For this study, the researchers designed three guide RNA strands that target different DNA sequences near the mutation that causes type I tyrosinemia, in a gene that codes for an enzyme called FAH. Patients with this disease, which affects about 1 in 100,000 people, cannot break down the amino acid tyrosine, which accumulates and can lead to liver failure. Current treatments include a low-protein diet and a drug called NTCB, which disrupts tyrosine production.
In experiments with adult mice carrying the mutated form of the FAH enzyme, the researchers delivered RNA guide strands along with the gene for Cas9 and a 199-nucleotide DNA template that includes the correct sequence of the mutated FAH gene.
Using this approach, the correct gene was inserted in about one of every 250 hepatocytes the cells that make up most of the liver. Over the next 30 days, those healthy cells began to proliferate and replace diseased liver cells, eventually accounting for about one-third of all hepatocytes. This was enough to cure the disease, allowing the mice to survive after being taken off the NCTB drug.
"We can do a one-time treatment and totally reverse the condition," says Hao Yin, a postdoc at the Koch Institute and one of the lead authors of the Nature Biotechnology paper.
To deliver the CRISPR components, the researchers employed a technique known as high-pressure injection, which uses a high-powered syringe to rapidly discharge the material into a vein. This approach delivers material successfully to liver cells, but Anderson envisions that better delivery approaches are possible. His lab is now working on methods that may be safer and more efficient, including targeted nanoparticles.
|Contact: Sarah McDonnell|
Massachusetts Institute of Technology