Sustained receptor activation and hyperproliferation in response to granulocyte colony-stimulating factor (G-CSF) in mice with a severe congenital neutropenia/acute myeloid leukemia-derived mutation in the G-CSF receptor gene

Leukemia-associated truncation of granulocyte colony-stimulating factor receptor impacts granulopoiesis throughout the life-course

Introduction

The granulocyte colony-stimulating factor receptor (G-CSFR), encoded by the CSF3R gene, is involved in the production and function of neutrophilic granulocytes. Somatic mutations in CSF3R leading to truncated G-CSFR forms are observed in acute myeloid leukemia (AML), particularly those subsequent to severe chronic neutropenia (SCN), as well as in a subset of patients with other leukemias.

Methods

This investigation introduced equivalent mutations into the zebrafish csf3r gene via genome editing and used a range of molecular and cellular techniques to understand the impact of these mutations on immune cells across the lifespan.

Results

Zebrafish harboring truncated G-CSFRs showed significantly enhanced neutrophil production throughout successive waves of embryonic hematopoiesis and a neutrophil maturation defect in adults, with the mutations acting in a partially dominant manner.

Discussion

This study has elucidated new insights into the impact of G-CSFR truncations throughout the life-course and created a bone fide zebrafish model for further investigation.

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.

A Systematic Review of the Role of Runt-Related Transcription Factor 1 (RUNX1) in the Pathogenesis of Hematological Malignancies in Patients With Inherited Bone Marrow Failure Syndromes

Somatic runt-related transcription factor 1 (RUNX1) mutations are the most common mutations in various hematological malignancies, such as myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Mono-allelic RUNX1 mutations in germline cells may cause familial platelet disorder (FPD), an inherited bone marrow failure syndrome (IBMFS) associated with an increased lifetime risk of AML. It is suspected that additional RUNX1 mutations may play a role in the pathogenesis of hematological malignancies in IBMFS. This review aims to study the role of RUNX1 mutations in the pathogenesis of hematological malignancies in patients with IBMFS. A PubMed database search was conducted using the following medical subject heading (MeSH) terms: “inherited bone marrow failure syndromes,” “hematological neoplasms,” “gene expression regulation, leukemic,” “RUNX1 protein, human,” “RUNX1 protein, mouse,” and “Neutropenia, Severe Congenital, Autosomal recessive.” Three studies published in 2020 were identified as meeting our inclusion and exclusion criteria. Leukemic progression in severe congenital neutropenia was used as a disease model to evaluate the clinical, molecular, and mechanistic basis of RUNX1 mutations identified in hematological malignancies. Studies in mice and genetically reprogrammed or induced pluripotent stem cells (iPSCs) have shown that isolated RUNX1 mutations are weakly leukemogenic and only initiate hyperproduction of immature hematopoietic cells when in combination with granulocyte colony-stimulating factor 3 receptor (GCSF3R) mutations. Despite this, whole-exome sequencing (WES) performed on leukemogenic transformed cells revealed that all AML cells had an additional mutation in the CXXC finger protein 4 (CXXC4) gene that caused hyperproduction of the ten-eleven translocation (TET2) protein. This protein causes inflammation in cells with RUNX1 mutations. This process is thought to be critical for clonal myeloid malignant transformation (CMMT) of leukemogenic cells. In conclusion, the combinations of GCSF3R and RUNX1 mutations have a prominent effect on myeloid differentiation resulting in the hyperproduction of myeloblasts. In other studies, it has been noted that the mutations in GCSF3R and RUNX1 genes are not sufficient for the full transformation of leukemogenic cells to AML, and an additional clonal mutation in the CXXC4 gene is essential for full transformation to occur. These data have implicitly demonstrated that RUNX1 mutations are critical in the pathogenesis of various hematological malignancies, and further investigations into the role of RUNX1 are paramount for the development of new cancer treatments.

What Clonal Hematopoiesis Can Teach Us About MDS

Clonal hematopoiesis (CH), defined as the clonal expansion of mutated hematopoietic stem and progenitor cells (HSPCs), is a common aging process. CH is a risk factor for the development of hematologic malignancies, most commonly myeloid neoplasms (MNs) including acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), and myeloproliferative neoplasm (MPN). Recent work has elucidated how the development and cellular fitness of CH is shaped by aging, environmental exposures, and the germline (inherited) genetic background of an individual. This in turn has provided valuable insights into the pathogenesis of MNs including MDS. Here, in this review, we discuss the genetic origins of CH, the environmental stressors that influence CH, and the implications of CH on health outcomes including MDS. Since MNs have shared risk factors and underlying biology, most of our discussion regarding the implications of CH surrounds MN in general rather than focusing specifically on MDS. We conclude with future directions and areas of investigation including how intervention studies of CH might inform future therapeutic approaches to MN including MDS.

Impaired myelopoiesis in congenital neutropenia: insights into clonal and malignant hematopoiesis

A common feature of both congenital and acquired forms of bone marrow failure is an increased risk of developing acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS). Indeed, the development of MDS or AML is now the major cause of mortality in patients with congenital neutropenia. Thus, there is a pressing clinical need to develop better strategies to prevent, diagnose early, and treat MDS/AML in patients with congenital neutropenia and other bone marrow failure syndromes. Here, we discuss recent data characterizing clonal hematopoiesis and progression to myeloid malignancy in congenital neutropenia, focusing on severe congenital neutropenia (SCN) and Shwachman-Diamond syndrome. We summarize recent studies showing excellent outcomes after allogenic hematopoietic stem cell transplantation for many (but not all) patients with congenital neutropenia, including patients with SCN with active myeloid malignancy who underwent transplantation. Finally, we discuss how these new data inform the current clinical management of patients with congenital neutropenia.

New insight into the mechanism of granulocyte colony-stimulating factor (G-CSF) that induces the mobilization of neutrophils

Over the past 20 years, granulocyte colony-stimulating factor (G-CSF) has driven the attention of researchers as a therapeutic agent for curing patients suffering from neutropenia. Despite the successful use of G-CSF, it currently requires daily injections, which are inconvenient, expensive, and distressing for children. Therefore, an alternative strategy for using G-CSF for treatment is needed. Understanding the G-CSF structure, expression, mechanism of action, and how it induces neutrophils mobilization is crucial to producing promising cancer therapy. The ability of G-CSF to mobilize hematopoietic stem cells from the bone marrow into the blood circulation was consequently exploited and altered the practice of hematopoietic stem cell transplantation. This is the motivation for the current review, which sheds light on the history of G-CSF and then focuses on the mechanism of action upon binding to its receptor (G-CSFR) and how that had led to the stimulation of neutrophils mobilization. The findings of this review show new insight into the mechanism of G-CSF that induces neutrophils mobilization. Thus, Understanding the G-CSF will provide a more effective treatment for all neutropenia patients.

