The recently discovered p53-related genes, p73 and p63, express multiple splice variants and N-terminally truncated forms initiated from an alternative promoter in intron 3. To date, no alternative promoter and multiple splice variants have been described for the p53 gene. In this study, we show that p53 has a gene structure similar to the p73 and p63 genes. The human p53 gene contains an alternative promoter and transcribes multiple splice variants. We show that p53 variants are expressed in normal human tissue in a tissue-dependent manner. We determine that the alternative promoter is conserved through evolution from Drosophila to man, suggesting that the p53 family gene structure plays an essential role in the multiple activities of the p53 family members. Consistent with this hypothesis, p53 variants are differentially expressed in human breast tumors compared with normal breast tissue. We establish that p53␤ can bind differentially to promoters and can enhance p53 target gene expression in a promoter-dependent manner, while ⌬133p53 is dominant-negative toward full-length p53, inhibiting p53-mediated apoptosis. The differential expression of the p53 isoforms in human tumors may explain the difficulties in linking p53 status to the biological properties and drug sensitivity of human cancer.[Keywords: Splice; promoter; Drosophila; cancer; p73; p63] Supplemental material is available at http://www.genesdev.org.

Abstractp63, p73 and p53 compose a family of transcription factors involved in cell response to stress and development. p53 is the most frequently mutated gene in cancer (50%) and loss of p53 activity is considered to be ubiquitous to all cancers. Recent publications may have a profound impact on our understanding of p53 tumour suppressor activity. p63, p73 and p53 genes have a dual gene structure conserved in drosophila, zebrafish and man. They encode for multiple p63, p73 or p53 proteins containing different protein domains (isoforms) due to multiple splicing, alternative promoter and alternative initiation of translation. In this review, we describe the different isoforms of p63, p73, p53 and their roles in development and cancer. The changes in the interactions between p53, p63 and p73 isoforms are likely to be fundamental to our understanding in the transition between normal cell cycling and the onset of tumour formation. Cell Death and Differentiation (2006) Introduction p53 was discovered in 1979 as a protein interacting with the oncogenic T antigen from SV40 virus. 1-5 p53 protein is the product of a pivotal tumor-suppressor gene whose inactivation by mutation or interaction with viral or overexpressed cellular proteins occurs in almost all cancers. 6 The p53 protein integrates multiple cellular stress signals assessing cellular damages to trigger either cell-cycle arrest or programmed cell death (apoptosis). 7 These effects are predominantly due to p53's ability to bind DNA through p53-responsive element (p53RE) 8,9 and regulate the transcription of genes involved in these processes, such as p21 10 (cell cycle arrest), Puma 11 or Scotin 12 (apoptosis). Thereby, p53 prevents proliferation of genetically abnormal cells and thus cancer formation.Two p53-related genes, p63 and p73, were identified in 1997. 13,14 The high level of sequence similarity between p63, p73 and p53 proteins, particularly in the DNA binding domain, allows p63 and p73 to transactivate p53-responsive genes causing cell cycle arrest and apoptosis. Therefore, p63, p73 and p53 genes form a family of transcription factor. However, they are not functionally entirely redundant and the primary role of each p53 family member -as determined by transgenetic knockout mice -illustrates that each protein has its own unique functions.We recently published that the p53 gene family has a dual gene structure conserved from drosophila to man. 15 Like most of the genes in the human genome, 16 p53 gene family members express multiple mRNA variants due to multiple splicing and alternative promoters. Hence, p53 gene family members express different forms of p53 protein containing different domain of the protein (isoforms). In this review, we will summarise the different isoforms of p63, p73 and p53 expressed in human, mouse and drosophila. p63The mouse p63 gene is composed of 15 exons spanning over 208 000 bp (GenBank Accession Number: AF533892) on chromosome 16, while the human p63 gene is composed of 15 exons, spanning over 270 000 bp on chromosome 3q2...

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