Expression of constitutively active Notch4 (Int-3) modulates myeloid proliferation and differentiation and promotes expansion of hematopoietic progenitors

Inhibition of NOTCH4 sensitizes FLT3/ITD acute myeloid leukemia cells to FLT3 tyrosine kinase inhibition

Internal tandem duplication mutations of FLT3 (FLT3/ITD) confer poor prognosis in AML. FLT3 tyrosine kinase inhibitors (TKIs) alone have limited and transient clinical efficacy thus calling for new targets for more effective combination therapy. In a loss-of-function RNAi screen, we identified NOTCH4 as one such potential target whose inhibition proved cytotoxic to AML cells, and also sensitized them to FLT3 inhibition. Further investigation found increased NOTCH4 expression in FLT3/ITD AML cell lines and primary patient samples. Inhibition of NOTCH4 by shRNA knockdown, CRISPR-Cas9-based knockout or γ-secretase inhibitors synergized with FLT3 TKIs to kill FLT3/ITD AML cells in vitro. NOTCH4 inhibition sensitized TKI-resistant FLT3/ITD cells to FLT3 TKI inhibition. The combination reduced phospho-ERK and phospho-AKT, indicating inhibition of MAPK and PI3K/AKT signaling pathways. It also led to changes in expression of genes involved in regulating cell cycling, DNA repair and transcription. A patient-derived xenograft model showed that the combination reduced both the level of leukemic involvement of primary human FLT3/ITD AML cells and their ability to engraft secondary recipients. In summary, these results demonstrate that NOTCH4 inhibition synergizes with FLT3 TKIs to eliminate FLT3/ITD AML cells, providing a new therapeutic target for AML with FLT3/ITD mutations.

Role of Notch Receptors in Hematologic Malignancies

Notch receptors are single-pass transmembrane proteins that play a critical role in cell fate decisions and have been implicated in the regulation of many developmental processes. The human Notch family comprises of four receptors (Notch 1 to 4) and five ligands. Their signaling can regulate extremely basic cellular processes such as differentiation, proliferation and death. Notch is also involved in hematopoiesis and angiogenesis, and increasing evidence suggests that these genes are involved and frequently deregulated in several human malignancies, contributing to cell autonomous activities that may be either oncogenic or tumor suppressive. It was recently proposed that Notch signaling could play an active role in promoting and sustaining a broad spectrum of lymphoid malignancies as well as mutations in Notch family members that are present in several disorders of T- and B-cells, which could be responsible for altering the related signaling. Therefore, different Notch pathway molecules could be considered as potential therapeutic targets for hematological cancers. In this review, we will summarize and discuss compelling evidence pointing to Notch receptors as pleiotropic regulators of hematologic malignancies biology, first describing the physiological role of their signaling in T- and B-cell development and homeostasis, in order to fully understand the pathological alterations reported.

All-trans retinoic acid enhances, and a pan-RAR antagonist counteracts, the stem cell promoting activity of EVI1 in acute myeloid leukemia

Ecotropic virus integration site 1 (EVI1), whose overexpression characterizes a particularly aggressive subtype of acute myeloid leukemia (AML), enhanced anti-leukemic activities of all-trans retinoic acid (atRA) in cell lines and patient samples. However, the drivers of leukemia formation, therapy resistance, and relapse are leukemic stem cells (LSCs), whose properties were hardly reflected in these experimental setups. The present study was designed to address the effects of, and interactions between, EVI1 and retinoids in AML LSCs. We report that Evi1 reduced the maturation of leukemic cells and promoted the abundance, quiescence, and activity of LSCs in an MLL-AF9-driven mouse model of AML. atRA further augmented these effects in an Evi1 dependent manner. EVI1 also strongly enhanced atRA regulated gene transcription in LSC enriched cells. One of their jointly regulated targets, Notch4, was an important mediator of their effects on leukemic stemness. In vitro exposure of leukemic cells to a pan-RAR antagonist caused effects opposite to those of atRA. In vivo antagonist treatment delayed leukemogenesis and reduced LSC abundance, quiescence, and activity in Evi1^high AML. Key results were confirmed in human myeloid cell lines retaining some stem cell characteristics as well as in primary human AML samples. In summary, our study is the first to report the importance of EVI1 for key properties of AML LSCs. Furthermore, it shows that atRA enhances, and a pan-RAR antagonist counteracts, the effects of EVI1 on AML stemness, thus raising the possibility of using RAR antagonists in the therapy of EVI1^high AML.

