COMPARISON OF THE SEGREGATION OF THE RYR1 C1840T MUTATION WITH SEGREGATION OF THE CAFFEINE HALOTHANE CONTRACTURE TEST-RESULTS FOR MALIGNANTHYPERTHERMIA SUSCEPTIBILITY IN A LARGE MANITOBA MENNONITE FAMILY

Background: Malignant hyperthermia (MH) is an important cause of anest hesia-induced death. Malignant hyperthermia susceptibility is diagnose d using the in vitro caffeine/halothane contracture test (CHCT) in fre sh muscle biopsy specimens. The CHCT test is highly invasive, expensiv e, and lacks 100% specificity. Genetic and biochemical evidence provid e strong support for the view that the substitution of cysteine for ar ginine 614 (Arg614Cys) in the human ryanodine receptor gene is one of several mutations that are likely to cause human MH. DNA testing was c ompared with CHCT as a means of predicting MH susceptibility in a larg e MH family in which the Arg614Cys mutation was detected, Methods: A c omparison of CHCT and DNA-based diagnosis was conducted in a large Man itoba Mennonite MH kindred identified by an index patient who died at age 45 yr of an MH crisis after general anesthesia. The presence of th e Arg614Cys mutation was detected through a combination of polymerase chain reaction and restriction endonuclease digestion. Blood samples f or DNA analysis were obtained from 68 family members, including 19 who had undergone muscle biopsies and 1 who had a documented crisis but d id not undergo biopsy. Family members were classified as MH-susceptibl e or MH-normal on the basis of the CHCT. Results: Twenty-two persons w ere 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 ho mozygous for the normal allele, Of these, ten had been classified as M H-normal and five as MH-susceptible on the basis of the CHCT. On reeva luation of the data obtained in our earlier CHCT diagnoses, we found t hat the condition of the muscle was poor, with no twitch, for three of five individuals homozygous for the normal allele but originally clas sified as MH-susceptible and for one who was homozygous for the normal allele and originally classified as MH-normal. Caffeine/halothane con tracture test results for these four persons were considered invalid, The twitch response was good for the two remaining persons who were ho mozygous for the normal allele but classified as MH-susceptible, becau se contracture was observed with appropriately low levels of both caff eine and halothane, Conclusions: An absolute correlation between DNA t est results and CHCT assignment could not be made in this kindred. Pos sible explanations for discordance are that the Arg614Cys mutation is not linked to MH, that a second MH mutation is segregating in the fami ly, or that there are errors in the CHCT. Because there is strong evid ence supporting the causal nature of the Arg614Cys mutation, the disco rdant persons are not closely related within the pedigree as they woul d 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-pos itive diagnoses by the CHCT.