Comparison Evaluation of Automated Nucleated Red Blood Cell Enumeration by Sysmex XN 1000 in Comparison With Microscopic Reference in Children Under 1 Year.

BACKGROUND: In newborns, elevated nucleated red blood cell (NRBC) levels can be associated with enhanced erythropoietic stress and might be predictive for adverse outcome. Also, the presence of NRBC in peripheral blood might lead to erroneous enumeration results of white blood cells in automated hematology analyzers. We aimed to assess the comparability of the Sysmex XN 1000 to manual slide reviews and correlation of NRBC with inflammation markers. METHODS: Specimens of 3397 children under 1 year were compared by automated and microscopic NRBC enumeration. Additionally, potential correlations between NRBC and age and inflammation markers were examined. RESULTS: Overall, there was good correlation (r = 0.97) between automated (range: 0%-3883%) and microscopic enumeration (range: 0%-3694%) of NRBC with high comparability up to a NRBC value of 200% and an increase in the variation between the two methods with increasing NRBC numbers. When 94 samples with ≤ 200% NRBC and ≥ 30% divergence between methods were separately reanalyzed with respect to overlapping cell populations in their scattergrams, Sysmex would have generated unrecognized incorrect automated results in 47 samples, corresponding to 1.4% of total study samples. NRBC counts were negatively correlated to age, but not to inflammation markers. CONCLUSION: Sysmex XN 1000 is highly precise in the enumeration of NRBC in children under 1 year up to counts of 200% and might replace time-intense manual counting in routine diagnostics. In the setting of neonatal and intensive care diagnostics, microscopic control and supervision of scattergrams are highly recommended for any automated NRBC enumeration processes.

Comparative analysis of feature-based ML and CNN for binucleated erythroblast quantification in myelodysplastic syndrome patients using imaging flow cytometry data.

Myelodysplastic syndrome is primarily characterized by dysplasia in the bone marrow (BM), presenting a challenge in consistent morphology interpretation. Accurate diagnosis through traditional slide-based analysis is difficult, necessitating a standardized objective technique. Over the past two decades, imaging flow cytometry (IFC) has proven effective in combining image-based morphometric analyses with high-parameter phenotyping. We have previously demonstrated the effectiveness of combining IFC with a feature-based machine learning algorithm to accurately identify and quantify rare binucleated erythroblasts (BNEs) in dyserythropoietic BM cells. However, a feature-based workflow poses challenges requiring software-specific expertise. Here we employ a Convolutional Neural Network (CNN) algorithm for BNE identification and differentiation from doublets and cells with irregular nuclear morphology in IFC data. We demonstrate that this simplified AI workflow, coupled with a powerful CNN algorithm, achieves comparable BNE quantification accuracy to manual and feature-based analysis with substantial time savings, eliminating workflow complexity. This streamlined approach holds significant clinical value, enhancing IFC accessibility for routine diagnostic purposes.

let-7 miRNAs repress HIC2 to regulate BCL11A transcription and hemoglobin switching.

ABSTRACT: The switch from fetal hemoglobin (γ-globin, HBG) to adult hemoglobin (ß-globin, HBB) gene transcription in erythroid cells serves as a paradigm for a complex and clinically relevant developmental gene regulatory program. We previously identified HIC2 as a regulator of the switch by inhibiting the transcription of BCL11A, a key repressor of HBG production. HIC2 is highly expressed in fetal cells, but the mechanism of its regulation is unclear. Here we report that HIC2 developmental expression is controlled by microRNAs (miRNAs), as loss of global miRNA biogenesis through DICER1 depletion leads to upregulation of HIC2 and HBG messenger RNA. We identified the adult-expressed let-7 miRNA family as a direct posttranscriptional regulator of HIC2. Ectopic expression of let-7 in fetal cells lowered HIC2 levels, whereas inhibition of let-7 in adult erythroblasts increased HIC2 production, culminating in decommissioning of a BCL11A erythroid enhancer and reduced BCL11A transcription. HIC2 depletion in let-7-inhibited cells restored BCL11A-mediated repression of HBG. Together, these data establish that fetal hemoglobin silencing in adult erythroid cells is under the control of a miRNA-mediated inhibitory pathway (let-7 ⊣ HIC2 ⊣ BCL11A ⊣ HBG).

