Nasopharyngeal carcinoma (NPC) is a distinct type of head and neck cancer arising from the epithelium of nasopharynx. While the incidence rate of NPC is low in Western countries, it is prevalent in southern China, south-east Asia and northern Africa. Generally, it is believed that multiple factors, including genetic predisposition, environmental carcinogens and Epstein-Barr virus infection, contribute to the tumor initiation and progression of NPC. Similar to other solid tumors in which genetic rearrangements play causal role in the genesis of tumors, our earlier cytogenetic and spectral karyotyping have demonstrated the prevalence of chromosomal translocations in NPC tumors. These evidences suggest that novel fusion gene product arises from chromosomal translocation may contribute to the pathogenesis of the disease.
Using paired-end whole-transcriptome sequencing, we discovered a novel fusion transcript composed of the exon 1 of the ubiquitin protein ligase E3 component n-recognin 5 (UBR5) on 8q22.3 and exon 7-9 of zinc finger protein 423 (ZNF423) on 16q12.1 from the NPC cell line C666-1. Moreover, FISH analysis using break-apart probes validated the UBR5-ZNF423 fusion. Furthermore reverse transcription-PCR (RT-PCR) confirmed the expression of the corresponding fusion transcript. Intriguingly, the fusion transcript was recurrently detected in 12/144 (8.3%) of primary tumors by RT-PCR, which indicates the UBR5-ZNF423 fusion gene may contribute to the genesis of a subset of NPCs.
The growth of C666-1 cells with UBR5-ZNF423 rearrangement is dependent on expression of this fusion protein. Knock-down of UBR5-ZNF423 by RNA interference inhibited the cell proliferation of C666-1 cells. The transforming ability of UBR5-ZNF423 fusion was also confirmed in NIH3T3 mouse fibroblasts by soft agar colony formation assay. Moreover, NIH3T3 cells stably expressing the fusion protein induced tumor formation in nude mice. Furthermore, using an inducible shRNA (Lac operon-driven) targeting the fusion transcript, the growth of C666-1 tumor was suppressed in nude mice. These findings suggest that expression of UBR5-ZNF423 fusion protein contributes to the transformation of a subset of NPCs. Currently, we are working on the functional role(s) of the fusion protein in C666-1 cells. Preliminary results demonstrated that the fusion protein interacts with the Early B-cell Factor 3 (EBF3) and may deregulate EBF3 transcriptional activity. Since EBF3 is a tumor suppressor gene, deregulating EBF3 downstream gene expression by UBR5-ZNF423 may confer to its oncogenic function on NPCs.
In conclusion, we have successfully identified the UBR5-ZNF423 fusion transcript in a subset of NPC tumors. By understanding the molecular mechanisms of this fusion protein on NPCs, novel therapeutic interventions may be implemented on treating a subsets of NPC tumors expressing this oncogenic fusion protein.
Citation Format: Pok Man Hau, Grace Chung, Raymond Lung, Samantha Lun, Chit Chow, Alice Wong, Fei-Fei Liu, George Tsao, Kevin Yip, Ka Fai To, Kwok Wai Lo. The identification of UBR5-ZNF423 recurrent fusion gene in EBV-associated nasopharyngeal carcinoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4064.