Multiple functions of a glioblastoma fusion oncogene

Abstract

RNA sequencing facilitates the discovery of novel gene fusions in cancer. In this issue of the JCI, Parker et al. identify an FGFR3-TACC3 fusion oncogene in glioblastoma and demonstrate a novel mechanism of pathogenicity. A miR-99a binding site within the 3′–untranslated region (3′-UTR) of FGFR3 is lost, releasing FGFR3 signaling from miR-99a–dependent inhibition and greatly enhancing tumor progression relative to WT FGFR3. These results provide compelling insight into the pathogenicity of a novel fusion oncogene and suggest new therapeutic approaches for a subset of glioblastomas.

Genomic technologies are transforming our knowledge about the mutational landscape of cancer. For glioblastoma, the most common and lethal form of adult primary brain cancer, integrated DNA, transcriptional, and epigenetic analyses have identified copy number alterations, mutations, tumor transcriptional and epigenetic subclasses, and potential new drug targets (1–5). Recent progress in next-generation sequencing technologies, including RNA sequencing, provides a powerful new platform adding to this integrated toolkit. Researchers are now able to identify and quantify changes in both coding and noncoding RNA; identify alternative splicing events; and detect expressed mutations, SNPs, gene translocations, and fusion transcripts. Importantly, the identification of novel fusion proteins may provide new insights into the biology of this dreadful disease.

Gene fusions and cancer

Fusion genes occur when parts of two genes combine during a chromosomal rearrangement, resulting in expression of a chimeric protein, a process whose importance in cancer is well recognized. BCR-ABL1 in chronic myelogenous leukemia (CML; ref. 6), PML-RARA in acute promyelocytic leukemia (APL; reviewed in refs. 7, 8), EML4-Alk in non–small-cell lung cancer (NSCLC; reviewed in ref. 9), EWS-FLI in Ewing sarcoma (10), and TMPRSS2-ERG in prostate cancer (11) are paradigmatic examples. Fusion oncogenes are not common in cancer in general, but their importance in understanding cancer biology is disproportionately large, providing some of the most compelling examples of successful targeted therapies for selected cancer subtypes. Mechanistic insights gleaned by studying the BCR-ABL1 and PML-RARA oncogenes have translated into near-cures for two previously deadly types of cancer: imatinib for the treatment of CML and combined arsenic trioxide and retinoic acid for the treatment of APL (6, 7). Of note, the finding of EML4-Alk fusion and its rapid translation into clinical benefit for NSCLC patients treated with crizotinib brings new hope that the insights gained from studying fusions may not be limited to the rarer types of molecularly homogeneous hematopoietic cancers, such as CML and APL. Thus, discovery of a new fusion in glioblastoma is an exciting and important development.

FGFR3-TACC3 gene fusion in glioblastoma

In this issue, Parker et al. used whole-transcriptome RNA sequencing (RNA-seq) to look for fusions formed as a result of chromosomal translocations (12). Analysis of 48 glioblastoma samples and 43 low-grade glioma samples obtained from the United States and China, as well as analysis of the large Cancer Genome Atlas dataset, revealed the presence of a fusion composed of FGF receptor 3 (FGFR3) with transforming acidic coiled-coil 3 (TACC3). The authors demonstrated that fusion occurred via a tandem duplication event and detected it in 8.3% of glioblastoma patients. It was not detected in any of the low grade glioma samples. Importantly, FGFR3-TACC3 fusions were mutually exclusive with EGFR, PDGFR, or MET genetic alterations, the receptor tyrosine kinase alterations commonly detected in glioblastoma. Taken together, these findings suggest an important and specific role for FGFR3-TACC3 in promoting glioblastoma growth.

This study by Parker and colleagues independently validates the very recent report by Singh et al. (13), which identified FGFR3-TACC fusions in a small subset of glioblastomas. While independent validation is essential, the real excitement of this study lies more in its identification of the novel and potentially targetable mechanism of pathogenicity created by the FGFR3-TACC3 fusion.

FGFR3 encodes a receptor tyrosine kinase that is commonly mutated in bladder and cervical cancer (14). FGFR3 engages downstream signaling cascades that are commonly activated in cancer, including PI3K-Akt and Ras-Mek-Erk signaling (15). In glioblastoma, a role for FGFR3 had not been previously established, although recent work suggests that it can phosphorylate PTEN to promote glioblastoma resistance to EGFR tyrosine kinase inhibitors (16). TACC3 encodes a centrosomal protein involved in mitosis (17) that is overexpressed in lung and colon carcinomas and in multiple myeloma (18). What are the mechanisms of its pathogenicity, and in what ways are the activities of this fusion protein greater than the sum of its parts?

