Haploinsufficiency of AML1 affects the temporal and spatial generation of hematopoietic stem cells in the mouse embryo

Bloodhounds chasing the origin of blood cells

The generation of blood cells during embryonic development involves a process resembling lineage reprogramming, where specialized cells within the vasculature become blood forming, or hemogenic. These hemogenic cells undergo rapid transcriptional and morphological changes as they appear to switch from an endothelial to blood identity. What controls this process and the exact nature of the hemogenic cells remains debated, with evidence supporting several hypotheses. In this opinion, we synthesize current knowledge and propose a model reconciling conflicting observations, integrating evolutionary and mechanistic insights into blood cell emergence.

Sox17 and Other SoxF-Family Proteins Play Key Roles in the Hematopoiesis of Mouse Embryos

During mouse development, hematopoietic cells first form in the extraembryonic tissue yolk sac. Hematopoietic stem cells (HSCs), which retain their ability to differentiate into hematopoietic cells for a long time, form intra-aortic hematopoietic cell clusters (IAHCs) in the dorsal aorta at midgestation. These IAHCs emerge from the hemogenic endothelium, which is the common progenitor of hematopoietic cells and endothelial cells. HSCs expand in the fetal liver, and finally migrate to the bone marrow (BM) during the peripartum period. IAHCs are absent in the dorsal aorta in mice deficient in transcription factors such as Runx-1, GATA2, and c-Myb that are essential for definitive hematopoiesis. In this review, we focus on the transcription factor Sry-related high mobility group (HMG)-box (Sox) F family of proteins that is known to regulate hematopoiesis in the hemogenic endothelium and IAHCs. The SoxF family is composed of Sox7, Sox17, and Sox18, and they all have the HMG box, which has a DNA-binding ability, and a transcriptional activation domain. Here, we describe the functional and phenotypic properties of SoxF family members, with a particular emphasis on Sox17, which is the most involved in hematopoiesis in the fetal stages considering that enhanced expression of Sox17 in hemogenic endothelial cells and IAHCs leads to the production and maintenance of HSCs. We also discuss SoxF-inducing signaling pathways.

Minimal Requirements for Cancer Initiation: A Comparative Consideration of Three Prototypes of Human Leukemia

Even if its completed form is complex, cancer originates from one or two events that happened to a single cell. A simplified model can play a role in understanding how cancer initiates at the beginning. The pathophysiology of leukemia has been studied in the most detailed manner among all human cancers. In this review, based on milestone papers and the latest research developments in hematology, acute promyelocytic leukemia (APL), chronic myeloid leukemia (CML), and acute myeloid leukemia (AML) with RUNX1-RUNX1T1 are selected to consider minimal requirements for cancer initiation. A one-hit model can be applied to the initiation of APL and CML whereas a two-hit model is more suitable to the initiation of AML with RUNX1-RUNX1T1 and other AMLs. Even in cancer cells with multiple genetic abnormalities, there must be a few mutant genes critical for the mutant clone to survive and proliferate. Such genes should be identified and characterized in each case in order to develop individualized target therapy.

Capturing embryonic hematopoiesis in temporal and spatial dimensions

Hematopoietic stem cells (HSCs) possess the ability to sustain the continuous production of all blood cell types throughout an organism's lifespan. Although primarily located in the bone marrow of adults, HSCs originate during embryonic development. Visualization of the birth of HSCs, their developmental trajectory, and the specific interactions with their successive niches have significantly contributed to our understanding of the biology and mechanics governing HSC formation and expansion. Intravital techniques applied to live embryos or non-fixed samples have remarkably provided invaluable insights into the cellular and anatomical origins of HSCs. These imaging technologies have also shed light on the dynamic interactions between HSCs and neighboring cell types within the surrounding microenvironment or niche, such as endothelial cells or macrophages. This review delves into the advancements made in understanding the origin, production, and cellular interactions of HSCs, particularly during the embryonic development of mice and zebrafish, focusing on studies employing (live) imaging analysis.

FLI1 is associated with regulation of DNA methylation and megakaryocytic differentiation in FPDMM caused by a RUNX1 transactivation domain mutation

Familial platelet disorder with associated myeloid malignancies (FPDMM) is an autosomal dominant disease caused by heterozygous germline mutations in RUNX1. It is characterized by thrombocytopenia, platelet dysfunction, and a predisposition to hematological malignancies. Although FPDMM is a precursor for diseases involving abnormal DNA methylation, the DNA methylation status in FPDMM remains unknown, largely due to a lack of animal models and challenges in obtaining patient-derived samples. Here, using genome editing techniques, we established two lines of human induced pluripotent stem cells (iPSCs) with different FPDMM-mimicking heterozygous RUNX1 mutations. These iPSCs showed defective differentiation of hematopoietic progenitor cells (HPCs) and megakaryocytes (Mks), consistent with FPDMM. The FPDMM-mimicking HPCs showed DNA methylation patterns distinct from those of wild-type HPCs, with hypermethylated regions showing the enrichment of ETS transcription factor (TF) motifs. We found that the expression of FLI1, an ETS family member, was significantly downregulated in FPDMM-mimicking HPCs with a RUNX1 transactivation domain (TAD) mutation. We demonstrated that FLI1 promoted binding-site-directed DNA demethylation, and that overexpression of FLI1 restored their megakaryocytic differentiation efficiency and hypermethylation status. These findings suggest that FLI1 plays a crucial role in regulating DNA methylation and correcting defective megakaryocytic differentiation in FPDMM-mimicking HPCs with a RUNX1 TAD mutation.

The role of m6A mRNA modification in normal and malignant hematopoiesis

Hematopoiesis is a highly orchestrated biological process sustaining the supply of leukocytes involved in the maintenance of immunity, O2 and CO2 exchange, and wound healing throughout the lifetime of an animal, including humans. During early hematopoietic cell development, several waves of hematopoiesis require the precise regulation of hematopoietic ontogeny as well as the maintenance of hematopoietic stem and progenitor cells in the hematopoietic tissues, such as the fetal liver and bone marrow. Recently, emerging evidence has suggested the critical role of m6A messenger RNA (mRNA) modification, an epigenetic modification dynamically regulated by its effector proteins, in the generation and maintenance of hematopoietic cells during embryogenesis. In the adulthood, m6A has also been demonstrated to be involved in the functional maintenance of hematopoietic stem and progenitor cells in the bone marrow and umbilical cord blood, as well as the progression of malignant hematopoiesis. In this review, we focus on recent progress in identifying the biological functions of m6A mRNA modification, its regulators, and downstream gene targets during normal and pathological hematopoiesis. We propose that targeting m6A mRNA modification could offer novel insights into therapeutic development against abnormal and malignant hematopoietic cell development in the future.

