Rescue of early-stage myelodysplastic syndrome-deriving erythroid precursors by the ectopic expression of a dominant-negative form of FADD

Treatment with the apoptosis inhibitor Asunercept reduces clone sizes in patients with lower risk Myelodysplastic Neoplasms

In low-risk Myelodysplastic Neoplasms (MDS), increased activity of apoptosis-promoting factors such as tumor necrosis factor (TNFα) and pro-apoptotic Fas ligand (CD95L) have been described as possible pathomechanisms leading to impaired erythropoiesis. Asunercept (APG101) is a novel therapeutic fusion protein blocking CD95, which has previously shown partial efficacy in reducing transfusion requirement in a clinical phase I trial for low-risk MDS patients (NCT01736436; 2012-11-26). In the current study we aimed to evaluate the effect of Asunercept therapy on the clonal bone marrow composition to identify potential biomarkers to predict response. Bone marrow samples of n = 12 low-risk MDS patients from the above referenced clinical trial were analyzed by serial deep whole exome sequencing in a total of n = 58 time points. We could distinguish a mean of 3.5 molecularly defined subclones per patient (range 2-6). We observed a molecular response defined as reductions of dominant clone sizes by a variant allele frequency (VAF) decrease of at least 10% (mean 20%, range: 10.5-39.2%) in dependency of Asunercept treatment in 9 of 12 (75%) patients. Most of this decline in clonal populations was observed after completion of 12 weeks treatment. Particularly early and pronounced reductions of clone sizes were found in subclones driven by mutations in genes involved in regulation of methylation (n = 1 DNMT3A, n = 1 IDH2, n = 1 TET2). Our results suggest that APG101 could be efficacious in reducing clone sizes of mutated hematopoietic cells in the bone marrow of Myelodysplastic Neoplasms, which warrants further investigation.

Erythropoiesis in lower-risk myelodysplastic syndromes and beta-thalassemia

The hematologic disorders myelodysplastic syndromes and beta-thalassemia are characterized by ineffective erythropoiesis and anemia, often managed with regular blood transfusions. Erythropoiesis, the process by which sufficient numbers of functional erythrocytes are produced from hematopoietic stem cells, is highly regulated, and defects can negatively affect the proliferation, differentiation, and survival of erythroid precursors. Treatments that directly target the underlying mechanisms of ineffective erythropoiesis are limited, and management of anemia with regular blood transfusions imposes a significant burden on patients, caregivers, and health care systems. There is therefore a strong unmet need for treatments that can restore effective erythropoiesis. Novel therapies are beginning to address this need by targeting a variety of mechanisms underlying erythropoiesis. Herein, we provide an overview of the role of ineffective erythropoiesis in myelodysplastic syndromes and beta-thalassemia, discuss unmet needs in targeting ineffective erythropoiesis, and describe current management strategies and emerging treatments for these disorders.

Luspatercept (RAP-536) modulates oxidative stress without affecting mutation burden in myelodysplastic syndromes

In low-risk myelodysplastic syndrome (LR-MDS), erythropoietin (EPO) is widely used for the treatment of chronic anemia. However, initial response to EPO has time-limited effects. Luspatercept reduces red blood cell transfusion dependence in LR-MDS patients. Here, we investigated the molecular action of luspatercept (RAP-536) in an in vitro model of erythroid differentiation of MDS, and also in a in vivo PDX murine model with primary samples of MDS patients carrying or not SF3B1 mutation. In our in vitro model, RAP-536 promotes erythroid proliferation by increasing the number of cycling cells without any impact on apoptosis rates. RAP-536 promoted late erythroid precursor maturation while decreasing intracellular reactive oxygen species level. RNA sequencing of erythroid progenitors obtained under RAP-536 treatment showed an enrichment of genes implicated in positive regulation of response to oxidative stress and erythroid differentiation. In our PDX model, RAP-536 induces a higher hemoglobin level. RAP-536 did not modify variant allele frequencies in vitro and did not have any effect against leukemic burden in our PDX model. These results suggest that RAP-536 promotes in vivo and in vitro erythroid cell differentiation by decreasing ROS level without any remarkable impact on iron homeostasis and on mutated allele burden.

