Inhibition of the BRAF(V600E) oncoprotein by the small-molecule drug PLX4032 (vemurafenib) is highly effective in the treatment of melanoma1. However, colon cancer patients harbouring the same BRAF(V600E) oncogenic lesion have poor prognosis and show only a very limited response to this drug2,3,4. To investigate the cause of the limited therapeutic effect of PLX4032 in BRAF(V600E) mutant colon tumours, here we performed an RNA-interference-based genetic screen in human cells to search for kinases whose knockdown synergizes with BRAF(V600E) inhibition. We report that blockade of the epidermal growth factor receptor (EGFR) shows strong synergy with BRAF(V600E) inhibition. We find in multiple BRAF(V600E) mutant colon cancers that inhibition of EGFR by the antibody drug cetuximab or the small-molecule drugs gefitinib or erlotinib is strongly synergistic with BRAF(V600E) inhibition, both in vitro and in vivo. Mechanistically, we find that BRAF(V600E) inhibition causes a rapid feedback activation of EGFR, which supports continued proliferation in the presence of BRAF(V600E) inhibition. Melanoma cells express low levels of EGFR and are therefore not subject to this feedback activation. Consistent with this, we find that ectopic expression of EGFR in melanoma cells is sufficient to cause resistance to PLX4032. Our data suggest that BRAF(V600E) mutant colon cancers (approximately 8–10% of all colon cancers2,3,5), for which there are currently no targeted treatment options available, might benefit from combination therapy consisting of BRAF and EGFR inhibitors.
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We thank P. Kumar, V. Gambino and W. Grernrum for assistance with experiments. We are grateful to C. Pochet for support. Major financial support for this work was provided by the EU Seventh Framework Programme, grant agreement 259015 (to A.B. and R.B.). Additional funding was provided by a European Research Council grant to R.B.; The Cancer Systems Biology Center grant by the Netherlands Organisation for Scientific Research (NWO); The Dutch Cancer Society; The Netherlands Genomics Initiative (NGI); The Associazione Italiana per la Ricerca sul Cancro (AIRC), 2010 Special Program Molecular Clinical Oncology 5x1000, project 9970; AIRC Investigator Grant (to A.B.); Italian Ministry of University and Research; Italian Ministry of Health, Regione Piemonte (to A.B. and F.D.N.); and Intramural Grant, 5xmille 2008; and Fondazione Piemontese per la Ricerca sul Cancro (to A.B. and F.D.N.).
The authors declare no competing financial interests.
The file contains Supplementary Figures 1-6 with legends. (PDF 2490 kb)
This file shows kinases screened for enhancers of PLX4032 sensitivity. Listed are the gene symbols for the genes tested in the PLX4032 sensitivity screen and the number of shRNAs for each gene present in the library. (XLS 349 kb)
This file shows the top shRNA candidates from the 'dropout' RNAi screen for enhancers of PLX4032 sensitivity. Listed are the 22 top shRNA candidates that had been sequenced at least 300 times and which were depleted at least five-fold by the drug treatment. (XLS 37 kb)
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Prahallad, A., Sun, C., Huang, S. et al. Unresponsiveness of colon cancer to BRAF(V600E) inhibition through feedback activation of EGFR. Nature 483, 100–103 (2012). https://doi.org/10.1038/nature10868
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