Mitchell J. Weiss

Abstract Sickle-cell disease (SCD) is caused by an A·T-to-T·A transversion mutation in the β -globin gene ( HBB ). Here we show that prime editing can correct the SCD allele ( HBB S ) to wild type ( HBB A ) at frequencies of 15%–41% in haematopoietic stem and progenitor cells (HSPCs) from patients with SCD. Seventeen weeks after transplantation into immunodeficient mice, prime-edited SCD HSPCs maintained HBB A levels and displayed engraftment frequencies, haematopoietic differentiation and lineage maturation similar to those of unedited HSPCs from healthy donors. An average of 42% of human erythroblasts and reticulocytes isolated 17 weeks after transplantation of prime-edited HSPCs from four SCD patient donors expressed HBB A , exceeding the levels predicted for therapeutic benefit. HSPC-derived erythrocytes carried less sickle haemoglobin, contained HBB A -derived adult haemoglobin at 28%–43% of normal levels and resisted hypoxia-induced sickling. Minimal off-target editing was detected at over 100 sites nominated experimentally via unbiased genome-wide analysis. Our findings support the feasibility of a one-time prime editing SCD treatment that corrects HBB S to HBB A , does not require any viral or non-viral DNA template and minimizes undesired consequences of DNA double-strand breaks.

Globin proteins exist in every cell type of the vasculature, from erythrocytes to endothelial cells, vascular smooth muscle cells, and peripheral nerve cells. Many globin subtypes are also expressed in muscle tissues (including cardiac and skeletal muscle), in other organ-specific cell types, and in cells of the central nervous system (CNS). The ability of each of these globins to interact with molecular oxygen (O 2 ) and nitric oxide (NO) is preserved across these contexts. Endothelial α-globin is an example of extraerythrocytic globin expression. Other globins, including myoglobin, cytoglobin, and neuroglobin, are observed in other vascular tissues. Myoglobin is observed primarily in skeletal muscle and smooth muscle cells surrounding the aorta or other large arteries. Cytoglobin is found in vascular smooth muscle but can also be expressed in nonvascular cell types, especially in oxidative stress conditions after ischemic insult. Neuroglobin was first observed in neuronal cells, and its expression appears to be restricted mainly to the CNS and the peripheral nervous system. Brain and CNS neurons expressing neuroglobin are positioned close to many arteries within the brain parenchyma and can control smooth muscle contraction and thus tissue perfusion and vascular reactivity. Overall, reactions between NO and globin heme iron contribute to vascular homeostasis by regulating vasodilatory NO signals and scavenging reactive species in cells of the mammalian vascular system. Here, we discuss how globin proteins affect vascular physiology, with a focus on NO biology, and offer perspectives for future study of these functions.

Resistance artery vasodilation in response to hypoxia is essential for matching tissue oxygen and demand. In hypoxia, erythrocytic hemoglobin tetramers produce nitric oxide through nitrite reduction. We hypothesized that the alpha subunit of hemoglobin expressed in endothelium also facilitates nitrite reduction proximal to smooth muscle. Here, we create two mouse strains to test this: an endothelial-specific alpha globin knockout (EC Hba1Δ/Δ) and another with an alpha globin allele mutated to prevent alpha globin's inhibitory interaction with endothelial nitric oxide synthase (Hba1WT/Δ36-39). The EC Hba1Δ/Δ mice had significantly decreased exercise capacity and intracellular nitrite consumption in hypoxic conditions, an effect absent in Hba1WT/Δ36-39 mice. Hypoxia-induced vasodilation is significantly decreased in arteries from EC Hba1Δ/Δ, but not Hba1WT/Δ36-39 mice. Hypoxia also does not lower blood pressure in EC Hba1Δ/Δ mice. We conclude the presence of alpha globin in resistance artery endothelium acts as a nitrite reductase providing local nitric oxide in response to hypoxia.

ABSTRACT: Genetic depletion of the transcriptional repressor BCL11A in red blood cell precursors alleviates β-hemoglobinopathies by inducing the fetal γ-globin genes. However, additional erythroid genes are regulated by BCL11A and the effects of its deficiency on erythropoiesis are insufficiently described. We discovered that Cas9 disruption of the BCL11A intron 2 erythroid enhancer in CD34+ hematopoietic stem and progenitor cells using a clinically approved strategy caused impaired expansion and apoptosis of erythroid precursors in vitro and reduced repopulation of the erythroid compartment after xenotransplantation into immunodeficient mice. Mutant colony-forming unit erythroid cells, proerythroblasts, and basophilic erythroblasts exhibited dysregulation of 94 genes (more than twofold change, false discovery rate < 0.05), 25 of which are likely direct targets of BCL11A. Differentially expressed genes were associated with a range of biological pathways that affect cell expansion and survival. Our findings reveal that BCL11A regulates additional aspects of erythropoiesis beyond γ-globin gene repression, with unknown clinical consequences.