The TNF Family of Ligands and Receptors: Communication Modules in the Immune System and Beyond

The tumor necrosis factor (TNF) and TNF receptor (TNFR) superfamilies (TNFSF/TNFRSF) include 19 ligands and 29 receptors that play important roles in the modulation of cellular functions. The communication pathways mediated by TNFSF/TNFRSF are essential for numerous developmental, homeostatic, and stimulus-responsive processes in vivo. TNFSF/TNFRSF members regulate cellular differentiation, survival, and programmed death, but their most critical functions pertain to the immune system. Both innate and adaptive immune cells are controlled by TNFSF/TNFRSF members in a manner that is crucial for the coordination of various mechanisms driving either co-stimulation or co-inhibition of the immune response. Dysregulation of these same signaling pathways has been implicated in inflammatory and autoimmune diseases, highlighting the importance of their tight regulation. Investigation of the control of TNFSF/TNFRSF activities has led to the development of therapeutics with the potential to reduce chronic inflammation or promote anti-tumor immunity. The study of TNFSF/TNFRSF proteins has exploded over the last 30 yr, but there remains a need to better understand the fundamental mechanisms underlying the molecular pathways they mediate to design more effective anti-inflammatory and anti-cancer therapies.

Astroglia contribute to the pathogenesis of spinocerebellar ataxia Type 1 (SCA1) in a biphasic, stage-of-disease specific manner

Spinocerebellar ataxia type 1 (SCA1) is a fatal, dominantly inherited neurodegenerative disease caused by the expansion of CAG repeats in the Ataxin-1 (ATXN1) gene. SCA1 is characterized by balance and coordination deficits due to the predominant loss of Purkinje neurons in the cerebellum. We previously demonstrated that cerebellar astrogliosis beings during the early stages of SCA1, prior to onset of motor deficits and loss of Purkinje neurons. We communicate here that cerebellar astrogliosis contributes to SCA1 pathogenesis in a biphasic, stage of disease dependent manner. We modulated astrogliosis by selectively reducing pro-inflammatory transcriptional regulator nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) signaling in astroglia via a Cre-lox mouse genetic approach. Our results indicate that inhibition of astroglial NF-κB signaling, prior to motor deficit onset, exacerbates disease severity. This is suggestive of a neuroprotective role mediated by astroglia during early stage SCA1. In contrast, inhibition of astroglial NF-κB signaling during late stage of disease ameliorated motor deficits, indicating a potentially harmful role of astroglia late in SCA1. These results indicate that astrogliosis may have a critical and dual role in disease. If so, our results imply that anti-inflammatory astroglia-based therapeutic approaches may need to consider disease progression to achieve therapeutic efficacy.

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.

Epoetin β pegol (continuous erythropoietin receptor activator, CERA) is another choice for the treatment of anemia in myelodysplastic syndrome: a case report

Background

In most patients, anemia is present when myelodysplastic syndrome is diagnosed. Although darbepoetin α is the first-choice supportive therapy for low-risk myelodysplastic syndrome, half of all patients develop a loss of response to darbepoetin α within 12 months. However, few reports have described supportive therapy after the loss of response to darbepoetin α.

Case presentation

We herein present a case involving a 65-year-old Japanese woman with low-risk myelodysplastic syndrome whose erythropoiesis-stimulating agent treatment was switched from darbepoetin α to epoetin β pegol (continuous erythropoietin receptor activator) to treat transfusion-dependent anemia. The frequent transfusions required to treat the anemia resulted in transfusion-associated circulatory overload. The transfusion-dependent anemia was initially treated with darbepoetin α, which negated the requirement for transfusion. However, after 12 months of darbepoetin α therapy, the hemoglobin concentration sharply declined. We switched her therapy from darbepoetin α to continuous erythropoietin receptor activator to avoid transfusion. After initiation of continuous erythropoietin receptor activator therapy, the hemoglobin concentration gradually increased and transfusion was not required. At the time of writing, no progression of the anemia had occurred.

Conclusions

Although darbepoetin α is the first-choice supportive therapy for low-risk myelodysplastic syndrome, continuous erythropoietin receptor activator might be considered the second-choice therapy.

