Mutations inRYR1in malignant hyperthermia and central core disease

Molecular aberrations, targeted therapy, and renal cell carcinoma: current state-of-the-art

Renal cell carcinoma (RCC) is among the most prevalent malignancies in the USA. Most RCCs are sporadic, but hereditary syndromes associated with RCC account for 2-3 % of cases and include von Hippel-Lindau, hereditary leiomyomatosis, Birt-Hogg-Dube, tuberous sclerosis, hereditary papillary RCC, and familial renal carcinoma. In the past decade, our understanding of the genetic mutations associated with sporadic forms of RCC has increased considerably, with the most common mutations in clear cell RCC seen in the VHL, PBRM1, BAP1, and SETD2 genes. Among these, BAP1 mutations are associated with aggressive disease and decreased survival. Several targeted therapies for advanced RCC have been approved and include sunitinib, sorafenib, pazopanib, axitinib (tyrosine kinase inhibitors (TKIs) with anti-vascular endothelial growth factor (VEGFR) activity), everolimus, and temsirolimus (TKIs that inhibit mTORC1, the downstream part of the PI3K/AKT/mTOR pathway). High-dose interleukin 2 (IL-2) immunotherapy and the combination of bevacizumab plus interferon-α are also approved treatments. At present, there are no predictive genetic markers to direct therapy for RCC, perhaps because the vast majority of trials have been evaluated in unselected patient populations, with advanced metastatic disease. This review will focus on our current understanding of the molecular genetics of RCC, and how this may inform therapeutics.

A CASQ1 founder mutation in three Italian families with protein aggregate myopathy and hyperCKaemia

BACKGROUND

Protein aggregate myopathies are increasingly recognised conditions characterised by a surplus of endogenous proteins. The molecular and mutational background for many protein aggregate myopathies has been clarified with the discovery of several underlying mutations. Familial idiopathic hyperCKaemia is a benign genetically heterogeneous condition with autosomal dominant features in a high proportion of cases.

METHODS

In 10 patients from three Italian families with autosomal dominant benign vacuolar myopathy and hyperCKaemia, we performed linkage analysis and exome sequencing as well as morphological and biochemical investigations.

RESULTS AND CONCLUSIONS

We show, by Sanger and exome sequencing, that the protein aggregate myopathy with benign evolution and muscle inclusions composed of excess CASQ1, affecting three Italian families, is due to the D244G heterozygous missense mutation in the CASQ1 gene. Investigation of microsatellite markers revealed a common haplotype in the three families indicating consanguinity and a founder effect. Results from immunocytochemistry, electron microscopy, biochemistry and transfected cell line investigations contribute to our understanding of pathogenetic mechanisms underlining this defect. The mutation is common to other Italian patients and is likely to share a founder effect with them. HyperCKaemia in the CASQ1-related myopathy is common and sometimes the sole overt manifestation. It is likely that CASQ1 mutations may remain undiagnosed if a muscle biopsy is not performed, and the condition could be more common than supposed.

Divergent Activity Profiles of Type 1 Ryanodine Receptor Channels Carrying Malignant Hyperthermia and Central Core Disease Mutations in the Amino-Terminal Region

The type 1 ryanodine receptor (RyR1) is a Ca²⁺ release channel in the sarcoplasmic reticulum of skeletal muscle and is mutated in several diseases, including malignant hyperthermia (MH) and central core disease (CCD). Most MH and CCD mutations cause accelerated Ca²⁺ release, resulting in abnormal Ca²⁺ homeostasis in skeletal muscle. However, how specific mutations affect the channel to produce different phenotypes is not well understood. In this study, we have investigated 11 mutations at 7 different positions in the amino (N)-terminal region of RyR1 (9 MH and 2 MH/CCD mutations) using a heterologous expression system in HEK293 cells. In live-cell Ca²⁺ imaging at room temperature (~25 °C), cells expressing mutant channels exhibited alterations in Ca²⁺ homeostasis, i.e., an enhanced sensitivity to caffeine, a depletion of Ca²⁺ in the ER and an increase in resting cytoplasmic Ca²⁺. RyR1 channel activity was quantitatively evaluated by [³H]ryanodine binding and three parameters (sensitivity to activating Ca²⁺, sensitivity to inactivating Ca²⁺ and attainable maximum activity, i.e., gain) were obtained by fitting analysis. The mutations increased the gain and the sensitivity to activating Ca²⁺ in a site-specific manner. The gain was consistently higher in both MH and MH/CCD mutations. Sensitivity to activating Ca²⁺ was markedly enhanced in MH/CCD mutations. The channel activity estimated from the three parameters provides a reasonable explanation to the pathological phenotype assessed by Ca²⁺ homeostasis. These properties were also observed at higher temperatures (~37 °C). Our data suggest that divergent activity profiles may cause varied disease phenotypes by specific mutations. This approach should be useful for diagnosis and treatment of diseases with mutations in RyR1.

Pharmacogenetics in electroconvulsive therapy and adjunctive medications

Electroconvulsive therapy (ECT) has shown apparent efficacy in treatment of patients with depression and other mental illnesses who do not respond to psychotropic medications or need urgent control of their symptoms. Pharmacogenetics contributes to an individual's sensitivity and response to a variety of drugs. Clinical insights into pharmacogenetics of ECT and adjunctive medications not only improves its safety and efficacy in the indicated patients, but can also lead to the identification of novel treatments in psychiatric disorders through understanding of potential molecular and biological mechanisms involved. In this review, we explore the indications of pharmacogenetics role in safety and efficacy of ECT and present the evidence for its role in patients with psychiatric disorders undergoing ECT.

RYR1-related myopathies: a wide spectrum of phenotypes throughout life

BACKGROUND AND PURPOSE

Although several recent studies have implicated RYR1 mutations as a common cause of various myopathies and the malignant hyperthermia susceptibility (MHS) trait, many of these studies have been limited to certain age groups, confined geographical regions or specific conditions. The aim of the present study was to investigate the full spectrum of RYR1-related disorders throughout life and to use this knowledge to increase vigilance concerning malignant hyperthermia.

METHODS

A retrospective cohort study was performed on the clinical, genetic and histopathological features of all paediatric and adult patients in whom an RYR1 mutation was detected in a national referral centre for both malignant hyperthermia and inherited myopathies (2008-2012).

RESULTS

The cohort of 77 non-related patients (detection rate 28%) included both congenital myopathies with permanent weakness and 'induced' myopathies such as MHS and non-anaesthesia-related episodes of rhabdomyolysis or hyperCKemia, manifested throughout life and triggered by various stimuli. Sixty-one different mutations were detected, of which 24 were novel. Some mutations are present in both dominant (MHS) and recessive modes (congenital myopathy) of inheritance, even within families. Histopathological features included an equally wide spectrum, ranging from only subtle abnormalities to prominent cores.

CONCLUSIONS

This broad range of RYR1-related disorders often presents to the general paediatric and adult neurologist. Its recognition is essential for genetic counselling and improving patients' safety during anaesthesia. Future research should focus on in vitro testing by the in vitro contracture test and functional characterization of the large number of RYR1 variants whose precise effects currently remain uncertain.

