Abstract 4129: Novel oncogenic BRAF fusions and impact on targeted therapies

Abstract

Abstract BRAF mutations are driver events in a number of cancers including thyroid cancer and melanoma. The most common, BRAF V600E, alters normal BRAF protein activity in the mitogen-activated protein kinase (MAPK) pathway by constitutively activating BRAF and inducing proliferative signaling and tumor growth. Small molecule tyrosine kinase inhibitors targeting tumors with the V600E mutation have been evaluated in clinical trials and are now approved for melanoma. While BRAF missense mutations have been extensively characterized for oncogenic potential and actionability in genomically-guided therapy, BRAF gene fusions have been underappreciated for not only their functional role in cancer but also in differential drug response. More recently, data suggest that alternative approaches may be needed for treatment of patients with BRAF fusion-containing tumors. We have identified two novel BRAF fusions in tumors from patients with papillary thyroid cancer and melanoma. Both fusions result in an in-frame fusion of a novel gene partner at the 5’ end of the fusion, an intact BRAF kinase domain at the 3’ end, and loss of the BRAF auto-inhibitory domain. We hypothesized that these novel BRAF fusions act as oncogenic drivers, and the mechanism of BRAF activation differs from that caused by V600E mutations and may be fusion partner-specific. These fusions have been engineered in the laboratory and tested for tumorigenic potential and functional activity. BRAF fusion expression in non-transformed cells induces colony formation similar to the V600E mutation indicating tumorigenic potential. These BRAF fusions also constitutively activate the MAPK pathway in the absence of stimulation as demonstrated by phosphorylated ERK and MEK proteins. Additionally, BRAF fusion-expressing cells form tumors in vivo similarly to the BRAF V600E-expressing cells. These tumors are highly proliferative as demonstrated by strong Ki67 immunohistochemical staining and display MAPK pathway activation as evidenced by phosphorylated ERK. BRAF fusion-expressing cells have differential sensitivity to MAPK pathway inhibitors compared to cells with the V600E mutation as measured by reduced MAPK signaling. Inhibition of the MAPK pathway is relevant in targeting BRAF fusion-containing cells but may not follow the same paradigm as point mutations. Collectively, our data suggest that BRAF fusions are functional and represent novel therapeutic targets, but may need an alternative approach as compared to tumors with BRAF missense mutations. Citation Format: Sonia C. Dolfi, Ann Silk, Bhavna Paratala, Whitney Petrosky, Srilatha Simhadri, Atul Kulkarni, Shridar Ganesan, Kim M. Hirshfield. Novel oncogenic BRAF fusions and impact on targeted therapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4129. doi:10.1158/1538-7445.AM2017-4129