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Background. Polypharmacy and potentially inappropriate medications (PIMs) are common among older adults with blood cancer and can lead to adverse effects and poor outcomes. Polypharmacy is commonly defined as taking ≥5 or ≥8 medications, depending on the population. PIMs can cause adverse side effects for certain patients, e.g. diphenhydramine and benzodiazepines. We sought to define the prevalence of polypharmacy and PIMs in older adults with blood cancers, and to examine the association between both with cognitive impairment and frailty in this population. Methods. From February 2015 to November 2019, all transplant-ineligible patients ages 75 and older who presented for initial consultation for hematologic malignancy at the Dana-Farber Cancer Institute (Boston, MA) were approached by a research assistant (RA) for a 15-minute screening geriatric assessment. The RA assessed 42 aging-related health deficits using patient-reported and objective performance measures spanning the domains of function, cognition, comorbidity, and mobility. Patients were determined to be frail, pre-frail or robust via two approaches: deficit accumulation approach (Rockwood, JGMedSci 2007) and phenotypic approach (Fried, JGMedSci 2001). Cognition was measured using the delayed recall section of the Montreal Cognitive Assessment (MoCA; Nasreddine, JAGS 2005) and Clock-In-Box test (CIB; Chester, Am J Med 2011). In addition, we collected data via electronic medical record review of all prescribed and over-the-counter medications patients were taking at the time of initial consultation. These data were reconciled and reviewed for quality by two board-certified geriatricians. The geriatricians identified 2 types of PIMs: anticholinergic PIMs per the Anticholinergic Risk Scale (Rudolph, Arch Intern Med 2008) and cancer-specific PIMs per the National Cancer Care Network Medications of Concern (NCCN Older Adult Oncology 2020). For patients recommended for active cancer treatment, the association between polypharmacy and PIMs with frailty was assessed using ordinal logistic regression. The association between polypharmacy and PIMs with cognitive impairment (by MoCA delayed recall and CIB) was assessed using logistic regression. All models controlled for age, gender, and comorbidity (via Charlson Comorbidity Index). Results. In this patient cohort (N=785), 286 (36%) were female with 240 (30%) in the leukemia disease group, 272 (35%) lymphoma and 273 (35%) multiple myeloma. 603 (77%) patients had polypharmacy (≥5 medications) and 421 (54%) were taking ≥8 medications. 201 (25%) patients were taking at least one PIM based on the Anticholinergic Risk Scale (Rudolph) and 343 (44%) based on the NCCN guidelines. Overall, 131 (17%) were frail, 457 (58%) pre-frail and 197 (25%) robust. 541 (69%) patients had Charlson Co-morbidity Index ≥3; 111 (14%) patients had "probable" cognitive impairment by MoCA Delayed Recall and 147 (19%) had "probable" cognitive impairment by CIB. In the 468 (60%) patients on active cancer treatment, there was an association of frailty with polypharmacy defined by a cutoff of ≥8 (adjusted odds ratio [aOR]=2.82, 95% confidence interval [CI] 1.92-4.17), but not ≥5 medications (aOR=1.42, 95% CI 0.91-2.22; Table 1). With each additional medication on a patient's medication list, their odds of being more frail increased by 8% (aOR=1.08, 95% CI 1.04-1.12). With each one-point increase on the Anticholinergic Risk Scale, odds of being more frail increased by 19% (aOR=1.19, 95% CI 1.03-1.39). With each additional PIM based on NCCN guidelines, odds of being more frail increased by 65% (aOR=1.65, 95% CI 1.34-2.04). Polypharmacy and PIMs were not associated with cognitive impairment by either MoCA Delayed Recall or CIB. Conclusion. Polypharmacy and PIMs are prevalent among older patients with blood cancers and are strongly associated with frailty but not cognitive impairment, independent of comorbidity. Increasing number of anticholinergic and especially cancer-specific PIMs have a stronger association with frailty compared to increasing number of medications in general. Our findings highlight that the types of medications contributing to polypharmacy may be more important than number of total medications. This suggests the need for streamlined ways of identifying specific PIMs in practice to deprescribe medications that may be associated with cumulative harm in older adults with cancer. Figure 1 Figure 1. Disclosures Stone: Aprea: Consultancy; Boston Pharmaceuticals: Consultancy; BerGen Bio: Membership on an entity's Board of Directors or advisory committees; Arog: Consultancy, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees; OncoNova: Consultancy; Syndax: Membership on an entity's Board of Directors or advisory committees; Syntrix/ACI: Membership on an entity's Board of Directors or advisory committees; Jazz: Consultancy; Macrogenics: Consultancy; Novartis: Consultancy, Research Funding; Glaxo Smith Kline: Consultancy; Innate: Consultancy; Janssen: Consultancy; Elevate Bio: Membership on an entity's Board of Directors or advisory committees; Foghorn Therapeutics: Consultancy; Gemoab: Membership on an entity's Board of Directors or advisory committees; Astellas: Membership on an entity's Board of Directors or advisory committees; Agios: Consultancy, Research Funding; Actinium: Membership on an entity's Board of Directors or advisory committees; Bristol Meyers Squibb: Consultancy; Celgene: Consultancy; Abbvie: Consultancy; Syros: Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy. Soiffer: Rheos Therapeutics, USA: Consultancy; Kiadis, Netherlands: Membership on an entity's Board of Directors or advisory committees; Juno Therapeutics, USA: Other: Data Safety Monitoring Board; Precision Biosciences, USA: Consultancy; Jazz Pharmaceuticals, USA: Consultancy; Takeda: Consultancy; Jasper: Consultancy; Gilead, USA: Other: Career Development Award Committee; NMPD - Be the Match, USA: Membership on an entity's Board of Directors or advisory committees.

Galectin-3-binding protein (gal3bp) and its receptor/ligand, galectin-3 (gal3), are secreted proteins that initiate signaling cascades in several diseases, and recent human proteomic data suggest they may play a role in venous thrombosis (VT). We hypothesized that gal3bp and gal3 may promote VT. Using a mouse stasis model of VT, we found that gal3bp and gal3 were localized on vein wall, red blood cells, platelets, and microparticles, whereas leukocytes expressed gal3 only. Gal3 was dramatically increased during early VT and gal3bp:gal3 colocalized in the leukocyte/endothelial cell interface, where leukocytes were partially attached to the vein wall. Thrombus size correlated with elevated gal3 and interleukin-6 (IL-6) vein wall levels. Recombinant gal3 promoted VT and increased vein wall IL-6 mRNA. Although recombinant gal3 restored the VT size in gal3(-/-) mice, it had no effect on IL6(-/-) mice, suggesting that gal3:gal3bp promotes VT through IL-6. Moreover, significantly fewer activated neutrophils were present in the gal3(-/-) vein walls. In a group of human patients, elevated circulating gal3bp correlated with acute VT. In conclusion, gal3bp:gal3 play a critical role in VT, likely via IL-6 and PMN-mediated thrombotic mechanisms, and may be a potential biomarker in human VT.

