A team led by researchers at the Broad Institute and St. Jude Children’s Research Hospital has now demonstrated that a base editor can efficiently correct the sickle cell disease mutation in mice.
Sickle cell disease
Sickle cell disease (SCD) is a rare autosomal recessive disorder that is caused by mutation of the HBB gene. This results in mutant β-globin, which at low oxygen concentrations, causes haemoglobin polymerisation within red blood cells. This ultimately leads to the characteristic sickle-shaped red blood cells. Symptoms of SCD include anaemia, severe acute and chronic pain, immunodeficiency, multi-organ failure and sadly, early death.
While allogeneic hematopoietic stem cell (HSC) transplantation can cure SCD, finding an optimally matched donor is difficult. In many cases, donors are not available, and the procedure results in graft rejection or graft-versus-host disease. As a result, researchers’ attention has shifted to ex vivo modification of autologous HSCs. Base editors are of particular interest as they do not require double-strand breaks (DSBs) or donor DNA template, with minimal formation of indels.
Base editor for sickle cell
In a recent study, published in Nature, researchers generated an adenine base editor (ABE8e-NRCH) that converts the SCD allele to a non-pathogenic allele. This particular base editor converts targeted A•T base pairs to G•C.
The team delivered mRNA encoding the base editor with a targeted guide RNA into haematopoietic stem and progenitor cells (HSPCs) from patients with SCD ex vivo. This resulted in 80% conversion of the SCD allele to the non-pathogenic allele. After sixteen weeks, the frequency of the non-pathogenic allele was 68%. In addition, the base editor reduced hypoxia-induced sickling by fivefold. This indicates a durable gene editing response.
Next, the researchers delivered the base editor into HSPCs from a humanised SCD mouse and then transplanted the cells into irradiated mice. After sixteen weeks, the non-pathogenic allele represented 79% of β-globin protein in the blood, and hypoxia-induced sickling was reduced threefold. The mice that received the base-edited HSPCs showed near-normal haematological parameters and also reduced splenic pathology.
To confirm durable editing, the researchers performed a secondary transplant. Here, they found that the edited cells continued to perform similarly to healthy blood cells. The team found that editing at least 20% of pathogenic haemoglobin genes was sufficient for phenotypic rescue.
Overall, these findings demonstrate that base editors could provide a one-time treatment for SCD that eliminates the pathogenic allele, generates a non-pathogenic allele and minimises the undesired effects of DSBs.
Image credit: By Science Photo Library – canva