Heterozygous loss of Srp72 in mice is not associated with major hematological phenotypes

OBJECTIVES

Familial cases of hematological malignancies are associated with germline mutations. In particular, heterozygous mutations of SRP72 correlate with the development of myelodysplasia and bone marrow aplasia in two families. The signal recognition particle 72 kDa protein (SRP72) is part of the SRP complex, responsible for targeting of proteins to the endoplasmic reticulum. The main objective of this study is to investigate the role of SRP72 in the hematopoietic system, thus explaining why a reduced dose could increase susceptibility to hematological malignancies.

METHODS

We developed an Srp72 null mouse model and characterized its hematopoietic system using flow cytometry, bone marrow transplantations, and gene expression analysis.

RESULTS

Heterozygous loss of Srp72 in mice is not associated with major changes in hematopoiesis, although causes mild reductions in blood and BM cellularity and minor changes within the stem/progenitor compartment. We did not observe any hematological disorder. Interestingly, gene expression analysis demonstrated that genes encoding secreted factors, including cytokines and receptors, were transcriptionally down-regulated in Srp72^+/- animals.

CONCLUSIONS

The Srp72^+/- mouse model only partially recapitulates the phenotype observed in families with inherited SRP72 lesions. Nonetheless, these results can provide mechanistic insights into why SRP72 mutations are associated with aplasia and myelodysplasia in humans.

The novel three-way variant t(6;17;15)(p21;q21;q22) in acute promyelocytic leukemia with an FLT3-ITD mutation: A case report

Acute promyelocytic leukemia (APL) is characterized by the reciprocal translocation t(15;17)(q22;q21), resulting in the fusion of the promyelocytic leukemia gene at 15q22 with the retinoic acid receptor α at 17q21. Additionally, all patients with APL who have additional chromosome abnormalities (ACA) and gene mutations are resistant to all-trans retinoic acid (ATRA), the drug that causes disease regression specifically in patients with APL globally. The present study describes a case of a 19-year-old female with APL carrying a novel complex variant translocation t(6;17;15)(p21;q21;q22), add(7)(q32) and an FMS-related tyrosine kinase 3 internal tandem duplication (FLT3-ITD) mutation. Complete remission was attained following a course of chemotherapy with ATRA and arsenic trioxide. To the best of our knowledge, this is the first report of a novel three-way translocation of 6p21 and a FLT3-ITD mutation involved with APL.

Recombinant TAT-BMI-1 fusion protein induces ex vivo expansion of human umbilical cord blood-derived hematopoietic stem cells

Transplantation of hematopoietic stem cells (HSCs) is a well-established therapeutic approach for numerous disorders. HSCs are typically derived from bone marrow or peripheral blood after cytokine-induced mobilization. Umbilical cord blood (CB) represents an appealing alternative HSC source, but the small amounts of the individual CB units have limited its applications. The availability of strategies for safe ex vivo expansion of CB-derived HSCs (CB-HSCs) may allow to extend the use of these cells in adult patients and to avoid the risk of insufficient engraftment or delayed hematopoietic recovery.

Here we describe a system for the ex vivo expansion of CB-HSCs based on their transient exposure to a recombinant TAT-BMI-1 chimeric protein. BMI-1 belongs to the Polycomb family of epigenetic modifiers and is recognized as a central regulator of HSC self-renewal. Recombinant TAT-BMI-1 produced in bacteria was able to enter the target cells via the HIV TAT-derived protein transduction peptide covalently attached to BMI-1, and conserved its biological activity. Treatment of CB-CD34⁺ cells for 3 days with repeated addition of 10 nM purified TAT-BMI-1 significantly enhanced total cell expansion as well as that of primitive hematopoietic progenitors in culture. Importantly, TAT-BMI-1-treated CB-CD34⁺ cells displayed a consistently higher rate of multi-lineage long-term repopulating activity in primary and secondary xenotransplants in immunocompromised mice. Thus, recombinant TAT-BMI-1 may represent a novel, effective reagent for ex vivo expansion of CB-HSC for therapeutic purposes.