BloodChIP Xtra: an expanded database of comparative genome-wide transcription factor binding and gene-expression profiles in healthy human stem/progenitor subsets and leukemic cells

The BloodChIP Xtra database (http://bloodchipXtra.vafaeelab.com/) facilitates genome-wide exploration and visualization of transcription factor (TF) occupancy and chromatin configuration in rare primary human hematopoietic stem (HSC-MPP) and progenitor (CMP, GMP, MEP) cells and acute myeloid leukemia (AML) cell lines (KG-1, ME-1, Kasumi1, TSU-1621-MT), along with chromatin accessibility and gene expression data from these and primary patient AMLs. BloodChIP Xtra features significantly more datasets than our earlier database BloodChIP (two primary cell types and two cell lines). Improved methodologies for determining TF occupancy and chromatin accessibility have led to increased availability of data for rare primary cell types across the spectrum of healthy and AML hematopoiesis. However, there is a continuing need for these data to be integrated in an easily accessible manner for gene-based queries and use in downstream applications. Here, we provide a user-friendly database based around genome-wide binding profiles of key hematopoietic TFs and histone marks in healthy stem/progenitor cell types. These are compared with binding profiles and chromatin accessibility derived from primary and cell line AML and integrated with expression data from corresponding cell types. All queries can be exported to construct TF-gene and protein-protein networks and evaluate the association of genes with specific cellular processes.

Proteogenomic analysis reveals cytoplasmic sequestration of RUNX1 by the acute myeloid leukemia-initiating CBFB::MYH11 oncofusion protein

Several canonical translocations produce oncofusion genes that can initiate acute myeloid leukemia (AML). Although each translocation is associated with unique features, the mechanisms responsible remain unclear. While proteins interacting with each oncofusion are known to be relevant for how they act, these interactions have not yet been systematically defined. To address this issue in an unbiased fashion, we fused a promiscuous biotin ligase (TurboID) in-frame with 3 favorable-risk AML oncofusion cDNAs (PML::RARA, RUNX1::RUNX1T1, and CBFB::MYH11) and identified their interacting proteins in primary murine hematopoietic cells. The PML::RARA- and RUNX1::RUNX1T1-TurboID fusion proteins labeled common and unique nuclear repressor complexes, implying their nuclear localization. However, CBFB::MYH11-TurboID-interacting proteins were largely cytoplasmic, probably because of an interaction of the MYH11 domain with several cytoplasmic myosin-related proteins. Using a variety of methods, we showed that the CBFB domain of CBFB::MYH11 sequesters RUNX1 in cytoplasmic aggregates; these findings were confirmed in primary human AML cells. Paradoxically, CBFB::MYH11 expression was associated with increased RUNX1/2 expression, suggesting the presence of a sensor for reduced functional RUNX1 protein, and a feedback loop that may attempt to compensate by increasing RUNX1/2 transcription. These findings may have broad implications for AML pathogenesis.

Engineered hematopoietic and immune cells derived from human pluripotent stem cells

For the past decade, significant advances have been achieved in human hematopoietic stem cell (HSC) transplantation for treating various blood diseases and cancers. However, challenges remain with the quality control, amount, and cost of HSCs and HSC-derived immune cells. The advent of human pluripotent stem cells (hPSCs) may transform HSC transplantation and cancer immunotherapy by providing a cost-effective and scalable cell source for fundamental studies and translational applications. In this review, we discuss the current developments in the field of stem cell engineering for hematopoietic stem and progenitor cell (HSPC) differentiation and further differentiation of HSPCs into functional immune cells. The key advances in stem cell engineering include the generation of HSPCs from hPSCs, genetic modification of hPSCs, and hPSC-derived HSPCs for improved function, further differentiation of HPSCs into functional immune cells, and applications of cell culture platforms for hematopoietic cell manufacturing. Current challenges impeding the translation of hPSC-HSPCs and immune cells as well as further directions to address these challenges are also discussed.

Genome-wide transcription factor-binding maps reveal cell-specific changes in the regulatory architecture of human HSPCs

Hematopoietic stem and progenitor cells (HSPCs) rely on a complex interplay among transcription factors (TFs) to regulate differentiation into mature blood cells. A heptad of TFs (FLI1, ERG, GATA2, RUNX1, TAL1, LYL1, LMO2) bind regulatory elements in bulk CD34+ HSPCs. However, whether specific heptad-TF combinations have distinct roles in regulating hematopoietic differentiation remains unknown. We mapped genome-wide chromatin contacts (HiC, H3K27ac, HiChIP), chromatin modifications (H3K4me3, H3K27ac, H3K27me3) and 10 TF binding profiles (heptad, PU.1, CTCF, STAG2) in HSPC subsets (stem/multipotent progenitors plus common myeloid, granulocyte macrophage, and megakaryocyte erythrocyte progenitors) and found TF occupancy and enhancer-promoter interactions varied significantly across cell types and were associated with cell-type-specific gene expression. Distinct regulatory elements were enriched with specific heptad-TF combinations, including stem-cell-specific elements with ERG, and myeloid- and erythroid-specific elements with combinations of FLI1, RUNX1, GATA2, TAL1, LYL1, and LMO2. Furthermore, heptad-occupied regions in HSPCs were subsequently bound by lineage-defining TFs, including PU.1 and GATA1, suggesting that heptad factors may prime regulatory elements for use in mature cell types. We also found that enhancers with cell-type-specific heptad occupancy shared a common grammar with respect to TF binding motifs, suggesting that combinatorial binding of TF complexes was at least partially regulated by features encoded in DNA sequence motifs. Taken together, this study comprehensively characterizes the gene regulatory landscape in rare subpopulations of human HSPCs. The accompanying data sets should serve as a valuable resource for understanding adult hematopoiesis and a framework for analyzing aberrant regulatory networks in leukemic cells.

Dorsal aorta polarization and haematopoietic stem cell emergence

Recent studies have highlighted the crucial role of the aorta microenvironment in the generation of the first haematopoietic stem cells (HSCs) from specialized haemogenic endothelial cells (HECs). Despite more than two decades of investigations, we require a better understanding of the cellular and molecular events driving aorta formation and polarization, which will be pivotal to establish the mechanisms that operate during HEC specification and HSC competency. Here, we outline the early mechanisms involved in vertebrate aorta formation by comparing four different species: zebrafish, chicken, mouse and human. We highlight how this process, which is tightly controlled in time and space, requires a coordinated specification of several cell types, in particular endothelial cells originating from distinct mesodermal tissues. We also discuss how molecular signals originating from the aorta environment result in its polarization, creating a unique entity for HSC generation.

Increased expression of RUNX3 inhibits normal human myeloid development

RUNX3 is a transcription factor dysregulated in acute myeloid leukemia (AML). However, its role in normal myeloid development and leukemia is poorly understood. Here we investigate RUNX3 expression in both settings and the impact of its dysregulation on myelopoiesis. We found that RUNX3 mRNA expression was stable during hematopoiesis but decreased with granulocytic differentiation. In AML, RUNX3 mRNA was overexpressed in many disease subtypes, but downregulated in AML with core binding factor abnormalities, such as RUNX1::ETO. Overexpression of RUNX3 in human hematopoietic stem and progenitor cells (HSPC) inhibited myeloid differentiation, particularly of the granulocytic lineage. Proliferation and myeloid colony formation were also inhibited. Conversely, RUNX3 knockdown did not impact the myeloid growth and development of human HSPC. Overexpression of RUNX3 in the context of RUNX1::ETO did not rescue the RUNX1::ETO-mediated block in differentiation. RNA-sequencing showed that RUNX3 overexpression downregulates key developmental genes, such as KIT and RUNX1, while upregulating lymphoid genes, such as KLRB1 and TBX21. Overall, these data show that increased RUNX3 expression observed in AML could contribute to the developmental arrest characteristic of this disease, possibly by driving a competing transcriptional program favoring a lymphoid fate.

