Conventional gene therapy can only be used to add genes to DNA. But with gene editing, specific DNA sequences can be cut with “molecular scissors”, introducing mutations that disable a particular gene. Qasim’s molecular scissors were of a kind known as TALEN proteins.
But there was still another problem to overcome. The recipient’s immune system also recognises non-matched T-cells as foreign and will attack them. In leukaemia patients, this is not a problem because they are given drugs that destroy their immune system. Except, one of these drugs – an antibody – also destroys donor T-cells. So Qasim’s team also disabled a second gene in the donor T-cells, which made them invisible to the antibody.
At the time that Qasim was contacted by Layla’s doctors, his engineered T-cells, called UCART19 cells and developed in collaboration with New York biotech company Cellectis, had only ever been tested in mice. “It was scary to think the treatment had never been used in a human before,” said Layla’s father Ashleigh, “but there was no doubt we wanted to try the treatment. She was sick and in lots of pain, so we had to do something.” And it worked within weeks.
This is only the second time that gene-edited cells have been used in people. The first ever trial involved modifying T-cells in people with HIV to make them more resistant to the virus, although these participants were not in immediate danger of dying.
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