Long-term lymphoid progenitors independently sustain naïve T and NK cell production in humans.

Our mathematical model of integration site data in clinical gene therapy supported the existence of long-term lymphoid progenitors capable of surviving independently from hematopoietic stem cells. To date, no experimental setting has been available to validate this prediction. We here report evidence of a population of lymphoid progenitors capable of independently maintaining T and NK cell production for 15 years in humans. The gene therapy patients of this study lack vector-positive myeloid/B cells indicating absence of engineered stem cells but retain gene marking in both T and NK. Decades after treatment, we can still detect and analyse transduced naïve T cells whose production is likely maintained by a population of long-term lymphoid progenitors. By tracking insertional clonal markers overtime, we suggest that these progenitors can support both T and NK cell production. Identification of these long-term lymphoid progenitors could be utilised for the development of next generation gene- and cancer-immunotherapies.

Liver tumorigenesis is promoted by a high saturated fat diet specifically in male mice and is associated with hepatic expression of the proto-oncogene Agap2 and enrichment of the intestinal microbiome with Coprococcus.

Liver cancer results in a high degree of mortality, especially among men. As fatty liver disease is a risk factor for development of hepatocellular carcinoma, we investigated the role of dietary fat type in tumor promotion by high-fat diets in mice after initiation with the chemical carcinogen diethyl nitrosamine. Tumor incidence and multiplicity were significantly greater in males than those in females. In males, fat type had complex effects on tumorigenesis. Preneoplastic foci were most prevalent in mice fed a polyunsaturated fat diet enriched in docosahexaenoic acid, whereas carcinomas and large visible liver tumors were significantly greater in mice fed a saturated fat diet made with cocoa butter relative to mice fed mono- or polyunsaturated fats. Different mechanisms thus seemed involved in early and late tumor promotion. The hepatic transcriptome and gut microbiome were assessed for traits associated with tumorigenesis. Hepatic expression of more than 20% of all genes was affected by sex, whereas fat type affected fewer genes. In males, the saturated fat diet induced expression of the proto-oncogene Agap2 and affected the expression of several cytochrome P450 genes, and genes involved in lipid, bile acid and fatty acid metabolism. The gut microbiome had a higher level of genus Akkermansia and a lower level of Firmicutes in females than in males. Males fed saturated fat had an altered microbiome, including an enrichment of the genus Coprococcus. In conclusion, sex and the dietary fat type affect the gut microbiome, the hepatic transcriptome and ultimately hepatic tumor growth.

The proto-oncogene function of Mdm2 in bone.

Mouse double minute 2 (Mdm2) is a multifaceted oncoprotein that is highly regulated with distinct domains capable of cellular transformation. Loss of Mdm2 is embryonically lethal, making it difficult to study in a mouse model without additional genetic alterations. Global overexpression through increased Mdm2 gene copy number (Mdm2Tg ) results in the development of hematopoietic neoplasms and sarcomas in adult animals. In these mice, we found an increase in osteoblastogenesis, differentiation, and a high bone mass phenotype. Since it was difficult to discern the cell lineage that generated this phenotype, we generated osteoblast-specific Mdm2 overexpressing (Mdm2TgOb ) mice in 2 different strains, C57BL/6 and DBA. These mice did not develop malignancies; however, these animals and the MG63 human osteosarcoma cell line with high levels of Mdm2 showed an increase in bone mineralization. Importantly, overexpression of Mdm2 corrected age-related bone loss in mice, providing a role for the proto-oncogenic activity of Mdm2 in bone health of adult animals.

PIM1 induces cellular senescence through phosphorylation of UHRF1 at Ser311.

PIM1 is a proto-oncogene, encoding a serine/threonine protein kinase that regulates cell proliferation, survival, differentiation and apoptosis. Previous reports suggest that overexpression of PIM1 can induce cellular senescence. However, the molecular mechanism underlying this process is not fully understood. Here we report that UHRF1 is a novel substrate of PIM1 kinase, which could be phosphorylated at Ser311 and therefore promoted to degradation. Our data demonstrates that PIM1 destabilizes UHRF1, leading to DNA hypomethylation, which consequently results in genomic instability, increased p16 expression and subsequent induction of cellular senescence. Taken together, our results suggest that down-regulation of UHRF1 is an important mechanism of PIM1-mediated cellular senescence.

Evi1 regulates Notch activation to induce zebrafish hematopoietic stem cell emergence.

