TEL/PDGFbetaR fusion protein activates STAT1 and STAT5: a common mechanism for transformation by tyrosine kinase fusion proteins

Pharmacological Inhibition of Oncogenic STAT3 and STAT5 Signaling in Hematopoietic Cancers

Signal Transducer and Activator of Transcription (STAT) 3 and 5 are important effectors of cellular transformation, and aberrant STAT3 and STAT5 signaling have been demonstrated in hematopoietic cancers. STAT3 and STAT5 are common targets for different tyrosine kinase oncogenes (TKOs). In addition, STAT3 and STAT5 proteins were shown to contain activating mutations in some rare but aggressive leukemias/lymphomas. Both proteins also contribute to drug resistance in hematopoietic malignancies and are now well recognized as major targets in cancer treatment. The development of inhibitors targeting STAT3 and STAT5 has been the subject of intense investigations during the last decade. This review summarizes the current knowledge of oncogenic STAT3 and STAT5 functions in hematopoietic cancers as well as advances in preclinical and clinical development of pharmacological inhibitors.

A novel fusion gene involving PDGFRB and GCC2 in a chronic eosinophilic leukemia patient harboring t(2;5)(q37;q31)

Background

Platelet‐derived growth factor receptor beta (PDGFRB) rearrangement has been reported in a number of patients with chronic eosinophilic leukemia (CEL), B‐acute lymphoblastic leukemia, myeloproliferative neoplasms, and juvenile myelomonocytic leukemia. Here, we report a case of CEL carrying a novel fusion gene involving PDGFRB and GRIP and coiled‐coil domain containing 2 (GCC2).

Patient and methods

A 54‐year‐old man presenting with a cough and dyspnea was diagnosed with acute eosinophilic pneumonia. Cytogenetic analysis of the bone marrow revealed the presence of t(2;5)(q37;q31). Fluorescence in situ hybridization analysis in the peripheral blood leukocytes revealed the presence of a split signal at PDGFRB gene. Imatinib treatment was effective, and disappearance of t(2;5)(q37;q31) in the bone marrow was confirmed after three months of imatinib therapy. Whole‐genome sequencing was performed in peripheral blood leukocytes collected before imatinib therapy.

Results

A novel fusion gene between exon 22 of GCC2 and exon 12 of PDGFRB was detected and the presence of GCC2‐PDGFRB was confirmed by PCR.

Conclusion

This is the first case report demonstrating the GCC2 gene as a partner of PDGFRB in the pathogenesis of CEL.

The Cooperative Relationship between STAT5 and Reactive Oxygen Species in Leukemia: Mechanism and Therapeutic Potential

Reactive oxygen species (ROS) are now recognized as important second messengers with roles in many aspects of signaling during leukemogenesis. They serve as critical cell signaling molecules that regulate the activity of various enzymes including tyrosine phosphatases. ROS can induce inactivation of tyrosine phosphatases, which counteract the effects of tyrosine kinases. ROS increase phosphorylation of many proteins including signal transducer and activator of transcription-5 (STAT5) via Janus kinases (JAKs). STAT5 is aberrantly activated through phosphorylation in many types of cancer and this constitutive activation is associated with cell survival, proliferation, and self-renewal. Such leukemic activation of STAT5 is rarely caused by mutation of the STAT5 gene itself but instead by overactive mutant receptors with tyrosine kinase activity as well as JAK, SRC family protein tyrosine kinases (SFKs), and Abelson murine leukemia viral oncogene homolog (ABL) kinases. Interestingly, STAT5 suppresses transcription of several genes encoding antioxidant enzymes while simultaneously enhancing transcription of NADPH oxidase. By doing so, STAT5 activation promotes an overall elevation of ROS level, which acts as a feed-forward loop, especially in high risk Fms-related tyrosine kinase 3 (FLT3) mutant leukemia. Therefore, efforts have been made recently to target ROS in cancer cells. Drugs that are able to either quench ROS production or inversely augment ROS-related signaling pathways both have potential as cancer therapies and may afford some selectivity by activating feedback inhibition of the ROS-STAT5 kinome. This review summarizes the cooperative relationship between ROS and STAT5 and explores the pros and cons of emerging ROS-targeting therapies that are selective for leukemia characterized by persistent STAT5 phosphorylation.

Ph-like ALL-related novel fusion kinase ATF7IP-PDGFRB exhibits high sensitivity to tyrosine kinase inhibitors in murine cells

ATF7IP-PDGFRB is a novel PDGFRB-related fusion gene identified in B-cell precursor acute lymphoblastic leukemia (BCP-ALL) with a signature similar to that of Ph1 ALL, so-called Ph-like ALL. When we introduced ATF7IP-PDGFRB, murine Ba/F3 cells acquired the ability to proliferate in an interleukin (IL)-3-independent manner. On the contrary, the expression of wild-type PDGFRB is not sufficient to acquire the ability for IL-3-independent proliferation in Ba/F3 cells. The introduction of ATF7IP-PDGFRB also induces a typical gene expression profile for Ph1-ALL in Ba/F3 cells. A series of biochemical and cell biological experiments revealed the constitutive activation of ATF7IP-PDGFRB as well as downstream signaling molecules, including AKT and MAPK. Although the phosphoinositide 3-kinase inhibitor led to cell death in both cells into which ATF7IP-PDGFRB had been introduced and IL-3-maintained Mock cells, MEK inhibitor selectively led to cell death into which ATF7IP-PDGFRB had been introduced. The introduction of tyrosine to phenylalanine mutations at binding sites of adaptor molecules important in the MAPK pathway located in the PDGFRB portion abolished ATF7IP-PDGFRB-mediated cell transformation, suggesting that MAPK-mediated signals are critical in ATF7IP-PDGFRB-mediated cell transformation. On treatment with tyrosine kinase inhibitors, ATF7IP-PDGFRB-expressing, but not Mock, Ba/F3 cells underwent rapid apoptosis accompanied by reduced phosphorylation of MAPK. Importantly, the sensitivity of ATF7IP-PDGFRB-expressing Ba/F3 cells to imatinib is significantly higher than that of BCR-ABL1-transformed Ba/F3 cells, as assessed by the IC50. Taken together, ATF7IP-PDGFRB has transforming potential via the constitutive activation of MAPK and participates in the pathogenesis of Ph-like ALL. Our observations suggest the therapeutic importance of tyrosine kinase inhibitors and possibly MEK inhibitor for a subset of BCP-ALL harboring PDGFRB-related fusion kinases.

Constitutive activation of oncogenic PDGFRα-mutant proteins occurring in GIST patients induces receptor mislocalisation and alters PDGFRα signalling characteristics

Background

Gastrointestinal stromal tumours (GIST) are mainly characterised by the presence of activating mutations in either of the two receptor tyrosine kinases c-KIT or platelet-derived growth factor receptor-α (PDGFRα). Most mechanistic studies dealing with GIST mutations have focused on c-KIT and far less is known about the signalling characteristics of the mutated PDGFRα proteins. Here, we study the signalling capacities and corresponding transcriptional responses of the different PDGFRα proteins under comparable genomic conditions.

Results

We demonstrate that the constitutive signalling via the oncogenic PDGFRα mutants favours a mislocalisation of the receptors and that this modifies the signalling characteristics of the mutated receptors. We show that signalling via the oncogenic PDGFRα mutants is not solely characterised by a constitutive activation of the conventional PDGFRα signalling pathways. In contrast to wild-type PDGFRα signal transduction, the activation of STAT factors (STAT1, STAT3 and STAT5) is an integral part of signalling mediated via mutated PDGF-receptors. Furthermore, this unconventional STAT activation by mutated PDGFRα is already initiated in the endoplasmic reticulum whereas the conventional signalling pathways rather require cell surface expression of the receptor. Finally, we demonstrate that the activation of STAT factors also translates into a biologic response as highlighted by the induction of STAT target genes.