Neuroprotection through G-CSF: recent advances and future viewpoints

Granulocyte-colony stimulating factor (G-CSF), a member of the cytokine family of hematopoietic growth factors, is 19.6 kDa glycoprotein which is responsible for the proliferation, maturation, differentiation, and survival of neutrophilic granulocyte lineage. Apart from its proven clinical application to treat chemotherapy-associated neutropenia, recent pre-clinical studies have highlighted the neuroprotective roles of G-CSF i.e., mobilization of haemopoietic stem cells, anti-apoptotic, neuronal differentiation, angiogenesis and anti-inflammatory in animal models of neurological disorders. G-CSF is expressed by numerous cell types including neuronal, immune and endothelial cells. G-CSF is released in autocrine manner and binds to its receptor G-CSF-R which further activates numerous signaling transduction pathways including PI3K/AKT, JAK/STAT and MAP kinase, and thereby promote neuronal survival, proliferation, differentiation, mobilization of hematopoietic stem and progenitor cells. The expression of G-CSF receptors (G-CSF-R) in the different brain regions and their upregulation in response to neuronal insult indicates the autocrine protective signaling mechanism of G-CSF by inhibition of apoptosis, inflammation, and stimulation of neurogenesis. These observed neuroprotective effects of G-CSF makes it an attractive target to mitigate neurodegeneration associated with neurological disorders. The objective of the review is to highlight and summarize recent updates on G-CSF as a therapeutically versatile neuroprotective agent along with mechanisms of action as well as possible clinical applications in neurodegenerative disorders including AD, PD and HD.

Cooperating, congenital neutropenia-associated Csf3r and Runx1 mutations activate pro-inflammatory signaling and inhibit myeloid differentiation of mouse HSPCs

Patients with the pre-leukemia bone marrow failure syndrome called severe congenital neutropenia (CN) have an approximately 15% risk of developing acute myeloid leukemia (AML; called here CN/AML). Most CN/AML patients co-acquire CSF3R and RUNX1 mutations, which play cooperative roles in the development of AML. To establish an in vitro model of leukemogenesis, we utilized bone marrow lin^- cells from transgenic C57BL/6-d715 Csf3r mice expressing a CN patient-mimicking truncated CSF3R mutation. We transduced these cells with vectors encoding RUNX1 wild type (WT) or RUNX1 mutant proteins carrying the R139G or R174L mutations. Cells transduced with these RUNX1 mutants showed diminished in vitro myeloid differentiation and elevated replating capacity, compared with those expressing WT RUNX1. mRNA expression analysis showed that cells transduced with the RUNX1 mutants exhibited hyperactivation of inflammatory signaling and innate immunity pathways, including IL-6, TLR, NF-kappaB, IFN, and TREM1 signaling. These data suggest that the expression of mutated RUNX1 in a CSF3R-mutated background may activate the pro-inflammatory cell state and inhibit myeloid differentiation.

RUNX1 Mutations in the Leukemic Progression of Severe Congenital Neutropenia

Somatic RUNX1 mutations are found in approximately 10% of patients with de novo acute myeloid leukemia (AML), but are more common in secondary forms of myelodysplastic syndrome (MDS) or AML. Particularly, this applies to MDS/AML developing from certain types of leukemia-prone inherited bone marrow failure syndromes. How these RUNX1 mutations contribute to the pathobiology of secondary MDS/AML is still unknown. This mini-review focusses on the role of RUNX1 mutations as the most common secondary leukemogenic hit in MDS/AML evolving from severe congenital neutropenia (SCN).

Mechanisms of leukemic transformation in congenital neutropenia

PURPOSE OF REVIEW

The development of a myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) in patients with congenital neutropenia is now the major cause of mortality. Treatment options are limited and there are no effective prevention strategies. This review focuses on mechanisms of leukemic transformation in severe congenital neutropenia (SCN) and Shwachman-Diamond syndrome (SDS), the two most common types of congenital neutropenia.

RECENT FINDINGS

AML/MDS that develops in the setting of congenital neutropenia has distinct molecular features. Clonal hematopoiesis because of TP53 mutations is seen in nearly 50% of patients with SDS, but is not seen in patients with SCN. Accordingly, there is a very high frequency of TP53 mutations in AML/MDS arising in the setting of SDS but not SCN. The rate of mutation accumulation in hematopoietic stem cells (HSCs) from patients with congenital neutropenia is not increased.

SUMMARY

Both HSC cell-intrinsic and noncell-intrinsic changes contribute to the development of clonal hematopoiesis in congenital neutropenia and likely accounts for the high rate of leukemic transformation. In SCN, the persistently high levels of granulocyte colony-stimulating factor drive expansion of HSCs carrying truncation mutations of CSF3R. In SDS, impaired ribosome biogenesis induces p53-mediated growth inhibition and drives expansion of HSCs carrying TP53 mutations.

Ultra-Sensitive CSF3R Deep Sequencing in Patients With Severe Congenital Neutropenia

High frequency of acquired CSF3R (colony stimulating factor 3 receptor, granulocyte) mutations has been described in patients with severe congenital neutropenia (CN) at pre-leukemia stage and overt acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS). Here, we report the establishment of an ultra-sensitive deep sequencing of a CSF3R segment encoding the intracellular “critical region” of the G-CSFR known to be mutated in CN-MDS/AML patients. Using this method, we achieved a mutant allele frequency (MAF) detection rate of 0.01%. We detected CSF3R mutations in CN patients with different genetic backgrounds, but not in patients with other types of bone marrow failure syndromes chronically treated with G-CSF (e.g., Shwachman-Diamond Syndrome). Comparison of CSF3R deep sequencing results of DNA and cDNA from the bone marrow and peripheral blood cells revealed the highest sensitivity of cDNA from the peripheral blood polymorphonuclear neutrophils. This approach enables the identification of low-frequency CSF3R mutant clones, increases sensitivity, and earlier detection of CSF3R mutations acquired during the course of leukemogenic evolution of pre-leukemia HSCs of CN patients. We suggest application of sequencing of the entire CSF3R gene at diagnosis to identify patients with inherited lost-of-function CSF3R mutations and annual ultra-deep sequencing of the critical region of CSF3R to monitor acquisition of CSF3R mutations.

Time resolved quantitative phospho-tyrosine analysis reveals Bruton's Tyrosine kinase mediated signaling downstream of the mutated granulocyte-colony stimulating factor receptors

Granulocyte-colony stimulating factor receptor (G-CSFR) controls myeloid progenitor proliferation and differentiation to neutrophils. Mutations in CSF3R (encoding G-CSFR) have been reported in patients with chronic neutrophilic leukemia (CNL) and acute myeloid leukemia (AML); however, despite years of research, the malignant downstream signaling of the mutated G-CSFRs is not well understood. Here, we used a quantitative phospho-tyrosine analysis to generate a comprehensive signaling map of G-CSF induced tyrosine phosphorylation in the normal versus mutated (proximal: T618I and truncated: Q741x) G-CSFRs. Unbiased clustering and kinase enrichment analysis identified rapid induction of phospho-proteins associated with endocytosis by the wild type G-CSFR only; while G-CSFR mutants showed abnormal kinetics of canonical Stat3, Stat5, and Mapk phosphorylation, and aberrant activation of Bruton's Tyrosine Kinase (Btk). Mutant-G-CSFR-expressing cells displayed enhanced sensitivity (3-5-fold lower IC50) for ibrutinib-based chemical inhibition of Btk. Primary murine progenitor cells from G-CSFR-Q741x knock-in mice validated activation of Btk by the mutant receptor and retrovirally transduced human CD34⁺ umbilical cord blood cells expressing mutant receptors displayed enhanced sensitivity to Ibrutinib. A significantly lower clonogenic potential was displayed by both murine and human primary cells expressing mutated receptors upon ibrutinib treatment. Finally, a dramatic synergy was observed between ibrutinib and ruxolinitib at lower dose of the individual drug. Altogether, these data demonstrate the strength of unsupervised proteomics analyses in dissecting oncogenic pathways, and suggest repositioning Ibrutinib for therapy of myeloid leukemia bearing CSF3R mutations. Phospho-tyrosine data are available via ProteomeXchange with identifier PXD009662.

