Common Ancestry-Specific Ion Channel Variants Predispose to Drug-Induced Arrhythmias

Abstract

Background: Multiple reports associate the cardiac sodium channel gene (SCN5A) variants S1103Y and R1193Q with type 3 congenital long QT syndrome and drug-induced long QT syndrome. These variants are too common in ancestral populations to be highly arrhythmogenic at baseline, however: S1103Y allele frequency is 8.1% in African Americans and R1193Q 6.1% in East Asians. R1193Q is known to increase late sodium current (I_Na-L) in cardiomyocytes derived from induced pluripotent stem cells but the role of these variants in modulating repolarization remains poorly understood.

Methods: We determined the effect of S1103Y on QT intervals among African-American participants in a large electronic health record. Using cardiomyocytes derived from induced pluripotent stem cells carrying naturally occurring or genome-edited variants, we studied action potential durations (APDs) at baseline and after challenge with the repolarizing potassium current (I_Kr) blocker dofetilide and I_Na-L and I_Kr at baseline.

Results: In 1479 African-American participants with no confounding medications or diagnoses of heart disease, QT intervals in S1103Y carriers was no different from that in noncarriers. Baseline APD was no different in cells expressing the Y allele (SY, YY cells) compared with isogenic cells with the reference allele (SS cells). However, I_Na-L was increased in SY and YY cells and the I_Na-L blocker GS967 shortened APD in SY/YY but not SS cells (P<0.001). I_Kr was increased almost 2-fold in SY/YY cells compared with SS cells (tail current: 0.66±0.1 versus 1.2±0.1 pA/pF; P<0.001). Dofetilide challenge prolonged APD at much lower concentrations in SY (4.1 nmol/L [interquartile range, 1.5-9.3]; n=11) and YY (4.2 nmol/L [1.7-5.0]; n=5) than in SS cells (249 nmol/L [22.3-2905]; n=14; P<0.001 and P<0.01, respectively) and elicited afterdepolarizations in 8/16 SY/YY cells but only in 1/14 SS cells. R1193Q cells similarly displayed no difference in baseline APD but increased I_Kr and increased dofetilide sensitivity.

Conclusions: These common ancestry-specific variants do not affect baseline repolarization, despite generating increased I_Na-L. We propose that increased I_Kr serves to maintain normal repolarization but increases the risk of manifest QT prolongation with I_Kr block in variant carriers. Our findings emphasize the need for inclusion of diverse populations in the study of adverse drug reactions.

Keywords: Torsades de Pointes; drug-related side effects and adverse reactions; induced pluripotent stem cells; long QT syndrome; myocytes, cardiac; potassium channels; sodium channels.

Figure 1.. QTc intervals in African American subjects.

QTc intervals are plotted separately for females (left) and males (right). Open symbols denote intervals with the SS genotype, grey symbols with the SY genotype, and black symbols the YY genotype. N indicates the number of subjects included in the study. QTc intervals are presented in Supplemental Table S2.

Figure 2.. Action potential duration and dofetilide sensitivity in iPSC-CMs with S1103Y variant.

(A) Baseline action potential duration at 90% repolarization (APD₉₀) was similar among C1-SS (n=27), A1-SS (n=21), C1-SY (n=39), A1-SY (n=19), and C1-YY (n=15) cells. (B) Representative traces during dofetilide perfusion in iPSC-CMs with the SS (C1-SS) and YY (C1-YY) genotype. Traces include APD₉₀ at baseline, 10, and 30 nM dofetilide perfusion. The trace of C1-YY at 30 nM dofetilide was paced at 0.25 Hz due to extreme APD prolongation (#). (C) Dofetilide concentration that prolonged APD₉₀ by 100 ms ([Dof]_Δ100ms). (D) Absolute change in APD₉₀ at 10 nM dofetilide. Open symbols denote data obtained from cells with the SS genotype, grey symbols from the SY genotype, and black symbols the YY genotype. Statistical comparisons were between lines, and between genotypes by combining same genotypes.*p<0.05, **p<0.01, †p<0.001.

Figure 3.. Effect of S1103Y variant on late sodium current (I_Na-L) in iPSC-CMs.

Representative traces of late sodium current (I_Na-L) in C1-SS (A) and C1-YY cells (B). I_Na-L is presented as the percentage of peak I_Na recorded at −30 mV. (C) Summary data of I_Na-L among C1-SS (n=10), C1-SY (n=12), and C1-YY (n=10) cells from three independent differentiation batches for each line. (D) APD₉₀ relative to baseline in each line before perfusion with the I_Na-L blocker GS967 (baseline), at 0.1 μM and 1 μM GS967 perfusion, and after wash-out. *p<0.05, **p<0.01, †p<0.001.

Figure 4.. I_Kr in iPSC-CM.

(A) Steady-state activation of I_Kr, assessed as an E-4031 sensitive current, was measured at 1 sec depolarizing pulse at 0.5 Hz (depicted as a grey arrow in the inset). (B) Peak tail current was measured at −40 mV (depicted as a black arrow in the inset). Summary data for all lines are presented in Supplemental Table S4.

Figure 5.. Baseline APD₉₀ and dofetilide sensitivity in iPSC-CMs by R1193Q genotype

(A) APD₉₀ among cells homozygous for the reference allele (C2-RR, n=23), heterozygous for R1193Q (C2-RQ, n=16), and homozygous for the R1193Q variant (C2-QQ, n=15). (B) The dofetilide concentration that prolonged APD₉₀ by 100 ms ([Dof]_Δ100ms) among C2-RR (n=6), C2-RQ (n=6), and C2-QQ (n=5). (C) Absolute change in APD₉₀ at 10 nM dofetilide among C2-RR (n=6), C2-RQ (n=6), and C2-QQ (n=5).

Figure 6.. Summary of findings.

We find that while the common SCN5A variants S1103Y and R1193Q enhance pathogenic current I_Na-L, they also increase I_Kr. This “balanced repolarization” normalizes APD at baseline but manifests exaggerated APD prolongation and afterpolarizations when I_Kr is blocked.