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Characterisation of C-terminal RyR1 variants linked to neuromuscular disorders : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Genetics at Massey University, Manawatū, New Zealand
Intracellular calcium influences a large array of cellular processes in skeletal muscle cells and as a result the movement of free calcium is tightly-regulated by a diverse set of calcium channels and accessory proteins. The main store of calcium in skeletal muscle cells is the sarcoplasmic reticulum, from which the ryanodine receptor one calcium channel (RyR1) controls calcium release. Changes in calcium homeostasis often result in the manifestation of neuromuscular disorders, most notably central core disease (CCD) and malignant hyperthermia (MH). While CCD is usually apparent from the presence of certain physical characteristics, MH is typically asymptomatic unless exposed to a trigger, at which point the disease rapidly manifests as a crisis event which is potentially fatal. Currently, the diagnosis of these disorders requires the testing of a muscle biopsy, which is an expensive and invasive procedure, and thus a genetic test would be an ideal diagnostic alternative.
For the most part, CCD and MH cases are linked to the inappropriate release of calcium by defective RyR1 channels – located in the calcium storage organelle membrane – but both are complex disorders with variable penetrance and genetic heterogeneity. A hypoactive RyR1 is thought to cause CCD while a hyperactive RyR1 is thought to cause MH, and yet individuals have been observed to be carriers of both diseases. Most of these instances have been linked to variants in the C-terminal region of RyR1, corresponding to the transmembrane portion of the channel.
This research described in this thesis focused on the functional analysis of five C-terminal domain RyR1 variants identified in patients with neuromuscular disorders. The ability of the variant RyR1 channels to release calcium in response to a stimulus in a heterologous system was measured and compared with that of the wild type channel. Moreover, one of these variants was also examined in several B-lymphoblastoid cell lines taken from carriers of the variant. Of the five variants tested in the heterologous system, four different phenotypes were observed, reinforcing the theory that these disorders are caused by a variety of factors that combine to produce a complex phenotype.