Hypercoagulability and Migraine

BACKGROUND

A growing body of literature suggests that migraineurs, particularly those with aura, have an increased risk for ischemic stroke, but not via enhanced atherosclerosis. The theory that micro-emboli induced ischemia provokes cortical spreading depression (ie, symptomatic aura) in migraineurs but transient ischemic attacks in others highlights a potential role for hypercoagulability as a link between migraine (with aura) and stroke.

AIM

Our objective is to summarize the literature evaluating the association of migraine with various acquired or inheritable thrombophilic states, including those related to elevated estrogen levels, endothelial activation and dysfunction, antiphospholipid antibodies (aPL), deficiency of coagulation inhibitors, and presence of certain genetic polymorphisms.

FINDINGS

Although definitive studies are lacking, a preponderance of available evidence links migraine, and especially aura, to increased levels of estradiol (eg, oral contraceptive pill [OCP] use, pregnancy), thrombo- and erythrocytosis, von Willebrand factor (vWF) antigen, fibrinogen, tissue plasminogen activator (tPA) antigen, and endothelial microparticles. Studies of a link to migraine are conflicting for aPL, homocysteine, Protein S, and the methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism. No association with migraine was found in meta-analyses of Factor V Leiden, and of prothrombin gene mutation. Within a large, young ischemic stroke sample, migraine with aura was associated with a thrombophilic state and with patent foramen ovale (PFO). In the non-stroke population, meta-analyses show an association of PFO and migraine with aura (MA), but two population-based studies do not support the link.

RECOMMENDATIONS

For persons with MA and (1) a personal history or family history of thrombosis, or (2) MRI evidence of micro-vascular ischemia or of stroke, an evaluation for hypercoagulability is warranted. In cases of MA alone, consider screening for markers of endothelial activation (eg, vWF, high sensitivity c-reactive protein [hs CRP], and fibrinogen). Rigorous management of other stroke risk factors is paramount, but efficacy of anti-thrombotic agents in the treatment of migraine is unproven. Closure of PFO is not routinely recommended based on negative randomized trials.

UMG Lenti: novel lentiviral vectors for efficient transgene- and reporter gene expression in human early hematopoietic progenitors

Lentiviral vectors are widely used to investigate the biological properties of regulatory proteins and/or of leukaemia-associated oncogenes by stably enforcing their expression in hematopoietic stem and progenitor cells. In these studies it is critical to be able to monitor and/or sort the infected cells, typically via fluorescent proteins encoded by the modified viral genome. The most popular strategy to ensure co-expression of transgene and reporter gene is to insert between these cDNAs an IRES element, thus generating bi-cistronic mRNAs whose transcription is driven by a single promoter. However, while the product of the gene located upstream of the IRES is generally abundantly expressed, the translation of the downstream cDNA (typically encoding the reporter protein) is often inconsistent, which hinders the detection and the isolation of transduced cells. To overcome these limitations, we developed novel lentiviral dual-promoter vectors (named UMG-LV5 and –LV6) where transgene expression is driven by the potent UBC promoter and that of the reporter protein, EGFP, by the minimal regulatory element of the WASP gene. These vectors, harboring two distinct transgenes, were tested in a variety of human haematopoietic cell lines as well as in primary human CD34⁺ cells in comparison with the FUIGW vector that contains the expression cassette UBC-transgene-IRES-EGFP. In these experiments both UMG-LV5 and UMG–LV6 yielded moderately lower transgene expression than FUIGW, but dramatically higher levels of EGFP, thereby allowing the easy distinction between transduced and non-transduced cells. An additional construct was produced, in which the cDNA encoding the reporter protein is upstream, and the transgene downstream of the IRES sequence. This vector, named UMG-LV11, proved able to promote abundant expression of both transgene product and EGFP in all cells tested. The UMG-LVs represent therefore useful vectors for gene transfer-based studies in hematopoietic stem and progenitor cells, as well as in non-hematopoietic cells.

