Modelling Brugada Syndrome using induced pluripotent stem cells
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Objective: Brugada Syndrome is an autosomal dominant congenital heart disease that is responsible for 20% of sudden deaths of patients with structurally normal hearts. The majority of mutations involve the cardiac sodium channel gene SCN5A and give rise to classical symptoms, which include an abnormal electrocardiogram with ST segment elevation and a predisposition to ventricular fibrillation. To date, the implantation of a cardioverter defibrillator is the only proven effective treatment of the disease. The ability to reprogram dermal fibroblasts to induced pluripotent stem (iPS) cells and to differentiate these into cardiomyocytes with the same genetic background provides a novel approach to studying inherited cardiac channelopathies with advantages over existing model systems. Whilst this technique has enormous potential to model inherited channelopathies, such as Brugada Syndrome, the derived cells have not been fully characterised and compared to foetal and adult cardiomyocytes. Methods: Dermal fibroblasts from a patient with Brugada syndrome (SCN5A; c.1100G>A - pARG367HIS) and an age- and sex-matched control were reprogrammed using episomal vectors. All newly derived iPS cell lines were fully characterised using immunocytochemistry, flow cytometry, real-time quantitative reverse transcription PCR and single nucleotide polymorphism analysis and were compared to established human embryonic stem (hES) cell and in-house derived healthy control iPS cell lines. The same control cell lines were used to compare the efficiencies of several cardiac differentiation media. Spontaneously contracting areas, derived from control as well as patient iPS cell lines, were disaggregated and single cardiomyocytes were compared to foetal and adult cardiomyocytes isolated from primary human tissue using immunocytochemistry, transmission electron microscopy, membrane visualisation, calcium imaging and electrophysiology. Results: Comparison of cardiac differentiation protocols using healthy control hES and iPS cell lines found that despite significant inter-line variability with regard to efficiency of cardiac formation guided differentiation protocols could be used to reliably and efficiently generate beating bodies. Spontaneous contraction was observed in stem cell-derived cardiomyocytes and human foetal cardiomyocytes. Pluripotent stem cell-derived cardiomyocytes stained for markers of the cardiac contractile apparatus such as α-actinin, cardiac troponin I and cardiac troponin T. They also expressed functional voltage-activated sodium channels and exhibited action potential triggered calcium-induced calcium release. Stem cell-derived cardiomyocytes showed organisation of myofibrils, ultrastructure and calcium handling more similar to foetal than adult cardiomyocytes. Brugada Syndrome patient-specific cardiomyocytes were structurally indistinguishable from healthy control iPS cell line-derived cardiomyocytes. Electrophysiological analysis of sodium current density confirmed a ~50% reduction in patient-derived compared to healthy control-derived cardiomyocytes. Conclusion: Although iPS cells give rise to a mixture of immature and more mature cardiomyocytes, they all express typical cardiac proteins and have functional cardiac sodium channels. Results illustrate the ability of patient-specific iPS cell technology to model the abnormal functional phenotype of an inherited channelopathy that is independent of structural abnormalities and that the relative immaturity of iPS cell-derived cardiomyocytes does not prevent their use as an accurate model system for channelopathies affecting the cardiac sodium channel Nav1.5. This iPS cell based model system for classical Brugada Syndrome allows for the first time to study the mutation in its native environment and holds promise for further studies to investigate disease mechanisms of known and unknown mutations and to develop new therapies.