Aberrant overexpression and function of the miR-17-92 cluster in MLL -rearranged acute leukemia

Abstract

MicroRNA (miRNA)-17-92 cluster (miR-17-92), containing seven individual miRNAs, is frequently amplified and overexpressed in lymphomas and various solid tumors. We have found that it is also frequently amplified and the miRNAs are aberrantly overexpressed in mixed lineage leukemia (MLL)-rearranged acute leukemias. Furthermore, we show that MLL fusions exhibit a much stronger direct binding to the locus of this miRNA cluster than does wild-type MLL; these changes are associated with elevated levels of histone H3 acetylation and H3K4 trimethylation and an up-regulation of these miRNAs. We further observe that forced expression of this miRNA cluster increases proliferation and inhibits apoptosis of human cells. More importantly, we show that this miRNA cluster can significantly increase colony-forming capacity of normal mouse bone marrow progenitor cells alone and, particularly, in cooperation with MLL fusions. Finally, through combinatorial analysis of miRNA and mRNA arrays of mouse bone marrow progenitor cells transfected with this miRNA cluster and/or MLL fusion gene, we identified 363 potential miR-17-92 target genes that exhibited a significant inverse correlation of expression with the miRNAs. Remarkably, these potential target genes are significantly enriched (P < 0.01; >2-fold) in cell differentiation, hematopoiesis, cell cycle, and apoptosis. Taken together, our studies suggest that overexpression of miR-17-92 cluster in MLL-rearranged leukemias is likely attributed to both DNA copy number amplification and direct up-regulation by MLL fusions, and that the miRNAs in this cluster may play an essential role in the development of MLL-associated leukemias through inhibiting cell differentiation and apoptosis, while promoting cell proliferation, by regulating relevant target genes.cell apoptosis and viability | colony-forming/replating assay | MLL binding | gene regulation | miRNA target M icroRNAs (miRNAs, miRs) are endogenous ≈22 nucleotides (nt) noncoding RNAs that play important regulatory roles in animals and plants by binding with the 3′ UTRs of messenger RNAs (mRNAs) of target genes, leading to mRNA cleavage/degradation or translational repression (1-4). Rapidly accumulating evidence has revealed that miRNAs are strongly associated with cancer (3, 5-7). Recent studies suggest that a cluster of miRNAs, the miR-17-92 polycistron located at 13q31 [containing seven individual miRNAs including miR-17-5p (now named miR-17), miR-17-3p (now named miR-17*), miR-18a, miR-19a, miR-20a, miR-19b-1, and miR-92a-1], may function as an oncogene (8-12). Particularly, He et al. (8) showed that enforced expression of the miR-17-92 cluster cooperated with Myc expression to accelerate tumor development in a mouse model of human B-cell lymphoma. Its oncogenic function is further supported by the finding that miRNAs from this cluster are overexpressed in lung, breast, colon, pancreas, prostate tumors, and chronic leukemias, and that overexpression of these miRNAs is usually correlated with the amplification of its genomic lo...

MicroRNAs in acute leukemia: from biological players to clinical contributors

“…34 These potential target genes were all downregulated in MLL-rearranged ALL. 32,34,61 Two genes, that is, APP and RASSF2 were confirmed targets for miR-17-92 as shown by a luciferase 3 0 UTR-binding assay 34 and the expression of one gene (that is, TNFRSF21) inversely correlated with the expression of the miR-17-92 cluster. 32 The miR-17-92 cluster also has a oncogenic role in T-ALL.…”

“…Induced expression of the miRNA cluster enhanced the colony-forming capacity of mouse bone marrow progenitors especially when MLL fusion genes were co-expressed. 32,61 Increased expression of the miR-17-92 cluster was associated with a downregulation of p21 inhibitor of cell cycle progression. Complete knockdown of p21 mimicked the oncogenic effect of miR-17-92 in MLL-rearranged leukemia cells.…”

“…High expression of another set of miRNAs including miR-382 was unique for t(15;17)/PML-RARa-positive AML. 18,20 MLL-rearranged AML cases, the oncogenic miR-17-92 cluster was upregulated 21,34,32 whereas the tumor-suppressor miR-29 was downregulated (Table 2). 19,23 MiRNA expression levels are also associated with molecular subtypes of AML, the mutational status and associated gene expression pattern in AML (Table 2).…”

“…20,21,[32][33][34]61 Together with the fact that this polycistronic cluster cooperated with c-Myc to accelerate B-cell lymphoma formation in vivo 62 suggests that miR-17-92 acts as an oncogene in lymphoid tissue. Induced expression of the miRNA cluster enhanced the colony-forming capacity of mouse bone marrow progenitors especially when MLL fusion genes were co-expressed.…”

“…For example, amplification of the miR-17-92 gene cluster contributed to the high level of expression of this cluster in MLL-rearranged patients. 32 In addition, MLL-fusion proteins 32,61 and oncogene c-myc may be responsible for miR-17-92 cluster overexpression. 61 In T-ALL, the newly identified translocation t(13;14)(q32;q11) targeted the miR-17-92 gene cluster and possibly thereby facilitates its expression.…”