DLC-1 gene encoding for a Rho GTPase-activating protein with tumor suppressor function is located on chromosome 8p21-22, a region frequently deleted in tumors of the prostate and carrying several prostate cancer-related genes. Tumor suppressor genes that are genetically or epigenetically altered frequently localize at sites of recurrent deletions. To see if DLC-1 is disrupted in prostate cancer, we examined mRNA expression by quantitative real-time RT-PCR and detected downregulation or loss of DCL-1 expression in 6 of 20 primary carcinomas excised by radical prostatectomy, in 3 of 5 carcinoma cell lines and in 8 of 21 benign prostatic hyperplasias (BPHs). DLC-1 mRNA status correlates with Gleason score in prostate cancer samples (p=0.036). As DNA methylation of CpG sites is a common mechanism responsible for the inactivation of tumor suppressor genes, the methylation status of DLC-1 gene was assessed by methylation-specific PCR and by sequencing the sodium bisulfite-modified DNA. DLC-1 promoter hypermethylation was identified in 11 of 20 adenocarcinomas and in 15 of 21 BPHs, but not in 3 normal prostate tissues. Among the patients with a reduced or lacking DLC-1 mRNA expression, the promoter region was hypermethylated in 5 of 6 carcinomas and in 6 of 8 BPHs. DLC-1 methylation status correlates with PSA blood levels in BPH samples (p=0.029). DLC-1 inactivation due to hypermethylation was confirmed by the re-expression of the gene after exposure of LNCaP cells to 5-aza-2’-deoxycytidine, an inhibitor of DNA methyl-transferase. Histone deacetylation may be another epigenetic mechanism of DLC-1 gene silencing in prostate cancer.To test this notion, prostate carcinoma cell lines 22RV and PC-3M, that have unmethylated alleles and lack DLC-1 expression, were treated with trichostatin A (TSA), a deacetylase inhibitor. Treatment with TSA restored DLC-1 expression. These results demonstrate that the transcriptional silencing of DLC-1 mediated by two distinct epigenetic mechanisms is common and may play an important role in the pathogenesis of prostate cancer. Because aberrant methylation is one of the earliest alterations in the development of neoplasia, the detection of promoter hypermethylation of the DLC-1 gene in 70% of BPHs may have clinical application for the early detection of prostate cancer.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]