Malignant hyperthermia (MH) is an anesthetic-drug-induced, life-threatening hypermetabolic syndrome caused by abnormal calcium regulation in skeletal muscle. Often inherited as an autosomal dominant trait, MH has linkage to 30 different mutations in the RYR1 gene, which encodes a calcium-release-channel protein found in the sarcoplasmic reticulum membrane in skeletal muscle. All published RYR1 mutations exclusively represent single-nucleotide changes. The present report documents, in exon 44 of RYR1 in two unrelated, MH-susceptible families, a 3-bp deletion that results in deletion of a conserved glutamic acid at position 2347. This is the first deletion, in RYR1, found to be associated with MH susceptibility. MH susceptibility was confirmed among some family members by in vitro diagnostic pharmacological contracture testing of biopsied skeletal muscle. Although a single-amino-acid deletion appears to be a subtle change in the protein, the deletion of Glu2347 from RYR1 produces an unusually large electrically evoked contraction tension in MH-positive individuals, suggesting that this deletion produces an alteration in skeletal-muscle calcium regulation, even in the absence of pharmacological agents.
The molecular basis for the clinically distinct entity of deafness with Charcot-Marie-Tooth disease has not been established with certainty. The authors report deafness associated with a demyelinating neuropathy in three individuals of a family in whom a novel four-amino acid deletion in the PMP22 gene was identified. The data and review of literature suggest that in the PMP22 gene, some point mutations and small deletions in the transmembrane domain that are in close proximity to the extracellular component of the protein result in this clinically distinct entity.
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