Epigenetic Silencing of MicroRNA-34b/c and B-Cell Translocation Gene 4 Is Associated with CpG Island Methylation in Colorectal Cancer

Abstract

Altered expression of microRNA (miRNA) is strongly implicated in cancer, and recent studies have shown that, in cancer, expression of some miRNAs cells is silenced in association with CpG island hypermethylation. To identify epigenetically silenced miRNAs in colorectal cancer (CRC), we screened for miRNAs induced in CRC cells by 5-aza-2 ¶-deoxycytidine (DAC) treatment or DNA methyltransferase knockout. We found that miRNA-34b (miR-34b) and miR-34c, two components of the p53 network, are epigenetically silenced in CRC; that this down-regulation of miR-34b/c is associated with hypermethylation of the neighboring CpG island; and that DAC treatment rapidly restores miR-34b/c expression. Methylation of the miR34b/c CpG island was frequently observed in CRC cell lines (nine of nine, 100%) and in primary CRC tumors (101 of 111, 90%), but not in normal colonic mucosa. Transfection of precursor miR-34b or miR-34c into CRC cells induced dramatic changes in the gene expression profile, and there was significant overlap between the genes down-regulated by miR-34b/c and those down-regulated by DAC. We also found that the miR-34b/c CpG island is a bidirectional promoter which drives expression of both miR-34b/c and B-cell translocation gene 4 (BTG4); that methylation of the CpG island is also associated with transcriptional silencing of BTG4; and that ectopic expression of BTG4 suppresses colony formation by CRC cells. Our results suggest that miR-34b/c and BTG4 are novel tumor suppressors in CRC and that the miR-34b/c CpG island, which bidirectionally regulates miR34b/c and BTG4, is a frequent target of epigenetic silencing in

High-resolution genomic profiling reveals gain of chromosome 14 as a predictor of poor outcome in ileal carcinoids

“…BTG4 is a cell cycle regulator that induces G1 arrest. Loss of BTG4 is a common event in chronic lymphocytic leukaemia, and epigenetic silencing of BTG4 by CpG island methylation occurs in colorectal cancer (Auer et al 2005, Toyota et al 2008. PPP2R1B regulates RaIA GTPase and inhibits cell growth (Sablina et al 2007).…”

MicroRNAs: Biogenesis, Roles for Carcinogenesis and as Potential Biomarkers for Cancer Diagnosis and Prognosis

“…For example, miR-21 and miR-203, which were implanted in a region with CpG islands, equally decreased miR-124a expression, which was attributed to DNA hypermethylation in colon, breast, and lung carcinomas (Lujambio et al, 2008). Other studies have experimentally proven that potentially oncogenic miRNAs can be upregulated by DNA hypomethylation (Iorio et al, 2005), such as upregulation of hypermethylated tumour suppressing miRNAs by 5-aza-20deoxycytidine treatment in the case of as miR-127, miR-9-1 and miR-34b/c cluster (Toyota et al, 2008). Histone acetylation also represents a similar mechanism of epigenetic alteration in cancer by reducing the level of acetylated histones, which can diminish the expression of anti-oncogenic miRNAs by utilising histone deacetylase inhibitors; alteration of the miRNA levels have been observed during the treatment, which was among the evidence defined by Saito and colleagues (2006).…”

The miR-34 family in cancer and apoptosis

“…35 More recently, the epigenetic inactivation of miR-34a was identified in cell lines derived from some of the most common tumors (breast, lung, colon, kidney, bladder, pancreatic cancer and melanoma) and also in primary melanoma. 38 In addition, CpG methylation of miR-34b/c was found in colorectal cancer, 45 in oral squamous cell carcinoma 60 and in malignant melanoma in which it correlated with metastatic potential. 46 Furthermore, experimental animal models of liver carcinogenesis showed downregulation of miR-34a.…”