The discovery of inducing pluripotency in somatic cells has opened a very exciting and promising field with regard to potential applications in medical research. Although the hiPSC technology still needs improvements and refinements, its contributions to disease modelling, drug screening and discovery, toxicity tests as well as cell transplantation studies are already well-recognized (Bellin et al. 2012; Fig. 5).
Figure 5. Promises of hiPSCs in medical research. Generated hiPSCs from a patient with a degenerative disease could be used for cellular therapy by autologous transplantation to repair degenerated or damaged tissues. Another medical use is to derive hiPSCs from patients with genetically inherited or other disorders, differentiate them in vitro and get novel insights into the molecular mechanisms of the disease. Differentiated cells of interest can also provide platforms for toxicology testing and personalized drug development. (Figure taken from Bellin et al. 2012)
1.4.1 Cell replacement therapy
The hiPSC technology offers the possibility to treat many degenerative diseases, including diabetes, Alzheimer´s disease, Parkinson´s disease or cardiovascular diseases by autologous cell transplantation (Fig. 5). The risk of immune rejection after autologous transplantation would be minimized and the use of immunosuppressive drugs might become unnecessary. In contrast to human ESCs (hESCs), patient-specific hiPSCs circumvent ethical concerns
Introduction
First therapeutic application studies with a mouse model of sickle cell anemia revealed promising resultsand provided a proof-of-concept illustration of the therapeutic use of iPSCs (Hanna et al. 2007). In this study, mice suffering from this disease could be rescued by autologous transplantation of iPSC-derived hematopoietic progenitor cells after the correction of the mutated hemoglobin allele by homologous recombination.
Studies like this may support the idea of using hiPSCs for heart regeneration. Recent work showed that hiPSC-derived CMs from a heart failure patient were able to engraft, survive, and integrate structurally with the host CMs after transplantation into rat hearts (Zwi-Dantsis et al.
2013). However, the correction of gene defects in (h)iPSCs remains challenging. More research focus has to be applied to improve efficiencies of gene correction technologies such as the use of zinc-finger nucleases (Wang et al. 2012), transcription activator-like effector nucleases (Hockemeyer et al. 2011), or clustered regularly interspaced short palindromic repeats (Mali et al. 2013) that induce DNA double-stranded breaks, followed by subsequent homology directed repair. Further investigation is necessary to ensure that the use of hiPSCs in cellular therapy is safe for patients and applicable in future.
1.4.2 Disease modelling
Patient-specific hiPSCs as a renewable and unlimited source for CMs also provide the possibility to study the pathophysiology of specific genetically inherited cardiac diseases in vitro (Fig. 5). Here, patient-specific hiPSC-derived CMs can act as a complementary model system to get a deeper insight into the molecular and electrophysiological mechanisms of arrhythmic syndromes. Patient-specific hiPSCs have already been generated from a wide spectrum of cardiac channelopathies, including LQTS type 1 (Moretti et al. 2010), type 2 (Itzhaki et al. 2011; Matsa et al. 2011; Lahti et al. 2012), and type 3 (Ma et al. 2013;
Terrenoire et al. 2013), Timothy syndrome (Yazawa et al. 2011), and catecholaminergic polymorphic ventricular tachycardia (CPVT, Fatima et al. 2011; Novak et al. 2012). All of these hiPSC models showed that the patient-specific hiPSC-derived CMs could recapitulate the disturbed electrophysiological phenotype of the arrhythmia syndromes in vitro.
Currently, four different (h)iPSC models have been generated for studying SCN5A mutation-related sodium channelopathies. The first iPSC model was generated from mouse embryonic fibroblasts (MEFs) of a Scn5aΔ/+ mouse model (ΔKPQ), showing that Scn5aΔ/+ iPSC-derived CMs could recapitulate the typical pathophysiological phenotype of LQTS type 3 in vitro
Introduction
(Malan et al. 2011). In another study, Davis and colleagues generated iPSCs from tail tip fibroblasts of the Scn5a1798insD/+ mouse, and differentiated them into CMs. They showed that Scn5a1798insD/+ iPSC-derived CMs exhibited features of both loss-of-function (reduced INa
density) and gain-of-function (larger persistent INa), mirroring the defects observed in primary adult CMs isolated from the Scn5a1798insD/+ mouse (Davis et al. 2012). Subsequently, hiPSCs were generated from a patient carrying the equivalent SCN5A1795insD/+ mutation in this study, and patch clamp measurements on derivative CMs revealed the biophysical abnormalities similar to those in mouse Scn5a1798insD/+ iPSC-derived CMs. It is interesting to note that the SCN5A1795insD/+ mutation clinically gives rise to an overlap phenotype of LQTS type 3 and BrS with conduction defects due to both gain- and loss-of-function effects on Nav1.5 (Bezzina et al. 1999). Moreover, hiPSCs carrying two other SCN5A mutations (p.F1473C, p.V1763M) were derived from patients with LQTS type 3 (Ma et al. 2013; Terrenoire et al. 2013). CMs derived from these hiPSCs showed significantly prolonged APD and enhanced persistent INa, recapitulating the typical pathophysiological phenotype of LQTS type 3. All of these studies indicate that (h)iPSC-derived CMs are suitable for studying complex sodium channel mutations in vitro. To our knowledge, no hiPSCs models have been reported regarding BrS associated with a SCN5A mutation.
1.4.3 Drug discovery and toxicity tests
In the last decade, novel drug discovery, development, and safety testing consisted of an arduous and expensive process. In 2001, drug development was abandoned because of lack of efficacy in 30% of the medicines that entered clinical trials, and in another 30% because of safety concerns such as cardiotoxicity and hepatotoxicity (Laustriat et al. 2010).
One major reason for the difficult translation of drug discovery from molecular levels and animal models to human therapeutics is the lack of economical and reliable methods that can accurately mimic the human physiological response. So far, the success of preclinical phases of drug development is mainly based on animal models (Gunaseeli et al. 2010). For instance, a number of drugs have been developed that showed therapeutic effects in rodent models of amyotrophic lateral sclerosis. Unfortunately, all of them turned out to be ineffective in human patients, emphasizing the necessity of disease models using human cells (Groeneveld et al.
2003; Shefner et al. 2004).
Introduction
For the development of anti-arrhythmic drugs, hiPSC-derived CMs may be useful in filling the gap between animal models and clinical trials. Importantly, they exhibit many of the characteristics of normal in vivo CMs, including molecular, structural, and functional properties such as ion channel, transporter, and receptor expression, as well as similar electrophysiological properties and biochemical responses (Ma et al. 2011). Recent studies show that hiPSC-derived CMs respond to specific drugs in a similar way that the human heart responds (Dick et al. 2010). Due to the properties of disease-specific hiPSC-derived CMs (e. g. cells from patients with sodium channelopathies), their application would provide a unique and predictive model for the pre-clinical screening of candidate anti-arrhythmic pharmacological agents. In addition, the effective development of new drugs requires predictive toxicity assays of adequate accuracy during preclinical testing. Currently, CMs from animals are used in pre-clinical models for cardiac toxicity tests. However, pharmaceuticals are designed to act on human targets. Because of species-related differences, the increased risk of cardiotoxicity may not be recognized prior to clinical trials. Furthermore, the use of animals is costly and involves ethical concerns. Differentiated CMs derived from hiPSCs may provide an alternative source for cardiac toxicity tests.