Two new CRISPR-based tests developed by academic groups have each taken a step toward providing portable, low-cost, sensitive SARS-CoV-2 diagnostics. A team at MIT and the Wyss Institute led by James J. Collins, publishing in Science Advances, describes an at-home saliva test that detects and reports the presence of different viral variants within an hour. The other COVID-19 diagnostic, published in Nature Chemical Biology by Jennifer Doudna’s group at the University of California, Berkeley, is a point-of-care diagnostic that enables results using swab testing within 20 min.
The MIT team developed the miSHERLOCK (short for minimally instrumented SHERLOCK) test. It is based on the CRISPR platform developed by Sherlock Biosciences (co-founded by Collins and Feng Zhang from the Broad Institute). The new assay uses Cas12a guide RNA to distinguish different variants with a limit of detection comparable to that of the gold-standard RT-PCR test: 1 molecule per microliter in unprocessed saliva. The team aimed to show it was possible to build a low-cost CRISPR-based test involving minimal instrumentation to detect SARS-CoV2 in raw samples, with results delivered to a smartphone app.
The competing paper from the Doudna lab combines two unrelated CRISPR nucleases—RNA-guided Cas13 and Csm9—in a tandem assay to provide a simpler alternative to PCR-based methods in a portable microfluidic chip. The one-step assay, named Fast Integrated Nuclease Detection in Tandem, differs from other approved CRISPR-based COVID assays in that it does not require an amplification step. The tandem nuclease technology boosts detection and amplifies the signal such that it can detect 30 copies per microliter of target RNA, well below the threshold of 100 copies per microliter necessary for diagnostic surveillance.