Hematopoietic Stem Cell Failure Research Articles

The future of aplastic anemia treatment in Brazil: Lessons learned for global hematology

Aplastic anemia (AA) is a rare serious hematologic disorder caused by hematopoietic stem cell failure in maintaining hematopoiesis. AA is virtually fatal if not treated, and diagnosis and therapy require extensive hematologic infrastructure. Academic medical centers in Brazil have continuously and significantly contributed to diagnostic tools and therapy development, from novel transplant strategies to drug combinations and implementation science in the national public health system. In the present review, we discuss how the collaborative effort among academic centers in hematology has contributed to improving health care for patients with aplastic anemia. We also discuss what needs are still unmet and how to overcome these challenges.

Outcomes of patients after mobilization failure of hematopoietic stem cells for autologous stem cell transplantation

IntroductionAutologous hematopoietic stem cell transplantation (ASCT) is the long-term consolidation treatment for various hematological malignancies. The collection of hematopoietic stem cell yield is critical to successful ASCTs, but not always achieved due to hematopoietic stem cell mobilization failure (HSCMF). Details regarding the cell collection and outcomes of those who fail mobilization are still lacking. Therefore, this study aimed to yield data on clinical outcomes and cellular products after HSCMF. MethodsRetrospective, unicentric study assessing clinical outcomes and characteristics of collected progenitor cells. The data were collected from patient databases. The results were reported in median, rates and percentages and absolute values. Patients older than 18 years of age at the time of mobilization and HSCMF were included. ResultsFive hundred ninety-nine patients underwent mobilization protocols. Thirty-five (5.8%) of them failed in the mobilization and fourteen (40%) died. Median time to death was eight months. Disease progression and infection were responsible for all deaths. Median relapse-free survival was 6.5 months (20 patients, 57%). Seven (20%) survivors were receiving salvage therapy and five (14%) were being followed clinically. Six (20.6%) participants underwent collection by apheresis, with insufficient cell collection. The median quantity of peripheral CD34+ cells in those patients was 10.5/mm3. The median CD34+ quantity collected was 0.86 × 106 CD34+ cells/kg. ConclusionsThe mobilization failure was associated with limited survival. Nonetheless, collected products offered perspectives for ex vivo expansion. Further studies should investigate the feasibility of expanding collected CD34+ cells to use as grafts for ASCT.

Mcm2 hypomorph leads to acute leukemia or hematopoietic stem cell failure, dependent on genetic context.

Minichromosome maintenance proteins (Mcm2-7) form a hexameric complex that unwinds DNA ahead of a replicative fork. The deficiency of Mcm proteins leads to replicative stress and consequent genomic instability. Mice with a germline insertion of a Cre cassette into the 3'UTR of the Mcm2 gene (designated Mcm2Cre ) have decreased Mcm2 expression and invariably develop precursor T-cell lymphoblastic leukemia/lymphoma (pre-T LBL), due to 100-1000 kb deletions involving important tumor suppressor genes. To determine whether mice that were protected from pre-T LBL would develop non-T-cell malignancies, we used two approaches. Mice engrafted with Mcm2Cre/Cre Lin- Sca-1+ Kit+ hematopoietic stem/progenitor cells did not develop hematologic malignancy; however, these mice died of hematopoietic stem cell failure by 6 months of age. Placing the Mcm2Cre allele onto an athymic nu/nu background completely prevented pre-T LBL and extended survival of these mice three-fold (median 296.5 vs. 80.5 days). Ultimately, most Mcm2Cre/Cre ;nu/nu mice developed B-cell precursor acute lymphoblastic leukemia (BCP-ALL). We identified recurrent deletions of 100-1000 kb that involved genes known or suspected to be involved in BCP-ALL, including Pax5, Nf1, Ikzf3, and Bcor. Moreover, whole-exome sequencing identified recurrent mutations of genes known to be involved in BCP-ALL progression, such as Jak1/Jak3, Ptpn11, and Kras. These findings demonstrate that an Mcm2Cre/Cre hypomorph can induce hematopoietic dysfunction via hematopoietic stem cell failure as well as a "deletor" phenotype affecting known or suspected tumor suppressor genes.

Abstract 6196: Fanconi anemia pathway deficiency drives copy number variation in squamous cell carcinoma

