Mutations in cytoplasmic loops of the KCNQ1 channel and the risk of life-threatening events: implications for mutation-specific response to β-blocker therapy in type 1 long-QT syndrome

Abstract

Background: β-Adrenergic stimulation is the main trigger for cardiac events in type 1 long-QT syndrome (LQT1). We evaluated a possible association between ion channel response to β-adrenergic stimulation and clinical response to β-blocker therapy according to mutation location.

Methods and results: The study sample comprised 860 patients with genetically confirmed mutations in the KCNQ1 channel. Patients were categorized into carriers of missense mutations located in the cytoplasmic loops (C loops), membrane-spanning domain, C/N terminus, and nonmissense mutations. There were 27 aborted cardiac arrest and 78 sudden cardiac death events from birth through 40 years of age. After multivariable adjustment for clinical factors, the presence of C-loop mutations was associated with the highest risk for aborted cardiac arrest or sudden cardiac death (hazard ratio versus nonmissense mutations=2.75; 95% confidence interval, 1.29-5.86; P=0.009). β-Blocker therapy was associated with a significantly greater reduction in the risk of aborted cardiac arrest or sudden cardiac death among patients with C-loop mutations than among all other patients (hazard ratio=0.12; 95% confidence interval, 0.02-0.73; P=0.02; and hazard ratio=0.82; 95% confidence interval, 0.31-2.13; P=0.68, respectively; P for interaction=0.04). Cellular expression studies showed that membrane spanning and C-loop mutations produced a similar decrease in current, but only C-loop mutations showed a pronounced reduction in channel activation in response to β-adrenergic stimulation.

Conclusions: Patients with C-loop missense mutations in the KCNQ1 channel exhibit a high risk for life-threatening events and derive a pronounced benefit from treatment with β-blockers. Reduced channel activation after sympathetic activation can explain the increased clinical risk and response to therapy in patients with C-loop mutations.

Figure 1

Frequency and location of mutations in the KCNQ1 potassium channel. Diagramatic location of 99 different mutations in the KCNQ1 potassium channel involving 860 subjects. The α subunit involves the N-terminus (N), 6 membrane-spanning segments, 2 cytoplasmic loops (S2-S3 and S4-S5) and the C-terminus portion (C). The size of the circles reflects the number of subjects with mutations at the respective locations.

Figure 2

Kaplan-Meier estimates of cumulative probability of life threatening cardiac events by mutation location and type. ACA= aborted cardiac arrest. LQTS= long QT syndrome. The number in parentheses reflects the cumulative event rate at that point in time.

Figure 3

Risk for life threatening cardiac events by mutation location and ß-blocker treatment. 63 of the 125 (50%) subjects with C-loop missense mutations were treated with beta-blockers during a mean follow-up of 26.2 years; 305 of the 735 (42%) subjects with non-C-loop-missense mutations were treated by beta-blockers during a mean follow-up of 27.5 years. Event rates per 100 person-years were calculated by dividing the number of events during the period of β-blocker therapy or the absence of β-blocker therapy by person-years, and multiplying the results by 100. ACA= aborted cardiac death. LQTS= long QT syndrome.

Figure 4

Regulation of LQT1 mutant channels by PKA. A. Schematic representation of location of the mutations used in the study. B: Effect of each of the mutations studied in basal non stimulated cell currents. Average current measured for cells expressing WT and mutant subunits measured at +40 mV after 3sec depolarization. KCNQ1 and KCNE1 subunit were expressed at a ratio 0.5 KCNQ1WT: 0.5 KCNQ1mut : 1 KCNE1 or 0.5 KCNQ1WT : 0.5vector : 1 KCNE1 for wild-type haploinsuficient channels, *p<0.05 compared to WT. C: Top panel: typical ion channel current measured before and after 10 min application of the PKA activator forskolin (FK, 10 μM) for wild-type (WT) and WT and mutant subunits co-expressed. Scale bars in each panel are 10pA/pF and 2sec. Scale bars are the same for all constructs. Bottom panel: time course of current regulation by forskolin measured at +20 mV after 3sec depolarization for channels formed by either WT or mutant co-expressed with WT subunits, as indicated. Current was normalized to current in the absence of forskolin application. KCNQ1 and KCNE1 subunits were expressed at a ratio 0.5 KCNQ1WT : 0.5 KCNQ1mut : 1 KCNE1 or 1 KCNQ1WT : 1 KCNE1 for wild-type channels. Currents were activated by 4sec depolarizing steps to +20 mV from a -80mV holding potential. These were followed by a step to −20mV. D: Summary data for experiments done as in C, *p<0.05 compared to the current before stimulation (black bar) in each group.