Last week, the FDA granted emergency-use approval for Sherlock Bioscience’s (Cambridge, MA) new coronavirus diagnostic. The test uses CRISPR genome editing to detect the presence of Sars-Cov-2, the virus that causes COVID-19. With the ability to return results in about an hour, this new diagnostic could be a game-changer in the race to accurately diagnose and track new COVID-19 cases. Today’s WEEKLY takes a look at this latest application of CRISPR techology.
SHERLOCK: The RNA Game is Afoot!
CRISPR genome editing (for a quick review, click here) uses the Cas9 enzyme to precisely cut target DNA in one specific location. The CRISPR-based diagnostic is based on a second Cas enzyme, Cas13, which recongizes and chops up RNA. Cas13 chops up not just its target RNA, but any other nearby mRNA too. Researchers call this mass destruction “collateral cleavage.”
Scientist at Sherlock Biosciences have developed a Cas13-based diagnostic consisting of a Cas13/guide RNA combo that targets virus-associated sequences. In addition to Sars-Cov-2, some of the world’s most dangerous viruses, including Zika and Ebola, have RNA-based genomes.
If a sample contains a targeted sequence, Cas13 homes in on it and cuts it. Easy peasy, right? But how do diagnosticians know if the enzyme has done its job? Short answer: reporter RNA comes with a label that can glow!
A label? No, not made of paper but molecular—the reporter RNA releases and activates a fluorescent tag only if and when the viral sequence has been cut. That is, only if the Cas13 enzyme finds its target and is activated. The Sherlock team calls the new diagnostic “Specific High-sensitivity Enzymatic Reporter unLOCKing” or SHERLOCK. The test has been adapted for use as a “lateral flow assay”—similar to an at-home pregnancy test. Think “First Response” for COVID-19.
The technology was originally developed at the Broad Institute (Cambridge, MA), with a license for limited exclusive rights to develop the technology for commerical use in the developed world was granted to Sherlock Biosciences. The Broad Institute retains the rights to use the technology in the developing world. (Article continues below)
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UC Berkeley scientists have developed a similar tool: DETECTR. This diagnostic, DNA Endonuclease Targeted CRISPR TransReporter, works with another Cas enzyme, CAS12a. This enzyme cuts a specific DNA sequence with the help of guide RNA. Like Cas13, it then goes nuclear, attacking nearby DNA sequences. In this case, the destruction includes fluorescently-labeled DNA reporter segments. The Berkeley team has demonstrated DETECTR’s ability to identify the human papillomavirus (HPV) in a patient blood sample. DETECTR should also be able to detect cancer and other disease-associated mutations in a patient DNA sample. San Francisco-based Mammoth Biosciences was launched to commercialize the development of this exciting new diagnostic platform.
CRISPR-based diagnostics promise extreme sensitivity—both the ability to detect vanishingly small amounts of DNA or RNA—and specificity, detection based on a specific gene sequence. This potent combination of attributes lays the groundwork for finding infectious diseases and cancers faster.