In the last few years, a very active line of research took place after the first identification of SCN5A mutations associated with an inherited form of cardiac arrhythmias and sudden death, the LQT3 variant of the long QT syndrome. Subsequently, two allelic diseases additional to LQT3 were shown to be due to mutations in the same gene, the Brugada syndrome (BrS) and the Lev-Lenegre syndrome (progressive cardiac conduction defect). Genotype-phenotype correlation and in vitro expression studies provide evidence that structure-function relationships of the SCN5A protein are much more complex than initially anticipated. The biophysical characterization of the sodium channel defects associated with different phenotypes and the genotype-phenotype correlation studies brought to the attention of the scientific community a plethora of mechanisms by which even a single amino acid substitution may remarkably affect cardiac excitability. Finally, the evidence of patients harboring an SCN5A mutation and overlapping clinical presentations creates a need for a revision of the traditional classification of the above mentioned diseases. It is now appropriate to consider the "sodium channel syndrome" as a unique clinical entity that may manifest itself with a spectrum of possible phenotypes.
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