HLF expression defines the human hematopoietic stem cell state.

Hematopoietic stem cells (HSCs) sustain blood cell homeostasis throughout life and can regenerate all blood lineages after transplantation. Despite this clear functional definition, highly enriched isolation of human HSCs can currently only be achieved through combinatorial assessment of multiple surface antigens. Although several transgenic HSC reporter mouse strains have been described, no analogous approach to prospectively isolate human HSCs has been reported. To identify genes with the most selective expression in human HSCs, we profiled population and single-cell transcriptomes of unexpanded and ex vivo cultured cord blood-derived hematopoietic stem and progenitor cells as well as peripheral blood, adult bone marrow, and fetal liver. On the basis of these analyses, we propose the master transcription factor HLF (hepatic leukemia factor) as one of the most specific HSC marker genes. To directly track its expression in human hematopoietic cells, we developed a genomic HLF reporter strategy, capable of selectively labeling the most immature blood cells on the basis of a single engineered parameter. Most importantly, HLF-expressing cells comprise all stem cell activity in culture and in vivo during serial transplantation. Taken together, these results experimentally establish HLF as a defining gene of the human HSC state and outline a new approach to continuously mark these cells with high fidelity.

EPCR expression marks UM171-expanded CD34+ cord blood stem cells.

A small subset of human cord blood CD34+ cells express endothelial protein C receptor (EPCR/CD201/PROCR) when exposed to the hematopoietic stem cell (HSC) self-renewal agonist UM171. In this article, we show that EPCR-positive UM171-treated cells, as opposed to EPCR-negative cells, exhibit robust multilineage repopulation and serial reconstitution ability in immunocompromised mice. In contrast to other stem cell markers, such as CD38, EPCR expression is maintained when cells are introduced in culture, irrespective of UM171 treatment. Although engineered overexpression of EPCR fails to reproduce the effects of UM171 on HSC activity, its expression is required for the repopulating activity of human HSCs. Altogether, our results indicate that EPCR is a reliable and cell culture-compatible marker of UM171-expanded human cord blood HSCs.

Haploinsufficiency screen highlights two distinct groups of ribosomal protein genes essential for embryonic stem cell fate.

In a functional genomics screen of mouse embryonic stem cells (ESCs) with nested hemizygous chromosomal deletions, we reveal that ribosomal protein (RP) genes are the most significant haploinsufficient determinants for embryoid body (EB) formation. Hemizygocity for three RP genes (Rps5, Rps14, or Rps28), distinguished by the proximity of their corresponding protein to the ribosome's mRNA exit site, is associated with the most profound phenotype. This EB phenotype was fully rescued by BAC or cDNA complementation but not by the reduction of p53 levels, although such reduction was effective with most other RP-deleted clones corresponding to non-mRNA exit-site proteins. RNA-sequencing studies further revealed that undifferentiated ESCs hemizygous for Rps5 showed reduced expression levels of several mesoderm-specific genes as compared with wild-type counterparts. Together, these results reveal that RP gene dosage limits the differentiation, not the self-renewal, of mouse ESCs. They also highlight two separate mechanisms underlying this process, one of which is p53 independent.

RNAi screen identifies Jarid1b as a major regulator of mouse HSC activity.

Histone methylation is a dynamic and reversible process proposed to directly impact on stem cell fate. The Jumonji (JmjC) domain-containing family of demethylases comprises 27 members that target mono-, di-, and trimethylated lysine residues of histone (or nonhistone) proteins. To evaluate their role in regulation of hematopoietic stem cell (HSC) behavior, we performed an in vivo RNAi-based functional screen and demonstrated that Jarid1b and Jhdm1f play opposing roles in regulation of HSC activity. Decrease in Jarid1b levels correlated with an in vitro expansion of HSCs with preserved long-term in vivo lymphomyeloid differentiation potential. Through RNA sequencing analysis, Jarid1b knockdown was associated with increased expression levels of several HSC regulators (Hoxa7, Hoxa9, Hoxa10, Hes1, Gata2) and reduced levels of differentiation-associated genes. shRNA against Jhdmlf, in contrast, impaired hematopoietic reconstitution of bone marrow cells. Together, our studies identified Jarid1b as a negative regulator of HSC activity and Jhdmlf as a positive regulator of HSC activity.

SF3B1 mutations provide genetic vulnerability to copper ionophores in human acute myeloid leukemia.

