Mutations inRYR1in malignant hyperthermia and central core disease

Epigenetic allele silencing and variable penetrance of malignant hyperthermia susceptibility

BACKGROUND

Tissue-specific monoallelic silencing of the RYR1 gene has been proposed as an explanation for variable penetrance of dominant RYR1 mutations in malignant hyperthermia (MH). We examined the hypothesis that monoallelic silencing could explain the inheritance of an MH discordant phenotype in some instances.

METHODS

We analysed parent-offspring transmission data from MH kindreds to assess whether there was any deviation from the expected autosomal dominant Mendelian inheritance pattern. We also evaluated informative single-nucleotide polymorphism (SNP) genotypes in a cohort of unrelated MH patients using genomic DNA (gDNA, prepared from leucocytes) and coding DNA (cDNA, prepared from skeletal muscle). Finally, we examined the segregation of specific mutations at the gDNA and cDNA level within MH families where positive RYR1 gDNA genotype/normal MH phenotype discordance had been observed.

RESULTS

In 2113 transmissions from affected parents, there was a consistent parent-of-origin effect (P<0.001) with affected fathers having fewer affected daughters (20%, 95% CI 17-22%) than affected sons (25%, 95% CI 23-26%) or unaffected daughters (27%, 95% CI 25-30%). No discrepancies were observed between the RYR1 SNP genotypes recorded at the gDNA and cDNA levels. In 14 MH negative individuals from 11 discordant families, the familial mutation was detected in skeletal muscle cDNA in all cases.

CONCLUSIONS

Epigenetic allele silencing may play a role in the inheritance of MH susceptibility, but this is unlikely to involve silencing of RYR1.

Scientific advances in the genetic understanding and diagnosis of malignant hyperthermia

Malignant hyperthermia (MH), a potentially fatal disorder triggered by certain types of general anesthesia, has received much attention in the scientific literature. From the first case report in 1960 until the present, hundreds of studies have been conducted. The diagnosis of MH has evolved from subjective assumptions by family history and clinical diagnosis to more sophisticated laboratory testing. A genetic basis for MH was recognized in the early 1990s and, since then, complex genetic pathways have been demonstrated. The purpose of this paper is to summarize the research literature on what is known scientifically about the diagnosis and genetic basis of MH.

Mutation screening of the RYR1-cDNA from peripheral B-lymphocytes in 15 Swedish malignant hyperthermia index cases

BACKGROUND

Malignant hyperthermia (MH), linked to the ryanodine receptor 1 gene (RYR1) on chromosome 19, is a potentially lethal pharmacogenetic disorder which may lead to a disturbance of intracellular calcium homeostasis when susceptible individuals are exposed to halogenated anaesthetics, suxamethonium, or both. Central core disease (CCD) is a rare dominantly inherited congenital myopathy allelic to MH-susceptibility.

METHODS

In this study, 14 unrelated MH-susceptible probands and one CCD patient from Sweden were screened for mutations in the RYR1. Since the RYR1 is also expressed in B-lymphocytes, RYR1-cDNA was transcribed from total RNA extracted from white blood cells.

RESULTS

We detected two known RYR1 mutations and two previously described unclassified sequence variants. In addition, six novel sequence variants were detected. All mutations or sequence variants were verified on genomic DNA. Seven of the probands did not show any candidate mutation, although the total coding region of RYR1 was sequenced. Segregation data in in vitro contracture tested family members of three probands support a causative role of three of the novel sequence variants.

CONCLUSIONS

Our study contributes to the genetic aetiology of MH in Sweden, but also raises questions about the involvement of genes other than RYR1 since nearly half of the probands did not show any sequence variants in the total coding region of the RYR1.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Malignant hyperthermia: a pharmacogenetic disorder

Malignant hyperthermia (MH) is a pharmacogenetic disorder triggered by volatile anesthetics or depolarizing muscle relaxants in predisposed individuals. Exercise or stress-induced MH episodes, in the absence of any obvious pharmacological trigger, have been reported, but these are rare. A considerable effort has taken place over the last two decades to identify mutations associated with MH and characterize their functional effects. A number of different, but complementary systems, have been developed and implemented to this end. The results of such studies have identified commonalities in functional affects of mutations, and also uncovered unexpected complexities in both the structure and function of the skeletal muscle calcium-release channel. The following review is an attempt to provide a summary of the background to current MH research, and highlight some recent advances in our knowledge of the molecular basis of the phenotypic expression of this disorder.