Apoptosis-Related Gene Expression Profiling in Hematopoietic Cell Fractions of MDS Patients

Although the vast majority of patients with a myelodysplastic syndrome (MDS) suffer from cytopenias, the bone marrow is usually normocellular or hypercellular. Apoptosis of hematopoietic cells in the bone marrow has been implicated in this phenomenon. However, in MDS it remains only partially elucidated which genes are involved in this process and which hematopoietic cells are mainly affected. We employed sensitive real-time PCR technology to study 93 apoptosis-related genes and gene families in sorted immature CD34+ and the differentiating erythroid (CD71+) and monomyeloid (CD13/33+) bone marrow cells. Unsupervised cluster analysis of the expression signature readily distinguished the different cellular bone marrow fractions (CD34+, CD71+ and CD13/33+) from each other, but did not discriminate patients from healthy controls. When individual genes were regarded, several were found to be differentially expressed between patients and controls. Particularly, strong over-expression of BIK (BCL2-interacting killer) was observed in erythroid progenitor cells of low- and high-risk MDS patients (both p = 0.001) and TNFRSF4 (tumor necrosis factor receptor superfamily 4) was down-regulated in immature hematopoietic cells (p = 0.0023) of low-risk MDS patients compared to healthy bone marrow.

Unraveling the Pathogenesis of MDS: The NLRP3 Inflammasome and Pyroptosis Drive the MDS Phenotype

Myelodysplastic syndromes (MDS) are characterized by bone marrow cytological dysplasia and ineffective hematopoiesis in the setting of recurrent somatic gene mutations and chromosomal abnormalities. The underlying pathogenic mechanisms that drive a common clinical phenotype from a diverse array of genetic abnormalities have only recently begun to emerge. Accumulating evidence has highlighted the integral role of the innate immune system in upregulating inflammatory cytokines via NF-κB activation in the pathogenesis of MDS. Recent investigations implicate activation of the NLRP3 inflammasome in hematopoietic stem/progenitor cells as a critical convergence signal in MDS with consequent clonal expansion and pyroptotic cell death though caspase-1 maturation. Specifically, the alarmin S100A9 and/or founder gene mutations trigger pyroptosis through the generation of reactive oxygen species leading to assembly and activation of the redox-sensitive NLRP3 inflammasome and β–catenin, assuring propagation of the MDS clone. More importantly, targeted inhibition of varied steps in this pathway restore effective hematopoiesis. Together, delineation of the role of pyroptosis in the clinical phenotype of MDS patients has identified novel therapeutic strategies that offer significant promise in the treatment of 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.

Targeting apoptosis proteins in hematological malignancies

The apoptotic machinery plays a key role in hematopoietic cell homeostasis. Terminally differentiated cells are eliminated, at least in part, by apoptosis, whereas part of the apoptotic machinery, including one or several caspases, is required to go through very specific steps of the differentiation pathways. A number of hematological diseases involve a deregulation of this machinery, which in most cases is a decrease in cell sensitivity to pro-apoptotic signals through over-expression of anti-apoptotic molecules. In some situations however, e.g. in the erythroid lineage of low grade myelodysplastic syndromes, cell sensitivity to apoptosis is increased in a death receptor-dependent manner and cell death pathways are inhibited only when these diseases progress into high grade and acute leukemia. Therapeutic strategies targeting the apoptotic machinery specifically block cell death inhibitors that are over-expressed in transformed cells, mainly Bcl-2-related proteins and Inhibitor of Apoptosis Proteins (IAPs). Another strategy is the activation of the extrinsic pathway to apoptosis, mainly through the death receptor agonist Tumor necrosis factor-Related Apoptosis Inducing Ligand (TRAIL) or agonistic antibodies targeting TRAIL receptors. The use of inhibitors of death receptors could make sense when these receptors are involved in excessive cell death or activation of survival pathways. Most of the drugs targeting apoptotic pathways introduced in clinics have demonstrated their tolerability. Their efficacy, either alone or in combination with other drugs such as demethylating agents and histone deacetylase inhibitors, is currently tested in both myeloid and lymphoid hematological diseases.

