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A novel LQT3 mutation implicates the human cardiac sodium channel domain IVS6 in inactivation kinetics
W Antoinette Groenewegen1, Connie R Bezzina, J Peter van Tintelen
1Department of Medical Physiology, University Medical Center Utrecht, P.O. Box 85060, 3508 AB, Utrecht, The Netherlands. groenewegen@med.uu.nl <groenewegen@med.uu.nl>
Unlabelled:
The Long QT3 syndrome is associated with mutations in the cardiac sodium channel gene SCN5A.
Objective:
The aim of the present study was the identification and functional characterization of a mutation in a family with the long QT3 syndrome.
Methods:
The human cardiac sodium channel gene SCN5A was screened for mutations by single-stranded conformation polymorphism. The functional consequences of mutant sodium channels were characterized after expressing mutant and wild-type cRNAs in Xenopus oocytes by two-electrode voltage clamp measurements.
Results:
SCN5A screening revealed an A-->G substitution at codon 1768, close to the C-terminal end of domain IVS6, which changes an isoleucine to a valine. Functional expression of mutant I1768V-channels in Xenopus oocytes showed that the voltage-dependence and slope factors of activation and inactivation were unchanged compared to wild-type channels. No difference in persistent TTX-sensitive current could be detected between wild-type and I1768V channels, a channel feature often increased in LQT3 mutants. However, I1768V mutant channels recovered faster from inactivation (2.4 times) than wild-type channels and displayed less slow inactivation.
Conclusions:
We postulate that severe destabilization of the inactivated state leads to increased arrhythmogenesis and QT prolongation in I1768V mutation carriers in the absence of a persistent inward sodium current.