Hes1 immortalizes committed progenitors and plays a role in blast crisis transition in chronic myelogenous leukemia

Hairy enhancer of split 1 (Hes1) is a basic helix-loop-helix transcriptional repressor that affects differentiation and often helps maintain cells in an immature state in various tissues. Here we show that retroviral expression of Hes1 immortalizes common myeloid progenitors (CMPs) and granulocyte-macrophage progenitors (GMPs) in the presence of interleukin-3, conferring permanent replating capability on these cells. Whereas these cells did not develop myeloproliferative neoplasms when intravenously administered to irradiated mice, the combination of Hes1 and BCR-ABL in CMPs and GMPs caused acute leukemia resembling blast crisis of chronic myelogenous leukemia (CML), resulting in rapid death of the recipient mice. On the other hand, BCR-ABL alone caused CML-like disease when expressed in c-Kit-positive, Sca-1-positive, and lineage-negative hematopoietic stem cells (KSLs), but not committed progenitors CMPs or GMPs, as previously reported. Leukemic cells derived from Hes1 and BCR-ABL-expressing CMPs and GMPs were more immature than those derived from BCR-ABL-expressing KSLs. Intriguingly, Hes1 was highly expressed in 8 of 20 patients with CML in blast crisis, but not in the chronic phase, and dominant negative Hes1 retarded the growth of some CML cell lines expressing Hes1. These results suggest that Hes1 is a key molecule in blast crisis transition in CML.

Expression levels of histone deacetylases determine the cell fate of hematopoietic progenitors

Histone deacetylases (HDACs) are globally implicated in the growth and differentiation of mammalian cells; however, relatively little is known about their specific roles in hematopoiesis. In this study, we investigated the expression of HDACs in human hematopoietic cells and their functions during hematopoiesis. The expression of HDACs was very low in hematopoietic progenitor cells, which was accompanied by histone hyperacetylation. HDACs were detectable in more differentiated progenitors and erythroid precursors but down-regulated in mature myeloid cells especially granulocytes. In contrast, acute myeloid leukemias showed HDAC overexpression and histone hypoacetylation. Transcription of the HDAC1 gene was repressed by CCAAT/enhancer binding proteins during myeloid differentiation, and activated by GATA-1 during erythro-megakaryocytic differentiation. Small interfering RNA-mediated knockdown of HDAC1 enhanced myeloid differentiation in immature hematopoietic cell lines and perturbed erythroid differentiation in progenitor cells. Myeloid but not erythro-megakaryocytic differentiation was blocked in mice transplanted with HDAC1-overexpressing hematopoietic progenitor cells. These findings suggest that HDAC is not merely an auxiliary factor of genetic elements but plays a direct role in the cell fate decision of hematopoietic progenitors.

Mixed-lineage-leukemia (MLL) fusion protein collaborates with Ras to induce acute leukemia through aberrant Hox expression and Raf activation

Mixed-lineage-leukemia (MLL) fusion oncogenes are closely involved in infant acute leukemia, which is frequently accompanied by mutations or overexpression of FMS-like receptor tyrosine kinase 3 (FLT3). Earlier studies have shown that MLL fusion proteins induced acute leukemia together with another mutation, such as an FLT3 mutant, in mouse models. However, little has hitherto been elucidated regarding the molecular mechanism of the cooperativity in leukemogenesis. Using murine model systems of the MLL-fusion-mediated leukemogenesis leading to oncogenic transformation in vitro and acute leukemia in vivo, this study characterized the molecular network in the cooperative leukemogenesis. This research revealed that MLL fusion proteins cooperated with activation of Ras in vivo, which was substitutable for Raf in vitro, synergistically, but not with activation of signal transducer and activator of transcription 5 (STAT5), to induce acute leukemia in vivo as well as oncogenic transformation in vitro. Furthermore, Hoxa9, one of the MLL-targeted critical molecules, and activation of Ras in vivo, which was replaceable with Raf in vitro, were identified as fundamental components sufficient for mimicking MLL-fusion-mediated leukemogenesis. These findings suggest that the molecular crosstalk between aberrant expression of Hox molecule(s) and activated Raf may have a key role in the MLL-fusion-mediated-leukemogenesis, and may thus help develop the novel molecularly targeted therapy against MLL-related leukemia.

Etoposide induces MLL rearrangements and other chromosomal abnormalities in human embryonic stem cells

MLL rearrangements are hallmark genetic abnormalities in infant leukemia known to arise in utero. They can be induced during human prenatal development upon exposure to etoposide. We also hypothesize that chronic exposure to etoposide might render cells more susceptible to other genomic insults. Here, for the first time, human embryonic stem cells (hESCs) were used as a model to test the effects of etoposide on human early embryonic development. We addressed whether: (i) low doses of etoposide promote MLL rearrangements in hESCs and hESCs-derived hematopoietic cells; (ii) MLL rearrangements are sufficient to confer hESCs with a selective growth advantage and (iii) continuous exposure to low doses of etoposide induces hESCs to acquire other chromosomal abnormalities. In contrast to cord blood-derived CD34(+) and hESC-derived hematopoietic cells, exposure of undifferentiated hESCs to a single low dose of etoposide induced a pronounced cell death. Etoposide induced MLL rearrangements in hESCs and their hematopoietic derivatives. After long-term culture, the proportion of hESCs harboring MLL rearrangements diminished and neither cell cycle variations nor genomic abnormalities were observed in the etoposide-treated hESCs, suggesting that MLL rearrangements are insufficient to confer hESCs with a selective proliferation/survival advantage. However, continuous exposure to etoposide induced MLL breaks and primed hESCs to acquire other major karyotypic abnormalities. These data show that chronic exposure of developmentally early stem cells to etoposide induces MLL rearrangements and make hESCs more prone to acquire other chromosomal abnormalities than postnatal CD34(+) cells, linking embryonic genotoxic exposure to genomic instability.

