ZNF198-FGFR1 transforms Ba/F3 cells to growth factor independence and results in high level tyrosine phosphorylation of STATS 1 and 5

The 8p11 myeloproliferative syndrome: Genotypic and phenotypic classification and targeted therapy

EMS(8p11 myeloproliferative syndrome, EMS) is an aggressive hematological neoplasm with/without eosinophilia caused by a rearrangement of the FGFR1 gene at 8p11-12. It was found that all cases carry chromosome abnormalities at the molecular level, not only the previously reported chromosome translocation and insertion but also a chromosome inversion. These abnormalities produced 17 FGFR1 fusion genes, of which the most common partner genes are ZNF198 on 13q11-12 and BCR of 22q11.2. The clinical manifestations can develop into AML (acute myeloid leukemia), T-LBL (T-cell lymphoblastic lymphoma), CML (chronic myeloid leukemia), CMML (chronic monomyelocytic leukemia), or mixed phenotype acute leukemia (MPAL). Most patients are resistant to traditional chemotherapy, and a minority of patients achieve long-term clinical remission after stem cell transplantation. Recently, the therapeutic effect of targeted tyrosine kinase inhibitors (such as pemigatinib and infigratinib) in 8p11 has been confirmed in vitro and clinical trials. The TKIs may become an 8p11 treatment option as an alternative to hematopoietic stem cell transplantation, which is worthy of further study.

Mechanisms of resistance to FGFR1 inhibitors in FGFR1-driven leukemias and lymphomas: implications for optimized treatment

Myeloid and lymphoid neoplasms with eosinophilia and FGFR1 rearrangements (MLN-eo FGFR1) disease is derived from a pluripotent hematopoietic stem cell and has a complex presentation with a myeloproliferative disorder with or without eosinophilia and frequently presents with mixed lineage T- or B-lymphomas. The myeloproliferative disease frequently progresses to AML and lymphoid neoplasms can develop into acute lymphomas. No matter the cell type involved, or clinical presentation, chromosome translocations involving the FGFR1 kinase and various partner genes, which leads to constitutive activation of downstream oncogenic signaling cascades. These patients are not responsive to treatment regimens developed for other acute leukemias and survival is poor. Recent development of specific FGFR1 inhibitors has suggested an alternative therapeutic approach but resistance is likely to evolve over time. Mouse models of this disease syndrome have been developed and are being used for preclinical evaluation of FGFR1 inhibitors. Cell lines from these models have now been developed and have been used to investigate the mechanisms of resistance that might be expected in clinical cases. So far, a V561M mutation in the kinases domain and deletion of PTEN have been recognized as leading to resistance and both operate through the PI3K/AKT signaling axis. One of the important consequences is the suppression of PUMA, a potent enforcer of apoptosis, which operates through BCL2. Targeting BCL2 in the resistant cells leads to suppression of leukemia development in mouse models, which potentially provides an opportunity to treat patients that become resistant to FGFR1 inhibitors. In addition, elucidation of molecular mechanisms underlying FGFR1-driven leukemias and lymphomas also provides new targets for combined treatment as another option to bypass the FGFR1 inhibitor resistance and improve patient outcome.

Avadomide induces degradation of ZMYM2 fusion oncoproteins in hematologic malignancies

Thalidomide analogs exert their therapeutic effects by binding to the CRL4^CRBN E3 ubiquitin ligase, promoting ubiquitination and subsequent proteasomal degradation of specific protein substrates. Drug-induced degradation of IKZF1 and IKZF3 in B-cell malignancies demonstrates the clinical utility of targeting disease-relevant transcription factors for degradation. Here, we found that avadomide (CC-122) induces CRBN-dependent ubiquitination and proteasomal degradation of ZMYM2 (ZNF198), a transcription factor involved in balanced chromosomal rearrangements with FGFR1 and FLT3 in aggressive forms of hematologic malignancies. The minimal drug-responsive element of ZMYM2 is a zinc-chelating MYM domain and is contained in the N-terminal portion of ZMYM2 that is universally included in the derived fusion proteins. We demonstrate that avadomide has the ability to induce proteasomal degradation of ZMYM2-FGFR1 and ZMYM2-FLT3 chimeric oncoproteins, both in vitro and in vivo. Our findings suggest that patients with hematologic malignancies harboring these ZMYM2 fusion proteins may benefit from avadomide treatment.

FGFR1 fusion kinase regulation of MYC expression drives development of stem cell leukemia/lymphoma syndrome

Oncogenic transformation of hematopoietic stem cells by chimeric fusion kinases causing constitutive activation of FGFR1 leads to a stem cell leukemia/lymphoma (SCLL) syndrome, accompanied by widespread dysregulation of gene activity. We now show that FGFR1 activation is associated with upregulation of MYC and pharmacological suppression of FGFR1 activation leads to downregulation of MYC and suppression of MYC target genes. Luciferase reporter assays demonstrate that FGFR1 can directly regulate MYC expression and this effect is enhanced in the presence of chimeric FGFR1 kinases. In SCLL cells, a truncated form of FGFR1 is generated by granzyme B cleavage of the chimeric kinases, producing a nucleus-restricted derivative that can bind MYC regulatory regions. Mutation of the granzyme B cleavage site prevents relocation to the nucleus but does not suppress MYC activation, suggesting additional mechanisms of MYC activation in the presence of cytoplasm-restricted chimeric kinases. We show that one of these mechanisms involves activating cytoplasmic STAT5, which upregulates MYC independent of the truncated FGFR1 kinase. Targeting MYC function using shRNA knockdown and 10054-F8 in SCLL cells leads to inhibition of cell proliferation and synergizes with the BGJ398 FGFR1 inhibitor, suggesting a combination therapy that could be used in the treatment of SCLL.

Myeloid/lymphoid neoplasms with FGFR1 rearrangement

Myeloid/lymphoid neoplasms with FGFR1 rearrangement are a rare entity. We present a multicenter experience of 17 patients with FISH-confirmed FGFR1 rearrangement. The clinical presentation at diagnosis included myeloproliferative neoplasm (MPN) in 4 (24%) patients, acute leukemia (AL) in 7 (41%), and concomitant MPN with AL in 6 (35%). The two most frequently observed cytogenetic abnormalities were t(8;13)(p11.2;q12)(partner gene ZMYM2) and t(8;22)(p11.2; q11.2)(BCR). Seventy-eight percent of tested patients had a RUNX1 mutation, of whom all had AL. Overall response rate to frontline therapy was 69%, and 76% of patients subsequently received allogeneic stem cell transplant (ASCT). After a median follow-up of 11 months, median progression-free survival was 15 months and median overall survival was not reached. In conclusion, FGFR1-rearranged hematologic malignancies present with features of MPN and/or AL. FGFR1 and RUNX1 are therapeutic targets for ongoing and future clinical trials.