Significance of JAK2 and TET2 mutations in myelodysplastic syndromes

The pathogenesis of myelodysplastic syndromes involves a pattern of genetic, epigenetic, and immune-mediated mechanisms but little is known about what causes the specific disease features and promotes disease progression in the individual patient. The identification of JAK2 and MPL mutations, and more recently TET2, CBL and ASXL-1 mutations in these disorders provide a basis for increased understanding of disease biology and mechanisms behind progression. Such mutations are more commonly found in patients with a significant amount of marrow ring sideroblasts, and in patients belonging to the category of mixed myelodysplastic/myeloproliferative neoplasms, entities which are in focus for this review.

Unimpaired terminal erythroid differentiation and preserved enucleation capacity in myelodysplastic 5q(del) clones: a single cell study

BACKGROUND

Anemia is a characteristic of myelodysplastic syndromes, such as the rare 5q- syndrome, but its mechanism remains unclear. In particular, data are lacking on the terminal phase of differentiation of erythroid cells (enucleation) in myelodysplastic syndromes.

DESIGN AND METHODS

We used a previously published culture model to generate mature red blood cells in vitro from human hematopoietic progenitor cells in order to study the pathophysiology of the 5q- syndrome. Our model enables analysis of cell proliferation and differentiation at a single cell level and determination of the enucleation capacity of erythroid precursors.

RESULTS

The erythroid commitment of 5q(del) clones was not altered and their terminal differentiation capacity was preserved since they achieved final erythroid maturation (enucleation stage). The drop in red blood cell production was secondary to the decrease in the erythroid progenitor cell pool and to impaired proliferative capacity. RPS14 gene haploinsufficiency was related to defective erythroid proliferation but not to differentiation capacity.

CONCLUSIONS

The 5q- syndrome should be considered a quantitative rather than qualitative bone marrow defect. This observation might open the way to new therapeutic concepts.

Myelodysplastic syndromes: biology and treatment

Optimal management of patients with myelodysplastic syndromes (MDS) requires an insight into the biology of the disease and the mechanisms of action of the available therapies. This review focuses on low-risk MDS, for which chronic anaemia and eventual progression to acute myeloid leukaemia are the main concerns. We cover the updated World Health Organization classification, the latest prognostic scoring system, and describe novel findings in the pathogenesis of 5q- syndrome. We perform in depth analyses of two of the most widely used treatments, erythropoietin and lenalidomide, discussing mechanisms of action, reasons for treatment failure and influence on survival.

Inhibition of p38alpha MAPK disrupts the pathological loop of proinflammatory factor production in the myelodysplastic syndrome bone marrow microenvironment

Myelodysplastic syndromes (MDS) are common causes of ineffective hematopoiesis and cytopenias in the elderly. Various myelosuppressive and proinflammatory cytokines have been implicated in the high rates of apoptosis and hematopoietic suppression seen in MDS. We have previously shown that p38 MAPK is overactivated in MDS hematopoietic progenitors, which led to current clinical studies of the selective p38alpha inhibitor, SCIO-469, in this disease. We now demonstrate that the myelosuppressive cytokines TNFalpha and IL-1beta are secreted by bone marrow (BM) cells in a p38 MAPK-dependent manner. Their secretion is stimulated by paracrine interactions between BM stromal and mononuclear cells and cytokine induction correlates with CD34+ stem cell apoptosis in an inflammation-simulated in vitro bone marrow microenvironment. Treatment with SCIO-469 inhibits TNF secretion in primary MDS bone marrow cells and protects cytogenetically normal progenitors from apoptosis ex vivo. Furthermore, p38 inhibition diminishes the expression of TNFalpha or IL-1beta-induced proinflammatory chemokines in BM stromal cells. These data indicate that p38 inhibition has anti-inflammatory effects on the bone marrow microenvironment that complements its cytoprotective effect on progenitor survival. These findings support clinical investigation of p38alpha as a potential therapeutic target in MDS and other related diseases characterised by inflammatory bone marrow failure.

