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(Circulation Research. 2008;103:e81.)
© 2008 American Heart Association, Inc.

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Physiological Properties of hERG 1a/1b Heteromeric Currents and a hERG 1b-Specific Mutation Associated With Long-QT Syndrome

Harinath Sale^*, Jinling Wang^*, Thomas J. O'Hara^*, David J. Tester, Pallavi Phartiyal, Jia-Qiang He, Yoram Rudy, Michael J. Ackerman, Gail A. Robertson

From the Department of Physiology (H.S., J.W., P.P., J.-Q.H., G.A.R.), University of Wisconsin, Madison; Department of Biomedical Engineering (T.J.O., Y.R.), Washington University, St Louis, Mo; and Departments of Medicine, Pediatrics, and Pharmacology (D.J.T., M.J.A.), Mayo Clinic, Rochester, Minn.

Correspondence to Dr Gail A. Robertson, Department of Physiology Satellite Laboratories, 601 Science Dr, Madison, WI 53711. E-mail Robertson{at}physiology.wisc.edu

Cardiac I_Kr is a critical repolarizing current in the heart and a target for inherited and acquired long-QT syndrome (LQTS). Biochemical and functional studies have demonstrated that I_Kr channels are heteromers composed of both hERG 1a and 1b subunits, yet our current understanding of I_Kr functional properties derives primarily from studies of homooligomers of the original hERG 1a isolate. Here, we examine currents produced by hERG 1a and 1a/1b channels expressed in HEK-293 cells at near-physiological temperatures. We find that heteromeric hERG 1a/1b currents are much larger than hERG 1a currents and conduct 80% more charge during an action potential. This surprising difference corresponds to a 2-fold increase in the apparent rates of activation and recovery from inactivation, thus reducing rectification and facilitating current rebound during repolarization. Kinetic modeling shows these gating differences account quantitatively for the differences in current amplitude between the 2 channel types. Drug sensitivity was also different. Compared to homomeric 1a channels, heteromeric 1a/1b channels were inhibited by E-4031 with a slower time course and a corresponding 4-fold shift in the IC₅₀. The importance of hERG 1b in vivo is supported by the identification of a 1b-specific A8V missense mutation in 1/269 unrelated genotype-negative LQTS patients that was absent in 400 control alleles. Mutant 1bA8V expressed alone or with hERG 1a in HEK-293 cells dramatically reduced 1b protein levels. Thus, mutations specifically disrupting hERG 1b function are expected to reduce cardiac I_Kr and enhance drug sensitivity, and represent a potential mechanism underlying inherited or acquired LQTS.

Key Words: Kv11.1 • KCNH2 • ether-à-go-go • arrhythmia • potassium channels

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