Retroviral vectors are the most efficient means of stable gene delivery to hematopoietic stem cells (HSCs). However, transgene expression from retroviral vectors is frequently subject to the negative influence of chromosomal sequences flanking the site of integration. Toward the development of autonomous transgene expression cassettes, we inserted the human interferon-β scaffold attachment region (IFN-SAR) and the chicken β-globin 5′ DNase I hypersensitive site 4 (5′HS4) insulator both separately and together into a series of self-inactivating (SIN) lentiviral vector backbones. Transduced cells of the human CD34+ hematopoietic progenitor line KG1a—pooled populations as well as individual clones harboring single integrants—were analyzed for reporter expression during culture periods of up to 4 months. Vectors carrying both the 5′HS4 insulator and the IFN-SAR consistently outperformed control vectors without inserts as well as vectors carrying either element alone. The performance of a set of vectors containing the murine stem cell virus long terminal repeat as an internal promoter was subsequently assessed during in vitro monocytic differentiation of transduced primary human CD34+ cord blood cells. Similar to what was observed in the KG1a hematopoietic progenitor cell model, optimal reporter expression in primary monocytes was obtained with the vector bearing both regulatory elements. These findings indicate that the 5′HS4/IFN-SAR combination is particularly effective at maintaining open chromatin domains permissive for high-level transgene expression at early and late stages of hematopoietic development, and thus could be of utility in HSC-directed retroviral vector–mediated gene transfer applications.

Hematopoiesis initiates in the extraembryonic yolk sac. To isolate various types of precursor cells from this blood cell–forming tissue, yolk sac cells were immortalized by retroviral-mediated expression of the HOX11 homeobox-containing gene. Among the cell lines derived, some were able to spontaneously generate adherent stromal-like cells capable of taking up acetylated low-density lipoprotein, and they could be induced to form tubelike structures when cultured on Matrigel. Although these cell lines were negative for hematopoietic cell surface markers, they gave rise to hematopoietic colonies—containing cells belonging to the monocytic, megakaryocytic, and definitive erythroid lineages—when plated in methylcellulose medium supplemented with hematopoietic growth factors. Low amounts of Flk-1 mRNA could be detected in these cells, and they showed significant responsiveness to vascular endothelial growth factor, stem cell factor, basic fibroblast growth factor, and interleukin 6. They also expressed the transcription factors SCL, GATA2, GATA1, PU.1, and c-myb. These yolk sac–derived cell lines may represent a transitional stage of early hematopoietic development.

Gab2, a newly identified pleckstrin homology domain-containing docking protein, is a major binding protein of SHP-2 tyrosine phosphatase in interleukin (IL)-3–stimulated hematopoietic cells. Its signaling mechanism remains largely unknown. We report here an important regulatory role for Gab2 in β1 integrin signaling pathway that mediates hematopoietic cell adhesion and migration. Cross-linking of the β1 integrin on Ba/F3 cells induced rapid tyrosine phosphorylation of Gab2 and its association with Syk kinase, SHP-2 phosphatase, and the p85 subunit of phosphatidylinositol (PI)-3 kinase. In addition, Gab2 was also constitutively associated with SHP-1 phosphatase via its C-terminal Src homology 2 domain. Overexpression of the pleckstrin homology domain or a mutant Gab2 molecule lacking SHP-2 binding sites resulted in significant reductions in Ba/F3 cell adhesion and migration. Biochemical analyses revealed that enforced expression of Gab2 mutant molecules dramatically reduced β1-integrin ligation-triggered PI3 kinase activation, whereas Erk kinase activation remained unaltered. Furthermore, transduction of primary hematopoietic progenitor cells from viable motheaten mice with these mutant Gab2 molecules also significantly ameliorated their enhanced migration capacity associated with theSHP1 gene mutation. Taken together, these results suggest an important signaling role for Gab2 in regulating hematopoietic cell adhesion and migration.

Translocations involving fibroblast growth factor receptor 3 (fgfr3) have been identified in about 25% of patients with myeloma. To directly examine the oncogenic potential offgfr3, murine bone marrow (BM) cells were transduced with retroviral vectors containing either wild-type fgfr3 or an activated mutant form of the receptor, fgfr3-TD. Mice transplanted with FGFR3-TD–expressing BM developed a marked leukocytosis and lethal hematopoietic cell infiltration of multiple tissues within 6 weeks of transplantation. Secondary and tertiary recipients of spleen or BM from primary fgfr3-TD mice also developed tumors within 6 to 8 weeks. Analysis of the circulating tumor cells revealed a pre-B-cell phenotype in most mice, although immature T-lymphoid or mature myeloid populations also predominated in some animals. Enhanced lymphoid but not myeloid colony formation was observed in the early posttransplantation period and only interleukin 7 and FGF-responsive pre-B-cell lines could be established from tumors. Cell expansions in primary recipients appeared polyclonal, whereas tumors in later passages exhibited either clonal B- or T-cell receptor gene rearrangements. Mice transplanted with wild-type FGFR3-expressing BM developed delayed pro-B-cell lymphoma/leukemias approximately 1 year after transplantation. These studies confirm that FGFR3 is transforming and can produce lymphoid malignancies in mice.

flt3/flk-2 ligand (FL) is a cytokine that exhibits synergistic activities in combination with other early acting factors on subpopulations of hematopoietic stem/progenitor cells. In addition to normal hematopoietic precursors, expression of the FL receptor, flt3R, has been frequently demonstrated on the blast cells from patients with acute B-lineage lymphoblastic, myeloid, and biphenotypic (also known as hybrid or mixed) leukemias. Because many of these leukemic cell types express FL, the possibility has been raised that altered regulation of FL-mediated signaling might contribute to malignant transformation or expansion of the leukemic clone. In humans, FL is predominantly synthesized as a transmembrane protein that must undergo proteolytic cleavage to generate a soluble form. To investigate the consequences of constitutively expressing the analogous murine FL isoform in murine hematopoietic stem/progenitor cells, lethally irradiated syngeneic mice (18 total) were engrafted with post–5-fluorouracil–treated bone marrow cells transduced ex vivo with a recombinant retroviral vector (MSCV-FL) encoding murine transmembrane FL. Compared with control mice (8 total), MSCV-FL mice presented with a mild macrocytic anemia but were otherwise healthy for more than 5 months posttransplant (until 22 weeks). Subsequently, all primary MSCV-FL recipients observed for up to 1 year plus 83% (20 of 24) of secondary MSCV-FL animals that had received bone marrow from asymptomatic primary hosts reconstituted for 4 to 5 months developed transplantable hematologic malignancies (with mean latency periods of 30 and 23 weeks, respectively). Phenotypic and molecular analyses indicated that the tumor cells expressed flt3R and displayed B-cell and/or myeloid markers. These data, establishing that dysregulated expression of FL in primitive hematopoietic cells predisposes flt3R+ precursors to leukemic transformation, underscore a potential role of this cytokine/receptor combination in certain human leukemias. © 1998 by The American Society of Hematology.