Abstract Fanconi anemia (FA), a model syndrome of genome instability, is caused by a deficiency in DNA interstrand crosslink (ICL) repair resulting in chromosome breakage. The FA repair pathway comprises at least 22 FANC proteins including BRCA1 and BRCA2 and protects against carcinogenic endogenous and exogenous aldehydes. Individuals with FA are hundreds to thousands-fold more likely to develop head and neck (HNSCC), esophageal and anogenital squamous cell carcinomas (SCCs) with a median onset age of 31 years. The aggressive nature of these tumors and poor patient tolerance of platinum and radiation-based therapy have been associated with short survival in FA. Molecular studies of SCCs from individuals with FA (FA SCCs) have been limited, and it is unclear how they relate to sporadic HNSCCs primarily driven by tobacco and alcohol exposure or human papillomavirus (HPV) infection. Here, by sequencing FA SCCs, we demonstrate that the primary genomic signature of FA-deficiency is the presence of a high number of structural variants (SVs). SVs are enriched for small deletions, unbalanced translocations, and fold-back inversions that arise in the context of TP53 loss. The SV breakpoints preferentially localize to early replicating regions, common fragile sites, tandem repeats, and SINE elements. SVs are often connected forming complex rearrangements. Resultant genomic instability underlies elevated copy number alteration (CNA) rates of key HNSCC-associated genes, including PIK3CA, MYC, CSMD1, PTPRD, YAP1, MXD4, and EGFR. In contrast to sporadic HNSCC, we find no evidence of HPV infection in FA HNSCC, although positive cases were identified in gynecologic tumors. A murine allograft model of FA pathway-deficient SCC was enriched in SVs, exhibited dramatic tumor growth advantage, more rapid epithelial-to-mesenchymal transition, and enhanced autonomous inflammatory signaling when compared to an FA pathway-proficient model. In light of the protective role of the FA pathway against SV formation uncovered here, and recent findings of FA pathway insufficiency in the setting of increased formaldehyde load resulting in hematopoietic stem cell failure and carcinogenesis, we propose that high copy-number instability in sporadic HNSCC may result from functional overload of the FA pathway by endogenous and exogenous DNA crosslinking agents. Our work lays the foundation for improved FA patient treatment and demonstrates that FA SCC is a powerful model to study tumorigenesis resulting from DNA crosslinking damage. Citation Format: Andrew L. Webster, Mathijs A. Sanders, Krupa Patel, Ralf Dietrich, Raymond J. Noonan, Francis P. Lach, Ryan R. White, Audrey M. Goldfarb, Kevin Hadi, Matthew M. Edwards, Frank X. Donovan, Moonjung Jung, Sunandini Sridhar, Olivier Fedrigo, Huasong Tian, Joel Rosiene, Thomas Heineman, Jennifer Kennedy, Lorenzo Bean, Rasim O. Rosti, Rebecca Tryon, Ashlyn-Maree Gonzalez, Allana Rosenberg, Ji-Dung Luo, Thomas Carrol, Eunike Velleuer, Jeff C. Rastatter, Susanne I. Wells, Jordi Surrallés, Grover Bagby, Margaret L. MacMillan, John E. Wagner, Maria Cancio, Farid Boulad, Theresa Scognamiglio, Roger Vaughan, Amnon Koren, Marcin Imielinski, Settara Chandrasekharappa, Arleen D. Auerbach, Bhuvanesh Singh, David Kutler, Peter J. Campbell, Agata Smogorzewska. Fanconi anemia pathway deficiency drives copy number variation in squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6196.

Eltrombopag inhibits TET dioxygenase to contribute to hematopoietic stem cell expansion in aplastic anemia.

Eltrombopag, an FDA-approved non-peptidyl thrombopoietin receptor agonist, is clinically used for the treatment of aplastic anemia, a disease characterized by hematopoietic stem cell failure and pancytopenia, to improve platelet counts and stem cell function. Eltrombopag treatment results in a durable trilineage hematopoietic expansion in patients. Some of the eltrombopag hematopoietic activity has been attributed to its off-target effects, including iron chelation properties. However, the mechanism of action for its full spectrum of clinical effects is still poorly understood. Here, we report that eltrombopag bound to the TET2 catalytic domain and inhibited its dioxygenase activity, which was independent of its role as an iron chelator. The DNA demethylating enzyme TET2, essential for hematopoietic stem cell differentiation and lineage commitment, is frequently mutated in myeloid malignancies. Eltrombopag treatment expanded TET2-proficient normal hematopoietic stem and progenitor cells, in part because of its ability to mimic loss of TET2 with simultaneous thrombopoietin receptor activation. On the contrary, TET inhibition in TET2 mutant malignant myeloid cells prevented neoplastic clonal evolution in vitro and in vivo. This mechanism of action may offer a restorative therapeutic index and provide a scientific rationale to treat selected patients with TET2 mutant–associated or TET deficiency–associated myeloid malignancies.

Scribble Is a Negative Regulator of Interferon-I Dependent Notch1 Activation in Adult Hematopoietic Stem Cells and Progenitors

Hematopoietic stem cells (HSC) are highly quiescent cells with the ability to rapidly enter cell cycle and differentiate through changes in their polarity and the disposition of intracellular molecular fate determinants in response to microenvironment (ME) cues. Interferons type 1 (IFN-I) are ME cytokines produced during the physiological response mounted to combat a viral infection. In bone marrow hematopoiesis, IFN-I induces activation and proliferation of HSC. Clinically, patients treated with IFN-I, as well as individuals suffering from IFN-I associated chronic disease, often exhibit sustained hematological cytopenias and HSC failure. The precise molecular mechanisms that govern HSC behavior in response to IFN-I are still unclear. Our data highlights that Scribble deficient HSC are less sensitive to IFN-I mediated activation. By using hematopoietic specific deletion of Scribble in murine hematopoiesis (Vav-Cre;Scribble KO); we demonstrated that Scribble deficient LSK CD150 +/CD48 - HSC are less responsive to polyinositide-polycytidine (pI:C) induced IFN-I mediated activation and retain cellular quiescence (G0:45±5.4% vs 63±2.7% in WT and Scribble KO, respectively, p<0.05). IFN-I induced upregulation of Sca-1 expression was also significantly hampered in Scribble deficient HSC. Functionally, serial transplantation experiments demonstrated that in response to poly I:C, Scribble deficient HSC display increased competitive repopulating potential (26±1.3% vs 38±1.2% BM chimerism for WT and KO BM in secondary recipients and 38±2.5% vs 48±2.7% BM chimerism in tertiary recipients, p<0.01). The maintenance of cellular quiescence and function for Scribble deficient HSC are independent of canonical IFN-I driven STAT-1 signaling, as we report no differences in STAT-1 activation, nuclear translocation or the expression of STAT-1 canonical target genes in response to pI:C. Unsupervised transcriptomics analysis of Scribble-deficient HSC supported dysregulation of Notch signaling. Furthermore, Scribble deficiency in non-activated LSK HSC and progenitors (HSPC) was associated with constitutive activation and cleavage of Notch1 (Notch1 ICD;~3 fold) at levels comparable to IFN-I mediated activation of WT HSPC. However, Scribble deficient HSPC did not exhibit further Notch1 cleavage activation upon in vivo IFN-I induction. Pharmacological in vivo γ-secretase inhibition (YO-01027) prevented the protective effect of Scribble deficiency on IFN-I dependent loss of HSC quiescence. These data indicate that Notch1 activation, and subsequent cleavage, is indispensable for Scribble deficient HSC quiescence in response to IFN-I. Active Cdc42 is a critical regulator of HSC quiescence and fate, and previous studies have demonstrated that Scribble controls HSC asymmetric division potential and fate through the PDZ mediated scaffolding of cytosolic Yap1 with activated Cdc42 (Cdc42-GTP). Next to determine whether poly I:C mediated Notch1 cleavage linked with Cdc42 activity, we analyzed the protein interactions between cleaved Notch1 and Cdc42-GTP in relation with Scribble. Our findings revealed that Scribble associates with non-cleaved, membrane bound Notch but upon in vivo IFN-I induction Notch1 is cleaved, activated and translocates with Scribble-free, activated Cdc42 to the nucleus of HSC. Deletion of HSC Scribble associated with a reduced (~45%, p<0.001) proximity interaction between cleaved Notch1 and Cdc42-GTP. Collectively our findings identify that Scribble controls IFN-I mediated HSPC activation through induction of Notch1 cleavage and Cdc42 activity, and highlight such interaction as a new potential target to dampen inflammation driven HSC exhaustion. DisclosuresCancelas: Cerus Co: Research Funding; TerumoBCT: Research Funding; Hemanext: Membership on an entity's Board of Directors or advisory committees, Research Funding; Cytosorbents Inc: Research Funding; Fresenius-Kabi LLC: Research Funding; Westat Inc: Research Funding; Vascular Solutions Inc.: Research Funding; Hemerus LLC: Research Funding; University of South Florida/MEQU Inc: Research Funding.

