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The 22q11.2 deletion syndrome (22q11.2DS) is the most common deletion syndrome in humans (1:4000 live births). Mutation of TBX1, located within the deletion and encoding a T-box transcription factor, is sufficient to recapitulate most if not all the 22q11.2DS phenotype in patients. Tbx1 has a critical role in cardiovascular and pharyngeal development, and, in many tissues affected, there is reduced cell proliferation. Homozygous mutant mice show developmental anomalies of the pharyngeal apparatus and reduced cell proliferation. During mouse heart development, Tbx1 functions in cardiac progenitors where it modulates positively cell proliferation and negatively cell differentiation.We have identified distinct genetic pathways within which Tbx1 effects these basic cellular functions. We now aim at genetic and pharmacological strategies to rescue the mutant phenotype. We considered cell proliferation as a potential target. We found that the cell cycle inhibitor p21 is negatively regulated by Tbx1. p53 regulates positively cell cycle inhibitors and, therefore, it has been our first target. In addition, there is already a drug that inhibits p53 and that has been used for in vivo experiments in mice, Pifithrin. We therefore tested whether reduced dosage of p53 (either genetically, using a p53 mutant, or pharmacologically using Pifithrin) may modify the Tbx1 mutant phenotype in vivo. Results showed a near complete rescue of the haploinsufficient phenotype in E10.5 embryos using genetic ablation, and a less dramatic but still significant rescue using the drug.Next, we tested whether p53 mutation could rescue the Tbx1 homozygous mutant phenotype. We tested N Tbx1-/- embryos and N Tbx1-/-;p53 + /- embryos at E18.5. Results showed no phenotypic difference between the two genotypes. Thus, p53 heterozygous mutation (or its pharmacological inhibition) is sufficient to rescue the pharyngeal arch artery phenotype of Tbx1 + /- embryos, but it does not modify the more complex heart phenotype of null embryos. Experiments are in progress to establish the mechanisms by which p53 dosage interferes with Tbx1 functions.