Novel FGFR2-INA fusion identified in two low-grade mixed neuronal-glial tumors drives oncogenesis via MAPK and PI3K/mTOR pathway activation
As a group, mixed neuronal-glial tumors (MNGTs) exhibit genetic variability, including stable genomes, whole chromosome gains, BRAF-V600E, and FGFR1 mutations [8, 9, 11, 12]. While histologic criteria are described to distinguish MNGT types ganglioglioma (GG) and dysembryoplastic neuroepithelial tumor (DNT), non-specific features preclude confident classification in a high proportion of cases [2, 8, 10, 12]. Herein, we report the characterization of a novel FGFR2-INA fusion gene identified during clinical genomic profiling in two cases of MNGTs that could not be specifically classified as GG or DNT.
Targeted RNA-sequencing revealed a novel in-frame fusion between FGFR2 exon 17 and INA exon 2 (Fig. 1b) in both cases. Additional DNA sequence and copy number variants of clinical significance were also identified by targeted next-generation sequence panel [suppl. Tables 2, 3, 4 (Online Resource 1)] . FGFR2, a receptor kinase, regulates several growth-related signaling pathways implicated in cancer progression, including RAS-RAF-MAPK and PI3K/AKT/mTOR . INA encodes the alpha-internexin protein involved in cytoskeletal organization and neuronal morphogenesis . The novel fusion retains the extracellular immunoglobin-like and tyrosine kinase domains of FGFR2, suggesting oncogenic activation of downstream signaling, and the truncated coil 2 and tail region of INA, suggesting dimerization (Fig. 1c).
We cloned FGFR2-INA and stably expressed it in NIH/3T3 and Tp53-null primary mouse astrocytes (PMAs) [1, 5] [suppl. Figure 1 (Online Resource 2)]. In soft agar proliferation assays, FGFR2-INA expressing NIH/3T3 showed a significant increase in colony count over control, similar to BRAF V600E (p < 0.0005) (Fig. 1d). Next, we assessed the signaling potential of FGFR2-INA. In serum starved conditions, we observed high-level activation of both the MAPK and PI3 K/mTOR pathways assessed via elevated levels of phosphorylated-ERK and -S6, respectively, compared to vector-controlled cells (Fig. 1e). Mechanistically, we found that FGFR2-INA homo-dimerizes in co-immunoprecipitation assays suggesting dimerization-induced activation of FGFR2-INA (Fig. 1f). Using combinatorial targeting of downstream MAPK and PI3K/mTOR pathways with trametinib and everolimus, respectively, we could suppress FGFR2-INA-driven oncogenic signaling and growth (Fig. 1f, suppl. Figure 2 (Online Resource 3)).
We identify and characterize a novel FGFR2-INA fusion associated with unclassified MNGT in two patients lacking other reported driver alterations (BRAF-V600E and FGFR1). Other FGFR2 fusions have been identified in epileptogenic tumors of the young with some overlapping histologic features to the current two cases . It is possible that these tumors represent an emerging category of low-grade epileptogenic tumor. Our functional studies show that the FGFR2-INA fusion drives oncogenesis potentially via activation of the MAPK and PI3 K/mTOR pathways. Therefore, FGFR2-INA is the likely driver of tumorigenesis in at least a subset of MNGTs and is a potential target for small-molecule inhibitors.
This work was supported by Children’s Brain Tumor Tissue Consortium (CBTTC).
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