α1-Syntrophin Variant Identified in Drug-Induced Long QT Syndrome Increases Late Sodium Current

Abstract

Drug-induced long-QT syndrome (diLQTS) is often due to drug block of IKr, especially in genetically susceptible patients with subclinical mutations in the IKr-encoding KCHN2. Few variants in the cardiac NaV1.5 Na+ channel complex have been associated with diLQTS. We tested whether a novel SNTA1 (α1-syntrophin) variant (p.E409Q) found in a patient with diLQTS increases late sodium current (INa-L), thereby providing a disease mechanism. Electrophysiological studies were performed in HEK293T cells co-expressing human NaV1.5/nNOS/PMCA4b with either wild type (WT) or SNTA1 variants (A390V-previously reported in congenital LQTS; and E409Q); and in adult rat ventricular cardiomyocytes infected with SNTA1 expressing adenoviruses (WT or one of the two SNTA1 variants). In HEK293T cells and in cardiomyocytes, there was no significant difference in the peak INa densities among the SNTA1 WT and variants. However, both variants increased INa-L (% of peak current) in HEK293T cells (0.58 ± 0.10 in WT vs. 0.90 ± 0.11 in A390V, p = 0.048; vs. 0.88 ± 0.07 in E409Q, p = 0.023). In cardiomyocytes, INa-L was significantly increased by E409Q, but not by A390V compared to WT (0.49 ± 0.14 in WT vs.0.94 ± 0.23 in A390V, p = 0.099; vs. 1.12 ± 0.24 in E409Q, p = 0.019). We demonstrated that a novel SNTA1 variant is likely causative for diLQTS by augmenting INa-L. These data suggest that variants within the NaV1.5-interacting α1-syntrophin are a potential mechanism for diLQTS, thereby expanding the concept that variants within congenital LQTS loci can cause diLQTS.

Fig 1. A drug-induced long-QT syndrome patient with the p.E409Q variant in SNTA1.

(A) A 12-lead resting electrocardiogram (ECG) from the proband during the post-arrest period showed markedly prolonged QT intervals. The QTc was 562 ms, and the notched T wave was noted. (B) The QTc interval was normalized and the T wave notch disappeared 2 months after aborted sudden cardiac death. (C) Pedigree of the family. An arrow is the proband with p.E409 variant. (D) Sequence conservation across species for A390V and E409Q versus normal in SNTA1. SCA, sudden cardiac arrest; and VF, ventricular fibrillation.

Fig 2. Electrophysiological data of Na_V1.5 in HEK293T cells coexpressing PMCA4b, nNOS, and either WT or SNTA1 mutants.

(A) Representative traces of inward Na⁺ current for the 3 groups tested. (B) I-V curve. (C) Activation (G/Gmax). (D) Inactivation (I/Imax). (E) Recovery (P2/P1).

Fig 3. Late Na⁺ current in HEK293T cells.

(A) Representative late Na⁺ currents with WT and the SNTA1 variants (Fig 3A). (B) Both A390V-SNTA1 and E409Q-SNTA1 significantly increased I_Na-L in HEK293T cells compared to that of WT-SNTA1. * P<0.05 versus WT-SNTA1. I_Na indicates sodium current; TTX, tetrodotoxin. Mean and standard error of mean (SEM) are shown in the graph.

Fig 4. Electrophysiological data of Na+ current in adult rat cardiomyocytes which were infected with either WT or SNTA1 mutants.

(A) Representative traces of inward Na⁺ current for the 3 groups tested. (B) I-V curve. (C) Activation (G/Gmax). (D) Inactivation (I/Imax). (E) Recovery (P2/P1).

Fig 5. Late Na⁺ current in adult rat cardiomyocyte infected with adenoviruses expressing either the WT or one of the two SNTA1 mutants.

(A) Representative late Na⁺ currents in with WT and the SNTA1 variants (Fig 5A). (B) E409Q-SNTA1 significantly increased I_Na-L compared to that of WT-SNTA1. * P<0.05 versus WT-SNTA1. I_Na indicates sodium current; TTX, tetrodotoxin. Mean and standard error of mean (SEM) are shown in the graph.

Fig 6. Syntrophin mutation and late sodium current.

A schematic diagram shows variants in SNTA1 lead to disrupted binding to PMCA4b and released inhibition of nNOS, resulting in increased I_Na-L through S-nitrosylation of Na_V1.5.