Recurrent FGFR3-TACC3 fusion gene in nasopharyngeal carcinoma

Abstract

Nasopharyngeal carcinoma (NPC) is one of the most common head and neck malignancies and exhibits regional differences in incidence. Because many fusion genes have been discovered in different types of tumors over the past few years, we aimed to investigate the existence of a fusion gene in primary NPC patients using RNA-seq. In this study, for the first time, we found that fibroblast growth factor receptor 3-transforming acidic coiled-coil-containing protein 3 (FGFR3-TACC3) fusion transcripts are recurrently detected in NPC. The presence of this fusion gene was also detected in head and neck cancer, esophageal squamous cell carcinoma (ESCC), and lung cancer. Furthermore, we found certain new isoforms of the FGFR3-TACC3 fusion transcripts, such as a gene fusion between exon 18 of FGFR3 and exon 6 or exon 14 of TACC3 and agene fusion between exon 19 of FGFR3 and exon 11 of TACC3. In addition, we showed that the FGFR3-TACC3 fusion gene promotes cell proliferation, colony formation, and transforming ability in vitro, whereas the FGFR3-TACC3 K508M mutant or treatment with the FGFR inhibitor PD173074 abrogates these effects, suggesting that FGFR3-TACC3 most likely exerts its effects through activation of FGFR kinase activity. This activation likely leads to the development of NPC. Additionally, FGFR3-TACC3 could trigger activation of the ERK and Akt signaling pathways, whereas FGFR3-TACC3 K508M mutant could not, suggesting that these 2 signaling pathways might be involved in the function of FGFR3-TACC3. Taken together, our data demonstrated the oncogenic role of FGFR3-TACC3 in vitro, indicating that FGFR3-TACC3 may be useful as a diagnostic marker and therapeutic target in cancers.

Keywords: CCND1, cyclin D1; DMSO, dimethyl sulfoxide; DTT, DL-dithiothreitol; FBS, fetal bovine serum; FGFR3, fibroblast growth factor receptor 3; FGFR3-TACC3; LTBR, lymphotoxin β receptor; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide; NPC; NPC, nasopharyngeal carcinoma; PBS, phosphate-buffered saline; PI, propidium iodide; RT-PCR, reverse transcription-PCR; SDS, sodium dodecyl sulfate; TACC3, transforming acidic coiled-coil-containing protein 3; fusion gene; proliferation; tumorigenesis.

Figure 1.

RNA-seq to identify FGFR3-TACC3 fusion genes in NPC. Circos plot of FGFR3-TACC3 fusion genes, as detected by RNA-seq. Chromosome ideograms are shown in the outer layer. The expression level of the genes is shown in the middle layer using a heat map. The FGFR3-TACC3 fusion genes are shown in the central layer. (A) Detection of FGFR3-TACC3 fusion genes using transcriptome sequencing. The reads are aligned across the junction of the predicted fusion transcripts. (B) The genomic fusion point is between exon 19 of FGFR3 and intron 5 of TACC3. (C) Structure of the predicted fusion protein.

Figure 2.

Different forms of FGFR3-TACC3 fusion transcripts present in a variety of solid tumors. (A) Frequency and different forms of the FGFR3-TACC3 fusion gene identified in different types of cancers. (B) Identification of the fusion breakpoint using Sanger sequencing.

Figure 3.

Different forms of FGFR3-TACC3 fusion genes at the genomic DNA level. Different forms of FGFR3-TACC3 fusion transcripts at the genomic DNA level. Each rectangle indicates an exon of the FGFR3 (red) or TACC3 (blue) gene.

Figure 4.

The FGFR3-TACC3 fusion gene can promote the development of NPC. Representative pictures of the stable expression of wild-type FGFR3(F), wild-typeTACC3(T),FGFR3-TACC3(FT) fusion protein and the FGFR3-TACC3 fusion protein with the K508M mutation (FGFR3-TACC3 K508M) in HNE1 and HK1 NPC cell lines, as detected by western blotting. (A) MTT assay measuring the viability of the stable HNE1 and HK1 cell lines expressing wild-type FGFR3, wild-type TACC3, the FGFR3-TACC3 fusion protein, or FGFR3-TACC3 K508M, or the vector control. (B) MTT assay measuring the viability of the FGFR3-TACC3 fusion protein-expressing stable HNE1 and HK1 cell lines treated with 500 nM or 1000 nM PD173074. DMSO served as a negative control. The error bars denote the SEM. * refers to differences between DMSO and 500 nM PD173074 treatment, whereas # refers to differences between DMSO and 1000 nM PD173074 treatment. *** and ^### mean P < 0.001 by 2-way ANOVA. (C) Cell cycle analysis of the FGFR3-TACC3 fusion protein- or vectorcontrol-expressing stable HNE1 and HK1 cell lines. * refers to differences between FT and the pBabe vector, P < 0.05 by unpaired t-test. (D) Colony formation assay of the stable HNE1 and HK1 cell lines expressing the FGFR3-TACC3 fusion protein, the FGFR3-TACC3 fusion protein with the K508M mutation (FGFR3-TACC3 K508M), or the vector control. * refers to differences between FT and the pBabe vector or FT-K, * means P < 0.05, ** means P < 0.01, *** means P < 0.001 by paired t-test. (E) Anchorage-independent growth in soft agar by the stable HNE1 cell lines expressing the FGFR3-TACC3 fusion protein, the FGFR3-TACC3 fusion protein with the K508M mutation (FGFR3-TACC3 K508M), or the vector control. 40X magnification. (F) Representative pictures of phosphorylated Akt (p-Akt), total Akt (t-Akt), phosphorylated ERK (p-ERK), total ERK (t-ERK), FGFR3 and β-actin in stable HNE1 cell lines, as determined by protein gel blotting.