p-ERK1/2 is a predictive factor of response to erythropoiesis-stimulating agents in low/int-1 myelodysplastic syndromes

A Systematic Literature Review of Predictors of Erythropoiesis-Stimulating Agent Failure in Lower-Risk Myelodysplastic Syndromes

Erythropoiesis-stimulating agents (ESAs) are the first-line treatment option for anemia in patients with lower-risk myelodysplastic syndromes (LR-MDS). A systematic literature review was conducted to identify evidence of the association between prognostic factors and ESA response/failure in LR-MDS. MEDLINE, Embase, and relevant conferences were searched systematically for studies assessing the association between prognostic factors and ESA response/failure in adult patients. Of 1566 citations identified, 38 were included. Patient risk status in studies published from 2000 onwards was commonly assessed using the International Prognostic Scoring System (IPSS) or revised IPSS. ESA response was generally assessed using the International Working Group MDS criteria. Among the included studies, statistically significant relationships were found, in both univariate and multivariate analyses, between ESA response and the following prognostic factors: higher hemoglobin levels, lower serum erythropoietin levels, and transfusion independence. Furthermore, other prognostic factors such as age, bone marrow blasts, serum ferritin level, IPSS risk status, and karyotype status did not demonstrate statistically significant relationships with ESA response. This systematic literature review has confirmed prognostic factors of ESA response/failure. Guidance to correctly identify patients with these characteristics could be helpful for clinicians to provide optimal treatment.

Clinical application of flow cytometry in patients with unexplained cytopenia and suspected myelodysplastic syndrome: A report of the European LeukemiaNet International MDS-Flow Cytometry Working Group

This article discusses the rationale for inclusion of flow cytometry (FCM) in the diagnostic investigation and evaluation of cytopenias of uncertain origin and suspected myelodysplastic syndromes (MDS) by the European LeukemiaNet international MDS Flow Working Group (ELN iMDS Flow WG). The WHO 2016 classification recognizes that FCM contributes to the diagnosis of MDS and may be useful for prognostication, prediction, and evaluation of response to therapy and follow-up of MDS patients.

Lower-risk myelodysplastic syndromes: Current treatment options for anemia

Myelodysplastic syndromes (MDS) are a heterogeneous group of clonal hematological disorders. Treatment options are classified and defined by prognostic risk based on the International Prognostic Scoring System (IPSS) and, more recently, the revised IPSS (IPSS-R). The treatment goal for lower-risk MDS is to correct cytopenias or their consequences, with the goal of maintaining or improving quality of life. Erythropoiesis-stimulating agents (ESAs) play an important role in treating anemia. Individuals with MDS who have a 5q deletion are particularly sensitive to treatment with lenalidomide; however, this comprises the minority of patients with MDS. Luspatercept was recently approved in the United States and the European Union for the treatment of ESA-refractory MDS with ring sideroblasts. Research into new treatment options, especially after ESA failure, is needed. In this review, we will focus on the current therapeutic options for MDS-related anemia.

Oxidative Stress Parameters Can Predict the Response to Erythropoiesis-Stimulating Agents in Myelodysplastic Syndrome Patients

Oxidative stress has been implicated in the development of several types of cancer, including myelodysplastic syndromes (MDS), as well as in the resistance to treatment. In this work, we assessed the potential of oxidative stress parameters to predict the response to erythropoiesis-stimulating agents (ESAs) in lower-risk MDS patients. To this end, we analyzed the systemic levels of reactive species (peroxides and NO), antioxidant defenses (uric acid, vitamin E, vitamin A, GSH, GSSG, TAS, as well as GPX and GR activities], and oxidative damage (8-OH-dG and MDA) in 66 MDS patients, from those 44 have been treated with ESA. We also calculated the peroxides/TAS and NO/TAS ratios and analyzed the gene expression of levels of the redox regulators, NFE2L2 and KEAP1. We found that patients that respond to ESA treatment showed lower levels of plasma peroxides (p < 0.001), cellular GSH (p < 0.001), and cellular GR activity (p = 0.001) when compared to patients who did not respond to ESA treatment. ESA responders also showed lower levels of peroxides/TAS ratio (p < 0.001) and higher levels of the expression of the NFE2L2 gene (p = 0.001) than those that did not respond to ESA treatment. The levels of plasmatic peroxides shown to be the most accurate biomarker of ESA response, with good sensitivity (80%) and specificity (100%) and is an independent biomarker associated with therapy response. Overall, the present study demonstrated a correlation between oxidative stress levels and the response to ESA treatment in lower-risk MDS patients, with the plasmatic peroxides levels a good predictive biomarker of drug (ESA) response.

