Genome Engineering to Prospectively Investigate the Pathogenesis of MLL-AF9 Acute Leukemia

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 883-883
Author(s):  
Corina Buechele ◽  
Erin H. Breese ◽  
Dominik Schneidawind ◽  
Matthew H. Porteus ◽  
Michael L. Cleary
Keyword(s):  
Stem Cells ◽  
Flow Cytometry ◽  
Stem Cell ◽  
Hematopoietic Stem Cells ◽  
Cell Biology ◽  
Poor Prognosis ◽  
Stem Cell Biology ◽  
Hematopoietic Stem ◽  
Mll Gene ◽  
In Cells

Abstract Chromosomal rearrangements involving the MLL gene occur in both primary and treatment-related leukemias, and are associated with a poor prognosis. While animal models of MLL-AF9 translocations have improved our understanding of the role of MLL oncogenes in leukemia pathogenesis, in this study a more representative approach based on generation of endogenous activated oncogenes is used to more faithfully model the genomic events on human leukemia. This system allows for prospective investigation of the downstream effects of the initiating event on gene expression, epigenetic regulation, stem cell biology, and genome stability in order to understand the key steps critical for the pathogenesis of MLL-rearranged leukemias. We designed a set of TALENs that cut in intron 11 of the MLL gene (MLL-11 TALENs) to facilitate insertion of sequences encoding for AF9 and the fluorescent marker gene coding NeonGreen (knock-in approach). This resulted in MLL-AF9 expression controlled through the endogenous MLL promoter. Co-expression of MLL-11 TALENs with the AF9 knock-in template resulted in AF9 NeonGreen insertion both in K562 cells and in primary human hematopoietic stem cells isolated from umbilical cord blood. This approach showed a knock-in efficiency of ~20% (range 2-40%) in K562 and CD34+ cells based on NeonGreen expression detected by flow cytometry and confocal microscopy. Long-term culture of primary human hematopoietic cells showed increased frequency of knock-in cells suggesting a survival advantage. Remarkably, further enrichment of knock-in cells was promoted by colony-forming assays in semi-solid medium. Subsequent transplantation into NSG mice was performed to assess their leukemogenic potential. Strikingly, leukemia was induced within 8-14 weeks post transplantation. All mice presented with a similar disease profile that included peripheral blood blasts and splenomegaly. Histologic examination confirmed extensive replacement of bone marrow cells and splenic infiltration by leukemic blasts expressing MLL-AF9 detected by RT-PCR. The leukemic blasts showed increased levels of common MLL target genes (HoxA9, Meis1) compared to non-MLL leukemias and comparable levels to known MLL-AF9 cell lines (Mono Mac 6, THP-1). The blast cells displayed a pre-lymphoid phenotype with CD10+/CD19++/CD38++/CD34+ and were negative for mature B cell markers CD20 and IgM as measured by flow cytometry. Our studies establish a novel experimental model to generate MLL leukemia deriving from primary human hematopoietic stem cells expressing the fusion oncogene under control of the endogenous promoter. The model will allow for further prospective study of leukemia initiating and stem cell biology for a genetic subtype of poor prognosis leukemia. Disclosures No relevant conflicts of interest to declare.

Change in SP Distribution with Age Reflects Intrinsic Age-Based Changes in Stem Cell Biology: Implications for the Mechanism of Aging.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 4209-4209
Author(s):  
Daniel J. Pearce ◽  
Catherine Simpson ◽  
Kirsty Allen ◽  
Ayad Eddaoudi ◽  
Derek Davies ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Stem Cells ◽  
Stem Cell ◽  
Hematopoietic Stem Cells ◽  
Progenitor Cells ◽  
Cell Biology ◽  
Stem Cell Biology ◽  
Hematopoietic Stem ◽  
Stem And Progenitor Cells ◽  
The Absolute

