Receptor tyrosine kinases in normal and malignant haematopoiesis

Impact of the overexpression of the tyrosine kinase receptor RET in the hematopoietic potential of induced pluripotent stem cells (iPSCs)

INTRODUCTION

Previous studies have suggested that the tyrosine kinase receptor RET plays a significant role in the hematopoietic potential in mice and could also be used to expand cord-blood derived hematopoietic stem cells (HSCs). The role of RET in human iPSC-derived hematopoiesis has not been tested so far.

METHODS

To test the implication of RET on the hematopoietic potential of iPSCs, we activated its pathway with the lentiviral overexpression of RETWT or RETC634Y mutation in normal iPSCs. An iPSC derived from a patient harboring the RETC634Y mutation (iRETC634Y) and its CRISPR-corrected isogenic control iPSC (iRETCTRL) were also used. The hematopoietic potential was tested using 2D cultures and evaluated regarding the phenotype and the clonogenic potential of generated cells.

RESULTS

Hematopoietic differentiation from iPSCs with RET overexpression (WT or C634Y) led to a significant reduction in the number and in the clonogenic potential of primitive hematopoietic cells (CD34+/CD38-/CD49f+) as compared to control iPSCs. Similarly, the hematopoietic potential of iRETC634Y was reduced as compared to iRETCTRL. Transcriptomic analyses revealed a specific activated expression profile for iRETC634Y compared to its control with evidence of overexpression of genes which are part of the MAPK network with negative hematopoietic regulator activities.

CONCLUSION

RET activation in iPSCs is associated with an inhibitory activity in iPSC-derived hematopoiesis, potentially related to MAPK activation.

Strategy for Leukemia Treatment Targeting SHP-1,2 and SHIP

Protein tyrosine phosphatases (PTPs) are modulators of cellular functions such as differentiation, metabolism, migration, and survival. PTPs antagonize tyrosine kinases by removing phosphate moieties from molecular signaling residues, thus inhibiting signal transduction. Two PTPs, SHP-1 and SHP-2 (SH2 domain-containing phosphatases 1 and 2, respectively) and another inhibitory phosphatase, SH2 domain-containing inositol phosphatase (SHIP), are essential for cell function, which is reflected in the defective phenotype of mutant mice. Interestingly, SHP-1, SHP-2, and SHIP mutations are identified in many cases of human leukemia. However, the impact of these phosphatases and their mutations regarding the onset and progression of leukemia is controversial. The ambiguity of the role of these phosphatases imposes challenges on the development of targeting therapies for leukemia. This fundamental problem, confronted by the expanding investigational field of leukemia, will be addressed in this review, which will include a discussion of the molecular mechanisms of SHP-1, SHP-2, and SHIP in normal hematopoiesis and their role in leukemia. Clinical development of leukemic therapies achieved by targeting these phosphatases will be addressed as well.

Silencing of long noncoding RNA TUG1 inhibits viability and promotes apoptosis of acute myeloid leukemia cells by targeting microRNA-221-3p/KIT axis

OBJECTIVE: Acute myeloid leukemia (AML) is a hematological malignancy. This study was attempted to uncover the effects of long noncoding RNA taurine-upregulated gene1 (TUG1) on the viability and apoptosis of AML cells. METHODS: QRT-PCR was implemented to examine the expression of TUG1, miR-221-3p and KIT in AML. The correlation between TUG1 and clinicopathological features of AML patients was evaluated. The effect of TUG1 on AML cells were studied by RNA interference approach. AML cells were transfected with miR-221-3p mimic and miR-221-3p inhibitor, respectively. Then the viability and apoptosis of AML cells were examined by MTT and flow cytometry assay, respectively. Additionally, dual-luciferase reporter assay was used to confirm the interactions among TUG1, miR-221-3p and KIT. Western blot was applied to analyze protein expression of KIT. RESULTS: The expression of TUG1 and KIT was up-regulated in AML, but miR-221-3p was down-regulated. TUG1 expression had obviously correlation with World Health Organization (WHO) grade in AML patients. The functional experiment stated that TUG1 silencing suppressed the viability and accelerated the apoptosis of AML cells. Moreover, the mechanical experiment demonstrated that TUG1 and KIT were both targeted by miR-221-3p with the complementary binding sites at 3’UTR. Up-regulation of miR-221-3p inhibited the protein expression of KIT. Furthermore, in the feedback experiment, miR-221-3p inhibition or KIT overexpression reversed the repression of tumor behavior induced by TUG1 silencing. CONCLUSIONS: TUG1 silencing retarded viability and promoted apoptosis of AML cells via regulating miR-221-3p/KIT axis, providing a potential therapeutic target for AML.

Transcriptional Regulation of Natural Killer Cell Development and Functions

Natural killer (NK) cells are the major lymphocyte subset of the innate immune system. Their ability to mediate anti-tumor cytotoxicity and produce cytokines is well-established. However, the molecular mechanisms associated with the development of human or murine NK cells are not fully understood. Knowledge is being gained about the environmental cues, the receptors that sense the cues, signaling pathways, and the transcriptional programs responsible for the development of NK cells. Specifically, a complex network of transcription factors (TFs) following microenvironmental stimuli coordinate the development and maturation of NK cells. Multiple TFs are involved in the development of NK cells in a stage-specific manner. In this review, we summarize the recent advances in the understandings of TFs involved in the regulation of NK cell development, maturation, and effector function, in the aspects of their mechanisms, potential targets, and functions.

Application of targeted next generation sequencing for the mutational profiling of patients with acute lymphoblastic leukemia

Background

Acute lymphoblastic leukemia (ALL) is the most common cancer in children, whereas it is less common in adults. Identification of cytogenetic aberrations and a small number of molecular abnormalities are still the most important risk and therapy stratification methods in clinical practice today. Next generation sequencing (NGS) technology provides a large amount of data contributing to elucidation of mutational landscape of childhood (cALL) and adult ALL (aALL).

Methods

We analyzed DNA samples from 34 cALL and aALL patients, using NGS targeted sequencing TruSeq Amplicon – Cancer Panel (TSACP) which targets mutational hotspots in 48 cancer related genes.

Results

We identified a total of 330 variants in the coding regions, out of which only 95 were potentially protein-changing. Observed in individual patients, detected mutations predominantly disrupted Ras/RTK pathway (STK11,KIT,MET,NRAS,KRAS,PTEN). Additionally, we identified 5 patients with the same mutation inHNF1Agene, disrupting both Wnt and Notch signaling pathway. In two patients we detected variants inNOTCH1gene.HNF1AandNOTCH1variants were mutually exclusive, while genes involved in Ras/RTK pathway exhibit a tendency of mutation accumulation.

Conclusions

Our results showed that ALL contains low number of mutations, without significant differences between cALL and aALL (median per patient 2 and 3, respectively). Detected mutations affect few key signaling pathways, primarily Ras/RTK cascade. This study contributes to knowledge of ALL mutational landscape, leading to better understanding of molecular basis of this disease.

