Gene therapy

Gene therapy treats disease by acting on its genetic cause rather than its symptoms. The original approach, gene addition, delivers a working copy of a gene to make up for a faulty or missing one. Other approaches silence or inactivate a harmful gene, or add a new gene that helps the body fight disease. Newer genome-editing tools such as CRISPR-Cas9 go further and rewrite the patient's own DNA sequence directly.

Delivery is the main technical hurdle. The most common carriers, or vectors, are viruses modified so they can no longer cause disease, such as adeno-associated viruses and lentiviruses; plasmid DNA and lipid nanoparticles are also used. Treatment can be given in vivo, by putting the vector into the body, or ex vivo, by removing a patient's cells, altering them in the laboratory and returning them, as in CAR T-cell therapy for certain blood cancers. Current therapies target somatic, or body, cells, so the change is not inherited by the patient's children. Altering eggs, sperm or embryos, called germline gene therapy, is controversial and not permitted clinically in many countries.

Approved gene therapies now exist for conditions including an inherited form of blindness, spinal muscular atrophy, hemophilia and sickle cell disease. Risks include immune reactions against the vector and, with some vectors, DNA landing in places that disrupt other genes.