The rs13376333 risk allele mimics the effect of atrial fibrillation on SK-current density and afterdepolarizations in human atrial myocytes
Authors: Jimenez-Sabado V , Tarifa C , Casabella-Ramon S , Montiel J , Rodriguez-Font E , Olesen M S , Hove-Madsen L 
Published: October 2022

Abstract

Background
Various genome wide association studies have associated the single nucleotide polymorphism (SNP) rs13376333 with increased risk of atrial fibrillation. This SNP is intronic to the KCNN3 gene that codes for the small conductance calcium-activated potassium channel type 3 (SK3), but functional electrophysiological effects of the risk variant are not known.

Purpose
This study aimed to study the effects of atrial fibrillation and the rs13376333 risk SNP on the apamine sensitive SK-current and on afterdepolarizations in human atrial myocytes.

Methods
Myocytes were isolated from human right atrial samples, and subjected to perforated patch-clamp technique to measure the apamine (100 nM) sensitive SK-current elicited by a voltage-ramp protocol, spontaneous transient inward Na-Ca exchange currents (ITI) or afterdepolarizations at rest. To assess the impact of the rs13376333 risk variant, patients without atrial fibrillation were genotyped and divided into risk and control groups according to the genotype at rs13376333.

Results
At membrane potentials above −40 mV, the SK-current density increased with increasing membrane potentials. At +20mV the density was significantly smaller (0.18±0.04 pA/pF) in 5 patients with atrial fibrillation than in 24 patients without this arrhythmia (0.65±0.12 pA/pF, p&lt;0.01). On the other hand, the ITI density was larger in 10 patients with than 23 without atrial fibrillation (0.71±0.16 vs. 0.30±0.03 pA/pF, p&lt;0.01), supporting the notion that a smaller SK-current in myocytes from patients with atrial fibrillation cannot counter-balance the calcium-release induced ITI, resulting in a larger net ITI density and a higher incidence of detectable afterdepolarizations in these patients. In line with this notion, inhibition of the SK-current with 100 nM apamine increased the frequency of afterdepolarization between −85 and −75 mV from 1.2±0.3 to 2.5±0.5 events/min (p=0.03) in patients without atrial fibrillation. Analysis of the current-voltage relationship for the SK-current in atrial myocytes from patients with the rs13376333 risk variant showed that these patients presented a significantly smaller apamine-sensitive SK current (p&lt;0.001, n=15). At +20 mV the SK-density was 60% smaller in patients carrying a rs13376333 risk allele (0.43±0.18 vs. 1.08±0.27 pA/pF) mimicking the depression of the SK-current observed in the patients with atrial fibrillation.

Conclusion
Both the rs13376333 risk allele and atrial fibrillation is associated with a reduced SK-current in human atrial myocytes, which may contribute to the higher incidence of afterdepolarizations in these conditions. Thus, restoration of SK-channel function in patients with a rs1337633 risk SNP or with atrial fibrillation may help reduce the incidence of afterdepolarizations in these patients.

Funding Acknowledgement
Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Spanish Ministry of Science and Innovation