flt3/flk-2 ligand (FL) is a cytokine that exhibits synergistic activities in combination with other early acting factors on subpopulations of hematopoietic stem/progenitor cells. In addition to normal hematopoietic precursors, expression of the FL receptor, flt3R, has been frequently demonstrated on the blast cells from patients with acute B-lineage lymphoblastic, myeloid, and biphenotypic (also known as hybrid or mixed) leukemias. Because many of these leukemic cell types express FL, the possibility has been raised that altered regulation of FL-mediated signaling might contribute to malignant transformation or expansion of the leukemic clone. In humans, FL is predominantly synthesized as a transmembrane protein that must undergo proteolytic cleavage to generate a soluble form. To investigate the consequences of constitutively expressing the analogous murine FL isoform in murine hematopoietic stem/progenitor cells, lethally irradiated syngeneic mice (18 total) were engrafted with post–5-fluorouracil–treated bone marrow cells transduced ex vivo with a recombinant retroviral vector (MSCV-FL) encoding murine transmembrane FL. Compared with control mice (8 total), MSCV-FL mice presented with a mild macrocytic anemia but were otherwise healthy for more than 5 months posttransplant (until 22 weeks). Subsequently, all primary MSCV-FL recipients observed for up to 1 year plus 83% (20 of 24) of secondary MSCV-FL animals that had received bone marrow from asymptomatic primary hosts reconstituted for 4 to 5 months developed transplantable hematologic malignancies (with mean latency periods of 30 and 23 weeks, respectively). Phenotypic and molecular analyses indicated that the tumor cells expressed flt3R and displayed B-cell and/or myeloid markers. These data, establishing that dysregulated expression of FL in primitive hematopoietic cells predisposes flt3R+ precursors to leukemic transformation, underscore a potential role of this cytokine/receptor combination in certain human leukemias. © 1998 by The American Society of Hematology.

Primitive and definitive erythropoiesis represent distinct hematopoietic programs that differ with respect to stage of development, transcriptional control, and growth regulation. Although these differences have been recognized for some time, the relationship of the two erythroid lineages to each other is not well established. We have used a model system based on the hematopoietic development of embryonic stem (ES) cells in culture to investigate the origins of the earliest hematopoietic populations. Using ES cells transduced with a retrovirus that overexpresses the HOX11 gene, we have established factor-dependent hematopoietic cell lines that represent novel stages of embryonic hematopoiesis. Analysis of three of these cell lines indicates that they differ with respect to cytokine responsiveness, cell surface markers, and developmental potential. Two of the cell lines, EBHX1 and EBHX11, display the unique capacity to generate both primitive and definitive erythroid progeny as defined by morphology and expression of βH1 and βmajor globin. The third line, EBHX14, has definitive erythroid and myeloid potential, but is unable to generate cells of the primitive erythroid lineage. Analysis of the cytokine responsiveness of the two lines with primitive erythroid potential has indicated that exposure to leukemia inhibitory factor (LIF) results in the upregulation of βH1 and a change in cellular morphology to that of primitive erythrocytes. These findings are the first demonstration of a clonal cell line with primitive and definitive hematopoietic potential and support the interpretation that these lineages may arise from a common precursor in embryonic life. In addition, they suggest that LIF could play a role in the regulation of primitive erythropoiesis.© 1998 by The American Society of Hematology.

Secreted growth factors are integral components of the bone marrow (BM) niche and can regulate survival, proliferation, and differentiation of committed hematopoietic stem cells (HSCs). However, downstream genes activated in HSCs by early-acting cytokines are not well characterized. To better define intracellular cytokine signaling in HSC function, we have analyzed mice lacking expression of both signal transducer and activator of transcription 5a (STAT5a) and STAT5b (STAT5ab−/−). These studies specifically avoided possible autoimmune and/or splenomegaly disease-mediated indirect effects on HSC function by using 2 independent approaches: (1) by crossing onto the C57Bl/6 RAG2−/− background, and (2) by generation of wild-type chimeric mice reconstituted with transplanted STAT5ab−/− BM cells. These experiments demonstrated that STAT5-deficient HSCs have cell autonomous defects in competitive long-term repopulating activity. Furthermore, in the chimeric mice, injected wild-type BM cells showed a progressive multilineage competitive repopulating advantage in vivo, demonstrating that steady-state hematopoiesis was also highly STAT5-dependent. Consistent with the in vivo repopulating deficiency, when Sca-1+c-kit+lin− (KLS) cells were isolated and stimulated with growth factors in vitro, up to a 13-fold reduced expansion of total nucleated cells was observed in response to cocktails containing interleukin 3 (IL-3), IL-6, stem cell factor (SCF), Flt3 ligand, and thrombopoietin. Notably, a 10-fold reduction in expansion was observed with IL-3 and SCF. However, STAT5 activation was not required for regeneration of the KLS pool in vivo following transplant or for secondary repopulating ability. These studies support a major role for STAT5 activation as a cellular determinant of cytokine-mediated HSC repopulating potential but not self-renewal capacity.

The chemokine stromal cell-derived factor 1 (SDF-1) is essential for perinatal viability, B lymphopoiesis, and bone marrow myelopoiesis, and is a potent monocyte and T-lymphocyte chemoattractant. Interactions of SDF-1 with its receptor CXCR4 have been implicated in CD34(+) cell migration and homing. Here it is shown that human SDF-1beta (hSDF-1beta) alone secreted by hSDF-1beta-transduced tumor cells promotes efficacious antitumor responses. The murine C1498 leukemia and B16F1 melanoma models have been studied. For expression of hSDF-1beta by tumor cells (SDF-tumor cells), packaging cell lines secreting retroviruses encoding hSDF-1beta have been used. The results demonstrate that 50% (B16F1) and 90% (C1498) of naive mice injected with SDF-tumor cells reject their tumors. Prophylactic vaccination of naive mice with irradiated SDF-tumor cells leads to systemic immunity, and therapeutic vaccination leads to cure of established tumors. Mice that previously rejected live SDF-tumor cells are immune to the rejected tumor but susceptible to another tumor and have in vitro tumor-specific cytotoxic T lymphocyte (CTL) activity. SDF-tumor cells are not rejected by immunodeficient scid mice. Immunohistochemistry shows significant infiltration of SDF-1 tumors by T cells, and in vivo T-cell depletion studies indicate that CD4(+) T cells are required for SDF-mediated tumor rejection. In conclusion, the present data suggest that SDF-1/CXCR4 interactions have the potential to regulate efficacious antitumor immune responses; exploitation of these interactions may lead to novel therapeutic interventions.

