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In this work, we utilized a parameterization model of ektacytometry to quantify the bulk rigidity of the rigid red blood cell (RBC) population in sickle cell disease (SCD) patients. Current ektacytometry techniques implement laser diffraction viscometry to estimate the RBC deformability in a whole blood sample. However, the diffraction measurement is an average of all cells present in the measured sample. By coupling an existing parameterization model of ektacytometry to an artificially rigid RBC model, we formulated an innovative system for estimating the average rigidity of the rigid RBC population in SCD blood. We demonstrated that this method could more accurately determine the bulk stiffness of the rigid RBC populations. This information could potentially help develop the ektacytometry technique as a tool for assessing disease severity in SCD patients, offering novel insights into the disease pathology and treatment.

The hallmark of sickle cell disease (SCD) is polymerization of deoxygenated hemoglobin S (HbS), resulting in red blood cell (RBC) sickling, oxidative and membrane damage, hemolysis, vaso-occlusion, and end-organ damage. Exacerbating the pathogenesis of SCD, the sickle RBC (sRBC) has increased 2,3-DPG levels with decreased oxygen (O 2) affinity (increased P 50) and decreased ATP. Etavopivat, a small molecule activator of erythrocyte pyruvate kinase (PKR), increases PKR activity, resulting in decreased 2,3-DPG levels and increased ATP levels in RBCs. In a Phase 1 study in patients with SCD [NCT03815695], etavopivat significantly improved anemia and hemolysis after 2 weeks of treatment (Brown et al. Blood 2020). To evaluate the potential of etavopivat to reduce vaso-occlusive crises, exploratory studies were conducted to characterize the sRBC specific (intrinsic) and systemic effects of PKR activation. Patients with SCD received once daily etavopivat 300 or 600 mg for 2 weeks or 400 mg for up to 12 weeks. Peripheral blood was collected prior to treatment (ie, baseline), on treatment and 7-28 days post treatment. Studies to assess the sRBC intrinsic effects of PKR activation included evaluation of RBC parameters and reticulocyte counts (ADVIA ®), membrane deformability (Lorrca ®), enzyme function studies, and membrane markers by flow cytometry. Studies to assess the systemic effects of PKR activation included markers of coagulation, inflammation, and hypoxia in the 12-week cohort only. As of May 24, 2021, 15 patients who completed the 2-week dose cohorts and 7 patients treated in the 12-week dose cohort were evaluable for this analysis. The intrinsic effects of etavopivat on the sRBCs of patients receiving 2 weeks of treatment are summarized in Table 1. Etavopivat significantly increased Hb and reduced reticulocytes, including immature reticulocytes (CD71 +), suggesting that an etavopivat-mediated increase in sRBC lifespan is accompanied by a decrease in erythropoietic stress. In addition, etavopivat reduced 2,3-DPG levels thereby increasing HbS O 2 affinity (decreased P 50) resulting in a significant shift in the point of sickling (PoS) to a lower partial O 2 pressure. The deformability (EI max) of the sRBCs as measured by oxygenscan and osmoscan was significantly improved with etavopivat treatment, consistent with reduced membrane damage due to decreased HbS polymerization and improved membrane repair enabled by increased ATP production, collectively improving the health of the sRBC membrane. This improvement in membrane health was further supported by a significant reduction in the external expression of phosphatidylserine (PS) following etavopivat treatment. Finally, etavopivat significantly improved enzymatic activity not only of PKR but also superoxide dismutase and glutathione reductase, enzymes involved in reducing oxidative stress in sRBCs. This suggests that etavopivat-treated sRBCs may have an improved ability to inhibit and repair damage caused by reactive O 2 species, thereby improving overall sRBC health and function. Initial results on the effect of etavopivat on systemic biomarkers that are commonly elevated in SCD are shown in Table 2. In patients receiving etavopivat 400 mg once daily for up to 12 weeks, tumor necrosis factor-a and prothrombin 1.2, as systemic markers of inflammation and hypercoagulability, respectively, showed a significant decrease compared with baseline. Furthermore, a trend towards reduced erythropoietin levels suggests that etavopivat treatment may reduce tissue hypoxia. Patients with SCD treated with etavopivat for at least 2 weeks demonstrated a significant increase in RBC membrane deformability and improved antioxidant capacity that resulted in increased sRBC survival and decreased anemia. The reduced reticulocyte count and lowered PS surface membrane expression suggest that etavopivat-treated sRBC may have reduced adhesive properties and may thus be less likely to promote vaso-occlusion. Initial studies evaluating the downstream effects of up