Long-QT syndromes are heritable diseases associated with prolongation of the QT interval on an electrocardiogram and a high risk of sudden cardiac death due to ventricular tachyarrhythmia. In long-QT syndrome type 1, mutations occur in the KCNQ1 gene, which encodes the repolarizing potassium channel mediating the delayed rectifier I_Ks current.

Coordinated release of calcium (Ca²⁺) from the sarcoplasmic reticulum (SR) through cardiac ryanodine receptor (RYR2) channels is essential for cardiomyocyte function. In catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited disease characterized by stress‐induced ventricular arrhythmias in young patients with structurally normal hearts, autosomal dominant mutations in RYR2 or recessive mutations in calsequestrin lead to aberrant diastolic Ca²⁺ release from the SR causing arrhythmogenic delayed after depolarizations (DADs). Here, we report the generation of induced pluripotent stem cells (iPSCs) from a CPVT patient carrying a novel RYR2 S406L mutation. In patient iPSC‐derived cardiomyocytes, catecholaminergic stress led to elevated diastolic Ca²⁺ concentrations, a reduced SR Ca²⁺ content and an increased susceptibility to DADs and arrhythmia as compared to control myocytes. This was due to increased frequency and duration of elementary Ca²⁺ release events (Ca²⁺ sparks). Dantrolene, a drug effective on malignant hyperthermia, restored normal Ca²⁺ spark properties and rescued the arrhythmogenic phenotype. This suggests defective inter‐domain interactions within the RYR2 channel as the pathomechanism of the S406L mutation. Our work provides a new in vitro model to study the pathogenesis of human cardiac arrhythmias and develop novel therapies for CPVT.

Frameshift mutations in the TTN gene encoding titin are a major cause for inherited forms of dilated cardiomyopathy (DCM), a heart disease characterized by ventricular dilatation, systolic dysfunction, and progressive heart failure. To date, there are no specific treatment options for DCM patients but heart transplantation. Here, we show the beneficial potential of reframing titin transcripts by antisense oligonucleotide (AON)‐mediated exon skipping in human and murine models of DCM carrying a previously identified autosomal‐dominant frameshift mutation in titin exon 326. Correction of TTN reading frame in patient‐specific cardiomyocytes derived from induced pluripotent stem cells rescued defective myofibril assembly and stability and normalized the sarcomeric protein expression. AON treatment in Ttn knock‐in mice improved sarcomere formation and contractile performance in homozygous embryos and prevented the development of the DCM phenotype in heterozygous animals. These results demonstrate that disruption of the titin reading frame due to a truncating DCM mutation can be restored by exon skipping in both patient cardiomyocytes in vitro and mouse heart in vivo , indicating RNA‐based strategies as a potential treatment option for DCM.

Calmodulin (CaM) is a small protein, encoded by three genes (CALM1-3), exerting multiple Ca²⁺-dependent modulatory roles. A mutation (F142L) affecting only one of the six CALM alleles is associated with long QT syndrome (LQTS) characterized by recurrent cardiac arrests. This phenotypic severity is unexpected from the predicted allelic balance. In this work, the effects of heterozygous CALM1-F142L have been investigated in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) obtained from a LQTS patient carrying the F142L mutation, i.e. in the context of native allelic ratio and potential gene modifiers. Skin fibroblasts of the mutation carrier and two unrelated healthy subjects (controls) were reprogrammed to hiPSC and differentiated into hiPSC-CMs. Scanty I_K1 expression, an hiPSC-CMs feature potentially biasing repolarization, was corrected by addition of simulated I_K1 (Dynamic-Clamp). Abnormalities in repolarization rate-dependency (in single cells and cell aggregates), membrane currents and intracellular Ca²⁺dynamics were evaluated as putative arrhythmogenic factors. CALM1-F142L prolonged repolarization, altered its rate-dependency and its response to isoproterenol. This was associated with severe impairment of Ca²⁺-dependent inactivation (CDI) of I_CaL, resulting in augmented inward current during the plateau phase. As a result, the repolarization of mutant cells failed to adapt to high pacing rates, a finding well reproduced by using a recent hiPSC-CM action potential model. The mutation failed to affect I_Ks and I_NaL and changed I_f only marginally. Intracellular Ca²⁺dynamics and Ca²⁺store stability were not significantly modified. Mutation-induced repolarization abnormalities were reversed by verapamil. The main functional derangement in CALM1-F142L was prolonged repolarization with altered rate-dependency and sensitivity to β-adrenergic stimulation. Impaired CDI of I_CaL underlined the electrical abnormality, which was sensitive to I_CaL blockade. High mutation penetrance was confirmed in the presence of the native genotype, implying strong dominance of effects.