Ineffective erythropoiesis and its treatment

The erythroid marrow and circulating red blood cells (RBCs) are the key components of the human erythron. Abnormalities of the erythron that are responsible for anemia can be distinguished into 3 major categories, that is, erythroid hypoproliferation, ineffective erythropoiesis, and peripheral hemolysis. Ineffective erythropoiesis is characterized by erythropoietin-driven expansion of early-stage erythroid precursors, associated with apoptosis of late-stage precursors. This mechanism is primarily responsible for anemia in inherited disorders like β-thalassemia, inherited sideroblastic anemias, and congenital dyserythropoietic anemias, as well as in acquired conditions like some subtypes of myelodysplastic syndromes (MDS). The inherited anemias due to ineffective erythropoiesis are also defined as iron loading anemias because of the associated parenchymal iron loading caused by the release of erythroid factors that suppress hepcidin production. Novel treatments specifically targeting ineffective erythropoiesis are being developed. Iron restriction through enhancement of hepcidin activity or inhibition of ferroportin function has been shown to reduce ineffective erythropoiesis in murine models of β-thalassemia. Luspatercept is a TGF-β ligand trap that inhibits SMAD2/3 signaling. Based on pre-clinical and clinical studies, this compound is now approved for the treatment of anemia in adult patients with β-thalassemia who require regular RBC transfusions. Luspatercept is also approved for the treatment of transfusion-dependent anemia in patients with MDS with ring sideroblasts, most of whom carry a somatic SF3B1mutation. While long-term efficacy and safety of luspatercept need to be evaluated both in β-thalassemia and MDS, defining the molecular mechanisms of ineffective erythropoiesis in different disorders might allow the discovery of new effective compounds.

β-Thalassemia: evolving treatment options beyond transfusion and iron chelation

After years of reliance on transfusion alone to address anemia and suppress ineffective erythropoiesis in β-thalassemia, many new therapies are now in development. Luspatercept, a transforming growth factor–β inhibitor, has demonstrated efficacy in reducing ineffective erythropoiesis, improving anemia, and possibly reducing iron loading. However, many patients do not respond to luspatercept, so additional therapeutics are needed. Several medications in development aim to induce hemoglobin F (HbF): sirolimus, benserazide, and IMR-687 (a phosphodiesterase 9 inhibitor). Another group of agents seeks to ameliorate ineffective erythropoiesis and improve anemia by targeting abnormal iron metabolism in thalassemia: apotransferrin, VIT-2763 (a ferroportin inhibitor), PTG-300 (a hepcidin mimetic), and an erythroferrone antibody in early development. Mitapivat, a pyruvate kinase activator, represents a unique mechanism to mitigate ineffective erythropoiesis. Genetically modified autologous hematopoietic stem cell transplantation offers the potential for lifelong transfusion independence. Through a gene addition approach, lentiviral vectors have been used to introduce a β-globin gene into autologous hematopoietic stem cells. One such product, betibeglogene autotemcel (beti-cel), has reached phase 3 trials with promising results. In addition, 2 gene editing techniques (CRISPR-Cas9 and zinc-finger nucleases) are under investigation as a means to silence BCL11A to induce HbF with agents designated CTX001 and ST-400, respectively. Results from the many clinical trials for these agents will yield results in the next few years, which may end the era of relying on transfusion alone as the mainstay of thalassemia therapy.

Luspatercept for the treatment of β-thalassemia: from preclinical research to clinical practice and beyond

β-thalassemia is an inherited disease causing an impaired hemoglobin production, eventually leading to a severe chronic anemia. Resolutive treatments are still limited to a small number of patients and blood transfusions still represent the standard of care. In this scenario, luspatercept is the first approved drug able to significantly modify the disease phenotype. Developed as a fusion protein, it binds TGF-β ligands, contributing to a reduction of ineffective erythropoiesis. As shown by clinical trials in thalassemia, this effect determines an increase in mean hemoglobin levels and/or a decrease in transfusion burden. While some potential indications are still being evaluated in trials, luspatercept has recently entered the clinical practice for transfusion-dependent thalassemia patients.