Multistep pathogenesis of leukemia via the MLL-AF4 chimeric gene/Flt3 gene tyrosine kinase domain (TKD) mutation-related enhancement of S100A6 expression

OBJECTIVE

Concerning MLL-AF4 leukemogenesis, previous mouse models suggest that the tumorigenesis capacity of MLL-AF4 alone is insufficient for causing leukemia. Based on the finding that an Fms-like tyrosine kinase 3 (Flt3) gene mutation in the tyrosine kinase domain (TKD) was observed in approximately 15% of mixed lineage leukemia (MLL), we investigated synergistic leukemogenesis effects of the two genes in vitro.

MATERIALS AND METHODS

In a mouse interleukin-3 (IL-3)-dependent cell line, 32Dc, expression of MLL-AF4 and mutant Flt3 was induced using a lentiviral vector. We analyzed apoptosis induction in the absence of IL-3 and the granulocyte colony-stimulating factor-related induction of differentiation, gene expression profiling, and the mechanism involved in the synergistic effects of MLL-AF4 and Flt3-TKD.

RESULTS

Neither Flt3-expressing 32Dc (32Dc(Flt3-TKD)) nor MLL-AF4-expressing 32Dc (32Dc(MLL-AF4)) acquired IL-3-independent proliferative capacity in semisolid/liquid media. However, Flt3-TKD+MLL-AF4-expressing 32Dc (32Dc(Flt3-TKD+MLL-AF4)) acquired a non-IL-3-dependent proliferative capacity by inhibiting apoptosis in the two media. The 32Dc(Flt3-TKD) and 32Dc(MLL-AF4) cells differentiated into granulocytes in the presence of granulocyte colony-stimulating factor. However, in the 32Dc(Flt3-TKD+MLL-AF4) cells, there was no differentiation. Subsequently, we performed gene expression profiling. The enhancement of Hox genes expression was not identified. However, expression of S100A6 was synergistically enhanced in the presence of both MLL-AF4 and Flt3-TKD genes. Moreover, anti-S100A6 small interfering RNA downregulated leukemic proliferation.

CONCLUSION

We conclude that their synergistic enhancement of S100A6 expression plays an important role in MLL-AF4-associated leukemogenesis.

Molecular bases of myelodysplastic syndromes: lessons from animal models

Myelodysplastic syndrome (MDS) is a clonal disorder of hematopietic stem cells characterized by ineffective hematopoiesis, peripheral blood cytopenia, morphologic dysplasia, and susceptibility to acute myeloid leukemia. Several mechanisms have been suggested as causes of MDS: unbalanced chromosomal abnormalities reflecting a gain or loss of chromosomal material, point mutations of transcription factors, and inactivation of p53. However, appropriate animal models that mimic MDS have long been lacking. We recently reported a novel murine model of MDS that recapitulates trilineage dysplasia and transformation to AML. In this review, we summarize the animal models of MDS and discuss the molecular bases of MDS as well as those of leukemia and myeloproliferative disorders (MPD). J. Cell. Physiol. 219: 529-534, 2009. (c) 2009 Wiley-Liss, Inc.

Mouse models of human AML accurately predict chemotherapy response

The genetic heterogeneity of cancer influences the trajectory of tumor progression and may underlie clinical variation in therapy response. To model such heterogeneity, we produced genetically and pathologically accurate mouse models of common forms of human acute myeloid leukemia (AML) and developed methods to mimic standard induction chemotherapy and efficiently monitor therapy response. We see that murine AMLs harboring two common human AML genotypes show remarkably diverse responses to conventional therapy that mirror clinical experience. Specifically, murine leukemias expressing the AML1/ETO fusion oncoprotein, associated with a favorable prognosis in patients, show a dramatic response to induction chemotherapy owing to robust activation of the p53 tumor suppressor network. Conversely, murine leukemias expressing MLL fusion proteins, associated with a dismal prognosis in patients, are drug-resistant due to an attenuated p53 response. Our studies highlight the importance of genetic information in guiding the treatment of human AML, functionally establish the p53 network as a central determinant of chemotherapy response in AML, and demonstrate that genetically engineered mouse models of human cancer can accurately predict therapy response in patients.

Therapeutic targeting of MLL

Treatment of hematologic malignancies is evolving from a uniform approach to targeted therapies directed at the underlying molecular abnormalities of disease. The mixed lineage leukemia (MLL) proto-oncogene is a recurrent site of genetic rearrangements in acute leukemias; and since its discovery in 1992, many advances have been made in understanding its role in leukemogenesis. A variety of MLL translocation partners have been described, and detailed structure/function studies have identified functional domains that are required for transformation. Proteins associated with the MLL core complex or its fusion partners have been isolated and characterized for their critical roles in leukemia pathogenesis. Downstream mediators of MLL transcriptional regulation and multiple collaborating signaling pathways have been described and characterized. These advances in our understanding of MLL-related leukemogenesis provide a foundation for ongoing and future efforts to develop novel therapeutic strategies that will hopefully result in better treatment outcomes.

Acute myeloid leukemia induced by MLL-ENL is cured by oncogene ablation despite acquisition of complex genetic abnormalities

Chromosomal translocations involving 11q23 are frequent in infant acute leukemia and give rise to the formation of MLL fusion genes. The mechanism of leukemic transformation by these fusions has been the subject of numerous investigations. However, the dependence of acute leukemia on MLL fusion activity in vivo and the efficacy of targeting this activity to eliminate disease have not been established. We have developed a model for conditional expression of MLL-ENL in hematopoietic progenitor cells, in which expression of the fusion oncogene is turned off by doxycycline. Conditionally immortalized myeloblast cells derived from these progenitors were found to induce leukemia in vivo. Leukemic cells isolated from primary recipient mice were shown to have acquired additional genetic abnormalities and, when transplanted into secondary recipients, induced leukemia with shortened latencies. However, the leukemic cells remained dependent on MLL-ENL expression in vitro and in vivo, and its ablation resulted in regression of established leukemias. This study demonstrates that even genetically complex leukemias can be reversed on inactivation of the initiating MLL fusion and has important implications for the design of novel leukemia therapies.