Dissecting the retinoid-induced differentiation of F9 embryonal stem cells by integrative genomics

Retinoic acid (RA) triggers physiological processes by activating heterodimeric transcription factors (TFs) comprising retinoic acid receptor (RARα, β, γ) and retinoid X receptor (RXRα, β, γ). How a single signal induces highly complex temporally controlled networks that ultimately orchestrate physiological processes is unclear. Using an RA-inducible differentiation model, we defined the temporal changes in the genome-wide binding patterns of RARγ and RXRα and correlated them with transcription regulation. Unexpectedly, both receptors displayed a highly dynamic binding, with different RXRα heterodimers targeting identical loci. Comparison of RARγ and RXRα co-binding at RA-regulated genes identified putative RXRα-RARγ target genes that were validated with subtype-selective agonists. Gene-regulatory decisions during differentiation were inferred from TF-target gene information and temporal gene expression. This analysis revealed six distinct co-expression paths of which RXRα-RARγ is associated with transcription activation, while Sox2 and Egr1 were predicted to regulate repression. Finally, RXRα-RARγ regulatory networks were reconstructed through integration of functional co-citations. Our analysis provides a dynamic view of RA signalling during cell differentiation, reveals RAR heterodimer dynamics and promiscuity, and predicts decisions that diversify the RA signal into distinct gene-regulatory programs.

Identifying genetic loci and spleen gene coexpression networks underlying immunophenotypes in BXD recombinant inbred mice

The immune system plays a pivotal role in the susceptibility to and progression of a variety of diseases. Due to a strong genetic basis, heritable differences in immune function may contribute to differential disease susceptibility between individuals. Genetic reference populations, such as the BXD (C57BL/6J × DBA/2J) panel of recombinant inbred (RI) mouse strains, provide unique models through which to integrate baseline phenotypes in healthy individuals with heritable risk for disease because of the ability to combine data collected from these populations across both multiple studies and time. We performed basic immunophenotyping (e.g., percentage of circulating B and T lymphocytes and CD4(+) and CD8(+) T cell subpopulations) in peripheral blood of healthy mice from 41 BXD RI strains to define the immunophenotypic variation in this strain panel and to characterize the genetic architecture that underlies these traits. Significant QTL models that explained the majority (50-77%) of phenotypic variance were derived for each trait and for the T:B cell and CD4(+):CD8(+) ratios. Combining QTL mapping with spleen gene expression data uncovered two quantitative trait transcripts, Ptprk and Acp1, as candidates for heritable differences in the relative abundance of helper and cytotoxic T cells. These data will be valuable in extracting genetic correlates of the immune system in the BXD panel. In addition, they will be a useful resource for prospective, phenotype-driven model selection to test hypotheses about differential disease or environmental susceptibility between individuals with baseline differences in the composition of the immune system.

Notch signaling in pediatric malignancies

Notch signaling plays crucial roles in many developmental pathways, with Notch mutations linked to several developmental disorders. Because many pediatric malignancies arise from dysregulated development, roles for Notch signaling in these cancers are to be expected. Evidence to support this is now emerging as the Notch pathway is being explored in more pediatric cancers. Not surprisingly, Notch appears to play diverse roles in different malignancies, effecting differentiation, metastasis, cancer "stem cells," and angiogenesis. As examples, although activating mutations of Notch1 are found in the majority of T-cell acute lymphoblastic leukemia (ALL) cases, Notch/HES1 signaling appears to play a tumor suppressor role in precursor B-cell ALL; although Notch/HES1 signaling appears to contribute to osteosarcoma metastasis, Notch signaling also promotes medulloblastoma "stem cell" survival and contributes to angiogenesis in neuroblastoma. Further understanding of the roles of Notch signaling in specific pediatric cancers will provide a rationale for Notch-based therapeutic strategies.

Aberrant Notch3 and Notch4 expression in human hepatocellular carcinoma

BACKGROUND

Notch signalling is altered in several solid tumours and it plays a role in growth inhibition and apoptosis of hepatocellular carcinoma (HCC)-derived cell lines, bile duct development and hepatocyte regeneration.

