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 tissu e but downregulated during breast cancer progression. Hence it was pre viously identified as a candidate tumor suppressor gene. In this repor t, we investigated the molecular basis of S100A2 gene expression in no rmal and tumorigenic human breast epithelial cells. We cloned the gene coding for S100A2 including its 5' flanking region. To locate positiv ely or negatively acting elements responsible for transcriptional regu lation, promoter deletion studies were performed. Results from these e xperiments demonstrate that an enhancer element is located 1.2 kb upst ream of the transcription start site. This element contains two AP1-li ke binding sites suggesting that transcriptional activation of S100A2 might be mediated by immediate early genes. Interestingly, the enhance r stimulates transcription in both normal and tumorigenic cells, indic ating that repression of endogenous S100A2 transcription in tumorigeni c cells might lie at an epigenetic level. Indeed, the proximal promote r region was found, by genomic sequencing, to be unmethylated in norma l but hypermethylated in tumorigenic cells. Hypermethylation of the pr omoter at the same CpG sites was also found in a breast cancer biopsy. In addition, site specific in vitro methylation led to reduced expres sion of the S100A2 gene in normal cells. These experiments provide str ong 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 suppress or function.