In a phenotypical screen of 56 acute myeloid leukemia (AML) patient samples and using a library of 10,000 compounds, we identified a hit with increased sensitivity toward SF3B1-mutated and adverse risk AMLs. Through structure-activity relationship studies, this hit was optimized into a potent, specific, and nongenotoxic molecule called UM4118. We demonstrated that UM4118 acts as a copper ionophore that initiates a mitochondrial-based noncanonical form of cell death known as cuproptosis. CRISPR-Cas9 loss-of-function screen further revealed that iron-sulfur cluster (ISC) deficiency enhances copper-mediated cell death. Specifically, we found that loss of the mitochondrial ISC transporter ABCB7 is synthetic lethal to UM4118. ABCB7 is misspliced and down-regulated in SF3B1-mutated leukemia, creating a vulnerability to copper ionophores. Accordingly, ABCB7 overexpression partially rescued SF3B1-mutated cells to copper overload. Together, our work provides mechanistic insights that link ISC deficiency to cuproptosis, as exemplified by the high sensitivity of SF3B1-mutated AMLs. We thus propose SF3B1 mutations as a biomarker for future copper ionophore-based therapies.

Genome-wide interrogation of Mammalian stem cell fate determinants by nested chromosome deletions.

Understanding the function of important DNA elements in mammalian stem cell genomes would be enhanced by the availability of deletion collections in which segmental haploidies are precisely characterized. Using a modified Cre-loxP-based system, we now report the creation and characterization of a collection of ∼1,300 independent embryonic stem cell (ESC) clones enriched for nested chromosomal deletions. Mapping experiments indicate that this collection spans over 25% of the mouse genome with good representative coverage of protein-coding genes, regulatory RNAs, and other non-coding sequences. This collection of clones was screened for in vitro defects in differentiation of ESC into embryoid bodies (EB). Several putative novel haploinsufficient regions, critical for EB development, were identified. Functional characterization of one of these regions, through BAC complementation, identified the ribosomal gene Rps14 as a novel haploinsufficient determinant of embryoid body formation. This new library of chromosomal deletions in ESC (DelES: http://bioinfo.iric.ca/deles) will serve as a unique resource for elucidation of novel protein-coding and non-coding regulators of ESC activity.

HMGA2 expression defines a subset of human AML with immature transcriptional signature and vulnerability to G2/M inhibition.

High-mobility group AT-hook 2 (HMGA2) is a nonhistone chromatin-binding protein that is normally expressed in stem cells of various tissues and aberrantly detected in several tumor types. We recently observed that one-fourth of human acute myeloid leukemia (AML) specimens express HMGA2, which associates with a very poor prognosis. We present results indicating that HMGA2+ AMLs share a distinct transcriptional signature representing an immature phenotype. Using single-cell analyses, we showed that HMGA2 is expressed in CD34+ subsets of stem cells and early progenitors, whether normal or derived from AML specimens. Of interest, we found that one of the strongest gene expression signatures associated with HMGA2 in AML is the upregulation of G2/M checkpoint genes. Whole-genome CRISPR/Cas9 screening in HMGA2 overexpressing cells further revealed a synthetic lethal interaction with several G2/M checkpoint genes. Accordingly, small molecules that target G2/M proteins were preferentially active in vitro and in vivo on HMGA2+ AML specimens. Together, our findings suggest that HMGA2 is a key functional determinant in AML and is associated with stem cell features, G2/M status, and related drug sensitivity.

CDK7/12/13 inhibition targets an oscillating leukemia stem cell network and synergizes with venetoclax in acute myeloid leukemia.

The heterogeneous response of acute myeloid leukemia (AML) to current anti-leukemic therapies is only partially explained by mutational heterogeneity. We previously identified GPR56 as a surface marker associated with poor outcome across genetic groups, which characterizes two leukemia stem cell (LSC)-enriched compartments with different self-renewal capacities. How these compartments self-renew remained unclear. Here, we show that GPR56+ LSC compartments are promoted in a complex network involving epithelial-to-mesenchymal transition (EMT) regulators besides Rho, Wnt, and Hedgehog (Hh) signaling. Unexpectedly, Wnt pathway inhibition increased the more immature, slowly cycling GPR56+ CD34+ fraction and Hh/EMT gene expression, while Wnt activation caused opposite effects. Our data suggest that the crucial role of GPR56 lies in its ability to co-activate these opposing signals, thus ensuring the constant supply of both LSC subsets. We show that CDK7 inhibitors suppress both LSC-enriched subsets in vivo and synergize with the Bcl-2 inhibitor venetoclax. Our data establish reciprocal transition between LSC compartments as a novel concept underlying the poor outcome in GPR56high AML and propose combined CDK7 and Bcl-2 inhibition as LSC-directed therapy in this disease.

