Ring Sideroblasts In Bone Marrow Research Articles

Predicting survival in patients with myelodysplastic/myeloproliferative neoplasms with SF3B1 mutation and thrombocytosis

We investigated data from 180 consecutive patients with myelodysplastic/myeloproliferative neoplasms with SF3B1 mutation and thrombocytosis (MDS/MPN-SF3B1-T) who were diagnosed according to the 2022 World Health Organization (WHO) classification of myeloid neoplasms to identify covariates associated with survival. At a median follow-up of 48 months (95% confidence interval [CI] 35–61 months), the median survival was 69 months (95% CI 59–79 months). Patients with bone marrow ring sideroblasts (RS) < 15% had shorter median overall survival (OS) than did those with bone marrow RS ≥ 15% (41 months [95% CI 32–50 months] versus 76 months [95% CI 59–93 months]; P < 0.001). According to the univariable analyses of OS, age ≥ 65 years (P < 0.001), hemoglobin concentration (Hb) < 80 g/L (P = 0.090), platelet count (PLT) ≥ 800 × 10E + 9/L (P = 0.087), bone marrow RS < 15% (P < 0.001), the Revised International Prognostic Scoring System (IPSS-R) cytogenetic category intermediate/poor/very poor (P = 0.005), SETBP1 mutation (P = 0.061) and SRSF2 mutation (P < 0.001) were associated with poor survival. Based on variables selected from univariable analyses, two separate survival prediction models, a clinical survival model, and a clinical-molecular survival model, were developed using multivariable analyses with the minimum value of the Akaike information criterion (AIC) to specifically predict outcomes in patients with MDS/MPN-SF3B1-T according to the 2022 WHO classification.

Pathophysiology of sideroblastic anemia

Sideroblastic anemias (SAs) are a diverse group of congenital and acquired disorders, characterized by anemia and the presence of ring sideroblasts in bone marrow. Congenital SA is a rare disorder that results from genetic mutations that impair heme biosynthesis, iron-sulfur [Fe-S] cluster biosynthesis, and mitochondrial protein synthesis. The predominant type of congenital SA is X-linked sideroblastic anemia, caused by mutations in the erythroid-specific δ-aminolevulinate synthase (ALAS2) gene, a key enzyme in the heme biosynthesis pathway in erythroid cells. SAs can also arise due to exposure to certain drugs or alcohol or to copper deficiency (secondary SAs). They are also often associated with myelodysplastic syndrome (idiopathic SA), and idiopathic SAs are the most frequently encountered type. This review discusses the current understanding of the pathophysiology underlying SA.

Early-Onset Myelodysplastic Syndromes (MDS) with Ring Sideroblasts (RS) without SF3B1 Mutations in Adults: Enrichment with Germline Variants in Genes Responsible for Congenital Sideroblastic Anemias

Introduction: Acquired mutations in SF3B1 gene in myeloid neoplasms are classically associated with the presence of RS in Pearl Stain, ineffective erythropoiesis, and favorable prognosis. Nevertheless, in up to 20-25% of adults diagnosed with MDS with RS (WHO 2017), mutations in SF3B1 are not found, and molecular grounds for the presence of RS remain to be ascertained. The objective of our study was to investigate whether the presence of RS could be associated with germline variants in genes responsible for congenital sideroblastic anemia (CSA). Methods: Patients diagnosed with de novo MDS between 16-60 years of age without previous organ dysfunction were recruited from 32 centers of GEMSD since 2016. Whole exomes were sequenced using HiSeq4000-NovaSeq6000-Illumina, paired tumor-germline samples. Mean depth was 100x, with 150 million reads per sample and quality Q30a&amp;gt;95%. Variants were analyzed using a bioinformatics pipeline: filtering intronic and synonymous variants and those with a population frequency &amp;gt;1%. The mutational state of SF3B1 was determined by Sanger Sequencing and Next Generation Sequencing (NGS). Germline variants were categorized according to American College of Molecular Genetics (ACMG) criteria. The list of genes related to CSA explored in this study is shown in Table 1. Results: Among 239 cases of adults diagnosed with early-onset MDS (mean age at diagnosis: 48 years, range 16-60), 58 (24%) patients presented with RS in bone marrow (mean RS: 28%). Of these 58 patients, acquired mutations in SF3B1 were not found in 32 (55%). Nine out of these 32 (25%) harbored a germline variant (two variants in one case) in genes responsible for CSA (Table 2): SLC25A38 (n=2), STEAP3 (n=2) FECH, ALAS2, GLRX5, SLC19A2, TRNT1 and IARS2. This frequency was statistically higher than in the SF3B1 and RS mutated group (n=23), with only one case with a germline variant in a CSA gene (p=0.013). It was also higher than in the non-RS cases (n=181), with only one case in this group (p&amp;lt;0.001). Using the Fisher's exact test, commonly used to perform enrichment, the odds ratios were also significant (p=0.033 and p&amp;lt;0.0001), respectively. Among patients with RS, those carrying a CSA gene germline variant were younger (43 vs. 54 years, p=0.04), had a higher rate of neutropenia (1.8 vs. 2.6 x 10E9/L, p=0.02), and thrombocytopenia (151 vs. 259 x 10E9/L, p=0.03) than MDS-RS patients with SF3B1 mutated. Furthermore, MDS-RS with a germline variant in CSA genes had a lower mean percentage of RS than patients who acquired the mutation in SF3B1 (14% vs. 38%; p=0.003). Conclusions: In our series, the frequency of MDS-RS without SF3B1 mutations is higher in early-onset adult MDS than the one reported in MDS in advanced age. Whole exome analysis allowed us to describe, for the first time, a significant enrichment of variants in genes causing CSA in young adults with RS and without acquired mutation in SF3B1.