Treating Iron Overload Disorders with a Novel Therapeutic Antibody Targeting TMPRSS6

Iron is an essential element for almost all living organisms as it participates in a wide variety of metabolic processes. Disorders of iron metabolism are among the most prevalent human diseases, ranging from anemia to hemochromatosis. Excessive iron accumulations in major organs of iron overload patients can lead to high mortality. Hepcidin, a HAMP-encoded liver hormone, is the master regulator of iron homeostasis. By binding to the sole iron exporter ferroportin and causing internalization and degradation of the complex, hepcidin inhibits cellular iron efflux, thereby lowers plasma iron levels. Inappropriately suppressed/low hepcidin production is central to iron overload. Transmembrane protease serine-6 (TMPRSS6), a type II transmembrane serine protease primarily expressed in liver, downregulates hepcidin expression through BMP-SMAD pathway. TMPRSS6 deficiencies have been shown to cause hepcidin overexpression in both TMPRSS6-mutant mice and in patients with iron-refractory iron deficiency anemia (IRIDA). Therefore, TMPRSS6 is a viable therapeutic target for iron overload disorders. Here we report the generation of an anti-TMPRSS6 antibody through a hybridoma campaign using a DNA-based immunization approach, followed by humanization and sequence optimization. Lead antibody, hzMWTx-003 selectively binds human TMPRSS6 with low nanomolar affinity (KD: 7.6nM), and is cross-reactive to rodent (mouse and rat) and monkey (cynomolgus and rhesus) TMPRSS6. Single-dose injection of hzMWTx-003 was able to significantly elevate serum hepcidin and liver HAMP RNA levels in wildtype mice, resulting in significantly reduced serum iron level. The Hbb th3/+ mouse model of β-thalassemia, like its human counterpart, is characterized by iron overload, ineffective erythropoiesis and splenomegaly. Treatment of Hbb th3/+mice with MWTx-003 effectively increased hepcidin expression at both protein and RNA levels, leading to significantly reduced serum iron and liver non-heme iron content. MWTx-003 also dramatically improved anemia and ineffective erythropoiesis, and alleviated splenomegaly in these mice. CMC development of hzMWTx-003 confirms outstanding biophysical properties. Preliminary studies in cynomolgus monkey using GLP-grade material demonstrated good pharmacokinetics of hzMWTx-003 and expected pharmacodynamic response where reduction of serum iron could be sustained for 21 days after single dose administration. A single dose toxicology study in cynomolgus monkey revealed no safety concerns, and no production of anti-idiotype antibodies was detected. In summary, anti-TMPRSS6 antibody MWTx-003 represents a promising therapy for iron overload disorders such as β-thalassemia, and potentially other diseases where iron restriction is beneficial. Disclosures Chen: Mabwell Therapeutics Inc: Current Employment. Wang: Mabwell Therapeutics Inc: Current Employment. Zheng: Mabwell (Shanghai) Bioscience Co. Ltd: Current Employment. Huang: Mabwell Therapeutics Inc: Current Employment. Mao: Mabwell (Shanghai) Bioscience Co. Ltd.: Current Employment. Ouyang: Mabwell (Shanghai) Bioscience Co. Ltd.: Current Employment. Du: Mabwell Therapeutics Inc: Current Employment.

Long-Term Efficacy and Safety of the Oral Pyruvate Kinase Activator Mitapivat in Adults with Non-Transfusion-Dependent Alpha- or Beta-Thalassemia

