Characterization and subcellular localization of KCNQ1 with a heterozygousmutation in the C terminus

Numerous mutations in KCNQ1, a gene encoding the alpha -subunit of cardiac delayed rectifier potassium channels. have been found in long QT syndrome ( LQTS). Among them, several mutations in the C terminus have been shown to c ause autosomal recessive or subclinical autosomal dominant LQTS, Here, we r eport a heterozygous mutation, T587M, which is also in the KCNQ1 C-terminal domain. The same mutation was found in three independent probands that wer e clearly symptomatic with family history of cardiac sudden death. Function al assay using a heterologous expression system with a mammalian cell line (COS7 cells) revealed that the mutant displayed neither functional channels when expressed alone nor dominant-negative effect when co-expressed with w ild-type (WT) KCNQ1, To examine the cellular trafficking of KCNQ1, green fl uorescent protein (GFP) was tagged to the cytoplasmic C terminus of WT or m utant KCNQ1, This procedure did not affect the essential properties of expr essed WT KCNQ1 channels. On confocal microscopic images, GFP-tagged WT KCNQ 1 showed a plasma membrane fluorescence pattern, whereas the GFP-tagged mut ant showed a perinuclear fluorescence pattern. Go-expression of the mutant with GFP-tagged WT KCNQ1 did not influence its normal cellular transport, T herefore, the T587M mutant cannot traffic to the plasma membrane and may fo rm no subunit assembly with WT KCNQ1. These findings provide a novel molecu lar basis for the clinical finding that this C-terminal mutation produced a severe form of RWS-type LQTS. (C) 2001 Academic Press.