The t(4;14) translocation occurs in 25% of multiple myeloma (MM) and results in both the ectopic expression of fibroblast growth factor receptor 3 (FGFR3) from der4 and immunoglobulin heavy chain-MMSET hybrid messenger RNA transcripts from der14. The subsequent selection of activating mutations of the translocated FGFR3 by MM cells indicates an important role for this signaling pathway in tumor development and progression. To investigate the mechanism by which FGFR3 overexpression promotes MM development, interleukin-6 (IL-6)-dependent murine B9 cells were transduced with retroviruses expressing functional wild-type or constitutively activated mutant FGFR3. Overexpression of mutant FGFR3 resulted in IL-6 independence, decreased apoptosis, and an enhanced proliferative response to IL-6. In the presence of ligand, wild-type FGFR3-expressing cells also exhibited enhanced proliferation and survival in comparison to controls. B9 clones expressing either wild-type FGFR3 at high levels or mutant FGFR3 displayed increased phosphorylation of STAT3 and higher levels of bcl-xL expression than did parental B9 cells after cytokine withdrawal. The mechanism of the enhanced cell responsiveness to IL-6 is unknown at this time, but does not appear to be mediated by the mitogen-activated protein kinases SAPK, p38, or ERK. These findings provide a rational explanation for the mechanism by which FGFR3 contributes to both the viability and propagation of the myeloma clone and provide a basis for the development of therapies targeting this pathway.

Current procedures for the genetic manipulation of hematopoietic stem cells are relatively inefficient due, in part, to a poor understanding of the conditions for ex vivo maintenance or expansion of stem cells. We report improvements in the retroviral transduction of human stem cells based on the SCID-repopulating cell (SRC) assay and analysis of Lin− CD34+CD38−cells as a surrogate measure of stem cell function. Based on our earlier study of the conditions required for ex vivo expansion of Lin−CD34+ CD38− cells and SRC, CD34+–enriched lineage–depleted umbilical cord blood cells were cultured for 2 to 6 days on fibronectin fragment in MGIN (MSCV-EGFP-Neo) retroviral supernatant (containing 1.5% fetal bovine serum) and IL-6, SCF, Flt-3 ligand, and G-CSF. Both CD34+CD38− cells (20.8%) and CFC (26.3%) were efficiently marked. When the bone marrow of engrafted NOD/SCID mice was examined, 75% (12/16) contained multilineage (myeloid and B lymphoid) EGFP+ human cells composing as much as 59% of the graft. Half of these mice received a limiting dose of SRC, suggesting that the marked cells were derived from a single transduced SRC. Surprisingly, these culture conditions produced a large expansion (166-fold) of cells with the CD34+CD38− phenotype (n = 20). However, there was no increase in SRC numbers, indicating dissociation between the CD34+CD38− phenotype and SRC function. The underlying mechanism involved apparent downregulation of CD38 expression within a population of cultured CD34+CD38+ cells that no longer contained any SRC function. These results suggest that the relationship between stem cell function and cell surface phenotype may not be reliable for cultured cells. (Blood. 2000;95:102-110)

Interleukin-12 (IL-12) is a heterodimeric cytokine mediating a dynamic interplay between T cells and antigen-presenting cells (APCs). Preclinical studies have demonstrated that recombinant murine IL-12 (rmIL-12) promotes specific antitumor immunity mediated by T cells in several types of tumors. However, the in vivo antitumor properties of IL-12 in acute myeloid leukemia (AML) have not been previously reported. We show here in a murine AML model that systemic administration of rmIL-12 significantly delays tumor growth but is incapable of rescuing mice from lethal leukemia. In contrast, AML cells genetically modified to express IL-12 (IL12-AML) using murine stem cell virus (MSCV) p40 + p35 elicit very potent antileukemic activity. Vaccines with lethally irradiated IL12-AML cells protect naive mice against challenge with wild-type AML cells and, more importantly, can cure mice bearing a considerable leukemic burden. Immunized mice show no signs of systemic IL-12 toxicity and their spleen histology is comparable with naive mice spleen. In vivo depletion of IL-12, interferon-γ (IFN-γ), or CD8+ T cells after injections with live IL12-AML cells abrogates completely the antileukemia immune responses. Studies on the in vitro effects of IFN-γ on AML cells demonstrate enhanced expression of major histocompatibility complex (MHC) and accessory molecules and induction of the costimulatory molecules B7.1 and B7.2, but no significant direct antiproliferative effect. 51Cr release assays show that rejection of live IL12-AML cells supports the development of long-lasting leukemia-specific cytotoxic T lymphocyte (CTL) activity. In conclusion, our results demonstrate that IL12-AML vaccination is a safe and potent immunotherapeutic approach that has a great potential to eliminate minimal residual disease in patients with AML.

STAT5 is a member of the signal transducers and activation of transcription (STAT) family of latent transcription factors activated in a variety of cytokine signaling pathways. We introduced alanine substitution mutations in highly conserved regions of murine STAT5A and studied the mutants for dimerization, DNA binding, transactivation, and dominant negative effects on erythropoietin-induced STAT5-dependent transcriptional activation. The mutations included two near the amino-terminus (W255KR→AAA and R290QQ→AAA), two in the DNA-binding domain (E437E→AA and V466VV→AAA), and a carboxy-terminal truncation of STAT5A (STAT5A/▵53C) analogous to a naturally occurring isoform of rat STAT5B. All of the STAT mutant proteins were tyrosine phosphorylated by JAK2 and heterodimerized with STAT5B except for the WKR mutant, suggesting an important role for this region in STAT5 for stabilizing dimerization. The WKR, EE, and VVV mutants had no detectable DNA-binding activity, and the WKR and VVV mutants, but not EE, were defective in transcriptional induction. The VVV mutant had a moderate dominant negative effect on erythropoietin-induced STAT5 transcriptional activation, which was likely due to the formation of heterodimers that are defective in DNA binding. Interestingly, the WKR mutant had a potent dominant negative effect, comparable to the transactivation domain deletion mutant, ▵53C. Stable expression of either the WKR or ▵53C STAT5 mutants in the murine myeloid cytokine-dependent cell line 32D inhibited both interleukin-3–dependent proliferation and granulocyte colony-stimulating factor (G-CSF)–dependent differentiation, without induction of apoptosis. Expression of these mutants in primary murine bone marrow inhibited G-CSF–dependent granulocyte colony formation in vitro. These results demonstrate that mutations in distinct regions of STAT5 exert dominant negative effects on cytokine signaling, likely through different mechanisms, and suggest a role for STAT5 in proliferation and differentiation of myeloid cells.

