Background: Beta-thalassemia and sickle cell disease are genetic disorders caused by mutations in the beta-globin gene which lead to significant anemia and serious medical complications. Increases in fetal hemoglobin (HbF) have been linked to improved clinical outcomes in patients with beta-thalassemia (BT) and sickle cell disease (SCD).

Methods: We have developed engineered zinc finger nucleases (ZFNs) that precisely cleave and disrupt the erythroid enhancer of the BCL11A gene, which substantially boosts HbF production in erythroid progeny of genome-edited CD34+ hematopoietic stem/progenitor cells (HSPCs). The autologous modified HSPC drug product are named ST-400 (BT) and BIVV003 (SCD).

Results: We demonstrate that ST-400 / BIVV003 can be manufactured by reproducible, high-level, ZFN-driven modification in peripheral blood mobilized HSPCs at clinical production scale (>108 cells). The drug product is made in a GMP-compliant setting using a clinical-grade electroporation device to deliver the ZFN mRNAs ex vivo . Unbiased specificity studies of ST-400 / BIVV003 demonstrated an exquisite amount of specificity, with high levels of on-target modification (~80%). Erythroid colony genotyping in enhancer targeted cells, showed bi-allelic modification of the BCL11A erythroid enhancer in >50% of HSPCs, resulting in >4-fold higher levels of gamma globin mRNA and protein compared to controls. Similarly, we observed high levels of modification in research-scale preparations of HSPCs from patients with beta-thalassemia. Injection of ST-400 / BIVV003 into immune-deficient mice resulted in robust long-term (19 week) engraftment. Targeted gene modification was maintained through multi-lineage differentiation in the bone marrow and peripheral blood.

Conclusions: These results support further clinical development of ST-400 / BIVV003 as a potential therapy for beta-thalassemia and sickle cell disease.


No relevant conflicts of interest to declare.

Author notes


Asterisk with author names denotes non-ASH members.

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