C/EBPalpha and C/EBPvarepsilon induce the monocytic differentiation of myelomonocytic cells with the MLL-chimeric fusion gene

CCAAT/enhancer binding proteins (C/EBPs) have an important function in granulocytic differentiation, and are also involved in the leukemogenesis of acute myeloid leukemia (AML). Their involvement in myelomonocytic leukemia, however, is still unclear. Therefore, the expression and function of C/EBPs in myelomonocytic cells with MLL-fusion genes were investigated. Retinoic acid (RA) induced monocytic differentiation in the myelomonocytic cell lines with MLL-fusion genes, THP-1, MOLM-14 and HF-6 cells, accompanied by monocytic differentiation with the upregulation of C/EBPalpha and C/EBPepsilon. Monocytic differentiation by RA treatment was confirmed in primary AML cells using a clonogenic assay. When the activity of C/EBPalpha or C/EBPepsilon was introduced into HF-6 cells, their cellular growth was arrested through differentiation into monocytes with the concomitant marked downregulation of Myc. Cebpe mRNA was upregulated by the induction of C/EBPalpha-ER, but not vice versa, thus suggesting that C/EBPepsilon may have an important function in the differentiation process. Introduction of Myc isoforms into HF-6 cells partially antagonized the C/EBPs effects. These findings suggest that the ectopic expression of C/EBPepsilon, as well as C/EBPalpha, can induce the monocytic differentiation of myelomonocytic leukemic cells with MLL-fusion gene through the downregulation of Myc, thus providing insight into the development of novel therapeutic approaches.

A novel variant form of MLL-ELL fusion transcript with t(11;19)(q23;p13.1) in chronic myelomonocytic leukemia transforming to acute myeloid leukemia

MLL located at 11q23 is fused with a variety of partner genes by recurrent chromosomal translocations in acute leukemias. ELL, the MLL partner gene located on chromosome 19p13.1, encodes an RNA polymerase II transcriptional elongation factor, which also possesses the N-terminal region involved in the inhibition of transcription initiation. Here we report a case of chronic myelomonocytic leukemia (CMML) with a 46,XY,t(11;19)(q23;p13.1) karyotype that transformed to acute myeloid leukemia (AML) without showing any karyotypic evolution. Interphase fluorescent in situ hybridization analysis showed the split MLL signals in 95% of bone marrow cells when the diagnosis of CMML was made and the percentage of blasts was 1.2%. Sequence analysis of reverse-transcriptional polymerase chain reaction product revealed a novel variant form of MLL-ELL transcript in which MLL exon 10 was fused to ELL exon 3. MLL has been fused to ELL exon 2 in all the previously reported MLL-ELL transcripts, which have always been associated with AML. It is deduced that the variant form of MLL-ELL may be defective not only in inhibition of transcription initiation, but also in transcriptional elongation. Thus, a possibility is raised that the unique clinical presentation of the present case with t(11;19)(q23;p13.1) might be related to the variant form of MLL-ELL.

A variant-type MLL/SEPT9 fusion transcript in adult de novo acute monocytic leukemia (M5b) with t(11;17)(q23;q25)

As a result of recurrent chromosomal translocations in acute leukemias, the mixed-lineage-leukemia (MLL) gene fuses with a variety of partner genes, which include several members of the septin gene family. SEPT9 is a very rare but recurrent fusion partner of MLL, and has recently been implicated in the oncogenesis of various malignancies. Herein, we report a case of de novo acute monocytic leukemia (M5b) with t(11;17)(q23;q25). MLL involvement was revealed by fluorescent in situ hybridization (FISH) analysis, and an MLL/SEP9 fusion transcript was detected by RT-PCR. Sequencing analysis further showed that, in contrast to originally reported cases, MLL exon 8 was fused not with SEPT9 exon 3 but with exon 2, which codes for the unique N-terminal region of the SEPT9_v1 isoform, the region implicated in the regulation of gene expression and cell proliferation. We did not detect any mutation of FLT3, which was expressed at a relatively low level in the leukemic cells. Relapsing after a very short complete remission, the leukemia progressed rapidly and became fatal in spite of intensive therapies including hematopoietic stem cell transplantation. It is thus suggested that, in common with the original MLL/SEPT9 cases, monocytic differentiation and a poor prognosis may also be associated with acute myeloid leukemia with the variant MLL/SEPT9 fusion transcript.

FLT3 kinase inhibitors in the management of acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase (TK) expressed by immature hematopoietic cells and is important for the normal development of stem cells and the immune system. Mutations of the juxtamembranous and TK domain of the gene are described in 30%-35% of patients with acute myeloid leukemia (AML). These mutations alter the biologic properties of AML and are associated with prognosis. In recent years, there has been an enormous development of potential inhibitors of FLT3 mutations. These substances are now being studied in clinical protocols. The initial trials reveal that, unlike in patients with chronic myeloid leukemia, TK inhibitor (TKI) therapy in AML is more complex. To date, most FLT3 TKIs investigated in clinical studies show a favorable toxicity profile with considerable biologic activity. However, refractory disease and/or the rapid development of resistance toward these new drugs remain major challenges. Strategies to circumvent this unsatisfactory clinical potential of FLT3 TKIs are mainly based on the combination with cytotoxic chemotherapy. Herein, we summarize results from studies using FLT3 TKIs as single agents and report on the first clinical trials investigating FLT3 TKIs in combination with chemotherapy.

