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1.  Mouse Hematopoietic Stem Cells, Unlike Human and Mouse Embryonic Stem Cells, Exhibit Checkpoint–Apoptosis Coupling 
Stem Cells and Development  2008;17(5):1017-1020.
Previously, we reported that the spindle assembly checkpoint (SAC), which is coupled in somatic cells, is uncoupled from apoptosis-initiation in mouse and human embryonic stem cells (ESCs). This condition allows ESCs to tolerate and proliferate as polyploidy/aneuploid cells. Proper function of the SAC is vital to prevent polyploidy/aneuploidy during ex vivo hematopoietic stem cell (HSC) expansion. Here we address, for the first time, whether HSCs are more like ESCs or somatic cells with respect to SAC–apoptosis coupling. Using multi-parametric permeablized cell flow-cytometric analysis to identify and analyze the mouse sca 1+/c-kit+/lin− (LSK) population, we found the mitotic spindle checkpoint to be functional in primary murine LSK cells, a population enriched in primitive hematopoietic stem/progenitor cells, after prolonged activation of the SAC by microtubule-depolymerizing agents such as nocodazole. HSCs can efficiently initiate apoptosis after activation of the SAC in LSK cells as indicated by increased hypodiploidy and increased levels of activated caspase 3, suggesting that HSCs behave more like somatic cells instead of ESCs with respect to this important cell cycle checkpoint. We conclude that mouse HSCs are not subject to the same kinds of chromosomal instability as are ESCs, knowledge that might aid in optimizing in vitro culture and expansion of human bone marrow or cord blood HSC for clinical applications.
PMCID: PMC2818989  PMID: 18788999
2.  Mouse Hematopoietic Stem Cells, Unlike Human and Mouse Embryonic Stem Cells, Exhibit Checkpoint-Apoptosis Coupling 
Stem cells and development  2008;17(5):1017.
Previously we reported that the spindle assembly checkpoint (SAC), which is coupled in somatic cells, is uncoupled from apoptosis-initiation in mouse and human ESCs. This condition allows ESCs to tolerate and proliferate as polyploidy/aneuploid cells. Proper function of the SAC is vital to prevent polyploidy/aneuploidy during ex-vivo HSC expansion. Here we address, for the first time, whether hematopoietic stem cells are more like ESC or somatic cells with respect to SAC-apoptosis coupling. Using multiparametric permeablized cell flow-cytometric analysis to identify and analyze the mouse sca 1+/c-kit+/lin− (LSK) population, we found the mitotic spindle checkpoint to be functional in primary murine LSK cells, a population enriched in primitive hematopoietic stem/progenitor cells, after prolonged activation of the SAC by microtubule-depolymerizing agents such as nocodazole. HSCs can efficiently initiate apoptosis after activation of the SAC in LSK cells as indicated by increased hypopdiploidy and increased levels of activated caspase-3 suggesting that HSCs behave more like somatic cells instead of ESCs with respect to this important cell cycle checkpoint. We conclude that mouse HSCs are not subject to the same kinds of chromosomal-instability as are ESCs, knowledge that might aid in optimizing in-vitro culture and expansion of human bone marrow or cord blood HSC for clinical applications.
PMCID: PMC2818989  PMID: 18788999
Hematopoietic stem cell; cell cycle; cell cycle checkpoint; flow cytometry
3.  Isoform-Specific Potentiation of Stem and Progenitor Cell Engraftment by AML1/RUNX1  
PLoS Medicine  2007;4(5):e172.
AML1/RUNX1 is the most frequently mutated gene in leukaemia and is central to the normal biology of hematopoietic stem and progenitor cells. However, the role of different AML1 isoforms within these primitive compartments is unclear. Here we investigate whether altering relative expression of AML1 isoforms impacts the balance between cell self-renewal and differentiation in vitro and in vivo.
Methods and Findings
The human AML1a isoform encodes a truncated molecule with DNA-binding but no transactivation capacity. We used a retrovirus-based approach to transduce AML1a into primitive haematopoietic cells isolated from the mouse. We observed that enforced AML1a expression increased the competitive engraftment potential of murine long-term reconstituting stem cells with the proportion of AML1a-expressing cells increasing over time in both primary and secondary recipients. Furthermore, AML1a expression dramatically increased primitive and committed progenitor activity in engrafted animals as assessed by long-term culture, cobblestone formation, and colony assays. In contrast, expression of the full-length isoform AML1b abrogated engraftment potential. In vitro, AML1b promoted differentiation while AML1a promoted proliferation of progenitors capable of short-term lymphomyeloid engraftment. Consistent with these findings, the relative abundance of AML1a was highest in the primitive stem/progenitor compartment of human cord blood, and forced expression of AML1a in these cells enhanced maintenance of primitive potential both in vitro and in vivo.
These data demonstrate that the “a” isoform of AML1 has the capacity to potentiate stem and progenitor cell engraftment, both of which are required for successful clinical transplantation. This activity is consistent with its expression pattern in both normal and leukaemic cells. Manipulating the balance of AML1 isoform expression may offer novel therapeutic strategies, exploitable in the contexts of leukaemia and also in cord blood transplantation in adults, in whom stem and progenitor cell numbers are often limiting.
The truncated "a" isoform of AML1 is shown to have the capacity to potentiate stem and progenitor cell engraftment, both of which are required for successful clinical transplantation.
Editors' Summary
Blood contains red blood cells (which carry oxygen round the body), platelets (which help the blood to clot), and white blood cells (which fight off infections). All these cells, which are regularly replaced, are derived from hematopoietic stem cells, blood-forming cells present in the bone marrow. Like all stem cells, hematopoietic stem cells self-renew (reproduce themselves) and produce committed progenitor cells, which develop into mature blood cells in a process called hematopoiesis. Many proteins control hematopoiesis, some of which are called transcription factors; these factors bind to DNA through their DNA-binding domain and then control the expression of genes (that is, how DNA is turned into proteins) through particular parts of the protein (their transcription regulatory domains). An important hematopoietic transcription factor is AML1—a protein first identified because of its involvement in acute myelogenous leukemia (AML, a form of blood cancer). Mutations (changes) in the AML1 gene are now known to be present in other types of leukemia, which are often characterized by overproliferation of immature blood cells.
Why Was This Study Done?
Because of AML1′s crucial role in hematopoiesis, knowing more about which genes it regulates and how its activity is regulated could provide clues to treating leukemia and to improving hematopoietic cell transplantation. Many cancer treatments destroy hematopoietic stem cells, leaving patients vulnerable to infection. Transplants of bone marrow or cord blood (the cord that links mother and baby during pregnancy contains peripheral blood stem cells) can replace the missing cells, but cord blood in particular often contains insufficient stem cells for successful transplantation. It would be useful, therefore, to expand the stem cell content of these tissues before transplantation. In this study, the researchers investigated the effect of AML1 on self-renewal and differentiation of hematopoietic stem and progenitor cells in the laboratory (in vitro) and in animals (in vivo). In particular, they have asked how two isoforms (closely related versions) of AML1 affect the ability of these cells to grow and differentiate (engraft) in mice after transplantation.
