Multiple 4q25 risk variants impair calcium homeostasis and compromise left atrial function

Abstract Aims Single-nucleotide polymorphisms (SNPs) from distinct linkage blocks in the chromosomal region 4q25 (rs1448818, rs2200733 and rs10033464) are associated with increased risk of atrial fibrillation (AF), but their impact on cardiomyocyte and atrial function remain elusive. Here, we tested the hypothesis that these SNPs have differential effects on calcium homeostasis that may afford SNP-specific targets and help explain their impact on atrial function. Methods and results Analysis of 391,008 individuals from the UK biobank revealed that the three risk-alleles increased incident AF during 10-year period in a dose-dependent manner, and that genetic and clinical risk was additive for the rs2200733 risk variant. Analyses of PITX2C mRNA expression in human atrial tissue showed that the rs2200733 risk variant increased PITX2C expression. Moreover, patch-clamp analyses in human atrial myocytes from 66 patients without AF revealed that L-type calcium current (ICaL) was significantly reduced in carriers of the rs1448818 risk allele only. In contrast, the transient inward current (ITI) frequency was significantly higher in carriers of rs2200733 or rs10033464 risk alleles only. This concurred with increased sarcoplasmic reticulum calcium load in those with the rs10033464 risk allele, while myocytes with the rs2200733 risk allele had increased ryanodine receptor 2 phosphorylation at Ser2808 (n=119) and displayed pronounced beat-to-beat alternation when paced. Finally, linear regression analyses of cardiac MRI data from 39,391 individuals in the UK biobank without AF showed that beta-values for minimal and maximal left atrial volume were increased and active ejection fraction (LAAEF) decreased in carriers of rs1448818 or rs2200733 risk alleles but preserved in individuals with the rs10033464 risk allele. Conclusions Distinct 4q25 risk SNPs produce differential alterations in intracellular calcium homeostasis that may help understand their impact on atrial function or rhythm. Moreover, the findings suggest that genotype-tailored strategies aiming to restore ICaL density may be effective for rs1448818, while attenuation of spontaneous calcium release may be suitable for rs2200733 or rs10033464 variants.