Previous reports have shown that copper chelators effectively decrease the activity of the BRAF targets MEK1 and MEK2 (MEK1/2) kinases, which are key components of the oncogenic ERK pathway. Now, Brady et al. have shown that either decreasing levels of copper trans-porter 1 (CTR1) or mutating copper binding sites in MEK1 both decrease the activation of ERK1 and ERK2 (ERK1/2) in response to BRAF signalling and lead to decreased tumorigenesis in mice.

The authors first demonstrated that decreasing copper influx affected proliferation and tumori genesis driven by an oncogenic BRAF mutant (BRAFV600E) in mouse

embryonic fibroblasts (MEFs). MEFs expressing BRAFV600E that were also defective for Ctr1 (Ctr1–/–) had decreased proliferation and decreased phosphorylation of the MEK1/2 targets ERK1/2 in vitro. Moreover, xenografts derived from subcutaneous injection of mice with these BRAFV600E, Ctr1–/– MEFs had a lower tumour volume than xenografts derived from BRAFV600E MEFs with functional CTR1. These results were further validated using a spontaneous BRAFV600E-driven lung adenoma tumour mouse model. Therefore, copper influx directly affects BRAF signalling and tumorigenesis.

The authors then used targeted mutagenesis to show that mutations disrupting copper binding sites in MEK1 decreased ERK1/2 signal-ling in vitro and xenografts derived from MEFs carrying these mutated MEK1 proteins were significantly smaller. Conversely, they artificially engineered a MEK mutant whose activity was independent of copper by fusing the ERK-binding domain of MEK1 to the kinase domain of MEK5, which lacks two sites that are important for copper binding. This copper-independent MEK mutant restored ERK1/2 phos-phorylation in Ctr1–/– MEFs, and xenografts derived from these cells showed increased tumorigenesis compared with those derived from Ctr1–/– MEFs.

Copper chelation therapy with tetrathiomolybdate (TTM) and simi-lar chelators has been widely used in individuals with the copper overload disorder Wilson disease. Mice carry-ing tumours derived from BRAFV600E MEFs that were treated with TTM had significantly decreased tumour load, and this was further decreased when TTM was combined with a low copper diet.

Resistance to BRAF inhibitors is a crucial issue in the treatment of BRAFV600E-driven tumours, especially melanomas. To this end, the authors showed that the BRAF inhibitor-resistant mutant MEK1C121S still binds to copper and its kinase activity is inhibited in vitro by TTM. Moreover, mice carrying tumours derived from a BRAFV600E melanoma cell line expressing MEK1C121S and treated with TTM had a significantly lower tumour volume than those treated with either a BRAF inhibitor or a vehicle alone.

The copper chelators that are used to treat Wilson disease are generally safe and are widely avail-able. Therefore, the results of this study have potential therapeutic applications in the treatment of both BRAFV600E-driven and BRAF inhibitor-resistant tumours.

Isabel Lokody

S I G N A L L I N G

Inhibiting oncogenic BRAF signalling by copper depletion

ORIGINAL RESEARCH PAPER Brady, D. C. et al. Copper is required for oncogenic BRAF signalling and tumorigenesis. Nature http://dx.doi.org/10.1038/nature13180 (2014)

the results of this study have potential therapeutic applications in the treatment of both BRAFV600E-driven and BRAF inhibitor-resistant tumours

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NATURE REVIEWS | CANCER VOLUME 14 | JUNE 2014

Nature Reviews Cancer | AOP, published online 28 April 2014; doi:10.1038/nrc3745

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