What Three Gene-Therapy Deaths Have in Common

In recent months, news came out about several distressing cases that have underscored the risks of gene therapy, which delivers DNA to a patient’s cells to treat disease. All of these cases have one thing in common that’s not apparent at first glance.

A child in China, enrolled in a clinical trial for gene therapy, died of acute respiratory distress syndrome, in which fluid builds up in the lungs and oxygen cannot enter. In a separate clinical trial in China, a child died after clots formed in their small blood vessels, likely disrupting the delivery of oxygen to organs such as the kidneys and brain. And earlier this year, researchers reported that a 5-year-old boy treated in Philadelphia had a brain tumor that was likely caused by gene therapy. (He’s doing okay now.) In each of these cases, the gene therapies relied on a versatile but unpredictable type of virus: adeno-associated virus, or AAV for short.

AAVs are the workhorses of many of the most successful gene therapies in the world. Scientists modify them so that they can deliver...