Next-generation Sequencing of RYR1 and CACNA1S in Malignant Hyperthermia and Exertional Heat Illness

BACKGROUND

Variants in RYR1 are associated with the majority of cases of malignant hyperthermia (MH), a form of heat illness pharmacogenetically triggered by general anesthetics, and they have also been associated with exertional heat illness (EHI). CACNA1S has also been implicated in MH. The authors applied a targeted next-generation sequencing approach to identify variants in RYR1 and CACNA1S in a cohort of unrelated patients diagnosed with MH susceptibility. They also provide the first comprehensive report of sequencing of these two genes in a cohort of survivors of EHI.

METHODS

DNA extracted from blood was genotyped using a "long" polymerase chain reaction technique, with sequencing on the Illumina GAII or MiSeq platforms (Illumina Inc., USA). Variants were assessed for pathogenicity using bioinformatic approaches. For further follow-up, DNA from additional family members and up to 211 MH normal and 556 MH-susceptible unrelated individuals was tested.

RESULTS

In 29 MH patients, the authors identified three pathogenic and four novel RYR1 variants, with a further five RYR1 variants previously reported in association with MH. Three novel RYR1 variants were found in the EHI cohort (n = 28) along with two more previously reported in association with MH. Two other variants were reported previously associated with centronuclear myopathy. The authors found one and three rare variants of unknown significance in CACNA1S in the MH and EHI cohorts, respectively.

CONCLUSIONS

Targeted next-generation sequencing proved efficient at identifying diagnostically useful and potentially implicated variants in RYR1 and CACNA1S in MH and EHI.

Analysis of the entire ryanodine receptor type 1 and alpha 1 subunit of the dihydropyridine receptor (CACNA1S) coding regions for variants associated with malignant hyperthermia in Australian families

Defects in the genes coding for the skeletal muscle ryanodine receptor (RYR1) and alpha 1 subunit of the dihydropyridine receptor (CACNA1S) have been identified as causative for malignant hyperthermia (MH). Sixty-two MH susceptible individuals presenting to the same diagnostic centre had copy deoxyribonucleic acid, derived from muscle ribonucleic acid, sequenced to identify variants with the potential to be responsible for the MH phenotype in both RYR1 and CACNA1S. These genetic findings were combined with clinical episode details and in vitro contracture test results to improve our understanding of the Australian MH cohort. Twelve novel variants were identified in RYR1 and six in CACNA1S. Known RYR1 causative mutations were identified in six persons and novel variants in RYR1 and CACNA1S in a further 17 persons. Trends indicated higher mutation identification in those with more definitive clinical episodes and stronger in vitro contracture test responses.

Oxidative stress, mitochondrial damage, and cores in muscle from calsequestrin-1 knockout mice

Background

Mutations in the gene encoding ryanodine receptor type-1 (RYR1), the calcium ion (Ca²⁺) release channel in the sarcoplasmic reticulum (SR) of skeletal muscle, are linked to central core disease (CCD) and malignant hyperthermia (MH) susceptibility. We recently reported that mice lacking the skeletal isoform of calsequestrin (CASQ1-null), the primary Ca²⁺ buffer in the SR of skeletal muscle and a modulator of RYR1 activity, exhibit lethal heat- and anesthetic-induced hypermetabolic episodes that resemble MH events in humans.

Methods

We compared ultrastructure, oxidative status, and contractile function in skeletal fibers of extensor digitorum longus (EDL) muscles in wild type (WT) and CASQ1-null mice at different ages (from 4 to 27 months) using structural, biochemical, and functional assays.

Results

About 25% of fibers in EDL muscles from CASQ1-null mice of 14 to 27 months of age exhibited large areas of structural disarray (named core-like regions), which were rarely observed in muscle from age-matched WT mice. To determine early events that may lead to the formation of cores, we analyzed EDL muscles from adult mice: at 4 to 6 months of age, CASQ1-null mice (compared to WT) displayed significantly reduced grip strength (40 ± 1 vs. 86 ± 1 mN/gr) and exhibited an increase in the percentage of damaged mitochondria (15.1% vs. 2.6%) and a decrease in average cross-sectional fiber area (approximately 37%) in EDL fibers. Finally, oxidative stress was also significantly increased (25% reduction in ratio between reduced and oxidized glutathione, or GSH/GSSG, and 35% increase in production of mitochondrial superoxide flashes). Providing ad libitum access to N-acetylcysteine in the drinking water for 2 months normalized GSH/GSSG ratio, reduced mitochondrial damage (down to 8.9%), and improved grip strength (from 46 ± 3 to 59 ± 2 mN/gr) in CASQ1-null mice.

Conclusions

Our findings: 1) demonstrate that ablation of CASQ1 leads to enhanced oxidative stress, mitochondrial damage, and the formation of structural cores in skeletal muscle; 2) provide new insights in the pathogenic mechanisms that lead to damage/disappearance of mitochondria in cores; and 3) suggest that antioxidants may provide some therapeutic benefit in reducing mitochondrial damage, limiting the development of cores, and improving muscle function.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0035-9) contains supplementary material, which is available to authorized users.

Ryanodine receptors are targeted by anti-apoptotic Bcl-XL involving its BH4 domain and Lys87 from its BH3 domain

Anti-apoptotic B-cell lymphoma 2 (Bcl-2) family members target several intracellular Ca(2+)-transport systems. Bcl-2, via its N-terminal Bcl-2 homology (BH) 4 domain, inhibits both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs), while Bcl-XL, likely independently of its BH4 domain, sensitizes IP3Rs. It remains elusive whether Bcl-XL can also target and modulate RyRs. Here, Bcl-XL co-immunoprecipitated with RyR3 expressed in HEK293 cells. Mammalian protein-protein interaction trap (MAPPIT) and surface plasmon resonance (SPR) showed that Bcl-XL bound to the central domain of RyR3 via its BH4 domain, although to a lesser extent compared to the BH4 domain of Bcl-2. Consistent with the ability of the BH4 domain of Bcl-XL to bind to RyRs, loading the BH4-Bcl-XL peptide into RyR3-overexpressing HEK293 cells or in rat hippocampal neurons suppressed RyR-mediated Ca(2+) release. In silico superposition of the 3D-structures of Bcl-2 and Bcl-XL indicated that Lys87 of the BH3 domain of Bcl-XL could be important for interacting with RyRs. In contrast to Bcl-XL, the Bcl-XL(K87D) mutant displayed lower binding affinity for RyR3 and a reduced inhibition of RyR-mediated Ca(2+) release. These data suggest that Bcl-XL binds to RyR channels via its BH4 domain, but also its BH3 domain, more specific Lys87, contributes to the interaction.

Characterization of excitation–contraction coupling components in human extraocular muscles

We show that the expression level of RyR1 in human extraocular muscles (EOMs) is low and that these muscles express different levels of proteins involved in excitation–contraction coupling (ECC) compared with leg muscles (LMs).

Signaling in muscle contraction

Signaling pathways regulate contraction of striated (skeletal and cardiac) and smooth muscle. Although these are similar, there are striking differences in the pathways that can be attributed to the distinct functional roles of the different muscle types. Muscles contract in response to depolarization, activation of G-protein-coupled receptors and other stimuli. The actomyosin fibers responsible for contraction require an increase in the cytosolic levels of calcium, which signaling pathways induce by promoting influx from extracellular sources or release from intracellular stores. Rises in cytosolic calcium stimulate numerous downstream calcium-dependent signaling pathways, which can also regulate contraction. Alterations to the signaling pathways that initiate and sustain contraction and relaxation occur as a consequence of exercise and pathophysiological conditions.

