Identification of malignant hyperthermia-susceptible ryanodine receptor type 1 gene (RYR1) mutations in a child who died in a car after exposure to a high environmental temperature

[Trigger-free anesthesia : Indications and safe application]

The safe anesthesiological care of patients with neuromuscular diseases poses a particular challenge for anesthetists. Only a small group of muscle diseases and syndromes are associated with an increased risk of malignant hyperthermia and therefore require trigger-free anesthetic procedures avoiding volatile anesthetics and succinylcholine. These diseases are frequently associated with mutations in the RYR1, CACNA1S or STAC-3 genes. In other neuromuscular diseases, anesthetic-induced rhabdomyolysis can occur. Therefore, volatile anesthetics should be avoided in these patients in addition to the contraindication for succinylcholine. In other neuromuscular diseases the risk of a propofol infusion syndrome or myotonic crises can be elevated or the duration of the effect of non-depolarizing muscle relaxants can be changed in an incalculable way. In every case an individual anesthetic aproach including the avoidance of potential pharmacological or non-pharmacological triggers is essential for the safety of the patients.

Fatal postoperative malignant hyperthermia following rhinoplasty in a patient with one variant of the ryanodine receptor 1 (RYR1) gene

Malignant hyperthermia (MH) is characterized by a rapid rise in body temperature after using inhalational anesthetics and depolarizing muscle relaxants. A 19-year-old female had a rapidly developing fever up to 43.0 °C, after rhinoplasty surgery. Inhalational anesthetics and depolarizing muscle relaxants were used in general anesthesia. It was suspected that the patient died of MH. The medico-legal autopsy findings showed classical MH histopathological changes in the skeletal muscles, cardiac muscles, as well as lungs. Additionally, postmortem blood biochemical results indicated rhabdomyolysis. A combination of clinical records and autopsy revealed that MH might have caused the death. A diagnostic genetic testing was performed to confirm the existence of MH, and an MH diagnostic variant RYR1 c.7048G >A (p. A2350T) was detected. Eventually, the cause of death was determined as MH based on clinical records, autopsy, and genetic analysis. This case highlights that diagnostic genetic analysis plays a vital role in postmortem diagnosis of MH in routine medico-legal contexts.

The current status of malignant hyperthermia

Malignant hyperthermia (MH) is a rare and life-threatening pharmacogenetic disorder triggered by volatile anesthetics, the depolarizing muscle relaxant succinylcholine, and rarely by strenuous exercise or environmental heat. The exact prevalence of MH is unknown, and it varies from 1:16 000 in Denmark to 1:100 000 in New York State. The underlying mechanism of MH is excessive calcium release from the sarcoplasmic reticulum (SR), leading to uncontrolled skeletal muscle hyper-metabolism. Genetic mutations in ryanodine receptor type 1 ( RYR1) and CACNA1S have been identified in approximately 50% to 86% and 1% of MH-susceptible (MHS) individuals, respectively. Classic clinical symptoms of MH include hypercarbia, sinus tachycardia, masseter spasm, hyperthermia, acidosis, muscle rigidity, hyperkalemia, myoglobinuria, and etc. There are two types of testing for MH: a genetic test and a contracture test. Contracture testing is still being considered as the gold standard for MH diagnosis. Dantrolene is the only available drug approved for the treatment of MH through suppressing the calcium release from SR. Since clinical symptoms of MH are highly variable, it can be difficult to establish a diagnosis of MH. Nevertheless, prompt diagnosis and treatments are crucial to avoid a fatal outcome. Therefore, it is very important for anesthesiologists to raise awareness and understand the characteristics of MH. This review summarizes epidemiology, clinical symptoms, diagnosis and treatments of MH and any new developments.

Aging Effects of Caenorhabditis elegans Ryanodine Receptor Variants Corresponding to Human Myopathic Mutations

Delaying the decline in skeletal muscle function will be critical to better maintenance of an active lifestyle in old age. The skeletal muscle ryanodine receptor, the major intracellular membrane channel through which calcium ions pass to elicit muscle contraction, is central to calcium ion balance and is hypothesized to be a significant factor for age-related decline in muscle function. The nematode Caenorhabditis elegans is a key model system for the study of human aging, and strains were generated with modified C. elegans ryanodine receptors corresponding to human myopathic variants linked with malignant hyperthermia and related conditions. The altered response of these strains to pharmacological agents reflected results of human diagnostic tests for individuals with these pathogenic variants. Involvement of nerve cells in the C. elegans responses may relate to rare medical symptoms concerning the central nervous system that have been associated with ryanodine receptor variants. These single amino acid modifications in C. elegans also conferred a reduction in lifespan and an accelerated decline in muscle integrity with age, supporting the significance of ryanodine receptor function for human aging.

