Altered expression of miR-24, miR-126 and miR-365 does not affect viability of childhood TCF3-rearranged leukemia cells.

Among the microRNAs (miRNAs) that control different cellular processes, miR-24, miR-126 and miR-365 were shown to regulate cell cycle progression and apoptosis in various types of tumors. Interestingly, these three miRNAs were downregulated in pediatric TCF3-rearranged B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Here, we showed that individual or combined overexpression of miR-24, miR-126 and miR-365 can neither alter the cell cycle progression nor the amount of apoptosis in 697, KASUMI-2 or MHH-CALL-3 TCF3-rearranged leukemic cells. We further integrated the miRNA-mRNA expression data of 37 children with BCP-ALL to identify candidate target genes for these three miRNAs. However, the expression levels of selected candidate target genes (ELL, EBF3 and IRF4 for miR-24, PITPNC1 for miR-126 and ZAP-70 for miR-365) did not reduce upon miRNAs overexpression in MHH-CALL-3 TCF3-rearranged leukemic cells. Although the expression level of AURKB-a validated target for miR-24-was reduced upon miR-24 overexpression in hepatocarcinoma HEP-G2 cells, overexpression of miR-24 cannot alter AURKB expression levels in MHH-CALL-3 TCF3-rearranged leukemic cells. Taken together, our data suggest that miRNAs' function is highly tissue-dependent and that a defined biological target gene or function of one miRNA in a specific tissue cannot be extended as a generalized target/function for that miRNA in all types of cells/tissues.

MiR-125b, miR-100 and miR-99a co-regulate vincristine resistance in childhood acute lymphoblastic leukemia.

MicroRNA-125b (miR-125b), miR-99a and miR-100 are overexpressed in vincristine-resistant acute lymphoblastic leukemia (ALL). Cellular viability of ETV6-RUNX1-positive Reh cells significantly increased in presence of 9 ng/mL vincristine upon co-expression of miR-125b/miR-99a (91 ± 4%), miR-125b/miR-100 (93 ± 5%) or miR-125b/miR-99a/miR-100 (82 ± 17%) compared with miR-125b-transduced cells (38 ± 13%, P<0.05). Co-expression of these miRNAs resulted in downregulation of DNTT, NUCKS1, MALAT1, SNRPE, PNO1, SET, KIF5B, PRPS2, RPS11, RPL38 and RPL23A (fold-change 1.3-1.9, p<0.05). Similarly, 7 out of these genes are lower expressed in vincristine-resistant ALL cells of children (p<0.05). The concerted function of miR-125b in combination with miR-99a and/or miR-100 illustrates the complexity of vincristine-resistant pediatric ALL.

The hunting of targets: challenge in miRNA research.

MicroRNAs (miRNAs) are small noncoding RNAs that control the expression of around 60% of the human protein-coding genes. In the past decade, deregulation of miRNAs (by expression and/or function) has been associated with the pathogenesis, progression and prognosis of different diseases, including leukemia. The number of discovered genes encoding miRNAs has risen exponentially in this period, but the numbers of miRNA-target genes discovered and validated lag far behind. Scientists have gained more in-depth knowledge of the basic mechanism of action of miRNAs, but the main challenge still remaining is the identification of direct targets of these important 'micro-players', to understand how they fine-tune so many biological processes in both healthy and diseased tissue. Many technologies have been developed in the past few years, some with more potential than others, but all with their own pros and cons. Here, we review the most common and most potent computational and experimental approaches for miRNA-target gene discovery and discuss how the hunting of targets is challenging but possible by taking the experimental limitations in consideration and choosing the correct cellular context for identifying relevant target genes.

Discovery of new microRNAs by small RNAome deep sequencing in childhood acute lymphoblastic leukemia.

MicroRNAs (miRNAs) relevant to acute lymphoblastic leukemia (ALL) in children are hypothesized to be largely unknown as most miRNAs have been identified in non-leukemic tissues. In order to discover these miRNAs, we applied high-throughput sequencing to pooled fractions of leukemic cells obtained from 89 pediatric cases covering seven well-defined genetic types of ALL and normal hematopoietic cells. This resulted into 78 million small RNA reads representing 554 known, 28 novel and 431 candidate novel miR genes. In all, 153 known, 16 novel and 170 candidate novel mature miRNAs and miRNA-star strands were only expressed in ALL, whereas 140 known, 2 novel and 82 candidate novel mature miRNAs and miRNA-star strands were unique to normal hematopoietic cells. Stem-loop reverse transcriptase (RT)-quantitative PCR analyses confirmed the differential expression of selected mature miRNAs in ALL types and normal cells. Expression of 14 new miRNAs inversely correlated with expression of predicted target genes (-0.49 ≤ Spearman's correlation coefficients (Rs)≤ -0.27, P ≤ 0.05); among others, low levels of novel sol-miR-23 associated with high levels of its predicted (antiapoptotic) target BCL2 (B-cell lymphoma 2) in precursor B-ALL (Rs -0.36, P = 0.007). The identification of >1000 miR genes expressed in different types of ALL forms a comprehensive repository for further functional studies that address the role of miRNAs in the biology of ALL.