Platelet-derived growth factor receptor/platelet-derived growth factor (PDGFR/PDGF) system is a prognostic and treatment response biomarker with multifarious therapeutic targets in cancers

Progress in cancer biology has led to an increasing discovery of oncogenic alterations of the platelet-derived growth factor receptors (PDGFRs) in cancers. In addition, their overexpression in numerous cancers invariably makes PDGFRs and platelet-derived growth factors (PDGFs) prognostic and treatment markers in some cancers. The oncologic alterations of the PDGFR/PDGF system affect the extracellular, transmembrane and tyrosine kinase domains as well as the juxtamembrane segment of the receptor. The receptor is also involved in fusions with intracellular proteins and receptor tyrosine kinase. These discoveries undoubtedly make the system an attractive oncologic therapeutic target. This review covers elementary biology of PDGFR/PDGF system and its role as a prognostic and treatment marker in cancers. In addition, the multifarious therapeutic targets of PDGFR/PDGF system are discussed. Great potential exists in the role of PDGFR/PDGF system as a prognostic and treatment marker and for further exploration of its multifarious therapeutic targets in safe and efficacious management of cancer treatments.

The platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) are major players in oncogenesis, drug resistance, and attractive oncologic targets in cancer

The platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) play a key role in signaling pathways in oncogenesis. The overexpression of PDGFs and PDGFRs and the oncogenic alterations of these receptors have been implicated in human cancers and correlated significantly with poor outcomes. This review discusses the biology of the PDGF isoforms and receptors briefly, and their role in oncogenesis. Also, the attractiveness of targeting PDGFs and PDGFRs, based on a wide display of oncologic alterations in cancers, diverse therapeutic strategies, their roles in resistance to cancer treatments with prospects of overcoming drug resistance, and the extent to which validated biomarkers have been developed for effective PDGFs and PDGFRs-based cancer management are discussed.

Perspectives of differentiation therapies of acute myeloid leukemia: the search for the molecular basis of patients' variable responses to 1,25-dihydroxyvitamin d and vitamin d analogs

The concept of differentiation therapy of cancer is ~40 years old. Despite many encouraging results obtained in laboratories, both in vitro and in vivo studies, the only really successful clinical application of differentiation therapy was all-trans-retinoic acid (ATRA)-based therapy of acute promyelocytic leukemia (APL). ATRA, which induces granulocytic differentiation of APL leukemic blasts, has revolutionized the therapy of this disease by converting it from a fatal to a curable one. However, ATRA does not work for other acute myeloid leukemias (AMLs). Since 1,25-dihydroxyvitamin D₃ (1,25D) is capable of inducing monocytic differentiation of leukemic cells, the idea of treating other AMLs with vitamin D analogs (VDAs) was widely accepted. Also, some types of solid cancers responded to in vitro applied VDAs, and hence it was postulated that VDAs can be used in many clinical applications. However, early clinical trials in which cancer patients were treated either with 1,25D or with VDAs, did not lead to conclusive results. In order to search for a molecular basis of such unpredictable responses of AML patients toward VDAs, we performed ex vivo experiments using patient’s blast cells. Experiments were also performed using 1,25D-responsive and 1,25D-non-responsive cell lines, to study their mechanisms of resistance toward 1,25D-induced differentiation. We found that one of the possible reasons might be due to a very low expression level of vitamin D receptor (VDR) mRNA in resistant cells, which can be increased by exposing the cells to ATRA. Our considerations concerning the molecular mechanism behind the low VDR expression and its regulation by ATRA are reported in this paper.

Molecular therapeutic approaches to acute myeloid leukemia: targeting aberrant chromatin dynamics and signal transduction

Acute myeloid leukemia research and clinical management have greatly benefited from the achievements in molecular biology regarding the identification of the underlying pathogenetic mechanisms of transformation and resistance to therapy. In particular, two categories of alterations, the aberrant activity of transcription/chromatin-remodeling factors and the deregulated activation of signal transduction pathways, have been demonstrated to play a pivotal role in leukemic cell differentiation, proliferation and resistance to apoptosis. These molecular lesions have proven to be suitable therapeutic targets in acute promyelocytic leukemia and chronic myeloid leukemia and are now also seen as therapeutic targets for a wider group of leukemic disorders. The development of novel drugs such as histone deacetylase inhibitors, demethylating agents and inhibitors of receptor tyrosine kinases may potentially benefit acute myeloid leukemia patients.

Orphan receptor tyrosine kinases ROR1 and ROR2 in hematological malignancies

The receptor tyrosine kinase ROR1 has been shown to be overexpressed in chronic lymphocytic leukemia (CLL). The aim of this study was to further characterize the expression of ROR1 and the other member of the ROR family, ROR2, in other lymphoid and myeloid malignancies. Normal white blood cells and reactive lymph nodes were negative for ROR1 and ROR2. A significantly high and uniform surface expression of ROR1 was found in CLL/hairy cell leukemia (HCL) compared to mantle cell lymphoma (MCL), marginal zone lymphoma (MZL), diffuse large B-cell lymphoma (DLBCL), follicular lymphoma (FL), myelomas, acute lymphoblastic leukemia (ALL) and myeloid leukemias (p = 0.02 to < 0.001). The lowest proportion of ROR1+ cells was seen in FL, whereas CLL, HCL and CML had significantly higher numbers of ROR1+ cells. Longitudinal follow-up of individual patients with CLL revealed that ROR1+ cells remained stable over time in non-progressive patients, but increased when the disease progressed (p < 0.05). Thus, a variable staining pattern of ROR1 ranging from very high (CLL, HCL) and high (CML) to intermediate (myeloma and DLBCL) or low (FL) was noted. ROR2 was not detected in hematological malignancies.

Ras in cancer and developmental diseases

Somatic, gain-of-function mutations in ras genes were the first specific genetic alterations identified in human cancer about 3 decades ago. Studies during the last quarter century have characterized the Ras proteins as essential components of signaling networks controlling cellular proliferation, differentiation, or survival. The oncogenic mutations of the H-ras, N-ras, or K-ras genes frequently found in human tumors are known to throw off balance the normal outcome of those signaling pathways, thus leading to tumor development. Oncogenic mutations in a number of other upstream or downstream components of Ras signaling pathways (including membrane RTKs or cytosolic kinases) have been detected more recently in association with a variety of cancers. Interestingly, the oncogenic Ras mutations and the mutations in other components of Ras/MAPK signaling pathways appear to be mutually exclusive events in most tumors, indicating that deregulation of Ras-dependent signaling is the essential requirement for tumorigenesis. In contrast to sporadic tumors, separate studies have identified germline mutations in Ras and various other components of Ras signaling pathways that occur in specific association with a number of different familial, developmental syndromes frequently sharing common phenotypic cardiofaciocutaneous features. Finally, even without being a causative force, defective Ras signaling has been cited as a contributing factor to many other human illnesses, including diabetes and immunological and inflammatory disorders. We aim this review at summarizing and updating current knowledge on the contribution of Ras mutations and altered Ras signaling to development of various tumoral and nontumoral pathologies.