Abstract It has been postulated that as we age, accumulated damage causes stem cells to die by apoptosis. This could lead to a diminished stem cell pool and consequently a reduced organ regeneration potential that contributes to somatic senescence. Hematopoietic stem cells have evolved many mechanisms to cope with their exposure to toxins during life. Cell surface transporters and anti-toxic enzymes are highly expressed in hematopoietic stem cells. If toxins do get the opportunity to damage the DNA of stem cells then the cell is more likely to die by apoptosis than attempt DNA repair and risk an error. Summarised below are our results from an investigation of the frequency, phenotype, cell cycle status and repopulation potential (in young recipients) of C57BL6 side population (SP) cells from mice with a range of ages. The absolute frequency of SP cells increases with age (Figure-A). The proportion of the lineage negative, Sca-1+, c-kit+ (KLS) cell population that is an SP stem cell increases from ~1% to over 30% during the murine lifetime (red bars in Figure-B). These SP cells from older mice have a reduced 4-month competitive repopulation potential when compared to SP cells from younger mice but contain a similarly low proportion of phenotypically-defined mature cells (blue bars in Figure-B) and have a similar cell cycle profile and progenitor cell output (2% of 3 x 96 well plates for each). SP cells from older mice contained a higher proportion of SP cells with the highest efflux ability (61 vs 414 days, p=<0.001, n=6) Engrafted cells derived from old SP cells 4 months previously still displayed an increased SP frequency when compared to engrafted cells derived from SP cells of young mice. Hence, more progenitors or committed cells have not gained the SP ability; rather this difference in SP distribution reflects an age-dependent change in hematopoietic stem cell biology that is independent of the microenvironment. Specifically, the proportion of stem and progenitor cells (KLS) that is a stem cell (SP fraction of KLS) increases with age. We hypothesize that this may be a progressive enrichment of primitive cells over time via selection. As we age, accumulative damage to hematopoietic stem and progenitor cells causes more cells to die by apoptosis. It may be that the stem/progenitor cells with the lowest hoechst efflux ability are most susceptible to damage and hence most likely to die by apoptosis. Since the HSCs with the highest efflux of hoechst are thought to be the most primitive, it may be that there is an enrichment of primitive cells. This could account for the increased SP proportion observed within KLS cells. As there may be cells with ABC/G2 activity that is undetectable via the SP technique, selection of cells with a higher pump activity could also explain the increased SP frequency we observed. This hypothetical mechanism would also be independent of microenvirinment. In summary, we surmise that HSCs have a mechanism that copes with cellular damage while compensating for the reduced cellular output of HSCs with age by increasing the absolute number of HSCs. Figure Figure

Leukemic CD34+CD38- Cells Display Conserved Differential Expression of Many ATP Binding-Cassette Transporter Genes.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 1380-1380
Author(s):  
Marc H.G.P. Raaijmakers ◽  
Elke P.L.M. de Grouw ◽  
Louis T.F. van de Locht ◽  
Bert A. van der Reijden ◽  
Theo J.M. de Witte ◽  
...  
Keyword(s):  
Gene Expression ◽  
Stem Cells ◽  
Stem Cell ◽  
Abc Transporters ◽  
Cell Biology ◽  
Cell Fraction ◽  
Atp Binding ◽  
Stem Cell Biology ◽  
Hematopoietic Stem ◽  
Atp Binding Cassette

Abstract In most cases of acute myeloid leukemia (AML) CD34+CD38− cells are considered to be stem cells, responsible for the maintenance and relapse of AML. ATP binding cassette transporters function in the extrusion of xenobiotics and chemotherapeutical compounds, and may be involved in therapy resistance. Elucidation of mechanisms conferring drug resistance to CD34+CD38− cells is essential to provide novel targets for stem cell eradication in AML. We studied gene expression of all 45 transmembrane ABC transporters (the complete ABCA, B, C, D and G family) in human hematopoietic CD34+CD38− cells and more committed CD34+CD38+ progenitor cells, from healthy donors and patients with non-hematological diseases (N=11) and AML patients (N=11). Gene expression was assessed using a novel real-time RT-PCR approach with micro fluidic cards. In normal CD34+CD38− cells 36 ABC transporters were expressed, 22 of these displayed significant higher expression in the CD34+CD38− cell fraction compared to the CD34+CD38+ cell fraction. In addition to the known stem cell transporters (ABCB1, ABCC1 and ABCG2) these differential expressed genes included many members not previously associated with stem cell biology. In AML the ABC transporter expression profile was largely conserved, including expression of all 13 known drug transporters. These data suggest an important role for many ABC transporters in hematopoietic stem cell biology. In addition, the preferential expression of a high number of drug transport related transporters predicts that broad spectrum inhibition of ABC transporters is likely to be required for CD34+38− stem cell eradication in AML. This approach will, apart from affecting the leukemic stem cells, equally affect the normal stem cells.