JMJD6 promotes self-renewal and regenerative capacity of hematopoietic stem cells

AbstractLifelong multilineage hematopoiesis critically depends on rare hematopoietic stem cells (HSCs) that reside in the hypoxic bone marrow microenvironment. Although the role of the canonical oxygen sensor hypoxia-inducible factor prolyl hydroxylase has been investigated extensively in hematopoiesis, the functional significance of other members of the 2-oxoglutarate (2-OG)-dependent protein hydroxylase family of enzymes remains poorly defined in HSC biology and multilineage hematopoiesis. Here, by using hematopoietic-specific conditional gene deletion, we reveal that the 2-OG–dependent protein hydroxylase JMJD6 is essential for short- and long-term maintenance of the HSC pool and multilineage hematopoiesis. Additionally, upon hematopoietic injury, Jmjd6-deficient HSCs display a striking failure to expand and regenerate the hematopoietic system. Moreover, HSCs lacking Jmjd6 lose multilineage reconstitution potential and self-renewal capacity upon serial transplantation. At the molecular level, we found that JMJD6 functions to repress multiple processes whose downregulation is essential for HSC integrity, including mitochondrial oxidative phosphorylation (OXPHOS), protein synthesis, p53 stabilization, cell cycle checkpoint progression, and mTORC1 signaling. Indeed, Jmjd6-deficient primitive hematopoietic cells display elevated basal and maximal mitochondrial respiration rates and increased reactive oxygen species (ROS), prerequisites for HSC failure. Notably, an antioxidant, N-acetyl-l-cysteine, rescued HSC and lymphoid progenitor cell depletion, indicating a causal impact of OXPHOS-mediated ROS generation upon Jmjd6 deletion. Thus, JMJD6 promotes HSC maintenance and multilineage differentiation potential by suppressing fundamental pathways whose activation is detrimental for HSC function.

Abstract IA18: The PI3K isoforms in myeloid leukemia and hematopoietic stem cells

Abstract The Class IA PI3 kinase isoforms (p110α, β, and δ) transduce many growth factor signals that are important for the proliferation, differentiation, and self-renewal of hematopoietic stem cells (HSCs). Mutations in growth factor receptors or RAS proteins are commonly observed in patients with acute myeloid leukemia (AML), leading to activation of the PI3K/AKT pathway. Therefore, PI3K inhibition is an attractive therapeutic strategy for a large subset of AML patients. We previously reported that p110α is dispensable for HSC function, suggesting that redundancy exists between the Class IA isoforms in HSCs (Gritsman et al., J Clin Invest 2014). However, we have identified a specific role for p110α in RAS-mutated myeloid leukemia. Furthermore, we found that the p110α-selective inhibitor BYL-719 can sensitize RAS-mutated leukemic cells to the MEK inhibitor MEK-162. While PI3K inhibitors have multiple clinical indications, including in hematologic malignancies, it is still unclear whether PI3K plays an important role in normal HSC function. Upon environmental stresses like chemotherapy or infection, quiescent HSCs must enter the cell cycle to re-establish homeostasis and promote emergency myelopoiesis. To test for redundancy between p110α and p110δ in HSCs, we generated p110α and p110δ double knockout (DKO) mice, with germline deletion of p110δ and conditional deletion of p110α. DKO mice have leukopenia, anemia, and decreased numbers of lymphoid-primed multipotent progenitors (LMPPs) in the bone marrow, though HSC numbers are unchanged. In competitive repopulation assays, B-cell maintenance was severely impaired, while the myeloid and T-cell lineages were relatively preserved. This suggests that HSC function is not impaired at steady state. We performed transcriptome analysis of DKO, p110δ, and p110α KO HSCs and LMPPs after bone marrow transplantation. Gene set enrichment analysis revealed downregulation of gene sets associated with cell cycle progression. Interestingly, we also observed negative enrichment of inflammatory response gene sets in both p110δ KO and DKO HSCs and LMPPs. To examine the roles of p110δ and α in the hematopoietic stress response, we injected p110δ, DKO, and WT mice with 5-fluorouracil. We observed significantly decreased survival of 5-FU-treated DKO mice due to impaired hematopoietic recovery, associated with the failure of HSCs to enter the cell cycle. Consistent with this finding, we also observed a decrease in myeloid reconstitution by DKO HSCs in secondary competitive transplantation, suggesting that p110α and p110δ play redundant roles in emergency myelopoiesis. We also observed defects in downstream signaling in DKO HSCs and progenitors in response to IL1β or TNFα stimulation. Our results suggest that p110α and δ act in a redundant fashion to transduce specific inflammatory signals in HSPCs in response to hematopoietic stress. Our results have important implications for the use of PI3K inhibitors in combination with chemotherapy and in other hematopoietic stress conditions. Citation Format: Shayda Hemmati, Taneisha Sinclair, Meng Tong, Boris Bartholdy, Rachel Okabe, Kristina Ames, Leanne Ostrodka, Tamanna Haque, Imit Kaur, Anupriya Agarwal, Jean Zhao, Thomas Roberts, Kira Gritsman. The PI3K isoforms in myeloid leukemia and hematopoietic stem cells [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr IA18.

