Connecting the clinic to the bench : modelling the functional effects of a titin variant associated with atrial and ventricular arrhythmias

Atrial fibrillation (AF) is a common arrhythmia linked to increased risk of stroke, heart failure, and death. In AF patients without known clinical risk factors, genetic factors are likely contributors to disease pathogenesis. Studies into the genetic risk factors of AF showed a strong association between truncating variants in the titin gene and increased risk of AF in structurally normal hearts. Titin truncating variants (TTNtv) are a well-established cause of dilated cardiomyopathy (DCM), and recent studies have also shown an increased risk of atrial and ventricular fibrillation (VF) in DCM patients with TTNtv. The effect of TTNtv on risk of AF compared to modifiable risk factors is unclear. Furthermore, the cellular effects of TTNtv in atrial and ventricular arrhythmias without dilated cardiomyopathy is unknown. The objectives of this dissertation were 1) to evaluate the association between titin variants and increased risk of AF compared to modifiable risk factors and 2) to examine the cellular effects of TTNtv identified in atrial and ventricular fibrillation patients without structural disease. We used genetic epidemiology approaches and data from UK Biobank to assess the risk of AF in titin variant carriers, and we used patient-specific induced pluripotent stem cell-derived atrial- and ventricular-like cardiomyocytes and engineered heart tissue constructs to model a TTNtv implicated in AF and VF. We demonstrated an additive effect between titin variant status and modifiable risk factors on the risk of AF. In the AF model, the TTNtv led to sarcomere disarray, reduced contractility, increased arrhythmogenicity, and transcriptional changes implicating substrate alterations. In the VF model, the TTNtv resulted in calcium transient shortening and impaired electrophysiological response to β-adrenergic stimulation. These findings highlight the utility of genetic testing to identify TTNtv carriers who are at increased risk of developing AF and the importance of clinical risk factor management to reduce AF risk. Furthermore, we provide evidence of functional abnormalities associated with TTNtv in vitro that implicate atrial myopathic remodelling as well as altered electrophysiology and β-adrenergic response in patient-specific models of AF and VF, respectively, that deepen our understanding of titin’s role in the pathogenesis of atrial and ventricular arrhythmias.