MicroRNA-193b regulates c-Kit proto-oncogene and represses cell proliferation in acute myeloid leukemia

Mutations and/or overexpression of c-Kit proto-oncogene frequently occur in subsets of acute myeloid leukemia (AML) and contribute to abnormal cell proliferation and poor outcomes. We showed that c-Kit expression was subject to post-transcriptional regulation by microRNA (miRNA)-193b. Notably, miR-193b was significantly down-regulated in the examined AML cells and its levels were inversely correlated with c-Kit levels. Restoration of miR-193b expression in AML cells resulted in distinctly reduced c-Kit expression and inhibited cell growth. These data reveal a role for miR-193b dysregulation in myeloid leukemogenesis and the therapeutic promise of regulating miR-193b expression for c-Kit-positive AML.

MicroRNA-193a represses c-kit expression and functions as a methylation-silenced tumor suppressor in acute myeloid leukemia

Aberrant activation of c-kit proto-oncogene contributes to abnormal cell proliferation by altering the tyrosine kinase signaling and constitutes a crucial impetus for leukemogenesis. Epigenetic silencing of tumor-suppressive microRNAs (miRNAs) is a key oncogenic mechanism for the activation of oncogenes in tumors. In this study, several miRNAs potentially binding to the 3'-untranslated region of human c-kit mRNA were screened by luciferase reporter assays. Among these miRNAs, miR-193a was embedded in a CpG island and epigenetically repressed by promoter hypermethylation in acute myeloid leukemia (AML) cell lines and primary AML blasts, but not in normal bone marrow cells. Importantly, miR-193a levels were inversely correlated with c-kit levels measured in 9 leukemia cell lines and 27 primary AML samples. Restoring miR-193a expression in AML cells harboring c-kit mutation and/or overexpression, either by synthetic miR-193a transfection or by DNA hypomethylating agent 5-azacytidine (5-aza) treatment, resulted in a significant reduction in c-kit expression at both RNA and protein levels and inhibition of cell growth. The growth-inhibitory activity of miR-193a was associated with apoptosis and granulocytic differentiation. Moreover, 5-aza-induced c-kit reduction could be partially blocked by miR-193a inhibitor, leading to a reversal of antiproliferative and proapoptotic effects of 5-aza. These data reveal a critical role for methylation-repressed miR-193a in myeloid leukemogenesis and the therapeutic promise of upregulating miR-193a expression for c-kit-positive AML.

Stem cell factor expression after renal ischemia promotes tubular epithelial survival

Background

Renal ischemia leads to apoptosis of tubular epithelial cells and results in decreased renal function. Tissue repair involves re-epithelialization of the tubular basement membrane. Survival of the tubular epithelium following ischemia is therefore important in the successful regeneration of renal tissue. The cytokine stem cell factor (SCF) has been shown to protect the tubular epithelium against apoptosis.

Methodology/Principal Findings

In a mouse model for renal ischemia/reperfusion injury, we studied how expression of c-KIT on tubular epithelium and its ligand SCF protect cells against apoptosis. Administration of SCF specific antisense oligonucleotides significantly decreased specific staining of SCF following ischemia. Reduced SCF expression resulted in impaired renal function, increased tubular damage and increased tubular epithelial apoptosis, independent of inflammation. In an in vitro hypoxia model, stimulation of tubular epithelial cells with SCF activated survival signaling and decreased apoptosis.

Conclusions/Significance

Our data indicate an important role for c-KIT and SCF in mediating tubular epithelial cell survival via an autocrine pathway.

[Tyrosine kinases in etiopathogenesis and therapy of malignant diseases--C-kit activating mutations]

In the last 15 years, the introduction of molecular biology methods and techniques for identifying mutations and measuring gene expression levels of mutated genes since recently, have enabled precise molecular diagnostics, classification and assessment of prognosis and therapeutic response of malignant disease to specific therapies. The increased knowledge of the cancer genome and the introduction of multiple new technologies in cancer research have significantly improved the drug discovery process, leading to key success in targeted cancer therapeutics, including tyrosine kinase inhibitors. Tyrosine kinase inhibitors are the molecular targeted neoadjuvant and adjuvant therapy of various malignancies. Many more results which are expected from ongoing trials are necessary to specify the appropriate dosages, stages at which to start the treatment, and which therapeutic combinations to apply.

Quantitative assay for the detection of the V617F variant in the Janus kinase 2 (JAK2) gene using the Luminex xMAP technology

Background

The availability of clinically valid biomarkers contribute to improve the diagnosis and clinical management of diseases. A valine-to-phenylalanine substitution at position 617 (V617F) in the Janus kinase 2 (JAK2) gene has been recently associated with key signaling abnormalities in the transduction of haemopoietic growth-factor receptors and is now considered as a useful clinical marker of myeloproliferative neoplasms. Several methods have recently been reported to detect the JAK2 V617F point mutation and show variable sensitivity.

Methods

Using the Luminex xMAP technology, we developed a quantitative assay to detect the JAK2V617F variant. The method was based on polymerase chain reaction (PCR) followed by hybridization to specific probes coupled with internally dyed microspheres. The assay comprises 3 steps: genomic DNA extraction, end point PCR reaction, direct hybridization of PCR fragments and quantification. It has been tested with different sources of nucleic acid.

Results

Applied to whole blood samples, this quantitative assay showed a limit of detection of 2%. A highly sensitive allele-specific primer extension reaction performed in parallel allowed to validate the results and to identify the specimens with values below 2%.

Conclusion

Direct hybridization assay using the Luminex xMAP technology allows sensitive quantification of JAK2V617F from blood spots. It is simple and can be easily performed in a clinical setting.

Expression profiling of a hemopoietic cell survival transcriptome implicates osteopontin as a functional prognostic factor in AML

Deregulated cell survival programs are a classic hallmark of cancer. We have previously identified a serine residue (Ser585) in the βc subunit of the granulocyte-macrophage colony-stimulating factor receptor that selectively and independently promotes cell survival. We now show that Ser585 phosphorylation is constitutive in 20 (87%) of 23 acute myeloid leukemia (AML) patient samples, indicating that this survival-only pathway is frequently deregulated in leukemia. We performed a global expression screen to identify gene targets of this survival pathway and report a 138-gene βc Ser585-regulated transcriptome. Pathway analysis defines a gene network enriched for PI3-kinase target genes and a cluster of genes involved in cancer and cell survival. We show that one such gene, osteopontin (OPN), is a functionally relevant target of the Ser585-survival pathway as shown by siRNA-mediated knockdown of OPN expression that induces cell death in both AML blasts and CD34+CD38−CD123+ leukemic progenitors. Increased expression of OPN at diagnosis is associated with poor prognosis with multivariate analysis indicating that it is an independent predictor of overall patient survival in normal karyotype AML (n = 60; HR = 2.2; P = .01). These results delineate a novel cytokine-regulated Ser585/PI3-kinase signaling network that is deregulated in AML and identify OPN as a potential prognostic and therapeutic target.

JAK2 inhibitors: A reality? A hope?