Conclusion

We show that the overall oncogenic response is the result of different signatures emanating from different cellular compartments. Furthermore, STAT mediated responses are an integral part of mutated PDGFRα signalling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-015-0096-8) contains supplementary material, which is available to authorized users.

STAT signaling in the pathogenesis and treatment of myeloid malignancies

STAT transcription factors play a critical role in mediating the effects of cytokines on myeloid cells. As STAT target genes control key processes such as survival, proliferation and self-renewal, it is not surprising that constitutive activation of STATs, particularly STAT3 and STAT5, are common events in many myeloid tumors. STATs are activated both by mutant tyrosine kinases as well as other pathogenic events, and continued activation of STATs is common in the setting of resistance to kinase inhibitors. Thus, the targeting of STATs, alone or in combination with other drugs, will likely have increasing importance for cancer therapy.

Tyrosine kinase gene fusions in cancer: translating mechanisms into targeted therapies

Tyrosine kinase fusion genes represent an important class of oncogenes associated with leukaemia and solid tumours. They are produced by translocations and other chromosomal rearrangements of a subset of tyrosine kinase genes, including ABL, PDGFRA, PDGFRB, FGFR1, SYK, RET, JAK2 and ALK. Based on recent findings, this review discusses the common mechanisms of activation of these fusion genes. Enforced oligomerization and inactivation of inhibitory domains are the two key processes that switch on the kinase domain. Activated tyrosine kinase fusions then signal via an array of transduction cascades, which are largely shared. In addition, the fusion partner provides a scaffold for the recruitment of proteins that contribute to signalling, protein stability, cellular localization and oligomerization. The expression level of the fusion protein is another critical parameter. Its transcription is controlled by the partner gene promoter, while translation may be regulated by miRNA. Several mechanisms also prevent the degradation of the oncoprotein by proteasomes and lysosomes, leading to its accumulation in cells. The selective inhibition of the tyrosine kinase activity by adenosine-5'-triphosphate competitors, such as imatinib, is a major therapeutic success. Imatinib induces remission in leukaemia patients that are positive for BCR-ABL or PDGFR fusions. Recently, crizotinib produced promising results in a subtype of lung cancers with ALK fusion. However, resistance was reported in both cases, partially due to mutations. To tackle this problem, additional levels of therapeutic interventions are suggested by the complex mechanisms of fusion tyrosine kinase activation. New approaches include allosteric inhibition and interfering with oligomerization or chaperones.

Tyrosine kinase chromosomal translocations mediate distinct and overlapping gene regulation events

Background

Leukemia is a heterogeneous disease commonly associated with recurrent chromosomal translocations that involve tyrosine kinases including BCR-ABL, TEL-PDGFRB and TEL-JAK2. Most studies on the activated tyrosine kinases have focused on proximal signaling events, but little is known about gene transcription regulated by these fusions.

Methods

Oligonucleotide microarray was performed to compare mRNA changes attributable to BCR-ABL, TEL-PDGFRB and TEL-JAK2 after 1 week of activation of each fusion in Ba/F3 cell lines. Imatinib was used to control the activation of BCR-ABL and TEL-PDGFRB, and TEL-JAK2-mediated gene expression was examined 1 week after Ba/F3-TEL-JAK2 cells were switched to factor-independent conditions.

Results

Microarray analysis revealed between 800 to 2000 genes induced or suppressed by two-fold or greater by each tyrosine kinase, with a subset of these genes commonly induced or suppressed among the three fusions. Validation by Quantitative PCR confirmed that eight genes (Dok2, Mrvi1, Isg20, Id1, gp49b, Cxcl10, Scinderin, and collagen Vα1(Col5a1)) displayed an overlapping regulation among the three tested fusion proteins. Stat1 and Gbp1 were induced uniquely by TEL-PDGFRB.

Conclusions

Our results suggest that BCR-ABL, TEL-PDGFRB and TEL-JAK2 regulate distinct and overlapping gene transcription profiles. Many of the genes identified are known to be involved in processes associated with leukemogenesis, including cell migration, proliferation and differentiation. This study offers the basis for further work that could lead to an understanding of the specificity of diseases caused by these three chromosomal translocations.

Emerging targeted therapies for lymphoid malignancies

CONTEXT

Our understanding of molecular events in the pathogenesis of hematologic malignancies has evolved substantially. The research data gathered in the past 3 decades have led to the definition of neoplastic disorders based on specific genetic and molecular alterations, which is reflected in the current World Health Organization's classification of tumors of hematopoietic and lymphoid tissues. Moreover, there have been dramatic successes in the development and implementation of therapies that specifically target the proteins and signaling cascades affected by tumor-specific genetic alterations.

OBJECTIVE

To review the development of select, novel therapies for lymphoid malignancies.

DATA SOURCES

We examine examples from the recent literature in targeting 4 major regulatory pathways: tyrosine kinase activation, transcription factor activity, apoptotic signaling, and histone acetylation in both preclinical models and early-stage (stage 1 and 2) clinical trials.

CONCLUSION

Given the successes of novel compounds that target signaling pathways critical to the growth and survival of lymphoid tumor cells, the routine clinical use of molecularly targeted therapies for the treatment of lymphoid malignancies is likely in the near future.

Multiple oligomerization domains of KANK1-PDGFRβ are required for JAK2-independent hematopoietic cell proliferation and signaling via STAT5 and ERK

BACKGROUND

KANK1-PDGFRB is a fusion gene generated by the t(5;9) translocation between KANK1 and the platelet-derived growth factor receptor beta gene PDGFRB. This hybrid was identified in a myeloproliferative neoplasm featuring severe thrombocythemia, in the absence of the JAK2 V617F mutation.

DESIGN AND METHODS

KANK1-PDGFRB was transduced into Ba/F3 cells and CD34(+) human progenitor cells to gain insights into the mechanisms whereby this fusion gene transforms cells.

RESULTS

Although platelet-derived growth factor receptors are capable of activating JAK2, KANK1-PDGFRβ did not induce JAK2 phosphorylation in hematopoietic cells and a JAK inhibitor did not affect KANK1-PDGFRβ-induced cell growth. Like JAK2 V617F, KANK1-PDGFRβ constitutively activated STAT5 transcription factors, but this did not require JAK kinases. In addition KANK1-PDGFRβ induced the phosphorylation of phospholipase C-γ, ERK1 and ERK2, like wild-type PDGFRβ and TEL-PDGFRβ, another hybrid protein found in myeloid malignancies. We next tested various mutant forms of KANK1-PDGFRβ in Ba/F3 cells and human CD34(+) hematopoietic progenitors. The three coiled-coil domains located in the N-terminus of KANK1 were required for KANK1-PDGFRβ-induced cell growth and signaling via STAT5 and ERK. However, the coiled-coils were not essential for KANK1-PDGFRβ oligomerization, which could be mediated by another new oligomerization domain. KANK1-PDGFRβ formed homotrimeric complexes and heavier oligomers.

CONCLUSIONS

KANK1-PDGFRB is a unique example of a thrombocythemia-associated oncogene that does not signal via JAK2. The fusion protein is activated by multiple oligomerization domains, which are required for signaling and cell growth stimulation.

Adaptor protein Lnk binds to PDGF receptor and inhibits PDGF-dependent signaling

OBJECTIVE

Platelet-derived growth factor receptors α and β (PDGFRA, PDGFRB) are frequently expressed on hematopoietic cells and regulate cellular responses such as proliferation, differentiation, survival, and transformation. Stimulation by autocrine loops or activation by chromosomal translocation makes them important factors in development of hematopoietic disorders. Interaction with the ligand PDGF results in activation of the tyrosine kinase domain and phosphorylation of tyrosine residues, thereby creating binding sites for molecules containing Src homology 2 domains. We hypothesized that one such protein may be Lnk, a negative regulator of cytokine receptors, including Mpl, EpoR, c-Kit, and c-Fms.

