BRAF Mutation: A Promising Target in Colorectal Neuroendocrine Carcinoma

To determine the role of BRAF mutation and MAPK signaling as well as the effects of BRAF and MEK directed therapy in gastroenteropancreatic neuroendocrine neoplasia (GEP-NEN), with a focus on highly aggressive gastroenteropancreatic neuroendocrine carcinoma (GEP-NEC). Using Sanger sequencing of BRAF exon 15 we determined the frequency of BRAF mutations in 71 primary GEP-NENs. MEK phosphorylation was examined by immunohistochemistry in corresponding tissue samples. To evaluate the biological relevance of BRAF mutation and MAPK signaling in GEP-NECs, effects of a pharmacological BRAF and MEK inhibition were analyzed in NEC cell lines both in vitro and in vivo. BRAF mutation was detected in 9.9% of all GEP-NENs. Interestingly, only NECs of the colon harbored BRAF mutations, leading to a mutation frequency of 46.7% in this subgroup of patients. In addition, a BRAF mutation was significantly associated with high levels of MEK phosphorylation (pMEK) and advanced tumor stages. Pharmacological inhibition of BRAF and MEK abrogated NEC cell growth, inducing G1 cell cycle arrest and apoptosis only in BRAF mutated cells. BRAF inhibitor dabrafenib and MEK inhibitor trametinib prevented growth of BRAF positive NEC xenografts. High frequencies of BRAF mutation and elevated expression levels of pMEK were detected in biologically aggressive and highly proliferative colorectal NECs. We provide evidence that targeting BRAF oncogene may represent a therapeutic strategy for patients with BRAF mutant colorectal NECs.