Ιn a world first, the UK medicines regulator has approved a therapy that uses the CRISPR–Cas9 gene-editing tool as a treatment. The therapy, called Casgevy, will treat the blood conditions sickle-cell disease and β-thalassaemia. Sickle-cell disease, also known as sickle-cell anaemia, can cause debilitating pain, and people with β-thalassaemia often require regular blood transfusions.
Clinicians administer Casgevy by taking blood-producing stem cells out of the bone marrow of people with either disease, and using CRISPR–Cas9 to edit the genes encoding haemoglobin in those cells. The gene-editing tool relies on an RNA molecule that guides the Cas9 enzyme to the correct region of DNA, which the enzyme cuts.
Once Cas9 reaches the gene targeted by Casgevy, called BCL11A, it cuts both DNA strands. BCL11A usually prevents the production of a form of haemoglobin that is made only in fetuses. By disrupting this gene, Casgevy unleashes the production of fetal haemoglobin, which does not carry the same abnormalities as adult haemoglobin in people with sickle-cell disease or β-thalassaemia.