Somatic mutations and clonal hematopoiesis in congenital neutropenia

Severe congenital neutropenia (SCN) and Shwachman-Diamond syndrome (SDS) are congenital neutropenia syndromes with a high rate of leukemic transformation. Hematopoietic stressors may contribute to leukemic transformation by increasing the mutation rate in hematopoietic stem/progenitor cells (HSPCs) and/or by promoting clonal hematopoiesis. We sequenced the exome of individual hematopoietic colonies derived from 13 patients with congenital neutropenia to measure total mutation burden and performed error-corrected sequencing on a panel of 46 genes on 80 patients with congenital neutropenia to assess for clonal hematopoiesis. An average of 3.6 ± 1.2 somatic mutations per exome was identified in HSPCs from patients with SCN compared with 3.9 ± 0.4 for healthy controls (P = NS). Clonal hematopoiesis due to mutations in TP53 was present in 48% (13/27) of patients with SDS but was not seen in healthy controls (0/17, P < .001) or patients with SCN (0/40, P < .001). Our SDS cohort was young (median age 6.3 years), and many of the patients had multiple TP53 mutations. Conversely, clonal hematopoiesis due to mutations of CSF3R was present in patients with SCN but was not detected in healthy controls or patients with SDS. These data show that hematopoietic stress, including granulocyte colony-stimulating factor, do not increase the mutation burden in HSPCs in congenital neutropenia. Rather, distinct hematopoietic stressors result in the selective expansion of HSPCs carrying specific gene mutations. In particular, in SDS there is enormous selective pressure to expand TP53-mutated HSPCs, suggesting that acquisition of TP53 mutations is an early, likely initiating event, in the transformation to myelodysplastic syndrome/acute myeloid leukemia in patients with SDS.

Role of CSF3R mutations in the pathomechanism of congenital neutropenia and secondary acute myeloid leukemia

Acquired mutations in the intracellular part of CSF3R (colony stimulating factor 3 receptor, granulocyte) have been detected with a frequency of more than 30% in severe congenital neutropenia (CN) patients. CN is a preleukemic syndrome with a risk of approximately 20% to develop leukemia. More than 80% of CN patients who develop acute myeloid leukemia or myelodysplastic syndrome reveal CSF3R mutations, suggesting that they are involved in leukemogenesis. Using deep-sequencing technology, we were able to analyze large cohorts of CN patients for the entire CSF3R sequence as well as to identify cell clones carrying mutations in the intracellular part of CSF3R with very high sensitivity. Acquisition of CSF3R mutations is a CN-specific phenomenon and is associated with inherited mutations causing CN or cyclic neutropenia, such as ELANE mutations. In the group of CN patients negative for known germ-line mutations, biallelic CSF3R mutations were identified. In addition, CSF3R mutant clones are highly dynamic and may disappear and reappear during continuous granulocyte colony-stimulating factor (G-CSF) therapy. The time between the first detection of CSF3R mutations and overt leukemia is highly variable.

Severe congenital neutropenias

Severe congenital neutropenias are a heterogeneous group of rare haematological diseases characterized by impaired maturation of neutrophil granulocytes. Patients with severe congenital neutropenia are prone to recurrent, often life-threatening infections beginning in their first months of life. The most frequent pathogenic defects are autosomal dominant mutations in ELANE, which encodes neutrophil elastase, and autosomal recessive mutations in HAX1, whose product contributes to the activation of the granulocyte colony-stimulating factor (G-CSF) signalling pathway. The pathophysiological mechanisms of these conditions are the object of extensive research and are not fully understood. Furthermore, severe congenital neutropenias may predispose to myelodysplastic syndromes or acute myeloid leukaemia. Molecular events in the malignant progression include acquired mutations in CSF3R (encoding G-CSF receptor) and subsequently in other leukaemia-associated genes (such as RUNX1) in a majority of patients. Diagnosis is based on clinical manifestations, blood neutrophil count, bone marrow examination and genetic and immunological analyses. Daily subcutaneous G-CSF administration is the treatment of choice and leads to a substantial increase in blood neutrophil count, reduction of infections and drastic improvement of quality of life. Haematopoietic stem cell transplantation is the alternative treatment. Regular clinical assessments (including yearly bone marrow examinations) to monitor treatment course and detect chromosomal abnormalities (for example, monosomy 7 and trisomy 21) as well as somatic pre-leukaemic mutations are recommended.

Characterization of the leukemogenic potential of distal cytoplasmic CSF3R truncation and missense mutations

An increasing number of variants of unknown significance (VUS) are being identified in leukemia patients with the application of deep sequencing and these include CSF3R cytoplasmic mutations. Previous studies have demonstrated oncogenic potential of certain CSF3R truncation mutations prior to internalization motifs. However, the oncogenic potential of truncating the more distal region of CSF3R cytoplasmic domain as well as cytoplasmic missense mutations remains uncharacterized. Here we identified that CSF3R distal cytoplasmic truncation mutations (Q793–Q823) also harbored leukemogenic potential. Mechanistically, these distal cytoplasmic truncation mutations demonstrated markedly decreased receptor degradation, probably due to loss of the de-phosphorylation domain (residues N818–F836). Furthermore, all truncations prior to Q823 demonstrated increased expression of the higher molecular weight CSF3R band, which is shown to be essential for the receptor surface expression and the oncogenic potential. We further demonstrated that sufficient STAT5 activation is essential for oncogenic potential. In addition, CSF3R K704A demonstrated transforming capacity due to interruption of receptor ubiquitination and degradation. In summary, we have expanded the region of the CSF3R cytoplasmic domain in which truncation or missense mutations exhibit leukemogenic capacity, which will be useful for evaluating the relevance of CSF3R mutations in patients and helpful in defining targeted therapy strategies.

