Comparison of the segregation of the RYR1 C1840T mutation with segregation of the caffeine/halothane contracture test results for malignant hyperthermia susceptibility in a large Manitoba Mennonite family.
Malignant hyperthermia (MH) is an important cause of anesthesia-induced death. Malignant hyperthermia susceptibility is diagnosed using the in vitro caffeine/halothane contracture test (CHCT) in fresh muscle biopsy specimens. The CHCT test is highly invasive, expensive, and lacks 100% specificity. Genetic and biochemical evidence provide strong support for the view that the substitution of cysteine for arginine 614 (Arg614Cys) in the human ryanodine receptor gene is one of several mutations that are likely to cause human MH. DNA testing was compared with CHCT as a means of predicting MH susceptibility in a large MH family in which the Arg614Cys mutation was detected.
A comparison of CHCT and DNA-based diagnosis was conducted in a large Manitoba Mennonite MH kindred identified by an index patient who died at age 45 yr of an MH crisis after general anesthesia. The presence of the Arg614Cys mutation was detected through a combination of polymerase chain reaction and restriction endonuclease digestion. Blood samples for DNA analysis were obtained from 68 family members, including 19 who had undergone muscle biopsies and 1 who had a documented crisis but did not undergo biopsy. Family members were classified as MH-susceptible or MH-normal on the basis of the CHCT.
Twenty-two persons were found to be heterozygous for the Arg614Cys mutation. Five of these persons had prior positive CHCT results and one had an MH crisis but did not undergo biopsy. On DNA testing, 44 persons were found to be homozygous for the normal allele. Of these, ten had been classified as MH-normal and five as MH-susceptible on the basis of the CHCT. On reevaluation of the data obtained in our earlier CHCT diagnoses, we found that the condition of the muscle was poor, with no twitch, for three of five individuals homozygous for the normal allele but originally classified as MH-susceptible and for one who was homozygous for the normal allele and originally classified as MH-normal. Caffeine/halothane contracture test results for these four persons were considered invalid. The twitch response was good for the two remaining persons who were homozygous for the normal allele but classified as MH- susceptible, because contracture was observed with appropriately low levels of both caffeine and halothane.
An absolute correlation between DNA test results and CHCT assignment could not be made in this kindred. Possible explanations for discordance are that the Arg614Cys mutation is not linked to MH, that a second MH mutation is segregating in the family, or that there are errors in the CHCT. Because there is strong evidence supporting the causal nature of the Arg614Cys mutation, the discordant persons are not closely related within the pedigree as they would be if a second MH mutation were segregating, and the CHCT is not 100% accurate, we propose that the observed discordance between DNA test results and CHCT assignment in this kindred results from two false-positive diagnoses by the CHCT.