Background: The thalassemias are a group of red blood cell (RBC) disorders in which ineffective erythropoiesis and hemolysis occur due to imbalanced production and precipitation of globin chains. Thalassemic RBCs have insufficient levels of ATP to meet increased energy demands associated with globin chain imbalance, protein degradation, and cellular oxidative stress responses. Mitapivat (AG-348) is a first-in-class, small-molecule, oral activator of RBC pyruvate kinase (PKR), a key enzyme regulating ATP production via glycolysis. In a phase 2, open-label trial of mitapivat in adults with α- or β-non-transfusion-dependent (NTD) thalassemia (NCT03692052), 80.0% (16/20) of patients (pts) met the primary endpoint of a hemoglobin (Hb) response (increase ≥ 1.0 g/dL from baseline at 1 or more assessments between Wks 4-12, inclusive). Improvements in markers of hemolysis and erythropoiesis were also observed in pts and mitapivat was generally well tolerated. Methods: Pts aged ≥ 18 (yrs) with a known medical history of α- or β-thalassemia, Hb concentration of ≤ 10.0 g/dL, and ≤ 5 RBC units transfused in prior 24 wks and none in 8 wks prior to study drug were eligible for the study. All pts started mitapivat at 50 mg twice daily (BID), escalating to 100 mg BID based on individual safety and Hb assessments. After completion of the 24-wk core period, pts were continued on mitapivat treatment in the extension period if they had achieved a Hb response, or a delayed Hb response (Hb increase of ≥ 1.0 g/dL at ≥ 1 assessment after Wk 12), with no ongoing grade ≥ 3 treatment-emergent adverse events (AE) related to study drug. Eligible pts continued mitapivat at the dose received at their Wk 24 visit. Study visits are conducted every 12 wks and will continue for up to 10 yrs. The extension period of the study is ongoing, here we report data up to Wk 72 visit (data cutoff March 27, 2021). Results: Of the 19 pts who completed the core period, 17 entered the extension period. During the extension period, 16 pts received 100 mg BID mitapivat and 1 received 50 mg BID. As of the cutoff date, 1 pt had discontinued (patient decision). Median (range, in wks) duration of mitapivat treatment for pts who entered the extension period was 70.9; (54.7, 105.6), with 8 of 17 pts receiving 72 wks or more of treatment as of the cutoff date for this analysis. The Median age of pts who entered the extension period was 44 yrs (range 29, 67). Mean baseline (standard deviation [SD]) Hb, total bilirubin and lactate dehydrogenase (LDH) was 8.1 (1.2) g/dL, 40.1 (26.2) μmol/L and 272.4 (121.7) U/L, respectively. Median baseline erythropoietin (EPO) was 70.5 (range 15, 11191) IU/L. Improvements in Hb concentration achieved in the core period were sustained in the extension study (n = 8 at Wk 72; Figure 1). Mean Hb (SD) increase from baseline to Wk 60 (which includes 4 pts with α- and 9 with β-thalassemia) and Wk 72 (which includes 8 pts with β-thalassemia) were 1.5 (0.4) g/dL and 1.7 (0.5) g/dL, respectively. Improvements in markers of hemolysis and ineffective erythropoiesis observed in the core period were maintained in the extension period up to Wk 72 (mean [SD] bilirubin and LDH, -15.8 [16.6] μmol/L and -63.6 [216.0] U/L, respectively; median [range] EPO, -33.0 [-72.0, -16.0] IU/L). The safety profile was consistent with that observed during the core period. AEs occurring in ≥ 15% of pts during the extension period were headache (5/17) and back pain (3/17), none of which were grade ≥ 3. No notable trends for changes in bone mineral density were observed (Figure 2). There were no treatment-related serious AEs during the extension period. Conclusions: In pts with either α- or β-thalassemia, a favorable efficacy-safety profile was observed with long-term treatment with mitapivat. Results show sustained improvements in Hb, hemolysis and ineffective erythropoiesis - despite the globin genotypic heterogeneity of the cohort - and no new safety findings. Mitapivat, through its unique mechanism of action, may represent a novel therapeutic approach for this condition. Two phase 3 trials of mitapivat in α- and β-thalassemia, one in pts who are NTD and one in pts who are transfusion-dependent, will be initiated in 2021. Figure 1 Figure 1. Disclosures Kuo: Bluebird Bio: Consultancy; Alexion: Consultancy, Honoraria; Agios: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria; Apellis: Consultancy; Pfizer: Consultancy, Research Funding; Bioverativ: Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy. Layton: Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Agios Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Cerus: Membership on an entity's Board of Directors or advisory committees. Lal: Chiesi: Consultancy; Agios Pharmaceuticals: Consultancy; bluebird bio, Inc.: Research Funding; La Jolla Pharmaceutical Company: Research Funding; Terumo Corporations: Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Insight Magnetics: Research Funding; Protagonist Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Research Funding. Al-Samkari: Dova/Sobi: Consultancy, Research Funding; Moderna: Consultancy; Argenx: Consultancy; Rigel: Consultancy; Amgen: Research Funding; Novartis: Consultancy; Agios: Consultancy, Research Funding. Bhatia: Agios Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Kosinski: Agios Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Tong: Agios Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Lynch: Agios Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Uhlig: Agios Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Vichinsky: Agios Pharmaceuticals: Consultancy, Research Funding; Bluebird Bio: Consultancy, Research Funding; Global Blood Therapeutics: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding.