EPO receptor circuits for primary erythroblast survival

EPO functions primarily as an erythroblast survival factor, and its antiapoptotic actions have been proposed to involve predominantly PI3-kinase and BCL-X pathways. Presently, the nature of EPO-regulated survival genes has been investigated through transcriptome analyses of highly responsive, primary bone marrow erythroblasts. Two proapoptotic factors, Bim and FoxO3a, were rapidly repressed not only via the wild-type EPOR, but also by PY-deficient knocked-in EPOR alleles. In parallel, Pim1 and Pim3 kinases and Irs2 were induced. For this survival gene set, induction failed via a PY-null EPOR-HM allele, but was restored upon reconstitution of a PY343 STAT5-binding site within a related EPOR-H allele. Notably, EPOR-HM supports erythropoiesis at steady state but not during anemia, while EPOR-H exhibits near wild-type EPOR activities. EPOR-H and the wild-type EPOR (but not EPOR-HM) also markedly stimulated the expression of Trb3 pseudokinase, and intracellular serpin, Serpina-3G. For SERPINA-3G and TRB3, ectopic expression in EPO-dependent progenitors furthermore significantly inhibited apoptosis due to cytokine withdrawal. BCL-XL and BCL2 also were studied, but in highly responsive Kit(pos)CD71(high)Ter119(neg) erythroblasts, neither was EPO modulated. EPOR survival circuits therefore include the repression of Bim plus FoxO3a, and EPOR/PY343/STAT5-dependent stimulation of Pim1, Pim3, Irs2 plus Serpina-3G, and Trb3 as new antiapoptotic effectors.

Supportive care and use of hematopoietic growth factors in myelodysplastic syndromes

Supportive care constitutes the basis of the management of patients with myelodysplastic syndromes (MDS). Appropriate treatment of cytopenia, as well as of other related complications, not only improves quality of life but also may positively affect the overall survival of patients. Anemia is the most common cytopenia in MDS, and the requirement for regular transfusions is a major clinical problem for patients with low-risk MDS. An important therapeutic goal in this patient group is to maintain acceptable hemoglobin levels without transfusions. Today, this goal can be achieved by treatment with erythropoietin (Epo) +/- granulocyte colony-stimulating factor (G-CSF), or by more targeted treatment such as antithymocyte globulin or lenalidomide in around 50% of patients. For the remaining patients, and for those who lose their therapeutic response, chronic transfusion therapy, with or without the addition of chelating agents, is the only option and it is important that this treatment is scheduled to meet the needs of the individual patient. Severe thrombocytopenia has recently been reported to respond to thrombopoietic agents, such as AMG 531.

The role of JAK2 mutations in RARS and other MDS

Acquired sideroblastic anemia with unilineage dysplasia (WHO RARS) is a clonal stem cell disorder characterized by erythroid dysplasia, mitochondrial accumulation of mitochondrial ferritin, defective erythroid maturation and anemia. A fraction of these patients also show elevated platelet counts; since 2001 this has been defined as RARS with marked thrombocytosis (RARS-T). It has recently been described that around half of RARS-T patients, along with a small subset of other MDS and mixed myelodysplastic/ myeloproliferative disorders, carry the JAK2 mutation, and that MPL mutations are found in single patients. Clinically, RARS-T patients show features of both RARS, essential thrombocythmia (ET) and to some extent also myelofibrosis. However, the degree of anemia and overall survival is more similar to RARS than myeloproliferative disorders. The occurrence of JAK2 mutations and features of ET in RARS is too frequent to be the result of chance only, and it is possible that this link may provide a key to an increased understanding of the genetic abnormalities causing ring sideroblast formation.

Deglycosylated bleomycin induces apoptosis in lymphoma cell via c-jun NH2-terminal kinase but not reactive oxygen species

Bleomycin (BLM) has demonstrated potent activity in treating malignant lymphomas but its therapeutic efficacy is hampered by induction of lung fibrosis. This side effect is related to the ability of the drug to generate reactive oxygen species in lung cells. In the present study, we evaluated the consequences of deglycosylation of BLM in term of cytotoxic activity and generation of reactive oxygen species. When tested on U937 human lymphoma cells, both compounds generated a typical apoptotic phenotype. Cell death induction was associated with Bax oligomerization, dissipation of the mitochondrial membrane potential, release of cytochrome c, caspase activation, chromatin condensation and internucleosomal degradation. Whereas both reactive oxygen species and c-jun NH(2)-terminal kinase (JNK) inhibitors prevented BLM-induced U937 cell death, only JNK inhibition prevented deglycosylated BLM-mediated cell death. Both compounds induced clustering of TRAIL receptors (DR4 and DR5) and Fas at the cell surface but neither a chimeric soluble DR5 receptor that inhibits TRAIL-induced cell death nor a dominant negative version of the adaptor molecule Fas-associated death domain prevented BLM-induced cytotoxicity. These observations indicate that deglycosylation of BLM does not impair the ability of the drug to trigger cell death through activation of the intrinsic pathway but prevents induction of reactive oxygen species. This observation suggests that deglycosylated BLM could exhibit less toxic side effects and could warrant its use in clinic.

