Mutant p53 mediates survival of breast cancer cells

Background:

p53 is the most commonly mutated tumour-suppressor gene in human cancers. Unlike other tumour-suppressor genes, most p53 cancer mutations are missense mutations within the core domain, leading to the expression of a full-length mutant p53 protein. Accumulating evidence has indicated that p53 cancer mutants not only lose tumour suppression activity but also gain new oncogenic activities to promote tumourigenesis.

Methods:

The endogenous mutant p53 function in human breast cancer cells was studied using RNA interference (RNAi). Gene knockdown was confirmed by quantitative PCR and western blotting. Apoptosis was evaluated by morphological changes of cells, their PARP cleavage and annexin V staining.

Results:

We show that cancer-associated p53 missense mutants are required for the survival of breast cancer cells. Inhibition of endogenous mutant p53 by RNAi led to massive apoptosis in two mutant p53-expressing cell lines, T47D and MDA-MB-468, but not in the wild-type p53-expressing cells, MCF-7 and MCF-10A. Reconstitution of an RNAi-insensitive mutant p53 in MDA-MB-468 cells completely abolished the apoptotic effects after silencing of endogenous mutant p53, suggesting the specific survival effects of mutant p53. The apoptotic effect induced by mutant p53 ablation, however, is independent of p63 or p73 function.

Conclusion:

These findings provide clear evidence of a pro-survival ‘gain-of-function’ property of a subset of p53 cancer mutants in breast cancer cells.

p63/p73 expression mediates cisplatin sensitivity in a subset of triple-negative primary breast cancer: Implications for a new clinical trial

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Background: Triple-negative breast cancer (ER-, PgR- and HER2-negative) has a poor prognosis relative to other subtypes, even with the best available chemotherapy. We recently reported that in squamous cell cancers, p63 promotes survival by direct interaction with and inhibition of p73-dependent apoptosis. We now report on the role of p63/p73 in triple-negative breast cancer. Methods: We performed quantitative RT-PCR on primary breast cancer specimens to determine the expression of specific p63 and p73 isoforms. Cell culture experiments used MCF7, HCC-1937, MDA-MB-468, T47D, and MCF10A cell lines. Lentiviral p63 and p73 shRNA were used for knockdown experiments. Chemotherapy dose-response curves were generated using MTT assay. Results: In primary tumors, ΔNp63 and TAp73 isoforms are expressed exclusively within a subset of triple-negative tumors in which p53 mutations are commonly found. Knockdown of p63 in triple-negative cell lines, but not the ER-positive MCF7 line, induced apoptosis which was specifically dependent on p73 expression. Breast cancer cell lines expressing p63/p73 exhibited p73-dependent sensitivity to cisplatin. Knockdown of p73 resulted in a 10–100 fold increase in the IC50 of cisplatin, but no change in doxorubicin or paclitaxel sensitivity. Exogenous p73 expression in MCF10A cells, which have high levels of p63 but no p73, confers increased sensitivity to cisplatin. Conclusion: These results show that p63/p73expression promotes survival in a subset of breast cancers, and they provide a mechanism for cisplatin sensitivity. These findings suggest that p63/p73 expression may serve as a biomarker to predict cisplatin sensitivity in triple-negative breast cancer. Based on these results, we are conducting a phase II study of 39 patients evaluating cisplatin as first line therapy for metastatic triple-negative breast cancer. The primary endpoints are overall objective response rate and response rate in low versus high p63/p73 expressing subgroups.

No significant financial relationships to disclose.