Repression of the candidate tumor suppressor gene S100A2 in breast cancer is mediated by site-specific hypermethylation.

The calcium-binding protein S100A2 is expressed in normal breast tissue but downregulated during breast cancer progression. Hence it was previously identified as a candidate tumor suppressor gene. In this report, we investigated the molecular basis of S100A2 gene expression in normal and tumorigenic human breast epithelial cells. We cloned the gene coding for S100A2 including its 5' flanking region. To locate positively or negatively acting elements responsible for transcriptional regulation, promoter deletion studies were performed. Results from these experiments demonstrate that an enhancer element is located 1.2 kb upstream of the transcription start site. This element contains two AP1-like binding sites suggesting that transcriptional activation of S100A2 might be mediated by immediate early genes. Interestingly, the enhancer stimulates transcription in both normal and tumorigenic cells, indicating that repression of endogenous S100A2 transcription in tumorigenic cells might lie at an epigenetic level. Indeed, the proximal promoter region was found, by genomic sequencing, to be unmethylated in normal but hypermethylated in tumorigenic cells. Hypermethylation of the promoter at the same CpG sites was also found in a breast cancer biopsy. In addition, site specific in vitro methylation led to reduced expression of the S100A2 gene in normal cells. These experiments provide strong evidence that S100A2 repression in tumor cells is mediated by site-specific methylation. Since transcription of a number of known tumor suppressor genes is also repressed by methylation, our observation is consistent with the suggestion that S100A2 might have a tumor suppressor function.