AIMS

This study aims to analyse the expression of Notch3, Notch4 and HES1 and HES6 as Notch-target genes in HCC, matched non-neoplastic tissue and HEPG2 cells.

RESULTS

Notch3 and Notch4 are not expressed in normal liver and in chronic hepatitis surrounding HCC. Cirrhotic tissue stains negative for Notch3, while Notch4 is expressed by hepatocytes at the edge of regenerative nodules and in cell planes adjacent to fibrous septa. HCC tissue displays Notch3 and Notch4 abnormal accumulation, respectively, in 78% and 68% of the cases. The endothelium of hepatic veins with neoplastic permeation is frequently Notch4 positive. An upregulation of Notch3 mRNA was found in 95% of HCCs vs cirrhosis (P=0.0001), while Notch4 mRNA was downregulated in 80% of HCCs. HES6 mRNA expression was higher in HCC tissue when compared with cirrhosis (P=0.007), paralleling Notch3 mRNA expression. The HEPG2 cell line displays high Notch3 and low Notch4 protein and mRNA levels.

CONCLUSIONS

These descriptive findings suggest an aberrant expression of Notch3 and Notch4 in HCC and allow the hypothesis of an activation of Notch signalling by Notch3.

Evolutionary history of the ABCB2 genomic region in teleosts

Gene duplication, silencing and translocation have all been implicated in shaping the unique genomic architecture of the teleost MH regions. Previously, we demonstrated that trout possess five unlinked regions encoding MH genes. One of these regions harbors ABCB2 which in all other vertebrate classes is found in the MHC class II region. In this study, we sequenced a BAC contig for the trout ABCB2 region. Analysis of this region revealed the presence of genes homologous to those located in the human class II (ABCB2, BRD2, psiDAA), extended class II (RGL2, PHF1, SYGP1) and class III (PBX2, Notch-L) regions. The organization and syntenic relationships of this region were then compared to similar regions in humans, Tetraodon and zebrafish to learn more about the evolutionary history of this region. Our analysis indicates that this region was generated during the teleost-specific duplication event while also providing insight about potential MH paralogous regions in teleosts.

Effects of histamine on immunophenotype and notch signaling in human HL-60 leukemia cells

Surface molecules are important biomarkers for cell proliferation and differentiation and play important roles in cell function and cell interaction. Notch is a transmembrane receptor that regulates developmental processes and cell-fate decision. Histamine is used as an adjunct to immunotherapy in myelogenous leukemia, and regulates hematopoietic cell development. Thus, we investigated the effects of histamine on immunophenotype and Notch signaling in human HL-60 leukemia cells. Histamine (0.1-10 microM) inhibited the colony-forming efficiency of HL-60 cells in a dose-dependent fashion and shifted the growth curve to the right. HL-60 cells were treated with histamine 0.1-1.0 microM for 6 days, and surface molecules were analyzed by flow cytometry. Histamine decreased CD49d positive cells by 74% while increasing CD31 positive cells by 53% as compared to controls. Histamine did not affect the expression of CD11b, CD14, CD34, CD44, CD54, CD49e, and CD62L. To examine Notch signaling in histamine-induced immunophenotype alterations in HL-60 cells, total RNA was isolated, purified, and subjected to real-time RT-PCR analysis. The expressions of Notch1, Notch4, the ligands Jagged1, Delta4, and the downstream hairy enhancer of split 1 gene (HES1) were not significantly altered by histamine. In summary, this study demonstrated that histamine inhibited HL-60 cell growth and regulated immunophenotypes of CD49d and CD31. These effects are not mediated through the Notch signaling.

Hematopoietic stem cell self-renewal

Recent studies have begun to elucidate the mechanisms controlling hematopoietic stem cell (HSC) self-renewal. Self-renewal requires the integration of survival signals and proliferation controls with the maintenance of an undifferentiated state. This demands a complex crosstalk between extrinsic signals from the microenvironment and the cell-intrinsic regulators of self-renewal. The Polycomb protein Bmi1 is absolutely required for the maintenance of both adult HSCs and neural stem cells. Evidence from studies in murine and human embryonic stem cells indicates that Polycomb group proteins play a dynamic role in concert with master transcriptional regulators in actively maintaining an undifferentiated state, suggesting that this mechanism applies to multiple types of stem cell. Recently, various new players that regulate HSC maintenance (e.g. Mcl1, Tel/Etv6, Gfi1, Pten and Stat5) have been identified. In order to better understand HSC self-renewal, we need to understand how these pathways are coordinated.

