Most LQT2 Mutations Reduce Kv11.1 (hERG) Current by a Class 2 (Trafficking-Deficient) Mechanism

Abstract

Background— The KCNH2 or human ether-a-go-go related gene ( hERG ) encodes the Kv11.1 α-subunit of the rapidly activating delayed rectifier K + current ( I Kr ) in the heart. Type 2 congenital long-QT syndrome (LQT2) results from KCNH2 mutations that cause loss of Kv11.1 channel function. Several mechanisms have been identified, including disruption of Kv11.1 channel synthesis (class 1), protein trafficking (class 2), gating (class 3), or permeation (class 4). For a few class 2 LQT2-Kv11.1 channels, it is possible to increase surface membrane expression of Kv11.1 current ( I Kv11.1 ). We tested the hypotheses that (1) most LQT2 missense mutations generate trafficking-deficient Kv11.1 channels, and (2) their trafficking-deficient phenotype can be corrected. Methods and Results— Wild-type (WT)-Kv11.1 channels and 34 missense LQT2-Kv11.1 channels were expressed in HEK293 cells. With Western blot analyses, 28 LQT2-Kv11.1 channels had a trafficking-deficient (class 2) phenotype. For the majority of these mutations, the class 2 phenotype could be corrected when cells were incubated for 24 hours at reduced temperature (27°C) or in the drugs E4031 or thapsigargin. Four of the 6 LQT2-Kv11.1 channels that had a wild-type–like trafficking phenotype did not cause loss of Kv11.1 function, which suggests that these channels are uncommon sequence variants. Conclusions— This is the first study to identify a dominant mechanism, class 2, for the loss of Kv11.1 channel function in LQT2 and to report that the class 2 phenotype for many of these mutant channels can be corrected. This suggests that if therapeutic strategies to correct protein trafficking abnormalities can be developed, it may offer clinical benefits for LQT2 patients.

Asymmetry of parental origin in long QT syndrome: preferential maternal transmission of KCNQ1 variants linked to channel dysfunction

“…33,34 We found more maternal than paternal transmission of LQT2 variants but the differences were smaller than for LQT1 variants. The ratios were of 0.58 for missense variants and 0.54 for variants leading to haploinsufficiency, and a similar distortion was observed for missense variants located in the pore region of the protein (S5-pore-S6) compared with other missense variants.…”

Modulatory Proteins Can Rescue a Trafficking Defective Epileptogenic Nav1.1 Na+Channel Mutant

“…Trafficking defects are usually caused by the inability of the mutant protein to correctly fold and pass ER quality control system, and thus by its subsequent degradation (Cobbold et al, 2003;Sitia and Braakman, 2003). Incubation at permissive temperature typically rescues several of these misprocessed proteins (Denning et al, 1992;Payne et al, 1998;Halaban et al, 2000;Valdivia et al, 2002;Anderson et al, 2006). This has been interpreted as a kinetic effect because of more functional proteins able to correctly fold and escape the quality control of the ER because of the slowed kinetics of folding (Bernier et al, 2004).…”

Properties of WT and mutant hERG K+ channels expressed in neonatal mouse cardiomyocytes

“…Mutations and drug block cause a loss-of-function phenotype in hERG current (I hERG ). Several molecular mechanisms have been proposed for the loss-of-function phenotype in LQT2, including abnormalities in ion channel protein trafficking, abnormalities in ion channel gating or permeation, and nonsense-mediated decay of mRNA (3,14,20). The most common type of mutation in LQT2 is a single nucleotide change leading to a single amino acid substitution (missense mutation).…”