A trafficking-deficient KCNQ1 mutation, T587M, causes a severe phenotype of long QT syndrome by interfering with intracellular hERG transport.

BACKGROUND

KCNQ1-T587M is a C-terminal mutation correlated with severe phenotypes of long QT syndrome (LQTS). However, functional analysis of KCNQ1 channels with the T587M mutation showed a mild genotype in the form of haploinsufficiency in a heterologous expression system. This study sought to explore the molecular mechanism underlying the phenotype-genotype dissociation of LQTS patients carrying the KCNQ1-T587M mutation.

METHODS

cDNAs for wild-type (WT) and KCNQ1 mutations (R259C and T587M) were transiently transfected into HEK293 cells stably expressing hERG (hERG-HEK), and whole-cell patch-clamp technique was performed to examine the effect of KCNQ1 mutations on I-like currents. In addition, fluorescence resonance energy transfer (FRET) was conducted to demonstrate the molecular interaction between KCNQ1 and hERG when co-expressed in HEK293 cells.

RESULTS

KCNQ1-T587M mutation produced a significant (p<0.01) decrease in I-like tail current densities without affecting the gating kinetics, while KCNQ1-R259C mutation had no significant effect on the I-like tail current densities. Consistent with this result, FRET experiments demonstrated that both KCNQ1-WT and -R259C interacted with hERG in the cytosol and on the plasma membrane; however, the interaction between KCNQ1-T587M and hERG was observed only in the cytosol, and hERG proteins were seldom transported to the cell membrane, suggesting that the KCNQ1-T587M mutation impaired the trafficking of hERG to the cell membrane.

CONCLUSIONS

The disruption of hERG trafficking caused by the KCNQ1-T587M mutation is likely the reason why some patients exhibit severe LQTS phenotypes.