An hiPSC-CM approach for electrophysiological phenotyping of a patient-specific case of short-coupled TdP

A healthy young woman, age 26 without prior cardiac complications, experienced an out-of-hospital cardiac arrest caused by ventricular fibrillation (VF), which coincided with a fever. Comprehensive diagnostics including echo, CMR, exercise testing, and genetic sequencing, did not identify any potential cause. This led to the diagnosis of idiopathic VF and installment of an implantable cardioverter defibrillator, which six months later appropriately intervened another VF episode under conditions comparable to the first event. A second diagnostic opinion concluded short-coupled Torsade de Pointes (scTdP), and the patient was started on a verapamil treatment.

From this patient, human induced pluripotent stem cell cardiomyocyte (hiPSC-CM) lines were generated to study cellular electrophysiology. Without a known genetic pathogenic variation, no isogenic control line could be produced, therefore a healthy age- and sex-matched control hiPSC-CM line was used. Cellular electrophysiology was studied in these cardiomyocytes using calcium- and voltage sensitive fluorescent dyes and measurements were carried out at 37 °C and 39 °C, to mimic the condition of hyperthermia in the patient. mRNA expression of electrophysiologically relevant genes were analyzed to identify a potential underlying mechanism.

Calcium transients measured in patient lines at a physiological temperature indicated the occurrence of early after transients (EATs). Strikingly, at 39 °C the incidence of EATs further increased. Membrane potential data from the patient also revealed shorter action potentials that, combined with the EATs, indicate the premature release of calcium during diastole, which could be responsible for the extrasystoles in the patient. Gene expression profiles were mainly downregulated in the patient but could not clearly aid in unraveling a mechanism behind the occurrence of EATs. Pharmacological screening was performed to evaluate the treatment regimen and to determine a mechanism of action of the EATs. While verapamil, dantrolene, and flecainide did not decrease the incidence of EATs, calcium handling parameters were affected indicating functionality of the drugs.

This patient-specific case of electrophysiological phenotyping resulted in a hypothesis of the possible mechanism behind the scTdP arrhythmias, but also accentuates the applicability of patient-specific hiPSC-CM disease modeling and phenotyping.