Definition of an inhibitory juxtamembrane WW-like domain in the platelet-derived growth factor beta receptor

Cancer-type somatic mutations in saccular cerebral aneurysms

Intracranial aneurysms (IAs) are a major cause of subarachnoidal hemorrhage (SAH) which can have a significant morbidity and mortality. The processes underlying the aneurysm development remains unclear. We performed whole exome sequencing of DNA derived from 20 saccular cerebral aneurysms of 20 patients, followed by somatic variant calling. Eleven (55%) of the 20 patients had detectable nonsynonymous somatic mutations and in total, 48 mutations were detected in the aneurysm samples. The mutations were highly enriched in cancer-related genes and 77% were predictably deleterious. A p.Tyr562Asp somatic mutation was detected in the PDGFRB gene; somatic mutations at the same codon have been reported in fusiform cerebral aneurysms. These results widen the concept on the role of somatic mutations in cerebral aneurysms, indicating their possible role in the more common saccular aneurysm, similarly to the rarer fusiform aneurysm.

Somatic PDGFRB activating variants promote smooth muscle cell phenotype modulation in intracranial fusiform aneurysm

BACKGROUND

The fusiform aneurysm is a nonsaccular dilatation affecting the entire vessel wall over a short distance. Although PDGFRB somatic variants have been identified in fusiform intracranial aneurysms, the molecular and cellular mechanisms driving fusiform intracranial aneurysms due to PDGFRB somatic variants remain poorly understood.

METHODS

In this study, single-cell sequencing and immunofluorescence were employed to investigate the phenotypic changes in smooth muscle cells within fusiform intracranial aneurysms. Whole-exome sequencing revealed the presence of PDGFRB gene mutations in fusiform intracranial aneurysms. Subsequent immunoprecipitation experiments further explored the functional alterations of these mutated PDGFRB proteins. For the common c.1684 mutation site of PDGFRβ, we established mutant smooth muscle cell lines and zebrafish models. These models allowed us to simulate the effects of PDGFRB mutations. We explored the major downstream cellular pathways affected by PDGFRBY562D mutations and evaluated the potential therapeutic effects of Ruxolitinib.

RESULTS

Single-cell sequencing of two fusiform intracranial aneurysms sample revealed downregulated smooth muscle cell markers and overexpression of inflammation-related markers in vascular smooth muscle cells, which was validated by immunofluorescence staining, indicating smooth muscle cell phenotype modulation is involved in fusiform aneurysm. Whole-exome sequencing was performed on seven intracranial aneurysms (six fusiform and one saccular) and PDGFRB somatic mutations were detected in four fusiform aneurysms. Laser microdissection and Sanger sequencing results indicated that the PDGFRB mutations were present in smooth muscle layer. For the c.1684 (chr5: 149505131) site mutation reported many times, further cell experiments showed that PDGFRBY562D mutations promoted inflammatory-related vascular smooth muscle cell phenotype and JAK-STAT pathway played a crucial role in the process. Notably, transfection of PDGFRBY562D in zebrafish embryos resulted in cerebral vascular anomalies. Ruxolitinib, the JAK inhibitor, could reversed the smooth muscle cells phenotype modulation in vitro and inhibit the vascular anomalies in zebrafish induced by PDGFRB mutation.

CONCLUSION

Our findings suggested that PDGFRB somatic variants played a role in regulating smooth muscle cells phenotype modulation in fusiform aneurysms and offered a potential therapeutic option for fusiform aneurysms.

t(4;12)(q12;p13) ETV6-rearranged AML without eosinophilia does not involve PDGFRA: relevance for imatinib insensitivity

Apparent ETV6-PDGFRA fusions identified by FISH analysis in t(4;12)(q12;p13) AML should be confirmed by sequencing.

Sequence-confirmed ETV6-PDGFRA fusions have not been identified in patients with t(4;12)(q12;p13) AML without eosinophilia.

Acute myeloid leukemia (AML) with t(4;12)(q12;p13) translocation is rare and often associated with an aggressive clinical course and poor prognosis. Previous reports based on fluorescence in situ hybridization (FISH) analysis have suggested that ETV6::PDGFRA fusions are present in these patients, despite the absence of eosinophilia, which is typically found in other hematopoietic malignancies with PDGFRA-containing fusions. We first detected an ETV6-SCFD2 fusion by targeted RNA sequencing in a patient with t(4;12)(q12;p13) who had been diagnosed with an ETV6-PDGFRA fusion by FISH analysis but failed to respond to imatinib. We then retrospectively identified 4 additional patients with AML and t(4;12)(q12;p13) with apparent ETV6-PDGFRA fusions using chromosome and FISH analysis and applied targeted RNA sequencing to archival material. We again detected rearrangements between ETV6 and non-PDGFRA 4q12 genes, including SCFD2, CHIC2, and GSX2. None of the 3 patients who received imatinib based on the incorrect assumption of an ETV6-PDGFRA fusion responded. Our findings highlight the importance of using a sequencing-based assay to confirm the presence of targetable gene fusions, particularly in genomic regions, such as 4q12, with many clinically relevant genes that are too close to resolve by chromosome or FISH analysis. Finally, combining our data and review of the literature, we show that sequence-confirmed ETV6-PDGFRA fusions are typically found in eosinophilic disorders (3/3 cases), and patients with t(4;12)(q12;p13) without eosinophilia are found to have other 4q12 partners on sequencing (17/17 cases).

