Clonal hematopoiesis in children with predisposing conditions

Clonal hematopoiesis in children and young adults differs from that occuring in the older adult population. A variety of stressors drive this phenomenon, sometimes independent of age-related processes. For the purposes of this review, we adopt the term clonal hematopoiesis in predisposed individuals (CHIPI) to differentiate it from classical, age-related clonal hematopoiesis of indeterminate potential (CHIP). Stress-induced CHIPI selection can be extrinsic, such as following immunologic, infectious, pharmacologic, or genotoxic exposures, or intrinsic, involving germline predisposition from inherited bone marrow failure syndromes. In these conditions, clonal advantage relates to adaptations allowing improved cell fitness despite intrinsic defects affecting proliferation and differentiation. In certain contexts, CHIPI can improve competitive fitness by compensating for germline defects; however, the downstream effects of clonal expansion are often unpredictable - they may either counteract the underlying pathology or worsen disease outcomes. A more complete understanding of how CHIPI arises in young people can lead to the definition of preleukemic states and strategies to assess risk, surveillance, and prevention to leukemic transformation. Our review summarizes current research on stress-induced clonal dynamics in individuals with germline predisposition syndromes.

The metabolic basis of inherited neutropenias

Neutrophils are the shortest-lived blood cells, which requires a prodigious degree of proliferation and differentiation to sustain physiologically sufficient numbers and be poised to respond quickly to infectious emergencies. More than 107 neutrophils are produced every minute in an adult bone marrow-a process that is tightly regulated by a small group of cytokines and chemical mediators and dependent on nutrients and energy. Like granulocyte colony-stimulating factor, the primary growth factor for granulopoiesis, they stimulate signalling pathways, some affecting metabolism. Nutrient or energy deficiency stresses the survival, proliferation, and differentiation of neutrophils and their precursors. Thus, it is not surprising that monogenic disorders related to metabolism exist that result in neutropenia. Among these are pathogenic mutations in HAX1, G6PC3, SLC37A4, TAFAZZIN, SBDS, EFL1 and the mitochondrial disorders. These mutations perturb carbohydrate, lipid and/or protein metabolism. We hypothesize that metabolic disturbances may drive the pathogenesis of a subset of inherited neutropenias just as defects in DNA damage response do in Fanconi anaemia, telomere maintenance in dyskeratosis congenita and ribosome formation in Diamond-Blackfan anaemia. Greater understanding of metabolic pathways in granulopoiesis will identify points of vulnerability in production and may point to new strategies for the treatment of neutropenias.

Ataluren improves myelopoiesis and neutrophil chemotaxis by restoring ribosome biogenesis and reducing p53 levels in Shwachman-Diamond syndrome cells

Shwachman-Diamond syndrome (SDS) is characterized by neutropenia, exocrine pancreatic insufficiency and skeletal abnormalities. SDS bone marrow haematopoietic progenitors show increased apoptosis and impairment in granulocytic differentiation. Loss of Shwachman-Bodian-Diamond syndrome (SBDS) expression results in reduced eukaryotic 80S ribosome maturation. Biallelic mutations in the SBDS gene are found in ~90% of SDS patients, ~55% of whom carry the c.183-184TA>CT nonsense mutation. Several translational readthrough-inducing drugs aimed at suppressing nonsense mutations have been developed. One of these, ataluren, has received approval in Europe for the treatment of Duchenne muscular dystrophy. We previously showed that ataluren can restore full-length SBDS protein synthesis in SDS-derived bone marrow cells. Here, we extend our preclinical study to assess the functional restoration of SBDS capabilities in vitro and ex vivo. Ataluren improved 80S ribosome assembly and total protein synthesis in SDS-derived cells, restored myelopoiesis in myeloid progenitors, improved neutrophil chemotaxis in vitro and reduced neutrophil dysplastic markers ex vivo. Ataluren also restored full-length SBDS synthesis in primary osteoblasts, suggesting that its beneficial role may go beyond the myeloid compartment. Altogether, our results strengthened the rationale for a Phase I/II clinical trial of ataluren in SDS patients who harbour the nonsense mutation.

SBDSR126T rescues survival of sbds -/- zebrafish in a dose-dependent manner independently of Tp53

Defects in ribosomal biogenesis profoundly affect organismal development and cellular function, and these ribosomopathies produce a variety of phenotypes. One ribosomopathy, Shwachman-Diamond syndrome (SDS) is characterized by neutropenia, pancreatic exocrine insufficiency, and skeletal anomalies. SDS results from biallelic mutations in SBDS, which encodes a ribosome assembly factor. Some individuals express a missense mutation, SBDS R126T , along with the common K62X mutation. We reported that the sbds-null zebrafish phenocopies much of SDS. We further showed activation of Tp53-dependent pathways before the fish died during the larval stage. Here, we expressed SBDS R126T as a transgene in the sbds -/- background. We showed that one copy of the SBDS R126T transgene permitted the establishment of maternal zygotic sbds-null fish which produced defective embryos with cdkn1a up-regulation, a Tp53 target involved in cell cycle arrest. None survived beyond 3 dpf. However, two copies of the transgene resulted in normal development and lifespan. Surprisingly, neutropenia persisted. The surviving fish displayed suppression of female sex differentiation, a stress response in zebrafish. To evaluate the role of Tp53 in the pathogenesis of sbds -/- fish phenotype, we bred the fish with a DNA binding deficient allele, tp53 M214K Expression of the loss-of-function tp53 M214K did not rescue neutropenia or survival in sbds-null zebrafish. Increased expression of cdkn1a was abrogated in the tp53 M214K/M214K ;sbds -/- fish. We conclude that the amount of SBDSR126T protein is important for development, inactivation of Tp53 fails to rescue neutropenia or survival in the sbds-null background, and cdkn1a up-regulation was dependent on WT tp53 We hypothesize that additional pathways are involved in the pathophysiology of SDS.

Shwachman-Diamond syndromes: clinical, genetic, and biochemical insights from the rare variants

Shwachman-Diamond syndrome is a rare inherited bone marrow failure syndrome characterized by neutropenia, exocrine pancreatic insufficiency, and skeletal abnormalities. In 10-30% of cases, transformation to a myeloid neoplasm occurs. Approximately 90% of patients have biallelic pathogenic variants in the SBDS gene located on human chromosome 7q11. Over the past several years, pathogenic variants in three other genes have been identified to cause similar phenotypes; these are DNAJC21, EFL1, and SRP54. Clinical manifestations involve multiple organ systems and those classically associated with the Shwachman-Diamond syndrome (bone, blood, and pancreas). Neurocognitive, dermatologic, and retinal changes may also be found. There are specific gene-phenotype differences. To date, SBDS, DNAJC21, and SRP54 variants have been associated with myeloid neoplasia. Common to SBDS, EFL1, DNAJC21, and SRP54 is their involvement in ribosome biogenesis or early protein synthesis. These four genes constitute a common biochemical pathway conserved from yeast to humans that involve early stages of protein synthesis and demonstrate the importance of this synthetic pathway in myelopoiesis.