Myelofibrosis (MF; primary or post-polycythemia vera/essential thrombocythemia) carries the worst prognosis among BCR-ABL-negative myeloproliferative neoplasms (MPNs). Stem cell transplantation is the only curative approach but is hampered by significant nonrelapse mortality. Thus, effective, targeted therapies are needed. A mutated Janus kinase 2 (JAK2) gene (JAK2(V617F)), found in a significant portion of patients with MPN, results in increased JAK2 tyrosine kinase activity, leading to clonal proliferation; several small molecules inhibit the growth of hematopoietic colonies harboring JAK2(V617). Several JAK2 inhibitors have reached the clinical trial stage and are reviewed here. The most developed among them is INCB018424, which has demonstrated noteworthy clinical activity, with a rapid and profound reduction in splenomegaly and associated improvement in constitutional symptoms in MF patients receiving 10-25 mg orally twice daily, continuously. Thrombocytopenia (reversible) was the most common adverse event, seen in 30% of patients treated with 25 mg twice daily but not with 10 mg twice daily. Interestingly, INCB018424 was equally active in patients with and without JAK2 mutation. Other JAK2 inhibitors are less developed but show a similar type of clinical benefit. Conclusively, JAK2 inhibitors, particularly INCB018424, are clinically active in MF and are well tolerated. Whether they have an effect on the natural course of MF in treated patients remains to be elucidated.

Persistence of leukemia-initiating cells in a conditional knockin model of an imatinib-responsive myeloproliferative disorder

Despite remarkable responses to the tyrosine kinase inhibitor imatinib, CML patients are rarely cured by this therapy perhaps due to imatinib refractoriness of leukemia-initiating cells (LICs). Evidence for this is limited because of poor engraftment of human CML-LICs in NOD-SCID mice and nonphysiologic expression of oncogenes in retroviral transduction mouse models. To address these challenges, we generated mice bearing conditional knockin alleles of two human oncogenes: HIP1/PDGFbetaR (H/P) and AML1-ETO (A/E). Unlike retroviral transduction, physiologic expression of H/P or A/E individually failed to induce disease, but coexpression of both H/P and A/E led to rapid onset of a fully penetrant, myeloproliferative disorder, indicating cooperativity between these two alleles. Although imatinib dramatically decreased disease burden, LICs persisted, demonstrating imatinib refractoriness of LICs.

Translational research in complex etiopathogenesis and therapy of hematological malignancies: the specific role of tyrosine kinases signaling and inhibition

During the recent genomics and proteomics era, high-resolution, genome-wide approaches have revealed numerous promising new drug targets and disease biomarkers, accelerating and emphasizing the need for targeted molecular therapy compounds. Significant progress has been made in understanding the pathogenesis of hematological malignancies there by, revealing new drug targets. Introduction of multiple new technologies in cancer research have significantly improved the drug discovery process, leading to key success in targeted cancer therapeutics, including tyrosine kinase inhibitors. The studies of receptor tyrosine kinases and their role in malignant transformation are already translated from the preclinical level (cell-based and animal models) to clinical studies, enabling the more complete understanding of tumor cell biology and improvement of tumor therapy.

Multi-stage Friend murine erythroleukemia: molecular insights into oncogenic cooperation

The Friend virus SFFV (Spleen Focus Forming Virus) provokes an acute erythroblastosis in susceptible strains of mice that progresses to overt erythroleukemia by a multi-step process. For virologists, the Friend virus-induced disease has provided deep insights into the host mechanisms influencing susceptibility to retroviral infection and viremia. These insights have contributed to the understanding of HIV and other human retroviral infections. For cell biologists and oncologists, this leukemia has been a powerful experimental model to identify critical oncogenes involved in a multi-stage process, to understand the contribution of host genes to cancer development, and to investigate the mechanisms leading to cell growth autonomy. This model also provided an example of oncogenic reversion since Friend tumor cells can reinitiate their erythroid differentiation program when exposed in vitro to some chemical inducers. This review highlights recent findings demonstrating that the leukemic progression depends on the cooperation of at least two oncogenic events, one interfering with differentiation and one conferring a proliferative advantage. The Friend model of leukemia progression recapitulates the two phases of human acute myeloid leukemia (AML). Coupling of insights from studies on the Friend erythroleukemia with knowledge on AML might allow a better understanding of the molecular mechanisms involved in the evolution of leukemia in mice and men.

Nonclinical safety evaluation of sunitinib: a potent inhibitor of VEGF, PDGF, KIT, FLT3, and RET receptors

Sunitinib malate (SUTENT) is a multitargeted receptor tyrosine kinase (RTK) inhibitor that is approved multinationally for the treatment of imatinib-resistant/-intolerant gastrointestinal stromal tumor and advanced renal cell carcinoma. This paper characterizes the organ toxicity of sunitinib in Sprague-Dawley rats and cynomolgus monkeys, and the reversibility of any treatment-induced effects. Rats and monkeys received sunitinib (0-15 and 0-20 mg/kg/day, respectively) orally on a consecutive daily dosing schedule for thirteen weeks or on an intermittent daily dosing schedule for up to nine months. Clinical observations and laboratory parameters were recorded. Necropsy was conducted following treatment/recovery periods, and histologic examinations were performed. In rats, sunitinib was generally tolerated at 0.3 and 1.5 mg/kg/day, and findings were reversible. In monkeys, the level at which there were no observed adverse effects was 1.5 mg/kg/day, and findings were similarly reversible (except for uterine/ovarian weight changes and skin pallor). Data suggest that inhibition of multiple RTK pathways may induce pharmacologic effects on organ systems in nonclinical species. Key pharmacologic effects of sunitinib included reversible inhibition of neovascularization into the epiphyseal growth plate, and impaired corpora lutea formation and uterine development during estrus. Similar observations have been noted with this class of RTK signaling inhibitors and are consistent with pharmacologic perturbations of physiologic/angiogenic processes associated with the intended molecular targets.

Extrinsic and intrinsic regulation of early natural killer cell development

Natural killer (NK) cells are lymphocytes that play a critical role in both adaptive and innate immune responses. These cells develop from multipotent progenitors in the embryonic thymus and neonatal or adult bone marrow and recent evidence suggests that a subset of these cells may develop in the thymus. Thymus- and bone marrow-derived NK cells have unique phenotypes and functional abilities supporting the hypothesis that the microenvironment dictates the outcome of NK cell development. A detailed understanding of the mechanisms controlling this developmental program will be required to determine how alterations in NK cell development lead to disease and to determine how to harness this developmental program for therapeutic purposes. In this review, we discuss some of the known extrinsic stromal-cell derived factors and cell intrinsic transcription factors that function in guiding NK cell development.

Leukemic transformation in mice expressing a NUP98-HOXD13 transgene is accompanied by spontaneous mutations in Nras, Kras, and Cbl

The NUP98-HOXD13 (NHD13) fusion gene occurs in patients with myelodysplastic syndrome (MDS) and acute nonlymphocytic leukemia (ANLL). We reported that transgenic mice expressing NHD13 develop MDS, and that more than half of these mice eventually progress to acute leukemia. The latency period suggests a requirement for at least 1 complementary event before leukemic transformation. We conducted a candidate gene search for complementary events focused on genes that are frequently mutated in human myeloid leukemia. We investigated 22 ANLL samples and found a high frequency of Nras and Kras mutations, an absence of Npm1, p53, Runx1, Kit and Flt3 mutations, and a single Cbl mutation. Our findings support a working hypothesis that predicts that ANLL cases have one mutation which inhibits differentiation, and a complementary mutation which enhances proliferation or inhibit apoptosis. In addition, we provide the first evidence for spontaneous collaborating mutations in a genetically engineered mouse model of ANLL.

