Variant Intronic Enhancer Controls Expression and Heart Conduction.

Background: Genetic variants in , encoding the neuronal voltage-gated sodium channel Na1.8, are strongly associated with atrial fibrillation, Brugada syndrome, cardiac conduction velocities, and heart rate. The cardiac function of has not been resolved, however, and diverging mechanisms have been proposed. Here, we investigated the cardiac expression of and the function of a variant-sensitive intronic enhancer previously linked to the regulation of , encoding the major essential cardiac sodium channel Na1.5.

Methods: The expression of was investigated in mouse and human hearts. With the use of CRISPR/Cas9 genome editing, the mouse intronic enhancer was disrupted, and mutant mice were characterized by transcriptomic and electrophysiological analyses. The association of genetic variants at enhancer regions and gene expression were evaluated by genome-wide association studies single-nucleotide polymorphism mapping and expression quantitative trait loci analysis.

Results: We found that cardiomyocytes of the atria, sinoatrial node, and ventricular conduction system express a short transcript comprising the last 7 exons of the gene (). Transcription occurs from an intronic enhancer-promoter complex, whereas full-length transcript was undetectable in the human and mouse heart. Expression quantitative trait loci analysis revealed that the genetic variants in linkage disequilibrium with genetic variant rs6801957 in the intronic enhancer associate with transcript levels in the heart. Genetic modification of the enhancer in the mouse genome led to reduced cardiac expression in atria and ventricles, reduced cardiac sodium current in atrial cardiomyocytes, atrial conduction slowing and arrhythmia, whereas the expression of , the presumed enhancer target gene, remained unaffected. In patch-clamp transfection experiments, expression of -encoded Na1.8-short increased Na1.5-mediated sodium current. We propose that noncoding genetic variation modulates transcriptional regulation of in cardiomyocytes that impacts Na1.5-mediated sodium current and heart rhythm.

Conclusions: Genetic variants in and around modulate enhancer function and expression of a cardiac-specific transcript. We propose that noncoding genetic variation modulates transcriptional regulation of a functional C-terminal portion of Na1.8 in cardiomyocytes that impacts on Na1.5 function, cardiac conduction velocities, and arrhythmia susceptibility.