Structural studies of full-length receptor tyrosine kinases and their implications for drug design

Receptor tyrosine kinases (RTKs) are important drug targets for cancer and immunological disorders. Crystal structures of individual RTK domains have contributed greatly to the structure-based drug design of clinically used drugs. Low-resolution structures from electron microscopy are now available for the RTKs, EGFR, PDGFR, and Kit. However, there are still no high-resolution structures of full-length RTKs due to the technical challenges of working with these complex, membrane proteins. Here, we review what has been learned from structural studies of these three RTKs regarding their mechanisms of ligand binding, activation, oligomerization, and inhibition. We discuss the implications for drug design. More structural data on full-length RTKs may facilitate the discovery of druggable sites and drugs with improved specificity and effectiveness against resistant mutants.

Somatic PDGFRB Activating Variants in Fusiform Cerebral Aneurysms

The role of somatic genetic variants in the pathogenesis of intracranial-aneurysm formation is unknown. We identified a 23-year-old man with progressive, right-sided intracranial aneurysms, ipsilateral to an impressive cutaneous phenotype. The index individual underwent a series of genetic evaluations for known connective-tissue disorders, but the evaluations were unrevealing. Paired-sample exome sequencing between blood and fibroblasts derived from the diseased areas detected a single novel variant predicted to cause a p.Tyr562Cys (g.149505130T>C [GRCh37/hg19]; c.1685A>G) change within the platelet-derived growth factor receptor β gene (PDGFRB), a juxtamembrane-coding region. Variant-allele fractions ranged from 18.75% to 53.33% within histologically abnormal tissue, suggesting post-zygotic or somatic mosaicism. In an independent cohort of aneurysm specimens, we detected somatic-activating PDGFRB variants in the juxtamembrane domain or the kinase activation loop in 4/6 fusiform aneurysms (and 0/38 saccular aneurysms; Fisher's exact test, p < 0.001). PDGFRB-variant, but not wild-type, patient cells were found to have overactive auto-phosphorylation with downstream activation of ERK, SRC, and AKT. The expression of discovered variants demonstrated non-ligand-dependent auto-phosphorylation, responsive to the kinase inhibitor sunitinib. Somatic gain-of-function variants in PDGFRB are a novel mechanism in the pathophysiology of fusiform cerebral aneurysms and suggest a potential role for targeted therapy with kinase inhibitors.

BCR: a promiscuous fusion partner in hematopoietic disorders

Considerable advances have been made in our understanding of the molecular basis of hematopoietic cancers. The discovery of the BCR-ABL fusion protein over 50 years ago has brought about a new era of therapeutic progress and overall improvement in patient care, mainly due to the development and use of personalized medicine and tyrosine kinase inhibitors (TKIs). However, since the detection of BCR-ABL, BCR has been identified as a commonly occurring fusion partner in hematopoietic disorders. BCR has been discovered fused to additional tyrosine kinases, including: Fibroblast Growth Factor Receptor 1 (FGFR1), Platelet-derived Growth Factor Receptor Alpha (PDGFRA), Ret Proto-Oncogene (RET), and Janus Kinase 2 (JAK2). While BCR translocations are infrequent in hematopoietic malignancies, clinical evidence suggests that patients who harbor these mutations benefit from TKIs and additional personalized therapies. The improvement of further methodologies for characterization of these fusions is crucial to determine a patient’s treatment regimen, and optimal outcome. However, potential relapse and drug resistance among patients’ highlights the need for additional treatment options and further understanding of these oncogenic fusion proteins. This review explores the mechanisms behind cancer progression of these BCR oncogenic fusion proteins, comparing their similarities and differences, examining the significance of BCR as a partner gene, and discussing current treatment options for these translocation-induced hematopoietic malignancies.

Acute myeloid leukemia with t(4;12)(q12;p13): an aggressive disease with frequent involvement of PDGFRA and ETV6

We describe the clinical, morphologic, immunophenotypic and molecular genetic features of 15 cases of acute myeloid leukemia (AML) with t(4;12)(q12;p13). There were 9 men and 6 women, with a median age of 50 years (range, 17–76). Most patients had hypercellular bone marrow with a median blast count of 58% and multilineage dysplasia. Flow cytometry analysis showed myeloid lineage with blasts positive for CD13, CD33, CD34, CD38, CD117 and HLA-DR. Interestingly, aberrant CD7 expression was detected in 12/14 cases, and myeloperoxidase was either negative (3/15) or positive in only a small subset of the blasts (12/15). t(4;12)(q12;p13) was detected at time of initial diagnosis in 4 and at relapse or progression in 9 patients. The initial karyotype was unknown in 2 cases. FISH analysis showed PDGFRA-ETV6 rearrangement in all 7 cases assessed. FLT3 ITD was detected in 2/11 cases and IDH2 and JAK2 mutation were each detected in 1/2 cases assessed. There were no mutations of KRAS (0/8), NRAS (0/8), CEBPA (0/3), KIT (0/3), NPM1 (0/3) or IDH1 (0/2). All patients received aggressive multiagent chemotherapy; 7 patients additionally received stem cell transplantation. With a median follow-up of 10 months (range, 6–51), 13 patients died of AML, 1 patient had persistent disease, and 1 patient was lost to follow-up. In summary, AML with t(4;12)(q12;p13) is usually associated with myelodysplasia, aberrant CD7 expression, weak of absent myeloperoxidase expression, frequent PDGFRA-ETV6 fusion, and an aggressive clinical course. The molecular findings suggest that there may be a role for tyrosine kinase inhibitors in patient management.

