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Gene silencing by promoter DNA hypermethylation is common in PCA. The role of HXKy methylation as an up-/downstream or parallel silencing mechanism is not yet clear. We analyzed HXKy methylation at gene sites that are aberrantly DNA-methylated in PCA (i.e. GSTP1, RARβ, APC and PTGS2). Cancerous (PC3, DU145, LNCaP) and benign prostate hyperplasia (BPH1) cell lines were studied using chromatin immunoprecipitation and methylation specific PCR. Gene expression was analyzed using quantitative RT-PCR. In PCA, histone modifications characteristic of gene repression (di-/tri-methylated H3K9, H3K27, H4K20) were predominantly observed at the studied gene sites and paralleled aberrant DNA hypermethylation. In BPH, no aberrant DNA hypermethylation and mostly histone modifications supporting active gene expression (H3K4, H3K79 di-/tri methylation) were seen at the respective gene sites. Mono-methylated H3K4 and H3K9 modifications that have moderate effects on gene expression were frequent in both PCA and BPH. PC3 cells grown in the presence of the DNA methyltransferase inhibitor 5-aza-2′-deoxycytidine (aza) resulted in DNA demethylation and moderate gene reexpression whereas repressive HXKy di-methylation increased 1.5-3 fold and tri-methylation was mostly unchanged. Removal of aza reverses gene expression. These findings suggest that HXKy methylation is upstream to DNA hypermethylation in PCA. [ABSTRACT FROM AUTHOR]