Pediatric 8p11 eosinophilic myeloproliferative syndrome (EMS): A case report and review of the literature

The 8p11 eosinophilic myeloproliferative syndrome (EMS) is an aggressive neoplasm driven by translocation of the fibroblast growth factor receptor 1 and often transforms to leukemias and lymphomas that are refractory to treatment. The first case was identified in 1983, and to date over 70 cases have been reported in the literature. Despite those reports, no consensus exists on management of this condition, and inconsistency in treatment regimens is even more pronounced in the pediatric literature. We report a case of a male infant with the 8p11 EMS, review the published pediatric experience with EMS, and discuss treatment strategies for this enigmatic hematological disorder.

Myeloid neoplasms with eosinophilia

Molecular diagnostics has generated substantial dividends in dissecting the genetic basis of myeloid neoplasms with eosinophilia. The family of diseases generated by dysregulated fusion tyrosine kinase (TK) genes is recognized by the World Health Organization (WHO) category, “Myeloid/lymphoid neoplasms with eosinophilia and rearrangement of PDGFRA, PDGFRB, or FGFR1, or with PCM1-JAK2.” In addition to myeloproliferative neoplasms (MPN), these patients can present with myelodysplastic syndrome/MPN, as well as de novo or secondary mixed-phenotype leukemias or lymphomas. Eosinophilia is a common, but not invariable, feature of these diseases. The natural history of PDGFRA- and PDGFRB-rearranged neoplasms has been dramatically altered by imatinib. In contrast, patients with FGFR1 and JAK2 fusion TK genes exhibit a more aggressive course and variable sensitivity to current TK inhibitors, and in most cases, long-term disease-free survival may only be achievable with allogeneic hematopoietic stem cell transplantation. Similar poor prognosis outcomes may be observed with rearrangements of FLT3 or ABL1 (eg, both of which commonly partner with ETV6), and further investigation is needed to validate their inclusion in the current WHO-defined group of eosinophilia-associated TK fusion-driven neoplasms. The diagnosis chronic eosinophilic leukemia, not otherwise specified (CEL, NOS) is assigned to patients with MPN with eosinophilia and nonspecific cytogenetic/molecular abnormalities and/or increased myeloblasts. Myeloid mutation panels have identified somatic variants in patients with a provisional diagnosis of hypereosinophilia of undetermined significance, reclassifying some of these cases as eosinophilia-associated neoplasms. Looking forward, one of the many challenges will be how to use the results of molecular profiling to guide prognosis and selection of actionable therapeutic targets.

B-lymphoblastic leukemia/lymphoma associated with t(8;13)(p11;q12)/ ZMYM2 (ZNF198)-FGFR1 : rare case and review of the literature

Myeloid and lymphoid neoplasms with fibroblastic growth factor receptor-1 (FGFR1) abnormalities originate from mutated pluripotent stem cells and have a heterogeneous clinical presentation. There are 12 identified partner genes commonly involved in FGFR1 translocation at an 8p11 breakpoint. In FGFR1-related neoplasms, T-lymphoblastic lymphoma with eosinophilia is the most common clinical scenario, whereas acute B-lymphoblastic leukemia/lymphoma (B-ALL/LBL) is rare. To date, only 7 cases of B-ALL/LBL with FGFR1 abnormalities have been reported. Here, we report an additional case of a 64-year-old gentleman with leukocytosis, eosinophilia and diffuse mediastinal and general lymphadenopathy. Bone marrow examination showed patchy infiltrates of immature precursors/blasts, along with myeloid/eosinophilic hyperplasia. Immunophenotyping confirmed increased B lymphoblasts (30-40%). Karyotyping revealed cytogenetic abnormalities, including t(8;13)(p11;q12)/ZMYM2 (ZNF198)-FGFR1 and trisomy 21. The patient did not respond to hyper-CVAD chemotherapy and within 4 months developed acute myelomonocytic leukemia and expired 11 months after the initial diagnosis. Similar cases from the literature are reviewed.

Ponatinib as targeted therapy for FGFR1 fusions associated with the 8p11 myeloproliferative syndrome

The 8p11 myeloproliferative syndrome is a rare, aggressive myeloproliferative neoplasm characterized by constitutively active FGFR1 fusion proteins that arise from specific chromosomal translocations and which drive aberrant proliferation. Although FGFR1 inhibitors have shown in vitro activity against FGFR1 fusions, none are in use clinically and there is a need to assess additional compounds as potential therapy. Here we use cell lines and primary cells to investigate ponatinib (AP24534). Ponatinib-treated Ba/F3 cells transformed by ZMYM2-FGFR1 and BCR-FGFR1 and the FGFR1OP2-FGFR1 positive KG1A cell line showed reduced proliferation and decreased survival when compared to control cells. Inhibition induced apoptosis and reduced phosphorylation of the FGFR1 fusion proteins and substrates. Ponatinib-treated cells from 8p11 myeloproliferative syndrome patients (n=5) showed reduced colony growth compared to controls. In one evaluable patient, ponatinib specifically reduced numbers of FGFR1-fusion gene positive colonies. Ponatinib, therefore, shows considerable promise for the treatment of patients with 8p11 myeloproliferative syndrome.

Ruxolitinib as potential targeted therapy for patients with JAK2 rearrangements

JAK2 fusion genes are rare but recurrent abnormalities associated with diverse, clinically heterogeneous hematologic malignancies. Here we assess the JAK1/2 inhibitor ruxolitinib as therapy for patients with JAK2-rearrangement associated myeloproliferative neoplasms (MPN). Ruxolitinib-treated Ba/F3 cells transformed to IL3 independence by ETV6-JAK2 showed reduced proliferation and survival (IC(50) = 370 nM) compared with KG1A or Ba/F3 cells transformed by BCR-ABL1, SPBN1-FLT3 and ZMYM2-FGFR1 (IC(50) > 10 μM for all). Inhibition was associated with reduced phosphorylation of ETV6-JAK2, ERK, STAT5 and AKT. Primary cell growth from 2 patients with JAK2 rearrangement and one patient with JAK2 amplification was assessed in methylcellulose assays. Reduced colony growth was seen for all patients in ruxolitinib-treated cultures compared with healthy controls (n=7). Fluorescence in situ hybridization showed reduced growth of JAK2-rearrangement positive colonies compared to JAK2-rearrangement negative colonies. Our data, therefore, provide evidence that ruxolitinib is a promising therapy for treatment of patients with JAK2 fusion genes.