A retrograde signal from RyR1 alters DHP receptor inactivation and limits window Ca2+ release in muscle fibers of Y522S RyR1 knock-in mice

Malignant hyperthermia (MH) is a life-threatening hypermetabolic condition caused by dysfunctional Ca(2+) homeostasis in skeletal muscle, which primarily originates from genetic alterations in the Ca(2+) release channel (ryanodine receptor, RyR1) of the sarcoplasmic reticulum (SR). Owing to its physical interaction with the dihydropyridine receptor (DHPR), RyR1 is controlled by the electrical potential across the transverse tubular (TT) membrane. The DHPR exhibits both voltage-dependent activation and inactivation. Here we determined the impact of an MH mutation in RyR1 (Y522S) on these processes in adult muscle fibers isolated from heterozygous RyR1(Y522S)-knock-in mice. The voltage dependence of DHPR-triggered Ca(2+) release flux was left-shifted by approximately 8 mV. As a consequence, the voltage window for steady-state Ca(2+) release extended to more negative holding potentials in muscle fibers of the RyR1(Y522S)-mice. A rise in temperature from 20 degrees to 30 degrees C caused a further shift to more negative potentials of this window (by approximately 20 mV). The activation of the DHPR-mediated Ca(2+) current was minimally changed by the mutation. However, surprisingly, the voltage dependence of steady-state inactivation of DHPR-mediated calcium conductance and release were also shifted by approximately 10 mV to more negative potentials, indicating a retrograde action of the RyR1 mutation on DHPR inactivation that limits window Ca(2+) release. This effect serves as a compensatory response to the lowered voltage threshold for Ca(2+) release caused by the Y522S mutation and represents a novel mechanism to counteract excessive Ca(2+) leak and store depletion in MH-susceptible muscle.

Anesthetic- and heat-induced sudden death in calsequestrin-1-knockout mice

Calsequestrin-1 (CASQ1) is a moderate-affinity, high-capacity Ca(2+)-binding protein in the sarcoplasmic reticulum (SR) terminal cisternae of skeletal muscle. CASQ1 functions as both a Ca(2+)-binding protein and a luminal regulator of ryanodine receptor (RYR1)-mediated Ca(2+) release. Mice lacking skeletal CASQ1 are viable but exhibit reduced levels of releasable Ca(2+) and altered contractile properties. Here we report that CASQ1-null mice exhibit increased spontaneous mortality and susceptibility to heat- and anesthetic-induced sudden death. Exposure of CASQ1-null mice to either 2% halothane or heat stress triggers lethal episodes characterized by whole-body contractures, elevated core temperature, and severe rhabdomyolysis, which are prevented by prior dantrolene administration. The characteristics of these events are remarkably similar to analogous episodes observed in humans with malignant hyperthermia (MH) and animal models of MH and environmental heat stroke (EHS). In vitro studies indicate that CASQ1-null muscle exhibits increased contractile sensitivity to temperature and caffeine, temperature-dependent increases in resting Ca(2+), and an increase in the magnitude of depolarization-induced Ca(2+) release. These results demonstrate that CASQ1 deficiency alters proper control of RYR1 function and suggest CASQ1 as a potential candidate gene for linkage analysis in families with MH/EHS where mutations in the RYR1 gene are excluded.

[In-vivo diagnosis of malignant hyperthermia susceptibility: a microdialysis study]

BACKGROUND

In malignant hyperthermia (MH), volatile anesthetics induce hypermetabolism, lactic acidosis and rhabdomyolysis in predisposed patients. The authors hypothesized that intramuscular caffeine and halothane application would increase local lactate concentration in MH susceptible (MHS) individuals more than in non-susceptible (MHN) subjects without initiating the full MH syndrome.