Myelodysplastic syndromes: a review of therapeutic progress over the past 10 years

INTRODUCTION

Myelodysplastic syndromes (MDS) represent a range of bone marrow disorders, with patients affected by cytopenias and risk of progression to AML. There are limited therapeutic options available for patients, including hypomethylating agents (azacitidine/decitabine), growth factor support, lenalidomide, and allogeneic stem cell transplant.

AREAS COVERED

This review provides an overview of the progress made over the past decade for emerging therapies for lower- and higher-risk MDS (MDS-HR). We also cover advances in prognostication, supportive care, and use of allogeneic SCT in MDS.

EXPERT OPINION

While there have been no FDA-approved therapies for MDS in the past decade, we anticipate the approval of luspatercept based on results from the MEDALIST trial for patients with lower-risk MDS (MDS-LR) and ringed sideroblasts who have failed or are ineligible for erythropoiesis stimulating agents (ESAs). With growing knowledge of the biologic and molecular mechanisms underlying MDS, it is anticipated that new therapies will be approved in the coming years.

The prognostic value of serum erythropoietin in patients with lower-risk myelodysplastic syndromes: a review of the literature and expert opinion

Myelodysplastic syndromes (MDS) are hematopoietic stem cell malignancies associated with an erythroid maturation defect, resulting in anemia. Treatments for MDS include erythropoiesis-stimulating agents (ESAs). The identification of prognostic markers is important to help predict response and improve outcomes. Various scoring systems have been developed to help predict response to ESAs. Despite limitations in its assessment, serum erythropoietin (sEPO) level is an important predictor of hematologic response to ESAs in patients with lower-risk MDS. Numerous studies have reported significantly lower sEPO levels among responders versus non-responders. Furthermore, treatment response is significantly more likely among those with sEPO levels below versus those above various cutoffs. Other prognostic indicators for response to ESAs include lower transfusion requirement, fewer bone marrow blasts, higher hemoglobin, lower serum ferritin, lower-risk MDS, and more normal cytogenetics. Studies of other MDS therapies (e.g., lenalidomide and luspatercept) have also reported that lower sEPO levels are indicative of hematologic response. In addition, lower sEPO levels (up to 500 IU/L) have been included in treatment algorithms for patients with lower-risk MDS to define whether ESAs are indicated. Lower sEPO levels are predictive of hematologic response—particularly to ESAs. Further, clinical trials should use sEPO thresholds to ensure more homogeneous cohorts.

Setting Fire to ESA and EMA Resistance: New Targeted Treatment Options in Lower Risk Myelodysplastic Syndromes. Int J Mol Sci. 2019;20. [PMID: 31394818 PMCID: PMC6720617 DOI: 10.3390/ijms20163853]

During the last decade, substantial advances have been made in the understanding of the complex molecular, immunological and cellular disturbances involved in the initiation as well as evolution of myelodysplastic syndromes (MDS). In 85% of the mainly frail and older patient population, anemia is present at the time of diagnosis and is thus a major therapeutic challenge. High rates of primary resistance to erythropoiesis-stimulating agents (ESAs), the currently only approved standard therapy to treat anemia in lower-risk MDS, demand the development of novel and efficient drugs with a good safety profile. Luspatercept, a ligand trap of activin receptor II, is able to promote late stage erythropoiesis even in patients failing prior ESA treatment. The presence of ring sideroblastic phenotype defines a subgroup of patients with higher response rates. Additionally, recent developments in clinical research using HIF-1 or telomerase modulation by roxadustat or imetelstat are promising. Other areas of translational research involve targeting the inflammasome by anti-inflammatory drugs in order to improve anemia. These efforts will hopefully pave the way for new targeted treatment options for anemic low-risk MDS patients.

