Evidence for the ligand-induced conversion from a dimer to a tetramer of the granulocyte colony-stimulating factor receptor

Cytokine receptor signaling through the Jak–Stat–Socs pathway in disease

The complexity of multicellular organisms is dependent on systems enabling cells to respond to specific stimuli. Cytokines and their receptors are one such system, whose perturbation can lead to a variety of disease states. This review represents an overview of our current understanding of the cytokine receptors, Janus kinases (Jaks), Signal transducers and activators of transcription (Stats) and Suppressors of cytokine signaling (Socs), focussing on their contribution to diseases of an immune or hematologic nature.

Homodimeric cross-over structure of the human granulocyte colony-stimulating factor (GCSF) receptor signaling complex

A crystal structure of the signaling complex between human granulocyte colony-stimulating factor (GCSF) and a ligand binding region of GCSF receptor (GCSF-R), has been determined to 2.8 A resolution. The GCSF:GCSF-R complex formed a 2:2 stoichiometry by means of a cross-over interaction between the Ig-like domains of GCSF-R and GCSF. The conformation of the complex is quite different from that between human GCSF and the cytokine receptor homologous domain of mouse GCSF-R, but similar to that of the IL-6/gp130 signaling complex. The Ig-like domain cross-over structure necessary for GCSF-R activation is consistent with previously reported thermodynamic and mutational analyses.

Thermodynamic analysis of the activation mechanism of the GCSF receptor induced by ligand binding

The granulocyte colony-stimulating factor receptor (GCSFR), containing the Ig-like domain (Ig) and cytokine receptor homologous region (CRH), was prepared as a preformed dimer (Ig-CRH-Fc)(2) after fusion to the mouse Fc region via an eight-residue linker (approximately 55 A). Monomer Ig-CRH was also prepared after the Fc region was removed from (Ig-CRH-Fc)(2). GCSF binding to Ig-CRH and (Ig-CRH-Fc)(2) was investigated using light scattering and isothermal titration calorimetry. The average molecular mass determined by light scattering showed that both Ig-CRH and (Ig-CRH-Fc)(2) formed a 2:2 dimer with GCSF. Moreover, isothermal titration calorimetry showed that the thermodynamic parameters upon binding of GCSF to Ig-CRH and (Ig-CRH-Fc)(2) were comparable, suggesting a similar binding stoichiometry and interface [including similar buried surface area (5700-6000 A(2))] despite the presence of the eight-residue linker. The buried surface area is much larger than that calculated from our previous report of the crystal structure of the GCSF-CRH complex [Aritomi, M., et al. (1999) Nature 401, 713-717], suggesting a substantial contribution of the Ig domain to GCSF binding. The data also indicate that the distance (55 A) between two CRH domains in the 2:2 complex is much shorter than in our previous model (approximately 90 A) predicted from the same crystal structure of the GCSF-CRH complex.

Change in the accessibility of an epitope of the human granulocyte-colony stimulating factor after binding to receptors

In a previous work we demonstrated that monoclonal antibody (mAb) 8C2 recognized a human granulocyte-colony stimulating factor (hG-CSF) region left unmasked after binding to placenta receptors, whereas mAb 6E3 defined a receptor-buried epitope. Herein we examined the role of these antigenic regions on the proliferative response induced by hG-CSF on a myeloid leukaemia cell line. Both mAbs significantly inhibited the hG-CSF-induced cell growing, although epitope 8C2 but not 6E3 remained exposed in hG-CSF:cell receptor complexes. When cytokine:receptor complexes already formed at 4 degrees C were incubated 1 h at 37 degrees C under conditions preventing the internalization, a significant reduction in the amount of accessible 8C2 epitopes was evident. However, this effect was not observed when mAb 8C2:hG-CSF complexes previously bound to cells were incubated at 37 degrees C. Thus, results suggest that a receptor oligomerization process could account for the temperature-induced epitope 8C2 masking. The identification of epitope 8C2 accomplished by synthesis of overlapping octapeptides, revealed that it is formed by sequences 39-52 and 155-164, both in close proximity in the three-dimensional structure of the hG-CSF molecule. Since part of this region has been proposed as a second binding site to receptors, we infer that the change of epitope 8C2 accessibility could be the result of either receptor aggregation or epitope binding to another receptor. In addition, our data support the hypothesis that a ligand-induced receptor oligomerization is required for transduction of cytokine signals.

