Results from the first human trial of an experimental gene-editing therapy for Duchenne muscular dystrophy (DMD), in which a young Chinese boy died, have been published¹. Although the trial provides evidence that the treatment developed by HuidaGene, based in Shanghai, China, edited some target genes, the therapeutic benefit was limited and led to a catastrophic immune reaction in one participant. Researchers say there is an urgent need to establish safe dosages for these kinds of experimental treatment.

The trial gained attention when HuidaGene announced on 5 August that one participant, a boy, had died roughly a year earlier. Researchers criticized the company for waiting so long to publicly report the death — and the event put a spotlight on China’s fast-track clinical trials. On 27 August, HuidaGene told Nature that the firm wanted to “establish the clinical facts and undertake a comprehensive investigation” before communicating with the broader public. They said a manuscript detailing the boy’s death and post-mortem findings had been submitted to a journal in January, which has now been published in Cell.

The boy who died was one of four children to receive the experimental therapy for DMD, a progressive and ultimately fatal genetic disorder that mostly affects boys, causing difficulty in walking and gradual muscle weakening. Children with DMD have mutations in the DMD gene that affect their ability to produce dystrophin, a protein that strengthens muscle fibres. The therapy used an adeno-associated virus (AAV) to deliver a base editor — a form of CRISPR gene-editing technology — to muscles to correct the mutation. The boy who died and another boy received a high dose of the therapy in AAVs; two other boys were given a low dose.

Within 24 hours of receiving the therapy, the authors say that the boy who died experienced sudden, severe abdominal pain, followed by a fever that responded only partially to treatment. The team says his condition declined rapidly, and he developed discoloured skin due to a lack of oxygen in his blood, as well as a slow, irregular heartbeat. His heart eventually stopped, and doctors could not revive him. The authors report that an autopsy revealed that he died of acute respiratory distress syndrome, which was related to the treatment.

Ian Woodcock, a paediatric neurologist and researcher at Murdoch Children’s Research Institute in Melbourne, Australia, says the boy’s fast decline suggests it was a reaction to the dose he received. In general, a large dose of AAVs are required so the gene therapy reaches muscle cells across the body. But this increases the risk of severe immune reactions, including death, that have been reported in this and other gene therapy trials for DMD^2,3. “Even though the AAV doesn't cause disease in humans, this amount of foreign invasion really triggers the immune system,” he says.

Animal studies

The authors report that they performed safety testing in cynomolgus macaques (Macaca fascicularis) before the trial in people. No negative side effects were observed with either a high or low dose, but the monkeys did have a weak, short-lived immune reaction following treatment. The authors acknowledge that the monkey experiments could not detect rare side effects, such as the severe immune reaction experienced by the boy who died, because they only tested the treatment in four animals.

When asked whether they should have run tests in more animals, co-author Hui Yang, co-founder of HuidaGene, said that increasing animal numbers alone could not guarantee prediction of an uncommon reaction in people, particularly because different species have different immune responses.

The boy who died was the second to receive the high dose, and only after safety data for the other high-dose recipient and the two low-dose recipients were reviewed by the trial’s safety review committee, the authors said. The exact dosage the boys received was based on their weight.

The team said that the boy who died received a higher dose than the other boy in the high-dose group because he had a higher body mass index (BMI). But that does not necessarily mean that he had more lean muscle, which is the target of the therapy. Yang says that the boy’s muscles might have been exposed to a higher dose than in the other boys, but it is still unclear whether that could have triggered the severe immune reaction. The authors suggest that dosing on the basis of body weight might not be appropriate.

Yang says the team tried to reduced the risk of severe immune responses by giving the boys immunosuppressants, testing both doses in animals and staggering each boy’s treatment following a safety review. But given the boy’s death, those measures were insufficient, he says. “We therefore need to re-examine the assumptions underlying dose selection, the role of body composition,” and the predictive value of preclinical studies, he adds. Stronger independent safety oversight and ways of reducing the effective dose also need to be developed, he adds. “The most important lesson is that severe systemic immune toxicity following AAV administration remains difficult to predict.”

Next-generation AAVs that are better at penetrating muscles could mean that lower doses can achieve the same editing effect, says Woodcock. Or the team might consider an alternative to AAVs, he adds.