Gene therapy trials for sickle cell disease halted after two patients develop cancer

A company has stopped its clinical studies of a promising gene therapy for the blood disorder sickle cell disease after two people who participated developed leukemia-like cancer. Bluebird bio is now investigating whether a virus it uses to deliver a therapeutic gene caused the cancers, reviving old concerns about the risks of this approach.

It’s also possible the cancers stemmed from chemotherapy the patients received to prepare their bodies for the gene’s delivery. “This is really a sad development whatever the cause,” says Donald Kohn of the University of California, Los Angeles, who has led gene therapy trials for sickle cell and other diseases.

In the bluebird bio trials, scientists remove a patient’s blood stem cells and treat them in a dish with a modified virus related to HIV. It carries DNA encoding the oxygen-carrying protein hemoglobin and is intended to compensate for the patient’s defective gene for this molecule. After this step, called “ex vivo” because a patient’s cells are treated outside the body, doctors infuse the cells back into the person. Fourteen people who have received the latest...