Project description:Rationale: The atrioventricular conduction system controls ventricular activation and is delineated by expression of Tbx3. Genome-wide association studies identified genetic variants near TBX3 associated with conduction velocities (PR interval and QRS duration), suggesting minor changes in TBX3 dose affect conduction system function. Objective: To assess whether and how Tbx3 dose reduction affects the integrity of the atrioventricular conduction system. Methods and Results: Electrocardiograms revealed a PR interval shortening and prolonged QT interval and QRS duration in heterozygous Tbx3 mutants compared to wild-types. We observed that the atrioventricular bundle and proximal bundle branches of Tbx3+/- mice after birth became hypoplastic, whereas the size of the atrioventricular node was not affected. The transcriptomes of wild-type and Tbx3+/- atrioventricular nodes were analyzed using BAC-Tbx3-Egfp mice enabling specific isolation of the atrioventricular node by laser capture microdissection followed by RNA-sequencing. Hundreds of genes were slightly but consistently deregulated. Cross-referencing with transcriptome data of isolated cardiomyocytes of the conduction system and chamber myocardium derived from Tbx3+/Venus;BAC-Nppb-Katushka hearts revealed that a set of chamber-enriched genes, including Kcne1 (MinK), Ryr2, and Scn5a, were upregulated in Tbx3+/- atrioventricular nodes, whereas conduction system-enriched genes, including Hcn4 and Cacna2d2, were downregulated. We performed ATAC-sequencing on purified fetal Tbx3+ atrioventricular cardiomyocytes to identify potential atrioventricular-specific regulatory DNA elements on a genome-wide scale, and identified regulatory elements mediating the Tbx3-dependent regulation of Ryr2 and other target genes in the atrioventricular node. Conclusions: Tbx3 dose reduction results in deregulation of a large number of genes affecting the electrical properties of the atrioventricular node and causes failure to maintain the structural integrity of the atrioventricular bundle. These data provide a mechanism underlying differences in PR interval and QRS duration in individuals carrying associated variants in the TBX3 locus.

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