Abstract 314: A Genetic Pathway Including Id2, Tbx5, and Nkx2–5 Required for Cardiac Conduction System Development
BACKGROUND: The conduction system is composed of a specialized group of cardiomyocytes that initiate and propagate electrical impulses to coordinate myocardial contraction. Despite the anatomic description of the structures comprising the conduction system 100 years ago, a molecular description of the conduction system is presently lacking.
METHODS: The molecular composition of the mouse ventricular conduction system was defined by serial analysis of gene expression (SAGE). Murine knockouts of conduction system candidate genes were evaluated by surface electrocardiography (ECG) and in-vivo electrophysiology.
RESULTS: A novel conduction system micro-dissection approach was developed to isolate conduction system tissue. A SAGE library generated from conduction tissue produced a catalogue of the distribution of cardiac conduction system RNAs. Conduction system fate is described as a composite of cardiomyocyte and neuronal fate. Id2, encoding a transcriptional repressor, demonstrated conduction system-specific expression within the myocardium by SAGE and in-situ hybridization. Id2 is shown to be required for normal conduction system function. Severe functional conduction system abnormalities were observed in adult Id2 knockout mice (Id2(−/−)), including a significantly prolonged QRS interval and left bundle branch block, a functional failure of left bundle branch conduction. Id2(−/−) animals demonstrated structural abnormalities of the conduction system anticipated by the functional deficits, including malformations of the left bundle branch and atrioventricular bundle. In-silico profiling identified Id2 as a potential transcriptional target of Nkx2–5 and Tbx5. We find that Nkx2–5 and Tbx5 in combination are necessary and sufficient for Id2 conduction system-specific expression. Using functional, molecular marker, and cell cycle criteria, we show that embryonic specification of the ventricular conduction system requires the coordinated activity of Id2, Nkx2–5 and Tbx5.
CONCLUSIONS: A molecular pathway including Tbx5, Nkx2–5, and Id2 establishes the regionally restricted program of cardiac conduction system gene expression and is required for specification of the ventricular cardiac conduction system.