to 12 weeks of etavopivat treatment once daily provided evidence for a reduction in markers of inflammation and hypercoagulability, with improved O 2 delivery capacity. These initial results suggest that the multimodal effects of decreased 2,3-DPG and increased ATP by PKR activation with etavopivat may have an impact on both the anemia and vaso-occlusive events that characterize SCD. Figure 1 Figure 1. Disclosures Kalfa: Agios Pharmaceuticals, Inc.: Other: Steering Committee, Research Funding; FORMA Therapeutics, Inc: Research Funding. Telen: GlycoMimetics, Inc.: Consultancy; Novartis, Inc.: Other: Data Safety Monitoring Board; Forma Therapeutics, Inc.: Consultancy, Research Funding; CSL Behring, Inc.: Research Funding; Doris Duke Charitable Foundation: Research Funding; National Institutes of Health: Research Funding. Saraf: Global Blood Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Research Funding. Brown: Novo Nordisk: Consultancy; Imara: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Global Blood Therapeutics: Consultancy, Research Funding; Pfizer: Research Funding; Forma Therapeutics: Research Funding. Larkin: Forma Therapeutics, Inc.: Research Funding. Ribadeneira: Forma Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Schroeder: Forma Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Wu: Forma Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Kelly: Forma Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Kuypers: Forma Therapeutics, Inc.: Research Funding.

The two most common genotypes causing sickle cell disease (SCD) are homozygous bsbs producing hemoglobin S (HbSS), and compound heterozygous bsbc producing HbS and HbC (HbSC). HbS polymerizes in both, when deoxygenated. Patients with HbSC have distinct hematologic characteristics such as higher Hb levels but lower fetal hemoglobin (HbF); increased corpuscular Hb concentration, due to pronounced red blood cell (RBC) K+ loss and dehydration, contributes to HbS polymerization. While hydroxyurea (HU) as a HbF-inducing agent clearly improves HbSS phenotype, targeting of other pathogenetic mechanisms in SCD may offer additional benefit in both HbSS and HbSC patients. The RBC metabolism depends on anaerobic glycolysis for ATP production. Key RBC functions are intricately linked to metabolism: hemoglobin-oxygen (Hb-O2) affinity is modulated by 2,3-diphosphoglycerate (2,3-DPG), RBC membrane health depends on adequate synthesis of ATP, and heme-iron is maintained in the functional reduced state by the synthesis of antioxidants, such as GSH, supplying NADH to methemoglobin reductase. Erythrocyte pyruvate kinase (PKR) performs a rate limiting step in RBC glycolysis, converting phosphoenolpyruvate to pyruvate and generating ATP. Insufficient PKR activity due to any of a variety of causes, including excessive energy demand, can alter the abundance of metabolic products and intermediates, including ATP and 2,3-DPG, adversely affecting RBC function. We assessed baseline 2,3-DPG and ATP levels in whole blood of patients with HbSS (N=18) by LC/MS analysis. As well-reported, hemoglobin (Hb) levels were significantly decreased in patients with HbSS compared to healthy volunteers (9.68 ± 0.32 g/dL vs 14.88 ± 0.27 g/dL, p Whole blood samples in K2EDTA were collected from donors with confirmed HbSS or HbSC disease in accordance with an approved IRB protocol in Cincinnati Children's Hospital. % HbF and % F-cells were measured for each sample. Packed RBCs were washed, resuspended to 20% v/v (20% hematocrit), and then incubated for 4 hr at 37oC with 20 μM FT-4202 or equivalent volume of vehicle control DMSO. P50 representing the partial pressure of oxygen at which Hb is 50% saturated with oxygen, was determined using non-linear regression analysis of hemoglobin-oxygen equilibrium curves obtained on a HEMOX Analyzer (TCS Scientific). RBC deformability as a function of oxygen pressure, or Oxygenscan, was measured using Lorrca® (RR Mechatronics) (Rab et al., 2019, Am J Hematol). Point of Sickling (PoS) was calculated as the pO2 at which RBC deformability is decreased by 5% of maximum value during deoxygenation. The P50 and PoS values in FT-4202 versus DMSO-treated HbSC or HbSS RBCs from patients treated with or without HU were analyzed by paired Wilcoxon test. FT-4202 treatment significantly decreased P50 in both HU-treated and not HU-treated HbSS RBCs, as well as in HbSC RBCs, as detailed in Table below showing mean ± se (*p These data suggest that increasing flux through the glycolytic pathway by FT-4202 treatment may offer dual beneficial effects of increased Hb-O2 affinity and membrane flexibility in HbSC as well as in HbSS RBCs irrespective of HU use. Figure Disclosures Fulzele: FORMA Therapeutics, Inc: Current Employment, Other: Shareholder of Forma Therapeutics. Guichard:FORMA Therapeutics, Inc: Current Employment, Other: Shareholder of Forma Therapeutics; AstraZeneca: Other: Shareholder. Kalfa:Agios Pharmaceuticals, Inc: Consultancy, Research Funding; Forma Therapeutics, Inc: Research Funding.

