Na⁺ channel mutation leading to loss of function and non-progressive cardiac conduction defects

Background - We previously described a Dutch family in which congenital cardiac conduction disorder has clinically been identified. The ECG of the index patient showed a first-degree AV block associated with extensive ventricular conduction delay. Sequencing of the SCN5A locus coding for the human cardiac Na⁺ channel revealed a single nucleotide deletion at position 5280, resulting in a frame-shift in the sequence coding for the pore region of domain IV and a premature stop codon at the C-terminus. Methods and results - Wild type and mutant Na⁺ channel proteins were expressed in Xenopus laevis oocytes and in mammalian cells. Voltage clamp experiments demonstrated the presence of fast activating and inactivating inward currents in cells expressing the wild type channel alone or in combination with the β₁ subinut (SCN1B). In contrast, cells expressing the mutant channels did not show any activation of inward current with or without the β₁ subunit. Culturing transfected cells at 25°C did not restore the Na⁺ channel activity of the mutant protein. Transient expression of WT and mutant Na⁺ channels in the form of GFP fusion proteins in COS-7 cells indicated protein expression in the cytosol. But in contrast to WT channels were not associated with the plasma membrane. Conclusions - The SCN5A/5280delG mutation results in the translation into non-function channel proteins that do not reach the plasma membrane. This could explain the cardiac conduction defects in patients carrying the mutation.