Multiple MYO18A-PDGFRB fusion transcripts in a myeloproliferative neoplasm patient with t(5;17)(q32;q11)

BACKGROUND

Myeloproliferative neoplasms (MPNs), typically defined by myeloid proliferation and eosinophilia, and are only rarely caused by platelet-derived growth factor receptor beta (PDGFRB) gene rearrangements.

CASE PRESENTATION

Here, we report a unique case of MPN that is negative for eosinophilia and characterized by a novel PDGFRB rearrangement. After cytogenetic analysis revealed a karyotype of t(5;17) (q32;q11), we used fluorescence in situ hybridization to specifically identify the PDGFRB gene at 5q31-q33 as the gene that had been translocated. Subsequently, RNA sequencing identified a new MYO18A-PDGFRB gene fusion. This fusion presented a previously undescribed breakpoint composed of exon 37 of MYO18A and exon 13 of PDGFRB. Furthermore, both RT-PCR and Bi-directional Sanger sequencing confirmed this out-of-frame fusion. Interestingly, we simultaneously identified the presence of another three PDGFRB transcripts, all of which were in-frame fusions. After treating the patient with imatinib, the t(5;17) translocation was no longer detected by conventional cytogenetics or by FISH, and at the time of the last follow-up, the patient had been in complete remission for 26 months.

CONCLUSION

We prove that MYO18A-PDGFRB fusions are recurrent genetic aberrations involved in MPNs, and identify multiple fusion transcripts with novel breakpoints.

Receptor Tyrosine Kinases: Translocation Partners in Hematopoietic Disorders

Receptor tyrosine kinases (RTKs) activate various signaling pathways and regulate cellular proliferation, survival, migration, and angiogenesis. Malignant neoplasms often circumvent or subjugate these pathways by promoting RTK overactivation through mutation or chromosomal translocation. RTK translocations create a fusion protein containing a dimerizing partner fused to an RTK kinase domain, resulting in constitutive kinase domain activation, altered RTK cellular localization, upregulation of downstream signaling, and novel pathway activation. While RTK translocations in hematological malignancies are relatively rare, clinical evidence suggests that patients with these genetic abnormalities benefit from RTK-targeted inhibitors. Here, we present a timely review of an exciting field by examining RTK chromosomal translocations in hematological cancers, such as Anaplastic Lymphoma Kinase (ALK), Fibroblast Growth Factor Receptor (FGFR), Platelet-Derived Growth Factor Receptor (PDGFR), REarranged during Transfection (RET), Colony Stimulating Factor 1 Receptor (CSF1R), and Neurotrophic Tyrosine Kinase Receptor Type 3 (NTRK3) fusions, and discuss current therapeutic options.

The structures of the kinase domain and UBA domain of MPK38 suggest the activation mechanism for kinase activity

Murine protein serine/threonine kinase 38 (MPK38) is the murine orthologue of human maternal embryonic leucine-zipper kinase (MELK), which belongs to the SNF1/AMPK family. MELK is considered to be a promising drug target for anticancer therapy because overexpression and hyperactivation of MELK is correlated with several human cancers. Activation of MPK38 requires the extended sequence (ExS) containing the ubiquitin-associated (UBA) linker and UBA domain and phosphorylation of the activation loop. However, the activation mechanism of MPK38 is unknown. This paper reports the crystal structure of MPK38 (T167E), which mimics a phosphorylated state of the activation loop, in complex with AMP-PNP. In the MPK38 structure, the UBA linker forces an inward movement of the αC helix. Phosphorylation of the activation loop then induces movement of the activation loop towards the C-lobe and results in interlobar cleft closure. These processes generate a fully active state of MPK38. This structure suggests that MPK38 has a similar molecular mechanism regulating activation as in other kinases of the SNF1/AMPK family.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

Novel oncogenic PDGFRA mutations in pediatric high-grade gliomas

The outcome for children with high-grade gliomas (HGG) remains dismal, with a 2-year survival rate of only 10% to 30%. Diffuse intrinsic pontine glioma (DIPG) comprise a subset of HGG that arise in the brainstem almost exclusively in children. Genome-wide analyses of copy number imbalances previously showed that platelet-derived growth factor receptor α (PDGFRA) is the most frequent target of focal amplification in pediatric HGGs, including DIPGs. To determine whether PDGFRA is also targeted by more subtle mutations missed by copy number analysis, we sequenced all PDGFRA coding exons from a cohort of pediatric HGGs. Somatic-activating mutations were identified in 14.4% (13 of 90) of nonbrainstem pediatric HGGs and 4.7% (2 of 43) of DIPGs, including missense mutations and in-frame deletions and insertions not previously described. Forty percent of tumors with mutation showed concurrent amplification, whereas 60% carried heterozygous mutations. Six different mutations impacting different domains all resulted in ligand-independent receptor activation that was blocked by small molecule inhibitors of PDGFR. Expression of mutants in p53-null primary mouse astrocytes conferred a proliferative advantage in vitro and generated HGGs in vivo with complete penetrance when implanted into brain. The gene expression signatures of these murine HGGs reflected the spectrum of human diffuse HGGs. PDGFRA intragenic deletion of exons 8 and 9 were previously shown in adult HGG, but were not detected in 83 nonbrainstem pediatric HGG and 57 DIPGs. Thus, a distinct spectrum of mutations confers constitutive receptor activation and oncogenic activity to PDGFRα in childhood HGG.