METHODS

In 14 MHS, 12 MHN and 7 control individuals, microdialysis probes were placed in the rectus femoris muscle and perfused with Ringer's solution at 1 microl/min. After equilibration, 250 microl caffeine (80 mM) was injected through the first microdialysis probe, halothane 10 vol% dissolved in soybean oil was perfused through a second microdialysis probe and a third probe was used for control measurements. Dialysate samples were analyzed for lactate spectrophotometrically. Systemic hemodynamic and metabolic parameters were measured. Data are presented as median and quartiles.

RESULTS

Intramuscular caffeine and halothane significantly increased local peak concentrations of lactate in MHS probands [5.0 mM (3.4-8.1 mM) and 3.7 mM (2.6-5.0 mM), respectively] compared to MHN [1.6 mM (1.3-2.0 mM) and 1.9 mM (1.6-2.0 mM)] or control individuals [2.1 mM (1.9-2.3 mM) and 2.0 mM (1.6-2.1 mM)]. This was accompanied by a higher serum creatine kinase level in the MHS group. Hemodynamic and metabolic parameters were normal in the investigated groups.

CONCLUSION

Intramuscular caffeine and halothane application induces a temporary and abnormal increase of local lactate in MHS individuals. No serious systemic side effects occurred. This study presents evidence that metabolic monitoring with local stimulation by caffeine and halothane may allow a minimally invasive diagnosis of MH susceptibility.

[Malignant hyperthermia in children]

INTRODUCTION

Malignant hyperthermia is a rare disease which is mainly an inherited autosomal dominant trait. It is characteristic for muscle rigidity, metabolism or respiratory acidosis, high values of serum creatine kinase. Then appears myoglobinuria which can lead to tubular necrosis and acute renal failure.

CASE OUTLINE

The male child, ten years old, hospitalized because of the high temperature, exhaustion and cough. On the second day of hospitalization, he has hyperthermia (39.8 degrees C), dyspnoea, tachypnoea, was somnolent, occasionally raving, exhausted with the pains in the muscles which were rigid and painfully sensitive. During the night, the urine was dark red, but the diuresis was well. The next day, laboratory analyses showed high values of aspartate aminotransferase (4263 IU/l), alanine aminotransferase (1311 IU/l), lactate dehydrogenase (11787 IU/l), while the values of serum creatine kinase were so high that they could not be registered. The urine analysis showed the negative result on gall colours and haematuria and positive on myoglobin. During the following days, the patient had normal skin temperature, the pains in the muscles were gradually weakened and the urine cleared up. On the third day, the value of creatine kinase was measured and it was 178700 IU/l. During the next two weeks, the clinical finding was gradually normalized while the laboratory values of serum enzyme were gradually normalized only in twenty days.

CONCLUSION

Malignant hyperthermia is a serious clinical syndrome which can be found with, until then, a clinically healthy child (without chronic myopathy) and it appears with the high temperature or during the surgical interventions with anaesthetics, which is especially dangerous.

Ryanodine receptor activation by Ca v 1.2 is involved in dendritic cell major histocompatibility complex class II surface expression

Dendritic cells express the skeletal muscle ryanodine receptor (RyR1), yet little is known concerning its physiological role and activation mechanism. Here we show that dendritic cells also express the Ca(v)1.2 subunit of the L-type Ca(2+) channel and that release of intracellular Ca(2+) via RyR1 depends on the presence of extracellular Ca(2+) and is sensitive to ryanodine and nifedipine. Interestingly, RyR1 activation causes a very rapid increase in expression of major histocompatibility complex II molecules on the surface of dendritic cells, an effect that is also observed upon incubation of mouse BM12 dendritic cells with transgenic T cells whose T cell receptor is specific for the I-A(bm12) protein. Based on the present results, we suggest that activation of the RyR1 signaling cascade may be important in the early stages of infection, providing the immune system with a rapid mechanism to initiate an early response, facilitating the presentation of antigens to T cells by dendritic cells before their full maturation.

