Abstract P71: EZH2 Inhibition Overcomes Radiotherapy Resistance in Radioresistant Head and Neck Cancers (HNCs) by Modulating Heterochromatin-Mediated Epigenetic Silencing

Abstract

Radiotherapy is a primary treatment modality for head and neck cancer patients. However, tumour recurrence following radiotherapy and the existing knowledge gap in radioresistance mechanisms pose a major clinical challenge that leads to poor treatment responses. Our previous work identified heterochromatin modulation as a key mechanism underlying acquired radioresistance in HNCs, whereby increased heterochromatin formation leads to lower radiosensitivity upon RT and promotes radioresistance. In this study, we investigated the role of enhancer of zeste homolog 2 (EZH2), a histone methyltransferase responsible for the tri-methylation of H3K27me3, in the radioresistance mechanism using our established radioresistant (RR) model. FaDu-RR cell line was established through longitudinal low-dose fractionated irradiation via exposure to a daily 2 Gy irradiation dose until a cumulative dose of 90 Gy was achieved. At baseline, our RR cells demonstrated comparable EZH2 expression to their parental wild-type (WT), with higher H3K27me3 expression observed. Treatment with GSK126, an EZH2 inhibitor, significantly reduced cell viability in RR cells (59.8% [RR] vs 73.9% [WT]). Combination treatment of GSK126 with 4 Gy irradiation led to lower H3K27me3 expression in both models, with more pronounced EZH2 inhibition activity in RR cells. Furthermore, we observed higher γH2Ax and p-ATM expression in RR cells, indicating that the combination treatment increased irradiation-induced DNA damage. Simultaneously, increased p21 expression in both WT and RR cells, with higher p-p53 expression in the latter, suggests the activation of p53-mediated cell cycle arrest. Consistently, clonogenic assays confirmed the RR phenotype with increased baseline survivability in RR cells compared to WT. Combination treatment selectively improved radiosensitivity in RR cells (SF4Gy GSK126:Control=0.8 [WT]; SF4Gy GSK126:Control=0.4 [RR]), indicating the reversal of radioresistance. Our findings demonstrate that EZH2 is a promising therapeutic target for overcoming radioresistance, where its inhibition lowers heterochromatin formation activity and increases susceptibility to radiation-induced damage, thereby reversing radioresistance in HNCs.