MATERIALS AND METHODS

Interaction of Lnk with PDGFRA, PDGFRB, or leukemogenic FIP1L1-PDGFRA or TEL-PDGFRB was studied in cotransfected 293T cells. Effects of Lnk on PDGFR signaling were shown in 293T and NIH3T3 cells, whereas its influence on either PDGF-dependent or factor-independent growth was investigated using Ba/F3 or 32D cells expressing wild-type PDGFR, FIP1L1-PDGFRA, or TEL-PDGFRB.

RESULTS

We show that Lnk binds to PDGFR after exposure of cells to PDGF. Furthermore, Lnk can bind the FIP1L1-PDGFRA fusion protein. Mutation or deletion of the Lnk Src homology 2 domain completely abolished binding of Lnk to FIP1L1-PDGFRA, but just partly prevented binding to PDGFRA or PDGFRB. Expression of Lnk inhibited proliferation of PDGF-dependent Ba/F3 cells and diminished phosphorylation of Erk in PDGF-treated NIH3T3. 32D cells transformed by either FIP1L1-PDGFRA or TEL-PDGFRB stopped growing when Lnk was expressed.

CONCLUSIONS

Lnk is a negative regulator of PDGFR signaling. Development of Lnk mimetic drugs might provide a novel therapeutic strategy for myeloproliferative disorders.

Proteomic analysis reveals a novel mechanism induced by the leukemic oncogene Tel/PDGFRβ in stem cells: activation of the interferon response pathways

Objective proteomic analysis offers opportunities for hypothesis generation on molecular events associated with pathogenesis in stem cells. Relative quantification mass spectrometry was employed to identify pathways affected by Tel/PDGFRβ, an oncogene associated with myeloproliferative neoplasia (MPN). Its effects on over 1800 proteins were quantified with high confidence. Of those up-regulated by Tel/PDGFRβ several were involved in the interferon gamma (IFNγ) response. To validate these observations we employed embryonic and myeloid stem cells models which revealed Tel/PDGFRβ-induced STAT1 up-regulation and activation was responsible for modulating the interferon response. A STAT1 target highly up-regulated was ICSBP, a transcriptional regulator of myeloid and eosinophilic differentiation. ICSBP interacts with CBP/p300 and Ets transcription factors, to promote transcription of additional genes, including the Egr family, key regulators of myelopoiesis. These interferon responses were recapitulated using IFNγ stimulation of stem cells. Thus Tel/PDGFRβ induces aberrant IFN signaling and downstream targets, which may ultimately impact the hematopoietic transcriptional factor network to bias myelomonocytic differentiation in this MPN.

Reversible phosphorylation in haematological malignancies: potential role for protein tyrosine phosphatases in treatment?

Most aspects of leukocyte physiology are under the control of reversible tyrosine phosphorylation. It is clear that excessive phosphorylation of signal transduction elements is a pivotal element of many different pathologies including haematological malignancies and accordingly, strategies that target such phosphorylation have clinically been proven highly successful for treatment of multiple types of leukemias and lymphomas. Cellular phosphorylation status is dependent on the resultant activity of kinases and phosphatases. The cell biology of the former is now well understood; for most cellular phosphoproteins we now know the kinases responsible for their phosphorylation and we understand the principles of their aberrant activity in disease. With respect to phosphatases, however, our knowledge is much patchier. Although the sequences of whole genomes allow us to identify phosphatases using in silico methodology, whereas transcription profiling allows us to understand how phosphatase expression is regulated during disease, most functional questions as to substrate specificity, dynamic regulation of phosphatase activity and potential for therapeutic intervention are still to a large degree open. Nevertheless, recent studies have allowed us to make meaningful statements on the role of tyrosine phosphatase activity in the three major signaling pathways that are commonly affected in leukemias, i.e. the Ras-Raf-ERK1/2, the Jak-STAT and the PI3K-PKB-mTOR pathways. Lessons learned from these pathways may well be applicable elsewhere in leukocyte biology as well.

New insights into the mechanisms of hematopoietic cell transformation by activated receptor tyrosine kinases

A large number of alterations in genes encoding receptor tyrosine kinase (RTK), namely FLT3, c-KIT, platelet-derived growth factor (PDGF) receptors, fibroblast growth factor (FGF) receptors, and the anaplastic large cell lymphoma kinase (ALK), have been found in hematopoietic malignancies. They have drawn much attention after the development of tyrosine kinase inhibitors. RTK gene alterations include point mutations and gene fusions that result from chromosomal rearrangements. In both cases, they activate the kinase domain in the absence of ligand, producing a permanent signal for cell proliferation. Recently, this simple model has been refined. First, by contrast to wild-type RTK, many mutated RTK do not seem to signal from the plasma membrane, but from various locations inside the cell. Second, their signal transduction properties are altered: the pathways that are crucial for cell transformation, such as signal transducer and activator of transcription (STAT) factors, do not necessarily contribute to the physiologic functions of these receptors. Finally, different mechanisms prevent the termination of the signal, which normally occurs through receptor ubiquitination and degradation. Several mutations inactivating CBL, a key RTK E3 ubiquitin ligase, have been recently described. In this review, we discuss the possible links among RTK trafficking, signaling, and degradation in leukemic cells.

The fusion proteins TEL-PDGFRbeta and FIP1L1-PDGFRalpha escape ubiquitination and degradation

BACKGROUND

Chimeric oncogenes encoding constitutively active protein tyrosine kinases are associated with chronic myeloid neoplasms. TEL-PDGFRbeta (TPbeta, also called ETV6-PDGFRB) is a hybrid protein produced by the t(5;12) translocation, FIP1L1-PDGFRalpha (FPalpha) results from a deletion on chromosome 4q12 and ZNF198-FGFR1 is created by the t(8;13) translocation. These fusion proteins are found in patients with myeloid neoplasms associated with eosinophilia. Wild-type receptor tyrosine kinases are efficiently targeted for degradation upon activation, in a process that requires Cbl-mediated monoubiquitination of receptor lysines. Since protein degradation pathways have been identified as useful targets for cancer therapy, the aim of this study was to compare the degradation of hybrid and wild-type receptor tyrosine kinases.

DESIGN AND METHODS

We used Ba/F3 as a model cell line, as well as leukocytes from two patients, to analyze hybrid protein degradation.

RESULTS

In contrast to the corresponding wild-type receptors, which are quickly degraded upon activation, we observed that TPbeta, FPalpha and the ZNF198-FGFR1 hybrids escaped down-regulation in Ba/F3 cells. The high stability of TPbeta and FPalpha hybrid proteins was confirmed in leukocytes from leukemia patients. Ubiquitination of TPbeta and FPalpha was much reduced compared to that of wild-type receptors, despite marked Cbl phosphorylation in cells expressing hybrid receptors. The fusion of a destabilizing domain to TPbeta induced protein degradation. Instability was reverted by adding the destabilizing domain ligand, Shield1. The destabilization of this modified TPbeta reduced cell transformation and STAT5 activation.

CONCLUSIONS

We have shown that chimeric receptor tyrosine kinases escape ubiquitination and down-regulation and that their stabilization is critical to efficient stimulation of cell proliferation.

Tel/PDGFRbeta induces stem cell differentiation via the Ras/ERK and STAT5 signaling pathways

OBJECTIVE

Fusion genes involving the platelet-derived growth factor receptor-beta (PDGFRbeta) are found in a subgroup of myeloproliferative neoplasms, with one such fusion, Tel/PDGFRbeta found in a subset of chronic myelomonocytic leukemia patients. Tel/PDGFRbeta results in constitutive activation of several signaling pathways and induces a myeloproliferative disease in mice, with signals via tyrosines 579/581 identified as being important for this phenotype. In this study, we have used a tetracycline-regulated system to express wild-type and the mutated F2 Tel/PDGFRbeta to identify the key signaling pathways, which drive Tel/PDGFRbeta-induced differentiation of embryonic stem (ES) cells.

