Functional phenotype variations of two novel K_V 7.1 mutations identified in patients with Long QT syndrome

Background: The slow delayed rectifier potassium current I_Ks is crucial for the repolarization of the cardiac action potential. It is conducted by the voltage-gated channel K_V 7.1 encoded by KCNQ1, together with its β-subunit KCNE1. Loss-of-function (LOF) mutations in KCNQ1 have been associated with heritable cardiac arrhythmias such as Long QT syndrome (LQTS). This disease is characterized by prolonged ventricular repolarization and propensity to ventricular tachyarrhythmia that may lead to syncope, cardiac arrest, and sudden death. We aimed to functionally characterize two K_V 7.1 mutations (p.A150T and p.L374H) identified in two independent LQTS patients with different severity of disease phenotype, family history, and co-segregation of LQTS.

Methods: We performed whole-cell patch clamp recordings in CHO-K1 cells, and confocal imaging in Madin-Darby Canine Kidney (MDCK) cells.

Results: I_Ks -A150T showed significantly decreased current amplitudes from above +20 mV (approximately 52% decrease at +40 mV), but demonstrated cell membrane localization similar to wild-type (WT). I_Ks -L374H, however, exhibited a complete LOF compared to WT channels. Confocal imaging showed endoplasmic reticulum retention of the channel in MDCK cells. Mimicking the heterozygous state of the patients by co-expressing WT and mutant subunits resulted in an approximately 22% decrease in current at +40 mV for A150T. The L374H mutation showed a more pronounced effect (62% reduction at +40 mV compared to WT channel).

Conclusion: Both mutations, K_V 7.1 A150T and L374H, led to loss of channel function. The degree of LOF may mirror the disease phenotype observed in the patients.