Abnormal megakaryopoiesis in patients with myelodysplastic syndromes: analysis of cellular and humoral defects

JAK2V617F Mutant Megakaryocytes Contribute to Hematopoietic Aging in a Murine Model of Myeloproliferative Neoplasm

Megakaryocytes (MKs) is an important component of the hematopoietic niche. Abnormal MK hyperplasia is a hallmark feature of myeloproliferative neoplasms (MPNs). The JAK2V617F mutation is present in hematopoietic cells in a majority of patients with MPNs. Using a murine model of MPN in which the human JAK2V617F gene is expressed in the MK lineage, we show that the JAK2V617F-bearing MKs promote hematopoietic stem cell (HSC) aging, manifesting as myeloid-skewed hematopoiesis with an expansion of CD41+ HSCs, a reduced engraftment and self-renewal capacity, and a reduced differentiation capacity. HSCs from 2-year-old mice with JAK2V617F-bearing MKs were more proliferative and less quiescent than HSCs from age-matched control mice. Examination of the marrow hematopoietic niche reveals that the JAK2V617F-bearing MKs not only have decreased direct interactions with hematopoietic stem/progenitor cells during aging but also suppress the vascular niche function during aging. Unbiased RNA expression profiling reveals that HSC aging has a profound effect on MK transcriptomic profiles, while targeted cytokine array shows that the JAK2V617F-bearing MKs can alter the hematopoietic niche through increased levels of pro-inflammatory and anti-angiogenic factors. Therefore, as a hematopoietic niche cell, MKs represent an important connection between the extrinsic and intrinsic mechanisms for HSC aging.

Defective platelet aggregation in myelodysplastic syndromes

INTRODUCTION

Hemorrhagic tendency in patients with myelodysplastic syndrome (MDS) is mainly attributed to thrombocytopenia. However, platelet function in these patients has not been thoroughly investigated.

AIM

The aim of our study is to evaluate platelet function in patients with primary MDS.

METHODS

Platelet function was studied with aggregometry in response to ristocetin, collagen, ADP and adrenaline in 26 MDS patients and 15 healthy individuals.

RESULTS

Aggregation was defective in 21 patients (80.7%). Adrenaline was the agonist with the most profound defect (45.9%), followed by ADP (58.7%), whereas aggregation with ristocetin and collagen was at the borderline. Abnormal aggregation to all four agonists was detected in 6 patients (23%). On the contrary, aggregation results were normal in only 5 patients (19.2%). RAEB-t (refractory anemia with excess blasts in transformation) patients were most seriously affected.

CONCLUSIONS

MDS patients have impaired platelet aggregation in response to one or more stimulants. Platelet aggregation was not statistically different between MDS patients at early stages of the disease (<12 months) and those at later stages (>12 months). Defective platelet aggregation is strongly related to MDS of worse prognosis. None of our patients was detected to have hyperfunctional platelets, defined as platelets aggregating spontaneously. Functional defects in MDS do not elicit hemorrhagic tendency.

[The place of growth factors in the treatment of myelodysplastic syndromes]

The myelodysplastic syndromes (MDS) are clonal hematological disorders characterized by ineffective hematopoiesis manifested by anemia, neutropenia, thrombocytopenia or a combination. Correction of these cytopenia is a priority in MDS without excess of blasts. Treatment of anemia depends mainly on erythrocyte transfusions. However with the ability of recombinant human hematopoietic growth factor many trials have been promoted. In vitro, erythroid progenitors from MDS patients are able to differentiate but they require much higher concentrations of erythropoietin than normal progenitors. Trials using rHu-Epo alone are disappointing. Combining rHu-Epo and rHu-G-CSF induces more encouraging results showing a synergistic effect particularly clear in sideroblastic anemia. Patients with low endogenous Epo level and low transfusion need are more likely to respond. Clinician should be able in the future to identify MDS patients with a chance of reversal of anemia or transfusion dependency.