Reversible TET Inhibitor As a Hematopoietic Stem Cell Booster: A Novel Strategy to Improve Stem Cell Function

Hematopoietic stem cells (HSCs) are responsible for the adaptation capacity in times of need but are also subjected to disease processes, natural or iatrogenic damage, and age-related attrition. The latter can lead to deficient production, decreased compensatory capacity and degenerative diseases such as MDS. In childhood, hereditary BMF predominates but increasingly with age, acquired idiopathic AA is a leading cause of HSC failure. Throughout life iatrogenic cases and cumulative exposures to environmental toxicities may lead to failure of the HSC compartment. Pharmacologic HSC boosters capable of expanding HSCs would have a wide range of clinical applications in acquired and inherited BMF states, including reconstitution of exhausted hematopoiesis after chemotherapy, aging, or HSC grafting. Currently, hematopoietic growth factors (HGF) are used but lead to progenitor rather than HSC expansion despite their success in clinical application. Depletion of TET2 in murine models leads to impairment of cellular differentiation and increases the proportion of HSCs and progenitors suggesting that TET2 is a key regulator of hematopoietic homeostasis and HSC self-renewal. Alterations of TET2 via somatic mutations and/ or deletion are frequent in MDS whereby HSC and progenitor expansion may be a key component of neoplastic evolution. We hypothesized that chemical agents reversibly inhibiting TET2 activity might phenocopy HSC expansion due to mutations and be applied as HSC boosters. Using structure guided approaches, we designed, synthesized and subsequently optimized bioavailable TET inhibitors (TETi). Among them, the one of the most effective TETi, named TETi76, showed dose-dependent inhibitory activity against TET dioxygenases in in vitro cell-free and cell culture systems with 5hmC production as a read out. We developed esterified forms of TETi76 and used in our cellular experimental models. Esterified TETi is bioavailable and non-toxic to normal bone marrow (BM) cells in therapeutically effective doses. Treatment of TETi76 resulted in a 44+15% and 42+17% increase in the clonogenic potential of human and murine BM cells consistent with the proliferative advantage gained by loss of TET activity in HSCs. We then performed serial replating experiments to determine the effect of TET inhibition on immature hematopoietic progenitor cells. Over 3 consecutive passages, TETi76 treatment prolonged the durability and capacity of human HSCs to maintain colony-forming cells (155+24 vs.107+14 colonies per 1x105 P1 cells). As long-term culture initiating cells (LTC-IC) are the best in vitro surrogates of HSCs, we also investigated the effect of TETi76 in LTC-IC cultures (n=3). The weekly addition of 1μM-TETi76 resulted in nearly 2-fold increase in LTC-IC numbers at the end of the culture (116±27 vs. 64 ±26 colonies per 2x106 P1 cells, p=.011). Expansion of grafts e.g., in the setting of umbilical cord HSC transplant (UCHSC) could be an important medical area of application of TETi76. We performed suspension cultures (n=3) with an optimal cocktail of hematopoietic growth factors (HGF) in the presence or absence of TETi76 (1μM). In control cultures, total cellular output peaked on day 14, but in the presence of TETi, growth continued beyond day 28. Cumulatively, total cellular output per 106 input was 27+2.61 x106cells/mL in control cultures and 32+0.642 x106 cells/mL in TETi treated cultures on day 28. CD34+ cell output was significantly higher in cultures treated with TETi vs. vehicle (2.5x105vs. 0.9X106)CD34+ cells per 1x104 CD34+ cell input). Similar effects were observed in murine BM suspension cultures. BM cells from C57BL/6 mice (n=3) were supplemented with HGF± TETi76 (1μM). TETi treatment led to a 5-fold HSC expansion compared to vehicle treated cells at 20 days of culture. The effect of TETi76 was reversed by treatment with ascorbic acid (50 μM), a known TET activator. Cumulatively our in vitro results suggest that the presence of TETi prevents exhaustion of immature cells, observed with growth factor driven expansion. In summary, our study indicates that novel agents modulating TET activity prevent exhaustion of HSC and may help expand the HSC in vitro. In vivo experiments examining the effects of TETi on hematopoietic recovery following radiation-induced aplasia, and competitive transplant experiments of grafts exposed in vivo and in vitro to TETi are underway. Disclosures Sekeres: Millenium: Membership on an entity's Board of Directors or advisory committees; Syros: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees. Maciejewski:Novartis: Consultancy; Alexion: Consultancy.

Holding All the Cards-How Fanconi Anemia Proteins Deal with Replication Stress and Preserve Genomic Stability.

