Epigenetic regulation of bone morphogenetic protein-6 gene expression in breast cancer cells

Bone morphogenetic protein-6 (BMP-6) is closely correlated with tumor differentiation and skeletal metastasis. Our previous research found that BMP-6 gene expression can be activated dose-dependently by estrogen in estrogen receptor positive (ER⁺) breast cancer cell line MCF-7, but not in ER negative (ER⁻) cell line MDA-MB-231. This experiment is designed to investigate the epigenetic regulatory mechanism of the BMP-6 gene expression in breast cancer cell lines MDA-MB-231, MCF-7 and T47D with regard to the methylation status in the 5′ flanking region of the human BMP-6 gene. The endogenous level of BMP-6 mRNA in ER⁻ cell line MDA-MB-231 was relatively lower than that in ER⁺ MCF-7 and T47D cell lines. After the treatment with 5-aza-2′-deoxycytidine (5-aza-dC, especially in the concentration of 10 μM), the BMP-6 mRNA expression in MDA-MB-231 was obviously up-regulated. However, 5-aza-dC treatment failed to regulate the expression of BMP-6 in MCF-7 and T47D cells. Using enzyme restriction PCR (MSRE-PCR), as well as bisulfite sequencing (BSG), methylation of human BMP-6 gene promoter was detected in MDA-MB-231; while in MCF-7 and T47D, BMP-6 gene promoter remained demethylated status. In 33 breast tumor specimens, promoter methylation of BMP-6 was detected by methylation-specific PCR, hypermethylation of BMP-6 was observed in ER negative cases (16 of 16 cases (100%)), while obviously lower methylation frequency were observed in ER positive cases (3 of 17 cases (18%)), indicating that BMP-6 promoter methylation status is correlated with ER status in breast cancer.