Abstract P1089: Short-QT & Long-QT Associated Trpm4 Mutations

Cardiac arrhythmias are responsible for 200-300 thousand deaths/year. Despite considerable research effort, much remains to be elucidated concerning the underlying mechanisms of arrhythmogenesis. Mutations in transient receptor potential melastatin 4 (TRPM4), a widely expressed Ca 2+ -activated nonselective cation channel, have been associated with causing cardiac arrhythmias. However, direct genotype-phenotype correlation of arrhythmogenic TRPM4 mutant variants are often complicated, as this channel is not primary to the cardiac action potential. Here, we assessed the electrophysiological and post-transcriptional molecular properties of a single mutation (R892C)-TRPM4 associated with short QT syndrome (SQTS) and a triple mutation (R250C|A432T|G582S)-TRPM4 associated with long QT syndrome (LQTS), using stably transfected HEK293 cells. Overall, protein expression of the triple mutant was found to be significantly reduced, with increased proteasomal degradation, but enhanced SUMOylation, as compared to either WT or the single R892 mutant TRPM4 channel. Consequently, expression of R250C|A432T|G582S-TRPM4 was significantly lower at the cellular membrane than either R892C- and WT-TRPM4. In contrast, while total expression of TRPM4 was not significantly different between WT and the R892C single mutant, although the R892C exhibited increase aggregation. Patch-clamp cellular electrophysiology experiments indicated that both single and triple TRPM4 mutant channels could be activated by lower Ca 2+ concentrations compared to WT. However, R892C-TRPM4 channels inactivated faster, while R250C|A432T|G582S-TRPM4 channels inactivated much slower compared to WT. These data as obtained in our homologous recombinant overexpression system reveal that while the R892C-TRPM4 mutant variant exhibited normal-to-higher levels of expression and increased Ca 2+ -activation sensitivity, its tendency to aggregate combined with faster inactivation can result in overall loss-of-function compared to WT, correlative with SQTS. Conversely, while the R250C|A432T|G582S-TRPM4 mutant variant exhibited reduced expression and perturbed trafficking, its increased Ca 2+ -activation sensitivity and slow inactivation can result in overall gain-of-function compared to WT, correlative with LQTS.