MATERIALS AND METHODS

The leukemic oncogene Tel/PDGFRbeta and Tel/PDGFRbeta-F2 were inducibly expressed in ES cells and their effects on self-renewal, signal transduction, and gene expression patterns analyzed.

RESULTS

Tel/PDGFRbeta activated several major signal transduction pathways (signal transducers and activators of transcription [STAT] 3, STAT5, mitogen-activated protein kinases, phosphatidylinositol-3 kinase) in ES cells, but only specific inhibition of the mitogen-activated protein kinase kinase/extracellular regulated kinase (MEK/ERK) or STAT5 pathways was able to significantly prevent Tel/PDGFRbeta-induced differentiation and restore ES-cell self-renewal. Inhibiting the tyrosine kinase activity of the oncogene using Gleevec or PDGFRbeta inhibitor III also substantially prevented Tel/PDGFRbeta-induced differentiation and its ability to upregulate key genes involved in myelopoiesis. Tyrosines 579/581 played a critical role in mediating signals via the Ras/ERK and STAT5 pathways, with dual targeting of the tyrosine kinase activity of Tel/PDGFRbeta and the MEK/ERK pathway completely preventing Tel/PDGFRbeta-induced differentiation.

CONCLUSION

These findings suggest that targeted disruption of key signaling pathways in combination with the tyrosine kinase activity of leukemic oncogenes, such as Tel/PDGFRbeta, may result in more efficacious therapies for suppressing leukemic progression in the clinical setting.

Id1 is a common downstream target of oncogenic tyrosine kinases in leukemic cells

Oncogenic tyrosine kinases, such as BCR-ABL, TEL-ABL, TEL-PDGFbetaR, and FLT3-ITD, play a major role in the development of hematopoietic malignancy. They activate many of the same signal transduction pathways. To identify the critical target genes required for transformation in hematopoietic cells, we used a comparative gene expression strategy in which selective small molecules were applied to 32Dcl3 cells that had been transformed to factor-independent growth by these respective oncogenic alleles. We identified inhibitor of DNA binding 1 (Id1), a gene involved in development, cell cycle, and tumorigenesis, as a common target of these oncogenic kinases. These findings were prospectively confirmed in cell lines and primary bone marrow cells engineered to express the respective tyrosine kinase alleles and were also confirmed in vivo in murine models of disease. Moreover, human AML cell lines Molm-14 and K562, which express the FLT3-ITD and BCR-ABL tyrosine kinases, respectively, showed high levels of Id1 expression. Antisense and siRNA based knockdown of Id1-inhibited growth of these cells associated with increased p27(Kip1) expression and increased sensitivity to Trail-induced apoptosis. These findings indicate that Id1 is an important target of constitutively activated tyrosine kinases and may be a therapeutic target for leukemias associated with oncogenic tyrosine kinases.

THOC5 spliceosome protein: a target for leukaemogenic tyrosine kinases that affects inositol lipid turnover

The fusion protein TEL/PDGFRB is associated with chronic myelomonocytic leukaemia and has intrinsic tyrosine kinase activity. The effects of TEL/PDGFRB were assessed using the multipotent haemopoietic cell line FDCP-Mix. In the absence of growth factors, TEL/PDGFRB expression increased survival that was associated with elevated levels of phosphatidylinositol 3,4,5 trisphosphate (PIP3). Whilst TEL/PDGFRB had subtle effects on the growth factor requirements it had a profound effect on differentiation. The cells became refractory to cytokine-stimulated development, showing limited maturation but failing to produce fully mature cells. We have previously identified the spliceosome protein THOC5 as a target in macrophage colony-stimulating factor signalling and a protein involved in the regulation of transcription factor expression. TEL/PDGFRB expression increased the expression and phosphorylation of THOC5. Elevated expression of THOC5 increased PIP3 levels and decreased apoptosis. Mass spectrometry was used to identify a site for TEL/PDGFRB-mediated phosphorylation on THOC5, which was shown to be a target for a number of other leukaemogenic tyrosine kinases. Thus, THOC5 is a novel target for modulation of signal transduction with a potential role in leukaemogenesis.

Roles for deregulated receptor tyrosine kinases and their downstream signaling molecules in hematologic malignancies

Growth, survival and differentiation of hematopoietic cells are regulated by the interactions between hematopoietic growth factors and their receptors. The defect in these interactions results in a failure of hematopoiesis, while aberrantly elevated and/or sustained activation of these signals cause hematologic malignancies. Among them, constitutively activating mutations of the receptor tyrosine kinases (RTKs), such as c-Kit, platelet-derived growth factor receptor (PDGFR) and FLT3, are often involved in the pathogenesis of various types of hematologic malignancies. Constitutive activation of RTKs is provoked by several mechanisms including chromosomal translocations and various mutations involving their regulatory regions. Chromosomal translocations commonly generate chimeric proteins consisting of the cytoplasmic domain of RTKs and the dimerization or multimerization motif of the fusion partner, resulting in the constitutive dimerization of RTKs. On the other hand, missense, insertion or deletion mutations in the regulatory regions, such as juxtamembrane domain, activation loop, and extracellular domain, also cause constitutive activation of RTKs mainly by preventing the auto-inhibitory regulation. Oncogenic RTKs activate downstream signaling molecules such as Ras/MAPK, PI3-K/Akt/mTOR, and STATs as well as ligand-activated wild type RTKs. However, their signals are quantitatively and qualitatively different from wild type RTKs. Based on these findings, several agents that target oncogenic RTKs or their downstream molecules have been developed: imatinib and FLT3 inhibitors for RTKs themselves, farnesyltransferase inhibitors, mTOR inhibitors and MEK inhibitors for the downstream signaling molecules. As promising results have been obtained in several clinical trials using these agents, the establishment of these molecular targeted agents is expected.

Myeloproliferative disease induced by TEL-PDGFRB displays dynamic range sensitivity to Stat5 gene dosage

Expression of the constitutively activated TEL/PDGFbetaR fusion protein is associated with the t(5;12)(q33;p13) chromosomal translocation found in a subset of patients with chronic myelomonocytic leukemia. TEL/PDGFbetaR activates multiple signal transduction pathways in cell-culture systems, and expression of the TEL-PDGFRB fusion gene induces myeloproliferative disease (MPD) in mice. We used gene-targeted mice to characterize the contribution of signal transducer and activator of transcription (Stat) and Src family genes to TEL-PDGFRB-mediated transformation in methylcellulose colony and murine bone marrow transduction/transplantation assays. Fetal liver hematopoietic stem and progenitor cells harboring targeted deletion of both Stat5a and Stat5b (Stat5ab(null/null)) genes were refractory to transformation by TEL-PDGFRB in methylcellulose colony assays. Notably, these cell populations were maintained in Stat5ab(null/null) fetal livers and succumbed to transformation by c-Myc. Surprisingly, targeted disruption of either Stat5a or Stat5b alone also impaired TEL-PDGFRB-mediated transformation. Survival of TPiGFP-->Stat5a(-/-) and TPiGFP-->Stat5a(+/-) mice was significantly prolonged, demonstrating significant sensitivity of TEL-PDGFRB-induced MPD to the dosage of Stat5a. TEL-PDGFRB-mediated MPD was incompletely penetrant in TPiGFP-->Stat5b(-/-) mice. In contrast, Src family kinases Lyn, Hck, and Fgr and the Stat family member Stat1 were dispensable for TEL-PDGFRB disease. Together, these data demonstrate that Stat5a and Stat5b are dose-limiting mediators of TEL-PDGFRB-induced myeloproliferation.