Mitapivat Improves Transfusion Burden and Reduces Iron Overload in Thalassemic Mice

Mitapivat, an oral activator of pyruvate kinase (PK), was recently shown to improve b-thalassemic anemia with a reduction of ineffective erythropoiesis and an amelioration of b-thalassemic red cell features in a mouse model for b-thalassemia (Hbb 3th/+ mice).). These changes were also associated with a beneficial effect on iron homeostasis by modulation of duodenal DMT1 expression (Matte A et al JCI 2021). Two clinical studies have shown improvement of anemia and ineffective erythropoiesis with mitapivat treatment in patients with non-transfusion-dependent (NTD) thalassemia (Kuo et al. EHA 2021). Based on these results, Phase 3 studies in both NTD and TD thalassemia are currently on going. The objective of this preclinical study was to determine if treatment with mitapivat affects the length between transfusion of red blood cells (RBCs) and the liver iron concentration (LIC). Using a previously established murine model of RBCs transfusions (Park Y et al Blood 2020), in Hbb 3th/+ mice, we used Hb 10.5 g/dL as threshold for RBCs transfusion, with washed RBCs, at 40% Hct (400 uL total volume infused). The animals were divided into two groups: vehicle and mitapivat (50mg/Kg by gavage BID for up to 61 days).The length of the interval between transfusions increased in mitapivat treated compared to vehicle treated animals (transfusion interval: 13.8±1.0 days vs vehicle 10.5±1.0 days respectively n=4 and n=3). In both groups, the transfusion regimen induced a significant reduction in spleen weight/mouse weight ratio and in extramedullary erythropoiesis. We also found a significant reduction in liver iron content (LIC) in mitapivat treated compared to vehicle treated animals. We then evaluated the effects of mitapivat in combination with iron chelation using deferiprone (DFP,1.25 mg/mL, drinking water). Casu et al. have previously shown in the same mouse model for β-thal that DFP did not affect erythropoiesis. In the β-thal mice, we did not find negative effects on hematologic parameters when mitapivat (50 mg/Kg/d by gavage BID) was co-administrated with DFP for 28 days. LIC was reduced in mitapivat treated mice and in mitapivat +DFP treated β-thal mice was further decreased compared to vehicle treated animals. This allowed us to reduce DFP dosage from 1.25 to 0.8 mg/mL in mitapivat treated β-thal mice. These data show that in mouse model of transfused β-thalassemia, mitapivat increases the time interval between transfusions, reduces transfusion burden and allows a reduction of the dosing iron chelation with DFP. Thus, mitapivat might represent an interesting option in transfusion dependent β-thalassemic patients, being transfusion burden still an unmet need in this patient population. Disclosures Kosinski: Agios Pharmaceuticals: Current Employment, Current equity holder in publicly-traded company. Dang: Agios Pharmaceuticals, Inc.: Current Employment, Current holder of stock options in a privately-held company. De Franceschi: F. Hoffmann-La Roche Ltd: Consultancy.

