p73 shares substantial structural and functional homology with p53. p73 can integrate diverse incoming death signals in vivo including DNA damage, oncogene deregulation and activation of T cell receptors, and in response mediate apoptosis in primary and tumor cells. For example, we recently showed that endogenous p73 is induced and activated by the oncogenes E2F1, cMyc and EtA. Nevertheless, p73's precise role in tumorigenesis is unclear because current genetic and expression data do not support a classic Knudson-type suppressor role. The mouse p73 gene is regulated by two promoters P1 and P2, with P1 producing full length p73 and P2 producing dominant negative deltaNp73 that lacks the transactivation domain. In mouse, deltaNp73 plays an important anti-apoptotic role in counteracting p53-mediated neuronal death during the sculpting of the developing brain. We have evidence that human p73 also has a functional P2 promoter which is generating deltaNp73 transcripts in tumors. Here we hypothesize that human p73 has an anti-tumor safeguard role in vivo, albeit weaker than p53, which is epigenetically rather than genetically targeted in tumors. We further hypothesize that a main mechanism of epigenetic p73 targeting is mediated through dominant negative interactions between p73 and i) mutant p53 proteins in tumors with p53 mutations ('double hit') and ii) transactivation-deficient isoforms of p73 itself such as deltaNp73. This epigenetic model could explain i) the lack of p73 mutations in human cancer, ii) the frequent tumor-associated overexpression of the p73 gene, since the contribution by dominant negative isoforms is currently not known and iii) the failure of p73-deficient mice to develop spontaneous tumors, because the presence of p53 could substitute for the suppressor function of p73. We will test this notion using genetic, functional and biochemical approaches in human tumors, cells lines and by generating an inducible deltaNp73 transgenic mouse model. We will also explore whether other established human oncogenes can activate p73.
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