The Role of Nucleophosmin In Hematopoietic Stem Cells and the Pathogenesis of Myelodysplastic Syndrome

Blood ◽  
2010 ◽  
Vol 116 (21) ◽  
pp. 95-95 ◽  
Author(s):  
Keisuke Ito ◽  
Paolo Sportoletti ◽  
John G Clohessy ◽  
Grisendi Silvia ◽  
Pier Paolo Pandolfi
Keyword(s):  
Stem Cells ◽  
Stem Cell ◽  
Myelodysplastic Syndrome ◽  
Cell Biology ◽  
De Novo ◽  
Stem Cell Biology ◽  
Hematopoietic Stem

Abstract Abstract 95 Myelodysplastic syndrome (MDS) is an incurable stem cell disorder characterized by ineffective hematopoiesis and an increased risk of leukemia transformation. Nucleophosmin (NPM) is directly implicated in primitive hematopoiesis, the pathogenesis of hematopoietic malignancies and more recently of MDS. However, little is known regarding the molecular role and function of NPM in MDS pathogenesis and in stem cell biology. Here we present data demonstrating that NPM plays a critical role in the maintenance of hematopoietic stem cells (HSCs) and the transformation of MDS into leukemia. NPM is located on chromosome 5q and is frequently lost in therapy-related and de novo MDS. We have previously shown that Npm1 acts as a haploinsufficient tumor suppressor in the hematopoietic compartment and Npm1+/− mice develop a hematologic syndrome with features of human MDS, including increased susceptibility to leukemogenesis. As HSCs have been demonstrated to be the target of the primary neoplastic event in MDS, a functional analysis of the HSC compartment is essential to understand the molecular mechanisms in MDS pathogenesis. However, the role of NPM in adult hematopoiesis remains largely unknown as Npm1-deficiency leads to embryonic lethality. To investigate NPM function in adult hematopoiesis, we have generated conditional knockout mice of Npm1, using the Cre-loxP system. Analysis of Npm1 conditional mutants crossed with Mx1-Cre transgenic mice reveals that Npm1 plays a crucial role in adult hematopoiesis and ablation of Npm1 in adult HSCs leads to aberrant cycling and followed by apoptosis. Analysis of cell cycle status revealed that HSCs are impaired in their ability to maintain quiescence after Npm1-deletion and are rapidly depleted in vivo as well as in vitro. Competitive reconstitution assay revealed that Npm1 acts cell-autonomously to maintain HSCs. Conditional inactivation of Npm1 leads to an MDS phenotype including a profoundly impaired ability to differentiate into cells of the erythroid lineage, megakaryocyte dyspoiesis and centrosome amplification. Furthermore, Npm1 loss evokes a p53-dependent response and Npm1-deleted HSCs undergo apoptosis in vivo and in vitro. Strikingly, transfer of the Npm1 mutation into a p53-null background rescued the apoptosis of Npm1-ablated HSCs and resulted in accelerated transformation to an aggressive and lethal form of acute myeloid leukemia. Our findings highlight the crucial role of NPM in stem cell biology and identify a new mechanism by which MDS can progress to leukemia. This has important therapeutic implications for de novo MDS as well as therapy-related MDS, which is known to rapidly evolve to leukemia with frequent loss or mutation of TRP53. Disclosures: No relevant conflicts of interest to declare.