Arrhythmogenic mechanisms in ryanodine receptor channelopathies

Ryanodine receptors (RyRs) are the calcium release channels of sarcoplasmic reticulum (SR) that provide the majority of cal-cium ions (Ca²⁺) necessary to induce contraction of cardiac and skeletal muscle cells. In their intracellular environment, RyR channels are regulated by a variety of cytosolic and luminal factors so that their output signal (Ca²⁺) induces finely-graded cell contraction without igniting cellular processes that may lead to aberrant electrical activity (ventricular arrhythmias) or cellular remodeling. The importance of RyR dysfunction has been recently highlighted with the demonstration that point mutations in RYR2, the gene encoding for the cardiac isoform of the RyR (RyR2), are associated with catecholaminergic polymorphic ventricular tachycardia (CPVT), an arrhythmogenic syndrome characterized by the development of adrenergically-mediated ventricular tachycardia in individuals with an apparently normal heart. Here we summarize the state of the field in regards to the main arrhythmogenic mechanisms triggered by RyR2 channels harboring mutations linked to CPVT. Most CPVT mutations characterized to date endow RyR2 channels with a gain of function, resulting in hyperactive channels that release Ca²⁺ spontaneously, especially during diastole. The spontaneous Ca²⁺ release is extruded by the electrogenic Na⁺/Ca²⁺ exchanger, which depolarizes the external membrane (delayed afterdepolarization or DAD) and may trigger untimely action potentials. However, a rare set of CPVT mutations yield RyR2 channels that are intrinsically hypo-active and hypo-responsive to stimuli, and it is unclear whether these channels release Ca²⁺ spontaneously during diastole. We discuss novel cellular mechanisms that appear more suitable to explain ventricular arrhythmias due to RyR2 loss-of-function mutations.

The disorders of the calcium release unit of skeletal muscles: what have we learned from mouse models?

Calcium storage, release, and reuptake are essential for normal physiological function of muscle. Several human skeletal muscle disorders can arise from dysfunction in the control and coordination of these three critical processes. The release from the Sarcoplasmic Reticulum stores (SR) is handled by a multiprotein complex called Calcium Release Unit and composed of DiHydroPyridine Receptor or DHPR, Ryanodine Receptor or RYR, Calsequestrin or CASQ, junctin, Triadin, Junctophilin and Mitsugumin 29. Malignant hyperthermia (MH), Central Core Disease (CCD), Exertional/environmental Heat Stroke (EHS) and Multiminicore disease (MmD) are inherited disorders of calcium homeostasis in skeletal muscles directly related to mutations of genes coding for proteins of the CRU, primarily ryanodine receptor (RYR1). To understand the pathophysiology of MH and CCD, four murine lines carrying point mutations of human RYR1 have been developed: Y524S, R163C, I4898T and T4826I. Mice carrying those mutations show a phenotype with the traits of MH and/or CCD. Interestingly, also ablation of skeletal muscle calsequestrin (CASQ1) leads to a phenotype with MH-like lethal episodes in response to halothane and heat stress and development of central cores. In this review, we aim to describe the murine lines with RYR mutations or CASQ ablation, which show a phenotype similar to human MH or CCD, to underline their specific phenotypes and their differences and to discuss their contribution to the understanding of the pathophysiology of the disorders and the development of therapeutic strategies.

Next-generation DNA sequencing of a Swedish malignant hyperthermia cohort

Malignant hyperthermia (MH)-related mutations have been identified in the ryanodine receptor type 1 gene (RYR1) and in the dihydropyridine gene (CACNA1S), but about half of the patients do not have causative mutations in these genes. We wanted to study the contribution of other muscle genes to the RYR1 phenotypes. We designed a gene panel for sequence enrichment targeting 64 genes of proteins involved in the homeostasis of the striated muscle cell. Next-generation sequencing (NGS) resulted in >50,000 sequence variants which were further analyzed by software filtering criteria to identify causative variants. In four of five patients we identified previously reported RYR1 mutations while the fifth patient did not show any candidate variant in any of the genes investigated. In two patients pathogenic variants were found in other genes known to cause a muscle disorders. All but one patient carried likely benign rare polymorphisms. The NGS technique proved convenient in identifying variants in the RYR1. However, with a clinically variable phenotype-like MH, the pre-selection of genes poses problems in variant interpretation.

Triadopathies: an emerging class of skeletal muscle diseases

The triad is a skeletal muscle substructure responsible for the regulation of excitation-contraction coupling. It is formed by the close apposition of the T-tubule and the terminal sarcoplasmic reticulum. A rapidly growing list of skeletal myopathies, here referred to as triadopathies, are caused by gene mutations in components of the triad. These disorders, at their root, are caused by defects in excitation contraction coupling and intracellular calcium homeostasis. Secondary abnormalities in triad structure and/or function are also reported in several muscle diseases, most notably certain muscular dystrophies. This review highlights the current understanding of both primary and secondary triadopathies, and identifies important concepts yet to be fully addressed in the field. The emphasis of the review is both on the pathogenesis of triadopathies and their potential treatment.

Malignant hyperthermia and the clinical significance of type-1 ryanodine receptor gene (RYR1) variants: proceedings of the 2013 MHAUS Scientific Conference

The Malignant Hyperthermia Association of the United States and the Department of Anesthesia at the University of Toronto sponsored a Scientific Conference on November 1-2, 2013 in Toronto, ON, Canada. The multidisciplinary group of experts, including clinicians, geneticists, and physiologists involved in research related to malignant hyperthermia (MH), shared new insights into the pathophysiology of diseases linked to the type-1 ryanodine receptor gene (RYR1) as well as the relationship between MH and "awake MH" conditions, such as exertional rhabdomyolysis and exertional heat illness. In addition, the molecular genetics of MH and clinical issues related to the diagnosis and management of disorders linked to RYR1 were presented. The conference also honoured Dr. David H. MacLennan for his contributions to our understanding of the genetics, pathogenesis, and treatment of MH and other RYR1-related myopathies. This report represents a summary of the proceedings of this conference.

Silent polymorphisms in the RYR1 gene do not modify the phenotype of the p.4898 I>T pathogenic mutation in central core disease: a case report

Background

Central core disease is a congenital myopathy, characterized by presence of central core-like areas in muscle fibers. Patients have mild or moderate weakness, hypotonia and motor developmental delay. The disease is caused by mutations in the human ryanodine receptor gene (RYR1), which encodes a calcium-release channel. Since the RYR1 gene is huge, containing 106 exons, mutation screening has been limited to three ‘hot spots’, with particular attention to the C-terminal region. Recent next- generation sequencing methods are now identifying multiple numbers of variants in patients, in which interpretation and phenotype prevision is difficult.

