非家族性週期癱瘓相關之鉀離子通道 26 基因突變之發現與其功能性特色
Identification and Functional Characterization of Kir2.6 Mutations
Associated with Non-familial Periodic Paralysis
林石化 1、鄭智仁 1,2、楊松昇 1、許育瑞 1、黃朝龍 2
Shih-Hua Lin1, Chih-Jen Cheng1,2, Sung-Sen Yang1, Yu-Juei Hsu1, Chou-Long Huang2
1
Department of Medicine, Division of Nephrology, Tri-Service General Hospital, National Defense
Medical Center, Taipei 114, Taiwan,
2
Department of Medicine, Division of Nephrology, UT Southwestern Medical Center, Dallas, TX
75390-8856, USA,
Background: Hypokalemic periodic paralysis (HPP) characterized by episodic muscle
weakness and acute hypokalemia during attacks can be familial or non-familial including
thyrotoxic periodic paralysis (TPP) and sporadic periodic paralysis (SPP). Recently,
mutations of inwardly rectifying K+ channel, Kir2.6, are reported in some TPP patients and
believed to predispose them to attacks.
Purpose: The aim of this study was to evaluate whether Kir2.6 mutations are present in
other non-familial HPP and the functional characterization of identified mutant Kir2.6.
Patients and Methods:
We studied a large cohort of Taiwanese patients, 100 with TPP,
60 with SPP without hyperthyroidism and familial history of paralysis, and 100 unrelated
healthy subjects. All affected individuals have severe hypokalemia during attacks (K ≈ 2.2
mEq/l). Direct sequencing of Kir2.6 from the genomic DNA of blood leukocytes was
performed. Site-directed mutagenesis, protein expression, and electrophysiological study
were conducted for of the mutant Kir2.6 in vitro.
Results: Two TPP patients had the same heterozygous V168M mutation in the 2nd
transmembrane segment of Kir2.6. Two SPP patients had different heterozygous
mutations- R43C in the N- and A200P in the C-terminal cytoplasmic domain. Compared to
WT, whole-cell current density for R43C and V168M mutants were reduced by 80% and
40%, respectively. No current was for A200P mutant. Western blot of Kir2.6-expressed
HEK cell lysates revealed all 3 mutants express equally full-length protein as wild type
(WT). Mutations did not change surface abundance of Kir2.6 channel. Single channel
recording showed reduced single channel conductance in R43C mutant, and decreased
open probability in V168M mutant. Furthermore, all three mutations exerted
dominant-negative effects on WT channel.
Conclusion: Mutations of Kir2.6 are associated with SPP as well as TPP. The regulation
of Kir2.6 by thyroid hormone is not central for mutations of Kir2.6 to cause susceptibility to
HPP. Hypofunction of Kir2.6 may depolarize the cell membrane potential rendering Na+
channel unavailable (i.e., inactivated) for firing when action potential arrives.