AML1-Evi-1 specifically transforms hematopoietic stem cells through fusion of the entire Evi-1 sequence to AML1

The t(3;21) chromosomal translocation seen in blastic crisis of chronic myeloid leukemia and secondary leukemias results in a formation of a chimeric protein AML1-Evi-1, which suppresses wild-type AML1 function. Loss of AML1 function causes expansion of hematopoietic progenitor cells, whereas it is not sufficient for the development of leukemia. To identify essential mechanisms through which AML1-Evi-1 exerts full leukemogenic potential, we introduced AML1-Evi-1 and its mutants in murine bone marrow cells, and evaluated their transforming activities by colony replating assays. The transforming activity of AML1-Evi-1 was lost when any of the known functional domains of Evi-1 was deleted from the chimeric protein, and forced expression of Evi-1 did not transform the AML1-deleted bone marrow cells. Unlike the MLL-ENL and AML1-ETO leukemia-related chimeric proteins, AML1-Evi-1 could transform only the hematopoietic stem cell fraction. Moreover, AML1-Evi-1-transformed cells show a cell-marker profile distinct from that of the cells transformed by AML1-ETO, which also suppresses AML1 function. Thus, leukemogenic activity of AML1-Evi-1 may be due to activation of molecular mechanisms distinct from those activated by MLL-ENL or AML1-ETO in the hematopoietic stem cell fractions.

Review: genetic models of acute myeloid leukaemia

The use of genetically engineered mice (GEM) have been critical in understanding disease states such as cancer, and none more so than acute myelogenous leukaemia (AML), a disease characterized by over 100 distinct chromosomal translocations. A substantial proportion of cases exhibiting recurrent reciprocal translocations at diagnosis, such as t(8;21) or t(15;17) have been exhaustively studied and are currently employed in clinical diagnosis. However, a definitive conclusion regarding the leukaemogenic potential of defined transgenes for this disease remains elusive. While it is increasingly apparent that a number of cooperating mutations are necessary to develop a leukaemic phenotype, the number of models reflecting these synergisms remains few. Furthermore, little emphasis has been paid to the effect of chromosomal translocations other than recurrent genetic abnormalities, with no models reflecting the multiple abnormalities observed in high-risk cases of AML accounting for 8-10% of adult AML. Here we review the differing technologies employed in generation of GEM of AML. We discuss the relevance of GEM AML from embryonic stem cell-mediated (for example retinoic acid receptor-alpha fusions and AML1/ETO) models; through to the valuable retroviral-mediated gene transfer models. The latter have been used to great effect in defining the transforming properties of chromosomal translocation products such as MLL (found in 5-6% of all AML cases) and NUP98 (denoting poor prognosis in therapy-related disease) and particularly when co-transduced with bad prognostic factors such as Flt3 mutations. Finally, we comment on the emergence of newer transduction technologies, which can regulate the level of expression to defined cell lineages in both primary murine and human xenografts, and discuss how combining multiple genetic modalities, more relevant models of this complex disease are being generated.

Knock-in of an internal tandem duplication mutation into murine FLT3 confers myeloproliferative disease in a mouse model

Constitutive activation of FMS-like tyrosine kinase 3 (FLT3) by internal tandem duplication (ITD) mutations is one of the most common molecular alterations known in acute myeloid leukemia (AML). To investigate the role FLT3/ITD mutations play in the development of leukemia, we generated a FLT3/ITD knock-in mouse model by inserting an ITD mutation into the juxtamembrane domain of murine Flt3. FLT3wt/ITD mice developed myeloproliferative disease, characterized by splenomegaly, leukocytosis, and myeloid hypercellularity, which progressed to mortality by 6 to 20 months. Bone marrow (BM) and spleen from FLT3wt/ITD mice had an increased fraction of granulocytes/monocytes and dendritic cells, and a decreased fraction of B-lymphocytes. No sign of acute leukemia was observed over the lifetime of these mice. BM from FLT3wt/ITD mice showed enhanced potential to generate myeloid colonies in vitro. BM from FLT3wt/ITD mice also produced more spleen colonies in the in vivo colony-forming unit (CFU)-spleen assay. In the long-term competitive repopulation assay, BM cells from FLT3wt/ITD mice outgrew the wild-type competitor cells and showed increased myeloid and reduced lymphoid expansion activity. In summary, our data indicate that expression of FLT3/ITD mutations alone is capable of conferring normal hematopoietic stem/progenitor cells (HSPCs) with enhanced myeloid expansion. It also appears to suppress B lymphoid maturation. Additional cooperative events appear to be required to progress to acute leukemia.

AML1 mutations induced MDS and MDS/AML in a mouse BMT model

Myelodysplastic syndrome (MDS) is a hematopoietic stem-cell disorder characterized by trilineage dysplasia and susceptibility to acute myelogenous leukemia (AML). Analysis of molecular basis of MDS has been hampered by the heterogeneity of the disease. Recently, mutations of the transcription factor AML1/RUNX1 have been identified in 15% to 40% of MDS-refractory anemia with excess of blasts (RAEB) and MDS/AML. We performed mouse bone marrow transplantation (BMT) using bone marrow cells transduced with the AML1 mutants. Most mice developed MDS and MDS/AML-like symptoms within 4 to 13 months after BMT. Interestingly, among integration sites identified, Evi1 seemed to collaborate with an AML1 mutant harboring a point mutation in the Runt homology domain (D171N) to induce MDS/AML with an identical phenotype characterized by marked hepatosplenomegaly, myeloid dysplasia, leukocytosis, and biphenotypic surface markers. Collaboration between AML1-D171N and Evi1 was confirmed by a BMT model where coexpression of AML1-D171N and Evi1 induced acute leukemia of the same phenotype with much shorter latencies. On the other hand, a C-terminal truncated AML1 mutant (S291fsX300) induced pancytopenia with erythroid dysplasia in transplanted mice, followed by progression to MDS-RAEB or MDS/AML. Thus, we have developed a useful mouse model of MDS/AML that should help in the understanding of the molecular basis of MDS and the progression of MDS to overt leukemia.

