CRISPR-Cas Systems: Advanced Mechanisms and Engineering
CRISPR as a Programmable Platform
Mechanistic core
CRISPR systems use RNA-guided recognition to direct a nuclease, nickase, or catalytically dead effector to a defined DNA or RNA sequence.
Advanced concept
The guide RNA is not just a targeting molecule; it encodes specificity, influences kinetics, and can be engineered to tune activity.
Cas Diversity and Functional Variants
Common CRISPR tool classes
Cas9 nuclease
- Creates double-strand breaks
- Widely used for knockouts and HDR
- Requires PAM recognition
Cas12
- Distinct targeting and cleavage properties
- Useful for some multiplexing and diagnostics
- Different PAM requirements
What is the role of the PAM in many CRISPR systems?
The PAM is essential for Cas recognition and cleavage in many CRISPR systems.
Correct answer: It is a short motif required for target recognition by the Cas protein
Why can dCas9 be useful even though it does not cut DNA?
Catalytically dead Cas proteins retain targeting ability and can recruit functional effector domains.
Correct answer: It can be fused to regulators to control transcription or chromatin state
Specificity engineering
Specificity can be improved through careful guide selection, truncated guides, high-fidelity Cas variants, and delivery strategies that reduce exposure time.
Sources of CRISPR off-target risk
| Source | Consequence |
|---|---|
| Guide mismatch tolerance | Unintended cleavage at similar sequences |
| Prolonged exposure | More opportunity for off-target activity |
| High nuclease expression | Increased cutting probability at weak sites |
| Repetitive genome regions | Ambiguous targeting and mapping |
Which design choice can reduce off-target editing?
High-fidelity Cas proteins are engineered to preserve on-target activity while reducing off-target cleavage.
Correct answer: Using a high-fidelity Cas variant
What does 'multiplex editing' mean in a CRISPR context?
This is achieved by expressing multiple guides or targeting multiple sites in one construct.
Correct answer: Editing multiple loci simultaneously