Efficient terminal erythroid differentiation requires the APC/C cofactor Cdh1 to limit replicative stress in erythroblasts.

The APC/C-Cdh1 ubiquitin ligase complex drives proteosomal degradation of cell cycle regulators and other cellular proteins during the G1 phase of the cycle. The complex serves as an important modulator of the G1/S transition and prevents premature entry into S phase, genomic instability, and tumor development. Additionally, mounting evidence supports a role for this complex in cell differentiation, but its relevance in erythropoiesis has not been addressed so far. Here we show, using mouse models of Cdh1 deletion, that APC/C-Cdh1 activity is required for efficient terminal erythroid differentiation during fetal development as well as postnatally. Consistently, Cdh1 ablation leads to mild but persistent anemia from birth to adulthood. Interestingly, loss of Cdh1 seems to affect both, steady-state and stress erythropoiesis. Detailed analysis of Cdh1-deficient erythroid populations revealed accumulation of DNA damage in maturing erythroblasts and signs of delayed G2/M transition. Moreover, through direct assessment of replication dynamics in fetal liver cells, we uncovered slow fork movement and increased origin usage in the absence of Cdh1, strongly suggesting replicative stress to be the underlying cause of DNA lesions and cell cycle delays in erythroblasts devoid of Cdh1. In turn, these alterations would restrain full maturation of erythroblasts into reticulocytes and reduce the output of functional erythrocytes, leading to anemia. Our results further highlight the relevance of APC/C-Cdh1 activity for terminal differentiation and underscore the need for precise control of replication dynamics for efficient supply of red blood cells.

Erythroid differentiation in mouse erythroleukemia cells depends on Tmod3-mediated regulation of actin filament assembly into the erythroblast membrane skeleton.

Erythroid differentiation (ED) is a complex cellular process entailing morphologically distinct maturation stages of erythroblasts during terminal differentiation. Studies of actin filament (F-actin) assembly and organization during terminal ED have revealed essential roles for the F-actin pointed-end capping proteins, tropomodulins (Tmod1 and Tmod3). Tmods bind tropomyosins (Tpms), which enhance Tmod capping and F-actin stabilization. Tmods can also nucleate F-actin assembly, independent of Tpms. Tmod1 is present in the red blood cell (RBC) membrane skeleton, and deletion of Tmod1 in mice leads to a mild compensated anemia due to mis-regulated F-actin lengths and membrane instability. Tmod3 is not present in RBCs, and global deletion of Tmod3 leads to embryonic lethality in mice with impaired ED. To further decipher Tmod3's function during ED, we generated a Tmod3 knockout in a mouse erythroleukemia cell line (Mel ds19). Tmod3 knockout cells appeared normal prior to ED, but showed defects during progression of ED, characterized by a marked failure to reduce cell and nuclear size, reduced viability, and increased apoptosis. Tmod3 does not assemble with Tmod1 and Tpms into the Triton X-100 insoluble membrane skeleton during ED, and loss of Tmod3 had no effect on α1,ß1-spectrin and protein 4.1R assembly into the membrane skeleton. However, F-actin, Tmod1 and Tpms failed to assemble into the membrane skeleton during ED in absence of Tmod3. We propose that Tmod3 nucleation of F-actin assembly promotes incorporation of Tmod1 and Tpms into membrane skeleton F-actin, and that this is integral to morphological maturation and cell survival during erythroid terminal differentiation.

Scalable in vitro production of defined mouse erythroblasts.