MLL-AF9 and FLT3 cooperation in acute myelogenous leukemia: development of a model for rapid therapeutic assessment

Human leukemias harboring chromosomal translocations involving the mixed lineage leukemia (MLL, HRX, ALL-1) gene possess high-level expression, and frequent activating mutations of the receptor tyrosine kinase FLT3. We used a murine bone marrow transplant model to assess cooperation between MLL translocation and FLT3 activation. We demonstrate that MLL-AF9 expression induces acute myelogenous leukemia (AML) in approximately 70 days, whereas the combination of MLL-AF9 and FLT3-ITD does so in less than 30 days. Secondary transplantation of splenic cells from diseased mice established that leukemia stem cells are present at a very high frequency of approximately 1:100 in both diseases. Importantly, prospectively isolated granulocyte macrophage progenitors (GMPs) coinfected with MLL-AF9 and FLT3-ITD give rise to a similar AML, with shorter latency than from GMP transduced with MLL-AF9 alone. Cooperation between MLL-AF9 and FLT3-ITD was further verified by real-time assessment of leukemogenesis using noninvasive bioluminescence imaging. We used this model to demonstrate that MLL-AF9/FLT3-ITD-induced leukemias are sensitive to FLT3 inhibition in a 2-3 week in vivo assay. These data show that activated FLT3 cooperates with MLL-AF9 to accelerate onset of an AML from whole bone marrow as well as a committed hematopoietic progenitor, and provide a new genetically defined model system that should prove useful for rapid assessment of potential therapeutics in vivo.

Transgenic mice expressing Tel-FLT3, a constitutively activated form of FLT3, develop myeloproliferative disease

Evidence is continuing to accumulate that the FMS-like tyrosine kinase 3 (FLT3) receptor plays an important role in acute leukemias. Acute myeloid leukemia patients often express constitutive active mutant forms of the receptor in their leukemic cells. A t(12;13)(p13;q12) translocation between Tel and the FLT3 receptor was recently described in a patient with myeloproliferative disease (MPD). Here a Tel-FLT3 construct mimicking this fusion protein was used to generate transgenic mice. The fusion protein was previously found to constitutively activate FLT3 signaling and transform Ba/F3 cells. Expression of the fusion protein in the transgenic mice was found in all tissues assayed including spleen, bone marrow (BM), thymus and liver. These mice developed splenomegaly and had a high incidence of MPD with extramedullary hematopoiesis in the liver and lymph nodes. Spleens also had increased dendritic and natural killer cell populations. In vitro analysis of the hematopoietic progenitor cells derived from Tel-FLT3 transgenic mice showed a significant increase in the number of CFU-GM in the BM, and CFU-GM, BFU-E and CFU-GEMM in the spleen. BM also showed significant increases of in vivo CFU-S colonies. Thus, transgenic mice expressing constitutively activated Tel-FLT3 develop MPD with a long latency and also result in the expansion of the hematopoietic stem/progenitor cells.

Signaling pathways in self-renewing hematopoietic and leukemic stem cells: do all stem cells need a niche?

Many adult tissue stem cells, such as the cells of the hematopoietic system, gastrointestinal epithelium, brain, epidermis, mammary gland and lung have now been identified, all of them fulfilling a crucial role in supplying organisms with mature cells during normal homeostasis as well as in times of tissue generation or repair. Two unique features characterize adult stem cells: the ability to generate new pluripotent stem cells (to self-renew) and the ability to give rise to differentiated progeny that has lost its self-renewal capacity. Our understanding of the mechanisms that determine whether, where and when a stem cell will self-renew or differentiate is still limited, but recent advances have indicated that the stem cell microenvironment, or niche, provides essential cues that direct these cell fate decisions. Moreover, loss of control over these cell fate decisions might lead to cellular transformation and cancer. This review addresses the current understandings of the molecular mechanisms that regulate hematopoietic stem cell self-renewal in the niche and how leukemic transformation might change the dependency of leukemic stem cells on their microenvironment for self-renewal and survival.

Establishment of a Stroma-Dependent Human Acute Myelomonocytic Leukemia Cell Line, NAMO-2, with FLT3 Tandem Duplication

We have established a stroma-dependent myelomonocytic cell line, NAMO-2, with FLT3 internal tandem duplication (FLT3/ITD). Leukemia cells from a patient with acute myelomonocytic leukemia were administered to form subcutaneous tumors in nude mice, which were maintained successively, although we failed to establish continuously growing cells from the original leukemia cell culture. In the cultures of cells from subcutaneous tumors, there were stroma cells that had originated from the nude mice and showed continuous growth. The leukemia cells showed continuous growth dependent on this stroma, and this cell line was named NAMO-2. Detection of FLT3/ITD by the reverse transcriptase polymerase chain reaction (PCR) and genomic PCR showed that NAMO-2 was homozygous for FLT3/ITD. Constitutive activation of FLT3 was detected by Western blotting, and the phosphorylation of Akt, MEK, and STAT5 was also observed. FLT3 kinase inhibitor AG1296 specifically inhibited cell growth. NAMO-2 provides a useful tool to analyze adherence-dependent survival signaling of leukemia with FLT3/ITD and a model for the screening of FLT3 kinase inhibitors.

Establishment of a myeloid leukemia cell line, TRL-01, with MLL-ENL fusion gene

We established a leukemia cell line derived from therapy-related acute myeloid leukemia with the t(11;19) by xenotransplantation into the NOD/SCID mouse with IL-2Rgamma(c)-/- (NOG mouse). The cell line, TRL-01, could be serially transplanted from mouse to mouse and also grown in an adherence-dependent manner on a murine bone marrow stroma cell line, HESS-5. TRL-01 had the same immunophenotype as the original leukemia cells: positive for CD13, CD33, CD11a, CD18, CD29, CD49d, CD49e, CD54, CD62L, and CD117, and negative for CD3, CD4, CD8, CD19, CD34, CD41a, CD41b, CD135, and myeloperoxidase. Translocation (11;19)(q23;p13) in both the original sample and TRL-01 generated MLL-ENL chimeric transcripts joining exon 6 and exon 4, respectively, which has a novel isoform. In cultures of TRL-01, addition of GM-CSF, SCF, and G-CSF and adhesion to fibronectin-coated plates promoted transient proliferation and survival, although they did not support long-term culture. Subcutaneous injection caused a tumor to form only when HESS-5 was coinjected at the same site. These results suggest that TRL-01 is a useful cell line for studying not only the leukemia-related biology of MLL-ENL but also the intercellular association between leukemia and stroma.