APG101 efficiently rescues erythropoiesis in lower risk myelodysplastic syndromes with severe impairment of hematopoiesis

CD95, a member of the death receptor family initiates a caspase-dependent apoptosis, when activated by its ligand CD95L, thought to negatively regulate erythrocyte production in the bone marrow. We have previously shown that CD95 is overexpressed in two thirds of patients with a lower risk myelodysplastic syndrome (MDS) and that resistance to erythropoiesis-stimulating agents (ESA) is linked to poor residual erythropoiesis. In the present study, we show that CD95 overexpression and previous transfusion are independent predictive factors of ESA resistance. To investigate an alternative therapeutic strategy of anemia in ESA-resistant patients, we have conducted a preclinical study of the effects of APG101, a fusion protein consisting of the extracellular domain of human CD95 and the Fc region of human IgG1 on MDS erythropoiesis in vitro. APG101 increases the number of burst-forming unit-erythroid (BFU-E) progenitors derived from CD34+ progenitors in liquid culture and improves overall proliferation rate of erythroid precursors by inhibiting apoptosis. APG101 rescues BFU-E growth in MDS patients presenting with attrition of erythroid progenitors at baseline, independently of CD95 or CD95L expression level. Our data show that overexpression of CD95 at diagnosis is a hallmark of ESA resistance and that severe impairment of erythropoiesis is predictive of erythroid response to APG101 in vitro. These data provide a rationale for further clinical investigation of APG101 in an attempt to treat anemia in lower risk MDS patients.

Reactive oxygen species levels control NF-κB activation by low dose deferasirox in erythroid progenitors of low risk myelodysplastic syndromes

Anemia is a frequent cytopenia in myelodysplastic syndromes (MDS) and most patients require red blood cell transfusion resulting in iron overload (IO). Deferasirox (DFX) has become the standard treatment of IO in MDS and it displays positive effects on erythropoiesis. In low risk MDS samples, mechanisms improving erythropoiesis after DFX treatment remain unclear. Herein, we addressed this question by using liquid cultures with iron overload of erythroid precursors treated with low dose of DFX (3μM), which corresponds to DFX 5 mg/kg/day, an unusual dose used for iron chelation. We highlight a decreased apoptosis rate and an increased proportion of cycling cells, both leading to higher proliferation rates. The iron chelation properties of low dose DFX failed to activate the Iron Regulatory Proteins and to support iron depletion, but low dose DFX dampers intracellular reactive oxygen species. Furthermore low concentrations of DFX activate the NF-κB pathway in erythroid precursors triggering anti-apoptotic and anti-inflammatory signals. Establishing stable gene silencing of the Thioredoxin (TRX) 1 genes, a NF-κB modulator, showed that fine-tuning of reactive oxygen species (ROS) levels regulates NF-κB. These results justify a clinical trial proposing low dose DFX in MDS patients refractory to erythropoiesis stimulating agents.

Increasing the effectiveness of hematopoiesis in myelodysplastic syndromes: erythropoiesis-stimulating agents and transforming growth factor-β superfamily inhibitors

Patients with lower-risk myelodysplastic syndromes (MDS) are mainly affected by chronic anemia and fatigue. Treatment strategies aim to improve anemia and quality of life, as well as iron overload due to red blood cell transfusion support. To promote proliferation and differentiation of erythropoiesis, erythropoiesis-stimulating agents (ESAs) such as erythropoietin (EPO) and mimetics are applied as first-line therapy in a large fraction of lower-risk MDS patients. In general, ESAs yield favorable responses in about half of the patients, although responses are often short-lived. In fact, many ESA-refractory patients harbor defects in late-stage erythropoiesis downstream of EPO action. Novel transforming growth factor (TGF)-β superfamily inhibitors sotatercept and luspatercept represent a promising approach to alleviate anemia by stimulating erythroid differentiation.

Randomized Phase III Study of Lenalidomide Versus Placebo in RBC Transfusion-Dependent Patients With Lower-Risk Non-del(5q) Myelodysplastic Syndromes and Ineligible for or Refractory to Erythropoiesis-Stimulating Agents

PURPOSE

This international phase III, randomized, placebo-controlled, double-blind study assessed the efficacy and safety of lenalidomide in RBC transfusion-dependent patients with International Prognostic Scoring System lower-risk non-del(5q) myelodysplastic syndromes ineligible for or refractory to erythropoiesis-stimulating agents.