Mouse embryonic stem cells (mESCs) can be manipulated in vitro to recapitulate the process of erythropoiesis, during which multipotent cells undergo lineage specification, differentiation and maturation to produce erythroid cells. Although useful for identifying specific progenitors and precursors, this system has not been fully exploited as a source of cells to analyse erythropoiesis. Here, we establish a protocol in which characterised erythroblasts can be isolated in a scalable manner from differentiated embryoid bodies (EBs). Using transcriptional and epigenetic analysis, we demonstrate that this system faithfully recapitulates normal primitive erythropoiesis and fully reproduces the effects of natural and engineered mutations seen in primary cells obtained from mouse models. We anticipate this system to be of great value in reducing the time and costs of generating and maintaining mouse lines in a number of research scenarios.

EpoR stimulates rapid cycling and larger red cells during mouse and human erythropoiesis.

The erythroid terminal differentiation program couples sequential cell divisions with progressive reductions in cell size. The erythropoietin receptor (EpoR) is essential for erythroblast survival, but its other functions are not well characterized. Here we use Epor-/- mouse erythroblasts endowed with survival signaling to identify novel non-redundant EpoR functions. We find that, paradoxically, EpoR signaling increases red cell size while also increasing the number and speed of erythroblast cell cycles. EpoR-regulation of cell size is independent of established red cell size regulation by iron. High erythropoietin (Epo) increases red cell size in wild-type mice and in human volunteers. The increase in mean corpuscular volume (MCV) outlasts the duration of Epo treatment and is not the result of increased reticulocyte number. Our work shows that EpoR signaling alters the relationship between cycling and cell size. Further, diagnostic interpretations of increased MCV should now include high Epo levels and hypoxic stress.

Spatial Transcriptomics to define transcriptional patterns of zonation and structural components in the mouse liver.

Reconstruction of heterogeneity through single cell transcriptional profiling has greatly advanced our understanding of the spatial liver transcriptome in recent years. However, global transcriptional differences across lobular units remain elusive in physical space. Here, we apply Spatial Transcriptomics to perform transcriptomic analysis across sectioned liver tissue. We confirm that the heterogeneity in this complex tissue is predominantly determined by lobular zonation. By introducing novel computational approaches, we enable transcriptional gradient measurements between tissue structures, including several lobules in a variety of orientations. Further, our data suggests the presence of previously transcriptionally uncharacterized structures within liver tissue, contributing to the overall spatial heterogeneity of the organ. This study demonstrates how comprehensive spatial transcriptomic technologies can be used to delineate extensive spatial gene expression patterns in the liver, indicating its future impact for studies of liver function, development and regeneration as well as its potential in pre-clinical and clinical pathology.

Bone marrow sinusoidal endothelium controls terminal erythroid differentiation and reticulocyte maturation.

Within the bone marrow microenvironment, endothelial cells (EC) exert important functions. Arterial EC support hematopoiesis while H-type capillaries induce bone formation. Here, we show that BM sinusoidal EC (BM-SEC) actively control erythropoiesis. Mice with stabilized ß-catenin in BM-SEC (Ctnnb1OE-SEC) generated by using a BM-SEC-restricted Cre mouse line (Stab2-iCreF3) develop fatal anemia. While activation of Wnt-signaling in BM-SEC causes an increase in erythroblast subsets (PII-PIV), mature erythroid cells (PV) are reduced indicating impairment of terminal erythroid differentiation/reticulocyte maturation. Transplantation of Ctnnb1OE-SEC hematopoietic stem cells into wildtype recipients confirms lethal anemia to be caused by cell-extrinsic, endothelial-mediated effects. Ctnnb1OE-SEC BM-SEC reveal aberrant sinusoidal differentiation with altered EC gene expression and perisinusoidal ECM deposition and angiocrine dysregulation with de novo endothelial expression of FGF23 and DKK2, elevated in anemia and involved in vascular stabilization, respectively. Our study demonstrates that BM-SEC play an important role in the bone marrow microenvironment in health and disease.

Association of the Degree of Erythroid Expansion and Maturation Arrest with the Clinical Severity of ß0-Thalassemia/Hemoglobin E Patients.

