Silencing of Mir-22 Is Linked to H3K27 Trimethylation and Independent of DNA Methylation in Pre-B Acute Lymphoblastic Leukemia Cells.

The regulation of human microRNA (miRNA) expression is still poorly understood and aberrant epigenetic regulation has recently been implicated in the down-regulation of tumor suppressor miRNAs. In this study, we investigated whether histone modifications would contribute to the dysregulation of miRNAs in lymphoblastic leukemia cells. Using a precursor B-cell acute lymphoblastic leukemia cell line, NALM-6 cells, we demonstrated by miRNA microarray analysis that a specific histone deacetylases inhibitor, trichostatin A (TSA), induced a differential alteration in cellular miRNA expression. A total of 10 miRNAs were down-regulated and 31 up-regulated significantly following TSA treatment. Among TSA-up-regulated miRNAs, miR-22 is an extronic miRNA and resides in the second exon of the non-coding transcript MGC14376. Up-regulation of both miR-22 and MGC14376 was found in NALM-6 cells treated with TSA but not 5-AZA-2’-deoxycytidine, a DNA demethylating agent. Luciferase reporter analysis identified three regions in the promoter of miR-22 and MGC14376 that differentially regulated its transcriptional activation. Although there is a CpG island within the promoter of miR-22 and MGC14376, no obvious methylation was detected at this region in NALM-6 cells. Conversely, H3K27 trimethylation (H3K27triM)-associated histone modification was identified in the first intron of MGC14376 gene and was involved in TSA-induced miR-22 expression. Thus, miR-22 silencing in NALM-6 cells involves H3K27triM-associated histone modification but is independent of DNA methylation, suggesting that methylation-independent H3K27triM histone modification may be an important mechanism for miRNA dysregulation in cancer cells.