Computational modeling of timothy syndrome using a human induced pluripotent stem cell-derived ventricular cardiomyocyte model with improved Ca2+current dynamics
A Trancuccio, F Simone, J K Sochacki, C Martinez-Prieto, S G Priori, L F Pavarino, D J SantiagoAbstract
Background
Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) represent a powerful platform to model inherited arrhythmia syndromes(1). However, current in silico models incompletely reproduce intracellular Ca²⁺ handling, particularly L-type Ca²⁺ current (ICaL) dynamics (2), limiting their ability to simulate Ca²⁺-dependent arrhythmogenic mechanisms and channelopathies such as Timothy Syndrome (TS)(3).
Purpose
To develop and validate a novel ventricular hiPSC-CM computational model incorporating a Markovian formulation of ICaL, enabling improved representation of Ca²⁺ dynamics and mechanistic simulation of Timothy Syndrome.
Methods
Starting from the Paci2020 hiPSC-CM model(4), we replaced the Hodgkin–Huxley ICaL formulation with a Markovian scheme separating voltage-dependent and Ca²⁺-dependent inactivation. Model parameters were optimized against experimental biomarkers obtained from wild-type (WT) hiPSC-CMs, including spontaneous action potential (AP) morphology and Ca²⁺ transient (CaT) kinetics. Validation included: (a) simulation of dose-dependent ICaL blockade with nifedipine; (b) induction of Ca²⁺ overload to assess delayed afterdepolarizations (DADs); (c) implementation of the p.G406R CACNA1C mutation to reproduce the TS phenotype without additional parameter fitting. A population-based sensitivity analysis (n=500 variants) was performed to evaluate robustness and intercellular variability, as in Sobie et al. (5).
Results
The optimized model accurately reproduced spontaneous AP features (APD90 253 ms vs 255 ms experimental) and CaT biomarkers within the experimental mean ± SD range. Nifedipine simulations demonstrated dose-dependent ICaL reduction, AP shortening (−16.7% vs −17.2% experimental at 30 nM), and increased AP triangulation. Under Ca²⁺ overload, the model developed DAD-mediated triggered activity at extracellular Ca²⁺ ≥3.4 mM, reproducing expected NCX-mediated depolarizing mechanisms. Introduction of the TS mutation resulted in marked AP prolongation (APD90 +47.7% vs +43.2% experimental), increased sarcoplasmic reticulum Ca²⁺ load (+54%), and augmented CaT amplitude, capturing hallmark disease features without further calibration. Sensitivity analysis confirmed model stability and identified key modulators of repolarization and automaticity.
Conclusions
This novel Markovian hiPSC-CM model provides an improved mechanistic representation of ICaL and Ca²⁺ handling, enabling accurate simulation of pharmacological modulation, Ca²⁺-dependent arrhythmogenesis, and Timothy Syndrome. This robust framework allows mechanistic investigation of Ca²⁺-related arrhythmias and represents a potent platform for translational and personalized medicine applications.Figure