Defining the cellular phenotype of ankyrin-B syndrome variants -: Human ANK2 variants associated with clinical phenotypes display a spectrum of activities in cardiomyocytes

Background - Mutations in the ankyrin- B gene ( ANK2) cause type 4 long-QT syndrome and have been described in kindreds with other arrhythmias. The frequency of ANK2 variants in large populations and molecular mechanisms underlying the variability in the clinical phenotypes are not established. More importantly, there is no cellular explanation for the range of severity of cardiac phenotypes associated with specific ANK2 variants. Methods and Results - We performed a comprehensive screen of ANK2 in populations ( control, congenital arrhythmia, drug- induced long- QT syndrome) of different ethnicities to discover unidentified ANK2 variants. We identified 7 novel nonsynonymous ANK2 variants; 4 displayed abnormal activity in cardiomyocytes. Including the 4 new variants, 9 human ANK2 loss- of- function variants have been identified. However, the clinical phenotypes associated with these variants vary strikingly, from no obvious phenotype to manifest long- QT syndrome and sudden death, suggesting that mutants confer a spectrum of cellular phenotypes. We then characterized the relative severity of loss- of- function properties of all 9 nonsynonymous ANK2 variants identified to date in primary cardiomyocytes and identified a range of in vitro phenotypes, including wild- type, simple loss- of- function, and severe loss- of- function activity, seen with the variants causing severe human phenotypes. Conclusions - We present the first description of differences in cellular phenotypes conferred by specific ANK2 variants. We propose that the various degrees of ankyrin- B loss of function contribute to the range of severity of cardiac dysfunction. These data identify ANK2 variants as modulators of human arrhythmias, provide the first insight into the clinical spectrum of ankyrin- B syndrome, and reinforce the role of ankyrin- B - dependent protein interactions in regulating cardiac electrogenesis.