T-box genes coordinate regional rates of proliferation and regional specification during cardiogenesis

Abstract

Mutations in T-box genes are the cause of several congenital diseases and are implicated in cancer. Tbx20-null mice exhibit severely hypoplastic hearts and express Tbx2, which is normally restricted to outflow tract and atrioventricular canal, throughout the heart. Tbx20 mutant hearts closely resemble those seen in mice overexpressing Tbx2 in myocardium,suggesting that upregulation of Tbx2 can largely account for the cardiac phenotype in Tbx20-null mice. We provide evidence that Tbx2 is a direct target for repression by Tbx20 in developing heart. We have also found that Tbx2 directly binds to the Nmyc1 promoter in developing heart, and can repress expression of the Nmyc1 promoter in transient transfection studies. Repression of Nmyc1 (N-myc) by aberrantly regulated Tbx2 can account in part for the observed cardiac hypoplasia in Tbx20 mutants. Nmyc1 is required for growth and development of multiple organs, including the heart, and overexpression of Nmyc1 is associated with childhood tumors. Despite its clinical relevance, the factors that regulate Nmyc1 expression during development are unknown. Our data present a paradigm by which T-box proteins regulate regional differences in Nmyc1 expression and proliferation to effect organ morphogenesis. We present a model whereby Tbx2 directly represses Nmyc1 in outflow tract and atrioventricular canal of the developing heart, resulting in relatively low proliferation. In chamber myocardium, Tbx20 represses Tbx2,preventing repression of Nmyc1 and resulting in relatively high proliferation. In addition to its role in regulating regional proliferation, we have found that Tbx20 regulates expression of a number of genes that specify regional identity within the heart, thereby coordinating these two important aspects of organ development.

Tinman is a direct activator of midline in the drosophila dorsal vessel

“…They are also required for distinguishing between the valve and contractile cardioblast fates . In vertebrate model organisms, Tbx20 is also essential for heart development (Cai et al, 2005;Takeuchi et al, 2005) and mutations in human Tbx20 are associated with familial congenital heart defects (Kirk et al, 2007;Posch et al, 2010).…”

T-box 2, a mediator of Bmp-Smad signaling, induced hyaluronan synthase 2 and Tgfβ2 expression and endocardial cushion formation

“…Expression of chamber-specific myocardial genes, which include Nppa (encoding atrial natriuretic factor, ANF), Gja5 (encoding connexin 40, Cx40), and Gja1 (encoding connexin 43, Cx43), were repressed by Tbx2 (3)(4)(5). Tbx2 null mutant embryos exhibited small AVC and defective OFT septation (3), whereas Tbx2 transgenic expression blocked chamber formation (4) and cell proliferation in the OFT and AVC (6).…”

Probing chromatin landscape reveals roles of endocardial TBX20 in septation

“…Mice were maintained on a Black Swiss (NIHBL(S); Taconic Biosciences) outbred background. Tbx20-conditional and -null alleles and the Tbx20-GFP and Nfatc1-Cre mouse lines were generated in the authors' laboratories and have been previously described (8,16,31). Tie2-Cre (69), Rosa26 flox-stop-flox tdTomato (Rosa tdTom) (70), and Rosa26 membrane-targeted (m) tdTom flox-stop-flox mGFP (Rosa mT/ mGFP) (71) mice were obtained from The Jackson Laboratory.…”

“…Mice that are globally null for Tbx20 are embryonically lethal around E10 and show a severely hypomorphic heart that is poorly proliferative and lacks specialized chamber myocardium (7)(8)(9)(10). Early lethality of the global mutants prevents the study of later roles of Tbx20.…”