A recessive screen for genes regulating hematopoietic stem cells

Blood ◽  
2010 ◽  
Vol 116 (26) ◽  
pp. 5849-5858 ◽  
Author(s):  
Peter Papathanasiou ◽  
Robert Tunningley ◽  
Diwakar R. Pattabiraman ◽  
Ping Ye ◽  
Thomas J. Gonda ◽  
...  
Keyword(s):  
Stem Cells ◽  
Stem Cell ◽  
Hematopoietic Stem Cells ◽  
Cell Biology ◽  
Regulatory Networks ◽  
Novel Mutation ◽  
Chemical Mutagenesis ◽  
Myb Transcription Factor ◽  
Cell Sorter ◽  
Hematopoietic Stem

Abstract Identification of genes that regulate the development, self-renewal, and differentiation of stem cells is of vital importance for understanding normal organogenesis and cancer; such knowledge also underpins regenerative medicine. Here we demonstrate that chemical mutagenesis of mice combined with advances in hematopoietic stem cell reagents and genome resources can efficiently recover recessive mutations and identify genes essential for generation and proliferation of definitive hematopoietic stem cells and/or their progeny. We used high-throughput fluorescence-activated cell sorter to analyze 9 subsets of blood stem cells, progenitor cells, circulating red cells, and platelets in more than 1300 mouse embryos at embryonic day (E) 14.5. From 45 pedigrees, we recovered 6 strains with defects in definitive hematopoiesis. We demonstrate rapid identification of a novel mutation in the c-Myb transcription factor that results in thrombocythemia and myelofibrosis as proof of principal of the utility of our fluorescence-activated cell sorter–based screen. Such phenotype-driven approaches will provide new knowledge of the genes, protein interactions, and regulatory networks that underpin stem cell biology.

Differential Expression of SLAM Family Receptor Markers in Normal Human Hematopoietic Stem Cells and Their Malignant Counterpart in MDS and AML.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 1897-1897
Author(s):  
Ramon V. Tiu ◽  
Jennifer J. Powers ◽  
Abdo Haddad ◽  
Ying Jiang ◽  
Jaroslaw P. Maciejewski
Keyword(s):  
Stem Cells ◽  
Flow Cytometry ◽  
Stem Cell ◽  
Hematopoietic Stem Cells ◽  
Cell Population ◽  
Stem Cell Population ◽  
Leukemic Stem Cell ◽  
Hematopoietic Stem ◽  
Cd34 Cells ◽  
Slam Family

Abstract Members of the signaling lymphocytic activation molecule (SLAM) family, including CD150, CD48 and CD244, were shown to precisely distinguish more committed lineage restricted progenitor cells from pluripotent and multipotent murine hematopoietic stem cells (HSC; Kiel et al; 2005 Cell). Similar SLAM profiles may also be present on HSC subsets in humans. We hypothesized that these SLAM markers may be indicators not only of stem cell potential in normal hematopoiesis but also distinguish a subset of the most immature malignant precursors of leukemia. In agreement with the concept of a “cancer stem cell,” the presence of leukemic stem cell population may be an indicator of important clinical and biological properties. We first tested the distribution of CD150, CD48 and CD244 antigens on human CD34+ cells derived from 7 control individuals using 4-color flow cytometry. CD34+ cells were measured in the blast gate based on side scatter and CD45 expression. Within CD34+ blasts, expression of CD48, CD150, and CD244 was detected on 16.71%±9.69, 6.53%±2.93, and 26.92%±6.95 of cells respectively. Subsequently, we investigated SLAM expression in 9 immature leukemic cell lines, including KG-1, K562, U937, HEL, HL60, MKN-95, NB-4, Kasumi and UT7, and found increased expression of SLAM markers in KG-1 (CD48+, CD150+, CD244+) and Kasumi (CD48−, CD150−, CD244+). Consequently, none of the leukemic cells showed pluripotent/multipotent SLAM profiles. We then compared the SLAM marker expression on blasts from patients with AML and MDS with that of CD34+ cells from normal controls. We studied a total of 28 patients: 11 MDS (2 low grade, 5 advanced MDS, 3 MDS/MPD overlap) and 10 AML (FAB: 3 M1, 2 M2, 1 M3, 2 M4/M4E0 and 2 M6). In our cohort, 8/10 AML patients expressed one of the three SLAM markers; 6/10 were CD150−CD48−CD244+ (63.57%±6.96) and 2/10 were CD150+CD48−CD244−(46%±10.96) suggestive of the presence of either pluripotent or multipotent leukemic stem cell phenotype. In the MDS cohort, 8/11 patients expressed one of three SLAM markers, 7/11 were CD150−CD48−CD244+ (41.21% ± 8.9) and 1/11 were CD150+CD48−CD244− (1.26%±0.59) again consistent with a profile derived from either pluripotent or multipotent stem cells. None of the MDS and AML patients had either co-expression of CD244 and CD48 or increased expression of CD48 alone. Two of the M1 type AML patients with CD150−CD48−CD244+ phenotype received prior chemotherapy and achieved complete remission on bone marrow biopsy and flow cytometry using traditional blast markers. In some, we conclude that the SLAM receptor markers may be associated with certain types of leukemic blasts and may be useful in the identification of leukemic stem cell population in both MDS and AML.

