AbstractBackground and Objectives
Single nucleotide polymorphisms (SNPs) close to the PITX2 gene (paired-like homeodomain transcription factor 2) are associated with atrial fibrillation, including recurrent AF with therapy: Carriers of the minor allele of the most common polymorphism close to pitx2 respond well to flecainide therapy, but less well to sotalol.1 The mechanisms underlying these clinical observations are not well understood. Heterozygous deletion of PITX2 isoform c results in reduced left atrial pitx2c mRNA expression and predisposes mice to atrial fibrillation.2 We studied the effect of flecainide on atrial electrophysiology in PITX2c+/- mice and their wild type (WT) littermates.Methods
We studied 3–4 months old PITX2c+/- and WT mice, on an MF1 background. Hearts were rapidly excised and perfused with Krebs solution at 4ml/min, 37°C, on a modified upright Langendorff apparatus. Left atrial monophasic action potentials were recorded at baseline and during 1 µM flecainide acetate perfusion. Inter-atrial activation times (AT) and action potential durations (APD) were measured during right atrial pacing at 80–120 ms fixed-rate cycle lengths (CL). An 8-pulse S1 train, at 80–120 ms CL, followed by a single extrastimulus, S2, was delivered to measure effective refractory periods (ERP) and arrhythmia inducibility. We analysed arrhythmias, AT, ERP, APD at 70% repolarisation (APD70), and post-repolarisation refractoriness (PRR). All values are expressed as mean ± SEM.Results
Flecainide reduced atrial arrhythmias in PITX2c+/- (6/18, baseline vs 0/15, flecainide, p < 0.05) but not in WT hearts (3/15, baseline vs 3/12, flecainide). Flecainide increased AT in PITX2c+/- slightly more than WT hearts (WT 8 ± 3 ms, n = 7; PITX2c+/- 13 ± 3 ms, n.s), in a frequency-dependent manner. Flecainide did not differentially alter atrial APD (increase in APD70 at 100 ms CL; WT 0.3 ± 2, n = 8; PITX2c+/- 0.7 ± 2, n = 14). In contrast, flecainide caused a significantly greater, frequency-dependent, increase in ERP (PITX2c+/- 14 ± 2 ms, n = 13 vs WT 7 ± 2 ms, n = 10; p < 0.05) and post-repolarisation refractoriness in PITX2c+/- mice (PITX2c+/- 21 ± 3 ms, n = 6 vs WT 6 ± 4 ms, n = 6, p < 0.01). There were no mice with post repolarisation refractoriness that were susceptible to arrhythmias (0/17 mice).Conclusion
Flecainide is very effective in preventing atrial fibrillation in mice with reduced pitx2 mRNA expression. We propose that this genotype-specific antiarrhythmic effect is due to higher post-repolarisation refractoriness induced in PITX2c+/- atria. These electrophysiological data may explain why carriers of SNPs close to the PITX2 gene respond well to flecainide therapy. Funded by EU (EUTRAF) and BHF.