What Did the Researchers Do and Find?
The researchers artificially expressed AML1a and AML1b (both isoforms contain a DNA binding domain, but only AML1b has transcription regulatory domains) in mouse hematopoietic stem and progenitor cells and then tested the cells' ability to engraft in mice. AML1a-expressing cells engrafted better than unaltered cells and outgrew unaltered cells when transplanted as a mixture. AML1b-expressing cells, however, did not engraft. In vitro, AML1a-expressing cells grew more than AML1b-expressing cells, whereas differentiation was promoted in AML1b-expressing cells. To investigate whether the isoforms have the same effects in human cells, the researchers measured the amount of AML1a and AML1b mRNA (the template for protein production) made by progenitor cells in human cord blood. Although AML1b (together with AML1c, an isoform with similar characteristics) mRNA predominated in all the progenitor cell types, the relative abundance of AML1a was greatest in the stem and progenitor cells. Furthermore, forced expression of AML1a in these cells improved their ability to divide in vitro and to engraft in mice.
What Do These Findings Mean?
These findings indicate that AML1a expression increases the self-renewal capacity of hematopoietic stem and progenitor cells and consequently improves their ability to engraft in mice, whereas AML1b expression encourages the differentiation of these cell types. These activities are consistent with the expression patterns of the two isoforms in normal hematopoietic cells and in leukemic cells—the mutated AML made by many leukemic cells resembles AML1a. Because the AML1 isoforms were expressed at higher than normal levels in these experiments, the physiological relevance of these findings needs to be confirmed by showing that normal levels of AML1a and AML1b produce similar results. Nevertheless, these results suggest that manipulating the balance of AML1 isoforms made by hematopoietic cells might be useful clinically. In leukemia, a shift toward AML1b expression might slow the proliferation of leukemic cells and encourage their differentiation. Conversely, in cord blood transplantation, a shift toward AML1a expression might improve patient outcomes by expanding the stem and progenitor cell populations.
Additional Information.
Please access these Web sites via the online version of this summary at
Wikipedia has pages on hematopoiesis and hematopoietic stem cells (note: Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)
The US National Cancer Institute has a fact sheet on bone marrow and peripheral blood stem cell transplantation (in English and Spanish) and information for patients and professionals on leukemia (in English)
The American Society of Hematology provides patient information about blood diseases, including information on bone marrow and stem cell transplantation
PMCID: PMC1868041  PMID: 17503961
4.  Radiation Rescue: Mesenchymal Stromal Cells Protect from Lethal Irradiation 
PLoS ONE  2011;6(1):e14486.
Successful treatment of acute radiation syndromes relies on immediate supportive care. In patients with limited hematopoietic recovery potential, hematopoietic stem cell (HSC) transplantation is the only curative treatment option. Because of time consuming donor search and uncertain outcome we propose MSC treatment as an alternative treatment for severely radiation-affected individuals.
Methods and Findings
Mouse mesenchymal stromal cells (mMSCs) were expanded from bone marrow, retrovirally labeled with eGFP (bulk cultures) and cloned. Bulk and five selected clonal mMSCs populations were characterized in vitro for their multilineage differentiation potential and phenotype showing no contamination with hematopoietic cells. Lethally irradiated recipients were i.v. transplanted with bulk or clonal mMSCs. We found a long-term survival of recipients with fast hematopoietic recovery after the transplantation of MSCs exclusively without support by HSCs. Quantitative PCR based chimerism analysis detected eGFP-positive donor cells in peripheral blood immediately after injection and in lungs within 24 hours. However, no donor cells in any investigated tissue remained long-term. Despite the rapidly disappearing donor cells, microarray and quantitative RT-PCR gene expression analysis in the bone marrow of MSC-transplanted animals displayed enhanced regenerative features characterized by (i) decreased proinflammatory, ECM formation and adhesion properties and (ii) boosted anti-inflammation, detoxification, cell cycle and anti-oxidative stress control as compared to HSC-transplanted animals.
Our data revealed that systemically administered MSCs provoke a protective mechanism counteracting the inflammatory events and also supporting detoxification and stress management after radiation exposure. Further our results suggest that MSCs, their release of trophic factors and their HSC-niche modulating activity rescue endogenous hematopoiesis thereby serving as fast and effective first-line treatment to combat radiation-induced hematopoietic failure.
PMCID: PMC3016319  PMID: 21245929
5.  Suppression of Cytochrome P450 Reductase Enhances Long-Term Hematopoietic Stem Cell Repopulation Efficiency in Mice 
PLoS ONE  2013;8(7):e69913.
Bone marrow microenvironment (niche) plays essential roles in the fate of hematopoietic stem cells (HSCs). Intracellular and extracellular redox metabolic microenvironment is one of the critical factors for the maintenance of the niche. Cytochrome P450 reductase (CPR) is an obligate electron donor to all microsomal cytochrome P450 enzymes (P450 or CYP), and contributes to the redox metabolic process. However, its role in maintaining HSCs is unknown.
To examine the effects of low CPR expression on HSCs function using a mouse model of globally suppressed Cpr gene expression (Cpr Low, CL mice).
Hematopoietic cell subpopulations in bone marrow (BM) and peripheral blood (PB) from WT and CL mice were examined for their repopulation and differentiation ability upon BM competitive transplantation and enriched HSC (LKS+) transplantation. Effects of low CPR expression on hematopoiesis were examined by transplanting normal BM cells into CL recipients. Reactive oxygen species (ROS), cell cycle, and apoptosis in CL mice were analyzed by flow cytometry for DCF-DA fluorescence intensity, Ki67 protein, and Annexin-V, respectively.
The levels of ROS in BM cells, HPCs and HSCs were comparable between CL and WT mice. In comparison to WT mice, the number of LT-HSCs or ST-HSCs was lower in CL mice while CMPs, GMPs and MEPs in CL mice were higher than that in WT control. Competitive transplantation assay revealed enhanced repopulation capacity of HSCs with low CPR expression, but no difference in differentiation potential upon in vitro experiments. Furthermore, lymphoid differentiation of donor cells decreased while their myeloid differentiation increased under CL microenvironment although the overall level of donor hematopoietic repopulation was not significantly altered.
Our studies demonstrate that suppressing CPR expression enhances the repopulation efficiency of HSCs and a low CPR expression microenvironment favors the differentiation of myeloid over lymphoid lineage cells.
PMCID: PMC3724780  PMID: 23922855
6.  Using quantitative real-time PCR to determine donor cell engraftment in a competitive murine bone marrow transplantation model 
Determining donor cell engraftment presents a challenge in mouse bone marrow transplant models that lack well-defined phenotypical markers. We described a methodology to quantify male donor cell engraftment in female transplant recipient mice. This method can be used in all mouse strains for the study of HSC functions.