Molecular Basis of Hematological Disease Caused by Inherited or Acquired RUNX1 Mutations

The transcription factor RUNX1 is essential for correct hematopoietic development; in its absence in the germ line, blood stem cells are not formed. RUNX1 orchestrates dramatic changes in the chromatin landscape at the onset of stem cell formation, which set the stage for both stem self-renewal and further differentiation. However, once blood stem cells are formed, the mutation of the RUNX1 gene is not lethal but can lead to various hematopoietic defects and a predisposition to cancer. Here we summarize the current literature on inherited and acquired RUNX1 mutations, with a particular emphasis on mutations that alter the structure of the RUNX1 protein itself, and place these changes in the context of what is known about RUNX1 function. We also summarize which mutant RUNX1 proteins are actually expressed in cells and discuss the molecular mechanism underlying how such variants reprogram the epigenome setting stem cells on the path to malignancy.

MyD88-dependent TLR signaling oppositely regulates hematopoietic progenitor and stem cell formation in the embryo

Hemogenic endothelial (HE) cells in the dorsal aorta undergo an endothelial-to-hematopoietic transition (EHT) to form multipotent progenitors, lympho-myeloid biased progenitors (LMPs), pre-hematopoietic stem cells (pre-HSCs) and adult-repopulating HSCs. These briefly accumulate in intra-arterial hematopoietic clusters (IAHCs) before being released into the circulation. It is generally assumed that the number of IAHC cells correlates with the number of HSCs. Here, we show that changes in the number of IAHC cells, LMPs and HSCs can be uncoupled. Mutations impairing MyD88-dependent toll-like receptor (TLR) signaling decreased the number of IAHC cells and LMPs, but increased the number of HSCs in the aorta-gonad-mesonephros region of mouse embryos. TLR4-deficient embryos generated normal numbers of HE cells, but IAHC cell proliferation decreased. Loss of MyD88-dependent TLR signaling in innate immune myeloid cells had no effect on IAHC cell numbers. Instead, TLR4 deletion in endothelial cells (ECs) recapitulated the phenotype observed with germline deletion, demonstrating that MyD88-dependent TLR signaling in ECs and/or in IAHCs regulates the numbers of LMPs and HSCs.

Dynamic Runx1 chromatin boundaries affect gene expression in hematopoietic development

The transcription factor RUNX1 is a critical regulator of developmental hematopoiesis and is frequently disrupted in leukemia. Runx1 is a large, complex gene that is expressed from two alternative promoters under the spatiotemporal control of multiple hematopoietic enhancers. To dissect the dynamic regulation of Runx1 in hematopoietic development, we analyzed its three-dimensional chromatin conformation in mouse embryonic stem cell (ESC) differentiation cultures. Runx1 resides in a 1.1 Mb topologically associating domain (TAD) demarcated by convergent CTCF motifs. As ESCs differentiate to mesoderm, chromatin accessibility, Runx1 enhancer-promoter (E-P) interactions, and CTCF-CTCF interactions increase in the TAD, along with initiation of Runx1 expression from the P2 promoter. Differentiation to hematopoietic progenitor cells is associated with the formation of tissue-specific sub-TADs over Runx1, a shift in E-P interactions, P1 promoter demethylation, and robust expression from both Runx1 promoters. Deletion of promoter-proximal CTCF sites at the sub-TAD boundaries has no obvious effects on E-P interactions but leads to partial loss of domain structure, mildly affects gene expression, and delays hematopoietic development. Together, our analysis of gene regulation at a large multi-promoter developmental gene reveals that dynamic sub-TAD chromatin boundaries play a role in establishing TAD structure and coordinated gene expression.

The RUNX1b Isoform Defines Hemogenic Competency in Developing Human Endothelial Cells

The transcription factor RUNX1 is a master regulator of blood cell specification. During embryogenesis, hematopoietic progenitors are initially generated from hemogenic endothelium through an endothelium-to-hematopoietic transition controlled by RUNX1. Several studies have dissected the expression pattern and role of RUNX1 isoforms at the onset of mouse hematopoiesis, however the precise pattern of RUNX1 isoform expression and biological output of RUNX1-expressing cells at the onset of human hematopoiesis is still not fully understood. Here, we investigated these questions using a RUNX1b:VENUS RUNX1c:TOMATO human embryonic stem cell line which allows multi-parameter single cell resolution via flow cytometry and isolation of RUNX1b-expressing cells for further analysis. Our data reveal the sequential expression of the two RUNX1 isoforms with RUNX1b expressed first in a subset of endothelial cells and during the endothelial to hematopoietic transition while RUNX1c only becomes expressed in fully specified blood cells. Furthermore, our data show that RUNX1b marks endothelial cells endowed with hemogenic potential and that RUNX1b expression level determines hemogenic competency in a dose-dependent manner. Together our data reveal the dynamic of RUNX1 isoforms expression at the onset of human blood specification and establish RUNX1b isoform as the earliest known marker for hemogenic competency.

Recent Advances in Developmental Hematopoiesis: Diving Deeper With New Technologies

The journey of a hematopoietic stem cell (HSC) involves the passage through successive anatomical sites where HSCs are in direct contact with their surrounding microenvironment, also known as niche. These spatial and temporal cellular interactions throughout development are required for the acquisition of stem cell properties, and for maintaining the HSC pool through balancing self-renewal, quiescence and lineage commitment. Understanding the context and consequences of these interactions will be imperative for our understanding of HSC biology and will lead to the improvement of in vitro production of HSCs for clinical purposes. The aorta-gonad-mesonephros (AGM) region is in this light of particular interest since this is the cradle of HSC emergence during the embryonic development of all vertebrate species. In this review, we will focus on the developmental origin of HSCs and will discuss the novel technological approaches and recent progress made to identify the cellular composition of the HSC supportive niche and the underlying molecular events occurring in the AGM region.

AML1/ETO and its function as a regulator of gene transcription via epigenetic mechanisms

The chromosomal translocation t(8;21) and the resulting oncofusion gene AML1/ETO have long served as a prototypical genetic lesion to model and understand leukemogenesis. In this review, we describe the wide-ranging role of AML1/ETO in AML leukemogenesis, with a particular focus on the aberrant epigenetic regulation of gene transcription driven by this AML-defining mutation. We begin by analyzing how structural changes secondary to distinct genomic breakpoints and splice changes, as well as posttranscriptional modifications, influence AML1/ETO protein function. Next, we characterize how AML1/ETO recruits chromatin-modifying enzymes to target genes and how the oncofusion protein alters chromatin marks, transcription factor binding, and gene expression. We explore the specific impact of these global changes in the epigenetic network facilitated by the AML1/ETO oncofusion on cellular processes and leukemic growth. Furthermore, we define the genetic landscape of AML1/ETO-positive AML, presenting the current literature concerning the incidence of cooperating mutations in genes such as KIT, FLT3, and NRAS. Finally, we outline how alterations in transcriptional regulation patterns create potential vulnerabilities that may be exploited by epigenetically active agents and other therapeutics.