[Ligand recognition mechanism of G-CSF receptor and metabotropic glutamate receptor]

A three-dimensional view of ligand-receptor recognition at the atomic level is crucial to understand the molecular mechanism of receptor activation. This review describes the structure-function relationships of two receptors important for pharmaceutical science. Granulocyte colony-stimulating factor (G-CSF) is the principal growth factor regulating the maturation, proliferation, and differentiation of the precursor cells of neutrophilic granulocytes. We have determined the crystal structure of G-CSF complexed to the BN-BC domains, the principal ligand binding region of the G-CSF receptor. In a novel oligomerization scheme, the two receptor domains complex in a 2:2 ratio to the ligand, with a noncrystallographic pseudo-two-fold axis through primarily the interdomain region and secondarily the BC domain. This first structural view of a gp130-type receptor-ligand complex presents a new molecular basis for cytokine-receptor recognition. The metabotropic glutamate receptors (mGluRs) are key receptors in the modulation of excitatory synaptic transmission in the central nervous system. Three different crystal structures of the extracellular ligand-binding region (LBR) of mGluR1 have been determined, in a complex with glutamate and in two unliganded forms. They all showed disulfide-linked homo-dimers, of which the "active" and "resting" conformations are modulated through the novel dimeric interface by a packed alpha-helical structure. The bilobed protomer architectures flexibly change their domain arrangements between an "open" or "closed" conformation. Glutamate binding stabilizes both the "active" dimer and "closed" protomer in dynamic equilibrium. Four domain movements within the dimer affect the separation of the transmembrane and intracellular regions and thereby activate the receptor.

Expression, purification, and characterization of the active immunoglobulin-like domain of human granulocyte-colony-stimulating factor receptor in Escherichia coli

We succeeded in the expression, purification, and refolding of the immunoglobulin-like (Ig) domain of human granulocyte-colony-stimulating factor (G-CSF) receptor with amino-terminal His-tag in Escherichia coli. The refolded Ig domain bound to a G-CSF affinity column and could be eluted with free G-CSF as a receptor-ligand complex, demonstrating that the Ig domain has the information necessary for binding its ligand, G-CSF. The eluted His-Ig/G-CSF complex could be separated from excess G-CSF by Ni-NTA column chromatography. The yield of this active recombinant His-Ig protein is about 0.72 mg per liter of culture. Its small size and the ease of production make this receptor fragment a useful reagent for the structural analysis of its complex with G-CSF.

Delineation and mapping of Stat5 isoforms activated by granulocyte colony-stimulating factor in myeloid cells

Granulocyte colony-stimulating factor (G-CSF) is a cytokine critical for proliferation and differentiation of granulocytic precursors and neutrophil functions that has previously been demonstrated to activate Stat3 and Stat5, two members of the signal transducer and activator of transcription (STAT) protein family. Stat3 has been identified to be critical for G-CSF receptor (G-CSFR)-mediated signaling for granulocyte differentiation. Stat5 activation has been mapped to the proximal portion of the cytosolic region of the G-CSFR. However, delineation and mapping of the specific Stat5 isoforms activated by G-CSF in myeloid cells have not been reported. In this study, we demonstrated that G-CSF activated a Stat5 complex in human myeloid cells containing three isoforms of Stat5: Stat5A, Stat5B, and Stat5 p80. Activation of Stat5A and Stat5B maps to the proliferation-specific domain of the G-CSFR, whereas Stat5 p80 is recruited by phosphotyrosine-704 within the region of G-CSFR required for differentiation. G-CSF-activated Stat5A/B, but not Stat5 p80, formed a heterodimer with Stat3. The Stat5A/B-Stat3 heterodimer can bind to specific DNA sequences preferred by both Stat3 and Stat5. These findings are consistent with the possibility that Stat5 p80 contributes to G-CSF-induced myeloid differentiation.