Murine bone marrow transplantation models provide an important tool in measuring hematopoietic stem cell (HSC) functions and determining genes/molecules that regulate HSCs. In these transplant model systems, the function of HSCs is determined by the ability of these cells to engraft and reconstitute lethally irradiated recipient mice. Commonly, the donor cell contribution/engraftment is measured by antibodies to donor- specific cell surface proteins using flow cytometry. However, this method heavily depends on the specificity and the ability of the cell surface marker to differentiate donor-derived cells from recipient-originated cells, which may not be available for all mouse strains. Considering the various backgrounds of genetically modified mouse strains in the market, this cell surface/flow cytometry-based method has significant limitations especially in mouse strains that lack well-defined surface markers to separate donor cells from congenic recipient cells. Here, we reported a PCR-based technique to determine donor cell engraftment/contribution in transplant recipient mice. We transplanted male donor bone marrow HSCs to lethally irradiated congenic female mice. Peripheral blood samples were collected at different time points post transplantation. Bone marrow samples were obtained at the end of the experiments. Genomic DNA was isolated and the Y chromosome specific gene, Zfy1, was amplified using quantitative Real time PCR. The engraftment of male donor-derived cells in the female recipient mice was calculated against standard curve with known percentage of male vs. female DNAs. Bcl2 was used as a reference gene to normalize the total DNA amount. Our data suggested that this approach reliably determines donor cell engraftment and provides a useful, yet simple method in measuring hematopoietic cell reconstitution in murine bone marrow transplantation models. Our method can be routinely performed in most laboratories because no costly equipment such as flow cytometry is required.
PMCID: PMC3622101  PMID: 23525072
Bone marrow transplantation; Y chromosome specific gene; murine model; chimerism
7.  Endothelial cells mitigate DNA damage and promote the regeneration of hematopoietic stem cells after radiation injury 
Stem cell research  2013;11(3):1013-1021.
Endothelial cells (ECs) are an essential component of the hematopoietic microenvironment, which maintains and regulates hematopoietic stem cells (HSCs). Although ECs can support the regeneration of otherwise lethally-irradiated HSCs, the mechanisms are not well understood. To further understand this phenomenon, we studied HSC regeneration from irradiated bone marrow using co-culture with human aortic endothelial cells (HAECs). Co-culture with HAECs induced a 24-fold expansion of long-term HSCs (CD150+, lineagelo, Sca-1+, c-Kit+; CD150+LSK cells) in vitro. These cells gave rise to functional hematopoietic stem and progenitor cells (HSPCs) with colony-forming activity, multilineage reconstitution and serial transplantation potential. Furthermore, HAECs significantly reduced DNA damage in irradiated LSK cells within 24 hours. Remarkably, we were able to delay the exposure of irradiated bone marrow to the regenerative, HAEC-derived signals for up to 48 hours and still rescue functional HSCs. G-CSF is the gold standard for promoting hematopoietic regeneration in vivo. However, when compared to HAECs, in vitro G-CSF treatment promoted lineage differentiation and regenerated 5-fold fewer CD150+LSK cells. Together, our results show that HAECs are powerful, direct mitigators of HSC injury and DNA damage. Identification of the HAEC-derived factors that rescue HSCs may lead to improved therapies for hematopoietic regeneration after radiation injury.
PMCID: PMC4066846  PMID: 23939266
8.  Cav-1 deletion impaired hematopoietic stem cell function 
Bai, L | Shi, G | Zhang, L | Guan, F | Ma, Y | Li, Q | Cong, Y-S | Zhang, L
Cell Death & Disease  2014;5(3):e1140-.
A tightly controlled balance between hematopoietic stem and progenitor cell compartments is required to maintain normal blood cell homeostasis throughout life, and this balance is regulated by intrinsic and extrinsic cellular factors. Cav-1 is a 22-kDa protein that is located in plasma membrane invaginations and is implicated in regulating neural stem cell and embryonic stem cell proliferation. However, the role of Cav-1 in hematopoietic stem cell (HSC) function is largely unknown. In this study, we used Cav-1−/− mice to investigate the role of Cav-1 in HSCs function during aging. The results showed that Cav-1−/− mice displayed a decreased percentage of B cells and an increased percentage of M cells in the bone marrow and peripheral blood, and these changes were due to an increased number of HSCs. FACS analysis showed that the numbers of Lin−Sca1+c-kit+ cells (LSKs), long-term HSCs (LT-HSCs), short-term HSCs and multipotent progenitors were increased in Cav-1−/− mice compared with Cav-1+/+ mice, and this increase became more pronounced with aging. An in vitro clonogenic assay showed that LT-HSCs from Cav-1−/− mice had reduced ability to self-renew. Consistently, an in vivo competitive transplantation assay showed that Cav-1−/− mice failed to reconstitute hematopoiesis. Moreover, a Cav-1 deletion disrupted the quiescence of LSKs and promoted cell cycle progression through G2/M phase. In addition, we found that Cav-1 deletion impaired the ability of HSCs to differentiate into mature blood cells. Taken together, these data suggest that Cav-1-deficient cells impaired HSCs quiescence and induced environmental alterations, which limited HSCs self-renewal and function.
PMCID: PMC3973224  PMID: 24675458
Cav-1; HSC; quiescence
9.  Deficiency of GRP94 in the Hematopoietic System Alters Proliferation Regulators in Hematopoietic Stem Cells 
Stem Cells and Development  2013;22(23):3062-3073.
We have previously reported that acute inducible knockout of the endoplasmic reticulum chaperone GRP94 led to an expansion of the hematopoietic stem and progenitor cell pool. Here, we investigated the effectors and mechanisms for this phenomenon. We observed an increase in AKT activation in freshly isolated GRP94-null HSC-enriched Lin− Sca-1+ c-Kit+ (LSK) cells, corresponding with higher production of PI(3,4,5)P3, indicative of PI3K activation. Treatment of GRP94-null LSK cells with the AKT inhibitor MK2206 compromised cell expansion, suggesting a causal relationship between elevated AKT activation and increased proliferation in GRP94-null HSCs. Microarray analysis demonstrated a 97% reduction in the expression of the hematopoietic cell cycle regulator Ms4a3 in the GRP94-null LSK cells, and real-time quantitative PCR confirmed this down-regulation in the LSK cells but not in the total bone marrow (BM). A further examination comparing freshly isolated BM LSK cells with spleen LSK cells, as well as BM LSK cells cultured in vitro, revealed specific down-regulation of Ms4a3 in freshly isolated BM GRP94-null LSK cells. On examining cell surface proteins that are known to regulate stem cell proliferation, we observed a reduced expression of cell surface connexin 32 (Cx32) plaques in GRP94-null LSK cells. However, suppression of Cx32 hemichannel activity in wild-type LSK cells through mimetic peptides did not lead to increased LSK cell proliferation in vitro. Two other important cell surface proteins that mediate HSC-niche interactions, specifically Tie2 and CXCR4, were not impaired by Grp94 deletion. Collectively, our study uncovers novel and unique roles of GRP94 in regulating HSC proliferation.
PMCID: PMC3856911  PMID: 23859598
10.  The Cytosolic Protein G0S2 Maintains Quiescence in Hematopoietic Stem Cells 
PLoS ONE  2012;7(5):e38280.