The Congenital Dyserythropoietic Anemia Registry (CDAR, ClinicalTrials.gov Identifier: NCT02964494) was created to investigate the natural history, biology, and molecular pathogenetic mechanisms of CDA. To date, there are 6 genes known to cause CDA (CDAN1, C15orf41, SEC23B, KIF23, KLF1, GATA1). However, 57% of patients registered in CDAR so far (17 out of 33 patients) have an unidentified genetic cause. We have utilized whole exome sequencing (WES) in family-trio design to search for novel candidate gene mutations that may be responsible for the disease. Three unrelated patients with dyserythropoiesis, hemolytic anemia, and neurodevelopmental delay were found to have missense mutations in the gene VPS4A which encodes an ATPase that participates with the ESCRT III machinery in endosomal vesicle trafficking, centrosome localization, and the abscission step of cytokinesis. It has been shown to play an essential role in division of HeLa cells in vitro where it concentrates at the spindle poles during mitosis and at the midbody during cytokinesis. The aim of this work is to validate the pathogenetic role of these VPS4A variants in CDA and further investigate the role of VPS4A in erythropoiesis. Patients 1 and 3 had de novo mutations (R284W and G203A) and transfusion-dependent anemia with presence of binucleated erythroblasts in the bone marrow resembling CDA type I. Of note, the patients' erythroblasts exhibited cytoplasmic bridges (Figure 1A) rather than the nuclear chromatin bridges observed in CDA-I. Patient 2, offspring of consanguineous parents, presented with hemolytic anemia and was found to have a homozygous mutation (A28V) in a highly conserved alanine residue in the microtubule-interacting domain (MIT) of VPS4A. She had rare evidence of dyserythropoiesis with fewer than 3% binucleated erythroblasts in bone marrow studies. All three patients had significant neurodevelopmental delay with axial hypotonia and appendicular hypertonia. Flow cytometry analysis of peripheral blood from each of these patients revealed a unique cell population which is negative for RNA (by thiazole orange) but still CD71 positive suggesting that loss of VPS4A function also impacts reticulocyte maturation, likely because of defective endosomal vesicle trafficking. Using CD34+ cells in ex vivo erythropoiesis cultures, we first confirmed that VPS4A is expressed in human erythroblasts and localizes at the spindle poles and midbody during mitosis and cytokinesis in these cells. RNA isolated from reticulocytes from patients 1 and 2 was assessed for expression of VPS4A and the paralogous VPS4B. Samples from patient 1 had reduced expression of VPS4A ( In summary, VPS4A plays a critical role in human erythroblast mitosis, cytokinesis, and reticulocyte maturation and mutations in VPS4A cause congenital dyserythropoietic and hemolytic anemia. The CDA registry is critical to allow systematic gene and pathway evaluation, enable novel treatment considerations, and reveal molecular mechanisms essential for erythropoiesis. Disclosures Kalfa: Agios: Other: local PI of clinical research trial; FORMA: Other: sponsored research agreement.

