Correlations between genotype and pharmacological, histological, functional, and clinical phenotypes in malignant hyperthermia susceptibility

Abstract

Malignant hyperthermia susceptibility (MHS) is a subclinical pharmacogenetic disorder caused by an impairment of skeletal muscle calcium homeostasis in response to triggering agents. While in vitro contracture testing (IVCT) is the gold standard for defining MHS, molecular analysis is increasingly used to diagnosis MHS. Mutations associated with MHS have been reported in two genes: RYR1 and CACNA1S. Mutations in RYR1 are also responsible for central core disease (CCD), a myopathy that can be associated with a positive IVCT response. We report here the results of correlation studies performed with molecular, pharmacological, histological, and functional data obtained in 175 families (referred to as confirmed (129) or potential (46) MHS families). Extensive molecular analysis allowed us to identify a variant in 60% of the confirmed MHS families, and resulted in the characterization of 11 new variants in the RYR1 gene. Most mutations clustered to MH1 and MH2 domains of RYR1. Functional analysis allowed us to assign a causative role for seven MHS mutations that we propose to add to the panel of MHS mutations used for genetic testing. The use of genetic data to determine MHS status led to a 99.5% sensitivity for IVCT. IVCT-positive/mutation-negative diagnoses were analyzed not only in terms of specificity for IVCT, but also to assess the presence of a second MHS trait in families, and the genetic heterogeneity of the disease. Histological analyses revealed the presence of cores in more than 20% of muscle biopsies originating from 242 genotyped and tested MHS patients who did not present with clinical symptoms. This indicates that these patients must be considered as MHS patients with cores, and are clearly differentiated from CCD patients who have been tested positive for MHS.

The Genetics of Malignant Hyperthermia

“…A false-negative contracture test may occur when the muscle is not viable, when it is altered by concomitant medications, or if the concentration of agonists in the contracture test bath was suboptimal. The sensitivity of the contracture test, based on genetic findings in these 196 patients, was 99.5% [13]. This is similar to the sensitivity of the contracture test defined by clinical cases.…”

“…Discordance between MH phenotype, defined by the muscle contracture test, and accepted MH-causative RYR1 genotypes has been noted [11,13]. Of 363 individuals in 58 well-characterized MHS families, there was diagnostic disagreement, discordance, between muscle contracture result and genotype in 10 subjects [13].…”

“…Of 363 individuals in 58 well-characterized MHS families, there was diagnostic disagreement, discordance, between muscle contracture result and genotype in 10 subjects [13]. The RYR1 variants in these families were all functionally tested mutations that could cause clinical MH.…”

“…It is very difficult to diagnose MHS on the basis of clinical signs and nonspecific laboratory tests. When the same portions of RYR1 are examined, in families who had positive muscle contracture tests, 60% of 129 had RYR1 variants, whereas in families who had NOT had contracture testing, only 20% of 46 had RYR1 variants, a strongly significant difference [13]. In other populations [4], the yield of positive results, if genetic testing were the initial test in a family, would be even lower.…”

Ryanodine receptor type 1 gene mutations found in the Canadian malignant hyperthermia population

“…10 The p.Val4847Leu that co-segregated well with an MHS phenotype within a large French-Canadian family, C-27, is positioned within the putative transmembrane segment and maps to the HS3b ''mutation hot spot'' region in RyR1. 13,38 At least three other closely spaced MH/CCD mutations are located within the same transmembrane segment 33,39,40 (Fig. 3).…”