Dosage -dependent role of Tbx1 in 22q11 deletion syndrome mouse models
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The 22q11 deletion syndrome (22q11DS) is a congenital anomaly disorder affecting structures derived from the pharyngeal apparatus including the craniofacial region, thymus gland and heart. Most affected individuals have the same sized 3 Mb deletion in chromosome 22q11.2 region, yet the disorder varies in severity. Tbx1, a member of T-box containing transcription factor family, was recently identified to be the gene responsible for the etiology of the 22q11DS syndrome by mouse genetic studies. Tbx1 is expressed in the endoderm and the core mesoderm of the pharyngeal apparatus as well as the secondary heart field (SHF) during embryonic development. Tbx1 +/- mice have mild cardiovascular defects, while Tbx1 -/- mice show severe anomalies in most pharyngeal apparatus-derived organs and die at birth. Interestingly, bacterial artificial chromosome (BAC) transgenic mice overexpressing human TBX1 and three other transgenes have strikingly similar malformations as in the syndrome. To determine whether overexpression of TBX1 is responsible for the phenotype in BAC transgenic mice, I performed genetic complementation experiments by crossing the BAC transgenic mice into the Tbx1 null mutant background. I found that partial normalization of the Tbx1 dosage rescued most of the malformations in BAC transgenic mice, suggesting that altered TBX1 dosage and not overexpression of the other transgenes is responsible for the mutant phenotype. In order to delineate genes that act downstream of Tbx1, I performed a gene profiling study using Affymetrix microarrays on microdissected distal pharyngeal apparatus from wild type and Tbx1 -/- embryos at E8.75, E9.75 and E10.75. Based on their expression pattern and developmental functions, four groups of genes were identified and validated by quantitative RT-PCR (qRT-PCR) and in situ hybridization (ISH), including Nkx2-6, Pax9, Gcm2 in the pharyngeal endoderm, Msc and Myf5 in the core mesoderm, Aldh1a2, Tbx5, Gata4, Hod and some cardiac muscle differentiation marker genes in the SHF, as well as Lhx8 and Dlx genes in the neural crest cells (NCCs). My study contributes significantly to our understanding of dosage-dependent role of Tbx1 in the development of the pharyngeal apparatus and the pathogenesis of 22q11DS.
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