Sudden death–associated KCNH2 variants exert opposing effects on a nuclear subdomain of the cardiac potassium channel hERG1

Human Ether-à-go-go-Related Gene 1 (hERG1) nuclear peptide (hERG1_NP) is a recently discovered nonconducting subdomain of the full-length hERG1 channel that is expressed in the nuclei of developing cardiomyocytes. From the nucleus, hERG1_NP modulates the gating and expression of the full-length hERG1 channel, introducing an unexplored regulatory mechanism of cell physiology. hERG1_NP variants are linked with sudden death in the young, but the impact of these variants on hERG1_NP activity is unknown. To determine the effect of hERG1_NP variants on hERG1_NP activity, we measured hERG1_NP intracellular localization and modulation of membrane current from the full-length hERG1a channel in human embryonic kidney 293 cells. Wildtype hERG1_NP suppressed both hERG1a current (I_hERG) and protein. We then screened six hERG1_NP variants for changes in either intracellular targeting or modulation of hERG1a current. Two variants, R885C and R1047L, disrupted hERG1_NP activity by altering nuclear transport or abolishing both I_hERG current suppression and nuclear targeting, respectively. Two additional variants, G1036D and Q1068R, enhanced hERG1_NP activity by depolarizing hERG1a's voltage dependence of activation via accelerated channel deactivation and recovery from inactivation. Last, two variants had no effect: R1035W and R1069S. This work demonstrates that hERG1_NP variants associated with sudden death in the young can trigger both loss-of-function and gain-of-function effects on hERG1_NP. Additional work is needed to identify specific pathogenic mechanisms of hERG1_NP variants, but these data demonstrate that hERG1_NP variants may represent a novel mechanism of disease.