Identification of additional cytogenetic and molecular genetic abnormalities in acute myeloid leukaemia with t(8;21)/AML1-ETO

AML1-ETO collaborates with further genetic abnormalities to induce acute myeloid leukaemia (AML). We analysed 99 patients with an AML1-ETO rearrangement for additional aberrations. Frequent genetic abnormalities were, loss of a sex chromosome (56/99, 56.5%) and del(9)(q22) (24/99, 24.2%). The most frequent molecular aberrations were mutations of KITD816 (3/23, 13%) and NRAS (8/89, 8.9%). Further molecular abnormalities were FLT3 mutations (3/87, 3.4%), AML1 (1/26, 3.8%) and PU1 (1/14, 7.1%). MLL-PTD, KRAS and CEBPA mutations were not found. These clinical findings support the model that AML1-ETO collaborates with other genetic alterations, such as mutations of receptor tyrosine kinases, to induce AML.

The Mll-Een knockin fusion gene enhances proliferation of myeloid progenitors derived from mouse embryonic stem cells and causes myeloid leukaemia in chimeric mice

Rearrangement of the mixed lineage leukaemia (MLL) gene with extra eleven nineteen (EEN) was previously identified in an infant with acute myeloid leukaemia. Using homologous recombination, we have created a mouse equivalent of the human MLL-EEN allele and showed that when Mll(Een/+) embryonic stem (ES) cells were induced to differentiate in vitro into haemopoietic cells, there was increased proliferation of myeloid progenitors with self-renewal property. We also generated Mll(Een/+) chimeric mice, which developed leukaemia displaying enlarged livers, spleens, thymuses and lymph nodes owing to infiltration of Mll(Een/+)-expressing leukemic cells. Immunophenotyping of cells from enlarged organs and bone marrow (BM) of the Mll(Een/+) chimeras revealed an accumulation of Mac-1+/Gr-1- immature myeloid cells and a reduction in normal B- and T-cell populations. We observed differential regulation of Hox genes between myeloid cells derived from Mll(Een/+) ES cells and mouse BM leukemic cells which suggested different waves of Hox expression may be activated by MLL fusion proteins for initiation (in ES cells) and maintenance (in leukemic cells) of the disease. We believe studies of MLL fusion proteins in ES cells combined with in vivo animal models offer new approaches to the dissection of molecular events in multistep pathogenesis of leukaemia.

The Flt3 receptor tyrosine kinase collaborates with NUP98-HOX fusions in acute myeloid leukemia

In leukemogenesis, several genetic changes conferring a proliferative and/or survival advantage to hematopoietic progenitor cells in addition to a block in differentiation are required. Here, we demonstrate that overexpression of the wild-type (wt) Flt3 receptor tyrosine kinase collaborates with NUP98-HOX fusions (NUP98-HOXA10 and NUP98-HOXD13) to induce aggressive acute myeloid leukemia (AML). We used a mouse transplantation model to show their synergism in cotransduced bone marrow cells as well as in a cellular model of leukemic progression. Furthermore, our data support the finding that Meis1 overexpression leads to marked elevation in Flt3 transcription and extend it to the context of NUP98-HOX–induced leukemia. Together, these results support a multistep model where the synergism between NUP98-HOX and wt-Flt3 is the result of the ability of Flt3 to increase proliferation of myeloid progenitors blocked in differentiation by NUP98-HOX fusions and reveal a direct role for wt-Flt3 in the pathobiology of AML. Given the similarities in the leukemogenic role of native HOX and NUP98-fused HOX genes, our results underscore the clinical significance of the recurrent co-overexpression of wt-FLT3 and HOX in human leukemia and suggest that specific FLT3 inhibitors could be useful in treatment of HOX-induced AML or acute lymphoblastic leukemia (ALL).

Tyk2 Is Dispensable for Induction of Myeloproliferative Disease by Mutant FLT3

Internal tandem duplication of FLT3 tyrosine kinase (FLT3-ITD) is the most prevalent mutation found in acute myelogenous leukemia (AML), having been identified in 20% to 30% of all AML patients. We have previously shown that FLT3-ITD signals mainly through the signal transducer and activator of transcription 5 (STAT5) pathway and have suggested the possible involvement of Tyk2 in STAT5 activation by FLT3-ITD. The present study addressed the role of Tyk2 in FLT3-ITD signaling in a murine bone marrow transplantation (BMT) model. Transplantation of wild-type bone marrow cells transduced with the FLT3-ITD gene induced lethal myeloproliferative disease (MPD) in the recipient mice at a median latency of 89 days. Interestingly, some mice presented the proliferation of B- or T-lymphoid blasts in various organs, a presentation that resembled acute lymphoblastic leukemia (ALL). Mice that received Tyk2-deficient bone marrow cells transduced with FLT3-ITD developed lethal MPD with a disease latency (median, 100 days) and pathologic picture similar to those of mice that received wild-type bone marrow cells. These results indicate that (1) Tyk2 is not essential for MPD induction by FLT3-ITD and (2) FLT3-ITD by itself can induce ALL in a murine BMT model.

Chromosomal translocations in cancer and their relevance for therapy

PURPOSE OF REVIEW

Recurring chromosomal abnormalities are considered the primary genetic change in oncogenesis as well as an important indicator for tumor phenotype and clinical outcome. This review highlights recent findings regarding the genes associated with chromosomal translocations.

RECENT FINDINGS

A great number of novel fusion genes associated with chromosomal translocations have been cloned. These novel fusion genes are found in the smaller part of various malignancies, and it can be expected that the significance of novel fusion gene occurrence for oncogenesis will be clarified in the not too distant future. Observation of high frequencies of mutations in NOTCH1, NPM and JAK2 in T-cell acute lymphoblastic leukemia, acute myeloid leukemia with normal karyotype and myeloproliferative disorders (polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis) have provided important suggestions for a better understanding of chromosomal translocations. This is because all these genes had already been identified as genes associated with chromosomal translocations in a small subset of specific phenotypes of hematologic malignancies.

