Abstract 16047: Atypical Brugada Syndrome Sodium Channel Mutations Require Interaction with Wild-Type Channels to Produce Disease Phenotype
Introduction: Brugada syndrome (BrS) is an inherited cardiac disorder that can be caused by mutations in the cardiac sodium channel gene. Most often, mutations result in nonfunctional sodium channels with reduction in sodium currents. However, several sodium channel mutations identified in BrS patients show miminal biophysical defects questioning how such ‘atypical’ mutants may produce a BrS phenotype. We hypothesized that interactions between atypical BrS sodium channel mutants and wild-type channels (WT) lead to a reduction in sodium currents hence explaining the disease phenotype.
Methods: Nine BrS mutations with minimal biophysical defects were co-expressed with either WT or WT channels containing a mutated calmodulin binding IQ motif (IQAA) in HEK293 cells. Biophysical properties of mutants were investigated by patch clamp. Cell surface biotinylation and co-immunoprecipitation were performed to assess channel location and interaction.
Results: Mutations co-expressed with WT (mimicking the heterozygous patient genotype) showed dramatic reductions in sodium current densities ranging from 50% to 75% compared to mutant or WT channels expressed alone. Cell surface biotinylation showed a significant reduction of both WT and mutant channels at the plasma membrane on co-expression while co-immunoprecipitation experiments demonstrated interactions between WT and mutant channels. Our data suggest interactions between WT and mutant channels produce an induced trafficking defect. In attempts to locate an interaction site we mutated the calmodulin binding IQ motif of the sodium channel found in the C-terminal. Interestingly, in co-immunoprecipitations IQAA failed to interact with atypical BrS mutants and when BrS mutants were co-expressed with this IQAA channel, current amplitudes were restored to control levels abrogating any loss-of-current phenotype.
Conclusions: Our data suggest that ‘atypical’ BrS mutations suppress the expression of WT channels at the cell surface via interactions mediated by the calmodulin-binding (IQ) domain. This induced trafficking defect represents a novel mechanism resolving the in vitro genotype-phenotype discordance seen in BrS sodium channel mutations with apparently minimal biophysical defects.
- © 2012 by American Heart Association, Inc.