Objectives: To determine the causal genetic variant and gene and the mechanism for the atrial fibrillation (AF) genome wide association study (GWAS) locus on chromosome 5q31.

Background: FAM13B expression is strongly associated with the lead AF GWAS variant at 5q31.  However, the regulatory variant controlling FAM13B expression and the mechanism by which FAM13B impacts AF susceptibility are not known.

Methods: Bioinformatics, reporter gene transfections, gel shifts, and gene editing were used to identify the variant regulating FAM13B expression. RNAseq after FAM13B knockdown in stem cell-derived cardiomyocytes (iCMs) identified downstream processes. Patch clamp and calcium transient assays were used to assess downstream mechanisms. Fam13b knockout (KO) mice were studied for heart structural and functional changes, and pacing-induced arrhythmia.

Results: rs17171731 was identified as the regulatory variant controlling FAM13B expression, with decreased enhancer activity of the risk allele.  Knockdown of FAM13B in iCMs altered expression of >1000 genes including SCN2B and led to pro-arrhythmogenic changes in the late sodium current and Ca²⁺ cycling.  FAM13B is a member of the Rho GTPase-activating protein (RhoGAP) gene family, but failed to demonstrate RhoGAP activity.  GFP-tagged FAM13B expressed in iCMs localized at the Z-disc and plasma membrane. Fam13b knockout mice had increased basal p-wave duration and QT interval, and were more susceptible to pacing-induced arrhythmias vs. controls.

Conclusions: This study went from an AF GWAS locus to identify the causal variant and gene, mechanisms for this association, and confirmed arrhythmia susceptibility in Fam13b KO mice. FAM13B and downstream effectors are potential targets for patient-specific therapeutics.

Excel and proprietary software: Labscribe ECG Analysis software (iWorx RA834).