Cloning and characterization of zebrafish tbx1.

Tbx1 is one of the genes within the DiGeorge Critical Region (DGCR) and has been recently identified as the critical gene for the cardiovascular anomalies in the DiGeorge mouse models. We have cloned, sequenced and analyzed the zebrafish (Danio rerio) tbx1 cDNA. It encodes a protein of 460 amino acids that shares 64% identity and 67% similarity with the human TBX1 orthologue at the amino acid level. Although maternal expression was detected by RT-PCR, only zygotic expression could be detected by whole-mount in situ hybridization. Expression of zebrafish tbx1 by whole-mount in situ hybridization was first detected at 40% epiboly, 5.0 hours post fertilization (hpf) in the dorsal blastoderm margin. Through the stage of embryonic shield formation, tbx1 expression is restricted to the hypoblast, in the region of cells fated to become head and lateral plate mesoderm and pharyngeal endoderm. At 18 hpf, when the heart tube is beginning to assemble, three domains of tbx1 expression can be seen: cardiac precursors, pharyngeal arch precursors and otic vesicle. These three domains will remain the sites of tbx1 expression to varying degrees through at least 72 hpf. By 51 hpf, tbx1 expression can be seen in the cardiac outflow tract, the ventricle and the atrium, although by 72 hpf cardiac expression is strongest in the cardiac outflow tract. This newly identified tbx1 expression pattern in cardiac regions other than the cardiac outflow tract offers a new insight into the role of the tbx1 transcription factor in cardiac development.