Abstract 123: Mechanism of oncogenic signal activation by the novel fusion kinase FGFR3-BAIAP2L1

Recent cancer genome profiling studies have identified many novel genetic alterations, including rearrangements of genes encoding fibroblast growth factor receptor (FGFR) family members. However, most fusion genes are not functionally well characterized, and the oncogenicity of some fusions as well as their potential sensitivity to targeted therapy are still unclear. In a previous study, we investigated a recently discovered gene fusion between FGFR3 and BAI1-associated protein 2-like 1 (BAIAP2L1). The FGFR3-BAIAP2L1 fusion gene was identified in 4 bladder cancer patients and 2 lung cancer patients via screens involving PCR and a break-apart fluorescence in situ hybridization assay. The functional analysis of FGFR3-BAIAP2L1 transfectant in Rat-2 fibroblast cells (Rat-2_F3-B) indicated that FGFR3-BAIAP2L1 forms a dimer via the Bin-Amphiphysin-Rvs (BAR) BAIAP2L1 dimerization domain that constitutively activates its FGFR3 kinase activity. CH5183284/Debio 1347*, a selective FGFR inhibitor, effectively inhibits growth of Rat-2_F3-B both in vitro and in vivo, indicating that the FGFR3 kinase activity is critical for tumorigenic activity of this fusion. These results indicate a potential application of FGFR inhibitors to treat FGFR3-BAIAP2L1fusion gene positive patients. In this study, we further elucidate the mechanism of tumorigenic potential of FGFR3-BAIAP2L1A by a comprehensive gene expression analysis of 4 cell lines (Rat-2_mock, Rat-2_FGFR3, Rat-2_F3-B, and Rat-2_BAIAP2L1) using RNA sequencing. We identified 143 up-regulated and 67 down-regulated genes specifically engaged by FGFR3-BAIAP2L1. Our gene signature analysis with this gene set revealed that FGFR3-BAIAP2L1 activates growth signals, such as the mitogen-activated protein kinase pathway, and inhibits tumor-suppressive signals, such as the p53, RB1, and CDKN2A pathways. Analysis by Western blot in xenograft tissues confirmed the activation and inactivation status of those pathways. These data suggest that a concurrent regulation of an oncogenic pathway and a tumor-suppressive pathway results in the tumorigenic potential of FGFR3-BAIAP2L1. *CH5183284/Debio 1347 was discovered by Chugai Pharmaceutical Co., Ltd. and is being developed by Debiopharm International S.A. under an exclusive worldwide license. Citation Format: Yoshito Nakanishi, Nukinori Akiyama, Toshiyuki Tsukaguchi, Toshihiko Fujii, Yasuko Satoh, Hideaki Mizuno, Nobuya Ishii, Masahiro Aoki. Mechanism of oncogenic signal activation by the nov