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Description
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Thanks to advances in molecular biology and biochemistry, scientists have been able to consistently detect lower and lower concentration of molecules1, to the point that single molecules can be reliably recognized with methods such as polymerase chain reaction (PCR)2, fluorescence in situ hybridization (FISH)3 and enzyme-linked immunosorbent assays (ELISA)4. This has opened doors for synthetic biology to create better and more accurate diagnostic tests that use biomarkers like nucleic acids and proteins as targets5,6. Through such advances, the field of molecular diagnostics developed. Unfortunately, current standard methods require expensive equipment or trained personnel, which generally limits their usability to hospitals or laboratories. Recently, there has been a push to develop new tests that fuse the reliability of standard methods with affordable platforms such as lab-on-a-chip or paper strips to overcome this restrictions7-9. We wanted to help close this gap and set out to engineer a diagnosis principle for the detection of a wide array of targets that could be used without difficult-to-meet technical requirements..
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CascAID
Our project, which we named Cas13a controlled assay for infectious diseases (CascAID), features the recently identified CRISPR/Cas effector Cas13a10. Unlike other proteins in the familiy, Cas13a has the unique ability to bind and cleave specific RNA targets rather than DNA ones. Moreover, after cleaving its target, Cas13a is able to unspecifically cleave RNA molecules. By using this collateral activity from Cas13a, our system is capable of detecting virtually any RNA target. This is done by changing the crRNA in the protein, that is a short RNA sequence that determines what is recognized as target.
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Cas13a binds specific target RNA depending on the crRNA sequence. After activation, Cas13a cleaves RNA indiscriminately.
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We wanted to start our project by showing that Cas13a's collateral activity could be used to detect the presence of specific RNA. For this, we used the RNAse alert system, as done in a recent publication11, to detect RNA digestion. In this assay, the presence of RNAse-like activity is detected by an increase in green fluorescence. Our experiments yielded a convincing proof-of-principle which we went on to model. Moreover, CascAID can be used to detect a wide spectrum of pathogens, as our experiments with gram-positive and viral targets suggested. As we wanted to make CascAID available for everyone, we focused on building an inexpensive fluorescence detector to measure the presence of the target. Our detector “Lightbringer” was designed to be able to detect the fluorescence produced by the fluorescein in the Rnase alert system12, but we theorize that changing the filters allows detection of other fluorophores. In addition, we experimented with freeze-drying on paper to make CascAID durable and easily portable.
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Cas13a can be used to detect specific RNA sequences
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Picture of the Thermocycler
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For RNA extraction from the samples we tested three methods: extraction with silica beads, extraction with silica membrane and heat lysis. We custom-built an affordable thermocycler for signal amplification by RT-PCR to improve the detection limit. We explored recombinase polymerase amplification (RPA), an isothermal amplification procedure, to use over more conventional PCR methods as its simplicity makes it the more attractive option.
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Colorimetric read-outs
To couple CascAID with an easy read-out method we explored three colorimetric read-outs:
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AeBlue: The RNA strand in a specially designed RNA/DNA dimer is cut by Cas13a's collateral
activity. After digestion, the interaction between the two strands is too weak to hold the dimer and it
decays. We can then use the DNA-strand as template to translate the chromoprotein aeBlue.
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Diagram of aeBlue
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Intein-Extein: By binding TEV-protease with a RNA-linker we can use Cas13a's collateral activity
to regulate the protease's diffusion and use it to cleave a TEV tag separating the intein regions of a
modified chromophore. After the first cleavage, the intein segment excises itself, bringing together the
halves of the chromophore. Only then is the chromophore functional and produces the colorimetric
read-out.
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Diagram of Intein-Extein
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Gold nanoparticles: Gold nanoparticles coated with short DNA sequences are held closely
together by a complementary linker RNA, which makes the solution intense blue. Activated Cas13a cuts
the linker RNA, causing the nanoparticles to diffuse away from each other. This increase in distance
causes a color change to intense red. |
Gold nanoparticles
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Colorimetric read-outs
To help facilitate the design of crRNA, the sequences that give CascAID its specificity, we developed a
software tool that checks crRNA for unwanted secondary structures. This gives valuable insight on
whether the sequence is suited to use with Cas13a or whether some modifications are needed.
Together with Team Delft's software tool which designs the corresponding crRNA based on the target,
we collaborated to develop a powerful tool that suggests crRNA sequences and checks their usability
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