in Asian patients. However, no functional analysis of p.R2508C has been performed. We therefore conducted a functional analysis of the mutation by altering calcium homeostasis in human embryonic kidney (HEK) 293 cells transfected with the p.R2508C mutation in exon 47 of the ryanodine receptor 1 (RYR1).

METHODS

The entire RYR1 coding region from genomic DNA, which was extracted from the biopsied muscle specimens of two patients, was sequenced. The p.R2508C mutation was introduced into rabbit RYR1 cDNA, and wild-type or p.R2508C mutant cDNAs were transfected into HEK-293 cells. Using the calcium-sensitive probe Fura 2, we utilized the 340/380 nm ratio to analyze alterations in calcium homeostasis following treatment with caffeine and 4-chloro-m-cresol (4CmC).

RESULTS

Genetic analysis revealed a C-->T point mutation of RYR1 exon 47 at position 7522, resulting in an amino acid exchange of arginine for cysteine at amino acid 2508. The half-maximal activation concentrations (EC(50)) of caffeine and 4CmC for HEK-293 cells transfected with the p.R2508C mutation were 1.86 +/- 0.23 mM and 73.14 +/- 19.44 microM, while those for wild-type RYR1 were 2.62 +/- 0.23 mM and 179.31 +/- 35.23 microM, respectively.

CONCLUSION

We demonstrated that the transfected RYR1 mutant was more sensitive to caffeine and 4CmC than wildtype RYR1. These findings suggest that the p.R2508C mutation may be pathogenetic for susceptibility to MH.

Effects of propofol on calcium homeostasis in human skeletal muscle

Malignant hyperthermia is a pharmacogenetic skeletal muscle disorder of intracellular calcium (Ca2+) homeostasis with an autosomal dominant inheritance. The objective of this study was to investigate the safety of propofol by investigating its effects on calcium homeostasis and its effect sites in human skeletal muscles. Muscle specimens were obtained from 10 individuals with predisposition to malignant hyperthermia. In skinned fibre experiments, we measured the effects of propofol on the Ca(2+)-induced Ca2+ release and the uptake of Ca2+ into the sarcoplasmic reticulum. Ca2+ imaging in primary myotubes was employed to analyse propofol-mediated alternations in the Ca2+ regulation and propofol-induced Ca2+ responses in the presence of Ca2+ channel blocker or Ca(2+)-induced Ca2+ release inhibitor. Increased Ca2+ release from the sarcoplasmic reticulum and inhibition of Ca2+ uptake into the sarcoplasmic reticulum were not observed with 100 microM propofol. A rise of Ca2+ was not seen under 100 microM propofol and the EC50 value for propofol was 274.7 +/- 33.9 microM, which is higher than the clinical levels for anaesthesia. Propofol-induced Ca2+ responses were remarkably attenuated in the presence of Ca2+ channel blocker or Ca(2+)-induced Ca+ release inhibitor compared with the results obtained with caffeine. We conclude firstly that propofol is safe for individuals with predisposition to malignant hyperthermia when it is used within the recommended clinical dosage range, and secondly that its mode of action upon ryanodine receptors is likely to be different from that of caffeine.