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SHERLOCK (nucleic acid detection)

SHERLOCK (nucleic acid detection)

SHERLOCK (Specific High Sensitivity Enzymatic Reporter UnLOCKing) is a nucleic acid detection system based on CRISPR-Cas13a that can detect viruses, distinguish pathogenic bacteria, genotype human DNA and cell-free tumor DNA mutations.

The SHERLOCK system was developed by a team lead by Feng Zhang, assistant professor at the Broad Institute at MIT and Harvard described in Science in 2017. Collaborator Pardis Sabeti, a geneticist-virologist and professor at Harvard University, lead research published in 2018, which demonstrated the feasibility of using SHERLOCK for detecting Zika virus and dengue virus in field conditions, instrument-free, in patient body fluids (urine and saliva) at concentrations of 1 copy per microliter.SHERLOCK can quantify the amount of virus or cancer mutation in the sample. SHERLOCK can quickly be adapted to target new virus mutations when they arise. 



SHERLOCK works without a processing step, requiring lab equipment and personnel, to isolate DNA or RNA. A process called HUDSON (Heating Unextracted Diagnostic Samples to Obliterate Nucleases) was developed in the Sabeti lab which eliminates the need for the DNA/RNA extraction step by inactivating enzymes in bodily fluids that would otherwise degrade the DNA or RNA. 



SHERLOCK makes use of RNA guided RNase activity of Cas13a and collatoral RNase activity, which degrades non target RNA nearby. It uses the Leptotrichia wadei Cas13a ortholog, LwCas13a, along with the crRNA that targets the desired nucleic acid sequence. Either RNA or DNA viruses can be detected. Viral RNA from body fluids is first converted to DNA with reverse transcriptase. Sample DNA is amplified with recombinase polymerase amplification (RPA) followed by conversion to RNA with T7 RNA polymerase which is detected by the Cas13a-crRNA complex. Detection results in non-target RNA nearby, including a reporter RNA included in the SHERLOCK system. In the first version of CRISPR, the reporter RNA fluoresces when cleaved, indicating Cas13a-crRNA has found a matching target nucleic acid sequence. A second version of SHERLOCK uses paper strips where the readout can be viewed with the naked eye similar to pregnancy tests.The second version also uses three Cas13 enzymes and one Cas12a enzyme to sense multiple targets like dengue and Zika virus in a single reaction.

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Documentaries, videos and podcasts

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Sherlock: Detecting disease with CRISPR

13 April 2017

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