A clinical trial of a new gene therapy for sickle cell disease that's shown promise has been halted after two participants were diagnosed with cancer.

Therapy developer Bluebird Bio said a patient who was treated five years ago developed myelodysplastic syndrome, a form of cancer that's often a precursor to leukemia, while another developed acute myeloid leukemia, The New York Times reported.

Though it isn't yet clear what caused the cancers, one possibility is that the disabled virus used to deliver the gene therapy damaged crucial DNA in blood-forming cells in the patients' bone marrows. That would be the worst-case scenario, said Dr. John Tisdale, head of the cellular and molecular therapeutics branch at the National Heart, Lung and Blood Institute.

But there is also the likelihood that both cancers were caused by a powerful drug, busulfan, which is used to clear bone marrow to make space for new cells modified by gene therapy. Busulfan is known to raise blood cancer risk, Tisdale noted.

Muddying the issue even further is the fact that people with sickle cell disease are known to have an increased risk of leukemia, even without treatment. Still, no one would expect two patients in a small trial to get the disease, the Times said.

The news is a disappointment for those who hoped a cure for sickle cell disease — which mainly affects Black Americans — was on the way.

"It feels like the sickle cell disease community just can never get a break," Dr. Melissa J. Frei-Jones, a researcher at the University of Texas School of Medicine in San Antonio, told the Times.

"My other concern is that the Black community will lose faith or trust in research studies again after it has taken the medical community so long to even regain some degree of trust," she added.