Correlations between clinical and physiological consequences of the novel mutation R878C in a highly conserved pore residue in the cardiac Na+ channel

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, hNav1.5, 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-hNav1.5, whole-cell patch-clamped HEK293 cells expressing R878C-hNav1.5 showed no detectable Na+ currents (iNa), even with substitution of a similarly charged lysine residue. Voltage-clamped Xenopus oocytes injected with either 0.04 or 1.5 μg μL-1 R878C-hNav1.5 cRNA similarly showed no iNa, yet WT-hNav1.5 cRNA diluted to 0.0004-0.0008 ng μL-1resulted in expression of detectable i Na. iNa was simply determined by the amount of injected WT-hNav1.5: doubling the dose of WT-hNav1.5 cRNA doubled iNa. iNa amplitudes and activation and inactivation characteristics were similar irrespective of whether WT-hNav1.5 cRNA was given alone or combined with equal doses of R878C-hNav1.5 cRNA therefore excluding dominant negative phenotypic effects. Na+ channel function in HEK293 cells transfected with R878C-hNav1.5 was not restored by exposure to mexiletine (200 μm) and lidocaine (100 μm). Fluorescence confocal microscopy using E3-Nav1.5 antibody demonstrated persistent membrane expression of both WT and R878C-hNav1.5. Modelling studies confirmed that such iNa reductions reproduced the SSS phenotype. Conclusion: Clinical consequences of the novel R878C mutation correlate with results of physiological studies. © 2008 The Authors.