Inflammatory signaling regulates hematopoietic stem and progenitor cell development and homeostasis

Inflammation exerts multiple effects on the early hematopoietic compartment. Best studied is the role of proinflammatory cytokines in activating adult hematopoietic stem and progenitor cells to dynamically replenish myeloid lineage cells in a process known as emergency myelopoiesis. However, it is increasingly appreciated that the same proinflammatory signaling pathways are used in diverse hematopoietic scenarios. This review focuses on inflammatory signaling in the emergence of the definitive hematopoietic compartment during embryonic life, and tonic inflammatory signals derived from commensal microbiota in shaping the adult hematopoietic compartment in the absence of pathogenic insults. Insights into the unique and shared aspects of inflammatory signaling that regulate hematopoietic stem and progenitor cell function across the lifespan and health span of an individual will enable better diagnostic and therapeutic approaches to hematopoietic dysregulation and malignancies.

An update on the molecular pathogenesis and potential therapeutic targeting of AML with t(8;21)(q22;q22.1);RUNX1-RUNX1T1

Acute myeloid leukemia (AML) with t(8;21)(q22;q22.1);RUNX1-RUNX1T1, one of the core-binding factor leukemias, is one of the most common subtypes of AML with recurrent genetic abnormalities and is associated with a favorable outcome. The translocation leads to the formation of a pathological RUNX1-RUNX1T1 fusion that leads to the disruption of the normal function of the core-binding factor, namely, its role in hematopoietic differentiation and maturation. The consequences of this alteration include the recruitment of repressors of transcription, thus blocking the expression of genes involved in hematopoiesis, and impaired apoptosis. A number of concurrent and cooperating mutations clearly play a role in modulating the proliferative potential of cells, including mutations in KIT, FLT3, and possibly JAK2. RUNX1-RUNX1T1 also appears to interact with microRNAs during leukemogenesis. Epigenetic factors also play a role, especially with the recruitment of histone deacetylases. A better understanding of the concurrent mutations, activated pathways, and epigenetic modulation of the cellular processes paves the way for exploring a number of approaches to achieve cure. Potential approaches include the development of small molecules targeting the RUNX1-RUNX1T1 protein, the use of tyrosine kinase inhibitors such as dasatinib and FLT3 inhibitors to target mutations that lead to a proliferative advantage of the leukemic cells, and experimentation with epigenetic therapies. In this review, we unravel some of the recently described molecular pathways and explore potential therapeutic strategies.

First blood: the endothelial origins of hematopoietic progenitors

Hematopoiesis in vertebrate embryos occurs in temporally and spatially overlapping waves in close proximity to blood vascular endothelial cells. Initially, yolk sac hematopoiesis produces primitive erythrocytes, megakaryocytes, and macrophages. Thereafter, sequential waves of definitive hematopoiesis arise from yolk sac and intraembryonic hemogenic endothelia through an endothelial-to-hematopoietic transition (EHT). During EHT, the endothelial and hematopoietic transcriptional programs are tightly co-regulated to orchestrate a shift in cell identity. In the yolk sac, EHT generates erythro-myeloid progenitors, which upon migration to the liver differentiate into fetal blood cells, including erythrocytes and tissue-resident macrophages. In the dorsal aorta, EHT produces hematopoietic stem cells, which engraft the fetal liver and then the bone marrow to sustain adult hematopoiesis. Recent studies have defined the relationship between the developing vascular and hematopoietic systems in animal models, including molecular mechanisms that drive the hemato-endothelial transcription program for EHT. Moreover, human pluripotent stem cells have enabled modeling of fetal human hematopoiesis and have begun to generate cell types of clinical interest for regenerative medicine.

Transcription factors regulate early T cell development via redeployment of other factors: Functional dynamics of constitutively required factors in cell fate decisions

Establishment of cell lineage identity from multipotent progenitors is controlled by cooperative actions of lineage-specific and stably expressed transcription factors, combined with input from environmental signals. Lineage-specific master transcription factors activate and repress gene expression by recruiting consistently expressed transcription factors and chromatin modifiers to their target loci. Recent technical advances in genome-wide and multi-omics analysis have shed light on unexpected mechanisms that underlie more complicated actions of transcription factors in cell fate decisions. In this review, we discuss functional dynamics of stably expressed and continuously required factors, Notch and Runx family members, throughout developmental stages of early T cell development in the thymus. Pre- and post-commitment stage-specific transcription factors induce dynamic redeployment of Notch and Runx binding genomic regions. Thus, together with stage-specific transcription factors, shared transcription factors across distinct developmental stages regulate acquisition of T lineage identity.

The genome-wide impact of trisomy 21 on DNA methylation and its implications for hematopoiesis

Down syndrome is associated with genome-wide perturbation of gene expression, which may be mediated by epigenetic changes. We perform an epigenome-wide association study on neonatal bloodspots comparing 196 newborns with Down syndrome and 439 newborns without Down syndrome, adjusting for cell-type heterogeneity, which identifies 652 epigenome-wide significant CpGs (P < 7.67 × 10-8) and 1,052 differentially methylated regions. Differential methylation at promoter/enhancer regions correlates with gene expression changes in Down syndrome versus non-Down syndrome fetal liver hematopoietic stem/progenitor cells (P < 0.0001). The top two differentially methylated regions overlap RUNX1 and FLI1, both important regulators of megakaryopoiesis and hematopoietic development, with significant hypermethylation at promoter regions of these two genes. Excluding Down syndrome newborns harboring preleukemic GATA1 mutations (N = 30), identified by targeted sequencing, has minimal impact on the epigenome-wide association study results. Down syndrome has profound, genome-wide effects on DNA methylation in hematopoietic cells in early life, which may contribute to the high frequency of hematological problems, including leukemia, in children with Down syndrome.

Distinctive phenotypes in two children with novel germline RUNX1 mutations - one with myeloid malignancy and increased fetal hemoglobin

RUNX1 associated familial platelet disorder (FPD) is a rare autosomal dominant hematologic disorder characterized by thrombocytopenia and/or altered platelet function. There is an increased propensity to develop myeloid malignancy (MM) - acute myeloid leukemia, myeloproliferative neoplasms or myelodysplastic syndrome often in association with secondary somatic variants in other genes. To date, 23 FPD-MM pediatric cases have been reported worldwide. Here, we present two new kindreds with novel RUNX1 pathogenic variants in which children are probands. The first family is a daughter/mother diad, sharing a heterozygous frameshift variant in RUNX1 gene (c.501delT p.Ser167Argfs*9). The daughter, age 13 years, presented with features resembling juvenile myelomonocytic leukemia - severe anemia, thrombocytopenia, high white cell count with blast cells, monocytosis, increased nucleated red cells and had somatic mutations with high allele burden in CUX1, PHF6, and SH2B3 genes. She also had increased fetal hemoglobin and increased LIN28B expression. The mother, who had a long history of hypoplastic anemia, had different somatic mutations- a non-coding mutation in CUX1 but none in PHF6 or SH2B3. Her fetal hemoglobin and LIN28B expression were normal. In the second kindred, the proband, now 4 years old with thrombocytopenia alone, was investigated at 3 months of age for persistent neonatal thrombocytopenia with large platelets. Molecular testing identified a heterozygous intragenic deletion in RUNX1 encompassing exon 5. His father is known to have increased bruising for several years but is unavailable for testing. These two cases illustrate the significance of secondary mutations in the development and progression of RUNX1-FPD to MM.