Exploring the mechanistic link between SF3B1 mutation and ring sideroblast formation in myelodysplastic syndrome

Acquired sideroblastic anemia, characterized by bone marrow ring sideroblasts (RS), is predominantly associated with myelodysplastic syndrome (MDS). Although somatic mutations in splicing factor 3b subunit 1 (SF3B1), which is involved in the RNA splicing machinery, are frequently found in MDS-RS, the detailed mechanism contributing to RS formation is unknown. To explore the mechanism, we established human umbilical cord blood-derived erythroid progenitor-2 (HUDEP-2) cells stably expressing SF3B1K700E. SF3B1K700E expressing cells showed higher proportion of RS than the control cells along with erythroid differentiation, indicating the direct contribution of mutant SF3B1 expression in erythroblasts to RS formation. In SF3B1K700E expressing cells, ABCB7 and ALAS2, known causative genes for congenital sideroblastic anemia, were downregulated. Additionally, mis-splicing of ABCB7 was observed in SF3B1K700E expressing cells. ABCB7-knockdown HUDEP-2 cells revealed an increased frequency of RS formation along with erythroid differentiation, demonstrating the direct molecular link between ABCB7 defects and RS formation. ALAS2 protein levels were obviously decreased in ABCB7-knockdown cells, indicating decreased ALAS2 translation owing to impaired Fe–S cluster export by ABCB7 defects. Finally, RNA-seq analysis of MDS clinical samples demonstrated decreased expression of ABCB7 by the SF3B1 mutation. Our findings contribute to the elucidation of the complex mechanisms of RS formation in MDS-RS.

When Ring Sideroblasts on Bone Marrow Smears Are Inconsistent with the Diagnosis of Myelodysplastic Neoplasms.

Ring sideroblasts are commonly seen in myelodysplastic neoplasms and are a key condition for identifying distinct entities of myelodysplastic neoplasms according to the WHO classification. However, the presence of ring sideroblasts is not exclusive to myelodysplastic neoplasms. Ring sideroblasts are as well either encountered in non-clonal secondary acquired disorders, such as exposure to toxic substances, drug/medicine, copper deficiency, zinc overload, lead poison, or hereditary sideroblastic anemias related to X-linked, autosomal, or mitochondrial mutations. This review article will discuss diseases associated with ring sideroblasts outside the context of myelodysplastic neoplasms. Knowledge of the differential diagnoses characterized by the presence of ring sideroblasts in bone marrow is essential to prevent any misdiagnosis, which leads to delayed diagnosis and subsequent management of patients that differ in the different forms of sideroblastic anemia.

Phase II trial of luspatercept with or without hydroxyurea for the treatment of patients with myelodysplastic/myeloproliferative neoplasms with ring sideroblasts and thrombocytosis or unclassifiable with ring sideroblasts.

TPS7080 Background: Myelodysplastic syndrome/myeloproliferative neoplasms (MDS/MPN) are classified as a distinct category under the World Health Organization (WHO) classification of myeloid neoplasms. MDS/MPN with RS and thrombocytosis (MDS/MPN-RS-T) and MDS/MPN, unclassifiable with &gt; 15% bone marrow ring sideroblasts (MDS/MPN-U-RS) have similar clinical and pathological characteristics with symptomatic or transfusion-dependent anemia as the predominant morbidity. Luspatercept has been approved in myelodysplastic syndromes with ring sideroblasts (MDS-RS) and MDS/MPN overlap syndromes, based on the phase 3 MEDALIST clinical trial which primarily included MDS-RS patients with an objective erythroid response rate of approximately 40 per cent. In this trial, some MDS-RS patients also experienced an increase in neutrophil and platelet counts. This raises a safety concern for MDS/MPN patients with elevated platelet or WBC counts such as MDS/MPN-RS-T and MDS/MPN-U-RS. Previous studies have shown clinical and biological differences between MDS-RS and MDS/MPN-RS-T, with the latter group at a significantly elevated risk for thrombotic events. Additionally, several MDS/MPN-RS-T patients are on hydroxyurea which may blunt the erythroid response of luspatercept. Therefore, it is imperative to establish the safety and efficacy of luspatercept in this patient group. Methods: This is an investigator-initiated, prospective, phase II study of luspatercept in MDS/MPN overlap neoplasms with ring sideroblasts and thrombocytosis or unclassifiable with ring sideroblasts with 2 arms; hydroxyurea-independent (cohort A) and hydroxyurea-dependent (cohort B). Hydroxyurea and/or aspirin use is allowed as per investigator discretion. The primary goal is to study the efficacy and safety of luspatercept in MDS/MPN-RS-T or MDS/MPN-U-RS with symptomatic anemia. The primary endpoint is to assess erythroid response rate as per the 2015 International Working Group MDS/MPN response criteria. Secondary endpoints include response duration, time to acute myeloid leukemia (AML) transformation, thrombosis rate, AML-free and overall survival. Inclusion criteria include newly diagnosed or relapsed/refractory adult patients with WHO-defined diagnosis of MDS/MPN-RS-T or MDS/MPN-U-RS with symptomatic or transfusion-dependent anemia and unlikely to respond (EPO level &gt; 200 IU/L) or intolerant to erythropoiesis stimulating agent (ESA) therapy. Prior therapy with lenalidomide, hypomethylating agents or immunosuppressive therapy is allowed. The overall plan is to enroll 54 patients across the three Mayo Clinic sites, Minnesota, Arizona and Florida. Enrollment to the trial began in January 2022 with 1 patient enrolled at the time of abstract submission. Clinical trial information: NCT05005182.

Bone marrow ring sideroblasts in hematological diseases: an analysis of consecutive 1300 samples in a single institution.

The incidence of MDS-RS in Japan has been recognized as about 5% which is lower than that in European countries. Insufficient use of iron staining tests in Japan has been noted as one conceivable factor contributing to this apparently lower prevalence. To investigate this issue, we analyzed the proportion of ring sideroblasts (RS) in 1300 bone marrow samples from patients with hematological diseases at Kitasato University Hospital, including iron staining of all samples. Sixteen of 96 patients with MDS (16.7%) were diagnosed with MDS-RS, and this accounted for 26.2% of MDS without excess blasts. Some MDS-EB (22.9%) and AML-MRC (13.8%) patients also had ≥ 15% RS. In contrast, RS were rarely found in myeloid neoplasms without dysplasia and non-myeloid diseases: only 1 in 46 (2.2%) patients with AML without dysplasia, 2 in 93 (2.2%) with MPN, and 8 in 984 (0.8%) with non-myeloid diseases had ≥ 5% RS. These results indicate that prevalence of MDS-RS in Japan may be higher than conventionally recognized and that RS are principally restricted to myelodysplastic disorders. Further multicenter studies using consecutive bone marrow samples with iron staining tests will be required to confirm our findings.

