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Ryanodine Receptors (RyRs) are massive channels that release Ca 2+ from the endoplasmic and sarcoplasmic reticulum. Hundreds of mutations are linked to malignant hyperthermia (MH), myopathies, and arrhythmias. Here, we explore the first MH mutation identified in humans by providing cryo-EM snapshots of the pig homolog, R615C, showing that it affects an interface between three solenoid regions. We also show the impact of apo-calmodulin (apoCaM) and how it can induce opening by bending of the bridging solenoid, mediated by its N-terminal lobe. For R615C RyR1, apoCaM binding abolishes a pathological ‘intermediate’ conformation, distributing the population to a mixture of open and closed channels, both different from the structure without apoCaM. Comparisons show that the mutation primarily affects the closed state, inducing partial movements linked to channel activation. This shows that disease mutations can cause distinct pathological conformations of the RyR and facilitate channel opening by disrupting interactions between different solenoid regions. Ryanodine Receptors (RyRs) release Ca2+ from the endoplasmic and sarcoplasmic reticulum. Mutations in RyR are linked to malignant hyperthermia (MH), myopathies, and arrhythmias. Here, a collection of cryoEM structures provides insights into the molecular consequences of MHrelated RyR mutation R615C, and how apoCaM opens RyR1.

ObjectiveThe histopathological features of malignant hyperthermia (MH) and non-anaesthetic (mostly exertional) rhabdomyolysis (RM) due to RYR1 mutations have only been reported in a few cases.MethodsWe performed a retrospective multi-centre cohort study focussing on the histopathological features of patients with MH or RM due to RYR1 mutations (1987–2017). All muscle biopsies were reviewed by a neuromuscular pathologist. Additional morphometric and electron microscopic analysis were performed where possible.ResultsThrough the six participating centres we identified 50 patients from 46 families, including patients with MH (n = 31) and RM (n = 19). Overall, the biopsy of 90% of patients showed one or more myopathic features including: increased fibre size variability (n = 44), increase in the number of fibres with internal nuclei (n = 30), and type I fibre predominance (n = 13). Abnormalities on oxidative staining, generally considered to be more specifically associated with RYR1-related congenital myopathies, were observed in 52%, and included unevenness (n = 24), central cores (n = 7) and multi-minicores (n = 3). Apart from oxidative staining abnormalities more frequently observed in MH patients, the histopathological spectrum was similar between the two groups. There was no correlation between the presence of cores and the occurrence of clinically detectable weakness or presence of (likely) pathogenic variants.ConclusionsPatients with RYR1-related MH and RM exhibit a similar histopathological spectrum, ranging from mild myopathic changes to cores and other features typical of RYR1-related congenital myopathies. Suggestive histopathological features may support RYR1 involvement, also in cases where the in vitro contracture test is not informative.

Thermoregulation is an important aspect of human homeostasis, and high temperatures pose serious stresses for the body. Malignant hyperthermia (MH) is a life-threatening disorder in which body temperature can rise to a lethal level. Here we employ an optically controlled local heat-pulse method to manipulate the temperature in cells with a precision of less than 1 °C and find that the mutants of ryanodine receptor type 1 (RyR1), a key Ca2+ release channel underlying MH, are heat hypersensitive compared with the wild type (WT). We show that the local heat pulses induce an intracellular Ca2+ burst in human embryonic kidney 293 cells overexpressing WT RyR1 and some RyR1 mutants related to MH. Fluorescence Ca2+ imaging using the endoplasmic reticulum–targeted fluorescent probes demonstrates that the Ca2+ burst originates from heat-induced Ca2+ release (HICR) through RyR1-mutant channels because of the channels’ heat hypersensitivity. Furthermore, the variation in the heat hypersensitivity of four RyR1 mutants highlights the complexity of MH. HICR likewise occurs in skeletal muscles of MH model mice. We propose that HICR contributes an additional positive feedback to accelerate thermogenesis in patients with MH.