STAT5 is a member of the signal transducers and activation of transcription (STAT) family of latent transcription factors activated in a variety of cytokine signaling pathways. We introduced alanine substitution mutations in highly conserved regions of murine STAT5A and studied the mutants for dimerization, DNA binding, transactivation, and dominant negative effects on erythropoietin-induced STAT5-dependent transcriptional activation. The mutations included two near the amino-terminus (W255KR→AAA and R290QQ→AAA), two in the DNA-binding domain (E437E→AA and V466VV→AAA), and a carboxy-terminal truncation of STAT5A (STAT5A/▵53C) analogous to a naturally occurring isoform of rat STAT5B. All of the STAT mutant proteins were tyrosine phosphorylated by JAK2 and heterodimerized with STAT5B except for the WKR mutant, suggesting an important role for this region in STAT5 for stabilizing dimerization. The WKR, EE, and VVV mutants had no detectable DNA-binding activity, and the WKR and VVV mutants, but not EE, were defective in transcriptional induction. The VVV mutant had a moderate dominant negative effect on erythropoietin-induced STAT5 transcriptional activation, which was likely due to the formation of heterodimers that are defective in DNA binding. Interestingly, the WKR mutant had a potent dominant negative effect, comparable to the transactivation domain deletion mutant, ▵53C. Stable expression of either the WKR or ▵53C STAT5 mutants in the murine myeloid cytokine-dependent cell line 32D inhibited both interleukin-3–dependent proliferation and granulocyte colony-stimulating factor (G-CSF)–dependent differentiation, without induction of apoptosis. Expression of these mutants in primary murine bone marrow inhibited G-CSF–dependent granulocyte colony formation in vitro. These results demonstrate that mutations in distinct regions of STAT5 exert dominant negative effects on cytokine signaling, likely through different mechanisms, and suggest a role for STAT5 in proliferation and differentiation of myeloid cells.

Type I interferons (IFNs-α and IFN-β) bind to a common receptor to exert strong antiproliferative activity on a broad range of cell types, including interleukin-6 (IL-6)–dependent myeloma cells. In this study, we investigated the effect of IFN-β pretreatment on IL-6–stimulated mitogenic signaling in the human myeloma cell line U266. IL-6 induced transient tyrosine phosphorylation of the IL-6 receptor signal-transducing subunit gp130, the gp130-associated protein tyrosine kinases Jak1, Jak2, and Tyk2, the phosphotyrosine phosphatase PTP1D/Syp, the adaptor protein Shc and the mitogen-activated protein kinase Erk2, and accumulation of GTP-bound p21ras. Prior treatment of U266 cells with IFN-β downregulated IL-6–induced tyrosine phosphorylation of gp130, Jak2, PTP1D/Syp, Shc, and Erk2, and GTP-loading of p21ras. Further analysis indicated that treatment with IFN-β disrupted IL-6–induced binding of PTP1D/Syp to gp130 and the adaptor protein Grb2; IFN-β pretreatment also interfered with IL-6–induced interaction of Shc with Grb2 and a 145-kD tyrosine-phosphorylated protein. These results suggest a novel mechanism whereby type I IFNs interrupt IL-6–promoted mitogenesis of myeloma cells in part by preventing the formation of essential signaling complexes leading to p21ras activation.

We have previously shown that mice induced to overexpress thrombopoietin (TPO) by retroviral-mediated gene transfer into bone marrow (BM) cells develop myelofibrosis and osteosclerosis. It was speculated that these effects were secondary to TPO, resulting from high levels of megakaryocytes and platelets. Also, it was proposed that these mice represent a model for myelofibrosis and osteosclerosis. In this report, we show that levels of both transforming growth factor- beta 1 and platelet-derived growth factor are increased twofold to fivefold in the platelet-poor plasma of TPO overexpressing mice compared with control mice. These data suggest that the increased megakaryocytes produce elevated levels of these cytokines that lead to the pathogenesis of disease. Further, we retransplanted TPO overexpressing mice, at 40 to 42 weeks after primary transplantation, with normal BM cells. After the secondary transplantation, megakaryocytes and platelets returned to normal levels and the myelofibrosis and osteosclerosis were completely corrected. These data extend our initial studies of the effects of overexpression of TPO and show the potential use of this model to explore the underlying cause of myelofibrosis and osteosclerosis and potential treatments for these diseases.

Megakaryocyte growth and development factor (MGDF) has recently been identified as a ligand for the c-mpl receptor. Using retroviral- mediated gene transfer, MGDF has been overexpressed in mice to evaluate the systematic effects due to chronic exposure to this growth factor. MGDF overexpressing mice had more rapid platelet recovery than control mice after transplantation. Following this recovery, the platelet levels continued increasing to fourfold to eightfold above normal baseline levels and remained elevated (five-fold above control mice) in these animals, which are alive and well at more than 4 months posttransplantation. Increased megakaryocyte numbers were detected in a number of organs in these mice including bone marrow, spleen, liver, and lymph nodes. Prolonged overexpression of MGDF led to decreased marrow hematopoiesis, especially erythropoiesis, with a shift to extramedullary hematopoiesis in the spleen and liver. All the MGDF overexpressing mice analyzed to date developed myelofibrosis and osteosclerosis, possibly induced by megakaryocyte and platelet produced cytokines. No significant effect on other hematopoietic lineages was seen in the MGDF overexpressing mice, showing that the stimulatory effect of MGDF in vivo is restricted to the megakaryocyte lineage.