Bone marrow hematopoietic stem cells (HSCs) balance proliferation and differentiation by integrating complex transcriptional and post-translational mechanisms regulated by cell intrinsic and extrinsic factors. We found that transcripts of G0/G1 switch gene 2 (G0S2) are enriched in lineage− Sca-1+ c-kit+ (LSK) CD150+ CD48− CD41− cells, a population highly enriched for quiescent HSCs, whereas G0S2 expression is suppressed in dividing LSK CD150+ CD48− cells. Gain-of-function analyses using retroviral expression vectors in bone marrow cells showed that G0S2 localizes to the mitochondria, endoplasmic reticulum, and early endosomes in hematopoietic cells. Co-transplantation of bone marrow cells transduced with the control or G0S2 retrovirus led to increased chimerism of G0S2-overexpressing cells in femurs, although their contribution to the blood was reduced. This finding was correlated with increased quiescence in G0S2-overexpressing HSCs (LSK CD150+ CD48−) and progenitor cells (LS−K). Conversely, silencing of endogenous G0S2 expression in bone marrow cells increased blood chimerism upon transplantation and promoted HSC cell division, supporting an inhibitory role for G0S2 in HSC proliferation. A proteomic study revealed that the hydrophobic domain of G0S2 interacts with a domain of nucleolin that is rich in arginine-glycine-glycine repeats, which results in the retention of nucleolin in the cytosol. We showed that this cytosolic retention of nucleolin occurs in resting, but not proliferating, wild-type LSK CD150+ CD48− cells. Collectively, we propose a novel model of HSC quiescence in which elevated G0S2 expression can sequester nucleolin in the cytosol, precluding its pro-proliferation functions in the nucleolus.
PMCID: PMC3365016  PMID: 22693613
11.  CD81 Is Essential for the Re-entry of Hematopoietic Stem Cells to Quiescence following Stress-Induced Proliferation Via Deactivation of the Akt Pathway 
PLoS Biology  2011;9(9):e1001148.
A protein that is thought to orchestrate the distribution of other signaling molecules on the cell membrane, CD81, is critical to maintaining the functional integrity of hematopoietic stem cells during their regeneration.
The regulatory mechanisms governing the cell cycle progression of hematopoietic stem cells (HSCs) are well characterized, but those responsible for the return of proliferating HSCs to a quiescent state remain largely unknown. Here, we present evidence that CD81, a tetraspanin molecule acutely responsive to proliferative stress, is essential for the maintenance of long-term repopulating HSCs. Cd81−/− HSCs showed a marked engraftment defect when transplanted into secondary recipient mice and a significantly delayed return to quiescence when stimulated to proliferate with 5-fluorouracil (5FU). In addition, we found that CD81 proteins form a polarized patch when HSCs are returning to quiescence. Thus, we propose that the spatial distribution of CD81 during the HSC recovery phase drives proliferative HSC to quiescence, and is important to preserve the self-renewal properties. Here, we show that lack of CD81 leads to loss of HSC self-renewal, and the clustering of CD81 on HSC membrane results in deactivation of Akt, which subsequently leads to nuclear translocation of FoxO1a. Thus, CD81 functions as part of a previously undefined mechanism that prohibits excessive proliferation of HSCs exposed to environmental stress.
Author Summary
Hematopoietic stem cells (HSCs) remain dormant in the bone marrow until needed to replenish the hematopoietic system, at which point they are stimulated to proliferate extensively, undergoing both regeneration (self-renewal) and differentiation. Self-renewal is key to maintaining an adequate HSC reserve, and return to dormancy after such stimulation is critical, yet still poorly understood. In this study, we report that CD81, a transmembrane organizing protein, is a novel regulator involved in HSC self-renewal. Transplanting HSCs into mice that are lethally irradiated to remove their native HSCs stimulates the transplanted HSCs to proliferate to replenish the hematopoietic system, allowing us to examine whether and how HSCs return to quiescence. HSCs lacking CD81 take longer to return to quiescence after such stimulation, resulting in reduced stem cell function. Conversely, forced CD81 membrane clustering, using an antibody, promotes early return of proliferating stem cells to quiescence and nuclear localization of FoxO1a, a key protein that mediates the cell cycle arrest. CD81 clustering also constrains Akt activity, which orchestrates multiple pathways such as cell proliferation and responses to reactive oxygen species. Treatment of Cd81-deficient HSCs with an Akt inhibitor, perifosine, which bypasses the requirement for CD81 in this process, rescues the delay defect of Cd81-deficient HSCs. Together, our data demonstrate that CD81 is critical to maintaining the functional integrity of HSCs during regeneration, and it is acting through Akt to influence its downstream pathways that govern cell cycle progression.
PMCID: PMC3172193  PMID: 21931533
12.  Defective Hematopoietic Stem Cell and Lymphoid Progenitor Development in the Ts65Dn Mouse Model of Down Syndrome: Potential Role of Oxidative Stress 
Antioxidants & Redox Signaling  2011;15(8):2083-2094.
Down Syndrome (DS), a genetic disease caused by a triplication of chromosome 21, is characterized by increased markers of oxidative stress. In addition to cognitive defects, patients with DS also display hematologic disorders and increased incidence of infections and leukemia. Using the Ts65Dn mouse model of DS, the goal of this study was to examine hematopoietic stem and lymphoid progenitor cell function in DS.
Analysis of hematopoietic progenitor populations showed that Ts65Dn mice possessed fewer functional hematopoietic stem cells and a significantly decreased percentage of bone marrow lymphoid progenitors. Increased reactive oxygen species and markers of oxidative stress were detected in hematopoietic stem cell populations and were associated with a loss of quiescence. Bone marrow progenitor populations expressed diminished levels of the IL-7Rα chain, which was associated with decreased proliferation and increased apoptosis. Modulating oxidative stress in vitro suggested that oxidative stress selectively leads to decreased IL-7Rα expression, and inhibits the survival of IL-7Rα-expressing hematopoietic progenitors, potentially linking increased reactive oxygen species and immunopathology.
The study results identify a link between oxidative stress and diminished IL-7Rα expression and function. Further, the data suggest that this decrease in IL-7Rα is associated with defective hematopoietic development in Down Syndrome.
The data suggest that hematopoietic stem and lymphoid progenitor cell defects underlie immune dysfunction in DS and that increased oxidative stress and reduced cytokine signaling may alter hematologic development in Ts65Dn mice. Antioxid. Redox Signal. 15, 2083–2094.
