Elektrophysiologische Charakterisierung von KCNQ1-Mutationen bei Patienten mit dem Langen-QT-Syndrom

Patients with a Long QT syndrome (LQTS) have a disposition to ventricular tachyarrhythmia and sudden cardiac death. The LQTS is characterized by a prolongation of the QTc interval (corrected QT time according to Bazett formula) in the ECG. Mutations of the KCNQ1 gene are associated with the LQT1 syndrome. A loss of function in the KCNQ1 channel impairs the ventricular repolarization. Different potassium channels are responsible for repolarization: KCNH2 (IKr), KCNQ1 (IKs) and Kir2.x (IK1). The channel complex generating IKs (slowly activating current) is assembled by α- and β- subunits of KCNQ1 and KCNE1. In the present study five KCNQ1 mutations of patients with the LQTS were characterized functionally in Xenopus oocytes by TEVC (two-electrode- voltage-clamp). The KCNQ1 mutations c.556 561del, p.G269S, p.L273V and p.R539L were identified in a heterozygous manner and c.1892 1893insC in a homozygous manner. To study the effect of the KCNQ1 mutations, the mutated KCNQ1- and the cardiac IKs-channels (KCNQ1 and KCNE1) were examined and additionally in co-expression with KCNQ1 wild type (KCNQ1 WT). For the first time KCNQ1 mutations c.556 561del, p.L273V and p.R539L are reported in LQTS patients and are characterized functionally. The mutations p.G269S and c.1892 1893insC are already known and partly characterized, whereas p.G269S is measured for the first time in co-expression with KCNE1 and KCNQ1 wild type in Xenopus oocytes by TEVC and c.1892 1893insC is characterized electrophysiologically for the first time in Xenopus oocytes by TEVC. For the mutation c.1892 1893insC the autosomal recessive inheritance was clarified in my study. Mutations c.556 561del and p.G269S show a loss of function in the mutated KCNQ1- and IKs-channel and as well when co-expressed with KCNQ1 wild type. Mutation c.556 561del co-expressed with KCNQ1 wild type has a dominant negative effect. Furthermore there are two mutations p.L273V and p.R539L with a gain of function in the mutated KCNQ1 channel. Co- expression with KCNE1 turns them into loss of function mutations, explaining the association with a LQTS. Mutation p.L273V with KCNE1 presents a small, but significant current reduction and shows a strong shift of voltage dependance to more positive potentials as a special mechanism of loss of function. The patients carrying p.L273V, p.R539L and p.G269S are asymptomatic and have QTc-intervals between 0.44 and 0.49 sec. The heterozygous mutation carrier of c.556 561del is symptomatic (QTc 0.52 sec) and shows a noticeable current reduction in the electrophysiological measurements. The mutated KCNQ1 channel of c.1892 1893insC presents a loss of function. After co-assembly with KCNE1 there is still a strong loss of function, but co- expression with KCNE1 and wild type channels shows no signi cant current reduction. The homozygous mutation carrier of c.1892 1893insC presenting a QTc time of 0.56 sec is affected severely. According to my electrophysiological data the heterozygous family members are asymptomatic underlining the autosomal recessive inheritance. My study shows the importance of KCNE1 co-expression for drawing conclusions on clinical manifestation and inheritance. The electrophysiological characterization of KCNQ1 mutations can be used to predict the pathogenicity of mutations. However, for more accurate results I recommended the experiments to be performed in co-expression with KCNE1.