Identification of Biomarkers That Are Associated with Clinical Complications of Hemoglobin SC Disease and Sickle Cell Anemia

Introduction: Sickle cell disease (SCD) is an umbrella term used to describe inherited anemias that originate from a mutation in HBB, the gene coding for β-globin, that causes the formation of the abnormal hemoglobin S (HbS). The two most common genotypes are HbSS, termed sickle cell anemia (SCA) and HbSC, termed hemoglobin SC disease. Due to the higher prevalence and more severe clinical symptoms in SCA, attention is focused on this latter group of patients. Patients with HbSC disease who report significant SCD related symptoms are treated largely the same as SCA patients, despite differences in disease pathophysiology. Objective: To evaluate rheological differences in red blood cells (RBC) and whole blood from individuals with SCA and HbSC, and examine for associations with clinical complications. Methods: We analyzed an adult (n=30) and pediatric cohort (n=226) of SCA and HbSC individuals. Blood samples were evaluated for percent dense red blood cells (% DRBC), whole blood viscosity, and several ektacytometry-derived parameters, such as the cell membrane stability test, osmotic gradient ektacytometry and Point of Sickling (PoS): an oxygen gradient ektacytometry-derived biomarker that depicts the oxygen tension at which sickling is initiated. Subsequently, we assessed in the pediatric cohort (189 SCA and 37 HbSC individuals) if PoS, dense RBCs or blood viscosity are associated with clinical complications. Results: In particular (micro)dense RBCs (Figure 1A and B), blood viscosity (Figure 1C) and point of sickling (PoS, Figure 1D-F), are notably different in HbSC disease compared to SCA. In SCA, PoS was associated with occurrence of acute chest syndrome (ACS, for every 10mmHg increase OR 1.84, p<0.0001, adjusted OR 1.67, p=0.002), but no such association was found in children with HbSC. In contrast, we found an association of blood viscosity and ACS (OR 9.3, p=0.012; adjusted OR 13.5, p=0.015) in HbSC children, while we found no such association in SCA. No significant association of micro dense RBCs was found in SCA or HbSC children. Conclusion: We found that associations between PoS and ACS found in SCA were not found in HbSC. Instead, in HbSC ACS was associated with higher whole blood viscosity. We therefore conclude that sickling is a key factor in the pathophysiology of SCA, whereas in HbSC disease blood viscosity might play a crucial role in development of certain complications like ACS. This study suggests that complications in SCA and HbSC disease are driven by different aspects of blood abnormalities, and may warrant a distinct treatment approach. Figure 1 Figure 1. Disclosures Rab: Axcella Health: Research Funding; Agios Pharmaceuticals: Research Funding. Schutgens: Shire/Takeda: Research Funding; Pfizer: Research Funding; OctaPharma: Research Funding; Novo Nordisk: Research Funding; CSL Behring: Research Funding; Bayer: Research Funding. Wijk: Axcella health: Research Funding; Global Blood Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Agios Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding. Van Beers: Agios Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Research Funding; RR Mechatronics: Research Funding; Pfizer: Research Funding. Sheehan: Beam Therapeutics: Research Funding; Novartis: Research Funding; Forma Therapeutics: Research Funding.

Revisiting Spleen Function and Pneumococcal Risk in Children with Hemoglobin SC Disease

Spleen dysfunction and susceptibility to pneumococcal infection is a well known feature in homozygous sickle cell disease (HbSS), whilst to date splenic function in hemoglobin SC disease (HbSC) has been poorly investigated. The aim of this study was to analyze spleen function in children with HbSC disease using a high-throughput validated method (1) and to examine if the current recommendations regarding pneumococcal risk are appropriate in this population. Spleen function was evaluated using a flow cytometry quantification of red blood cells (RBCs) with Howell-Jolly bodies (HJBs), in a cross-sectional study of patients at steady state during an outpatient visit in an expert center. Quantification of HJB-RBCs was performed in children with HbSC disease aged < 10 years and compared to children with HbSS disease or healthy children of the same age groups, or splenectomized children. Additional exploratory analysis was performed according to age (under or above the age of 5 years old) and treatment group (hydroxyurea). The median (Q1-Q3) HJB-RBCs count was 16 (11-28.25) /100.000 RBCs in 40 HbSC children (Figure 1). This result was not statistically different from the control group of 22 healthy children (p=0.96) nor in subgroups < or ≥ 5 years old, indicating that children with HbSC under 10 years have a preserved splenic function. Expectedly, the HJB-RBCs counts differed significantly from splenectomized children (419 (296-489)/100.000 RBCs, n=15, p<0.0001). By contrast, among the 53 HbSS children, the median HJB-RBCs count was 134 (29-216) /100.000 RBCs, differing significantly from HbSC children (p<0.0001). In HbSS children, HJB-RBCs counts increased significantly with age (r=0.30, p=0.03), showing important variability among subjects but did not reach the level found in splenectomized patients suggesting that complete loss of spleen function occurs presumably later in a majority of children in this population. Treatment with hydroxyurea did not significantly impact HJB-RBCs counts in a subgroup analysis in HbSS children. The result of this study suggests that spleen function in children under 10 years old with HbSC is not altered. The routine administration of prophylactic penicillin to young children with SC disease may therefore be questioned. Similarly, fever in children with HbSC under 3 years old may not require parenteral antibiotics as it is generally currently recommended by analogy to children with HbSS. Functional or anatomical asplenia in children with HbSC is delayed compared to those with HbSS at least after the first decade of life. Future large cohort studies using similar methodology will allow better evaluation of the pneumococcal risk in adolescents and adults with Hb SC disease. Bibliography (1) El Hoss S, Dussiot M, Renaud O, Brousse V, El Nemer W. A novel non-invasive method to measure splenic filtration function in humans. Haematologica. oct 2018;103(10):e436-9. Figure 1 Figure 1. Disclosures El Nemer: Hemanext: Consultancy.