Targeting BRAF-mutant tumours with TGFBR1 inhibitors

(TGFβ) is a major contributing factor to many diseases including progressive cancers [1]. Given the normal homeostatic functions of TGFβ in controlling cell proliferation, immune surveillance and angiogenesis, however, it is much less clear which cancer patients would derive clinical benefit from the administration of anti-TGFβ targeted therapy. The use of these therapies is complicated not only by toxicity issues, but also by concerns that interfering with tumour suppressive functions of TGFβ may ultimately exacerbate disease. Thus, identifying when and where TGFβ signalling promotes tumour progression [2] is key to the success of numerous approaches to block TGFβ signalling using a range of humanised antibodies, small molecule inhibitors (SMIs) and antisense oligonucleotides currently being evaluated in clinical trials [3]. Our recent data now provides insight into when these therapies might succeed [4]. To investigate when anti-TGFβ targeted therapies might be used appropriately, we screened cancer cell lines carrying defined cancer-associated genetic abnormalities in the RAS/RAF/MEK/ERK signalling pathway for sensitivity to SMIs of the TGFβ type 1 receptor (TGFBR1). In assays specifically designed to measure cancer-stem cell like properties including clonogenicity and anchorage independent growth assays, we found that cells carrying mutant BRAF were dependent on TGFβ signalling for growth. Although cells carrying mutant KRAS (n=11), NRAS (n=4) and BRAF (n=7) all exhibit constitutive activation of the MAPK pathway, only mutant BRAF cells were universally inhibited by the TGFBR1 inhibitor. Our data were confirmed by siRNA knockdown of the receptor to ensure that the effects of the inhibitor were on-target. Additionally, the dependence of mutant BRAF cells on secreted autocrine TGFβ signalling for formation of xenograft tumours was demonstrated by stably expressing short-hairpin shRNA targeting TGFβ RNA in the cancer cells. The ability of these cells to establish tumours in mice was significantly impaired by TGFβ knockdown. The data suggests, therefore, that mutant BRAF cells are 'hard-wired' to depend on autocrine TGFβ signalling for growth in stressful conditions. The data also implies that the presence of mutant BRAF in Editorial sequenced tumour biopsies could act as a biomarker for stratifying patients for anti-TGFβ therapy. The mechanism of TGFβ-mediated tumour cell growth promotion remains elusive, but appears to be independent of canonical TGFβ signalling pathways involving the SMAD transcription factors and may involve activation of the small GTPase RHO-A. It might be misleading, therefore, to use the phosphorylation of SMADs downstream of TGFβ receptor activation as an additional biomarker in tumour tissue …