Case presentation

In a Brazilian Caucasian family, clinical, histopathological and molecular analysis identified a new case of central core disease in a 48-year female. Sanger sequencing of the C-terminal region of the RYR1 gene identified two different missense mutations: c.14256 A > C polymorphism in exon 98 and c.14693 T > C in exon 102, which have already been described as pathogenic. Trans-position of the 2 mutations was confirmed because patient’s daughter, mother and sister carried only the exon 98’s mutation, a synonymous variant that was subsequently found in the frequency of 013–0,05 of alleles. Further next generation sequencing study of the whole RYR1 gene in the patient revealed the presence of additional 5 common silent polymorphisms in homozygosis and 8 polymorphisms in heterozygosis.

Conclusions

Considering that patient’s relatives showed no pathologic phenotype, and the phenotype presented by the patient is within the range observed in other central core disease patients with the same mutation, it was concluded that the c.14256 A > C polymorphism alone is not responsible for disease, and the associated additional silent polymorphisms are not acting as modifiers of the primary pathogenic mutation in the affected patient. The case described above illustrates the present reality where new methods for wide genome screening are becoming more accessible and able to identify a great variety of mutations and polymorphisms of unknown function in patients and their families.

An association study of CASQ1 gene polymorphisms and heat stroke

Although molecular mechanisms of heat stroke under physiological and pathological conditions have not yet been elucidated, a novel disease-associated gene encoding a calcium-binding protein, calsequestrin-1 (CASQ1), was suggested relevant based on results from a transgenic murine model. Here, we show the association between single nucleotide polymorphisms (SNPs) of CASQ1 and physiological parameters for heat stroke from a study involving 150 patients. Pooled DNA from heat stroke patients were subjected to sequencing and 3 SNPs were identified. Genotypes were assigned for all patients according to g. 175A>G, one SNP which leads to a nonsynonymous substitution (N59D) in the first exon of human CASQ1 gene. We analyzed the genotypic data with a linear model based on significance scores between SNP (175A>G) and heat stroke parameters. As a result, we found a significant association between SNP A175G and heat stroke (P<0.05). Further bioinformatics analysis of the 1-Mb flanking sequence revealed the presence of two genes that encode DDB1 and CUL4 associated factor 8 (DCAF8), and peroxisomal biogenesis factor 19 (PEX19), respectively, which might be functionally related to CASQ1. Our results showed that the blood calcium of patients with allele D increased significantly, compared to patients with allele N (P<0.05), which may result from the decreased calcium in muscle, suggesting that N59D in CASQ1 might account for the dysfunction of CASQ1 in calcium regulation during heat stroke.

Ca2+ influx via the Na+/Ca2+ exchanger is enhanced in malignant hyperthermia skeletal muscle

Malignant hyperthermia (MH) is potentially fatal pharmacogenetic disorder of skeletal muscle caused by intracellular Ca(2+) dysregulation. NCX is a bidirectional transporter that effluxes (forward mode) or influxes (reverse mode) Ca(2+) depending on cellular activity. Resting intracellular calcium ([Ca(2+)]r) and sodium ([Na(+)]r) concentrations are elevated in MH susceptible (MHS) swine and murine muscles compared with their normal (MHN) counterparts, although the contribution of NCX is unclear. Lowering [Na(+)]e elevates [Ca(2+)]r in both MHN and MHS swine muscle fibers and it is prevented by removal of extracellular Ca(2+) or reduced by t-tubule disruption, in both genotypes. KB-R7943, a nonselective NCX3 blocker, reduced [Ca(2+)]r in both swine and murine MHN and MHS muscle fibers at rest and decreased the magnitude of the elevation of [Ca(2+)]r observed in MHS fibers after exposure to halothane. YM-244769, a high affinity reverse mode NCX3 blocker, reduces [Ca(2+)]r in MHS muscle fibers and decreases the amplitude of [Ca(2+)]r rise triggered by halothane, but had no effect on [Ca(2+)]r in MHN muscle. In addition, YM-244769 reduced the peak and area under the curve of the Ca(2+) transient elicited by high [K(+)]e and increased its rate of decay in MHS muscle fibers. siRNA knockdown of NCX3 in MHS myotubes reduced [Ca(2+)]r and the Ca(2+) transient area induced by high [K(+)]e. These results demonstrate a functional NCX3 in skeletal muscle whose activity is enhanced in MHS. Moreover reverse mode NCX3 contributes to the Ca(2+) transients associated with K(+)-induced depolarization and the halothane-triggered MH episode in MHS muscle fibers.

Management of malignant hyperthermia: diagnosis and treatment

Malignant hyperthermia is a potentially lethal inherited disorder characterized by disturbance of calcium homeostasis in skeletal muscle. Volatile anesthetics and/or the depolarizing muscle relaxant succinylcholine may induce this hypermetabolic muscular syndrome due to uncontrolled sarcoplasmic calcium release via functionally altered calcium release receptors, resulting in hypoxemia, hypercapnia, tachycardia, muscular rigidity, acidosis, hyperkalemia, and hyperthermia in susceptible individuals. Since the clinical presentation of malignant hyperthermia is highly variable, survival of affected patients depends largely on early recognition of the symptoms characteristic of malignant hyperthermia, and immediate action on the part of the attending anesthesiologist. Clinical symptoms of malignant hyperthermia, diagnostic criteria, and current therapeutic guidelines, as well as adequate management of anesthesia in patients susceptible to malignant hyperthermia, are discussed in this review.

Functional characterization of 2 known ryanodine receptor mutations causing malignant hyperthermia

BACKGROUND

Malignant hyperthermia (MH) is a potentially lethal pharmacogenetic disorder. More than 300 variants in the ryanodine receptor 1 (RYR1) have been associated with MH; however, only 31 have been identified as causative. To confirm a mutation in RYR1 as being causative for MH, segregation of the potential mutation in at least 2 unrelated families with MH susceptibility must be demonstrated and functional assays must show abnormal calcium release compared with wild-type RYR1.

METHODS

We used "Hot-spot" DNA screening to identify mutations in RYR1 in 3 New Zealand families. B-lymphoblastoid cells were used to compare the amount of calcium released on stimulation with 4-chloro-m-cresol between wild-type RYR1 cells and cells carrying the new variants in RYR1.

RESULTS

We identified a known RYR1 mutation (R2355W) in 2 families and another more recently identified (V2354M) mutation in another family. Both mutations segregated with MH susceptibility in the respective families. Cell lines carrying a mutation in RYR1 showed increased sensitivity to 4-chloro-m-cresol.

CONCLUSIONS

We propose that R2355W is confirmed as being an MH-causative mutation and suggest that V2354M is a RYR1 mutation likely to cause MH.

