MicroRNAs (miRNAs) play a pivotal role in the regulation of hematopoiesis and development of leukemia. Great interest emerged in modulating miRNA expression for therapeutic purposes. In order to identify miRNAs, which specifically suppress leukemic growth of acute myeloid leukemia (AML) with t(8;21), inv(16) or mixed lineage leukemia (MLL) rearrangement by inducing differentiation, we conducted a miRNA expression profiling in a cohort of 90 cytogenetically characterized, de novo pediatric AML cases. Four miRNAs, specifically downregulated in MLL-rearranged, t(8;21) or inv(16) AMLs, were characterized by their tumor-suppressive properties in cell lines representing those respective cytogenetic groups. Among those, forced expression of miR-9 reduced leukemic growth and induced monocytic differentiation of t(8;21) AML cell lines in vitro and in vivo. The tumor-suppressive functions of miR-9 were specifically restricted to AML cell lines and primary leukemic blasts with t(8;21). On the other hand, these functions were not evident in AML blasts from patients with MLL rearrangements. We showed that miR-9 exerts its effects through the cooperation with let-7 to repress the oncogenic LIN28B/HMGA2 axis. Thus, miR-9 is a tumor suppressor-miR which acts in a stringent cell context-dependent manner.

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We show aberrant expression of specific miRNAs in clinically relevant cytogenetic and molecular subgroups of pediatric acute myeloid leukemia, suggesting a role for these miRNAs in the underlying biology in these specific subgroups.

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Pediatric mixed-lineage leukemia (MLL)-rearranged acute monoblastic leukemia with t(9;11)(p22;q23) has a favorable outcome compared with other MLL-rearranged AML. The biologic background for this difference remains unknown. Therefore, we compared gene expression profiles (GEPs; Affymetrix HGU133 ؉ 2.0) of 26 t(9; 11)(p22;q23) patients with 42 other MLLrearranged AML patients to identify differentially expressed genes. IGSF4, a cellcell adhesion molecule, was found to be highly expressed in t(9;11)(p22;q23) patients, which was confirmed by real-time quantitative polymerase chain reaction and Western blot. IGSF4 expression within t(9;11)(p22;q23) patients was 4.9 times greater in French-American-British morphology classification (FAB)-M5 versus other FAB-types (P ‫؍‬ .001). Methylation status investigation showed that high IGSF4-expressing t(9;11)(p22;q23) patients with FAB-M5 have no promoter hypermethylation, whereas all other cases do. Cell-line incubation with demethylating agent decitabine resulted in promoter demethylation and increased expression of IGSF4. Down-regulation of IGSF4 by siRNA did not affect proliferation or drug sensitivity. In a cohort of 79 MLL-rearranged AML cases, we show significant better overall survival for cases with high IGSF4 expression (5-year overall survival 0.70 vs 0.37, P ‫؍‬ .03) In conclusion, we identified IGSF4 overexpression to be discriminative for t(9;11)(p22;q23) patients with FAB-M5, regulated partially by promoter methylation and resulting in survival benefit. (Blood. 2011;117(3):928-935)

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In acute myeloid leukemia (AML), specific genomic aberrations induce aberrant methylation, thus directly influencing the transcriptional programing of leukemic cells. Therefore, therapies targeting epigenetic processes are advocated as a promising therapeutic tool for AML treatment. However, to develop new therapies, a comprehensive understanding of the mechanism(s) driving the epigenetic changes as a result of acquired genetic abnormalities is necessary. This understanding is still lacking. In this study, we performed genome-wide CpG-island methylation profiling on pediatric AML samples. Six differentially methylated genomic regions within two genes, discriminating inv(16)(p13;q22) from non-inv(16) pediatric AML samples, were identified. All six regions had a hypomethylated phenotype in inv(16) AML samples, and this was most prominent at the regions encompassing the meningioma (disrupted in balanced translocation) 1 (MN1) oncogene. MN1 expression primarily correlated with the methylation level of the 3' end of the MN1 exon-1 locus. Decitabine treatment of different cell lines showed that induced loss of methylation at the MN1 locus can result in an increase of MN1 expression, indicating that MN1 expression is coregulated by DNA methylation. To investigate this methylation-associated mechanism, we determined the expression of DNA methyltransferases in inv(16) AML. We found that DNMT3B expression was significantly lower in inv(16) samples. Furthermore, DNMT3B expression correlated negatively with MN1 expression in pediatric AML samples. Importantly, depletion of DNMT3B impaired remethylation efficiency of the MN1 exon-1 locus in AML cells after decitabine exposure. These findings identify DNMT3B as an important coregulator of MN1 methylation. Taken together, this study shows that the methylation level of the MN1 exon-1 locus regulates MN1 expression levels in inv(16) pediatric AML. This methylation level is dependent on DNMT3B, thus suggesting a role for DNMT3B in leukemogenesis in inv(16) AML, through MN1 methylation regulation.

