Novel C-terminus frameshift mutation, 1122fs/147, of HERG in LQT2: additional amino acids generated by frameshift cause accelerated inactivation.
OBJECTIVE
The function of the C-terminus region of the human ether-a-go-go related gene (HERG) has not been well characterized except for its involvement in trafficking. To understand further the role of C-terminus region, we performed a functional analysis of a novel frameshift mutation (1122fs/147) identified in a Japanese long QT syndrome 2 (LQT2) patient who had recurrent episodes of syncope.


METHODS
Wild type (WT) and mutant HERG plasmids were transfected into human embryonic kidney (HEK-293) cells, and whole-cell current was recorded by the patch-clamp technique. Confocal microscopy was performed to examine the membrane distribution of channel protein using a green fluorescent protein tagged to the N-terminus of HERG.


RESULTS
The mutant 1122fs/147 alone could express current, but reduced density by 74% of control. No dominant negative effect was noted with co-expression of WT and 1122fs/147. Activation and deactivation time constants were not changed, while inactivation was accelerated in 1122fs/147 compared to WT, and V(1/2) of steady-state inactivation curve shifted by 11 mV in the negative direction. Current density of 1123stop mutant revealed 49% reduction compared to WT and showed no shift in steady-state inactivation. Confocal microscopy revealed reduced protein expression on the cell surface both in 1122fs/147 and 1123stop mutants compared to WT.


CONCLUSIONS
Frameshift mutation at the C-terminus region with additional 147 amino acids evoked a loss of function of the HERG channel. A negative shift in steady-state inactivation induced by the additional 147 amino acids and trafficking defect contribute to a reduced current amplitude of 1122fs/147.
Sasano
Tetsuo
T
Department of Cardiovascular Disease, Medical Research Institute, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan. tsasano@jhmi.edu
Ueda
Kazuo
K
Orikabe
Minako
M
Hirano
Yuji
Y
Kawano
Seiko
S
Yasunami
Michio
M
Isobe
Mitsuaki
M
Kimura
Akinori
A
Hiraoka
Masayasu
M
D000293:Adolescent; D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels; D016368:Frameshift Mutation; D006801:Humans; D007700:Kinetics; D008133:Long QT Syndrome / Q000235:genetics* / Q000378:metabolism; D008297:Male; D018613:Microscopy, Confocal; D018408:Patch-Clamp Techniques; D024642:Potassium Channels, Voltage-Gated / Q000235:genetics* / Q000378:metabolism; D014162:Transfection
D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels; C000606913:KCNH2 protein, human; D024642:Potassium Channels, Voltage-Gated