Atrio-ventricular conduction block : from disease models to novel therapeutic strategies

Abstract

Atrio-ventricular conduction block (AV block), for which the electrical conduction from the atria to the ventricle is blocked or delayed, is one of the most common forms of bradyarrhythmia encountered in clinical practice. Although cardiac pacing via an electronic pacemaker is widely used and well-established standard therapy for AV block, limitations of electronic pacemakers mean there is a great demand in developing novel pacing strategies. Nonetheless, due to a lack of consistent and reliable animal or cell models for disease recapitulation, the molecular basis of AV block remains obscure, thus hindering the investigation and assessment of potential therapeutic methods. This dissertation first generated and characterized a transgenic mouse model with homozygous or heterozygous Lamin A/C (Lmna) R225X mutation in cellular structure, molecular profile, and cardiac electrophysiological function. LMNA is the gene encoding the nuclear lamin protein and the mutation of which is related to familial dilated cardiomyopathy associated with conduction disorders in the clinic. Although the homozygous mutation of Lmna was lethal to neonatal mice, heterozygous mice were asymptomatic with a higher occurrence of AV block during aging according to the electrophysiology study. Histological and molecular profiles revealed an increase in apoptotic cells and accumulation of extracellular matrix in Lmna R225X mutant hearts, suggesting the possible mechanism and drug targets for treatment of AV block. Also, improvement of ventricular ejection fraction for Lmna R225X mutant mice after three-months of endurance swimming, provides a potential, non-pharmacological therapy for LMNA- related cardiomyopathy. Next, investigation of LMNA mutation was extended in patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Three iPSC lines were generated from two patients with nonsense mutation R225X and Q354X, and another patient with a frame-shift mutation T518fs of the LMNA gene. Treatment with PTC124, a small molecule drug that promotes selective read-through of the disease-causing premature stop codon, alleviated Lamin A/C expression and decreased the cellular senescence and apoptosis induced by the continuous electrical stimulation in the iPSC-CMs bearing LMNA R225X but not in the other two lines. These results demonstrated the feasibility of using PTC124 to treat the nonsense mutation-induced LMNA-related cardiomyopathy. The last project of the dissertation proposed a novel method to fabricate a biological pacemaker that could restore AV conduction during AV block. Cardiac muscle strips, intended to provide the biological AV bypass, were generated using iPSC-CMs and collagen I with a bio-printing and hydrogel technique. iPSC-CMs were well aligned and electrically coupled on muscle strips with spontaneous firing properties. When implanted onto rat hearts, the engineered cardiac muscle strips alleviated the transient AV block induced by a bolus injection of adenosine triphosphate, indicating that engineered muscle strips provided alternative electrical conduits between the atria and ventricles. Taken together, LMNA R225X mutant cells and mice model serve as consistent and reliable in vivo/in vitro models to explore the mechanism and therapeutic methods for AV block. Using the engineered cardiac muscle strips which work as a biological pacemaker, is a promising cardiac pacing strategy to provide alternative electrical conduction between the atria and ventricles during AV block.