Normal and Pathological Red Cell Physiology

Sickle Cell Disease (SCD) is a genetic hemoglobinopathy resulting from a unique mutation in the β-globin gene and characterized by hemolytic anemia, painful vaso-occlusive crises and progressive organ failure. Although a rare disease, SCD represents the first genetic disease in French metropolitan areas, with an estimation of 20,000 patients in 2020. In the French West Indies, the carrier frequency reaches around 12%^1,2. About 300,000 affected children are born every year with SCD. The global burden of disease study 2010 (GBD 2010) reported that the years lived with disability (YLDs) for hemoglobinopathies and SCD is 10197, which is a dramatic observation since the YLDs for cardiovascular disorders is 21985.

Blood rheological alterations, chronic vascular inflammation, abnormal adhesion and vascular dysfunction play important roles in the occurrence of SCD complications. However, it is still not clear how these abnormal processes interact to modulate the variable phenotypic expression of SCD and the genesis of vaso occlusive crisis. Furthermore, although RBC dysfunction is the major contributor to disease development and progression, other cell types that are not affected by the genetic mutation (endothelial cells, leukocytes, and platelets) represent key actors in the pathophysiology of SCD

The main goal of our project was to decipher the molecular events leading to vaso occlusion events and chronic complications, in order to develop new therapies. Studies were performed in Paris and in Pointe-à-Pitre and benefited from cohorts of patients that were constituted thanks to our strong collaboration with clinicians in Ile de France (Necker, European Georges Pompidou, Robert Debré hospitals) and in Guadeloupe (CHU Pointe à Pitre).

· Some aims in the SWG1 physiopathology of Sickle Cell Disease

• Exploring disease variability in sickle cell disease

SCD phenotype is highly variable among patients, with a wide spectrum from asymptomatic/mild to severe cases. Different factors have been considered as modulators of SCD such as age, co-inheritance of α-thalassemia, haplotypes and fetal hemoglobin (HbF) levels. We address:

Identification of early predictors in SCD to allow personalized medicine

Treatment intensifications are so far mostly proposed to symptomatic patients. It has been proposed to systematize them to all patients, with a risk of severe side effects occurring in potentially mild forms. We will address the variability among SCD patients in order to stratify risk factors. We will apply an approach combining the use of a microfluidic device with a panoply of tests (using The Red Cell Clinic)

Genetic modifiers of vascular complications in sub-Saharan Africa

The project aims at understanding SCD chronic vascular complications. It is a transversal case control study, nested in the CADRE cohort, built up by Xavier Jouven and African collaborators. CADRE is the largest ongoing epidemiological cohort, including 4,300 SCD patients and 1,000 controls in five west and central African countries. We tested novel markers and modifiers of hemolysis and heme metabolism and assess their relationship with inflammation and clinical vascular phenotypes in a unique genetic and epigenetic “african context”.

• Erythropoiesis in sickle cell disease

Chronic anemia contributes to stress erythropoiesis in SCD, which is reflected by HbF expression in young reticulocytes exiting the bone marrow. Increased production of reactive oxygen species (ROS) may exacerbate the chronic anemia by worsening intravascular hemolysis³⁸. The recent discovery of the abnormal presence of mitochondria in the mature RBCs of sickle cell patients³⁹ suggests mitophagy defects during erythroid differentiation.

We address the impact of hemolysis on stress erythropoiesis and potential defects in mitophagy that may explain the retention of mitochondria in the RBCs. Presence of mitochondria in mature RBCs will be assessed in a cohort of patients from Pointe-à-Pitre and correlate with markers of oxidative stress and hemolysis.

We investigate the role of hypoxia in erythropoiesis by focusing on the impact of HbS polymers during the terminal stage on apoptosis, enucleation and organelle sorting.

• Inflammation in sickle cell disease