King-Denborough Syndrome (KDS) is a congenital myopathy (CM) characterised by myopathy, dysmorphic features and susceptibility to malignant hyperthermia. The objective of this study was to investigate the genotype-phenotype correlation in Black African patients presenting with CM, specifically those with KDS-like phenotypes, who remained undiagnosed for over 25 years. A cohort of 67 Black African patients with CM was studied, of whom 44 were clinically evaluated and diagnosed with KDS. Whole-exome sequencing (WES) was performed as part of an international genomics study (ICGNMD) to identify potential pathogenic mutations. Genomic assessments focused on identifying relevant genes, including RYR1 and STAC3 , and establishing genotype-phenotype correlations. The study identified RYR1 and STAC3 mutations as the predominant genetic causes of KDS in this cohort, with mutations in both genes exhibiting autosomal recessive inheritance. While RYR1 has previously been linked to autosomal dominant mutations, STAC3 , which was formerly associated exclusively with Native American Myopathy/Bailey-Bloch Myopathy, congenital hypotonia, and susceptibility to malignant hyperthermia, is now newly associated with CM-KDS in this study. This establishes the first genotype-phenotype correlation for 44 Black African individuals with KDS. This study marks a significant milestone in research on understudied African populations with CM, emphasising the lengthy diagnostic journey these patients endured. The findings highlight the pressing need for improved access to genomic medicine in underserved regions and underscore the importance of expanding research and diagnostic capabilities in Africa. This work contributes to the advancement of genetic medicine in underrepresented populations, facilitating better diagnostic and therapeutic outcomes.

Most glucose is processed in muscle, for energy or glycogen stores. Malignant Hyperthermia Susceptibility (MHS) exemplifies muscle conditions that increase [Ca 2+ ] cytosol . 42% of MHS patients have hyperglycemia. We show that phosphorylated glycogen phosphorylase (GP a ), glycogen synthase (GS a ) – respectively activated and inactivated by phosphorylation – and their Ca 2+ -dependent kinase (PhK), are elevated in microsomal extracts from MHS patients’ muscle. Glycogen and glucose transporter GLUT4 are decreased. [Ca 2+ ] cytosol , increased to MHS levels, promoted GP phosphorylation. Imaging at ~100 nm resolution located GP a at sarcoplasmic reticulum (SR) junctional cisternae, and apo -GP at Z disk. MHS muscle therefore has a wide-ranging alteration in glucose metabolism: high [Ca 2+ ] cytosol activates PhK, which inhibits GS, activates GP and moves it toward the SR, favoring glycogenolysis. The alterations probably cause these patients’ hyperglycemia. For basic studies, MHS emerges as a variable stressor, which forces glucose pathways from the normal to the diseased range, thereby exposing novel metabolic links. Animals and humans move by contracting the skeletal muscles attached to their bones. These muscles take up a type of sugar called glucose from food and use it to fuel contractions or store it for later in the form of glycogen. If muscles fail to use glucose it can lead to excessive sugar levels in the blood and a condition called diabetes. Within muscle cells are stores of calcium that signal the muscle to contract. Changes in calcium levels enhance the uptake of glucose that fuel these contractions. However, variations in calcium have also been linked to diabetes, and it remained unclear when and how these ‘signals’ become harmful. People with a condition called malignant hyperthermia susceptibility (MHS for short) have genetic mutations that allow calcium to leak out from these stores. This condition may result in excessive contractions causing the muscle to over-heat, become rigid and break down, which can lead to death if left untreated. A clinical study in 2019 found that out of hundreds of patients who had MHS, nearly half had high blood sugar and were likely to develop diabetes. Now, Tammineni et al. – including some of the researchers involved in the 2019 study – have set out to find why calcium leaks lead to elevated blood sugar levels. The experiments showed that enzymes that help convert glycogen to glucose are more active in patients with MHS, and found in different locations inside muscle cells. Whereas the enzymes that change glucose into glycogen are less active. This slows down the conversion of glucose into glycogen for storage and speeds up the breakdown of glycogen into glucose. Patients with MHS also had fewer molecules that transport glucose into muscle cells and stored less glycogen. These changes imply that less glucose is being removed from the blood. Next, Tammineni et al. used a microscopy technique that is able to distinguish finely separated objects with a precision not reached before in living muscle. This revealed that when the activity of the enzyme that breaks down glycogen increased, it moved next to the calcium store. This effect was also observed in the muscle cells of MHS patients that leaked calcium from their stores. Taken together, these observations may explain why patients with MHS have high levels of sugar in their blood. These findings suggest that MHS may start decades before developing diabetes and blood sugar levels in these patients should be regularly monitored. Future studies should investigate whether drugs that block calcium from leaking may help prevent high blood sugar in patients with MHS or other conditions that cause a similar calcium leak.