A recessive ryanodine receptor 1 mutation in a CCD patient increases channel activity

Ryanodine receptors plays a crucial role in skeletal muscle excitation-contraction coupling by releasing calcium ions required for muscle contraction from the sarcoplasmic reticulum. At least three phenotypes associated with more than 100 RYR1 mutations have been identified; in order to elucidate possible pathophysiological mechanisms of RYR1 mutations linked to neuromuscular disorders, it is essential to define the mutation class by studying the functional properties of channels harbouring clinically relevant amino acid substitutions. In the present report we investigated the functional effects of the c.7304G>T RYR1 substitution (p.Arg2435Leu) found in a patient affected by central core disease. Both parents were heterozygous for the substitution while the proband was homozygous. We characterized Ca(2+) homeostasis in myoD transduced myotubes from controls, the heterozygous parents and the homozygous proband expressing the endogenous mutation. We also expressed the recombinant mutant channels in heterologous cells and characterized their [(3)H]ryanodine binding and single channel properties. Our results show that the p.Arg2435Leu substitution affects neither the resting [Ca(2+)], nor the sensitivity of the ryanodine receptor to pharmacological activators, but rather reduces the release of Ca(2+) from intracellular stores induced by pharmacological activators as well as by KCl via the voltage sensing dihydropyridine receptor.

Identification of genetic mutations in Australian malignant hyperthermia families using sequencing of RYR1 hotspots

Advances in analysis of the RYR1 gene (which encodes the skeletal muscle ryanodine receptor) show that genetic examination is a useful adjunct to the in vitro contracture test in the diagnosis of malignant hyperthermia, as defects in RYR1 have been shown to be responsible for malignant hyperthermia susceptibility. DNA from 34 malignant hyperthermia susceptible individuals and four malignant hyperthermia equivocal subjects was examined using direct sequencing of 'hot-spots' in the RYR1 gene to identify mutations associated with malignant hyperthermia. Seven different causative mutations (as defined by the European Malignant Hyperthermia Group) in nine malignant hyperthermia susceptible individuals were identified. In another six malignant hyperthermia susceptible individuals, five different published but as yet functionally uncharacterised mutations were identified. A further three as yet unpublished and functionally uncharacterised (novel) mutations were identified in three malignant hyperthermia susceptible samples. If the novel and previously published mutations prove to be functionally associated with calcium homeostasis, then this method of analysis achieved a mutation detection rate of 47%. Based on the number of relatives presenting to our unit in the study period, the muscle biopsy rate would have decreased by 25%. That we only identified a genetic defect in RYR1 in 47% of in vitro contracture test positive individuals suggests that there are other areas in RYR1 where pathogenic mutations may occur and that RYR1 may not be the sole gene associated with malignant hyperthermia. It may also reflect a less than 100% specificity of the in vitro contracture test.

An in-vivo metabolic test for detecting malignant hyperthermia susceptibility in humans: a pilot study

INTRODUCTION

In vitro contracture testing to diagnose malignant hyperthermia (MH) susceptibility requires a muscle biopsy, which may be associated with severe side effects for the patient. After investigation of several different protocols, we present a less invasive metabolic test that involves IM injection of caffeine and halothane, and subsequent measurement of interstitial lactate to differentiate between MH susceptible (MHS) and MH non-susceptible (MHN) individuals.

METHODS

Two microdialysis probes with attached microtubing for trigger injection were inserted into the lateral vastus muscle of eight previously diagnosed MHS patients (representing three genetic variants Gly2434Arg, Thr2206Met, and Arg614Cys), seven MHN patients, and seven control individuals. After equilibration and lactate baseline recording, a single bolus of 200 muL caffeine 80 mM and a suspension of 200 muL halothane 4%V/V in soy bean oil (triggers) were injected locally. Lactate was measured spectrophotometrically. Data are presented as medians and interquartile ranges.

RESULTS

Although baseline lactate values were similar in the investigated groups before trigger injection, caffeine increased local lactate in MHS patients significantly more (2.0 [1.8-2.6] mM) than in MHN (0.8 [0.6-1.1] mM) or in control individuals (0.8 [0.6-0.8 mM]). Similarly, halothane lead to a significant lactate increase in MHS compared to MHN and control individuals (8.6 [3.7-8.9] mM vs 0.9 [0.5-1.1] mM and 1.7 [0.9-2.3] mM, respectively). However, a relevant increase of lactate was observed in one MHN and in two control individuals. Systemic hemodynamic and metabolic variables did not differ between the investigated groups.