A role for caspases in the differentiation of erythroid cells and macrophages

Several cysteine proteases of the caspase family play a central role in many forms of cell death by apoptosis. Other enzymes of the family are involved in cytokine maturation along inflammatory response. In recent years, several caspases involved in cell death were shown to play a role in other cellular processes such as proliferation and differentiation. In the present review, we summarize the current knowledge of the role of caspases in the differentiation of erythroid cells and macrophages. Based on these two examples, we show that the nature of involved enzymes, the pathways leading to their activation in response to specific growth factors, and the specificity of the target proteins that are cleaved by the activated enzymes strongly differ from one cell type to another. Deregulation of these pathways is thought to play a role in the pathophysiology of low-grade myelodysplastic syndromes, characterized by excessive activation of caspases and erythroid precursor apoptosis, and that of chronic myelomonocytic leukemia, characterized by a defective activation of caspases in monocytes exposed to M-CSF, which blocks their differentiation.

Differentiating megakaryocytes in myelodysplastic syndromes succumb to mitochondrial derangement without caspase activation

Myelodysplastic syndromes (MDS) constitute a preneoplastic condition in which potentially malignant cancer stem cells continuously die during differentiation. This MDS-associated cell death often involves caspase-3 activation, yet can also occur without caspase activation, for instance in differentiating megakaryocytes (MK). We investigated, the mechanisms through which MK from MDS patients undergo premature cell death. While polyploid, mature MK from healthy subjects or MDS patients manifested caspase-3 activation during terminal differentiation, freshly isolated, immature MK from MDS died without caspase-3 activation. Similarly, purified bone marrow CD34(+) cells from MDS patients that were driven into MK differentiation in vitro died without caspase-3 activation at an immature stage, before polyploidization. The premature death of MDS MK was accompanied by the mitochondrial release of cytochrome c, Smac/DIABLO and endonuclease G, a caspase-independent death effector, as well loss of the mitochondrial membrane potential and plasma membrane phosphatidylserine exposure before definitive loss of viability. Thus, a stereotyped pattern of mitochondrial alterations accompanies differentiation-associated MK death in MDS.

Myelodysplastic syndromes: the complexity of stem-cell diseases

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

[Myelodysplastic syndromes in adults]

Myelodysplastic syndromes (MDS) are clonal hematological disorders characterized by ineffective hematopoiesis. Their increased prevalence is associated with the aging of the population. The aim of this article is to analyze the available literature about MDS - its classification, pathophysiology, prognosis and recent progress in treatment - and report our experience with it. Its diagnosis and classification are based on the morphological features of blood and bone marrow cells. The World Health Organization (WHO) working group has proposed a new classification to improve the homogeneity of the categories in the widely-used French-American-British classification in term of pathophysiology and prognosis. This new classification permits the consideration of clinical, etiologic, and cytogenetic data, including complex and heterogeneous cytogenetic abnormalities. The International Scoring System for evaluating Prognosis (IPSS), which has proven highly useful for this purpose, is a risk-based classification system for MDS, based on these cytogenetic abnormalities. It can be used to help guide therapeutic choices. For MDS with low IPSS scores (low-risk), appropriate treatment includes best supportive care, hematopoietic growth factors, and immunomodulatory drugs. For MDS with high IPSS scores, chemotherapy is appropriate at low or intense doses. For younger patients, allogeneic bone marrow transplantation, the only truly curative treatment, may be considered. Better understanding of the pathophysiology of MDS, including the role of oncogenes and various cytokines, is required to develop new treatments and treatment targets. The new classification and prognosis scoring system should lead to new therapeutic strategies that may modify the overall prognosis and quality of life of patients with MDS.

