NOVEL LQT-3 MUTATION AFFECTS NA-SUBUNIT AND BETA(1)-SUBUNIT( CHANNEL ACTIVITY THROUGH INTERACTIONS BETWEEN ALPHA)

The congenital long-QT syndrome (LQT), an inherited cardiac arrhythmia characterized in part by prolonged ventricular repolarization, has be en linked to 5 loci, 4 of which have been shown to harbor genes that e ncode ion channels. Previously studied LQT-3 mutations of SCN5A (or hH 1), the gene that encodes the human Na+ channel alpha-subunit, have be en shown to encode voltage-gated Na+ channels that reopen during prolo nged depolarization and hence directly contribute to the disease pheno type: delayed repolarization. Here, we report the functional consequen ces of a novel SCN5A mutation discovered in an extended LQT family, Th e mutation, a single A-->G base substitution at nucleotide 5519 of the SCN5A cDNA, is expected to cause a nonconservative change from an asp artate to a glycine at position 1790 (D1790G) of the SCN5A gene produc t. We investigated ion channel activity in human embryonic kidney (HEK 293) cells transiently transfected with wild-type (hH1) or mutant (D1 790G) cDNA alone or in combination with cDNA encoding the human Na+ ch annel beta(1)-subunit (h beta(1)) using whole-cell patch-clamp procedu res. Heteromeric channels formed by coexpression of alpha- and beta(1) -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 beta(1)-subunit isoform. We find no significant effect of D1790G on the biophysical properties of monomeric alpha- (h H1) channels. We conclude that the effects of the novel LQT-3 mutation on inactivation of heteromeric channels are due to D1790G-induced cha nges in alpha- and beta(1)-interactions.