Fanconi anemia (FA) is a hereditary chromosomal instability disorder often displaying congenital abnormalities and characterized by a predisposition to progressive bone marrow failure (BMF) and cancer. Over the last 25 years since the discovery of the first linkage of genetic mutations to FA, its molecular genetic landscape has expanded tremendously as it became apparent that FA is a disease characterized by a defect in a specific DNA repair pathway responsible for the correction of covalent cross-links between the two complementary strands of the DNA double helix. This pathway has become increasingly complex, with the discovery of now over 20 FA-linked genes implicated in interstrand cross-link (ICL) repair. Moreover, gene products known to be involved in double-strand break (DSB) repair, mismatch repair (MMR), and nucleotide excision repair (NER) play roles in the ICL response and repair of associated DNA damage. While ICL repair is predominantly coupled with DNA replication, it also can occur in non-replicating cells. DNA damage accumulation and hematopoietic stem cell failure are thought to contribute to the increased inflammation and oxidative stress prevalent in FA. Adding to its confounding nature, certain FA gene products are also engaged in the response to replication stress, caused endogenously or by agents other than ICL-inducing drugs. In this review, we discuss the mechanistic aspects of the FA pathway and the molecular defects leading to elevated replication stress believed to underlie the cellular phenotypes and clinical features of FA.

Targeting NAD+ Modulating Enzymes to Improve HSC Function in Aging and Bone Marrow Failure

Human hematopoietic system produces various types of differentiated and short-lived cells with specialized functions, which require continuous replenishment through the function of hematopoietic stem cells (HSC). HSC failure is a common distal endpoint of various pathogenic mechanisms in almost all bone marrow failure (BMF) syndromes and associated diseases. Hematopoietic growth factor cocktails (HGF) used in expanding bone marrow cells e.g., to increase cellularity of the HSC grafts, lead to differentiation and decreased HSC count. Theoretically, when used in vivo, they may act on progenitors rather than HSC and lead to stimulation of clonal outgrowth. Our current ability to stimulate HSC self-renewal to provide reconstitution of long-term hematopoiesis is limited.Nicotinamide adenine dinucleotide (NAD+) serves as an essential cofactor and substrate for a number of critical cellular processes. NAD+ depletion may occur in response to DNA damage due to free radical/ionizing radiation attack, resulting in significant activation of NAD+ consuming PARPs. Because of their long lifespan, maintenance of the genomic integrity of HSCs by efficient and accurate DNA repair to reduce the risk of BMF and cellular transformation is essential. NAD+ is also required for the maintenance of sirtuins activity, important class III HDAC essential for the prevention of senescence. Aging or chronic immune activation and inflammatory cytokine production result in upmodulation of NAD+ degrading enzyme CD38 that rapidly depletes cellular and extracellular levels of NAD+. Various lines of evidence suggest that regulation of CD38 NADase activity is essential for maintenance of physiologic NAD+ levels. Enhancing NAD+ level can profoundly reduce oxidative cell damage in catabolic tissue, including blood. Consequently, promotion of intracellular NAD+ by preventing NAD+ catabolism represents a promising therapeutic strategy for degenerative diseases in general, and BMF and associated diseases in particular. Therefore; CD38, a major NAD+ degrading enzyme, can be an excellent therapeutic target to increase the cellular levels of NAD+ and consequently improve the function of HSC. Here we report the development of inhibitors of CD38 NADase activity that extends the self-renewal and proliferative life span of HSC.We used structure-guided virtual screening followed by docking simulation to develop CD38 inhibitors. The compounds were synthesized using rational chemical synthesis and characterized by high-resolution mass spectroscopy and C13 & H1NMR. HPLC based assays were performed to assess the ability of compounds to inhibit NAD+ degradation by recombinant CD38. Using an iterative approach of synthesis characterization and activity, we selected the most potent compound, designated as ccf1172, for further studies. Docking simulations, surface plasmon resonance, and HPLC based assays demonstrate that ccf1172 binds (KD=12 nM) and inhibits CD38 (IC50=10 nM) (Fig.1B, C&D). To further characterize the ability of ccf1172, colony forming assays (CFU-A) and long-term culture-initiating cell assays (LTCIC-A) were performed with cord blood, human and murine bone marrows. No GF-like activity was observed, but in combination with GF mix ccf1172 increased the number of erythroid and myeloid colonies (n=9) in dose-dependent manner with a maximal effect seen at 100 nM in a serial replating assay. Significant extension of proliferative life span of hematopoietic progenitors (n=5) were observed (Fig 1E). When we studied the ability of CD38 inhibitor to expand LTCICs in stromal cultures (n=3) as best in vitro surrogates of HSC, ccf1172 increased LTCIC numbers 2.6-fold at 10 nM. The effect did not require the presence of accessory cells as ccf1172 treatment resulted in ~2-fold increase in CD34+Lin-/CD45+ cells in stem cell culture media supplemented with growth factors over a period of 25 days (Fig 1F). The CD38 inhibitor demonstrated cytotoxic effects on nine different leukemic cell lines with IC 50 ranging from 1 to 5 µM while no effect was observed on normal bone marrow.Here, we demonstrate that CD38 inhibition may be a potential therapeutic principle for ex vivo and in vivo expansion of HSC. Decreasing levels of NAD+ have been linked to aging and stem cell dysfunction, as a key aspect of various BMF syndromes. The strategy of CD38 inhibition to preserve NAD+ is innovative and relevant therapeutic strategy. [Display omitted] DisclosuresSaunthararajah:Novo Nordisk, A/S: Patents & Royalties; EpiDestiny, LLC: Patents & Royalties. Maciejewski:Apellis Pharmaceuticals: Consultancy; Ra Pharmaceuticals, Inc: Consultancy; Ra Pharmaceuticals, Inc: Consultancy; Alexion Pharmaceuticals, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Alexion Pharmaceuticals, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Apellis Pharmaceuticals: Consultancy.