Vesicular trafficking is a key determinant of the statin response in acute myeloid leukemia.

Cholesterol homeostasis has been proposed as one mechanism contributing to chemoresistance in AML and hence, inclusion of statins in therapeutic regimens as part of clinical trials in AML has shown encouraging results. Chemical screening of primary human AML specimens by our group led to the identification of lipophilic statins as potent inhibitors of AMLs from a wide range of cytogenetic groups. Genetic screening to identify modulators of the statin response uncovered the role of protein geranylgeranylation and of RAB proteins, coordinating various aspect of vesicular trafficking, in mediating the effects of statins on AML cell viability. We further show that statins can inhibit vesicle-mediated transport in primary human specimens, and that statins sensitive samples show expression signatures reminiscent of enhanced vesicular trafficking. Overall, this study sheds light into the mechanism of action of statins in AML and identifies a novel vulnerability for cytogenetically diverse AML.

UM171 Preserves Epigenetic Marks that Are Reduced in Ex Vivo Culture of Human HSCs via Potentiation of the CLR3-KBTBD4 Complex.

Human hematopoietic stem cells (HSCs) exhibit attrition of their self-renewal capacity when cultured ex vivo, a process that is partially reversed upon treatment with epigenetic modifiers, most notably inhibitors of histone deacetylases (HDACs) or lysine-specific demethylase LSD1. A recent study showed that the human HSC self-renewal agonist UM171 modulates the CoREST complex, leading to LSD1 degradation, whose inhibition mimics the activity of UM171. The mechanism underlying the UM171-mediated loss of CoREST function remains undetermined. We now report that UM171 potentiates the activity of a CULLIN3-E3 ubiquitin ligase (CRL3) complex whose target specificity is dictated by the poorly characterized Kelch/BTB domain protein KBTBD4. CRL3KBTBD4 targets components of the LSD1/RCOR1 corepressor complex for proteasomal degradation, hence re-establishing H3K4me2 and H3K27ac epigenetic marks, which are rapidly decreased upon ex vivo culture of human HSCs.

Genetic characterization of ABT-199 sensitivity in human AML.

Acute myeloid leukemias (AML) with mutations in the NPM1 gene (NPM1c+) represent a large AML subgroup with varying response to conventional treatment, highlighting the need to develop targeted therapeutic strategies for this disease. We screened a library of clinical drugs on a cohort of primary human AML specimens and identified the BCL2 inhibitor ABT-199 as a selective agent against NPM1c+ AML. Mutational analysis of ABT-199-sensitive and -resistant specimens identified mutations in NPM1, RAD21, and IDH1/IDH2 as predictors of ABT-199 sensitivity. Comparative transcriptome analysis further uncovered BCL2A1 as a potential mediator of ABT-199 resistance in AML. In line with our observation that RAD21 mutation confers sensitivity to ABT-199, we provide functional evidence that reducing RAD21 levels can sensitize AML cells to BCL2 inhibition. Moreover, we demonstrate that ABT-199 is able to produce selective anti-AML activity in vivo toward AML with mutations associated with compound sensitivity in PDX models. Overall, this study delineates the contribution of several genetic events to the response to ABT-199 and provides a rationale for the development of targeted therapies for NPM1c+ AML.

UM171 induces a homeostatic inflammatory-detoxification response supporting human HSC self-renewal.

Elucidation of the molecular cues required to balance adult stem cell self-renewal and differentiation is critical for advancing cellular therapies. Herein, we report that the hematopoietic stem cell (HSC) self-renewal agonist UM171 triggers a balanced pro- and anti-inflammatory/detoxification network that relies on NFKB activation and protein C receptor-dependent ROS detoxification, respectively. We demonstrate that within this network, EPCR serves as a critical protective component as its deletion hypersensitizes primitive hematopoietic cells to pro-inflammatory signals and ROS accumulation resulting in compromised stem cell function. Conversely, abrogation of the pro-inflammatory activity of UM171 through treatment with dexamethasone, cAMP elevating agents or NFkB inhibitors abolishes EPCR upregulation and HSC expansion. Together, these results show that UM171 stimulates ex vivo HSC expansion by establishing a critical balance between key pro- and anti-inflammatory mediators of self-renewal.

Integrin-α3 Is a Functional Marker of Ex Vivo Expanded Human Long-Term Hematopoietic Stem Cells.