Fusion tyrosine kinase mediated signalling pathways in the transformation of haematopoietic cells

The fusion tyrosine kinases (FTKs) are generated by chromosomal translocations creating bipartite proteins in which the kinase is hyperactivated by an adjoining oligomerization domain. Autophosphorylation of the FTK generates a 'signalosome', an ensemble of signalling proteins that transduce signals to downstream pathways. At the earliest stages of oncogenesis, FTKs can mimic mitogenic cytokine signalling pathways involving the GAB-2 adaptor protein and signal transducers and activators of transcription (STAT) factors, generating replicative stress and thereby promoting a mutator phenotype. In parallel, FTKs couple to survival pathways that upregulate prosurvival proteins such as Bcl-xL, so preventing DNA-damage-induced apoptosis. Following transformation, FTKs induce resistance to genotoxic attack by upregulating DNA repair mechanisms such as STAT5-dependent RAD51 transcription. The phenomenon of 'oncogene addiction' reflects the continued requirement of an active FTK 'signalosome' to mediate survival and mitogenic signals involving the PI 3-kinase and mitogen-activated protein stress-activated protein kinase pathways, and the nuclear factor-kappa B, activator protein 1 and STAT transcription factors. The available data so far suggest that FTKs, with some possible exceptions, induce and maintain the transformed state using similar panoplies of signals, a finding with important therapeutic implications. The FTK signalling field has matured to an exciting phase in which rapid advances are facilitating rational drug design.

Classification of chronic myeloid disorders: From Dameshek towards a semi-molecular system

Hematological malignancies are phenotypically organized into lymphoid and myeloid disorders, although such a distinction might not be precise from the standpoint of lineage clonality. In turn, myeloid malignancies are broadly categorized into either acute myeloid leukemia (AML) or chronic myeloid disorder (CMD), depending on the presence or absence, respectively, of AML-defining cytomorphologic and cytogenetic features. The CMD are traditionally classified by their morphologic appearances into discrete clinicopathologic entities based primarily on subjective technologies. It has now become evident that most CMD represent clonal stem cell processes where the primary oncogenic event has been characterized in certain instances; Bcr/Abl in chronic myeloid leukemia, FIP1L1-PDGFRA or c-kit(D816V) in systemic mastocytosis, rearrangements of PDGFRB in chronic eosinophilic leukemia, and rearrangements of FGFR1 in stem cell leukemia/lymphoma syndrome. In addition, Bcr/Abl-negative classic myeloproliferative disorders are characterized by recurrent JAK2(V617F) mutations, whereas other mutations affecting the RAS signaling pathway molecules have been associated with juvenile myelomonocytic leukemia. Such progress is paving the way for a transition from a histologic to a semi-molecular classification system that preserves conventional terminology, while incorporating new information on molecular pathogenesis.

ETV6: a versatile player in leukemogenesis

Alterations of the ets family transcription factor ETV6 (TEL) and the RUNT domain transcription factor RUNX1 (AML1) play pivotal roles in the leukemogenesis of various types of leukemia. While only three fusion partners of RUNX1 namely ETO, ETV6 and MTG16 have been described so far, there is a plethora of ETV6 fusion partners with about 20 partners described so far. Apart from forming fusion genes there are other genetic alterations of ETV6 including deletions, point mutations and possible alterations at the promoter level that might contribute to the malignant phenotype. This review will focus on ETV6 and on the different mechanisms that are used by this gene to cause leukemia.

STATs as critical mediators of signal transduction and transcription: lessons learned from STAT5

Signal transducers and activators of transcription (Stats) comprise a family of seven transcription factors that are activated by a variety of cytokines, hormones and growth factors. Stats are activated through tyrosine phosphorylation, mainly by Jak kinases, that lead to their dimerization, nuclear translocation and regulation of target gene expression. Stat5 was originally identified as a transcription factor that regulates the beta-casein gene in response to prolactin (PRL), but Stat5 is activated also by several other cytokines and growth factors. The molecular mechanisms that underlie Stat5-mediated transcription involve interactions and cooperation with sequence specific transcription factors and transcriptional coregulators. Our studies identified p100 protein as a coactivator for Stat5, and suggest the existence of a positive regulatory loop in PRL-induced transcription, where PRL stabilizes p100 protein, which in turn can cooperate with Stat5 in transcriptional activation. Suppressors of cytokine signaling (SOCS) proteins are important negative regulators of Stats. A target gene for Stat5, the serine/threonine kinase Pim-1, was found to cooperate with SOCS-1 and SOCS-3 to inhibit Stat5 activity suggesting that Pim-1 together with SOCS-1 and SOCS-3 are components of a negative feedback mechanism that allows Stat5 to regulate its own activation.

Global effects of BCR/ABL and TEL/PDGFRbeta expression on the proteome and phosphoproteome: identification of the Rho pathway as a target of BCR/ABL

Many leukemic oncogenes form as a consequence of gene fusions or mutation that result in the activation or overexpression of a tyrosine kinase. To identify commonalities and differences in the action of two such kinases, breakpoint cluster region (BCR)/ABL and TEL/PDGFRbeta, two-dimensional gel electrophoresis was employed to characterize their effects on the proteome. While both oncogenes affected expression of specific proteins, few common effects were observed. A number of proteins whose expression is altered by BCR/ABL, including gelsolin and stathmin, are related to cytoskeletal function whereas no such changes were seen in TEL/PDGFRbeta-transfected cells. Treatment of cells with the kinase inhibitor STI571 for 4-h reversed changes in expression of some of these cytoskeletal proteins. Correspondingly, BCR/ABL-transfected cells were less responsive to chemotactic and chemokinetic stimuli than non-transfected cells and TEL/PDGFRbeta-transfected Ba/F3 cells. Decreased motile response was reversed by a 16-h treatment with STI571. A phosphoprotein-specific gel stain was used to identify TEL/PDGFRbeta and BCR/ABL-mediated changes in the phosphoproteome. These included changes on Crkl, Ras-GAP-binding protein 1, and for BCR/ABL, cytoskeletal proteins such as tubulin, and Nedd5. Decreased phosphorylation of Rho-GTPase dissociation inhibitor (Rho GDI) was also observed in BCR/ABL-transfected cells. This results in the activation of the Rho pathway, and treatment of cells with Y27632, an inhibitor of Rho kinase, inhibited DNA synthesis in BCR/ABL-transfected Ba/F3 cells but not TEL/PDGFRbeta-expressing cells. Expression of a dominant-negative RhoA inhibited both DNA synthesis and transwell migration, demonstrating the significance of this pathway in BCR/ABL-mediated transformation.

Positive and negative regulatory roles of the WW-like domain in TEL-PDGFbetaR transformation

TEL-platelet-derived growth factor-beta receptor (TEL-PDGFbetaR) is expressed in chronic myelomonocytic leukemias associated with t(5;12)(q33;p13), and the fusion tyrosine kinase retains a conserved WW-like domain in the PDGFbetaR autoinhibitory juxtamembrane region. Here we report that mutation of the 2 conserved tryptophan residues of the WW-like domain has opposing effects on TELPDGFbetaR kinase activation. Alanine substitution of W593, essential for protein-protein interaction in the context of other WW domains, impaired TEL-PDGFbetaR-mediated transformation of hematopoietic cells due to inhibition of TEL-PDGFbetaR kinase activity. In contrast, alanine substitution of W566, essential for structural integrity of WW domain in other contexts, had no effect on TEL-PDGFbetaR activation and oncogenic activity. Surprisingly, however, the W566A mutation suppressed the W593A phenotype. Double mutant W566A/W593A was indistinguishable from the wild-type fusion protein with regard to kinase activity, ability to confer factor-independent growth to Ba/F3 cells, or ability to induce a myeloproliferative disease in mice. Additional mutational analysis identified other substitutions within the WW-like domain in addition to W566A that could also suppress the W593A phenotype, including mutations predicted to diminish the autoinhibitory function of the juxtamembrane region. Therefore, the WW-like domain in the context of TELPDGFbetaR may have both positive and negative regulatory roles in kinase activation.

