EGFR blockade prevents glioma escape from BRAFV600E targeted therapy

Mutational activation of BRAF (BRAF^V600E) occurs in pediatric glioma and drives aberrant MAPK signaling independently of upstream cues. Targeted monotherapy against BRAF^V600E displays efficacy in pre-clinical models of glioma, however xenograft tumors adapt rapidly and escape from the growth-inhibitory effects of BRAF-targeted therapy. Here, we show that intrinsic resistance to a BRAF^V600E specific inhibitor stems, in part, from feedback activation of EGFR and downstream signaling pathways. BRAF^V600E inhibition suppresses MAPK signaling, which in turn downregulates the EGFR phosphatase PTPN9, resulting in sustained EGFR phosphorylation and enhanced EGFR activity. We demonstrated that overexpression of PTPN9 reduces EGFR phosphorylation and cooperates with BRAF^V600E inhibitor PLX4720 to reduce MAPK and Akt signaling, resulting in decreased glioma cell viability. Moreover, pharmacologic inhibition of EGFR combined with inhibition of BRAF^V600E to reduce growth of glioma cell lines and orthotopic glioma xenograft by decreasing tumor cell proliferation while increasing apoptosis, with resultant significant extension of animal subject survival. Our data support clinical evaluation of BRAF^V600E and EGFR targeted therapy in treating BRAF^V600E glioma.