PI3Kinase Alpha and Delta Promote Hematopoietic Stem Activation Under Stress

Hematopoietic stem cells (HSCs) are a rare population in the bone marrow (BM) that mostly exists in a quiescent state. Upon environmental stresses such as infection, inflammatory signals are released and induce HSCs to proliferate to quickly re-establish homeostasis and maintain the blood system. Many hematopoietic growth factors and chemokines signal through PI3K pathway. In hematopoietic cells, 3 Class IA PI3K isoforms are expressed (p110α, β, and δ), each encoded by a different gene. While p110α and p110β are ubiquitously expressed, p110δ is enriched in leukocytes. We previously showed that p110α is dispensable for HSC function (Gritsman et al, J Clin Invest 2014 124:1794-1809), suggesting redundancy between Class I PI3K isoforms in HSCs.To test for potential redundancy between p110α and p110δ in HSCs, we generated mice with conditional deletion of p110α and germline deletion of p110δ (DKO mice). DKO mice have anemia, leukopenia and decreased BM cellularity. While there is no change in HSC numbers, the number of lymphoid-primed multipotent progenitors (LMPPs) is significantly decreased. In the primary competitive repopulation assay, DKO BM cells fail to reconstitute the B cell lineage, while the myeloid and T cell lineages were relatively preserved. However, in a secondary competitive transplantation setting, we also observed a significant decrease in myeloid reconstitution, suggesting that p110α and p110δ play redundant roles in emergency myelopoiesis. To examine effects of p110α and p110δ deletion on gene expression, we performed microarray analysis of WT, DKO, p110δ-/-, and p110α-/- hematopoietic stem and progenitor cells (HSPCs) after bone marrow transplantation. Gene set enrichment analysis revealed negative enrichment of gene sets associated with inflammatory response pathways in both DKO HSCs and LMPPs. In DKO HSCs, we also observed negative enrichment of gene sets associated with cell cycle progression.To further examine the roles of p110δ and p110α in the hematopoietic stress response, we injected DKO, p110δ-/-, and WT; Mx1-Cre mice with 5-fluorouracil (5-FU). We observed decreased survival of 5-FU-treated DKO mice associated with impaired hematopoietic recovery, and with the failure of HSCs to enter the cell cycle. Given the important roles of inflammatory signaling pathways in HSC activation and emergency myelopoiesis, we examined the roles of p110α and p110δ in signal transduction in HSPCs in response to IL1β or TNFα. Our phospho-flow cytometry analysis revealed a decrease in p38-MAPK phosphorylation in both p110δ-/- and DKO HSPCs, both at baseline and after stimulation with either IL1β or TNFα. To confirm these results, we stimulated p110δ KO and DKO cKit-enriched bone marrow cells with IL1β or TNFα. We observed a significant decrease in both p38-MAPK phosphorylation and phosphorylation of Akt at Ser473 in DKO cells, but not in p110δ-/- cells, both at baseline and with IL1-β or TNF-α stimulation. This suggests that both p110α and p110δ are required for optimal transduction of IL1β or TNFα in hematopoietic progenitors. Surprisingly, we found that DKO HSPCs can enter the cell cycle normally upon in vivo stimulation with lipopolysaccharide (LPS), which simulates bacterial infection. Our results suggest that p110α and δ act in a redundant fashion to transduce specific inflammatory signals in HSPCs, such as IL1β and TNFα, in response to hematopoietic stress. Our findings have important implications for the use of PI3K inhibitors in combination with chemotherapy and in the setting of infection or inflammation. DisclosuresNo relevant conflicts of interest to declare.

Spred1 Safeguards Hematopoietic Homeostasis against Diet-Induced Systemic Stress

Stem cell self-renewal is critical for tissue homeostasis, and its dysregulation can lead to organ failure or tumorigenesis. While obesity can induce varied abnormalities in bone marrow components, it is unclear how diet might affect hematopoietic stem cell (HSC) self-renewal. Here, we show that Spred1, a negative regulator of RAS-MAPK signaling, safeguards HSC homeostasis in animals fed a high-fat diet (HFD). Under steady-state conditions, Spred1 negatively regulates HSC self-renewal and fitness, in part through Rho kinase activity. Spred1 deficiency mitigates HSC failure induced by infection mimetics and prolongs HSC lifespan, but it does not initiate leukemogenesisdue to compensatory upregulation of Spred2.In contrast, HFD induces ERK hyperactivation and aberrant self-renewal in Spred1-deficient HSCs, resulting in functional HSC failure, severe anemia, and myeloproliferative neoplasm-like disease. HFD-induced hematopoietic abnormalities are mediated partly through alterations to the gut microbiota. Together, these findings reveal thatdiet-induced stress disrupts fine-tuningof Spred1-mediated signals to govern HSC homeostasis.

Eosinophil-derived CCL-6 impairs hematopoietic stem cell homeostasis

Eosinophils (Eos) have been long considered as end-stage effector cells in the hierarchical hematopoietic system. Numerous lines of evidence have suggested that Eos are multifunctional leukocytes with respect to the initiation, propagation and regulation of various inflammatory or immune reactions, especially in allergic diseases. Recent studies have shown that Eos are also required for maintenance of bone marrow plasma cells and differentiation of B cells. However, it remains unclear whether Eos contributes to regulation of hematopoietic stem cell (HSC) homeostasis. Here, we demonstrate that Eos disrupt HSC homeostasis by impairing HSC quiescence and reconstitution ability in wild-type mice following ovalbumin (OVA) challenge and even by causing bone marrow HSC failure and exhaustion in Cd3δ-Il-5 transgenic mice. The impaired maintenance and function of HSCs were associated with Eos-induced redox imbalance (increased oxidative phosphorylation and decreased anti-oxidants levels). More importantly, using mass spectrometry, we determined that CCL-6 is expressed at a high level under eosinophilia. We demonstrate that CCL-6 is Eos-derived and responsible for the impaired HSC homeostasis. Interestingly, blockage of CCL-6 with a specific neutralizing antibody, restored the reconstitution ability of HSCs while exacerbating eosinophilia airway inflammation in OVA-challenged mice. Thus, our study reveals an unexpected function of Eos/CCL-6 in HSC homeostasis.