INTRODUCTION: ß-Thalassemia/hemoglobin E represents one-half of all the clinically severe ß-thalassemias worldwide. Despite similar genetic backgrounds, patients show clinical heterogeneity ranging from nearly asymptomatic to transfusion-dependent thalassemia. The underlying disease modifying factors remain largely obscure. METHODS: To elucidate the correlation between ineffective erythropoiesis and ß0-thalassemia/hemoglobin E (HbE) disease severity, in vitro culture of erythroid cells derived from patients with different clinical symptoms was established. Cell proliferation, viability, and differentiation were investigated. To identify potential molecular mechanisms leading to the arrested erythroid maturation, the expression levels of erythropoiesis modifying factors were measured. RESULTS: The ß0-thalassemia/HbE cells exhibited enhanced proliferation, limited differentiation, and impaired erythroid terminal maturation but did not show accelerated erythroblast differentiation and increased cell death. Erythroblasts derived from mild patients showed the highest proliferation rate with a faster cell division time, while erythroblasts derived from severe patients displayed extremely delayed erythroid maturation. Downregulation of growth differentiation factor 11 and FOXO3a was observed in mild ß0-thalassemia/HbE erythroblasts, while upregulation of heat shock protein 70 and activin receptor 2A was revealed in severe erythroblasts. DISCUSSION/CONCLUSION: The degree of erythroid expansion and maturation arrest contributes to the severity of ß0-thalassemia/HbE patients, accounting for the disease heterogeneity. The findings suggest a restoration of erythroid maturation as a promising targeted therapy for severe patients.

Generation of 'designer erythroblasts' lacking one or more blood group systems from CRISPR/Cas9 gene-edited human-induced pluripotent stem cells.

Despite the recent advancements in transfusion medicine, red blood cell (RBC) alloimmunization remains a challenge for multiparous women and chronically transfused patients. At times, diagnostic laboratories depend on difficult-to-procure rare reagent RBCs for the identification of different alloantibodies in such subjects. We have addressed this issue by developing erythroblasts with custom phenotypes (Rh null, GPB null and Kx null/Kell low) using CRISPR/Cas9 gene-editing of a human induced pluripotent stem cell (hiPSC) parent line (OT1-1) for the blood group system genes: RHAG, GYPB and XK. Guide RNAs were cloned into Cas9-puromycin expression vector and transfected into OT1-1. Genotyping was performed to select puromycin-resistant hiPSC KOs. CRISPR/Cas9 gene-editing resulted in the successful generation of three KO lines, RHAG KO, GYPB KO and XK KO. The OT1-1 cell line, as well as the three KO hiPSC lines, were differentiated into CD34+ CD41+ CD235ab+ hematopoietic progenitor cells (HPCs) and subsequently to erythroblasts. Native OT1-1 erythroblasts were positive for the expression of Rh, MNS, Kell and H blood group systems. Differentiation of RHAG KO, GYPB KO and XK KO resulted in the formation of Rh null, GPB null and Kx null/Kell low erythroblasts, respectively. OT1-1 as well as the three KO erythroblasts remained positive for RBC markers-CD71 and BAND3. Erythroblasts were mostly at the polychromatic/ orthochromatic stage of differentiation. Up to ~400-fold increase in erythroblasts derived from HPCs was observed. The availability of custom erythroblasts generated from CRISPR/Cas9 gene-edited hiPSC should be a useful addition to the tools currently used for the detection of clinically important red cell alloantibodies.

Improving the efficiency of the autoverification workflow for nucleated red blood cell reporting in the hematology laboratory.