PMCID: PMC3166202  PMID: 21504363
13.  Co-transplantation with mesenchymal stem cells expressing a SDF-1/HOXB4 fusion protein markedly improves hematopoietic stem cell engraftment and hematogenesis in irradiated mice 
Introduction: Mesenchymal stem cells (MSCs) contribute to the engraftment of transplanted hematopoietic stem cells (HSCs). MSCs also accelerate hematological recovery by secreting SDF-1 and enabling HSCs to enter the bone marrow (BM) via the SDF-1/CXCR4 axis. HOXB4 has been shown to stimulate HSC self-renewal. In this study, we examined whether SDF-1 and HOXB4 expression in MSCs co-transplanted with HSCs could synergistically improve hematopoietic recovery in irradiated mice. Methods: Using recombinant adenoviruses, we generated genetically modified BM-MSCs that expressed SDF-1, HOXB4, and an SDF-1/HOXB4 fusion gene. We then co-transplanted these modified MSCs with HSCs and investigated blood cell counts, BM cellularity, degree of human HSC engraftment, and survival rate in irradiated mice. Results: We found that co-culturing the SDF-1/HOXB4 fusion gene-modified MSCs (SDF-1/HOXB4-MSCs) and human umbilical cord blood CD34+ cells significantly improved HSC cell expansion in vitro. More importantly, co-transplantation of CD34+ cells and SDF-1/HOXB4-MSCs markedly increased the hematopoietic potential of irradiated mice as evidenced by the rapid recovery of WBC, PLT and HGB levels in peripheral blood and of BM cellularity. Co-transplantation also markedly improved engraftment of human CD45+ cells in mouse BM. Conclusions: Our study demonstrates that SDF-1/HOXB4-MSCs markedly accelerate hematopoietic recovery and significantly improve survival among mice treated with a lethal dose of irradiation. Therefore, SDF-1/HOXB4-MSCs could have therapeutic value by improving the efficacy of clinical transplantations in patients with defective hematopoiesis.
PMCID: PMC4297337  PMID: 25628780
Mesenchymal stem cells; hematopoietic stem cells; SDF-1 gene; HOXB4 gene; irradiation; hematopoietic reconstitution; NOD/SCID mice
14.  Fak Depletion in Both Hematopoietic and Non-hematopoietic Niche cells Leads to Hematopoietic Stem Cell Expansion 
Experimental Hematology  2011;40(4):307-317.e3.
Hematopoietic stem cells (HSCs) reside in complex bone marrow (BM) microenvironments where niche-induced signals regulate hematopoiesis. Focal adhesion kinase (Fak) is a non-receptor protein tyrosine kinase, which plays an essential role in many cell types, where its activation controls adhesion, motility and survival. Fak expression is relatively increased in HSCs compared to progenitors and mature blood cells. Therefore we explored its role in HSC homeostasis. We have used the Mx1-Cre inducible conditional knockout mouse model to investigate the effects of Fak deletion in bone marrow compartments. Results. The total number as well as the fraction of cycling Lin-Sca-1+c-kit+ (LSK) cells is increased in Fak−/− mice, compared to controls, while hematopoietic progenitors and mature blood cells are unaffected. BM cells from Fak−/− mice exhibit enhanced, long-term (i.e. 20 week duration) engraftment in competitive transplantation assays. Intrinsic Fak function was assessed in serial transplantation assays, which showed that HSCs (Lin-Sca-1+c-kit +CD34-Flk-2-cells) sorted from Fak−/− mice have similar self-renewal and engraftment ability on a per cell basis as wild type HSCs. When Fak deletion is induced following engraftment of Fakfl/flMx1-Cre+ BM cells into wild type recipient mice, the number of LSKs is unchanged. In conclusion, Fak inactivation does not intrinsically regulate HSC behavior and is not essential for steady-state hematopoiesis. However, widespread Fak inactivation in the hematopoietic system induces an increased and activated HSC pool size, potentially as a result of altered reciprocal interactions between HSCs and their microenvironment.
PMCID: PMC3307834  PMID: 22155722
Focal adhesion kinase; Hematopoietic stem cell; Homeostasis; Engraftment
15.  Rapamycin enhances long-term hematopoietic reconstitution of ex vivo expanded mouse hematopoietic stem cells by inhibiting senescence 
Transplantation  2014;97(1):10.1097/TP.0b013e3182a7fcf8.
The mTOR is an important regulator of HSCs self-renewal and its overactivation contributes to HSCs premature exhaustion in part via induction of HSCs senescence. Inhibition of mTOR with rapamycin has the potential to promote long term hematopoiesis of ex vivo expanded HSCs to facilitate the clinical application of HSCs transplantation for various hematological diseases.
A well-established ex vivo expansion system for mouse bone marrow HSCs was utilized to investigate whether inhibition of overactivated mTOR with rapamycin can promote long term hematopoiesis of ex vivo expanded HSCs and to elucidate the mechanisms of action of rapamycin.
HSCs-enriched mouse bone marrow LSK cells exhibited a time-dependent activation of mTOR after ex vivo expansion in a serum-free medium supplemented with SCF, TPO and FL. The overactivation of mTOR was associated with induction of senescence but not apoptosis in LSK cells and a significant reduction in the ability of HSCs to produce long-term hematopoietic reconstitution. Inhibition of overactivated mTOR with rapamycin promoted ex vivo expansion and long term hematopoietic reconstitution of HSCs. The increase in long term hematopoiesis of expanded HSCs is likely attributable in part to rapamycin-mediated upregulation of Bmi1 and downregulation of p16, which prevent HSCs from undergoing senescence during ex vivo expansion.
These findings suggest that mTOR plays an important role in the regulation of HSCs self-renewal in vitro and inhibition of mTOR hyperactivation with rapamycin may represent a novel approach to promote ex vivo expansion and their long term hematopoietic reconstitution of HSCs.
PMCID: PMC3877183  PMID: 24092377
hematopoietic stem cells; ex vivo expansion; long term hematopoietic reconstitution; mTOR; rapamycin; senescence
16.  Inhibition of p38 Mitogen-Activated Protein Kinase Promotes Ex Vivo Hematopoietic Stem Cell Expansion 
Stem Cells and Development  2011;20(7):1143-1152.
Hematopoietic stem cell (HSC) self-renewal is tightly regulated by a complex crosstalk between many cell-intrinsic regulators and a variety of extrinsic signals from the stem cell niche. In this study, we examined whether the p38 mitogen-activated protein kinase (p38) is one of the intrinsic regulators that can negatively regulate HSC self-renewal in vitro and whether inhibition of p38 activity with a small molecule inhibitor can promote HSC expansion ex vivo. The results from this study showed that sorted mouse bone marrow Lin−Sca1+c-kit+ cells (LSK+ cells) exhibited selective activation of p38 after culture in a serum-free medium supplemented with 100 ng/mL stem cell factor, thrombopoietin, and Flt3 ligand. The activation of p38 was associated with a significant reduction in HSCs and induction of apoptosis and cellular senescence in LSK+ cells and their progeny. Addition of the specific p38 inhibitor SB203580 (SB, 5 μM) to the culture inhibited the activation of p38 in LSK+ cells, which led to increase in HSC self-renewal and ex vivo expansion as shown by the cobblestone area forming cell assay, competitive repopulation, and serial transplantation. The increase in HSC expansion is likely attributable to SB-mediated inhibition of HSC apoptosis and senescence and upregulation of HoxB4 and CXCR4. These findings suggest that p38 plays an important role in the regulation of HSC self-renewal in vitro and inhibition of p38 activation with a small molecule inhibitor may represent a novel approach to promote ex vivo expansion of HSCs.
