Glutathione peroxidase 7 has potential tumour suppressor functions that are silenced by location-specific methylation in oesophageal adenocarcinoma

OBJECTIVE: We investigated the potential tumor suppressor functions of glutathione peroxidase 7 (GPX7) and examined the interplay between epigenetic and genetic events in regulating its expression in oesophageal adenocarcinomas (OAC). DESIGN: In vitro and in vivo cell models were developed to investigate the biological and molecular functions of GPX7 in OAC. RESULTS: Reconstitution of GPX7 in OAC cell lines, OE33 and FLO-1, led to significant growth suppression as demonstrated by growth curve, colony formation and EdU proliferation assays. Meanwhile, GPX7-expressing cells displayed significant impairment in G1/S progression and an increase in cell senescence. Concordant with the above functions, Western blot analysis displayed higher levels of p73, p27, p21, and p16 with a decrease in phosphorylated RB; indicating its increased tumor suppressor activities. On the contrary, knockdown of GPX7 in HET1A cells (an immortalized normal esophageal cell line) rendered the cells growth advantage as indicated with a higher EdU rate, lower levels of p73, p27, p21, and p16 and an increase in phosphorylated RB. We confirmed the tumor suppressor function in vivo using GPX7-expressing OE33 cells in a mouse xenograft model. Pyrosequencing of the GPX7 promoter region (−162 to +138) demonstrated location-specific hypermethylation between +13 and +64 in OACs (69%, 54/78). This was significantly associated with the downregulation of GPX7 (p<0.01). Neither mutations in the coding exons of GPX7 nor DNA copy number losses were present in the OACs examined (<5%). CONCLUSIONS: Our data suggest that GPX7 possesses tumor suppressor functions in OAC and is silenced by location-specific promoter DNA methylation