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.

Dysferlin, annexin A1, and mitsugumin 53 are upregulated in muscular dystrophy and localize to longitudinal tubules of the T-system with stretch.

Mutations in dysferlin cause an inherited muscular dystrophy because of defective membrane repair. Three interacting partners of dysferlin are also implicated in membrane resealing: caveolin-3 (in limb girdle muscular dystrophy type 1C), annexin A1, and the newly identified protein mitsugumin 53 (MG53). Mitsugumin 53 accumulates at sites of membrane damage, and MG53-knockout mice display a progressive muscular dystrophy. This study explored the expression and localization of MG53 in human skeletal muscle, how membrane repair proteins are modulated in various forms of muscular dystrophy, and whether MG53 is a primary cause of human muscle disease. Mitsugumin 53 showed variable sarcolemmal and/or cytoplasmic immunolabeling in control human muscle and elevated levels in dystrophic patients. No pathogenic MG53 mutations were identified in 50 muscular dystrophy patients, suggesting that MG53 is unlikely to be a common cause of muscular dystrophy in Australia. Western blot analysis confirmed upregulation of MG53, as well as of dysferlin, annexin A1, and caveolin-3 to different degrees, in different muscular dystrophies. Importantly, MG53, annexin A1, and dysferlin localize to the t-tubule network and show enriched labeling at longitudinal tubules of the t-system in overstretch. Our results suggest that longitudinal tubules of the t-system may represent sites of physiological membrane damage targeted by this membrane repair complex.

Myocardial membrane injury in pediatric cardiac surgery: An animal model.

OBJECTIVE: Reduced myocardial performance invariably follows pediatric cardiac surgery and is manifested by a low cardiac output state in its severest form. The role of myocardial membrane proteins in this setting is unknown. Dystrophin and dysferlin are involved in membrane integrity, whereas aquaporins selectively transport water. These proteins were examined in a model of pediatric cardiac surgery, together with a trial of poloxamer 188, which may reduce membrane injury. METHODS: Eight lambs were randomized to saline with or without poloxamer 188. Lambs underwent 2 hours of cardiopulmonary bypass and aortic crossclamping. After a further 9 hours of monitoring, the hearts were assessed for water content, capillary leak, and protein expression. RESULTS: Dystrophin expression was unaffected by ischemia/reperfusion, but dysferlin expression was reduced. Aquaporin 1 protein increased after ischemia/reperfusion. Poloxamer 188 administration was associated with supranormal levels of dystrophin, preservation of dysferlin expression, and normalization of aquaporin 1 expression. Poloxamer 188 was associated with less capillary leak, maintained colloid osmotic pressure, and less hemodilution. Poloxamer 188 was associated with an improved hemodynamic profile (higher blood pressure, higher venous saturation, and lower lactate), although the heart rate tended to be higher. CONCLUSIONS: Changes in protein expression within the myocardial membrane were found in a clinically relevant model of pediatric cardiac surgery. Indicators of reduced performance, such as lower blood pressure and lower oxygen delivery, were lessened in association with the administration of the membrane protecting poloxamer 188. Poloxamer 188 was also associated with potentially beneficial changes in membrane protein expression, reduced capillary leakage, and less hemodilution.

Myocardial ischemia is more important than the effects of cardiopulmonary bypass on myocardial water handling and postoperative dysfunction: a pediatric animal model.

OBJECTIVES: Low cardiac output state is the principal cause of morbidity after surgical intervention for congenital heart disease. Myocardial ischemia-reperfusion injury, apoptosis, capillary leak syndrome, and myocardial edema are associated factors. We established a clinically relevant model to examine relationships between myocardial ischemia, edema, and cardiac dysfunction and to assess the role of the water transport proteins aquaporins. METHODS: Sixteen lambs were studied. Seven were control animals not undergoing cardiopulmonary bypass, and 9 underwent bypass. Six had 90 minutes of aortic crossclamping with blood cardioplegia and moderate hypothermia. The remaining 3 underwent cardiopulmonary bypass without aortic crossclamping. Hemodynamic and biochemical data were recorded, and myocardial edema, apoptotic markers, and aquaporin expression were determined after death. RESULTS: The group undergoing cardiopulmonary bypass with aortic crossclamping had a low cardiac output state, with early postoperative tachycardia, hypotension, increased serum lactate levels, and impaired tissue oxygen delivery (P < .05) compared with the group undergoing cardiopulmonary bypass without aortic crossclamping. The lambs undergoing cardiopulmonary bypass with aortic crossclamping had increased myocardial water (P < .05) compared with those not undergoing cardiopulmonary bypass and a 2-fold increase in aquaporin 1 mRNA expression (P < .05) compared with those not undergoing cardiopulmonary bypass and those undergoing cardiopulmonary bypass without aortic crossclamping. CONCLUSIONS: A temporal association between hemodynamic dysfunction, myocardial edema, and increased aquaporin 1 expression was demonstrated. Cardiopulmonary bypass without ischemia was associated with minimal edema, negligible myocardial dysfunction, and static aquaporin expression. Ischemic reperfusion injury is the main cause of myocardial edema and myocardial dysfunction, but a causal relationship between edema and dysfunction remains to be proved.