Granulocyte colony-stimulating factor receptor signaling in severe congenital neutropenia, chronic neutrophilic leukemia, and related malignancies

Granulocyte colony-stimulating factor is a hematopoietic cytokine that stimulates neutrophil production and hematopoietic stem cell mobilization by initiating the dimerization of homodimeric granulocyte colony-stimulating factor receptor. Different mutations of CSF3R have been linked to a unique spectrum of myeloid disorders and related malignancies. Myeloid disorders caused by the CSF3R mutations include severe congenital neutropenia, chronic neutrophilic leukemia, and atypical chronic myeloid leukemia. In this review, we provide an analysis of granulocyte colony-stimulating factor receptor, various mutations, and their roles in the severe congenital neutropenia, chronic neutrophilic leukemia, and malignant transformation, as well as the clinical implications and some perspective on approaches that could expand our knowledge with respect to the normal signaling mechanisms and those associated with mutations in the receptor.

A Truncated Granulocyte Colony-stimulating Factor Receptor (G-CSFR) Inhibits Apoptosis Induced by Neutrophil Elastase G185R Mutant: IMPLICATION FOR UNDERSTANDING CSF3R GENE MUTATIONS IN SEVERE CONGENITAL NEUTROPENIA

Mutations in ELANE encoding neutrophil elastase (NE) have been identified in the majority of patients with severe congenital neutropenia (SCN). The NE mutants have been shown to activate unfolded protein response and induce premature apoptosis in myeloid cells. Patients with SCN are predisposed to acute myeloid leukemia (AML), and progression from SCN to AML is accompanied by mutations in CSF3R encoding the granulocyte colony-stimulating factor receptor (G-CSFR) in ∼80% of patients. The mutations result in the expression of C-terminally truncated G-CSFRs that promote strong cell proliferation and survival. It is unknown why the CSF3R mutations, which are rare in de novo AML, are so prevalent in SCN/AML. We show here that a G-CSFR mutant, d715, derived from an SCN patient inhibited G-CSF-induced expression of NE in a dominant negative manner. Furthermore, G-CSFR d715 suppressed unfolded protein response and apoptosis induced by an SCN-derived NE mutant, which was associated with sustained activation of AKT and STAT5, and augmented expression of BCL-XL. Thus, the truncated G-CSFRs associated with SCN/AML may protect myeloid precursor cells from apoptosis induced by the NE mutants. We propose that acquisition of CSF3R mutations may represent a mechanism by which myeloid precursor cells carrying the ELANE mutations evade the proapoptotic activity of the NE mutants in SCN patients.

Enhanced MAPK signaling is essential for CSF3R-induced leukemia

Both membrane-proximal and truncation mutations in CSF3R have recently been reported to drive the onset of chronic neutrophilic leukemia (CNL). Here we show that although truncation mutation alone can not induce leukemia, both proximal and compound mutations (proximal and truncation mutations on same allele) are leukemogenic with a disease latency of 90 and 23 days, respectively. Comparative whole-genome expression profiling and biochemical experiments revealed that induced expression of Mapk adaptor protein Ksr1 and enhanced Mapk signaling are crucial to leukemogenesis by CSF3R proximal and compound mutants. Moreover, inhibition of Mek1/2 by trametinib alone is sufficient to suppress leukemia induced by both CSF3R proximal and ruxolitinib-resistant compound mutations. Together, these findings elucidate a Mapk-dependent mechanism of CSF3R-induced pathogenesis, and they establish the rationale for clinical evaluation of MEK1/2 inhibition in CNL.

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.

The importance of the granulocyte-colony stimulating factor in oncology

Granulocyte-colony stimulating factor (G-CSF) is a glycoprotein, the second CSF, sharing some common effects with granulocyte macrophage-colony stimulating factor (GM-CSF), interleukin-3 (IL-3) and interleukin-5 (IL-5). G-CSF is mainly produced by fibroblasts and endothelial cells from bone marrow stroma and by immunocompetent cells (monocytes, macrophages). The receptor for G-CSF (G-CSFR) is part of the cytokine and hematopoietin receptor superfamily and G-CSFR mutations cause severe congenital neutropenia. The main action of G-CSF - G-CSFR linkage is stimulation of the production, mobilization, survival and chemotaxis of neutrophils, but there are many other G-CSF effects: growth and migration of endothelial cells, decrease of norepinephrine reuptake, increase in osteoclastic activity and decrease in osteoblast activity. In oncology, G-CSF is utilized especially for the primary prophylaxis of chemotherapy-induced neutropenia, but it can be used for hematopoietic stem cell transplantation, it can produce monocytic differentiation of some myeloid leukemias and it can increase some drug resistance. The therapeutic indications of G-CSF are becoming more and more numerous: non neutropenic patients infections, reproductive medicine, neurological disturbances, regeneration therapy after acute myocardial infarction and of skeletal muscle, and hepatitis C therapy.

CSF3R and CALR mutations in paediatric myeloid disorders and the association of CSF3R mutations with translocations, including t(8; 21)

Mutations in the colony-stimulating factor 3 receptor (CSF3R) and calreticulin (CALR) genes have been reported in a proportion of adults with myeloproliferative disease. However, little is known about CSF3R or CALR mutations in paediatric myeloid disorders. We analysed CSF3R exons 14 and 17, and CALR exon 9, using direct sequencing in samples of paediatric acute myeloid leukaemia (AML; n = 521), juvenile myelomonocytic leukaemia (JMML; n = 40), myelodysplastic syndrome (MDS; n = 20) and essential thrombocythaemia (ET; n = 21). CSF3R mutations were found in 10 (1.2%) of 521 patients with AML; two in exon 14 (both missense mutations resulting in p.T618I) and eight in exon 17 (three frameshift mutations: p.S715X, p.Q774R, and p.S783Q; and five novel missense mutations: p.Q754K, p.R769H, p.L777F, p.T781I, and S795R). All of the patients with mutations in CSF3R exon 17 had chromosomal translocations, including four with t(8;21). At the time of reporting, seven of these ten patients are alive; three have died, due to side effects of chemotherapy. No CSF3R mutations were found in cases of MDS, JMML or ET. The only mutation found in the CALR gene was a frameshift (p.L367 fs) in one ET patient. We discuss the potential impact of these findings for the leukaemogenesis and clinical features of paediatric myeloid disorders.

Game of clones: the genomic evolution of severe congenital neutropenia

Severe congenital neutropenia (SCN) is a genetically heterogeneous condition of bone marrow failure usually diagnosed in early childhood and characterized by a chronic and severe shortage of neutrophils. It is now well-established that mutations in HAX1 and ELANE (and more rarely in other genes) are the genetic cause of SCN. In contrast, it has remained unclear how these mutations affect neutrophil development. Innovative models based on induced pluripotent stem cell technology are being explored to address this issue. These days, most SCN patients receive life-long treatment with granulocyte colony-stimulating factor (G-CSF, CSF3). CSF3 therapy has greatly improved the life expectancy of SCN patients, but also unveiled a high frequency of progression toward myelodysplastic syndrome (MDS) and therapy refractory acute myeloid leukemia (AML). Expansion of hematopoietic clones with acquired mutations in the gene encoding the G-CSF receptor (CSF3R) is regularly seen in SCN patients and AML usually descends from one of these CSF3R mutant clones. These findings raised the questions how CSF3R mutations affect CSF3 responses of myeloid progenitors, how they contribute to the pre-leukemic state of SCN, and which additional events are responsible for progression to leukemia. The vast (sub)clonal heterogeneity of AML and the presence of AML-associated mutations in normally aged hematopoietic clones make it often difficult to determine which mutations are responsible for the leukemic process. Leukemia predisposition syndromes such as SCN are unique disease models to identify the sequential acquisition of these mutations and to interrogate how they contribute to clonal selection and leukemic evolution.