The Molecular and Genetic Mechanisms of Inherited Bone Marrow Failure Syndromes: The Role of Inflammatory Cytokines in Their Pathogenesis

Inherited bone marrow failure syndromes (IBMFSs) include Fanconi anemia, Diamond-Blackfan anemia, Shwachman-Diamond syndrome, dyskeratosis congenita, severe congenital neutropenia, and other rare entities such as GATA2 deficiency and SAMD9/9L mutations. The IBMFS monogenic disorders were first recognized by their phenotype. Exome sequencing has validated their classification, with clusters of gene mutations affecting DNA damage response (Fanconi anemia), ribosome structure (Diamond-Blackfan anemia), ribosome assembly (Shwachman-Diamond syndrome), or telomere maintenance/stability (dyskeratosis congenita). The pathogenetic mechanisms of IBMFSs remain to be characterized fully, but an overarching hypothesis states that different stresses elicit TP53-dependent growth arrest and apoptosis of hematopoietic stem, progenitor, and precursor cells. Here, we review the IBMFSs and propose a role for pro-inflammatory cytokines, such as TGF-β, IL-1β, and IFN-α, in mediating the cytopenias. We suggest a pathogenic role for cytokines in the transformation to myeloid neoplasia and hypothesize a role for anti-inflammatory therapies.

Alternatively spliced CSF3R isoforms in SRSF2 P95H mutated myeloid neoplasms

Alternatively spliced colony stimulating factor 3 receptor (CSF3R) isoforms Class III and Class IV are observed in myelodysplastic syndromes (MDS), but their roles in disease remain unclear. We report that the MDS-associated splicing factor SRSF2 affects the expression of Class III and Class IV isoforms and perturbs granulopoiesis. Add-back of the Class IV isoform in Csf3r-null mouse progenitor cells increased granulocyte progenitors with impaired neutrophil differentiation, while add-back of the Class III produced dysmorphic neutrophils in fewer numbers. These CSF3R isoforms were elevated in patients with myeloid neoplasms harboring SRSF2 mutations. Using in vitro splicing assays, we confirmed increased Class III and Class IV transcripts when SRSF2 P95 mutations were co-expressed with the CSF3R minigene in K562 cells. Since SRSF2 regulates splicing partly by recognizing exonic splicing enhancer (ESE) sequences on pre-mRNA, deletion of either ESE motifs within CSF3R exon 17 decreased Class IV transcript levels without affecting Class III. CD34+ cells expressing SRSF2 P95H showed impaired neutrophil differentiation in response to G-CSF and was accompanied by increased levels of Class IV. Our findings suggest that SRSF2 P95H promotes Class IV splicing by binding to key ESE sequences in CSF3R exon 17, and that SRSF2, when mutated, contributes to dysgranulopoiesis.

G-CSF, the guardian of granulopoiesis

A considerable amount of continuous proliferation and differentiation is required to produce daily a billion new neutrophils in an adult human. Of the few cytokines and factors known to control neutrophil production, G-CSF is the guardian of granulopoiesis. G-CSF/CSF3R signaling involves the recruitment of non-receptor protein tyrosine kinases and their dependent signaling pathways of serine/threonine kinases, tyrosine phosphatases, and lipid second messengers. These pathways converge to activate the families of STAT and C/EBP transcription factors. CSF3R mutations are associated with human disorders of neutrophil production, including severe congenital neutropenia, neutrophilia, and myeloid malignancies. More than three decades after their identification, cloning, and characterization of G-CSF and G-CSF receptor, fundamental questions remain about their physiology.

Predicting Time to Relapse in Acute Myeloid Leukemia through Stochastic Modeling of Minimal Residual Disease Based on Clonality Data

Event-free and overall survival remain poor for patients with acute myeloid leukemia. Chemoresistant clones contributing to relapse arise from minimal residual disease (MRD) or newly-acquired mutations. However, the dynamics of clones comprising MRD is poorly understood. We developed a predictive stochastic model, based on a multitype age-dependent Markov branching process, to describe how random events in MRD contribute to the heterogeneity in treatment response. We employed training and validation sets of patients who underwent whole genome sequencing and for whom mutant clone frequencies at diagnosis and relapse were available. The disease evolution and treatment outcome are subject to stochastic fluctuations. Estimates of malignant clone growth rates, obtained by model fitting, are consistent with published data. Using the estimates from the training set, we developed a function linking MRD and time of relapse, with MRD inferred from the model fits to clone frequencies and other data. An independent validation set confirmed our model. In a third data set, we fitted the model to data at diagnosis and remission and predicted the time to relapse. As a conclusion, given bone marrow genome at diagnosis and MRD at or past remission, the model can predict time to relapse, and help guide treatment decisions to mitigate relapse.

Effect of the unfolded protein response and oxidative stress on mutagenesis in CSF3R: a model for evolution of severe congenital neutropenia to myelodysplastic syndrome/acute myeloid leukemia

Severe congenital neutropenia (SCN) is a rare blood disorder characterised by abnormally low levels of circulating neutrophils. The most common recurrent mutations that cause SCN involve neutrophil elastase (ELANE). The treatment of choice for SCN is the administration of granulocyte-colony stimulating factor (G-CSF), which increases the neutrophil number and improves the survival and quality of life. Long-term survival is however linked to the development of myelodysplastic syndrome/acute myeloid leukemia (MDS/AML). About 70% of MDS/AML patients acquire nonsense mutations affecting the cytoplasmic domain of CSF3R (the G-CSF receptor). About 70% of SCN patients with AML harbour additional mutations in RUNX1. We hypothesised that this coding region of CSF3R constitutes a hotspot vulnerable to mutations resulting from excessive oxidative stress or endoplasmic reticulum (ER) stress. We used the murine Ba/F3 cell line to measure the effect of induced oxidative or ER stress on the mutation rate in our hypothesised hotspot of the exogenous human CSF3R, the corresponding region in the endogenous Csf3r, and Runx1. Ba/F3 cells transduced with the cDNA for partial C-terminal of CSF3R fused in-frame with a green fluorescent protein (GFP) tag were subjected to stress-inducing treatment for 30 days (~51 doubling times). The amplicon-based targeted deep sequencing data for days 15 and 30 samples show that although there was increased mutagenesis observed in all the three genes of interest (partial CSF3R, Csf3r and Runx1), there were more mutations in the GFP region compared with the partial CSF3R region. Our findings also indicate that there is no correlation between the stress-inducing chemical treatments and mutagenesis in Ba/F3 cells. Our data suggest that oxidative or ER stress induction does not promote genomic instability, affecting partial C-terminal of the transduced CSF3R, the endogenous Csf3R and the endogenous Runx1 in Ba/F3 cells that could account for these targets to being mutational hotspots. We conclude that other mechanisms to acquire mutations of CSF3R that help drive the evolution of SCN to MDS/AML.

