Genome editing: cutting and pasting corrects blood disease

This week, scientists reveal how the targeted editing of genes has fixed hemophilic mice.

People with the blood-clotting disorder – such as Queen Victoria’s youngest son – can bleed to death after just simple injuries.

Direct genome editing offers a lot of promise for treating many genetic disorders by simply correcting dysfunctional genes to restore normal function. In particular, those with hemophilia B, about 20%, have multiple mutations on the F9 gene, causing them to lack a blood-clotting factor made by the liver.

“I always say that someone with mild hemophilia can play on the college tennis team, but they cannot play on the football team,” says study author Katherine High of the Children’s Hospital of Philadelphia. A person with severe hemophilia, on the other hand, may bleed in the knee by just sitting in a chair.

By replacing genes and correcting mutations – all without removing cells from the body – scientists managed to correct clotting times in mice with hemophilia B.

Enzymes called zinc finger nucleases cause breaks in specifically selected genomic locations. These molecular scissors replace specific DNA sequences by cutting through the double helix, after which the cell's repair machinery fixes the break. The Scientists explains:

By offering a template of a desired correction or revision, researchers can effectively trick the cell’s own repair mechanisms into inserting the new code in the place of the defective gene. Getting the cell itself to do the job is optimal, because the replacement gene will come ‘under the control of the normal regulatory elements’ that ensure it will work as it should.

Previously, scientists have used these to stimulate genome editing, but only in cells that have been taken out of the body, specially cultured, and then put back in.

So High and her team show, for the first time, that zinc finger nucleases can work inside a live animal – that is, in vivo – as well.

1. The team engineered mice to carry the faulty F9 gene.
2. They injected them with zinc finger nucleases that cleaved the genome at the start of the F9 sequence and inserted a normal version of the gene. This fixed all the mutations at once.
3. Their blood clotted in 44 seconds, according to Nature News, compared with more than a minute in untreated mice.

That gene correction sufficiently prevented hemophilia: their blood then contained as much as 7% of the normal amount of the missing human blood coagulation factor IX. This translates to mild bleeding in humans.

There were no signs of toxicity or changes in liver function in the mice over the next 8 months. And when the researchers removed part of the 10-week-old mice’s livers, their factor IX levels continued to go strong at 30 weeks.

This new method really broadens the range of diseases treatable with gene therapy. The zinc finger technique was developed by Sangamo BioSciences (“the zinc finger company”).

As the New York Times reports, zinc fingers are being used as a treatment for AIDS in early clinical trials. The fingers disrupt a gene called CCR5, so people without a CCR5 receptor are naturally immune to AIDS. The hope is that patients will acquire the same immunity after their immune cells are similarly treated.

The study was published in Nature yesterday.

Image: Queen Victoria at the V&A

This post was originally published on Smartplanet.com