Structural and functional properties of platelet-derived growth factor and stem cell factor receptors

The receptors for platelet-derived growth factor (PDGF) and stem cell factor (SCF) are members of the type III class of PTK receptors, which are characterized by five Ig-like domains extracellularly and a split kinase domain intracellularly. The receptors are activated by ligand-induced dimerization, leading to autophosphorylation on specific tyrosine residues. Thereby the kinase activities of the receptors are activated and docking sites for downstream SH2 domain signal transduction molecules are created; activation of these pathways promotes cell growth, survival, and migration. These receptors mediate important signals during the embryonal development, and control tissue homeostasis in the adult. Their overactivity is seen in malignancies and other diseases involving excessive cell proliferation, such as atherosclerosis and fibrotic diseases. In cancer, mutations of PDGF and SCF receptors-including gene fusions, point mutations, and amplifications-drive subpopulations of certain malignancies, such as gastrointestinal stromal tumors, chronic myelomonocytic leukemia, hypereosinophilic syndrome, glioblastoma, acute myeloid leukemia, mastocytosis, and melanoma.

ETV6 fusion genes in hematological malignancies: a review

Translocations involving band 12p13 are one of the most commonly observed chromosomal abnormalities in human leukemia and myelodysplastic syndrome. Their frequently result in rearrangements of the ETV6 gene. At present, 48 chromosomal bands have been identified to be involved in ETV6 translocations, insertions or inversions and 30 ETV6 partner genes have been molecularly characterized. The ETV6 protein contains two major domains, the HLH (helix-loop-helix) domain, encoded by exons 3 and 4, and the ETS domain, encoded by exons 6 through 8, with in between the internal domain encoded by exon 5. ETV6 is a strong transcriptional repressor, acting through its HLH and internal domains. Five potential mechanisms of ETV6-mediated leukemogenesis have been identified: constitutive activation of the kinase activity of the partner protein, modification of the original functions of a transcription factor, loss of function of the fusion gene, affecting ETV6 and the partner gene, activation of a proto-oncogene in the vicinity of a chromosomal translocation and dominant negative effect of the fusion protein over transcriptional repression mediated by wild-type ETV6. It is likely that ETV6 is frequently involved in leukemogenesis because of the large number of partners with which it can rearrange and the several pathogenic mechanisms by which it can lead to cell transformation.

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.

Critical role of the platelet-derived growth factor receptor (PDGFR) beta transmembrane domain in the TEL-PDGFRbeta cytosolic oncoprotein

The fusion of TEL with platelet-derived growth factor receptor (PDGFR) beta (TPbeta) is found in a subset of patients with atypical myeloid neoplasms associated with eosinophilia and is the archetype of a larger group of hybrid receptors that are produced by rearrangements of PDGFR genes. TPbeta is activated by oligomerization mediated by the pointed domain of TEL/ETV6, leading to constitutive activation of the PDGFRbeta kinase domain. The receptor transmembrane (TM) domain is retained in TPbeta and in most of the described PDGFRbeta hybrids. Deletion of the TM domain (DeltaTM-TPbeta) strongly impaired the ability of TPbeta to sustain growth factor-independent cell proliferation. We confirmed that TPbeta resides in the cytosol, indicating that the PDGFRbeta TM domain does not act as a transmembrane domain in the context of the hybrid receptor but has a completely different function. The DeltaTM-TPbeta protein was expressed at a lower level because of increased degradation. It could form oligomers, was phosphorylated at a slightly higher level, co-immunoprecipitated with the p85 adaptor protein, but showed a much reduced capacity to activate STAT5 and ERK1/2 in Ba/F3 cells, compared with TPbeta. In an in vitro kinase assay, DeltaTM-TPbeta was more active than TPbeta and less sensitive to imatinib, a PDGFR inhibitor. In conclusion, we show that the TM domain is required for TPbeta-mediated signaling and proliferation, suggesting that the activation of the PDGFRbeta kinase domain is not enough for cell transformation.

Screening for diverse PDGFRA or PDGFRB fusion genes is facilitated by generic quantitative reverse transcriptase polymerase chain reaction analysis

BACKGROUND

Rapid identification of diverse fusion genes with involvement of PDGFRA or PDGFRB in eosinophilia-associated myeloproliferative neoplasms is essential for adequate clinical management but is complicated by the multitude and heterogeneity of partner genes and breakpoints.

DESIGN AND METHODS

We established a generic quantitative reverse transcriptase polymerase chain reaction to detect overexpression of the 3'-regions of PDGFRA or PDGFRB as a possible indicator of an underlying fusion.