DNA testing for malignant hyperthermia: the reality and the dream

The advent of the polymerase chain reaction and the availability of data from various global human genome projects should make it possible, using a DNA sample isolated from white blood cells, to diagnose rapidly and accurately almost any monogenic condition resulting from single nucleotide changes. DNA-based diagnosis for malignant hyperthermia (MH) is an attractive proposition, because it could replace the invasive and morbid caffeine-halothane/in vitro contracture tests of skeletal muscle biopsy tissue. Moreover, MH is preventable if an accurate diagnosis of susceptibility can be made before general anesthesia, the most common trigger of an MH episode. Diagnosis of MH using DNA was suggested as early as 1990 when the skeletal muscle ryanodine receptor gene (RYR1), and a single point mutation therein, was linked to MH susceptibility. In 1994, a single point mutation in the α 1 subunit of the dihydropyridine receptor gene (CACNA1S) was identified and also subsequently shown to be causative of MH. In the succeeding years, the number of identified mutations in RYR1 has grown, as has the number of potential susceptibility loci, although no other gene has yet been definitively associated with MH. In addition, it has become clear that MH is associated with either of these 2 genes (RYR1 and CACNA1S) in only 50% to 70% of affected families. While DNA testing for MH susceptibility has now become widespread, it still does not replace the in vitro contracture tests. Whole exome sequence analysis makes it potentially possible to identify all variants within human coding regions, but the complexity of the genome, the heterogeneity of MH, the limitations of bioinformatic tools, and the lack of precise genotype/phenotype correlations are all confounding factors. In addition, the requirement for demonstration of causality, by in vitro functional analysis, of any familial mutation currently precludes DNA-based diagnosis as the sole test for MH susceptibility. Nevertheless, familial DNA testing for MH susceptibility is now widespread although limited to a positive diagnosis and to those few mutations that have been functionally characterized. Identification of new susceptibility genes remains elusive. When new genes are identified, it will be the role of the biochemists, physiologists, and biophysicists to devise functional assays in appropriate systems. This will remain the bottleneck unless high throughput platforms can be designed for functional work. Analysis of entire genomes from several individuals simultaneously is a reality. DNA testing for MH, based on current criteria, remains the dream.

Whole-exome sequencing in patients with inherited neuropathies: outcome and challenges

Inherited peripheral neuropathies (IPN) are one of the most frequent inherited causes of neurological disability characterized by considerable phenotypic and genetic heterogeneity. Based on clinical and electrophysiological properties, they can be subdivided into three main groups: HMSN, dHMN, and HSN. At present, more than 50 IPN genes have been identified. Still, many patients and families with IPN have not yet received a molecular genetic diagnosis because clinical genetic testing usually only covers a subset of IPN genes. Moreover, a considerable proportion of IPN genes has to be identified. Here we present results of WES in 27 IPN patients excluded for mutations in many known IPN genes. Eight of the patients received a definite diagnosis. While six of these patients carried bona fide pathogenic mutations in known IPN genes, two patients had mutations in genes known to be involved in other types of neuromuscular disorders. A further group of eight patients carried sequence variations in IPN genes that could not unequivocally be classified as pathogenic. In addition, combining data of WES and linkage analysis identified SH3BP4, ITPR3, and KLHL13 as novel IPN candidate genes. Moreover, there was evidence that particular mutations in PEX12, a gene known to cause Zellweger syndrome, could also lead to an IPN phenotype. We show that WES is a useful tool for diagnosing IPN and we suggest an expanded phenotypic spectrum of some genes involved in other neuromuscular and neurodegenerative disorders. Nevertheless, interpretation of variants in known and potential novel disease genes has remained challenging.

The ryanodine receptor agonist 4-chloro-3-ethylphenol blocks ORAI store-operated channels

BACKGROUND

Depletion of the Ca(2+) store by ryanodine receptor (RyR) agonists induces store-operated Ca(2+) entry (SOCE). 4-Chloro-3-ethylphenol (4-CEP) and 4-chloro-m-cresol (4-CmC) are RyR agonists commonly used as research tools and diagnostic reagents for malignant hyperthermia. Here, we investigated the effects of 4-CEP and its analogues on SOCE.

EXPERIMENTAL APPROACH

SOCE and ORAI1-3 currents were recorded by Ca(2+) imaging and whole-cell patch recordings in rat L6 myoblasts and in HEK293 cells overexpressing STIM1/ORAI1-3.

KEY RESULTS

4-CEP induced a significant release of Ca(2+) in rat L6 myoblasts, but inhibited SOCE. The inhibitory effect was concentration-dependent and more potent than its analogues 4-CmC and 4-chlorophenol (4-ClP). In the HEK293 T-REx cells overexpressing STIM1/ORAI1-3, 4-CEP inhibited the ORAI1, ORAI2 and ORAI3 currents evoked by thapsigargin. The 2-APB-induced ORAI3 current was also blocked by 4-CEP. This inhibitory effect was reversible and independent of the Ca(2+) release. The two analogues, 4-CmC and 4-ClP, also inhibited the ORAI1-3 channels. Excised patch and intracellular application of 4-CEP demonstrated that the action site was located extracellularly. Moreover, 4-CEP evoked STIM1 translocation and subplasmalemmal clustering through its Ca(2+) store-depleting effect via the activation of RyR, but no effect on STIM1 redistribution was observed in cells co-expressing STIM1/ORAI1-3.

CONCLUSION AND IMPLICATIONS

4-CEP not only acts as a RyR agonist to deplete the Ca(2+) store and trigger STIM1 subplasmalemmal translocation and clustering, but also directly inhibits ORAI1-3 channels. These findings demonstrate a novel pharmacological property for the chlorophenol derivatives that act as RyR agonists.

Mapping the sevoflurane-binding sites of calmodulin

General anesthetics, with sevoflurane (SF) being the first choice inhalational anesthetic agent, provide reversible, broad depressor effects on the nervous system yet have a narrow margin of safety. As characterization of low-affinity binding interactions of volatile substances is exceptionally challenging with the existing methods, none of the numerous cellular targets proposed as chief protagonists in anesthesia could yet be confirmed. The recognition that most critical functions modulated by volatile anesthetics are under the control of intracellular Ca²⁺ concentration, which in turn is primarily regulated by calmodulin (CaM), motivated us for characterization of the SF–CaM interaction. Solution NMR (Nuclear Magnetic Resonance) spectroscopy was used to identify SF-binding sites using chemical shift displacement, NOESY and heteronuclear Overhauser enhancement spectroscopy (HOESY) experiments. Binding affinities were measured using ITC (isothermal titration calorimetry). SF binds to both lobes of (Ca²⁺)₄-CaM with low mmol/L affinity whereas no interaction was observed in the absence of Ca²⁺. SF does not affect the calcium binding of CaM. The structurally closely related SF and isoflurane are shown to bind to the same clefts. The SF-binding clefts overlap with the binding sites of physiologically relevant ion channels and bioactive small molecules, but the binding affinity suggests it could only interfere with very weak CaM targets.

Approach to the diagnosis of congenital myopathies

Over the past decade there have been major advances in defining the genetic basis of the majority of congenital myopathy subtypes. However the relationship between each congenital myopathy, defined on histological grounds, and the genetic cause is complex. Many of the congenital myopathies are due to mutations in more than one gene, and mutations in the same gene can cause different muscle pathologies. The International Standard of Care Committee for Congenital Myopathies performed a literature review and consulted a group of experts in the field to develop a summary of (1) the key features common to all forms of congenital myopathy and (2) the specific features that help to discriminate between the different genetic subtypes. The consensus statement was refined by two rounds of on-line survey, and a three-day workshop. This consensus statement provides guidelines to the physician assessing the infant or child with hypotonia and weakness. We summarise the clinical features that are most suggestive of a congenital myopathy, the major differential diagnoses and the features on clinical examination, investigations, muscle pathology and muscle imaging that are suggestive of a specific genetic diagnosis to assist in prioritisation of genetic testing of known genes. As next generation sequencing becomes increasingly used as a diagnostic tool in clinical practise, these guidelines will assist in determining which sequence variations are likely to be pathogenic.