Thyroid hormones and retinoids: a possible link between genes and environment in schizophrenia

Phenotypic discordance for schizophrenia in monozygotic twins clearly indicates involvement of environmental factors as key determinants in disease development. Positive findings from genome scans, linkage and association studies apply in only a minority of those affected, while post-mortem brain investigations reveal altered expression of genes and proteins involved in numerous neurodevelopmental, metabolic and neurotransmitter pathways. Such altered expressions could result, on the one hand, from mutations in coding regions or polymorphisms in the promoter and regulatory regions in genes within those areas identified by gene searches or, on the other hand, from inadequate amounts of modulators, transporters and synthesizers of transcription factors necessary for regulation of the putative genes. Hormones and vitamins are such modulators. They could serve as bridges between genes and environment in schizophrenia. Multiple evidence supports the suggestion of retinoids and thyroid hormones as plausible actors in these roles. Both are not only essential for normal development of the central nervous system but also regulate the expression of many neurotransmitters, their synthesizing enzymes and receptors, and other genes in broader signaling transduction cascades affecting pathways that are altered in response to treatment. Functional and positional candidate genes include retinoic acid and thyroid hormone receptors, retinaldehyde dehydrogenases and deiodinases, which synthesize the powerful morphogens, retinoic acid and triiodothyronine, and the enzymes involved in their inactivation. This review highlights selective evidence supporting the retinoid and thyroid hormone hypotheses of schizophrenia.

Notch Signaling Requires GATA-2 to Inhibit Myelopoiesis from Embryonic Stem Cells and Primary Hemopoietic Progenitors

The bone marrow and thymus, although both hemopoietic environments, induce very distinct differentiation outcomes. The former supports hemopoietic stem cell self-renewal and multiple hemopoietic lineages, while the latter supports T lymphopoiesis almost exclusively. This distinction suggests that the thymic environment acts to restrict the hemopoietic fates available to thymic immigrants. In this study, we demonstrate that the addition of the Notch ligand Delta-like-1 (Dll-1) to an in vitro system that otherwise supports myelopoiesis, greatly reduces the myelopoietic potential of stem cells or uncommitted progenitors. In contrast, committed myeloid progenitors mature regardless of the presence of Dll-1. The block in myelopoiesis is the direct result of Notch signaling within the hemopoietic progenitor, and Dll-1-induced signals cause a rapid increase in the expression of the zinc finger transcription factor GATA-2. Importantly, in the absence of GATA-2, Dll-1-induced signals fail to inhibit commitment to the myeloid fate. Taken together, our results support a role for GATA-2 in allowing Dll-1 to restrict non-T cell lineage differentiation outcomes.

Activation of vascular endothelial growth factor receptor-2 in bone marrow leads to accumulation of myeloid cells: role of granulocyte-macrophage colony-stimulating factor

Vascular endothelial growth factor (VEGF) is a secreted cytokine that plays a major role in the formation and maintenance of the hemopoietic and vascular compartments. VEGF and its receptors, VEGFR-1 and VEGFR-2, have been found to be expressed on subsets of normal and malignant hemopoietic cells, but the role of the individual receptors in hemopoiesis requires further study. Using a VEGFR-2 fusion protein that can be dimerized with a synthetic drug, we were able to specifically examine the effects of VEGFR-2 signaling in hemopoietic cells in vivo. Mice transplanted with bone marrow transduced with this inducible VEGFR-2 fusion protein demonstrated expansion of myeloid cells (Gr-1+, CD11b+). Levels of myeloid progenitors were also increased following VEGFR-2 activation, through autocrine and paracrine mechanisms, as measured by clonogenic progenitor assays. VEGFR-2 activation induced expression of GM-CSF and increased serum levels in vivo. Abrogation of GM-CSF activity, either with neutralizing Abs or by using GM-CSF-null hemopoietic cells, inhibited VEGFR-2-mediated myeloid progenitor activity. Our findings indicate that VEGF signaling through VEGFR-2 promotes myelopoiesis through GM-CSF-dependent and -independent mechanisms.