During development, hematopoietic stem cells (HSCs) emerge from aortic endothelial cells (ECs) through an intermediate stage called hemogenic endothelium by a process known as endothelial-to-hematopoietic transition (EHT). While Notch signaling, including its upstream regulator Vegf, is known to regulate this process, the precise molecular control and temporal specificity of Notch activity remain unclear. Here, we identify the zebrafish transcriptional regulator evi1 as critically required for Notch-mediated EHT In vivo live imaging studies indicate that evi1 suppression impairs EC progression to hematopoietic fate and therefore HSC emergence. evi1 is expressed in ECs and induces these effects cell autonomously by activating Notch via pAKT Global or endothelial-specific induction of notch, vegf, or pAKT can restore endothelial Notch and HSC formations in evi1 morphants. Significantly, evi1 overexpression induces Notch independently of Vegf and rescues HSC numbers in embryos treated with a Vegf inhibitor. In sum, our results unravel evi1-pAKT as a novel molecular pathway that, in conjunction with the shh-vegf axis, is essential for activation of Notch signaling in VDA endothelial cells and their subsequent conversion to HSCs.

Involvement of JunB Proto-Oncogene in Tail Formation During Early Xenopus Embryogenesis.

Integration of signaling pathways is important for the establishment of the body plan during embryogenesis. However, little is known about how the multiple signals interact to regulate morphogenesis. Here, we show that junb is expressed in the posterior neural plate and the caudal fin during Xenopus embryogenesis and that overexpression of wild-type JunB induces small head phenotypes and ectopic tail-like structures. A mutant form of JunB that lacked GSK3 and MAPK phosphorylation sites showed stronger tail-like structure-inducing activity than wild-type JunB. Moreover, the mutant JunB induced expression of tailbud and neural marker genes, but not somite and chordoneural hinge (CNH) marker genes in ectopic tail-like structures. In ectodermal explants of Xenopus embryos, overexpression of JunB increased the expression of tailbud and posterior marker genes including fgf3, xbra (t) and wnt8. These results indicate that JunB is capable of inducing the ectopic formation of tissues similar to the tailbud, and that the tailbud-inducing activity of JunB is likely to be regulated by FGF and Wnt pathways. Overall, our results suggest that JunB is a regulator of tail organization possibly through integration of several morphogen signaling pathways.

The cnidarian origin of the proto-oncogenes NF-κB/STAT and WNT-like oncogenic pathway drives the ctenophores (Review).

The cell survival pathways of the diploblastic early multicellular eukaryotic hosts contain and operate the molecular machinery resembling those of malignantly transformed individual cells of highly advanced multicellular hosts (including Homo). In the present review, the STAT/NF-κB pathway of the cnidarian Nematostella vectensis is compared with that of human tumors (malignant lymphomas, including Reed-Sternberg cells) pointing out similarities, including possible viral initiation in both cases. In the ctenophore genome and proteome, ß-catenin gains intranuclear advantages due to a physiologically weak destructive complex in the cytoplasm, and lack of natural inhibitors (the dickkopfs). Thus, a scenario similar to what tumor cells initiate and achieve is presented through several constitutive loss-of-function type mutations in the destructive complex and in the elimination of inhibitors. Vice versa, malignantly transformed individual cells of advanced multicellular hosts assume pheno-genotypic resemblance to cells of unicellular or early multicellular hosts, and presumably to their ancient predecessors, by returning to the semblance of immortality and to the resumption of the state of high degree of resistance to physicochemical insults. Human leukemogenic and oncogenic pathways are presented for comparisons. The supreme bioengineers RNA/DNA complex encoded both the malignantly transformed immortal cell and the human cerebral cortex. The former generates molecules for the immortality of cellular life in the Universe. The latter invents the inhibitors of the process in order to gain control over it.

Reg3g Promotes Pancreatic Carcinogenesis in a Murine Model of Chronic Pancreatitis.