A crucial role for the homeodomain transcription factor Hhex in lymphopoiesis

AbstractThe hematopoietically expressed homeobox gene, Hhex, is a transcription factor that is important for development of definitive hematopoietic stem cells (HSCs) and B cells, and that causes T-cell leukemia when overexpressed. Here, we have used an Hhex inducible knockout mouse model to study the role of Hhex in adult hematopoiesis. We found that loss of Hhex was tolerated in HSCs and myeloid lineages, but resulted in a progressive loss of B lymphocytes in the circulation. This was accompanied by a complete loss of B-cell progenitors in the bone marrow and of transitional B-cell subsets in the spleen. In addition, transplantation and in vitro culture experiments demonstrated an almost complete failure of Hhex-null HSCs to contribute to lymphoid lineages beyond the common lymphoid precursor stage, including T cells, B cells, NK cells, and dendritic cells. Gene expression analysis of Hhex-deleted progenitors demonstrated deregulated expression of a number of cell cycle regulators. Overexpression of one of these, cyclin D1, could rescue the B-cell developmental potential of Hhex-null lymphoid precursors. Thus, Hhex is a key regulator of early lymphoid development, functioning, at least in part, via regulation of the cell cycle.

Role of Casein Kinase 1A1 in the Biology and Targeted Therapy of del(5q) MDS

The casein kinase 1A1 gene (CSNK1A1) is a putative tumor suppressor gene located in the common deleted region for del(5q) myelodysplastic syndrome (MDS). We generated a murine model with conditional inactivation of Csnk1a1 and found that Csnk1a1 haploinsufficiency induces hematopoietic stem cell expansion and a competitive repopulation advantage, whereas homozygous deletion induces hematopoietic stem cell failure. Based on this finding, we found that heterozygous inactivation of Csnk1a1 sensitizes cells to a CSNK1 inhibitor relative to cells with two intact alleles. In addition, we identified recurrent somatic mutations in CSNK1A1 on the nondeleted allele of patients with del(5q) MDS. These studies demonstrate that CSNK1A1 plays a central role in the biology of del(5q) MDS and is a promising therapeutic target.

Deletion of the histone methyltransferase Setdb1 during hematopoiesis results in hematopoietic stem cell failure and abrogates B cell development

Hematopoietic stem cells (HSCs) give rise to all blood lineages. The fact that transcription factors (TFs) determine gene expression profiles during hematopoiesis has been broadly demonstrated. However, we still lack a detailed understanding about how the chromatin context influences the ability of TFs in activating or repressing genes. Because chromatin modifications might play significant roles for TF binding and function, we investigate the importance of H3K9me3, a repressive chromatin mark established by the histone methyltransferase Setdb1.

Immune Injury by Allogeneic CD4+ T Cells Leads to Host Hematopoietic Stem Cell Dormancy and Prevents Engraftment of Donor Cells.

Abstract 2366The bone marrow (BM) is a complex microsystem that supports lifelong blood production and dynamically responds to injury, blood cell loss and senescence. At steady-state most hematopoietic stem cells (HSC) are quiescent. However, in situations of increased demand, their activation is triggered by an array of signals, such as cytokines released during infections. The susceptibility of BM to injury by T cells (TC) is long recognized, e.g. in aplastic anemia, and BM failure syndromes, and the inhibitory effect of IFNg on the proliferative activity of (HSC) is established. How donor TC given as part of an allogeneic hematopoietic cell transplantation (HCT) affect the recipient BM microenvironment and influence HSC activity has not been elucidated. Yet, graft TC are believed to be essential to overcome host barriers, especially following lower intensity (non-myeloablative) conditioning that spares host HSC and residual host immunity. We examined in an MHC-matched, minor-antigen-mismatched mouse model of non-myeloablative HCT the impact of graft TC subsets on HSC and hematopoietic reconstitution. BALB.K (H2k) and BALB.B (H2b) mice were prepared with sublethal radiation at a dose that allows donor cells (AKR/J [H2k] and C57BL/6 [H2b]) to engraft but also permits survival of host HSC and immune populations. Grafts consisted of purified HSC (KTLS; cKit+Thy1.1loLin−Sca-1+) given alone or supplemented with donor TC or TC subsets. Rather than improve engraftment, donor TC, specifically CD4+ cells, resulted in detainment of host HSC in their niches and consequent failure of donor HSC to engraft. These events appeared to be initiated specifically by conventional alloreactive donor CD4+CD25− cells. These CD4+CD25− cells interacted with immune-competent host cells, stimulating CD11c+ dendritic cells to upregulate the activation markers CD40 and MHC II. Following their activation dendritic cells produced excessive amounts of IL-12. IL-12, in turn, activated donor CD4+ cells to produce IFNg. This Th1-response was prominent in BM but absent in the spleen. In this Th1-cytokine environment, high levels of quiescent host HSC were observed, arrested at the short-term HSC stage, while more mature, multipotent progenitors were absent. Moreover, HSC arrest correlated with marrow aplasia, analogous to that seen in BM failure syndromes. This hypocellularity was only observed in recipients of CD4+ containing grafts, while recipients given HSC alone or HSC in combination with CD8+ TC had prompt blood regeneration. When resting HSC were FACS-purified from the BM of recipients of HSC+CD4+ and infused into secondary recipients they promptly expanded and gave rise to all blood lineages. Although recipients of HSC+CD4+ cells ultimately recovered blood formation, B lymphopoiesis was markedly delayed. No clinical signs of graft-versus-host disease were noted in these mice. Lymphocytes that recovered in these recipients of HSC+CD4+ cells were of host type only. In contrast, recipients of pure HSC demonstrated prompt donor cell engraftment, and mixed chimerism in all lymphoid lineages, including T cells. Our model corroborates the importance of the cytokine microenvironment in hematopoietic reconstitution and marrow failure post-transplant. Moreover, our findings underscore the importance of IFNg in BM aplasia and support the idea that IFNg can, under physiologic conditions, exert regulatory effects on hematopoiesis by controlling the exit of ST-HSC out of the marrow niche. Our studies shed new light on understanding the physiology of engraftment and marrow failure after HCT using nonmyeloablative conditioning, and indicate that the composition of the graft is of critical importance due to its effects on the marrow microenvironment We postulate that in some naturally occurring settings of BM inflammation (e.g., infections) detainment of resting HSC in their niche may provide protection of hematopoiesis and that comparable events are relevant for engraftment post-HCT. Disclosures:No relevant conflicts of interest to declare.