The Role of Signal Transducer and Activator of Transcription Factors in Leukemogenesis

Human leukemias are frequently associated with the aberrant expression of activated fusion tyrosine kinases or activated protein tyrosine kinases carrying insertional or point mutations. The activated kinase enzymes typically phosphorylate one or more signal transducer and activator of transcription (STAT) factors, which translocate to the cell nucleus and regulate the expression of genes associated with survival and proliferation. The phosphorylation and activation of STAT family members has been described in a wide range of human leukemias. Furthermore, animal models of leukemia have demonstrated the pivotal contribution of STAT activation to leukemic pathogenesis. This review discusses evidence for the functional importance of STAT activation in the biology of leukemia and current opportunities for modulating STAT proteins in the therapy of this group of diseases.

TEL, a putative tumor suppressor, induces apoptosis and represses transcription of Bcl-XL

The ETS family transcriptional repressor TEL is frequently disrupted by chromosomal translocations, including the t(12;21) in which the second allele of TEL is deleted in up to 90% of the cases. Consistent with its role as a putative tumor suppressor, TEL expression inhibits colony formation by Ras-transformed NIH 3T3 cells and hinders proliferation of a variety of cell types. Although we observed no alteration in the cell cycle of TEL-expressing cells, we did find a marked increase in apoptosis of serum-starved TEL-expressing NIH 3T3 cells. This decrease in cell survival required the DNA binding domain of TEL, suggesting that TEL repressed an anti-apoptotic gene. These observations prompted us to search for genes regulated by ETS family proteins that regulate apoptosis. The anti-apoptotic molecule Bcl-XL contains multiple ets-factor binding sites within its promoters, and TEL repressed a Bcl-XL promoter-linked reporter gene. Moreover, the enforced expression of TEL decreased the endogenous expression of both Bcl-XL mRNA and protein. TEL-mediated repression of Bcl-XL likely affects cell survival via regulation of the apoptotic pathway.

The coupling of TEL/PDGFbetaR to distinct functional responses is modulated by the presence of cytokine: involvement of mitogen-activated protein kinases

The TEL/PDGFbetaR oncogenic fusion protein is the product of the t(5;12)(q33; p13) translocation recurrently found in patients with chronic myelomonocytic leukemia (CMML). To investigate the coupling of molecular signaling events activated by TEL/PDGFbetaR to functional responses, we expressed TEL/PDGFbetaR in interleukin 3 (IL-3)-dependent BaF/3 cells using the tetracycline-regulated expression system. Induction of TEL/PDGFbetaR expression led to increased cell survival following IL-3 withdrawal and constitutive activation of protein kinase B (PKB), signal transducer and activator of transcription 5 (STAT5), extracellular signal-regulated kinases 1/2 (ERK1/2), Jun N-terminal kinases 1/2 (JNK1/2), and p38 mitogen-activated protein kinase (MAPK) pathways. However, inducible expression of TEL/PDGFbetaR failed to generate factor-independent cells, whereas constitutive expression of TEL/PDGFbetaR did, albeit at low frequency, suggesting the duration of TEL/PDGFbetaR expression is important for transformation. Surprisingly, in cells induced to express TEL/PDGFbetaR, IL-3-dependent growth was dramatically reduced as a result of increased apoptosis of cells receiving combined IL-3 and TEL/PDGFbetaR signals. We demonstrate that TEL/PDGFbetaR expression augmented IL-3-induced activation of PKB, STAT5, ERK1/2, p38, and JNK1/2. Inhibition of neither phosphoinositide-3 kinases nor p38 MAPKs reduced the inhibition of IL-3-driven proliferation observed when TEL/PDGFbetaR was expressed. However, inhibition of MEKs or JNKs partially reversed the combined inhibitory effects of TEL/PDGFbetaR and IL-3 on proliferation and survival. These results suggest that the combination of TEL/PDGFbetaR and IL-3-induced signals activate apoptosis through ERK and JNK MAPK-dependent pathways. Given that in vivo hematopoietic cells are in contact with a variety of cytokines, our results have important implications for cellular responses in the pathogenesis of CMML.

Systemic and primary cutaneous anaplastic large cell lymphomas

Anaplastic large cell lymphoma (ALCL) is a neoplasm of activated lymphocytes, commonly expressing T-cell antigens and cytotoxic proteins. Histopathology reveals distinctive infiltration of sinuses and paracortical T-cell-rich regions of lymph nodes by tumor cells which have abundant cytoplasm and large irregular/convoluted nuclei, and which are frequently multinucleated with prominent nucleoli. ALCL often presents in advanced clinical stages with B symptoms; extranodal disease occurs in 40% of patients. The pathogenesis of systemic ALCL is linked to phosphorylation of a tyrosine kinase (ALK) resulting in unregulated growth of affected lymphoid cells. ALK is activated through chromosomal translocations/inversions with any of several partner genes, most commonly nucleophosmin (NPM). Downstream signal transduction pathway(s) are not fully defined but appear to involve phospholipase Cgamma, phosphatidylinositol (PI)3K/Akt, and STAT-3 and STAT-5 proteins. Primary cutaneous ALCL appears to have a different pathogenesis and better prognosis than does systemic ALCL, presenting as one or more skin tumors, usually localized. Excision or local irradiation is usually effective treatment. A clinically benign variant of primary cutaneous ALCL is lymphomatoid papulosis (LyP), characterized by recurrent crops of papules/nodules up to 2 cm in diameter which undergo spontaneous regression. LyP is managed by observation, ultraviolet light therapy, or low-dose methotrexate. LyP patients have a predisposition to develop malignant lymphomas, including Hodgkin's lymphoma, mycosis fungoides, and non-Hodgkin's lymphoma, by as yet unknown mechanisms. The prognosis for patients with LyP is otherwise excellent.

Genetics of myeloid leukemias

Human leukemias are typified by acquired recurring chromosomal translocations. Cloning of these translocation breakpoints has provided important insights into pathogenesis of disease as well as novel therapeutic approaches. Chronic myelogenous leukemias (CML) are caused by constitutively activated tyrosine kinases, such as BCR/ABL, that confer a proliferative and survival advantage to hematopoietic progenitors but do not affect differentiation. These activated kinases are validated targets for therapy with selective tyrosine kinase inhibitors, a paradigm that may have broad applications in treatment of hematologic malignancies as well as solid tumors. Chromosomal translocations in acute myeloid leukemias (AML) most often result in loss-of-function mutations in transcription factors that are required for normal hematopoietic development. These latter mutations, however, are not sufficient to cause AML. The available evidence indicates that activating mutations in the hematopoietic tyrosine kinases FLT3 and c-KIT, and in N-RAS and K-RAS, confer proliferative advantage to hematopoietic progenitors and cooperate with loss-of-function mutations in hematopoietic transcription factors to cause an acute leukemia phenotype characterized by proliferation and impaired differentiation. The data supporting this hypothesis and the clinical and therapeutic implications of these observations are reviewed.

Identification of orally active, potent, and selective 4-piperazinylquinazolines as antagonists of the platelet-derived growth factor receptor tyrosine kinase family

We have previously found that the 4-[4-(N-substituted carbamoyl)-1-piperazinyl]-6,7-dimethoxyquinazolines can function as potent and selective inhibitors of platelet-derived growth factor receptor (PDGFR) phosphorylation. A series of highly potent, specific, orally active, small molecule kinase inhibitors directed against members of PDGFR receptor have been developed through modifications of the novel quinazoline template I. Systematic modifications in the A-bicyclic ring and D-rings of protype I were carried out to afford potent analogues, which display IC(50) values of <250 nM in cellular betaPDGFR phosphorylation assays. An optimized analogue in this series, 75 (CT53518), inhibits Flt-3, betaPDGFR, and c-Kit receptor phosphorylation with IC(50) values of 50-200 nM, whereas 15-20-fold less potent activity against CSF-1R was observed. This analogue also inhibits autophosphorylation of Flt-3 ligand-stimulated wild-type Flt-3 and a constitutively activated Flt-3/internal tandem duplication (ITD) with IC(50) values of 30-100 nM. Through this optimization process, 75 was found to be metabolically stable and has desirable pharmacokinetic properties in all animal species studied (F% > 50%, T(1/2) > 8 h). Oral administration of 75 promotes mice survival and significantly delayed disease progression in a Flt-3/ITD-mediated leukemia mouse model and shows efficacy in a nude mouse model of chronic myelomonocytic leukemia.