BACKGROUND: Regenerating islet-derived 3 (Reg3) is abnormally expressed in several human digestive system diseases, including chronic pancreatitis (CP) and pancreatic cancer (PC). AIM: The purpose of this study was to clarify the mechanism of the enhanced expression of Reg3 in inflammation-induced PC. METHODS: C57BL/6 mice were treated with caerulein for 6 weeks to induce CP and then injected with pReg3g--a lentivirus system encoding for murine Reg3g--accompanied by dimethylbenzanthracene to induce PC. We detected pancreatic histopathological characteristics, tumor-related gene expression, inflammation-associated pathway activation, serum biochemical indicators, and immunological cell activities. RESULTS: The mice that developed CP after caerulein treatment were marked by pronounced histologic lesions, elevated serum amylase levels, and activation of inflammation-related pathways. Mice given a high dose of pReg3g developed PC by 16 weeks, with recognizable tumors in the pancreas. While, both the low and high doses of pReg3g produced higher transcription of c-fos, k-ras, cytokeratin-19, and proliferating cell nuclear antigen, and a lower expression of caspase-3 compared to pNEG controls. Additionally, the higher dose of pReg3g increased the expressions of pSTAT3, NFκB (p65), and SOCS3 methylation during PC development. In addition, mice treated with pReg3g displayed higher levels of serum IL10 and TGFß and suppressed T lymphocyte proliferation and DC function. CONCLUSION: The comprehensive analysis suggests enhanced Reg3g expression exacerbates PC in inflammation-associated cancer progression. Reg3g appears to promote CP-related PC in mice through multiple mechanisms, involving enhanced transcription of pancreatic tumor markers, repression of anti-tumor immunity, and activation of STAT3/p65 signal transduction pathways.

Analysis of EGFR and HER-2 expressions in ductal carcinomas in situ in canine mammary glands / Análise da expressão de EGFR e HER-2 em carcinomas ductais in situ da glândula mamária canina

Biomolecular evidence has shown that ductal carcinoma in situ (DCIS) may develop into invasive carcinoma of the canine mammary gland, and mutations in proto-oncogenes HER2 and EGFR; two members of the family of epidermal growth factor receptors, may be involved in this process. The purpose of this study was the characterization of the immunohistochemical expression of the EGFR and HER2 proteins in the process of neoplastic transformation, supposedly present in ductal carcinomas in situ in canine mammary glands. Fifteen cases of DCIS were evaluated, with a higher expression of HER2 and EGFR being observed in low-grade carcinomas when compared with high-grade neoplasms, and with a high positive statistical correlation in the latter. Results suggest that aggressive tumors tend to lose the expression of EGFR and HER2 simultaneously. The loss of the expression of these markers may be related to the process of neoplastic progression in canine mammary tumors...

Evidências biomoleculares sugerem que o carcinoma ductal in situ (CDIS) pode progredir para carcinoma invasor na mama canina e que mutações nos proto-oncogenes HER-2 e EGFR, dois membros da família de receptores para fatores de crescimento epidérmicos, podem estar envolvidas neste processo. A partir disso, este trabalho teve por objetivo caracterizar a expressão imuno-histoquímica das proteínas EGFR e HER-2 no processo de transformação neoplásica supostamente presente em carcinomas ductais in situ da glândula mamária canina. Foram avaliados 15 casos de CDIS, sendo observada maior expressão de HER-2 e EGFR em carcinomas de baixo grau em comparação às neoplasias de alto grau, com correlação estatística positiva alta nestes últimos. Os resultados sugerem que tumores mais agressivos tendem a perder, simultaneamente, a expressão de EGFR e HER-2. A perda na expressão desses marcadores pode estar envolvida no processo de progressão neoplásica em tumores mamários caninos...

Hypomethylation of long interspersed nuclear element-1 (LINE-1) leads to activation of proto-oncogenes in human colorectal cancer metastasis.