Atomic structure of the GCSF-receptor complex showing a new cytokine-receptor recognition scheme

Granulocyte colony-stimulating factor (GCSF) is the principal growth factor regulating the maturation, proliferation and differentiation of the precursor cells of neutrophilic granulocytes and is used to treat neutropenia. GCSF is a member of the long-chain subtype of the class 1 cytokine superfamily, which includes growth hormone, erythropoietin, interleukin 6 and oncostatin M. Here we have determined the crystal structure of GCSF complexed to the BN-BC domains, the principal ligand-binding region of the GCSF receptor (GCSFR). The two receptor domains form a complex in a 2:2 ratio with the ligand, with a non-crystallographic pseudo-twofold axis through primarily the interdomain region and secondarily the BC domain. This structural view of a gp130-type receptor-ligand complex presents a new molecular basis for cytokine-receptor recognition.

Novel point mutation in the extracellular domain of the granulocyte colony-stimulating factor (G-CSF) receptor in a case of severe congenital neutropenia hyporesponsive to G-CSF treatment

Severe congenital neutropenia (SCN) is a heterogeneous condition characterized by a drastic reduction in circulating neutrophils and a maturation arrest of myeloid progenitor cells in the bone marrow. Usually this condition can be successfully treated with granulocyte colony-stimulating factor (G-CSF). Here we describe the identification of a novel point mutation in the extracellular domain of the G-CSF receptor (G-CSF-R) in an SCN patient who failed to respond to G-CSF treatment. When this mutant G-CSF-R was expressed in myeloid cells, it was defective in both proliferation and survival signaling. This correlated with diminished activation of the receptor complex as determined by signal transducer and activator of transcription (STAT) activation, although activation of STAT5 was more affected than STAT3. Interestingly, the mutant receptor showed normal affinity for ligand, but a reduced number of ligand binding sites compared with the wild-type receptor. This suggests that the mutation in the extracellular domain affects ligand-receptor complex formation with severe consequences for intracellular signal transduction. Together these data add to our understanding of the mechanisms of cytokine receptor signaling, emphasize the role of GCSFR mutations in the etiology of SCN, and implicate such mutations in G-CSF hyporesponsiveness.

A small, nonpeptidyl mimic of granulocyte-colony-stimulating factor [see commetns]

A nonpeptidyl small molecule SB 247464, capable of activating granulocyte-colony-stimulating factor (G-CSF) signal transduction pathways, was identified in a high-throughput assay in cultured cells. Like G-CSF, SB 247464 induced tyrosine phosphorylation of multiple signaling proteins and stimulated primary murine bone marrow cells to form granulocytic colonies in vitro. It also elevated peripheral blood neutrophil counts in mice. The extracellular domain of the murine G-CSF receptor was required for the activity of SB 247464, suggesting that the compound acts by oligomerizing receptor chains. The results indicate that a small molecule can activate a receptor that normally binds a relatively large protein ligand.

A non-peptide compound which can mimic the effect of thrombopoietin via c-Mpl

Thrombopoietin (TPO) is a cytokine which plays a central role in megakaryopoiesis and platelet production by binding to its cell surface receptor, termed c-Mpl. In the present study, two benzodiazepinones that compete with the binding of TPO to the extracellular region of c-Mpl were identified, and one of them stimulated the proliferation of a human TPO-dependent megakaryocytic cell line, UT-7/TPO. It stimulated the activation of signal transducer and activator of transcription 5 in UT-7/TPO cells. These results suggest that a non-peptide compound can mimic the effect of TPO via c-Mpl.