Transplantation of expanded hematopoietic stem cells (HSCs) and gene therapy based on HSC engineering have emerged as promising approaches for the treatment of hematological diseases. Nevertheless, the immunophenotype of cultured HSCs remains poorly defined. Here, we identify Integrin-α3 (ITGA3) as a marker of cultured human HSCs. Exploiting the pyrimidoindole derivative UM171 to expand cord blood (CB) cells, we show that ITGA3 expression is sufficient to separate the primitive EPCR+CD90+CD133+CD34+CD45RA- HSC population into two functionally distinct fractions presenting mostly short-term (ITGA3-) and both short-term and long-term (ITGA3+) repopulating potential. ITGA3+ cells exhibit robust multilineage differentiation potential, serial reconstitution ability in immunocompromised mice, and an HSC-specific transcriptomic signature. Moreover, ITGA3 expression is functionally required for the long-term engraftment of CB cells. Altogether, our results indicate that ITGA3 is a reliable marker of cultured human long-term repopulating HSCs (LT-HSCs) and represents an important tool to improve the accuracy of prospective HSC identification in culture.

Mubritinib Targets the Electron Transport Chain Complex I and Reveals the Landscape of OXPHOS Dependency in Acute Myeloid Leukemia.

To identify therapeutic targets in acute myeloid leukemia (AML), we chemically interrogated 200 sequenced primary specimens. Mubritinib, a known ERBB2 inhibitor, elicited strong anti-leukemic effects in vitro and in vivo. In the context of AML, mubritinib functions through ubiquinone-dependent inhibition of electron transport chain (ETC) complex I activity. Resistance to mubritinib characterized normal CD34+ hematopoietic cells and chemotherapy-sensitive AMLs, which displayed transcriptomic hallmarks of hypoxia. Conversely, sensitivity correlated with mitochondrial function-related gene expression levels and characterized a large subset of chemotherapy-resistant AMLs with oxidative phosphorylation (OXPHOS) hyperactivity. Altogether, our work thus identifies an ETC complex I inhibitor and reveals the genetic landscape of OXPHOS dependency in AML.

Transcriptomic landscape of acute promyelocytic leukemia reveals aberrant surface expression of the platelet aggregation agonist Podoplanin.

Acute promyelocytic leukemia (APL) is a medical emergency because of associated lethal early bleeding, a condition preventable by prompt diagnosis and therapeutic intervention. The mechanisms underlying the hemostatic anomalies of APL are not completely elucidated. RNA-sequencing-based characterization of APL (n = 30) was performed and compared to that of other acute myeloid leukemia (n = 400) samples and normal promyelocytes. Perturbations in the transcriptome of coagulation and fibrinolysis-related genes in APL extend beyond known culprits and now include Thrombin, Factor X and Urokinase Receptor. Most intriguingly, the Podoplanin (PDPN) gene, involved in platelet aggregation, is aberrantly expressed in APL promyelocytes and is the most distinctive transcript for this disease. Using an antibody panel optimized for AML diagnosis by flow cytometry, we also found that PDPN was the most specific surface marker for APL, and that all-trans retinoic acid therapy rapidly decreases its expression. Functional studies showed that engineered overexpression of this gene in human leukemic cells causes aberrant platelet binding, activation and aggregation. PDPN-expressing primary APL cells, but not PDPN-negative primary leukemias, specifically induce platelet binding, activation and aggregation. Finally, PDPN expression on leukemia cells in a xenograft model was associated with thrombocytopenia and prolonged bleeding time in vivo. Together our results suggest that PDPN may contribute to the hemostatic perturbations found in APL.

Chemogenomic Landscape of RUNX1-mutated AML Reveals Importance of RUNX1 Allele Dosage in Genetics and Glucocorticoid Sensitivity.

Purpose:RUNX1-mutated (RUNX1mut) acute myeloid leukemia (AML) is associated with adverse outcome, highlighting the urgent need for a better genetic characterization of this AML subgroup and for the design of efficient therapeutic strategies for this disease. Toward this goal, we further dissected the mutational spectrum and gene expression profile of RUNX1mut AML and correlated these results to drug sensitivity to identify novel compounds targeting this AML subgroup.Experimental Design: RNA-sequencing of 47 RUNX1mut primary AML specimens was performed and sequencing results were compared to those of RUNX1 wild-type samples. Chemical screens were also conducted using RUNX1mut specimens to identify compounds selectively affecting the viability of RUNX1mut AML.Results: We show that samples with no remaining RUNX1 wild-type allele are clinically and genetically distinct and display a more homogeneous gene expression profile. Chemical screening revealed that most RUNX1mut specimens are sensitive to glucocorticoids (GCs) and we confirmed that GCs inhibit AML cell proliferation through their interaction with the glucocorticoid receptor (GR). We observed that specimens harboring RUNX1 mutations expected to result in low residual RUNX1 activity are most sensitive to GCs, and that coassociating mutations as well as GR levels contribute to GC sensitivity. Accordingly, acquired glucocorticoid sensitivity was achieved by negatively regulating RUNX1 expression in human AML cells.Conclusions: Our findings show the profound impact of RUNX1 allele dosage on gene expression profile and glucocorticoid sensitivity in AML, thereby opening opportunities for preclinical testing which may lead to drug repurposing and improved disease characterization. Clin Cancer Res; 23(22); 6969-81. ©2017 AACR.

