Heidi Ledford in Nature:
A tweak to a technique that edits DNA with pinpoint precision has boosted its ability to correct defective genes in people. Called CRISPR, the method is already used in the lab to insert and remove genome defects in animal embryos. But the genetic instructions for the machinery on which CRISPR relies — a gene-editing enzyme called Cas9 and RNA molecules that guide it to its target — are simply too large to be efficiently ferried into most of the human body’s cells. This week, researchers report a possible way around that obstacle: a Cas9 enzyme that is encoded by a gene about three-quarters the size of the one currently used.
…Gene-therapy researchers often harness a virus called AAV to shuttle foreign genes into mature human cells. However, most laboratories use a gene encoding the Cas9 protein that is too large to fit in the snug confines of the AAV genome alongside the extra sequences necessary for Cas9 function. Feng Zhang of the Broad Institute of MIT and Harvard in Cambridge, Massachusetts, and his colleagues decided to raid bacterial genomes for a solution, because the CRISPR system is derived from a process that bacteria use to snip unwanted DNA sequences out of their genomes. Zhang’s team analysed genes encoding more than 600 Cas9 enzymes from hundreds of bacteria in search of a smaller version that could be packaged in AAV and delivered to mature cells. The gene encoding Cas9 in Staphylococcus aureus — a bacterium best known for causing skin infections and food poisoning — was more than 1,000 DNA letters smaller than the one for the commonly used Cas9. The researchers packed it into AAV along with RNAs that would target the enzyme to modify a cholesterol regulatory gene in the liver. Within a week of injecting mice with the modified virus, the team found that more than 40% of liver cells contained the modified gene.