Installation of HbG-Makassar by base editing restores hemoglobin function: a transformative therapy for sickle cell disease

Adenine base editing offers a viable gene-based therapy for sickle cell disease (SCD), converting sickle hemoglobin (HbS, βΕ6V) to G-Makassar hemoglobin (HbG, βE6A), a naturally occurring, non-pathogenic variant. However, HbG functionality alone and with HbS has been largely uncharacterized. We present a mouse model used to characterize purified HbG-Makassar as well as HbGG and HbGS red blood cell function. Purified HbG-Makassar behaves as a functional hemoglobin, including no polymerization under hypoxia. Structural characterization of oxy and deoxy states of HbG-Makassar showed no change in the topology of the hemoglobin fold with the βΕ6Α mutation. Red blood cell function assays, sickling propensity under hypoxia, blood counts, and mitochondrial retention measures place HbGS RBCs as intermediate in severity between HbAS and HbSS, organ function was comparable to HbAS. HbGG resembled HbAA for most metrics. Taken together our results suggest direct correction of HbS to HbG-Makassar could provide a transformative therapy for SCD.