Gain-of-function enhancer variant near KCNB1 causes familial ST-depression syndrome.

BACKGROUND AND AIMS Familial ST-depression syndrome (FSTD) is a recently identified inherited cardiac disease associated with arrhythmias and systolic dysfunction. The underlying genetic aetiology has remained elusive. This study aimed at finding the causative variant. METHODS A total of 67 FSTD patients (20 families) were studied. Linkage analysis and whole-genome sequencing (WGS) were initially performed. An identified non-coding variant was functionally characterized in AC16 human cardiomyocytes, muscle tissue, and human myocardium. In silico analyses, luciferase and dCas9-activator/repressor assays, protein-DNA experiments, chromosome conformation capture (4C), and RNA sequencing were also performed. RESULTS The electrocardiographic (ECG) phenotype was inherited in an autosomal dominant manner in all families. Linkage analysis revealed a single peak on chromosome 20, and WGS identified a single, rare, non-coding variant located 18 kb downstream of KCNB1 on chromosome 20 in all affected individuals. Perfect co-segregation with the ECG phenotype was observed together with full penetrance in all families. The variant creates a MEF2-binding site and presence of the variant allele or MEF2 co-expression enhanced transcriptional activity. dCas9-activator/repressor assays showed that KCNB1 was the only gene consistently regulated by the locus and 4C experiments in AC16 cells and human muscle tissue confirmed the locus-KCNB1 promoter interaction. Expression analysis in human endocardial tissue did not document any change in gene expression likely explained by expressional heterogeneity. CONCLUSIONS A gain-of-function enhancer variant creates a hyperactive regulatory locus that interacts with the KCNB1 promoter and causes FSTD. This is the first time that KCNB1 has been implicated in human cardiac electrophysiology and arrhythmogenesis.