Different mutant RUNX1 oncoproteins program alternate haematopoietic differentiation trajectories

Using integrated genome-wide and phenotypic methods this study investigates four different mutant RUNX1 oncoproteins and reveals how they differentially contribute to aberrant haematopoiesis.

Mutations of the haematopoietic master regulator RUNX1 are associated with acute myeloid leukaemia, familial platelet disorder and other haematological malignancies whose phenotypes and prognoses depend upon the class of the RUNX1 mutation. The biochemical behaviour of these oncoproteins and their ability to cause unique diseases has been well studied, but the genomic basis of their differential action is unknown. To address this question we compared integrated phenotypic, transcriptomic, and genomic data from cells expressing four types of RUNX1 oncoproteins in an inducible fashion during blood development from embryonic stem cells. We show that each class of mutant RUNX1 deregulates endogenous RUNX1 function by a different mechanism, leading to specific alterations in developmentally controlled transcription factor binding and chromatin programming. The result is distinct perturbations in the trajectories of gene regulatory network changes underlying blood cell development which are consistent with the nature of the final disease phenotype. The development of novel treatments for RUNX1-driven diseases will therefore require individual consideration.

TET2/IDH1/2/WT1 and NPM1 Mutations Influence the RUNX1 Expression Correlations in Acute Myeloid Leukemia

Background and objectives: Mutational analysis has led to a better understanding of acute myeloid leukemia (AML) biology and to an improvement in clinical management. Some of the most important mutations that affect AML biology are represented by mutations in genes related to methylation, more specifically: TET2, IDH1, IDH2 and WT1. Because it has been shown in numerous studies that mutations in these genes lead to similar expression profiles and phenotypes in AML, we decided to assess if mutations in any of those genes interact with other genes important for AML. Materials and Methods: We downloaded the clinical data, mutational profile and expression profile from the TCGA LAML dataset via cBioPortal. Data were analyzed using classical statistical methods and functional enrichment analysis software represented by STRING and GOrilla. Results: The first step we took was to assess the 196 AML cases that had a mutational profile available and observe the mutations that overlapped with TET2/IDH1/2/WT1 mutations. We observed that RUNX1 mutations significantly overlap with TET2/IDH1/2/WT1 mutations. Because of this, we decided to further investigate the role of RUNX1 mutations in modulating the level of RUNX1 mRNA and observed that RUNX1 mutant cases presented higher levels of RUNX1 mRNA. Because there were only 16 cases of RUNX1 mutant samples and that mutations in this gene determined a change in mRNA expression, we further observed the correlation between RUNX1 and other mRNAs in subgroups regarding the presence of hypermethylating mutations and NPM1. Here, we observed that both TET2/IDH1/2/WT1 and NPM1 mutations increase the number of genes negatively correlated with RUNX1 and that these genes were significantly linked to myeloid activation. Conclusions: In the current study, we have shown that NPM1 and TET2/IDH1/2/WT1 mutations increase the number of negative correlations of RUNX1 with other transcripts involved in myeloid differentiation.

Developmental trajectory of prehematopoietic stem cell formation from endothelium

Hematopoietic stem and progenitor cells (HSPCs) in the bone marrow are derived from a small population of hemogenic endothelial (HE) cells located in the major arteries of the mammalian embryo. HE cells undergo an endothelial to hematopoietic cell transition, giving rise to HSPCs that accumulate in intra-arterial clusters (IAC) before colonizing the fetal liver. To examine the cell and molecular transitions between endothelial (E), HE, and IAC cells, and the heterogeneity of HSPCs within IACs, we profiled ∼40 000 cells from the caudal arteries (dorsal aorta, umbilical, vitelline) of 9.5 days post coitus (dpc) to 11.5 dpc mouse embryos by single-cell RNA sequencing and single-cell assay for transposase-accessible chromatin sequencing. We identified a continuous developmental trajectory from E to HE to IAC cells, with identifiable intermediate stages. The intermediate stage most proximal to HE, which we term pre-HE, is characterized by increased accessibility of chromatin enriched for SOX, FOX, GATA, and SMAD motifs. A developmental bottleneck separates pre-HE from HE, with RUNX1 dosage regulating the efficiency of the pre-HE to HE transition. A distal candidate Runx1 enhancer exhibits high chromatin accessibility specifically in pre-HE cells at the bottleneck, but loses accessibility thereafter. Distinct developmental trajectories within IAC cells result in 2 populations of CD45+ HSPCs; an initial wave of lymphomyeloid-biased progenitors, followed by precursors of hematopoietic stem cells (pre-HSCs). This multiomics single-cell atlas significantly expands our understanding of pre-HSC ontogeny.

The Genetics and Mechanisms of T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy derived from early T-cell progenitors. The recognition of clinical, genetic, transcriptional, and biological heterogeneity in this disease has already translated into new prognostic biomarkers, improved leukemia animal models, and emerging targeted therapies. This work reviews our current understanding of the molecular mechanisms of T-ALL.

RUNX1 Dosage in Development and Cancer

The transcription factor RUNX1 first came to prominence due to its involvement in the t(8;21) translocation in acute myeloid leukemia (AML). Since this discovery, RUNX1 has been shown to play important roles not only in leukemia but also in the ontogeny of the normal hematopoietic system. Although it is currently still challenging to fully assess the different parameters regulating RUNX1 dosage, it has become clear that the dose of RUNX1 can greatly affect both leukemia and normal hematopoietic development. It is also becoming evident that varying levels of RUNX1 expression can be used as markers of tumor progression not only in the hematopoietic system, but also in non-hematopoietic cancers. Here, we provide an overview of the current knowledge of the effects of RUNX1 dosage in normal development of both hematopoietic and epithelial tissues and their associated cancers.

