Novel LQT-3 Mutation Affects Na⁺ Channel Activity Through Interactions Between - and ß₁-Subunits

From the Department of Pharmacology, College of Physicians and Surgeons of Columbia University, New York, NY (R.H.A., X.L.W., R.S.K.), and the Department of Genetics, Hebrew University (B.K., M.G.), and Heiden Department of Cardiology, Bikur Cholim Hospital (J.B., A.M.), Jerusalem, Israel.

Correspondence to Robert S. Kass, PhD, Department of Pharmacology, College of Physicians and Surgeons of Columbia University, 630 W 168th St, New York, NY 10032.

Abstract—The congenital long-QT syndrome (LQT), an inherited cardiac arrhythmia characterized in part by prolonged ventricular repolarization, has been linked to 5 loci, 4 of which have been shown to harbor genes that encode ion channels. Previously studied LQT-3 mutations of SCN5A (or hH1), the gene that encodes the human Na⁺ channel -subunit, have been shown to encode voltage-gated Na⁺ channels that reopen during prolonged depolarization and hence directly contribute to the disease phenotype: delayed repolarization. Here, we report the functional consequences of a novel SCN5A mutation discovered in an extended LQT family. The mutation, a single AG base substitution at nucleotide 5519 of the SCN5A cDNA, is expected to cause a nonconservative change from an aspartate to a glycine at position 1790 (D1790G) of the SCN5A gene product. We investigated ion channel activity in human embryonic kidney (HEK 293) cells transiently transfected with wild-type (hH1) or mutant (D1790G) cDNA alone or in combination with cDNA encoding the human Na⁺ channel ß₁-subunit (hß₁) using whole-cell patch-clamp procedures. Heteromeric channels formed by coexpression of - and ß₁-subunits are affected: steady-state inactivation is shifted by –16 mV, but there is no D1790G-induced sustained inward current. This effect is independent of the ß₁-subunit isoform. We find no significant effect of D1790G on the biophysical properties of monomeric - (hH1) channels. We conclude that the effects of the novel LQT-3 mutation on inactivation of heteromeric channels are due to D1790G-induced changes in - and ß₁-interactions.

Key Words: long-QT syndrome • genetics • Na⁺ channel

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