Background
Sixteen genes encoding for cardiac ion channels and associated proteins have been linked to long-QT-syndrome (LQTS) in humans. So far, no pathogenic variant in the KCND3 gene coding for Kv4.3, the main component of the transient-outward potassium current (Ito), has been described in LQTS.
Purpose
This study presents a novel KCND3 variant in a patient with aborted sudden cardiac death with LQTS as the most likely clinical diagnosis and aimed to identify the underlying genetic and biophysical basis for arrhythmogenesis.
Methods
The index-patient and index-family were examined by electrocardiography, echocardiography and cardiac magnetic resonance imaging. Whole-genome sequencing identified a novel KCND3-C1469G variant, which was generated by in-vitro mutagenesis and heterologously expressed in Xenopus laevis oocytes and CHO cells for studies using dual-electrode voltage clamp, patch-clamp, western blot and immunofluorescence staining. Current measurements in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) of the index-patient were compared to a genotype-negative family member.
Results
QTc prolongation and T wave alterans led to the clinical diagnosis of LQTS in the index-patient. Brugada syndrome and cardiac structural abnormalities were ruled out. The KCND3-C1469G (rs1256867747) variant resulted in a serine-to-cysteine substitution at position 490 in the Kv4.3 C-terminus. Heterologous expression of Kv4.3-S490C revealed reduced Ito amplitude and a change in Kv4.3 channel inactivation kinetics. A comparison of protein expression levels between wildtype and mutant samples isolated from injected Xenopus laevis oocytes showed no significant difference. We propose the formation of a new disulfide bond as potential biophysical mechanism, as in-vitro mutagenesis of serine-457 partially restored current amplitudes. Reduced Ito amplitudes, identified as a specific electrophysiological alteration underlying LQTS, were confirmed in hiPSC-CM derived from the index-patient compared to a genotype-negative family member.
Conclusion
The novel KCND3-C1469G (rs1256867747) variant leads to reduced Ito amplitude and is linked to LQTS and aborted sudden cardiac death. Further studies are needed to identify the arrhythmogenic mechanism in more detail.
