A Novel Mouse Model of Hemoglobin SC Disease Reveals Mechanisms Underlying Beneficial Effects of Hydroxyurea

• Hemoglobin-SC disease is the second most common form of sickle cell disease, which has largely been excluded from research and therapies.

• We developed humanized hemoglobin-SC mice that help identify the mechanism of hydroxyurea benefit in hemoglobin-SC disease.

Sickle cell hemoglobin-C (HbSC) disease results from compound heterozygosity of hemoglobin-S (HbS) and hemoglobin-C (HbC), comprising 30% of sickle cell disease (SCD). HbC induces RBC dehydration/xerocytosis, which promotes sickling. HbSC-SCD causes significant morbidity despite being milder than homozygous HbSS-SCD. Current research/treatment strategies have focused on HbSS-SCD, while HbSC patients are deprived of disease-modifying/transformative therapies due to lack of preclinical models. We generated HbSC mice, which resemble human HbSC-SCD: HbSC erythrocytes showed marked xerocytosis. Anemia, hemolysis, inflammation and organ damage were milder than HbSS mice, but hypoxia/reperfusion injury was similar. Retinopathy developed at higher frequency than HbSS mice (66.7% versus 16.7%; p<0.05), like HbSC-SCD patients. While HbSC RBCs sickled at lower pO2 than HbSS RBCs (43mmHg versus 24mmHg; P<0.0001), they did not completely recover deformability after hypoxia-reoxygenation. Using the HbSC mice developed herein, we studied the mechanism by which hydroxyurea causes significant clinical benefit in HbSC-SCD patients, despite minimal/modest increases in fetal-hemoglobin (HbF). We found hydroxyurea had distinct non-HbF and HbF effects. Hydroxyurea did not increase HbF in adult HbSC/HbSS mice, but reduced RBC ROS, Ferryl-hemoglobin, Heinz-body formation, thereby reducing membrane damage; however, RBC hydration was unaffected. When given to unborn pups before they switch off g-globin expression and continued postnatally, we could induce HbF in both HbSC/HbSS mice (higher HbF in HbSS versus HbSC mice). Minimal increases in HbF (~1%) improved HbSC RBC hydration. Peak HbF levels of 7% in HbSC mice abrogated sickling. Overall, this HbSC model will help bridge the knowledge-gap in mechanistic/therapeutic studies in this neglected disease.