Comparative pre-clinical evaluation of receptor tyrosine kinase inhibitors for the treatment of multiple myeloma

BACKGROUND

Fibroblast growth factor receptor 3 (FGFR3) is up-regulated as a result of the t(4;14)(p16;q32) translocation that occurs in up to 20% of multiple myeloma (MM) patients. Recent studies have demonstrated that up-regulation of FGFR3 promotes cell survival, growth and drug resistance in malignant plasma cells, both in vitro and in vivo. Therefore, inhibition of FGFR3 signalling is potential target for the chemotherapeutic intervention in t(4;14) MM.

METHODS

Small molecule receptor tyrosine kinase inhibitors (PD173074, sunitinib (SU-11248), vandetanib (ZD6474) and vatalanib (PTK-787)) with varying degrees of inhibitory activity and selectivity against FGFR, were assessed in Ba/f3 cells expressing ZNF198-FGFR1 and MM cell lines. Cell viability, FGFR3 and ZNF198-FGFR1 phosphorylation and apoptosis were evaluated by growth inhibition assays, immunoblotting and fluorescence-activated cell sorting analysis, respectively. An in vivo study was performed with sunitinib in t(4;14)-positive and t(4;14)-negative human MM tumour xenograft models.

RESULTS

PD173074 and sunitinib differentially inhibited the growth of Ba/f3 cells expressing ZNF198-FGFR1 (GI(50)=10 nM and 730 nM, versus GI(50) >1 μM and 2.7 μM for parental cells; p<0.0001) and t(4;14) positive MM cell lines (GI(50)=4-10 μM and 1-3 μM, versus GI(50)=14-15 μM and 4-5 μM for t(4;14) negative MM cells; p≤0.002). In addition, both PD173074 and sunitinib inhibited the activation of FGFR3 in t(4;14)-positive MM cells. PD173074 and sunitinib induced an apoptotic response in a concentration and time-dependent manner in a t(4;14)-positive (PD174073 and sunitinib) but not a t(4;14)-negative MM cell line (sunitinib only); however, in in vivo tumours derived from the same cell lines, sunitinib was only active in the t(4;14)-negative model.

CONCLUSIONS

These data demonstrate that PD173074 and sunitinib are inhibitors of FGFR3 in MM cell lines, and that sunitinib has in vivo activity in a human MM tumour xenograft model. However, caution should be exercised in using the t(4;14) translocation as a predictive biomarker for patient selection in clinical trials with sunitinib.

Identification of protein tyrosine kinases with oncogenic potential using a retroviral insertion mutagenesis screen

Protein tyrosine kinases form a large family of signaling proteins implicated in both normal and malignant cell signaling. The aim of this study was to identify protein tyro-sine kinases that can transform hematopoietic cells to growth factor independent proliferation when constitutively activated by homodimerization. We used a modified retroviral insertion mutagenesis screen with a retroviral vector containing the homodimerization domain of ETV6 followed by an artificial splice donor site. Integration of this retroviral vector within a gene of the host genome would generate a fusion transcript containing the dimerization domain and part of the disrupted gene. Using this strategy with the IL3 dependent Ba/F3 cell line, we identified 8 different protein tyrosine kinases (Abl1, Fgfr1, Hck, Jak2, Lck, Mertk, Mst1r, Tnk1) that transformed the cells. These results characterize HCK, MERTK, MST1R and TNK1 as potential oncogenes and describe a method to identify gain-of-function fusion genes using a retroviral insertion screen.

Targeting mutant fibroblast growth factor receptors in cancer

Fibroblast growth factor receptors (FGFRs) play diverse roles in the control of cell proliferation, cell differentiation, angiogenesis and development. Activating the mutations of FGFRs in the germline has long been known to cause a variety of skeletal developmental disorders, but it is only recently that a similar spectrum of somatic FGFR mutations has been associated with human cancers. Many of these somatic mutations are gain-of-function and oncogenic and create dependencies in tumor cell lines harboring such mutations. A combination of knockdown studies and pharmaceutical inhibition in preclinical models has further substantiated genomically altered FGFR as a therapeutic target in cancer, and the oncology community is responding with clinical trials evaluating multikinase inhibitors with anti-FGFR activity and a new generation of specific pan-FGFR inhibitors.

Modeling the human 8p11-myeloproliferative syndrome in immunodeficient mice

The 8p11 myeloproliferative syndrome (EMS), also referred to as stem cell leukemia/lymphoma, is a chronic myeloproliferative disorder that rapidly progresses into acute leukemia. Molecularly, EMS is characterized by fusion of various partner genes to the FGFR1 gene, resulting in constitutive activation of the tyrosine kinases in FGFR1. To date, no previous study has addressed the functional consequences of ectopic FGFR1 expression in the potentially most relevant cellular context, that of normal primary human hematopoietic cells. Herein, we report that expression of ZMYM2/FGFR1 (previously known as ZNF198/FGFR1) or BCR/FGFR1 in normal human CD34(+) cells from umbilical-cord blood leads to increased cellular proliferation and differentiation toward the erythroid lineage in vitro. In immunodeficient mice, expression of ZMYM2/FGFR1 or BCR/FGFR1 in human cells induces several features of human EMS, including expansion of several myeloid cell lineages and accumulation of blasts in bone marrow. Moreover, bone marrow fibrosis together with increased extramedullary hematopoiesis is observed. This study suggests that FGFR1 fusion oncogenes, by themselves, are capable of initiating an EMS-like disorder, and provides the first humanized model of a myeloproliferative disorder transforming into acute leukemia in mice. The established in vivo EMS model should provide a valuable tool for future studies of this disorder.

Molecular monitoring of 8p11 myeloproliferative syndrome in an infant

The 8p11 myeloproliferative syndrome is a rare hematologic malignancy derived from a pluripotent hematopoietic stem cell associated with rearrangements involving the fibroblast growth factor receptor 1 (FGFR1) gene located on chromosome 8p11. The most common translocation, t(8;13) (p11;q13), results in a ZNF198-FGFR1 fusion gene and constitutively active FGFR1 tyrosine kinase activity. Typical pathologic findings include myeloid hyperplasia, lymphadenopathy, precursor T-lymphoblastic lymphoma, and eosinophilia. The disease is usually associated with an aggressive course and progression to acute myeloid leukemia is frequent. We report here the first case of 8p11 myeloproliferative syndrome in an infant and demonstrate the value of molecular testing in the diagnosis and minimal disease monitoring of this rare disease.

