Genome editing reactivates zeta-globin gene in alpha-thalassaemia

Switching back on a gene that is normally silenced early in development has improved features of severe alpha-thalassaemia in mice and restored globin production in cells from patients.

The approach, developed at the University of Oxford, targets the regulatory DNA controlling gene expression rather than correcting the faulty genes themselves. Alpha-thalassaemia is one of the most widespread inherited blood disorders. It is caused by the loss of some or all of the four functional alpha-globin genes, which encode the alpha-globin chains that make up half of the haemoglobin molecule, which is essential for carrying oxygen in red blood cells. Instead of repairing them, the team used genome editing to switch on zeta-globin, an embryonic alpha-like globin gene that is normally switched off around six to seven weeks after conception.

'Our study shows that small changes to regulatory DNA can reactivate an embryonic gene that is normally silenced after early development,' said Dr Siyu Liu, first author of the study published in Nature Genetics. 'This offers a promising foundation for developing future gene therapies for patients with severe alpha-thalassaemia.'

Working first in mouse models, the team screened regulatory DNA sequences involved in silencing the zeta-globin gene. They identified two sites that help keep the zeta-globin gene switched off and found that disrupting both sites together reactivated the gene. In a mouse model of haemoglobin Bart's hydrops fetalis syndrome (BHFS) – the most severe form of alpha-thalassaemia, in which all four alpha-globin genes are missing and which is usually fatal before birth – edited embryos survived to late pregnancy. The mouse embryos developed near-normal red blood cells, whereas unedited embryos developed severe anaemia and hydrops fetalis, characterised by an abnormal accumulation of fluid.

The team then made the equivalent edits in blood stem cells taken from patients, including a fetus with BHFS and patients with haemoglobin H disease. The edited cells reactivated zeta-globin expression, which helped compensate for the missing alpha-globin. The researchers found that the edited cells produced around half the normal level of alpha-like globin. They suggest this could be enough to reduce the effects of severe alpha-thalassaemia, although this has not yet been demonstrated in patients.

The research needs further preclinical studies before it can progress towards clinical trials. The authors caution that the safety of the strategy in human blood stem cells has yet to be established, including off-target effects and whether edited cells can successfully engraft and survive long-term in the body. Physiological questions also remain about the resulting edited haemoglobin, which binds oxygen more tightly than the adult form.

'Our next goal is to advance this approach towards clinical application by establishing its safety, efficacy and suitability for future clinical trials,' said Professor Douglas Higgs, who supervised the study.

Genome editing is already used to reactivate the related fetal haemoglobin gene in sickle cell disease and beta-thalassaemia (see BioNews 1319, 1108 and 1052). However, there are currently no equivalent genome-editing treatments for alpha-thalassaemia.