Obligate N-Terminal but Not C-Terminal Monoferric Transferrin Ameliorates Anemia in β-Thalassemic Mice

Transferrin (TF) is a bilobed 80kD glycoprotein with N- and C-lobe iron binding sites. TF circulates as four forms: unbound to iron (apo-TF), iron bound to the N-lobe (monoferric N-TF), the C-lobe (monoferric-C), or to both lobes (diferric-TF). Most circulating TF under physiological conditions is monoferric. The iron-bound TF forms interact with TF receptor-1 (TFR1), which is ubiquitously expressed and serves as the main mechanism for cellular iron delivery. Iron-bound TF also interacts with TF receptor-2 (TFR2) which is expressed on hepatocytes, erythroblasts, and bone cells. Whereas TFR1 serves primarily as a cargo receptor, TFR2 serves primarily to influence cellular signaling events regulating hepcidin expression, erythropoiesis, and bone formation. We proposed that different transferrin forms provide differential signaling properties in this regulation. We thus generated TF mutant mice in which all iron-containing TF was either monoferric N (Tf monoN) or monoferric C (Tf monoC). Compared with Tf monoC mice, the Tf monoN mice demonstrated increased RBC production and increased hepcidin expression relative to iron status (Parrow et al. Blood). Based on observations in β-thalassemic mice treated with exogenous TF (Li et al. Nat Med), we hypothesized that β-thalassemic mice obligate for monoN TF would demonstrate improved erythropoietic and iron parameters compared with β-thalassemic mice obligate for monoC TF. To address this hypothesis, we crossed Hbb th3/+ mice (a mouse model of β-thalassemia intermedia) with Tf monoN and Tf monoC mice. Compared with Hbb th3Tf +/+mice, in Hbb th3/+Tf monoN mice demonstrated significantly increased RBC counts, elevated hemoglobin, improved erythrocyte morphology (Figure 1A-B), decreased splenomegaly, fewer bone marrow erythroblasts, and improvement of ineffective erythropoiesis (as measured by the ratio of progenitors to RBC in the bone marrow). Additionally, serum ERFE was significantly reduced and hepcidin levels were increased in Hbb th3/+Tf monoN relative to Hbb th3/+Tf +/+controls. Conversely, hematological parameters from Hbb th3/+Tf monoC mice were comparable to Hbb th3/+Tf +/+ mice. Similarly, Hbb th3/+Tf monoCmice had no improvements in markers of ineffective erythropoiesis in the bone marrow compared with Hbb th3/+Tf +/+ mice. In summary, we demonstrate that the differential regulatory effects of monoN and monoC TF on erythropoiesis are relevant not only in steady-state, but also in the ineffective erythropoiesis that is characteristic of β-thalassemia. Because both monoN and monoC TF forms can deliver only one iron atom per TF-TFR1 binding event, our findings suggest that the improvements observed only in the Hbb th3/+Tf monoN mice were not due to iron restriction alone. We are now elucidating the mechanisms by which the two TF lobes exert their differential effects on ineffective erythropoiesis and exploring the translational potential of obligate monoN TF in the treatment of β-thalassemia. Figure 1 Figure 1. Disclosures Rivella: Ionis Pharmaceuticals: Consultancy; Meira GTx: Consultancy.

Vitamin B5 and Succinyl-CoA Improve Ineffective Erythropoiesis in SF3B1 Mutated Myelodysplasia