Functional and genetic characterization of clinical malignant hyperthermia crises: a multi-centre study

BACKGROUND

Malignant hyperthermia (MH) is a rare pharmacogenetic disorder which is characterized by life-threatening metabolic crises during general anesthesia. Classical triggering substances are volatile anesthetics and succinylcholine (SCh). The molecular basis of MH is excessive release of Ca2+ in skeletal muscle principally by a mutated ryanodine receptor type 1 (RyR1). To identify factors explaining the variable phenotypic presentation and complex pathomechanism, we analyzed proven MH events in terms of clinical course, muscle contracture, genetic factors and pharmocological triggers.

METHODS

In a multi-centre study including seven European MH units, patients with a history of a clinical MH episode confirmed by susceptible (MHS) or equivocal (MHE) in vitro contracture tests (IVCT) were investigated. A test result is considered to be MHE if the muscle specimens develop pathological contractures in response to only one of the two test substances, halothane or caffeine. Crises were evaluated using a clinical grading scale (CGS), results of IVCT and genetic screening. The effects of SCh and volatile anesthetics on Ca2+ release from sarcoplasmic reticulum (SR) were studied in vitro.

RESULTS

A total of 200 patients met the inclusion criteria. Two MH crises (1%) were triggered by SCh (1 MHS, 1 MHE), 18% by volatile anesthetics and 81% by a combination of both. Patients were 70% male and 50% were younger than 12 years old. Overall, CGS was in accord with IVCT results. Crises triggered by enflurane had a significantly higher CGS compared to halothane, isoflurane and sevoflurane. Of the 200 patients, 103 carried RyR1 variants, of which 14 were novel. CGS varied depending on the location of the mutation within the RyR1 gene. In contrast to volatile anesthetics, SCh did not evoke Ca2+ release from isolated rat SR vesicles.

CONCLUSIONS

An MH event could depend on patient-related risk factors such as male gender, young age and causative RyR1 mutations as well as on the use of drugs lowering the threshold of myoplasmic Ca2+ release. SCh might act as an accelerant by promoting unspecific Ca2+ influx via the sarcolemma and indirect RyR1 activation. Most MH crises develop in response to the combined administration of SCh and volatile anesthetics.

Neurology of pregnancy

Pregnancy creates alterations in maternal physiology which predispose to unique neurologic disorders. Pre-eclampsia, eclampsia, certain types of ischemic and hemorrhagic stroke, reversible cerebral vasoconstriction syndrome, posterior reversible encephalopathy syndrome, and thunderclap headache all appear to share a common origin from vascular endothelial dysfunction, with overlapping clinical presentations. Multiple sclerosis often improves during pregnancy. Compression mononeuropathies may occur in the extremities. Myasthenia gravis may affect second stage labor. Various inflammatory peripheral neuropathies, dystrophies, myopathies may occur during pregnancy. The safety of specific immune suppressants is reviewed. Epilepsy does not have a significant effect upon the course of pregnancy, albeit there is a modest increase in the need for cesarean section. Certain antiepileptic drugs may produce fetal malformations, most notably valproic acid. Brain tumors are rare during pregnancy, but may increase in size due to activation of hormonal receptors on tumor cells surfaces, water retention, and engorged blood vessels.

Using exome data to identify malignant hyperthermia susceptibility mutations

BACKGROUND

Malignant hyperthermia susceptibility (MHS) is a life-threatening, inherited disorder of muscle calcium metabolism, triggered by anesthetics and depolarizing muscle relaxants. An unselected cohort was screened for MHS mutations using exome sequencing. The aim of this study was to pilot a strategy for the RYR1 and CACNA1S genes.

METHODS

Exome sequencing was performed on 870 volunteers not ascertained for MHS. Variants in RYR1 and CACNA1S were annotated using an algorithm that filtered results based on mutation type, frequency, and information in mutation databases. Variants were scored on a six-point pathogenicity scale. Medical histories and pedigrees were reviewed for malignant hyperthermia and related disorders.

RESULTS

The authors identified 70 RYR1 and 53 CACNA1S variants among 870 exomes. Sixty-three RYR1 and 41 CACNA1S variants passed the quality and frequency metrics but the authors excluded synonymous variants. In RYR1, the authors identified 65 missense mutations, one nonsense, two that affected splicing, and one non-frameshift indel. In CACNA1S, 48 missense, one frameshift deletion, one splicing, and one non-frameshift indel were identified. RYR1 variants predicted to be pathogenic for MHS were found in three participants without medical or family histories of MHS. Numerous variants, previously described as pathogenic in mutation databases, were reclassified by the authors as being of unknown pathogenicity.

CONCLUSIONS

Exome sequencing can identify asymptomatic patients at risk for MHS, although the interpretation of exome variants can be challenging. The use of exome sequencing in unselected cohorts is an important tool to understand the prevalence and penetrance of MHS, a critical challenge for the field.

Monogene Ionenkanalerkrankungen der Skelettmuskulatur

Muskuläre Kanalopathien wie Myotonien, dyskaliämische periodische Paralysen (PP), maligne Hyperthermie (MH) und Core-Myopathien sind durch Mutationen in Na+-, K+-, Ca2+- und Cl−-Kanälen bedingt. Eine leichte Membrandepolarisation verursacht myotone Aktivität. Eine vermehrte Depolarisation kann die Über- in Untererregbarkeit umkehren und eine transiente Schwäche hervorrufen. Eine länger anhaltende Depolarisation der Plasma- und T-tubulären Membran ist für die dominanten dyskaliämischen PP die gemeinsame Basis der Schwäche. Der Kaliumspiegel im Serum beeinflusst das Ruhemembranpotenzial, weshalb Abweichungen vom physiologischen Bereich (z. B. durch Schilddrüsendysfunktion) Membrandepolarisation und Muskelschwäche begünstigen. Bei der Anlage zu MH, einer potenziell lebensbedrohlichen hypermetabolen Krise, erhöhen dominante Mutationen im zytoplasmatischen Teil des Ca2+-Freisetzungskanals des sarkoplasmatischen Retikulums die Empfindlichkeit für volatile Anästhetika. Dagegen führen dominante oder rezessive Mutationen im sarkoplasmatischen Teil zur Entleerung der Ca2+-Speicher und so zu Schwäche und Core-Myopathie.

Themes and variations in ER/SR calcium release channels: structure and function

Calcium (Ca(2+)) release from reticular stores is a vital regulatory signal in eukaryotes. Recent structural data on large NH(2)-terminal regions of IP(3)Rs and RyRs and their tetrameric arrangement in the full-length context reveal striking mechanistic similarities in Ca(2+) release channel function. A common ancestor found in unicellular genomes underscores the fundamentality of these elements to Ca(2+) release channels.

Exome sequencing reveals novel rare variants in the ryanodine receptor and calcium channel genes in malignant hyperthermia families

BACKGROUND

About half of malignant hyperthermia (MH) cases are associated with skeletal muscle ryanodine receptor 1 (RYR1) and calcium channel, voltage-dependent, L type, α1S subunit (CACNA1S) gene mutations, leaving many with an unknown cause. The authors chose to apply a sequencing approach to uncover causal variants in unknown cases. Sequencing the exome, the protein-coding region of the genome, has power at low sample sizes and identified the cause of over a dozen Mendelian disorders.

