DNA promoter methylation in urothelial cancer: a feature of the invasive phenotype and a novel therapeutic target

Many molecular events in cancer occur at the epigenetic rather than genetic level. The best studied of these is DNA cytosine methylation. This occur most frequently in DNA promoter regions and results in loss of gene expression. Tumour suppressor gene silencing by aberrant hypermethylation has been demonstrated in several carcinogenic pathways. Here we present the latest results of our studies regarding promoter methylation in urothelial cancer (UC). Firstly, using 280 UC of the upper and lower urinary tracts we show that promoter methylation (at 11 tumour suppressor genes) occurs frequently (86%) and in an anatomically distinct pattern. Higher methylation levels are seen in the upper (94%) than lower urinary (76%) tracts (P < 0.0001), explaining the distinct pattern of microsatellite instability seen in UC. Furthermore, when tumour grade, stage and behaviour analysed, aggressive tumours (rapidly progressing and poorest differentiation, P = 0.001) have the highest levels of methylation. Secondly, studying 196 corresponding samples from 104 patients with invasive UC, we show that methylation occurs early in the invasive pathway and can be detected in both carcinoma in situ and normal urothelial samples from these patients. Once again the presence of methylation in either CIS or normal urothelium predicts rapid tumour progression (T test P < 0.0001). Finally, we use bladder cancer cell lines to show that epigenetic treatment (with 5-azacytidine) removes the methyl groups from methylated cytosine residues and allows the re-expression of previously silenced tumour suppressor genes. This induces cell apoptosis and reduces growth rates, suggesting a novel therapeutic role.