RYR1-related rhabdomyolysis: A common but probably underdiagnosed manifestation of skeletal muscle ryanodine receptor dysfunction

Mutations in the skeletal muscle ryanodine receptor (RYR1) gene are associated with a wide spectrum of inherited myopathies presenting throughout life. Malignant hyperthermia susceptibility (MHS)-related RYR1 mutations have emerged as a common cause of exertional rhabdomyolysis, accounting for up to 30% of rhabdomyolysis episodes in otherwise healthy individuals. Common triggers are exercise and heat and, less frequently, viral infections, alcohol and drugs. Most subjects are normally strong and have no personal or family history of malignant hyperthermia. Heat intolerance and cold-induced muscle stiffness may be a feature. Recognition of this (probably not uncommon) rhabdomyolysis cause is vital for effective counselling, to identify potentially malignant hyperthermia-susceptible individuals and to adapt training regimes. Studies in various animal models provide insights regarding possible pathophysiological mechanisms and offer therapeutic perspectives.

Hypothesis: exertional heat stroke-induced myopathy and genetically inherited malignant hyperthermia represent the same disorder, the human stress syndrome

Exertional heat stroke is usually experienced as a result of a prolonged and intensive exercise. It is a life-threatening condition that is characterized by an increase in core body temperature and rhabdomyolysis. The associated hyperkalemia and metabolic acidosis may lead to an acute renal, cardiac, and hemostatic failure. Exactly, the same symptoms are noticed in case of the anesthesia-induced malignant hyperthermia (MH), an inherited disorder of the skeletal muscle ryanodine receptor. This receptor is a Ca(2+) channel that is activated by the volatile anesthetic agents and depolarizing muscle relaxant. The presence of MH-associated ryanodine receptor variant in the individuals who suffered from EH and improvement of the symptoms with dantrolene has frequently raised the question as to whether the two disorders actually represent one and the same disease. Nevertheless, an exact explanation of the susceptibility of the genetically predisposed MH individuals to ER remains elusive. We have attempted to review the published clinical reports to explore the possibility that ER and EH represent one and the same disorder.

Identification of arrhythmogenic right ventricular cardiomyopathy-causing gene mutations in young sudden unexpected death autopsy cases

Arrhythmogenic right ventricular cardiomyopathy (ARVC) results in an increased risk of sudden death. We sought mutations of desmoglein-2 (DSG2), desmoplakin (DSP), and plakophilin-2 (PKP2) in 15 cases of sudden death whose causes of death could not be determined at autopsy. In three victims, mutations were identified in DSP. Two of these mutations were novel; one had previously been reported in a patient with ARVC that had been diagnosed clinically. Histological findings were not typical of ARVC; however, it was notable that these mutations were present in three of 15 cases, a relatively high proportion. The causal relationship between the mutations and ARVC is unclear, but the mutations might have been associated with faulty desmosomal proteins resulting in fatal arrhythmia. Combining information gathered by the traditional means of gross and histological examination with postmortem genetic analysis of young victims would assist in identifying their cause of death.

Effects of ulinastatin administered at different time points on the pathological morphologies of the lung tissues of rats with hyperthermia

Hyperthermia not only directly induces cell injury of body tissues, but also causes the body to release large amounts of inflammatory mediators and cells with extensive biological activities to induce a systemic inflammatory response and immune dysfunction. Thus, hyperthermia causes systemic inflammatory response syndrome, aggravating injuries to various organs. This study aimed to observe the effects of ulinastatin (UTI) administered at different time points on the cellular morphologies of the lung tissues of rats with systemic hyperthermia. A total of 40 male Sprague Dawley rats were randomly divided into five groups: The normal control group (C group), the hyperthermia group without medication (H group), the hyperthermia and UTI pre-treatment group (HU group), the group treated with UTI at 1 h after hyperthermia (HU1 group), and the group treated with UTI at 2 h after hyperthermia (HU2 group). The systemic hyperthermia rat model was established in a heating chamber with a biological oxygen supply. For the HU, HU1 and HU2 groups, UTI (5×10⁴ U/kg) was administered at different time points. For the C and H groups, an equivalent volume of normal saline was administered. During heating, the respiratory frequency and rectal temperature were measured and recorded once every 30 min. After 2.5 h of heating, the wet/dry weight (W/D) ratio of the lung tissues of the rats was measured. Additionally, the cellular morphologies of the lung tissues were observed under light and electron microscopes. The respiratory frequencies and lung tissue W/D ratios of the rats in the various hyperthermia groups were significantly higher than those of the rats in the C group (all P<0.05). The respiratory frequencies and lung tissue W/D values of the HU and HU1 groups were significantly lower than those of the H group (all P<0.05). Under the light microscope, the bronchial surrounding tissues of the HU and HU1 groups were loose, and the majority of the pulmonary alveolar structures were normal; the H and HU2 groups presented a number of changes, including pulmonary interstitial hyperemia, alveolar epithelial swelling and emphysema. Under the electron microscope, it was observed in the type II epithelial cells of the pulmonary alveoli of the H group that the mitochondria were swollen, the cell ridges were shortened, the microvilli were thin and increased, and the alveolar wall was thickened. Also, an increased number of infiltrating neutrophils were visible. In addition, the type II epithelial cells of the HU2 group also presented these changes to different extents and the changes in the HU and HU1 groups were the mildest. These results indicate that the early application of UTI relieves edema and the extent of cell injury of the lung tissue in rats with systemic hyperthermia.

