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224 Fetal Hemoglobin Rescues Ineffective Erythropoiesis in Sickle Cell Disease

Program: Oral and Poster Abstracts
Type: Oral
Session: 113. Hemoglobinopathies, Excluding Thalassemia—New Genetic Approaches to Sickle Cell Disease: New Insights Into Sickle Cell Disease Pathophysiology
Hematology Disease Topics & Pathways:
Anemias, sickle cell disease, Diseases, Hemoglobinopathies
Saturday, December 5, 2020: 2:00 PM

Sara El Hoss, PhD1,2,3*, Sylvie Cochet1,2,3*, Auria Godard, MS1,2,3*, Hongxia Yan, MS4*, Michaël Dussiot, PhD1,3,5*, Giacomo Frati, PhD1,6*, Bénédicte Boutonnat-Faucher, MD7*, Sandrine Laurance, PhD1,2,3*, Olivier Renaud, PhD8*, Laure Joseph, MD9*, Annarita Miccio, PhD1,6, Valentine Brousse, MD/PhD1,2,3,7*, Mohandas Narla, DSc4 and Wassim El Nemer, PhD1,2,3*

1University of Paris, Paris, France
2Integrated Biology of the Red Cell, INSERM Unit 1134, Paris, France
3Laboratory of Excellence GR-Ex, Paris, France
4Laboratory of Red Cell Physiology, New York Blood Center, New York, NY
5Laboratory of Cellular and Molecular Mechanisms of Hematological Disorders and Therapeutic Implications, Institut Imagine U1163, INSERM, Paris, France
6Imagine Institute, Laboratory of chromatin and gene regulation during development, INSERM UMR1163, Paris, France
7Pediatrics Department, Hopital Universitaire Necker-Enfants Malades, Paris, France
8Paris Sciences et Lettres Research University, Institut Curie - CNRS, Paris, France
9Biotherapy Department, Necker Children's Hospital, Assistance Publique-Hôpitaux de Paris, Paris, France

Sickle cell disease (SCD) is an autosomal hereditary recessive disorder caused by a point mutation in the β globin gene resulting in a Glu-to-Val substitution at the 6th position of the β globin protein. The resulting abnormal hemoglobin (HbS) polymerizes under hypoxic conditions driving red blood cell (RBC) sickling (Pauling et al., 1949). While pathobiology of circulating RBCs has been extensively analyzed in SCD, erythropoiesis is surprisingly poorly documented. In β-thalassemia, ineffective erythropoiesis is characterized by high levels of apoptotic erythroblasts during the late stages of terminal differentiation, due to an accumulation of free β-globin chains (Arlet et al., 2016). Ineffective erythropoiesis is the major cause of anemia in β-thalassemia patients. In contrast, a marked decrease in life span of circulating red cells, a feature of sickle red cells, is considered to be the major determinant of chronic anemia in SCD. It is generally surmised that ineffective erythropoiesis contributes little to anemia. The bone marrow environment has been well documented to be hypoxic (0.1 to 6% O2) (Mantel et al., 2015). As hypoxia induces HbS polymerization, we hypothesized that cell death may occur in vivo because of HbS polymer formation in the late stages of differentiation characterized by high intracellular hemoglobin concentration.

In the present study, using both in vitro and in vivo derived human erythroblasts we assessed the extent of ineffective erythropoiesis in SCD. We explored the mechanistic basis of the ineffective erythropoiesis in SCD using biochemical, cellular and imaging techniques.

In vitro erythroid differentiation using CD34+ cells isolated from SCD patients and from healthy donors was performed. A 2-phase erythroid differentiation protocol was used and cultures were performed at two different oxygen conditions, i.e. normoxia and partial hypoxia (5% O2). We found that hypoxia induces cell death of sickle erythroblasts starting at the polychromatic stage, positively selecting cells with high levels of fetal hemoglobin (HbF). This inference was supported by flow cytometry data showing higher percentages of dead cells within the non-F-cell population as compared to the F-cell population for SCD cells. Moreover, SCD dead cells showed higher levels of chaperon protein HSP70 in the cytoplasm than live cells, while no difference was detected between both subpopulations for control cells, suggesting that cell death of SCD erythroblasts was probably due to HSP70 cytoplasmic sequestration. This was supported by western-blot experiments showing less HSP70 in the nucleus of SCD erythroblasts under hypoxia, associated with decreased levels of GATA-1. At the molecular level, HSP70 was co-immunoprecipitated with HbS under hypoxia indicating that both proteins were in the same complex and suggesting interaction between HSP70 and HbS polymers in the cyotplasm. Importantly, we confirm these results in vivo by showing that in bone marrow of SCD patients (n = 5) cell loss occurs during terminal erythroid differentiation, with a significant drop in the cell count between the polychromatic and the orthochromatic stages (Figure 1).

In order to specifically address the role of HbF in cell survival, we used a CRISPR-Cas9 approach to mimic the effect of hereditary persistence of fetal hemoglobin (HPFH). CD34+ cells were transfected either with a gRNA targeting the LRF binding site (-197) or a gRNA targeting an unrelated locus (AAVS1) (Weber, Frati, et al. 2020). As expected, the disruption of the LRF binding site resulted in HbF induction as shown by higher %F-cells compared to AAVS1 control. These higher levels of F-cells resulted in decreased apoptosis, under both normoxic and hypoxic conditions, clearly demonstrating the positive and selective effect of HbF on SCD cell survival (Figure 2).

In summary, our study shows that HbF has a dual beneficial effect in SCD by conferring a preferential survival of F-cells in the circulation and by decreasing ineffective erythropoiesis. These findings thus bring new insights into the role of HbF in modulating clinical severity of anemia in SCD by both regulating red cell production and red cell destruction.

Disclosures: No relevant conflicts of interest to declare.

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