A patient who was treated [with gene therapy for sickle cell disease] five and a half years ago has developed myelodysplastic syndrome, a form of cancer that is often a precursor to leukemia, Bluebird Bio reported, while another has developed acute myeloid leukemia.
Concerned by the diagnoses, the National Institutes of Health has halted a similar trial at Boston’s Children’s Hospital pending further investigation.
It is not clear whether the cancers are linked to the experimental gene therapy. But the sudden setback is a disappointment to many sickle cell experts and patients, mostly African-Americans, who had hoped that a cure was on the horizon.
It is not yet clear what caused the cancers. One possibility is that the disabled virus used to deliver the gene therapy treatment damaged crucial DNA in blood-forming cells in the patients’ bone marrows. That would be the worst-case scenario, said Dr. John F. Tisdale.
But there is also the likelihood that both cancers were caused by a powerful drug, busulfan, which is used to clear bone marrow in order to make space for new cells modified by gene therapy. Busulfan is known to confer a blood cancer risk, Dr. Tisdale noted. If it turns out to be the culprit in Bluebird Bio’s trials, “We are back to what we know,” he said.