The project, which also involved electrical engineers, is aimed at searching the human genome for genes, and the associated protein products, that can be targeted by drugs to treat a variety of illnesses. This is normally a very lengthy task, but research leaders Shana Kelley and Jason Moffat of the University of Toronto reasoned that combining the techniques they were working on – respectively, a magnetic sorting technique and gene-editing using CRISPR – might speed the process up. As they report in a paper in Nature Biomedical Engineering, their hunch was correct.
Both researchers were working on a large multi-centre project called Medicine by Design, with Kelley, a pharmacist, leading a team that was building microfluidic devices which use tiny magnets incorporated into cells to sort large mixed populations of cells. Moffat, a cellular and biomedical research specialist, was using CRISPR, a powerful technique for identifying and manipulating specific genes in cells, to study how the body's immune system is triggered to attack certain cells but not others. A conversation in a corridor led researchers to combine their research strands, resulting in what Kelley calls "an engine for the discovery of new therapeutic targets in cells."
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The team's paper describes how they used CRISPR to reveal promising drug targets by switching off genes that produce proteins that help cancer to spread. Using techniques developed by Kelley, the researchers bound tiny magnetic particles to the target proteins which reside on the surface of the cells that produce them, and funnelled the entire population of cells into a device about half the size of a credit card, streaked with strips of magnetic material that capture the marked cells into collection channels corresponding to the amount of magnetic material on the surface, which corresponded to the concentration of the target protein.
To test the method, they focused on cancer immunotherapy, a technique which tricks the immune system into attacking mutated cancer cells (normally, these would be ignored, leading to growth and spread of the cancer). Using CRISPR, they identified a gene that produces a protein known as CD47, which signals immune cells not to attack – cancer cells often hijack this process to escape detection. Previous research had indicated that blocking CD47 directly with drugs leads to harmful side effects, so just tricking the cell to produce less might be a more effective treatment. The CRISPR screen identified an enzyme that helps camouflage the protein from the immune system, and could be blocked with an off-the-shelf drug, and the microfluidic device successfully sorted cells with the gene producing the enzyme from a mixed population of cells.
"As many as one billion cells can travel down this highway of magnetic guides at once and we can process that in one hour," says Kelley." It's a huge gamechanger for CRISPR screens." Using current sorting techniques, which employ fluorescent markers picked out by lasers, the same sorting procedure would take 20-30 hours, making drug discovery an expensive and arduous task.
Kelley and Moffat also hope the technique can be used in regenerative medicine, to identify genes that activate stem cells to transform into specific cell types, which would make it easier to harvest the right sort of cells for therapies.