Myelodysplastic syndromes (MDS) are a group of disorders characterized by variable cytopenias and ineffective hematopoiesis. Hematopoietic stem cells (HSCs) and myeloid progenitors in MDS have not been extensively characterized. We transplanted purified human HSCs from MDS samples into immunodeficient mice and show that HSCs are the disease-initiating cells in MDS. We identify a recurrent loss of granulocyte-macrophage progenitors (GMPs) in the bone marrow of low risk MDS patients that can distinguish low risk MDS from clinical mimics, thus providing a simple diagnostic tool. The loss of GMPs is likely due to increased apoptosis and increased phagocytosis, the latter due to the up-regulation of cell surface calreticulin, a prophagocytic marker. Blocking calreticulin on low risk MDS myeloid progenitors rescues them from phagocytosis in vitro. However, in the high-risk refractory anemia with excess blasts (RAEB) stages of MDS, the GMP population is increased in frequency compared with normal, and myeloid progenitors evade phagocytosis due to up-regulation of CD47, an antiphagocytic marker. Blocking CD47 leads to the selective phagocytosis of this population. We propose that MDS HSCs compete with normal HSCs in the patients by increasing their frequency at the expense of normal hematopoiesis, that the loss of MDS myeloid progenitors by programmed cell death and programmed cell removal are, in part, responsible for the cytopenias, and that up-regulation of the "don't eat me" signal CD47 on MDS myeloid progenitors is an important transition step leading from low risk MDS to high risk MDS and, possibly, to acute myeloid leukemia.myelodysplasia | blood disorders | aging | monosomy 7 | cancer stem cell T he World Health Organization (WHO) defines myelodysplastic syndromes (MDS) as a heterogeneous group of related clonal diseases characterized by variable cytopenias due to ineffective hematopoiesis and increased risk of progression to acute myeloid leukemia (AML) (1-4). To date, functional and diagnostic evaluations of immature hematopoietic cells in MDS have predominantly relied on unpurified or partially purified bone marrow cells (most frequently CD34 + cells; reviewed in ref. 5), which have limited power to identify cell lineage specific alterations. We have taken advantage of the purification of highly enriched HSCs and committed myeloid progenitors (6-9) via fluorescence activated cell sorting (FACS) to characterize hematopoietic subsets in primary MDS patient bone marrow samples with the goal of increasing the understanding of MDS pathogenesis.Recent fluorescence in situ hybridization (FISH) and gene expression data from purified HSCs from 5q-MDS patients have suggested an HSC origin for MDS (10-13). HSCs from MDS patients (MDS HSCs) are relatively resistant to lenalidomide and decitabine treatment, because even patients with cytogenetic remission, as determined by FISH on peripheral blood cells, can maintain a significant MDS HSC burden (12, 13). Nevertheless, HSCs from MDS patients have not been shown to initiat...

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The role of heme oxygenase (HO)-1 was evaluated in the oxygen-resistant hamster fibroblast cell line, O 2 R95, which moderately overexpress HO when compared with the parental cell line, HA-1. To suppress HO-1 expression, O 2 R95 were transfected with HO-1 antisense oligonucleotide or treated with tin-mesoporphyrin (SnMP). To increase HO-1 expression, cells were transfected with HO-1 cDNA in a pRC/cytomegalovirus (CMV) vector. All cells were challenged with a 48-h exposure to 95% O 2 (hyperoxia). When HO activity was suppressed, O 2 R95 cells had significantly decreased cell viability, increased susceptibility to lipid peroxidation, and increased protein oxidation in hyperoxia. In contrast, further overexpression of HO-1 did not improve resistance to oxygen toxicity. Antisense-transfected cells and SnMP-treated cells with lowered HO activity showed increased levels of cellular heme compared with controls. In the HO-1 cDNA-transfected O 2 R95 cells, cellular heme was lowered compared with controls; however, cellular redox active iron levels were increased. We conclude that HO mediates cytoprotection to oxygen toxicity within a narrow range of expression. We speculate that this protective effect may be mediated in part through increased metabolism of the pro-oxidant heme but that higher levels of HO activity obviate protection by increased redox active iron release.Heme oxygenase (HO-1), 1 the rate-limiting enzyme in the conversion of heme to bilirubin, is known to be induced by various oxidant stresses. However, it is not clear whether HO serves in protection against hyperoxia and, if so, by which mechanisms. HO-1 antisense transfection experiments have shown that higher HO-1 protein levels were associated with protection against UVA radiation (1). Furthermore, transfection of coronary vessel endothelial cells with an overexpression vector containing HO-1 cDNA resulted in resistance against hemoglobin-induced injury (2). We have previously shown that HA-1 hamster fibroblasts made stably resistant to oxygen toxicity (O 2 R95) had 1.8-fold higher HO activity (3), suggesting that moderate increases in HO activity may be beneficial in resistance to oxygen toxicity. However, O 2 R95 cells have increases in other antioxidants that may also contribute to their resistance to oxygen toxicity (4), and no direct evidence currently exists linking HO to resistance to oxygen toxicity.Investigators have hypothesized that HO may serve a role in protection against oxidative injury by forming the antioxidant molecules biliverdin and bilirubin (5, 6). Additionally, induction of ferritin with enhanced HO activity has been observed (1). This could lead to sequestration of redox active iron, thereby conferring protection against oxidative stress (7). Another possible antioxidant mechanism of HO could involve the destruction of heme itself. Heme and hemoproteins have been shown by several investigators to be instrumental in exacerbating oxidative injury (8, 9). This has lead to the hypothesis that reduction of the cellular heme pool by ...