Application of the Moran Model in Estimating Selection Coefficient of Mutated CSF3R Clones in the Evolution of Severe Congenital Neutropenia to Myeloid Neoplasia

Bone marrow failure (BMF) syndromes, such as severe congenital neutropenia (SCN) are leukemia predisposition syndromes. We focus here on the transition from SCN to pre-leukemic myelodysplastic syndrome (MDS). Stochastic mathematical models have been conceived that attempt to explain the transition of SCN to MDS, in the most parsimonious way, using extensions of standard processes of population genetics and population dynamics, such as the branching and the Moran processes. We previously presented a hypothesis of the SCN to MDS transition, which involves directional selection and recurrent mutation, to explain the distribution of ages at onset of MDS or AML. Based on experimental and clinical data and a model of human hematopoiesis, a range of probable values of the selection coefficient s and mutation rate μ have been determined. These estimates lead to predictions of the age at onset of MDS or AML, which are consistent with the clinical data. In the current paper, based on data extracted from published literature, we seek to provide an independent validation of these estimates. We proceed with two purposes in mind: (i) to determine the ballpark estimates of the selection coefficients and verify their consistency with those previously obtained and (ii) to provide possible insight into the role of recurrent mutations of the G-CSF receptor in the SCN to MDS transition.

Loss of Sbds in zebrafish leads to neutropenia and pancreas and liver atrophy

Shwachman-Diamond syndrome (SDS) is characterized by exocrine pancreatic insufficiency, neutropenia, and skeletal abnormalities. Biallelic mutations in SBDS, which encodes a ribosome maturation factor, are found in 90% of SDS cases. Sbds^–/– mice are embryonic lethal. Using CRISPR/Cas9 editing, we created sbds-deficient zebrafish strains. Sbds protein levels progressively decreased and became undetectable at 10 days postfertilization (dpf). Polysome analysis revealed decreased 80S ribosomes. Homozygous mutant fish developed normally until 15 dpf. Mutant fish subsequently had stunted growth and showed signs of atrophy in pancreas, liver, and intestine. In addition, neutropenia occurred by 5 dpf. Upregulation of tp53 mRNA did not occur until 10 dpf, and inhibition of proliferation correlated with death by 21 dpf. Transcriptome analysis showed tp53 activation through upregulation of genes involved in cell cycle arrest, cdkn1a and ccng1, and apoptosis, puma and mdm2. However, elimination of Tp53 function did not prevent lethality. Because of growth retardation and atrophy of intestinal epithelia, we studied the effects of starvation on WT fish. Starved WT fish showed intestinal atrophy, zymogen granule loss, and tp53 upregulation — similar to the mutant phenotype. In addition, there was reduction in neutral lipid storage and ribosomal protein amount, similar to the mutant phenotype. Thus, loss of Sbds in zebrafish phenocopies much of the human disease and is associated with growth arrest and tissue atrophy, particularly of the gastrointestinal system, at the larval stage. A variety of stress responses, some associated with Tp53, contribute to pathophysiology of SDS.

Loss of ribosome maturation factor sbds in the zebrafish phenocopies human Shwachman-Diamond syndrome and is associated with p53 activation, but lethality cannot be rescued by p53 mutation.

Inducible expression of a disease-associated ELANE mutation impairs granulocytic differentiation, without eliciting an unfolded protein response

Severe congenital neutropenia (SCN) is characterized by a near absence of neutrophils, rendering individuals with this disorder vulnerable to recurrent life-threatening infections. The majority of SCN cases arise because of germline mutations in the gene elastase, neutrophil-expressed (ELANE) encoding the neutrophil granule serine protease neutrophil elastase. Treatment with a high dose of granulocyte colony-stimulating factor increases neutrophil production and reduces infection risk. How ELANE mutations produce SCN remains unknown. The currently proposed mechanism is that ELANE mutations promote protein misfolding, resulting in endoplasmic reticulum stress and activation of the unfolded protein response (UPR), triggering death of neutrophil precursors and resulting in neutropenia. Here we studied the ELANE mutation p.G185R, often associated with greater clinical severity (e.g. decreased responsiveness to granulocyte colony-stimulating factor and increased leukemogenesis). Using an inducible expression system, we observed that this ELANE mutation diminishes enzymatic activity and granulocytic differentiation without significantly affecting cell proliferation, cell death, or UPR induction in murine myeloblast 32D and human promyelocytic NB4 cells. Impaired differentiation was associated with decreased expression of genes encoding critical hematopoietic transcription factors (Gfi1, Cebpd, Cebpe, and Spi1), cell surface proteins (Csf3r and Gr1), and neutrophil granule proteins (Mpo and Elane). Together, these findings challenge the currently prevailing model that SCN results from mutant ELANE, which triggers endoplasmic reticulum stress, UPR, and apoptosis.

Peripheral blood immunophenotyping in a large cohort of patients with Shwachman-Diamond syndrome

Shwachman-Diamond syndrome (SDS) is one of the more common inherited bone marrow failure syndromes, characterized by neutropenia, occasional thrombocytopenia, and anemia. Bone marrow evaluation reveals an increased number of monocytes and mature B cells along with decreased granulocytes. However, little is known about the subpopulations of peripheral blood cells, and few previous publications have been based on a small number of patients. Here, we report a comprehensive immunophenotypic analysis from a cohort of 37 SDS patients who display impairment mostly in the myeloid compartment with a deficiency also in the number of B cells and CD4/CD8 double-negative T cells.

Mutation, drift and selection in single-driver hematologic malignancy: Example of secondary myelodysplastic syndrome following treatment of inherited neutropenia

Cancer development is driven by series of events involving mutations, which may become fixed in a tumor via genetic drift and selection. This process usually includes a limited number of driver (advantageous) mutations and a greater number of passenger (neutral or mildly deleterious) mutations. We focus on a real-world leukemia model evolving on the background of a germline mutation. Severe congenital neutropenia (SCN) evolves to secondary myelodysplastic syndrome (sMDS) and/or secondary acute myeloid leukemia (sAML) in 30–40%. The majority of SCN cases are due to a germline ELANE mutation. Acquired mutations in CSF3R occur in >70% sMDS/sAML associated with SCN. Hypotheses underlying our model are: an ELANE mutation causes SCN; CSF3R mutations occur spontaneously at a low rate; in fetal life, hematopoietic stem and progenitor cells expands quickly, resulting in a high probability of several tens to several hundreds of cells with CSF3R truncation mutations; therapeutic granulocyte colony-stimulating factor (G-CSF) administration early in life exerts a strong selective pressure, providing mutants with a growth advantage. Applying population genetics theory, we propose a novel two-phase model of disease development from SCN to sMDS. In Phase 1, hematopoietic tissues expand and produce tens to hundreds of stem cells with the CSF3R truncation mutation. Phase 2 occurs postnatally through adult stages with bone marrow production of granulocyte precursors and positive selection of mutants due to chronic G-CSF therapy to reverse the severe neutropenia. We predict the existence of the pool of cells with the mutated truncated receptor before G-CSF treatment begins. The model does not require increase in mutation rate under G-CSF treatment and agrees with age distribution of sMDS onset and clinical sequencing data.

