Rare Duplication in the RYR1 Gene Causing Malignant Hyperthermia and Clinical Variability

<b>Background/Objectives</b>: Variants in the <i>RYR1</i> gene are associated mainly with Malignant Hyperthermia. Missense variants are largely the most common, while insertions and duplications account for less than 10%. We aimed to investigate the effect of a rare duplication in the <i>RYR1</i> gene with the variability of the Malignant Hyperthermia susceptibility phenotype. <b>Methods</b>: We used exome variant screening, in vitro contracture test, anatomopathological examination of the muscle biopsy, RT-qPCR analysis for <i>RYR1</i> relative expression. <b>Results</b>: We identified a family with two affected siblings carrying an insertion of 18 pair bases in exon 91 of the <i>RYR1</i> gene, resulting in an in-frame duplication of 6 amino acids (c.12835_12852 dupGAGGGCGCGGCGGGGCTC: 162 p.G4279_T4284insAAGLEG). This variant was found at a frequency of 0.0007% in gnomAD and was absent in 1200 Brazilian controls. First classified as a Variant of Uncertain Significance (VUS), with the molecular and physiological data from our family, we were able to reclassify it, reaching 5 points, which is still a VUS but borderline likely pathogenic. Muscle relative mRNA expression of <i>RYR1</i> in the two patients identified a ~50% reduction, suggesting a possible hypomorphic allele. <b>Conclusions</b>: The pathomechanisms of <i>RYR1</i> gene variants in Malignant Hyperthermia are mainly associated with gain-of-function mechanisms, but small insertions can often lead to loss of function or improper folding protein. This study adds evidence to the possibility that duplication in this region can cause structural defects and a more severe phenotype in the patients.