Expression of DLK1 in hematopoietic cells results in inhibition of differentiation and proliferation

The Delta-like (DLK1) gene is overexpressed in CD34+ cells from myelodysplastic syndrome (MDS) patients. DLK1 encodes an EGF-like homeotic transmembrane protein homologous to the notch/delta/serrate family. Although exogenous DLK1 promotes maintenance of murine hematopoietic stem cells, the functional effects of DLK1 overexpression in hematopoietic cells are unknown. We show that ectopically expressed DLK1 significantly inhibits differentiation and proliferation of human promyelocytic HL-60 cells. Unlike preadipocytes, where proteolytic processing of membrane-bound protein and release of a soluble form mediates differentiation inhibition, proteolytic release of the extracellular domain was not required for inhibition of hematopoietic cell differentiation. However, intracellular domain interactions were critical to this DLK1 function. We conclude that DLK1 overexpression in hematopoietic cells has important functional consequences. Our studies identify novel molecular mechanisms and indicate that DLK1 has activity both as a soluble and a transmembrane expressed protein. Our results support further investigation of the role of DLK1 in abnormal hematopoiesis in MDS.

Converging pathways in leukemogenesis and stem cell self-renewal

Studies over the last 40 years have led to an understanding of the hierarchical organization of the hematopoietic system and the role of the pluripotential hematopoietic stem cell. Earlier recognition of the importance of bone marrow hematopoietic microenvironments has evolved into the recognition of specific niches that regulate stem cell pool size, proliferative status, mobilization, and differentiation. The discovery of the role of multiple hematopoietic growth factors and their receptors in the orchestration of stem cell self-renewal and differentiation has been followed by recognition of the importance of the Notch and Wnt pathways. The homeobox family of transcription factors serve as master regulators of development and are increasingly found to be critical regulators of hematopoiesis. In parallel with this understanding of normal hematopoiesis has come a recognition that stem cell dysregulation at various levels is involved in leukemogenesis. Furthermore, the progression from chronic leukemia or myelodysplasia to acute leukemia involves accumulation of at least two mutational events that lead to enhancement of stem cell proliferation, or acquisition of stem cell behavior by a progenitor cell, coupled with maturation inhibition. Translocations resulting in development of oncogenic fusion genes are found in AML and the transforming potential of two of these, AML1-ETO and NUP98-HOXA9, will be discussed. Secondary, constitutively activating mutations of the Flt3 and c-kit receptors and of K- and N-ras are found with high frequency in AML, and the transforming potential of mutated FLT3 and the role of STAT5A activation in human stem cell transformation will be reviewed.

Fbxw7/Cdc4 is a p53-dependent, haploinsufficient tumour suppressor gene

The FBXW7/hCDC4 gene encodes a ubiquitin ligase implicated in the control of chromosome stability. Here we identify the mouse Fbxw7 gene as a p53-dependent tumour suppressor gene by using a mammalian genetic screen for p53-dependent genes involved in tumorigenesis. Radiation-induced lymphomas from p53+/- mice, but not those from p53-/- mice, show frequent loss of heterozygosity and a 10% mutation rate of the Fbxw7 gene. Fbxw7+/- mice have greater susceptibility to radiation-induced tumorigenesis, but most tumours retain and express the wild-type allele, indicating that Fbxw7 is a haploinsufficient tumour suppressor gene. Loss of Fbxw7 alters the spectrum of tumours that develop in p53 deficient mice to include a range of tumours in epithelial tissues such as the lung, liver and ovary. Mouse embryo fibroblasts from Fbxw7-deficient mice, or wild-type mouse cells expressing Fbxw7 small interfering RNA, have higher levels of Aurora-A kinase, c-Jun and Notch4, but not of cyclin E. We propose that p53-dependent loss of Fbxw7 leads to genetic instability by mechanisms that might involve the activation of Aurora-A, providing a rationale for the early occurrence of these mutations in human cancers.