Abstract 2769: Response to 5FU therapy in colorectal cancer is affected by EZH2-mediated BclX splicing

Background and Aims: The chromatin modifier EZH2 is remarkably up-regulated in many cancer types including colon cancer. High levels of EZH2 have been correlated with tumor metastasis and regulation of tumor-initiating cell subpopulation. Since, DZNep, an EZH2 inhibitor, can inhibit the growth capability of colon cancer by inducing apoptosis; down-regulation of EZH2 seems to be a promising therapeutic approach. Our preliminary data showed that EZH2 levels were remarkably increased in blood samples from metastatic colorectal cancer patients who did not respond to the combination therapy with bevacizumab-irinotecan-5FU in comparison to patients who were responders and developed a stable disease. Many apoptosis-related proteins possess a dual function (pro- and anti-apoptotic) due to alternative splicing. Therefore we aimed to find possible apoptosis-associated EZH2 targets to explain these interesting findings. Methods: HCT116 colon cancer cells were used for DZNep and 5FU treatment and its efficiency was verified by Western blot analysis. In vivo splicing assay was performed using BclX minigene construct. The mRNA expression of EZH2, caspase 2, Death-associated protein kinase (DAPK) and BclX isoforms was assessed by quantitative real-time PCR in cells (BclX also in blood of patients) and normalized to the GAPDH. Cell viability was examined by Crystal Violet staining. Results: Western Blot for caspase 3 cleavage indicated increased apoptotic levels after DZNep treatment. Combined DZNep and 5FU treatment of cells led to a synergistic effect in reducing cell viability. DZNep treatment specifically promoted pro-apoptotic BclXs isoform, but did not affect the shift of pro-apoptotic isoforms of caspase 2 and DAPK. Using a BclX minigene transfection we confirmed this effect exogenously. The ratio BclXs/Xl was significantly altered by overexpression of different splicing factors such as ASF/SF2, Sam68, after DZnep treatment. Moreover, patients who did not respond to the combination therapy had the highest EZH2 and BclXl isoform levels. Conclusion: Our data indicate a new interesting link between epigenetic regulation and the splicing machinery. Detailed investigation of EZH2-mediated regulation of BclXs/Xl ratio could have the potential for prediction of 5FU-based therapy response in colon cancer patients. Citation Format: Natalya Benderska, Maria Gazouli, Gerasimos Aravantinos, Arndt Hartmann, Regine Schneider-Stock. Response to 5FU therapy in colorectal cancer is affected by EZH2-mediated BclX splicing. [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 2769.