For this first human application of base-editing technology, the researchers used healthy donor T cells from a registry to generate banks of universal chimeric antigen receptor (CAR) T cells. They made changes to the cells using base editing, which works by chemically converting single nucleotide bases (letters of the DNA code) that carry instructions for a protein expressed in the leukemia, to prevent it from being produced. The steps included removing existing receptors and identifying flags, and adding chimeric antigen receptors, which enabled the new base-edited T cells to recognize and fight T-cell leukemia.

Last year a 13-year-old girl from Leicester, England was the world's first person to be treated in the trial for T-cell acute lymphoblastic leukemia, a white blood cell cancer. Usually treated with chemotherapy, this leukemia can be very difficult to treat once it returns. No other treatment options remained for the girl. However, within four weeks of receiving the new base-edited cells, the girl's leukemia was undetectable. Following a successful bone marrow transplant, she is currently doing well at home almost a year later.

A second teenager, whose leukemia was cleared within a similar period, is now recovering at home after a transplant. A third child, whose course was unfortunately complicated by serious infections, moved to palliative care.

The U.K.'s National Health Service (NHS) is the government-funded medical and health care system available to all U.K. residents. The still-open clinical trial seeks to recruit up to 10 NHS patients with T-cell leukemia who have exhausted conventional treatment options, referred by NHS children's leukemia specialists.

If successful, the team hopes the treatment can be offered to more children earlier on, when they are not as sick; they also hope to eventually make it available to adults. The researchers believe the base editing technique could also be used for other conditions.

"The technology itself could also have wide reaching applications for corrections of certain inherited conditions such as sickle cell disease. As the technology matures and is shown to be safe, it could be applied quite widely," co-author Waseem Qasim, cell and gene therapy professor at UCL, said in a statement.

"It is really crucial that children affected by cancer who failed standard of care have access to innovative strategies in the context of clinical trials such as this," co-author Dr. Robert Chiesa, of GOSH's bone marrow transplant and hematology teams, added in a statement. "Research hospitals such as GOSH offer the ideal setting for developing experimental approaches that might offer hope to children with otherwise very poor prognoses."