OBJECTIVE: Hypomethylation of LINE-1 elements has emerged as a distinguishing feature in human cancers. Limited evidence indicates that some LINE-1 elements encode an additional internal antisense promoter, and increased hypomethylation of this region may lead to inadvertent activation of evolutionarily methylation-silenced downstream genes. However, the significance of this fundamental epigenetic mechanism in colorectal cancer (CRC) has not been investigated previously. DESIGN: We analysed tissue specimens from 77 CRC patients with matched sets of normal colonic mucosa, primary CRC tissues (PC), and liver metastasis tissues (LM). LINE-1 methylation levels were determined by quantitative bisulfite pyrosequencing. MET, RAB3IP and CHRM3 protein expression was determined by western blotting and IHC. MET proto-oncogene transcription and 5-hydroxymethylcytosine (5-hmc) were evaluated by quantitative real-time-PCR. RESULTS: Global LINE-1 methylation levels in LM were significantly lower compared with the matched PC (PC=66.2% vs LM=63.8%; p<0.001). More importantly, we observed that specific LINE-1 sequences residing within the intronic regions of multiple proto-oncogenes, MET (p<0.001), RAB3IP (p=0.05) and CHRM3 (p=0.01), were significantly hypomethylated in LM tissues compared with corresponding matched PC. Furthermore, reduced methylation of specific LINE-1 elements within the MET gene inversely correlated with induction of MET expression in CRC metastases (R=-0.44; p<0.0001). Finally, increased 5-hmc content was associated with LINE-1 hypomethylation. CONCLUSIONS: Our results provide novel evidence that hypomethylation of specific LINE-1 elements permits inadvertent activation of methylation-silenced MET, RAB3IP and CHRM3 proto-oncogenes in CRC metastasis. Moreover, since 5-hmc content inversely correlated with LINE-1 hypomethylation in neoplastic tissues, our results provide important mechanistic insights into the fundamental processes underlying global DNA hypomethylation in human CRC.

ELL inhibits E2F1 transcriptional activity by enhancing E2F1 deacetylation via recruitment of histone deacetylase 1.

ELL (eleven-nineteen lysine-rich leukemia protein) was first identified as a translocation partner of MLL in acute myeloid leukemia; however, the exact mechanism of its action has remained elusive. In this study, we identified ELL as a direct downstream target gene of E2F1. Coimmunoprecipitation assays showed that ELL interacted with E2F1 in vitro and in vivo, leading to inhibition of E2F1 transcriptional activity. In addition, ELL enhanced E2F1 deacetylation via recruitment of histone deacetylase 1 (HDAC1). Notably, the MLL-ELL fusion protein lost the inhibitory role of ELL in E2F1 transcriptional activity. Furthermore, DNA damage induced ELL in an E2F1-dependent manner and ELL protected cells against E2F1-dependent apoptosis. Our findings not only connect ELL to E2F1 function and uncover a novel role of ELL in response to DNA damage but also provide an insight into the mechanism for MLL-ELL-associated leukemogenesis.

miR-9 is an essential oncogenic microRNA specifically overexpressed in mixed lineage leukemia-rearranged leukemia.

MicroRNAs (miRNAs), small noncoding RNAs that regulate target gene mRNAs, are known to contribute to pathogenesis of cancers. Acute myeloid leukemia (AML) is a group of heterogeneous hematopoietic malignancies with various chromosomal and/or molecular abnormalities. AML with chromosomal translocations involving the mixed lineage leukemia (MLL) gene are usually associated with poor survival. In the present study, through a large-scale, genomewide miRNA expression assay, we show that microRNA-9 (miR-9) is the most specifically up-regulated miRNA in MLL-rearranged AML compared with both normal control and non-MLL-rearranged AML. We demonstrate that miR-9 is a direct target of MLL fusion proteins and can be significantly up-regulated in expression by the latter in human and mouse hematopoietic stem/progenitor cells. Depletion of endogenous miR-9 expression by an appropriate antagomiR can significantly inhibit cell growth/viability and promote apoptosis in human MLL-rearranged AML cells, and the opposite is true when expression of miR-9 is forced. Blocking endogenous miR-9 function by anti-miRNA sponge can significantly inhibit, whereas forced expression of miR-9 can significantly promote, MLL fusion-induced immortalization/transformation of normal mouse bone marrow progenitor cells in vitro. Furthermore, forced expression of miR-9 can significantly promote MLL fusion-mediated leukemogenesis in vivo. In addition, a group of putative target genes of miR-9 exhibited a significant inverse correlation of expression with miR-9 in a series of leukemia sample sets, suggesting that they are potential targets of miR-9 in MLL-rearranged AML. Collectively, our data demonstrate that miR-9 is a critical oncomiR in MLL-rearranged AML and can serve as a potential therapeutic target to treat this dismal disease.

The shortest isoform of C/EBPß, liver inhibitory protein (LIP), collaborates with Evi1 to induce AML in a mouse BMT model.