A Mouse Model of Hematopoietic Stem Cell Failure Associated with p53-Loss and Defective Fbw7-Dependent Cyclin E Regulation.

AbstractAbstract 2297Recent findings have challenged the notion that increased proliferation of hematopoietic stem cells (HSCs) necessarily restricts their self-renewal capacity. We have studied the physiologic consequences to HSCs of ablating a key cell cycle regulatory mechanism, Fbw7-dependent cyclin E ubiquitination, using germline knock-in of a cyclin ET74A T393A allele. Fbw7 is a tumor suppressor that regulates the abundance of several oncoprotein substrates by ubiquitin-mediated proteolysis, including cyclin E, Notch, and c-Myc. Cyclin E overexpression in vivo is associated with increased proliferation in some cellular contexts as well as a variety of deleterious consequences, including genomic instability, senescence, or apoptosis. In HSCs, Fbw7-loss has been shown to induce self-renewal and multi-lineage reconstitution defects, and the effect of Fbw7-loss in HSCs has been ascribed to dysregulated Myc and Notch expression. Using the cyclin ET74A T393A mouse model, we tested the hypothesis that impaired Fbw7-mediated regulation of cyclin E, specifically, promotes HSC exhaustion due to loss of self-renewal capacity. We first examined bone marrow HSC counts and their cell cycle kinetics in cyclin E knock-in and wild-type control mice at steady state and following hematologic injury induced by 5-fluorouracil treatment. We found that cyclin E dysregulation reduces numbers of quiescent HSCs and increases cells in S/G2/M-phases, while decreasing total numbers of HSCs, phenotypes made more severe after recovery from hematologic stress. Using bromodeoxyuridine labeling studies, we found that excess cyclin E activity causes DNA hyper-replication in cyclin ET74A T393A HSCs in a cell autonomous manner. By enumerating multi-potent progenitors (MPPs), we ruled out increased rate of transit from HSC-to-MPP as a cause of the apparent exhaustion of cyclin E knock-in HSCs. Thus, dysregulated cyclin E in HSCs promotes both increased proliferation and depletion of the HSC pool. Serial transplantation further revealed peripheral blood reconstitution defects associated with cyclin ET74A T393A HSCs. Recently, we have found that p53 is activated by dysregulated cyclin E in hematopoietic cells in vivo, in association with phosphorylation of both p53 and Chk1 proteins, resembling a DNA damage-type response. Interestingly, p53-loss has been found to be associated with a gain of HSC self-renewal activity. We therefore hypothesized that p53-loss would rescue the self-renewal defect of cyclin E knock-in HSCs. Surprisingly, we discovered that cyclin ET74A T393A; p53-null HSCs showed evidence of significantly worse self-renewal and peripheral reconstitution, compared to p53-null HSCs, defects that are more severe than those associated with impaired Fbw7-mediated cyclin E control in the setting of wild-type p53 (Chi-squared test, p<0.0001). Thus, our data are consistent with the concept that intact p53 function, in the setting of oncogenic insult, can preserve partial HSC self-renewal capacity, and its loss in vivo is detrimental to HSC viability when accompanied by defects in cell cycle control mechanisms. Disclosures:No relevant conflicts of interest to declare.

Perturbation of the Hematopoietic System during Embryonic Liver Development Due to Disruption of Polyubiquitin Gene Ubc in Mice

Disruption of the polyubiquitin gene Ubc leads to a defect in fetal liver development, which can be partially rescued by increasing the amount of ubiquitin. However, it is still not known why Ubc is required for fetal liver development and the nature of the defective cell types responsible for embryonic lethality have not been characterized. In this study, we assessed the cause of embryonic lethality with respect to the fetal liver hematopoietic system. We found that Ubc was highly expressed in the embryonic liver, and the proliferation capacity of fetal liver cells was reduced in Ubc−/− embryos. Specifically, Ubc was most highly expressed in hematopoietic cells, and the proliferation capacity of hematopoietic cells was significantly impaired in Ubc−/− embryos. While hematopoietic cell and hematopoietic stem cell (HSC) frequency was maintained in Ubc−/− embryos, the absolute number of these cells was diminished because of reduced total liver cell number in Ubc−/− embryos. Transplantations of fetal liver cells into lethally irradiated recipient mice by non-competitive and competitive reconstitution methods indicated that disruption of Ubc does not significantly impair the intrinsic function of fetal liver HSCs. These findings suggest that disruption of Ubc reduces the absolute number of HSCs in embryonic livers, but has no significant effect on the autonomous function of HSCs. Thus, the lethality of Ubc−/− embryos is not the result of intrinsic HSC failure.