A Rare SK2 Channel Variant Confers a Gain-of-Function Phenotype

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Sequencing the DNA from a 52-year-old female with modest QTc interval prolongation (460 ms) revealed pro-arrhythmic or drug-induced QTc-prolonging variants in the genes KCNE1 (p.S38G), encoding for the beta subunit (minK) of a slowly activated cardiac potassium channel, and ANK2 (p.P2835S), encoding for the adapter protein ankyrin-B, respectively, as well as KCNN2 (p.F503L), encoding for the pore-forming subunit of the type 2 small conductance, calcium-activated potassium (SK2) channel. Whereas electrophysiological consequences of KCNE1 and ANK2 variants have been described previously, the impact of KCNN2 variants, specifically F503L, on SK2 channel function is unknown. We first characterized the properties of F503L KCNN2 by heterologous expression of the variant and wild type in HEK293 cells. F503L KCNN2 expression led to a ∼22% decrease in the average EC50 for Ca2+ activation of SK2 channels (478±25 nM vs 610±47 nM; mean ± SEM; P 0.05). We next tested the impact on macroscopic SK currents in cardiomyocytes isolated from “knock-in” mice heterozygous for the KCNN2 variant. Whole-cell SK current density-voltage relationships recorded from 7 wild type- and 9 variant-expressing myocytes exhibited strong inward rectification at potentials positive to the K+ equilibrium potential (−78 mV). Average inward SK current density was ∼2.5-fold larger in variant compared to wild type myocytes (e.g., at −120 mV, 6.1±0.7 pA/pF versus 2.4±0.4 pA/pF; P<0.05; intrapipette free [Ca2+] was 1 μmol). Outward SK currents at potentials between 0 and +40 mV were markedly increased in variant compared to wild type cells (e.g., at +40 mV, 1.0±0.3 pA/pF vs 0.2±0.1 pA/pF; P<0.05). These findings support the intriguing possibility that an increase in outward SK currents associated with a gain-of-function KCNN2 variant could mitigate the possible combined effect of pro-arrhythmic or drug-induced QTc-prolonging variants in KCNE1 and/or ANK2.