Ryanodine receptor gene (RYR1) mutations for diagnosing susceptibility to malignant hyperthermia

Sir, The report by Rueffert et al. (1) adds another piece to the puzzle of the complex genetics of malignant hyperthermia (MH). To date, more than 30 mutations have been reported in the major susceptibility gene, the skeletal muscle calcium channel RYR1, and linkage data in individual families have demonstrated the involvement of several other loci (for review see 2, 3).The present paper reports a mutation screen conducted in 56 German MH index patients and 265 relatives. All the index patients had good clinical presentations of MH (rank D5-6) (4), and strong in vitro contracture test (IVCT) results. Molecular genetic analysis of regions of the RYR1 gene detected 13 different mutations, four of which are listed in the European MH Group (EMHG) guidelines (5) and three of which were novel. Two of these novel mutations were detected in single families in whom only three individuals each had also been screened using the IVCT. One mutation was found in two families involving eight individuals. One objective of genetic studies in MH is the hope that a DNA-based test could eventually replace the invasive biopsy procedure required for the IVCT. Given the observed genetic heterogeneity, it is evident that no single genetic test will enable a quick and simple screen for MH before anesthesia. Nevertheless, a number of MH families can benefit from genetic testing under certain circumstances. Guidelines for genetic testing have been defined by the EMHG (5), and stipulate that for MH diagnoses based on genetic data (1), ‘only mutations with experimental evidence indicating that normal RYR1 function is altered can be regarded as causative for MH’ (2); concordance between IVCT results and genetic data within families must be observed. The combination of both criteria has been chosen because discordance between genetic and IVCT results is apparent and may be as high as 5% (6, 7). The reasons for these discrepancies are not well understood but ‘poor’ IVCT results are clearly not the only contributing factor. Rueffert et al. only selected index patients with IVCT muscle contracture threshold values significantly greater than those recommended by the EMHG protocol (8), and in so doing reduced the likelihood of detecting discordance between IVCT and mutation data. Indeed, no discordance was detected in their sample. For any novel RYR1 mutation, the evidence for genotype–phenotype concordance is initially based on a very small sample size, e.g. in the study by Rueffert et al., even for the most prevalent mutation Thr2206Met (not presently listed in the EMHG guidelines), the concordance data was based on 24 individuals only. On the basis of these findings, they applied genetic testing to family members in order to study the segregation of mutations. Although the authors did not offer a genetic MH diagnosis to relatives using the novel mutations characterized, they did not consider it necessary to biopsy individuals found to carry mutations Thr2206Met and Arg2454His because of the ‘perfect’ concordance observed between the genotype and the IVCT phenotype in their own sample. They further proposed to include these mutations in the guidelines for genetic testing, even without functional studies. We would like to add a note of caution to these conclusions and suggestions. From the clinical viewpoint, it may be safe to regard a mutation carrier as at risk for MH. Our concern, however, is with the relatives who do not carry the familial RYR1 mutation. Given the percentage of discordant results observed in the study of larger sample sizes, we think it prudent to request additional, independent (i.e. functional) evidence for the causative role of a mutation. We think it is premature to change the cautious approach to genetic testing for MH as currently outlined (5), which has been adopted in view of the observed level of genotype–phenotype discordance. The request of additional functional evidence is in the interest of the patients' safety. It is the policy of the EMHG to review and update annually the list of mutations suitable for diagnosis of MH as new scientific evidence comes to light. The mutations described in this paper will be thus reviewed. New causative mutations will be included in an updated list of diagnostic MH mutations (http://www.emhg.org). For a correct clinical diagnosis for potentially MH-susceptible subjects and for their safety during anesthesia, it is strongly recommended that diagnostic DNA testing for MH is carried out according to the EMHG guidelines. Address: C. MuellerGenetics Section of EMHG Wuerzberg Germany