METHODS

The authors considered four families with multiple MH cases lacking mutations in RYR1 and CACNA1S by Sanger sequencing of complementary DNA. Exome sequencing in two affecteds per family, chosen for maximum genetic distance, were compared. Variants were ranked by allele frequency, protein change, and measures of conservation among mammals to assess likelihood of causation. Finally, putative pathogenic mutations were genotyped in other family members to verify cosegregation with MH.

RESULTS

Exome sequencing revealed one rare RYR1 nonsynonymous variant in each of three families (Asp1056His, Val2627Met, Val4234Leu), and one CACNA1S variant (Thr1009Lys) in the fourth family. These were not seen in variant databases or in our control population sample of 5,379 exomes. Follow-up sequencing in other family members verified cosegregation of alleles with MH.

CONCLUSIONS

The authors found that using both exome sequencing and allele frequency data from large sequencing efforts may aid genetic diagnosis of MH. In a sample selected by the authors, this technique was more sensitive for variant detection in known genes than Sanger sequencing of complementary DNA, and allows for the possibility of novel gene discovery.

Conformational dynamics inside amino-terminal disease hotspot of ryanodine receptor

The N-terminal region of both skeletal and cardiac ryanodine receptor is a disease mutation hotspot. Recently, a crystal structure of the RyR1 fragment (residues 1-559) was solved. This N-terminal structure contains three separate domains, A, B, and C, and was docked into a central vestibule in a full-length RyR1 cryo-EM map. Here, we reconstructed three-dimensional cryo-EM structures of two GFP-tagged RyR2s with GFP inserted after residue Glu-310 and Ser-437, respectively. The structures of RyR2E310-GFP and RyR2S437-GFP displayed an extra mass on domain B and C, directly validating the predicted docking model. Next, we revealed domain movements in molecular dynamics flexible fitting models in both the closed and open state cryo-EM maps. To further probe the conformational changes, we generated FRET pairs by inserting CFP or YFP in two selected domains, FRET studies of three dual-insertion pairs and three co-expressed single-insertion pairs showed the dynamic structural changes within the N-terminal domains.

[Malignant hyperthermia: multifactorial events leading to a crisis]

This case-report focused on the occurrence of a crisis of malignant hyperthermia on a young biker (21 y.o.) victim of a severe traffic accident. This observation gives some key points: the delayed occurrence of the crisis after 4 hours of surgery, the multi-factorial events that triggered the crisis; the choice to perform a quick diagnosis through gene analysis in comparison to the standard tests of caffeine on a muscular biopsy. These different points are discussed with a proposal of algorithm in the situation.

Evaluation of suspected malignant hyperthermia events during anesthesia

Background

Malignant hyperthermia (MH), a metabolic myopathy triggered by volatile anesthetics and depolarizing muscle relaxants, is a potentially lethal complication of general anesthesia in susceptible patients. The implementation of modern inhalation anesthetics that research indicates as less potent trigger substances and the recommended limitations of succinylcholine use, suggests there may be considerable decline of fulminant MH cases. In the presented study, the authors analyzed suspected MH episodes during general anesthesia of patients that were referred to the Wuerzburg MH unit between 2007 and 2011, assuming that MH is still a relevant anesthetic problem in our days.

Methods

With approval of the local ethics committee data of patients that underwent muscle biopsy and in vitro contracture test (IVCT) between 2007 and 2011 were analyzed. Only patients with a history of suspected MH crisis were included in the study. The incidents were evaluated retrospectively using anesthetic documentation and medical records.

Results

Between 2007 and 2011 a total of 124 patients were tested. 19 of them were referred because of suspected MH events; 7 patients were diagnosed MH-susceptible, 4 MH-equivocal and 8 MH-non-susceptible by IVCT. In a majority of cases masseter spasm after succinylcholine had been the primary symptom. Cardiac arrhythmias and hypercapnia frequently occurred early in the course of events. Interestingly, dantrolene treatment was initiated in a few cases only.

Conclusions

MH is still an important anesthetic complication. Every anesthetist must be aware of this life-threatening syndrome at any time. The rapid onset of adequate therapy is crucial to avoid major harm and possibly lethal outcome. Dantrolene must be readily available wherever MH triggering agents are used for anesthesia.

Core myopathies and malignant hyperthermia susceptibility: a review

The core myopathies are a subset of myopathies that present in infancy with hypotonia and muscle weakness. They were formerly considered a rare type of congenital myopathy but are now recognized as being more prevalent. Due to their genetic linkage to mutations in the ryanodine receptor gene (RYR1), core myopathies (in particular, central core disease) carry a high risk of malignant hyperthermia susceptibility. In this review article, we describe the phenotypical, genetic, and histopathological characteristics of core myopathies and further describe the currently understood nature of their risk of malignant hyperthermia. We also review the level of suspicion a clinician should exhibit with a child who has a possible core myopathy or other congenital myopathy presenting for an anesthetic prior to a definitive genetic analysis. For this review article, we performed literature searches using the key words anesthesiology, core myopathies, pediatric neurology, malignant hyperthermia, genetics, ryanodine receptor, and molecular biology. We also relied on literature accumulated by the two authors, who served as hotline consultants for the Malignant Hyperthermia Hotline of the Malignant Hyperthermia Association of the United States (MHAUS) for the past 12 years.

Genotype-phenotype correlations in recessive RYR1-related myopathies

Background

RYR1 mutations are typically associated with core myopathies and are the most common overall cause of congenital myopathy. Dominant mutations are most often associated with central core disease and malignant hyperthermia, and genotype-phenotype patterns have emerged from the study of these mutations that have contributed to the understanding of disease pathogenesis. The recent availability of genetic testing for the entire RYR1 coding sequence has led to a dramatic expansion in the identification of recessive mutations in core myopathies and other congenital myopathies. To date, no clear patterns have been identified in these recessive mutations, though no systematic examination has yet been performed.

Methods

In this study, we investigated genotype-phenotype correlations in a large combined cohort of unpublished (n = 14) and previously reported (n = 92) recessive RYR1 cases.

Results

Overall examination of this cohort revealed nearly 50% of cases to be non-core myopathy related. Our most significant finding was that hypomorphic mutations (mutations expected to diminish RyR1 expression) were enriched in patients with severe clinical phenotypes. We also determined that hypomorphic mutations were more likely to be encountered in non-central core myopathies. With analysis of the location of non-hypomorphic mutations, we found that missense mutations were generally enriched in the MH/CCD hotspots and specifically enriched in the selectivity filter of the channel pore.

Conclusions

These results support a hypothesis that loss of protein function is a key predictive disease parameter. In addition, they suggest that decreased RyR1 expression may dictate non-core related pathology though, data on protein expression was limited and should be confirmed in a larger cohort. Lastly, the results implicate abnormal ion conductance through the channel pore in the pathogenesis in recessive core myopathies. Overall, our findings represent a comprehensive analysis of genotype-phenotype associations in recessive RYR1-myopathies.