Activity of STI571 in chronic myelomonocytic leukemia with a platelet-derived growth factor beta receptor fusion oncogene

Platelet-derived growth factor beta receptor (PDGFbetaR) fusion genes have been shown to be critical transforming oncogenes in a subset of patients with chronic myelomonocytic leukemia (CMML). The sensitivity of dysregulated tyrosine kinase oncogenes to the tyrosine kinase inhibitor STI571 (imatinib mesylate) makes it a potentially attractive treatment option in this subset of patients. We have recently cloned a novel member of the PDGFbetaR fusion oncogene family, rabaptin-5-PDGFbetaR. A patient with CMML carrying the rabaptin-5-PDGFbetaR fusion gene underwent allogeneic stem cell transplantation (SCT) and was monitored closely with a sensitive reverse transcriptase-polymerase chain assay to detect the novel fusion gene transcript. After achieving a molecular remission at 5 months after transplantation, 15 months after SCT the patient showed persistent and progressive evidence of molecular relapse. After demonstrating in vitro that cells transformed with this specific fusion oncogene are efficiently killed by STI571, the patient was started on STI571. The patient responded rapidly and entered molecular remission after 6 weeks of therapy, and he continues to be in remission 6 months later. These results suggest that STI571 may be an effective targeted therapy in patients with CMML related to PDGFbetaR fusion oncogenes.

Tyrosine kinase fusion genes in chronic myeloproliferative diseases

With the exception of chronic myeloid leukemia (CML), chronic myeloproliferative disorders (CMPDs) are a heterogeneous spectrum of conditions for which the molecular pathogenesis is not well understood. Most cases have a normal or aneuploid karyotype, but a minority present with a reciprocal translocation that disrupts specific tyrosine kinase genes, most commonly PDGFRB or FGFR1. These translocations result in the production of constitutively active tyrosine kinase fusion proteins that deregulate hemopoiesis in a manner analogous to BCR-ABL. With the advent of targeted signal transduction therapy, an accurate clinical and molecular diagnosis of CMPDs has become increasingly important. Currently, patients with PDGFRB or ABL fusion genes are candidates for treatment with Imatinib (STI571), but it is likely that alternative strategies will be necessary for the treatment of most other patients.

Molecular, cytogenetic and genetic abnormalities in MDS and secondary AML

Myelodysplasia (MDS) is a clonal disease, which increases with age, suggesting that multiple steps are required for the evolution of the condition. Approximately 30% of MDS evolve into acute myelogenous leukemia (AML). In this review, we intend to delineate the genetic events, which may drive this sequence and therefore we will focus primarily on cytogenetic abnormalities where the genes have been identified and oncogenes and suppressor genes that have been implicated. In terms of the biological mechanisms, which characterise this process, it is generally thought that the MDS cell has impaired differentiation, and has increased apoptosis. As the disease progresses in addition, the cells have increased proliferation. As the disease evolves, the population of cells, which predominate remain immature, have decreased apoptosis and in many cases, upregulate anti-apoptotic genes and have deregulated proliferation as the number of blast cells increase. Etiological factors, which contribute to the development of leukemia, include therapeutic agents administered for a primary malignancy. The cytogenetic abnormalities, predisposition factors and genes involved in secondary leukemia will also be reviewed.

TEL/platelet-derived growth factor receptor beta activates phosphatidylinositol 3 (PI3) kinase and requires PI3 kinase to regulate the cell cycle

TEL/platelet-derived growth factor receptor beta (PDGF beta R) is the protein product of the t(5;12) translocation in chronic myelomonocytic leukemia. TEL/PDGF beta R transforms interleukin-3 (IL-3)-dependent Ba/F3 and 32D cells to IL-3 independence and induces a murine myeloproliferative disease in a bone marrow transplantation model of leukemogenesis. The fusion protein encodes a constitutively activated, cytoplasmic tyrosine kinase that activates multiple signal transduction pathways. To identify the signaling pathways that are necessary for transformation by TEL/PDGF beta R, transformed Ba/F3 and 32D cells were studied. TEL/PDGF beta R activates the kinase activity of phosphatidylinositol-3 (PI3) kinase and stimulates phosphorylation of its downstream substrates, including Akt and p70S6 kinase. Activation of this pathway requires the kinase activity of TEL/PDGF beta R and is inhibited by the PDGF beta R inhibitor, STI571. Furthermore, inhibition of PI3 kinase with the pharmacologic inhibitor, LY294002, inhibits growth of the transformed cells. Treated cells arrest in the G1 phase of the cell cycle within 16 hours but do not undergo apoptosis. To study the mechanism of cell cycle arrest by LY294002, the activity of the cdk4 complex, which regulates the transit of cells from the G1 to S phase in hematopoietic cells, was examined. Both STI571 and LY294002 lead to a decrease in the activity of cdk4 kinase activity and a decrease in expression of both Cyclin D2 and Cyclin E within several hours. These studies demonstrate the presence of a signaling pathway from TEL/PDGF beta R to PI3 kinase and subsequently to regulation of the cdk4 kinase complex. Activation of this pathway is necessary for transformation by TEL/PDGF beta R.

The TEL/PDGFbetaR fusion in chronic myelomonocytic leukemia signals through STAT5-dependent and STAT5-independent pathways

The TEL/PDGFbetaR gene, which encodes a fusion protein containing the ETS-family member TEL fused to the protein-tyrosine kinase domain of the platelet-derived growth factor receptor-beta (PDGFbetaR), confers interleukin 3 (IL-3)-independent growth on Ba/F3 hematopoietic cells. TEL/PDGFbetaR mutants have been generated that contain tyrosine-to-phenylalanine (Tyr-->Phe) substitutions at phosphorylation sites present in the native PDGFbetaR to assess the role of these sites in cell transformation by TEL/PDGFbetaR. Similar to previous findings in a murine bone marrow transplantation model, full transformation of Ba/F3 cells to IL-3-independent survival and proliferation required the TEL/PDGFbetaR juxtamembrane and carboxy terminal phosphorylation sites. In contrast to previous reports concerning comparable mutants in the native PDGFbetaR, each of the TEL/PDGFbetaR mutants is fully active as a protein-tyrosine kinase. Expression of the TEL/PDGFbetaR fusion protein causes hyperphosphorylation and activation of signal transducer and activator of transcription (STAT5), and this activation of STAT5 requires the juxtamembrane Tyr579 and Tyr581 in the TEL/PDGFbetaR fusion. Hyperphosphosphorylation of phospholipase Cgamma (PLCgamma) and the p85 subunit of phosphatidylinositol 3-kinase (PI3K) requires the carboxy terminal tyrosine residues of TEL/PDGFbetaR. Thus, full transformation of Ba/F3 cells by TEL/PDGFbetaR requires engagement of PI3K and PLCgamma and activation of STAT5. Taken together with the growth properties of cells transformed by the TEL/PDGFbetaR variants, these findings indicate that a minimal combination of these signaling intermediates contributes to hematopoietic transformation by the wild-type TEL/PDGFbetaR fusion. (Blood. 2001;98:3390-3397)

Transformation of interleukin-3-dependent cells without participation of Stat5/bcl-xL: cooperation of akt with raf/erk leads to p65 nuclear factor kappaB-mediated antiapoptosis involving c-IAP2