In vitro proliferation and differentiation of megakaryocytic progenitors in patients with aplastic anemia, paroxysmal nocturnal hemoglobinuria, and the myelodysplastic syndromes

It has previously been shown that patients with aplastic anemia (AA) have a stem cell defect both of proliferation and differentiation. This has been shown by long-term bone marrow (BM) culture, long-term initiating cell assays, and committed progenitor assays. We present, for the first time, data on megakaryocyte (Mk) colony formation from purified BM CD34(+) cells from patients with AA. The results are compared with those from normal controls and from patients with paroxysmal nocturnal hemoglobinuria (PNH) and the myelodysplastic syndromes (MDSs). Those treated for AA had previously received immunosuppression (antithymocyte globulin and/or cyclosporin). No patients had received bone marrow transplantation. A total of 13 AA patients (five untreated, eight treated), six PNH, six MDS, and 13 normal donors were studied. BM CD34(+) cells were purified by indirect labeling and then cultured in a collagen-based Mk assay kit (MegaCult-C, StemCell Technologies). The cultures were fixed on day 12, and the Mk colonies were identified by the alkaline phosphatase anti-alkaline phosphatase technique using the monoclonal antibody CD41 (GP IIb/IIIa). The slides were scored for Mk colony-forming units (CFU-Mks) (3-20 and >20 cells), Mk burst-forming units (BFU-Mks) (>50 cells), and mixed colonies. The results show that total Mk colony formation in AA was significantly lower than in normal donors (p<0.0001), both in untreated patients/nonresponders to treatment (p = 0.0001) and in complete/partial responders (p<0.002). There was no significant difference in Mk colony formation in treated and untreated patients (p = 0.05). Patients with AA had a lower total colony formation than PNH patients (p = 0.0002). PNH patients exhibited lower colony formation than normal controls (p = 0.03), as shown by MDS patients, although the considerable number of variables resulted in a lack of statistically significant difference from normal controls (p = 0.2). We have now shown that Mk colony formation from purified BM CD34(+) cells is significantly reduced, supporting previous evidence that AA results from a stem cell defect.

Colony-forming unit-megakaryocyte (CFR-meg) numbers and serum thrombopoietin concentrations in thrombocytopenic disorders: an inverse correlation in myelodysplastic syndromes

We studied both serum-free colony-forming unit-megakaryocyte (CFU-meg) numbers and serum thrombopoietin (TPO) levels in 14 patients with aplastic anemia (AA), 37 patients with myelodysplastic syndromes (MDS) and 23 patients with idiopathic thrombocytopenic purpura (ITP) to assess thrombopoiesis in these thrombocytopenic disorders. The mean CFU-meg numbers were lower in AA and MDS patients (10.7 +/- 11.4 and 42.3 +/- 58.5/10(5) BMLD cells) than in healthy controls (103.1 +/- 57.3/10(5) BMLD cells) (P < 0.0001 and P= 0.0053, respectively), although they were distributed variably in MDS. ITP patients showed higher CFU-meg numbers (223.2 +/- 143.5/10(5) BMLD cells) (P= 0.017). The mean TPO concentrations were higher in both AA (986.8 +/- 500.8 pg/ml) and MDS patients (838.2 +/- 639.1 pg/ml) than in healthy controls (80.7 +/- 38.8 pg/ml) (P < 0.0001), although they were distributed from high to low in MDS. ITP patients showed a slight elevation of TPO (123.1 +/- 55.3 pg/ml) P = 0.0106). The TPO levels was inversely correlated to both platelet counts and CFU-meg numbers (correlative coefficient (CC): -0.719 and -0.682, P < 0.0001) in AA, but not in ITP. In MDS, the inverse correlation to TPO was stronger in CFU-meg (CC: -0.678, P < 0.0001) than in platelet counts (CC: -0.538, P = 0.0014), suggesting that CFU-meg plays an important role in regulating TPO production in this heterogenous disorder. CFU-meg and TPO may provide useful information for understanding thrombopoiesis of MDS, especially for application of TPO.

