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Stabilised mutant p53 is an important determinant of tumour survival and a valid drug target

by ecancer reporter Clare Sansom

Missense mutations in the tumour suppressor gene TP53 are found in 40-50% of all human tumours.

Some mutated forms of the p53 protein also acquire tumour-promoting functions, promoting tumour progression, invasion, metastasis or resistance to chemotherapy.

These gain-of-function mutations have been demonstrated in mouse models and in human patients with Li-Fraumeni syndrome, a rare disorder linked to germline mutations in TP53.

Li-Fraumeni patients with gain-of-function p53 mutations live shorter lives and have a higher tumour burden than those with loss of p53 expression.

A mutant p53 protein must be stabilised in order to function, and, in fact, constituent stabilisation of aberrant p53 protein is a characteristic feature of tumours with gain-of-function TP53 mutations.

Knockdown of mutant p53 using RNA interference is cytotoxic in human tumour cell lines and mouse xenograft models.

These findings suggest that destabilisation of p53 carrying gain-of-function mutations might be a useful strategy against tumours with this type of mutation.

A group of researchers led by Ute Moll of Stony Brook School of Medicine, Stony Brook, New York, USA has tested this hypothesis in a novel transgenic mouse model.

The researchers generated a strain of mice that carried a form of the gain-of-function p53 mutation R248Q (floxQ or Q/-) that could be inactivated using tamoxifen.

Without tamoxifen treatment, these mice develop tumours in the same way and to the same time-scale as mice carrying a constitutive R248Q mutation in p53.

Ablation of the mutant p53 in these mice with tamoxifen significantly reduced tumour growth and prolonged the survival of the mice compared to untreated controls.

P53 deletion caused the tumours in these mice to regress or stop growing, and this was shown to be due to tumour cell apoptosis rather than cell cycle arrest.

Ablation of mutant p53 also prevented lung metastases.

The lifespan of Q/- mice treated with tamoxifen at 10 weeks of age, before their tumours had spread, was lengthened to be comparable with that of p53-null mice.

Tumours taken from tamoxifen-treated Q/-mice at death were found to be composed of cells that over-expressed mutant p53, suggesting that carcinogenesis in these mice is driven by a small proportion of cells in which p53 is not knocked down and that deletion of the mutant p53 allele might further improve survival.

Moll and her co-workers among others have shown previously that the HSP90 chaperone machinery is involved in p53 stabilisation, and that cancer cells are less tolerant of HSP90 inhibition than normal cells.

Other experiments have indicated that the oncogenicity of another gain-of-function p53 mutation, R172H, but not that of p53 deletion, depends on the heat shock response and the Hsp90 chaperone system.

Inhibitors of Hsp90 and of histone deacetylase, such as 17DMAG and SAHA respectively, are toxic to cancer cells carrying gain-of-function TP53 mutations largely through destabilising the aberrant p53.

The researchers treated mice bearing the R172H mutation (H/H mice) with a combination of 17DMAG and SAHA throughout their lives, starting at 8 weeks before any solid tumours had developed.

This treatment extended the lifespan of the mice to a median of 180 days compared to 140 days for similar untreated mice.

The lifespan of mice bearing p53-null mutations was unaffected by drug treatment and was less than that of the treated H/H mice.

Analysis of thymic tissues at death showed that T-cell lymphomas only occurred in a few mice in which the drugs had failed to completely inhibit HSP90.

Ganetespib is a small-molecule Hsp90 inhibitor that is in Phase III trials for a number of cancer types.

Finally, Moll and her co-workers treated Q/-, H/H and p53-null mice with weekly doses of this drug throughout their lives, again starting at 8 weeks of age.

The drug increased the median lifespan of the Q/- mice from 146 to 232 days (59%) and that of the H/H mice from 157 to 233 days (48%) but had no effect on that of the p53-null mice.

Again, successful inhibition of Hsp90 prevented the development of T-cell lymphomas.

Taken together, these results suggest that tumours in mice carrying gain-of-function p53 mutations are highly dependent on p53 stabilisation via the Hsp90 chaperone system, and that Hsp90 inhibitors might be useful drugs for patients whose tumours carry such mutations.

Reference

Alexandrova, Yallowitz, Li et al. Improving survival by exploiting tumour dependence on stabilised mutant p53 for treatment. Nature, published online ahead of print 25 May 2015. 

 

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