DISCUSSION

Metabolic monitoring of IM lactate after local caffeine and halothane injection may allow less invasive testing to detect MH susceptibility, without systemic side effects.

Reduced threshold for luminal Ca2+ activation of RyR1 underlies a causal mechanism of porcine malignant hyperthermia

Naturally occurring mutations in the skeletal muscle Ca(2+) release channel/ryanodine receptor RyR1 are linked to malignant hyperthermia (MH), a life-threatening complication of general anesthesia. Although it has long been recognized that MH results from uncontrolled or spontaneous Ca(2+) release from the sarcoplasmic reticulum, how MH RyR1 mutations render the sarcoplasmic reticulum susceptible to volatile anesthetic-induced spontaneous Ca(2+) release is unclear. Here we investigated the impact of the porcine MH mutation, R615C, the human equivalent of which also causes MH, on the intrinsic properties of the RyR1 channel and the propensity for spontaneous Ca(2+) release during store Ca(2+) overload, a process we refer to as store overload-induced Ca(2+) release (SOICR). Single channel analyses revealed that the R615C mutation markedly enhanced the luminal Ca(2+) activation of RyR1. Moreover, HEK293 cells expressing the R615C mutant displayed a reduced threshold for SOICR compared with cells expressing wild type RyR1. Furthermore, the MH-triggering agent, halothane, potentiated the response of RyR1 to luminal Ca(2+) and SOICR. Conversely, dantrolene, an effective treatment for MH, suppressed SOICR in HEK293 cells expressing the R615C mutant, but not in cells expressing an RyR2 mutant. These data suggest that the R615C mutation confers MH susceptibility by reducing the threshold for luminal Ca(2+) activation and SOICR, whereas volatile anesthetics trigger MH by further reducing the threshold, and dantrolene suppresses MH by increasing the SOICR threshold. Together, our data support a view in which altered luminal Ca(2+) regulation of RyR1 represents a primary causal mechanism of MH.

Halothane modulation of skeletal muscle ryanodine receptors: dependence on Ca2+, Mg2+, and ATP

Malignant hyperthermia (MH) susceptibility is a genetic disorder of skeletal muscle associated with mutations in the ryanodine receptor isoform 1 (RyR1) of sarcoplasmic reticulum (SR). In MH-susceptible skeletal fibers, RyR1-mediated Ca(2+) release is highly sensitive to activation by the volatile anesthetic halothane. Indeed, studies with isolated RyR1 channels (using simple Cs(+) solutions) found that halothane selectively affects mutated but not wild-type RyR1 function. However, studies in skeletal fibers indicate that halothane can also activate wild-type RyR1-mediated Ca(2+) release. We hypothesized that endogenous RyR1 agonists (ATP, lumenal Ca(2+)) may increase RyR1 sensitivity to halothane. Consequently, we studied how these agonists affect halothane action on rabbit skeletal RyR1 reconstituted into planar lipid bilayers. We found that cytosolic ATP is required for halothane-induced activation of the skeletal RyR1. Unlike RyR1, cardiac RyR2 (much less sensitive to ATP) responded to halothane even in the absence of this agonist. ATP-dependent halothane activation of RyR1 was enhanced by cytosolic Ca(2+) (channel agonist) and counteracted by Mg(2+) (channel inhibitor). Dantrolene, a muscle relaxant used to treat MH episodes, did not affect RyR1 or RyR2 basal activity and did not interfere with halothane-induced activation. Studies with skeletal SR microsomes confirmed that halothane-induced RyR1-mediated SR Ca(2+) release is enhanced by high ATP-low Mg(2+) in the cytosol and by increased SR Ca(2+) load. Thus, physiological or pathological processes that induce changes in cellular levels of these modulators could affect RyR1 sensitivity to halothane in skeletal fibers, including the outcome of halothane-induced contracture tests used to diagnose MH susceptibility.

