A Novel SCN5A Mutation Associated with Drug Induced Brugada Type ECG

Abstract

Background: Class IC antiarrhythmic agents may induce acquired forms of Brugada Syndrome. We have identified a novel mutation in SCN5A, the gene that encodes the α-subunit of the human cardiac sodium channel (hNav1.5), in a patient who exhibited Brugada- type ECG changes during pharmacotherapy of atrial arrhythmias.

Objective: To assess whether the novel mutation p.V1328M can cause drug induced Brugada Syndrome.

Methods: Administration of pilsicainide, a class IC antiarrhythmic agent, caused Brugada- type ST elevation in a 66-year-old Japanese male who presented with paroxysmal atrial fibrillation (PAF), type I atrial flutter and inducible ventricular fibrillation (VF) during electrophysiological study. Genetic screening using direct sequencing identified a novel SCN5A variant, p.V1328M. Electrophysiological parameters of WT and p.V1328M and their effects on drug pharmacokinetics were studied using the patch-clamp method.

Results: Whole-cell sodium current densities were similar for WT and p.V1328M channels. While p.V1328M mutation did not affect the voltage-dependence of the activation kinetics, it caused a positive shift of voltage-dependent steady-state inactivation by 7 mV. The tonic block in the presence of pilsicainide was similar in WT and p.V1328M, when sodium currents were induced by a low frequency pulse protocol (q15s). On the contrary, p.V1328M mutation enhanced pilsicainide induced use-dependent block at 2 Hz. (Ki: WT, 35.8 μM; V1328M, 19.3 μM).

Conclusion: Our study suggests that a subclinical SCN5A mutation, p.V1328M, might predispose individuals harboring it to drug-induced Brugada Syndrome.

Fig 1. ECG phenotypes.

(A) Type I atrial flutter. (B) PAF.

Fig 2. ECG phenotypes on pilsicainide.

Left panels show baseline ECGs and the right panels show the ECGs after intravenous injection of pilsicainide (0.8 mg/kg).

Fig 3. Genetic analysis.

(A) Family tree. The filled square indicates the proband. (B) Abnormal migration pattern identified with DHPLC in the proband. (C) A nucleotide change (3982G>A) identified in the exon 23 of SCN5A (black arrow). (D) Schematic illustration of the SCN5A structure. The red closed circle with the arrowhead indicates the location of p.V1328M mutation.

Fig 4. Properties of control and mutant SCNA5 channels expressed in HEK-293 cells.

(A) Superimposed whole-cell current traces recorded in response to step changes in membrane voltage. 20 ms were applied in 5 mV increments from -100 to +75 mV from a holding potential (HP) of –140 mV. (B) I-V relationships. (C) Voltage-dependence of peak conductance and steady-state inactivation. Conductance G(V) was calculated by the equation: G(V) = I / (V_m—E_rev), where I is the peak currents, E_rev is the measured reversal potential, and V_m is the membrane potential. Normalized peak conductance was plotted against membrane potentials. Steady-state inactivation was measured using a protocol consisting of a 500 ms pre-pulse ranging from -120 to -20 mV in 5 mV increments followed by a 20 ms testing pulse to -20 mV; HP = -140 mV. Normalized peak currents at test pulses were plotted as a function of membrane potentials. Voltage-dependence of activation and inactivation were fitted with the Boltzmann equation. (D) Recovery from fast inactivation estimated by a double pulse protocol (shown in the inset). The currents at the second pulse were normalized to the currents at pre-pulse and plotted against intervals between the two pulses. The time course was fitted with a sum of two exponentials.

Fig 5. Tonic block of WT and V1328M by pilsicainide.

(A) Representative traces in WT and V1328M in the absence or presence of 50 μM pilsicainide. Currents were elicited every 15 seconds by 150 ms pulses to -20 mV from a holding potential of -120 mV. (B) Dose-response relationship of the tonic block. The data were fitted with the Hill equation: y = 1/[1 + (x/Ki)ⁿ], where y represents the fractional block; x is the concentration of quinidine; Ki is the half-maximal concentration of inhibition; and n is the Hill coefficient. Numbers in parentheses are the number of patches.

Fig 6. Use-dependent block of WT and V1328M by pilsicainide.

(A) Superimposed current traces in the absence or presence of 10 μM pilsicainide at pulse number 1, 50 and 100 (P1, P50 and P100). Currents were elicited every 0.5 second by 150 ms pulses to -20 mV from a holding potential of -120 mV. (B) Normalized currents were plotted against the pulse number. (C) Dose-response relationships of the use-dependent block. Fractional block was estimated at pulse #100 in the absence or presence of the drug. The data were fitted with the Hill equation.