Functional analysis of RYR1 variants in patients with confirmed susceptibility to malignant hyperthermia.

BACKGROUND: A major bottleneck to the introduction of noninvasive presymptomatic diagnostic tests for the pharmacogenetic disorder malignant hyperthermia is the lack of functional data for associated variants. METHODS: We screened 50 genes having a potential role in skeletal muscle calcium homeostasis using the HaloPlex™ (Agilent Technologies, Santa Clara, CA, USA) target enrichment system and next-generation sequencing. Twenty-one patients with a history of a clinical malignant hyperthermia reaction together with a positive in vitro contracture test were included. Eight variants in RYR1 were subsequently introduced into the cDNA for the human ryanodine receptor gene and tested in cultured human embryonic kidney (HEK293) cells for their effect on calcium release from intracellular stores in response to the ryanodine receptor-1 agonist 4-chloro-m-cresol using fura-2 as calcium indicator. Each variant was subjected to in silico curation using the European Malignant Hyperthermia Group scoring matrix and ClinGen RYR1 variant curation expert panel guidelines. RESULTS: Potentially causative RYR1 variants were identified in 15 patients. Of these, two families carried two RYR1 variants, five variants had been previously reported as 'pathogenic', two variants had been previously reported as 'likely benign', and eight were of 'uncertain significance'. Of these eight variants, four showed hypersensitivity to 4-chloro-m-cresol. Three variants were reclassified as either 'pathogenic' or 'likely pathogenic'. Two were classified as 'benign', whilst three remained of 'uncertain significance'. CONCLUSIONS: Three (p.Tyr1711Cys, p.Val2280Ile, and p.Arg4737Gln) additional variants can be added to the list of RYR1 disease-associated variants managed by the European Malignant Hyperthermia Group. These can therefore be used diagnostically in the future. Three variants (p.Glu2348Gly, p.Asn2634Lys, and p.Arg3629Trp) that remained classified as of uncertain significance require further family studies or a different functional test to determine clinical relevance in malignant hyperthermia.

Investigating the genetic susceptibility to exertional heat illness.

BACKGROUND: We aimed to identify rare (minor allele frequency ≤1%), potentially pathogenic non-synonymous variants in a well-characterised cohort with a clinical history of exertional heat illness (EHI) or exertional rhabdomyolysis (ER). The genetic link between malignant hyperthermia (MH) and EHI was investigated due to their phenotypic overlap. METHODS: The coding regions of 38 genes relating to skeletal muscle calcium homeostasis or exercise intolerance were sequenced in 64 patients (mostly military personnel) with a history of EHI, or ER and who were phenotyped using skeletal muscle in vitro contracture tests. We assessed the pathogenicity of variants using prevalence data, in silico analysis, phenotype and segregation evidence and by review of the literature. RESULTS: We found 51 non-polymorphic, potentially pathogenic variants in 20 genes in 38 patients. Our data indicate that RYR1 p.T3711M (previously shown to be likely pathogenic for MH susceptibility) and RYR1 p.I3253T are likely pathogenic for EHI. PYGM p.A193S was found in 3 patients with EHI, which is significantly greater than the control prevalence (p=0.000025). We report the second case of EHI in which a missense variant at CACNA1S p.R498 has been found. Combinations of rare variants in the same or different genes are implicated in EHI. CONCLUSION: We confirm a role of RYR1 in the heritability of EHI as well as ER but highlight the likely genetic heterogeneity of these complex conditions. We propose defects, or combinations of defects, in skeletal muscle calcium homeostasis, oxidative metabolism and membrane excitability are associated with EHI.

The influence of different intensity of treatment on hormonal markers of gonadal function in acute lymphoblastic leukemia survivors.

