Ryanodine receptor type 1 gene variants in the malignant hyperthermia-susceptible population of the United States

Abstract

Background: Mutations in the ryanodine receptor type 1 gene (RYR1) that encodes the skeletal muscle-specific intracellular calcium (Ca(2+)) release channel are a cause of malignant hyperthermia (MH). In this study, we examined RYR1 mutations in a large number of North American MH-susceptible (MHS) subjects without prior genetic diagnosis.

Methods: RYR1 was examined in 120 unrelated MHS subjects from the United States in a tiered manner. The α-1 subunit of the dihydropyridine receptor gene (CACNA1S) was screened for 4 variants in subjects in whom no abnormality was found in ≥ 100 exons of RYR1.

Results: Ten known causative MH mutations were found in 26 subjects. Variants of uncertain significance in RYR1 were found in 36 subjects, 16 of which are novel. Novel variants in both RYR1 and CACNA1S were found in the 1 subject who died of MH. Two RYR1 variants were found in 4 subjects. Variants of uncertain significance were found outside and inside the hotspots of RYR1. Maximal contractures in the caffeine-halothane contracture test were greater in those who had a known MH mutation or variant of uncertain significance in RYR1 than in those who did not.

Conclusions: The identification of novel RYR1 variants and previously observed RYR1 variants of uncertain significance in independent MHS families is necessary for demonstrating the significance of these variants for MH susceptibility and supports the need for functional studies of these variants. Continued reporting of the clinical phenotypes of MH is necessary for interpretation of genetic findings, especially because the pathogenicity of most of these genetic variants associated with MHS remains to be elucidated.

Figure 1

This flow chart illustrates the outcomes of DNA sequencing for subjects with and without prior caffeine-halothane contracture test (CHCT). A variant of uncertain significance (VUS) is a variant that has been previously reported, but it is not yet proven to be malignant hyperthermia (MH)-causative. A novel variant is a variant that has not been previously reported, and it has not been proven to be MH-causative. A polymorphism is a change in the amino acid sequence of the gene that has been found at least once in 100 normal subjects and is not expected to be pathogenic. The numbers in parentheses are the numbers of subjects found to have this type of change in RYR1. Two novel variants in the ryanodine receptor type one gene (RYR1) were found in one subject. A novel variant and a VUS in RYR1 were found in one subject. Two different VUS were found in 2 subjects. A VUS and a polymorphism were found in one subject. A novel variant and a polymorphism in RYR1 were found in another subject. One subject had 2 polymorphisms in RYR1. In summary, there were 7 subjects who had 2 different variants when polymorphisms in RYR1 are included in this count. Thus, the sum of the numbers in the sub-boxes is 7 more than the number of subjects in this study reported to have mutations, VUS, novel variants and/or polymorphisms. (see Table 4 and the Web Supplement)