Cooperating gene mutations in acute myeloid leukemia: a review of the literature

Acute myeloid leukemia (AML) is a heterogeneous group of neoplastic disorders with great variability in clinical course and response to therapy, as well as in the genetic and molecular basis of the pathology. Major advances in the understanding of leukemogenesis have been made by the characterization and the study of acquired cytogenetic abnormalities, particularly reciprocal translocations observed in AML. Besides these major cytogenetic abnormalities, gene mutations also constitute key events in AML pathogenesis. In this review, we describe the contribution of known gene mutations to the understanding of AML pathogenesis and their clinical significance. To gain more insight in this understanding, we clustered these alterations in three groups: (1) mutations affecting genes that contribute to cell proliferation (FLT3, c-KIT, RAS, protein tyrosine standard phosphatase nonreceptor 11); (2) mutations affecting genes involved in myeloid differentiation (AML1 and CEBPA) and (3) mutations affecting genes implicated in cell cycle regulation or apoptosis (P53, NPM1). This nonexhaustive review aims to show how gene mutations interact with each other, how they contribute to refine prognosis and how they can be useful for risk-adapted therapeutic management of AML patients.

Ezrin is a target for oncogenic Kit mutants in murine erythroleukemia

The model of erythroleukemia caused by Spi-1/PU.1 transgenesis in mice is a multistage disease. A preleukemic step is characterized by an acute proliferation of proerythroblasts due to the arrest of differentiation provoked by Spi-1/PU.1. Later on, a blastic crisis occurs associated with somatic oncogenic mutations in the stem cell factor (SCF) receptor kit. To gain insights into the mechanisms of the leukemic progression, we performed proteomic profiling analyses of proerythroblasts isolated at the 2 stages of the disease. Our results indicate that the level of ezrin, a membrane cytoskeletal crosslinker, is increased in the leukemic cells. We show that Kit oncogenic forms are responsible for ezrin phosphorylation and that phosphorylation rather than overexpression is essential in the leukemic proerythroblasts. Using expression of dominant-negative forms of ezrin, we show that phosphorylation of ezrin on residue Y353 participates in apoptosis resistance, whereas phosphorylation on residue Y145 promotes proliferation of the leukemic cells in vitro and in vivo. Another recurrent oncogenic form of tyrosine kinases (Flt3) most frequently involved in human myeloid leukemia was also able to phosphorylate ezrin. These findings point to a new role for ezrin as signaling player in the development of leukemia, being a downstream effector of oncogenic tyrosine kinases in leukemic blasts.

Heterozygous kit mutants with little or no apparent anemia exhibit large defects in overall hematopoietic stem cell function

OBJECTIVE

The evolutionarily conserved Kit receptor is vital for function of hematopoietic stem cells (HSC). Kit(W-41) (W-41) and Kit(W-42) (W-42) are single residue changes in the KIT intracellular phosphotransferase domain, while Kit(W-v) (W-v) is a single residue change in the ATP binding domain. This study tests how each mutation affects HSC function.

METHODS

Cells in mutant and C57BL/6J(+/+) blood and marrow were compared. Overall HSC function was measured by competitive repopulation. Functions of specific progenitor populations were tested with stage-specific competitive repopulation and standard colony-forming unit assays.

RESULTS

Bone marrow cells from these Kit mutants are severely defective at reconstituting peripheral blood lineages and bone marrow of irradiated recipients, when compared to +/+ control marrow. These defects increased with time. Marrow from W-41/+ and W-v/+ functions similarly but better than marrow from W-41/W-41 and W-42/+, to repopulate the erythroid and lymphoid lineages. Long-term (LT) and short-term (ST) HSC from W-v/+, W-41/W-41, and W-42/+ are more defective at reconstituting bone marrow than LT- and ST-HSC from W-41/+ and +/+. Common myeloid progenitor (CMP) cells from W-42/+ and W-41/W-41 are more defective at producing spleen colonies than CMP from W-v/+ and W-41/+.

CONCLUSION

Heterozygous Kit mutants with little or no apparent anemia exhibit surprisingly large defects in overall HSC function. Multiplying the fractional defects in LT-HSC, ST-HSC, and CMP can account for overall effects of W-v/+, but does not completely account for the defects observed with W-41/+, W-42/+, and W-41/W-41.

Structural basis for stem cell factor-KIT signaling and activation of class III receptor tyrosine kinases

Stem cell factor (SCF) binds to and activates the KIT receptor, a class III receptor tyrosine kinase (RTK), to stimulate diverse processes including melanogenesis, gametogenesis and hematopoeisis. Dysregulation of KIT activation is associated with many cancers. We report a 2.5 A crystal structure of the functional core of SCF bound to the extracellular ligand-binding domains of KIT. The structure reveals a 'wrapping' SCF-recognition mode by KIT, in which KIT adopts a bent conformation to facilitate each of its first three immunoglobulin (Ig)-like domains to interact with SCF. Three surface epitopes on SCF, an extended loop, the B and C helices, and the N-terminal segment, contact distinct KIT domains, with two of the epitopes undergoing large conformational changes upon receptor binding. The SCF/KIT complex reveals a unique RTK dimerization assembly, and a novel recognition mode between four-helix bundle cytokines and Ig-family receptors. It serves as a framework for understanding the activation mechanisms of class III RTKs.

CD133-positive hematopoietic stem cell "stemness" genes contain many genes mutated or abnormally expressed in leukemia

Affymetrix human Hu133A oligonucleotide arrays were used to study the expression profile of CD133+ cord blood (CB) and peripheral blood (PB) using CD133 cell-surface marker. An unsupervised hierarchical clustering of 14,025 valid probe sets showed a clear distinction between the CD133+ cells representing the hematopoietic stem cell (HSC) population and CD133-differentiated cells. Two hundred forty-four genes were found to be upregulated by at least twofold in the CD133-positive cells of both CB and PB compared with the CD133-negative cells. These genes represent the hematopoietic "stemness," whereas the 218 and 304 upregulated genes exclusively in PB and CB, respectively, represent tissue specificity. Some of the stemness genes were also common to HSC genes found to be upregulated in several recently published studies. Among these common stemness genes, we identified several groups of genes that have an important role in hematopoiesis: growth factor receptors, transcription factors, genes that have an important role in development, and genes involved in cell growth. Sixteen selected stemness genes are known to be mutated or abnormally regulated in acute leukemias. It can be suggested that key hematopoietic stemness machinery genes may lead to abnormal proliferation and leukemia upon mutation or change of their expression.

