IDENTIFICATION OF HETEROZYGOUS AND HOMOZYGOUS INDIVIDUALS WITH THE NOVEL RYR1 MUTATION CYS35ARG IN A LARGE KINDRED

Background: Malignant hyperthermia (MH) is a potentially fatal, often autosomal dominant, disorder of skeletal muscle and is triggered in su sceptible people by all commonly used inhalational anesthetics. In thi s article, the authors describe a malignant hyperthermia susceptible ( MHS) kindred in which both parents of the proband are MHS and are firs t-degree cousins. Haplotype analysis in this kindred with chromosome 1 9 linked markers revealed that the proband and another sibling were ho mozygous for the affected RYR1 allele. Methods: Eighteen members of th is large pedigree were investigated, with a clinical examination for s igns of a myopathy, a caffeine halothane contracture test, a histo-enz ymologic study on the muscle biopsies, and linkage analysis on genomic DNA isolated from family blood samples. RYR1 cDNA was amplified by po lymerase chain reaction and was cloned and sequenced, facilitating mut ation detection. Results: Linkage analysis demonstrated linkage betwee n RYR1-linked markers and MH susceptibility in this family. DNA Sequen cing identified a T to C transition at nucleotide position 103, result ing in the substitution of an arginine for cysteine 35, representing t he most N-terminal mutation reported to date in the RYR1 gene, This mu tation segregates fully with the MHS trait, generating a lod score of 4.65 in favor of linkage to MHS at a recombination frequency of 0.0. C onclusions: The proband in this kindred is the first reported homozygo te to have presented with an MH episode. The homozygotes in this pedig ree do not have an overt myopathy. The sensitivity of muscle samples t o caffeine clearly distinguished the two homozygotes from other hetero zygous-susceptible individuals. No clear differentiation was observed with the halothane contracture results.