Accelerated activation of SOCE current in myotubes from two mouse models of anesthetic- and heat-induced sudden death

Store-operated calcium entry (SOCE) channels play an important role in Ca²⁺ signaling. Recently, excessive SOCE was proposed to play a central role in the pathogenesis of malignant hyperthermia (MH), a pharmacogenic disorder of skeletal muscle. We tested this hypothesis by characterizing SOCE current (I_SkCRAC) magnitude, voltage dependence, and rate of activation in myotubes derived from two mouse models of anesthetic- and heat-induced sudden death: 1) type 1 ryanodine receptor (RyR1) knock-in mice (Y524S/+) and 2) calsequestrin 1 and 2 double knock-out (dCasq-null) mice. I_SkCRAC voltage dependence and magnitude at -80 mV were not significantly different in myotubes derived from wild type (WT), Y524S/+ and dCasq-null mice. However, the rate of I_SkCRAC activation upon repetitive depolarization was significantly faster at room temperature in myotubes from Y524S/+ and dCasq-null mice. In addition, the maximum rate of I_SkCRAC activation in dCasq-null myotubes was also faster than WT at more physiological temperatures (35-37°C). Azumolene (50 µM), a more water-soluble analog of dantrolene that is used to reverse MH crises, failed to alter I_SkCRAC density or rate of activation. Together, these results indicate that while an increased rate of I_SkCRAC activation is a common characteristic of myotubes derived from Y524S/+ and dCasq-null mice and that the protective effects of azumolene are not due to a direct inhibition of SOCE channels.

Exercise-induced rhabdomyolysis and stress-induced malignant hyperthermia events, association with malignant hyperthermia susceptibility, and RYR1 gene sequence variations

Exertional rhabdomyolysis (ER) and stress-induced malignant hyperthermia (MH) events are syndromes that primarily afflict military recruits in basic training and athletes. Events similar to those occurring in ER and in stress-induced MH events are triggered after exposure to anesthetic agents in MH-susceptible (MHS) patients. MH is an autosomal dominant hypermetabolic condition that occurs in genetically predisposed subjects during general anesthesia, induced by commonly used volatile anesthetics and/or the neuromuscular blocking agent succinylcholine. Triggering agents cause an altered intracellular calcium regulation. Mutations in RYR1 gene have been found in about 70% of MH families. The RYR1 gene encodes the skeletal muscle calcium release channel of the sarcoplasmic reticulum, commonly known as ryanodine receptor type 1 (RYR1). The present work reviews the documented cases of ER or of stress-induced MH events in which RYR1 sequence variations, associated or possibly associated to MHS status, have been identified.

Disease mutations in the ryanodine receptor N-terminal region couple to a mobile intersubunit interface

Ryanodine receptors are large channels that release Ca(2+) from the endoplasmic and sarcoplasmic reticulum. Hundreds of RyR mutations can cause cardiac and skeletal muscle disorders, yet detailed mechanisms explaining their effects have been lacking. Here we compare pseudo-atomic models and propose that channel opening coincides with widening of a cytoplasmic vestibule formed by the N-terminal region, thus altering an interface targeted by 20 disease mutations. We solve crystal structures of several disease mutants that affect intrasubunit domain-domain interfaces. Mutations affecting intrasubunit ionic pairs alter relative domain orientations, and thus couple to surrounding interfaces. Buried disease mutations cause structural changes that also connect to the intersubunit contact area. These results suggest that the intersubunit contact region between N-terminal domains is a prime target for disease mutations, direct or indirect, and we present a model whereby ryanodine receptors and inositol-1,4,5-trisphosphate receptors are activated by altering domain arrangements in the N-terminal region.