Gata3 targets Runx1 in the embryonic haematopoietic stem cell niche

Runx1 is an important haematopoietic transcription factor as stressed by its involvement in a number of haematological malignancies. Furthermore, it is a key regulator of the emergence of the first haematopoietic stem cells (HSCs) during development. The transcription factor Gata3 has also been linked to haematological disease and was shown to promote HSC production in the embryo by inducing the secretion of important niche factors. Both proteins are expressed in several different cell types within the aorta-gonads-mesonephros (AGM) region, in which the first HSCs are generated; however, a direct interaction between these two key transcription factors in the context of embryonic HSC production has not formally been demonstrated. In this current study, we have detected co-localisation of Runx1 and Gata3 in rare sub-aortic mesenchymal cells in the AGM. Furthermore, the expression of Runx1 is reduced in Gata3 -/- embryos, which also display a shift in HSC emergence. Using an AGM-derived cell line as a model for the stromal microenvironment in the AGM and performing ChIP-Seq and ChIP-on-chip experiments, we demonstrate that Runx1, together with other key niche factors, is a direct target gene of Gata3. In addition, we can pinpoint Gata3 binding to the Runx1 locus at specific enhancer elements which are active in the microenvironment. These results reveal a direct interaction between Gata3 and Runx1 in the niche that supports embryonic HSCs and highlight a dual role for Runx1 in driving the transdifferentiation of haemogenic endothelial cells into HSCs as well as in the stromal cells that support this process.

Analysis of Runx1 Using Induced Gene Ablation Reveals Its Essential Role in Pre-liver HSC Development and Limitations of an In Vivo Approach

Hematopoietic stem cells (HSCs) develop in the embryonic aorta-gonad-mesonephros (AGM) region and subsequently relocate to fetal liver. Runx1 transcription factor is essential for HSC development, but is largely dispensable for adult HSCs. Here, we studied tamoxifen-inducible Runx1 inactivation in vivo. Induction at pre-liver stages (up to embryonic day 10.5) reduced erythromyeloid progenitor numbers, but surprisingly did not block the appearance of Runx1-null HSCs in liver. By contrast, ex vivo analysis showed an absolute Runx1 dependency of HSC development in the AGM region. We found that, contrary to current beliefs, significant Cre-inducing tamoxifen activity persists in mouse blood for at least 72 hr after injection. This deferred recombination can hit healthy HSCs, which escaped early Runx1 ablation and result in appearance of Runx1-null HSCs in liver. Such extended recombination activity in vivo is a potential source of misinterpretation, particularly in analysis of dynamic developmental processes during embryogenesis.

•

Runx1 ablation induced in vivo at the AGM stage yields null HSCs in fetal liver

•

Controlled Runx1 ablation in cultured AGM region blocks HSC development

•

Discrepancy is explained by persistence of Cre activity in vivo for at least 3 days

•

Runx1 is essential at pre-liver stage of HSC development

RUNX transcription factors: orchestrators of development

RUNX transcription factors orchestrate many different aspects of biology, including basic cellular and developmental processes, stem cell biology and tumorigenesis. In this Primer, we introduce the molecular hallmarks of the three mammalian RUNX genes, RUNX1, RUNX2 and RUNX3, and discuss the regulation of their activities and their mechanisms of action. We then review their crucial roles in the specification and maintenance of a wide array of tissues during embryonic development and adult homeostasis.

RUNX1-dependent mechanisms in biological control and dysregulation in cancer

The RUNX1 transcription factor has recently been shown to be obligatory for normal development. RUNX1 controls the expression of genes essential for proper development in many cell lineages and tissues including blood, bone, cartilage, hair follicles, and mammary glands. Compromised RUNX1 regulation is associated with many cancers. In this review, we highlight evidence for RUNX1 control in both invertebrate and mammalian development and recent novel findings of perturbed RUNX1 control in breast cancer that has implications for other solid tumors. As RUNX1 is essential for definitive hematopoiesis, RUNX1 mutations in hematopoietic lineage cells have been implicated in the etiology of several leukemias. Studies of solid tumors have revealed a context-dependent function for RUNX1 either as an oncogene or a tumor suppressor. These RUNX1 functions have been reported for breast, prostate, lung, and skin cancers that are related to cancer subtypes and different stages of tumor development. Growing evidence suggests that RUNX1 suppresses aggressiveness in most breast cancer subtypes particularly in the early stage of tumorigenesis. Several studies have identified RUNX1 suppression of the breast cancer epithelial-to-mesenchymal transition. Most recently, RUNX1 repression of cancer stem cells and tumorsphere formation was reported for breast cancer. It is anticipated that these new discoveries of the context-dependent diversity of RUNX1 functions will lead to innovative therapeutic strategies for the intervention of cancer and other abnormalities of normal tissues.

A new member of the runt domain family from Pacific oyster Crassostrea gigas (CgRunx) potentially involved in immune response and larvae hematopoiesis

The Runx family is a kind of heteromeric transcription factors, which is defined by the presence of a runt domain. As transcriptional regulator during development and cell fate specification, Runx is best known for its critical roles in hematopoiesis. In the present study, a Runx transcription factor (designed as CgRunx) was identified and characterized from the oyster Crassostrea gigas. The complete coding sequence of CgRunx was of 1638 bp encoding a predicted polypeptide of 545 amino acids with one conserved runt domain, which shared high similarity with other reported Runx proteins. CgRunx was highly expressed in hemocytes, gill and mantle both at the protein and nucleic acid levels. CgRunx protein was localized specifically in the cell nuclei of hemocytes, and distributed at the tubule lumen of gill filament. During the larval developmental stages, the mRNA transcripts of CgRunx gradually increased after fertilization, reached to a relative high level at the 8 cell embryos and the blastula stage of 2-4 hpf (hours post fertilization) (about 40-fold), and peaked at early trochophore larvae (10 hpf) (about 60-fold). Whole-mount immunofluorescence assay further revealed that the abundant immunofluorescence signals of CgRunx distributed through the whole embryo at blastula stage (5 hpf), and progressively reduced with the development to a ring structure around the dorsal region in trochophore larvae (10 hpf). Scattered positive immunoreactivity signals finally appeared in the velum region of D-veliger larvae. After LPS and Vibrio splendidus stimulations, the expression levels of CgRunx mRNA in hemocytes were up-regulated significantly compared with that in the control (0 h), which were 2.98- and 2.46-fold (p < 0.05), 2.67- and 1.5-fold (p < 0.05), 2.36- and 1.38-fold (p < 0.05) at 3 h, 6 h and 12 h, respectively. These results collectively suggested that CgRunx involved in immune response and might participate in larvae hematopoiesis in oyster.

Endothelial-to-haematopoietic transition: an update on the process of making blood

The first definitive blood cells during embryogenesis are derived from endothelial cells in a highly conserved process known as endothelial-to-haematopoietic transition (EHT). This conversion involves activation of a haematopoietic transcriptional programme in a subset of endothelial cells in the major vasculature of the embryo, followed by major morphological changes that result in transitioning cells rounding up, breaking the tight junctions to neighbouring endothelial cells and adopting a haematopoietic fate. The whole process is co-ordinated by a complex interplay of key transcription factors and signalling pathways, with additional input from surrounding tissues. Diverse model systems, including mouse, chick and zebrafish embryos as well as differentiation of pluripotent cells in vitro, have contributed to the elucidation of the details of the EHT, which was greatly accelerated in recent years by sophisticated live imaging techniques and advances in transcriptional profiling, such as single-cell RNA-Seq. A detailed knowledge of these developmental events is required in order to be able to apply it to the generation of haematopoietic stem cells from pluripotent stem cells in vitro - an achievement which is of obvious clinical importance. The aim of this review is to summarise the latest findings and describe how these may have contributed towards achieving this goal.

