Amino acid substitution in the S1 or CH1-CH2 linker domain of TRPM4: Two newTRPM4variants found in complete heart block patients lead to gain of expression and gain of current

Abstract

Background: TRPM4 is a Ca2+-activated ion channel permeable to monovalent cations. Its expression in the heart has been confirmed, with most data supporting its presence in the conductive cardiac tissue. Numerous investigations have linked TRPM4 mutations to conduction disorders such as right bundle branch block and complete heart block. More than 25 different TRPM4 variants have been reported in patients with conduction abnormalities. Interestingly, both gain- and loss-of-function variants were shown to exhibit similar pathological phenotypes. Aims: To study the effect of two new TRPM4 variants found in patients suffering from complete heart block. Methods: We transfected HEK-293 cells either with wildtype (WT) TRPM4 or one of these new variants: p.Tyr790Cys or p.Leu1113Val. We analysed total and membrane protein expression 48 hours after transfection using biotinylation assay and western blotting. To assess TRPM4 function, electrophysiological recordings were performed in the inside-out patch-clamp configuration. Results: We found both variants (p.Tyr790Cys and p.Leu1113Val) to show a strong overall gain-of-expression, specifically at the plasma membrane. This correlated with an increased current in patch-clamp recordings. Conclusions: Two novel TRPM4 variants (p.Tyr790Cys and p.Leu1113Val) responsible for complete heart block in young patients show a significant gain-of-function secondary to enhanced cell membrane cellular expression.