AICAR prevents heat-induced sudden death in RyR1 mutant mice independent of AMPK activation

Mice with a knock-in mutation (Y524S) in the type I ryanodine receptor (RyR1) die when exposed to short periods of temperature elevation (≥ 37 °C). We demonstrate that treatment with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) prevents heat-induced sudden death in Y524S mice. The AICAR protection is independent of AMPK activation and results from a newly identified action on the mutant RyR1 to reduce Ca²⁺ leak, preventing Ca²⁺ dependent increases in both reactive oxygen and reactive nitrogen species that act to further increase resting Ca²⁺ concentrations. If unchecked, the temperature driven increases in resting Ca²⁺ and ROS/RNS create an amplifying cycle that ultimately triggers sustained muscle contractions, rhabdomyolysis and death. Although antioxidants are effective in reducing this cycle in vitro, only AICAR prevents the heat induced death in vivo. Our findings suggest that AICAR is likely to be effective in prophylactic treatment of humans with enhanced susceptibility to exercise/heat-induced sudden death associated with RyR1 mutations.

Identical de novo mutation in the type 1 ryanodine receptor gene associated with fatal, stress-induced malignant hyperthermia in two unrelated families

BACKGROUND

Mutations in the type 1 ryanodine receptor gene (RYR1) result in malignant hyperthermia, a pharmacogenetic disorder typically triggered by administration of anesthetics. However, cases of sudden death during exertion, heat challenge, and febrile illness in the absence of triggering drugs have been reported. The underlying causes of such drug-free fatal "awake" episodes are unknown.

METHODS

De novo R3983C variant in RYR1 was identified in two unrelated children who experienced fatal, nonanesthetic awake episodes associated with febrile illness and heat stress. One of the children also had a second novel, maternally inherited D4505H variant located on a separate haplotype. Effects of all possible heterotypic expression conditions on RYR1 sensitivity to caffeine-induced Ca release were determined in expressing RYR1-null myotubes.

RESULTS

Compared with wild-type RYR1 alone (EC50 = 2.85 ± 0.49 mM), average (± SEM) caffeine sensitivity of Ca release was modestly increased after coexpression with either R3983C (EC50 = 2.00 ± 0.39 mM) or D4505H (EC50 = 1.64 ± 0.24 mM). Remarkably, coexpression of wild-type RYR1 with the double mutant in cis (R3983C-D4505H) produced a significantly stronger sensitization of caffeine-induced Ca release (EC50 = 0.64 ± 0.17 mM) compared with that observed after coexpression of the two variants on separate subunits (EC50 = 1.53 ± 0.18 mM).

CONCLUSIONS

The R3983C mutation potentiates D4505H-mediated sensitization of caffeine-induced RYR1 Ca release when the mutations are in cis (on the same subunit) but not when present on separate subunits. Nevertheless, coexpression of the two variants on separate subunits still resulted in a ∼2-fold increase in caffeine sensitivity, consistent with the observed awake episodes and heat sensitivity.

Postmortem genetic analysis for a sudden death case complicated with Marfan syndrome

We report here a sudden death case of a patient previously diagnosed as Marfan syndrome (MFS). The victim was dead on the wheel and the cause of death was diagnosed to be a rupture of the thoracic aorta by autopsy findings. MFS is an autosomal dominant disorder of the connective tissue and can be a cause of sudden death. Postmortem genetic analysis revealed a heterozygous p.C1307Y of the FBN1 gene, which is responsible for pathogenesis of MFS, was evident. This substitution was not found in 400 alleles from control individuals. In addition, the position 1307 is highly conserved among species. Because the position 1307 serves as part of the Cys1307-Cys1320 disulfide bond of the fibrillin-1, the p.C1307Y substitution results in loss of the intramolecular disulfide bond. The p.C1307Y substitution may be associated with the pathology of the present case, and show a higher risk for aortic rupture and subsequent sudden death.

Effect of prior exercise on thermal sensitivity of malignant hyperthermia-susceptible muscle

Malignant hyperthermia (MH) episodes may occur upon exposure to halogenated anesthetics, during resistance and endurance exercise, and in response to thermal stress. The purpose of this study was to investigate the effects of prior eccentric and concentric (i.e., wheel running) exercise on the thermal sensitivity of isolated MH-susceptible mouse muscle (RyR1(Y522S/wt)). Eccentric, but not concentric, exercise attenuated the thermal sensitivity of MH-susceptible muscle.