JAK2 V617F in myeloid disorders: molecular diagnostic techniques and their clinical utility: a paper from the 2005 William Beaumont Hospital Symposium on Molecular Pathology

In early 2005, several groups of investigators studying myeloid malignancies described a novel somatic point mutation (V617F) in the conserved autoinhibitory pseudokinase domain of the Janus kinase 2 (JAK2) protein, which plays an important role in normal hematopoietic growth factor signaling. The V617F mutation is present in blood and marrow from a large proportion of patients with classic BCR/ABL-negative chronic myeloproliferative disorders and of a few patients with other clonal hematological diseases such as myelodysplastic syndrome, atypical myeloproliferative disorders, and acute myeloid leukemia. The JAK2 V617F mutation causes constitutive activation of the kinase, with deregulated intracellular signaling that mimics continuous hematopoietic growth factor stimulation. Within 7 months of the first electronic publication describing this new mutation, clinical molecular diagnostic laboratories in the United States and Europe began offering JAK2 mutation testing on a fee-for-service basis. Here, I review the various techniques used by research groups and clinical laboratories to detect the genetic mutation underlying JAK2 V617F, including fluorescent dye chemistry sequencing, allele-specific polymerase chain reaction (PCR), real-time PCR, DNA-melting curve analysis, pyrosequencing, and others. I also discuss diagnostic sensitivity, performance, and other practical concerns relevant to the clinical laboratorian in addition to the potential diagnostic utility of JAK2 mutation tests.

Mutational analysis of the KIT gene in myelodysplastic syndrome (MDS) and MDS-derived leukemia

The progenitor cells of myelodysplastic syndrome (MDS) are thought to undergo a multistep process during their transformation into overt acute leukemia. In this study, the role of mutation of the KIT gene in the extracellular membrane, juxtamembrane and tyrosine kinase domains was investigated in 75 patients with MDS or MDS-derived leukemia (MDS-AML). Mutation was detected in 2 of 15 (13.3%) patients with refractory anemia with excess blasts transformation (RAEB-T), in 1 of 15 (6.6%) patients with chronic myelomonocytic leukemia (CMML), and in 5 of 26 (19.2%) patients with MDS-AML. However, no mutation was found in any of the nine patients with refractory anemia (RA) or the 10 patients with refractory anemia with excess blasts (RAEB). Of the mutations, five patients had changes at the same codon in tyrosine kinase domain, Asp816, while the remainder had unique mutations. These observations suggest that KIT gene mutations identified in the advanced stage of MDS, and genetic abnormality in the KIT gene, particularly at codon 816, might be additional events that contribute to the progression of MDS to AML.

Co expression of SCF and KIT in gastrointestinal stromal tumours (GISTs) suggests an autocrine/paracrine mechanism

KIT is a tyrosine kinase receptor expressed by several tumours, which has for specific ligand the stem cell factor (SCF). KIT is the main oncogene in gastrointestinal stromal tumours (GISTs), and gain-of-function KIT mutations are present in 70% of these tumours. The aim of the study was to measure and investigate the mechanisms of KIT activation in 80 KIT-positive GIST patients. KIT activation was quantified by detecting phosphotyrosine residues in Western blotting. SCF production was determined by reverse transcriptase-PCR, ELISA and/or immunohistochemistry. Primary cultures established from three GISTs were also analysed. The results show that KIT activation was detected in all cases, even in absence of KIT mutations. The fraction of activated KIT was not correlated with the mutational status of GISTs. Membrane and soluble isoforms of SCF mRNA were present in all GISTs analysed. Additionally, SCF was also detected in up to 93% of GISTs, and seen to be present within GIST cells. Likewise, the two SCF mRNA isoforms were found to be expressed in GIST-derived primary cultures. Thus, KIT activation in GISTs may in part result from the presence of SCF within the tumours.

The role of Janus kinases in haemopoiesis and haematological malignancy

The production of blood cells is regulated by a number of protein growth factors and cytokines that influence cell survival, proliferation and differentiation. Many of these molecules bind to cell surface receptors, which belong to a family of closely related cytokine receptors that lack intrinsic catalytic activity but are intimately associated with tyrosine kinases of the Janus kinase (JAK) family. Ligand binding induces the activation of JAKs, which sit at the apex of a signalling cascade in which a key role is played by members of the signal transducers and activators of transcription (STAT) group. Congenital deficiencies in JAK-STAT signalling are associated with immunodeficiency states and acquired activating mutations and translocations are involved in the pathophysiology of haematological malignancy. The latter findings have raised hopes that drugs that target aberrant JAK-STAT signalling may be useful for the treatment of human disease.

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.

Analysis of c-kit expression in small cell lung cancer: prevalence and prognostic implications

c-kit, a growth factor receptor with tyrosine kinase activity, plays an important role in the biology of cancer. Its expression has been documented in several malignancies. We performed a retrospective study in 85 patients diagnosed with small cell lung cancer (SCLC) to determine the prevalence and role of c-kit as a possible prognostic marker in this lung cancer malignancy. Demographic and clinical data were obtained from patient charts. c-kit, analyzed as immunohistochemical expression in paraffin-embedded tumour tissues, was observed in 60% of patients. All patients were former or present smokers. At diagnosis, 46% of the patients had limited disease (LD) and 54% extended disease (ED). c-kit expression was observed in 59% of LD and 61% of ED patients (p=0.4). Patients received a median of 4 cycles first-line combination chemotherapy (platinum and etoposide). In LD patients, time to progression (TTP) was 11.5 months in c-kit (+) versus 5.9 in c-kit (-) patients (p=0.14), and median survival 15.4 and 12.8 months, respectively (p=0.33). In the ED group, TTP was 5.5 months in c-kit (+) versus 3.8 in c-kit (-) patients (p=0.34), whereas median survival was 6.3 and 7.9 months, respectively (p=0.45). With the limited number of patients in mind, our findings tended towards an association between c-kit expression and survival in the LD group.

KIT-D816 mutations in AML1-ETO-positive AML are associated with impaired event-free and overall survival

Mutations in codon D816 of the KIT gene represent a recurrent genetic alteration in acute myeloid leukemia (AML). To clarify the biologic implication of activation loop mutations of the KIT gene, 1940 randomly selected AML patients were analyzed. In total, 33 (1.7%) of 1940 patients were positive for D816 mutations. Of these 33 patients, 8 (24.2%) had a t(8;21), which was significantly higher compared with the subgroup without D816 mutations. Analyses of genetic subgroups showed that KIT-D816 mutations were associated with t(8;21)/AML1-ETO and other rare AML1 translocations. In contrast, other activating mutations like FLT3 and NRAS mutations were very rarely detected in AML1-rearranged leukemia. KIT mutations had an independent negative impact on overall (median 304 vs 1836 days; P = .006) and event-free survival (median 244 vs 744 days; P = .003) in patients with t(8;21) but not in patients with a normal karyotype. The KIT-D816V receptor expressed in Ba/F3 cells was resistant to growth inhibition by the selective PTK inhibitors imatinib and SU5614 but fully sensitive to PKC412. Our findings clearly indicate that activating mutations of receptor tyrosine kinases are associated with distinct genetic subtypes in AML. The KIT-D816 mutations confer a poor prognosis to AML1-ETO-positive AML and should therefore be included in the diagnostic workup. Patients with KIT-D816-positive/AML1-ETO-positive AML might benefit from early intensification of treatment or combination of conventional chemotherapy with KIT PTK inhibitors.