Propofol-induced changes in myoplasmic calcium concentrations in cultured human skeletal muscles from RYR1 mutation carriers

Malignant hyperthermia is a pharmacogenetic disorder caused by autosomal dominant mutations in the ryanodine receptor type 1 gene. Propofol has been reported as a safe anaesthetic for malignant hyperthermia susceptible patients but has not been tested on cultured cells from patients with the ryanodine receptor type 1 mutation. The aim of this study was to determine whether propofol could trigger abnormal calcium fluxes in human myotubes isolated from malignant hyperthermia susceptible patients harbouring the native ryanodine receptor type 1 mutation. Muscle specimens were obtained from the patients to diagnose malignant hyperthermia disposition and the calcium-induced calcium release test and molecular genetic analyses were performed. Using the calcium sensitive probe Fura 2, we determined the 340/380 nm wave-length ratios by measuring alterations in calcium homeostasis in isolated myotubes from cultured skeletal muscle specimens. Two patients, one with ryanodine receptor type 1 R2508C and one with the L4838V mutation had accelerated calcium-induced calcium release rates. The 340/380 nm ratios increased when the propofol concentration exceeded 100 microM. The half-maximal activation concentrations (EC50) for propofol from patients 1 and 2 were 181.1 and 420.5 microM, respectively. Increases in calcium concentrations in response to propofol dosage were limited to doses at least 100-fold greater than those used in clinical settings. These observations correlate well with clinical observations that propofol does not trigger malignant hyperthermia in susceptible humans.

Single channel properties of heterotetrameric mutant RyR1 ion channels linked to core myopathies

Skeletal muscle excitation-contraction coupling involves activation of homotetrameric ryanodine receptor ion channels (RyR1s), resulting in the rapid release of Ca(2+) from the sarcoplasmic reticulum. Previous work has shown that Ca(2+) release is impaired by mutations in RyR1 linked to Central Core Disease and Multiple Minicore Disease. We studied the consequences of these mutations on RyR1 function, following their expression in human embryonic kidney 293 cells and incorporation in lipid bilayers. RyR1-G4898E, -G4898R, and -DeltaV4926/I4927 mutants in the C-terminal pore region of RyR1 and N-terminal RyR1-R110W/L486V mutant all showed negligible Ca(2+) permeation and loss of Ca(2+)-dependent channel activity but maintained reduced K(+) conductances. Co-expression of wild type and mutant RyR1s resulted in Ca(2+)-dependent channel activities that exhibited intermediate Ca(2+) selectivities compared with K(+), which suggested the presence of tetrameric RyR1 complexes composed of wild type and mutant subunits. The number of wild-type subunits to maintain a functional heterotetrameric channel differed among the four RyR1 mutants. The results indicate that homozygous RyR1 mutations associated with core myopathies abolish or greatly reduce sarcoplasmic reticulum Ca(2+) release during excitation-contraction coupling. They further suggest that in individuals, expressing wild type and mutant alleles, a substantial portion of RyR1 channels is able to release Ca(2+) from sarcoplasmic reticulum.

Identification of ryanodine receptor 1 single-nucleotide polymorphisms by high-resolution melting using the LightCycler 480 System

High-resolution melting (HRM) allows single-nucleotide polymorphism (SNP) detection/typing using inexpensive generic heteroduplex-detecting double-stranded DNA (dsDNA) binding dyes. Until recently HRM has been a post-PCR process. With the LightCycler 480 System, however, the entire mutation screening process, including post-PCR analysis, can be performed using a single instrument. HRM assays were developed to allow screening of the ryanodine receptor gene (RYR1) for potential mutations causing malignant hyperthermia (MH) and/or central core disease (CCD) using the LightCycler 480 System. The assays were validated using engineered plasmids and/or genomic DNA samples that are either homozygous wild type or heterozygous for one of three SNPs that lead to the RyR1 amino acid substitutions T4826I, H4833Y, and/or R4861H. The HRM analyses were conducted using two different heteroduplex-detecting dsDNA binding dyes: LightCycler 480 HRM dye and LCGreen Plus. Heterozygous samples for each of the HRM assays were readily distinguished from homozygous samples with both dyes. By using engineered plasmids, it was shown that even homozygous sequence variations can be identified by using either small amplicons or the addition of exogenous DNA after PCR. Thus, the LightCycler 480 System provides a novel, integrated, real-time PCR/HRM platform that allows high throughput, inexpensive SNP detection, and genotyping based on high-resolution amplicon melting.

