Atrial-specific reduction of Kir2.1 channel pore diameter in addition to loss of inward-going rectification underlies inducible atrial fibrillation in a mouse model of short QT syndrome type 3

Introduction

Short QT Syndrome Type 3 (SQTS3) is an extremely rare arrhythmogenic disease caused by gain-of-function mutations in the KCNJ2 gene coding the inward rectifier potassium channel Kir2.1. We investigated arrhythmogenic mechanisms associated with a de-novo mutation (E299V) in Kir2.1 in an 11-year-old boy presenting an extremely abbreviated QT interval, paroxysmal atrial fibrillation, and mild left ventricular dysfunction. Amino acid E299 in the Kir2.1 sequence is necessary for polyamine binding induced inward rectification.

Purpose

Test the hypothesis that Kir2.1E299V induces reduced conductance and lack of rectification that causes electrical defects in atrial cardiomyocytes, predisposing patients to atrial arrhythmias.

Methods

We used intravenous adeno-associated virus-mediated gene transfer to generate mice expressing wild-type (WT) and the E299V mutant protein. We used ECG, intracardiac stimulation, patch-clamp, molecular biology and computational modelling to characterize the models and study arrhythmia mechanisms in the atria and ventricles.

Results

We confirmed WT or mutant Kir2.1 gene expression specifically in the mouse heart. On ECG, the corrected QT (QTc) interval of Kir2.1E299V mice was significantly shorter than Kir2.1WT mice (p<0.0001). The PR interval in Kir2.1E299V was also significantly shorter than WT mice (p<0.0001). On intracardiac stimulation, the largest proportion of arrhythmic events occurred in the atria, as 7 out of 9 Kir2.1E299V mice presented >1 second atrial flutter/fibrillation, while only 2 out of 10 Kir2.1WT mice showed this type of arrhythmia (p=0.023). On patch clamping, both atrial and ventricular cardiomyocytes expressing Kir2.1E299V had extremely abbreviated action potential durations (APD90) at all frequencies studied (p<0.0001). The current/voltage relation of ventricular Kir2.1E299V cardiomyocytes revealed an absence of inward-going rectification and increased IK1 at voltages positive to −80 mV compared to Kir2.1WT cardiomyocytes (p<0.0001). In contrast, while in the atrial Kir2.1E299V cardiomyocytes the outward IK1 was increased at voltages positive to −80 mV with loss of rectification, IK1 was significantly reduced at voltages negative to −80 mV (p<0.0001), suggesting a loss of function leading to atrial arrhythmia inducibility. A higher proportion of Kir2.2 at atrial level and atomic in-silico 3D simulations suggested that the mutation impaired polyamine block of the Kir2.1E299V-Kir2.2 channel while reducing the pore diameter.

Conclusions

This first in-vivo mouse model of cardiac-specific SQTS3 recapitulates the electrophysiological phenotype of a patient with the Kir2.1E299V mutation. The mutation results in a Kir2.1 gain-of-function mediated by and absence of rectification. The predominant arrhythmias induced in these SQTS3 mice were supraventricular likely due to the combined lack of inward rectification and atrial-specific reduced pore diameter of the Kir2.1E299V-Kir2.2 channel.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): La Caixa FoundationLa Maratό TV3 Foundation

Three dimensional modelling of mutant Kir2.1 channel PIP2 interactions help stratify arrhythmia severity in Andersen Tawil syndrome type 1

Background

Andersen-Tawil type 1 (ATS1) is associated with loss-of-function mutations in the inward rectifier potassium channel Kir2.1, which controls cardiac excitability and impulse conduction. Phosphatidylinositol-4,5-bisphosphate (PIP2) acts as an essential cofactor regulating the opening of Kir2.1 channels. Fifty percent of reported ATS1 mutations affect Kir2.1-PIP2 interactions, leading to ECG defects, ventricular arrhythmias and sudden cardiac death (SCD) by mechanisms that are poorly understood.

Purpose

To test the hypothesis that the degree of arrhythmogenic severity of ATS1 mutations disrupting PIP2-Kir2.1 binding may be predicted by the level of polarization of the mutant Kir2.1 channel pore.

Methods

We first used a statistical mean value approach to classify the 40 known arrhythmogenic ATS1 mutations impacting Kir2.1-PIP2 interaction (N=260 individuals) according to arrhythmogenic severity, ranging from SCD through ventricular bigeminy and QT prolongation. We then generated 3D in-silico atomic models of the wildtype channel and the 10 mutant channels with the most severe arrhythmic phenotype to assess the mechanism of the structural defects associated with Kir2.1-PIP2 disruption.

Results

Our cardiac lethality scoring stratifying Kir2.1 mutations according to arrhythmogenic severity was validated by three additional biostatical quantitative measures. On in-silico modelling, wildtype Kir2.1 channels without PIP2 binding had transmembrane and cytoplasmic pore radius of 1.5 and 3 Å, respectively. Kir2.1-PIP2 interactions increased transmembrane and cytoplasmic pore radius to 3 and 6 Å, respectively. All 10 Kir2.1 mutations had similar transmembrane and cytoplasmic pore radius of ∼1.0 and ∼3.0 Å, respectively. The most severe mutations yielded pore channels with highly polarized electrostatic forces. Remarkably, simulations showed a descending electrostatic pattern at the transmembrane region of PIP2 binding, where the more severe the mutation, the more positive that region was. Structural changes produced by mutations correlated with cardiac severity (R2=0.51; p<0.005) in that the most drastically altered protein structure correlated with the most severe arrhythmic phenotype.

Conclusions

Computer simulations of mutant Kir2.1 channel structure from the most arrhythmogenic to the least arrhythmogenic predict a gradual decrease in polarization of electrostatic forces along the Kir2.1 channel pore. The results reveal a novel mechanistic stratification of arrhythmogenic severity of ATS1 mutant Kir2.1 channel-PIP2 interactions and open new pathways for developing more personalized ATS1 patient therapies.

Funding Acknowledgement

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): La Caixa Banking Foundation under the project code HR18-00304Fundaciόn La Marato TV3: Ayudas a la investigaciόn en enfermedades raras 2020