Background : Selectins, among other adhesion-mediated functions, facilitate and augment thrombosis; standard anticoagulants address thrombosis but also increase bleeding risk. Previous work in animal models showed inhibiting E-selectin decreases venous thrombosis (VTE) and vein wall inflammation without adverse bleeding events, making E-selectin inhibition a favorable therapeutic candidate for VTE. GMI-1271 is a potent, specific E-selectin antagonist. Here we report final analysis of safety, PK, biomarker and bleeding risk profile for GMI-1271 in 2 phase 1 studies of healthy subjects. Methods: The first study was a blinded single ascending dose (SAD) evaluation of 2, 5, 20, or 40mg/kg of GMI-1271 (n=4/cohort), vs placebo control saline (n=4) or active control Lovenox 1mg/kg (n=4). The second was an open-label multiple ascending dose (MAD) study of 10 (n=3) or 20mg/kg (n=3) of GMI-1271 QD d 1-5 vs Lovenox 1mg/kg (n=2) d 1-5. Assessments included safety (adverse events [AEs], clinical labs, bleeding time, PT/PTT, vitals, exam); PK (plasma and urine); and biomarkers. Biomarkers included ELISAs (CRP, D-dimer, IL-10, MPO, Prothrombin fragment 1.2, soluble E-selectin (sEsel), soluble P-selectin (sPsel), sICAM, Thrombin-antithrombin complex (TAT), Tissue Factor (TF), TNFα, VWF activity, and sCD40L); PicoGreen Assay for circulating DNA; flow cytometry (Platelet Monocyte Aggregates (PMA), Mac-1, LFA-1, and CD44) and Thromboelastography (TEG). See Table 1 for functional description. SAD remains blinded to GMI-1271 or placebo (GMI-1271/p). In SAD, we measured biomarker values at baseline, 8 and 24 h after dosing. Analysis was performed of biomarkers at each dose level and then pooled by GMI-1271/p vs Lovenox. In MAD, we measured biomarker values at baseline and day 4. Comparisons were made using unpaired t-test. Results: In total, 32 subjects enrolled and received GMI-1271/placebo (GMI-1271/p; 20), GMI-1271 (6) or Lovenox (L; 6). Safety: All subjects completed dosing uneventfully. Safety findings for GMI-1271/p were unremarkable. No moderate or serious AE were seen. AE overall were as expected in healthy volunteers. In SAD, only 1/20 GMI-1271/p subjects experienced an AE possibly related to study drug (mild transient headache); 0/6 in MAD. In the L group we saw expected mild transaminitis and injection site bruising. In all studies, GMI-1271 had no effect on bleeding time, PT, or PTT. PK: Plasma levels, Cmax, and AUC increased in a linear manner. Cl, Vz, and t ½ were not dose dependent; no accumulation was seen with multiple dosing (Fig 1 and 2). TEG: In SAD, there was a tendency to increase R, K, and decrease A and MA with no change in % lysis in L vs GMI-1271/p. In MAD, we saw a similar trend as SAD except for an increase in % lysis in L. In the pooled SAD analysis, there was a statistically significant difference between GMI-1271/p and L (higher values) for R (p sEsel: In SAD, there was a trend for sEsel to decrease with treatment in all cohorts (combined GMI-1271/p vs L p= 0.017). In MAD, there was non-significant trend for sEsel to decrease between Day 0 and Day 4 in GMI-1271; sEsel levels trended upward with L in comparison. MPO: In SAD, there was a trend for increased values in L vs GMI-1271/p, except for the 40mg/kg cohort. When pooled, there was a significant difference between GMI-1271/p vs L (higher levels) p MAC-1 In SAD, there was no change. In MAD, there was a significant decrease in MAC-1 at the 10mg/kg dose (p No other notable trend was seen in the other biomarkers measured. Conclusion: We report a favorable safety, biomarker and bleeding profile for the E-selectin antagonist GMI-1271 in healthy subjects. There is no signal to suggest GMI-1271 increases bleeding potential based on adverse events, PT, PTT, bleeding time, and TEG values, unlike traditional anticoagulants. An additional unblinded analysis of the biomarkers will be presented at ASH. We note a trend for sE-sel to decrease with GMI-1271 treatment even in this healthy volunteer population, consistent with previous experience and as expected based on mechanism of action. A phase 1 study of GMI-1271 is currently ongoing to evaluate the safety and efficacy of E-selectin antagonism for the treatment of patients with calf vein DVT. PK Figures: Figure 1: SAD; Figure 2: MAD. Figure 1 Figure 1. Figure 2 Figure 2. Disclosures Hemmer: GlycoMimetics, Inc.: Employment, Equity Ownership. Flanner:GlycoMimetics, Inc.: Employment. Parker:GlycoMimetics, Inc.: Employment. Li:GlycoMimetics, Inc.: Employment, Equity Ownership. Froehlich:Novartis: Consultancy; Janssen: Consultancy; Pfizer: Membership on an entity's Board of Directors or advisory committees; Boehringer-Ingelheim: Membership on an entity's Board of Directors or advisory committees; Fibromuscular Disease Society of America: Research Funding; Blue Cross/Blue Shield of Michigan: Research Funding; Merck: Consultancy. Magnani:GlycoMimetics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Thackray:GlycoMimetics: Employment, Equity Ownership. Sood:Bayer: Research Funding.

Introduction: Despite efforts toward prevention, thousands of patients suffer from venous thromboembolic disease (VTE), consisting of deep vein thrombosis (DVT) and pulmonary embolism (PE), each year; this results in significant morbidity and mortality. Clinical prediction rules, such as the Wells score, in conjunction with biomarkers like the D-dimer, are suggested by national guidelines to aide in evaluating for DVT. Substantial interest exists in developing improved diagnostic testing strategies and algorithms to limit the need for more costly, time consuming, and/or invasive testing. Novel biomarkers, like the soluble P selectin (sP-Sel), a cell adhesion molecule that functions to mediate thrombus formation and amplification, have shown promise in this area. While cancer patients suffer from a high burden of VTE, the D-dimer and Wells score are less helpful in this group. This is due to non-specific D-dimer elevations in these patients, and cancer being part of the Wells risk stratification schema. We sought to identify a more specific combination of biomarkers and Wells score, for the diagnosis of DVT that would apply to cancer patients as well. Methods: We previously performed a prospective cohort study of adults presenting with symptoms of DVT afflicting the upper or lower extremities (Vandy et al. J Vasc Surg Venous Lymphat Disord., 2013). Patients were enrolled from December 2008 to July 2013. Those with isolated calf DVT, superficial venous thrombosis, pregnant or breastfeeding, on therapeutic anticoagulation, or with symptoms of simultaneous upper and lower DVT were excluded. After informed consent was obtained, clinical characteristics and biomarkers (D-dimer, C-reactive protein (CRP), Von Willebrand Factor activity (VWF), and sP-Sel) were collected, and duplex ultrasonography was performed to determine if DVT was present. In this subset analysis, cancer patients were compared to non-cancer patients with regards to thrombotic risk factors, biomarker values, and to assess the test characteristics of various combinations of biomarkers and Wells score. Results: A total of 373 patients were enrolled, 151 (40%) in the cancer group, compared to 222 (60%) in the non-cancer group. Cancer patients were more likely to have a DVT on confirmatory testing (58.9% vs. 43.2%). Cancer patients tended to be older, male, have a lower BMI, more recent acute illness, and more central lines, relative to the non-cancer group. Non-cancer patients were more likely to have other potential risk factors for thrombosis, i.e. recent surgery, oral contraceptive use, and a family history of thrombosis. Biomarker values for CRP and sP-Sel were similar, however VWF and D-dimer values were significantly higher in the cancer patients (see table). D-Dimer > 500 with Wells score ≥ 2 was less specific for the diagnosis of DVT among cancer patients compared to non-cancer patients (p=0.003). However, D-Dimer ≥ 500 with sP-Sel ≥ 90 showed not only high specificity, but that specificity was not different between the cancer and non-cancer populations (p=0.88). sP-Sel ≥ 90 and Wells ≥ 2 had similar performance characteristics in both groups as well (p=0.54 for specificity, p=0.14 for positive predictive value (PPV)). Conclusion: Co-morbid risk factors for thrombosis among cancer patients included an older age, male gender, and an increased use of central lines. This group was less likely to have other potential thrombotic risk factors like family history or recent surgery. Our results concur with the finding that the D-dimer, combined with a clinical prediction rule, is not as helpful for DVT in cancer patients. Moreover, this study further supports sP-Sel as a specific test for DVT, that can be combined with clinical information (Wells score) or other laboratory data (D-dimer) to reflect the presence of DVT and potentially rule in clot; the test seems equally useful for both cancer and non-cancer populations. This could potentially be used to guide initial patient management in scenarios where imaging or further testing is not immediately available, such as in an outpatient clinic or rural area. Further exploration of the optimal strategy for utilizing biomarkers and clinical prediction rules in the diagnosis of DVT in cancer is needed. Table Table. Disclosures Sood: Bayer: Research Funding.