An Ryr1I4895T mutation abolishes Ca2+ release channel function and delays development in homozygous offspring of a mutant mouse line

A heterozygous Ile4898 to Thr (I4898T) mutation in the human type 1 ryanodine receptor/Ca(2+) release channel (RyR1) leads to a severe form of central core disease. We created a mouse line in which the corresponding Ryr1(I4895T) mutation was introduced by using a "knockin" protocol. The heterozygote does not exhibit an overt disease phenotype, but homozygous (IT/IT) mice are paralyzed and die perinatally, apparently because of asphyxia. Histological analysis shows that IT/IT mice have greatly reduced and amorphous skeletal muscle. Myotubes are small, nuclei remain central, myofibrils are disarranged, and no cross striation is obvious. Many areas indicate probable degeneration, with shortened myotubes containing central stacks of pyknotic nuclei. Other manifestations of a delay in completion of late stages of embryogenesis include growth retardation and marked delay in ossification, dermatogenesis, and cardiovascular development. Electron microscopy of IT/IT muscle demonstrates appropriate targeting and positioning of RyR1 at triad junctions and a normal organization of dihydropyridine receptor (DHPR) complexes into RyR1-associated tetrads. Functional studies carried out in cultured IT/IT myotubes show that ligand-induced and DHPR-activated RyR1 Ca(2+) release is absent, although retrograde enhancement of DHPR Ca(2+) conductance is retained. IT/IT mice, in which RyR1-mediated Ca(2+) release is abolished without altering the formation of the junctional DHPR-RyR1 macromolecular complex, provide a valuable model for elucidation of the role of RyR1-mediated Ca(2+) signaling in mammalian embryogenesis.

Enhanced excitation-coupled calcium entry in myotubes is associated with expression of RyR1 malignant hyperthermia mutations

Myotubes expressing wild type RyR1 (WT) or RyR1 with one of three malignant hyperthermia mutations R615C, R2163C, and T4826I (MH) were exposed sequentially to 60 mm KCl in Ca(2+)-replete and Ca(2+)-free external buffers (Ca+ and Ca-, respectively) with 3 min of rest between exposures. Although the maximal peak amplitude of the Ca(2+) transients during K(+) depolarization was similar for WT and MH in both external buffers, the rate of decay of the sustained phase of the transient during K(+) depolarization (decay rate) in Ca+ was 50% slower for MH. This difference was eliminated in Ca-, and the relative decay rates were faster for both genotypes than in Ca+. The integrated Ca(2+) transient in Ca-compared with Ca+ was reduced by 50-60% for MH and 20% for WT. The decay rate was not affected by [K(+)] x [Cl(-)] product or NiCl(2) (2 mm) supplementation of Ca-. The addition of La(2+) (0.1 mm), or SKF 96365 (20 microm) to Ca+ significantly accelerated decay rates for both WT and MH, but their effect was significantly greater in MH. Nifedipine (1 microm) had no effect, suggesting that the mechanism for this difference was not a reduction in L-type Ca(2+) channel Ca(2+) current. These data strongly suggest: 1) the decay rate in skeletal myotubes is related in part to Ca(2+) entry through the ECCE channel; 2) the MH mutations enhance ECCE compared with wild type; and 3) the increased Ca(2+) entry might play a significant role in the pathophysiology of MH.

Congenital myopathies

PURPOSE OF REVIEW

The aim of this review is to provide an up-to-date personal analysis of current congenital myopathy research.

RECENT FINDINGS

In the past year novel congenital myopathies have been suggested, genes have been discovered for some of the congenital myopathies for the first time (beta-tropomyosin in cap disease and perhaps skeletal muscle alpha-actin in Zebra body myopathy), further genes have been identified for congenital myopathies where other genes had already been found (cofilin in nemaline myopathy, selenoprotein N in congenital fibre type disproportion) and recessive myosin storage myopathy was associated with homozygous mutation of slow-skeletal/beta-cardiac myosin which was already known to be mutated in dominant myosin storage myopathy. There has been further clarification of the pathobiology of the congenital myopathies, including determination of the basis of epigenetic effects: silencing of the normal allele in recessive central core disease and persistence of cardiac (fetal) alpha-actin in nemaline myopathy patients with no skeletal actin.

