The p53 is the most commonly mutated tumour suppressor gene in human cancers. Unlike other tumour suppressor genes, most p53 cancer mutations are missense mutations leading to the expression of full-length mutant p53 protein. Accumulating evidence has indicated that p53 cancer mutants not only lose the tumour suppression activity, but also gain new oncogenic activities to promote tumourigenesis. Given the active role of mutant p53 proteins in promoting tumourigenesis and their very common occurrence in breast cancer, identification of pathways regulated by distinct classes of mutant p53 proteins may provide new therapeutic targets that would improve the treatment of patients with p53-mutated breast cancers. By employing RNA interference (RNAi) approach, this study has provided direct evidence to support the oncogenic role of mutant p53 in human breast cancers. This study shows that silencing of endogenous p53 R 273H contact mutant in MDA-MB-46 8 cells induced massive apoptosis while no apoptosis was observed in the silencing of the p53 R175H structural mutant in SKBR-3 cells. Further analysis showed that the apoptotic effects of mutant p53 knock-down involved activation of caspase 9 and mitochondria depolarisation, suggesting a role of mutant p53 in suppressing the intrinsic apoptotic pathway. Indeed, a BCL-2 family member, BCL-2 modifying factor (BMF), was reproducibly up-regulated following knock-down of mutant p53 in MDA- MB-468 cells while no significant changes were observed in other BCL-2 family members. The functional role of this up-regulation was further interrogated by ectopic expression of BMF in MDA-MB- 468 cells which was shown to be capable of inducing massive apoptosis. Finally, depletion of BMF in MDA-MB-468 cells completely abrogated the apoptotic effects of mutant p53 knock-down, suggesting a role of mutant p53 in mediating the survival of breast cancer cells through the suppression of BMF expression.

The p53 tumour suppressor is the most frequently mutated gene in human cancers. Unlike other tumour-suppressor genes, p53 mutations mainly occur as missense mutations within the DNA-binding domain, leading to the stable expression of full-length mutant p53 proteins. A wide range of studies shows that mutant p53 proteins not only lose their tumour suppressor function, but may also actively promote tumourigenesis through gain-of-function (GOF) mechanisms. As such, expression of mutant p53 proteins is commonly associated with poor clinical prognosis and high metastatic rate. Anoikis is a form of apoptosis in response to loss of cell adhesion or inappropriate cell adhesion. Gaining resistance to anoikis may be a general necessity for the tumour cells to metastasise. Here, we showed that p53 R273H contact mutant, but not p53 R175H conformation mutant, suppressed anoikis by down-regulation of BCL2 modifying factor (BMF) expression. Ectopic expression of the activated form of AKT in MDA-MB-468 completely „rescued‟ the apoptotic effects following depletion of endogenous mutant p53, suggesting that p53 R273H mutant promotes anoikis resistance in breast cancer cells through suppression of BMF expression by activating AKT signalling pathway. In addition, our findings also demonstrated that p53 R273H contact mutant regulate the expression of miR-19 and miR-200 family member, suggesting that mutant p53 R273H might promote tumourigenesis through modulation of microRNAs expression. Finally, we showed that ectopic expression of wild-type and mutant p53 in p53-null cells confers differential regulation of metabolic genes, suggesting that wild-type and mutant p53 play a distinct role in regulating cellular metabolism in tumour cells in a cell type dependent context. In summary, our findings suggest a mechanism by which p53 mutants might promote tumorigenesis through 1) regulation of cancer cell survival via AKT-dependent suppression of BMF in a wide range of tumour types; 2) regulation of miRNA expression; and 3) modulation of cancer cell metabolism.