Identification of a Cys motif in the common beta chain of the interleukin 3, granulocyte-macrophage colony-stimulating factor, and interleukin 5 receptors essential for disulfide-linked receptor heterodimerization and activation of all three receptors

The human interleukin 3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors undergo covalent dimerization of the respective specific alpha chains with the common beta subunit (betac) in the presence of the cognate ligand. We have now performed alanine substitutions of individual Cys residues in betac to identify the Cys residues involved and their contribution to activation of the IL-3, GM-CSF, and IL-5 receptors. We found that substitution of Cys-86, Cys-91, and Cys-96 in betac but not of Cys-100 or Cys-234 abrogated disulfide-linked IL-3 receptor dimerization. However, although Cys-86 and Cys-91 betac mutants retained their ability to form non-disulfide-linked dimers with IL-3Ralpha, substitution of Cys-96 eliminated this interaction. Binding studies demonstrated that all betac mutants with the exception of C96A supported high affinity binding of IL-3 and GM-CSF. In receptor activation experiments, we found that betac mutants C86A, C91A, and C96A but not C100A or C234A abolished phosphorylation of betac in response to IL-3, GM-CSF, or IL-5. These data show that although Cys-96 is important for the structural integrity of betac, Cys-86 and Cys-91 participate in disulfide-linked receptor heterodimerization and that this linkage is essential for tyrosine phosphorylation of betac. Sequence alignment of betac with other cytokine receptor signaling subunits in light of these data shows that Cys-86 and Cys-91 represent a motif restricted to human and mouse beta chains, suggesting a unique mechanism of activation utilized by the IL-3, GM-CSF, and IL-5 receptors.

Identification of a ligand-binding site on the granulocyte colony-stimulating factor receptor by molecular modeling and mutagenesis

Granulocyte colony-stimulating factor (G-CSF) initiates its effects on cells of the neutrophil lineage by inducing formation of a homodimeric receptor complex. The structure of the G-CSF receptor has not yet been determined, therefore we used molecular modeling to identify regions of the receptor that were likely to be involved in ligand binding. The G-CSF receptor sequence was aligned with all the available sequences of the gp130 and growth hormone receptor families and a model of the cytokine receptor homologous domain was constructed, based on the growth hormone receptor structure. Alanine substitution mutagenesis was performed on loops and individual residues that were predicted to bind ligand. Mutant receptors were expressed in factor-dependent Ba/F3 cells and assessed for proliferation response and ligand binding. Six residues were identified that significantly reduced receptor function, with Arg288 in the F'-G' loop having the greatest effect. These residues formed a binding face on the receptor model resembling the growth hormone receptor site, which suggests that the model is reasonable. However, electrostatic analysis of the model provided further evidence that the mechanism of receptor dimerization is different from that of the growth hormone receptor.

Ligand-independent dimerization of the extracellular domain of the leptin receptor and determination of the stoichiometry of leptin binding

The leptin receptor is a class I transmembrane protein with either a short or a long cytoplasmic domain. Using chemical cross-linking we have analyzed the binding of leptin to its receptor. Cross-linking of radiolabeled leptin to different isoforms of the leptin receptor expressed on COS-1 cells reveals leptin receptor monomer, homodimer, and oligomer complexes. Cotransfection of the long and short form of the leptin receptor did not provide any evidence for the formation of heterodimer complexes. Soluble forms consisting of either the entire extracellular domain or the two cytokine receptor homologous domains of the leptin receptor were purified to homogeneity from recombinant baculovirus-infected insect cells by leptin affinity chromatography. Gel filtration chromatography showed that these proteins exist in a dimeric form. Analysis of the complex formed between soluble leptin receptor and leptin by native polyacrylamide gel electrophoresis, and data obtained from the amino acid composition of the complex provide direct evidence that the extracellular domain of the leptin receptor binds leptin in a 1:1 ratio.

