Cas9 & Cpf1 secretion
and activity
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<div class="page-heading">Integrated human practices: design of OUTCASST</div> | <div class="page-heading">Integrated human practices: design of OUTCASST</div> | ||
Altogether, we gathered information from many different fields and perspectives. All of these views helped us to optimize our design to use OUTCASST as a tool for diagnosing Chagas disease. Since we want our tool to be easy to use in the field and rural areas, there are different aspects that should be taken into account. Robustness and resistance of the toolkit to temperature fluctuations and humidity are chief among these. In addition, it is important not to rely on material and storage containers such as fridges or freezers (Marit de Wit, Doctors without borders). | Altogether, we gathered information from many different fields and perspectives. All of these views helped us to optimize our design to use OUTCASST as a tool for diagnosing Chagas disease. Since we want our tool to be easy to use in the field and rural areas, there are different aspects that should be taken into account. Robustness and resistance of the toolkit to temperature fluctuations and humidity are chief among these. In addition, it is important not to rely on material and storage containers such as fridges or freezers (Marit de Wit, Doctors without borders). | ||
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+ | The OUTCASST toolkit consists of dried cells in a closed off compartment, two medium compartments (green) and a lysis and reset buffer compartment (purple). | ||
+ | <br> | ||
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+ | First, the seal on one of the medium compartments is broken so medium goes onto the cells. The cells will now be rejuvenated and allowed to acclimatize in 12 hours. | ||
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+ | |||
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+ | After 12 hours, a patient's sample can be added to the test kit. The sample will enter the filtering compartment, which contains chemicals that bind all sorts of unwanted chemicals. | ||
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+ | The seal on the lysis buffer compartment is broken. The lysis buffer causes any intact cells to burst, releasing their DNA and internal contents. Unwanted cell debris is bound by the filter molecules, coated to the inside of the filtering compartment. | ||
+ | <br> | ||
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+ | The seal on the reset buffer compartment is broken. Reset buffer, sample and lysis buffer mingle, neutralizing each other and resulting in an isotonic mixture that will not harm the sensor cells. | ||
+ | <br> | ||
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+ | The second medium pocket is used to resuspend the gRNA. We kept the gRNA dry until now to prevent degradation of this sensitive chemical. | ||
+ | <br> | ||
+ | <br> | ||
+ | <a href="javascript: void(0);" class="btn blue" id="goto-7" style="margin: 0; padding: 10px 25px; font-size: 18px;">Next</a> | ||
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+ | |||
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+ | The dissolved gRNA is released onto the sensor cells. The protein chains on the surface of these cells can now bind with the gRNA, making the sensor cells specific for DNA that is complementary to the gRNA they were provided with. | ||
+ | <br> | ||
+ | <br> | ||
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+ | |||
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+ | The seal to the waste compartment is broken. As this compartment was under a slight vacuum, a part of the medium is sucked away from the cells, making room for the sample. | ||
+ | <br> | ||
+ | <br> | ||
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+ | |||
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+ | The seal to the waste compartment is closed again and so are the seals to the medium pockets. The lysed and filtered sample, containing the patient's DNA, is now brought to the cells. If the right DNA sequence is present, the cells will detect it and give an output signal. | ||
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The tool should be cheap to produce and easy to use. To achieve this goal, the tool needs to have a closed box design wherein only the blood sample has to be added and a simple protocol can be followed to perform the test (Jet Bliek and Ruud van den Bogaard, Academical Medical Center Amsterdam: clinical genetics). Disposal of the system also needs to be considered (collaboration RIVM National Insitute for Public Health and the Environment). | The tool should be cheap to produce and easy to use. To achieve this goal, the tool needs to have a closed box design wherein only the blood sample has to be added and a simple protocol can be followed to perform the test (Jet Bliek and Ruud van den Bogaard, Academical Medical Center Amsterdam: clinical genetics). Disposal of the system also needs to be considered (collaboration RIVM National Insitute for Public Health and the Environment). | ||
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Revision as of 02:29, 2 November 2017
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There are also some things in the OUTCASST toolkit that need to be changed in comparison to the experimental approach in order to prepare the system for diagnosing Chagas disease. One of these things is the use of HEK293T cells, which need a very stable environment to stay alive. In the eventual tool, we will need to use cells that are more resistant to environmental fluctuations yet still cannot survive outside of the device (Patrick van Zon and Pieter-Jaap Krijtenburg, University Medical Center of Utrecht: genome diagnostics.) We also used a fluorescence signal as output in the experiments, which requires a fluorescence microscope to analyse the test results. To avoid the need of these and other equipment, we would ideally use an output signal in the form of visible light or, more promisingly, a change of color that is visible to the naked eye. Another thing we should keep in mind is the time it takes to get the results from our test device (Marit de Wit, Doctors without Borders).
There are also technical aspects that should be considered, like the method used for lysis of the parasites in the sample. Lysis needs to occur to get free DNA, i.e. a hypotonic solution (Jaap van Hellemond, Erasmus University Medical Center Rotterdam: parasitology). If a colorant is used as reporter mechanism, we need to remove the red color of heme groups from red blood cells, too, as it would interfere with the output signal.