OBJECTIVE: Although the microscopic manual count is considered the standard method for NRBC enumeration, modern hematology analyzers can perform this task automatically with reliable accuracy and efficiency. This study aims to evaluate the diagnostic performance of the Sysmex XN hematology analyzer and to construct the optimal workflow for accurate and efficient NRBC reporting. METHODS: Specimens containing different levels of NRBC were included. Analytical performance was evaluated via method comparison with flow cytometry (FCM) and manual count (MC). Clinical sensitivity was analyzed by ROC analysis using manual count as the standard method. RESULTS: Correlation study of %NRBC with FCM and MC demonstrated an r value of 0.925 (95% CI 0.905 to 0.942) and 0.990 (95% CI 0.987 to 0.992) with a mean difference of -0.8 (95%CI: -6.7 to +5.0) and +0.50 (95% CI: -6.7 to +7.7), respectively. When the automated NRBC count was equal to zero and >0.07 × 109 /L, the false-negative rate and false-positive rate were 100%, respectively; hence, manual slide review could be omitted. A false-positive rate of 72.7% was noted in specimens containing NRBC count less than 0.07 × 109 /L. CONCLUSION: The Sysmex XN can help improve the efficiency of NRBC enumeration owing to its accuracy, rapidity, and automation. However, further studies are required to improve the accuracy of detection in specimens containing a very low level of NRBC.

Regulation of RNA polymerase II activity is essential for terminal erythroid maturation.

The terminal maturation of human erythroblasts requires significant changes in gene expression in the context of dramatic nuclear condensation. Defects in this process are associated with inherited anemias and myelodysplastic syndromes. The progressively dense appearance of the condensing nucleus in maturing erythroblasts led to the assumption that heterochromatin accumulation underlies this process, but despite extensive study, the precise mechanisms underlying this essential biologic process remain elusive. To delineate the epigenetic changes associated with the terminal maturation of human erythroblasts, we performed mass spectrometry of histone posttranslational modifications combined with chromatin immunoprecipitation coupled with high-throughput sequencing, Assay for Transposase Accessible Chromatin, and RNA sequencing. Our studies revealed that the terminal maturation of human erythroblasts is associated with a dramatic decline in histone marks associated with active transcription elongation, without accumulation of heterochromatin. Chromatin structure and gene expression were instead correlated with dynamic changes in occupancy of elongation competent RNA polymerase II, suggesting that terminal erythroid maturation is controlled largely at the level of transcription. We further demonstrate that RNA polymerase II "pausing" is highly correlated with transcriptional repression, with elongation competent RNA polymerase II becoming a scare resource in late-stage erythroblasts, allocated to erythroid-specific genes. Functional studies confirmed an essential role for maturation stage-specific regulation of RNA polymerase II activity during erythroid maturation and demonstrate a critical role for HEXIM1 in the regulation of gene expression and RNA polymerase II activity in maturing erythroblasts. Taken together, our findings reveal important insights into the mechanisms that regulate terminal erythroid maturation and provide a novel paradigm for understanding normal and perturbed erythropoiesis.

RNA-regulatory exosome complex confers cellular survival to promote erythropoiesis.

Cellular differentiation requires vast remodeling of transcriptomes, and therefore machinery mediating remodeling controls differentiation. Relative to transcriptional mechanisms governing differentiation, post-transcriptional processes are less well understood. As an important post-transcriptional determinant of transcriptomes, the RNA exosome complex (EC) mediates processing and/or degradation of select RNAs. During erythropoiesis, the erythroid transcription factor GATA1 represses EC subunit genes. Depleting EC structural subunits prior to GATA1-mediated repression is deleterious to erythroid progenitor cells. To assess the importance of the EC catalytic subunits Dis3 and Exosc10 in this dynamic process, we asked if these subunits function non-redundantly to control erythropoiesis. Dis3 or Exosc10 depletion in primary murine hematopoietic progenitor cells reduced erythroid progenitors and their progeny, while sparing myeloid cells. Dis3 loss severely compromised erythroid progenitor and erythroblast survival, rendered erythroblasts hypersensitive to apoptosis-inducing stimuli and induced γ-H2AX, indicative of DNA double-stranded breaks. Dis3 loss-of-function phenotypes were more severe than those caused by Exosc10 depletion. We innovated a genetic rescue system to compare human Dis3 with multiple myeloma-associated Dis3 mutants S447R and R750K, and only wild type Dis3 was competent to rescue progenitors. Thus, Dis3 establishes a disease mutation-sensitive, cell type-specific survival mechanism to enable a differentiation program.

