Epigenetic silencing of the candidate tumor suppressor gene PROX1 in sporadic breast cancer

Extensive hypermethylation and consecutive transcriptional silencing of turnorsuppressor genes have been documented in multiple tumor entities including breast cancer. In a microarray based genome-wide methylation analysis of five sporadic breast carcinomas we identified a hypermethylated CpG island within the first intron of the prospero related homeobox gene 1 (PROX1). We, therefore, investigated CpG island methylation of PROX 1 in a series of 33 pairs of primary breast cancer and corresponding normal tissue samples by bisulfite sequencing and COBRA analyses. Seventeen of these (52%) breast cancer samples revealed a significant accumulation of methylated CpG sites along with a significant reduction of PROXI transcription compared to normal breast tissues of the same patients. Frequent methylation was also observed in brain metastases from primary breast cancer (21/37 = 57% of cases). Secondary, we analysed 38 brain metastases of primary breast carcinomas and detected a significantly reduced expression of PROXI compared to normal breast tissue (p < 0.001) and primary breast carcinomas (p < 0.05), respectively. Additionally, treatment of breast cancer cell lines with demethylating agents could reactivate PROXI transcription. In summary, we have identified PROXI as a novel target gene that is hypermethylated and transcriptionally silenced in primary and metastatic breast cancer. (c) 2007 Wiley-Liss, Inc.