Abstract 783: Transcription Factor Tbx3 Delineates the Forming AV-bundle and Bundle Branches
Background: The mechanisms that underlie specification and formation of the atrioventricular bundle (AVB) and bundle branches (BB) are largely unknown. Tbx3 is a transcriptional repressor that is expressed in all components of the central conduction system of the heart from the earliest stages of its development onwards. Previous analysis has revealed that Tbx3 controls sinoatrial node formation and the pacemaker gene program, indicating that Tbx3 may be important in the formation of the ventricular conduction system as well.
Objective and methods: We analyzed Tbx3-deficient mice, which express the Cre gene in the Tbx3 pattern, to assess the function of Tbx3 in the developing AVB and BB. Knock-out (KO) and wild-type (WT) littermate embryos of different ages were analyzed for known and suspected target genes of Tbx3 by immunohistochemistry and in situ-hybridization.
Results: KO embryos die between 12 and 15 days of gestation. We observed ventricular septal and outflow defects in nearly all KO embryos (11 of 14 analyzed). In WT embryos, expression of Cx43, ANF, Tbx18 and Tbx20 was specifically absent from the forming AVB and BB. All of these genes were ectopically expressed in the developing Cre-expressing AVB and BB of KO embryos. Also, Cx40 and Nav1.5 (SCN5a), which are suppressed by Tbx3, but are induced in the AVB and BB during development, were precociously up-regulated in KO embryos. Finally, while the forming AVB and BB show very low rates of proliferation compared to the adjacent myocardium, the proliferation rate of the AVB and BB in KOs was increased to levels comparable to the ventricular myocardium.
Discussion and conclusions: These data demonstrate that Tbx3 is required for the specification and formation of the AVB and BB, and for the organization of ventricular septal development. Tbx3 expression delineates the forming AVB and BB. It suppresses the developing working myocardial phenotype by suppression of working myocardial genes and proliferation. Doing so, Tbx3 diverts the phenotype of the cells within its expression domain towards cells of the ventricular conduction system.