A new cell line, SBH-1, with the morphologic, immunophenotypic, and karyotypic features consistent with those of Reed-Sternberg (RS) and Hodgkin (H) cells, has been established from the pleural effusion of a patient. The cytologic appearance of SBH-1 cells is characteristic of multinucleate RS and mononuclear H cells, all containing inclusion-like nucleoli. The SBH-1 cells express CD30, CD15, CD25, CD71, CD45, CD20, CD22, and bcl-2 protein and are negative for epithelial membrane antigen. Cytogenetic analysis showed multiple clonal abnormalities with breakpoints at 14q32, 6q21, and 11q23. The Ig heavy chain genes and both Ig light chain genes were rearranged in SBH-1 cells, whereas the bcl-2 gene was in germline configuration. Messages for the cytokines interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha, and transforming growth factor-beta and the cytokine receptors IL-2R, IL-4R, IL-6R, and IL-7R were detected by reverse transcription-polymerase chain reaction analysis. Xenotransplantation of SBH-1 cells into severe combined immunodeficient (SCID) mice led to local and disseminated tumor growth. The cytologic, histologic, and immunohistochemical features of SBH-1 cells in SCID mouse tumors were typical of RS and H cells. The SBH-1 cell line will be useful in the study of RS and H cell biology, inasmuch as it represents a cell line obtained from a previously untreated patient.

flt3/flk-2 ligand (FL) is a cytokine that exhibits synergistic activities in combination with other early acting factors on subpopulations of hematopoietic stem/progenitor cells. In addition to normal hematopoietic precursors, expression of the FL receptor, flt3R, has been frequently demonstrated on the blast cells from patients with acute B-lineage lymphoblastic, myeloid, and biphenotypic (also known as hybrid or mixed) leukemias. Because many of these leukemic cell types express FL, the possibility has been raised that altered regulation of FL-mediated signaling might contribute to malignant transformation or expansion of the leukemic clone. In humans, FL is predominantly synthesized as a transmembrane protein that must undergo proteolytic cleavage to generate a soluble form. To investigate the consequences of constitutively expressing the analogous murine FL isoform in murine hematopoietic stem/progenitor cells, lethally irradiated syngeneic mice (18 total) were engrafted with post-5-fluorouracil-treated bone marrow cells transduced ex vivo with a recombinant retroviral vector (MSCV-FL) encoding murine transmembrane FL. Compared with control mice (8 total), MSCV-FL mice presented with a mild macrocytic anemia but were otherwise healthy for more than 5 months posttransplant (until 22 weeks). Subsequently, all primary MSCV-FL recipients observed for up to 1 year plus 83% (20 of 24) of secondary MSCV-FL animals that had received bone marrow from asymptomatic primary hosts reconstituted for 4 to 5 months developed transplantable hematologic malignancies (with mean latency periods of 30 and 23 weeks, respectively). Phenotypic and molecular analyses indicated that the tumor cells expressed flt3R and displayed B-cell and/or myeloid markers. These data, establishing that dysregulated expression of FL in primitive hematopoietic cells predisposes flt3R+ precursors to leukemic transformation, underscore a potential role of this cytokine/receptor combination in certain human leukemias.

Primitive and definitive erythropoiesis represent distinct hematopoietic programs that differ with respect to stage of development, transcriptional control, and growth regulation. Although these differences have been recognized for some time, the relationship of the two erythroid lineages to each other is not well established. We have used a model system based on the hematopoietic development of embryonic stem (ES) cells in culture to investigate the origins of the earliest hematopoietic populations. Using ES cells transduced with a retrovirus that overexpresses the HOX11 gene, we have established factor-dependent hematopoietic cell lines that represent novel stages of embryonic hematopoiesis. Analysis of three of these cell lines indicates that they differ with respect to cytokine responsiveness, cell surface markers, and developmental potential. Two of the cell lines, EBHX1 and EBHX11, display the unique capacity to generate both primitive and definitive erythroid progeny as defined by morphology and expression of betaH1 and betamajor globin. The third line, EBHX14, has definitive erythroid and myeloid potential, but is unable to generate cells of the primitive erythroid lineage. Analysis of the cytokine responsiveness of the two lines with primitive erythroid potential has indicated that exposure to leukemia inhibitory factor (LIF) results in the upregulation of betaH1 and a change in cellular morphology to that of primitive erythrocytes. These findings are the first demonstration of a clonal cell line with primitive and definitive hematopoietic potential and support the interpretation that these lineages may arise from a common precursor in embryonic life. In addition, they suggest that LIF could play a role in the regulation of primitive erythropoiesis.