C-kit receptor tyrosine kinase (CD117) expression and its positive predictive value for the diagnosis of Thai adult acute myeloid leukemia

We examined the expression of c-kit receptor tyrosine kinase in 195 Thai adult patients with acute leukemia and determined its specificity and predictive values for the diagnosis of adult acute myeloid leukemia (AML). CD117 was used to detect c-kit expression on CD45 and side-scatter-gated blast cells by flow cytometry. Of 163 AML cases, 67% expressed CD117. None of acute lymphoid leukemia (ALL) had CD117 expression, except one case of T-ALL. The majority of AML patients carrying t(8;21), inv(16), and t(15;17) had high CD117 expression. High proportion of AML cases without c-kit expressed monocytic markers. Significant associations between CD117 and CD34 (P<0.001), CD13 (P=0.006), CD7 (P=0.034), and CD19 (P<0.001) were found in AML cases. The calculated specificity of CD117 for the diagnosis of AML was 0.97, which was higher than CD13 (0.78) and CD33 (0.75) but comparable to MPO (0.97). The positive predictive value (PPV) of CD117 for AML was 0.99, with the negative predictive value of 0.35. In conclusion, the majority of Thai adult AML cases expressed c-kit. C-kit is infrequently expressed in ALL and appeared to be specific for AML with high PPV. Future targeting therapy using c-kit as a therapeutic target should benefit the majority of Thai AML patients who had high c-kit expression.

Aberrant methylation of the negative regulators RASSFIA, SHP-1 and SOCS-1 in myelodysplastic syndromes and acute myeloid leukaemia

Mutations in the receptor tyrosine kinase (RTK/RAS) signalling pathway frequently provide a proliferative signal in myeloid malignancies. However, the role of RASSF1A, SHP-1 and SOCS-1, negative regulators of RTK/RAS signalling, has not been extensively investigated in the myelodysplastic syndromes (MDS) or acute myeloid leukaemia (AML). This study employed methylation-specific polymerase chain reaction (MS-PCR) to determine if aberrant promotor methylation of RASSF1A, SHP-1 and SOCS-1 is involved in the pathogenesis of myeloid malignancies. Patients with MDS (n = 107), AML (n = 154) and juvenile myelomonocytic leukaemia (JMML, n = 5) were investigated, together with 15 normal controls. Primers were located in the promotor region of each gene as well as within exon 2 of SOCS-1. Methylation of RASSF1A was found in five of 55 (9%) MDS cases, but not in any of 57 AML cases studied. RASSF1A methylation was present in one case (20%) of JMML. SHP-1 methylation was present in 13 of 121 (11%) AML cases but was not found in MDS or JMML. SOCS-1 promoter methylation was present in eight of 74 (11%) MDS patients but was not seen in JMML or AML. Importantly, RAS mutations and RASSF1A and SOCS-1 methylation were mutually exclusive indicating that approximately 30% of MDS cases had a defect of the RTK/RAS pathway and its negative regulation. Finally, SOCS-1 exon 2 methylation may not be pathogenetically relevant, since it was detected in samples from normal individuals and did not correlate with promotor methylation.

In vivoTarget Modulation and Biological Activity of CHIR-258, a Multitargeted Growth Factor Receptor Kinase Inhibitor, in Colon Cancer Models

PURPOSE

To evaluate the therapeutic and biological effects of CHIR-258, an orally bioavailable, potent inhibitor of class III-V receptor tyrosine kinases, in colon cancer models.

EXPERIMENTAL DESIGN

The pharmacologic activity of CHIR-258 was characterized by monitoring target modulation as well as by evaluating the antitumor and antiangiogenic effects in human colon xenograft models.

RESULTS

CHIR-258 inhibits vascular endothelial growth factor receptor 1/2, fibroblast growth factor receptor 1/3, and platelet-derived growth factor receptor beta (PDGFRbeta) and shows both antitumor and antiangiogenic activities in vivo. Treatment of KM12L4a human colon cancer cells with CHIR-258 resulted in a dose-dependent inhibition of vascular endothelial growth factor receptor 1 and PDGFRbeta phosphorylation and reduction of phosphorylated extracellular signal-regulated kinase (ERK) levels, indicating modulation of target receptors and downstream signaling. In vivo administration of CHIR-258 resulted in significant tumor growth inhibition and tumor regressions, including large, established tumors (500-1,000 mm(3)). Immunohistochemical analysis showed a reduction of phosphorylated PDGFRbeta and phosphorylated ERK in tumor cells after oral dosing with CHIR-258 compared with control tumors. These changes were accompanied by decreased tumor cell proliferation rate and reduced intratumoral microvessel density. CHIR-258 inhibited the phosphorylation of PDGFRbeta and ERK phosphorylation in tumors within 2 hours following dosing and the inhibitory activity was sustained for >24 hours. Significant antitumor activity was observed with intermittent dosing schedules, indicating a sustained biological activity.

CONCLUSION

These studies provide evidence that biological activity of CHIR-258 in tumors correlates with efficacy and aids in the identification of potential biomarkers of this multitargeted receptor tyrosine kinase inhibitor. CHIR-258 exhibits properties that make it a promising candidate for clinical development in a variety of solid and hematologic malignancies.

KIT exon 8 mutations associated with core-binding factor (CBF)–acute myeloid leukemia (AML) cause hyperactivation of the receptor in response to stem cell factor

KIT exon 8 mutations are located in the extracellular portion of the receptor and are strongly associated with core-binding factor (CBF)-acute myeloid leukemia (AML). To characterize the functional role of these mutants, we analyzed the proproliferative and antiapoptotic potential of 3 KIT exon 8 mutations in interleukin 3 (IL-3)-dependent Ba/F3 cells. All KIT exon 8 mutants induced receptor hyperactivation in response to stem cell factor (SCF) stimulation in terms of proliferation and resistance toward apoptotic cell death. A representative KIT exon 8 mutant showed spontaneous receptor dimerization, phosphorylation of mitogen-activated protein kinase (MAPK), and conferred IL-3-independent growth to Ba/F3 cells. MAPK and phosphatidylinositol 3-kinase (PI3-kinase) activation was essential for the phenotype of this mutant. Additionally, imatinib inhibited proliferation of KIT exon 8 mutant-expressing Ba/F3 cells. Our data show that KIT exon 8 mutations represent gain-of-function mutations and might represent a new molecular target for treatment of CBF leukemias. (Blood. 2005;105:3319-3321)

Acquired mutation of the tyrosine kinase JAK2 in human myeloproliferative disorders

BACKGROUND

Human myeloproliferative disorders form a range of clonal haematological malignant diseases, the main members of which are polycythaemia vera, essential thrombocythaemia, and idiopathic myelofibrosis. The molecular pathogenesis of these disorders is unknown, but tyrosine kinases have been implicated in several related disorders. We investigated the role of the cytoplasmic tyrosine kinase JAK2 in patients with a myeloproliferative disorder.