PMCID: PMC3121934  PMID: 21198398
17.  Loss of Aryl Hydrocarbon Receptor Promotes Gene Changes Associated with Premature Hematopoietic Stem Cell Exhaustion and Development of a Myeloproliferative Disorder in Aging Mice 
Stem Cells and Development  2013;23(2):95-106.
Loss of immune function and increased hematopoietic disease are among the most clinically significant consequences of aging. Hematopoietic stem cells (HSCs) from mice lacking aryl hydrocarbon receptor (AhR) have high rates of cell division. Studies were designed to test the hypothesis that aging AhR-null allele (AhR-KO) mice develop premature HSC exhaustion, and changes leading to hematological disease. Compared to wild-type, aging AhR-KO mice showed a decreased survival rate, splenomegaly, increased circulating white blood cells, hematopoietic cell accumulation in tissues, and anemia. Analysis of bone marrow indicated increased numbers of stem/progenitor and lineage-committed cells, but decreased erythroid progenitors. There was also decreased self-renewal capacity of HSCs determined by competitive repopulation and serial transplantation. HSCs also showed increased levels of reactive oxygen species (ROS), Ki-67, and γ-H2A.X, but decreased p16Ink4a. Splenic cells from aging KO mice had abnormal expression of genes, including Gata-1, Sh2d3c, Gfi-1, p21, and c-myc, involved in trafficking and associated with leukemia. HSCs from AhR-KO mice had gene changes related to HSC maintenance and consistent with phenotype observed. The most prominent gene changes (overexpression of Srpk2, Creb1, Hes1, mtor, pdp1) have been associated with HSC hyperproliferation, leukemia, and accelerated aging. Pathway analyses also indicated an enrichment of genes associated with oxidative stress, acute myelogenous leukemia, aging, and heat shock response, and the β-catenin/Wnt pathways. These data indicate that loss of AhR and associated changes in multiple signaling pathways promote premature HSC exhaustion and development of a myeloproliferative disorder. They also implicate a critical role of the AhR in the regulation of HSCs.
PMCID: PMC3887405  PMID: 24138668
18.  Circulation and Chemotaxis of Fetal Hematopoietic Stem Cells 
PLoS Biology  2004;2(3):e75.
The major site of hematopoiesis transitions from the fetal liver to the spleen and bone marrow late in fetal development. To date, experiments have not been performed to evaluate functionally the migration and seeding of hematopoietic stem cells (HSCs) during this period in ontogeny. It has been proposed that developmentally timed waves of HSCs enter the bloodstream only during distinct windows to seed the newly forming hematopoietic organs. Using competitive reconstitution assays to measure HSC activity, we determined the localization of HSCs in the mid-to-late gestation fetus. We found that multilineage reconstituting HSCs are present at low numbers in the blood at all timepoints measured. Seeding of fetal bone marrow and spleen occurred over several days, possibly while stem cell niches formed. In addition, using dual-chamber migration assays, we determined that like bone marrow HSCs, fetal liver HSCs migrate in response to stromal cell-derived factor-1α (SDF-1α); however, unlike bone marrow HSCs, the migratory response of fetal liver HSCs to SDF-1α is greatly increased in the presence of Steel factor (SLF), suggesting an important role for SLF in HSC homing to and seeding of the fetal hematopoietic tissues. Together, these data demonstrate that seeding of fetal organs by fetal liver HSCs does not require large fluxes of HSCs entering the fetal bloodstream, and that HSCs constitutively circulate at low levels during the gestational period from 12 to 17 days postconception. Newly forming hematopoietic tissues are seeded gradually by HSCs, suggesting initial seeding is occurring as hematopoietic niches in the spleen and bone marrow form and become capable of supporting HSC self-renewal. We demonstrate that fetal and adult HSCs exhibit specific differences in chemotactic behavior. While both migrate in response to SDF-1α, fetal HSCs also respond significantly to the cytokine SLF. In addition, the combination of SDF-1α and SLF results in substantially enhanced migration of fetal HSCs, leading to migration of nearly all fetal HSCs in this assay. This finding indicates the importance of the combined effects of SLF and SDF-1α in the migration of fetal HSCs, and is, to our knowledge, the first demonstration of a synergistic effect of two chemoattractive agents on HSCs.
New results on the migratory behavior of blood cell precursors in the early embryo might be relevant to bone marrow transplants and other clinical therapies
PMCID: PMC368169  PMID: 15024423
19.  Altered Hematopoiesis in Mice Lacking DNA Polymerase μ Is Due to Inefficient Double-Strand Break Repair 
PLoS Genetics  2009;5(2):e1000389.
Polymerase mu (Polμ) is an error-prone, DNA-directed DNA polymerase that participates in non-homologous end-joining (NHEJ) repair. In vivo, Polμ deficiency results in impaired Vκ-Jκ recombination and altered somatic hypermutation and centroblast development. In Polμ−/− mice, hematopoietic development was defective in several peripheral and bone marrow (BM) cell populations, with about a 40% decrease in BM cell number that affected several hematopoietic lineages. Hematopoietic progenitors were reduced both in number and in expansion potential. The observed phenotype correlates with a reduced efficiency in DNA double-strand break (DSB) repair in hematopoietic tissue. Whole-body γ-irradiation revealed that Polμ also plays a role in DSB repair in non-hematopoietic tissues. Our results show that Polμ function is required for physiological hematopoietic development with an important role in maintaining early progenitor cell homeostasis and genetic stability in hematopoietic and non-hematopoietic tissues.
Author Summary
Double-strand breaks (DSB) in DNA are a highly deleterious type of genetic damage, potentially causing genomic rearrangements or cell death if unrepaired. DSB can be triggered by environmental factors (such as electromagnetic radiation or clastogenic chemicals) or normal cell metabolism. The main mechanism of DSB repair in mammals is thought to be the non-homologous end-joining (NHEJ) pathway. Our article describes how DNA polymerase mu (Polμ), a recently identified component of the NHEJ machinery, is required for hematopoiesis—the process that generates and maintains the correct balance of the millions of blood cells needed to sustain life and defend against infection. Hematopoietic stem cells (HSC) divide asymmetrically, yielding another HSC and a progenitor cell. These progenitors proliferate and differentiate, their progeny eventually generating mature blood cells. In mice in which Polμ is genetically eliminated, we found that hematopoietic progenitors proliferate slowly and are functionally impaired. The incidence of DSB in hematopoietic cells from these mice is increased, suggesting that reduced DNA repair may be the cause of the hematopoietic defects. DNA damage was also increased in tissues unrelated to hematopoiesis, including liver, kidney, lung, and mouse embryonic fibroblasts. Thus, these results demonstrate that Polμ plays an important role in general DSB repair in many cell lineages.
PMCID: PMC2638008  PMID: 19229323
20.  The Distinct Metabolic Profile of Hematopoietic Stem Cells Reflects Their Location in a Hypoxic Niche 
Cell stem cell  2010;7(3):380-390.