Granulocyte colony-stimulating factor receptor mutations in myeloid malignancy

Granulocyte colony-stimulating factor is a cytokine able to stimulate both myelopoiesis and hematopoietic stem cell mobilization, which has seen it used extensively in the clinic to aid hematopoietic recovery. It acts specifically via the homodimeric granulocyte colony-stimulating factor receptor (G-CSFR), which is principally expressed on the surface of myeloid and hematopoietic progenitor cells. A number of pathogenic mutations have now been identified in CSF3R, the gene encoding G-CSFR. These fall into distinct classes, each of which is associated with a particular spectrum of myeloid disorders, including malignancy. This review details the various CSF3R mutations, their mechanisms of action, and contribution to disease, as well as discussing the clinical implications of such mutations.

Cooperativity of RUNX1 and CSF3R mutations in severe congenital neutropenia: a unique pathway in myeloid leukemogenesis

CN/AML patients have a high frequency of CSF3R and RUNX1 mutations. CSF3R and RUNX1 mutations induce elevated proliferation of CD34+ cells.

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.

GCSF-R expression in myelodysplastic and myeloproliferative disorders and blast dysmaturation in CML

OBJECTIVES

To characterize granulocyte colony-stimulating factor receptor (CD114) expression in normal (n = 20), myelodysplastic (n = 34), and chronic myelogenous leukemia (CML; n = 5) bone marrow by flow cytometry.

METHODS

Clinical bone marrow samples were analyzed using CD33/CD114/CD34/CD117/CD45. CD114 density (mean fluorescence intensity) and cellular distribution were evaluated on early blasts (CD33-), late blasts (CD33+), promyelocytes, and granulocytes.

RESULTS

Normal CD114 acquisition occurred on early blasts, peaked on promyelocytes, and decreased on granulocytes. Forty percent of CD34+ blasts expressed CD114 and one-third were early blasts. In myelodysplastic syndromes, altered CD114 distribution was more informative than density changes. In CML, CD114 density was significantly decreased on early blasts and expression was essentially limited to late blasts. We observed a specific blast dysmaturation pattern in CML involving CD33, CD34, and CD114 that was 83% sensitive and 100% specific in initial diagnosis.

CONCLUSIONS

CD114 provides useful additional detail in phenotypic assessment of hematopoietic precursor maturation.

Granulocyte colony-stimulating factor receptor signaling: implications for G-CSF responses and leukemic progression in severe congenital neutropenia

Following activation by their cognate ligands, cytokine receptors undergo intracellular routing toward lysosomes, where they are degraded. This review focuses on the signaling function of the G-CSFR in relation to the dynamics of endosomal routing of the G-CSFR. Mechanisms involving receptor lysine ubiquitination and redox-controlled phosphatase activities are discussed. Specific attention is paid to the consequences of G-CSFR mutations, acquired in patients with severe congenital neutropenias who receive G-CSF therapy, particularly in the context of leukemic transformation, a major clinical complication of the disease.

Engineering mouse models with myelodysplastic syndrome human candidate genes; how relevant are they?

Myelodysplastic syndromes represent particularly challenging hematologic malignancies that arise from a large spectrum of genetic events resulting in a disease characterized by a range of different presentations and outcomes. Despite efforts to classify and identify the key genetic events, little improvement has been made in therapies that will increase patient survival. Animal models represent powerful tools to model and study human diseases and are useful pre-clinical platforms. In addition to enforced expression of candidate oncogenes, gene inactivation has allowed the consequences of the genetic effects of human myelodysplastic syndrome to be studied in mice. This review aims to examine the animal models expressing myelodysplastic syndrome-associated genes that are currently available and to highlight the most appropriate model to phenocopy myelodysplastic syndrome disease and its risk of transformation to acute myelogenous leukemia.

Androgen receptor influences on body defense system via modulation of innate and adaptive immune systems: lessons from conditional AR knockout mice

Upon insult, such as infection or tissue injury, the innate and adaptive immune systems initiate a series of responses to defend the body. Recent studies from immune cell-specific androgen receptor (AR) knockout mice demonstrated that androgen and its receptor (androgen/AR) play significant roles in both immune regulations. In the innate immunity, androgen/AR is required for generation and proper function of neutrophils; androgen/AR also regulates wound healing processes through macrophage recruitment and proinflammatory cytokine production. In adaptive immunity, androgen/AR exerts suppressive effects on development and activation of T and B cells. Removal of such suppression causes thymic enlargement and excessive export of immature B cells. Altogether, androgen/AR plays distinct roles in individual immune cells, and targeting androgen/AR may help in treatment and management of immune-related diseases.

From famine to feast: sending out the clones

In this issue of Blood, Beekman et al provide compelling evidence for the multistep evolution of acute myeloid leukemia (AML) from severe congenital neutropenia (SCN) over a 17-year period. Moreover, they found that 5 different gain-of-function mutations in the granulocyte colony-stimulating factor receptor (GCSFR) arose during this transformation, suggesting that 2 mutations behaved as drivers for clonal outgrowth, while 3 others did not.1

Sequential gain of mutations in severe congenital neutropenia progressing to acute myeloid leukemia

Severe congenital neutropenia (SCN) is a BM failure syndrome with a high risk of progression to acute myeloid leukemia (AML). The underlying genetic changes involved in SCN evolution to AML are largely unknown. We obtained serial hematopoietic samples from an SCN patient who developed AML 17 years after the initiation of G-CSF treatment. Next- generation sequencing was performed to identify mutations during disease progression. In the AML phase, we found 12 acquired nonsynonymous mutations. Three of these, in CSF3R, LLGL2, and ZC3H18, co-occurred in a subpopulation of progenitor cells already in the early SCN phase. This population expanded over time, whereas clones harboring only CSF3R mutations disappeared from the BM. The other 9 mutations were only apparent in the AML cells and affected known AML-associated genes (RUNX1 and ASXL1) and chromatin remodelers (SUZ12 and EP300). In addition, a novel CSF3R mutation that conferred autonomous proliferation to myeloid progenitors was found. We conclude that progression from SCN to AML is a multistep process, with distinct mutations arising early during the SCN phase and others later in AML development. The sequential gain of 2 CSF3R mutations implicates abnormal G-CSF signaling as a driver of leukemic transformation in this case of SCN.

Hematopoietic stem cell transplantation for severe congenital neutropenia

PURPOSE OF REVIEW

Hematopoietic stem cell transplantation (HCT) is the only curative option for patients with severe congenital neutropenia (SCN). Transplant success is dependent on identifying at-risk patients and proceeding to transplant before the development of severe infections or malignant transformation. This review focuses on recent advancements in risk stratification of SCN patients, indications for HCT, and review of published transplant studies.

