The biosafety of our Prime Editing complex has been a top priority throughout the entire development process. We have therefore tried to optimise all parts that influence the biosecurity of our system as much as possible. To ensure maximum biosecurity, we have created and tested many designs, as well as extensively researched alternatives and/or additional elements that contribute to biosecurity.
</p>
<H4title="PAM disrupt"></H4>
<H4text="PAM disrupt"></H4>
<p>
A key safety mechanism incorporated in our design of the Prime Editing complex is the disruption of the PAM sequence. For the nickase enzyme to function properly, it must bind directly to the DNA strand, a process that is facilitated by the presence of a specific sequence called the PAM (Protospacer Adjacent Motif). This critical interaction occurs through the recognition of the PAM sequence by the nickase itself. To achieve PAM disruption, the pegRNA (prime editing guide RNA) is specifically designed in a way so that the PAM sequence is situated within the reverse transcription template (RTT) of the pegRNA. By introducing a silent mutation within the RT template into the PAM sequence. Therefore the PAM sequence is effectively eliminated after the gene editing process is successfully completed [1]. As a result of that, the PAM sequence is no longer present on the DNA strand, preventing the nickase from binding again at the same location. This reduction in repeated or undesired binding of the nickase enhances the safety of our prime editing complex, minimizing the risk of unintended edits or off-target effects in subsequent steps. Ultimately, this feature contributes very much to the overall safety and reliability of the prime editing process.
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<H4title="pegRNA design - Spacer"></H4>
<H4text="pegRNA design - Spacer"></H4>
<p>
Biosafety is also guaranteed by the careful selection of the spacer, which plays a critical role in guiding the complex to its intended target site [2]. To ensure both precision and safety, we meticulously chose and rigorously checked the spacer using the CRISPick software [3]. This allowed us to evaluate whether our Spacer would be likely to target other regions than our target site and therefore allowing us to analyse and predict potential off-target effects, ensuring that erroneous edits are minimised. By optimising the spacer selection, we have not only significantly enhanced the overall editing efficiency, striking a balance between precision and performance, but especially ensured the utmost accuracy in directing the Prime Editor, further contributing to the safety of the editing process. [Bild 1]
</p>
<H4title="Riboswitch"></H4>
<H4text="Riboswitch"></H4>
<p>
Riboswitches are segments of an RNA strand that bind to small molecules, causing them to change their secondary structure by forming hairpin structures. This process regulates gene expression at the translation level by preventing ribosomes from binding at the RBS and translating the coding region on the RNA strand. 0For our project we also considered an ion-sensitive riboswitch, specifically dependent on sodium ions (Na⁺), as a regulatory mechanism. The secondary structure of this riboswitch prevents the binding of ribosomes to the ribosome binding site (RBS) under normal conditions, thus inhibiting the translation of the subsequent mRNA. When sodium ions bind to the riboswitch, a structural change occurs, exposing the RBS, which allows for the translation of the mRNA and the production of our fusion protein which is the main component of our prime editing system and therefore of enormous importance for it to work [4]. In the context of the CFTR mutation and its effects on the cell, the elevated Na⁺ levels play a crucial role. Due to the dysfunctional CFTR channel, which fails to properly function as a chloride channel, the ENaC channel (epithelial sodium channel) becomes upregulated. This upregulation results in an increased transport of sodium ions into the cell, leading to a higher intracellular sodium concentration. This elevated Na⁺ concentration creates a specific ionic environment that could potentially be utilized to regulate our Prime-Editing complex in a targeted manner. Given these specific ionic changes in the cell, we could have a disease-specific regulation of our Prime-Editing system based on the ionic situation typical of this condition. However, despite the initial promise of this approach, after further research, we concluded that the riboswitch, even considering the ion levels within epithelial cells, is overall too nonspecific and therefore too unreliable as a regulatory mechanism. Although the ion levels in CFTR cells are much lower, there are still low concentrations of sodium ions, which can lead to the riboswitch not being completely switched off.
[Bild 2]
As a further approach to developing alternative riboswitch variants, we considered the possibility of an RNA-regulated riboswitch targeting the defective mRNA sequence of the genetically defective CFTR gene. The basic idea behind this concept was that the riboswitch specifically binds to a region on the CFTR mRNA containing the F508Δ mutation. This binding should induce a structural change in the riboswitch on our prime editing complex’s mRNA that ultimately leads to exposure of the RBS to allow translation of the downstream sequence. This mechanism would be designed to react specifically to the defective CFTR mRNA and only cause a change in the secondary structure in the presence of the specific mutation. The riboswitch could thus ensure selective and disease-specific activation of our prime editing complex, which would be of particular interest in the context of genetic diseases such as cystic fibrosis. However, we did not pursue this approach any further. A major reason for this was the lack of sufficient literature providing a sound scientific basis for this specific application of a riboswitch. In
addition, our research steered us in a different direction, particularly with regard to the alternative mechanism involving the XBP1 intron to regulate the prime editing system. This alternative seemed more promising and was based on an established regulatory mechanism that is triggered by cellular stress and specifically responds to misfolding processes.