Ecotropic viral integration site 1 (Evi1) is one of the master regulators in the development of acute myeloid leukemia (AML) and myelodysplastic syndrome. High expression of Evi1 is found in 10% of patients with AML and indicates a poor outcome. Several recent studies have indicated that Evi1 requires collaborative factors to induce AML. Therefore, the search for candidate factors that collaborate with Evi1 in leukemogenesis is one of the key issues in uncovering the mechanism of Evi1-related leukemia. Previously, we succeeded in making a mouse model of Evi1-related leukemia using a bone marrow transplantation (BMT) system. In the Evi1-induced leukemic cells, we identified frequent retroviral integrations near the CCAAT/enhancer-binding protein ß (C/EBPß) gene and overexpression of its protein. These findings imply that C/EBPß is a candidate gene that collaborates with Evi1 in leukemogenesis. Cotransduction of Evi1 and the shortest isoform of C/EBPß, liver inhibitory protein (LIP), induced AML with short latencies in a mouse BMT model. Overexpression of LIP alone also induced AML with longer latencies. However, excision of all 3 isoforms of C/EBPß (LAP*/LAP/LIP) did not inhibit the development of Evi1-induced leukemia. Therefore, isoform-specific intervention that targets LIP is required when we consider C/EBPß as a therapeutic target.

PNUTS functions as a proto-oncogene by sequestering PTEN.

PTEN is a well-defined tumor suppressor gene that antagonizes the PI3K/Akt pathway to regulate a multitude of cellular processes, such as survival, growth, motility, invasiveness, and angiogenesis. While the functions of PTEN have been studied extensively, the regulation of its activity during normal and disease conditions still remains incompletely understood. In this study, we identified the protein phosphatase-1 nuclear targeting subunit PNUTS (PPP1R10) as a PTEN-associated protein. PNUTS directly interacted with the lipid-binding domain (C2 domain) of PTEN and sequestered it in the nucleus. Depletion of PNUTS leads to increased apoptosis and reduced cellular proliferation in a PTEN-dependent manner. PNUTS expression was elevated in certain cancers compared with matched normal tissues. Collectively, our studies reveal PNUTS as a novel PTEN regulator and a likely oncogene.

Activation of Evi1 inhibits cell cycle progression and differentiation of hematopoietic progenitor cells.

The transcription factor Evi1 has an outstanding role in the formation and transformation of hematopoietic cells. Its activation by chromosomal rearrangement induces a myelodysplastic syndrome with progression to acute myeloid leukemia of poor prognosis. Similarly, retroviral insertion-mediated upregulation confers a competitive advantage to transplanted hematopoietic cells, triggering clonal dominance or even leukemia. To study the molecular and functional response of primary murine hematopoietic progenitor cells to the activation of Evi1, we established an inducible lentiviral expression system. EVI1 had a biphasic effect with initial growth inhibition and retarded myeloid differentiation linked to enhanced survival of myeloblasts in long-term cultures. Gene expression microarray analysis revealed that within 24 h EVI1 upregulated 'stemness' genes characteristic for long-term hematopoietic stem cells (Aldh1a1, Abca1, Cdkn1b, Cdkn1c, Epcam, among others) but downregulated genes involved in DNA replication (Cyclins and their kinases, among others) and DNA repair (including Brca1, Brca2, Rad51). Cell cycle analysis demonstrated EVI1's anti-proliferative effect to be strictly dose-dependent with accumulation of cells in G0/G1, but preservation of a small fraction of long-term proliferating cells. Although confined to cultured cells, our study contributes to new hypotheses addressing the mechanisms and molecular targets involved in preleukemic clonal dominance or leukemic transformation by Evi1.

[Nature of cancer explored from the perspective of the functional evolution of proto-oncogenes].

The products of proto-oncogene play critical roles in the development or maintenance of multicellular societies in animals via strict regulatory systems. When these regulatory systems are disrupted, proto-oncogenes can become oncogenes, and thereby induce cell transformation and carcinogenesis. To understand the molecular basis for development of the regulatory system of proto-oncogenes during evolution, we screened for ancestral proto-oncogenes from the unicellular choanoflagellate Monosiga ovata (M. ovata) by monitoring their transforming ability in mammalian cells; consequently, we isolated a Pak gene ortholog, which encodes a serine/threonine kinase as a 'primitive oncogene'. We also cloned Pak orthologs from fungi and the multicellular sponge Ephydatia fluviatilis, and compared their regulatory features with that of M. ovata Pak (MoPak). MoPak is constitutively active and induces cell transformation in mammalian cells. In contrast, Pak orthologs from multicellular animals are strictly regulated. Analyses of Pak mutants revealed that structural alterations in the auto-inhibitory domain (AID) are responsible for the enhanced kinase activity and the oncogenic activity of MoPak. Furthermore, we show that Rho family GTPases-mediated regulatory system of Pak kinase is conserved throughout the evolution from unicellular to multicellular animals, but the MoPak is more sensitive to the Rho family GTPases-mediated activation than multicellular Pak. These results show that maturation of AID function was required for the development of the strict regulatory system of the Pak proto-oncogene, and support the potential link between the development of the regulatory system of proto-oncogenes and the evolution of multicellularity. Further analysis of oncogenic functions of proto-oncogene orthologs in the unicellular genes would provide some insights into the mechanisms of the destruction of multicellular society in cancer.