Phosphorylation of the SSBP2 and ABL proteins by the ZNF198-FGFR1 fusion kinase seen in atypical myeloproliferative disorders as revealed by phosphopeptide-specific MS

The ZNF198-fibroblast growth factor receptor-1 (FGFR1) fusion kinase is a constitutively activated tyrosine kinase associated with a specific atypical myeloproliferative disease. The chimeric protein localizes to the cytoplasm, unlike the wild type FGFR1 receptor kinase, and presumably inappropriately phosphorylates specific targets as part of the oncogenic signaling cascade. Other than known targets of the FGFR1 kinase itself, few specific targets of ZNF198-FGFR1 have been identified. Using a genetically engineered HEK 293 cell system, we have identified proteins that are specifically phosphorylated in the presence of the fusion kinase using anti-phosphotyrosine immunoprecipitation and MS. Compared with 293 cells expressing exongenous wild type FGFR1, ZNF198-FGFR1 is associated with phosphorylation of several proteins including SSBP2, ABL, FLJ14235, CALM and TRIM4 proteins. The specificity of the phosphorylation events in the SSBP2 and ABL proteins, which have previously been implicated in leukemogenesis, was further confirmed independently using immunoprecipitation with protein-specific antibodies and Western blotting. The MS analysis also identified the phosphorylation events in the ZNF198 moiety in the chimeric protein that might be related to its function. These studies identify the intersection of several different leukemia-related pathways in the development of this myeloproliferative disorder and provide new insights into the substrates of FGFR1 under defined conditions.

HSPA1A is an important regulator of the stability and function of ZNF198 and its oncogenic derivative, ZNF198-FGFR1

Mass spectroscopy analysis demonstrated that the HSPA1A protein is found in complex with the ZNF198 protein which is involved in a chromosome rearrangement with the FGFR1 gene in an atypical myeloproliferative disease. HSPA1A is a member of the HSP70 family of genes which has been shown to be inducible in a variety of circumstances. Exogenous expression of the ZNF198-FGFR1 fusion kinase gene as well as ZNF198 in a model cell system results in a large (>650-fold) increase in HSP70 mRNA levels. Using KNK437, a specific inhibitor of HSP70 transcription, we have demonstrated that an important function of HSPA1A is to stabilize the ZNF198 and ZNF198-FGFR1 proteins. In the absence of HSPA1A, specific functions of ZNF198-FGFR1 such as STAT3 phosphorylation is also lost. Treatment of cells with KNK437 in the presence of MG132, an inhibitor of proteasomal degradation of proteins, suggested that only the ZNF198-FGFR1 protein is subject to the proteasomal degradation pathway, while ZNF198 is not. These observations suggest an important role for HSPA1A in ZNF198 and ZNF198-FGFR1 mediated cellular function.

Stat5 as a diagnostic marker for leukemia

The Jak-Stat-Socs pathway is an important component of cytokine receptor signaling. Not surprisingly, perturbation of this pathway is implicated in diseases of hematopoietic and immune origin, including leukemia, lymphoma and immune deficiencies. This review examines the role of a key component of this pathway, Stat5. This has been shown to be activated in a variety of leukemias and myeloproliferative disorders, including downstream of a range of key oncogenes where it has been shown to play an important role in mediating their effects. Therefore, Stat5 represents a useful pan-leukemia/myeloproliferative disorder diagnostic marker and key therapeutic end point, as well as representing an attractive therapeutic target for these disorders.

The 8p11 myeloproliferative syndrome: review of literature and an illustrative case report

The 8p11 myeloproliferative syndrome (EMS), also called stem cell leukemia/lymphoma (SCLL), is a relatively rare condition characterized in its typical form by the occurrence, either simultaneously or sequentially, of a bcr/abl-negative myeloproliferative disorder and a lymphoma, usually a precursor T lymphoblastic lymphoma. The disease most often terminates in acute myeloid leukemia which is resistant to conventional chemotherapy. The defining cytogenetic abnormality, a translocation at the 8p11 locus, always involves the fibroblast growth factor 1 (FGFR1) gene. To date, eight partner genes have been identified in association with FGFR1 rearrangements. The most frequent FGFR1 translocation partner is the zinc finger gene ZNF198 located at 13q11. The t(8;13)(p11;q11) disrupts intron 8 of the FGFR1 gene and fuses proline-rich and zinc finger domains of the ZNF198 gene with the cytoplasmic tyrosine kinase domain of FGFR1. Oligomerization of the fusion protein occurs, with subsequent activation of downstream signal transduction pathways, culminating in neoplastic cell transformation. This review describes the historical development of the EMS/SCLL and outlines its cytogenetic abnormalities and molecular mechanisms with an illustrative case.

Activity of TKI258 against primary cells and cell lines with FGFR1 fusion genes associated with the 8p11 myeloproliferative syndrome

The 8p11 myeloproliferative syndrome (EMS) is an aggressive, atypical stem cell myeloproliferative disorder associated with chromosome translocations that disrupt and constitutively activate FGFR1 by fusion to diverse partner genes. To explore the possibility of targeted therapy for EMS, we have investigated the use of TKI258, a multitargeted receptor tyrosine kinase inhibitor with activity against FGFR, VEGFR, PDGFR, FLT3, and KIT that is currently being assessed for the treatment of a variety of malignancies in phase 1 clinical studies. The viability of Ba/F3 cells transformed to IL3 independence by ZNF198-FGFR1 or BCR-FGFR1 was specifically inhibited by TKI258 with IC50 values of 150 nM and 90 nM, respectively. Inhibition was accompanied by dose-dependent inhibition of phosphorylation of each fusion gene, ERK, and STAT5. TKI258 also specifically inhibited proliferation and survival of the FGFR1OP2-FGFR1–positive KG1 and KG1A cell lines, resulting in increased levels of apoptosis. Primary cells from EMS patients showed significant, dose-dependent responses in liquid culture and in methylcellulose colony assays compared with controls. This work provides evidence that targeted therapy may be beneficial for patients with EMS.