Integrative Analysis of Primary SF3B1 mt Ring Sideroblasts Provides Fundamental Insights into MDS-RS Pathogenesis and Dyserythropoiesis

Myelodysplastic syndromes (MDS) constitute a heterogeneous group of clonal hematopoietic stem cell (HSC) disorders characterized by aberrant HSC differentiation, cytopenia, and an increased risk of progression to leukemia. The low-risk subtype MDS with ring sideroblasts (MDS-RS) is specifically characterized by expanded and ineffective erythropoiesis, with more than 80% of patients displaying mutations in the core spliceosome component SF3B1 (SF3B1 mt).A hallmark of the MDS-RS bone marrow (BM) is the progressive accumulation of ring sideroblasts (RS), erythroblasts displaying perinuclear mitochondria loaded with aberrant ferritin-iron complexes. Whilst several in vitro and in vivo model systems exist for studying the impact of SF3B1 mt on erythropoiesis and RS development, primary SF3B1 mt erythroid biology remains largely unexplored due to the inability to purify live SF3B1 mt cells or fully replicate BM conditions in vitro.To address this issue, we developed an innovative two-step method to isolate live ring sideroblasts from SF3B1 mt MDS-RS BM aspiration material with extremely high purity (as determined through droplet digital PCR-based genotyping [Fig. 1A] and morphology-based detection through Prussian blue staining [Fig. 1B,C]). Unexpectedly, evaluation of matching peripheral blood samples showed that circulating ring sideroblasts are strikingly common in MDS-RS (Fig. 1D), with their abundance being significantly positively associated with clinically-determined BM RS frequencies and serum erythropoietin levels, as well as negatively associated with hemoglobin levels.Through high-throughput Chromium 3'-based single-cell RNA sequencing (scRNAseq) analysis of purified RS, we then showed that these cells comprise a heterogeneous population encompassing all stages of the erythroid differentiation continuum, from early progenitors to orthochromatic erythroblasts (Fig. 1E). The RS transcriptome was shown to be dynamically regulated towards the maintenance of cell survival during late terminal erythroid differentiation (exemplified through parkin 1 [PINK1] expression), with SF3B1 K700E erythroblasts employing multiple strategies to preserve homeostasis despite undergoing extreme oxidative stress. These observations were confirmed through a parallel whole-transcript RNAseq investigation comprising CD34 + and GPA +-enriched samples obtained from normal bone marrow (NBM) donors and SF3B1 K700E MDS-RS patients, as well as purified RS samples. This bulk RNAseq experiment validated the RS transcriptomic signature observed in scRNAseq (Fig. 1F) and allowed for a detailed investigation of RNA splicing. SF3B1 K700E-associated alternative splicing in CD34 + and RS was consistent with previous literature, but also highly context-dependent and with substantial changes in scope and magnitude throughout erythroid differentiation (Fig. 1G-I).Finally, we substantiated these RNAseq results through Tandem Mass Tag-based semi-quantitative proteomic analysis of purified RS and GPA-enriched cells from NBM donors and MDS-RS patients. We confirmed that ring sideroblast survival is heavily dependent on redox balance modulation and suppression of ER stress via an increased dependence on glutamine, mirroring the molecular mechanisms observed in malignancy. Additionally, our data strongly indicate that the RS population is a major modulator of the MDS-RS BM microenvironment due to expression of stress factors (with particular emphasis on GDF15, erythroferrone and IL-18). In conclusion, our integrative analysis of primary RS constitutes a unique platform for the study of MDS-RS, with special interest for the investigation of potential drivers of disease severity or treatment avenues. [Display omitted] DisclosuresKretzschmar: Vanadis Diagnostics, a PerkinElmer company.: Current Employment.

Establishment and Characterization of in Vitro Model of X-Linked Sideroblastic Anemia

(Background) Congenital sideroblastic anemia (CSA) is an inherited microcytic anemia characterized by the presence of bone marrow ring sideroblasts. The most common form of CSA is X-linked sideroblastic anemia (XLSA), which is attributed to mutations in the X-linked gene erythroid-specific 5-aminolevulinate synthase (ALAS2) . ALAS2 encodes the enzyme that catalyzes the first and rate-limiting steps in the heme biosynthesis pathway in erythroid cells. This pathway converts glycine and acetyl-CoA to 5-aminolevulinic acid (ALA), which requires pyridoxal 5'-phosphate (PLP) as a cofactor. Although PLP has been used for treating XLSA, a marked proportion of patients with XLSA remain refractory to treatment. Therefore, to elucidate the details of the underlying molecular mechanisms that contribute to ring sideroblast formation for XLSA, we succeeded to induce ALAS2 gene mutation in human induced pluripotent stem cell-derived erythroid progenitor (HiDEP) cells by CRISPR/Cas9 (European Hematology Association 2017). Recently we have further extended the study; we succeeded to establish ring sideroblast by newly optimized in vitro co-culture system and conducted its molecular characterization.(Method) We targeted the GATA-1-binding region of intron 1 of the human ALAS2 gene in HiDEP cells. The mutation diminished the binding of transcription factor GATA-1, which lead to decreased transcription of the ALAS2 gene, thereby causing XLSA (Kaneko et al . Haematologica 2014). Co-culture with OP9 stromal cells (ATCC) was conducted with IMDM medium supplemented with FBS, erythropoietin, dexamethasone, doxycycline, MTG, Insulin-Transferrin-Selenium, and L-ascorbic acid. Prussian blue staining and electron microscopy were used to assess iron deposition in erythroblasts. For transcription profiling, Human Oligo chip 25K (Toray) was used. Apoptotic cell was determined with annexin V.(Results) The established XLSA clone harbored 19-bp deletion within the intron 1 enhancer region of ALAS2, including GATA binding domain. The XLSA clone appeared pink/pale color, accompanied by the decreased intracellular heme concentration. Quantitative ChIP analysis demonstrated that the chromatin occupancies of GATA-1 and its cofactor TAL1 were significantly abrogated at the ALAS2 intron 1 loci in the XLSA clone. Quantitative RT-PCR analysis demonstrated significant downregulation of ALAS2 as well as globin genes in the XLSA clone. Microarray analysis revealed >2-fold up- and down-regulation of 619 and 274 genes caused by the 19-bp deletion, respectively. The downregulated gene ensemble included globins as well as genes involved in iron/heme metabolism (ALAS2, TFRC, CPOX, and MFRN1). GO analysis revealed significant enrichment of genes involved in cellular iron ion homeostasis, regulation of transcription, and regulation of apoptosis. In support of the observation, the apoptotic cell frequency was increased in the XLSA clone than the wild-type cells.When the XLSA clone was induced to undergo erythroid differentiation by co-culture with OP9 cells in the medium supplemented with 100 uM sodium ferrous citrate (SFC), a majority of the erythroblasts exhibited aberrant mitochondrial iron deposition (Figure). Intriguingly, the co-culture without SFC nor even with transferrin-loaded iron did not induce ring sideroblasts. By the co-culture with SFC, ALAS2 as well as globin mRNA levels in the XLSA clone was obviously induced to the level comparable to the wild-type cells. Quantitative ChIP-based mechanistic analysis at the ALAS2 locus demonstrated that GATA-1 occupancy was still abrogated at intron 1, while it was significantly increased at intron 8, in the XLSA clone-derived ring sideroblasts. Noticeably, when SFC was supplemented to induce ring sideroblasts, the apoptotic cell frequency was significantly reduced as compared with the XLSA clone co-cultured without SFC. Also, microarray analysis identified upregulation of anti-apoptotic genes (i.e. HSP70) in the ring sideroblast, implying that ring sideroblast formation might be a consequence of cytoprotective reaction. Finally, ALA treatment significantly improved the consequences compromised heme synthesis in the XLSA clone, implying that ALA may represent a therapeutic option for XLSA.(Conclusion) Further characterization of the XLSA model would help to clarify its molecular etiology as well as to establish novel therapeutic strategies. [Display omitted] DisclosuresFukuhara:Eisai, Janssen, Takeda, Ono and Zenyaku Kogyo: Honoraria; Nihon Ultmarc, Astellas, AbbVie, Alexionpharma, Bayer Yakuhin, Bristol-Myers Squibb, Baxalta, Celgene, Chugai, Daiichi-Sankyo, Toehringer Ingelheim, Eisai, GlaxoSmithKline, Janssen, Japan Blood Products Organization, Kyowa Hakko Kirin, Mitsubishi Tanabe, : Research Funding.