Background: GMI-1271 is a potent, specific E-selectin antagonist which targets interactions between cancer cells and the bone marrow niche thereby increasing tumor sensitivity to chemotherapy; avoids HSC mobilization; and inhibits thrombosis without increasing bleeding risk. Delayed inflammatory cell recruitment into the vein wall post thrombus induction indicates a possible decrease in leukocyte activation, which may be crucial in treating both acute and cancer associated thrombosis. Here we report first-in-human evaluation of GMI-1271 for safety (including bleeding), PK, and biomarker data for effects on adhesion and mobilization. Methods: Two Phase I single dose escalation DBRCT in healthy subjects evaluated 2, 5, 10, and 20 mg/kg IV doses of GMI-1271. Study 1 is complete and unblinded (GMI-1271 vs placebo). Study 2 is ongoing, remains blinded (GMI-1271 vs placebo) with open-label positive control (Lovenox), and is reported in aggregate. In both studies, assessments included safety (adverse events [AEs], clinical labs, bleeding time, PT/PTT, vitals, and exam); PK (drug levels in plasma and urine); and biomarkers (plasma soluble E-selectin [sEsel], sPsel, sICAM-1 levels; and blood CD34+ counts). Only sEsel and CD34+ are reported for Study 1; sEsel, sPsel, sICAM-1 for Study 2. Comparisons to baseline were made by ANOVA and paired t-test models; PK analyses were for total clearance (CL), volume of distribution (Vz), elimination half-life (t½), fraction excreted (Fe), and renal clearance (CLr). Results: Forty-six subjects enrolled; 30 received GMI-1271 (10 each at 2 and 5 mg/kg; 6 at 10 mg/kg; and 4 at 20 mg/kg) and 16 received control (13 placebo, 3 Lovenox). Safety: All subjects completed the studies; labs, vitals, and exam were unremarkable. Bleeding times and PT/PTT were unaffected by E-selectin inhibition. No serious AEs were seen. For Study 1, AEs were seen in 12/18 (66.7%) of those on GMI-1271 and 5/10 (50%) on placebo. AEs were mostly mild events at the infusion site (i.e. tenderness or erythema), and occurred in both groups. One moderate AE occurred (vasovagal reaction) in the 2 mg/kg GMI-1271 cohort. Three AEs were possibly related to drug: 2 in the 2 mg/kg cohort (infusion site bruise, lightheaded), and 1 in the 5 mg/kg cohort (metallic taste). For Study 2, AEs were seen in 12/15 (80%) of those on GMI-1271/placebo and 3/3 (100%) on Lovenox. All AEs were mild. In GMI-1271/placebo group, all AEs were considered unrelated to study drug; 3 were infusion site bruise (1 per dose level). In the Lovenox group, AEs were bruise at injection site (related) and blood in urine (possibly related). PK: Plasma levels, Cmax, and AUC increased in a consistent dose-related manner in both studies (figure). CL, Vz, and t½ were not dose dependent; the latter averaged ~2.3 hours. ~66% of the dose was excreted unchanged in the urine independent of dose level, and CLr averaged 86 mL/min, less than estimated CrCl, suggesting tubular reabsorption is one component of CLr. Biomarkers: In Study 1, the absolute CD34+ and %CD34+ cell counts were normal across all cohorts. There was no dose-response for changes in peripheral CD34+ counts after administration of GMI-1271 (doses 2-10 mg/kg). Plasma concentrations of sEsel were relatively constant in the placebo and GMI-1271 2 mg/kg cohorts, with significant reduction (vs baseline, p=0.012) at 48hrs post-2mg/kg dose only. In the 5 and 10 mg/kg cohorts sEsel decreased significantly post-dose (p Conclusion: First-in-human experience with the potent E-selectin antagonist GMI-1271 demonstrated favorable safety and PK at single doses up to 20 mg/kg. These findings are consistent with the glycomimetic class for safety at anticipated therapeutic levels. Biomarkers of physiologic effect demonstrated clear reductions in sEsel/sPsel; decreased leukocyte adhesion with lower sICAM-1 levels; and no CD34+ rise indicating no mobilization of HSCs. Studies of GMI-1271 are ongoing to evaluate activity in hematologic malignancies and thrombosis. Figure 1. Plasma concentrations and Cmax are consistent for GMI-1271 between Phase 1 studies. Figure 1. Plasma concentrations and Cmax are consistent for GMI-1271 between Phase 1 studies. Disclosures Sood: Bayer: Research Funding. Hemmer:GlycoMimetics: Employment, Equity Ownership. Flanner:GlycoMimetics: Employment, Equity Ownership. Kramer:GlycoMimetics: Consultancy. Nietubicz:GlycoMimetics: Employment, Equity Ownership. Myers:GlycoMimetics: Research Funding. Wakefield:GlycoMimetics: Research Funding. Magnani:GlycoMimetics: Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Thackray:GlycoMimetics: Employment, Equity Ownership.

HOX11 encodes a homeodomain protein that is aberrantly expressed in T-cell acute lymphoblastic leukemia as a consequence of the t(10;14) and t(7;10) chromosomal translocations. We previously reported that HOX11 immortalizes murine hematopoietic progenitors and induces pre–T-cell tumors in mice after long latency. It has been demonstrated in a number of studies that HOX11, similar to other homeodomain proteins, binds DNA and transactivates transcription. These findings suggest that translocation-activated HOX11 functions as an oncogenic transcription factor. Here we report that HOX11 represses transcription through both TATA-containing and TATA-less promoters. Interestingly, transcriptional repression by HOX11 is independent of its DNA binding capability. Moreover, a systematic mutational analysis indicated that repressor activity was separable from immortalizing function, which requires certain residues within the HOX11 homeodomain that make base-specific or phosphate-backbone contacts with DNA. We further showed that the pathologic action of HOX11 involves DNA binding-dependent transcriptional pathways that are distinct from those controlling expression of a chromosomal target gene (Aldh-1) . We conclude that dysregulated expression of a particular set of downstream target genes by DNA binding via the homeodomain is of central importance for leukemia initiation mediated by HOX11.

Signal transducers and activators of transcription (STATs) are intracellular mediators of cytokine receptor signals. Because many early-acting growth factors have been implicated in STAT5 activation, this study sought to investigate whether STAT5 may be a transcriptional regulator of hematopoietic stem cell (HSC) long-term repopulating activity. To test this possibility, bone marrow (BM) and fetal liver (FL) cells from mice containing homozygous deletions of both STAT5a and STAT5b genes (STAT5ab−/−) were characterized for hematopoietic repopulating activities. BM and FL grafts were capable of repopulating lymphoid and myeloid lineages of lethally irradiated primary and secondary hosts, with defects observed primarily in T-lymphocyte engraftment. Because only a fraction of normal HSC function is required to reconstitute hematopoiesis, competitive repopulation assays of adult BM or FL cells were used against wild type adult BM or FL cells to quantitate stem cell function. In these analyses, average 25-, 28-, 45-, and 68-fold decreases in normal repopulating activity were evident in granulocyte (Gr-1+), macrophage (Mac-1+), erythroid progenitor (Ter119+), and B-lymphocyte (B220+) populations, respectively, with T lymphocytes (CD4+) always undetectable from the STAT5ab−/− graft. Consistent with previous reports of divergence between stem cell phenotype and function in cases of perturbed hematopoiesis, the absolute number of cells within Sca-1+c-kit+lin− or lin− Hoechst 33342 side population fractions was not significantly different between wild type and STAT5ab−/−BM or FL cells. These results demonstrate that a significant proportion of the growth factor signals required for multilineage reconstitution potential of HSCs is STAT5 dependent.