Breaking: Base Editing Rescues Severe β-Thalassaemia Mutations

The approach was also applied to sickle-cell β-thalassaemia, where repairing the β-thalassaemic allele shifts cells toward the benign sickle-trait state. Deoxygenation assays showed a substantial reduction in sickling compared to untreated cells, with levels lower than in asymptomatic carriers.

»To our knowledge, our study describes for the first time the efficacy of such β-thalassemia mutation correction approaches to correct also the sickle phenotype associated with the sickle cell-β-thalassemia genotype, further expanding the scope of clinical application of these base editing strategies,« says Giulia Hardouin.

Safety was assessed across multiple complementary assays. GUIDE-seq and computational prediction identified candidate off-targets, which were validated in primary HSPCs. Structural integrity of the HBB locus was preserved after base editing, contrasting with deletion-rich alleles generated by Cas9 nuclease. Whole-exome sequencing in vitro and after xenograft engraftment showed no detectable increase in mutation burden, and clonal analyses indicated preserved stem-cell diversity.

»We used a combination of cutting-edge assays to confirm the safety of the highly processive ABE8e usage in patients’ HSPCs, in terms of genotoxicity and specificity,« Giulia Hardouin explains.

RNA-seq showed minimal transcriptome perturbation beyond the targeted correction, with no increase in A>G RNA deamination. Bystander edits – including benign Hb San Bruno and Hb-Nîmes – were functionally neutral in engineered HSPCs. A further screen identified an ABE variant capable of reducing C-bystander activity while maintaining high correction efficiency.

To gauge clinical reach, the authors examined the French NaThalY registry, Giulia Hardouin explains:

»Epidemiological studies are key to refining the population that could benefit from these approaches. Here, we took advantage of the French national thalassemia registry (NaThalY) and estimated that >40% of TDT patients who have not received an allogeneic HSC transplantation could benefit from such a treatment.«

The work was led by Annarita Miccio at the Imagine Institute (INSERM UMR1163) in Paris and conducted by a collaborative team of researchers at the Imagine Institute, the San Raffaele Telethon Institute for Gene Therapy in Milan, Genethon in Evry, and clinical partners in Marseille, with contributions from the University of Padova. It was published today, 26 November, in Science Translational Medicine.