Fibroblast growth factor receptor-induced phosphorylation of STAT1 at the Golgi apparatus without translocation to the nucleus

STAT transcription factors signal from the plasma membrane to the nucleus in response to growth factors and cytokines, but little is known about activation of STAT1 from intracellular sites. Here we show that transient transfection of COS cells with fibroblast growth factor receptors (FGFRs) led to ligand-independent phosphorylation of the receptors, including intracellular immature forms. FGF-independent activation of STAT1 was demonstrated at the Golgi apparatus where it was colocalized with FGFRs. Both FGFR1 and FGFR2 induced strong phosphorylation of STAT1 causing redistribution of the Golgi apparatus, while FGFR3 and FGFR4 induced less phosphorylation of STAT1 and little or no redistribution of the Golgi apparatus. Upon expression of a cytosolic mutant of FGFR4 lacking the transmembrane as well as the extracellular region (CytR4), STAT1 was phosphorylated and transferred to the nucleus. The results indicate that immature forms of FGFRs form incomplete signaling complexes on Golgi membranes trapping phospho-STAT1 on this organelle.

14-3-3 Integrates prosurvival signals mediated by the AKT and MAPK pathways in ZNF198-FGFR1-transformed hematopoietic cells

Human 8p11 stem cell leukemia/lymphoma syndrome usually presents as a myeloproliferative disorder (MPD) that evolves to acute myeloid leukemia and/or lymphoma. The syndrome associated with t(8;13)(p11;q12) results in expression of the ZNF198-FGFR1 fusion tyrosine kinase that plays a pathogenic role in hematopoietic transformation. We found that ZNF198-FGFR1 activated both the AKT and mitogen activated protein kinase (MAPK) prosurvival signaling pathways, resulting in elevated phosphorylation of the AKT target FOXO3a at T32 and BAD at S112, respectively. These phosphorylated residues subsequently sequestered the proapoptotic FOXO3a and BAD to 14-3-3 to prevent apoptosis. We used a peptide-based 14-3-3 competitive antagonist, R18, to disrupt 14-3-3-ligand association. Expression of R18 effectively induced apoptosis in hematopoietic Ba/F3 cells transformed by ZNF198-FGFR1 compared with control cells. Moreover, purified recombinant transactivator of transcription (TAT)-conjugated R18 proteins effectively transduced into human leukemia cells and induced significant apoptosis in KG-1a cells expressing FGFR1OP2-FGFR1 fusion tyrosine kinase but not in control HL-60 and Jurkat T cells. Surprisingly, R18 was only able to dissociate FOXO3a, but not BAD as previously proposed, from 14-3-3 binding and induced apoptosis partially through liberation and reactivation of FOXO3a. Our findings suggest that 14-3-3 integrates prosurvival signals in FGFR1 fusion-transformed hematopoietic cells. Disrupting 14-3-3-ligand association may represent an effective therapeutic strategy to treat 8p11 stem cell MPD.

Fusion gene-mediated truncation ofRUNX1 as a potential mechanism underlying disease progression in the 8p11 myeloproliferative syndrome

The 8p11 myeloproliferative syndrome (EMS) is a chronic myeloproliferative disorder molecularly characterized by fusion of various 5' partner genes to the 3' part of the fibroblast growth factor receptor 1 (FGFR1) gene at 8p, resulting in constitutive activation of the tyrosine kinase activity contained within FGFR1. EMS is associated with a high risk of transformation to acute myeloid leukemia (AML), but the mechanisms underlying the disease progression are unknown. In the present study, we have investigated a case of EMS harboring a t(8;22)(p11;q11)/BCR-FGFR1 rearrangement as well as a t(9;21)(q34;q22) at the time of AML transformation. FISH and RT-PCR analyses revealed that the t(9;21) leads to a fusion gene consisting of the 5' part of RUNX1 (exons 1-4) fused to repetitive sequences of a gene with unknown function on chromosome 9, adding 70 amino acids to RUNX1 exon 4. The t(9;21) hence results in a truncation of RUNX1. No point mutations were found in the other RUNX1 allele. The most likely functional outcome of the rearrangement was haploinsufficiency of RUNX1, which thus may be one mechanism by which EMS transforms to AML.

Eosinophilic disorders: molecular pathogenesis, new classification, and modern therapy

Before the 1990s, lack of evidence for a reactive cause of hypereosinophilia or chronic eosinophilic leukemia (e.g. presence of a clonal cytogenetic abnormality or increased blood or bone marrow blasts) resulted in diagnosticians characterizing such nebulous cases as 'idiopathic hypereosinophilic syndrome (HES)'. However, over the last decade, significant advances in our understanding of the molecular pathophysiology of eosinophilic disorders have shifted an increasing proportion of cases from this idiopathic HES 'pool' to genetically defined eosinophilic diseases with recurrent molecular abnormalities. The majority of these genetic lesions result in constitutively activated fusion tyrosine kinases, the phenotypic consequence of which is an eosinophilia-associated myeloid disorder. Most notable among these is the recent discovery of the cryptic FIP1L1-PDGFRA gene fusion in karyotypically normal patients with systemic mast cell disease with eosinophilia or idiopathic HES, redefining these diseases as clonal eosinophilias. Rearrangements involving PDGFRA and PDGFRB in eosinophilic chronic myeloproliferative disorders, and of fibroblast growth factor receptor 1 (FGFR1) in the 8p11 stem cell myeloproliferative syndrome constitute additional examples of specific genetic alterations linked to clonal eosinophilia. The identification of populations of aberrant T-lymphocytes secreting eosinophilopoietic cytokines such as interleukin-5 establish a pathophysiologic basis for cases of lymphocyte-mediated hypereosinophilia. This recent revival in understanding the biologic basis of eosinophilic disorders has permitted more genetic specificity in the classification of these diseases, and has translated into successful therapeutic approaches with targeted agents such as imatinib mesylate and recombinant anti-IL-5 antibody.

