Switch From Fetal to Adult SCN5A Isoform in Human Induced Pluripotent Stem Cell–Derived Cardiomyocytes Unmasks the Cellular Phenotype of a Conduction Disease–Causing Mutation

Abstract

Background Human induced pluripotent stem cell–derived cardiomyocytes (hi PSC ‐ CM s) can recapitulate features of ion channel mutations causing inherited rhythm disease. However, the lack of maturity of these cells is considered a significant limitation of the model. Prolonged culture of hi PSC ‐ CM s promotes maturation of these cells. We studied the electrophysiological effects of the I230T mutation in the sodium channel gene SCN 5A in hi PSC ‐ CM s generated from a homozygous (I230T homo ) and a heterozygous (I230T het ) individual from a family with recessive cardiac conduction disease. Since the I230T mutation occurs in the developmentally regulated “adult” isoform of SCN 5A, we investigated the relationship between the expression fraction of the adult SCN 5A isoform and the electrophysiological phenotype at different time points in culture. Methods and Results After a culture period of 20 days, sodium current ( I N a ) was mildly reduced in I230T homo hi PSC ‐ CM s compared with control hi PSC ‐ CM s, while I230T het hi PSC ‐ CM s displayed no reduction in I N a . This coincided with a relatively high expression fraction of the “fetal” SCN 5A isoform compared with the adult isoform as measured by quantitative polymerase chain reaction. Following prolonged culture to 66 days, the fraction of adult SCN 5A isoform increased; this was paralleled by a marked decrease in I N a in I230T homo hi PSC ‐ CM s, in line with the severe clinical phenotype in homozygous patients. At this time in culture, I230T het hi PSC ‐ CM s displayed an intermediate loss of I N a , compatible with a gene dosage effect. Conclusions Prolonged culture of hi PSC ‐ CM s leads to an increased expression fraction of the adult sodium channel isoform. This new aspect of electrophysiological immaturity should be taken into account in studies that focus on the effects of SCN 5A mutations in hi PSC ‐ CM s.