BCL11A enhancer editing for transfusion-dependent beta-thalassemia: exa-cel CLIMB-111 results

CRISPR-Cas9-mediated BCL11A erythroid enhancer disruption in autologous CD34+ HSPCs (exa-cel/Casgevy) has demonstrated transfusion independence in the majority of treated transfusion-dependent beta-thalassemia (TDT) patients. The approach reactivates fetal hemoglobin to compensate for deficient beta-globin, making it effective across all HBB genotypes including splice variants such as c.92+5G>C (IVS-I position +5).

The CLIMB-111 pivotal trial (NCT03655678, completed) enrolled TDT patients requiring at least 100 mL/kg/year of packed RBCs. Published NEJM data (PMID:38661449) reported that 32 of 35 evaluable patients achieved transfusion independence at 12+ months with total hemoglobin consistently above 11 g/dL, sustained by high HbF fractions (mean approximately 40 percent). The c.92+5G>C variant (ClinVar VCV000015447) is a pathogenic splice donor variant at the IVS-I +5 position causing aberrant splicing and severely reduced beta-globin mRNA — one of the most common beta-thalassemia mutations in Mediterranean populations. FDA approved exa-cel for TDT in January 2024. A pediatric extension study (NCT05356195) is active. A separate CRISPR-Cas12b approach targeting HBG1/HBG2 promoters has also shown promise (NCT06040620).

Myeloablative busulfan conditioning is required with associated gonadotoxicity, mucositis, and hepatic sinusoidal obstruction syndrome risk. Patients with chronic iron overload from years of transfusion therapy require continued chelation even after achieving transfusion independence. The c.92+5G>C variant is common in Mediterranean and Middle Eastern populations where access to the complex ex vivo manufacturing process (apheresis, CD34+ selection, electroporation, cryopreservation) may be limited. Compound heterozygotes carrying this splice variant plus a different severe HBB allele may respond differently. Long-term follow-up beyond 3 years remains limited, and the durability of edited HSC engraftment in myeloablated marrow niches needs further characterization.