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Epigenetic Modifications of RASSF1A Gene through Chromatin Remodeling in Prostate Cancer

Authors' Affiliations: ¹ Department of Urology, Veterans Affairs Medical Center and University of California School of Medicine, San Francisco, California; ² Department of Urology, Shimane University, Faculty of Medicine, Izumo 693-8501, Japan; and ³ Department of Urology, Graduate School of Medical and Dental Sciences, Kagoshima University, Kagoshima 890-8520, Japan

Requests for reprints: Rajvir Dahiya, Urology Research Center (112F), Veterans Affairs Medical Center and University of California School of Medicine, San Francisco, 4150 Clement Street, San Francisco, CA 94121. Phone: 415-750-6964; E-mail: rdahiya{at}urology.ucsf.edu.

Purpose: The RAS-association domain family 1, isoform A (RASSF1A) gene is shown to be inactivated in prostate cancers. However, the molecular mechanism of silencing of the RASSFIA gene is not fully understood. The present study was designed to investigate the mechanisms of inactivation of the RASSF1A gene through the analysis of CpG methylation and histone acetylation and H3 methylation associated with the RASSF1A promoter region.

Experimental Design: Methylation status of the RASSF1A gene was analyzed in 131 samples of prostate cancer, 65 samples of benign prostate hypertrophy (BPH), and human prostate cell lines using methylation-specific PCR. Histone acetylation (acetyl-H3, acetyl-H4) and H3 methylation (dimethyl-H3-K4, dimethyl-H3-K9) status associated with the promoter region in prostate cells were analyzed by chromatin immunoprecipitation (ChIP) assay.

Results: Aberrant methylation was detected in 97 (74.0%) prostate cancer samples and 12 (18.5%) BPH samples. The methylation frequency of RASSF1A showed a significant increase with high Gleason sum and high stage. The ChIP assays showed enhancement of histone acetylation and dimethyl-H3-K4 methylation on the unmethylated RASSF1A promoter. TSA alone was unable to alter key components of the histone code. However, after 5-aza-2'-deoxy-cytidine treatment, there was a complete reversal of the histone components in the hypermethylated promoter. Levels of acetyl-H3, acetyl-H4, and dimethyl-H3-K4 became more enriched, whereas H3K9me2 levels were severely depleted.

Conclusions: This is the first report suggesting that reduced histone acetylation or H3K4me2 methylation and increased dimethyl-H3-K9 methylation play a critical role in the maintenance of promoter DNA methylation–associated RASSF1A gene silencing in prostate cancer.

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