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<H4title="XBP1 Intron"></H4>
<H4text="XBP1 Intron"></H4>
<p>After extensive research, we discovered a regulatory system in eukaryotic cells, the XBP1 mechanism. The activation of XBP1 is an important mechanism that occurs as part of the Unfolded Protein Response (UPR), a cellular stress response triggered by the accumulation of misfolded proteins in the endoplasmic reticulum (ER). The ER is a key cellular component responsible for protein folding and transport. When many misfolded proteins accumulate in the ER, a specific regulatory mechanism is activated to reduce the stress on the ER. XBP1 activation is controlled by a protein called IRE1α, which is embedded in the ER membrane. IRE1α acts as a sensor for protein misfolding stress in the ER. Once IRE1α detects misfolded proteins, it dimerizes and becomes activated through autophosphorylation. This activation switches on the endoribonuclease activity of IRE1α, which is a crucial step in the activation of XBP1. The mRNA for XBP1 is continuously transcribed in the nucleus and transported to the cytoplasm, where it contains an intron that is not normally spliced out. This intron contains a stop codon, preventing the translation of a functional XBP1 protein. However, when ER stress activates IRE1α, the endoribonuclease domain of IRE1α splices this intron out of the XBP1 mRNA. This is an unconventional splicing event, as it occurs in the cytoplasm rather than in the nucleus. Once the intron is removed, the spliced XBP1 mRNA can be translated into a functional XBP1 protein. This activated XBP1 acts as a transcription factor, turning on genes that increase the protein-folding capacity of the ER and promote the degradation of misfolded proteins. In this way, XBP1 helps the cell cope with ER stress and restore balance in the protein-folding process. Thus, this mechanism originally functions within the cell in the context of ER stress to maintain ER function when protein folding is disrupted. [5] [6] Our idea was therefore to integrate this intron into the mRNA encoding our prime-editing complex and thus use this mechanism to ensure that a functional prime editor is only synthesized when there is a high accumulation of misfolded proteins in the cell (similar to F508del). This would therefore represent an optimal safety aspect, as our fusion protein, which is essential for prime editing, cannot be fully synthesised as long as the genetic defect is not present in the cell. Accordingly, this provides the security that no healthy cells, as well as correctly edited cells, cannot be edited, which is an enormous contribution to biosafety. However, there was too much uncertainty about the extent to which other factors, such as misfolded proteins that are not associated with the CFTR protein, play a role in this mechanism. And since we could not and did not want to take the risk of such factors initiating the system, we decided against using it. To clarify this unknown correlation, we have considered a future experiment in which we want to switch this intron in front of a fluorescent marker and express it in cells with defective CFTR in order to confirm/investigate the dependence of intron splicing and the presence of CFTR F508del.</p>
</Subesction>
<Subesctiontitle="Safety aspects of our Airbuddy"id="Biosafety2">
<H4title="SORT LNP and Cytotoxicity"></H4>
<H4text="SORT LNP and Cytotoxicity"></H4>
<p>
We have carefully considered the biosafety aspects of our delivery system, starting with the decision between Adeno-associated viruses (AAV) or lipid nanoparticles (LNPs) as delivery systems. Our comparison revealed that the biocompatibility and safety of LNPs are paramount for our approach. That is why we chose selective organ-targeting (SORT) lipid nanoparticles (LNPs) [1] in the context of targeted pulmonary mRNA delivery. One of our primary concerns with the LNP was the potential cytotoxicity of polyethylene glycol (PEG), a common stabilizing agent in LNP formulations. Aware of the immune responses PEG can trigger, potentially leading to cytotoxicity [2], we aimed at optimizing its concentration in our SORT LNPs to minimize such reactions while maintaining therapeutic efficacy. By the use of low molecular weight PEG, we addressed this problem. To test weather our approach succeeded, we conducted MTT and proliferation assays to ensure that our LNP posed no cytotoxicity risks.
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<H4title="Precision of our SORT LNP"></H4>
<H4text="Precision of our SORT LNP"></H4>
<p>
To further improve safety, we focused on reducing off-target effects. By incorporating specific SORT molecules, such as permanently cationic lipids like DOTAP, we ensured that the nanoparticles are systematically directed to the lungs. This precise targeting is particularly beneficial for respiratory diseases, as it enhances therapeutic effectiveness while limiting the impact on non-target organs. Our outlook of antibody conjugation as surface modification of our LNP for cell type-specific delivery, more exactly club cells [3] and ionocytes [4] as CFTR-expressing lung epithelial cells, would round off this aspect.
