ABSTRACT Background Variants in KCNH2 , encoding the hERG channel which is responsible for the rapid component of the cardiac delayed rectifier K + current (I Kr ), are causal to Long QT Syndrome type 2 (LQTS2). We identified eight index patients with a new variant of unknown significance (VUS), KCNH2 :c.2717C&gt;T:p.(Ser906Leu). We aimed to elucidate the biophysiological effect of this variant, to enable reclassification and consequent clinical decision-making. Methods A genotype-phenotype overview of the patients and relatives was created. The biophysiological effects were assessed by manual whole-cell patch-clamp using HEK293a cells expressing: (I) wild type (WT) KCNH2 , (II) KCNH2 -p.S906L alone (homozygous, Hm) or (III) KCNH2 -p.S906L in combination with WT (1:1) (heterozygous, Hz). A calibrated automated patch-clamp assay using Flp-In HEK293 was used to follow up on the functional data. Results Incomplete penetrance of LQTS2 in KCNH2 :p.(Ser906Leu) carriers was observed. In addition, some patients were heterozygous for other VUSs in CACNA1C, PKP2, RYR2 , or AKAP9 . The phenotype of carriers of KCNH2 :p.(Ser906Leu) ranged from asymptomatic to life-threatening arrhythmic events. Manual patch-clamp showed a reduced current density by 69.8%, and 60.4% in KCNH2-p.S906L-Hm and KCNH2 -p.S906L-Hz, respectively. The time constant of activation was significantly increased with 80.1% in KCNH2 -p.S906L-Hm compared to KCNH2 -WT. Assessment of KCNH2-p.S906L-Hz, by calibrated automatic patch-clamp showed a reduction in current density by 35.6%. Conclusion The reduced current density in the KCNH2 -p.S906L-Hz indicates a moderate loss of function. Combined with the reduced penetrance and variable phenotype, we conclude that KCNH2 :p.(Ser906Leu) is a low penetrant likely pathogenic variant for LQTS2.

Reclassification of a likely pathogenic Dutch founder variant in KCNH2; implications of reduced penetrance