Significance and context
The human p53 protein is the most studied tumor suppressor protein and yet many questions still remain regarding its regulation and function. The activity of p53 is induced by DNA damage and genotoxic stress, initiating two potential outcomes - apoptosis or reversible cell-cycle arrest. These functions are thought to account for its tumor suppressor properties. The p53 protein is a transcription factor, containing an acidic transactivation domain, a sequence-specific DNA-binding domain and a tetramerization domain. Cancer-associated mutations frequently map to the DNA-binding region, creating effective dominant-negative polypeptides. p53 regulates the expression of many target genes, some of which partially explain its role in apoptosis and growth arrest. Examples include the p21/Cip1 cyclin-dependent kinase inhibitor that leads to G1/S arrest, and apoptosis-inducing genes such as fas and bax. Brodsky et al. describe the characterization of a Drosophila p53 homolog which regulates the expression of reaper (rpr), one of several fly genes associated with caspase-dependent apoptosis.
The authors used information generated by the Drosophila genome sequencing project to identify both genomic and cDNA clones and expressed sequence tags representing a Drosophilap53 homolog (Dmp53). Dmp53 is the most divergent p53 family member to be characterized, with the highest similarities in the DNA-binding region. Comparison with the human Hp53 protein revealed that residues critical for DNA binding are conserved and the most frequent tumor-associated mutation hotspots were identical or similar in fly and human sequences. The authors demonstrate functional conservation by showing that Dmp53 can bind to human p53 consensus binding sites and can activate transcription. Point mutants of Dmp53 corresponding to cancer mutational hotspots in Hp53 abolished DNA binding and inhibited transactivation. The authors used these dominant-negative forms in vivo (using the elegant GAL4-UAS overexpression system) to test their effect during wing development. Dominant-negative Dmp53 mutants had no effect on developmental apoptosis, but dramatically reduced apoptosis in the posterior wing imaginal disk following X-irradiation. Surprisingly, Dmp53 appears not to be involved in radiation-induced cell-cycle arrest. The authors identified a 20 bp p53-response element (p53RE) within the upstream region of the pro-apoptotic rpr gene. This enhancer region was shown to be p53-responsive in yeast and multimers of the p53RE were sufficient to direct radiation-responsive transactivation in transgenic flies in vivo.
Brodsky et al. conclude that Dmp53 is clearly a functional homolog of the Hp53 tumor suppressor. Like its mammalian counterpart, Dmp53 is important for the cellular stress response, but is not essential for normal development. Unlike the mammalian protein, Dmp53 regulates radiation-induced apoptosis but not radiation-induced cell-cycle arrest. The isolation of null mutants for the Dmp53 gene will provide further insights into p53 function and the identification of other pro-apoptotic target genes.
The identification of a fruit fly p53 homolog and the recent sequencing of the Drosophila genome offer an exciting opportunity for using the fly as a model organism to understand p53 regulation and function. The application of powerful fly genetics together with whole-genome expression analysis will undoubtedly lead to the identification of novel upstream regulators and downstream target genes. These insights may give new clues to therapeutic strategies to treat p53-associated malignancies.