ZNF198, a zinc finger protein rearranged in myeloproliferative disease, localizes to the PML nuclear bodies and interacts with SUMO-1 and PML

The ZNF198/FGFR1 fusion gene in atypical myeloproliferative disease produces a constitutively active cytoplasmic tyrosine kinase, unlike ZNF198 which is normally a nuclear protein. We have now shown that the ZNF198/FGFR1 fusion kinase interacts with the endogenous ZNF198 protein suggesting that the function of ZNF198 may be compromised in cells expressing it. Little is currently known about the endogenous function of ZNF198 and to investigate this further we performed a yeast two-hybrid analysis and identified SUMO-1 as a binding partner of ZNF198. These observations were confirmed using co-immunoprecipitation which demonstrated that ZNF198 is covalently modified by SUMO-1. Since many of the SUMO-1-modified proteins are targeted to the PML nuclear bodies we used confocal microscopy to show that SUMO-1, PML and ZNF198 colocalize to punctate structures, shown by immunocytochemistry to be PML bodies. Using co-immunoprecipitation we now show that PML and sumoylated ZNF198 can be found in a protein complex in the cell. Mutation of the SUMO-1 binding site in wild-type ZNF198 resulted in loss of distinct PML bodies, reduced PML levels and a more dispersed nuclear localization of the PML protein. In cells expressing ZNF198/FGFR1, which also lack the SUMO-1 binding site, SUMO-1 is preferentially localized in the cytoplasm, which is associated with loss of distinct PML bodies. Recently, arsenic trioxide (ATO) was proposed as an alternative therapy for APL that was resistant to traditional therapy. Treatment of cells expressing ZNF198/FGFR1 with ATO demonstrated reduced autophosphorylation of the ZNF198/FGFR1 protein and induced apoptosis, which is not seen in cells expressing wild-type ZNF198. Overall our results suggest that the sumoylation of ZNF198 is important for PML body formation and that the abrogation of sumoylation of ZNF198 in ZNF198/FGFR1 expressing cells may be an important mechanism in cellular transformation.

STAT5 acts as a repressor to regulate early embryonic erythropoiesis

STAT5 regulates definitive (adult stage) erythropoiesis through its ability to transduce signals from the erythropoietin receptor. A function for STAT-dependent signaling during primitive (embryonic) erythropoiesis has not been analyzed. We tested this in the Xenopus system, because STAT5 is expressed at the right time and place to regulate development of the embryonic primitive ventral blood island. Depletion of STAT5 activity results in delayed accumulation of the first globin-expressing cells, indicating that the gene does regulate primitive erythropoiesis. Our results suggest that in this context STAT5 functions as a repressor, since forced expression of an activator isoform blocks erythropoiesis, while embryos expressing a repressor isoform develop normally. The erythroid phenotype caused by the activator isoform of STAT5 resembles that caused by overexpression of fibroblast growth factor (FGF). We show that STAT5 isoforms can function epistatic to FGF and can be phosphorylated in response to hyperactivated FGF signaling in Xenopus embryos. Therefore, our data indicate that STAT5 functions in both primitive and definitive erythropoiesis, but by different mechanisms.

Expression of a homodimeric type I cytokine receptor is required for JAK2V617F-mediated transformation

A recurrent somatic activating mutation in the nonreceptor tyrosine kinase JAK2 (JAK2V617F) occurs in the majority of patients with the myeloproliferative disorders polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia, and, less commonly, chronic myelomonocytic leukemia. We do not understand the basis for the specificity of the JAK2V617F mutation in clonal disorders of the myeloid, but not lymphoid, lineage, nor has the basis for the pleiotropic phenotype of JAK2V617F-associated myeloproliferative disorders been delineated. However, the presence of the identical mutation in patients with related, but clinicopathologically distinct, myeloid disorders suggests that interactions between the JAK2V617F kinase and other signaling molecules may influence the phenotype of hematopoietic progenitors expressing JAK2V617F. Here, we show that coexpression of the JAK2V617F mutant kinase with a homodimeric Type I cytokine receptor, the erythropoietin receptor (EpoR), the thrombopoietin receptor, or the granulocyte colony-stimulating-factor receptor, is necessary for transformation of hematopoietic cells to growth-factor independence and for hormone-independent activation of JAK-STAT signaling. Furthermore, EpoR mutations that impair erythropoietin-mediated JAK2 or STAT5 activation also impair transformation mediated by the JAK2V617F kinase, indicating that JAK2V617F requires a cytokine receptor scaffold for its transforming and signaling activities. Our results reveal the molecular basis for the prevalence of JAK2V617F in diseases of myeloid lineage cells that express these Type I cytokine receptors but not in lymphoid lineage cells that do not.

Molecular classification and pathogenesis of eosinophilic disorders: 2005 update

Use of the term "idiopathic hypereosinophilic syndrome (HES)" has highlighted our basic lack of understanding of the molecular pathophysiology of eosinophilic disorders. However, over the last 10 years, the study of hypereosinophilia has enjoyed a revival. This interest has been rekindled by two factors: (1) the development of increasingly sophisticated molecular biology techniques that have unmasked recurrent genetic abnormalities linked to eosinophilia, and (2) the successful application of targeted therapy with agents such as imatinib to treat eosinophilic diseases. To date, most of these recurrent molecular abnormalities have resulted in constitutively activated fusion tyrosine kinases whose phenotypic consequence is an eosinophilia-associated myeloid disorder. Most notable among these are rearrangements of platelet-derived growth factor receptors alpha and beta (PDGFRalpha, PDGFRbeta), which define a small subset of patients with eosinophilic chronic myeloproliferative disorders (MPDs) and/or overlap myelodysplastic syndrome/MPD syndromes, including chronic myelomonocytic leukemia. Discovery of the cryptic FIP1L1-PDGFRA gene fusion in cytogenetically normal patients with systemic mast cell disease with eosinophilia or idiopathic HES has redefined these diseases as clonal eosinophilias. A growing list of fibroblast growth factor receptor 1 fusion partners has similarly emerged in the 8p11 myeloproliferative syndromes, which are often characterized by elevated eosinophil counts. Herein the focus is on the molecular gains made in these MPD-type eosinophilias, and the classification and clinicopathological issues related to hypereosinophilic syndromes, including the lymphocyte variant. Success in establishing the molecular basis of a group of once seemingly heterogeneous diseases has now the laid the foundation for establishing a semi-molecular classification scheme of eosinophilic disorders.