RESULTS

At diagnosis, all patients with known fusion genes involving PDGFRA (n=5; 51 patients) or PDGFRB (n=5; 7 patients) showed significantly increased normalized expression levels compared to 191 patients with fusion gene-negative eosinophilia or healthy individuals (PDGFRA/ABL: 0.73 versus 0.0066 versus 0.0064, P<0.0001; PDGFRB/ABL: 196 versus 3.8 versus 5.85, P<0.0001). The sensitivity and specificity of the activation screening test were, respectively, 100% and 88.4% for PDGFRA and 100% and 94% for PDGFRB. Furthermore, significant overexpression of PDGFRB was found in a patient with an eosinophilia-associated myeloproliferative neoplasm with uninformative cytogenetics and an excellent response to imatinib. Subsequently, a new SART3-PDGFRB fusion gene was identified by 5'-rapid amplification of cDNA ends polymerase chain reaction (5'-RACE-PCR).

CONCLUSIONS

Quantitative reverse transcriptase polymerase chain reaction analysis is a simple and useful adjunct to standard diagnostic assays to detect clinically significant overexpression of PDGFRA and PDGFRB in eosinophilia-associated myeloproliferative neoplasms or related disorders.

Imatinib sensitivity as a consequence of a CSF1R-Y571D mutation and CSF1/CSF1R signaling abnormalities in the cell line GDM1

Imatinib is usually a highly effective treatment for myeloproliferative neoplasms (MPNs) associated with ABL, PDGFRA or PDGFRB gene fusions; however, occasional imatinib-responsive patients have been reported without abnormalities of these genes. To identify novel imatinib-sensitive lesions, we screened 11 BCR-ABL-negative cell lines and identified GDM1, derived from a patient with an atypical MPN (aMPN), as being responsive to imatinib. Screening of genes encoding known imatinib targets revealed an exon 12 mutation in the colony-stimulating factor 1 receptor (CSF1R; c-FMS) with a predicted Y571D amino-acid substitution. CSF1R in GDM1 was constitutively phosphorylated, but rapidly dephosphorylated on exposure to imatinib. Y571D did not transform FDCP1 cells to growth factor independence, but resulted in a significantly increased colony growth compared with controls, constitutive CSF1R phosphorylation and elevated CSF1R signaling. We found that GDM1 expresses CSF1, and CSF1 neutralization partially inhibited proliferation, suggesting the importance of both autocrine and intrinsic mechanisms of CSF1R activation. An extensive screen of CSF1R in aMPNs and acute myeloid leukemia identified three additional novel missense variants. None of these variants were active in transformation assays and are therefore likely to be previously unreported rare polymorphisms or non-pathogenic passenger mutations.

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.

Patterns of somatic mutation in human cancer genomes

Cancers arise owing to mutations in a subset of genes that confer growth advantage. The availability of the human genome sequence led us to propose that systematic resequencing of cancer genomes for mutations would lead to the discovery of many additional cancer genes. Here we report more than 1,000 somatic mutations found in 274 megabases (Mb) of DNA corresponding to the coding exons of 518 protein kinase genes in 210 diverse human cancers. There was substantial variation in the number and pattern of mutations in individual cancers reflecting different exposures, DNA repair defects and cellular origins. Most somatic mutations are likely to be 'passengers' that do not contribute to oncogenesis. However, there was evidence for 'driver' mutations contributing to the development of the cancers studied in approximately 120 genes. Systematic sequencing of cancer genomes therefore reveals the evolutionary diversity of cancers and implicates a larger repertoire of cancer genes than previously anticipated.

The 2.7 Å Crystal Structure of the Autoinhibited Human c-Fms Kinase Domain

c-Fms, a member of the Platelet-derived Growth Factor (PDGF) receptor family of receptor tyrosine kinases (RTKs), is the receptor for macrophage colony stimulating factor (CSF-1) that regulates proliferation, differentiation and survival of cells of the mononuclear phagocyte lineage. Abnormal expression of c-fms proto-oncogene is associated with a significant number of human pathologies, including a variety of cancers and rheumatoid arthritis. Accordingly, c-Fms represents an attractive therapeutic target. To further understand the regulation of c-Fms, we determined the 2.7 A resolution crystal structure of the cytosolic domain of c-Fms that comprised the kinase domain and the juxtamembrane domain. The structure reveals the crucial inhibitory role of the juxtamembrane domain (JM) that binds to a hydrophobic site immediately adjacent to the ATP binding pocket. This interaction prevents the activation loop from adopting an active conformation thereby locking the c-Fms kinase into an autoinhibited state. As observed for other members of the PDGF receptor family, namely c-Kit and Flt3, three JM-derived tyrosine residues primarily drive the mechanism for autoinhibition in c-Fms, therefore defining a common autoinhibitory mechanism within this family. Moreover the structure provides an understanding of c-Fms inhibition by Gleevec as well as providing a platform for the development of more selective inhibitors that target the inactive conformation of c-Fms kinase.

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.