Development of the hematopoietic system: Role of inflammatory factors

Hematopoietic stem cells (HSCs) have two defining features, multipotency and self-renewal, both of which are tightly controlled by cell autonomous programs and environmental factors throughout the lifetime of an organism. During development, HSCs are born in the aorta-gonad-mesonephros region, and migrate to distinct hematopoietic organs such as the placenta, fetal liver and spleen, continuously self-renewing and expanding to reach a homeostatic number. HSCs ultimately seed the bone marrow around the time of birth and become dormant to sustain lifelong hematopoiesis. In this review, we will summarize the recent findings on the role of inflammatory factors regulating HSC development, that is, emergence, trafficking and differentiation. An understanding of HSC kinetics during developmental processes will provide useful knowledge on HSC behavior under physiological and pathophysiological conditions. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Environmental Control of Stem Cells.

Transcription Factor PU.1 Represses and Activates Gene Expression in Early T Cells by Redirecting Partner Transcription Factor Binding

Transcription factors normally regulate gene expression through their action at sites where they bind to DNA. However, the balance of activating and repressive functions that a transcription factor can mediate is not completely understood. Here, we showed that the transcription factor PU.1 regulated gene expression in early T cell development both by recruiting partner transcription factors to its own binding sites and by depleting them from the binding sites that they preferred when PU.1 was absent. The removal of partner factors Satb1 and Runx1 occurred primarily from sites where PU.1 itself did not bind. Genes linked to sites of partner factor "theft" were enriched for genes that PU.1 represses despite lack of binding, both in a model cell line system and in normal T cell development. Thus, system-level competitive recruitment dynamics permit PU.1 to affect gene expression both through its own target sites and through action at a distance.

RUNX1-ETO: Attacking the Epigenome for Genomic Instable Leukemia

Oncogenic fusion protein RUNX1-ETO is the product of the t(8;21) translocation, responsible for the most common cytogenetic subtype of acute myeloid leukemia. RUNX1, a critical transcription factor in hematopoietic development, is fused with almost the entire ETO sequence with the ability to recruit a wide range of repressors. Past efforts in providing a comprehensive picture of the genome-wide localization and the target genes of RUNX1-ETO have been inconclusive in understanding the underlying mechanism by which it deregulates native RUNX1. In this review; we dissect the current data on the epigenetic impact of RUNX1 and RUNX1-ETO. Both share similarities however, in recent years, research focused on epigenetic factors to explain their differences. RUNX1-ETO impairs DNA repair mechanisms which compromises genomic stability and favors a mutator phenotype. Among an increasing pool of mutated factors, regulators of DNA methylation are frequently found in t(8;21) AML. Together with the alteration of both, histone markers and distal enhancer regulation, RUNX1-ETO might specifically disrupt normal chromatin structure. Epigenetic studies on the fusion protein uncovered new mechanisms contributing to leukemogenesis and hopefully will translate into clinical applications.

RUNX1 and the endothelial origin of blood

The transcription factor RUNX1 is required in the embryo for formation of the adult hematopoietic system. Here, we describe the seminal findings that led to the discovery of RUNX1 and of its critical role in blood cell formation in the embryo from hemogenic endothelium (HE). We also present RNA-sequencing data demonstrating that HE cells in different anatomic sites, which produce hematopoietic progenitors with dissimilar differentiation potentials, are molecularly distinct. Hemogenic and non-HE cells in the yolk sac are more closely related to each other than either is to hemogenic or non-HE cells in the major arteries. Therefore, a major driver of the different lineage potentials of the committed erythro-myeloid progenitors that emerge in the yolk sac versus hematopoietic stem cells that originate in the major arteries is likely to be the distinct molecular properties of the HE cells from which they are derived. We used bioinformatics analyses to predict signaling pathways active in arterial HE, which include the functionally validated pathways Notch, Wnt, and Hedgehog. We also used a novel bioinformatics approach to assemble transcriptional regulatory networks and predict transcription factors that may be specifically involved in hematopoietic cell formation from arterial HE, which is the origin of the adult hematopoietic system.

The multi-faceted role of Gata3 in developmental haematopoiesis

The transcription factor Gata3 is crucial for the development of several tissues and cell lineages both during development as well as postnatally. This importance is apparent from the early embryonic lethality following germline Gata3 deletion, with embryos displaying a number of phenotypes, and from the fact that Gata3 has been implicated in several cancer types. It often acts at the level of stem and progenitor cells in which it controls the expression of key lineage-determining factors as well as cell cycle genes, thus being one of the main drivers of cell fate choice and tissue morphogenesis. Gata3 is involved at various stages of haematopoiesis both in the adult as well as during development. This review summarizes the various contributions of Gata3 to haematopoiesis with a particular focus on the emergence of the first haematopoietic stem cells in the embryo—a process that appears to be influenced by Gata3 at various levels, thus highlighting the complex nature of Gata3 action.

Poly(C)-Binding Protein Pcbp2 Enables Differentiation of Definitive Erythropoiesis by Directing Functional Splicing of the Runx1 Transcript

Formation of the mammalian hematopoietic system is under a complex set of developmental controls. Here, we report that mouse embryos lacking the KH domain poly(C) binding protein, Pcbp2, are selectively deficient in the definitive erythroid lineage. Compared to wild-type controls, transcript splicing analysis of the Pcbp2-/- embryonic liver reveals accentuated exclusion of an exon (exon 6) that encodes a highly conserved transcriptional control segment of the hematopoietic master regulator, Runx1. Embryos rendered homozygous for a Runx1 locus lacking this cassette exon (Runx1ΔE6) effectively phenocopy the loss of the definitive erythroid lineage in Pcbp2-/- embryos. These data support a model in which enhancement of Runx1 cassette exon 6 inclusion by Pcbp2 serves a critical role in development of hematopoietic progenitors and constitutes a critical step in the developmental pathway of the definitive erythropoietic lineage.

Mouse Yolk Sac Hematopoiesis

The yolk sac is the first observed site of hematopoiesis during mouse ontogeny. Primitive erythroid cells are the most well-recognized cell lineages produced from this tissue. In addition to primitive erythroid cells, several types of hematopoietic cells are present, including multipotent hematopoietic progenitors. Yolk sac-derived blood cells constitute a transient wave of embryonic and fetal hematopoiesis. However, recent studies have demonstrated that some macrophage and B cell lineages derived from the early yolk sac may persist to adulthood. This review discusses the cellular basis of mouse yolk sac hematopoiesis and its contributions to embryonic and adult hematopoietic systems.