SUMMARY

This review summarizes recent findings associated with chromosomal translocations including newly identified fusion genes, a novel mechanism of fusion gene formation and their relevance for novel targeted therapies. Continuing attempts to identify genes associated with chromosomal translocations can be expected to provide further insights into the significance of various gene alterations in cancer and the development of novel targeted therapies.

SEPT2 is a new fusion partner of MLL in acute myeloid leukemia with t(2;11)(q37;q23)

We have identified a new mixed lineage leukemia (MLL) gene fusion partner in a patient with treatment-related acute myeloid leukemia (AML) presenting a t(2;11)(q37;q23) as the only cytogenetic abnormality. Fluorescence in situ hybridization demonstrated a rearrangement of the MLL gene and molecular genetic analyses identified a septin family gene, SEPT2, located on chromosome 2q37, as the fusion partner of MLL. RNA and DNA analyses showed the existence of an in-frame fusion of MLL exon 7 with SEPT2 exon 3, with the genomic breakpoints located in intron 7 and 2 of MLL and SEPT2, respectively. Search for DNA sequence motifs revealed the existence of two sequences with 94.4% homology with the topoisomerase II consensus cleavage site in MLL intron 7 and SEPT2 intron 2. SEPT2 is the fifth septin family gene fused with MLL, making this gene family the most frequently involved in MLL-related AML (about 10% of all known fusion partners). The protein encoded by SEPT2 is highly homologous to septins 1, 4 and 5 and is involved in the coordination of several key steps of mitosis. Further studies are warranted to understand why the septin protein family is particularly involved in the pathogenesis of MLL-associated leukemia.

K-Ras mutations and N-Ras mutations in childhood acute leukemias with or without mixed-lineage leukemia gene rearrangements

BACKGROUND

It is believed that Ras mutations drive the proliferation of leukemic cells. The objective of this study was to investigate the association of Ras mutations with childhood acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) with special reference to the presence or absence of mixed-lineage leukemia gene (MLL) rearrangements.

METHODS

Bone marrow samples from 313 children with B-precursor ALL and 130 children with de novo AML were studied at diagnosis. Southern blot analysis was used to detect MLL rearrangements, and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis was used to detect common MLL fusion transcripts. Complementary DNA panhandle PCR was used to identify the infrequent or unknown MLL partner genes. DNA PCR or RT-PCR followed by direct sequencing was performed to detect mutations at codons 12, 13, and 61 of the N-Ras and K-Ras genes.

RESULTS

Twenty of 313 patients with B-precursor ALL and 17 of 130 patients with de novo AML had MLL rearrangements. N-Ras mutations were detected in 2 of 20 patients with MLL-positive ALL and in 27 of 293 patients with MLL-negative ALL (P = 1.000). N-Ras mutations were detected in 2 of 17 patients with MLL-positive AML and in 14 of 113 patients with MLL-negative AML (P = 1.000). K-Ras mutations were present in 8 of 20 patients with MLL-positive ALL compared with 32 of 293 patients with MLL-negative ALL (P = 0.001). K-Ras mutations were detected in 3 of 17 patients with MLL-positive AML compared with 5 of 113 patients with MLL-negative AML (P = 0.069).

CONCLUSIONS

Ras mutations were detected in 20.8% of patients with childhood B-precursor ALL and in 17.7% of patients with childhood AML. MLL-positive B-precursor ALL was associated closely with Ras mutations (50%), especially with K-Ras mutations (40%), whereas MLL-positive AML was not associated with Ras mutations.

Disruption of Sept6, a fusion partner gene of MLL, does not affect ontogeny, leukemogenesis induced by MLL-SEPT6, or phenotype induced by the loss of Sept4

Septins are evolutionarily conserved GTP-binding proteins that can heteropolymerize into filaments. Recent studies have revealed that septins are involved in not only diverse normal cellular processes but also the pathogenesis of various diseases, including cancer. SEPT6 is ubiquitously expressed in tissues and one of the fusion partner genes of MLL in the 11q23 translocations implicated in acute leukemia. However, the roles of this septin in vivo remain elusive. We have developed Sept6-deficient mice that exhibited neither gross abnormalities, changes in cytokinesis, nor spontaneous malignancy. Sept6 deficiency did not cause any quantitative changes in any of the septins evaluated in this study, nor did it cause any additional changes in the Sept4-deficient mice. Even the depletion of Sept11, a close homolog of Sept6, did not affect the Sept6-null cells in vitro, thus implying a high degree of redundancy in the septin system. Furthermore, a loss of Sept6 did not alter the phenotype of myeloproliferative disease induced by MLL-SEPT6, thus suggesting that Sept6 does not function as a tumor suppressor. To our knowledge, this is the first report demonstrating that a disruption of the translocation partner gene of MLL in 11q23 translocation does not contribute to leukemogenesis by the MLL fusion gene.

Cancer Stem Cells and Differentiation Therapy

Cancers arise from stem cells in adult tissues and the cells that make up a cancer reflect the same stem cell --> progeny --> differentiation progression observed in normal tissues. All adult tissues are made up of lineages of cells consisting of tissue stem cells and their progeny (transit-amplifying cells and terminally differentiated cells); the number of new cells produced in normal tissue lineages roughly equals the number of old cells that die. Cancers result from maturation arrest of this process, resulting in continued proliferation of cells and a failure to differentiate and die. The biological behavior, morphological appearance, and clinical course of a cancer depend on the stage of maturation at which the genetic lesion is activated. This review makes a comparison of cancer cells to embryonic stem cells and to adult tis sue stem cells while addressing two basic questions: (1) Where do cancers come from?, and (2) How do cancers grow? The answers to these questions are critical to the development of approaches to the detection, prevention, and treatment of cancer.

