| - BACKGROUND: Although sudden cardiac death (SCD) from primary ion channelopathies accounts for a relatively small proportion of SCD cases in the general population, the investigation of causative (ion channel gene) mutations provides an important tool for the study of arrhythmia mechanisms. The functional consequences of such mutations can be complex, resolved only by combining appropriate clinical, experimental and theoretical approaches. Mutations in the cardiac Na+ channel gene (SCN5A) have been linked to 3 forms of arrhythmogenic disorders, namely Long QT Syndrome (LQTS), Brugada Syndrome (BS) and Conduction Disease (CD). Overlap in clinical presentation of these SCN5A-linked disorders has been recognized. At times overlap is more pronounced leading to so-called 'overlap syndromes'. We have reported such an SCN5A-linked channelopathy in a very extended Dutch kindred, wherein features of LQTS, BS and CD were detected within individuals carrying the SCN5A 1795insD mutation. - HYPOTHESIS / AIM: In order to gain further insight into the mechanism(s) of arrhythmia within this highly malignant family and to shed more light on mechanisms of cardiac Na+ channelopathy in general, we have generated a mouse carrying the Scn5a 1798insD mutation, the mouse equivalent of the human 1795insD mutation. In this project, we propose to fully characterize this mouse with respect to its cardiac electrophysiology, in particular with respect to the properties of the mutant channel in the native myocyte, and the mechanism of arrhythmia. Furthermore, since one form of SCN5A-linked CD is progressive in nature, presumably due to progressive fibrosis, we will compare fibrosis in hearts from wild-type and mutation-carrying mice at various ages. In addition, we will seek to reproduce the 'structural remodelling' phenotype observed in hearts of some patients with mutation in SCN5A, and shed light on the underlying mechanism(s). - METHODS: We will study transgenic mice and their wild-type littermates by (a) telemetric ECG characterization in freely-moving mice, (b) electrophysiological characterization of the Langendorff-perfused whole-heart by multi-electrode activation mapping and optical mapping, (c) analysis of Na+ channel density, kinetics and action potential morphology by patch-clamp analysis in isolated ventricular and sino-atrial node myocytes, and (d) characterization of Na+ channel expression and distribution by RT-PCR, Western blotting and immunohistochemistry. Additionally by immunohistochemical techniques, we will investigate the possible occurrence of fibrosis with increasing age. Finally we will investigate the occurrence of structural remodeling in the 1798insD mouse and attempt to reproduce the structural remodeling phenotype (namely fibrosis) observed in hearts with abnormal Na+ channel function through the crossing of mice obtained from other investigators (the SCN5A delta-KPQ mouse and the knock-out mouse). - EXPECTED RESULTS: Through this study we will shed light on mechanisms of cardiac Na+ channelopathy at the electrophysiological and structural level. These findings are not expected to impact only on the management of patients with this specific mutation but also on patients with other SCN5A mutations, as well as in other (more common) disorders such as ischemia and heart failure wherein Na+ channel function is also compromised. - OBJECTIVES: The major objectives of the project are to: (i) Investigate the arrhythmia mechanism and cause of death in transgenic mice carrying the Scn5a 1798insD mutation. Secondary objectives are to: (ii) Identify ionic remodeling events associated with cardiac Na+ channelopathy. (iii) Investigate possible occurrence of fibrosis with advancing age in mutant mice. (iv) Reproduce structural abnormalities that likely occur secondary to cardiac Na+ channelopathy (as seen in hearts of patients with compromised Na+ channel function) and investigate possible underlying mechanisms. - RELEVANCE FOR CARDIOVASCULAR DISEASES: This project aims to elucidate mechanisms of arrhythmias and mode of death in carriers of the 1795insD cardiac Na+ channel mutation. This will provide insight into the mechanism of Na+ channel-related arrhythmias both in patients with monogenic (primary) arrhythmias syndromes but also in the more common cardiac arrhythmias that occur in the context of ischemia and heart failure, wherein Na+ channel function is also altered. The elucidation of mechanisms responsible for arrhythmias and sudden cardiac death could result in better patient management by enabling optimisation of treatment. Furthermore it could open new research areas into the development of new therapeutic modalities. |
