The Polycistronic miRNA Cluster miR-17-92 Is Over-Expressed in Early Phase Chronic Myeloid Leukemia (CML) CD34+ Cells.

Micro RNAs (miRNAs) are small non-coding, single-stranded RNAs regulating gene expression by hybridization to complementary sequences in target mRNAs. Aberrant miRNA expression has been found in a variety of human malignancies including B-cell leukemias and lymphomas. The polycistronic miR-17–92 cluster, which includes seven miRNAs (miR-17-5p, miR-17-3p, miR-18a, miR-19a, miR-20a, miR-19b-1, miR-92-1), resides in intron 3 of the C13orf25 gene located on human chromosome 13 and has recently been described to harbor oncogenic potential in a murine transplantation model. In addition, expression of the polycistron can be regulated by c-myc, which is also activated by the Bcr-Abl oncogene product. To study miRNA expression in CML, K562 cells were analyzed by microarray analysis (miCHIP) for normalized expression of 244 miRNAs following treatment either with

1. imatinib to inhibit Bcr-Abl kinase activity or with

2. specific anti bcr-abl shRNA to knock-down gene expression.

Both of these treatment strategies led to a 2–4-fold reduction of six mature miRNAs (miR-17-5p, miR-17-3p, miR-18a, miR-20a miR-19b-1, miR-92–1) as compared to untreated control cells. The reduction of the miR-17–92 miRNA cluster expression was confirmed for each individual mature miRNA species encoded in the cluster by real-time RT-PCR. Treatment with either imatinib or anti-bcr-abl shRNA also down-regulates expression of mature miRNA miR-19a (>3-fold) in K562 cells as quantified by real-time RT-PCR. We next used PCR based quantification to analyze miR-17–92 expression in purified normal (n=4) and CML CD34+ cells harvested at diagnosis of chronic phase disease (n=16). Individual mature miRNAs of the cluster were up-regulated (2–6-fold) in CML as compared to normal CD34+ cells. Additionally, the pri-miRNA for the miR-17–92 cluster was up-regulated in 70% (7/10) of CML samples. Interestingly, retrovirus-mediated over-expression of the miR-17-19b-1 cluster in K562 cells mediates enhanced colony-formation of transduced cells. Furthermore, K562 cells expressing high levels of the miR-17-19b-1 cluster are more resistant to proliferation inhibition induced by specific anti-myc RNAi, indicating some functional role of the miR-17-92 cluster on proliferation and survival in this model. In summary, the polycistronic miR-17–92 cluster is up-regulated in early chronic phase CML CD34+ cells and may contribute to cellular transformation by the Bcr-Abl oncogene product.