Gene therapy cures haemophilia in mice

Doctors have treated a life-threatening blood disease by repairing flaws in the genetic code of a living animal.

The pioneering work raises the prospect of powerful new therapies that can target and repair the genetic defects behind a wide range of human diseases.

Called genome editing, it holds particular promise for a group of illnesses that run in families and are caused by faults in genes that underpin the working of the immune system, bone marrow and liver.

To demonstrate the therapy, researchers treated mice that were bred to develop haemophilia B, an inherited disorder that destroys the body’s ability to form blood clots. Normally, when the body suffers a cut or graze, proteins called clotting factors combine with platelet cells in the blood to make it sticky and form a clot that stops any bleeding.

But people born with haemophilia B carry a defect in a gene that makes clotting proteins, leaving them vulnerable to excessive bleeding, even when they have not sustained injury.

In a report in the journal Nature, a team led by Katherine High at the Children’s Hospital of Philadelphia describes how genome editing restored blood-clotting times in mice to near-normal without causing any apparent side effects.

First, the mice were injected with enzymes hidden in harmless virus shells that find their way to the liver where blood-clotting proteins are made. Once there, the viruses smuggle the enzymes into liver cells, where they hone in on the specific gene sequence that is defective in haemophilia B. Having arrived at their target, the enzymes slice through the faulty gene, causing the cell to launch an emergency repair effort.

At this point, the second stage of the therapy swings into action. Injected with the enzymes are DNA templates of the healthy blood clotting gene. These feed into the cell’s repair machinery and, instead of simply fixing the sliced gene, a new working copy is created.

The work is a welcome breakthrough in a field that has struggled to deliver on the promise many scientists believe it holds. —

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