SUMMARY

The increased understanding of the genes and pathobiology of the congenital myopathies that is developing should ultimately lead to effective treatments.

CK-MM and ACE genotypes and physiological prediction of the creatine kinase response to exercise

Exertional rhabdomyolysis (ERB) is a syndrome of severe skeletal muscle breakdown. Blood levels of creatine kinase (CK) are widely used as a marker to reflect muscle breakdown. Some individuals exhibit extreme increases in blood CK after exercise and have been characterized as high responders (HR), but no clinical definition of HR exists and reasons for the HR phenomenon are not understood. This study investigated possible associations between the magnitude of the CK response to exercise and polymorphisms of two genes: muscle-specific creatine kinase (CK-MM) NcoI and angiotensin-converting enzyme (ACE) I/D. An exercise test for defining HR was also investigated. Participants (n = 88) underwent an exercise test that included stepping up and down two stairs for 5 min followed by 15 squats while wearing a backpack weighted at 30% of their body weight. CK levels were measured before, immediately after, and 48 and 72 h after the test. Nine participants (10.2%) were defined as HR. Participants with the CK-MM NcoI AA genotype had a sixfold higher risk of being HR compared with GG and AG genotypes (P = 0.031). No significant differences were found for the ACE I/D polymorphism. Percent body fat was an independent predictor of being a HR. We conclude that the CK-MM AA genotype and percent body fat may be part of the constellation of mechanisms that explain susceptibility to ERB. A physiological test that may assist in predicting ERB is also presented.

Malignant hyperthermia

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

Diseases associated with altered ryanodine receptor activity

Mutations in two intracellular Ca2+ release channels or ryanodine receptors (RyR1 and RyR2) are associated with a number of human skeletal and cardiac diseases. This chapter discusses these diseases in terms of known mechanisms, controversies, and unanswered questions. We also compare the cardiac and skeletal muscle diseases to explore common mechanisms.

Identification of cysteines involved in S-nitrosylation, S-glutathionylation, and oxidation to disulfides in ryanodine receptor type 1

The skeletal muscle Ca(2+)-release channel (ryanodine receptor type 1 (RyR1)) is a redox sensor, susceptible to reversible S-nitrosylation, S-glutathionylation, and disulfide oxidation. So far, Cys-3635 remains the only cysteine residue identified as functionally relevant to the redox sensing properties of the channel. We demonstrate that expression of the C3635A-RyR1 mutant in RyR1-null myotubes alters the sensitivity of the ryanodine receptor to activation by voltage, indicating that Cys-3635 is involved in voltage-gated excitation-contraction coupling. However, H(2)O(2) treatment of C3635A-RyR1 channels or wild-type RyR1, following their expression in human embryonic kidney cells, enhances [(3)H]ryanodine binding to the same extent, suggesting that cysteines other than Cys-3635 are responsible for the oxidative enhancement of channel activity. Using a combination of Western blotting and sulfhydryl-directed fluorescent labeling, we found that two large regions of RyR1 (amino acids 1-2401 and 3120-4475), previously shown to be involved in disulfide bond formation, are also major sites of both S-nitrosylation and S-glutathionylation. Using selective isotopecoded affinity tag labeling of RyR1 and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy, we identified, out of the 100 cysteines in each RyR1 subunit, 9 that are endogenously modified (Cys-36, Cys-315, Cys-811, Cys-906, Cys-1591, Cys-2326, Cys-2363, Cys-3193, and Cys-3635) and another 3 residues that were only modified with exogenous redox agents (Cys-253, Cys-1040, and Cys-1303). We also identified the types of redox modification each of these cysteines can undergo. In summary, we have identified a discrete subset of cysteines that are likely to be involved in the functional response of RyR1 to different redox modifications (S-nitrosylation, S-glutathionylation, and oxidation to disulfides).