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Impaired ion channel function related to a common KCNQ1 mutation - implications for risk stratification in long QT syndrome 1.
Long QT syndrome (LQTS) 1 is the most common type of inherited LQTS and is linked to mutations in the KCNQ1 gene. We identified a KCNQ1 missense mutation, KCNQ1 G325R, in an asymptomatic patient presenting with significant QT prolongation (QTc, 448-600ms). Prior clinical reports revealed phenotypic variability ranging from the absence of symptoms to syncope among KCNQ1 G325R mutation carriers. The present study was designed to determine the G325R ion channel phenotype and its association with the clinical LQTS presentation. Electrophysiological testing was performed using the Xenopus oocyte expression system. KCNQ1 G325R channels were non-functional and suppressed wild type (WT) currents by 71.1%. In the presence of the native cardiac regulatory ß-subunit, KCNE1, currents conducted by G325R and WT KCNQ1 were reduced by 52.9%. Co-expression of G325R and WT KCNQ1 with KCNE1 shifted the voltage-dependence of I(Ks) activation by 12.0mV, indicating co-assembly of mutant and WT subunits. The dysfunctional biophysical phenotype validates the pathogenicity of the KCNQ1 G325R mutation and corresponds well with the severe clinical presentation revealed in some reports. However, the index patient and other mutation carriers were asymptomatic, highlighting potential limitations of risk assessment schemes based on ion channel data.
Identification of the KCNQ1 G325R mutation. (A) Pedigree of the LQTS1 family analyzed in this study (arrow indicates the index patient). Closed symbols denote patients exhibiting QT prolongation, open symbols indicate unaffected family members. Circles refer to women, squares indicate men. (B) Resting ECG of the index patient (III.4; QTc, 452 ms). (C) Schematic representation of a heteromeric IKs channel (left), comprising four KCNQ1 α‐subunits (gray) and the maximum occupancy of four KCNE1 β‐subunits (yellow) ( Bokil et al., 2010, Nakajo et al., 2010 and Wang et al., 2011). The membrane folding model (right) of a single KCNQ1 subunit indicates the location of the mutated amino acid residue, G325. Conservation of residue G325 is demonstrated by amino acid sequence alignments of pore helix, potassium selectivity filter and S5-/S6-linker for human (h) KCNQ family potassium channels KCNQ1-5, Caenorhabditis elegans (n) KCNQ homologs KQT1-3, and human voltage gated potassium (Kv) channels. The respective GenBank accession numbers are AF000571, NM_172107, NM_004519, NM_004700, NM_019842 for KCNQ1-5; NM_171709, NM_076991, NM_064474 for KQT1-3; and NM_004974 (hKv1.2; KCNA2), NM_002234 (hKv1.5; KCNA5), NM_004975 (hKv2.1; KCNB1), NM_004976 (hKv3.1; KCNC1), NM_012281 (hKv4.2; KCND2), NM_000238 (hERG; KCNH2), AF078741 (hEAG; KCNH1), respectively. Amino acids forming the channel pore are highlighted by gray color, and G325 and its homologs are boxed and highlighted in red.
K+ currents induced by expression of KCNQ1 α-subunits in Xenopus oocytes. Representative current traces recorded from cells expressing WT KCNQ1 (A) or KCNQ1 G325R (B) channels are shown. (C) Currents induced by co-expression of equal amounts of WT KCNQ1 and KCNQ1 G325R subunits. (D) Currents recorded from a non-injected control cell. (E) Mean peak tail current amplitudes (n = 24 cells per column; ***p < 0.001 compared to WT KCNQ1).
Analysis of IKs currents. IKs was generated by co-expression of WT KCNQ1 (A) or KCNQ1 G325R (B) in combination with KCNE1 cRNA. (C) IKs currents resulting from co-expression of KCNE1 with equal amounts of WT KCNQ1 and KCNQ1 G325R subunits. (D) Currents induced by expression of KCNE1 β-subunits. (E) Currents recorded from a non-injected control cell. (F) Mean peak tail current amplitudes (n = 36 cells per column; ***p < 0.001 compared to WT KCNQ1 + KCNE1).
I–V relationships of IKs tail currents. (A) Mean current amplitudes. (B) Current amplitudes normalized to maximum current values obtained after data fitting to the Boltzmann function. Mean values ± SEM are shown (n = 9).