Abstract Fri100: A novel LMNA mutant (R225X) leads to cardiac conduction disorders

Background: Cardiac conduction disorders (CCDs), including sick sinus syndrome, atrioventricular block (AVB), and bundle branch block, are common arrhythmic disorders. LMNA mutations (e.g. LMNA.pR225X), affecting the nuclear envelope proteins lamin A and lamin C, have been linked to CCDs in patients, yet the mechanistic connection remains largely unexplored. Objective: To uncover the mechanisms by which the LMNA.pR225X mutation contributes to CCDs. Methods: The LMNA-R225X knock-in and cardiac conduction system-specific LMNA-deficient (CCS-KD) mice were generated using CRISPR/Cas9 and Cre-loxP technology, respectively. Cardiac phenotype was assessed through surface ECG, telemetry ECG, programmed electrical stimulation, and echocardiography. Histology, Ca imaging, and RNA sequencing were performed to elucidate the mechanisms. Results: A heterozygous nonsense mutation (c.C673T, p.R225X) in LMNA was identified in a kindred and associated with progressive CCDs and atrial arrhythmias. In LMNA-R225X KI model, R225X-homozygous mice displayed bradycardia and first-degree AVB prior to death at the age of 2 weeks. R225X-heterozygous mice exhibited age-dependent progressive AVB ranging from first to second degree and increased susceptibility to pacing-induced atrial fibrillation, preceding cardiomyopathy. Similar CCDs phenotypes were confirmed in CCS-KD mice. Histology showed increased atrioventricular node (AVN) fibrosis and cellular/nuclear morphological changes in AVN pacemaker cells of R225X-heterzygous mice. Ca imaging demonstrated a reduced spontaneous firing rate of the AVN pacemaker cells. Bulk RNA-seq of the AVN tissue identified downregulation of Kcnn3 mRNA, encoding the SK3 channel, in R225X KI mice. Administration of SK3 agonists improved LMNA-related AVB and restored the spontaneous firing rate of AVN pacemaker cells. Conclusion: The LMNA-R225X mutation leads to progressive AVB prior to cardiomyopathy, primarily associated with increased AVN fibrosis and decreased SK3 channel expression in AVN pacemaker cells. This results in disrupted electrical conduction pathways and diminished pacemaker cell automaticity in the AVN. SK3 agonists improve LMNA-related AVB, presenting a potential therapeutic approach.