PMC4789166_01
Presenting ECGs and identification of SCN5A p.R1309H variant. A, Presenting rhythm strip ECG showing wide complex tachycardia with varying QRS durations interspersed with sinus tachycardia. B, The ECG shows sinus bradycardia at 90 beats/min, first degree AV block (PR interval 220 ms), prolonged P wave duration (120 ms), ST elevation in lead V1 and RSr' pattern in lead V2. C, Family pedigree (arrow indicates proband). D, Schematic structure of NaV1.5 in which the positive charged mutant arginine in DIII/S4 is highlighted in red.
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Hospital of Eastern Ontario emergency room with an 8 hour history of lethargy, irritability and vomiting. His initial rhythm on electrocardiogram (ECG) was a non-sustained and sustained wide complex tachycardia at 225 beats/minute with varying QRS durations interspersed with some sinus tachycardia (Figure 1A). The first ECG obtained in sinus rhythm showed a heart rate of 90 min-1, first degree AV block (PR 220 ms), prolonged P wave duration (120 ms), ST elevation in lead V1, and RSr' pattern in V1 suggestive of a depolarization defect (Figure 1B). There is no evidence that it is related to sodium channelopathy (Figure 1C). Genetic testing for Brugada syndrome with a 7-gene GeneDx panel (GeneDx, Inc., Gaithersburg, MD) revealed a homozygous and previously unreported missense mutation in exon 22 of the SCN5A gene (c.3926 G>A; p.Arg1309His, Figure 1D), which was considered to be "likely disease-causing". Cascade screening was offered to both parents and the 6 living siblings (Figure 1C); the father declined genetic testing. Although neither parent nor any of the heterozygous siblings have experienced syncope or known arrhythmias, the presence of the p.R1309H variant in the heterozygous state correlated with an abnormal ECG in those individuals for whom ECGs were able to be obtained (see Supplemental Figure 1A-C). In one homozygous sibling for whom an ECG was available, we observed multiple abnormalities including sinus bradycardia, P wave prolongation, first degree AV block and QRS prolongation (Supplemental Figure 1D).
Kinetics of NaV1.5 channel activation. A, Exemplar current traces recorded using a depolarization protocol from -100 to +60 mV with 5 mV increment at holding potential of -120 mV. B, Summary current density. C, Exemplar current traces showing activation and inactivation phases of the NaV1.5 currents induced by a depolarization step at -25 mV. D. Exemplar peak currents of WT (left) and p.R1309H mutant channels showing reduction in current amplitude by 0.1 mM lidocaine. E, Voltage dependence of activation fitted with a Boltzmann function. F, Relationship between voltage and tau for activation.
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Subsequently, variable morphologic patterns were noted on the baseline 12 lead ECGs (Supplemental Figure 2A-C).
Inactivation kinetics. A, Relationship between voltage and tau for inactivation. B, Normalized currents induced by the second pulse were plotted as function of the first pulse and fitted by Boltzmann function. C and D, Effect of 0.1 mM lidocaine on the inactivation kinetics. Dashed lines indicate the data (in the absence of lidocaine) repeated from panel B. E, Exemplar traces from a standard two-pulse protocol testing recovery from inactivation with 7.5, 10, 20, 30 or 40 ms interval between P1 and P2. F, Summarized data for recovery from inactivation (WT, n=34; p.R1309H, n=37). G, Examples of transient peak (upper) and late (lower) Na+ currents induced by a 200 ms depolarization to -10 mV. H, Ratio of late current to transient peak current.
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Surveillance Holter recordings demonstrated brief episodes of wide complex rhythm as well as occasional episodes of definite atrial tachycardia with narrow complex QRS (Supplemental Figure 3A-B).
Voltage fluorometry analysis. A, Exemplar current traces recorded at -140 mV, -80 mV, -20 mV, and 40 mV. B, Voltage dependent activation and inactivation relationships. C, Exemplar fluorescence signals recorded simultaneously with current traces shown in A. Percentage of fluorescence change was calculated as DeltaF/F0. D, Voltage dependence of steady state fluorescence signal change (F-V). The amplitude of fluorescence signal was determined as the mean of the signal in a 5 ms period between starting and end of the depolarizing voltage pulse. E and F, Activation and deactivation time constants (tau) of fluorescent signal showing movement of the S4. Activation time constants were obtained by fitting fluorescence trace activation during channel activation in response to voltage pulse of -80 mV and deactivation time constants were calculated by fitting fluorescence trace deactivation at voltage pulse of 40 mV.
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Because of a documented sustained wide complex arrhythmia noted on Holter (Supplemental Figure 4) and the pending lack of availability of the original quinidine sulfate formulation, he was admitted for transition to a powdered formulation of quinidine sulfate from a new supplier.
Inward gating pore currents. A and B, Exemplar current traces and summarized data (WT, n=33; p.R1309H, n=38) of gating pore currents recorded with Na+-free TEA external solution (see ref.). C and D, Gating pore currents with Na+-free TEA external solution were blocked by 6 mM Ba2+ (WT, n=5; p.R1309H, n=6) (C) and reduced by 0.1 mM lidocaine (WT, n=5; p.R1309H, n=9) (D). E and F, Absence of outward gating pore currents recorded with a low Na+ external solution (see ref.) (WT, n=11; p.R1309H, n=10). G and H, Summary of inward gating pore currents recorded with low Na+ solution (WT, n=11; p.R1309H, n=10).
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Within 48 hours of stopping the quinidine, sustained atrial arrhythmias occurred (Supplemental Figure 5), followed by further episodes of sustained wide complex tachycardia.