In a murine model of calmodulinopathy, mutant N98S-CALM protein accumulated in an allele dosage-dependent manner, exceeding wild-type CALM levels and correlating with arrhythmia severity.
The study demonstrates that decreased degradation and subsequent enrichment of mutant CALM protein in the heart drives the severe electrophysiological phenotype in calmodulinopathy.
Heterozygosity for missense mutations in 1 of 3 seemingly redundant calmodulin-encoding (CALM-encoding) genes can cause life-threatening arrhythmias, suggesting that small fractions of mutant CALM protein suffice to cause a severe phenotype. However, the exact molar ratios of wild-type to mutant CALM protein in calmodulinopathy hearts remain unknown. The aim of the present study was to quantitate mutant versus wild-type CALM transcript and protein levels in hearts of knockin mice harboring the p.N98S mutation in the Calm1 gene. We found that the transcripts from the mutant Calm1 allele were the least abundantly expressed Calm transcripts in both hetero- and homozygous mutant hearts, while mutant hearts accumulated high levels of N98S-CALM protein in a Calm1N98S allele dosage-dependent manner, exceeding those of wild-type CALM protein. We further show that the severity of the electrophysiological phenotype incrementally increased with the graded increase in the mutant/wild-type CALM protein expression ratio seen in homozygous versus heterozygous mutant mice. We finally show a decrease in N98S-CALM protein degradation, suggesting that mutant CALM stabilization contributed to its enrichment in the heart. Our results support what we believe to be a novel mechanism by which a mutation in a single Calm gene can give rise to a severe phenotype.
Tsai et al. (Thu,) conducted a other in Calmodulinopathy (n=1,465). Calm1 p.N98S mutation vs. Wild-type (Calm1 +/+) was evaluated on Relative abundance of mutant over total CALM protein. In a murine model of calmodulinopathy, mutant N98S-CALM protein accumulated in an allele dosage-dependent manner, exceeding wild-type CALM levels and correlating with arrhythmia severity.