Fig. 4. Proposed gene-targeting experiments to determine whether TP53-mediated upregulation of a specific direct TP53 target gene is responsible for tumour suppression.

TP53 acts as a tumour suppressor through transcriptional induction of target genes. Binding of TP53 to TP53 binding sites in the promoter and intronic regions induces upregulation of a TP53 target gene. This allows the protein encoded by this TP53 target gene to be upregulated and exert its function that contributes to TP53-mediated tumour suppression. Mutation of TP53 binding sites in target genes can identify the critical genes important for TP53-mediated tumour suppression. Gene targeting can be used to mutate the TP53 binding site(s) in a gene of interest. In cells derived from such genetically modified mice, the TP53 target gene of interest cannot be upregulated by activated TP53, but it can still be regulated by other transcription factors that bind to other sequences in its promoter or intronic regions. If these genetically modified mice, for example after introducing a potent oncogene, such as c-MYC or mutant RAS, will show accelerated tumour development, this would demonstrate that TP53-mediated upregulation of this gene is critical for tumour suppression. If these genetically modified mice are not tumour prone, this would indicate that transcription factors in addition to TP53 can also induce the expression of this protein at levels sufficient for effective tumour suppression.