N-Terminal LQT2 Nonsense Mutations Cause a Dominant-Negative Destabilization of WT hERG Subunits

Abstract

Familial mutations in hERG, the Human Ether-a-go-go Related Gene, disrupt cardiac IKr, leading to Type 2 Long QT Syndrome and potentially fatal ventricular arrhythmias. Of the more than 200 mutations identified to date, eight are nonsense mutations in the hERG 1a amino terminus (NT). How these mutations cause disease is unknown. We found that constructs corresponding to six of these LQT2-linked mutations encoded stable, truncated NT fragments when expressed in HEK cells. We coexpressed these fragments with WT hERG 1a and 1b subunits to mimic heterozygous expression in LQT2 patients and assessed their effects on WT hERG trafficking and expression levels. Western blots revealed that the polypeptides significantly decreased WT 1a and 1b protein expression levels. We hypothesized N-N terminal interactions may mediate the observed reduction and indeed found levels of N-deleted hERG 1a subunit (N-del) were unaffected by the LQT2 polypeptides. Surprisingly, the LQT2 fragments promoted 1a N-del maturation. This indicates the polypeptides are not inherently misfolded and can rescue 1a N-del trafficking, possibly by interacting with downstream regions of the subunit. Reduction of WT protein levels by LQT2 polypeptides therefore requires an intact N-terminus. To determine if the LQT2 polypeptides destabilize WT subunits, we blocked protein synthesis using cycloheximide and tracked WT protein levels over time. Co-expression with LQT2 polypeptides caused a rapid loss of both immature and mature hERG 1a and 1b compared with control indicating accelerated degradation in the ER. In summary, this study demonstrates a novel disease mechanism in which LQT2-linked nonsense mutations in the hERG 1a N-terminus act in a dominant-negative manner by destabilizing ER-resident WT subunits.Support: HL081780 (GAR) and AHA predoctoral fellowship and T32HL007936 (ECRR).