A Novel KCNJ2 Nonsense Mutation, S369X, Impedes Trafficking and Causes a Limited Form of Andersen-Tawil Syndrome

Abstract

Background-Mutations in KCNJ2, a gene encoding the inward rectifier K ϩ channel Kir2.1, are associated with Andersen-Tawil syndrome (ATS), which is characterized by (1) ventricular tachyarrhythmias associated with QT (QU)-interval prolongation, (2) periodic paralysis, and (3) dysmorphic features. Methods and Results-We identified a novel KCNJ2 mutation, S369X, in a 13-year-old boy with prominent QU-interval prolongation and mild periodic paralysis. The mutation results in the truncation at the middle of the cytoplasmic C-terminal domain that eliminates the endoplasmic reticulum (ER)-to-Golgi export signal. Current recordings from Chinese hamster ovary cells transfected with KCNJ2-S369X exhibited significantly smaller K ϩ currents compared with KCNJ2 wild type (WT) (1 g each) (Ϫ84Ϯ14 versus Ϫ542Ϯ46 picoamperes per picofarad [pA/pF]; Ϫ140 mV; PϽ0.0001). Coexpression of the WT and S369X subunits did not show a dominant-negative suppression effect but yielded larger currents than those of WTϩS369X (Ϫ724Ϯ98 pA/pFϾϪ[84ϩ542] pA/pF; 1 g each; Ϫ140 mV). Confocal microscopy analysis showed that the fluorescent protein-tagged S369X subunits were predominantly retained in the ER when expressed alone; however, the expression of S369X subunits to the plasma membrane was partially restored when coexpressed with WT. Fluorescence resonance energy transfer analysis demonstrated direct proteinprotein interactions between WT and S369X subunits in the intracellular compartment. Conclusions-The S369X mutation causes a loss of the ER export motif. However, the trafficking deficiency can be partially rescued by directly assembling with the WT protein, resulting in a limited restoration of plasma membrane localization and channel function. This alleviation may explain why our patient presented with a relatively mild ATS phenotype. (Circ Cardiovasc Genet. 2011;4:253-260.)

Phenotype Variability in Patients Carrying KCNJ2 Mutations

“…32,33 More recently, we reported an S369X mutation located close to this ER export signal that impedes ER-Golgi transport. 7 We tested the trafficking function of four mutations, and only G144D mutation showed a trafficking defect ( Figure 5). Our results suggest that the phenotype expression variability of KCNJ2 mutations may be influenced by the topological location of mutations; however, the other possibilities, for example, environmental factors, modifier genes, or SNPs, 34 remain unstudied.…”

“…3 Since the first discovery of a mutation in this disease in 2001, 4 we have extensively examined KCNJ2 mutations in patients suspected of having ATS. [5][6][7] In 2007, we described both the clinical and genetic features of 23 patients (13 probands and 10 family members) and reported that the identification rate of KCNJ2 mutation in this cohort was 100% if the patients satisfied Ն2 features of ATS. 8 On a closer inspection, however, we noticed that Ϸ30% of the KCNJ2 mutation carriers lacked 2 of the ATS features: frequent PVC, bidirectional or polymorphic ventricular tachycardias (bVT or pVT), with QT or QU prolongation, without periodic paralysis or dysmorphic features.…”

Phenotypic Manifestations of Mutations in Genes Encoding Subunits of Cardiac Potassium Channels

“…133 More recently, a naturally occurring KCNJ2 mutation in the C terminus (S369X), located immediately upstream of this endoplasmic reticulum export signal, was shown to cause a limited form of Andersen-Tawil syndrome (LQT7) by impeding transportation from the endoplasmic reticulum to Golgi (step 5 in Figure 3). 134 Most KCNH2 mutations have been reported to reduce hERG currents by a trafficking-deficient mechanism (step 6 in Figure 3). 131 Several trafficking-refractory KCNQ1mutations are also known, of which T587M in the C-terminal region was the first reported.…”

Novel Insights into the Pathomechanisms of Skeletal Muscle Channelopathies

“…New mutations continue to be identified [25][26][27][28]. The recently described S369X mutation causes a loss of the endoplasmic reticulum export motif and impaired trafficking of the channel to the plasma membrane [28].…”