Cancer develops by multistep acquisition of mutations in a progenitor cell and its daughter cells. Severe congenital neutropenia (SCN) manifests itself through an inability to produce enough granulocytes to prevent infections. SCN commonly results from a germline ELANE mutation. Large doses of the blood growth factor granulocyte colony-stimulating factor (G-CSF) rescue granulocyte production. However, SCN frequently transforms to a myeloid malignancy, commonly associated with a somatic mutation in CSF3R, the gene encoding the G-CSF Receptor. We built a mathematical model of evolution for CSF3R mutation starting with bone marrow expansion at the fetal development stage and continuing with postnatal competition between normal and malignant bone marrow cells. We employ tools of probability theory such as multitype branching processes and Moran models modified to account for expansion of hematopoiesis during human development. With realistic coefficients, we obtain agreement with the age range at which malignancy arises in patients. In addition, our model predicts the existence of a pool of cells with mutated CSF3R before G-CSF treatment begins. Our findings may be clinically applied to intervene more effectively and selectively in SCN patients.

Peering through zebrafish to understand inherited bone marrow failure syndromes

Inherited bone marrow failure syndromes are experiments of nature characterized by impaired hematopoiesis with cancer and leukemia predisposition. The mutations associated with inherited bone marrow failure syndromes affect fundamental cellular pathways, such as DNA repair, telomere maintenance, or proteostasis. How these disturbed pathways fail to produce sufficient blood cells and lead to leukemogenesis are not understood. The rarity of inherited cytopenias, the paucity of affected primary human hematopoietic cells, and the sometime inadequacy of murine or induced pluripotential stem cell models mean it is difficult to acquire a greater understanding of them. Zebrafish offer a model organism to study gene functions. As vertebrates, zebrafish share with humans many orthologous genes involved in blood disorders. As a model organism, zebrafish provide advantages that include rapid development of transparent embryos, high fecundity (providing large numbers of mutant and normal siblings), and a large collection of mutant and transgenic lines useful for investigating the blood system and other tissues during development. Importantly, recent advances in genomic editing in zebrafish can speedily validate the new genes or novel variants discovered in clinical investigation as causes for marrow failure. Here we review zebrafish as a model organism that phenocopies Fanconi anemia, Diamond-Blackfan anemia, dyskeratosis congenita, Shwachman-Diamond syndrome, congenital amegakaryocytic thrombocytopenia, and severe congenital neutropenia. Two important insights, provided by modeling inherited cytopenias in zebrafish, widen understanding of ribosome biogenesis and TP53 in mediating marrow failure and non-hematologic defects. They suggest that TP53-independent pathways contribute to marrow failure. In addition, zebrafish provide an attractive model organism for drug development.

Captopril mitigates splenomegaly and myelofibrosis in the Gata1low murine model of myelofibrosis

Allogeneic stem cell transplantation is currently the only curative therapy for primary myelofibrosis (MF), while the JAK2 inhibitor, ruxolitinib. Has been approved only for palliation. Other therapies are desperately needed to reverse life-threatening MF. However, the cell(s) and cytokine(s) that promote MF remain unclear. Several reports have demonstrated that captopril, an inhibitor of angiotensin-converting enzyme that blocks the production of angiotensin II (Ang II), mitigates fibrosis in heart, lung, skin and kidney. Here, we show that captopril can mitigate the development of MF in the Gata1low mouse model of primary MF. Gata1low mice were treated with 79 mg/kg/d captopril in the drinking water from 10 to 12 months of age. At 13 months of age, bone marrows were examined for fibrosis, megakaryocytosis and collagen expression; spleens were examined for megakaryocytosis, splenomegaly and collagen expression. Treatment of Gata1low mice with captopril in the drinking water was associated with normalization of the bone marrow cellularity; reduced reticulin fibres, splenomegaly and megakaryocytosis; and decreased collagen expression. Our findings suggest that treating with the ACE inhibitors captopril has a significant benefit in overcoming pathological changes associated with MF.

New monogenic disorders identify more pathways to neutropenia: from the clinic to next-generation sequencing

Neutrophils are the most common type of leukocyte in human circulating blood and constitute one of the chief mediators for innate immunity. Defined as a reduction from a normal distribution of values, neutropenia results from a number of congenital and acquired conditions. Neutropenia may be insignificant, temporary, or associated with a chronic condition with or without a vulnerability to life-threatening infections. As an inherited bone marrow failure syndrome, neutropenia may be associated with transformation to myeloid malignancy. Recognition of an inherited bone marrow failure syndrome may be delayed into adulthood. The list of monogenic neutropenia disorders is growing, heterogeneous, and bewildering. Furthermore, greater knowledge of immune-mediated and drug-related causes makes the diagnosis and management of neutropenia challenging. Recognition of syndromic presentations and especially the introduction of next-generation sequencing are improving the accuracy and expediency of diagnosis as well as their clinical management. Furthermore, identification of monogenic neutropenia disorders is shedding light on the molecular mechanisms of granulopoiesis and myeloid malignancies.

Monosomy 7/del (7q) in inherited bone marrow failure syndromes: A systematic review

Inherited bone marrow failure syndromes (IBMFS) are rare cancer predisposition syndromes with an especially high risk of transformation to myelodysplastic syndrome (MDS) and/or acute myeloid leukemia (AML). We performed a retrospective systematic review of reported MDS/AML arising in the eight most common IBMFS to determine the frequency and outcome of chromosome 7 abnormalities. We identified 738 MDS/AML cases of 4,293 individuals. Monosomy 7 or del (7q) occurred in ∼17%. Greater understanding of the roles played by sequential acquisition of genetic and cytogenetic changes will provide insights into myeloid leukemogenesis and improve the surveillance and hopefully outcomes for individuals with IBMFS.

Synonymous mutation in TP53 results in a cryptic splice site affecting its DNA-binding site in an adolescent with two primary sarcomas

Pathologic variants in TP53 are known risk factors for the development of cancer. We report a 17-year-old male who presented with two primary sarcomas. Germline sequencing revealed a novel TP53 c.672 G>A mutation. Sequencing revealed wild-type TP53 in the parents, and there was no history of cancer in first-degree relatives. This de novo synonymous germline mutation results in a 5' cryptic splice site that is bound by U1, resulting in a shift of the splice site by 5 base pairs. The frame shift results in a truncated protein at residue 246, which disrupts the DNA-binding domain of p53.

G-CSF and GM-CSF in Neutropenia

G-CSF and GM-CSF are used widely to promote the production of granulocytes or APCs. The U.S. Food and Drug Administration approved G-CSF (filgrastim) for the treatment of congenital and acquired neutropenias and for mobilization of peripheral hematopoietic progenitor cells for stem cell transplantation. A polyethylene glycol-modified form of G-CSF is approved for the treatment of neutropenias. Clinically significant neutropenia, rendering an individual immunocompromised, occurs when their number is <1500/μl. Current guidelines recommend their use when the risk for febrile neutropenia is >20%. GM-CSF (sargramostim) is approved for neutropenia associated with stem cell transplantation. Because of its promotion of APC function, GM-CSF is being evaluated as an immunostimulatory adjuvant in a number of clinical trials. More than 20 million persons have benefited worldwide, and >$5 billion in sales occur annually in the United States.

