Conclusion
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.
Methods
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.
Results
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.