RECENT FINDINGS

Patients with poor neutrophil response despite high doses of granulocyte colony-stimulating factor (G-CSF) are at greatest risk for malignant transformation. Other studies demonstrate elevated risk with mutations in the G-CSF receptor gene and a specific mutation in the ELANE gene. These patients are at high-risk of sepsis or leukemia development and should proceed to transplant with best available donor. As recent published studies demonstrate, HCT is highly successful in patients without leukemia and, therefore, may be considered in selected low-risk patients given the life-long risk of malignancy and infection.

SUMMARY

The decision whether to proceed to HCT in healthy patients maintained on G-CSF is difficult. As transplant-related mortality continues to decrease, the role of transplant in SCN is likely to expand to more patients.

Investigation and analysis of single nucleotide polymorphisms in Janus kinase/signal transducer and activator of transcription genes with leukemia

Aberrant activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway may predispose to leukemia due to deregulation of proliferation, differentiation or apoptosis. This study was conducted to investigate whether any association exists between genetic polymorphisms in the JAK2, STAT3 and STAT5 genes and individual susceptibility to leukemia. A case-control study was carried out using a Chinese sample set with 344 cases of leukemia and 346 controls matched by age and ethnicity. Genomic DNA was assayed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF) on 13 single nucleotide polymorphisms (SNPs). Genotype analyses showed that two SNPs, namely rs17886724 and rs2293157 located in STAT3 and STAT5, respectively, were significantly associated with leukemia (p < 0.05 for all). Interaction analyses of SNPs (rs17886724|rs2293157; rs11079041| rs2293157) showed that there were inferior associations in chronic lymphocytic leukemia (CLL) and acute myeloid leukemia (AML) compared to the control group (0.1 > p > 0.05). Linkage disequilibrium existed between rs11079041 and rs2293157 in both leukemia and control groups (r(2) = 0.7). The haplotypes displayed significant association between rs11079041 and rs2293157 in both leukemia and control groups (p < 0.05). The accuracy rate of the support vector machine (SVM) classification model in making a prediction of leukemia was 97%. The results indicated that STAT3 and STAT5 gene SNPs may be prognostic of leukemia.

Mutations in the transmembrane and juxtamembrane domains enhance IL27R transforming activity

Cytokines and their receptors regulate haemopoiesis by controlling cellular growth, survival and differentiation. Thus it is not surprising that mutations of cytokine receptors contribute to the formation of haemopoietic disorders, including cancer. We recently identified transforming properties of IL27R, the ligand-binding component of the receptor for interleukin-27. Although wild-type IL27R exhibits transforming properties in haemopoietic cells, in the present study we set out to determine if the transforming activity of IL27R could be enhanced by mutation. We identified three mutations of IL27R that enhance its transforming activity. One of these mutations is a phenylalanine to cysteine mutation at residue 523 (F523C) in the transmembrane domain of the receptor. The two other mutations identified involve deletions of amino acids in the cytoplasmic juxtamembrane region of the receptor. Expression of each of these mutant IL27R proteins led to rapid cytokine-independent transformation in haemopoietic cells. Moreover, the rate of transformation induced by these mutants was significantly greater than that induced by wild-type IL27R. Expression of these IL27R mutants also induced enhanced activation of JAK (Janus kinase)/STAT (signal transducer and activator of transcription) signalling compared with wild-type. An activating deletion mutation of IL27R enhanced homodimerization of the receptor by a mechanism that may involve disulfide bonding. These transforming IL27R mutants displayed equal or greater transforming activity than bona fide haemopoietic oncogenes such as BCR-ABL (breakpoint cluster region-Abelson murine leukaemia viral oncogene homologue) and JAK2-V617F. Since IL27R is expressed on haemopoietic stem cells, lymphoid cells and myeloid cells, including acute myeloid leukaemia blast cells, mutation of this receptor has the potential to contribute to a variety of haemopoietic neoplasms.

A truncation mutant of Csf3r cooperates with PML-RARα to induce acute myeloid leukemia in mice

Severe congenital neutropenia is associated with a marked propensity to develop myelodysplasia or acute myeloid leukemia (AML). Truncation mutations of CSF3R, encoding the granulocyte colony-stimulating factor receptor (G-CSFR), are associated with development of myelodysplasia/AML in severe congenital neutropenia. However, a causal relationship between CSF3R mutations and leukemic transformation has not been established. Herein, we show that truncated G-CSFR cooperates with the PML-RARα oncogene to induce AML in mice. Expression of truncated G-CSFR significantly shortens the latency of AML in a G-CSF-dependent fashion and it is associated with a distinct AML presentation characterized by higher blast counts and more severe myelosuppression. Basal and G-CSF-induced signal transducer and activator of transcription 3, signal transducer and activator of transcription 5, and extracellular signal-regulated kinase 1/2 phosphorylation were highly variable but similar in leukemic blasts expressing wild-type and truncated G-CSFR. These data provide new evidence suggesting a causative role for CSF3R mutations in human AML.

The deubiquitinating enzyme DUB2A enhances CSF3 signalling by attenuating lysosomal routing of the CSF3 receptor

Ubiquitination of the CSF3R [CSF3 (colony-stimulating factor 3) receptor] occurs after activated CSF3Rs are internalized and reside in early endosomes. CSF3R ubiquitination is crucial for lysosomal routing and degradation. The E3 ligase SOCS3 (suppressor of cytokine signalling 3) has been shown to play a major role in this process. Deubiquitinating enzymes remove ubiquitin moieties from target proteins by proteolytic cleavage. Two of these enzymes, AMSH [associated molecule with the SH3 domain of STAM (signal transducing adaptor molecule)] and UBPY (ubiquitin isopeptidase Y), interact with the general endosomal sorting machinery. Whether deubiquitinating enzymes control CSF3R trafficking from early towards late endosomes is unknown. In the present study, we asked whether AMSH, UBPY or a murine family of deubiquitinating enzymes could fulfil such a role. This DUB family (deubiquitin enzyme family) comprises four members (DUB1, DUB1A, DUB2 and DUB2A), which were originally described as being haematopoietic-specific and cytokine-inducible, but their function in cytokine receptor routing and signalling has remained largely unknown. We show that DUB2A expression is induced by CSF3 in myeloid 32D cells and that DUB2 decreases ubiquitination and lysosomal degradation of the CSF3R, leading to prolonged signalling. These results support a model in which CSF3R ubiquitination is dynamically controlled at the early endosome by feedback mechanisms involving CSF3-induced E3 ligase (SOCS3) and deubiquitinase (DUB2A) activities.

Hematopoietic stem cell transplantation in severe congenital neutropenia

BACKGROUND

Severe congenital neutropenia (SCN) is an immunodeficiency characterized by disturbed myelopoiesis and an absolute neutrophil count (ANC) <0.5 × 10(9)/L. SCN is also a premalignant condition; a significant proportion of patients develop myelodysplastic syndrome or leukemia (MDS/L). Allogeneic hematopoietic stem cell transplantation (HSCT) is the only curative treatment for SCN.

