Epigenetic silencing of miR-335 and its host gene MEST in hepatocellular carcinoma

MicroRNAs (miRNAs) are small non-coding RNAs that function as endogenous silencers of target genes. Some tumor-suppressive miRNAs are known to be epigenetically silenced by promoter DNA methylation in cancer. In the present study, we aimed to identify miRNA genes that are silenced by DNA hypermethylation in hepatocellular carcinoma (HCC). We screened for miRNA genes with promoter DNA hypermethylation using a genome-wide methylation microarray analysis in HCC cells. It was found that miR-335, which is harbored within an intron of its protein-coding host gene, MEST, was downregulated by aberrant promoter hypermethylation via further methylation assays, including methylation-specific PCR, combined bisulfite and restriction analysis, bisulfite sequencing analysis and 5-aza-2'-deoxycytidine treatment. The expression levels of miR-335 significantly correlated with those of MEST, supporting the notion that the intronic miR-335 is co-expressed with its host gene. The levels of miR-335/MEST methylation were significantly higher in 18 (90%) out of 20 primary HCC tumors, compared to their non-tumor tissue counterparts (P<0.001). The expression levels of miR-335 were significantly lower in 25 (78%) out of 32 primary HCC tumors, compared to their non-tumor tissue counterparts (P=0.001). Furthermore, the expression levels of miR-335 were significantly lower in HCC tumors with distant metastasis compared to those without distant metastasis (P=0.02). In conclusion, our results indicate that expression of miR-335 is reduced by aberrant DNA methylation in HCC.