Functional Characterization of an Immortalized Human Muscle Cell Line Carrying the Heterozygous RyR1 p.T4288_A4290dup Associated With Rhabdomyolysis

The ryanodine receptor 1 plays an essential role in skeletal muscle by releasing the calcium necessary for contraction from the sarcoplasmic reticulum and mutations in RYR1 , the gene encoding the ryanodine receptor 1 are associated with a variety of neuromuscular phenotypes, ranging from core myopathies and centronuclear myopathy to the pharmacogenetic disorder malignant hyperthermia. The muscle phenotype of the patient mainly depends on whether the mutations are dominant or recessive and on their location within the gene. Gain of function RYR1 mutations usually affect the biophysical properties of the calcium channel and are associated with malignant hyperthermia susceptibility trait with or without a myopathy. From a functional point of view, these mutations render the calcium channel oversensitive thereby facilitating its opening and allowing more calcium to be released from the sarcoplasmic reticulum. More recently exertional rhabdomyolysis has been observed in particularly athletic individuals who also carry gain of function RYR1 variants, but so far no functional studies have been performed investigating if and how such variants affect ryanodine receptor 1 calcium channels. In the present report we used an immortalized muscle cell line from an individual with exertional rhabdomyolysis carrying the heterozygous RyR1 duplication p.T4288_A4290dup in exon 91 and show that the presence of this variant is associated with morphometric changes in myotubes, enhanced myotube maturation and enhanced calcium release following membrane depolarization and pharmacological RyR1 activation. The enhanced calcium release would lead to more efficient muscle contraction but would also result in increased activation of SERCA pumps, more ATP consumption and increased enzymatic activity of the calcium dependent protease calpain 3. We hypothesize that these events are potentially the underlying (co)causes of exercise‐induced rhabdomyolysis in individuals carrying this RyR1 variant.