Abstract B2-50: Stochastic population genetics modeling of the evolution of the myelodysplastic syndrome

We are developing new models to understand the evolution of Myelodysplastic Syndromes (MDS), a group of pre-leukemic blood diseases, which are increasing in incidence as the population is aging and which remain difficult to treat. In the early stage of MDS, exaggerated apoptosis of myeloid blood cells occurs, but as the disease evolves, survival of undifferentiated cells dominates. As a model we study progression to MDS/AML from severe congenital neutropenia (SCN), an inherited bone marrow failure with known germ-line mutations, e.g. ELANE which encodes the neutrophil elastase. Administration of granulocyte colony-stimulating factor (GCSF) improves the neutropenia but is associated with transformation to MDS and acute myeloid leukemia (AML), defined by greater than 20% blasts in the marrow. Of note, ~ 70% of SCN patients who progress to MDS acquire a truncating D715 mutation in the GCSF Receptor. We previously developed a model of the SCN -&gt; MDS -&gt; AML sequence [1], which however did not include the MDS stage.

We hypothesize that the propagation of the D715 mutant in MDS secondary to SCN follows a newly developed Moran model with directional selection and co-localization. Selection is caused by alterations in the STAT/SOCS feedback loop with the truncated GCSF receptor, leading to changes in proliferation and differentiation [2]. Usually, mathematical models assume that bone marrow is a well-mixed environment and the normal and malignant precursors freely interact. However, these subpopulations occupy distinct niches [3] and their competition is limited by physical separation, which is a factor in our variant of the Moran model. We estimate the selective advantage of mutant cells based on competitive repopulation assay and flow cytometry cell-cycle distribution and find the time at onset of MDS consistent with the published median of 13 years [1].

We built an ODE model of the STAT/SOCS circuit, and investigated its sensitivity to reduced binding of JAK/STAT complexes to the D715 mutant receptor, even that the circuit exhibits extremely strong stabilizing properties. A link to cell cycle characteristics via distributions of cell-cycle-phase dependent activated STATs suggests connection with differentiation and proliferation. Further, to investigate a fuller model in which a number of driver mutations may occur at a different order, we developed a new version of the infinite-allele branching process of Griffiths and Pakes [4, 5]. Mathematical modeling of SCN-&gt;MDS -&gt; AML will lead to a greater understanding of homeostatic and aberrant myelopoiesis as well as designing better predictive biomarkers for malignant clonal evolution.

References:

[1] Kimmel, M., and Corey, S. (2013). Stochastic hypothesis of transition from inborn neutropenia to AML. Frontiers in oncology, 3.

[2] Whichard, Z. L., Sarkar, C. A., Kimmel, M., and Corey, S. J. (2010). Hematopoiesis and its disorders: a systems biology approach. Blood, 115(12), 2339-2347.

[3] Colmone, A., Amorim, M., Pontier, A. L., Wang, S., Jablonski, E., and Sipkins, D. A. (2008). Leukemic cells create bone marrow niches that disrupt the behavior of normal hematopoietic progenitor cells. Science, 322(5909), 1861-1865.

[4] Griffiths, R. C., and Pakes, A. G. (1988). An infinite-alleles version of the simple branching process. Advances in applied probability, 20(3), 489-524.

[5] McDonald, T. A., and Kimmel, M. (2014) A multitype infinite-allele branching process with applications to cancer evolution. Journal of applied probability, accepted for publication.

Note: This abstract was not presented at the conference.

Citation Format: Tomasz Wojdyla, Hrishikesh Mehta, Taly Glaubach, Thomas McDonald, Seth J. Corey, Marek Kimmel. Stochastic population genetics modeling of the evolution of the myelodysplastic syndrome. [abstract]. In: Proceedings of the AACR Special Conference on Computational and Systems Biology of Cancer; Feb 8-11 2015; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(22 Suppl 2):Abstract nr B2-50.

Expanding the scope of drug repurposing in pediatrics: the Children's Pharmacy Collaborative

Drug repurposing is the use of 'old' drugs for new indications, avoiding the need for time- and cost-intensive toxicity studies. This approach should be particularly attractive for pediatrics, but its use in this population has been limited. One obstacle has been the lack of a comprehensive database of drugs for which there already is at least one indication in children. We describe the development of The Children's Pharmacy Collaborative, which should grow over time, serve as a resource for professionals and families, and stimulate drug-repurposing efforts for a range of pediatric disorders.

Shockingly: the loss of Lyn leads to leakiness

In this issue of Blood, Han et al demonstrate that endotoxin-induced mortality in a murine model of acute lung injury (ALI) was associated with increased vascular permeability attributable to loss of the Src family kinase (SFK) Lyn.

Alternatively spliced, truncated GCSF receptor promotes leukemogenic properties and sensitivity to JAK inhibition

Granulocyte colony-stimulating factor (GCSF) drives the production of myeloid progenitor and precursor cells toward neutrophils via the GCSF receptor (GCSFR, gene name CSF3R). Children with severe congenital neutropenia chronically receive pharmacologic doses of GCSF, and ∼30% will develop myelodysplasia/acute myeloid leukemia (AML) associated with GCSFR truncation mutations. In addition to mutations, multiple isoforms of CSF3R have also been reported. We found elevated expression of the alternatively spliced isoform, class IV CSF3R in adult myelodysplastic syndrome/AML patients. Aside from its association with monosomy 7 and higher rates of relapse in pediatric AML patients, little is known about the biology of the class IV isoform. We found developmental regulation of CSF3R isoforms with the class IV expression more representative of a progenitor cell stage. Striking differences were found in phosphoprotein signaling involving Janus kinase (JAK)-signal transducer and activator of transcription (STAT) and cell cycle gene expression. Enhanced proliferation by class IV GCSFR was associated with diminished STAT3 and STAT5 activation, yet showed sensitivity to JAK2 inhibitors. Alterations in the C-terminal domain of the GCSFR result in leukemic properties of enhanced growth, impaired differentiation and resistance to apoptosis, suggesting that they can behave as oncogenic drivers, sensitive to JAK2 inhibition.

Biochemical and functional significance of F-BAR domain proteins interaction with WASP/N-WASP

The Bin-Amphiphysin-Rvs (BAR) domain family of proteins includes groups which promote positive (classical BAR, N-BAR, and F-BAR) and negative (I-BAR) membrane deformation. Of these groups, the F-BAR subfamily is the most diverse in its biochemical properties. F-BAR domain proteins dimerize to form a tight scaffold about the membrane. The F-BAR domain provides a banana-shaped, alpha-helical structure that senses membrane curvature. Different types of F-BAR domain proteins contain tyrosine kinase or GTPase activities; some interact with phosphatases and RhoGTPases. Most possess an SH3 domain that facilitates the recruitment and activation of WASP/N-WASP. Thus, F-BAR domain proteins affect remodeling of both membrane and the actin cytoskeleton. The purpose of this review is to highlight the role of F-BAR proteins in coupling WASP/N-WASP to cytoskeletal remodeling. A role for F-BAR/WASP interaction in human diseases affecting nervous, blood, and neoplastic tissues is discussed.