PROCEDURE

Since 2004, eight HSCT have been performed in seven patients at our center. The indications were transformation to MDS/L (n = 2), granulocyte colony-stimulating factor receptor (CSF3R) mutation(s) (n = 2), granulocyte colony-stimulating factor (G-CSF) resistance (n = 2), and at the patient's own request (n = 1).

RESULTS

The mean age at transplantation was 13 years (2.8-28 years) (mean follow-up 32 months, range 21-60). Three patients harbored ELANE mutations, three HAX1 mutations, and in one patient no causative mutation was identified. Two of the ELANE mutations were novel mutations. Three patients initially received myeloablative conditioning and four had reduced intensity conditioning (RIC). Three grafts were from HLA-identical siblings, three from matched unrelated donors and two were cord blood units. Engraftment occurred in all patients. Two of seven (29%) patients died; both had MDS/L and both were among the three that underwent myeloablative conditioning. One patient has chronic GVHD 2 years post-transplant.

CONCLUSIONS

The role of HSCT should be explored further in patients with SCN. In particular, the influence of the conditioning regime needs to be evaluated in a larger cohort of patients.

Lineage-instructive function of C/EBPα in multipotent hematopoietic cells and early thymic progenitors

Hematopoiesis is tightly controlled by transcription regulatory networks, but how and when specific transcription factors control lineage commitment are still largely unknown. Within the hematopoietic stem cell (Lin(-)Sca-1(+)c-Kit(+)) compartment these lineage-specific transcription factors are expressed at low levels but are up-regulated with the process of lineage specification. CCAAT/enhancer binding protein α (C/EBPα) represents one of these factors and is involved in myeloid development and indispensable for formation of granulocytes. To track the cellular fate of stem and progenitor cells, which express C/EBPα, we developed a mouse model expressing Cre recombinase from the Cebpa promoter and a conditional EYFP allele. We show that Cebpa/EYFP(+) cells represent a significant subset of multipotent hematopoietic progenitors, which predominantly give rise to myeloid cells in steady-state hematopoiesis. C/EBPα induced a strong myeloid gene expression signature and down-regulated E2A-induced regulators of early lymphoid development. In addition, Cebpa/EYFP(+) cells compose a fraction of early thymic progenitors with robust myeloid potential. However, Cebpa/EYFP(+) multipotent hematopoietic progenitors and early thymic progenitors retained the ability to develop into erythroid and T-lymphoid lineages, respectively. These findings support an instructive but argue against a lineage-restrictive role of C/EBPα in multipotent hematopoietic and thymic progenitors.

G-CSF and its receptor in myeloid malignancy

Granulocyte colony-stimulating factor (G-CSF) has been used in the clinic for more than 2 decades to treat congenital and acquired neutropenias and to reduce febrile neutropenia before or during courses of intensive cytoreductive therapy. In addition, healthy stem cell donors receive short-term treatment with G-CSF for mobilization of hematopoietic stem cells. G-CSF has also been applied in priming strategies designed to enhance the sensitivity of leukemia stem cells to cytotoxic agents, in protocols aimed to induce their differentiation and accompanying growth arrest and cell death, and in severe aplastic anemia and myelodysplastic syndrome (MDS) to alleviate anemia. The potential adverse effects of G-CSF administration, particularly the risk of malignant transformation, have fueled ongoing debates, some of which can only be settled in follow-up studies extending over several decades. This specifically applies to children with severe congenital neutropenia who receive lifelong treatment with G-CSF and in which the high susceptibility to develop MDS and acute myeloid leukemia (AML) has now become a major clinical concern. Here, we will highlight some of the controversies and challenges regarding the clinical application of G-CSF and discuss a possible role of G-CSF in malignant transformation, particularly in patients with neutropenia harboring mutations in the gene encoding the G-CSF receptor.

Severe congenital neutropenia

Congenital neutropenia (CN) is a genetically heterogeneous bone marrow failure syndrome characterized by a maturation arrest of myelopoiesis at the level of the promyelocyte/myelocyte stage with peripheral blood absolute neutrophil counts below 0.5 x 10(9)/L. From early infancy patients who have CN suffer from bacterial infections. Leukemias occur in both the autosomal dominant and recessive subtypes of CN. The individual risk for each genetic subtype needs to be evaluated further, because the number of patients tested for the underlying genetic defect is still limited. Acquired G-CSFR (CSF3R) mutations are detected in approximately 80% of patients who had CN and who developed acute myeloid leukemia, suggesting that these mutations are involved in leukemogenesis.

Site-specific ubiquitination determines lysosomal sorting and signal attenuation of the granulocyte colony-stimulating factor receptor

Ubiquitination of cytokine receptors controls intracellular receptor routing and signal duration, but the underlying molecular determinants are unclear. The suppressor of cytokine signaling protein SOCS3 drives lysosomal degradation of the granulocyte colony-stimulating factor receptor (G-CSFR), depending on SOCS3-mediated ubiquitination of a specific lysine located in a conserved juxtamembrane motif. Here, we show that, despite ubiquitination of other lysines, positioning of a lysine within the membrane-proximal region is indispensable for this process. Neither reallocation of the motif nor fusion of ubiquitin to the C-terminus of the G-CSFR could drive lysosomal routing. However, within this region, the lysine could be shifted 12 amino acids toward the C-terminus without losing its function, arguing against the existence of a linear sorting motif and demonstrating that positioning of the lysine relative to the SOCS3 docking site is flexible. G-CSFR ubiquitination peaked after endocytosis, was inhibited by methyl-beta-cyclodextrin as well as hyperosmotic sucrose and severely reduced in internalization-defective G-CSFR mutants, indicating that ubiquitination mainly occurs at endosomes. Apart from elucidating structural and spatio-temporal aspects of SOCS3-mediated ubiquitination, these findings have implications for the abnormal signaling function of G-CSFR mutants found in severe congenital neutropenia, a hematopoietic disorder with a high leukemia risk.

Neutropenia with impaired host defense against microbial infection in mice lacking androgen receptor

Neutrophils, the major phagocytes that form the first line of cell-mediated defense against microbial infection, are produced in the bone marrow and released into the circulation in response to granulocyte-colony stimulating factor (G-CSF). Here, we report that androgen receptor knockout (ARKO) mice are neutropenic and susceptible to acute bacterial infection, whereas castration only results in moderate neutrophil reduction in mice and humans. Androgen supplement can restore neutrophil counts via stabilizing AR in castrated mice, but not in ARKO and testicular feminization mutant (Tfm) mice. Our results show that deletion of the AR gene does not influence myeloid lineage commitment, but significantly reduces the proliferative activity of neutrophil precursors and retards neutrophil maturation. CXCR2-dependent migration is also decreased in ARKO neutrophils as compared with wild-type controls. G-CSF is unable to delay apoptosis in ARKO neutrophils, and ARKO mice show a poor granulopoietic response to exogenous G-CSF injection. In addition, AR can restore G-CSF–dependent granulocytic differentiation upon transduction into ARKO progenitors. We further found that AR augments G-CSF signaling by activating extracellular signal-regulated kinase 1/2 and also by sustaining Stat3 activity via diminishing the inhibitory binding of PIAS3 to Stat3. Collectively, our findings demonstrate an essential role for AR in granulopoiesis and host defense against microbial infection.

