PKC412 overcomes resistance to imatinib in a murine model of FIP1L1-PDGFRα-induced myeloproliferative disease

Stable expression of small interfering RNA sensitizes TEL-PDGFbetaR to inhibition with imatinib or rapamycin

Small molecule inhibitors, such as imatinib, are effective therapies for tyrosine kinase fusions BCR-ABL-TEL-PDGFbetaR-mediated human leukemias, but resistance may develop. The unique fusion junctions of these molecules are attractive candidates for molecularly targeted therapeutic intervention using RNA interference (RNAi), which is mediated by small interfering RNA (siRNA). We developed a retroviral system for stable expression of siRNA directed to the unique fusion junction sequence of TEL-PDGFbetaR in transformed hematopoietic cells. Stable expression of the siRNA resulted in approximately 90% inhibition of TEL-PDGFbetaR expression and its downstream effectors, including PI3K and mammalian target of rapamycin (mTOR). Expression of TEL-PDGFbetaR-specific siRNA (TPsiRNA) significantly attenuated the proliferation of TEL-PDGFbetaR-transformed Ba/F3 cells or disease latency and penetrance in mice induced by intravenous injection of these Ba/F3 cells. Although a 90% reduction in TEL-PDGFbetaR expression was insufficient to induce cell death, stable siRNA expression sensitized transformed cells to the PDGFbetaR inhibitor imatinib or to the mTOR inhibitor rapamycin. TPsiRNA also inhibited an imatinib-resistant TEL-PDGFbetaR mutant, and the inhibition was enhanced by siRNA in combination with PKC412, another PDGFbetaR inhibitor. Although siRNA delivery in vivo is a challenging problem, stable expression of siRNA, which targets oncogenic fusion genes, may potentiate the effects of conventional therapy for hematologic malignancies.

Platelet-derived growth factor receptor inhibition to treat idiopathic hypereosinophilic syndrome

Hypereosinophilic syndrome (HES) is a heterogeneous group of rare disorders characterized by peripheral blood and tissue eosinophilia leading to end-organ damage. Hypereosinophilic syndrome can be fatal, particularly in patients with endomyocardial fibrosis, and treatment has traditionally been palliative or preventive. The disease shares features with myeloproliferative disorders, such as chronic myeloid leukemia, including responsiveness to hydroxyurea and interferon. The tyrosine kinase inhibitor imatinib, a highly effective treatment for chronic myeloid leukemia, has shown efficacy in normalizing eosinophil counts and resolving signs and symptoms in some HES patients. Fusion of the Fip1-like 1 gene (FIP1L1) and the platelet-derived growth factor receptor alpha gene (PDGFRA) was discovered in the majority of patients with imatinib-sensitive HES, and all patients with the fusion responded to imatinib. The product of this fusion gene, FIP1L1-PDGFRalpha, is a constitutively active protein-tyrosine kinase capable of transforming hematopoietic cells. The efficacy of relatively low imatinib concentrations in HES, mediated by inhibition of FIP1L1-PDGFRalpha kinase activity, causally implicates FIP1L1-PDGFRA in the pathogenesis in certain HES patients.

Imatinib Therapy in Clonal Eosinophilic Disorders, Including Systemic Mastocytosis

Primary (nonreactive) eosinophilia is operationally classified as either a "clonal" or an "idiopathic" process. Clonal eosinophilia stipulates the presence of cytogenetic, molecular, or bone marrow histologic evidence of acute leukemia or a chronic myeloid disorder. Idiopathic eosinophilia is a diagnosis of exclusion that is made after ruling out both "secondary" (reactive) and clonal eosinophilia. Hypereosinophilic syndrome is a subclass of idiopathic eosinophilia that requires the documentation of both sustained eosinophilia (> or = 1500/microL for at least 6 months) and target-organ damage. A series of novel observations in the last 5 years have warranted a refined approach to the diagnosis as well as the treatment of clonal eosinophilic disorders, including systemic mastocytosis. At the center of these new developments are mutations involving the platelet-derived growth factor receptor genes (PDGFRA and PDGFRB), which have been pathogenetically linked to clonal eosinophilia, and their presence predicts complete as well as durable treatment responses to imatinib mesylate. The bone marrow histologic phenotype of these imatinib-sensitive eosinophilic disorders includes systemic mastocytosis, chronic eosinophilic leukemia, chronic myelomonocytic leukemia, and atypical chronic myeloproliferative disorder.