KIT Mutations in Mastocytosis and Their Potential as Therapeutic Targets

Deregulation of the KIT receptor TK by the prevalent activation loop mutation D816V has served as a focal point in therapeutic strategies aimed curbing neoplastic mast cell growth. Perhaps the most important development in this era of targeted therapy, and certainly relevant to KIT-driven diseases like mastocytosis, is the realization that small molecule inhibitors with varied chemical structure (eg, PKC412, dasatinib, AP23464) can circumvent the resistance of TKs to first-generation agents such as imatinib. Genuine opportunity now exists to effectively treat mastocytosis, and the arsenal consists of several orally bioavailable drugs with promising preclinical activity against D816V and other KIT mutants that promote mast cell growth. Because KIT mutations may not act as fully transforming oncogenic events in SM, it is prudent to evaluate combinations of TK inhibitors with drugs with activity in mast cell disease, such as cladribine, interferon-alpha, and corticosteroids. The identification of novel "drug-able" targets within mast cells should aid in the development of complementary therapies that promote enhanced cytotoxicity of mast cells through blockade of nonredundant signaling pathways. In addition, the generation of murine models that recapitulate human mastocytosis should accelerate preclinical testing of novel agents.

Molecular regulation of receptor tyrosine kinases in hematopoietic malignancies

Dysregulation of receptor tyrosine kinase (RTK) activity has been implicated in the progression of a variety of human leukemias. Most notably, mutations and chromosomal translocations affecting regulation of tyrosine kinase activity in the Kit receptor, the Flt3 receptor, and the PDGFbeta/FGF1 receptors have been demonstrated in mast cell leukemia, acute myeloid leukemia (AML), and chronic myelogenous leukemias (CML), respectively. In addition, critical but non-overlapping roles for the Ron and Kit receptor tyrosine kinases in the progression of animal models of erythroleukemia have been demonstrated [Persons, D., Paulson, R., Loyd, M., Herley, M., Bodner, S., Bernstein, A., Correll, P. and Ney, P., 1999. Fv2 encodes a truncated form of the Stk receptor tyrosine kinase. Nat. Gen. 23, 159-165.; Subramanian, A., Teal, H.E., Correll, P.H. and Paulson, R.F., 2005. Resistance to friend virus-induced erythroleukemia in W/Wv mice is caused by a spleen-specific defect which results in a severe reduction in target cells and a lack of Sf-Stk expression. J. Virol. 79 (23), 14586-14594.]. The various classes of RTKs implicated in the progression of leukemia have been recently reviewed [Reilly, J., 2003. Receptor tyrosine kinases in normal and malignant haematopoiesis. Blood Rev. 17 (4), 241-248.]. Here, we will discuss the mechanism by which alterations in these receptors result in transformation of hematopoietic cells, in the context of what is known about the molecular regulation of RTK activity, with a focus on our recent studies of the Ron receptor tyrosine kinase.

Activation of FIP1L1-PDGFRalpha requires disruption of the juxtamembrane domain of PDGFRalpha and is FIP1L1-independent

Genetic abnormalities that result in expression of chimeric tyrosine kinase proteins such as BCR-ABL1 and ETV6-PDGFRbeta are common causes of hematopoietic malignancies. The paradigm for constitutive activation of these fusion tyrosine kinases is enforced homodimerization by self-association domains present in the fusion partner proteins. The unique interstitial deletion on chromosome 4q12 that leads to expression of the FIP1L1-PDGFRalpha fusion tyrosine kinase was recently identified as a cause of chronic eosinophilic leukemia. In this report, we demonstrate that FIP1L1 is completely dispensable for PDGFRalpha activation in vitro and in vivo. Instead, truncation of PDGFRalpha between two conserved tryptophan residues in the juxtamembrane (JM) domain is required for kinase activation and transforming potential of FIP1L1-PDGFRalpha. The presence of a complete JM domain in FIP1L1-PDGFRalpha is inhibitory, but this autoinhibition can be overcome by enforced homodimerization. Similar effects of the JM domain in the context of PDGFRbeta were observed. These results suggest that disruption of the autoinhibitory JM domain is an alternative, dimerization-independent mechanism by which chimeric tyrosine kinases are constitutively activated and induce leukemogenesis.

Point mutations in the juxtamembrane domain of FLT3 define a new class of activating mutations in AML

In acute myeloid leukemia (AML), two clusters of activating mutations are known in the FMS-like tyrosine kinase-3 (FLT3) gene: FLT3-internal tandem duplications (FLT3-ITDs) in the juxtamembrane (JM) domain in 20% to 25% of patients, and FLT3 point mutations in the tyrosine-kinase domain (FLT3-TKD) in 7% to 10% of patients, respectively. Here, we have characterized a new class of activating point mutations (PMs) that cluster in a 16-amino acid stretch of the juxtamembrane domain of FLT3 (FLT3-JM-PMs). Expression of 4 FLT3-JM-PMs in interleukin-3 (IL-3)-dependent Ba/F3 cells led to factor-independent growth, hyperresponsiveness to FLT3 ligand, and resistance to apoptotic cell death. FLT3-JM-PM receptors were autophosphorylated and showed a higher constitutive dimerization rate compared with the FLT3-wild-type (WT) receptor. As a molecular mechanism, we could show activation of STAT5 and up-regulation of Bcl-x(L) by all FLT3-JM-PMs. The FLT3 inhibitor PKC412 abrogated the factor-independent growth of FLT3-JM-PM-expressing cells. Compared with FLT3-ITD and FLT3-TKD mutants, the FLT3-JM-PMs showed a weaker transforming potential related to lower autophosphorylation of the receptor and its downstream target STAT5.

Mapping of the FLT3-JM-PMs on the crystal structure of FLT3 showed that these mutations reduce the stability of the autoinhibitory JM domain, and provides a structural basis for the transforming capacity of this new class of gain-of-function mutations of FLT3.