Marking of definitive HSC precursors in E7.5-E8.5 embryos using an Abcg2-CreER lineage-tracing mouse model

Abcg2, a member of the ATP-binding cassette transporter family, is expressed in adult hematopoietic stem cells (HSCs) and is required for the side population phenotype of adult bone marrow HSCs and other adult tissue-specific stem cells. Lineage tracing in adult mice using the Abcg2-Cre mouse model showed that Abcg2 marks HSCs, intestinal stem cells, and spermatogonial stem cells. It is unclear whether definitive HSCs or their precursors in early embryonic development can be marked by Abcg2 expression. Here, we treated pregnant Abcg2 Cre/Cre RosaLSL-YFP mice with a single injection of 4-hydroxytamoxifen at embryonic day 7.5. Four months after birth, a small yellow fluorescent protein-positive (YFP⁺) cell population could be detected in all of the major white blood cell lineages and this was stable for 8 months. Transplant of bone marrow cells or Sca1⁺YFP⁺ cells from these mice showed continued multilineage marking in recipient mice at 4 months. These results demonstrate that Abcg2 expression marks precursors to adult long-term repopulating HSCs at E7.5 to E8.5 and contributes to a stable subpopulation of HSCs well into adulthood.

Single-cell transcriptomics reveal the dynamic of haematopoietic stem cell production in the aorta

Haematopoietic stem cells (HSCs) are generated from haemogenic endothelial (HE) cells via the formation of intra-aortic haematopoietic clusters (IAHCs) in vertebrate embryos. The molecular events controlling endothelial specification, endothelial-to-haematopoietic transition (EHT) and IAHC formation, as it occurs in vivo inside the aorta, are still poorly understood. To gain insight in these processes, we performed single-cell RNA-sequencing of non-HE cells, HE cells, cells undergoing EHT, IAHC cells, and whole IAHCs isolated from mouse embryo aortas. Our analysis identified the genes and transcription factor networks activated during the endothelial-to-haematopoietic switch and IAHC cell maturation toward an HSC fate. Our study provides an unprecedented complete resource to study in depth HSC generation in vivo. It will pave the way for improving HSC production in vitro to address the growing need for tailor-made HSCs to treat patients with blood-related disorders.

Regulation of RUNX1 dosage is crucial for efficient blood formation from hemogenic endothelium

During ontogeny, hematopoietic stem and progenitor cells arise from hemogenic endothelium through an endothelial-to-hematopoietic transition that is strictly dependent on the transcription factor RUNX1. Although it is well established that RUNX1 is essential for the onset of hematopoiesis, little is known about the role of RUNX1 dosage specifically in hemogenic endothelium and during the endothelial-to-hematopoietic transition. Here, we used the mouse embryonic stem cell differentiation system to determine if and how RUNX1 dosage affects hemogenic endothelium differentiation. The use of inducible Runx1 expression combined with alterations in the expression of the RUNX1 co-factor CBFβ allowed us to evaluate a wide range of RUNX1 levels. We demonstrate that low RUNX1 levels are sufficient and necessary to initiate an effective endothelial-to-hematopoietic transition. Subsequently, RUNX1 is also required to complete the endothelial-to-hematopoietic transition and to generate functional hematopoietic precursors. In contrast, elevated levels of RUNX1 are able to drive an accelerated endothelial-to-hematopoietic transition, but the resulting cells are unable to generate mature hematopoietic cells. Together, our results suggest that RUNX1 dosage plays a pivotal role in hemogenic endothelium maturation and the establishment of the hematopoietic system.

Interleukin-2-regulatory T cell axis critically regulates maintenance of hematopoietic stem cells

The role of IL-2 in HSC maintenance is unknown. Here we show that Il2^−/− mice develop severe anomalies in HSC maintenance leading to defective hematopoiesis. Whereas, lack of IL-2 signaling was detrimental for lympho- and erythropoiesis, myelopoiesis was enhanced in Il2^−/− mice. Investigation of the underlying mechanisms of dysregulated hematopoiesis in Il2^−/− mice shows that the IL-2-T_reg cell axis is indispensable for HSC maintenance and normal hematopoiesis. Lack of T_reg activity resulted in increased IFN-? production by activated T cells and an expansion of the HSCs in the bone marrow (BM). Though, restoring T_reg population successfully rescued HSC maintenance in Il2^−/− mice, preventing IFN-? activity could do the same even in the absence of T_reg cells. Our study suggests that equilibrium in IL-2 and IFN-? activity is critical for steady state hematopoiesis, and in clinical conditions of BM failure, IL-2 or anti-IFN-? treatment might help to restore hematopoiesis.

Tracking the origin, development, and differentiation of hematopoietic stem cells

PURPOSE OF REVIEW

The hierarchical nature of the hematopoietic system provides an ideal model system to illustrate the features of lineage tracing. We have outlined the utility of lineage tracing methods in establishing the origin and development of hematopoietic cells.

RECENT FINDINGS

Methods such as CRISPR/Cas9, Polylox barcoding, and single-cell RNA-sequencing have improved our understanding of hematopoiesis.

SUMMARY

This review chronicles the fate of the hematopoietic cells emerging from the mesoderm that subsequently develops into the adult blood lineages. Specifically, we explain classic techniques utilized in lineage tracing for the hematopoietic system, as well as novel state-of-the-art methods to elucidate clonal hematopoiesis and cell fate mapping at a single-cell level.

Regulation of embryonic haematopoietic multipotency by EZH1

All haematopoietic lineages circulating in the blood of adult mammals derive from multipotent haematopoietic stem cells (HSCs)¹. Haematopoiesis in the mammalian embryo stands in stark contrast, with lineage-restricted progenitors arising first, independently of HSCs, and HSCs emerging only later in gestation^2,3. As best defined in the mouse, “primitive” progenitors first appear in the yolk sac (YS) at 7.5 days post-coitum (dpc)^2,3. Subsequently, erythroid-myeloid progenitors (EMPs) that express fetal hemoglobin⁴, as well as fetal lymphoid progenitors⁵ develop in the YS and the embryo proper, but these cells lack HSC potential. Ultimately, “definitive” HSCs with long-term, multilineage potential and the capacity to engraft irradiated adults emerge at 10.5 dpc from arterial endothelium in the aorta-gonad-mesonephros (AGM) and other haemogenic vasculature³. The molecular mechanisms for reverse progression of haematopoietic ontogeny remain unexplained. We hypothesized that the definitive haematopoietic program might be actively repressed in early embryogenesis via epigenetic silencing⁶, and that alleviating this repression would elicit multipotency in otherwise restricted haematopoietic progenitors. Here, we demonstrate that reduced expression of the Polycomb group protein EZH1 uncovers multi-lymphoid output from human pluripotent stem cells (PSCs) and precocious emergence of functional definitive HSCs at sites of primitive and/or EMP-biased haematopoiesis in vivo, identifying EZH1 as a repressor of haematopoietic multipotency in the early mammalian embryo.

RUNX1 Mutations in Inherited and Sporadic Leukemia

RUNX1 is a recurrently mutated gene in sporadic myelodysplastic syndrome and leukemia. Inherited mutations in RUNX1 cause familial platelet disorder with predisposition to acute myeloid leukemia (FPD/AML). In sporadic AML, mutations in RUNX1 are usually secondary events, whereas in FPD/AML they are initiating events. Here we will describe mutations in RUNX1 in sporadic AML and in FPD/AML, discuss the mechanisms by which inherited mutations in RUNX1 could elevate the risk of AML in FPD/AML individuals, and speculate on why mutations in RUNX1 are rarely, if ever, the first event in sporadic AML.