CDAR (ClinicalTrials.gov Identifier: NCT02964494), a registry for patients with Congenital Dyserythropoietic Anemia (CDA) in North America, has been created with the goal to provide a longitudinal database and associated biorepository to facilitate natural history studies and research on the molecular pathways involved in the pathogenesis of CDAs. A 1 y.o. female patient with non-immune hemolytic anemia with suboptimal reticulocytosis, requiring frequent transfusions, and with the pathologic diagnosis of CDA was enrolled in CDAR. Her father had a similar phenotypical presentation in early childhood and underwent splenectomy at 3 years of age. Since then, he has rarely required transfusions but he continues to have a mild anemia at baseline with characteristics of hemolysis and with suboptimal reticulocytosis; at the time of enrollment, he had hemoglobin of 9.3 g/dL with absolute reticulocyte count of 115 x 106 cells/µl. Next Generation sequencing and deletion/duplication assay for the known CDA-associated genes (CDAN1, C15ORF41, SEC23B, KIF23, GATA1) identified no mutations. Whole-exome sequencing for the patient and her parents (family-trio design) revealed a novel PRDX2 missense variant (c.154C>T; p.Pro52Ser) present in heterozygous state in both proband and her father; no mutation in this gene was present in the asymptomatic mother. In silico prediction programs suggest that this variant is probably damaging and deleterious, causing a non-conservative substitution of a phylogenetically highly-conserved amino acid (down to Baker's yeast), and located in an enzymatically active protein domain, adjacent to the active Cys51, with the potential to change its conformation. Peroxiredoxin II is highly expressed during terminal erythropoiesis and is one of the most abundant proteins after hemoglobin in erythroblasts and mature erythrocytes. It is an antioxidant enzyme that reduces the reactive oxygen species (ROS), like hydrogen peroxide and alkyl hydroperoxides readily produced within the erythroid cells due to the presence of heme iron and oxygen. In addition, PRDX2 has been implicated in intracellular signaling, cellular proliferation and differentiation, and as a regulator of iron homeostasis. PRDX2-/- mice were found to have hemolytic anemia with evidence of oxidative damage of the erythrocyte proteins resulting to decreased red blood cell (RBC) survival. The aim of this work is to validate the pathogenetic role of the PRDX2 variant found in this family as the molecular cause of this dominantly-inherited CDA and further investigate the role of PRDX2 in human terminal erythropoiesis. Central review of the patient's bone marrow aspirate and biopsy slides, according to the CDAR protocol, revealed erythroid hyperplasia with dyserythropoiesis, including megaloblastoid changes, nuclear lobation and fragmentation, and binucleated erythroblasts (less than 10%), compatible with atypical CDA. There were rare erythroids with cytoplasmic bridging but no nuclear bridges. Review of the peripheral blood smear showed significant poikilocytosis, mild polychromasia, and the presence of blister and ghost cells reminiscent of G6PD deficiency, pointing to RBC damage by oxidative stress. Induced pluripotent stem cells (iPSCs) and EBV-immortalized lymphocytes were generated from the patients' peripheral blood mononuclear cells after informed consent per CDAR protocol, to allow further in vitro studies of the peroxiredoxin II-deficiency. Flow cytometry confirmed significantly increased ROS in the patients' derived versus control EBV-immortalized lymphocytes as well as in the reticulocytes and mature erythrocytes of the proband and her father, indicating that their PRDX2 variant is causing loss-of-function of the enzyme and increased oxidative stress. Further work is ongoing to explore the mechanisms of pathogenicity of peroxiredoxin II deficiency towards human dyserythropoiesis and decreased erythrocyte lifespan. To our knowledge, this is the first case of anemia described in humans associated with PRDX2 mutation implicating this gene as a novel candidate gene for atypical, dominantly-inherited CDA. Disclosures No relevant conflicts of interest to declare.

