Membrane vesicles delivered a CRISPR DNA editor to the brains of two macaques and altered the MSH3 gene

Membrane vesicles delivered a CRISPR DNA editor to the brains of two macaques and altered the MSH3 gene

On August 6, the authors of a preprint reported injecting modified extracellular vesicles, which are tiny membrane-bound sacs, into two brain regions in two adult macaques. The vesicles delivered a fully assembled CRISPR DNA editor targeting the MSH3 gene into cells. DNA analysis of tissue from the injection sites confirmed that editing had occurred.

A DNA editor can alter a specific sequence, but it must first reach the target cells. Delivery to the liver is already possible, but the brain requires a different approach. A viral vector carries genetic instructions for an editor into a cell, which then continues to produce the editor after administration. In the preprint, the authors took a different approach. Producer cells first assemble Cas9, the protein that cuts DNA, with a guide RNA, the molecule that directs Cas9 to the target sequence. The cells then package the assembled complex inside an extracellular vesicle.

Once delivered, this complex remains active for only a limited time. The researchers modified the guide RNA to improve its binding to Cas9 and delayed the release of the complex until it had been packaged. In cell experiments, these changes increased editing efficiency by approximately 300-fold. The system was first tested in cells and organoids, which are small models of the human brain, and then in mouse brains. The researchers subsequently tested it in primates.

For the primate experiment, the researchers selected the MSH3 gene. In Huntington's disease, CAG repeats, which are repeated DNA sequences, lengthen with age in neurons of a brain region called the striatum. The MSH3 protein participates in a DNA repair process associated with this expansion. The experiment therefore tested both whether the editor could be delivered into the tissue of a large animal and whether it could alter a gene involved in the disease mechanism.

The vesicles were injected into the caudate nucleus and putamen, two parts of the striatum. Imaging showed which areas of the striatum were reached by the vesicle solution. DNA analysis of individual tissue samples detected up to 60–75% MSH3 editing in some areas. When the analysis included only tissue reached by the solution, the editing rate exceeded 40% in the putamen and 60% in the caudate nucleus.

“Until now, there had been no direct demonstration of DNA editing in the brain of nonhuman primates, a critical prerequisite for testing in humans.”