Bone marrow transplantation is the primary therapy for numerous hematopoietic disorders. The efficiency of bone marrow transplantation depends on the function of long-term hematopoietic stem cells (LT-HSCs), which is markedly influenced by their hypoxic niche. Survival in this low-oxygen microenvironment requires significant metabolic adaptation. Here, we show that LT-HSCs utilize glycolysis instead of mitochondrial oxidative phosphorylation to meet their energy demands. We used flow cytometry to identify a unique low mitochondrial activity/glycolysis-dependent subpopulation that houses the majority of hematopoietic progenitors and LT-HSCs. Finally, we demonstrate that Meis1 and Hif-1α are markedly enriched in LT-HSCs and that Meis1 regulates HSC metabolism through transcriptional activation of Hif-1α. These findings reveal an important transcriptional network that regulates HSC metabolism.
PMCID: PMC4159713  PMID: 20804973
21.  Aging Hematopoietic Stem Cells Decline in Function and Exhibit Epigenetic Dysregulation 
PLoS Biology  2007;5(8):e201.
Age-related defects in stem cells can limit proper tissue maintenance and hence contribute to a shortened lifespan. Using highly purified hematopoietic stem cells from mice aged 2 to 21 mo, we demonstrate a deficit in function yet an increase in stem cell number with advancing age. Expression analysis of more than 14,000 genes identified 1,500 that were age-induced and 1,600 that were age-repressed. Genes associated with the stress response, inflammation, and protein aggregation dominated the up-regulated expression profile, while the down-regulated profile was marked by genes involved in the preservation of genomic integrity and chromatin remodeling. Many chromosomal regions showed coordinate loss of transcriptional regulation; an overall increase in transcriptional activity with age and inappropriate expression of genes normally regulated by epigenetic mechanisms was also observed. Hematopoietic stem cells from early-aging mice expressing a mutant p53 allele reveal that aging of stem cells can be uncoupled from aging at an organismal level. These studies show that hematopoietic stem cells are not protected from aging. Instead, loss of epigenetic regulation at the chromatin level may drive both functional attenuation of cells, as well as other manifestations of aging, including the increased propensity for neoplastic transformation.
Author Summary
Aging is marked by a decline in function of the entire organism. The effect of age on the regenerative capacity of adult stem cells, which should rejuvenate tissues throughout life, is poorly understood. Bone marrow stem cells, also known as hematopoietic stem cells (HSCs), continuously regenerate the cells that comprise the blood, including the immune system, which fails with age. Here, we show that older HSCs were less able to regenerate the blood system than young HSCs. Paradoxically, the HSC number increased concomitantly, leading to no major difference in overall blood production, even though the immune system did exhibit some defects. To determine why these changes occurred, we looked at global patterns of gene expression in young versus old HSC. Stem cells exhibited an elevated inflammatory response and a decline in factors, called chromatin regulators, that orchestrate DNA accessibility and gene expression. Additional evidence supports the idea that loss of overall gene regulation (epigenetic regulation) is a major event during aging. Whereas much of aging research is concentrated on accumulation of mutations in DNA rather than on global regulatory mechanisms, we speculate that these epigenetic changes could drive many of the manifestations of age. This view also may explain the increased incidence of cancer with age.
In highly purified hematopoietic stem cells from mice aged 2 to 21 months, gene expression analysis indicates a deficit in function yet an increase in stem cell number with advancing age.
PMCID: PMC1925137  PMID: 17676974
22.  Inflammatory ROS promote and cooperate with the Fanconi anemia mutation for hematopoietic senescence 
Journal of cell science  2007;120(Pt 9):1572-1583.
The proinflammatory cytokine tumor necrosis factor α (TNFα) inhibits hematopoietic stem cell (HSC) expansion, interferes with HSC self-renewal and compromises the ability of HSC to reconstitute hematopoiesis. We have investigated mechanisms by which TNFα suppresses hematopoiesis using the genomic instability syndrome Fanconi anemia mouse model deficient for the complementation-group-C gene (Fancc). Examination of senescence makers, such as senescence-associated β-galactosidase, HP1-γ, p53 and p16INK4A shows that TNFα induces premature senescence in bone marrow HSCs and progenitor cells as well as other tissues of Fancc–/– mice. TNFα-induced senescence correlates with the accumulation of reactive oxygen species (ROS) and oxidative DNA damage. Neutralization of TNFα or deletion of the TNF receptor in Fancc–/– mice (Fancc–/–;Tnfr1–/–) prevents excessive ROS production and hematopoietic senescence. Pretreatment of TNFα-injected Fancc–/– mice with a ROS scavenger significantly reduces oxidative base damage, DNA strand breaks and senescence. Furthermore, HSCs and progenitor cells from TNFα-treated Fancc–/– mice show increased chromosomal aberrations and have an impaired oxidative DNA-damage repair. These results indicate an intimate link between inflammatory reactive oxygen species and DNA-damage-induced premature senescence in HSCs and progenitor cells, which may play an important role in aging and anemia.
PMCID: PMC2857731  PMID: 17405815
DNA damage and repair; Fanconi anemia; Genomic instability; Hematopoietic stem cells; Inflammation; Reactive oxygen species
23.  Acellular Bone Marrow Extracts Significantly Enhance Engraftment Levels of Human Hematopoietic Stem Cells in Mouse Xeno-Transplantation Models 
PLoS ONE  2012;7(7):e40140.
Hematopoietic stem cells (HSC) derived from cord blood (CB), bone marrow (BM), or mobilized peripheral blood (PBSC) can differentiate into multiple lineages such as lymphoid, myeloid, erythroid cells and platelets. The local microenvironment is critical to the differentiation of HSCs and to the preservation of their phenotype in vivo. This microenvironment comprises a physical support supplied by the organ matrix as well as tissue specific cytokines, chemokines and growth factors. We investigated the effects of acellular bovine bone marrow extracts (BME) on HSC in vitro and in vivo. We observed a significant increase in the number of myeloid and erythroid colonies in CB mononuclear cells (MNC) or CB CD34+ cells cultured in methylcellulose media supplemented with BME. Similarly, in xeno-transplantation experiments, pretreatment with BME during ex-vivo culture of HSCs induced a significant increase in HSC engraftment in vivo. Indeed, we observed both an increase in the number of differentiated myeloid, lymphoid and erythroid cells and an acceleration of engraftment. These results were obtained using CB MNCs, BM MNCs or CD34+ cells, transplanted in immuno-compromised mice (NOD/SCID or NSG). These findings establish the basis for exploring the use of BME in the expansion of CB HSC prior to HSC Transplantation. This study stresses the importance of the mechanical structure and soluble mediators present in the surrounding niche for the proper activity and differentiation of stem cells.
PMCID: PMC3388059  PMID: 22768336
24.  Bid is a positive regulator for donor-derived lymphoid cell regeneration in γ-irradiated recipients 
Experimental hematology  2011;39(9):947-957.e1.
Hematopoietic regeneration is regulated by cell survival proteins such as the Bcl-2 family. Bid, a BH3-only protein of the Bcl-2 family, has multiple cellular functions and is involved in a variety of physiological or pathological conditions. We attempted to define its role in hematopoietic cell repopulation under the stress condition of bone marrow transplantation (BMT).
