Prostate cancer is phenotypically, genetically and epigenetically heterogeneous. In the present study we have dissected the epigenetic heterogeneity of prostate cancer by analyzing DNA methylation patterns in pure subpopulations of primary cancer cells, on the basis of their differentiation status. GSTP1, an enzyme involved in intracellular detoxification, is highly expressed in prostate basal epithelial cells but is down-regulated in luminal cells by a mechanism independent of DNA methylation. The GSTP1 promoter becomes frequently hypermethylated in prostate cancer, where the majority of cells bear a luminal-like phenotype. However, a small subpopulation of basal cells (<1%) persists within prostate cancer and is hypothesized to contain stem-like cells that give rise to aberrantly differentiated cancer cells. By analyzing the DNA methylation patterns in basal and luminal like prostate cancer cells, it has been possible to unveil the origin of promoter hypermethylation in prostate cancer that ultimately leads to epigenetic heterogeneity. In established cell lines, GSTP1 was actively transcribed (measured by RT-PCR) and not hypermethylated (measured by pyrosequencing methylation assay) in basal-like cancer cells, while it was hypermethylated and down-regulated in luminal-like cancer cells. MACs selection of Lin/CD31/CD24+ cells from prostate primary tissues highly enriched for AR+/KRT8+/GSTP1low cells bearing a luminal-like phenotype, while generation of prostate primary epithelial cultures gave rise to cells with basal phenotype. In luminal-like cells, GSTP1 was hypermethylated in cancer samples compared to benign controls. However, no significant hypermethylation of GSTP1 was found in the basal-like cells, where the gene was actively expressed in both benign and cancer samples. Lack of GSTP1 promoter methylation was also found in tumor xenografts generated in Rag2−/+gammaC−/+ mice from primary prostate cancer tissues. These xenografts do not undergo complete differentiation and show an intermediate phenotype expressing both basal and luminal markers. Moreover, in BPH-1 cells, a fast cycling immortalized cell line, expression and promoter methylation of GSTP1 correlated with the differentiation status of the cells, being hypermethylated in more differentiated cells. Our results strongly indicate that within prostate cancer there is a subpopulation of undifferentiated basal-like cells that do not hypermethylate the GSTP1 promoter. We hypothesize that these cells can differentiate into luminal-like cancer cells, which down-regulate GSTP1 and hypermethylate its promoter as a consequence of aberrant proliferation. Future work will include determination of the mechanism of GSTP1 down-regulation in normal and malignant prostate epithelial differentiation, and whether this mechanism is shared with other genes frequently hypermethylated in prostate cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5209. doi:1538-7445.AM2012-5209