Variable myopathic presentation in a single family with novel skeletal RYR1 mutation

We describe an autosomal recessive heterogeneous congenital myopathy in a large consanguineous family. The disease is characterized by variable severity, progressive course in 3 of 4 patients, myopathic face without ophthalmoplegia and proximal muscle weakness. Absence of cores was noted in all patients. Genome wide linkage analysis revealed a single locus on chromosome 19q13 with Zmax = 3.86 at θ = 0.0 and homozygosity of the polymorphic markers at this locus in patients. Direct sequencing of the main candidate gene within the candidate region, RYR1, was performed. A novel homozygous A to G nucleotide substitution (p.Y3016C) within exon 60 of the RYR1 gene was found in patients. ARMS PCR was used to screen for the mutation in all available family members and in an additional 150 healthy individuals. This procedure confirmed sequence analysis and did not reveal the A to G mutation (p.Y3016C) in 300 chromosomes from healthy individuals. Functional analysis on EBV immortalized cell lines showed no effect of the mutation on RyR1 pharmacological activation or the content of intracellular Ca²⁺ stores. Western blot analysis demonstrated a significant reduction of the RyR1 protein in the patient’s muscle concomitant with a reduction of the DHPRα1.1 protein. This novel mutation resulting in RyR1 protein decrease causes heterogeneous clinical presentation, including slow progression course and absence of centrally localized cores on muscle biopsy. We suggest that RYR1 related myopathy should be considered in a wide variety of clinical and pathological presentation in childhood myopathies.

CASQ1 gene is an unlikely candidate for malignant hyperthermia susceptibility in the North American population

BACKGROUND

Malignant hyperthermia (MH, MIM# 145600) is a complex pharmacogenetic disorder that is manifested in predisposed individuals as a potentially lethal reaction to volatile anesthetics and depolarizing muscle relaxants. Studies of CASQ1-null mice have shown that CASQ1, encoding calsequestrin 1, the major Ca2+ binding protein in the lumen of the sarcoplasmic reticulum, is a candidate gene for MH in mice. The aim of this study was to establish whether the CASQ1 gene is associated with MH in the North American population.

METHODS

The entire coding region of CASQ1 in 75 unrelated patients diagnosed by caffeine-halothane contracture test as MH susceptible (MHS) was analyzed by DNA sequencing. Subsequently, three groups of unrelated individuals (130 MHS, 100 MH negative, and 192 normal controls) were genotyped for a variant that was identified by sequencing. Levels of CASQ1 expression in the muscle from unrelated MHS and MH negative individuals were estimated by Western blotting.

RESULTS

Screening of the entire coding sequence of the CASQ1 gene in 75 MHS patients revealed a single variant c.260T > C (p.Met87Thr) in exon 1. This variant is unlikely to be pathogenic, because its allele frequency in the MHS group was not significantly different from that of controls. There was also no difference in calsequestrin 1 protein levels between muscle samples from MHS and controls, including those carrying the p.Met87Thr variant.

CONCLUSIONS

This study revealed a low level of protein coding sequence variability within the human CASQ1 gene, indicating that CASQ1 is not a major MHS locus in the North American population.

An integrated diagnosis strategy for congenital myopathies

Congenital myopathies are severe muscle disorders affecting adults as well as children in all populations. The diagnosis of congenital myopathies is constrained by strong clinical and genetic heterogeneity. Moreover, the majority of patients present with unspecific histological features, precluding purposive molecular diagnosis and demonstrating the need for an alternative and more efficient diagnostic approach. We used exome sequencing complemented by histological and ultrastructural analysis of muscle biopsies to identify the causative mutations in eight patients with clinically different skeletal muscle pathologies, ranging from a fatal neonatal myopathy to a mild and slowly progressive myopathy with adult onset. We identified RYR1 (ryanodine receptor) mutations in six patients and NEB (nebulin) mutations in two patients. We found novel missense and nonsense mutations, unraveled small insertions/deletions and confirmed their impact on splicing and mRNA/protein stability. Histological and ultrastructural findings of the muscle biopsies of the patients validated the exome sequencing results. We provide the evidence that an integrated strategy combining exome sequencing with clinical and histopathological investigations overcomes the limitations of the individual approaches to allow a fast and efficient diagnosis, accelerating the patient's access to a better healthcare and disease management. This is of particular interest for the diagnosis of congenital myopathies, which involve very large genes like RYR1 and NEB as well as genetic and phenotypic heterogeneity.

Gain of function in the immune system caused by a ryanodine receptor 1 mutation

Mutations in RYR1, the gene encoding ryanodine receptor 1, are linked to a variety of neuromuscular disorders including malignant hyperthermia (MH), a pharmacogenetic hypermetabolic disease caused by dysregulation of Ca(2+) in skeletal muscle. RYR1 encodes a Ca(2+) channel that is predominantly expressed in skeletal muscle sarcoplasmic reticulum, where it is involved in releasing the Ca(2+) necessary for muscle contraction. Other tissues, however, including cells of the immune system, have been shown to express ryanodine receptor 1; in dendritic cells its activation leads to increased surface expression of major histocompatibility complex II molecules and provides synergistic signals leading to cell maturation. In the present study, we investigated the impact of an MH mutation on the immune system by studying the RYR1Y522S knock-in mouse. Our results show that there are subtle but significant differences both in resting 'non-challenged' mice as well as in mice treated with antigenic stimuli, in particular the knock-in mice: (i) have dendritic cells that are more efficient at stimulating T cell proliferation, (ii) have higher levels of natural IgG1 and IgE antibodies, and (iii) are faster and more efficient at mounting a specific immune response in the early phases of immunization. We suggest that some gain-of-function MH-linked RYR1 mutations might offer selective immune advantages to their carriers. Furthermore, our results raise the intriguing possibility that pharmacological activation of RyR1 might be exploited for the development of new classes of vaccines and adjuvants.

Ryanodine receptor type 1 gene variants in the malignant hyperthermia-susceptible population of the United States

BACKGROUND

Mutations in the ryanodine receptor type 1 gene (RYR1) that encodes the skeletal muscle-specific intracellular calcium (Ca(2+)) release channel are a cause of malignant hyperthermia (MH). In this study, we examined RYR1 mutations in a large number of North American MH-susceptible (MHS) subjects without prior genetic diagnosis.

METHODS

RYR1 was examined in 120 unrelated MHS subjects from the United States in a tiered manner. The α-1 subunit of the dihydropyridine receptor gene (CACNA1S) was screened for 4 variants in subjects in whom no abnormality was found in ≥ 100 exons of RYR1.

RESULTS

Ten known causative MH mutations were found in 26 subjects. Variants of uncertain significance in RYR1 were found in 36 subjects, 16 of which are novel. Novel variants in both RYR1 and CACNA1S were found in the 1 subject who died of MH. Two RYR1 variants were found in 4 subjects. Variants of uncertain significance were found outside and inside the hotspots of RYR1. Maximal contractures in the caffeine-halothane contracture test were greater in those who had a known MH mutation or variant of uncertain significance in RYR1 than in those who did not.

CONCLUSIONS

The identification of novel RYR1 variants and previously observed RYR1 variants of uncertain significance in independent MHS families is necessary for demonstrating the significance of these variants for MH susceptibility and supports the need for functional studies of these variants. Continued reporting of the clinical phenotypes of MH is necessary for interpretation of genetic findings, especially because the pathogenicity of most of these genetic variants associated with MHS remains to be elucidated.