Tyrosine kinase oncogenes such as p210BCR-ABL activate multiple signal pathways. As a result, it is difficult to infer the functional relevance of a pathway acting alone or in cooperation with another. One or 2 second-tier kinases represented in the p21ras and phosphatidylinositol-3-kinase (PI-3-kinase) pathways (activated RafCAAX and gag-akt, respectively) were expressed in parental H7 interleukin-3 (IL-3)-dependent myeloid cells. IL-3-dependent cells served, independently, as recipients of p210BCR-ABL, which activated p21ras and PI-3-kinase pathways, including raf/erk and akt, respectively, en route to transformation. By contrast, neither RafCAAX nor gag-akt when expressed in parental cells in isolation produced factor-independent cells. On the other hand, H7 cells expressing both RafCAAX and gag-akt (H7gag-akt/RafCAAX) were transformed. Such transformation in H7gag-akt/RafCAAX was accomplished in the absence of active versions of Shc or cbl, and there was no evidence of Stat activity and only modest amounts of bcl-xL, a Stat5 transcriptional target protein, all of which characterized the cells transformed by BCR-ABL. However, H7gag-akt/RafCAAX cells and H7BCR-ABL cells cultured in the absence of IL-3 shared strikingly increased p65 nuclear factor kappaB (NFkappaB) activity. Treatment of cells with a specific NFkappaB inhibitor, parthenolide, led to loss of NFkappaB activity and down-regulation of antiapoptotic c-IAP2. In cells with only gag-akt/RafCAAX, this was sufficient to allow polyADP ribosyltransferase (PARP)-degradative apoptosis, but in cells with p210BCR-ABL, apoptosis was blocked, possibly by a Stat5/bcl-xL-dependent mechanism. Therefore, one hematopoietic antiapoptotic program, among others, available to certain tyrosine kinase oncogenes involves a cooperative response between raf/erk and akt, unambiguous components of p21ras and PI-3-kinase pathways, to induce p65 NFkappaB and c-IAP2.

Regulation of the Src homology 2-containing inositol 5-phosphatase SHIP1 in HIP1/PDGFbeta R-transformed cells

It has been shown previously that the Huntingtin interacting protein 1 gene (HIP1) was fused to the platelet-derived growth factor beta receptor gene (PDGFbetaR) in leukemic cells of a patient with chronic myelomonocytic leukemia. This resulted in the expression of the chimeric HIP1/PDGFbetaR protein, which oligomerizes, is constitutively tyrosine-phosphorylated, and transforms the Ba/F3 murine hematopoietic cell line to interleukin-3-independent growth. Tyrosine phosphorylation of a 130-kDa protein (p130) correlates with transformation by HIP1/PDGFbetaR and related transforming mutants. We report here that the p130 band is immunologically related to the 125-kDa isoform of the Src homology 2-containing inositol 5-phosphatase, SHIP1. We have found that SHIP1 associates and colocalizes with the HIP1/PDGFbetaR fusion protein and related transforming mutants. These mutants include a mutant that has eight Src homology 2-binding phosphotyrosines mutated to phenylalanine. In contrast, SHIP1 does not associate with H/P(KI), the kinase-dead form of HIP1/PDGFbetaR. We also report that phosphorylation of SHIP1 by HIP1/PDGFbetaR does not change its 5-phosphatase-specific activity. This suggests that phosphorylation and possible PDGFbetaR-mediated sequestration of SHIP1 from its substrates (PtdIns(3,4,5)P(3) and Ins(1,3,4,5)P(4)) might alter the levels of these inositol-containing signal transduction molecules, resulting in activation of downstream effectors of cellular proliferation and/or survival.

Socs-1 inhibits TEL-JAK2-mediated transformation of hematopoietic cells through inhibition of JAK2 kinase activity and induction of proteasome-mediated degradation

TEL-JAK2 fusion proteins, which are a result of t(9;12)(p24;p13) translocations associated with human leukemia, activate Stat5 in vitro and in vivo and cause a myelo- and lymphoproliferative disease in a murine bone marrow transplant model. We report that Socs-1, a member of the SOCS family of endogenous inhibitors of JAKs and STATs, inhibits transformation of Ba/F3 cells by TEL-JAK2 but has no effect on Ba/F3 cells transformed by BCR-ABL, TEL-ABL, or TEL-platelet-derived growth factor receptor beta. TEL-JAK2, in addition to activating Stat5, associates with Shc and Grb2 and induces activation of Erk2, and expression of Socs-1 inhibits engagement of each of these signaling molecules. TEL-JAK2 kinase activity is inhibited by Socs-1, as assessed by in vitro kinase assays. In addition, Socs-1 induces proteasomal degradation of TEL-JAK2. Mutational analysis indicates that the SOCS box of Socs-1 is required for proteasomal degradation and for abrogation of growth of TEL-JAK2-transformed cells. Furthermore, murine bone marrow transplant assays demonstrate that expression of Socs-1 prolongs latency of TEL-JAK2-mediated disease in vivo. Collectively, these data indicate that Socs-1 inhibits TEL-JAK2 in vitro and in vivo through inhibition of kinase activity and induction of TEL-JAK2 protein degradation.

Analysis of the molecular genetics of acute promyelocytic leukemia in mouse models

Acute promyelocytic leukemia (APL) is characterized by reciprocal chromosomal translocations that always Involve the retinoic acid receptor-alpha (RARalpha) gene on chromosome 17. RARalpha variably fuses to the PML, PLZF, NPM, NuMA, and STAT 5b genes (X genes), leading to the generation of X-RARalpha and RARalpha-X fusion genes. The aberrant X-RARalpha proteins retain the dimerization domains of their parental proteins and therefore can act as dominant negative oncogenic products on both RARalpha/RXR and X pathways. Studies in transgenic mice harboring X-RARalpha and RARalpha-X fusion genes and In mice lacking X genes have helped unravel the molecular mechanisms underlying APL leukemogenesis, which lead to the development of novel therapeutic strategies. Moreover, transgenic mouse models of APL were useful to test in vivo the efficacy of these novel therapeutic approaches as well as of drug combinations such as retinoic acid and As2O3 that were previously known to be effective as single agents in human APL.

Modeling Acute Promyelocytic Leukemia in the Mouse: New Insights in the Pathogenesis of Human Leukemias

Acute promyelocytic leukemia (APL) is characterized by the expansion of malignant myeloid cells blocked at the promyelocytic stage of differentiation and is associated with reciprocal chromosomal translocations always involving the retinoic acid receptor alpha (RARalpha) gene on chromosome 17. As a consequence of the translocation, RARalpha variably fuses to the PML, PLZF, NPM, NuMA, and Stat5b genes (X genes), respectively, leading to the generation of RARalpha-X and X-RARalpha fusion genes. The aberrant chimeric proteins encoded by these genes, as well as the inactivation of the X and RARalpha functions, may exert a crucial role in leukemogenesis. To define the molecular genetics of APL and the contribution of each molecular event in APL pathogenesis, we have generated transgenic mice harboring X-RARalpha and/or RARalpha-X genes as well as mice where the various X genes have been inactivated by homologous recombination. Here we show that while the X-RARalpha fusion gene is crucial for leukemogenesis, the presence of RARalpha-X and the inactivation of X function are critical in modulating the onset as well as the phenotype of the leukemia.

Kinases: positive and negative regulators of apoptosis

Cells sense and respond to extracellular factors via receptors on the cell surface that trigger intracellular signaling pathways. The signals received by the receptors on hematopoietic cells often determine if the cell proliferates, survives or undergoes apoptosis. Apoptosis can be induced by almost any cytotoxic stimuli. These stimuli may be an absence of signals arising from cellular receptors, stimulation of specific ligand receptors on the cell surface, chemotherapeutic agents, and ionizing radiation or oxygen radicals, as well as a number of other factors. Cellular kinases and phosphatases participate in signaling cascades that influence this process. We review the ability of the calmodulin-dependent-kinases, I-kappaB kinases, PI3-kinases, Jakkinases, PKC, PKA, and MAP kinase signaling pathways (Erk, Jnk, and p38), to influence the apoptotic process. In addition, we discuss the cross-talk that exists between signaling cascades that are pro-apoptotic and anti-apoptotic.