We analyzed DNA methyltransferase (Dnmt) protein expression and DNA methylation 
patterns during four progressive stages of prostate cancer in the transgenic 
adenocarcinoma of mouse prostate (TRAMP) model, including prostatic 
intraepithelial neoplasia, well-differentiated tumors, early poorly 
differentiated tumors, and late poorly differentiated tumors. Dnmt1, Dnmt3a, and 
Dnmt3b protein expression were increased in all stages; however, after 
normalization to cyclin A to account for cell cycle regulation, Dnmt proteins 
remained overexpressed in prostatic intraepithelial neoplasia and 
well-differentiated tumors, but not in poorly differentiated tumors. Restriction 
landmark genomic scanning analysis of locus-specific methylation revealed a high 
incidence of hypermethylation only in poorly differentiated (early and late) 
tumors. Several genes identified by restriction landmark genomic scanning showed 
hypermethylation of downstream regions correlating with mRNA overexpression, 
including p16INK4a, p19ARF, and Cacna1a. Parallel gene expression and DNA 
methylation analyses suggests that gene overexpression precedes downstream 
hypermethylation during prostate tumor progression. In contrast to gene 
hypermethylation, genomic DNA hypomethylation, including hypomethylation of 
repetitive elements and loss of genomic 5-methyldeoxycytidine, occurred in both 
early and late stages of prostate cancer. DNA hypermethylation and DNA 
hypomethylation did not correlate in TRAMP, and Dnmt protein expression did not 
correlate with either variable, with the exception of a borderline significant 
association between Dnmt1 expression and DNA hypermethylation. In summary, our 
data reveal the relative timing of and relationship between key alterations of 
the DNA methylation pathway occurring during prostate tumor progression in an in 
vivo model system.