The isoforms of the p53 protein

Abstract

p53 is a transcription factor with a key role in the maintenance of genetic stability and therefore preventing cancer formation. It belongs to a family of genes composed of p53, p63, and p73. The p63 and p73 genes have a dual gene structure with an internal promoter in intron-3 and together with alternative splicing, can express 6 and 29 mRNA variants, respectively. Such a complex expression pattern had not been previously described for the p53 gene, which was not consistent with our understanding of the evolution of the p53 gene family. Consequently, we revisited the human p53 gene structure and established that it encodes nine different p53 protein isoforms because of alternative splicing, alternative promoter usage, and alternative initiation sites of translation. Therefore, the human p53 gene family (p53, p63, and p73) has a dual gene structure. We determined that the dual gene structure is conserved in Drosophila and in zebrafish p53 genes. The conservation through evolution of the dual gene structure suggests that the p53 isoforms play an important role in p53 tumor-suppressor activity. We and others have established that the p53 isoforms can regulate cell-fate outcome in response to stress, by modulating p53 transcriptional activity in a promoter and stress-dependent manner. We have also shown that the p53 isoforms are abnormally expressed in several types of human cancers, suggesting that they play an important role in cancer formation. The determination of p53 isoforms' expression may help to link clinical outcome to p53 status and to improve cancer patient treatment.

Figure 1.

Schema of the p53 genes of (A) Homo sapiens (human), (B) Danio rerio (zebrafish), and (C) Drosophila. Promoters (P1, P1′, and P2) are indicated as ↱. Exons are numbered, noncoding exons are represented by black boxes, and coding exons are represented by white boxes. The size of the boxes is not proportional to the size of the exons. Note: An intraexon splicing of Homo sapien (human) p53 mRNA has been reported through a noncanonical splicing mechanism of the exon-7 to exon-9, leading to a p53 isoform deleted of the conserved box V in the DNA binding domain Δp53 (Rohaly et al. 2005). However, expression of Δp53 at the mRNA and protein levels is still controversial and has not been confirmed by other laboratories.

Figure 2.

Schema of the p53 isoform proteins encoded by Homo sapiens (human) (A), Danio rerio (zebrafish) (B), and Drosophila (C) p53 genes. (TAD) transactivation domain (TAD1 and TAD2); (PrD) proline domain; (NLS) nuclear localization signal; (OD) oligomerization domain; (BR) basic region. The black boxes represent amino-acid domains conserved through evolution. The amino-acid positions defining p53 domains are indicated. Carboxy-terminal amino-acid sequences of Homo sapien (human) p53β and p53γ are indicated. p53 domains for Drosophila p53 are based on protein alignment and need to be confirmed by experimental evidence.

Figure 3.

Regions of Homo sapien (human) p53 protein isoforms specifically recognized by p53 antibodies. (A) Epitope map for commercially available p53 antibodies. Epitopes of the mouse monoclonal antibodies DO-1, DO-7, 1801, DO-12, BP53.10, 421, and ICA-9 are indicated in violet, epitopes of the rabbit polyclonal antibody CM-1 raised against full-length recombinant Homo sapien (human) p53 protein are indicated in blue, and epitopes of the sheep polyclonal antibody SAPU raised against full-length recombinant Homo sapien (human) p53 protein are indicated in brown. (TAD) transactivation domain; (PrD) proline domain; (NLS) nuclear localization signal; (OD) oligomerization domain; (BR) basic region. The amino-acid positions defining p53 domain are indicated. The carboxy-terminal domains of p53β and p53γ are indicated with a yellow and green box, respectively. The molecular weight of each Homo sapien (human) p53 isoform is indicated (kDa). (B, C) p53 isoforms specifically recognized by the rabbit polyclonal antibodies specific for β isoforms (KJC8) and Δ133p53 isoforms (MAP4.9), respectively.