Materials and Methods
We performed conventional or competitive BMT with donor hematopoietic cells from Bid−/− or Bid+/+ mice. Flow Cytometry was used for quantification of hematopoietic stem cells (HSCs), hematopoietic progenitor cells (HPCs) and differentiated cells in different lineages (T, B and Myeloid cells). Single cell culture and homing assays were performed to further evaluate HSC functions. HPCs were also measured by the colony-forming cell (CFC) culture.
Contrary to the widely-recognized role of Bid as a pro-apoptotic protein, the absence of Bid significantly reduced the reconstitution of donor hematopoietic cells in γ-irradiated recipients. Interestingly, however, numbers of HSCs and HPCs, and their functions were not overtly altered. Instead, the regeneration of donor T and B cells was significantly impaired in the absence of Bid. Further analysis indicated an accumulation of the triple negative T cell population in the thymus, and pro-B cells in the bone marrow.
Our current study demonstrates a positive impact of Bid on hematopoietic regeneration mainly due to its unique effects on donor lymphopoiesis in the transplant recipients.
PMCID: PMC3156351  PMID: 21703985
Bid; apoptosis; transplantation; hematopoietic stem and progenitor cells; T cells; B cells
25.  An Erythroid Differentiation Signature Predicts Response to Lenalidomide in Myelodysplastic Syndrome  
PLoS Medicine  2008;5(2):e35.
Lenalidomide is an effective new agent for the treatment of patients with myelodysplastic syndrome (MDS), an acquired hematopoietic disorder characterized by ineffective blood cell production and a predisposition to the development of leukemia. Patients with an interstitial deletion of Chromosome 5q have a high rate of response to lenalidomide, but most MDS patients lack this deletion. Approximately 25% of patients without 5q deletions also benefit from lenalidomide therapy, but response in these patients cannot be predicted by any currently available diagnostic assays. The aim of this study was to develop a method to predict lenalidomide response in order to avoid unnecessary toxicity in patients unlikely to benefit from treatment.
Methods and Findings
Using gene expression profiling, we identified a molecular signature that predicts lenalidomide response. The signature was defined in a set of 16 pretreatment bone marrow aspirates from MDS patients without 5q deletions, and validated in an independent set of 26 samples. The response signature consisted of a cohesive set of erythroid-specific genes with decreased expression in responders, suggesting that a defect in erythroid differentiation underlies lenalidomide response. Consistent with this observation, treatment with lenalidomide promoted erythroid differentiation of primary hematopoietic progenitor cells grown in vitro.
These studies indicate that lenalidomide-responsive patients have a defect in erythroid differentiation, and suggest a strategy for a clinical test to predict patients most likely to respond to the drug. The experiments further suggest that the efficacy of lenalidomide, whose mechanism of action in MDS is unknown, may be due to its ability to induce erythroid differentiation.
Using gene expression profiling, Azra Raza and colleagues identified a molecular signature that predicts response to lenalidomide in patients without Chromosome 5q deletions, which suggests that these patients have a defect in erythroid differentiation.
Editors' Summary
Myelodysplastic syndrome (MDS) is a group of disorders in which the bone marrow (the spongy material found inside bones) does not make enough healthy blood cells. Normally, immature cells in the bone marrow called hematopoietic stem cells mature (differentiate) into three types of blood cells: red blood cells (which carry oxygen around the body; people with too few red blood cells are “anemic”), white blood cells (which fight off infections), and platelets (which prevent bleeding by forming blood clots). In patients with MDS, the production of these mature cell types is defective. In addition, immature cells called leukemic blasts sometimes accumulate in the bone marrow and blood. Thus, although MDS itself is not a type of cancer, it often develops into leukemia (blood cancer). The cause of most cases of MDS, which affects mainly elderly people, is not known. Its symptoms include tiredness and breathlessness (signs of anemia), frequent infections, and easy bruising or bleeding. Patients are usually given supportive care to relieve their symptoms (for example, blood transfusions to top up their red blood cells). Chemotherapy can sometimes delay the progression of MDS to leukemia and a few patients can be helped with bone marrow transplantation.
Why Was This Study Done?
Recently, researchers have discovered that some people with MDS respond very well to a drug called lenalidomide. Three-quarters of patients whose MDS is characterized by the loss of a small part of Chromosome 5 need fewer blood transfusions after being given lenalidomide but only a quarter of people without this chromosomal defect respond to the drug. Unfortunately, most patients with MDS do not have this chromosome abnormality and there is no way to predict which of these patients are likely to respond to lenalidomide. Lenalidomide is a toxic drug that damages white blood cells and platelets, so it is important not to give it to people who might not benefit. In this study, the researchers have used gene expression profiling (a technique that catalogs all the genes expressed by a cell) to try to develop a way of predicting who will respond to lenalidomide
What Did the Researchers Do and Find?
The researchers obtained pre-treatment bone marrow samples from patients enrolled in two clinical trials of lenalidomide and compared the gene expression profiles of the bone marrow cells from the patients who subsequently responded to the drug with the profiles of cells from nonresponding patients. In all, 47 genes were more highly expressed in nonresponders than in responders. The researchers then asked whether the expression of any gene sets (collections of genes that code for proteins that work in a single pathway) was greater in the nonresponders than in the responders. This analysis revealed a “signature” of lenalidomide response consisting of a set of genes normally expressed during the differentiation of red blood cells (an “erythroid differentiation signature”). Decreased expression of this signature was associated with a response to lenalidomide in an independent set of patients (validation set). The researchers then used the response signature and the original set of samples to develop a single score that could distinguish individual responders from nonresponders. This score accurately predicted the response of three-quarters of the patients in the validation set to lenalidomide. Finally, the researchers showed that lenalidomide promotes the erythroid maturation of normal human hematopoietic stem cells grown in dishes and stimulates the expression of the lenalidomide response signature in these cells.
What Do These Findings Mean?
These findings indicate that patients with MDS who respond to lenalidomide have defective red blood cell differentiation. In addition, they suggest that it might be possible to use the response signature to develop a test that can predict which patients with MDS will benefit from treatment with lenalidomide. However, the preliminary predictive test described here will need to be tested in many more patients before it can be used as a routine clinical test. Finally, the researchers' last experiment suggests that lenalidomide may help people with MDS because it induces red blood cell differentiation. Lenalidomide therapy might, therefore be useful in other disorders in which red blood cell maturation is defective, including some forms of anemia.
Additional Information.
Please access these Web sites via the online version of this summary at
See a related PLoS Medicine Perspective article
The US National Cancer Institute provides information for patients about myelodysplastic syndrome (in English and Spanish)
The UK charity Cancerbackup provides information about myelodysplastic syndrome
The American Cancer Society and the Leukemia and Lymphoma Society provide additional information about myelodysplastic syndrome
The US Food and Drug Administration provides information for patients on lenalidomide
PMCID: PMC2235894  PMID: 18271621

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