CRISPR-Cas9 editing of BCL11A erythroid enhancer as a variant-agnostic strategy for HBB c.92+5G>C beta-thalassemia

Exagamglogene autotemcel (Casgevy), the first CRISPR-Cas9-based therapy approved by the FDA (December 2023) and EMA, targets the BCL11A erythroid enhancer to reactivate fetal hemoglobin (HbF) production. This approach is mechanistically variant-agnostic: rather than correcting the specific HBB splice site mutation c.92+5G>C, it compensates by upregulating gamma-globin expression. Clinical data show transfusion independence in a high proportion of treated beta-thalassemia patients.

The pivotal CLIMB THAL-111 trial demonstrated that exagamglogene autotemcel achieved transfusion independence in 29 of 32 evaluable patients (90.6%) with transfusion-dependent beta-thalassemia at a median follow-up of approximately 20 months. Total hemoglobin levels rose to >11 g/dL in most responders, with HbF constituting >30% of total hemoglobin. The c.92+5G>C splice site variant disrupts the intron 1 donor site of HBB, leading to aberrant splicing and reduced or absent beta-globin chain production. Since Casgevy bypasses the defective HBB gene entirely by derepressing gamma-globin via BCL11A disruption, the specific HBB variant does not affect therapeutic mechanism. A 2026 review (PMID: 41919270) and CRISPR research trend analysis (PMID: 41787916) confirm the growing evidence base supporting this approach. Process standardization efforts (PMID: 41352680) using FMEA methodology are addressing manufacturing consistency.

Treatment requires myeloablative busulfan conditioning, carrying risks of infertility, veno-occlusive disease, and secondary malignancies. Long-term durability beyond 3-4 years is still being evaluated. Off-target CRISPR editing remains a theoretical concern despite no clinically significant off-target events reported to date. The autologous ex vivo approach requires viable CD34+ stem cell collection, which may be challenging in heavily transfused patients with iron overload. Cost is estimated at $2.2 million per patient, and manufacturing complexity limits scalability. Variant-specific efficacy data for c.92+5G>C carriers within the trial cohort have not been separately reported.