Difference between revisions of "Team:Munich/Results"

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 targets^5,6. 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 restrictions^7-9. 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.

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.

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 to determine the theoretical detection limit of our system. Moreover, CascAID can be used to detect a wide spectrum of pathogens, as our experiments with gram-positive and viral targets suggested.

Cas13a can be used to detect specific RNA sequences.

Colorimetric read-outs

To couple CascAID with an easy read-out method we explored three colorimetric read-outs:

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.

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 itself¹³, bringing together the halves of the chromophore. Only then is the chromophore functional and produces the colorimetric read-out.

Software

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

References

1. Cohen, Limor, and David R. Walt. "Single-Molecule Arrays for Protein and Nucleic Acid Analysis." Annual Review of Analytical Chemistry 0 (2017).
2. Nakano, Michihiko, et al. "Single-molecule PCR using water-in-oil emulsion." Journal of biotechnology 102.2 (2003): 117-124.
3. Taniguchi, Yuichi, et al. "Quantifying E. coli proteome and transcriptome with single-molecule sensitivity in single cells." science 329.5991 (2010): 533-538.
4. Rissin, David M., et al. "Single-molecule enzyme-linked immunosorbent assay detects serum proteins at subfemtomolar concentrations." Nature biotechnology 28.6 (2010): 595-599.
5. Pardee, Keith, et al. "Rapid, low-cost detection of Zika virus using programmable biomolecular components." Cell 165.5 (2016): 1255-1266.
6. Slomovic, Shimyn, Keith Pardee, and James J. Collins. "Synthetic biology devices for in vitro and in vivo diagnostics." Proceedings of the National Academy of Sciences 112.47 (2015): 14429-14435.
7. Tang, Ruihua, et al. "A fully disposable and integrated paper-based device for nucleic acid extraction, amplification and detection." Lab on a Chip 17.7 (2017): 1270-1279.
8. Vashist, Sandeep Kumar, et al. "Emerging technologies for next-generation point-of-care testing." Trends in biotechnology 33.11 (2015): 692-705.
9. Gubala, Vladimir, et al. "Point of care diagnostics: status and future." Analytical chemistry 84.2 (2011): 487-515.
10. Abudayyeh, Omar O., et al. "C2c2 is a single-component programmable RNA-guided RNA-targeting CRISPR effector." Science 353.6299 (2016): aaf5573.
11. Gootenberg, Jonathan S., et al. "Nucleic acid detection with CRISPR-Cas13a/C2c2." Science (2017): eaam9321.
12. https://www.idtdna.com/pages/docs/technical-reports/in_vitro_nuclease_detectionD325FDB69855.pdf (retrieved: 13.10.17)
13. Anraku, Yasuhiro, Ryuta Mizutani, and Yoshinori Satow. "Protein splicing: its discovery and structural insight into novel chemical mechanisms." IUBMB life 57.8 (2005): 563-574.
14. Link, Stephan, and Mostafa A. El-Sayed. "Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles." The Journal of Physical Chemistry B 103.21 (1999): 4212-4217.
15. Zhao, W., Ali, M.M., Aguirre, S.D., Brook, M.A., and Li, Y. (2008). "Paper-based bioassays using gold nanoparticle colorimetric probes." Analytical Chemistry 80, 8431–8437.