[Physiopathological, diagnostic and therapeutic impacts of chromosomal translocations in hematological malignancies]. / Translocations récurrentes en onco-hématologie : physiopathologie, intérêt clinique et thérapeutique.

Oncogenesis is correlated with the occurrence of multiple genomic events allowing cancer cells to acquire new properties, including the capacity of survival and proliferation with down regulated control signals. Among those genomic events, the study of recurrent translocations, particularly common in oncohematology, has allowed for a better understanding of leucemogenesis and lymphomagenesis mechanisms. These translocations are classically distinguished depending on their physiopathologic consequences. It may encode for a fusion gene leading to a chimeric protein, which exhibits a new activity or an aberrant one, corresponding in most cases to the constitutive activation of a proto-oncogene. In other cases, these translocations may cause abnormal expression of a proto-oncogene with a regular structure by a transcriptional deregulation. Beyond this highlighting recurrent translocations and understanding better the physiopathologic consequences of these chromosomal modifications has a real impact on patients. These cytogenetic anomalies represent an essential diagnostic tool for some hematologies; and pave the way for a better evaluation of the prognosis and thus, a better adaptation of the therapeutic strategy. They also contributed to improve survival with the development of targeted therapies. Finally, thanks to cytogenetic techniques combined to molecular biology techniques, cytogenetic aberrations can be used as a marker of response, which allowed a monitoring of residual disease.

EVI1-mediated down regulation of MIR449A is essential for the survival of EVI1 positive leukaemic cells.

Chromosomal rearrangements involving the MECOM (MDS1 and EVI1 complex) locus are recurrent genetic events in myeloid leukaemia and are associated with poor prognosis. In this study, we assessed the role of MECOM locus protein EVI1 in the transcriptional regulation of microRNAs (miRNAs) involved in the leukaemic phenotype. For this, we profiled expression of 366 miRNAs in 38 MECOM-rearranged patient samples, normal bone marrow controls and MECOM (EVI1) knock down/re-expression models. Cross-comparison of these miRNA expression profiling data showed that MECOM rearranged leukaemias are characterized by down regulation of MIR449A. Reconstitution of MIR449A expression in MECOM-rearranged cell line models induced apoptosis resulting in a strong decrease in cell viability. These effects might be mediated in part by MIR449A regulation of NOTCH1 and BCL2, which are shown here to be bona fide MIR449A targets. Finally, we confirmed that MIR449A repression is mediated through direct promoter occupation of the EVI1 transcriptional repressor. In conclusion, this study reveals MIR449A as a crucial direct target of the MECOM locus protein EVI1 involved in the pathogenesis of MECOM-rearranged leukaemias and unravels NOTCH1 and BCL2 as important novel targets of MIR449A. This EVI1-MIR449A-NOTCH1/BCL2 regulatory axis might open new possibilities for the development of therapeutic strategies in this poor prognostic leukaemia subgroup.

The heterogeneity of pediatric MLL-rearranged acute myeloid leukemia.

Translocations involving the mixed-lineage leukemia (MLL) gene, localized at 11q23, comprise 15 to 20% of all pediatric acute myeloid leukemia (AML) cases. This review summarizes current knowledge about the etiology, biology, clinical characteristics and differences in outcome in MLL-rearranged pediatric AML. Furthermore, we discuss the role of cooperating events in MLL-rearranged pediatric AML, and future therapeutic strategies to improve outcome. We conclude that MLL-rearranged pediatric AML is a heterogeneous disease, and prognosis depends on various factors, for example, translocation partner, age, WBC and additional cytogenetic aberrations. The relationship of outcome with specific translocation partners requires that they be searched for in the diagnostic work-up of AML. To achieve further improvements in outcome, unraveling the biology of MLL-rearranged pediatric AML is warranted.