Pluripotent stem cells, derived from skin fibroblasts of two relatives with familial long-QT syndrome and induced to differentiate into cardiac myocytes, exhibited the electrophysiologic traits of the disorder, showing for the first time that stem cell models “can recapitulate aspects of genetic cardiac diseases,” according to a small study reported in the Oct. 7 issue of the New England Journal of Medicine.
The research findings have far-reaching implications for cardiac research in general, not just for the relatively uncommon long-QT syndrome, including opening the door to in vitro development and testing of new cardiac medications, said Alessandra Moretti, Ph.D., and her associates at the Technical University of Munich.
They obtained skin fibroblasts from an 8-year-old boy when clinical evaluation for attention-deficit/hyperactivity disorder revealed a prolonged QT interval on his electrocardiogram, and subsequent genotyping revealed that he carried a mutation of the KSNQ1 gene that characterizes long-QT syndrome type 1. Skin fibroblasts also were obtained from the patient’s father, who also carried the same mutation.
The researchers grew pluripotent stem cell colonies from these cells, as well as from the cells of two healthy control subjects. The cells were induced to differentiate into the cardiac lineages. Then spontaneously beating cells were subjected to pacing, then separated into three distinct types of action potentials corresponding to “ventricular,” “atrial,” and “nodal” myocytes.
When compared with myocytes derived from the control subjects, the patients’ myocytes showed prolongation of the action potential, altered IKs activation and deactivation, and an abnormal response to catecholamine stimulation – all of which are characteristic of the cardiac abnormalities in long-QT syndrome. Further characterization of the KSNQ1 mutation showed that “a dominant negative trafficking defect” was associated with the 70%-80% reduction in IKs current and altered properties governing channel activation and deactivation.
The investigators then assessed the effects of adrenergic stimulation by exposing the beating myocytes to isoproterenol. This shortened the action potential of the myocytes from the control subjects but lengthened that of the myocytes from affected patients, “exacerbating the long-QT syndrome type 1 phenotype and increasing the risk of arrhythmic events” in the cells.
Pretreating the myocytes by exposing them to the beta-blocker propranolol induced a protective effect, much as beta-blocker therapy protects affected patients from the effects of abnormally prolonged ventricular repolarization phase: a propensity for polymorphic ventricular tachycardia and sudden cardiac death.
“Our findings suggest that there may be alternative approaches to the development of candidate drugs” to treat this and other genetic cardiac disorders, Dr. Moretti and her colleagues said (N. Engl. J. Med. 2010;363:1397-409).
In particular, “the observed protective effects of beta-blockade show that it is possible to investigate the therapeutic action of medications for treating human cardiac disease in vitro with the use of patient-specific cells. This approach is particularly attractive because of the pluripotent nature of these cells and the potentially unlimited number of induced cardiomyocytes available for high-throughput drug development,” they noted.
Until now, research on the pathogenesis of the long-QT syndrome has relied primarily on genetic animal models. But because of the differences among species in the chemical channels that generate cardiac repolarizing currents, “none of the available [animal] models of the long-QT syndrome fully emulate the human disease phenotype.”
This study thus demonstrates “the importance of alternative systems in which human genetic disorders can be studied in the physiologic and disease-causing contexts on a patient-specific level,” the researchers said.
This work was supported by grants from the European Research Council, the German Research Foundation, and the German Ministry for Education and Research. One of the investigators also reported receiving Deutsche Forschungsgemeinschaft grant support.