What is a Photo-Controlled CRISPR-based Diagnostic?
Photo-controlled CRISPR is an advanced diagnostic method using light to turn CRISPR activity on or off with high precision in time and space. Light-sensitive molecules, also known as photocages or photo-responsive nanomaterials, enable optochemical control of CRISPR systems by temporarily blocking or inactivating guide RNA (crRNA, sgRNA) or Cas proteins. Photo-controlled CRISPR systems enable the design of potent and dependable tools in modern nucleic acid detection, due to their high specificity, rapidity, sensitivity, ease of use, and broad applicability.
Exposing photo-caged systems to ultraviolet (UV) or near-infrared (NIR) light removes the photo-blockage, triggering release of the active CRISPR complex and activation of CRISPR-based gene editing or diagnostics precisely where and when the light is targeted, reducing unwanted off-target effects.
Light-activated crRNA improves one-pot diagnostic assays by preventing premature reactions during amplification, lowering contamination risks, and boosting sensitivity. Photo-controlled CRISPR can act as a powerful molecular switch, enhancing traditional CRISPR biosensing via "one-pot" diagnostics.
Near-infrared light systems enable targeted control of gene expression and multiplex editing in specific cells or tissues without complex genetic engineering. Nanoparticle-based, light-activated CRISPR systems also enable light activation within tumor environments when triggered by external light sources.
Traditional rapid diagnostic tests usually require two steps:
(1) nucleic acid amplification of target DNA/RNA sequence and
(2) CRISPR detection, selectively finding the target and generating a signal.
In classical "one-pot" assays, the active CRISPR enzymes, such as Cas12a, prematurely digest the DNA primers and templates, drastically reducing amplification efficiency and sensitivity. By attaching light-sensitive chemical modifications to the guide RNA, the CRISPR system remains 100% inactive during amplification. However, once amplification finishes, light activation instantly activates the CRISPR enzymes to read out the signal without any interference.
One challenge in existing photo-controlled CRISPR diagnostics is the reliance on modifications to the crRNA spacer region, which must be tailored to each target sequence. This approach requires optimizing caging sites, particularly because chemical modifications like NPOM-dT are most effective on thymidine (T) or uracil (U) bases. Spacer sequences lacking sufficient U residues limit the design of light-activated CRISPR systems. To overcome this constraint, Tian et al. focused on the conserved direct repeat (DR) region of the crRNA, which is essential for Cas12a activity. The DR contains a repeat recognition sequence (RRS) forming a pseudoknot structure critical for crRNA processing and function. Systematic mutation analysis identified the third and fourth nucleotide of the RRS as essential for both cis- and trans-cleavage activities. Single-base mutations at these positions abolished cleavage without disrupting target binding or recognition, suggesting a distinct mechanism for activity loss. These observations led to a photo-regulation strategy that introduced NPOM caging at the RRS-4 position (U+4), which effectively blocks Cas12a activity and is fully restored upon UV exposure.
Unlike spacer-based caging, this approach is universally applicable, as the DR sequence is conserved across targets.
A further improvement is split crRNA design, in which the DR and spacer regions are synthesized separately. A 10+26 nt split crRNA retains full activity and allows pre-preparation of the caged DR fragment, reducing costs and simplifying adaptation to new targets.
A split-Cas9/dCas9 system in which activation is achieved through a near-infrared photo-cleavable dimerization complex can be used in vivo in humans. According to Zhang et al., this system is adaptable to different split-Cas9/dCas9 systems, enabling rapid, spatially precise light activation across various cell types.
In a minimalist plug-and-play approach, the repeat region (rRNA) of a split crRNA is modified with photolabile groups. Since the modifications are done post-synthetically and independent of the spacer region (sRNA), every rRNA, regardless of its target sequence, can be activated by light irradiation alone. This method is called POIROTv2 (Photo-Initiated CRISPR-Cas12a system for Robust One-pot Testing, version 2).
Types of Photo-controlled CRISPR systems
- PC-linker RNA caged
- NPOM caged
- G4 caged
- PC and PS DNA caged
- ACR caged
References
Photocleavable-guide-RNAs-(pcRNAs)-enable-light-mediated,-rapid,-and-complete-deactivation-of-CRISPR-Cas9-systems
Use-of-Photocleavable-Linkers-in-CRISPR-Guide-RNAs
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