Anti-cancer treatment in children can deteriorate gonadal function and affect future fertility. We analyzed the hormonal markers of gonadal function in adolescent leukemia survivors, treated in childhood with different levels of aggressiveness. We analyzed hormone levels in 69 adolescents and young adults, leukemia survivors stratified into standard (SR), intermediate (IR), and high (HR) risk groups, and in 80 healthy controls (38 men) at a similar age. We assessed follicular stimulating hormone (FSH), luteinizing hormone (LH), and inhibin B in the whole group, testosterone in males, and E2 and anti-Müllerian hormone (AMH) in females. Males classified into HR group presented, in comparison to control, higher levels of FSH, LH, lower inhibin B, and normal testosterone, whereas in SR and IR group, the hormonal values were comparable to the control. In females, in all risk groups, the levels of FSH, LH, E2, and inhibin B were comparable with the control, but the mean AMH levels were slightly lowered. We did not observe the effect of prophylactic cranial irradiation (12 or 18 Gy) or the time of treatment (before vs. during puberty) on hormone levels. In females, a positive correlation was found between the time interval after the end of treatment and AMH levels. Male leukemia survivors having undergone more intensive chemotherapy show the symptoms of disturbed spermatogenesis and need to be followed-up in the future. Women, irrespective of the risk group, can develop the signs of preterm ovarian insufficiency. They should be informed about the impact of the treatment on gonadal function.

Permeabilised skeletal muscle reveals mitochondrial deficiency in malignant hyperthermia-susceptible individuals.

BACKGROUND: Individuals genetically susceptible to malignant hyperthermia (MH) exhibit hypermetabolic reactions when exposed to volatile anaesthetics. Mitochondrial dysfunction has previously been associated with the MH-susceptible (MHS) phenotype in animal models, but evidence of this in human MH is limited. METHODS: We used high resolution respirometry to compare oxygen consumption rates (oxygen flux) between permeabilised human MHS and MH-negative (MHN) skeletal muscle fibres with or without prior exposure to halothane. A substrate-uncoupler-inhibitor titration protocol was used to measure the following components of the electron transport chain under conditions of oxidative phosphorylation (OXPHOS) or after uncoupling the electron transport system (ETS): complex I (CI), complex II (CII), CI+CII and, as a measure of mitochondrial mass, complex IV (CIV). RESULTS: Baseline comparisons without halothane exposure showed significantly increased mitochondrial mass (CIV, P=0.021) but lower flux control ratios in CI+CII(OXPHOS) and CII(ETS) of MHS mitochondria compared with MHN (P=0.033 and 0.005, respectively) showing that human MHS mitochondria have a functional deficiency. Exposure to halothane triggered a hypermetabolic response in MHS mitochondria, significantly increasing mass-specific oxygen flux in CI(OXPHOS), CI+CII(OXPHOS), CI+CII(ETS), and CII(ETS) (P=0.001-0.012), while the rates in MHN samples were unaltered by halothane exposure. CONCLUSIONS: We present evidence of mitochondrial dysfunction in human MHS skeletal muscle both at baseline and after halothane exposure.

Functional analysis of RYR1 variants linked to malignant hyperthermia.

Malignant hyperthermia manifests as a rapid and sustained rise in temperature in response to pharmacological triggering agents, e.g. inhalational anesthetics and the muscle relaxant suxamethonium. Other clinical signs include an increase in end-tidal CO2, increased O2 consumption, as well as tachycardia, and if untreated a malignant hyperthermia episode can result in death. The metabolic changes are caused by dysregulation of skeletal muscle Ca2+ homeostasis, resulting from a defective ryanodine receptor Ca2+ channel, which resides in the sarcoplasmic reticulum and controls the flux of Ca2+ ions from intracellular stores to the cytoplasm. Most genetic variants associated with susceptibility to malignant hyperthermia occur in the RYR1 gene encoding the ryanodine receptor type 1. While malignant hyperthermia susceptibility can be diagnosed by in vitro contracture testing of skeletal muscle biopsy tissue, it is advantageous to use DNA testing. Currently only 35 of over 400 potential variants in RYR1 have been classed as functionally causative of malignant hyperthermia and thus can be used for DNA diagnostic tests. Here we describe functional analysis of 2 RYR1 variants (c. 7042_7044delCAG, p.ΔGlu2348 and c.641C>T, p.Thr214Met) that occur in the same malignant hyperthermia susceptible family. The p.Glu2348 deletion, causes hypersensitivity to ryanodine receptor agonists using in vitro analysis of cloned human RYR1 cDNA expressed in HEK293T cells, while the Thr214Met substitution, does not appear to significantly alter sensitivity to agonist in the same system. We suggest that the c. 7042_7044delCAG, p.ΔGlu2348 RYR1 variant could be added to the list of diagnostic mutations for susceptibility to malignant hyperthermia.