Down-regulation of CXCL12 mRNA expression by promoter hypermethylation and its association with metastatic progression in human breast carcinomas

Abstract

Not only is the expression of CXCR4 on breast cancers a key determinant of tumor metastasis, CXCL12 exhibiting peak levels of constitutive expression in organs representing the first destinations of cancer metastasis, but is proposed to be also essential for the organ-specific metastatic process. In this study, the expressions of CXCR4 and CXCL12 were investigated using quantitative RT-PCR and immunohistochemistry in samples of 63 primary breast carcinomas and 20 normal breast tissues. Using methylation-specific PCR, we also analyzed the methylation status of CXCL12. Both up-regulation of CXCR4 and down-regulation of CXCL12 were observed in primary breast carcinomas. Over-expression of CXCR4 mRNA was significantly related to lymph node metastasis status and strong Her-2 expression, while decreased expression of CXCL12 mRNA was significantly associated with positive lymph node metastasis and estrogen receptor negativity. Methylation-specific PCR showed that 52.4% of breast tumors were hypermethylated in the CXCL12 promoter region. The expression levels of DNA methyltransferase (DNMT) 1 and DNMT3B were significantly higher in the CXCL12-methylated breast carcinomas than in the CXCL12-unmethylated ones. In summary, DNA hypermethylation of CXCL12 plays an important role in the down-regulation of CXCL12 expression in breast carcinomas. Cancer cells lacking expression of CXCL12, but maintaining over-expression of CXCR4, can selectively spread to target organs in which the ligand is highly secreted.