Prognostic impacts of peripheral blood erythroblasts after single-unit cord blood transplantation.

INTRODUCTION: The appearance of erythroblasts (EBLs) in peripheral blood occurs in a variety of serious conditions and has been associated with mortality in critically ill patients. However, the incidence, risk factor, and outcomes of EBLs after cord blood transplantation (CBT) remain unclear. METHODS: We have investigated the impact of EBLs on transplant outcomes on 225 adult patients who underwent single-unit CBT at our single institute. RESULTS: The cumulative incidences of EBL ≥200 × 106 /L and EBL ≥1000 × 106 /L at 60 days after CBT were 17% and 4%, respectively, detected after a median of 35 days and 36.5 days. Multivariate analysis using erythroblastosis as time-dependent covariates demonstrated the significant association of EBL ≥1000 × 106 /L, but not EBL ≥200 × 106 /L, with the development of grade III-IV acute graft-versus-host disease (GVHD, hazard ratio [HR]: 18.56; P < .001), higher nonrelapse mortality (HR: 13.38; P < .001), and overall mortality (HR: 4.97; P = .001). CONCLUSION: These data suggested that higher levels of EBLs were recognized as a significant risk factor for severe acute GVHD and mortality after single-unit CBT. Higher levels of EBLs may serve as a surrogate marker for poor single CBT outcomes.

Impairment of human terminal erythroid differentiation by histone deacetylase 5 deficiency.

Histone deacetylases (HDACs) are a group of enzymes that catalyze the removal of acetyl groups from histone and nonhistone proteins. HDACs have been shown to have diverse functions in a wide range of biological processes. However, their roles in mammalian erythropoiesis remain to be fully defined. This study showed that, of the 11 classic HDAC family members, 6 (HDAC1, -2, -3, and HDAC5, -6, -7) are expressed in human erythroid cells, with HDAC5 most significantly upregulated during terminal erythroid differentiation. Knockdown of HDAC5 by either short hairpin RNA or small interfering RNA in human CD34+ cells followed by erythroid cell culture led to increased apoptosis, decreased chromatin condensation, and impaired enucleation of erythroblasts. Biochemical analyses revealed that HDAC5 deficiency resulted in activation of p53 in association with increased acetylation of p53. Furthermore, although acetylation of histone 4 (H4) is decreased during normal terminal erythroid differentiation, HDAC5 deficiency led to increased acetylation of H4 (K12) in late-stage erythroblasts. This increased acetylation was accompanied by decreased chromatin condensation, implying a role for H4 (K12) deacetylation in chromatin condensation. ATAC-seq and RNA sequencing analyses revealed that HDAC5 knockdown leads to increased chromatin accessibility genome-wide and global changes in gene expression. Moreover, pharmacological inhibition of HDAC5 by the inhibitor LMK235 also led to increased H4 acetylation, impaired chromatin condensation, and enucleation. Taken together, our findings have uncovered previously unrecognized roles and molecular mechanisms of action for HDAC5 in human erythropoiesis. These results may provide insights into understanding the anemia associated with HDAC inhibitor treatment.

PPARγ is essential for the development of bone marrow erythroblastic island macrophages and splenic red pulp macrophages.

