Cardiac Na + Channel Dysfunction in Brugada Syndrome Is Aggravated by β 1 -Subunit

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Abstract
Background—Mutations in the gene encoding the human cardiac Na+ channel α-subunit (hH1) are responsible for chromosome 3–linked congenital long-QT syndrome (LQT3) and idiopathic ventricular fibrillation (IVF). An auxiliary β1-subunit, widely expressed in excitable tissues, shifts the voltage dependence of steady-state inactivation toward more negative potentials and restores normal gating kinetics of brain and skeletal muscle Na+ channels expressed in Xenopus oocytes but has little if any functional effect on the cardiac isoform. Here, we characterize the altered effects of a human β1-subunit (hβ1) on the heterologously expressed hH1 mutation (T1620M) previously associated with IVF. Methods and Results—When expressed alone in Xenopus oocytes, T1620M exhibited no persistent currents, in contrast to the LQT3 mutant channels, but the midpoint of steady-state inactivation (V1/2) was significantly shifted toward more positive potentials than for wild-type hH1. Coexpression of hβ1 did not significantly alter current decay or recovery from inactivation of wild-type hH1; however, it further shifted the V1/2 and accelerated the recovery from inactivation of T1620M. Oocyte macropatch analysis revealed that the activation kinetics of T1620M were normal. Conclusions—It is suggested that coexpression of hβ1 exposes a more severe functional defect that results in a greater overlap in the relationship between channel inactivation and activation (window current) in T1620M, which is proposed to be a potential pathophysiological mechanism of IVF in vivo. One possible explanation for our finding is an altered α-/β1-subunit association in the mutant.