Sickle cell disease (SCD)

Sickle cell disease (SCD) is an inherited blood disorder caused by abnormal hemoglobin, the protein in red blood cells that carries oxygen to tissues in the body. While red blood cells are normally smooth, disk-shaped and flexible, SCD red blood cells are stiff and sticky and form a sickle or crescent shape. In SCD, red blood cells stick together, which can block small blood vessels, hinder the movement of oxygen-carrying blood, and cause pain. Symptoms of SCD include anemia, pain, acute chest syndrome, a damaged spleen, stroke, jaundice, and priapism.

SCD is an autosomal recessive genetic disorder, meaning that the disease manifests in a person who inherits two copies of a mutated gene, one from each parent. Approximately 100,000 people in the United States are affected by the disease and millions are affected worldwide. People that carry one copy of the mutation have sickle cell trait (SCT) do not usually have symptoms of SCD. The term SCD is used collectively for all mutations in the β-globin gene that cause the same clinical syndrome. Sickle cell anemia is the most common form of SCD, which results from a homozygous mutation termed the beta-S (β^S) allele located on chromosome 11p15.5 and makes up 70% of SCD cases in patients of African ethnicity. In sickle cell anemia, a different nucleotide residue in the genetic code at a single position changes the sequence from GAG (normal β^A allele) to GTG (β^S), which translates to a change in the protein sequence substituting glutamic acid (Glu) to valine (Val). Valine is hydrophobic (repelled by water), which changes the behavior of this version of hemoglobin in the body. The normal adult hemoglobin (HbA) is a tetramer protein and comprised of two alpha polypeptide chains and two beta polypeptide chains. The red blood cells or erythrocytes of individuals with SCD have the two β-subunits of the hemoglobin (Hb) tetramer mutated, and this is referred to as sickle hemoglobin or HbS. Other mutations that cause different forms of SCD include coinheritance of the β^S mutation, with other mutations such as β^C (HbSC), β^D (HbSD), β^O (HbSO/Arab), β^E (HbSE), or a β-thalassemia allele (HbS/β-thal⁰ or HbS/β-thal⁺).

The three main pathobiological processes are hemoglobin tetramer HbS polymerization, vaso-occlusion, and hemolysis-mediated endothelial dysfunction. A fourth disease pathway is sterile inflammation.

HbS polymerize and form bundles when oxygen is released. Upon oxygen release, the hydrophobic valine residue becomes exposed in the watery cellular environment and avoids water by sticking to a hydrophobic area of a nearby hemoglobin molecule. This leads to bundles of polymerized hemoglobin that deform the shape of the red blood cell. Upon returning to the lungs to pick up oxygen, the red blood cells return to their normal shape; but over time, repeated distortions cause permanent changes to the cell membrane creating the sickle shape. Abnormally shaped cells accumulate and aggregate with other types of blood cells in deoxygenated areas of the body, such as capillaries and veins where they obstruct blood flow and damage organs and tissues.

HbS polymerizing causes blood to stop flowing in some areas, which is called vaso-occlusion. Vaso-occlusion promotes ischemia-reperfusion injury, since restored blood flow to tissues previously lacking oxygen (ischaemic) paradoxically causes cellular dysfunction and tissue damage.

Hemolysis or lysis of red blood cells is promoted by polymer bundles and this releases cell-free hemoglobin into blood circulation. Oxygenated hemoglobin damages cells that line the heart and blood vessels, called endothelial cells, through chemical reactions that result in the depletion of endothelial nitric oxide (NO) reserves. Endothelial cells produce nitric oxide gas, which functions to maintain vascular homeostasis through regulation of vascular dilation and local cell growth as well as protection of blood vessels from the wear and tear caused by circulating platelets and blood cells.

The release of cell-free hemoglobin upon hemolysis reacts to form methemoglobin (Fe³⁺), which releases cell-free heme, a damage-associated molecular pattern (DAMP) in red blood cells. DAMPs are intracellular factors normally hidden from the host immune system that are released into the extracellular environment due to cell necrosis and trigger inflammation. This is termed sterile inflammation as it occurs in the absence of microorganisms.

Red blood cell transfusions are used in acute and chronic management of SCD complications.

