A new gene-editing protocol has the potential to treat sickle cell disease (SCD) using corrected hematopoietic stem cells (HSCs) reimplanted into patients, as published in Science. Using the protocol, HSCs from a patient could be harvested, genetically corrected, and then reimplanted into the same patient.

During the study, CD34+ HSCs were collected from a patient with SCD before being corrected using an optimized Cas9 ribonucleic protein and single-stranded oligonucleotide donor before being xenografted into NOD.Cg-KitW−41JTyr+PrkdcscidIl2rgtm1Wjl/ThomJ mice.

Analysis of marrows collected from the mice following long-term engraftment revealed a correction in >20% of HBB alleles in hematopoietic cells. Some cells were heterozygous and it was ultimately found that about 30% of cells contained at least 1 corrected allele.

Read more about SCD pathophysiology

Based on previous studies involving allogeneic bone marrow transplants for SCD and β-thalassemia, there is a progressive enrichment of correctly functioning donor cells as they progress from early erythroid progenitor cells up to mature red blood cells. In these studies, the healthy donor cells made up between 10%-30% of bone marrow after transplantation and having as little as 20% donor cells resulted in donor red blood cells dominating in circulation.

“Clinical evidence suggests that a manufacturing protocol capable of achieving mono- or biallelic correction in as few as 20% of the autologous HSC is very likely to eliminate the signs and symptoms of SCD (and β-thalassemia),” the authors concluded.

“Based on this observation, we conclude that our manufacturing method exceeds this threshold for generating a significant clinical benefit, and is suitable for testing in a clinical trial.”

The authors cautioned about the potential of the nonhomologous end-joining repair leading to the null HBB genes and effectively creating β-thalassemia cells. The risk of this is mitigated by the fact that β-thalassemia is recessive to the corrected alleles and cells which are heterozygous with a corrected and null allele would function as normal erythrocytes.

Reference

Magis W, DeWitt MA, Wyman SK, et al. High-level correction of the sickle mutation is amplified in vivo during erythroid differentiation. iScience. 2022;25(6):104374. doi:10.1016/j.isci.2022.104374