Tissue-resident macrophages play a crucial role in maintaining homeostasis. Macrophage progenitors migrate to tissues perinatally, where environmental cues shape their identity and unique functions. Here, we show that the absence of PPARγ affects neonatal development and VCAM-1 expression of splenic iron-recycling red pulp macrophages (RPMs) and bone marrow erythroblastic island macrophages (EIMs). Transcriptome analysis of the few remaining Pparg-deficient RPM-like and EIM-like cells suggests that PPARγ is required for RPM and EIM identity, cell cycling, migration, and localization, but not function in mature RPMs. Notably, Spi-C, another transcription factor implicated in RPM development, was not essential for neonatal expansion of RPMs, even though the transcriptome of Spic-deficient RPMs was strongly affected and indicated a loss of identity. Similarities shared by Pparg- and Spic-deficient RPM-like cells allowed us to identify pathways that rely on both factors. PPARγ and Spi-C collaborate in inducing transcriptional changes, including VCAM-1 and integrin αD expression, which could be required for progenitor retention in the tissue, allowing access to niche-related signals that finalize differentiation.

Hypoxia promotes erythroid differentiation through the development of progenitors and proerythroblasts.

Oxygen is a critical noncellular component of the bone marrow microenvironment that plays an important role in the development of hematopoietic cell lineages. In this study, we investigated the impact of low oxygen (hypoxia) on ex vivo myeloerythroid differentiation of human cord blood-derived CD34+ hematopoietic stem and progenitor cells. We characterized the culture conditions to demonstrate that low oxygen inhibits cell proliferation and causes a metabolic shift in the stem and progenitor populations. We found that hypoxia promotes erythroid differentiation by supporting the development of progenitor populations. Hypoxia also increases the megakaryoerythroid potential of the common myeloid progenitors and the erythroid potential of megakaryoerythroid progenitors and significantly accelerates maturation of erythroid cells. Specifically, we determined that hypoxia promotes the loss of CD71 and the appearance of the erythroid markers CD235a and CD239. Further, evaluation of erythroid populations revealed a hypoxia-induced increase in proerythroblasts and in enucleation of CD235a+ cells. These results reveal the extensive role of hypoxia at multiple steps during erythroid development. Overall, our work establishes a valuable model for further investigations into the relationship between erythroid progenitors and/or erythroblast populations and their hypoxic microenvironment.

SETD2 is essential for terminal differentiation of erythroblasts during fetal erythropoiesis.

SET domain-containing 2 (SETD2), the primary methyltransferase for histone 3 lysine-36 trimethylation (H3K36me3) in mammals, is associated with many hematopoietic diseases when mutated. Previous works have emphasized its role in maintaining adult hematopoietic stem cells or tumorigenesis, however, whether and how SETD2 regulates erythropoiesis during embryonic development is relatively unexplored. In this study, using a conditional SETD2 knockout (KO) mouse model, we reveal that SETD2 plays an essential role in fetal erythropoiesis. Loss of Setd2 in hematopoietic cells ablates H3K36me3, and leads to anemia with a significant decrease in erythroid cells in the peripheral blood at E18.5. This is due to impaired erythroblast differentiation in both spleen and liver. We also find increased proportions of nucleated erythrocytes in the blood of Setd2 KO embryos. Lastly, we ascribe embryonic erythropoiesis-related genes Vegfc, Vegfr3, and Prox1, as likely downstream targets of SETD2 regulation. Our study reveals a critical role of SETD2 in fetal erythropoiesis that precedes adult hematopoiesis, and provide unique insights into the defects in erythroid lineages, such as anemia.

Erythroid enucleation: a gateway into a "bloody" world.

Erythropoiesis is an intricate process starting in hematopoietic stem cells and leading to the daily production of 200 billion red blood cells (RBCs). Enucleation is a greatly complex and rate-limiting step during terminal maturation of mammalian RBC production involving expulsion of the nucleus from the orthochromatic erythroblasts, resulting in the formation of reticulocytes. The dynamic enucleation process involves many factors ranging from cytoskeletal proteins to transcription factors to microRNAs. Lack of optimum terminal erythroid maturation and enucleation has been an impediment to optimum RBC production ex vivo. Major efforts in the past two decades have exposed some of the mechanisms that govern the enucleation process. This review focuses in detail on mechanisms implicated in enucleation and discusses the future perspectives of this fascinating process.