Mammalian erythropoiesis has long been established to occur within erythroblastic islands (EBIs), niches where erythroblasts differentiate in close contact with a central macrophage. While it is generally accepted that EBI macrophages play an important role in regulation of erythropoiesis, very little is known about the specific macrophage populations involved in EBI formation, the regulation that occurs within EBIs, or how this niche fits into the broader context of hematopoiesis. We analyzed native EBIs isolated from mouse bone marrow using multispectral imaging flow cytometry (Seu et. al. Front Immunol 2017). Consistent with historical observations, the EBIs were heterogeneous and many contained a number of closely CD11b+ cells in addition to erythroblasts and a central F4/80+ macrophage. Flow cytometry analysis of cells dissociated from native bone marrow EBIs indicated these niches are also enriched 2-3 fold in myeloblasts and granulocytic precursors up to metamyelocytes relative to the total bone marrow while they are depleted of mature granulocytes (bands and segmented cells). Bulk RNAseq of the CD11b+ population isolated from EBIs showed high expression of genes characteristic of the granulocytic lineage (e.g. Elane, Mpo, Gfi1, Cebpe, Camp, and Mmp9), indicating the EBI macrophages may regulate myelopoiesis along with erythropoiesis and that EBIs should really be considered as erythro-myeloblastic islands (EMBIs). To critically document the various hematopoietic cell populations that constitute EMBIs, we used the 10x Genomics Chromium system to obtain single cell gene expression data on ~3,500 total cells from isolated EMBIs along with at least 1,000 sorted cells from each of the 3 major EMBI-associated populations (F4/80+, CD71+, and CD11b+) (Fig 1a, b). The data were analyzed using 10x Genomics' Loupe cell Browser and Iterative Clustering and Guide-gene Selection (ICGS, http://www.AltAnalyze.org, Olsson et. al. Nature 2016). From the ICGS analysis, ~30% of the total EMBI-associated cells were myeloid cells that segregated into at least 3 transcriptionally distinct clusters representing granulocytic progenitors and precursors. As expected, erythroblasts with a progressive maturation pattern made up the bulk (60%) of the EMBI-associated cells, while up to 10% were a heterogeneous population of cells that exhibited expression of macrophage markers such as Csf1R and Irf8, along with genes previously described to characterize resident macrophages, such as Fn1and Fsp1/S100A4 (Fig 1c). In order to investigate the balance of myeloid cells with erythroid cells within the EMBIs, we examined the ratio of CD71+ cells to CD11b+ and how this ratio changes in models of altered granulopoiesis. While the number of myeloid cells at any island varied, the overall ratio of CD11b+ area to CD71+ within the EMBIs was relatively constant at steady state. In three different murine models of anemia of inflammation (AoI), we found that this ratio of CD11b+ to CD71+ cells within the EMBI increases dramatically indicating that the increased granulopoiesis and suppression of erythropoiesis noted in AoI is a result of altered balance of the hematopoiesis within the EMBI unit. Similarly, stimulation of granulopoiesis with GCSF also results in a shift within the EMBIs to CD11b+ myeloid cells and suppression of erythroid cells. Alternatively, in gfi1 KO mice, a model of congenital neutropenia in which granulopoiesis fails at an early stage, the ratio shifts toward CD71+ erythroid cells with paucity of the granulocytic precursors that are typically found at the EMBIs. Taken together, these data indicate that granulocyte progenitors and precursors are specifically associated with EMBI macrophages in the mouse bone marrow. The preferential localization of myeloid precursors within EMBIs suggests this niche is a site for granulopoiesis as well as erythropoiesis and production of these lineages is dynamically regulated within this niche. Our work with multiple murine models of altered granulopoiesis demonstrates that pathological expansion of one of the lineages within this niche may suppress the other and that the interactions within the EMBI could be a useful therapeutic target for AoI. These novel findings significantly broaden our understanding of the role of this hematopoietic niche in the regulated development of lineage committed erythroid and myeloid cells. Disclosures No relevant conflicts of interest to declare.

Hereditary xerocytosis (HX) is a rare autosomal dominant hemolytic anemia caused by mutations in the mechanosensitive cation channel PIEZO1 or, less commonly, in the Ca2+-gated K+ channel KCNN4 (Gardos channel). It is a clinically heterogeneous condition, characterized by erythrocyte dehydration. As erythrocytes traverse narrow capillaries and sinusoids, PIEZO1 is thought to be activated by mechanical stimuli, leading to increased intracellular Ca2+ which then may activate KCNN4, causing K+ efflux and water loss. The p.R2456H and p.L2495_E2496dup PIEZO1 mutations are likely the most common of the PIEZO1 disease-causing mutations and both result in delayed channel inactivation (Glogowska et al., 2017, Blood 130:1845). The prolonged channel activity in response to mechanical stimulation is likely to contribute to erythrocyte dehydration. Ten patients with PIEZO1-associated HX (5 with p.R2456H and 5 