FMS-like tyrosine kinase 3 in normal hematopoiesis and acute myeloid leukemia

Ligand-mediated activation of the FMS-like tyrosine kinase 3 (FLT3) receptor is important for normal proliferation of primitive hematopoietic cells. However, activating mutations in FLT3 induce ligand-independent downstream signaling that promotes oncogenesis through pathways involved in proliferation, differentiation, and survival. FLT3 mutations are identified as the most frequent genetic abnormality in acute myeloid leukemia and are also observed in other leukemias. Multiple small-molecule inhibitors are under development to target aberrant FLT3 activity that confers a poor prognosis in patients.

Redox regulation of platelet-derived-growth-factor-receptor: Role of NADPH-oxidase and c-Src tyrosine kinase

This study identifies some early events contributing to the redox regulation of platelet-derived growth factor receptor (PDGFr) activation and its signalling in NIH3T3 fibroblasts. We demonstrate for the first time that the redox regulation of PDGFr tyrosine autophosphorylation and its signalling are related to NADPH oxidase activity through protein kinase C (PKC) and phosphoinositide-3-kinase (PI3K) activation and H2O2 production. This event is also essential for complete PDGF-induced activation of c-Src kinase by Tyr416 phosphorylation, and the involvement of c-Src kinase on H2O2-induced PDGFr tyrosine phosphorylation is demonstrated, suggesting a role of this kinase on the redox regulation of PDGFr activation. Finally, it has been determined that not only PI3K activity, but also PKC activity, are related to NADPH oxidase activation due to PDGF stimulation in NIH3T3 cells, as it occurs in non-phagocyte cells. Therefore, we suggest a redox circuit whereby, upon PDGF stimulation, PKC, PI3K and NADPH oxidase activity contribute to complete c-Src kinase activation, thus promoting maximal phosphorylation and activation of PDGFr tyrosine phosphorylation.

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.

A Gain of Function Mutation in the Activation Loop of Plateletderived Growth Factor β-Receptor Deregulates Its Kinase Activity

The platelet-derived growth factor receptors (PDGFRs) are receptor tyrosine kinases implicated in multiple aspects of cell growth, differentiation, and survival. Recently, a gain of function mutation in the activation loop of the human PDGFRalpha has been found in patients with gastrointestinal stromal tumors. Here we show that a mutation in the corresponding codon in the activation loop of the murine PDGFRbeta, namely an exchange of asparagine for aspartic acid at amino acid position 849 (D849N), confers transforming characteristics to embryonic fibroblasts from mutant mice, generated by a knock-in strategy. By comparing the enzymatic properties of the wild-type versus the mutant receptor protein, we demonstrate that the D849N mutation lowers the threshold for kinase activation, causes a dramatic alteration in the pattern of tyrosine phosphorylation kinetics following ligand stimulation, and induces a ligand-independent phosphorylation of several tyrosine residues. These changes result in deregulated recruitment of specific signal transducers. The GTPase-activating protein for Ras (RasGAP), a negative regulator of the Ras mitogenic pathway, displayed a delayed binding to the mutant receptor. Moreover, we have observed enhanced ligand-independent ERK1/2 activation and an increased proliferation of mutant cells. The p85 regulatory subunit of the phosphatidylinositol 3 '-kinase was constitutively associated with the mutant receptor, and this ligand-independent activation of the phosphatidylinositol 3'-kinase pathway may explain the observed strong protection against apoptosis and increased motility in cellular wounding assays. Our findings support a model whereby an activating point mutation results in a deregulated PDGFRbeta with oncogenic predisposition.

A juxtamembrane tyrosine in the colony stimulating factor-1 receptor regulates ligand-induced Src association, receptor kinase function, and down-regulation

Recent literature implicates a regulatory function of the juxtamembrane domain (JMD) in receptor tyrosine kinases. Mutations in the JMD of c-Kit and Flt3 are associated with gastrointestinal stromal tumors and acute myeloid leukemias, respectively. Additionally, autophosphorylated Tyr559 in the JMD of the colony stimulating factor-1 (CSF-1) receptor (CSF-1R) binds to Src family kinases (SFKs). To investigate SFK function in CSF-1 signaling we established stable 32D myeloid cell lines expressing CSF-1Rs with mutated SFK binding sites (Tyr559-TFI). Whereas binding to I562S was not significantly perturbed, Y559F and Y559D exhibited markedly decreased CSF-1-dependent SFK association. All JMD mutants retained intrinsic kinase activity, but Y559F, and less so Y559D, showed dramatically reduced CSF-1-induced autophosphorylation. CSF-1-mediated wild-type (WT)-CSF-1R phosphorylation was not markedly affected by SFK inhibition, indicating that lack of SFK binding is not responsible for diminished Y559F phosphorylation. Unexpectedly, cells expressing Y559F were hyperproliferative in response to CSF-1. Hyperproliferation correlated with prolonged activation of Akt, ERK, and Stat5 in the Y559F mutant. Consistent with a defect in receptor negative regulation, c-Cbl tyrosine phosphorylation and CSF-1R/c-Cbl co-association were almost undetectable in the Y559F mutant. Furthermore, Y559F underwent reduced multiubiquitination and delayed receptor internalization and degradation. In conclusion, we propose that Tyr559 is a switch residue that functions in kinase regulation, signal transduction and, indirectly, receptor down-regulation. These findings may have implications for the oncogenic conversion of c-Kit and Flt3 with JMD mutations.