Children with Down Syndrome (DS) have an increased risk of developing leukemia, including both acute myeloid (ML DS), as well as acute lymphoblastic leukemia (DSALL). Recently, Izraeli et al. reported on an activating mutation (R683) localized in exon 16 of the Janus Kinase 2 (JAK2) gene, in 18% of DS-ALL (n=16) patients collected from 9 European study groups (ASH 2007). Screening of other leukemia subsets showed that this mutation was exclusive for DS-ALL patients. This JAK2 mutation differs from the V617F exon 14 mutation found in myeloproliferative diseases. JAK2 is located on chromosome 9p24, and belongs to a family of intracellular non-receptor protein tyrosine kinases that transduce cytokine-mediated signals via the JAK-STAT pathway. It plays an important role in regulating the processes of cell proliferation, differentiation and apoptosis in response to cytokines and growth factors. Between 1991 and 2007, 45 children with DS ALL were treated in the Netherlands, according to the DCOG protocols ALL 7–10. Of 36 children samples were available in the DCOG cell bank, on which we performed JAK2 mutation screening of the pseudokinase and kinase domains of JAK2 by direct sequencing. All 36 patients were classified as BCP-ALL. Mutations in JAK2 exon 16 were identified in 6 (16.6%) DS-ALL patients. In five patients a point mutation resulted in substitution of Arginine at position 683, the same as was described by Izraeli et al. In one patient an insertion was found. JAK2 mutated patients did not differ in age at diagnosis (3.3 vs. 5.1 years, p=0.08) or in sex (p= 0.8) compared to non-mutated DSALL patients. The diagnostic WBC for DS-ALL patients with a JAK2 exon 16 mutation was lower than for non-mutated patients (3.6×109/L vs. 12.1×109/L; p=0.04). Ploidy status based on karyotyping was known in 29/36 patients. None of the JAK2 mutated samples was hyperdiploid (>52 chromosomes) vs. one in the non-mutated samples (p=0.89). TEL/AML rearrangements were screened in 23/36 samples, and 3/23 (13%) samples showed a TEL-AML rearrangement. None of the JAK2 mutated samples was TEL-AML rearranged (p= 0.76). One JAK2 mutated patient had a normal karyotype, the other JAK2 mutated patients had random cytogenetic abnormalities. We next analyzed the prognostic significance of JAK2 mutated DS-ALL children versus the other patients. The median follow up time for all patients was 3.1 years (range 0.1–15.1 years). Interestingly, none of the JAK2 mutated patients relapsed, versus 4/30 wild type JAK2 patients. The differences between pOS (100% vs. 83.3% p=0.41), pEFS (100% vs. 80%, p=0.38) and pDFS (100% vs. 82.8%, p=0.44) were not statistically significant, probably due to small numbers. Since DS-ALL children are more sensitive to the side effects of chemotherapy, and have relatively high toxic mortality rates, reduction of therapy intensity might be an option for DS-ALL children with a JAK2 exon 16 mutation, if our results could be confirmed in larger series. The development of specific JAK2 inhibitors may allow further reduction of chemotherapy.