Myelodysplastic syndrome (MDS) is a hematological clonal stem cell disease. Recurrent splicing factors mutations are reported in 50% of MDS. Interestingly, mutations in the splicing factor gene SF3B1 are over-represented in MDS with ring sideroblasts (MDS-RS), co-occurring in up to 90% of patients. In MDS-RS, anemia is the major clinical manifestation. Erythropoiesis stimulating agents (ESAs) are used to treat anemia; however, the overall response rates are 20% to 40% with a duration of response of 18-24 months. New therapeutic options are needed to improve response to ESAs treatment and delay red blood cell transfusion, which are associated with acute myeloid leukemia progression and increase in morbidity. Mutations in SF3B1 modify the recognition pattern of the 3' splice site and lead to subsequent mis-splicing of its targets. To identify critical mis-splicing events involved in the erythroid differentiation blockage, we performed splicing analysis on RNA sequencing generated from hematopoietic stem/progenitor cells undergoing differentiation. Three MDS primary samples harboring SF3B1 mutations and three age-matched healthy donors cultured under normoxia and hypoxia conditions were initially used for the analysis. High depth RNA sequencing and differential splicing analyses using rMATS identified 2,845 mis-spliced events including 200 shared between hypoxia and normoxia conditions. Here, using a cohort of 42 MDS samples, we report the mis-splicing of the coenzyme A synthase (COASY) transcript. Heme synthesis relies on succinyl-CoA synthesis, and its production itself depends on the availability of cellular CoA. We thus hypothesised that COASY mis-splicing is a key driver of ineffective erythropoiesis in MDS-RS patients. In primary hematopoietic cells, COASY is upregulated during erythroid differentiation and its silencing in CD34 + cells severely impedes the generation of mature erythroid cells CD71 - CD235a + and causes disruption in heme production. Functional characterisations of the CRISPR-CAS9 edited K562 SF3B1K700E and the SF3B1-mutated HNT-34 cell lines confirmed that COASY mis-splicing impairs COASY protein synthesis that ultimately results in 60% loss of the protein. Metabolomic analysis showed that COASY mis-splicing depletes cells in CoA and succinyl-CoA metabolites, however this phenotype can be rescued by supplementation with vitamin B5, a CoA precursor. Consequently, we showed in vitro that saturating the 40% of remaining COASY enzyme with vitamin B5 or supplementing medium with its downstream by-product, succinyl-CoA, improved erythropoietic differentiation in MDS SF3B1mut patients. In summary, our results for the first time show that SF3B1 mutations induce coenzyme A synthase (COASY) transcript mis-splicing, that consequently leads to measurable defects in metabolites essential for heme biosynthesis. Our report reveals a novel critical role of COASY in regulating normal bone marrow erythropoiesis through control of succinyl-coA during human erythroid differentiation. Remarkably, partial loss of the coenzyme A synthase in MDS-RS patients leads to disruption in the erythroid lineage as well as heme deficiency, that can be rescued by exogenous treatment with vitamin B5 or succinyl-CoA. Therefore, vitamin B5 could represent a very attractive agent to combine with existing treatments in order to increase erythroid maturation and delay red blood cell transfusion dependency in MDS-RS patients. Graphical representation: SF3B1 mutant causes mis-splicing in COASY that results in loss of protein. Deficiency in COASY triggers a downregulation of succinyl-CoA that is involved in the rate limiting step of heme synthesis. Heme deficiency subsequently impairs erythroid differentiation. Treatment of MDS SF3B1 mutant cells with vitamin B5 (precursor of CoA), or succinyl-CoA, rescues erythroid differentiation. Figure 1 Figure 1. Disclosures Platzbecker: Geron: Honoraria; Takeda: Honoraria; Janssen: Honoraria; Celgene/BMS: Honoraria; Novartis: Honoraria; AbbVie: Honoraria. Wiseman: Bristol Myers Squibb: Consultancy; Novartis: Consultancy; StemLine: Consultancy; Takeda: Consultancy; Astex: Research Funding. Gribben: Abbvie: Honoraria; AZ: Honoraria, Research Funding; BMS: Honoraria; Gilead/Kite: Honoraria; Janssen: Honoraria, Research Funding; Morphosys: Honoraria; Novartis: Honoraria; Takeda: Honoraria; TG Therapeutis: Honoraria.

