On December 8, 2023, the FDA approved the first CRISPR-based genomic editing medicine for clinical use in humans: exagamglogene autotemcel (Casgevy, BioNTech/CRISPR Therapeutics) for sickle cell disease and transfusion-dependent beta-thalassemia. This historic approval, followed by EMA approval in February 2024, marked the beginning of CRISPR's therapeutic era.
Hemoglobinopathies: a global problem
Sickle cell disease (SCD) and beta-thalassemia are the most prevalent monogenic diseases worldwide, with 7 million people affected and 300,000 new cases annually. Both are caused by mutations in the beta-globin gene (HBB) on chromosome 11. In SCD, the E6V mutation generates hemoglobin S (HbS), which polymerizes under deoxygenation conditions, producing falciform deformation of erythrocytes and vascular occlusion.
Casgevy's CRISPR approach: fetal hemoglobin reactivation
Casgevy's strategy doesn't directly correct the causal mutation but rather reactivates fetal hemoglobin (HbF, γ-globin) production, which is functionally normal and silenced in adults by the BCL11A transcriptional repressor. CRISPR-Cas9 edits the BCL11A gene specifically in erythroid cells, de-repressing HbF production.
In the pivotal CLIMB-SCD-121 trial (n=45), 93.5% of patients had no severe vaso-occlusive crisis in the 12 months following transplant. In CLIMB-THAL-111 (n=54), 93% achieved transfusion independence. 24-month follow-up data presented at ASH 2024 confirm results durability.
Comparison with allogeneic stem cell transplant
Allogeneic hematopoietic stem cell transplantation (allo-HSCT) with HLA-identical donor remains curative in SCD and beta-thalassemia with 85-95% success rates in children. However, access to a compatible donor is limited (fewer than 25% of patients have an HLA-identical sibling), and graft-versus-host disease (GVHD) is the most serious complication. Autologous gene therapy avoids GVHD risk and requires no donor, but the cost of Casgevy ($2.2 million per patient) is the most relevant access barrier.
The horizon: in vivo editing with LNPs
The next frontier is to avoid the ex vivo process by delivering the CRISPR-Cas9 complex directly in vivo via lipid nanoparticles (LNPs) or AAV vectors targeted to hematopoietic stem cells. If efficient in vivo HSC editing is achieved, the process could be simplified to an IV infusion without cell extraction.