Donor-Cell Derived Aplastic Anemia (AA) with a Small Population of CD55−CD59− Blood Cells after Allogeneic Stem Cell Transplantation: Evidence for the Immune System Attack Against Normal Hematopoietic Stem Cells in AA.

Blood ◽  
2007 ◽  
Vol 110 (11) ◽  
pp. 3682-3682
Author(s):  
Chiharu Sugimori ◽  
Kanako Mochizuki ◽  
Hirohito Yamazaki ◽  
Shinji Nakao
Keyword(s):  
Stem Cells ◽  
Flow Cytometry ◽  
Stem Cell ◽  
Immune System ◽  
Stem Cell Transplantation ◽  
Hematopoietic Stem Cells ◽  
Blood Cells ◽  
Donor Cell ◽  
Small Population ◽  
Hematopoietic Stem

Abstract Acquired aplastic anemia (AA) is thought to be caused by the immune system attack against hematopoietic stem cells. However, there is no direct evidence that an immune system attack against normal hematopoietic stem cells leads to development of AA. The immune system attack may be directed toward abnormal stem cells given the fact that some patients with myelodysplastic syndrome respond to immunosuppressive therapy. Although the presence of a small population of CD55−CD59− blood cells represents a reliable marker for the immune pathophysiology of AA, little is known regarding when and how such paroxysmal nocturnal hemoglobinuria (PNH)-type cells appear in patients with AA. The development of AA with a small population of PNH-type cells was recently observed in an allogeneic stem cell transplant (SCT) recipient. This patient, a 59-year-old male, who had been treated with allogeneic peripheral blood stem cell transplantation (PBSCT) from an HLA-compatible sibling for treatment of very severe AA in March 2002, developed severe pancytopenia in December 2005. Late graft failure (LGF) without residual recipient cells was diagnosed based on the results of a chimerism analysis. Sensitive flow cytometry failed to reveal any increase in the proportion of CD55−CD59− PNH-type blood cells. The patient underwent a second PBSCT from the original donor without preconditioning in February 2006. Although his pancytopenia was completely resolved by day 20, his blood counts gradually decreased from day 60 without any apparent complications. Flow cytometry revealed small populations of PNH-type granulocytes in his peripheral blood (Figure 1). Both the PNH-type and normal phenotype granulocytes were of donor origin. PIG-A gene analyses showed the PNH-type granulocytes in the patient to be a clonal stem cell with an insertion of thymine at position 593 (codon 198). Similar results were obtained from the sorted PNH-type granulocytes obtained 6 months later. The patient was treated with horse antithymocyte globulin and cyclosporine. The patient required no further transfusions after 88 days of the therapy and remains well as of August, 2007. The small population of PNH-type cells was not detectable in any of 50 SCT recipients showing stable engraftment or in an AA patient suffering graft rejection after a SCT. These findings suggest that some factors expressed by the patient induced an immune system attack against autologous hematopoietic cells, leading to de novo development of donor-cell derived AA. This is the first evidence that an immune system attack against normal hematopoietic stem cells results in AA associated with a clonal expansion of a PIG-A mutant which may originally be present in the donor bone marrow. Figure Figure

Superior Impact of p18INK4c over p27kip1 on Long-Term Repopulating Ability of Hematopoietic Stem Cells.