Allogeneic hematopoietic stem cell (HSC) transplantation, where bone marrow cells are replaced with bone marrow cells from a healthy individual that produce red blood cells with normal hemoglobin, provides a curative option for SCD. HSC transplantation has a high success rate; however, it requires a suitably matched sibling donor, which is not available for more than 80% of candidates. Patients have a risk of graft rejection, graft versus host disease, and transplant-related mortality. Before receiving transplanted cells, patients may take drugs to decrease their own bone marrow cells, called myeloablative conditioning. Nonmyeloablative conditioning regimens may alternatively be used, which can result in the recipient carrying a stable mixture of their cells and donor-derived blood cells.

An option that allows SCD patients to use their own cells for cell therapy is the gene editing or introduction of a gene into a patient's SCD cells so they produce functional hemoglobin and put them back into the patient.

Targeting a BCL11A enhancer with CRISPR-Cas9 was shown to enable the fetal hemoglobin gene to produce γ-globin, which it is normally downregulated after birth. Gene editing components were introduced into patient CD34+ hematopoietic stem and progenitor cells (HSPCs) by electroporation and returned to the patient by intravenous infusion. The cell therapy approach called CTX001 was sponsored by CRISPR Therapeutics and Vertex Pharmaceuticals is part of the CLIMB SCD-121 clinical trials.

BCL11A mRNA was targeted and knocked down by short hairpin RNA (shRNA) embedded in microRNA (shmiR) and introduced by lentiviral vector into autologous CD34+ cells. Knocking down the BCL11A repressor of γ-globin allowed this form of hemoglobin to be produced in the adult erythrocytes. This clinical study was funded by the NIH and sponsored by David Williams at Boston Children’s Hospital.

Novartis in collaboration with Intellia Therapeutics has two genome-edited autologous HSPC products in clinical trials, OTQ923 and HIX763 that repress activity of BCL11A using CRISPR-Cas9 resulting in increased production of fetal hemoglobin.

An anti-sickling form of gamma globin gene is introduced into SCD patient CD34+ cells for autologous cell therapy, a treatment called ARU-1801 and developed by Aruvant, is under clinical evaluation.

An anti-sickling form of gamma globin (γ^G16D) is introduced into patient CD34+ cells in the product CSL200 by CSL Behring.

Drug candidate BIVV003 developed by Sangamo Therapeutics and Sanofi is composed of autologous CD34+ SCD patient cells transfected with zinc finger nuclease mRNAs SB-mRENH1 and SB-mRENH2 which target the BCL11A locus to increase production of fetal hemoglobin (HbF).

Bluebird Bio, co-lead by John Tisdale of NIH, is conducting clinical studies for the approach of introducing the adult hemoglobin form of β-globin that reduces polymerization of the sickling form (anti-sickling) introduced by lentiviral vector in ex vivo gene therapy of autologous CD34+ cells. The LentiGlobin drug product contains the genetic change designated β^A-T87Q (Threonine at codon 87 replaced by Glutamine) that confers the anti-sickling effect. In 2019, the Bluebird treatment was approved in Europe for certain patients with the blood disorder beta-thalassemia. In February, 2021 clinical studies were halted after two participants developed leukemia-like cancer. The European Medicines Agency (EMA) safety committee, Pharmacovigilance Risk Assessment Committee (PRAC) reviewed the two cases of acute myelogenous leukemia (AML) in patients treated with LentiGlobin for SCD and found that the viral vector was unlikely to be the cause. PRAC concluded that the AML was more likely caused by conditioning treatment used to clear out bone marrow cells in preparation for therapy or that individuals living with SCD are at increased risk for blood cancer.

The drug product DREPAGLOBE containing the anti-sickling beta globin product β^AS3 introduced with the GLOBE1 lentiviral vector into CD34+ cells is in clinical trials investigated by Pablo Bartulocci and colleagues in France.

Beam Therapeutics developed a CRISPR base editing tool, a form of CRISPR editing that does not introduce double strand breaks, which is being tested in mouse models for treatment of SCD. The adenine base editor tool called BEAM-102 edits the sickle hemoglobin point mutation that results in the HbS protein and recreates HbG-Makassar, a naturally occurring normal hemoglobin variant that shows that same function as the wild-type HbA version.

Novartis, with funding from the Bill and Melinda Gates Foundation, has stated that it is developing in vivo gene therapy for SCD. In vivo gene therapy delivers the gene directly into the patient, rather than into removed cells from the patient as is done for ex vivo gene therapy.