Singh et al. demonstrated that the transforming capacity of FGFR3-TACC3 is related to its localization to the mitotic spindle, where it causes chromosomal missegregation and aneuploidy. Importantly, they showed that this requires FGFR kinase activity, because a pan-FGFR tyrosine kinase inhibitor abrogated the fusion protein’s effects on chromosomal instability, reversing aneuploidy (13). Thus, Singh and colleagues identified one intriguing mechanism by which FGFR3-TACC3 fusion can promote tumorigenicity.

In the present study, Parker and colleagues provide an alternative and entirely new view of a mechanism driving FGFR3-TACC3 pathogenicity (Figure 1). They conclude that the pathogenicity of FGFR3-TACC3 is mediated, at least in part, through loss of the miR-99a binding site. MicroRNAs typically bind the 3′–untranslated region (3′-UTR) of a transcript and can repress translation and/or promote degradation of that transcript (19). Changes in the 3′-UTR, due to alternative splicing or shortening through alternative cleavage, can significantly affect mRNA translation (20, 21), resulting in enhanced expression of transcripts insensitive to microRNA regulation. This may promote tumor development and/or progression (20, 21). Parker and colleagues demonstrated that a miR-99a binding site in the 3′-UTR of FGFR3 was lost during fusion of FGFR3 with TACC3, causing greatly increased FGFR3 expression, an effect that was counteracted by reintroduction of the 3′-UTR of FGFR3 in the presence of miR-99a (12). Importantly, FGFR3-TACC3 fusion was demonstrated to preferentially engage ERK and STAT3 signaling and to enhance tumor progression in vivo relative to WT FGFR3, which suggests that the fusion creates a specific gain of function.

Figure 1

A tandem duplication event results in the formation of an FGFR3-TACC3 fusion product. Here, Parker et al. demonstrate that the fusion transcript lacks a miR-99a binding site, resulting in increased expression (12). In addition, FGFR3-TACC3 fusion activates ERK and STAT3 signaling and enhances tumor progression. Previous work also demonstrated that localization of this fusion protein to the mitotic spindles promotes aneuploidy (13).

New approaches

Glioblastoma is now one of the most intensely studied of all cancers at the molecular level; however, the mapping of the mutational landscape has yet to be successfully leveraged to yield better treatment for patients. Obtaining a mechanistic understanding of this mutational landscape, particularly with regard to clarifying function of specific mutations, may go a long way toward transforming basic science knowledge into clinical benefit for patients. Here, Parker and colleagues have taken important steps toward developing a functional understanding of the consequences of the FGFR3-TACC3 fusion in glioblastoma (12). Their study, taken together with the work of Singh et al. (13), demonstrates the importance of the FGFR3-TACC3 fusion in glioblastoma, which suggests that the gain of function created by the FGFR3-TACC3 fusion is greater than the sum of its parts.

The current study also raises several questions for further investigation. First, what are the mechanisms by which the FGFR3 component of the fusion protein differentially engages downstream signaling, enabling it to more effectively activate the ERK and STAT3 signaling cascades? Second, the data from Singh et al. suggest a critical role for the TACC3 component of the fusion protein in regulating aneuploidy in a fashion dependent on FGFR3 signaling; how is that regulated? Finally, what are the therapeutic implications? Will glioblastomas bearing FGFR3-TACC3 fusions respond to pharmacologic inhibition with FGFR3 inhibitors, when they become available? Efforts to elucidate the mechanisms by which fusion oncogenes promote their oncogenic effects, even if they are rare, have yielded remarkable therapeutic insights. We expect that similar deep investigation into these pathogenic mechanisms, and others that will be discovered through RNA sequencing, will also reveal new therapeutic strategies.

Acknowledgments

This work is supported by grants from National Institute for Neurological Diseases and Stroke (NS73831), the National Cancer Institute (CA119347), and the Ben and Catherine Ivy Foundation.

Address correspondence to: Paul S. Mischel, Ludwig Institute for Cancer Research, University of California at San Diego, La Jolla, California 92093, USA. Phone: 858.534.6080; Fax: 858.534.7750; E-mail: pmischel@ucsd.edu.

Footnotes

Conflict of interest: The authors have declared that no conflict of interest exists.

Reference information: J Clin Invest. 2013;123(2):548–551. doi:10.1172/JCI67658.

See the related article at The tumorigenic FGFR3-TACC3 gene fusion escapes miR-99a regulation in glioblastoma.

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