ELANE mutations in cyclic and severe congenital neutropenia: genetics and pathophysiology

The 2 main forms of hereditary neutropenia are cyclic (CN) and severe congenital (SCN) neutropenia. CN is an autosomal dominant disorder in which neutrophil counts fluctuate with 21-day periodicity. SCN consists of static neutropenia, with promyelocytic maturation arrest in the bone marrow. Unlike CN, SCN displays frequent acquisition of somatic mutations in the gene CSF3R. CN is caused by heterozygous mutations in the gene ELANE, encoding neutrophil elastase. SCN is genetically heterogeneous but is most frequently associated with ELANE mutations. We discuss how the mutations provide clues into the pathogenesis of neutropenia and describe current hypotheses for its molecular mechanisms.

Lysosomal disruption preferentially targets acute myeloid leukemia cells and progenitors

Despite efforts to understand and treat acute myeloid leukemia (AML), there remains a need for more comprehensive therapies to prevent AML-associated relapses. To identify new therapeutic strategies for AML, we screened a library of on- and off-patent drugs and identified the antimalarial agent mefloquine as a compound that selectively kills AML cells and AML stem cells in a panel of leukemia cell lines and in mice. Using a yeast genome-wide functional screen for mefloquine sensitizers, we identified genes associated with the yeast vacuole, the homolog of the mammalian lysosome. Consistent with this, we determined that mefloquine disrupts lysosomes, directly permeabilizes the lysosome membrane, and releases cathepsins into the cytosol. Knockdown of the lysosomal membrane proteins LAMP1 and LAMP2 resulted in decreased cell viability, as did treatment of AML cells with known lysosome disrupters. Highlighting a potential therapeutic rationale for this strategy, leukemic cells had significantly larger lysosomes compared with normal cells, and leukemia-initiating cells overexpressed lysosomal biogenesis genes. These results demonstrate that lysosomal disruption preferentially targets AML cells and AML progenitor cells, providing a rationale for testing lysosomal disruption as a novel therapeutic strategy for AML.

Two SHIPs passing in the middle of the immune system

Immunity requires a complex, multiscale system of molecules, cells, and cytokines. In this issue of the European Journal of Immunology, Collazo et al. [Eur. J. Immunol. 2012. 42: 1785-1796] provide evidence that links the lipid phosphatase SHIP1 with the coordination of interactions between regulatory T (Treg) cells and myeloid-derived suppressor cells (MDSCs). Using conditional knockouts of SHIP1 in either the myeloid or T-cell-lineage of mice, the authors show that the regulated development of Treg cells is controlled directly by cell-intrinsic SHIP1, and indirectly by extrinsic SHIP1 control of an unknown myeloid cell. Regulation of MDSCs is also determined by SHIP1 in an extrinsic manner, again via an as-yet-unknown myeloid cell. Furthermore, this extrinsic control of Treg cells and MDSCs is mediated in part by increased production of G-CSF, a growth factor critical for the production of neutrophils, in SHIP1-deficient mice. Thus, a physiologically important implication of this report is the collaboration between the innate and adaptive immune systems in fine tuning of Treg cells as discussed in this commentary.

Loss of SHIP-1 protein expression in high-risk myelodysplastic syndromes is associated with miR-210 and miR-155

The myelodysplastic syndromes (MDSs) comprise a group of disorders characterized by multistage progression from cytopenias to acute myeloid leukemia (AML). They display exaggerated apoptosis in early stages, but lose this behavior during evolution to AML. The molecular basis for loss of apoptosis is unknown. To investigate this critical event, we analyzed phosphatidylinositol (PI) 3'kinase signaling, implicated as a critical pathway of cell survival control in epithelial and hematological malignancies. PI 3'kinase activates Akt through its production of 3' phosphoinositides. In turn, the phosphoinositides are dephosphorylated by two lipid phosphatases, PTEN and SHIP-1, in myeloid cells. We studied primary MDS-enriched bone marrow cells and bone marrow sections by western blotting, immunohistochemistry, immunocytochemistry and quantitative PCR for components of the SHIP/PTEN/PI 3'kinase signaling circuit. We reported constitutively activated Akt, variable levels of PTEN and uniformly decreased SHIP-1 expression in MDS progenitor cells. Overexpression of SHIP-1, but not the phosphatase-deficient form, inhibited myeloid leukemic growth. Levels of microRNA (miR)-210 and miR-155 transcripts, which target SHIP-1, were increased in CD34(+) MDS cells compared with their normal counterparts. Direct binding of miR-210 to the 3' untranslated region of SHIP-1 was confirmed by luciferase reporter assay. Transfection of a myeloid cell line with miR-210 resulted in loss of SHIP-1 protein expression. These data suggest that miR-155 and miR-210/SHIP-1/Akt pathways could serve as clinical biomarkers for disease progression, and that miR-155 and miR-210 might serve as novel therapeutic targets in MDS.

Drug repurposing in pediatrics and pediatric hematology oncology

Drug 'repurposing', that is, using old drugs for new indications, has been proposed as a more efficient strategy for drug development than the current standard of beginning with novel agents. In this review, we explore the scope of drug repurposing in pediatric hematology oncology and in pediatrics in general. Drugs commonly used in children were identified using the Harriet Lane Handbook (HLH) and searched in PubMed for different uses. Additional drugs were identified by searching PubMed and Google.com for 'drug repurposing' or 'drug repositioning'. Almost 10% of drugs with primary uses in pediatrics have been repurposed in pediatric hematology oncology or pediatrics. The observant clinician, pharmacologist and translational bioinformatician, as well as structural targeting, will have a role in discovering new repurposing opportunities.

BAR proteins in cancer and blood disorders

Remodeling of the membrane and cytoskeleton is involved in a wide range of normal and pathologic cellular function. These are complex, highly-coordinated biochemical and biophysical processes involving dozens of proteins. Serving as a scaffold for a variety of proteins and possessing a domain that interacts with plasma membranes, the BAR family of proteins contribute to a range of cellular functions characterized by membrane and cytoskeletal remodeling. There are several subgroups of BAR proteins: BAR, N-BAR, I-BAR, and F-BAR. They differ in their ability to induce angles of membrane curvature and in their recruitment of effector proteins. Evidence is accumulating that BAR proteins contribute to cancer cell invasion, T cell trafficking, phagocytosis, and platelet production. In this review, we discuss the physiological function of BAR proteins and discuss how they contribute to blood and cancer disorders.

Slowly produced microRNAs control protein levels

Proteins are the primary agents of function in biological systems, and their levels are critical control elements, reflecting the interplay between transcription, translation, and protein degradation. Here, we consider the role of microRNAs (miRNAs) in the post-transcriptional regulation of protein synthesis. To determine their impact on protein concentration, we constructed a mechanistic model consisting of four state variables and nine kinetic parameters that account for transcript sequestration and degradation via miRNA-mRNA complex formation. Our dynamical model predicts that, even when present in low copy number, miRNAs can exert potent effects on protein concentration. Sensitivity analysis of the steady-state solution indicates that miRNA synthesis commonly acts to fine-tune protein concentrations. However, the same analysis shows that for a small subset of miRNA-mRNA pairs characterized by slowly produced miRNAs, the miRNA synthesis rate is the dominant control element. Our model equations provide a tool to evaluate the importance of particular miRNAs on their target proteins and promote the development of miRNA-based therapies that target proteins associated with cancer, inflammation, and metabolic disorders.

