The genes that encode the p53 family members p63 and p73 enable the production of several different protein isoforms. In light of this fact, Jean-Christophe Bourdon, David Lane and colleagues have re-investigated the gene structure of the founding member, TP53. They found that TP53 in fact encodes at least six different p53 mRNA isoforms, some of which are differentially regulated in cancer cells.

Mammalian genomes contain three members of the TP53 family, yet only one form exists in invertebrates, implying that the mammalian members are derived from the triplication of one ancestral gene. If this hypothesis is correct, it is somewhat strange that TP53 does not share the complexity of TP63 and TP73, both of which can be transcribed from an alternative internal promoter and express at least 3 and 11 alternatively spliced isoforms, respectively. TP53, on the other hand, was thought to have a much simpler structure with only one promoter that transcribes three mRNA splice variants.

To assess TP53 and all of its encoded mRNAs, Bourdon et al. used GeneRacer PCR, a technique that amplifies only capped mRNA transcripts and so allows the detection of the transcription initiation site. They also designed specific primers for exons four and five in an attempt to identify any transcripts that might be generated from an internal promoter.

The authors found that, altogether, TP53 can theoretically transcribe nine different p53 isoforms. These are full length p53, p53β and p53γ, Δ133p53, Δ133p53β and Δ133p53γ (Δ133 owing to alternative internal promoters in intron four), and Δ40p53, Δ40p53β and Δ40p53γ (Δ40 owing to the alternative splicing of intron two or use of an alternative translation-initiation site). The β and γ isoforms arise from alternative splicing of intron nine. All of these mRNAs can be detected in normal human tissue samples in a tissue-specific manner and all of these mRNAs lead to protein expression. Endogenous p53β isoforms were detected by specific antibodies. However, isoform-specific antibodies still need to be generated to detect endogenous p53γ and Δ133p53 protein isoforms.

Do any of these isoforms affect p53 function? Further investigations showed that the p53β isoform binds the BAX promoter more readily than the MDM2 promoter (or the CDKN1A promoter), whereas p53 preferentially binds MDM2 over BAX. The authors show that this causes p53β to enhance p53-mediated BAX promoter activity, but that this does not seem to affect the level of apoptosis induced in cells expressing both p53 and p53β. On the other hand, Δ133p53 inhibits p53-mediated apoptosis, indicating that it can function as a dominant negative.

The authors also assessed expression of the mRNA isoforms in human breast tumour samples. None of the 30 samples expressed the same combination of p53 isoforms that is seen in normal breast tissue. For example, TP53γ and TP53β, which are expressed in normal breast tissue, were either not detected or detected in only ten samples, respectively. Δ133TP53, which is not expressed in normal breast tissue, was detected in 24 samples. Notably, only five of the tumours expressed a mutant form of p53.

On the basis of these findings, the authors conclude that the regulation of expression of the p53 isoforms seems to be altered in breast cancer, a finding that could be relevant in tumours that express wild-type p53.