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1 Department of Medicine, University of Wisconsin-Madison, Madison, WI, USA
2 Department of Medicine, Pediatrics, Molecular Pharmacology and Experimental Therapeutics, Mayo Clinic, Rochester, MN, USA
* To whom correspondence should be addressed. E-mail: bda{at}medicine.wisc.edu.
Long QT syndrome (LQTS) is a cardiac repolarization disorder that can lead to arrhythmias and sudden death. Chromosome-7 linked inherited LQTS (LQT2) is caused by mutations in HERG (KCNH2), while drug-induced LQTS is caused primarily by HERG channel block. Many common polymorphisms are functionally silent and have been traditionally regarded as benign and without physiological consequence. However, the identification of common nonsynonymous single nucleotide polymorphisms (nSNPs, i.e. amino-acid coding variants) with functional phenotypes in the SCN5A Na+ channel and MiRP1 K+ channel beta subunit have challenged this viewpoint. In this report, we test the hypothesis that common missense HERG polymorphisms alter channel physiology. Comprehensive mutational analysis of HERG was performed on genomic DNA derived from a population-based cohort of sudden infant death syndrome, and two reference allele cohorts derived from 100 African American and 100 Caucasian individuals. Amino-acid encoding variants were considered common polymorphisms if they were present in at least two of the three study cohorts with an allelic frequency greater than 0.5%. Four nSNPs were identified: K897T, P967L, R1047L, and Q1068R. Wild type (WT) and polymorphic channels were heterologously expressed in human embryonic kidney cells, and biochemical and voltage clamp techniques were used to characterize their functional properties. All channel types were processed similarly, but several electrophysiological differences were identified; 1. K897T current density was lower than the other polymorphic channels, 2. K897T channels activated at more negative potentials than WT and R1047L, 3. K897T and Q1068R channels inactivated and recovered from inactivation faster than WT, P967L and R1047L channels, 4. K897T channels showed subtle differences compared to WT when stimulated with an action potential waveform. In contrast to K897T and Q1068R channels, P967L and R1047L channels were electrophysiologically indistinguishable from WT. All HERG 2 channels had similar sensitivity to block by cisapride. Therefore, some HERG polymorphic channels are electrophysiologically different than WT channels.
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