Myelodysplastic syndromes with ring sideroblasts (MDS-RS) and MDS/myeloproliferative neoplasm with RS and thrombocytosis (MDS/MPN-RS-T) - "2021 update on diagnosis, risk-stratification, and management".

Ring sideroblasts (RS) are erythroid precursors with abnormal perinuclear mitochondrial iron accumulation. Two myeloid neoplasms defined by the presence of RS, include myelodysplastic syndromes with RS (MDS-RS) and MDS/myeloproliferative neoplasm with RS and thrombocytosis (MDS/MPN-RS-T). MDS-RS is a lower risk MDS, with single or multilineage dysplasia (MDS-RS-SLD/MLD), <5% bone marrow (BM) blasts, <1% peripheral blood blasts and ≥15% BM RS (≥5% in the presence of SF3B1 mutations). MDS/MPN-RS-T, now a formal entity in the MDS/MPN overlap syndromes, has diagnostic features of MDS-RS-SLD, along with a platelet count ≥450 × 109 /L and large atypical megakaryocytes. Mutations in SF3B1 are seen in ≥80% of patients with MDS-RS-SLD and MDS/MPN-RS-T, and strongly correlate with the presence of BM RS; MDS/MPN-RS-T patients also demonstrate JAK2V617F (50%), DNMT3A, TET2 and ASXL1 mutations. Cytogenetic abnormalities are uncommon in both. Most patients with MDS-RS-SLD are stratified into lower risk groups by the revised-IPSS. Disease outcome in MDS/MPN-RS-T is better than that of MDS-RS-SLD, but worse than that of essential thrombocythemia (MPN). Both diseases are associated with a low risk of leukemic transformation. Anemia and iron overload are complications seen in both and are managed similar to lower risk MDS and MPN. Luspatercept, a first-in-class erythroid maturation agent is now approved for the management of anemia in patients with MDS-RS and MDS/MPN-RS-T. Aspirin therapy is reasonable in MDS/MPN-RS-T, especially in the presence of JAK2V617F, but the value of platelet-lowering drugs remains to be defined.

Characterization of Congenital Sideroblastic Anemia Model Due to ABCB7 Defects: How Do Defects in Iron-Sulfur Cluster Metabolism Lead to Ring Sideroblast Formation?