with p.L2495_E2496dup PIEZO1 heterozygous mutations) had their diagnosis established utilizing a Next Generation sequencing panel for hereditary hemolytic and congenital dyserythropoietic anemias (HHA/CDA panel). These patients, along with family members who had similar clinical characteristics (a total of 8 patients with p.R2456H and 6 patients with p.L2495_E2496dup) ranging from 2 - 63 years of age, were enrolled in an IRB-approved study to explore the phenotype-genotype correlation of the disease. We collected patient and family histories and laboratory data and performed phenotypic testing including ektacytometry and erythrocyte cation determinations by atomic spectroscopy. Most of the patients in our cohort had compensated hemolysis without anemia at the time of evaluation and had not required any transfusions. All of the patients had elevated reticulocyte counts ranging from 5.7 to 15.7 %. Reticulocyte counts of individuals with the p.R2456H mutation were significantly higher than those with the p.L2495_E2496dup mutation (Student's t test, p=0.01). Some of the patients had splenomegaly, but it was not a universal finding. Although elevated MCHC is often considered a hallmark of HX, most of the individuals in this cohort had increased MCV and MCH but only high-normal MCHC. Varying percentages of hyper dense cells were detected in blood samples analyzed using an Advia Hematology System and stomatocytes were notable in most peripheral blood smears. Several complications have been associated with HX including hydrops fetalis, hemolytic episodes, thrombosis (especially after splenectomy), gallstones, and iron overload. One patient in our cohort had experienced ascites and a pleural effusion in the perinatal period and one was diagnosed with gallstones at age 11. Nearly all the patients had an increase in total bilirubin. Ferritin levels were increased in the majority of the patients tested after 15 years of age. The three patients older than 40 years of age had iron overload verified by T2* MRI imaging of the liver. HX presents several diagnostic challenges due to its heterogeneity. Two of the subjects in our study had been previously diagnosed with hereditary spherocytosis (HS) and one had the diagnosis of congenital dyserythropoietic anemia (CDA) prior to genetic evaluation. Misdiagnosis of individuals with HX as having HS is especially problematic since splenectomy is contraindicated in HX due to a greatly increased incidence of thromboembolic complications. Another diagnostic consideration concerns the use of osmotic fragility tests. Although osmotic fragility tests are frequently used to diagnose HX, ektacytometry provides greater accuracy. In the present study two patients who had ektacytometry results characteristic of HX had reportedly normal osmotic fragility tests in previous work-up. All individuals in our cohort with confirmed disease-causing mutations in PIEZO1 demonstrated the classic left shifted ektacytometry osmoscan indicative of erythrocyte dehydration. In addition, all individuals with these mutations demonstrated the expected decrease in erythrocyte potassium content not associated with a proportional increase in sodium. This illustrates the importance of specialized testing (ektacytometry and/or intracellular cation determinations) and the use of appropriately designed genetic panels to establish an accurate diagnosis in patients with hemolytic anemia of ambiguous phenotype, especially when splenectomy is being contemplated. Disclosures Quinn: Amgen: Research Funding; Global Blood Therapeutics: Research Funding; Silver Lake Research Corporation: Research Funding.

The Hereditary Hemolytic Anemias (HHAs) are a genetically heterogeneous group of anemias characterized by decreased red blood cell (RBC) survival because of defects in hemoglobin, RBC membrane proteins or enzymes. The diagnosis of this group of disorders is complex and challenging requiring analysis of the morphology of RBCs, hemoglobin electrophoresis, and a battery of phenotypic assays. The phenotypic analysis is often problematic in transfusion dependent patients or at times of presentation with a hemolytic crisis as transfused blood or reticulocytosis confounds diagnostic testing. Molecular genetic testing has grown in popularity in the diagnosis of hereditary hemolytic anemias as it is not affected by transfusions or other clinical variables and provides additional insight into the mechanism of the disease. We have developed a Next Generation Sequencing (NGS) panel for HHA due to RBC membrane disorders and enzymopathies and congenital dyserythropoietic anemias (CDA). CDAs, although collectively rare, are included in the panel as they are occasionally misdiagnosed as hereditary spherocytosis (HS) due to their clinical characteristics of hemolysis, increased osmotic fragility, and splenomegaly albeit with inadequate reticulocytosis We reviewed the results of 282 sequential HHA/CDA panels testing for patients with suspected HHA or CDA diagnosis, performed and interpreted at Cincinnati Children's Hospital Medical Center between 1/2013-5/2016. Forty-three samples were omitted from the final analysis due to diagnosis of other disorders, indicating that negative results were true-negatives. For the analysis of the remaining 239 panels, all results were reviewed and categorized based on the type of testing