Mutation location and IKs regulation in the arrhythmic risk of long QT syndrome type 1: the importance of the KCNQ1 S6 region

Abstract

Aims: Mutation type, location, dominant-negative IKs reduction, and possibly loss of cyclic adenosine monophosphate (cAMP)-dependent IKs stimulation via protein kinase A (PKA) influence the clinical severity of long QT syndrome type 1 (LQT1). Given the malignancy of KCNQ1-p.A341V, we assessed whether mutations neighbouring p.A341V in the S6 channel segment could also increase arrhythmic risk.

Methods and results: Clinical and genetic data were obtained from 1316 LQT1 patients [450 families, 166 unique KCNQ1 mutations, including 277 p.A341V-positive subjects, 139 patients with p.A341-neighbouring mutations (91 missense, 48 non-missense), and 900 other LQT1 subjects]. A first cardiac event represented the primary endpoint. S6 segment missense variant characteristics, particularly cAMP stimulation responses, were analysed by cellular electrophysiology. p.A341-neighbouring mutation carriers had a QTc shorter than p.A341V carriers (477 ± 33 vs. 490 ± 44 ms) but longer than the remaining LQT1 patient population (467 ± 41 ms) (P < 0.05 for both). Similarly, the frequency of symptomatic subjects in the p.A341-neighbouring subgroup was intermediate between the other two groups (43% vs. 73% vs. 20%; P < 0.001). These differences in clinical severity can be explained, for p.A341V vs. p.A341-neighbouring mutations, by the p.A341V-specific impairment of IKs regulation. The differences between the p.A341-neighbouring subgroup and the rest of LQT1 mutations may be explained by the functional importance of the S6 segment for channel activation.

Conclusion: KCNQ1 S6 segment mutations surrounding p.A341 increase arrhythmic risk. p.A341V-specific loss of PKA-dependent IKs enhancement correlates with its phenotypic severity. Cellular studies providing further insights into IKs-channel regulation and knowledge of structure-function relationships could improve risk stratification. These findings impact on clinical management.

Keywords: Genetics; Long QT syndrome; Sudden cardiac death.

Upper panel: Topology of KCNQ1 missense and non-missense mutations identified in all studied long QT syndrome type 1 patients (n = 1316), according to their amino acid position along the KCNQ1 channel. Location of p.A341V and p.A341-neighbouring mutations in the KCNQ1-S6 segment is highlighted in red and blue, respectively. Exemplary I  _Ks traces for p.A341V (red) and p.A341V-neighbouring mutation (blue), with and without cyclic adenosine monophosphate/okadaic acid are depicted on the left. Left lower panel: structural image of the pore of the I  _Ks channel. Amino acid position A341 is indicated in red and the A341-neighbouring amino acids in blue. Right lower panel: Cumulative event-free survival of patients with p.A341V (red; n = 269), p.A341-neighbouring mutations (blue; n = 91), and all other KCNQ1-missense mutations located outside the S6 region (black; n = 611).

Figure 1

Topology of KCNQ1 missense and non-missense mutations identified in the total study population of 1316 long QT syndrome type 1 patients, according to their amino acid position along the KCNQ1 channel.

Figure 2

Proportion of missense mutation carriers with a QTc ≤440 and >500 ms in the three study groups. LQT1, long QT syndrome type 1.

Figure 3

Cumulative event-free survival of patients for all long QT syndrome type 1 missense mutations, i.e. p.A341V, p.A341-neighbouring mutations, and all the other KCNQ1 mutations located outside the S6 region. Top: hazard ratio with 95% confidence interval for the first occurrence of cardiac events in p.A341V and p.A341-neighbouring mutation vs. other missense long QT syndrome type 1 mutation carriers. Bottom: patients at risk per decade of age since birth up to 40 years, according to the genetic subgroup. Discrepancies in numbers of subjects between this figure and Table 2 are due to the lack of precise time at the first event for a few patients. CI, confidence interval; HR, hazard ratio; LQT1, long QT syndrome type 1.

Figure 4

Cumulative event-free survival of genotype-positive probands carrying long QT syndrome type 1 missense mutations: namely, p.A341V, p.A341-neighbouring mutations, and all other KCNQ1 mutations located outside the S6 region. LQT1, long QT syndrome type 1.

Figure 5

Cumulative event-free survival of patients with p.A341V-neighbouring mutations—either missense or non-missense—compared with all other variants elsewhere located in KCNQ1, also by mutation type.

Figure 6

Cumulative event-free survival of patients for all mutations per KCNQ1 region, i.e. p.A341V, p.A341V-neighbouring mutations, other membrane-spanning segments (transmembrane), C-loops (S2–S3 and S4–S5), also by protein kinase A-dependent I  _Ks-channel activation, C/N termini, and non-missense variants.

Figure 7

Protein kinase A-dependent regulation of mutant I  _Ks for p.A341V vs. p.A341-neigboring mutations. (A) Localization of p.A341V (red) and A341-neighbouring mutations (blue) on S6 in KCNQ1. (B) Averaged I  _Ks-density traces for wild type, p.S338C, p.F339S, p.A341V, p.L342F, p.P343R, p.A344V, and p.G345R, all heterozygously expressed in the absence (n = 11, 8, 8, 9, 19, 6, 11, and 21 cells, respectively) or presence (+; n = 15, 6, 9, 12, 16, 8, 6, and 14 cells, respectively) of intrapipette cyclic adenosine monophosphate/okadaic acid during a 5-s depolarizing pulse to +60 mV (holding potential −80 mV). Blue arrows indicate increased tail current density; red arrow indicates lack of tail current enhancement. (C) I  _Ks-tail densities for wild type and all heterozygously expressed mutations in the absence of cyclic adenosine monophosphate. Multigroup comparison using one-way ANOVA for wild type vs. p.S338C, P = 0.325; vs. p.F339S, P = 0.637; vs. p.A341V, P = 0.018; vs. p.L342F, P = 0.008; vs. p.P343R, P = 0.0001; vs. p.A344V, P = 0.420; and vs. p.G345R, P = 0.001. (D) Relative tail current densities compared to normalized values without cyclic adenosine monophosphate. All mutant channels responded to cyclic adenosine monophosphate/okadaic acid, except for p.A341V_het. Statistical comparison between the absence and presence of cyclic adenosine monophosphate/okadaic acid: wild type, P = 0.034; p.S338C, P = 0.018; p.F339S, P = 0.031; p.A341V, P = 0.202; p.L342F, P = 0.015; p.P343R, P = 0.020; p.A344V, P = 0.036; and p.G345R, P = 0.022. *Statistically significant difference. cAMP/OA, cyclic adenosine monophosphate/okadaic acid; WT, wild type.