Backgroun d: The sideroblastic anemias (SAs) are a group of congenital and acquired bone marrow disorderscharacterized by bone marrow ring sideroblasts (RSs). The disease commonly presents as myelodysplastic syndrome with RS (MDS-RS), known as an acquired clonal SA that is strongly correlated with a specific somatic mutation inSF3B1 (splicing factor 3b subunit 1), which is involved in RNA splicing machinery. Thus far, several studies have consistently revealed compromised splicing and/or expression of ABCB7 (ATP-binding cassette subfamily B member 7) in MDS-RS harboring the SF3B1 mutation. ABCB7 encodes an ATP-binding cassette family transporter localizing to the inner mitochondrial membrane, and its loss-of-function mutation causes a syndromic form of congenital SA, which is associated with cerebellar ataxia. The substrates transported by ABCB7 are predicted to be iron-sulfur clusters (ISCs), which are essential for the function of multiple mitochondrial and extramitochondrial proteins, such as ferrochelatase and aconitase (its apo-form without ISC is called IRP1; iron regulatory protein 1). However, the detailed molecular mechanisms by which defects in ISC metabolism resulting from ABCB7 defects contribute to RS formation remains to be fully elucidated. Methods: Endogenous ABCB7 was depleted based on pGIPZ lentiviral shRNAmir (Dharmacon) in human umbilical cord blood-derived erythroid progenitor (HUDEP)-2 cells (Kurita et al., PLoS ONE, 2013). Puromycin (Sigma) was used for the selection of transduced cells. To induce terminal erythroid differentiation, HUDEP-2 cells were co-cultured with OP9 stromal cells (ATCC) in Iscove's modified Dulbecco's medium supplemented with fetal bovine serum, erythropoietin, dexamethasone, monothioglycerol, insulin-transferrin-selenium, ascorbic acid, and sodium ferrous citrate (Saito and Fujiwara et al., MCB, 2019). For transcription profiling, Human Oligo Chip 25K (Toray) was used. Results: We first conducted ABCB7 knockdown in HUDEP-2 cells based on two independent shRNA plasmids. When the knockdown cells were induced to undergo erythroid differentiation,the majority of the erythroblasts exhibited aberrant mitochondrial iron deposition. Thus, we sought to clarify the potential causative link between ABCB7 defects and RS formation. Expression profiling revealed &gt;1.5-fold up- and down-regulation of 33 and 44 genes, respectively, caused by the ABCB7 knockdown. Intriguingly, 43% of the downregulated gene ensemble (19/44 genes) included multiple ribosomal genes, such as RPS2, RPL11,and RPS12. The downregulated genes also included HMOX1 (heme oxygenase 1), implying that heme biosynthesis would be compromised by the knockdown. Gene ontology (GO) analysis revealed significant (p&lt; 0.01) enrichment of genes associated with nuclear-transcribed mRNA catalytic process, cytoplasmic translation, and cellular iron ion homeostasis. Whereas the mRNA expression for ALAS2 (erythroid-specific 5-aminolevulinate synthase), encoding a rate-limiting enzyme of heme biosynthesis and one of the responsible genes for congenital SA, was not affected, its protein expression was noticeably decreased by ABCB7 knockdown, indicating that compromised transport of ISC from mitochondria to the cytosol may result in decreased ALAS2 translation by the binding of IRP1 to the iron-responsive element located in the 5'-UTR of ALAS2 mRNA.We are currently conducting detailed biological analyses to elucidate the causative link between defects in ISC metabolism due to ABCB7 defects and RS formation. Conclusion: We have first demonstrated the emergence of RS by ABCB7 depletion in human erythroblasts. Further characterization of the established SA model would aid in the clarification of its molecular etiology and the establishment of novel therapeutic strategies. Furthermore, our results may lead to a better understanding of the role of ISC in affecting cerebellar symptoms. Disclosures Fukuhara: Gilead: Research Funding; Nippon Shinkyaku: Honoraria; Zenyaku: Honoraria; AbbVie: Research Funding; Takeda Pharmaceutical Co., Ltd.: Honoraria, Research Funding; Mundi: Honoraria; Ono Pharmaceutical Co., Ltd.: Honoraria; Bayer: Research Funding; Celgene Corporation: Honoraria, Research Funding; Chugai Pharmaceutical Co., Ltd.: Honoraria; Eisai: Honoraria, Research Funding; Janssen Pharma: Honoraria; Kyowa-Hakko Kirin: Honoraria; Mochida: Honoraria; Solasia Pharma: Research Funding. Onishi:Novartis Pharma: Honoraria; Otsuka Pharmaceutical Co., Ltd.: Honoraria; Astellas Pharma Inc.: Honoraria; ONO PHARMACEUTICAL CO., LTD.: Honoraria; Bristol-Myers Squibb: Honoraria, Research Funding; Janssen Pharmaceutical K.K.: Honoraria; MSD: Honoraria, Research Funding; Sumitomo Dainippon Pharma: Honoraria; Chugai Pharmaceutical Co., Ltd.: Honoraria; Takeda Pharmaceutical Co., Ltd.: Research Funding; Nippon Shinyaku: Honoraria; Pfizer Japan Inc.: Honoraria; Kyowa-Hakko Kirin: Honoraria; Celgene: Honoraria. Yokoyama:Astellas: Other: Travel expenses.

Molecular Characterization and Novel Therapeutic Strategy for X-Linked Sideroblastic Anemia Associated with ALAS2 Missense Variants

Background: Congenital sideroblastic anemia (CSA) is an inherited anemia characterized by the presence of ring sideroblasts (RSs) in bone marrow. X-linked sideroblastic anemia (XLSA) is the most common form of CSA and is caused by germline mutations in the erythroid-specific 5-aminolevulinate synthase (ALAS2) gene. Each ALAS2 mutation has a variable effect on the protein's enzymatic function, and only half of XLSA patients are responsive to pyridoxal 5′-phosphate (PLP) treatment. Thus, novel therapeutic strategies should be explored. We have established an in vitro XLSA model in human induced pluripotent stem cell-derived erythroid progenitors (HiDEP-1) by disrupting GATA-1-binding intronic enhancer region of ALAS2, which accounts for a minor XLSA subtype with PLP refractoriness (Saito and Ono et al. MCB 2019; ASH 2018). However, evidence regarding molecular characterization of ALAS2 missense variants is lacking, presumably due to difficulties in introducing the desired missense mutations in primary erythroblasts. In this study, we optimized a protocol for introducing point mutations in human umbilical cord blood-derived erythroid progenitor (HUDEP)-2 cells and subsequently characterized XLSA models harboring ALAS2 missense variants. Methods: Based on homology-directed CRISPR/Cas9 system, we introduced an ALAS2 mutation from arginine at amino acid residue 170, one of the XLSA hot spots, into leucine (R170L) or histidine (R170H). A twenty-nucleotide targeted genomic sequence, located upstream of protospacer adjacent motif (PAM) near ALAS2 R170, was cloned into plasmid vectors expressing Cas9 nuclease and green fluorescent protein (GFP). Each homologous recombinant template was designed as a 127 single-stranded oligodeoxynucleotide (ssODN) asymmetrically distributed from the Cas9-mediated cleavage site. PAM silent mutations were added to avoid continuous cleavage after successful mutation. HUDEP-2 cells were co-transduced with gRNA/Cas9/GFP-expressing vectors and ssODNs using Amaxa Nucleofector 2b (Lonza). After 48 hours, cells were isolated by GFP-positive expression; subsequently, single-cell dilution was performed, followed by clonal analysis. Results: Both XLSA clones (ALAS2 R170L and R170H) appeared pink/pale colored, reflecting impaired hemoglobin biosynthesis. Consistently, quantitative RT-PCR analysis demonstrated decreased globin gene (HBB) expression. On the other hand, XLSA clones did not show RSs and were morphologically similar to wild-type controls. When XLSA clones were induced to undergo erythroid differentiation by co-culturing with OP9 cells in a 100 mM sodium ferrous citrate (SFC)-supplemented medium, increased numbers of RSs were observed in mutant clones, mimicking findings in XLSA patients. Electron microscopy confirmed an aberrant mitochondrial iron deposit in XLSA clones. Also, HBB expression and intracellular heme concentration were significantly lower than those in wild-type controls. Intriguingly, while OP9 co-culture and SFC addition promoted erythroid differentiation in both wild-type and XLSA clones, XLSA clones exhibited a more immature morphological phenotype than wild-type controls. Expression profiling revealed that 317 and 86 genes were commonly up- and downregulated &amp;gt;2-fold in XLSA clones compared with those in wild-type controls. Gene ontology analysis showed significant (p &amp;lt; 0.01) enrichment of genes associated with mitochondrial gene expression and organization, suggesting that mitochondrial alteration is involved in XLSA pathogenesis. On the other hand, 188 genes in total were differentially expressed between ALAS2 R170L and R170H. Genes associated with ferroptosis, such as GPX4 and GCLC, showed a significantly lower expression in ALAS2 R170H clones, possibly accounting for phenotypic differences among patients with ALAS2 R170L or R170H mutation. Finally, supplementation with 5-aminolevulinic acid (ALA) significantly improved the compromised heme biosynthesis in XLSA clones, suggesting ALA treatment to be a promising therapeutic option for XLSA. Conclusion: The established models may act as useful tools for exploring the precise molecular mechanisms of XLSA harboring missense mutations and for drug testing. Also, our homology-directed CRISPR/Cas9-based protocol can be applied to establish a wide variety of CSA as well as congenital anemia models. Disclosures Fukuhara: Zenyaku: Honoraria; Eisai: Honoraria, Research Funding; Takeda Pharmaceutical Co., Ltd.: Honoraria, Research Funding; Janssen Pharma: Honoraria; Chugai Pharmaceutical Co., Ltd.: Honoraria; Gilead: Research Funding; Ono Pharmaceutical Co., Ltd.: Honoraria; Nippon Shinkyaku: Honoraria; Celgene Corporation: Honoraria, Research Funding; Mundi: Honoraria; Kyowa-Hakko Kirin: Honoraria; Mochida: Honoraria; AbbVie: Research Funding; Bayer: Research Funding; Solasia Pharma: Research Funding. Onishi:Celgene: Honoraria; Kyowa-Hakko Kirin: Honoraria; Chugai Pharmaceutical Co., Ltd.: Honoraria; Novartis Pharma: Honoraria; Pfizer Japan Inc.: Honoraria; Bristol-Myers Squibb: Honoraria, Research Funding; Janssen Pharmaceutical K.K.: Honoraria; MSD: Honoraria, Research Funding; Astellas Pharma Inc.: Honoraria; Nippon Shinyaku: Honoraria; Takeda Pharmaceutical Co., Ltd.: Research Funding; Sumitomo Dainippon Pharma: Honoraria; ONO PHARMACEUTICAL CO., LTD.: Honoraria; Otsuka Pharmaceutical Co., Ltd.: Honoraria.