ordered: comprehensive HHA/CDA (32 genes), RBC membrane disorders (13 genes), RBC enzyme disorders (14 genes), or CDA (6 genes). The protein-coding exons plus 25 bases of exon-intron junction as well as promoter sequences were included in the design. Genomic DNA was isolated from blood and target regions were enriched using the Haloplex technology. Enriched samples were then sequenced on an Illumina MiSeq benchtop sequencer with 150 base pair, paired-end reads. Sequencing reads were aligned to the human genome reference sequence and analysis of coverage and variants was completed using NextGENe software. All positive findings were confirmed by Sanger sequencing. These 239 panels included 159 (66.5%) comprehensive HHA/CDA panels, 41 (17.2%) RBC membrane disorder panels, 10 (4.2%) RBC enzyme disorder panels, and 29 (12.1%) CDA panels. Overall, a diagnosis was confirmed or identified in 135 (56.5%) patients with specific genotype of hereditary spherocytosis in 52 patients; hereditary elliptocytosis in 15 patients; hereditary pyropoikilocytosis in 7 patients; hereditary stomatocytosis/xerocytosis in 12 patients; South East Asian Ovalocytosis in 1 patient; G6PD deficiency in 15 patients; pyruvate kinase deficiency in 17 patients; other rare RBC enzymopathies in 6 patients; and CDA in 10 patients. The clinical performance of RBC membrane disorder and RBC enzyme disorder panels were comparable between 68-70% in reaching a final diagnosis, while CDA panel confirmed final diagnosis in only 20% of suspected cases. The overall low prevalence, complexity of diagnosis with findings of dyserythropoiesis in bone marrow studies in patients with severe HHA, and evidence of locus heterogeneity in CDA might explain this result. Among patients with suspected RBC membrane disorders, approximately 14% were eventually diagnosed with hereditary xerocytosis (HX). HX diagnosis is critical to make in such patients since splenectomy is contraindicated due to the high risk of life-threatening thrombophilia complications. In more than half (56.5%) of all cases with suspected hereditary hemolytic anemia, genetic testing provided or confirmed the diagnosis and optimized patients' clinical management. Further genetic counseling and testing for other at-risk family members was made possible by achieving molecular diagnosis. Genetic testing substantially altered management in approximately 14% of cases with suspected RBC membrane disorders due to the diagnosis of HX. In conclusion, genetic testing has a significant clinical utility and may facilitate and improve diagnosis, prognosis and management considerations in patients with hereditary hemolytic or dyserythropoietic anemia. Figure 1 Figure 1. Figure 2 Figure 2. Disclosures No relevant conflicts of interest to declare.

Although Sickle Cell Anemia (SCA) is caused by a single nucleotide mutation in the beta globin gene, there is broad phenotypic variability in affected individuals. It would be highly advantageous to be able to predict which SCA patients are most likely to suffer severe complications and which are likely to benefit from specific treatments. Since two of the major pathologic mechanisms in SCA are erythrocyte dehydration and chronic inflammation, alterations in the expression and/or function of proteins affecting these processes may be responsible for heterogeneity in SCA phenotype. We have developed a 23-gene Next-Generation sequencing panel to identify variants in genes involved in erythrocyte hydration and in reactive oxygen species (ROS) generation and reduction in patients with SCA. We have collected blood samples from an initial cohort of 18 SCA patients with severe phenotype, defined by history of stroke or abnormal transcranial Doppler velocities, and 13 SCA patients with mild phenotype for DNA preparation and for specialized testing to evaluate erythrocyte hydration status and ROS generation. Advia Automated Cell Counter results provided the clearest indications of erythrocyte dehydration in patient blood samples. The ability to examine reticulocyte parameters made it possible to evaluate hydration regardless of the frequency of transfusion. Reticulocyte CHCM, MCV, and percent hyperdense cells were significantly different between the two groups and indicative of a greater degree of dehydration in the severe phenotype group. Osmoscans (using a LoRRca ektacytometer) demonstrated a highly significant left shift indicative of erythrocyte dehydration in both mild and severe groups relative to controls (O min and O hyper values significantly decreased). Both phenotypic groups also demonstrated a significantly lower EI max in relation to controls, indicating decreased deformability of sickle erythrocytes. Deformability scans revealed significant increases in SS ½ and decreases in EI max in both groups in relation to the controls. There were significant differences between mild and severe phenotype groups in osmoscan O min and O hyper values and in deformability assay SS ½ and EI max values. The severe group had values closer to the normal values, most likely due to the contribution of transfused blood in the severe phenotype group. There were no significant differences between the mild and severe SCA phenotype groups in intracellular cation levels (K+ and Na+) measured by flame emission spectroscopy, although both groups had significantly higher intracellular