Epigenetic inactivation of inhibitor of differentiation 4 ( I d4) correlates with prostate cancer

The inhibitor of DNA‐binding (Id) proteins, Id1–4 are negative regulators of basic helix‐loop‐helix ( bHLH ) transcription factors. As key regulators of cell cycle and differentiation, expression of Id proteins are increasingly observed in many cancers and associated with aggressiveness of the disease. Of all the four Id proteins, the expression of Id1, Id2, and to a lesser extent, Id3 in prostate cancer and the underlying molecular mechanism is relatively well known. On the contrary, our previous results demonstrated that Id4 acts as a potential tumor suppressor in prostate cancer. In the present study, we extend these observations and demonstrate that Id4 is down‐regulated in prostate cancer due to promoter hypermethylation. We used prostate cancer tissue microarrays to investigate I d4 expression. Methylation specific PCR on bisulfite treated DNA was used to determine methylation status of Id4 promoter in laser capture micro‐dissected normal, stroma and prostate cancer regions. High Id4 expression was observed in the normal prostate epithelial cells. In prostate cancer, a stage‐dependent decrease in I d4 expression was observed with majority of high grade cancers showing no Id4 expression. Furthermore, I d4 expression progressively decreased in prostate cancer cell line LNC a P and with no expression in androgen‐insensitive LNC a P ‐ C 81 cell line. Conversely, I d4 promoter hypermethylation increased in LNC a P ‐ C 81 cells suggesting epigenetic silencing. In prostate cancer samples, loss of I d4 expression was also associated with promoter hypermethylation. Our results demonstrate loss of I d4 expression in prostate cancer due to promoter hypermethylation. The data strongly support the role of I d4 as a tumor suppressor.