Background: Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle triggered by certain anesthetic agents. While Ryanodine Receptor 1 (RYR1) and Calcium Voltage-Gated Channel Subunit Alpha1 S (CACNA1S) are well-established susceptibility genes, the complete genetic basis of MH remains unclear, particularly in Asian populations. Methods: We conducted gene panel testing targeting 24 calcium-related genes in 338 individuals from 247 Japanese families with suspected or confirmed MH. Variants were analyzed on a gene-by-gene basis, and their pathogenicity was assessed using in silico prediction tools. Additionally, patients were classified into subgroups based on the results of the calcium-induced calcium release (CICR) assay and the Clinical Grading Scale (CGS) score. Results: Candidate pathogenic variants were identified in 118 families (48.2%), including 73 (29.8%) in RYR1, 16 (6.5%) in CACNA1S, and 62 (25.3%) in other genes. Among CICR-positive families, RYR1 and CACNA1S variants were detected in 42.0% and 5.3% of cases, respectively. In individuals with high CGS scores (Ranks 5–6), RYR1 and CACNA1S variants were observed in 56.0% and 12.0%, respectively. Variants in other genes such as STAC3, CASQ1, ATP2A1, ASPH, HRC and TRPV1 were also detected. Conclusions: Our findings confirm the predominant role of RYR1 and CACNA1S in MH susceptibility in the Japanese population and highlight additional candidate genes that may contribute to the condition. Broader genetic screening and functional validation studies are warranted to further elucidate the polygenic nature of MH.

Background/Objectives: Malignant hyperthermia (MH) is a pharmacogenetic hypermetabolic syndrome triggered by halogenated agents/succinylcholine. Most families present variants in the RYR1 and, rarely, in other genes (CACNA1S/STAC3/ASPH). However, each country or region presents differences in the type and frequency of MH variants. Objective: To present the genetic characteristics of Brazilian individuals with MH history. Methods: We reviewed clinical and laboratory data from all families referred for evaluation in the Brazilian MH unit due to a personal or family history of MH during anesthesia. Demographic and clinical data were collected, as well as serum creatine kinase (CK) levels, in vitro contracture test (IVCT) results, and the results of anatomopathological studies of skeletal muscle. Molecular analysis was performed using whole-exome sequencing (WES). Patients with and without variants were compared. Results: WES analysis was available for 61 patients (29 patients who survived an MH crisis and 32 relatives). Variants in the RYR1 were found in 38 patients (62.2%), and no variants were identified in 20 patients (32.7%). More than one variant in the RYR1 was found in six individuals. Variants in the CACNA1S were found in three patients (4.9%), all of them with concomitant variants in the RYR1. Three patients presented variants in the STAC3 (4.9%). Comparing the groups of patients with variants in the RYR1 with the one with no variants in this gene, it was observed that the first group showed higher levels of serum CK, a greater frequency of ptosis, strabismus, and cores, and a higher amplitude of contracture in the IVCT after caffeine or halothane. Conclusion: In this preliminary evaluation of Brazilian individuals with MH history, the frequency of RYR1 variants was similar to those of previous reports in other countries, but there was a higher frequency of STAC3 and CACNA1S variants.