PATIENTS AND METHODS

In total, 239 patients were randomly assigned (2:1) to treatment with lenalidomide (n = 160) or placebo (n = 79) once per day (on 28-day cycles). The primary end point was the rate of RBC transfusion independence (TI) ≥ 8 weeks. Secondary end points were RBC-TI ≥ 24 weeks, duration of RBC-TI, erythroid response, health-related quality of life (HRQoL), and safety.

RESULTS

RBC-TI ≥ 8 weeks was achieved in 26.9% and 2.5% of patients in the lenalidomide and placebo groups, respectively (P < .001). Ninety percent of patients achieving RBC-TI responded within 16 weeks of treatment. Median duration of RBC-TI with lenalidomide was 30.9 weeks (95% CI, 20.7 to 59.1). Transfusion reduction of ≥ 4 units packed RBCs, on the basis of a 112-day assessment, was 21.8% in the lenalidomide group and 0% in the placebo group. Higher response rates were observed in patients with lower baseline endogenous erythropoietin ≤ 500 mU/mL (34.0% v 15.5% for > 500 mU/mL). At week 12, mean changes in HRQoL scores from baseline did not differ significantly between treatment groups, which suggests that lenalidomide did not adversely affect HRQoL. Achievement of RBC-TI ≥ 8 weeks was associated with significant improvements in HRQoL (P < .01). The most common treatment-emergent adverse events were neutropenia and thrombocytopenia.

CONCLUSION

Lenalidomide yields sustained RBC-TI in 26.9% of RBC transfusion-dependent patients with lower-risk non-del(5q) myelodysplastic syndromes ineligible for or refractory to erythropoiesis-stimulating agents. Response to lenalidomide was associated with improved HRQoL. Treatment-emergent adverse event data were consistent with the known safety profile of lenalidomide.

Anemia as the Main Manifestation of Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are a constellation of different diseases sharing anemia in the great majority of cases, and this cytopenia defines these pathologies and their most dramatic clinical manifestations. Anemia in MDS is due to ineffective erythropoiesis, with a high degree of apoptosis of marrow erythroid progenitors. These progenitors show distinctive dysplastic features that consent diagnosis, and are recognizable and differentiated, although not easily, from other morphologic alterations present in other types of anemia. Reaching the diagnosis of MDS in a macrocytic anemia and alleviating the symptoms of anemia are therefore an essential objective of the treating physician. In this work, the signs and symptoms of anemia in MDS, as well as its peculiar pathophysiology, are discussed. Erythopoietic stimulating agents (ESAs) are providing the best treatment for anemic MDS patients, but their use is still not approved by health agencies. While still waiting for this waiver, their clinical use is widespread and their effectivness is well known, as well as the dismal prognosis of patients who do not respond to ESAs and require transfusions. MDS with del5q constitute a unique model of anemia whose complex pathophysiology has been clarified at least partially, defining its link to ribosomal alterations likewise what observed in hereditary anemias like Blackfan Diamond anemia. Lenalidomide is the agent that has shown striking and specific erythropoietic activity in del5q MDS, and the basis of this response is starting to be understood. Several new agents are under evaluation for ESA refractory/relapsed MDS patients, targeting different putative mechanisms of ineffective erythropoiesis, and are here reviewed.

Myelodysplasia is in the niche: novel concepts and emerging therapies

Myelodysplastic syndromes (MDSs) represent clonal disorders mainly of the elderly that are characterized by ineffective hematopoiesis and an increased risk of transformation into acute myeloid leukemia. The pathogenesis of MDS is thought to evolve from accumulation and selection of specific genetic or epigenetic events. Emerging evidence indicates that MDS is not solely a hematopoietic disease but rather affects the entire bone marrow microenvironment, including bone metabolism. Many of these cells, in particular mesenchymal stem and progenitor cells (MSPCs) and osteoblasts, express a number of adhesion molecules and secreted factors that regulate blood regeneration throughout life by contributing to hematopoietic stem and progenitor cell (HSPC) maintenance, self-renewal and differentiation. Several endocrine factors, such as erythropoietin, parathyroid hormone and estrogens, as well as deranged iron metabolism modulate these processes. Thus, interactions between MSPC and HSPC contribute to the pathogenesis of MDS and associated pathologies. A detailed understanding of these mechanisms may help to define novel targets for diagnosis and possibly therapy. In this review, we will discuss the scientific rationale of ‘osteohematology' as an emerging research field in MDS and outline clinical implications.