Caspase-8 prevents sustained activation of NF-kappaB in monocytes undergoing macrophagic differentiation

Caspases have demonstrated several nonapoptotic functions including a role in the differentiation of specific cell types. Here, we show that caspase-8 is the upstream enzyme in the proteolytic caspase cascade whose activation is required for the differentiation of peripheral-blood monocytes into macrophages. On macrophage colony-stimulating factor (M-CSF) exposure, caspase-8 associates with the adaptor protein Fas-associated death domain (FADD), the serine/threonine kinase receptor-interacting protein 1 (RIP1) and the long isoform of FLICE-inhibitory protein FLIP. Overexpression of FADD accelerates the differentiation process that does not involve any death receptor. Active caspase-8 cleaves RIP1, which prevents sustained NF-kappaB activation, and activates downstream caspases. Together these data identify a role for caspase-8 in monocytes undergoing macrophagic differentiation, that is, the enzyme activated in an atypical complex down-regulates NF-kappaB activity through RIP1 cleavage.

Mitochondria in hematopoiesis and hematological diseases

Mitochondria are involved in hematopoietic cell homeostasis through multiple ways such as oxidative phosphorylation, various metabolic processes and the release of cytochrome c in the cytosol to trigger caspase activation and cell death. In erythroid cells, the mitochondrial steps in heme synthesis, iron (Fe) metabolism and Fe-sulfur (Fe-S) cluster biogenesis are of particular importance. Mutations in the specific delta-aminolevulinic acid synthase (ALAS) 2 isoform that catalyses the first and rate-limiting step in heme synthesis pathway in the mitochondrial matrix, lead to ineffective erythropoiesis that characterizes X-linked sideroblastic anemia (XLSA), the most common inherited sideroblastic anemia. Mutations in the adenosine triphosphate-binding cassette protein ABCB7, identified in XLSA with ataxia (XLSA-A), disrupt the maturation of cytosolic (Fe-S) clusters, leading to mitochondrial Fe accumulation. In addition, large deletions in mitochondrial DNA, whose integrity depends on a specific DNA polymerase, are the hallmark of Pearson's syndrome, a rare congenital disorder with sideroblastic anemia. In acquired myelodysplastic syndromes at early stage, exacerbation of physiological pathways involving caspases and the mitochondria in erythroid differentiation leads to abnormal activation of a mitochondria-mediated apoptotic cell death pathway. In contrast, oncogenesis-associated changes at the mitochondrial level can alter the apoptotic response of transformed hematopoietic cells to chemotherapeutic agents. Recent findings in mitochondria metabolism and functions open new perspectives in treating hematopoietic cell diseases, for example various compounds currently developed to trigger tumor cell death by directly targeting the mitochondria could prove efficient as either cytotoxic drugs or chemosensitizing agents in treating hematological malignancies.

Inhibition of overactivated p38 MAPK can restore hematopoiesis in myelodysplastic syndrome progenitors

The myelodysplastic syndromes (MDSs) are collections of heterogeneous hematologic diseases characterized by refractory cytopenias as a result of ineffective hematopoiesis. Development of effective treatments has been impeded by limited insights into any unifying pathogenic pathways. We provide evidence that the p38 MAP kinase is constitutively activated or phosphorylated in MDS bone marrows. Such activation is uniformly observed in varied morphologic subtypes of low-risk MDS and correlates with enhanced apoptosis observed in MDS hematopoietic progenitors. Most importantly, pharmacologic inhibition of p38alpha by a novel small molecule inhibitor, SCIO-469, decreases apoptosis in MDS CD34+ progenitors and leads to dose-dependant increases in erythroid and myeloid colony formation. Down-regulation of the dominant p38alpha isoform by siRNA also leads to enhancement of hematopoiesis in MDS bone marrow progenitors in vitro. These data implicate p38 MAPK in the pathobiology of ineffective hematopoiesis in lowrisk MDS and provide a strong rationale for clinical investigation of SCIO-469 in MDS.