Clinical and molecular features of FIP1L1-PDFGRA (+) chronic eosinophilic leukemias

Detection of the FIP1L1-PDGFRA fusion gene or the corresponding cryptic 4q12 deletion supports the diagnosis of chronic eosinophilic leukemia (CEL) in patients with chronic hypereosinophilia. We retrospectively characterized 17 patients fulfilling WHO criteria for idiopathic hypereosinophilic syndrome (IHES) or CEL, using nested RT-PCR and interphase fluorescence in situ hybridization (FISH). Eight had FIP1L1-PDGFRA (+) CEL, three had FIP1L1-PDGFRA (-) CEL and six had IHES. FIP1L1-PDGFRA (+) CEL responded poorly to steroids, hydroxyurea or interferon-alpha, and had a high probability of eosinophilic endomyocarditis (n=4) and disease-related death (n=4). In FIP1L1-PDGFRA (+) CEL, palpable splenomegaly was present in 5/8 cases, serum vitamin B(12) was always markedly increased, and marrow biopsies revealed a distinctively myeloproliferative aspect. Imatinib induced rapid complete hematological responses in 4/4 treated FIP1L1-PDGFRA (+) cases, including one female, and complete molecular remission in 2/3 evaluable cases. In the female patient, 1 log reduction of FIP1L1-PDGFRA copy number was reached as by real-time quantitative PCR (RQ-PCR). Thus, correlating IHES/CEL genotype with phenotype, FIP1L1-PDGFRA (+) CEL emerges as a homogeneous clinicobiological entity, where imatinib can induce molecular remission. While RT-PCR and interphase FISH are equally valid diagnostic tools, the role of marrow biopsy in diagnosis and of RQ-PCR in disease and therapy monitoring needs further evaluation.

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.

Targeted treatment of hypereosinophilic syndromes and chronic eosinophilic leukemias with imatinib mesylate

Idiopathic hypereosinophilic syndrome (HES) and chronic eosinophilia leukemia (CEL) represent the most recent additions to the list of molecularly defined chronic myeloproliferative disorders. Beginning with the observation that imatinib mesylate (Gleevec) could elicit rapid and complete hematologic remissions in a proportion of patients with HES, a reverse bedside-to-bench translational research effort led to the discovery of FIP1L1-PDGFRA, a novel fusion gene on chromosome 4q12 whose product is an imatinib-sensitive protein tyrosine kinase. FIP1L1-PDGFRA is the first description of a gain-of-function fusion gene derived from an interstitial chromosomal deletion rather than a reciprocal translocation. Empiric use of imatinib in HES and CEL provides a dramatic example of how the development of targeted therapeutics can provide tremendous insight into the molecular etiology of what appear to be a diverse and otherwise indecipherable collection of diseases. In this review, we discuss the role of imatinib in HES/CEL and other malignancies characterized by constitutively activated tyrosine kinases, and examine molecular features of the FIP1L1-PDGFRA fusion.

The EOL-1 cell line as an in vitro model for the study of FIP1L1-PDGFRA-positive chronic eosinophilic leukemia

We recently identified the chimeric kinase FIP1L1-platelet-derived growth factor receptor alpha (PDGFRalpha) as a cause of the hypereosinophilic syndrome and of chronic eosinophilic leukemia. To investigate the role of FIP1L1-PDGFRA in the pathogenesis of acute leukemia, we screened 87 leukemia cell lines for the presence of FIP1L1-PDGFRA. One cell line, EOL-1, expressed the FIP1L1-PDGFRA fusion. Three structurally divergent kinase inhibitors--imatinib (STI-571), PKC412, and SU5614--inhibited the growth of EOL-1 cells. These results indicate that the fusion of FIP1L1 to PDGFRA occurs rarely in leukemia cell lines, but they identify EOL-1 as an in vitro model for the study of FIP1L1-PDGFRA-positive chronic eosinophilic leukemia and for the analysis of small molecule inhibitors of FIP1L1-PDGFRalpha.

The FIP1L1-PDGFRalpha fusion tyrosine kinase in hypereosinophilic syndrome and chronic eosinophilic leukemia: implications for diagnosis, classification, and management

Idiopathic hypereosinophilic syndrome (HES) and chronic eosinophilic leukemia (CEL) comprise a spectrum of indolent to aggressive diseases characterized by unexplained, persistent hypereosinophilia. These disorders have eluded a unique molecular explanation, and therapy has primarily been oriented toward palliation of symptoms related to organ involvement. Recent reports indicate that HES and CEL are imatinib-responsive malignancies, with rapid and complete hematologic remissions observed at lower doses than used in chronic myelogenous leukemia (CML). These BCR-ABL-negative cases lack activating mutations or abnormal fusions involving other known target genes of imatinib, implicating a novel tyrosine kinase in their pathogenesis. A bedside-to-benchtop translational research effort led to the identification of a constitutively activated fusion tyrosine kinase on chromosome 4q12, derived from an interstitial deletion, that fuses the platelet-derived growth factor receptor-alpha gene (PDGFRA) to an uncharacterized human gene FIP1-like-1 (FIP1L1). However, not all HES and CEL patients respond to imatinib, suggesting disease heterogeneity. Furthermore, approximately 40% of responding patients lack the FIP1L1-PDGFRA fusion, suggesting genetic heterogeneity. This review examines the current state of knowledge of HES and CEL and the implications of the FIP1L1-PDGFRA discovery on their diagnosis, classification, and management.

Targeting oncogene dependence and resistance

Our expanding experience with imatinib mesylate provides instructive lessons on the power and pitfalls of targeted therapy. The often impressive initial clinical responses seen with imatinib in a variety of malignancies inevitably give way to the emergence of resistant disease. Recent findings reveal several mechanisms of resistance and suggest ways to overcome them.