Refractory anemia with ring sideroblasts (RARS) and RARS with thrombocytosis: "2019 Update on Diagnosis, Risk-stratification, and Management".

Ring sideroblasts (RS) are erythroid precursors with abnormal perinuclear mitochondrial iron accumulation. Two myeloid neoplasms defined by the presence of RS, include refractory anemia with ring sideroblasts (RARS), now classified under myelodysplastic syndromes with RS (MDS-RS) and RARS with thrombocytosis (RARS-T); now called myelodysplastic/myeloproliferative neoplasm with RS and thrombocytosis (MDS/MPN-RS-T). MDS-RS is a lower-risk MDS, with single or multilineage dysplasia (SLD/MLD), <5% bone marrow (BM) blasts and ≥ 15% BM RS (≥5% in the presence of SF3B1 mutations). MDS/MPN-RS-T, now a formal entity in the MDS/MPN overlap syndromes, has diagnostic features of MDS-RS-SLD, along with a platelet count ≥450 × 10(9)/L and large atypical megakaryocytes. Mutations in SF3B1 are seen in ≥80% of patients with MDS-RS-SLD and MDS/MPN-RS-T, and strongly correlate with the presence of BM RS; MDS/MPN-RS-T patients also demonstrate JAK2V617F, ASXL1, DNMT3A, SETBP1, and TET2 mutations. Cytogenetic abnormalities are uncommon in both. Most patients with MDS-RS-SLD are stratified into lower-risk groups by the revised-IPSS. Disease outcome in MDS/MPN-RS-T is better than that of MDS-RS-SLD, but worse than that of essential thrombocythemia. Both diseases have a low risk of leukemic transformation. Anemia and iron overload are complications seen in both and are managed similar to lower-risk MDS and MPN. The advent of luspatercept, a first-in-class erythroid maturation agent will tremendously boost the ability to manage anemia. Aspirin therapy is reasonable in MDS/MPN-RS-T, especially in the presence of JAK2V617F, but the value of platelet-lowering drugs remains uncertain.

FLOW CYTOMETRY IN MYELODYSPLASTIC SYNDROME. STEP FORWARD.

Myelodysplastic syndrome (MDS) represents a heterogeneous group of clonal diseases originating from altered (malignant) hematopoietic stem cells. Clinically, it manifests as cytopenia in peripheral blood, dysplastic changes in one or more cell lines in bone marrow and increased risk of evolution to acute myeloid leukemia (AML). Acquired chromosomal aberrations are detected in 40-50% of MDS patients. According to WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues (Lion 2008), MDS diagnosis is based on the morphological interpretation of found dysplastic changes, the number of blasts, the presence of ring sideroblasts in bone marrow and the established cytogenetic abnormalities. The progress made in recent years in immunophenotyping of hematopoietic progenitor cells and mature cells of dysplastic bone marrow gives to multiparameter flow cytometric analysis (MFC), although initially included as an optional technique, an opportunity to become a standard part of the integrated MDS diagnosis and prognosis.

