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Abstract
To identify methylation-silenced genes in prostate cancers, a microarray analysis for genes up-regulated by treatment with a demethylating agent, 5-aza-2'-deoxycytidine, was performed using three rat prostate cancer cell lines. Eight genes (Aebp1, Dysf, Gas6, LOC361288, Nnat, Ocm, RGD1308119, and Tgfbr2) were re-expressed at 16-fold or more, and their promoter CpG islands were shown to be densely methylated in the cancer cell lines. From the eight genes, Tgfbr2, a key mediator of transforming growth factor-beta (TGF-beta) signaling that has been strongly implicated in human and rat prostate carcinogenesis, was selected, and its silencing in primary samples was analyzed further. Tgfbr2 was methylated and markedly down-regulated in three of seven 3,2'-dimethyl-4-aminobiphenyl-induced invasive adenocarcinomas in the dorsolateral lobe of the rat prostate. In humans, marked down-regulation of TGFBR2 protein was observed in 12 of 20 high-grade prostatic intraepithelial neoplasia and 36 of 60 prostate cancers. DNA methylation of the human TGFBR2 promoter CpG islands repressed transcription, if present, but neither methylation nor mutation were detected in 27 human prostate cancers analyzed. Methylation silencing of rat Tgfbr2 was associated with histone H3 lysine 9 trimethylation, whereas decreased expression of human TGFBR2 was mainly due to decreased transcription activity, sometimes in concert with histone deacetylation and H3 lysine 27 trimethylation. The identification of methylation silencing of Tgfbr2 in rat prostate cancers, in accordance with TGFBR2 down-regulation in human prostate cancers, will enable us to analyze how aberrant methylation is induced in vivo and identify factors that promote and suppress the induction of aberrant methylation.
Highlights
Gene silencing due to DNA methylation of promoter CpG islands (CGIs) is one of the major mechanisms of tumor-suppressor gene inactivation, along with mutations and loss of heterozygosity [1]
If methylation-silenced genes involved in prostate carcinogenesis are found in rat prostate cancers, they will enable us to analyze the molecular processes of how aberrant methylation is induced in vivo as well as the factors, including hormones, that influence the process
To examine whether the induction of these genes by 5-aza-dC treatment was due to demethylation of promoter CGIs, the methylation statuses of the putative promoter CGIs were analyzed by Methylation-specific PCR (MSP)
Summary
Gene silencing due to DNA methylation of promoter CpG islands (CGIs) is one of the major mechanisms of tumor-suppressor gene inactivation, along with mutations and loss of heterozygosity [1]. Chronic inflammation is known to be an inducer of aberrant methylation in humans [3], the exact effector cells and molecular changes in target cells are unknown. To address these questions, animal models are indispensable. Because we select models by the presence of dense methylation of a promoter CGI in cancer and by the meaningful expression of its downstream gene in the corresponding normal tissue, only a limited number of methylation-silenced genes have far been identified in animal models [4,5,6,7]
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