Blood ◽  
2005 ◽  
Vol 106 (11) ◽  
pp. 796-796
Author(s):  
Hui Yu ◽  
Hongmei Shen ◽  
Xianmin Song ◽  
Paulina Huang ◽  
Tao Cheng
Keyword(s):  
Stem Cells ◽  
Bone Marrow ◽  
Stem Cell ◽  
Hematopoietic Stem Cells ◽  
Cell Biology ◽  
Wild Type ◽  
Self Renewal ◽  
Hematopoietic Stem ◽  
The Impact

Abstract The G1-phase is a critical window during the cell cycle in which stem cell self-renewal may be balanced with differentiation and apoptosis. Increasing evidence suggests that the cyclin-dependent kinase inhibitors (CKIs) such as p21Cip1/Waf1, p27kip1, p16INK4A, and p18INK4C (p21, p27, p16 and p18 hereafter) are involved in stem cell self-renewal, as largely demonstrated in murine hematopoietic stem cells (HSCs). For example, we have recently demonstrated a significant increase of HSC self-renewal in the absence of p18 (Yuan et al, Nature Cell Biology 2004). But the actual roles of these CKIs in HSCs appear to be distinct as p21 and p18 have opposite effects (Yu H et al, ASH 2004) whereas p16 has a limited effect (Stepanova et al, Blood 2005) on HSC exhaustion after serial bone marrow transfer. Like p18, however, p27 was recently reported to also inhibit HSC self-renewal due to the fact that the competitive repopulating units (CRUs) were increased in p27−/− mouse bone marrow (Walkley et al, Nature Cell Biology 2005) in contrast to the results in a previous report (Cheng T et al, Nature Medicine 2000). To further gauge the impact of p18 versus p27 on the long-term repopulating ability (LTRA) of HSCs, we have generated different congenic strains (CD45.1 and CD45.2) of p18−/− or p27−/− mice in the C57BL/6 background, allowing us to compare them with the competitive repopulation model in the same genetic background. The direct comparison of LTRA between p18−/− and p27−/− HSCs was assessed with the competitive bone marrow transplantation assay in which equal numbers of p18−/− (CD45.2) and p27−/− cells (CD45.1) were co-transplanted. Interestingly, the p18−/− genotype gradually dominated the p27−/− genotype in multiple hematopoietic lineages and p18−/− HSCs showed 4-5 times more LTRA than p27−/− HSCs 12 months after cBMT. Further self-renewal potential of HSCs was examined with secondary transplantation in which primarily transplanted p18−/− or p27−/− cells were equally mixed with wild-type unmanipulated cells. Notably, while the p18−/− cells continued to outcompete the wild-type cells as we previously observed, the p27−/− cells did not behave so in the secondary recipients. Based on the flow cytometric measurement and bone marrow cellularity, we estimated that transplanted p18−/− HSCs (defined with the CD34−LKS immunophenotype) had undergone a 230-fold expansion, while transplanted p27−/− and wild-type HSCs had only achieved a 6.6- and 2.4-fold expansion in the secondary recipients respectively. We further calculated the yield of bone marrow nucleated cells (BMNCs) per HSC. There were approximately 44 x 103, 20.6 x 103, and 15 x 103 BMNCs generated per CD34−LKS cell in p18−/−, p27−/− and wild-type transplanted recipients respectively. Therefore, the dramatic expansion of p18−/− HSCs in the hosts was not accompanied by decreased function per stem cell. Our current study demonstrates that hematopoietic engraftment in the absence of p18 is more advantageous than that in the absence of p27, perhaps due to a more specific role of p18 on self-renewal of the long-term repopulating HSCs.

Identification of a Neoplastic Stem Cell in Human Mast Cell Leukemia

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 817-817 ◽  
Author(s):  
Gregor Eisenwort ◽  
Barbara Peter ◽  
Katharina Blatt ◽  
Sabine Cerny-Reiterer ◽  
Gregor Hoermann ◽  
...  
Keyword(s):  
Stem Cells ◽  
Flow Cytometry ◽  
Stem Cell ◽  
Poor Prognosis ◽  
Systemic Mastocytosis ◽  
Human Mast Cell ◽  
Hematopoietic Stem ◽  
Organ Systems ◽  
Cd34 Cells ◽  
Cell Fractions

