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Background  In cancer cells, transcriptional gene silencing has been associated with genetic and epigenetic defects. The disruption of DNA methylation patterns and covalent histone marks has been associated with cancer development. Until recently, microRNA (miRNA) gene silencing was not well understood. In particular, miR-125b1 has been suggested to be an miRNA with tumor suppressor activity, and it has been shown to be deregulated in various human cancers. In the present study, we evaluated the DNA methylation at the CpG island proximal to the transcription start site of  miR-125b1  in cancer cell lines as well as in normal tissues and gynecological tumor samples. In addition, we analyzed the association of CTCF and covalent histone modifications at the  miR-125b1  locus.    Methods  To assess the DNA methylation status of the miR-125b1, genomic DNA was transformed with sodium bisulfite, and then PCR-amplified with modified primers and sequenced. The  miR-125b1  gene expression was analyzed by qRT-PCR using U6 as a control for constitutive gene expression. CTCF repressive histone marks abundance was evaluated by chromatin immunoprecipitation assays.    Results  The disruption of CTCF in breast cancer cells correlated with the incorporation of repressive histone marks such H3K9me3 and H3K27me3 as well as with aberrant DNA methylation patterns. To determine the effect of DNA methylation at the CpG island of  miR-125b1  on the expression of this gene, we performed a qRT-PCR assay. We observed a significant reduction on the expression of  miR-125b1  in cancer cells in comparison with controls, suggesting that DNA methylation at the CpG island might reduce  miR-125b1  expression. These effects were observed in other gynecological cancers, including ovarian and cervical tumors.    Conclusions  A reduction of  miR-125b1  expression in cancers, correlated with methylation, repressive histone marks and loss of CTCF binding at the promoter region.

Disruption of CTCF at the miR-125b1 locus in gynecological cancers