| Identified Mutation |
ILE4898THR; In a large Mexican kindred in which all affected members through 4 generations suffered from an unusually severe, highly penetrant form of CCD (117000), Lynch et al. (1999) identified an ile4898-to-thr (I4898T) mutation in the C-terminal transmembrane region of the RYR1 protein. In 2 family members tested, malignant hyperthermia (145600) was also present. Lynch et al. (1999) noted that all previously reported RYR1 mutations had been located either in the cytoplasmic N-terminus or in a central cytoplasmic region of the protein. Introduction of the I4898T mutation into a rabbit RYR1 cDNA and expression in HEK293 cells resulted in abolition of response to the agonists halothane and caffeine. Coexpression of normal and mutant RYR1 cDNAs in a 1:1 ratio, however, produced RYR1 channels with normal halothane and caffeine sensitivities, but maximal levels of Ca(2+) release were reduced by 67%. Binding of [3H]ryanodine indicated that the heterozygous channel was activated by Ca(2+) concentrations 4-fold lower than normal. Single-cell analysis of cotransfected cells showed a significantly increased resting cytoplasmic Ca(2+) level and a significantly reduced luminal Ca(2+) level. These data indicated a leaky channel, possibly caused by a reduction in the Ca(2+) concentration required for channel activation. Comparison with 2 other coexpressed mutant/normal channels suggested that the I4898T mutation produces one of the most abnormal RYR1 channels that had been investigated, and this level of abnormality was reflected in the severe and penetrant phenotype of affected CCD individuals in the pedigree.
Avila et al. (2001) expressed the analogous rabbit mutation (I4897T) in skeletal myotubes derived from Ryr1-knockout mice. They found that homozygous expression of I4897T in myotubes resulted in a complete uncoupling of sarcolemmal excitation from voltage-gated sarcoplasmic reticulum (SR) calcium ion release without significantly altering resting cytosolic calcium ion levels, sarcoplasmic reticulum calcium ion content, or Ryr1-mediated enhancement of dihydropyridine receptor (DHPR) channel activity. Coexpression of both I4897T and wildtype Ryr1 resulted in a 60% reduction in voltage-gated SR calcium ion release, again without altering resting cytosolic calcium ion levels, SR calcium ion content, or DHPR channel activity. These findings indicated that muscle weakness suffered by individuals possessing the I4898T mutation involves a functional uncoupling of sarcolemmal excitation from SR calcium ion release, rather than the expression of overactive or leaky SR calcium ion release channels.
Tilgen et al. (2001) identified the I4898T mutation, resulting from a 14693T-C transition, in 3 of 25 unrelated individuals with CCD. The isoleucine residue is highly conserved and is located in the C-terminal hydrophobic membrane-spanning region of the protein. |