METHODS

We obtained DNA samples from patients with polycythaemia vera, essential thrombocythaemia, or idiopathic myelofibrosis. The coding exons of JAK2 were bidirectionally sequenced from peripheral-blood granulocytes, T cells, or both. Allele-specific PCR, molecular cytogenetic studies, microsatellite PCR, Affymetrix single nucleotide polymorphism array analyses, and colony assays were undertaken on subgroups of patients.

FINDINGS

A single point mutation (Val617Phe) was identified in JAK2 in 71 (97%) of 73 patients with polycythaemia vera, 29 (57%) of 51 with essential thrombocythaemia, and eight (50%) of 16 with idiopathic myelofibrosis. The mutation is acquired, is present in a variable proportion of granulocytes, alters a highly conserved valine present in the negative regulatory JH2 domain, and is predicted to dysregulate kinase activity. It was heterozygous in most patients, homozygous in a subset as a result of mitotic recombination, and arose in a multipotent progenitor capable of giving rise to erythroid and myeloid cells. The mutation was present in all erythropoietin-independent erythroid colonies.

INTERPRETATION

A single acquired mutation of JAK2 was noted in more than half of patients with a myeloproliferative disorder. Its presence in all erythropoietin-independent erythroid colonies demonstrates a link with growth factor hypersensitivity, a key biological feature of these disorders.

RELEVANCE TO PRACTICE

Identification of the Val617Phe JAK2 mutation lays the foundation for new approaches to the diagnosis, classification, and treatment of myeloproliferative disorders.

Oncogenes as Novel Targets for Cancer Therapy (Part I)

In the past 10 years, progress made in cancer biology, genetics, and biotechnology has led to a major transition in cancer drug design and development. There has been a change from an emphasis on non-specific, cytotoxic agents to specific, molecular-based therapeutics. Mechanism-based therapy is designed to act on cellular and molecular targets that are causally involved in the formation, growth, and progression of human cancers. These agents, which may have greater selectivity for cancer versus normal cells, and which may produce better anti-tumor efficacy and lower host toxicity, can be small molecules, natural or engineered peptides, proteins, antibodies, or synthetic nucleic acids (e.g. antisense oligonucleotides, ribozymes, and siRNAs). Novel targets are identified and validated by state-of-the-art approaches, including high-throughput screening, combinatorial chemistry, and gene expression arrays, which increase the speed and efficiency of drug discovery and development. Examples of oncogene-based, molecular therapeutics that show promising clinical activity include trastuzumab (Herceptin), imatinib (Gleevec), and gefitinib (Iressa). However, the full potential of oncogenes as novel targets for cancer therapy has not been realized and many challenges remain, from the validation of novel targets, to the design of specific agents, to the evaluation of these agents in both preclinical and clinical settings. In maximizing the benefits of molecular therapeutics in monotherapy or combination therapy of cancer, it is necessary to have an understanding of the underlying molecular abnormalities and mechanisms involved. This is the first part of a four-part review in which we discuss progress made in the last decade as it relates to the discovery of novel oncogenes and signal transduction pathways, in the context of their potential as targets for cancer therapy. This part delineates the latest discoveries about the potential use of growth factors and protein tyrosine kinases as targets for therapy. Later parts focus on intermediate signaling pathways, transcription factors, and proteins involved in cell cycle, DNA damage, and apoptotic pathways.

Activating mutations in the extracellular domain of the melanoma inducing receptor Xmrk are tumorigenicin vivo

Mutated versions or overexpression of receptor tyrosine kinases such as the epidermal growth factor receptor are found frequently in various cancers. In Xiphophorus the formation of hereditary melanoma is caused by the overexpression of the Xmrk oncogene locus. Xmrk is a mutationally altered version of the epidermal growth factor receptor. Two amino acid changes in the extracellular domain of the receptor were shown in vitro to be responsible for a constitutive, ligand-independent activity of Xmrk. To analyze whether these two mutations are indeed responsible for the in vivo oncogenic activity of the receptor, both were independently introduced into the wild-type, non-oncogenic Xiphophorus EGF-receptor and tested in Medaka embryos for their tumorigenic capacity. Both mutations were sufficient to induce tumors after short latency periods and at a comparable frequency as the native Xmrk oncogene. The G359R mutation led to a significantly higher tumor rate than the C578S mutation. Our study shows that subtle point mutations of the EGF-receptor can lead to a highly tumorigenic oncoprotein.

Concepts of human leukemic development

Two fundamental problems in cancer research are identification of the normal cell within which cancer initiates and identification of the cell type capable of sustaining the growth of the neoplastic clone. There is overwhelming evidence that virtually all cancers are clonal and represent the progeny of a single cell. What is less clear for most cancers is which cells within the tumor clone possess tumorigenic or 'cancer stem cell' (CSC) properties and are capable of maintaining tumor growth. The concept that only a subpopulation of rare CSC is responsible for maintenance of the neoplasm emerged nearly 50 years ago. Testing of this hypothesis is most advanced for the hematopoietic system due to the establishment of functional in vitro and in vivo assays for stem and progenitor cells at all stages of development. This body of work led to conclusive proof for CSC with the identification and purification of leukemic stem cells capable of repopulating NOD/SCID mice. This review will focus on the historical development of the CSC hypothesis, the mechanisms necessary to subvert normal developmental programs, and the identification of the cell in which these leukemogenic events first occur.

Mechanisms and implications of imatinib resistance mutations in BCR-ABL

PURPOSE OF REVIEW

Aside from bone marrow transplantation, a definitive cure for Philadelphia (Ph) chromosome-positive chronic myeloid leukemia (CML) has yet to be developed. Although Imatinib, the first molecularly targeted drug developed for CML has achieved a remarkable success, the emergence of resistance to this agent mitigates the prospect of a cure for this leukemia. Though a variety of resistance mechanisms can arise, in the majority of patients resistance coincides with reactivation of the tyrosine kinase activity of the BCR-ABL fusion oncoprotein. This can result from gene amplification and, more importantly, point mutations that disrupt the bind of imatinib to BCR-ABL itself. In this review, we aim to define and illuminate mechanisms of resistance and describe how drug resistance is shedding new light on kinase domain regulation.

RECENT FINDINGS

In light of recent studies and publications, it is now clear that Imatinib exerts its inhibitory action by stabilizing the inactive non ATP-binding conformation of BCR-ABL and that mutations even outside the kinase domain can lead to enhanced autophosphorylation of the kinase, thereby stabilizing the active conformation that resists imatinib binding. So far, 25 different substitutions of 21 amino acid residues of BCR-ABL have been detected in CML patients. In addition, it has been recently illustrated that mutations preexist the onset of treatment and that some confer a more aggressive disease phenotype. Finally it has been shown that molecular remission is almost never reached through Imatinib therapy.

SUMMARY

The most common mechanism of relapse for CML patients treated with Imatinib is the appearance of point mutations in the BCR-ABL oncogene that confer resistance to this drug. Insights into the emerging problem of resistance should promote the rational development of alternative, synergistic, and potentially curative treatment strategies.