PKC412 inhibits the zinc finger 198-fibroblast growth factor receptor 1 fusion tyrosine kinase and is active in treatment of stem cell myeloproliferative disorder

Human stem cell leukemia-lymphoma syndrome usually presents itself as a myeloproliferative disorder (MPD) that evolves to acute myeloid leukemia and/or lymphoma. The syndrome associated with t(8;13)(p11;q12) results in expression of the ZNF198-fibroblast growth factor receptor (FGFR) 1 fusion tyrosine kinase. Current empirically derived cytotoxic chemotherapy is inadequate for treatment of this disease. We hypothesized that small-molecule inhibitors of the ZNF198-FGFR1 fusion would have therapeutic efficacy. We characterized the transforming activity of ZNF198-FGFR1 in hematopoietic cells in vitro and in vivo. Expression of ZNF198-FGFR1 in primary murine hematopoietic cells caused a myeloproliferative syndrome in mice that recapitulated the human MPD phenotype. Transformation in these assays, and activation of the downstream effector molecules PLC-gamma, STAT5, and phosphatidylinositol 3-kinase/AKT, required the proline-rich domains, but not the ZNF domains, of ZNF198. A small-molecule tyrosine kinase inhibitor, PKC412 (N-benzoyl-staurosporine) effectively inhibited ZNF198-FGFR1 tyrosine kinase activity and activation of downstream effector pathways, and inhibited proliferation of ZNF198-FGFR1 transformed Ba/F3 cells. Furthermore, treatment with PKC412 resulted in statistically significant prolongation of survival in the murine model of ZNF198-FGFR1-induced MPD. Based in part on these data, PKC412 was administered to a patient with t(8;13)(p11;q12) and was efficacious in treatment of progressive myeloproliferative disorder with organomegaly. Therefore, PKC412 may be a useful therapy for treatment of human stem cell leukemia-lymphoma syndrome.

Targeting FGFR3 in multiple myeloma: inhibition of t(4;14)-positive cells by SU5402 and PD173074

The t(4;14)(p16.3;q32), associated with 10-20% of cases of multiple myeloma (MM), deregulates the expression of MMSET and FGFR3. To assess the potential of FGFR3 as a drug target, we evaluated the effects of selective inhibitors on MM and control cell lines. SU5402 and PD173074 specifically inhibited the growth of the two t(4;14)-positive MM lines, KMS-11 and OPM-2. Importantly, inhibition was still observed in the presence of IL-6, a growth factor known to play an important role in MM. Both compounds induced a dose-dependent reduction in cell viability and an increase in apoptosis, accompanied by a decrease in extracellular signal-related kinase phosphorylation. In contrast, no inhibition was seen with either compound against t(4;14)-negative cell lines or NCI-H929, a t(4;14)-positive, FGFR3-negative MM cell line. FGFR3 is thus a plausible candidate for targeted therapy in a subset of MM patients.

Distinct stem cell myeloproliferative/T lymphoma syndromes induced by ZNF198-FGFR1 and BCR-FGFR1 fusion genes from 8p11 translocations

8p11 myeloproliferative syndrome (EMS) is a hematopoietic stem cell disorder characterized by myeloid hyperplasia and non-Hodgkin's lymphoma with chromosomal translocations fusing several genes, most commonly ZNF198, to fibroblast growth factor receptor-1 (FGFR1). However, patients with BCR-FGFR1 fusion present with typical chronic myeloid leukemia (CML). We demonstrate that ZNF198-FGFR1 induces EMS-like disease in mice, with myeloproliferation and T lymphoma arising from common multipotential progenitors. Mutation of FGFR1 Tyr766 attenuates both myeloid and lymphoid diseases, identifying phospholipase C-gamma1 as a downstream effector. Bcr-FGFR1 binds Grb2 via Bcr Tyr177 and induces CML-like leukemia in mice, whereas Bcr-FGFR1/Y177F lacks Grb2 binding and causes EMS-like disease. These results implicate different signaling pathways originating from both kinase and fusion partner in the pathogenesis of CML and EMS.

Critical Role of STAT5 Activation in Transformation Mediated by ZNF198-FGFR1

The 8p11 myeloproliferative syndrome is an aggressive disorder caused by FGFR1 fusion proteins resulting from a subset of acquired translocations that target chromosome band 8p11. These chimeric proteins have constitutive FGFR1 tyrosine kinase activity and are believed to deregulate hemopoietic development in a manner analogous to BCR-ABL in chronic myeloid leukemia. Here we have studied the role of STAT proteins in transformation mediated by the most common of these fusions, ZNF198-FGFR1. We found that STATs 1, 3, and 5 were activated constitutively in ZNF198-FGFR1-transformed Ba/F3 cells and that STATs 2, 4, and 6 were also tyrosine-phosphorylated. Induction of dominant negative STAT mutants showed that activation of STAT5, but not STATs 1 or 3, was essential for the anti-apoptotic effect of ZNF198-FGFR1 and that STAT5 activation is essential for the elevated levels of BclXL in transformed cells. STAT5 activation was also shown to be required for continued cell cycle progression of BaF3/ZNF198-FGFR1 cells in conditions of cytokine deprivation and for up-regulation of the DNA repair protein Rad51. These findings suggest a critical role of STAT5 activation in transformation mediated by ZNF198-FGFR1.

Roles of FGF receptors in mammalian development and congenital diseases

Four fibroblast growth factor receptors (FGFR1-4) constitute a family of transmembrane tyrosine kinases that serve as high affinity receptors for at least 22 FGF ligands. Gene targeting in mice has yielded valuable insights into the functions of this important gene family in multiple biological processes. These include mesoderm induction and patterning; cell growth, migration, and differentiation; organ formation and maintenance; neuronal differentiation and survival; wound healing; and malignant transformation. Furthermore, discoveries that mutations in three of the four receptors result in more than a dozen human congenital diseases highlight the importance of these genes in skeletal development. In this review, we will discuss recent progress on the roles of FGF receptors in mammalian development and congenital diseases, with an emphasis on signal transduction pathways.

Identification of potential anticancer drug targets through the selection of growth-inhibitory genetic suppressor elements

To identify human genes required for tumor cell growth, transcriptome-scale selection was used to isolate genetic suppressor elements (GSEs) inhibiting breast carcinoma cell growth. Growth-inhibitory GSEs (cDNA fragments that counteract their cognate gene) were selected from 57 genes, including known positive regulators of cell growth or carcinogenesis as well as genes that have not been previously implicated in cell proliferation. Many GSE-cognate genes encode transcription factors (such as STAT and AP-1) and signal transduction proteins. Monoclonal antibodies against a cell surface protein identified by GSE selection, neural cell adhesion molecule L1CAM, strongly inhibited the growth of several tumor cell lines but not of untransformed cells. Hence, selection for growth-inhibitory GSEs allows one to find potential targets for new anticancer drugs.