Impact of growth factors in the regulation of apoptosis in low-risk myelodysplastic syndromes

Increased apoptosis of hematopoietic progenitors is a hallmark of myelodysplastic syndromes (MDS) and results in ineffective hematopoiesis. Erythroid apoptosis is thought to be the main mechanism underlying the severe anemia observed in the low-risk subgroups, refractory anemia (RA) and RA with ringed sideroblasts (RARS). Treatment with erythropoietin (Epo) alone or in combination with granulocyte colony-stimulating factor (G-CSF) may significantly improve anemia and reduce bone marrow apoptosis. A synergistic effect between Epo and G-CSF has been observed in the clinic, in particular in RARS. However, the molecular mechanisms beyond the anti-apoptotic effect of these growth factors have not been fully understood. This paper outlines the potential mechanisms underlying the augmented apoptosis during the erythroid differentiation in low-risk MDS as well as the anti-apoptotic effect of the growth factors.

Suppression of Fas-FasL coexpression by erythropoietin mediates erythroblast expansion during the erythropoietic stress response in vivo

Erythropoietin (Epo) is the principal regulator of the erythropoietic response to hypoxic stress, through its receptor, EpoR. The EpoR signals mediating the stress response are largely unknown, and the spectrum of progenitors that are stress responsive is not fully defined. Here, we used flow cytometry to identify stress-responsive Ter119+CD71highFSChigh early erythroblast subsets in vivo. In the mouse spleen, an erythropoietic reserve organ, early erythroblasts were present at lower frequencies and were undergoing higher rates of apoptosis than equivalent cells in bone marrow. A high proportion of splenic early erythroblasts coexpressed the death receptor Fas, and its ligand, FasL. Fas-positive early erythroblasts were significantly more likely to coexpress annexin V than equivalent, Fas-negative cells, suggesting that Fas mediates early erythroblast apoptosis in vivo. We examined several mouse models of erythropoietic stress, including erythrocytosis and beta-thalassemia. We found a dramatic increase in the frequency of splenic early erythroblasts that correlated with down-regulation of Fas and FasL from their cell surface. Further, a single injection of Epo specifically suppressed early erythroblast Fas and FasL mRNA and cell-surface expression. Therefore, Fas and FasL are negative regulators of erythropoiesis. Epo-mediated suppression of erythroblast Fas and FasL is a novel stress response pathway that facilitates erythroblast expansion in vivo.

The myelodysplastic syndromes: diagnosis and treatment

The myelodysplastic syndromes (MDSs) are common, acquired, clinically challenging hematologic conditions that are characterized by bone marrow failure and a risk of progression to acute leukemia. These disorders can arise de novo, especially in elderly patients or, less often, as a consequence of prior chemotherapy or radiotherapy for an unrelated disease. The MDS classification systems were revised recently and updated. These refined classification and prognostic schemes help stratify patients by their risk of leukemia progression and death; this knowledge can help clinicians select appropriate therapy. Although many treatments for MDS have been proposed and evaluated, at present, only hematopoietic stem cell transplantation offers any real hope for cure, and no available therapy beyond general supportive care offers benefit to more than a minority of patients. However, recent clinical trials enrolling patients with MDS have reported encouraging results with use of newer drugs, including lenalidomide, decitabine, and darbepoetin alfa. Other exciting treatment regimens are being tested. Here, we present a contemporary, practical clinical approach to the diagnosis and risk-stratified treatment of MDS. We review when to suspect MDS, detail how to evaluate patients who may have a form of the condition, explain key features of treatments that are currently available in the United States, and summarize a general, common-sense therapeutic approach to patients with MDS.

On the TRAIL of a new therapy for leukemia

The cytokine TRAIL (tumor necrosis factor alpha-related apoptosis-inducing ligand) as well as agonistic antibodies that bind to the TRAIL receptors, death receptor 4 (DR4) and DR5, are undergoing preclinical and early clinical evaluation as potential therapeutic agents for a variety of hematological and nonhematological malignancies. Here, we briefly review the normal biological function of TRAIL, the mechanism of cytotoxicity of TRAIL receptor ligands, and their effects on normal myeloid progenitors, myelodysplastic marrow and leukemic cells, including acute myelogenous leukemia (AML) and chronic lymphocytic leukemia (CLL), in vitro. Recent observations suggesting that DR4 is the predominant receptor for the cytotoxic effects of TRAIL in CLL and that histone deacetylase inhibitors synergize with TRAIL in CLL in vitro are described and discussed. Collectively, the reviewed studies not only illustrate the potential therapeutic usefulness of TRAIL and the agonistic antibodies, but also highlight the need for additional preclinical evaluation of these agents.