Cdc42-interacting protein 4 promotes breast cancer cell invasion and formation of invadopodia through activation of N-WASp

In the earliest stages of metastasis, breast cancer cells must reorganize the cytoskeleton to affect cell shape change and promote cell invasion and motility. These events require the cytoskeletal regulators Cdc42 and Rho, their effectors such as N-WASp/WAVE, and direct inducers of actin polymerization such as Arp2/3. Little consideration has been given to molecules that shape the cell membrane. The F-BAR proteins CIP4, TOCA-1, and FBP17 generate membrane curvature and act as scaffolding proteins for activated Cdc42 and N-WASp. We found that expression of CIP4, but not TOCA-1 or FBP17, was increased in invasive breast cancer cell lines in comparison with weakly or noninvasive breast cancer cell lines. Endogenous CIP4 localized to the leading edge of migrating cells and to invadopodia in cells invading gelatin. Because CIP4 serves as a scaffolding protein for Cdc42, Src, and N-WASp, we tested whether loss of CIP4 could result in decreased N-WASp function. Interaction between CIP4 and N-WASp was epidermal growth factor responsive, and CIP4 silencing by small interfering RNA caused decreased tyrosine phosphorylation of N-WASp at a Src-dependent activation site (Y256). CIP4 silencing also impaired the migration and invasion of MDA-MB-231 cells and was associated with decreased formation of invadopodia and gelatin degradation. This study presents a new role for CIP4 in the promotion of migration and invasion of MDA-MB-231 breast cancer cells and establishes the contribution of F-BAR proteins to cancer cell motility and invasion.

The Cdc42-interacting protein-4 (CIP4) gene knock-out mouse reveals delayed and decreased endocytosis

The newly described F-BAR (Fer/CIP4 and Bin, amphiphysin, Rvs) family of proteins includes Cdc42-interacting protein-4 (CIP4), formin-binding protein-17 (FBP-17) and transactivator of cytoskeletal assembly-1 (Toca-1), and drives membrane deformation and invagination. Membrane remodeling affects endocytosis, vesicle budding, and cargo selection. The F-BAR family presents a novel family of proteins, which little is known about their in vivo function. We investigated the physiological role of CIP4, by creating Cip4-null mice through homologous recombination. Compared with their wild-type littermates, the Cip4-null mice displayed lower early post-prandial glucose levels. Adipocytes isolated from Cip4-null mice exhibited increased [(14)C]2-deoxyglucose uptake compared with cells from wild-type mice. The enhanced insulin sensitivity was not due to higher levels of insulin or phospho-Akt, a critical player in insulin signaling. However, higher glucose transporter 4 (GLUT4) levels were detected in muscle membrane fractions in Cip4-null mice under insulin stimulation. Mouse embryonic fibroblasts from Cip4-null mice demonstrated decreased transferrin uptake, fluorescein isothiocyanate-dextran, and horseradish peroxidase uptake, indicating that CIP4 affects multiple modes of endocytosis. These studies demonstrate a physiological role for CIP4 in endocytosis leading to a whole animal phenotype.

Dasatinib inhibits the growth of molecularly heterogeneous myeloid leukemias

PURPOSE

Dasatinib is a dual Src/Abl inhibitor recently approved for Bcr-Abl+ leukemias with resistance or intolerance to prior therapy. Because Src kinases contribute to multiple blood cell functions by triggering a variety of signaling pathways, we hypothesized that their molecular targeting might lead to growth inhibition in acute myeloid leukemia (AML).

EXPERIMENTAL DESIGN

We studied growth factor-dependent and growth factor-independent leukemic cell lines, including three cell lines expressing mutants of receptor tyrosine kinases (Flt3 or c-Kit) as well as primary AML blasts for responsiveness to dasatinib.

RESULTS

Dasatinib resulted in the inhibition of Src family kinases in all cell lines and blast cells at approximately 1 x 10(-9) mol/L. It also inhibited mutant Flt3 or Kit tyrosine phosphorylation at approximately 1 x 10(-6) mol/L. Mo7e cells expressing the activating mutation (codon 816) of c-Kit were most sensitive to growth inhibition with a GI(50) of 5 x 10(-9) mol/L. Primary AML blast cells exhibited a growth inhibition of <1 x 10(-6) mol/L. Cell lines that showed growth inhibition at approximately 1 x 10(-6) mol/L showed a G(1) cell cycle arrest and correlated with accumulation of p21 and p27 protein. The addition of rapamycin or cytotoxic agents enhanced growth inhibition. Dasatinib also caused the apoptosis of Mo7e cells expressing oncogenic Kit.

CONCLUSIONS

Although all of the precise targets for dasatinib are not known, this multikinase inhibitor causes either growth arrest or apoptosis in molecularly heterogeneous AML. The addition of cytotoxic or targeted agents can enhance its effects.

Flow cytometric determination of Src phosphorylation in pediatric patients treated with dasatinib

Tyrosine kinase inhibitors, such as imatinib, have dramatically improved the outcomes for patients with selected cancers. For imatinib, western blotting of phospho-CrkL was an insensitive, indirect, and descriptive method to determine drug efficacy. Greater use of targeted therapies should involve more quantitative evaluation of the target's dose-inhibition. The Src/Abl kinase inhibitor dasatinib has recently been approved for use in Ph+ leukemias after failure with imatinib. Src family kinases (SFK) also play a critical role in nonhematologic cancers. We have developed a flow cytometric assay to measure SFK autophosphorylation levels in blood mononuclear cells and observed a direct correlation between its inhibition and patient dosage. This method provides a sensitive, quick, and quantitative tool to assess drug efficacy.

The F-BAR protein CIP4 promotes GLUT4 endocytosis through bidirectional interactions with N-WASp and Dynamin-2

F-BAR proteins are a newly described family of proteins with unknown physiological significance. Because F-BAR proteins, including Cdc42 interacting protein-4 (CIP4), drive membrane deformation and affect endocytosis, we investigated the role of CIP4 in GLUT4 traffic by flow cytometry in GLUT4myc-expressing L6 myoblasts (L6 GLUT4myc). L6 GLUT4myc cells express CIP4a as the predominant F-BAR protein. siRNA knockdown of CIP4 increased insulin-stimulated (14)C-deoxyglucose uptake by elevating cell-surface GLUT4. Enhanced surface GLUT4 was due to decreased endocytosis, which correlated with lower transferrin internalization. Immunoprecipitation of endogenous CIP4 revealed that CIP4 interacted with N-WASp and Dynamin-2 in an insulin-dependent manner. FRET confirmed the insulin-dependent, subcellular properties of these interactions. Insulin exposure stimulated specific interactions in plasma membrane and cytosolic compartments, followed by a steady-state response that underlies the coordination of proteins needed for GLUT4 traffic. Our findings reveal a physiological function for F-BAR proteins, supporting a previously unrecognized role for the F-BAR protein CIP4 in GLUT4 endocytosis, and show that interactions between CIP4 and Dynamin-2 and between CIP4 and NWASp are spatially coordinated to promote function.