Background/Objectives: Variants in the RYR1 gene are associated mainly with Malignant Hyperthermia. Missense variants are largely the most common, while insertions and duplications account for less than 10%. We aimed to investigate the effect of a rare duplication in the RYR1 gene with the variability of the Malignant Hyperthermia susceptibility phenotype. Methods: We used exome variant screening, in vitro contracture test, anatomopathological examination of the muscle biopsy, RT-qPCR analysis for RYR1 relative expression. Results: We identified a family with two affected siblings carrying an insertion of 18 pair bases in exon 91 of the RYR1 gene, resulting in an in-frame duplication of 6 amino acids (c.12835_12852 dupGAGGGCGCGGCGGGGCTC: 162 p.G4279_T4284insAAGLEG). This variant was found at a frequency of 0.0007% in gnomAD and was absent in 1200 Brazilian controls. First classified as a Variant of Uncertain Significance (VUS), with the molecular and physiological data from our family, we were able to reclassify it, reaching 5 points, which is still a VUS but borderline likely pathogenic. Muscle relative mRNA expression of RYR1 in the two patients identified a ~50% reduction, suggesting a possible hypomorphic allele. Conclusions: The pathomechanisms of RYR1 gene variants in Malignant Hyperthermia are mainly associated with gain-of-function mechanisms, but small insertions can often lead to loss of function or improper folding protein. This study adds evidence to the possibility that duplication in this region can cause structural defects and a more severe phenotype in the patients.

Mutations in the skeletal muscle ryanodine receptor ( RYR1 ) gene are a common cause of inherited neuromuscular disorders and have been associated with a wide clinical spectrum, ranging from various congenital myopathies to the malignant hyperthermia susceptibility (MHS) trait without any associated weakness. RYR1 -related myopathies are usually of early-childhood onset. Here we present 11 patients from 8 families with a late-onset axial myopathy associated with RYR1 variants. Patients presented between the third and seventh decade of life to neuromuscular centres in Norway, the Netherlands and the United Kingdom with predominant axial muscle involvement, comprising variable degrees of lumbar hyperlordosis, scapular winging and/or camptocormia. Marked myalgia was commonly associated. Serum creatine kinase levels were normal or moderately elevated. Muscle imaging showed consistent involvement of the lower paravertebral muscles and the posterior thigh. Muscle biopsy findings were often discrete, featuring variability in fibre size, increased internal nuclei and unevenness of oxidative enzyme staining, but only rarely overt cores. RYR1 sequencing revealed heterozygous missense variants, either previously associated with the MHS trait or localizing to known MHS mutational hotspots. These findings indicate that MHS-related RYR1 mutations may present later in life with prominent axial weakness but not always typical histopathological features. We propose a combined effect of RyR1 dysfunction, aging and particular vulnerability of axial muscle groups as a possible pathogenic mechanism. RYR1 is a candidate for cases with “idiopathic” camptocormia or bent spine syndrome (BSS).

Background/Objectives: Malignant hyperthermia (MH) is a life-threatening pharmacogenetic disorder of skeletal muscle calcium regulation and commonly associated with pathogenic variants in the RYR1 gene. Interpretation of rare RYR1 variants remains challenging, particularly when classified as variants of uncertain significance (VUS). This study describes the clinical, functional, and genetic evaluation of a patient with suspected MH susceptibility carrying a rare RYR1 mutation. Methods: We report a retrospective case evaluation of a 32-year-old female referred for MH assessment following a prior peri-operative hypermetabolic event. Clinical records were reviewed, and MH susceptibility was assessed using the caffeine–halothane contracture test (CHCT). Genetic testing was performed using a targeted MH susceptibility gene panel, including RYR1, CACNA1S, and STAC3. Variant classification was conducted following American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) guidelines. Results: The patient demonstrated a positive CHCT, consistent with MH susceptibility. Genetic analysis identified a rare heterozygous RYR1 missense variant. No pathogenic or likely pathogenic variants were detected in CACNA1S or STAC3. Based on ACMG/AMP criteria, the RYR1 p.Gln474His variant is currently classified as a VUS. However, its localization within the N-terminal regulatory region of RyR1 and concordance with abnormal CHCT findings provide supportive functional context. Conclusions: This case underscores the importance of integrating clinical history, functional contracture testing, and genetic data in the evaluation of MH susceptibility. While functional findings may support biological plausibility, definitive pathogenic classification of rare RYR1 variants requires additional segregation data or mechanistic studies.