Generation and Molecular Characterization of Human Ring Sideroblasts

(Background) Sideroblastic anemias are heterogeneous congenital and acquired refractory anemias characterized by bone marrow ring sideroblasts, reflecting excess mitochondrial iron deposition. While the disease is commonly associated with myelodysplastic syndrome, the congenital forms of sideroblastic anemias comprise a diverse class of syndromic and non-syndromic disorders, which are caused by the germline mutation of genes involved in iron-heme metabolism in erythroid cells. Although the only consistent feature of sideroblastic anemia is the bone marrow ring sideroblasts, evidence on the detailed molecular characteristics of ring sideroblasts is scarce owing to a lack of the biological models. We have recently established ring sideroblasts by inducing ALAS2 gene mutation based on human-induced pluripotent stem cell-derived erythroid progenitor (HiDEP) cells (ASH 2017) and have further extended the molecular characterization of human ring sideroblasts to gain new biological insights.(Method) We targeted the GATA-1-binding region of intron 1 of the human ALAS2 gene in HiDEP cells and established two independent clones [X-linked sideroblastic anemia (XLSA) clones]. A co-culture with OP9 stromal cells (ATCC) was conducted with IMDM medium supplemented with FBS, erythropoietin, dexamethasone, MTG, insulin-transferrin-selenium, and ascorbic acid. To obtain human primary erythroblasts, CD34-positive cells isolated from cord blood were induced in a liquid suspension culture (Fujiwara et al. JBC 2014). Bone marrow glycophorin A (GPA)-positive erythroblasts of patients with XLSA and normal individuals were separated using the MACS system (Miltenyi Biotec GmbH, Bergisch Gladbach, Germany) after obtaining written informed consent. For transcription profiling, Human Oligo chip 25K (Toray) was used.(Results) We previously demonstrated that co-culture with OP9 cells in the medium supplemented with 100 uM sodium ferrous citrate (SFC) promoted erythroid differentiation of XLSA clones, which enabled the establishment of ring sideroblasts (ASH 2017). To confirm the importance of SFC in terminal erythroid differentiation, we further demonstrated that the addition of SFC, and not transferrin-loaded iron, induced the frequency of GPA+ cells and TfR1-GPA+ mature erythroid population, based on primary erythroblasts derived from human CD34-positive cells. Subsequently, to reveal the molecular mechanism by which abnormal iron mitochondrial iron accumulation occurs by co-culture with SFC, we evaluated the expressions of various metal transporters, demonstrating that the addition of SFC significantly increased the expressions of mitoferrin 1 (MFRN1; a ferrous iron transporter in mitochondria), divalent metal transporter 1 (DMT1), and Zrt- and Irt-like protein 8 (ZIP8; a transmembrane zinc transporter, recently known as a ferrous iron transporter) in the XLSA clone than the wild-type cells, which would have contributed to the formation of ring sideroblasts.Moreover, we performed expression analyses to elucidate the biochemical characteristics of ring sideroblasts. After co-culture with OP9 in the presence of SFC, ring sideroblasts exhibited more than two-fold upregulation and downregulation of 287 and 143 genes, respectively, than the wild-type cells. Interestingly, when compared with the expression profiling results before co-culture (ASH 2017), we noticed prominent upregulation of gene involved in anti-apoptotic process (p = 0.000772), including HSPA1A, superoxide dismutase (SOD) 1, and SOD2. In addition, we conducted a microarray analysis based on GPA-positive erythroblasts from an XLSA patient and a normal individual. The analysis revealed significant upregulation of genes involved in the apoptosis process, as represented by apoptosis enhancing nuclease, DEAD-box helicase 47, and growth arrest and DNA-damage-inducible 45 alpha, and anti-apoptotic genes, such as HSPA1A and SOD2. Concomitantly, when the XLSA clone was co-cultured with OP9 in the presence of SFC, the apoptotic cell frequency as well as DNA fragmentation were significantly reduced compared with the XLSA clone co-cultured without SFC, indicating that ring sideroblasts avoid cell death by inducing anti-apoptotic properties.(Conclusion) Further characterization of the XLSA model would help clarify its molecular etiology as well as establish novel therapeutic strategies. DisclosuresFukuhara:Celgene: Research Funding; Chugai: Research Funding; Daiichi-Sankyo: Research Funding; Boehringer Ingelheim: Research Funding; Eisai: Honoraria, Research Funding; GlaxoSmithKline: Research Funding; Janssen: Honoraria, Research Funding; Japan Blood Products Organization: Research Funding; Kyowa Hakko Kirin: Honoraria, Research Funding; Mitsubishi Tanabe: Research Funding; Mundipharma: Honoraria, Research Funding; MSD: Research Funding; Nippon-shinyaku: Research Funding; Novartis pharma: Research Funding; Ono: Honoraria, Research Funding; Otsuka Pharmaceutical: Research Funding; Pfizer: Research Funding; Sanofi: Research Funding; Symbio: Research Funding; Solasia: Research Funding; Sumitomo Dainippon: Research Funding; Taiho: Research Funding; Teijin Pharma: Research Funding; Zenyaku Kogyo: Honoraria, Research Funding; Takeda: Honoraria; Baxalta: Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding; Bayer Yakuhin: Research Funding; Alexionpharma: Research Funding; AbbVie: Research Funding; Astellas: Research Funding; Nihon Ultmarc: Research Funding.

Prognostic value and clinical feature of SF3B1 mutations in myelodysplastic syndromes: A meta-analysis.

SF3B1 gene mutations are the most frequent mutations found in myelodysplastic syndromes (MDS), and the prognostic implication of these mutations remains controversial. We conducted a meta-analysis of studies assessing the prognostic impact and clinical feature of SF3B1 mutations in MDS patients. The overall hazard ratio for overall survival (OS) was 0.90 (95% confidence interval 0.60-1.35, P = 0.61) in MDS patients with SF3B1 mutations compared to those without. Lower leukemia-free survival was associated with SF3B1 mutations. Subgroup analyses showed that Asian cohorts and Illumina HiSeq 2000 methods were significantly associated with OS. Furthermore, SF3B1 mutations were significantly correlated with a lower level of blast cells and a high level of platelet counts and bone marrow ring sideroblasts. Thus, the current meta-analysis suggests that SF3B1 mutations have no significant impact on the OS of MDS patients, and the hematologic parameters of SF3B1 mutations identify a distinct subset of MDS patients with homogeneous features.

Establishment of a cell model of X-linked sideroblastic anemia using genome editing

ALAS2 gene mutations cause X-linked sideroblastic anemia. The presence of ring sideroblasts in a patient's bone marrow is the hallmark of sideroblastic anemia, but the precise mechanisms underlying sideroblast formation are largely unknown. Using a genome-editing system, a mutation was introduced in the erythroid-specific enhancer of the ALAS2 gene in HUDEP2 cells, which were derived from human umbilical stem cells and can produce erythrocytes. The established cell line, termed HA2low, expressed less ALAS2 mRNA than did wild-type cells, even after erythroid differentiation. Although the mRNA expression of α-globin, β-globin, and the mitochondrial iron importer mitoferrin-1 was induced similarly in wild-type and HA2low cells, hemoglobinization of differentiated cells was limited in HA2low cells compared with wild-type cells. Importantly, Prussian blue staining revealed that approximately one-third of differentiated HA2low cells exhibited intracellular iron deposition and these cells looked like ring sideroblasts. Electron microscopy confirmed that the mitochondria in HA2low cells contained high-density deposits that might contain iron. Ring sideroblastic cells appeared among HA2low cells only after differentiation, whereas the induced expression of mitochondrial ferritin was observed in both cell types during differentiation. These results suggest that the induction of mitochondrial ferritin expression might be essential for, but not the primary cause of, ring sideroblast formation. Our results also suggest that the insufficient supply of protoporphyrin IX due to ALAS2 deficiency in combination with increased iron import into mitochondria during erythroid differentiation results in the formation of ring sideroblasts. Furthermore, HA2low cells are a useful tool for characterizing ring sideroblasts in vitro.

