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The idea behind gene therapy is seductively simple: if a faulty gene causes disease, deliver a working copy and let the body heal itself. Scientists first attempted the feat in 1990, when a four-year-old girl with a severe immune disorder received an infusion of her own cells, corrected in the laboratory. The results were encouraging but modest, and the field's confidence collapsed nine years later when Jesse Gelsinger, an eighteen-year-old volunteer in a trial at the University of Pennsylvania, died from a catastrophic immune reaction to the virus used to ferry the corrective gene. Funding evaporated almost overnight, and gene therapy acquired a reputation as a beautiful idea that biology stubbornly refused to honour.
The central technical problem has always been delivery. Genes are large, fragile molecules that cannot simply be injected into the bloodstream; they need a vehicle, or vector, to carry them into cells. The most successful vectors are adeno-associated viruses, small viruses that infect humans readily without causing illness. Stripped of their own genes and loaded with therapeutic ones, they act as molecular couriers. Early versions provoked dangerous immune responses or inserted their cargo at random positions in the genome, occasionally triggering leukaemia in trial participants. Decades of engineering have produced newer generations that are safer, more precise and capable of targeting particular organs.
Two broad strategies have emerged. In ex vivo therapy, cells are removed from the patient, corrected in a dish and returned; this approach underpins treatments for blood disorders such as sickle cell disease, in which a patient's own stem cells are edited and reinfused. In vivo therapy delivers the gene directly into the body, as with Luxturna, approved in 2017, which restores partial vision in people with a rare inherited form of blindness by injecting the vector beneath the retina. Zolgensma, a single intravenous dose for infants with spinal muscular atrophy, has allowed children who would once have died in early childhood to sit, stand and in some cases walk.
The clinical scorecard is lengthening steadily. Haemophilia patients, who once injected clotting factor several times a week, have remained free of bleeding episodes for years after a single treatment. Trials for inherited deafness have restored hearing in profoundly deaf children, and early results in metabolic disorders of the brain suggest that some conditions once regarded as untreatable may yet yield to intervention. Regulators have now approved more than a dozen gene therapies, and several hundred trials are under way worldwide, spanning diseases from muscular dystrophy to heart failure and even some cancers.
Formidable obstacles remain. Because the vector is a virus, many patients carry pre-existing antibodies that neutralise it, excluding them from treatment, and those who do receive it usually cannot be dosed a second time. Some early benefits fade as corrected cells die off, and nobody yet knows whether a single dose given to a growing child will protect an adult for decades. Then there is cost: Zolgensma launched at roughly 2.1 million dollars per patient, making it, at the time, the most expensive medicine ever sold, and insurers in poorer countries have largely refused to pay for it.
The ethical terrain is equally uneven. Editing the genomes of sperm, eggs or embryos, so-called germline editing, would make changes heritable, passing them to generations who cannot consent to the procedure. The 2018 announcement by He Jiankui that he had produced gene-edited babies in China drew near-universal condemnation and a prison sentence, and most scientific bodies now call for a moratorium on clinical germline use. Somatic editing, which alters only the treated patient, is considered ethically far safer, though even it raises uncomfortable questions about access when a single cure costs more than a lifetime of conventional care.
Proponents argue that the field has finally crossed from promise to practice, and that costs will fall as manufacturing matures and competition arrives. Sceptics note that nearly every approved therapy treats a rare disease, and that extending the approach to common conditions such as diabetes or Alzheimer disease will demand delivery systems far more sophisticated than anything now in the clinic. Both camps agree on one point: the teenage tragedy of 1999 is no longer the whole story. Gene therapy has survived its troubled adolescence; the question now is what kind of adult it will become.