Cancer is primarily a disease that consists of the accumulation of genetic and epigenetic alterations. The epigenetic alterations can lead to the inactivation of tumor suppressor genes and the activation of oncogenes. Because some miRNAs target genes involved in cell cycle control, they have been associated with the functions of tumor suppressor genes and oncogenes [22
]. Currently, there is experimental evidence that aberrant patterns of DNA methylation and repressive covalent histone modifications in the promoter regions of miRNAs associated with CpG islands can lead to epigenetic silencing [4
]. Some studies have suggested that the loss of DNA methylation in the promoters of some miRNAs (e.g., miR-21, miR-203
) in ovarian cancer results in their aberrant overexpression [23
]. On the other hand, it has been shown that DNA hypermethylation in the promoters of miR-124a
leads to their transcriptional silencing in colon cancer models and influences the expression of two oncogenes, such as BCL6 and CDK6 [4
]. The epigenetic silencing of miRNAs has been frequently observed in early stages of breast cancer, including miR-9-1 and the miRNAs of the Let-7 family. Therefore, miRNAs could be good candidates for early tumor markers [24
]. Furthermore, some miRNAs have been associated with invasion and metastasis in breast cancer [25
]. One of the miRNAs that are deregulated in breast cancer is miR-125b1, and its expression is reduced in this type of cancer, suggesting its importance [10
In the present study, we showed that methylation of DNA in a CpG island upstream of the transcription start site of miR-125b1 and the gain of repressive histone marks, such as H3K9me3 and H3K27me3, is a mechanism for the epigenetic silencing of this miRNA. We found that DNA methylation at the CpG island of miR-125b1 is present in the evaluated gynecological tumors. Our results suggest that this methylation negatively affects the miRNA gene expression, indicating that it could be in part responsible for the gene inactivation (Figures and ).
One of the most studied proteins in vertebrates is the nuclear factor CTCF [27
]. This protein has the ability to delimit chromosomal borders, which affect the distinction of transcriptionally active regions from more compact regions in which transcription is repressed. It is currently accepted that CTCF can contribute to the establishment of chromatin domains. This organization promotes the creation of a local chromatin that facilitates communication between regulatory elements, such as promoters or enhancers [28
]. Because CTCF has been related with protection against DNA methylation and repressive histone marks propagation, we investigated whether the presence of CTCF is associated with the transcription of miR125-b1
. Interestingly, in normal breast cells, CTCF is distributed along the locus miR-125b1
, whereas in primary breast tumors, we observed the absence of this factor. These data suggest that the absence of CTCF may be related to a gain of aberrant DNA methylation in the CpG island and a gain of repressive histone marks along the four genomic regions studied (Figure ).
Therefore, we propose that the absence of CTCF along a locus may result in the loss of transcriptionally active borders, and together with DNA methylation and some histone repressive marks, such as H3K27me3, could promote the epigenetic silencing of the miRNA (Figure ).
Figure 4 Model of miR-125b1 gene silencing, in which the absence of the nuclear factor CTCF is associated with DNA methylation of the CpG island, and the enrichment of repressive histone marks. In normal breast, CTCF might prevent the recruitment of the epigenetic (more ...)
Some authors have referred to CTCF as the "glue" that connects the ends of domains, both intra- and inter-chromosomally, thus suggesting that deregulation of this factor leads to the aberrant silencing of multiple genes, including miRNAs [29
]. These observations are consistent with published data that demonstrate that miR-125b1
can be silenced by DNA methylation, which may lead to a worse prognosis in cancer patients [8