A Small Internal Deletion in Titin Yields Ion Channel-Dependent Cardiomyopathy and Atrial Fibrillation

Genetic variants may lead to cardiac developmental defects, and the most common type of arrhythmia, Atrial fibrillation (AF), has been shown to be associated occasionally with cardiac developmental defects. However, the mechanisms contributing to this association are unclear. One of the major sarcomere structural proteins, titin, may help reveal these mechanisms because of its roles in both cardiac development and regulating cardiac function. In a screening of CRISPR/Cas9-mediated mutagenesis in zebrafish, I identified a small internal in-frame deletion of 9 amino acids in the Immunoglobin-like domain 139 (Ig-139) of the zebrafish titin A-band region (ttnaΔ9). Compared to previous studies focusing on titin truncating variants that remove large regions and multiple domains, our mutant titin with the small internal deletion may reveal a role of this specific region (Ig-139 domain) in the pathogenesis of atrial fibrillation. Briefly, ttnaΔ9/Δ9 zebrafish exhibit a transient ventricular function defect during development, followed by AF and cardiomyopathy in the adult. Meanwhile, atria exhibit enlargement and reduced contraction in both embryos and adults compared to wild-type clutchmates. How does this transient ventricular defect contribute to atrial fibrillation in adults? I hypothesized that atrial natriuretic peptide (ANP) upregulation plays a role in this mechanism because: (1) ANP secretion is upregulated in response to cardiac dysfunctions and (2) previous studies showed ANP overexpression leads to AF through increased potassium slow rectifier channel current (IKs). I found ttnaΔ9/Δ9 embryos show ANP upregulation and ANP-dependent IKs channel remodeling indicated by the downregulation of the IKs channel regulatory subunit kcne1. Specific blocking of the IKs channel with a chemical inhibitor in both the ttnaΔ9/Δ9 zebrafish model and TTNΔ9/Δ9 human iPSC-derived atrial cardiomyocytes reduced arrhythmia and improved atrial contraction, suggesting that the ANP-dependent IKs channel remodeling and increased activity contribute to the pathogenesis of AF and cardiomyopathy in the mutant adult. In conclusion, ttnaΔ9/Δ9 zebrafish exhibit a phenotype suggesting that subtle and transient cardiovascular defects during development may lead to a higher risk of atrial fibrillation in adults. The Δ9 mutation in the Ig-139 domain of titin A-band triggers an ANP-regulated IKs channel remodeling, which may be part of the mechanism and provide potential treatment targets for developmental defects-related adult cardiac diseases.