Erythropoiesis stimulating agents and other growth factors in low-risk MDS

Anemia and transfusion need constitute major problems for patients with myelodysplastic syndromes (MDS) and are associated with reduced quality of life, poorer survival and an increased risk for transformation to AML. Treatment with erythropoiesis-stimulating agents (ESAs) is first-line treatment for the anemia of most patients with MDS. Erythropoietin acts synergistically with G-CSF to inhibit erythroid apoptosis and promote erythrocyte production. The median duration of response is 2-3 years, with patients responding for more than a decade. Onset of a permanent transfusion need is delayed if treatment is introduced early after the onset of symptomatic anemia. A positive effect on long-term outcome has been suggested by several large epidemiological studies, with no difference in the rate of leukemic transformation between treated and untreated patients. Moreover, responding patients show improvement of quality of life and exercise capacity. Response to treatment can be predicted by combining serum erythropoietin, transfusion rate, and flow cytometry profiling.

Age-related changes of healthy bone marrow cell signaling in response to growth factors provide insight into low risk MDS

BACKGROUND

Single Cell Network Profiling (SCNP) is a multiparametric flow cytometry-based assay that quantifiably and simultaneously measures changes in intracellular signaling proteins in response to in vitro extracellular modulators at the single cell level. Myelodysplastic syndrome (MDS) is a heterogeneous clonal disorder of hematopoietic stem cells that occurs in elderly subjects and is characterized by dysplasia and ineffective hematopoiesis. The functional responsiveness of MDS bone marrow (BM) hematopoietic cells, including functionally distinct myeloid and erythroid precursor subsets, to hematopoietic growth factors (HGF) and the relationship of modulated signaling to disease characteristics is poorly understood.

METHODS

SCNP was used first to examine the effects of age on erythropoietin (EPO) and granulocyte colony stimulating factor (GCSF)-induced signaling in myeloid, nucleated red blood cells (nRBC), and CD34 expressing cell subsets in healthy BM (n = 15). SCNP was then used to map functional signaling profiles in low risk (LR) MDS (n = 7) for comparison to signaling in samples from healthy donors and to probe signaling associations within clinically defined subgroups.

RESULTS

In healthy BM samples, signaling responses to HGF were quite homogeneous (i.e., tightly regulated) with age-dependent effects observed in response to EPO but not to GCSF. Despite the relatively small number of samples assayed in the study, LR MDS could be classified into distinct subgroups based on both cell subset frequency and signaling profiles.

CONCLUSIONS

As a correlate of underlying genetic abnormalities, signal transduction analyses may provide a functional and potentially clinically relevant classification of MDS. Further evaluation in a larger cohort is warranted.

Treatment of low-risk myelodysplastic syndrome: hematopoietic growth factors erythropoietins and thrombopoietins

The use of erythropoietic growth factors has become standard of care in many countries for lower risk myelodysplastic syndrome (MDS) patients. Throughout a large number of clinical trials, therapy with erythropoietic agents has consistently shown improvement of anemia and reduction of transfusion dependence. There is currently no evidence of safety issues of erythropoietins in MDS, including thrombosis, polycythemia, and progressive disease. Large retrospective comparative analyses have shown no increase in mortality in erythropoietin (EPO)-treated MDS patients. Doses of up to 80,000 IU/wk have successfully been employed and the addition of granulocyte colony-stimulating factor (G-CSF) can benefit previously unresponsive patients. Although several other combination therapies have been tested, apart from G-CSF, none has gained wide clinical acceptance. Thrombopoietic agents can alleviate thrombocytopenia and bleeding symptoms in lower risk MDS patients. However, concerns regarding a higher rate of transformation to acute myeloid leukemia and the fear of increased bone marrow fibrosis during treatment have hampered their clinical development.

