The role of TNNI3K in cardiac disease Clinical and molecular perspectives

Troponin I-interacting kinase (TNNI3K), a protein kinase predominantly expressed in the heart, has been associated with various cardiac pathologies. Yet, the underlying disease mechanisms remain unclear. This thesis explores the role of TNNI3K in cardiac disease from both clinical and molecular perspectives.
Our research identified that TNNI3K missense, rather than loss-of-function, variants are significantly associated with dilated cardiomyopathy and arrhythmias. Moreover, we identified two novel TNNI3K variants with reduced kinase activity in families with congenital junctional ectopic tachycardia. These findings also suggest that altered TNNI3K kinase function, rather than protein absence, is a key driver of pathogenicity.
To further explore the TNNI3K disease mechanisms, we created mouse models homologous to two pathogenic human TNNI3K variants with enhanced or reduced kinase activity. These models exhibited prolonged PR intervals and early-stage dilated cardiomyopathy. Phosphoproteomic profiling of cardiac tissue revealed tissue-specific alterations in biological processes.
Next, we presented PR interval prolongation in mice expressing TNNI3K, which was not observed in mice expressing kinase-dead TNNI3K. This prolongation occurred independently of atrial electrical remodeling or structural remodeling in the atrioventricular node. Additionally, connexin-45 was identified as a novel target, with reducing gap junctional conductance and increasing protein expression in presence of TNNI3K. Further phosphoproteomic studies uncovered additional interactors and mechanisms of action, identifying Hcn4^Ser719 as a promising phosphorylation target regulated by TNNI3K.
Overall, this thesis provides novel insights into the clinical and molecular mechanisms of TNNI3K in cardiac disease, highlighting its potential as a therapeutic target for cardiac conduction disorders and cardiomyopathies.