by ecancer reporter Clare Sansom

The range of treatment options for cutaneous melanoma, the most deadly form of skin cancer, is now increasing rapidly.

In particular, the selective BRAF inhibitor vemurafenib has shown very promising results in the large subset of patients in which this protein has the common mutation V600E.

However, melanomas often acquire resistance to this drug, so further treatment options for this intractable tumour are still needed. 

The tumour suppressor gene TP53 is inactivated in many cancers and reactivating it has emerged as an attractive mechanism for anti-cancer drug development.

Mutations in this gene are rare in melanoma, but there is evidence that its function is nevertheless suppressed in this tumour through alterations in genes encoding proteins that act in the same pathway.

Jean-Christophe Marine of the Center for Human Genetics, Leuven, Belgium and his colleagues have now shown that p53 activity is suppressed in a majority of melanomas through upregulation of a negative regulator known as MDM4, and thus suggested that it may prove an important therapeutic target for this disease.

The researchers assessed expression levels of this protein, Mdm4 p53 binding protein homolog (MDM4), in melanomas and paired normal skin tissue using immuno-fluorescence.  Expression levels of MDM4 were increased compared to normal melanocytes in 68.5% of melanomas at all stages, with high expression levels correlated with high-grade tumours and metastatic disease. There was no correlation between the likelihood of MDM4 over-expression and the mutation status of either NRAS or BRAF. In contrast, the related p53 binding protein MDM2 was over-expressed in only a small percentage of metastatic melanomas.

Next, Marine and his co-workers examined the interplay between MDM4 and p53 in several transgenic mouse models in which the mice spontaneously developed melanoma. Mice in which the orthologous protein Mdm4 had been overexpressed in melanocytes were found to develop melanomas more quickly than those with normal Mdm4 expression levels and the same genetic background. Further analysis suggested that the mechanism through which Mdm4 promoted melanoma formation involves cooperation with the oncoprotein NRAS.

To test the effect of high levels of MDM4 on human melanoma proliferation and survival, the researchers knocked down the MDM4 gene in several widely used melanoma cell lines, all with wild type p53, and showed that its knockdown inhibited tumour growth in all lines. Interestingly, knocking down both p53 and MDM4 in the same cell lines rescued the melanoma cells from apoptosis but did not restore proliferation, suggesting that the main function of the MDM4-p53 interaction in tumour cells is to protect them from apoptosis.

These findings suggested to Marine and his colleagues that it might be possible to induce apoptosis in melanoma cells by inhibiting the interaction between MDM4 and p53. They tested this hypothesis using a peptide, SAH-p53-8, that is known to bind to the p53-binding pocket on MDM4 and disrupt the protein-protein interaction. They treated several melanoma cell lines with this peptide and found, as expected, that growth was inhibited only in those lines with wild type p53. Cell lines bearing the BRAF V600E mutation were sensitive to the peptide, regardless of whether they had acquired resistance to BRAF inhibitors such as vemurafenib. Furthermore, co-treatment of BRAF inhibitor sensitive cell lines with both SAH-p53-8 and vemurafenib showed synergy between the two agents.

These results show that inhibiting the interaction between MDM4 and p53 is likely to be a useful strategy for restoring the pro-apoptotic function of p53 in melanomas without mutations in the latter protein. The in vitro results in particular suggest that it may be equally successful in tumours with and without acquired resistance to BEAF inhibitors, and thus that MDM4 should therefore be an extremely promising target for melanoma therapy.

Reference

Gembarska, A., Luciani, F., Fedele, C. and 19 others (2012). MDM4 is a key therapeutic target in cutaneous melanoma. Nature Medicine, published online ahead of print 22 July 2012. doi:10.1038/nm.2863