Revisiting use of growth factors in myelodysplastic syndromes

Myelodysplastic syndromes (MDS) represent a heterogeneous group of clonal hematologic neoplasms characterized by morphologic dysplasia, aberrant hematopoiesis and peripheral blood refractory cytopenias. MDS is recognized to be associated with an increased risk of symptomatic anemia, infectious complications and bleeding diathesis, as well as a risk of progression to acute myeloid leukemia, particularly in patients with a high IPSS score. The advent of use of hematopoietic growth factors such as granulocyte colony-stimulating factor (G-CSF) and recombinant erythropoietin (EPO) has improved symptoms in MDS patients in addition to some data that suggest there might be an improvement in survival. G-CSF is an effective therapeutic option in MDS patients, and it should be considered for the management of refractory symptomatic cytopenias. G-CSF and EPO in combination can improve outcomes in appropriate MDS patients such as those with lower-risk MDS and refractory anemia with ring sideroblasts (RARS) . This article reviews use of growth factors for lower-risk MDS patients, and examines the data for G-CSF, EPO and thrombopietic growth factors (TPO) that are available or being developed as therapeutic modalities for this challenging disease.

Novel therapeutic strategies: hypomethylating agents and beyond

The treatment of symptomatic and high-risk myelodysplastic syndrome (MDS) spans several therapeutic goals and options. Key to the successful therapy of these heterogeneous diseases is careful characterization and diagnosis, including clinical, cytogenetic, biological, and molecular evaluation of individual patients. Any novel management strategy in MDS must be based on accepted and validated prognostic scoring systems, such as the International Prognostic Scoring System (IPSS), and should take into account predictive parameters of response to the available therapeutic agents and individual comorbidities. For IPSS lower-risk MDS patients, several first-line options are available, including erythropoietic stimulating agents, lenalidomide, and immunosuppressive drugs. Sequential therapy is advisable whenever response is lost, and the activity of azacitidine and decitabine in first- or second-line therapy is relevant, especially in patients with symptomatic cytopenias and anemia. Hypomethylating agents have a central role in therapy of IPSS higher-risk MDS patients. These agents include azacitidine and decitabine, which allow treatment of very elderly and frail patients, resulting in hematological improvement and transfusion independency in roughly half, and for azacitidine a demonstrated significant prolongation of survival. Because hypomethylating agents are not curative, they are not satisfactory for younger MDS patients, for whom a transplantation strategy should be planned. Although hypomethylating agent therapy is used extensively, a growing number of MDS patients fail to respond or progress. The future challenge is not only to find treatment regimens that target the dysplastic clone(s) so that durable remissions are achieved (particularly in high-risk patients with short survival and/or increased leukemic transformation rates), but also to also identify active salvage regimens.

Rationale for the clinical application of flow cytometry in patients with myelodysplastic syndromes: position paper of an International Consortium and the European LeukemiaNet Working Group

An international working group within the European LeukemiaNet gathered, aiming to determine the role of flow cytometry (FC) in myelodysplastic syndromes (MDS). It was agreed that FC has a substantial application in disease characterization, diagnosis and prognosis. FC may also be useful in predicting treatment responses and monitoring novel and standard therapeutic regimens. In this article the rationale is discussed that flow cytometry should be integrated as a part of diagnostic and prognostic scoring systems in MDS.

Defective nuclear localization of Hsp70 is associated with dyserythropoiesis and GATA-1 cleavage in myelodysplastic syndromes

Normal human erythroid cell maturation requests the transcription factor GATA-1 and a transient activation of caspase-3, with GATA-1 being protected from caspase-3-mediated cleavage by interaction with the chaperone heat shock protein 70 (Hsp70) in the nucleus. Erythroid cell dysplasia observed in early myelodysplastic syndromes (MDS) involves impairment of differentiation and excess of apoptosis with a burst of caspase activation. Analysis of gene expression in MDS erythroblasts obtained by ex vivo cultures demonstrates the down-regulation of a set of GATA-1 transcriptional target genes, including GYPA that encodes glycophorin A (GPA), and the up-regulation of members of the HSP70 family. GATA-1 protein expression is decreased in MDS erythroblasts, but restores in the presence of a pan-caspase inhibitor. Expression of a mutated GATA-1 that cannot be cleaved by caspase-3 rescues the transcription of GATA-1 targets, and the erythroid differentiation, but does not improve survival. Hsp70 fails to protect GATA-1 from caspases because the protein does not accumulate in the nucleus with active caspase-3. Expression of a nucleus-targeted mutant of Hsp70 protects GATA-1 and rescues MDS erythroid cell differentiation. Alteration of Hsp70 cytosolic-nuclear shuttling is a major feature of MDS that favors GATA-1 cleavage and differentiation impairment, but not apoptosis, in dysplastic erythroblasts.