β-thalassemia, a disease of the red blood cells, is a genetic disorder characterized by a total (β⁰) or partial (β⁺) deficiency in the synthesis of the β-globin chains of hemoglobin. Patients with β-thalassemia major produce abnormally low (or even no) β-globin chains, and require regular transfusions to overcome their deficiencies in red blood cells and "adult" hemoglobin (consisting of two α-globin chains and two β-globin chains).

The CEA-Jacob researchers developed a lentiviral vector expressing the β-globin gene to treat these patients by gene therapy. Clinical trials conducted between 2010 and 2019 resulted in the drug being approved for patients expressing non-zero amounts of β-globin chains (β⁺).

Building on these strong results, the researchers have continued their efforts to make this therapeutic approach by gene transfer more efficient. The aim is to treat patients that do not express any β-globin chain (β⁰), but also to reduce the number of vector copies per cell in treated individuals, thereby diminishing the risk of insertional mutagenesis.

In β-thalassemic patients, the maturation of erythroid cells (red blood cell precursors) is blocked, not because of the deficiency in β-globin chains, but because the excess α-globin chains are highly unstable and toxic when not bound to β-globin chains. For this reason, individuals in which one or two of the four copies of the HBA1 and HBA2 genes (which encode α-globins) are nonfunctional produce more red blood cells than people with a normal number of copies, and their red blood cells are fully functional.

To improve the effectiveness of the gene therapy vector, the researchers inserted a sequence encoding an anti-HBA2 RNAi into one of the introns of the gene coding for the therapeutic β-globin. They showed that the number of gene copies required to restore the balance between α- and β-globin chains in the cells of β-thalassemic individuals is nearly two times lower than the number of copies required by the current vector on the market.

Moreover, they made sure that the vector is as stable as the original one, that it produces just as much therapeutic β-globin chains, and that the quantity of α-globin chains cannot be reduced by more than 50% of the normal level, since the RNAi specifically targets HBA2 and not HBA1.

These very encouraging results hint at new therapeutic perspectives for the most severe β-thalassemic patients (β⁰), along with an increase in the risk-benefit ratio for all β-thalassemic patients (β⁺ and β⁰) treated with this vector.

This study was conducted in collaboration with the Hudson Institute of Medical Research (Australia) and Mahidol University (Thailand).