Characterization of the receptor binding determinants of granulocyte colony stimulating factor

We performed a series of experiments using alanine-scanning mutagenesis to locate side chains within human granulocyte colony-stimulating factor (G-CSF) that are involved in human G-CSF receptor binding. We constructed a panel of 28 alanine mutants that examined all surface exposed residues on helices A and D, as well as all charged residues on the surface of G-CSF. The G-CSF mutants were expressed in a transiently transfected mammalian cell line and quantitated by a sensitive biosensor method. We measured the activity of mutant proteins using an in vitro proliferation assay and an ELISA binding competition assay. These studies show that there is a region of five charged residues on helices A and C employed by G-CSF in binding its receptor, with the most important residue in this binding patch being Glu 19. Both wild-type G-CSF and the E19A mutant were expressed in E. coli. The re-folded proteins were found to have proliferative activities similar to the analogous proteins from mammalian cells: furthermore, biophysical analysis indicated that the E19A mutation does not cause gross structural perturbations in G-CSF. Although G-CSF is likely to signal through receptor homo-dimerization, we found no compelling evidence for a second receptor binding region. We also found no evidence of self-antagonism at high G-CSF concentrations, suggesting that, in contrast to human growth hormone (hGH) and erythropoietin (EPO), G-CSF probably does not signal via a pure 2:1 receptor ligand complex. Thus, G-CSF, while having a similar tertiary structure to hGH and EPO, uses different areas of the four helix bundle for high-affinity interaction with its receptor.

Neutralising antibodies to the granulocyte colony-stimulating factor receptor recognise both the immunoglobulin-like domain and the cytokine receptor homologous domain

To define regions of the granulocyte colony-stimulating factor (G-CSF) receptor that are important for ligand binding, neutralising monoclonal antibodies to the human receptor have been produced. Eleven antibodies recognised six different receptor epitopes. Antibodies from three of the epitope groups were able to detect the receptor by western blotting but did not inhibit G-CSF binding. The other three antibody groups inhibited G-CSF binding either completely (groups 1 and 2) or partially (group 3). All the antibodies inhibited proliferation of BA/F3 cells expressing the G-CSF receptor to varying extents. By using human-marine chimeric receptors, the binding sites of the antibodies were mapped to the immunoglobulin-like domain (groups 1 and 3), the cytokine receptor homologous domain (group 2) or the fibronectin type III domains (groups 4 to 6). These results show that the immunoglobulin-like and cytokine receptor homologous domains of the receptor are important for ligand binding and subsequent signalling.

Affinity chromatography with immunochemical detection applied to the analysis of human methionyl granulocyte colony stimulating factor in serum

An on-line, automated, HPLC method was developed for the separation and detection of recombinant human methionyl granulocyte colony stimulating factor (GCSF) and GCSF modified with poly(ethylene glycol) (PEG-GCSF) in rat serum. The automated method consists of an initial immunoaffinity chromatography step, a microbore reverse phase separation, and postcolumn immunochemical detection. Calibration curves were constructed with a lower limit of 1.5 ng of GCSF (80 fmol) and 7.5 ng of PEG-GCSF in 120 microL of rat serum. In vivo serum samples from rats dosed with PEG-GCSF were also analyzed. The development of the method is discussed as well as its general application to the detection of metabolites of recombinant proteins in body fluids.

