Studying the Determinant of Sickle Cell Disease Vasculopathy in Sub-Saharan Africa : The Biocadre Study

Although most individuals with sickle cell disease (SCD) live in sub-Saharan Africa, the history of the disease in this continent remains largely unknown. SCD is characterized by chronic hemolytic anemia, acute-vaso occlusive events and progressive vascular organ damage. The CADRE study is a large cohort of SCD patients in five countries of West and Central Africa aiming at studying SCD-related vascular complications. The inclusion data of this study did not match the hyper-hemolysis paradigm (Dubert et al. Blood 2018), but several methodological limitations were raised, including probable mortality bias and questionable reliability of classical hemolysis markers in Africa. For the 5-year follow-up of the CADRE study, we designed a case-control study nested in the cohort, based on extreme phenotypes, to look for new markers of vasculopathy, including new markers of hemolysis.

Patients and Methods

SS adult patients of the CADRE cohort were selected in the centres of Dakar (Senegal) and Bamako (Mali), depending on the presence of none or at least one of the following complications at inclusion: tricuspid regurgitant jet velocity (TRJV)>2,5 m/s (which may indicate pulmonary hypertension), albuminuria/creatininuria>100 mg/g, leg ulcer, priapism, aseptic osteonecrosis and retinopathy. We chose the youngest patients with a vascular complication and the oldest without any complication. Overall, 6 groups of 40 SS patients with extreme phenotypes were constituted (20 in each centre). Patients were called for a specific visit and investigated at steady state. Besides clinical examination, usual laboratory blood tests and albuminuria measure, additional plasma and saliva samples were collected. A trained investigator isolated microparticles immediately after blood sampling by successive centrifugations, measured blood and plasma viscosities and assessed microcirculation function using peripheral arterial tonometry. A cardio-echography was performed by a trained cardiologist. Plasma samples were stored at -80 °C and shipped to Paris. High technology tests were performed in Paris, including blood cell derived microparticles, free hemoglobin, inflammatory cytokines, neutrophile extracellular trap (NETs). Using saliva DNA, we also genotyped the known SCD genetic modifiers and new candidate genes implicated in the catabolism of hemoglobin. Potential associations between those markers, usual hematological parameters, and the vascular complications were assessed statistically .

Results

We recalled 240 selected patients 5 years after their first visit: 38 could not be retrieved, 21 had deceased, 62 had at least one new complication, and only 15 still had no complication. Therefore, we selected 56 more patients to obtain at least 30 patients in each group. 237 SS adults were eventually investigated and the plasma samples of 232 SS patients were analyzable in Paris (106 from Bamako and 126 from Dakar). In these patients, at a mean age of 29 years (+/- 11), high TRJV was present in 58, macroalbuminuria in 33, leg ulcers in 36, priapism in 43, aseptic osteonecrosis in 45 and retinopathy in 31 whereas 28 had no vascular complication. A principal component analysis found no cluster of complications. Among patients with one "hyper-viscous" complication (retinopathy or osteonecrosis) or more, 78% also had at least one "hyper-haemolytic" complication (high TRJV, albuminuria, leg ulcer or priapism) and 49% of patients with a "hyper-haemolytic" complication also had a "hyper-viscous" complication. The microvascular function was not associated with any of the complications, whereas higher blood viscosity was associated with retinopathy. The results of the associations between the vascular complications and specific biological tests are presented in other publications.

Conclusion

This study illustrates the feasibility of high-technology experiments in SCD patients living in sub-Saharan Africa, but was particularly challenging because of the difficulty to prepare, store and transport frozen plasma samples. Moreover, in agreement with previously published data from the CADRE study, we found that the dichotomization of vascular complications into hyperhemolytic and hyperviscous subgroups is not clinically relevant in Africa. Other simple predictive markers of vascular complication are needed to optimize the follow-up of African patients with SCD.