Key result
SCN5A R878C mutation eliminates cardiac Na+ currents, driving clinical bradycardia and conduction slowing.
Why the study?
Does the R878C mutation in the SCN5A gene reduce Na+ current and cause cardiac conduction disease in affected individuals?
Does the R878C mutation in the SCN5A gene reduce Na+ current and cause cardiac conduction disease in affected individuals?
Answers are grounded in this paper and cite their sources.
The novel R878C mutation in SCN5A causes a complete loss of Na+ channel function, explaining the clinical phenotype of sick sinus syndrome and conduction slowing in affected patients.
AIM: We compared the clinical and physiological consequences of the novel mutation R878C in a highly conserved pore residue in domain II (S5-S6) of human, hNa(v)1.5, cardiac Na(+) channels. METHODS: Full clinical evaluation of pedigree members through three generations of a Chinese family combined with SCN5A sequencing from genomic DNA was compared with patch and voltage-clamp results from two independent expression systems. RESULTS: The four mutation carriers showed bradycardia, and slowed sino-atrial, atrioventricular and intraventricular conduction. Two also showed sick sinus syndrome; two had ST elevation in leads V1 and V2. Unlike WT-hNa(v)1.5, whole-cell patch-clamped HEK293 cells expressing R878C-hNa(v)1.5 showed no detectable Na(+) currents (i(Na)), even with substitution of a similarly charged lysine residue. Voltage-clamped Xenopus oocytes injected with either 0.04 or 1.5 microg microL(-1) R878C-hNa(v)1.5 cRNA similarly showed no i(Na), yet WT-hNa(v)1.5 cRNA diluted to 0.0004-0.0008 ng microL(-1)resulted in expression of detectable i(Na). i(Na) was simply determined by the amount of injected WT-hNa(v)1.5: doubling the dose of WT-hNa(v)1.5 cRNA doubled i(Na). i(Na) amplitudes and activation and inactivation characteristics were similar irrespective of whether WT-hNa(v)1.5 cRNA was given alone or combined with equal doses of R878C-hNa(v)1.5 cRNA therefore excluding dominant negative phenotypic effects. Na(+) channel function in HEK293 cells transfected with R878C-hNa(v)1.5 was not restored by exposure to mexiletine (200 microM) and lidocaine (100 microM). Fluorescence confocal microscopy using E3-Nav1.5 antibody demonstrated persistent membrane expression of both WT and R878C-hNa(v)1.5. Modelling studies confirmed that such i(Na) reductions reproduced the SSS phenotype. CONCLUSION: Clinical consequences of the novel R878C mutation correlate with results of physiological studies.
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Zhang et al. (2008) studied Cardiac conduction disease (Sick sinus syndrome) (n=217). R878C mutation in SCN5A vs. Wild-type SCN5A was evaluated on Na+ current (iNa) and ECG parameters. The R878C mutation in the cardiac Na+ channel resulted in a complete loss of detectable Na+ currents, correlating with clinical bradycardia and slowed conduction.
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