Abstract Leukemic stem cells (LSCs) have recently been identified as an important target of therapy in various human leukemias and related blood cell disorders. Systemic mastocytosis (SM) is a rare hematologic neoplasm characterized by abnormal growth and accumulation of mast cells (MCs) in various organ systems, including the bone marrow (BM). Whereas patients with indolent SM (ISM) have a normal life-expectancy, patients with more advanced forms of SM have a poor prognosis. In these patients, neoplastic MCs are usually resistant against conventional drugs and various targeted drugs. MC leukemia (MCL) is the rare leukemic variant of advanced SM, defined by a rapidly devastating expansion of immature MCs in various hematopoietic organs and a poor prognosis with short survival times. Although MCL is considered a stem cell disease, little is known about the origin and phenotype of MCL-initiating LSCs. We examined the phenotypic and functional characteristics of putative LSCs in patients with aggressive SM (ASM, n=12) and MCL (n=6). Putative LSCs were identified and characterized phenotypically by flow cytometry. Highly enriched, sorted LSCs were injected into NOD-SCID-IL-2Rγ-/- mice exhibiting a 220 amino acid isoform of human membrane-bound hSCF (NSGSCF). We found that disease-initiating and propagating LSCs reside within a CD34+ fraction of the MCL clone. Whereas cell fractions containing CD34+ cells as well as highly enriched CD34+ cells produced engraftment in NSGSCF mice with a MCL-like disease (43-77% human MCL cells in mouse BM after 10-22 weeks), no substantial engraftment was produced by MC-rich but stem cell-depleted, KIT+/CD34─ cell fractions obtained from the same patients (<1% engraftment in mouse BM). In dilution experiments, engraftment of CD34+ cells was documented down to a minimum of 50 cells per mouse. The identity of engrafting MCL cells was confirmed by morphology, phenotyping and molecular studies demonstrating the presence of KIT mutations that were initially detected in the primary MCL samples used. Moreover, we were able to confirm long-term engraftment by successful serial transplantations into secondary recipient mice. In consecutive experiments, we were able to show that CD45+/CD34+/CD38─ cells also produce leukemic engraftment in NSGSCF mice. As assessed by flow cytometry, these CD34+/CD38─ MCL LSCs were found to express several stem cells markers, including aminopeptidase-N (CD13), leukosialin (CD43), Pgp-1 (CD44), the IL-3R alpha-chain (CD123), AC133 (CD133) and CXCR4 (CD184). In addition, in most patients examined, MCL LSCs were found to display IL-1RAP, a surface antigen that is otherwise expressed in CML LSCs but is not expressed in normal stem cells. In addition, MCL LSCs were found to express various cell surface targets, including CD33 and CD52. By contrast, MCL LSCs did not express CD2, CD25, CD26 and CLL-1. The more mature progenitor cell fractions (CD34+/CD38+) were found to stain positive for CD13, CD33, CD43, CD44, CD90, CD117, CD123, CD133 and CD184. Mature clonal MCs expressed a similar phenotype, including molecular markers and targets, such as CD13, CD30 CD33, CD52 and CD184. In patients with ISM and aggressive SM (ASM), the CD34+/CD38─ stem cells exhibited a similar surface marker profile compared to MCL, but expressed lower levels of CD133 and did not express IL-1RAP. In the validation phase of our study, we examined the effects of target-specific antibodies. As assessed by flow cytometry, the CD52-targeting antibody alemtuzumab was found to induce complement-dependent lysis of CD34+ and CD34+/CD38─ cells in all MCL samples analysed. Furthermore, pre-incubation of MCL cells with alemtuzumab prior to injection into NSGSCF mice resulted in a significantly reduced engraftment (2.7±4.1%) after 22 weeks. In conclusion, our data show that the MCL clone originates from a primitive hematopoietic stem cell that co-expresses CD34, CD123, CD133 and IL-1RAP but lacks CD25 and CD26. In addition, our data show that MCL LSC express a number of clinically relevant surface targets, including CD33, CD52 and CD117 (KIT). These observations may facilitate LSC detection and isolation in MCL and may lead to the development of novel LSC-eradicating treatment concepts in this highly aggressive and drug-resistant form of leukemia. Disclosures Valent: Novartis: Consultancy, Honoraria, Research Funding.

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