Base Editing-Mediated Dissection of the -200 Region of the γ-Globin Promoters to Induce Fetal Hemoglobin and Rescue Sickle Cell Disease and β-Thalassemia

β-hemoglobinopathies are caused by mutations affecting the adult hemoglobin production. In sickle cell disease (SCD), the β6 Glu→Val substitution leads to sickle hemoglobin (HbS) polymerization and red blood cell (RBC) sickling. In β-thalassemia, reduced β-globin production leads to precipitation of uncoupled α-chains causing ineffective erythropoiesis and the production of poorly hemoglobinized RBCs. Transplantation of autologous, genetically modified hematopoietic stem/progenitor cells (HSPCs) is an attractive therapeutic option. The clinical severity of β-hemoglobinopathies is alleviated by the co-inheritance of mutations causing hereditary persistence of fetal Hb (HPFH). HPFH mutations clustering 200 nucleotides upstream of the TSS of the fetal γ-globin (HBG) genes either disrupt the binding site (BS) of the fetal Hb (HbF) repressor LRF or generate a de novo BS for the KLF1 activator. To reactivate γ-globin expression, nuclease-based approaches have been explored. However, nucleases generate double-strand breaks (DSBs), raising safety concerns for clinical applications. Base editing (BE) allows the introduction of point mutations without generating DSBs. In this study, we designed BE systems to introduce a variety of HPFH or HPFH-like mutations in the -200 region of the HBG promoters. First, we screened in erythroid cell lines known and novel BEs, and we selected combinations of BEs and guide RNAs that edit alternative bases of the -200 region. We then developed a clinically-relevant protocol based on RNA-transfection to deliver the BE system to HSPCs. The expression profile of genes activated by RNA stimuli revealed no immune response in HSPCs. A progenitor assay indicated no alteration in the growth and multilineage differentiation of edited HSPCs. We applied this protocol to SCD and β-thalassemia HSPCs, achieving editing efficiencies up to ~70% of the HBG promoters. In RBCs differentiated from edited SCD HSPCs, RT-qPCR, HPLC and flow cytometry showed a potent γ-globin reactivation with a high frequency of HbF + cells and a concomitant decrease in the HbS content/cell. Importantly, the pathological RBC sickling phenotype was corrected in the samples derived from edited HSPCs. Similarly, in β-thalassemia samples, RT-qPCR and HPLC analyses showed strong γ-globin induction and decrease of the α-globin precipitates. HbF expression rescued the delay in erythroid differentiation and ineffective erythropoiesis characterizing β-thalassemia, as demonstrated by the increased RBC enucleation rate and the reduced apoptosis and oxidative stress. We then compared BE strategies that either disrupt the LRF BS or create a de novo KLF1 BS in single colonies derived from erythroid progenitors. Generation of the KLF1 BS was associated with higher levels of HbF compared to the LRF BS disruption. These results suggest that eviction of the LRF repressor is sufficient to reactivate HBG genes, but recruitment of an activator is more effective to achieve high levels of gene expression. HbF expression induced by both LRF BS disruption and KLF1 BS generation was sufficient to rescue the SCD cell phenotype, but higher HbF levels - achieved only through KLF1 BS generation - were necessary to fully correct the β-thalassemia phenotype. In the majority of cases, we detected no DSB-induced insertions, deletions, or large genomic rearrangements in base-edited samples. Accordingly, DSB-induced DNA damage response (DDR) was absent in base-edited HSPCs, as measured by evaluating the expression of p21, a readout of p53-induced DDR. DNA off-target activity was assessed by GUIDE-seq and targeted sequencing of the potential off-target sites in edited HSPCs, while RNA off-target activity was evaluated by RNA-seq in HSPCs. Finally, BE-treated HSPCs were transplanted in immunodeficient mice to evaluate the engraftment and differentiation capability of edited HSCs. We detected good frequencies of human cells with up to ~60% of edited promoters in the peripheral blood of transplanted mice. In conclusion, we developed a clinically-relevant strategy to perform efficient BE in the HBG promoters that led to therapeutically-relevant HbF levels and rescued both the SCD and β-thalassemia phenotypes, thus providing sufficient proof of efficacy and safety to enable the clinical development of base-edited HSPCs for the therapy of β-hemoglobinopathies. Disclosures El Nemer: Hemanext: Consultancy.