Human interleukin-3 (IL-3) induces disulfide-linked IL-3 receptor alpha- and beta-chain heterodimerization, which is required for receptor activation but not high-affinity binding

The human interleukin-3 receptor (IL-3R) is a heterodimer that comprises an IL-3 specific alpha chain (IL-3R alpha) and a common beta chain (beta C) that is shared with the receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-5. These receptors belong to the cytokine receptor superfamily, but they are structurally and functionally more related to each other and thus make up a distinct subfamily. Although activation of the normal receptor occurs only in the presence of ligand, the underlying mechanisms are not known. We show here that human IL-3 induces heterodimerization of IL-3R alpha and beta c and that disulfide linkage of these chains is involved in receptor activation but not high-affinity binding. Monoclonal antibodies (MAb) to IL-3R alpha and beta c were developed which immunoprecipitated, in the absence of IL-3, the respective chains from cells labelled with 125I on the cell surface. However, in the presence of IL-3, each MAb immunoprecipitated both IL-3R alpha and beta c. IL-3-induced receptor dimers were disulfide and nondisulfide linked and were dependent on IL-3 interacting with both IL-3R alpha and beta c. In the presence of IL-3 and under nonreducing conditions, MAb to either IL-3R alpha or beta c immunoprecipitated complexes with apparent molecular weights of 215,000 and 245,000 and IL-3R alpha and beta c monomers. Preincubation with iodoacetamide prevented the formation of the two high-molecular-weight complexes without affecting noncovalent dimer formation or high-affinity IL-3 binding. Two-dimensional gel electrophoresis and Western blotting (immunoblotting) demonstrated the presence of both IL-3R alpha and beta c in the disulfide-linked complexes. IL-3 could also be coimmunoprecipitated with anti-IL-3R alpha or anti-beta c MAB, but it was not covalently attached to the receptor. Following IL-3 stimulation, only the disulfide-linked heterodimers exhibited reactivity with antiphosphotyrosine antibodies, with beta c but not IL-3R alpha being the phosphorylated species. A model of IL-3R activation is proposed which may be also applicable to the related GM-CSF and IL-5 receptors.

Dimerization of the extracellular domain of granuloycte-colony stimulating factor receptor by ligand binding: a monovalent ligand induces 2:2 complexes

Granulocyte-colony stimulating factor (G-CSF) binds to a specific cell surface receptor and induces signals for growth and differentiation in cells of granulocyte hematopoietic lineage. In order to understand how G-CSF binding initiates signals into these cells, we have studied its interactions with the entire extracellular domain of the receptor (sG-CSFR). The sG-CSFR was purified from CHO cell conditioned media with a G-CSF affinity column, resting in a preparation fully competent for ligand binding. However, when sG-CSFR was purified by conventional means, i.e., without affinity chromatography, only about half was competent. Therefore, all studies were carried out using affinity-purified material. The sG-CSFR exhibited a weak self-association into a dimer with a dissociation constant of 200microM in the absence of G-CSF. Addition of G-CSF dimerizes the receptor, with a preferred stoichiometry of 2 G-CSF molecules plus 2 receptors. Unexpectedly, receptor-receptor interactions rather than through two receptors binding to the same G-CSF molecule; i.e., G-CSF is a monovalent ligand. G-CSF binding to the receptor monomer occurs with high affinity. The binding of G-CSF also enhances the receptor-receptor dimerization; when G-CSF is bound to both receptors, dimerization is enhanced 2000-fold, while the interaction of a 1:1 receptor-ligand complex with a second ligand-free receptor is enhanced 80-fold. Thus, the mechanism of receptor dimerization is fundamentally different than that of related cytokine receptors such as growth hormone and erythropoietin receptors. Circular dichroic spectra showed a small but significant conformational change of receptor upon binding G-CSF. This is consistent with the idea that G-CSF binding alters the conformation of the receptor, resulting in an increase in receptor-receptor interactions.

The extracellular region of granulocyte colony-stimulating factor receptor in solution has multiple oligomerization states without ligand

Expression and purification of the extracellular portion of granulocyte colony-stimulating factor (G-CSF) receptor, which contains an immunoglobulin-like (Ig) domain and the cytokine receptor homologous (CRH) region, using a baculovirus secretion system have shown that a tetrameric Ig-CRH protein (about 200 kDa) existed in addition to the dimer (85 kDa) [7]. Scatchard analysis revealed that the tetramer had ligand binding affinity, with a dissociation constant of about 2.5 nM. The tetramer dissociated into monomers at pH 2 and was re-formed at pH7, in contrast, the dimer was re-dimerized with the same treatment. These observations led us to hypothesize the existence of conformational heterogeneity, which leads to tetramer as well as dimer formation, in the soluble state of the Ig-CRH protein.

