This study reports for the first time the critical role of the Na(v)1.5 N-terminal region in channel function and the dominant-negative effect of trafficking-defective channels occurring through α-subunit interaction.

Simultaneous ion conductance and confocal microscopy in live cells reveal a new form of asymmetric clathrin-coated pit closure.

Background Mutations in the SCN5A gene, encoding the α-subunit of the cardiac Na+ channel, Nav1.5, can result in several life-threatening arrhythmias. Objective To characterize a distal truncating SCN5A mutation, R1860Gfs*12, identified in a family with different phenotypes including sick sinus syndrome (SSS), atrial fibrillation (AF), atrial flutter and atrioventricular-block. Methods Patch-clamp and biochemical analysis were performed in HEK293 cells transfected with wild-type (WT) and/or mutant channels. Results The mutant channel expressed alone caused a 70% reduction in INa density compared to WT currents, consistent with its partial proteasomal degradation. It led also to a negative shift of steady-state inactivation and to a persistent current. When mimicking the heterozygous state of the patients by co-expressing WT and R1860Gfs*12 channels, the biophysical properties of INa were still altered, and the mutant channel α-subunits still interacted with the WT ones. Since the proband developed paroxysmal AF at young age, we screened 17 polymorphisms associated with AF risk in this family, and showed that the proband carries at-risk polymorphisms upstream of PITX2, a gene widely associated with AF development. In addition, when mimicking the difference in resting membrane potentials between cardiac atria and ventricles in HEK293 cells, or using computer-model simulation, R1860Gfs*12 induced a more drastic decrease in INa at the atrial potential. Conclusion We have identified a distal truncated SCN5A mutant associated with gain- and loss-of-function effects, leading to SSS and atrial arrhythmias. A constitutively higher susceptibility to arrhythmias of atrial tissues and genetic variability could explain the complex phenotype observed in this family.