Tfr2 Genetic Deletion Makes Transfusion-Independent a Murine Model of Transfusion-Dependent β-Thalassemia

β-thalassemia is an autosomal recessive disorder due to mutations in the β-globin gene, that leads to defective production of hemoglobin (Hb) and red blood cells (RBC). The main features of the disease are anemia, ineffective erythropoiesis and iron overload. Patients affected by the most severe form of β-thalassemia are transfusion-dependent (TDT) and require lifelong blood transfusions and iron chelation, symptomatic treatments that affect the quality of life. The only curative option, unfortunately limited by the insufficient number of HLA-matched donors, is allogenic bone marrow (BM) transplantation. Other recently approved treatments (i.e. Luspatercept and gene therapy) are only partially effective and/or suitable for selected patients. Thus, the identification of novel therapeutic approaches is a clinical need. Transferrin receptor 2 (TFR2) contributes to the transcriptional activation of hepcidin, the master regulator of iron homeostasis, in the liver and is a brake of Erythropoietin signaling in erythroid cells, thus balancing RBC production and iron availability. We have recently proved that BM Tfr2 deletion enhances erythropoiesis in wild-type mice (Nai et al., Blood 2015) and ameliorates anemia in non-TDT mice models, both alone (Artuso et al., Blood 2018) and in combination with iron-restricting agents (Casu, Pettinato et al., Blood 2020). Here we aim at investigating whether Tfr2 targeting might be beneficial also for TDT. To this purpose we generated TDT mice (Hbb th1/th2; Casu et al., Haematologica 2020) with heterozygous (Tfr2 BMhetero/Hbb th1/th2) and homozygous (Tfr2 BMKO/Hbb th1/th2) BM Tfr2 deletion by transplantation of Hbb th1/th2, Tfr2-hetero/Hbb th1/th2 and Tfr2-ko/Hbb th1/th2 fetal liver cells (FLT) from day E14.5 embryos into lethally irradiated wild-type mice. BM Tfr2 deficient mice have increased RBC count and Hb levels and decreased percentage of reticulocytes with a gene dosage effect 8 weeks after FLT, before the onset of transfusion-dependance. At this time-point, complete BM Tfr2 deletion ameliorates ineffective erythropoiesis, decreasing the percentage of polychromatic erythroblasts and increasing orthochromatic erythroblasts and mature RBCs both in the BM and in the spleen. The improved anemia was also accompanied by a partial correction of two debilitating TDT complications, iron overload and cardiomegaly. The beneficial effect of Tfr2 deletion was maintained over time. Indeed, Hbb th1/th2 mice became transfusion-dependent 14 weeks after FLT, when Hb levels drop below 5.5 g/dL, requiring transfusions of an average 108.75±56.87μL of blood/animal/week. On the contrary, animals with both heterozygous and homozygous BM Tfr2 deletion are still non-transfusion-dependent at 20 weeks, maintaining Hb levels above 7 and 9 g/dL respectively. Overall, our results prove that, despite the persistence of the genetic defect, hematopoietic Tfr2 deletion ameliorates anemia, ineffective erythropoiesis and secondary complications also in the most severe form of β-thalassemia. This improvement is associated to a substantial increase in transfusion-free survival of both Tfr2 BMhetero/Hbb th1/th2 and Tfr2 BMKO/Hbb th1/th2 mice, which are transfusion-independent 6 weeks after the time of blood transfusion requirement in Hbb th1/th2 animals. The difference of blood transfusions needs will be evaluated over time for at least 10 additional weeks. Our findings demonstrate that TFR2 targeting represents a new promising therapeutic opportunity for the management of β-thalassemia, worth to be tested both as a monotherapy and in association with available treatments. Disclosures Rivella: Incyte: Consultancy; MeiraGTx: Consultancy, Membership on an entity's Board of Directors or advisory committees; Forma Theraputics: Consultancy; Keros Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees; Disc Medicine: Consultancy, Membership on an entity's Board of Directors or advisory committees; Ionis Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Consultancy.