Targeting Lyn inhibits tumor growth and metastasis in Ewing's sarcoma

Src family tyrosine kinases (SFK) play an important role in growth and metastasis of many types of human malignancies. However, their significance in Ewing's sarcoma remains to be elucidated. The purpose of this study was to evaluate the role of Lyn, one member of the SFK, in Ewing's sarcoma growth and metastasis and to determine whether a SFK inhibitor can induce Ewing's tumor regression. Lyn was expressed and activated in TC71, A4573, and SK-ES human Ewing's sarcoma cells. Lyn expression was seen in 13 of 15 patient tumor samples, 6 of which showed Lyn activation. Specific inhibition of Lyn using small interfering RNA significantly decreased primary tumor growth and lytic activity, and also reduced lung metastases in vivo. Down-regulation of Lyn resulted in decreased invasive capacity of tumor cells in vitro. AP23994, a small-molecule SFK inhibitor, decreased Lyn kinase activity and suppressed TC71 cell growth in vitro in a dose-dependent manner. Furthermore, treatment of mice bearing s.c. TC71 tumors with AP23994 or with polyethylenimine/Lyn-small interfering RNA gene therapy resulted in reduced Lyn kinase activity and significant tumor growth suppression. EWS/FLI-1, which is translocation fusion protein associated with Ewing's sarcoma, regulated Lyn gene expression and kinase activity. These data suggest that targeting Lyn may be a new therapeutic approach in treatment of Ewing's sarcoma.

A chemical biology screen identifies glucocorticoids that regulate c-maf expression by increasing its proteasomal degradation through up-regulation of ubiquitin

The oncogene c-maf is frequently overexpressed in multiple myeloma cell lines and patient samples and contributes to increased cellular proliferation in part by inducing cyclin D2 expression. To identify regulators of c-maf, we developed a chemical screen in NIH3T3 cells stably overexpressing c-maf and the cyclin D2 promoter driving luciferase. From a screen of 2400 off-patent drugs and chemicals, we identified glucocorticoids as c-maf–dependent inhibitors of cyclin D2 transactivation. In multiple myeloma cell lines, glucocorticoids reduced levels of c-maf protein without influencing corresponding mRNA levels. Subsequent studies demonstrated that glucocorticoids increased ubiquitination-dependent degradation of c-maf and up-regulated ubiquitin C mRNA. Moreover, ectopic expression of ubiquitin C recapitulated the effects of glucocorticoids, demonstrating regulation of c-maf protein through the abundance of the ubiquitin substrate. Thus, using a chemical biology approach, we identified a novel mechanism of action of glucocorticoids and a novel mechanism by which levels of c-maf protein are regulated by the abundance of the ubiquitin substrate.

Myelodysplastic syndromes: the complexity of stem-cell diseases

The prevalence of patients with myelodysplastic syndromes (MDS) is increasing owing to an ageing population and increased awareness of these diseases. MDS represent many different conditions, not just a single disease, that are grouped together by several clinical characteristics. A striking feature of MDS is genetic instability, and a large proportion of cases result in acute myeloid leukaemia (AML). We Review three emerging principles of MDS biology: stem-cell dysfunction and the overlap with AML, genetic instability and the deregulation of apoptosis, in the context of inherited bone marrow-failure syndromes, and treatment-related MDS and AML.

Src kinases in G-CSF receptor signaling

The Granulocyte Colony-Stimulating Factor (G-CSF) receptor, a member of the hematopoietin cytokine receptor superfamily, functions as a homodimer and requires the recruitment of cytosolic protein tyrosine kinases (PTKs) to transduce its signal. At least two cytosolic PTKs are primarily involved: Jak2, a member of the Janus family, and Lyn, a member of the Src family. Through poorly understood mechanisms, these kinases functionally interact with the G-CSF receptor. Jak2 primarily enlists members of the signal transducer and activator of transcription (STAT) family and Lyn phosphorylates a number of adaptor molecules, which link the G-CSF receptor to phosphatidylinositol (PI) 3'-kinase and extracellular signal-regulated kinases (Erk) pathways. This review presents evidence that the Src kinases play a major role in the pathways of G-CSF-mediated proliferation, survival, and differentiation. Identification of Src-dependent pathways provides drug targets useful in the treatment of myeloid leukemias.

Engagement of the B-cell antigen receptor activates STAT through Lyn in a Jak-independent pathway

Engagement of the B-cell antigen receptor (BCR) initiated by the Src kinase Lyn triggers rapid signaling cascades, leading to proliferation, differentiation or growth arrest of B cells. The Janus kinase (JAK)-STAT (signal transducer and activator of transcription) pathway, activated through cytokine receptors, mediates similar responses. Hypothesizing that Src and JAK pathways engage in crosstalk in B-cell signaling, we studied wild-type and Lyn-null B-cell lines, which express BCR. We found that activated BCR results in tyrosine phosphorylation of JAK-STAT, which required Lyn. To confirm that STAT activation is not due to JAK, we cloned the chicken homologs of JAK1 and JAK2 and made their antisense constructs. In cells expressing antisense JAK1 and JAK2, tyrosine phosphorylation of STAT was not inhibited following BCR stimulation. Using activation loop-specific phosphotyrosine antibodies, we did not detect phospho-JAK1 and phospho-JAK2 after BCR stimulation. The JAK inhibitor AG490 did not inhibit the tyrosine phosphorylation of Lyn or STAT after BCR simulation. An in vitro phosphorylation assay showed that Lyn directly phosphorylates STAT3. In an electrophoretic mobility shift assay, BCR stimulation led to enhanced DNA binding of the STAT3 in DT40, but not in the Lyn-null cells. We conclude that BCR engagement activates the STAT pathway via Lyn, independent of JAK.

The Integrin alpha9beta1 contributes to granulopoiesis by enhancing granulocyte colony-stimulating factor receptor signaling

The integrin alpha9beta1 is widely expressed on neutrophils, smooth muscle, hepatocytes, endothelia, and some epithelia. We now show that mice lacking this integrin have a dramatic defect in neutrophil development, with decreased numbers of granulocyte precursors in bone marrow and impaired differentiation of bone marrow cells into granulocytes. In response to granulocyte colony-stimulating factor (G-CSF), alpha9-deficient bone marrow cells or human bone marrow cells incubated with alpha9beta1-blocking antibody demonstrated decreased phosphorylation of signal transducer and activator of transcription 3 and extracellular signal-regulated protein kinase. These effects depended on the alpha9 subunit cytoplasmic domain, which was required for formation of a physical complex between alpha9beta1 and ligated G-CSF receptor. Integrin alpha9beta1 was required for granulopoiesis and played a permissive role in the G-CSF-signaling pathway, suggesting that this integrin could play an important role in disorders of granulocyte development and other conditions characterized by defective G-CSF signaling.