Association of red cell distribution width with clinical outcomes in myelodysplastic syndrome

Studies showed red cell distribution width (RDW) can improve the detection of morphological changes in red blood cells and the understanding of their contribution to dyserythropoiesis in myelodysplastic syndrome (MDS). The purpose of the study was to evaluate dyserythropoiesis in MDS by RDW analysis and to explore the utility of RDW in clinical practice. We retrospectively analyzed laboratory and clinical data of 101 patients (59 patients was refractory anemia (RA) according to the French-American-British (FAB) classification). In patients with RA, RDW was showed weak inverse correlation with both hemoglobin concentration (Hb) (rs = −0.37, P = 0.0035) and mean corpuscular hemoglobin concentration (MCHC) (rs = −0.36, P = 0.0047). On the other hand, RDW was showed weak correlation with the number of ringed sideroblasts in bone marrow (rs = 0.31, P = 0.023). The increased RDW (≥15.0%) was associated with shorter overall survival (OS) (P = 0.0086). In patients with refractory anemia with excess blasts (RAEB) and RAEB in transformation (RAEB-t), effect of RDW on OS was less evident. These results suggested that increased RDW might reflect dyserythropoiesis, associated with deregulated hemoglobin synthesis and iron metabolism in MDS. Furthermore, increased RDW may have potential to be a prognostic significance in RA.

Molecular pathophysiology of sideroblastic anemia

Sideroblastic anemias (SAs) are heterogeneous congenital and acquired disorders characterized by anemia and the presence of ring sideroblasts in bone marrow. Congenital sideroblastic anemia (CSA) is a rare disease caused by mutations in genes that are involved in heme biosynthesis, iron-sulfur [Fe-S] cluster biosynthesis, and mitochondrial protein synthesis. The most common form of CSA is X-linked sideroblastic anemia; it occurs because of mutations in the erythroid-specific δ-aminolevulinate synthase gene (ALAS2), which is the first enzyme of the heme biosynthesis pathway in erythroid cells. Additionally, SAs can occur after exposure to certain drugs or alcohol and with copper deficiency (secondary SA) ; they are also detected in association with myelodysplastic syndrome (idiopathic SA). Among all types of SAs, idiopathic SA is the most common form. This review encompasses the current understanding of the molecular pathophysiology of SA.

Hscb, a Mitochondrial Iron-Sulfur Cluster Assembly Co-Chaperone, Is a Novel Candidate Gene for Congenital Sideroblastic Anemia

Congenital sideroblastic anemias (CSAs) are uncommon inherited diseases resulting from defects in heme biosynthesis, mitochondrial translation or mitochondrial iron-sulfur cluster (ISC) assembly. CSAs are characterized by pathological mitochondrial iron deposits in bone marrow erythroblasts. Recently, mutations in mitochondrialheat shock protein 70 (HSPA9), a critical chaperone involved in mitochondrial ISC assembly, have been reported as a cause of non-syndromic CSA. Human heat shock cognate protein 20 (HSCB), a highly conserved mitochondrial co-chaperone, is the primary binding partner of HSPA9. HSCB allows the transfer of nascent ISC to HSPA9 and stimulates its ATPase activity, promoting ISC transfer to target proteins.To identify novel genes responsible for CSA, we performed whole exome sequencing on more than 75 CSA probands and their family members. In one patient, a young woman, with pancytopenia characterized by a normocytic anemia with numerous bone marrow ringed sideroblasts, we identified two variants in HSCB : a paternally-inherited promoter variant (c.-134C>A) predicted to disrupt a conserved ETS transcription factor binding site, and a maternally-inherited frameshift (c.259dup, p.T87fs*27). A fibroblast cell-line derived from the proband showed a decrease in HSCB expression, but normal HSPA9 expression compared to healthy, unrelated controls. Impairment of ETS1-dependent transcriptional activation of the promoter variant was demonstrated in K562 cells transfected with an HSCB-luciferase reporter construct.K562 cells were also employed to determine if reduced expression of HSCB could result in impaired erythroid metabolism, maturation, or proliferation. K562 cells infected with shRNA directed against HSCB were deficient in multiple mitochondrial respiratory complexes, had abnormal iron metabolism and a defect of protein lipoylation, all consistent with defective ISC metabolism. In addition, both IRP1 and IRP2 expression were decreased and cell surface transferrin receptor 1 (TFR1) expression was enhanced, suggesting disturbed cellular iron metabolism. Nevertheless, cells lacking HSCB partially retained an ability to respond to iron chelation and iron overload. Cells lacking HSCB lose their ability to hemoglobinize in response to sodium butyrate treatment (Figure 1A). This defect was confirmed in vivo using a morpholino strategy in zebrafish, as fish lacking HSCB are also unable to hemoglobize (Fig 1B).We generated an Hscb conditional mouse to better elucidate the underlying pathophysiology of the disease. Heterozygous (Hscb+/-) animals have no discernable phenotype; however, null animals die prior to embryonic day E7.5. Thus, to avoid this lethality, we employed Vav-cre animals (Tg(Vav1-cre)1Graf) to evaluate the loss of HSCB specifically in the hematopoietic compartment. Hscbc/- Vav-cre+ pups are pale and growth retarded compared to control littermates and die at approximately p10 with severe pancytopenia. To assess the loss of HSCB specifically in the erythroid lineage, we bred conditional animals to EpoR-cre (Eportm1(EGFP/cre)Uk) mice. Hscbc/- EpoR-cre+ mice die at approximately E12.5 due to a complete failure of erythropoiesis (Figure 1C). Finally, temporally inducible, hematopoietic-specific deletion animals were generated by transplantation of fetal livers from Mx-Cre (Tg(Mx1-cre)1Cgn) positive Hscbc/- animals. After polyinosinic:polycytidylic acid (pIpC) induction, global defects of hematopoiesis were observed in Mx-Cre+ animals, leading to their death 3-weeks post-induction from profound pancytopenia. A transient siderocytosis was seen in the peripheral blood between days 6-8 post-pIpC. Flow cytometry using FSC-TER119-CD44 gating strategy confirmed the defect in erythropoiesis.Taken together, these data demonstrate that HSCB is essential for hematopoiesis; both whole animal and in vitro cell culture models recapitulate the patient's phenotype, suggesting that the two patient mutations are likely disease-causing. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.