Pharmacologic rationale for early G-CSF prophylaxis in cancer patients and role of pharmacogenetics in treatment optimization

The use of recombinant human granulocyte colony stimulating factors (G-CSF) has become an integral part of supportive care during cytotoxic chemotherapy. Current guidelines recommend the use of G-CSF in patients with substantial risk of febrile neutropenia. However, little consensus exists about optimal timing and tailoring of this therapy. Based on the known effects of chemotherapy and G-CSF on bone marrow compartments, we propose a model that supports the prophylactic rather than therapeutic use of G-CSF therapy. In addition, several genetic alterations in G-CSF signalling pathway have been described. These genetic variants may predict the risk of febrile neutropenia and response to G-CSF. Thus, future pharmacogenetic/omics studies in this field are warranted. Through the identification of patients at risk and the knowledge of biological basis for optimal timing, hopefully we should soon be able to improve the application of the existing guidelines for G-CSF therapy and patient's prognosis.

Clinical implications of ELA2-, HAX1-, and G-CSF-receptor (CSF3R) mutations in severe congenital neutropenia

Congenital Neutropenia (CN) is a heterogeneous bone marrow failure syndrome characterized by a maturation arrest of myelopoiesis at the level of the promyelocyte/myelocyte stage with peripheral blood absolute neutrophil counts below 0.5 x 10(9)/l. There are two major subtypes of CN as judged by inheritance: an autosomal dominant subtype, e.g. defined by neutrophil elastase mutations (approximately 60% of patients) and an autosomal recessive subtype (approximately 30% of patients), both presenting with the same clinical and morphological phenotype. Different mutations have been described (e.g. HAX1, p14 etc) in autosomal recessive CN, with HAX1 mutations in the majority of these patients. CN in common is considered as a preleukemic syndrome, since the cumulative incidence for leukemia is more than 25% after 20 years of observation. Leukemias occur in both, the autosomal dominant and recessive subtypes of CN. The individual risk for each genetic subtype needs to be further evaluated. Numbers of patients tested for the underlying genetic defect are still limited. Acquired G-CSFR (CSF3R) mutations are detected in approximately 80% of CN patients who developed acute myeloid leukemia independent of the ELA2 or HAX1 genetic subtype, suggesting that these mutations are involved in leukemogenesis. As the majority of patients benefit from G-CSF administration, HSCT should be restricted to non-responders and patients with leukaemic transformation.

G-CSFR ubiquitination critically regulates myeloid cell survival and proliferation

The granulocyte colony-stimulating factor receptor (G-CSFR) is a critical regulator of granulopoiesis. Mutations in the G-CSFR in patients with severe congenital neutropenia (SCN) transforming to acute myelogenous leukemia (AML) have been shown to induce hypersensitivity and enhanced growth responses to G-CSF. Recent studies have demonstrated the importance of the ubiquitin/proteasome system in the initiation of negative signaling by the G-CSFR. To further investigate the role of ubiquitination in regulating G-CSFR signaling, we generated a mutant form of the G-CSFR (K762R/G-CSFR) which abrogates the attachment of ubiquitin to the lysine residue at position 762 of the G-CSFR that is deleted in the Δ716 G-CSFR form isolated from patients with SCN/AML. In response to G-CSF, mono-/polyubiquitination of the G-CSFR was impaired in cells expressing the mutant K762R/G-CSFR compared to cells transfected with the WT G-CSFR. Cells stably transfected with the K762R/G-CSFR displayed a higher proliferation rate, increased sensitivity to G-CSF, and enhanced survival following cytokine depletion, similar to previously published data with the Δ716 G-CSFR mutant. Activation of the signaling molecules Stat5 and Akt were also increased in K762R/G-CSFR transfected cells in response to G-CSF, and their activation remained prolonged after G-CSF withdrawal. These results indicate that ubiquitination is required for regulation of G-CSFR-mediated proliferation and cell survival. Mutations that disrupt G-CSFR ubiquitination at lysine 762 induce aberrant receptor signaling and hyperproliferative responses to G-CSF, which may contribute to leukemic transformation.

Divergent pathways in COS-7 cells mediate defective internalization and intracellular routing of truncated G-CSFR forms in SCN/AML

Background

Expression of truncated G-CSFR forms in patients with SCN/AML induces hyperproliferation and prolonged cell survival. Previously, we showed that ligand internalization is delayed and degradation of truncated G-CSFR forms is defective in patients with SCN/AML.

Methodology/Principal Findings

In this study, we investigated the potential roles of dileucine and tyrosine-based motifs within the cytoplasmic domain of the G-CSFR in modulating ligand/receptor internalization. Using standard binding assays with radiolabeled ligand and COS-7 cells, substitutions in the dileucine motif or deletion of tyrosine residues in the G-CSFR did not alter internalization. Attachment of the transferrin receptor YTRF internalization motif to a truncated G-CSFR form from a patient with SCN/AML corrected defective internalization, but not receptor degradation suggesting that receptor internalization and degradation occur independently via distinct domains and/or processes.

Conclusions

Our data suggest that distinct domains within the G-CSFR mediate separate processes for receptor internalization and degradation. Our findings using standard binding assays differ from recently published data utilizing flow cytometry.

Role of the proteasome in modulating native G-CSFR expression

The granulocyte colony-stimulating factor receptor (G-CSFR) is a critical regulator of granulopoiesis, but the mechanisms controlling its surface expression are poorly understood. Recent studies using transfected cell lines have suggested the activated G-CSFR is routed to the lysosome and not the proteasome. Here, we examined the role of the ubiquitin/proteasome system in regulating G-CSFR surface expression in both ts20 cells that have a temperature-sensitive E1 ubiquitin-activating enzyme and in primary human neutrophils. We show that the G-CSFR is constitutively ubiquitinated, which increases following ligand binding. In the absence of a functional E1 enzyme, ligand-induced internalization of the receptor is inhibited. Pre-treatment of ts20 transfectants with either chloroquine or MG132 inhibited ligand-induced G-CSFR degradation, suggesting a role for both lysosomes and proteasomes in regulating G-CSFR surface expression in this cell line. In neutrophils, inhibition of the proteasome but not the lysosome was found to inhibit internalization/degradation of the activated G-CSFR. Collectively, these data demonstrate the requirement for a functional ubiquitin/proteasome system in G-CSFR internalization and degradation. Our results suggest a prominent role for the proteasome in physiologic modulation of the G-CSFR, and provide further evidence for the importance of the ubiquitin/proteasome system in the initiation of negative signaling by cytokine receptors.