Sickle cell disease (SCD) is an inherited blood disorder associated with a debilitating chronic illness. SCD is caused by a point mutation in the β-globin gene (HBB). A single nucleotide substitution converts glutamic acid to a valine that leads to the production of sickle hemoglobin (HbS), which impairs the function of red blood cells. Here we show that delivery of Streptococcus pyogenes (Sp) Cas9 protein and CRISPR guide RNA as a ribonucleoprotein complex (RNP) together with a short single-stranded DNA donor (ssODN) template into CD34+ hematopoietic stem and progenitor cells (HSPCs) from SCD patients' bone marrow (BM) was able to correct the sickling HBB mutation, with up to 33% homology directed repair (HDR) without selection. Further, CRISPR/Cas9 cutting of HBB in SCD HSPCs induced gene conversion between the HBB sequences in the vicinity of the target locus and the homologous region in δ-globin gene (HBD), with up to 4.4% additional gene correction mediated by the HBD conversion in cells with Cas9 cutting only. The erythrocytes derived from gene-edited cells showed a marked reduction of the HbS level, increased expression of normal adult hemoglobin (HbA), and a complete loss of cell sickling, demonstrating the potential in curing SCD.

We performed extensive off-target analysis of gene-edited SCD HSPCs using the in-silico prediction tool COSMID and unbiased, genome-wide assay Guide-Seq, revealing a gross intrachromosomal rearrangement event between the on- and off-target Cas9 cutting sites. We used a droplet digital PCR assay to quantify deletion and inversion events from Day 2 to Day 12 after RNP delivery, and found that large chromosomal deletion decreased from 1.8% to 0.2%, while chromosomal inversion maintained at 3.3%. We demonstrated that the use of high-fidelity SpCas9 (HiFi Cas9 by IDT) significantly reduced off-target effects and completely eliminated the intrachromosome rearrangement events, while maintaining the same level of on-target gene editing, leading to high-efficiency gene correction with increased specificity.

In order to determine if gene-corrected SCD HSPCs retain the ability to engraft, CD34+ cells from the BM of SCD patients were treated with Cas9/gRNA RNP and ssODN donor for HBB gene correction, cryopreserved at Day 2 post genome editing, then intravenously transplanted into NSG mice shortly after thawing. These mice were euthanized at Week 16 after transplantation, and the BM was harvested to determine the engraftment potential. An average of 7.5 ±5.4% of cells were double positive for HLA and hCD45 in mice injected with gene-edited CD34+ cells, compared to 16.8 ±9.3% with control CD34+ cells, indicating a good level of engraftment of gene-corrected SCD HSPCs. A higher fraction of human cells were positive for CD19 (66 ±28%), demonstrating lymphoid lineage bias. DNA was extracted from unsorted cells, CD19 or CD33 sorted cells for gene-editing analysis; the HBB editing rates were respectively 29.8% HDR, 2.4% HBD conversion, and 42.8% non-homologous end joining (NHEJ) pretransplantation, and editing rates at Week 16 posttransplantation were respectively 8.8 ±12% HDR, 1.8 ±1.7% HBD conversion, and 24.5 ±12% NHEJ. The highly variable editing rate and indel diversity in gene-edited cells at Week 16 in all four transplanted mice suggest clonal dominance of a limited number of HSPCs after transplantation.

Taken together, our results demonstrate highly efficient gene and phenotype correction of the sickling mutation in BM HSPCs from SCD patients mediated by HDR and HBD conversion, and the ability of gene-edited SCD HSPCs to engraft in vivo. We also demonstrate the importance of genome-wide analysis for off-target analysis and the use of HiFi Cas9. Our results provide further evidence for the potential of moving genome editing-based SCD treatment into clinical practice.

Acknowledgments: This work was supported by the Cancer Prevention and Research Institute of Texas grants RR140081 and RP170721 (to G. B.), and the National Heart, Lung and Blood Institute of NIH (1K08DK110448 to V.S.)


Porteus:CRISPR Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees.

Author notes


Asterisk with author names denotes non-ASH members.