Reversible and adaptive resistance to BRAF(V600E) inhibition in melanoma

Patients with melanomas carrying an activating BRAF mutation respond to treatment with BRAF inhibitors although resistance to the inhibitor usually emerges; this resistance is shown to arise through increased expression of receptor tyrosine kinases such as EGFR; however, these changes decrease cell fitness and during a break from inhibitor treatment these cells are selected against, revealing that some patients who acquire EGFR expression may benefit from inhibitor re-treatment after a drug holiday. Melanomas carrying a mutant BRAF gene generally respond to treatment with BRAF inhibitors, but in the majority of cases resistant cancer clones emerge. It has been shown that such resistant clones can nevertheless exhibit reduced fitness when the drug is removed. This paper demonstrates a molecular mechanism underlying this observation. Rene Bernards and colleagues show that a signalling cascade leading from suppression of SOX10 to increased expression of the EGFR (epidermal growth factor receptor) gene confers resistance to BRAF inhibitors and at the same time reduces melanoma cell proliferation and induces senescence in the absence of inhibitors. With preliminary evidence that this pathway is induced in patients who have developed resistance, the authors suggest that temporary withdrawal of BRAF inhibitors — a drug holiday — would reverse induced EGFR expression and thus may re-sensitize melanoma cells to BRAF inhibition when treatment is reinstated. Treatment of BRAF(V600E) mutant melanoma by small molecule drugs that target the BRAF or MEK kinases can be effective, but resistance develops invariably1,2. In contrast, colon cancers that harbour the same BRAF(V600E) mutation are intrinsically resistant to BRAF inhibitors, due to feedback activation of the epidermal growth factor receptor (EGFR)3,4. Here we show that 6 out of 16 melanoma tumours analysed acquired EGFR expression after the development of resistance to BRAF or MEK inhibitors. Using a chromatin-regulator-focused short hairpin RNA (shRNA) library, we find that suppression of sex determining region Y-box 10 (SOX10) in melanoma causes activation of TGF-β signalling, thus leading to upregulation of EGFR and platelet-derived growth factor receptor-β (PDGFRB), which confer resistance to BRAF and MEK inhibitors. Expression of EGFR in melanoma or treatment with TGF-β results in a slow-growth phenotype with cells displaying hallmarks of oncogene-induced senescence. However, EGFR expression or exposure to TGF-β becomes beneficial for proliferation in the presence of BRAF or MEK inhibitors. In a heterogeneous population of melanoma cells having varying levels of SOX10 suppression, cells with low SOX10 and consequently high EGFR expression are rapidly enriched in the presence of drug, but this is reversed when the drug treatment is discontinued. We find evidence for SOX10 loss and/or activation of TGF-β signalling in 4 of the 6 EGFR-positive drug-resistant melanoma patient samples. Our findings provide a rationale for why some BRAF or MEK inhibitor-resistant melanoma patients may regain sensitivity to these drugs after a ‘drug holiday’ and identify patients with EGFR-positive melanoma as a group that may benefit from re-treatment after a drug holiday.