Requirement for the immunoglobulin-like domain of granulocyte colony-stimulating factor receptor in formation of a 2:1 receptor-ligand complex

The extracellular portion of the granulocyte colony-stimulating factor (G-CSF) receptor has a mosaic structure of six domains (each approximately 100 amino acid residues) consisting of an immunoglobulin-like (Ig) domain, a cytokine receptor homologous region subdivided into amino-terminal (BN) and carboxyl-terminal (BC) domains, and three fibronectin type III repeats. In the present study, we expressed the Ig-BN and the BN-BC regions and purified them to homogeneity as monomers using G-CSF affinity column chromatography. Using gel filtration high performance liquid chromatography, we investigated the molecular composition of receptor-ligand complexes formed between G-CSF and purified BN-BC or Ig-BN domains. In contrast to the well characterized example of the human growth hormone (GH) receptor, in which the BN-BC.GH complex shows a 2:1 receptor-ligand complex stoichiometry, the BN-BC domain of the G-CSF receptor formed a 1:1 complex. The isolated Ig-BN domain also formed a 1:1 complex with G-CSF. However, in the presence of both Ig-BN and BN-BC domains, we detected a 1:1:1 Ig-BN.G-CSF.BN-BC complex corresponding to the 2:1 receptor: ligand stoichiometry. These results suggest that 1) the Ig domain and both the BN and the BC domains are required for oligomerization of the G-CSF receptor, 2) G-CSF contains two binding sites for its receptor, and 3) there are two ligand binding sites on the G-CSF receptor, one site on the BN-BC domain and one on the Ig-BN domain.

Mutations in the gene for the granulocyte colony-stimulating-factor receptor in patients with acute myeloid leukemia preceded by severe congenital neutropenia

BACKGROUND

In severe congenital neutropenia the maturation of myeloid progenitor cells is arrested. The myelodysplastic syndrome and acute myeloid leukemia develop in some patients with severe congenital neutropenia. Abnormalities in the signal-transduction pathways for granulocyte colony-stimulating factor (G-CSF) may play a part in the progression to acute myeloid leukemia.

METHODS

We isolated genomic DNA and RNA from hematopoietic cells obtained from two patients with acute myeloid leukemia and histories of severe congenital neutropenia. The nucleotide sequences encoding the cytoplasmic domain of the G-CSF receptor were amplified by means of the polymerase chain reaction and sequenced. Murine myeloid 32D.C10 cells were transfected with complementary DNA encoding the wild-type or mutant G-CSF receptors and tested for their responses to G-CSF.

RESULTS

Point mutations in the gene for the G-CSF receptor were identified in both patients. The mutations, a substitution of thymine for cytosine at the codon for glutamine at position 718 (Gln718) in one patient and at the codon for glutamine at position 731(Gln731) in the other, caused a truncation of the C-terminal cytoplasmic region of the receptor. Both mutant and wild-type genes for the G-CSF receptor were present in leukemic cells from the two patients. In one patient, the mutation was also found in the neutropenic stage, before the progression to acute myeloid leukemia. The 32D.C10 cells expressing mutant receptors had abnormally high proliferative responses but failed to mature when cultured in G-CSF. The mutant G-CSF receptors also interfered with terminal maturation mediated by the wild-type G-CSF receptor in the 32D.C10 cells that coexpressed the wild-type and mutant receptors.

CONCLUSIONS

Mutations in the gene for the G-CSF receptor that interrupt signals required for the maturation of myeloid cells are involved in the pathogenesis of severe congenital neutropenia and associated with the progression to acute myeloid leukemia.