Novel Arrhythmogenic Mechanism Revealed by a Long-QT Syndrome Mutation in the Cardiac Na ⁺ Channel

Abstract

Abstract-Variant 3 of the congenital long-QT syndrome (LQTS-3) is caused by mutations in the gene encoding the ␣ subunit of the cardiac Na ϩ channel. In the present study, we report a novel LQTS-3 mutation, E1295K (EK), and describe its functional consequences when expressed in HEK293 cells. The clinical phenotype of the proband indicated QT interval prolongation in the absence of T-wave morphological abnormalities and a steep QT/R-R relationship, consistent with an LQTS-3 lesion. However, biophysical analysis of mutant channels indicates that the EK mutation changes channel activity in a manner that is distinct from previously investigated LQTS-3 mutations. The EK mutation causes significant positive shifts in the half-maximal voltage (V 1/2 ) of steady-state inactivation and activation (ϩ5.2 and ϩ3.4 mV, respectively). These gating changes shift the window of voltages over which Na ϩ channels do not completely inactivate without altering the magnitude of these currents. The change in voltage dependence of window currents suggests that this alteration in the voltage dependence of Na ϩ channel gating may cause marked changes in action potential duration because of the unique voltage-dependent rectifying properties of cardiac K ϩ channels that underlie the plateau and terminal repolarization phases of the action potential. Na ϩ channel window current is likely to have a greater effect on net membrane current at more positive potentials (EK channels) where total K ϩ channel conductance is low than at more negative potentials (wild-type channels), where total K ϩ channel conductance is high. These findings suggest a fundamentally distinct mechanism of arrhythmogenesis for congenital LQTS-3. (Circ Res. 2001;88:740-745.)

Regulated and aberrant glycosylation modulate cardiac electrical signaling

“…Such a shift in steady-state activation and inactivation curves would lead to depolarization of the window current, defined as the voltage range at which a small percentage of channels are persistently active. Previous reports suggest that point mutations of two Na v isoforms that cause similar shifts in window current voltage range may be responsible for such maladies as epilepsy and LQTS (25,26). Such changes in window current will cause a shift in the voltages at which a persistent Na ϩ current exists relative to the threshold and resting membrane potentials, and this could increase susceptibility to altered excitability.…”

Phosphorylation of the A-kinase-anchoring Protein Yotiao Contributes to Protein Kinase A Regulation of a Heart Potassium Channel

“…CHO cells were transfected with channel subunits, Yotiao and CD8, using Lipofectamine and Plus reagent (Invitrogen) when cells reached 20 -30% confluence as described previously (12,20). Transfected cells were identified by Dynabeads M-450 anti-CD8 beads (Dynal, Oslo, Norway).…”

A Novel mutation L619F in the cardiac Na channel SCN5A associated with long-QT syndrome (LQT3): a role for the I-II linker in inactivation gating

“…However, the L619F mutation also causes an increase in Na + channel window current, which enhances Na + channel activity in a different voltage range. Whereas an increase in the magnitude of window current has been previously reported for LQT3 mutations (Abriel et al, 2001;Wedekind et al, 2001), a combination phenotype of increased sustained current and enhanced window current is new. Increased Na + channel activity underlies prolongation of the ventricular action potential, which is manifested as lengthening of the QT interval on the electrocardiogram of the mutation carriers.…”

“…In experiments recording I/V curves, external Na + was reduced to 30 mmol/L using n-methyl-glucamine as a Na + substitute. The use of these solutions in our experiments allowed us to make very stable recordings with minimal time-dependent changes in voltageparameters of activation and inactivation (Abriel et al, 2001). Currents elicited with the ramp protocol (ranging from -100 mV to +50 mV in 2s) were measured before and after application of 30 µM tetrodotoxin (TTX), as previously described (Abriel et al, 2001).…”

“…This leads to persistent I Na during the action potential (AP) plateau, which results in prolongation of the AP duration (APD), reflected as an increased QT-interval on the ECG. We have recently reported other arrhythmogenic mechanisms that may cause the LQT3 syndrome, suggesting that multiple distinct channel defects can result in the same clinical disease (Wehrens et al, 2000;Abriel et al, 2001). Here we report a novel LQT3 mutation, L619F (LF), in the SCN5A gene, leading to an amino acid substitution in the intracellular loop connecting domains I and II the cardiac Na + channel.…”