Sickle cell disease (SCD) is a severe, life-threatening blood disorder that manifests in episodes of excruciating pain and organ damage, with limited treatment options. It is caused by a mutation in the β-globin gene (HBB), resulting in a structurally abnormal haemoglobin tetramer that distorts the shape of red blood cells. One potential treatment is restoring the production of foetal haemoglobin (HbF) to prevent abnormal haemoglobin formation.
Mitchell Weiss and colleagues investigated a new gene therapy approach to allow production of HbF by overcoming repression of γ-globin (HBG1 and HBG2; HBG) in two established SCD mouse models: Berkeley and Townes mice. First, the authors characterised the genetic background of the mice to find that Berkeley mice had four to 22 randomly arranged, fragmented copies of the human mutated HBB gene and the foetal HBG genes. By contrast, Townes mice had single copies of the human mutated HBB gene and the foetal HBG gene with proximal but not distal regulatory elements.
The authors then CRISPR edited the HBG gene in mouse haematopoietic pluripotent stem cells (HPSCs) to prevent the gene from binding to its repressor, with the aim of increasing HbF production. The gene editing efficiency upon implantation in Berkeley mice was only 3.1%, and the gene therapy proved lethal, with only 30% survival. This was due to extensive DNA damage that was amplified by the high copy number of human haemoglobin transgenes.
In Townes mice, the authors successfully edited HBG in 57% of HPSCs, and there were no significant effects on mouse survival. However, the levels of foetal haemoglobin were 7- to 10-fold lower than those seen when this approach is undertaken in human cells in vitro, and the gene therapy was not able to improve SCD biomarkers in Townes mice. This was potentially due to the lack of HBG distal enhancers in these mice.
This work demonstrates how essential the selection of accurate model systems is when developing and validating gene therapy. For HBG-based SCD gene therapy in particular, the results highlight the importance of distal regulatory elements for the success of the gene therapy. Further preclinical studies are essential to optimise and refine HBG-based treatments prior to clinical trials, which can hopefully provide more effective options for patients with this devastating disease.
