Mechanisms underlying the phenotypic diversity in RYR1-associated Malignant Hyperthermia

Malignant hyperthermia (MH) is a potential fatal hereditary skeletal muscle disorder that occurs upon exposure to certain anaesthetic agents. Susceptibility is predominantly conferred by variants in the RYR1 gene encoding the type 1 ryanodine receptor (RYR1). A common feature observed in MH susceptible patients and in animal models, is a RYR1-leak dependent elevation in the intracellular Ca2+ concentration ([Ca2+]i) in the non-triggered state. However, it is not fully understood whether extracellular Ca2+ entry plays an important role in maintaining this elevated resting [Ca2+]i. It is also becoming increasingly evident that RYR1 variants can produce different MH phenotypes, for example the variant p.G2434R (c.7300G>A) is the major MH variant found globally, but has a weaker clinical phenotype relative to rarer variants such as p.R2454H (c.7361G>A). Live-cell calcium imaging was used to examine different aspects of the molecular mechanisms underlying RYR1-associated MH. The first was to use HEK293 cells to explore the caffeine sensitivity of novel potentially pathogenic RYR1 variants. Next skeletal muscle myotubes derived from MH patients with p.G2434R or p.R2454H, were found to have an enhanced sensitivity to caffeine, and a TRPC3/6 mediated increased sarcolemmal cationic influx relative to non-susceptible controls. There was no significant difference between the two MH associated RYR1 variants. Myotubes from a novel MH mouse model containing the equivalent variant to human p.G2434R, were also found to have an elevated sensitivity to RYR1 agonists. This model has further confirmed that the TRPC3/6 mediated enhanced cationic entry is conserved between the two species, and this entry can be reduced to control levels by blocking the RYR1-dependent leak. Taken together, the data presented in this thesis furthers our understanding of the molecular mechanisms that underlie the perturbed Ca2+ handling observed in MH tissue, and provides new avenues of research into MH and related RYR1 disorders as well as a potential target for novel therapies.