Intragenic Rescue of the Function of Long QT Syndrome-Causing hERG Mutant Channels

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The human-ether-a-go-go-related gene (hERG) encodes the pore forming subunit of the rapidly activating delayed rectifier K+ current (IKr) which is important for the repolarization phase of cardiac action potentials. A reduction in IKr due to a loss of hERG function can lead to long QT syndrome (LQTS). There are a variety of loss-of-function hERG mutations known to cause LQTS, the majority of which are thought to be trafficking deficient, including G601S. It has been shown that the plasma membrane (PM) expression of the G601S hERG mutant can be rescued by reduced temperature culture. We have shown that wild-type (WT) hERG channels require extracellular K+ for their stability in the PM. They undergo internalization under 0 mM K+ culture conditions, which can be prevented by reduced temperature culture. We also identified that the S624T mutation makes hERG insensitive to extracellular K+, thereby preventing hERG channel internalization under 0 mM K+ culture conditions. We hypothesize that certain hERG mutants, including G601S, may not be able to sense extracellular K+, making them unstable in the PM, leading to a loss of PM expression. In this study, we added a secondary mutation, S624T, which does not depend on extracellular K+ for its PM expression, to the G601S mutant hERG channel. The addition of S624T suppressed the loss-of-function G601S phenotype, and rescued its current to a level similar to WT channels. The addition of S624T also rescued LQTS-causing hERG mutants T474I and P596R. Our data reveals the most effective intragenic rescue of LQTS-causing hERG mutants to date and lends insight into the mechanisms through which these mutations confer the loss-of-function phenotype.