ZNF198 protein, involved in rearrangement in myeloproliferative disease, forms complexes with the DNA repair-associated HHR6A/6B and RAD18 proteins

A highly specific t(8;13)(p11;q12) translocation has been consistently identified in bone marrow cells from patients with an atypical myeloproliferative disease that is associated with peripheral blood eosinophila and T- or B-cell leukemias. In all patients analysed to date, the translocation event results in a chimeric gene in which the atypical zinc-finger domain of ZNF198 is fused to the N-terminal end of the catalytic domain of the FGFR1 receptor tyrosine kinase. To understand more about the consequences of this rearrangement we have investigated the normal function of the ZNF198 gene. Using yeast two-hybrid analysis we identified HHR6 as a protein binding partner and confirmed this using immunoprecipitation studies. The ZNF198/FGFR1 fusion protein also binds to HHR6. We demonstrate here that the human RAD18 is also present in the ZNF198/HHR6 protein complex, although it does not coimmunoprecipitate with the fusion kinase. Cells expressing the fusion kinase gene show a marked increased sensitivity to UVB irradiation, suggesting that it acts in a dominant-negative way to affect DNA repair. These observations support the idea that ZNF198, through its interaction with HHR6 and RAD18, may be involved in the DNA repair process.

The oncogenic fusion protein-tyrosine kinase ZNF198/fibroblast growth factor receptor-1 has signaling function comparable with interleukin-6 cytokine receptors

The reciprocal t(8;13) chromosome translocation results in a fusion gene (FUS) in which the N-terminal half of the zinc finger protein ZNF198 is combined with the cytoplasmic domain of the fibroblast growth factor receptor-1 (FGFR1). Expression of FUS is suggested to provide growth-promoting activity to myeloid cells similar to the activity of hematopoietic cytokine receptors. This study determined the specificity of FUS to activate signal transduction pathways. Because no tumor cell line expressing FUS was available, the mode of FUS action was identified in cells transiently and stably transfected with an expression vector for FUS. FUS acted as a constitutively active protein-tyrosine kinase and mediated phosphorylation of STAT1, 3, and 5 but not STAT4 and 6. The same specificity but lower activity was determined for normal FGFR1. STAT activation by FUS, similar to that by interleukin-6-type cytokines, promoted STAT-specific induction of genes. The functionality of FUS, as well as the relative recruitment of STAT isoforms, was determined by the dimerizing function of the zinc finger domain. Replacement of the ZNF198 portion by the Bcr portion as present in the t(8;22) translocation shifted the signaling toward a more prominent STAT5 activation. This study documents that both gene partners forming the fusion oncogene define the activity and the signaling specificity of the protein-tyrosine kinase of FGFR1.

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.

The t(8;22) in chronic myeloid leukemia fuses BCR to FGFR1: transforming activity and specific inhibition of FGFR1 fusion proteins

This report describes 2 patients with a clinical and hematologic diagnosis of chronic myeloid leukemia (CML) in chronic phase who had an acquired t(8;22)(p11;q11). Analysis by fluorescence in situ hybridization (FISH) and reverse transcription-polymerase chain reaction (RT-PCR) indicated that both patients were negative for the BCR-ABL fusion, but suggested that the BCR gene was disrupted. Further FISH indicated a breakpoint within fibroblast growth factor receptor 1 (FGFR1), the receptor tyrosine kinase that is known to be disrupted in a distinctive myeloproliferative disorder, most commonly by fusion to ZNF198. RT-PCR confirmed the presence in both cases of an in-frame messenger RNA fusion between BCR exon 4 and FGFR1 exon 9. Expression of BCR-FGFR1 in the factor-dependent cell line Ba/F3 resulted in interleukin 3-independent clones that grew at a comparable rate to cells transformed with ZNF198-FGFR1. The growth of transformed cells was inhibited by the phosphatidylinositol 3-kinase inhibitor LY294002, the farnesyltransferase inhibitors L744832 and manumycin A, the p38 inhibitors SB202190 and SB203580 but not by the MEK inhibitor PD98059. The growth of BaF3/BCR-FGFR1 and BaF3/ZNF198-FGFR1 was not significantly inhibited by treatment with STI571, but was inhibited by SU5402, a compound with inhibitory activity against FGFR1. Inhibition with this compound was associated with decreased phosphorylation of ERK1/2 and BCR-FGFR1 or ZNF198-FGFR1, and was dose dependent with an inhibitory concentration of 50% of approximately 5 microM. As expected, growth of BaF3/BCR-ABL was inhibited by STI571 but not by SU5402. The study demonstrates that the BCR-FGFR1 fusion may occur in patients with apparently typical CML. Patients with constitutively active FGFR1 fusion genes may be amenable to treatment with specific FGFR1 inhibitors.

Identification of four new translocations involving FGFR1 in myeloid disorders

The 8p11 myeloproliferative syndrome (EMS) is associated with three translocations, t(8;13)(p11;q12), t(8;9)(p11;q33), and t(6;8)(q27;p11), that fuse unrelated genes (ZNF198, CEP110, and FOP, respectively) to the entire tyrosine kinase domain of FGFR1. In all cases thus far examined (n = 10), the t(8;13) results in an identical mRNA fusion between ZNF198 exon 17 and FGFR1 exon 9. To determine if consistent fusions are also seen in the variant translocations, we performed RT-PCR on four cases and sequenced the products. For two patients with a t(8;9), we found that CEP110 exon 15 was fused to FGFR1 exon 9. For two patients with a t(6;8), we found that FOP exon 5 (n = 1) or exon 7 (n = 1) was fused to FGFR1 exon 9. To determine if FGFR1 might be involved in other myeloid disorders with translocations of 8p, we developed a two-color FISH assay using two differentially labeled PAC clones that flank FGFR1. Disruption of this gene was indicated in a patient with a t(8;17)(p11;q25) and Ph-negative chronic myeloid leukemia in association with systemic malignant mast cell disease, a patient with acute myeloid leukemia with a t(8;11)(p11;p15), and two cases with T-cell lymphoma, myeloproliferative disorder, and marrow eosinophilia with a t(8;12)(p11;q15) and ins(12;8)(p11;p11p21), respectively. For the patient with the t(8;11), the chromosome 11 breakpoint was determined to be in the vicinity of NUP98. We conclude that 1) all mRNA fusions in EMS result in splicing to FGFR1 exon 9 but breakpoints in FOP are variable, 2) two-color FISH can identify patients with EMS, and 3) the t(8;17)(p11;q25), t(8;11)(p11;p15), t(8;12)(p11;q15), and ins(12;8)(p11;p11p21) are novel karyotypic changes that most likely involve FGFR1.