Abstract Body: Introduction: Antisense Oligonucleotides (ASOs) are an emerging class of drugs that target RNAs to impede their production, stability, and translation. Calmodulinopathies are severe cardiac arrhythmias caused by dominant mutations in CALM1, CALM2, or CALM3. Each of these genes encodes the identical calmodulin (CaM) protein. Leveraging this genetic redundancy, we hypothesized that ASO-mediated knockdown of the affected CALM gene would treat arrhythmia while functional redundancy of the other CALM genes would maintain CaM protein level and prevent toxicity.

Methods: Cells from a patient expressing the heterozygous CALM1- F142L mutation were reprogrammed into induced pluripotent stem cells (iPSCs; P-CALM1^F142L/+). Additionally, the variant or a nonsense codon were introduced into the wild-type reference line, WTC11, to yield isogenic disease (E-CALM1^F142L/+) and CALM1 null (CALM1^Δ/Δ) lines respectively. iPSCs were differentiated into cardiomyocytes (iPSC-CMs) to assess their action potentials and Ca²⁺ transients using multielectrode arrays and fluorescent voltage- and calcium-sensitive dyes. Calm1^N98S/+ mice, previously shown to develop catecholamine-induced ventricular tachycardia, were used for in vivo studies. Using these model systems, we tested ASOs that selectively depleted human or murine CALM1.

Results: Both E-CALM1^F142L/+ and P-CALM1^F142L/+ iPSC-CMs exhibited prolonged action potentials, consistent with QT prolongation observed in the proband. CALM1^Δ/Δ iPSC-CMs expressed normal levels of CaM protein and exhibited no change in action potential duration. ASO that selectively depleted CALM1 transcripts by ~50% did not alter CaM protein level but normalized action potential and Ca²⁺ transient duration in CALM1^F142L/+ iPSC-CMs. In Calm1^N98S/+ mice, ASO selectively depleted Calm1 transcripts by ~90% and prevented catecholamine-induced ventricular tachycardia. Calm1 ASO did not alter CaM protein level, cardiac function, or treadmill endurance, or cause significant histopathological changes.

Conclusion: ASO-based therapy safely and effectively treated arrhythmias caused by dominant CALM1 variants, suggesting that ASOs are a promising potential therapy for calmodulinopathy.