Defective megakaryocytic development in myelodysplastic syndromes

Megakaryocytic proliferation and differentiation is typically abnormal in patients with myelodysplastic syndromes (MDS). The underlying mechanisms for this finding are not known, but may involve defects at the level of the thrombopoietin-receptor (c-mpl) or post-receptor signaling pathways in megakaryocyte progenitor cells. Premature apoptosis of the bone marrow cells and inhibitory effects of cytokines such as tumor necrosis factor alpha have been implicated as contributing to altered megakaryopoiesis in MDS, but their significance remains unclear. The availability of thrombopoietin (TPO) has facilitated more detailed analysis of megakaryocytic biology using several experimental in-vitro systems. However numerous studies have shown that the developmental abnormalities of MDS megakaryocytes could not be corrected by TPO. Increasing investigations are being extended to the evaluation of signal transduction pathways of c-mpl both in cell lines and human hematopoietic cells in order to identify the molecular mechanisms responsible for the defective megakaryocytic development in MDS.

Analysis of megakaryocyte growth and development factor (thrombopoietin) effects on blast cell and megakaryocyte growth in myelodysplasia

Thrombocytopenia is a frequent feature of myelodysplastic syndromes (MDS) that could be improved by the use of recombinant human megakaryocyte growth and development factor (rHuMGDF). Using short-term liquid cultures and progenitor assays, we have found that rHuMGDF stimulated DNA synthesis and potentiated leukemic cluster growth of bone marrow mononuclear cells in 10/38 MDS cases (26%). Cytogenetically malignant colonies were detectable in rHuMGDF-stimulated cultures (n=3) by fluorescence in situ hybridization. rHuMGDF was able to stimulate CFU-MK formation in 45% of the samples tested. Finally, rHuMGDF-induced blast cell proliferation correlated with elevated expression of c-MPL, previously identified as a bad prognosis factor in MDS.

Recombinant human interleukin-8 restores function in neutrophils from patients with myelodysplastic syndromes without stimulating myeloid progenitor cells

Prognosis in myelodysplastic syndrome (MDS) is not only correlated closely with blast cell count in bone marrow and chromosomal abnormalities but also correlated with decreased leucocyte count and function leading to acquisition of lethal infections. Recently, clinical trials in MDS have focused on the application of haemopoietic growth factors such as G-CSF or GM-CSF, which have proven to increase neutrophil count and function. However, these cytokines carry the risk of stimulating the malignant clone, particularly in patients with increased blast cell count. Therefore, investigation of cytokines which are able to stimulate neutrophil function without the potential risk of stimulating haemopoietic progenitor cells may be relevant for MDS. As the stimulatory effect of interleukin-8 on neutrophil function is well known, we investigated whether recombinant human IL-8 is also able to improve the function of neutrophils gained from patients with MDS. Using three different techniques--the E. coli killing assay (8 patients), the production of reactive oxygen as determined by cytochrome c reduction (7 patients) and chemiluminescence (8 patients)--a significant stimulation of neutrophil function at a concentration of 10 nm IL-8 was found in all test systems. No correlation with FAB classification was evident. On the other hand, IL-8 only mildly stimulated growth of myeloid progenitor cells in bone marrow culture of healthy individuals and MDS patients. This minimal stimulation was blocked by a neutralizing antibody directed against GM-CSF, suggesting an indirect effect of IL-8 via secondary GM-CSF release. Thus, IL-8 is able in vitro to repair the functional abnormalities of neutrophils from patients with MDS but has only a marginal influence on myeloid progenitor cells.

Endogenous serum levels and surface receptor expression of GM-CSF and IL-3 in patients with myelodysplastic syndromes

The majority of patients suffering from myelodysplastic syndromes (MDS) die of complications due to cytopenia. Clinical trials have demonstrated that in an essential number of MDS patients cytopenia can be ameliorated by exogenously supplied growth factors. We investigated endogenous serum levels of GM-CSF and IL-3 in 15 healthy individuals and 34 patients suffering from MDS. No circulating growth factors were detected in the serum of healthy controls, nor was IL-3 measurable in MDS patients. GM-CSF serum levels, however, were elevated in a significant number of patients (26.5%). Levels did not correlate with FAB classification, leukocyte count or chromosomal abnormalities. No significant differences in GM-CSF or IL-3 receptor expression were detected between healthy individuals and MDS patients. One patient with increased endogenous GM-CSF serum level and normal surface receptor expression responded to exogenously applied GM-CSF. In the light of these results, a functional alteration of growth factor receptors or disturbances of signal transduction pathways must be discussed for MDS.