title: Dysfunctional potassium channel subunit interaction as a novel mechanism of long QT syndrome.
abstract: BACKGROUND
  The slowly-activating delayed rectifier current IKs contributes to repolarization of the cardiac action potential, and is composed of a pore-forming α-subunit, KCNQ1, and a modulatory β-subunit, KCNE1. Mutations in either subunit can cause long QT syndrome, a potentially fatal arrhythmic disorder. How KCNE1 exerts its extensive control over the kinetics of IKs remains unresolved
  
  
  OBJECTIVE
  To evaluate the impact of a novel KCNQ1 mutation on IKs channel gating and kinetics
  
  
  METHODS
  KCNQ1 mutations were expressed in Xenopus oocytes in the presence and absence of KCNE1. Voltage clamping and MODELLER software were used to characterize and model channel function. Mutant and wt genes were cloned into FLAG, Myc and HA expression vectors to achieve differential epitope tagging, and expressed in HEK293 cells for immunohistochemical localization and surface biotinylation assay.
  
  
  RESULTS
  We identified 2 adjacent mutations, S338F and F339S, in the KCNQ1 S6 domain in unrelated probands. The novel KCNQ1 S338F mutation segregated with prolonged QT interval and torsade de pointes; the second variant, F339S, was associated with fetal bradycardia and prolonged QT interval, but no other clinical events. S338F channels expressed in Xenopus oocytes had slightly increased peak conductance relative to wild type, with a more positive activation voltage. F339S channels conducted minimal current. Unexpectedly, S338F currents were abolished by co-expression with intact WT KCNE1 or its C-terminus (aa63-129), despite normal membrane trafficking and surface co-localization of KCNQ1 S338F and wt KCNE1. Structural modeling indicated that the S338F mutation specifically alters the interaction between the S6 domain of one KCNQ1 subunit and the S4-S5 linker of another, inhibiting voltage-induced movement synergistically with KCNE1 binding.
  
  
  CONCLUSIONS
  A novel KCNQ1 mutation specifically impaired channel function in the presence of KCNE1. Our structural model shows that this mutation effectively immobilizes voltage gating by an inhibitory interaction that is additive with that of KCNE1. Our findings illuminate a previously unreported mechanism for LQTS, and validate recent theoretical models of the closed state of the KCNQ1:KCNE1 complex.
authors.affiliation: Department of Medicine, University of Miami Miller School of Medicine, Miami, Florida 33101, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Hoosien
authors.forename: Mary Ellen
authors.initials: ME
authors.lastname: Ahearn
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Myerburg
authors.forename: Thai V
authors.initials: TV
authors.lastname: Pham
authors.forename: Todd E
authors.initials: TE
authors.lastname: Miller
authors.forename: Marcel J
authors.initials: MJ
authors.lastname: Smets
authors.forename: Lisa
authors.initials: L
authors.lastname: Baumbach-Reardon
authors.forename: Ming-Lon
authors.initials: ML
authors.lastname: Young
authors.forename: Amjad
authors.initials: A
authors.lastname: Farooq
authors.forename: Nanette H
authors.initials: NH
authors.lastname: Bishopric
mesh_terms: D000818:Animals; D002648:Child; D004562:Electrocardiography; D006801:Humans; D007231:Infant, Newborn; D051657:KCNQ1 Potassium Channel / Q000235:genetics* / Q000378:metabolism; D008133:Long QT Syndrome / Q000235:genetics* / Q000378:metabolism; D008297:Male; D009154:Mutation; D010375:Pedigree; D024642:Potassium Channels, Voltage-Gated / Q000235:genetics* / Q000378:metabolism; D014981:Xenopus
chemical_list: C489622:KCNE1 protein, human; D051657:KCNQ1 Potassium Channel; D024642:Potassium Channels, Voltage-Gated