Alyssa, who is 13 and from Leicester, England, was diagnosed with T-cell acute lymphoblastic leukemia last May. Her cancer was aggressive — chemotherapy, and then a bone marrow transplant, were unable to rid it from her body.
What happened next would have been unthinkable just a few years ago and has been made possible by incredible advances in genetics. Doctors at Great Ormond Street Hospital used “base editing” to perform a feat of biological engineering to build her a new living drug.
Base editing allows scientists to zoom to a precise part of the genetic code and then alter the molecular structure of just one base, converting it into another and changing the genetic instructions. The four types of base – adenine (A), cytosine (C), guanine (G) and thymine (T) – are the building blocks of our genetic code.
The large team of doctors and scientists used this tool to engineer a new type of T-cell that was capable of hunting down and killing Alyssa’s cancerous T-cells. They started with healthy T-cells that came from a donor and set about modifying them.
– The first base edit disabled the T-cells targeting mechanism so they would not assault Alyssa’s body.
– The second removed a chemical marking, called CD7, which is on all T-cells.
– The third edit was an invisibility cloak that prevented the cells being killed by a chemotherapy drug.
– The final stage of genetic modification instructed the T-cells to go hunting for anything with the CD7 marking on it so that it would destroy every T-cell in her body, including the cancerous ones. (This marking has to be removed from the therapy, otherwise it would destroy itself.)
Alyssa spent 16 weeks in hospital and couldn’t see her brother in case he brought germs in. Her three-month check-up found signs of the cancer again, but her two most recent investigations have been clear.
“It’s just amazing I’ve been able to have this opportunity, I’m very thankful for it,” Alyssa says. She’s getting ready for Christmas, being a bridesmaid at her aunt’s wedding, getting back on her bike, going back to school and “just doing normal people stuff”.
Dr Robert Chiesa, from the bone-marrow transplant department at Great Ormond Street Hospital, said: “It is extremely exciting. Obviously, this is a new field in medicine and it’s fascinating that we can redirect the immune system to fight cancer.”
Dr. David Liu, one of the inventors of base editing at the Broad Institute, told me it was “a bit surreal” that people were being treated just six years after the technology was invented.
The family hope the cancer will never return, but are already grateful for the time it has bought them. Her mum, Kiona, said this time last year she had been dreading Christmas, “thinking this is our last with her.” And through her daughter’s 13th birthday in January, she “just cried.”