Na+ compared to normal controls. Erythrocyte ROS detection was performed using a flow cytometry DCFDA assay. Both mild and severe phenotype groups demonstrated a highly significant increase in ROS compared to controls. However there was not a significant difference in erythrocyte ROS between the mild and severe phenotype groups. DNA from patient blood samples has been sequenced using the Haloplex target enrichment system and Illumina high-throughput sequencing. We analyzed 23 genes that are likely candidates to be involved in erythrocyte hydration (ABCB6, ABCG5, ABCG8, AQP1, AQP3, ATP2B4, KCNN4, PIEZO1, RHAG, SLC9A1, SLC12A4, SLC12A6, SLC12A7, SLC2A1, SLC4A1, STOM, TRPC6, XK) or in ROS production or reduction (G6PD, NOX1, CYBB, NOX4, NOX5). At least 19 variants classified as possibly damaging have been detected in 10 of the genes. Future study will be directed toward exploring the phenotypic implications of identified variants. Results of this study should further our understanding of the pathogenesis of SCD and help identify biomarkers of disease severity which may ultimately help guide clinical management for individual patients. They may also suggest novel targets for development of new therapies for SCD based on the modulation of erythrocyte hydration and inflammation. Disclosures Quinn: Amgen: Research Funding; Eli Lilly: Research Funding; Silver Lake Research Corporation: Consultancy. Joiner:Global Blood Therapeutics: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Erythroblastic islands (EBIs) are a hallmark of mammalian erythropoiesis consisting of a central macrophage surrounded by and interacting closely with maturing erythroblasts. While it is generally accepted that the island macrophages play an important role in erythropoiesis, the inability to identify and isolate this macrophage subpopulation has limited our understanding of their functional involvement. Previous studies have relied on immunohistochemistry/immunofluorescence in situ or in vitro. More recently, flow cytometry was used to characterize EBI formation and the immunophenotype of the central macrophages in murine erythroblastic islands. These approaches provide either morphological/structural information or high-throughput quantification, but not both, and often carry the expectation that all EBI macrophages have similar phenotype (F4/80+/CD169+/VCAM1+ for example), and thus potentially overlook critical information about the nature and biology of the islands and the central macrophages. We have developed a novel method for analysis and characterization of EBI macrophages from hematopoietic tissues using multispectral imaging flow cytometry, which combines the high-throughput advantage of flow cytometry with the morphology and fluorescence details obtained from microscopy. This method allows automated, non-biased evaluation of the EBIs recovered from a sample, their number, mean size, as well as structural and morphological details of the central macrophages and associated erythroblasts. Most importantly, the images, combined with the fluorescence similarity feature, enables the evaluation of co-expression of any phenotypic markers that may be used to identify the macrophages which is crucial since some antigens used to identify macrophages (e.g. CD45, CD11b) may also be expressed on non-erythroid cells associated with the islands instead of, or in addition to, the central macrophage itself. We used this method to confirm the expression of various markers previously reported to be expressed on the erythroblastic island macrophages by flow, including CD11b, VCAM1, F4/80, CD169, and CD163, in mouse, rat, and human bone marrow. Indeed, while a large number of studies have focused on murine erythropoiesis, the identity and role of the EBIs in other species is much less known. We confirmed expression of CD169 and VCAM1 on the F4/80+ central macrophages of murine EBIs and also identified a population of VCAM+/F4/80- central cells associated with developing erythroblasts. CD11b is abundantly expressed by non-erythroid, non-macrophage cells associated with the islands, but is not expressed significantly on the central macrophages (Figure 1). CD163, a marker of EBI macrophages in rat and human, was not detected in the murine EBIs by imaging flow cytometry, but this may be due to limitation of the antibodies tested. In contrast, anti-CD163 stained well rat and human EBI macrophages but CD11b or VCAM1 were not detected in EBIs from rat and human bone marrow respectively, raising the question of a species-specificity regarding the macrophage heterogeneity and satellite cells present within erythroblastic islands. In summary, the data presented herein demonstrate the effectiveness of this method for the analysis and characterization of EBIs and establish a new tool for future investigations of EBIs and their central macrophages in the nurturing of erythropoiesis. Figure 1 Representative image of an erythroblastic island harvested from murine bone marrow stained with F4/80-AF488 (green), CD11b-PE (blue), and CD71-BV421 (red) and analyzed by imaging flow cytometry. Figure 1. Representative image of an erythroblastic island harvested from murine bone marrow stained with F4/80-AF488 (green), CD11b-PE (blue), and CD71-BV421 (red) and analyzed by imaging flow cytometry. Disclosures No relevant conflicts of interest to declare.