RNA-Seq reveals spliceosome and proteasome genes as most consistent transcripts in human cancer cells.

Accurate quantification of gene expression by qRT-PCR relies on normalization against a consistently expressed control gene. However, control genes in common use often vary greatly between samples, especially in cancer. The advent of Next Generation Sequencing technology offers the possibility to better select control genes with the least cell to cell variability in steady state transcript levels. Here we analyze the transcriptomes of 55 leukemia samples to identify the most consistent genes. This list is enriched for components of the proteasome (ex. PSMA1) and spliceosome (ex. SF3B2), and also includes the translation initiation factor EIF4H, and many heterogeneous nuclear ribonucleoprotein genes (ex. HNRNPL). We have validated the consistency of our new control genes in 1933 cancer and normal tissues using publically available RNA-seq data, and their usefulness in qRT-PCR analysis is clearly demonstrated.

Urinary monocyte chemotactic protein 1: marker of renal function decline in diabetic and nondiabetic proteinuric renal disease.

BACKGROUND: Reliable biomarkers are needed to identify patients with glomerular disease at risk of progression. Transforming growth factor beta 1 (TGF-ß1) and monocyte chemotactic protein 1 (MCP-1) play key roles in promoting renal tissue injury. Whether their urinary measurement adds value to current predictors of progression is uncertain. METHODS: We enrolled patients with diabetic (n=53) and nondiabetic (n=47) proteinuric renal disease and retrospectively studied their rate of renal function decline over a defined period of 2 years. We simultaneously measured urinary protein, MCP-1 and TGF-ß1, standardized to urinary creatinine. RESULTS: The initial estimated glomerular filtration rate, proteinuria and rate of renal function decline (slope) were 36 ml/min per 1.73 m2, 1.1 g/day and -4.0 ± 7.2 ml/ min per 1.73 m2 year. Median urinary TGF-ß1 and MCP- 1 levels were 0.3 (range 0.0-28.1) and 18 (range 3-370) ng/mmol of creatinine, respectively. Urinary protein and MCP-1 to creatinine ratios were associated with slope, and this applied to both diabetic and nondiabetic patients separately. Urinary TGF-ß1 showed no relation to slope. However, the majority of its measurements were below the suggested reproducibility threshold. Using linear regression, both normalized urinary protein and MCP-1 were independently associated with the slope. Adding urinary MCP-1 to the model statistically raised the adjusted R2 from 0.35 to 0.40, refining patient risk stratification. Using cutoffs for urinary protein and MCP-1 obtained by receiver operating characteristic curves, the risk of progression was confidently determined in 80% of patients. CONCLUSION: Urinary MCP-1 is a marker of renal function decline in diabetic and nondiabetic proteinuric renal disease, independent of and additive to proteinuria.

An RNAi screen identifies Msi2 and Prox1 as having opposite roles in the regulation of hematopoietic stem cell activity.

In this study, we describe an in vivo RNA interference functional genetics approach to evaluate the role of 20 different conserved polarity factors and fate determinants in mouse hematopoietic stem cell (HSC) activity. In total, this screen revealed three enhancers and one suppressor of HSC-derived reconstitution. Pard6a, Prkcz, and Msi2 shRNA-mediated depletion significantly impaired HSC repopulation. An in vitro promotion of differentiation was observed after the silencing of these genes, consistent with their function in regulating HSC self-renewal. Conversely, Prox1 knockdown led to in vivo accumulation of primitive and differentiated cells. HSC activity was also enhanced in vitro when Prox1 levels were experimentally reduced, identifying it as a potential antagonist of self-renewal. HSC engineered to overexpress Msi2 or Prox1 showed the reverse phenotype to those transduced with corresponding shRNA vectors. Gene expression profiling studies identified a number of known HSC and cell cycle regulators as potential downstream targets to Msi2 and Prox1.