MLL chimeric protein activation renders cells vulnerable to chromosomal damage: An explanation for the very short latency of infant leukemia

MLL fusion genes are a predominant feature of acute leukemias in infants and in secondary acute myeloid leukemia (AML) associated with prior chemotherapy with topo-II poisons. The former is considered to possibly arise in utero via transplacental chemical exposure. A striking feature of these leukemias is their malignancy and remarkably brief latencies implying the rapid acquisition of any necessary additional mutations. We have suggested that these coupled features might be explained if MLL fusion gene encoded proteins rendered cells more vulnerable to further DNA damage and mutation in the presence of chronic exposure to the agent(s) that induced the MLL fusion itself. We have tested this idea by exploiting a hormone regulated MLL-ENL (MLLT1) activation system and show that MLL-ENL function in normal murine progenitor cells substantially increases the incidence of chromosomal abnormalities in proliferating cells that survive exposure to etoposide VP-16. This phenotype is associated with an altered pattern of cell cycle arrest and/or apoptosis.

Continuous MLL-ENL expression is necessary to establish a "Hox Code" and maintain immortalization of hematopoietic progenitor cells

The t[(11;19)(p22;q23)] translocation, which gives rise to the MLL-ENL fusion protein, is commonly found in infant acute leukemias of both the myeloid and lymphoid lineage. To investigate the molecular mechanism of immortalization by MLL-ENL we established a Tet-regulatable system of MLL-ENL expression in primary hematopoietic progenitor cells. Immortalized myeloid cell lines were generated, which are dependent on continued MLL-ENL expression for their survival and proliferation. These cells either terminally differentiate or die when MLL-ENL expression is turned off with doxycycline. The expression profile of all 39 murine Hox genes was analyzed in these cells by real-time quantitative PCR. This analysis showed that loss of MLL-ENL was accompanied by a reduction in the expression of multiple Hoxa genes. By comparing these changes with Hox gene expression in cells induced to differentiate with granulocyte colony-stimulating factor, we show for the first time that reduced Hox gene expression is specific to loss of MLL-ENL and is not a consequence of differentiation. Our data also suggest that the Hox cofactor Meis-2 can substitute for Meis-1 function. Thus, MLL-ENL is required to initiate and maintain immortalization of myeloid progenitors and may contribute to leukemogenesis by aberrantly sustaining the expression of a "Hox code" consisting of Hoxa4 to Hoxa11.

Mechanisms of therapy-related carcinogenesis

Therapy-related cancers, defined as second primary cancers that arise as a consequence of chemotherapy and/or radiotherapy, are unusual in that they have a well-defined aetiology. Knowledge of the specific nature of the initiating exposure and exactly when it occurred has made it easier to identify crucial genetic events and to model these in vitro and in vivo. As such, the study of therapy-related cancers has led to the elucidation of discrete mechanisms of carcinogenesis, including DNA double-strand-break-induced gene translocation and genomic instability conferred by loss of DNA repair. Unsurprisingly, some of these mechanisms seem to operate in the development of sporadic cancers.

D324N single-nucleotide polymorphism in theFLT3gene is associated with higher risk of myeloid leukemias

Mutations within the FLT3 gene are of growing importance for classification, risk assessment, and therapeutic targeting of acute myeloid leukemia (AML). We analyzed 656 AML patients for a recently described single-nucleotide polymorphism (SNP) in the third immunoglobulin-like domain of the extracellular region of FLT3. The FLT3 D324N variant was present in 42 cases (6.4%), but it was not associated with a specific AML subtype and did not show an elevated leukocyte count, as do other FLT3 mutations. In remission samples, a 50% ratio of the normal to the D324N variant was detectable. Stably expressed in IL-3 dependent Ba/F3 cells, the D324N variant did not confer receptor autophosphorylation, factor independent growth, or increased resistance to apoptotic cell death in response to varying doses of FLT3 ligand. In 400 healthy donors, the FLT3 D324N variant was detected in 6 cases (1.5%) and segregated in a family. Thus, it was shown to be a polymorphism with a lower frequency in healthy controls than in patients with AML (P < 0.001). In addition, 21 of 234 CML (9.0%) and 7 of 155 ALL (4.5%) cases carried the FLT3 D324N. Our data suggest that the FLT3 D324N variant might be associated with a predisposition to different subtypes of leukemia.

Do septins have a role in cancer?

Septins are an evolutionarily conserved family of genes that encode a P loop-based GTP-binding domain flanked by a polybasic domain and (usually) a coiled-coil region. They have roles in cytokinesis, vesicle trafficking, polarity determination, and can form membrane diffusion barriers, as well as in microtubule and actin dynamics. Septins can form hetero-oligomeric complexes and possibly function as dynamic protein scaffolds. Recently, it has been shown that there are at least 13 human septin genes that exhibit extensive alternate splicing. There are complex patterns of human septin gene expression and recently it has been found that alterations in septin expression are seen in human diseases including neoplasia. This review summarises the essential properties of septins and outlines the accumulating evidence for their involvement in human neoplasia. Septins may belong to the class of cancer critical genes where alteration in expression profile (including alterations in the spectrum of transcripts expressed) may underpin their role in neoplasia as opposed to specific mutational events.

The AML1-ETO fusion gene and the FLT3 length mutation collaborate in inducing acute leukemia in mice

The molecular characterization of leukemia has demonstrated that genetic alterations in the leukemic clone frequently fall into 2 classes, those affecting transcription factors (e.g., AML1-ETO) and mutations affecting genes involved in signal transduction (e.g., activating mutations of FLT3 and KIT). This finding has favored a model of leukemogenesis in which the collaboration of these 2 classes of genetic alterations is necessary for the malignant transformation of hematopoietic progenitor cells. The model is supported by experimental data indicating that AML1-ETO and FLT3 length mutation (FLT3-LM), 2 of the most frequent genetic alterations in AML, are both insufficient on their own to cause leukemia in animal models. Here we report that AML1-ETO collaborates with FLT3-LM in inducing acute leukemia in a murine BM transplantation model. Moreover, in a series of 135 patients with AML1-ETO-positive AML, the most frequently identified class of additional mutations affected genes involved in signal transduction pathways including FLT3-LM or mutations of KIT and NRAS. These data support the concept of oncogenic cooperation between AML1-ETO and a class of activating mutations, recurrently found in patients with t(8;21), and provide a rationale for therapies targeting signal transduction pathways in AML1-ETO-positive leukemias.