Molecular and cytogenetic characterization of a novel translocation t(4;22) involving the breakpoint cluster region and platelet-derived growth factor receptor-alpha genes in a patient with atypical chronic myeloid leukemia

We report a case of BCR-ABL-negative atypical chronic myeloid leukemia (CML) with translocation t(4;22) (q12;q11.2) juxtaposing the breakpoint cluster region (BCR) and platelet-derived growth factor receptor-alpha (PDGFRA) genes. The patient was a 57-year-old man with a history of stage IV diffuse large B-cell lymphoma, status post-6 cycles of combination chemotherapy in 1999, who presented in August 2002 with enlarged lymph nodes, anemia, and marked leukocytosis (50 x 10(9) g/dL) consistent with a myeloproliferative disorder (MPD). A bone marrow biopsy showed granulocytic hyperplasia, neutrophilia, and mild eosinophilia. Initial cytogenetic evaluation by interphase FISH for BCR-ABL, to rule out a translocation 9;22, showed a variant signal pattern consistent with rearrangement of BCR at 22q11.2, but not ABL at 9q34. Analysis of the patient's cDNA by polymerase chain reaction (PCR) for BCR-ABL was negative. Cytogenetic analysis showed an abnormal karyotype with rearrangement of chromosomes 4 and 22. PCR amplification and subsequent sequence analysis demonstrated an in-frame 5'-BCR/3'-PDGFRA fusion in the patient's cDNA. PDGFRA encodes a receptor tyrosine kinase and shares structural and organizational homology with the KIT and CSf1R receptor genes. However, although the incidence of MPD involving translocations of PDGFRB has been well established, to our knowledge there are only two previous reports describing a BCR-PDGFRA fusion gene, in 3 patients diagnosed with atypical CML. Here, we report the molecular and cytogenetic characterization of a patient with BCR-PDGFRA-positive MPD who had a complete hematologic response after treatment with imatinib mesylate.

Structural basis for the autoinhibition and STI-571 inhibition of c-Kit tyrosine kinase

The activity of the c-Kit receptor protein-tyrosine kinase is tightly regulated in normal cells, whereas deregulated c-Kit kinase activity is implicated in the pathogenesis of human cancers. The c-Kit juxtamembrane region is known to have an autoinhibitory function; however the precise mechanism by which c-Kit is maintained in an autoinhibited state is not known. We report the 1.9-A resolution crystal structure of native c-Kit kinase in an autoinhibited conformation and compare it with active c-Kit kinase. Autoinhibited c-Kit is stabilized by the juxtamembrane domain, which inserts into the kinase-active site and disrupts formation of the activated structure. A 1.6-A crystal structure of c-Kit in complex with STI-571 (Imatinib or Gleevec) demonstrates that inhibitor binding disrupts this natural mechanism for maintaining c-Kit in an autoinhibited state. Together, these results provide a structural basis for understanding c-Kit kinase autoinhibition and will facilitate the structure-guided design of specific inhibitors that target the activated and autoinhibited conformations of c-Kit kinase.

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 structural basis for autoinhibition of FLT3 by the juxtamembrane domain

FLT3 is a type III receptor tyrosine kinase that is thought to play a key role in hematopoiesis. Certain classes of FLT3 mutations cause constitutively activated forms of the receptor that are found in significant numbers of patients with acute myelogenous leukemia (AML). The mutations occur either in the activation loop, for example, as point mutations of Asp835 or as internal tandem duplication (ITD) sequences in the juxtamembrane (JM) domain. To further understand the nature of FLT3 autoinhibition and regulation, we have determined the crystal structure of the autoinhibited form of FLT3. This structure shows the autoinhibitory conformation of a complete JM domain in this class of receptor tyrosine kinases. The detailed inhibitory mechanism of the JM domain is revealed, which is likely utilized by other members of type III receptor tyrosine kinases.

Autoinhibition of the platelet-derived growth factor beta-receptor tyrosine kinase by its C-terminal tail

In this report, we investigated the role of the C-terminal tail of the platelet-derived growth factor (PDGF) beta-receptor in the control of the receptor kinase activity. Using a panel of PDGF beta-receptor mutants with progressive C-terminal truncations, we observed that deletion of the last 46 residues, which contain a proline- and glutamic acid-rich motif, increased the autoactivation velocity in vitro and the V(max) of the phosphotransfer reaction, in the absence of ligand, as compared with wild-type receptors. By contrast, the kinase activity of mutant and wild-type receptors that were pre-activated by treatment with PDGF was comparable. Using a conformation-sensitive antibody, we found that truncated receptors presented an active conformation even in the absence of PDGF. A soluble peptide containing the Pro/Glu-rich motif specifically inhibited the PDGF beta-receptor kinase activity. Whereas deletion of this motif was not enough to confer ligand-independent transforming ability to the receptor, it dramatically enhanced the effect of the weakly activating D850N mutation in a focus formation assay. These findings indicate that allosteric inhibition of the PDGF beta-receptor by its C-terminal tail is one of the mechanisms involved in keeping the receptor inactive in the absence of ligand.

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.