Public health officials universally agree that the world needs much more coronavirus testing before we can safely ease current lock-down restrictions. Even at the low end, experts say we’ll need to do hundreds of thousands more daily tests, but the equipment and resources to make that happen are in short supply. An MIT spin-off company called Sherlock Biosciences has gotten FDA approval to begin using its CRISPR-based COVID-19 test, which promises to be faster and easy to perform without access to a full lab.
Current coronavirus testing is based on PCR (polymerase chain reaction), the same technology used in DNA tests. This involves repeatedly heating the sample to amplify the genetic material so technicians can detect viral RNA. Sequencing those samples to hunt for viral genes requires expensive machines that many facilities don’t have, but the Sherlock method relies on a device similar to a pregnancy test. MIT’s Broad Institute developed Sherlock as a way to identify diseases with the clever addition of a reporter molecule with a DNA segment. Sherlock Biosciences now develops tests with this technology for specific diseases like COVID-19.
CRISPR/Cas9 has gained fame as a powerful tool for genetic engineering, but that’s slightly different than the system devised by Sherlock. CRISPR is the sequence that guides Cas9 to the specific genetic code where you want to make a cut (known as cleaving), but scientists can also pair CRISPR with other members of the Cas family. This system uses a related enzyme called Cas13 that doesn’t just cleave the specified sequence — when CRISPR activates Cas13, it cleaves any RNA it touches in a process called collateral cleaving. Some of that genetic material will be in the reporter molecule, and that’s how you detect disease.
In a sample positive for coronavirus, the CRISPR sequence will detect the virus’ genetic material and activate Cas13. This leaves plenty of sliced up reporter molecules in the sample, but a negative test would have all intact reporters. Technicians can run the sample across a test strip with two electrochemical “zones.” Intact reporters stick to the first zone, but cleaved reporters stick to the second. That location of the visible line tells you whether the patient’s sample contains coronavirus.
The FDA is sufficiently confident in Sherlock’s test that it has granted an Emergency Use Authorization (EUA). This marks the first time a CRISPR-based tool has gotten any regulatory approval for use in humans. Sherlock Biosciences says it’s now working to manufacture and distribute the kits, which could get us one step closer to closing the testing gap.
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