Lastly, we should consider the target DNA we want to use to detect the parasites. Things that require careful consideration are GC-content, which has influence on binding affinity and specificity of the guide RNA. Specificity needs to be mutation specific as a strand with different base pairs should, ideally, not activate the system (Hans Bos and Hugo Snippert, University Medical Center Utrecht: cancer research).
Toolkit design solutions
The OUTCASST toolkit has a closed box design, wherein all the components to perform the test are present in distinct compartments, separated by seals. These seals can be broken by applying pressure on them.As was stated earlier, a lot of variables need to be kept constant to keep the HEK293T cells alive. Because of this, it is not feasible to use these cells in our design. Instead, we opt to use air-dried cells from the anhydrobiotic insect, Polypelidum vanderplanki, which can be stored at room temperature for 251 days and can restart proliferating again after rehydration 1. This way the shelf life of our tool can also be prolonged. To prevent the risk of our GMO getting out in the environment, several mechanisms and kill-switches will be incorporated in the cells, so they can only survive in our closed box system, in their resurgent state. This can be done by manipulating the metabolism, so that the cells can’t produce a crucial substance for survival, e.g. an amino acid, which will be added in the toolkit medium. In case the cells get out of the toolkit, they will die because of the absence of the crucial substance.
Rehydration can be done with a suitable medium. This has to be done one hour before use. The seal between the dried insect cells and the medium can be broken to pump the medium manually to the cells. After rehydration, the medium can be manually pumped to the waste compartment.
The next step is to add the two guide RNA’s to the revived cells. The gRNA’s are present in the design as dry powder to prevent premature degradation. This time, two seals need to be broken. First, the gRNA needs to be dissolved with the contents of another medium compartment. Then the medium with gRNA can be pumped to the cells where they will bind to dCas9 and dCpf1 on the extracellular cell membrane. This process takes about 10 minutes and after that, the medium with excessive gRNA can also be pumped to the waste compartment.
These are the preparation steps before the real diagnosis can start. First off, a blood sample has to be taken from a patient that might be infected with Chagas disease. To prevent the blood from clotting, heparin or EDTA can be added to the sample. The blood sample can then be introduced to the tool, after which the device needs to be sealed. To get access to the parasite DNA, all cells need to be lysed, including the red blood cells. This is done with a lysis buffer, a hypotonic solution.
The next step is to pump everything to a next compartment, wherein there are heme-binding compounds (such as HEBP) linked to the inside surface to decolorize the sample. Then a hypertonic resetting buffer is added to return the sample to isotonic levels, in order to prevent damage to the detector cells.
Now, the seal separating the sample from the cells can be broken and the sample can be introduced to the actual sensor. A color signal will appear after about 10 to 12 hours in case the patiënt is infected and will continue to become more visible after that.
The output signal will be a blue chromoprotein. This way, the sample color will become blue (or purple if there is still a little bit of heme in the sample) upon detection of the targeted DNA sequence.
After use, the tool should be disposed of in a safe manner preventing it to end up in the environment. Therefore, there will be a disposal guideline added in the toolkit manual. The test can be disposed of in a self-sealing bag, which can be boiled after the test is completed to minimize the risks.
Additional considerations
There are still a lot of things that should be considered to make the OUTCASST tool optimal for diagnosing Chagas disease.The first thing we still need to consider is the blood sample size needed to perform the test. From the patients aspect it would be best to take as little as possible. A smaller blood sample would also mean that the device can be made smaller, which in turn also makes the production costs for one test cheaper. However, there needs to be enough pathogen DNA in the blood sample to make sure that the test gives the right results. It would be possible to pretreat a larger sample to concentrate it before applying, increasing the chance of correct diagnosis, but this would again require skilled professionals and materials.
We have also thought about a question that was raised at the University Medical Center at the Cancer department. The question was why we wanted to express our system on the membrane of eukaryotic cells and not just express it intracellularly in bacteria. Then a blood sample could be added and the bacteria can be heat shocked to get the pathogen DNA intracellular, activating the binding of the two proteins. In this case, there would be a loss of the amplification step, since the transcription factor is then able to activate the reporter gene without a signal or cleavage of the transcription factor. Since we don’t know what the minimum amount of blood needed is, we wanted to design it in the way we can get the most signal, which is to include the amplification step. If it would prove that this amplification step is not needed, we could also just put the proteins in the tool and use a split reporter. On the other hand, the tool would not rely on use of living cells, which would make the use of our tool a whole lot safer.
We should also consider the material that the device is going to be made of. It should be of sturdy quality to prevent contamination of the environment with the device’s content. From the production perspective, the costs to produce it should be as low as possible to make the tool affordable. A main issue with costs, currently, is the production of the gRNA as it is expensive to synthesize.
We have also heard that the tool should have a low incidence of false positive and negative results and that our device should distinguish DNA strands with one different base pair. We want to take this information into account to design the target DNA. There are two possibilities from which we can choose. The first option would be to permit certain mutations in the target DNA, to prevent getting a false negative result in some cases. The second option would be to use a very conserved domain as target DNA and don’t allow any mismatches. From our perspective, we think the second option would be more suitable, since the specificity in our system is a very valuable aspect of the design. We have chosen to use the satellite DNA, which is present in the T. cruzi parasite as a 195 base pair repeat with about 100,000 copies (Aldert Bart, Academical Medical Center Amsterdam: Clinical molecular parasitologist).