Common Ancestry-specific Ion Channel Variants Predispose to Drug-induced Arrhythmias
Background: Multiple reports associate the cardiac sodium channel gene ( SCN5A ) variants S1103Y and R1193Q with type 3 congenital long QT syndrome (LQTS) and drug-induced LQTS. These variants are, however, too common in ancestral populations to be highly arrhythmogenic at baseline: S1103Y allele frequency is 8.1% in Africans 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 (iPSC-CMs) but the role of these variants in modulating repolarization remains poorly-understood. Methods: We determined the effect of S1103Y on QT intervals among Africans in a large electronic health record. Using iPSC-CMs 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 subjects with no confounding medications or diagnoses of heart disease, QT in S1103Y carriers was no different from that in non-carriers. Similarly, baseline APD was no different in cells expressing the Y allele (SY, YY cells) compared to 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 to SS cells (tail current: 0.66±0.1 vs 1.2±0.1 pA/pF, p<0.001). Dofetilide challenge prolonged APD at much lower concentrations in SY (4.1 nM [IQR 1.5-9.3], n=11) and YY (4.2 nM [1.7- 5.0], n=5) than in SS cells (249 nM [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 further emphasize the need for inclusion of diverse populations in the study of adverse drug reactions.