Loss of the repressive H3K27me3 mark from the promoters of squamous differentiation genes has been shown to drive squamous differentiation in normal keratinocytes. The polycomb repressor complex two (PRC2) member, EZH2, is a methyltransferase that trimethylates lysine 27 on histone H3 (H3K27me3) and induces transcriptional repression. We examined whether dysregulation of the H3K27me3 mark on the promoters of squamous differentiation genes could explain the differentiation-refractory behaviour observed in head and neck squamous cell carcinoma (HNSCC). Therefore, we investigated the effects of inhibiting EZH2, by RNA interference or pharmacologically, on H3K27me3 binding to differentiation gene promoters, HNSCC growth, viability and differentiation in vivo and in vitro.
EZH2 was highly expressed in HNSCC cell lines in vitro and tissue microarray analysis revealed high expression in (n= 59) in situ relative to normal oral epithelium (n=12). Strikingly, inhibition of EZH2 with siRNA could induce expression of differentiation genes in differentiation-refractory squamous cell carcinoma cell lines which exhibited aberrant recruitment of H3K27me3 to differentiation gene loci. Using the homocysteine hydrolase inhibitor 3-Deazaneplanocin A (DZNep), we inhibited EZH2 and H3K27me3 pharmacologically in HNSCC cells and normal keratinocytes and demonstrate that DZNep causes cancer-cell specific apoptosis in addition to a profound reduction in colony forming efficiency and induction of some squamous differentiation genes.
We investigated the anti-cancer effects of DZNep in a pre-clinical xenograft model of HNSCC and show attenuation of tumour growth in two different xenograft models, intra-tumour inhibition of EZH2 and induction of differentiation gene transcripts in situ. Collectively, these data suggest that the aberrant differentiation may be attributed to epigenetic dysregulation and suggests that inhibition of PRC2-mediated gene repression may represent a potential therapeutic target in SCC.